Optical Fiber Communication System

Objective: This course is designed to develop proficiency in profession

in telecom sector. More specifically for an Optical Fiber Network engineer who

is responsible for efficient Installation, Testing and Splicing of optical fiber

cable. It aims to aware them of step by step proces s for laying optical fiber

cable, Testing and fault finding by using an OTDR.

This Course is divided into three Modules:

Module 1: Technical Knowledge of Optical Fiber S ystems

Module 2: Installation of Optical Fiber Cable

Module 3: Testing and Splicing of OFC

Module 1: Technical Knowledge of Optical Fiber Sys tems

Contents

1. Overview of Optical Fiber Communications

2. Key Advantages of fiber over copper wire

3. Structure of fiber optics cable

4. Principle of fiber communications

5. Types of optical fiber cable

6. Optical fiber characteristics

7. Fiber Optic Connector Types

8. Common Optical Parameters (KPI)

9. Test equipments and Functionality

10. Analyzing test values

1. Overview of Optical Fiber Communications

One often sees articles written about fiber optic communications networks that imply that fiber optics is "new." That is hardly the case. The first fiber optic link was installed in Chicago in 1976. By 1980, commercial long distance links were in use and fiber optic data links for RS-232 were available. Since that beginning, fiber has become very commonplace - one should say dominant - in the communications infrastructure.

If you make a long distance call today, you are undoubtedly talking on fiber optics, since it has replaced over 90% of all the voice circuits for long distance communications. Most large office buildings have fiber in the building itself. Only the last link to the home, office and phone are not fiber and installations of fiber to the home are growing rapidly.

CATV also has discovered fiber optics, along with compressed digital video. Most large city CATV systems have been converted to fiber optic backbones which allow voice and data transmission in addition to video.

The LAN backbone also has become predominately fiber-based. The back-end of mainframes and storage area networks (SANs) are almost totally fiber. Only the desktop is a holdout, currently a battlefield between the copper and fiber contingents.

Fiber optics offers an unrivaled level of security. It cannot be easily jammed or tapped and is immune to interference. It is widely used for security cameras, perimeter alarms and other critical systems in military, government, utility and civilian applications.

Keeping in mind the user demand for high speed packet services internet architecture that is now at a gradual stage of evolution by resorting to fiber optics technology; subscriber will receive the best quality data transmission that is accessible through television, internet and home phone.

Fiber optics is replacing copper wire as an appropriate means of transmission medium. A fiber-optic system is similar to the copper wire system the only difference being fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Organizations today are looking to control hardware expenses i.e. upgrading existing hardware, limiting e-mail storage, qualifying new vendors, outsourcing backups etc. With a number of networking equipments and cabling options available, one should plan well and identify the best infrastructure to maximize your return on investment.

2. Key Advantages of Fiber over Copper Wire

• All Dielectric

o Low Signal Radiation

o Secure Transmission

o RFI and EMI Immunity

o High Voltage Installations

• Small Size

o Less Duct Space

o Fewer Additional ducts Installed

• Low Attenuation

o Greater Distance/Fewer Repeaters

o Less Installation and maintenance

• Optical Signal

o No ground Loops

o No Spark Hazard

o Operation in Flammable Area

• High Bandwidth

o Future Signal Capability Expansion

2.1 Secure communication (low signal radiation):

Data traveling over fiber is very secure. No electronic eves dropping can be used making it extremely difficult to “Tap” fiber optic cable. Also, if tapped it is very easy to determine that cable is tapped.

2.2 Immunity from EMI/RFI/High Voltage/Lightning:

Fiber Resulting in clean, error free data transmission over long distances bit error rates of better than 10-9. Fiber is made of glass or plastic all dielectric material which is not affected by proximity to high voltage or lightning. Fiber optic cable can be routed in same ducts or cable trays as high voltage or power cables. Saving cost and providing ease of installation.

2.3 Small Size:

Fiber optic cables are physically very small and light weight, saving space, weight, and allowing ease of installation.

2.4 Lightning damage:

Particularly outdoors between buildings etc. can be avoided with use of F.O cable. If lightning strikes one plant it does not travel to other parts of same plant as F.O cable provides electrical isolation.

2.5 No Ground Loops:

Expensive grounding and shielding not required – saving on installation cost. Also, protects equipment from high voltage damage.

2.6 Low Loss (low signal attenuation):

Data can be sent long distances error free. With proper F.O. equipment and cabling 100 Km is possible without repeaters.

2.7 High data rate and High bandwidth:

Large data files such as engineering drawings can be transmitted in seconds. This allows main computers or process controllers to be in a more controlled and safe area away from plant. Also, future proofs installation.

2.8 No Spark Hazard:

Extremely important in chemical plants, oil refineries and other hazardous locations, since fiber cable is made of glass or plastic, if cut no electrical sparks. This way cable cut cannot start a fire.

2.9 Low Maintenance Cost:

The fiber optics is not water or chemical sensitive as they are made of glass and are generally free from corrosion. There is no risk of being destroyed by any harsh elements and can bear the living conditions that conventional copper cables fail to, for instance direct contact with the soil. The maintenance and service expenses are much lower than the copper wires owing to this.

2.10 Safety and Efficiency:

The fiber optics has what is called unlimited bandwidth that helps in making the transmission of data with greater flexibility. It provides a bandwidth that is 1000 times more than the copper wires. As a result, what you have is a fast connection that operates in circles in and around the bandwidth that is given by the cable connections. At the same time, the fiber optics provides greater data safety.

2.11 Picture Quality:

Fiber optics generally use advanced technology that is powerful than the copper cables. The GBIC transceiver deserves a mention here. Users can receive good picture quality of high definition.

3. Structure of fiber optics cable

An optical fiber is a cylindrical dielectric waveguide (non-conducting waveguide) that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber. Structure of Fiber-Optic Cables is made up of:

• Core

• Cladding

• Coating

The typical fibers today are made out of glass or plastic since it is possible to make them thin and long. Also both glass and plastic are transparent at particular wavelengths, which allow the fibber to guide light efficiently. The fiber is constructed with a core with a high index surrounded by a layer of cladding at a lower index. The core and the cladding can be made out of both plastic and glass. For plastics, the core can be polystyrene or polymethylmethacrylate and the cladding is generally silicone or Teflon. For glasses both the cladding and the core are made out of Silica with small amounts of dopants such as boron, germanium to change its index. Major differences exist between the two materials when it comes to making the optical fiber. In plastic core fibers they are more flexible and inexpensive compared to glass fibers. They are easier to install and can withstand greater stresses and weight 60% less than glass fibers. However, they transmit light less efficiently leading to high losses, giving them very limited use in communication applications. Such plastic fibers are practical for short runs such as within buildings. Therefore, due to their restrictive nature glass core fibers are much more widely used because they are capable of transmitting light effectively over large distances.

4. Principle of fiber communications

When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle" for the boundary), the light will be completely reflected. This effect is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA.

4.1 Refraction

Total Internal Reflection is the basis of fiber-optic communication. Total Internal Reflection may be considered to be an extreme case of refraction. When a light ray strikes a boundary of two materials with different RIs, it bends, or in other terms, refracts to an extent that depends on the ratio of the RIs of the two materials. The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

4.2 Refractive Index

Refractive index of an optical medium = Speed of light in a vacuum (300,000,000 meters per

second) / speed of light in the optical medium

5. Types of optical fiber cable

• Single-mode fiber

• Multi-mode fiber

• Step index fiber

• Graded index fiber

5.1 Single-mode fiber

Single mode fiber optic cable has a small diametric core that allows only one mode of light to propagate. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the ability for the signal to travel faster, further. This application is typically used in long distance, higher bandwidth links. Single mode fiber is usually 9/125 in construction. This means that the core to cladding diameter ratio is 9 microns to 125 microns.

A peculiar property of single-mode fibers is that the transverse intensity profile at the fiber output has a fixed shape, which is independent of the launch conditions and the spatial properties of the injected light, assuming that no cladding modes can carry substantial power to the fiber end. The launch conditions only influence the efficiency with which light can be coupled into the guided mode. Intermodal dispersion can of course not occur in single-mode fibers. This is an important advantage for the application in optical fiber communications at high data rates (multiple Gbit/s), particularly for long distances. Essentially for that reason, and partly because of their tentatively lower propagation losses, single-mode fibers are exclusively used for long-haul data transmission, and nearly always for outdoor applications even over shorter distances. For short-distance indoor use, multimode fibers are more common, mostly because that allows the use of cheaper multimode data transmitters based on light-emitting diodes instead of laser diodes.

Conditions for Efficiently Launching Light into a S ingle-mode Fiber:

Efficiently launching light into a single-mode fiber requires that the transverse complex amplitude profile of that light at the fiber's input end matches that of the guided mode. This implies

• that the light source has a high beam quality (with M2 ≈ 1) • that the light has a focus at the fiber's input end (for matching the plane wavefronts of

the fiber mode) • that the beam profile has the correct size and shape and is precisely aligned

(concerning position and direction) to the core. More precisely, the error in position must be well below the beam radius, and the angular misalignment must be small compared with the beam divergence of the mode.

Generally, a long-term stable efficient launch of a free-space laser beam into a single-mode fiber requires well designed mechanical parts, which allow precisely aligning and keeping fixed the focusing lens and the fiber end while not exhibiting excessive thermal drifts.

5.2 Multi-mode fiber

Multimode fibers are optical fibers which support multiple transverse guided modes for a given optical frequency and polarization. The number of guided modes is determined by the wavelength and the refractive index profile. For step-index fibers, the relevant quantities are the core radius and the numerical aperture, which in combination determine the V number. For large V values, the number of modes is proportional to V2. Particularly for fibers with a relatively large the number of supported modes can be very high. Such fibers can guide light with poor beam quality (e.g. generated with a high-power diode bar), but for preserving the beam quality of a light source with higher brightness it can be better to use a fiber with smaller core and moderate numerical aperture, even though efficient launching can then be more difficult. Compared with standard single-mode fibers, multimode fibers usually have significantly larger core areas, but also generally a higher numerical aperture of e.g. 0.2–0.3. The latter leads to robust guidance, even under conditions of tight bending, but also to higher propagation losses without bending, as irregularities at the core–cladding interface can scatter light more effectively. A basic specification of a multimode fiber contains the core diameter and the outer diameter of a multimode fiber. Common types for fiber-optic communications are 50/125 µm and 62.5/125 µm fibers, having a core diameter of 50 µm or 62.5 µm, respectively, and a cladding diameter of 125 µm. Such fibers support hundreds of guided modes. There are also large-core fibers with even substantially larger core diameters of hundreds of micrometers. Launching light into a multimode fiber is comparatively easy, because there are larger tolerances concerning the location and propagation angle of incident light, compared with a single-mode fiber. On the other hand, the spatial coherence of the fiber output is reduced, and the output field pattern can hardly be controlled, for reasons explained below.

5.3 Step index fiber

Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fiber is best suited for transmission over short distances.

Optical fibers can have different refractive index profiles. Apart from such fibers where light

is guided at the air–glass interface, the simplest index profile is a rectangular one, where the refractive index is constant within the fiber core, and is higher than in the cladding. Fibers of that kind are called step-index fibers. The assumption of a step-index profile is often used for calculations in fiber optics, even though standard fabrication techniques often lead to significant deviations from this simple situation. In particular, preferential evaporation of the dopant during the collapse of the preform (assuming that the preform is made with chemical vapor deposition) often leads to a pronounced dip of the refractive index profile at the center. For profiles deviating from a step-index profile, an effective step-index profile may be defined which leads to similar mode properties as the actual profile.

The propagation modes of step-index fibers can be described with functions belonging to the family of Bessel functions, multiplied by an exponential phase factor exp(i β z) for the longitudinal phase variation. Concerning the radial dependence, the field strength in the core is proportional to the zero-order Bessel function of the first kind, and the cladding part is given by a modified Bessel function of the second kind. The mode function and its first derivative are always continuous at the core–cladding interface. 5.4 Graded index fiber

Contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Due to the graded index, light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion. This type of fiber is best suited for local-area networks.

Difference between Single and Multimode Fiber

6. Optical fiber characteristics

Optical-fiber systems have many advantages over metallic-based communication systems. These advantages include interference, attenuation, and bandwidth characteristics. Furthermore, the relatively smaller cross section of fiber-optic cables allows room for substantial growth of the capacity in existing conduits. Fiber-optic characteristics can be classified as linear and nonlinear. Nonlinear characteristics are influenced by parameters, such as bit rates, channel spacing, and power levels.

6.1 Interference

Light signals traveling via a fiber-optic cable are immune from electromagnetic interference (EMI) and radio-frequency interference (RFI). Lightning and high-voltage interference is also eliminated. A fiber network is best for conditions in which EMI or RFI interference is heavy or safe operation free from sparks and static is a must. This desirable property of fiber-optic cable makes it the medium of choice in industrial and biomedical networks. It is also possible to place fiber cable into natural-gas pipelines and use the pipelines as the conduit.

6.2 Linear Characteristics

Linear characteristics include attenuation, chromatic dispersion (CD), polarization mode dispersion (PMD), and optical signal-to-noise ratio (OSNR).

6.3 Attenuation

Several factors can cause attenuation, but it is generally categorized as either intrinsic or extrinsic. Intrinsic attenuation is caused by substances inherently present in the fiber, whereas extrinsic attenuation is caused by external forces such as bending. The attenuation coefficient α is expressed in decibels per kilometer and represents the loss in decibels per kilometer of fiber.

6.4 Intrinsic Attenuation

Intrinsic attenuation results from materials inherent to the fiber. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities. When a light signal hits an impurity in the fiber, one of two things occurs: It scatters or it is absorbed. Intrinsic loss can be further characterized by two components:

o Material absorption

o Rayleigh scattering

Material Absorption :

Material absorption occurs as a result of the imperfection and impurities in the fiber. The most common impurity is the hydroxyl (OH-) molecule, which remains as a residue despite stringent manufacturing techniques. Figure shows the variation of attenuation with wavelength

measured over a group of fiber-optic cable material types. The three principal windows of operation include the 850-nm, 1310-nm, and 1550-nm wavelength bands. These correspond to wavelength regions in which attenuation is low and matched to the capability of a transmitter to generate light efficiently and a receiver to carry out detection.

Attenuation versus Wavelength

The OH- symbols indicate that at the 950-nm, 1380-nm, and 2730-nm wavelengths, the presence of hydroxyl radicals in the cable material causes an increase in attenuation. These radicals result from the presence of water remnants that enter the fiber-optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The variation of attenuation with wavelength due to the water peak for standard, single-mode fiber-optic cable occurs mainly around 1380 nm. Recent advances in manufacturing have overcome the 1380-nm water peak and have resulted in zero-water-peak fiber (ZWPF). Examples of these fibers include SMF-28e from Corning and the Furukawa-Lucent OFS AllWave. Absorption accounts for three percent to five percent of fiber attenuation. This phenomenon causes a light signal to be absorbed by natural impurities in the glass and converted to vibration energy or some other form of energy such as heat. Unlike scattering, absorption can be limited by controlling the amount of impurities during the manufacturing process. Because most fiber is extremely pure, the fiber does not heat up because of absorption. Rayleigh Scattering:

As light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fiber. Rayleigh scattering accounts for about 96 percent of attenuation in optical fiber. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, however, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of

the propagation path and escapes from the fiber core. Some scattered light is reflected back toward the light source. This is a property that is used in an optical time domain reflectometer (OTDR) to test fibers. The same principle applies to analyzing loss associated with localized events in the fiber, such as splices.

Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below 800 nm is unusable for optical communication because attenuation due to Rayleigh scattering is high. At the same time, propagation above 1700 nm is not possible due to high losses resulting from infrared absorption.

6.5 Extrinsic Attenuation

Extrinsic attenuation can be caused by two external mechanisms: macro bending or micro bending. Both cause a reduction of optical power. If a bend is imposed on an optical fiber, strain is placed on the fiber along the region that is bent. The bending strain affects the refractive index and the critical angle of the light ray in that specific area. As a result, light traveling in the core can refract out, and loss occurs. A macro bend is a large-scale bend that is visible, and the loss is generally reversible after bends are corrected. To prevent macro bends, all optical fiber has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fiber can withstand before experiencing problems in optical performance or mechanical reliability.

The second extrinsic cause of attenuation is a micro bend. Micro bending is caused by imperfections in the cylindrical geometry of fiber during the manufacturing process. Micro bending might be related to temperature, tensile stress, or crushing force. Like macro bending, micro bending causes a reduction of optical power in the glass. Micro bending is very localized, and the bend might not be clearly visible on inspection. With bare fiber, micro bending can be reversible.

Chromatic Dispersion

Chromatic dispersion is the spreading of a light pulse as it travels down a fiber. Light has a dual nature and can be considered from an electromagnetic wave as well as quantum perspective. This enables us to quantify it as waves as well as quantum particles. During the propagation of light, all of its spectral components propagate accordingly. These spectral components travel at different group velocities that lead to dispersion called group velocity dispersion (GVD). Dispersion resulting from GVD is termed chromatic dispersion due to its wavelength dependence. The effect of chromatic dispersion is pulse spread.

As the pulses spread, or broaden, they tend to overlap and are no longer distinguishable by the receiver as 0s and 1s. Light pulses launched close together (high data rates) that spread too much (high dispersion) result in errors and loss of information. Chromatic dispersion occurs as a result of the range of wavelengths present in the light source. Light from lasers and LEDs

consists of a range of wavelengths, each of which travels at a slightly different speed. Over distance, the varying wavelength speeds cause the light pulse to spread in time. This is of most importance in single-mode applications. Modal dispersion is significant in multimode applications, in which the various modes of light traveling down the fiber arrive at the receiver at different times, causing a spreading effect. Chromatic dispersion is common at all bit rates. Chromatic dispersion can be compensated for or mitigated through the use of dispersion-shifted fiber (DSF). DSF is fiber doped with impurities that have negative dispersion characteristics. Chromatic dispersion is measured in ps/nm-km. A 1-dB power margin is typically reserved to account for the effects of chromatic dispersion. Polarization Mode Dispersion

Polarization mode dispersion (PMD) is caused by asymmetric distortions to the fiber from a perfect cylindrical geometry. The fiber is not truly a cylindrical waveguide, but it can be best described as an imperfect cylinder with physical dimensions that are not perfectly constant. The mechanical stress exerted upon the fiber due to extrinsically induced bends and stresses caused during cabling, deployment, and splicing as well as the imperfections resulting from the manufacturing process are the reasons for the variations in the cylindrical geometry.

Single-mode optical fiber and components support one fundamental mode, which consists of two orthogonal polarization modes. This asymmetry introduces small refractive index differences for the two polarization states. This characteristic is known as birefringence. Birefringence causes one polarization mode to travel faster than the other, resulting in a difference in the propagation time, which is called the differential group delay (DGD). DGD is the unit that is used to describe PMD. DGD is typically measured in picoseconds. A fiber that acquires birefringence causes a propagating pulse to lose the balance between the polarization components. This leads to a stage in which different polarization components travel at different velocities, creating a pulse spread as shown in Figure 3-13. PMD can be classified as first-order PMD, also known as DGD, and second-order PMD (SOPMD). The SOPMD results from dispersion that occurs because of the signal's wavelength dependence and spectral width.

PMD is not an issue at low bit rates but becomes an issue at bit rates in excess of 5 Gbps. PMD is noticeable at high bit rates and is a significant source of impairment for ultra-long-haul systems. PMD compensation can be achieved by using PMD compensators that contain dispersion-maintaining fibers with degrees of birefringence in them. The introduced birefringence negates the effects of PMD over a length of transmission. For error-free transmission, PMD compensation is a useful technique for long-haul and metropolitan-area networks running at bit rates greater than 10 Gbps. Note in Figure 3-13 that the DGD is the

difference between Z1 and Z2. The PMD value of the fiber is the mean value over time or frequency of the DGD and is represented as ps/ km. A 0.5-dB power margin is typically reserved to account for the effects of PMD at high bit rates.

Polarization Dependent Loss

Polarization dependent loss (PDL) refers to the difference in the maximum and minimum variation in transmission or insertion loss of an optical device over all states of polarization (SOP) and is expressed in decibels. A typical PDL for a simple optical connector is less than .05 dB and varies from component to component. Typically, the PDL for an optical add/drop multiplexer (OADM) is around 0.3 dB. The complete polarization characterization of optical signals and components can be determined using an optical polarization analyzer.

Optical Signal-to-Noise Ratio

The optical signal-to-noise ratio (OSNR) specifies the ratio of the net signal power to the net noise power and thus identifies the quality of the signal. Attenuation can be compensated for by amplifying the optical signal. However, optical amplifiers amplify the signal as well as the noise. Over time and distance, the receivers cannot distinguish the signal from the noise, and the signal is completely lost. Regeneration helps mitigate these undesirable effects before they can render the system unusable and ensures that the signal can be detected at the receiver. Optical amplifiers add a certain amount of noise to the channel. Active devices, such as lasers, also add noise. Passive devices, such as taps and the fiber, can also add noise components. In the calculation of system design, however, optical amplifier noise is considered the predominant source for OSNR penalty and degradation.

OSNR is an important and fundamental system design consideration. Another parameter considered by designers is the Q-factor. The Q-factor, a function of the OSNR, provides a qualitative description of the receiver performance. The Q-factor suggests the minimum signal-to-noise ratio (SNR) required to obtain a specific BER for a given signal. OSNR is measured in decibels. The higher the bit rate, the higher the OSNR ratio required. For OC-192 transmissions, the OSNR should be at least 27 to 31 dB compared to 18 to 21 dB for OC-48. Nonlinear Characteristics Nonlinear characteristics include self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). Self-Phase Modulation

Phase modulation of an optical signal by itself is known as self-phase modulation (SPM). SPM is primarily due to the self-modulation of the pulses. Generally, SPM occurs in single-wavelength systems. At high bit rates however, SPM tends to cancel dispersion. SPM increases with high signal power levels. In fiber plant design, a strong input signal helps overcome linear attenuation and dispersion losses. However, consideration must be given to receiver saturation and to nonlinear effects such as SPM, which occurs with high signal levels.

SPM results in phase shift and a nonlinear pulse spread. As the pulses spread, they tend to overlap and are no longer distinguishable by the receiver. The acceptable norm in system design to counter the SPM effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of SPM at high bit rates and power levels. Cross-Phase Modulation

Cross-phase modulation (XPM) is a nonlinear effect that limits system performance in wavelength-division multiplexed (WDM) systems. XPM is the phase modulation of a signal caused by an adjacent signal within the same fiber. XPM is related to the combination (dispersion/effective area). CPM results from the different carrier frequencies of independent channels, including the associated phase shifts on one another. The induced phase shift is due to the walkover effect, whereby two pulses at different bit rates or with different group velocities walk across each other. As a result, the slower pulse sees the walkover and induces a phase shift. The total phase shift depends on the net power of all the channels and on the bit output of the channels. Maximum phase shift is produced when bits belonging to high-powered adjacent channels walk across each other. XPM can be mitigated by carefully selecting unequal bit rates for adjacent WDM channels. XPM, in particular, is severe in long-haul WDM networks, and the acceptable norm in system design to counter this effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of XPM in WDM fiber systems. Four-Wave Mixing

FWM can be compared to the intermodulation distortion in standard electrical systems. When three wavelengths (λ1, λ 2, and λ 3) interact in a nonlinear medium, they give rise to a fourth wavelength (λ 4), which is formed by the scattering of the three incident photons, producing the fourth photon. This effect is known as four-wave mixing (FWM) and is a fiber-optic characteristic that affects WDM systems. The effects of FWM are pronounced with decreased channel spacing of wavelengths and at high signal power levels. High chromatic dispersion also increases FWM effects. FWM also causes inter-channel cross-talk effects for equally spaced WDM channels. FWM can be mitigated by using uneven channel spacing in WDM systems or nonzero dispersion-shifted fiber (NZDSF). A 0.5-dB power margin is typically reserved to account for the effects of FWM in WDM systems. Stimulated Raman Scattering

When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects, such as stimulated Raman scattering (SRS), in the forward and reverse directions of propagation along the fiber. This results in a sporadic distribution of energy in a random direction.

SRS refers to lower wavelengths pumping up the amplitude of higher wavelengths, which results in the higher wavelengths suppressing signals from the lower wavelengths. One way to mitigate the effects of SRS is to lower the input power. In SRS, a low-wavelength wave called Stoke's wave is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Raman amplification. The Raman gain can extend most of the operating band (C- and L-band) for WDM networks. SRS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB. Stimulated Brillouin Scattering

Stimulated Brillouin scattering (SBS) is due to the acoustic properties of photon interaction with the medium. When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects such as SBS in the reverse direction of propagation along the fiber. In SBS, a low-wavelength wave called Stoke's wave is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Brillouin amplification. The Brillouin gain peaks in a narrow peak near the C-band. SBS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB. 7. Fiber Optic Connector Types SMA — Due to its stainless steel structure and low-precision threaded fiber locking mechanism, this connector is used mainly in applications requiring the coupling of high-power laser beams into large-core multimode fibers. Typical applications include laser beam delivery systems in medical, bio-medical, and industrial applications. The typical insertion loss of an SMA connector is greater than 1 dB. ST — The ST connector is used extensively both in the field and in indoor fiber optic LAN applications. Its high-precision, ceramic ferrule allows its use with both multimode and single-mode fibers. The bayonet style, keyed coupling mechanism featuring push and turn locking of the connector, prevents over tightening and damaging of the fiber end. The insertion loss of the ST connector is less than 0.5 dB, with typical values of 0.3 dB being routinely achieved. Drilled-out, metallic ST connectors, with insertion losses of >1 dB, are used with Newport’s large-core (>140 µm) fibers. FC —The FC has become the connector of choice for single-mode fibers and is mainly used in fiber-optic instruments, SM fiber optic components, and in high-speed fiber optic communication links. This high-precision, ceramic ferrule connector is equipped with an anti-rotation key, reducing fiber end face damage and rotational alignment sensitivity of the fiber. The key is also used for repeatable alignment of fibers in the optimal, minimal-loss position. Multimode versions of this connector are also available. The typical insertion loss of the FC connector is around 0.3 dB. Drilled-out, metallic FC connectors, having insertion losses of >1 dB are being used with Newport’s large-core (>140 µm) fibers.

SC —The SC connector is becoming increasingly popular in single-mode fiber optic telecom and analog CATV, field deployed links. The high-precision, ceramic ferrule construction is optimal for aligning single-mode optical fibers. The connectors’ outer square profile combined with its push-pull coupling mechanism, allow for greater connector packaging density in instruments and patch panels. The keyed outer body prevents rotational sensitivity and fiber end face damage. Multimode versions of this connector are also available. The typical insertion loss of the SC connector is around 0.3 dB.

8. Common Optical Parameters

Port Configuration: Number of input ports x number of output ports. e.g. 2 x 2

Coupling Ratio: The ratio of the power at an output port to the launched power, expressed in

dB is equal to 10 log (P2/P1).

Isolation: The ratio of the power at an output port in the transmitted wavelength band to that in

the extinguished wavelength band, expressed in dB.

Directivity: The ratio of the power returned to any other input port to the launched power,

expressed in dB. e.g.- 10 log (P4/P1).

Bandwidth: The range of operating wavelengths over which performance parameters are

specified.

Excess Loss: The ratio of the total power at all output ports to the launched power, expressed

in dB. e.g. -10 log [(P2+P3)/P1].

Uniformity: The difference between maximum and minimum insertion losses.

Extinction Ratio: The ratio of the residual power in an extinguished polarization state to the

transmitted power, expressed in dB.

Return Loss: The ratio of the power returned to the input port to the launched power,

expressed in dB. e.g.-10 log (P5/P1).

Polarization-Dependent Loss (PDL): The maximum (peak-to-peak) variation in insertion loss

as the input polarization varies, expressed in dB.

9. Test Equipments and Functionality

9.1 The need for testing:

• Ensure the fiber has been Properly installed

• Check IL and reflectance of all splices and connections

• Providers must maintain high quality of customer service

• Quickly locate problem areas for repair

9.2 Types of Testing:

• End to End loss testing

• Cable Commissioning

• Splice and connector optimization

• System ORL characterization

• Dispersion verification and wavelength measurements

• End to End loss Testing:

o Both units must be referenced with the test jumpers

o Confirm power source is connected to test fiber

o Reading is only accurate if both units at same wavelength

9.3 Testing devices

1. Optical Time Domain Reflectometer

o Measures reflectance and ORL

o Can characterize connectors and mechanical splices

o Accuracy of plus/minus 2dB

2. Optical Continuous Wave Reflectometer

o Also measures reflectance and ORL

o Some units measure bi-directional loss

o Accuracy of plus/minus 0.4 dB to 1.0 dB

Module 2: Installation of Optical Fiber Cable

Contents

1. Planning for Installation

2. Installation Requirements / General Guidelines

3. Fiber Optic Components: Cables

4. Installing Fiber Optic Cable

5. Fiber Optic Splicing and Termination Hardware

6. Testing the Installed Fiber Optic Cable Plant

7. Administration, Management, and Documentation

1. Introduction

This module describes the procedures for installing and testing cabling networks that

use fiber optic cables and related components to carry signals for communications, security,

control and similar purposes. It defines procedures that should provide a high level of quality for

fiber optic cable installations. This document covers fiber optic cabling installed indoors

(premises installations) with the addition of outside plant (OSP) applications involved in campus

installations where the fiber optic cabling extends between buildings.

2. Planning for the Installation

Final planning for the installation is a critical phase of any project as it involves coordinating

activates of many people and companies. The best way to keep everything straight is probably

to develop a checklist based on the design path:

2.1 Pre-Installation checklist:

a) Main point of contact/project manager chosen

b) Link communications requirements set

c) Equipment requirements set and vendors chosen

d) Link route chosen, permits obtained

e) Cable plant components and vendors chosen

f) Coordination with facilities and electrical personnel complete

g) Documentation ready for installation, preliminary restoration plans ready

h) Test plan complete

i) Schedule and start date set for installation, all parties notified

j) Components ordered and delivery date set, plans made for receiving materials

(time, place,) arrange security if left outside or on construction site

k) Contractor/installer chosen and start date set

l) Link route tour with contractor

m) Construction plans reviewed with contractor

n) Components chosen reviewed with contractor

o) Schedule reviewed with contractor

p) Safety rules reviewed with contractor

q) Excess materials being kept for restoration reviewed with contractor

r) Test plan reviewed with contractor End to End loss testing

2.2 Before starting the installation:

a) All permits available for inspection

b) Sites prepared, power available

c) All components on site, inspected, security arranged if necessary

d) Contractor available

e) Relevant personnel notified

f) Safety rules posted on the job site and reviewed with all supervisors and installation personnel

2.3 During the Installation:

a) Inspect workmanship

b) Daily review of process, progress, test data

c) Immediate notification and solution of problems, shortages, etc.

2.4 After completion of cable plant installation:

a) Inspect workmanship

b) Review test data on cable plant

c) Set up and test communications system

d) Update and complete documentation

e) Update and complete restoration plan

f) Store restoration plan, documentation, components, etc.

3. Installation Requirements / General Guidelines

3.1 Receiving Fiber Optic Cabling and Equipment on Site

a) Fiber Optic equipment and components are subject to damage by improper handling and

must be handled accordingly.

b) When initially received on the job site all fiber optic components should be carefully

inspected for damage and tested for continuity or loss if damage is suspected.

c) Ensure that all components and parts have been shipped, received, match quantities

ordered (e.g. fiber optic cable contains the number and type of fiber ordered and is the length

ordered), and that any discrepancies or damaged goods are noted, the supplier notified and

replaced as required.

d) All equipment and cabling shall be stored in a clean and dry location, protected from harsh

environments and extremes of cold and heat.

3.2 Handling Fiber Optic Cables

a) Handle reels of fiber optic cable with care. All reels, regardless of size or length, must have

both ends of the cable available for the testing. A fiber tracer or visual fault locator and bare

fiber adapters can be used for continuity testing.

b) Move small, lightweight spools of fiber optic cable by hand. Move larger reels with

appropriate lifting equipment or using two or more installers skilled in the moving operation.

c) Lifting equipment shall only must reels with a matched set of slings or chokers, attached to

an appropriately sized piece of pipe inserted into the hole in the center of the reel. Slings and

chokers shall never be attached around the spooled area of the reel. The cable reels shall be

moved carefully to avoid damage to the cable.

3.3 Support Structures

a) Install support structures for fiber optic cable installations before the installation of the fiber

optic cable itself.

b) Allow for future growth in the quantity and size of cables when determining the size of the

pathway bend radius requirements.

c) Do not install a fiber optic cable in a conduit or duct that already contains cabling, regardless

of the cable type. Existing or new empty ductwork can be modified to accept several different

installations by the placement of inner duct within it.

3.4 Removal of Abandoned Cables

Unless directed by the owner or other agency that unused cables are reserved for future use,

remove abandoned optical fiber cable (cable that is not terminated at equipment other than a

connector and not identified for future use with a tag) as required by the National Electrical

Code.

3.5 Fire Stopping

a) All telecommunications fire stopping shall comply with applicable codes and standards,

including TIA/EIA 569 and NECA/BICSI 568.

b) All penetrations shall be protected by approved fire stops. Fire stopping compounds and

devices shall be used whenever a fire separation has been breached by an installation.

c) In most geographical locals the breaching of a fire separation will require physical monitoring

until it has been repaired.

d) Check with the “Authority Having Jurisdiction” for specific requirements on the project before

commencing work.

3.6 Grounding and Bonding

a) Ground systems shall be designed as specified by the NEC and other applicable codes and

standards (ANSI/TIA/EIA 607, NECA-BICSI-568).

b) Although most fiber optic cables are not conductive, any metallic hardware used in fiber optic

cabling systems (such as wall-mounted termination boxes, racks, and patch panels) must be

grounded.

c) Conductive cables require proper grounding and bonding for applicable conductors.

4. Fiber Optic Components

4.1 Cables

Fiber optic cables are available in many types, for different applications. Premises cables are

usually tight buffer designs that include jackets rated for flammability. OSP cables are generally

loose tube designs with water blocking and may also have an armored jacket.

Some cables are usable for either OSP or premises applications. These include dry-water

blocked tight buffered cables that can be used for short outdoor runs and double jacketed OSP

cables that have a removable outside jacket and an inner jacket that is rated a flame retardant.

4.2 Fiber Optic Cables by Fiber Types

Fiber optic cables may contain multimode fibers, single mode fibers or a combination of the two,

in which case it is referred to as a “hybrid” cable. The type of cable shall be positively identified

and , if hybrid, the type of each fiber, since multimode and single mode fiber are terminated in

different manners. See the section on termination below.

4.3 Fiber Optic Cables by Construction Type

a) Tight Buffered Cables

Tight buffered fiber optic cable contains fiber with a soft 900-micron diameter coating that

protects the fiber and is color-coded for identification.

Simplex, Zip Cord: Tight buffered fibers are cabled with strength members (usually aramid

fibers) in simplex or zip cord cables for use as patch cords.

Distribution: Multiple tight buffered fibers may be cabled with aramid fiber strength members and

a central stiffener in a cable type called a distribution cable, often used for premises backbones,

horizontal runs or general building cabling.

Breakout: Several simplex cables can be bundled in a single cable called a breakout cable.

Simplex, zip cord and breakout cables may be directly terminated for connection to a patch

panel or network equipment as the cable provides adequate protection for the fibers.

Fibers in distribution cables are terminated directly, but the lack of protection for the individual

fibers requires they be placed inside patch panels or wall-mounted boxes.

b) Loose Tube Cable

Loose tube (also called loose buffer) fiber optic cable consists of one or more protective tubes,

each containing one or more fibers with only 250-micron primary coating over the fiber or

ribbons typically containing 12 fibers per ribbon. Loose-tube cable is primarily used for outside

plant installations where low attenuation and high cable pulling strength are required.

Many fibers can be incorporated into the same tube, providing a small-size, high-fiber density

construction. The tubes are usually filled with a gel, which prevents water from entering the

cable, although dry water-blocking compounds are becoming more common. The fibers in loose

tube cables are protected from the outside environment and can be installed with higher pulling

tensions than tight-buffered cables.

Fiber in loose tube cables may be spliced directly and placed in appropriate protective

enclosures. Fibers in loose tube cables which have only the 250 micron primary coating should

be sleeved with a break out kit for protection before termination and placed in patch panels or

wall-mounted boxes for protection.

Ribbon Cable is a special type of cable that combines 12 or 24 fibers into a single ribbon.

Ribbons can be cabled in loose tubes or in slots in a plastic core of the cable. Ribbon cables

have typical fiber counts of 144, 288, 864 or more. Water blocking, armoring, etc. is done as

with any loose tube cable.

4.4 Flammability - Cable Ratings and Markings

All premises cables shall be listed and have flammability ratings per NEC 770.50. Cables

without markings should never be installed inside buildings, as they do not comply with the

National Electrical Code. Optical cable markings are as follows:

OFN Optical fiber nonconductive OFC Optical fiber conductive OFNG or OFCG General purpose OFNR or OFCR Riser rated cable for vertical runs OFNP or OFCP Plenum rated cables for use in air-handling plenums OFN-LS Low smoke density

4.5 Color Codes: Fiber Optic Cable

Cable Jackets

Colors of cable jackets for identifying indoor fiber optic cable are not standardized. Typical

colors are as follows:

Premises cables (Per TIA-598C ):

Fiber Types Color Codes Non-Military Military Printed On Cable Multimode (50/125) (TIA- 492AAAB) (OM2)

Orange Orange 50/125

Multimode (50/125) (850 nm Laser-optimized) (TIA-492AAAC) (OM3/4)

Aqua Undefined 850 LO 50/125

Multimode (62.5/125) (TIA-492AAAA) (OM1)

Orange Slate 62.5/125

Multimode (100/140) (Obsolete)

Orange Green 100/140

Single-mode (TIA-492C000 / TIA-492E000)

Yellow Yellow SM/NZDS or SM

Polarization Maintaining Single-mode

Blue Undefined Undefined

NOTES:

1) Natural jackets with colored tracers may be used instead of solid-color jackets.

2) Because of the limited number of applications for these fibers, print nomenclature is to be

agreed upon between manufacturer and end-user

3) Other colors may be used providing that the print on the outer jacket identifies fiber

classifications per sub-clause 4.3.3.

4) For some Premises Cable functional types (e.g., plenum cables), colored jacketing material

may not be available. Distinctive jacket colors for other fiber types may be considered for

addition to Table 3 at some future date.

Color Codes: Connectors

Since the earliest days of fiber optics, orange, black or gray was multimode and yellow single

mode. However, the advent of metallic connectors like the FC and ST made color coding

difficult, so colored boots were often used. The TIA 568 color code for connector bodies and/or

boots is Beige for multimode fiber, Blue for single mode fiber, and Green for APC (angled)

connectors.

4.6 Outside plant cables:

These are typically black to prevent UV radiation damage.

Some indoor cables are black or other colors. Refer to manufacturer’s datasheets or cable

jacket markings to determine the fibers in the cable.

Color Codes: Fiber

Fiber color codes are specified by TIA/EIA 598-A. In loose tube cables, this color code will be

used for tubes as well as fibers within the tubes and sub-groups.

Fiber No. Color

1 Blue

2 Orange

3 Green

4 Brown

5 Slate

6 White

7 Red

8 Black

9 Yellow

10 Violet

11 Rose

12 Aqua

5. Installing Fiber Optic Cable

Fiber optic cable may be installed indoors or outdoors using several different installation

processes. Outdoor cable may be direct buried, pulled or blown into conduit or inner duct, or

installed aerially between poles. Indoor cables can be installed in raceways, cable trays, placed

in hangers, pulled into conduit or inner duct or blown though special ducts with compressed gas.

The installation process will depend on the nature of the installation and the type of cable being

used. Installation methods for both wire and optical fiber communications cables are similar.

Fiber cable is designed to be pulled with much greater force than copper wire if pulled correctly,

but excess stress may harm the fibers, potentially causing eventual failure.

5.1 Installation Guidelines

a) Follow the cable manufacturer's recommendations. Fiber optic cable is often custom-

designed for the installation and the manufacturer may have specific instructions on its

installation.

b) Check the cable length to make sure the cable being pulled is long enough for the run to

prevent having to splice fiber and provide special protection for the splices.

c) Try to complete the installation in one pull. Prior to any installation, assess the route carefully

to determine the methods of installation and obstacles likely to be encountered.

• Pulling tension

a) Cable manufacturers install special strength members, usually aramid yarn, for pulling. Fiber

optic cable should only be pulled by these strength members. Any other method may put stress

on the fibers and harm them.

b) Swivel pulling eyes should be used to attach the pulling rope or tape to the cable to prevent

cable twisting during the pull.

c) Cables should not be pulled by the jacket unless it is specifically approved by the cable

manufacturers and an approved cable grip is used.

d) Tight buffer cable can be pulled by the jacket in premises applications if a large (~40 cm, 8

in.) spool is used as a pulling mandrel. Wrap the cable around the spool 5 times and hold

gently when pulling.

e) Do not exceed the maximum pulling tension rating. Consult the cable manufacturer and

suppliers of conduit, inner duct, and cable lubricants for guidelines on tension ratings and

lubricant use.

f) On long runs (up to approximately 3 miles or 5 kilometers), use proper lubricants and make

sure they are compatible with the cable jacket. If possible, use an automated puller with tension

control and/or a breakaway pulling eye. On very long runs (farther than approximately 2.5 miles

or 4 kilometers), pull from the middle out to both ends or use an automated fiber puller at

intermediate point for a continuous pull.

g) When laying loops of fiber on a surface during a pull, use “figure-8” loops to prevent twisting

the cable.

• Bend radius

a) Do not exceed the cable bend radius. Fiber optic cable can be broken when kinked or bent

too tightly, especially during pulling.

b) If no specific recommendations are available from the cable manufacturer, the cable should

not be pulled over a bend radius smaller than twenty (20) times the cable diameter.

c) After completion of the pull, the cable should not have any bend radius smaller than ten (10)

times the cable diameter.

• Twisting cable

a) Do not twist the cable. Twisting the cable can stress the fibers. Tension on the cable and

pulling ropes can cause twisting.

b) Use a swivel pulling eye to connect the pull rope to the cable to prevent pulling tension

causing twisting forces on the cable.

c) Roll the cable off the spool instead of spinning it off the spool end to prevent putting a twist in

the cable for every turn on the spool.

d) When laying cable out for a long pull, use a "figure 8" on the ground to prevent twisting. The

figure 8 puts a half twist in on one side of the 8 and takes it out on the other, preventing twists.

• Vertical cable runs

a) Drop vertical cables down rather than pulling them up whenever possible.

b) Support cables at frequent intervals to prevent excess stress on the jacket. Support can be

provided by cable ties (tightened snugly, not tightly enough to deform the cable jacket) or

Kellems grips.

c) Use service loops can to assist in gripping the cable for support and provide cable for future

repairs or rerouting.

• Use Of Cable Ties

Fiber optic cables, like all communications cables, are sensitive to compressive or crushing

loads. Cable ties used with many cables, especially when tightened with an installation tool, are

harmful to fiber optic cables, causing attenuation and potential fiber breakage.

a) When used, cable ties should be hand tightened to be snug but loose enough to be moved

along the cable by hand. Then the excess length of the tie should be cut off to prevent future

tightening.

b) Hook-and-loop fastener ties are preferred for fiber optic cables, as they cannot apply crush

loads sufficient to harm the cable.

• Cable Plant Hardware

All premises hardware and support structures should follow the recommendations of TIA/EIA

569. Outside plant hardware and installation equipment should follow manufacturer’s

recommendations.

• Cable Racks, Trays, Conduit and Inner duct

a) Outside plant cables can be installed underground in conduit or inner duct or direct buried,

depending on the cable type. Aerial cable installation may use cables lashed to a support

messenger cable or be self-supporting. Underwater cable installation must follow

manufacturer’s directions.

b) Premises cabling can be installed in cable trays, ladder racks, J-hooks, or other appropriate

support structures.

c) Building cables can be installed directly, but installing them inside plenum-rated inner duct

provides extra protection for the fiber cable. Inner duct is bright orange and will provide a good

way to identify fiber optic cable and protect it from damage.

• Fiber Optic Splicing and Termination Hardware

a) Breakout kits: The fibers in loose tube cables have only the 250 micron primary buffer

coating. Use breakout kits to separate and protect individual fibers in a loose tube cable for

termination directly on the fibers.

b) Splice enclosures: For long cable runs outside, splices are necessary to connect lengths of

cable. Splices require protection that is provided by a sealed splice closure. Choose closures

with adequate space for the number of fibers in the cables and port locations appropriate for the

final mounting. Splice closures can be sealed and buried in the ground, placed in a vault or

suspended aerially. Outside plant cables terminated in buildings may also need closures if they

are terminated by splicing on pre-terminated pigtails as is common.

c) Splice panels and patch panels: Terminate or splice distribution cables inside panels or

boxes to protect the fibers from damage. Boxes or panels may be rack- or wall-mounted. All

should have locks to prevent unauthorized entry.

d) Racks and cabinets: Enclosures for patch panels and splice panels are used to terminate

and organize cables. Use appropriate cable management hardware on the racks to route and

separate cables to minimize potential for damage and facilitate moves, adds and changes.

e) Take care with all splicing and termination hardware to maintain cable bend radiuses,

prevent pinching or kinking of fibers and separate fibers to allow for future restoration, moves or

other work.

6. Fiber Optic Termination

Fiber optic termination processes vary according to the types of fiber being terminated, the style

of connectors or splices used and the termination process appropriate for that connector. Fiber

optic cable can be terminated in two ways, using:

1) Connectors that mate two fibers to create a temporary joint and/or connect the optical fiber to

network equipment.

2) Splices which create a permanent joint between two fibers.

The decision whether to use connectors or splices depends on the application. All terminations

must be of the right style, installed in a manner that provides low light loss and back reflection

and protected against the expected environment, dirt or damage while in use.

6.1 Choice of connector

Fiber optic connectors are manufactured in a number of different styles (e.g., ST, SC, LC, MT-

RJ) that attach to the fibers in a fiber optic cable by a number of different methods (e.g., epoxy

polish, prepolished/splice, etc.)

The connectors used in the cable plant being installed should:

1) Be compatible with the fiber optic cabling,

1) Be compatible with the equipment intended for use on the cabling

2) Provide adequate optical performance (loss and return loss)

3) Be compatible with the operating environment (temperature, humidity, etc.) of the installation

and

4) Be compatible with like style connectors. All fiber optic connectors used should have a

reference FOCIS document (Fiber Optic Connector Intermateability Standard) published by

TIA/EIA.

Fiber optic connectors may be field installed by direct attachment to the cable or by splicing pre-

terminated pigtails onto the installed cable. Multimode connectors are generally installed directly

onto fibers in the field while single mode cables are more likely to be terminated by splicing on

pre-terminated pigtails.

6.2 Termination types

Several different types of terminations are available for optical fibers. Follow the manufacturer’s

directions exactly for the termination process used to ensure best connector performance and

reliability.

a) Adhesive Terminations

Many connectors use epoxies or other adhesives to hold the fiber in the connector. Use only

the specified epoxy, as the fiber-to-ferrule bond is critical for low loss and long term reliability.

1. Epoxy/Polish: The fiber is glued into the connector with two-part epoxy and the end

polished with special polishing film. This method provides the most reliable connection and

lowest losses. The epoxy can be allowed to set overnight or cured in a special oven. A "heat

gun" should not be used to cure the epoxy as the uneven heat may not cure all the epoxy or

may overheat it which will prevent curing.

2. Hot Melt: This connector is similar to the epoxy/polish connector but already has the

adhesive (a heat set glue) inside the connector. The adhesive is liquefied in an oven before the

fiber can be inserted. The fiber is secured when the adhesive cools.

3. Anaerobic Adhesives: These connectors use a quick-setting adhesive instead of the epoxy.

They may use a single part adhesive or an adhesive and setting agent. Some adhesives do not

have the wide temperature range of epoxies, so they should only be used indoors unless

otherwise specified.

b) Crimp/Polish or Crimp/Cleave Terminations

These connectors use a crimp on the fiber to hold it in the connector ferrule. The fiber can be

polished like an adhesive connector or cleaved with a special tool. Ensure the crimp is made

properly to prevent fiber pistoning (pulling back or pushing forward in the connector ferrule.)

c) Pre-polished/Splice

These connectors have a short stub of fiber already epoxied into the ferrule and polished.

Termination requires cleaving a fiber, inserting it into the back of the connector like a splice and

crimping. The loss of these connectors will generally be higher than adhesive connectors, since

they include a connector loss plus a splice loss in every connector. To achieve low loss, the

fiber must be cleaved properly, which requires a good cleaver (preferably the type used with

fusion splicers) and good technique. Ensure the crimp is made properly to prevent fiber

pistoning (pulling back in the connector ferrule.) The termination process can be monitored with

a visual fault locator.

6.3 Termination process

a) Whichever process is used for termination, follow the manufacturer’s instructions carefully.

b) Use only adhesives approved by the manufacturer, and employ adhesive curing times in

accordance with the manufacturer’s instructions.

c) When special tools are required, use them in the appropriate manner.

d) Once installation is completed, connectors should be covered with an appropriate dust cap

and stored in a safe location waiting testing or connection to network equipment.

Connector performance

Connector performance shall be within industry normal limits or as specified in TIA/EIA 568.

Connector performance may be specified by end users at a different value, and if so, those

values shall be used for acceptance. Since TIA 568 includes adhesive/polish and pre-

polished/splice connectors as well as multi-fiber connectors, the limit is set high to

accommodate all types at 0.75 dB. Adhesive polish connectors will generally have losses well

under 0.5 dB.

Performance verification

Following completed installation and termination, all terminated cables must be tested. The

section on testing below provides more detail on testing requirements at the conclusion of

installation.

a) Examine all connectors requiring polishing with a microscope for proper end finish, cracks,

scratches or dirt per FOTP-57.

b) Test all fibers in all cables for loss using an OLTS power meter and source. Test multimode

cables using TIA/EIA 526-14, and single mode cables using TIA/EIA 526- 7 (single mode). Total

loss shall be less than the calculated maximum loss for the cable based on appropriate

standards or customer specifications as determined in a loss budget analysis done in the design

phase.

6.4 Fiber polarization/End To End Connections

In fiber networks, separate fibers are typically used for transmission in each direction, therefore

it is necessary to identify the fiber connected to the transmitter and receiver at each end.

a) Duplex connectors such as the duplex SC or MT-RJ are polarized, that is they are keyed to

allow connection in only one orientation. Follow the polarization rules given in TIA/EIA 568-B3,

Section 5.2.4.

b) Simplex connectors should be documented for connections and when allocated to the

transceiver of networking equipment, marked for transmit and receive at each end of the link.

6.5 Fiber Optic Splices

Types of splices

Splices are a permanent joint or connection between two fibers. There are two basic types of

splices, fusion and mechanical.

a) Fusion Splices

These "weld" the two fibers together usually in an electric arc. Fusion splicers are generally

automated and produce splices that have minimal losses. Fusion splicing should not be

performed in a dusty or explosive atmosphere as the electric arc may cause an explosion or fire.

b) Mechanical Splices

These align two fibers in a ferrule or v-groove with index-matching gel or adhesive between the

fibers to reduce loss and back reflection. Mechanical splices are used for temporary restoration

as well as permanent joints.

Splice performance

Splice performance shall be within industry accepted limits or as specified in TIA/EIA 568. While

TIA-568 specifies 0.3 dB loss for both multimode and single mode splices, single mode fusion

splices are typically under 0.1 dB. If splice performance may be specified by end users at a

different value, and if so, those values shall be used for acceptance.

Splice performance verification

End-to-end tests of fiber optic cable loss include the losses caused by splices. If the cable loss

exceeds the calculated maximum value, or if the customer requires splice loss verification, test

the cable with an OTDR to analyze the loss of individual components (fiber, connectors, and

splices) in the cable. Test splice loss in both directions and average the measured values to

reduce the directional effects of OTDR measurements.

7. Testing the Installed Fiber Optic Cable Plant

During the design phase, each cable run should have a “Loss Budget” calculated based on

component specifications. After installation, test each fiber in all fiber optic cables for verification

of proper installation by comparing measured loss to the calculated loss from the Loss Budget.

Perform the following tests:

a) Continuity testing to determine that the fiber routing and/or polarization is correct and

documentation is proper.

b) End-to-end insertion loss using an OLTS power meter and source. Test multimode cables

using TIA/EIA 526-14, and single mode cables using TIA/EIA 526-7 (single mode). Total loss

shall be less than the calculated maximum loss for the cable based on Loss Budget calculations

using appropriate standards or customer specifications.

c) Optional OTDR testing may be used to verify cable installation and splice performance.

However, OTDR testing should not be used to determine cable loss, especially on longer

cables. Use of an OTDR in premises applications may be inappropriate if cables are too short.

d) If the design documentation does not include cable plant length, and this is not recorded

during installation, read the length from the distance marking on the cable jacket or test the

length of the fiber using the length feature available on an OTDR, or some OLTSs.

e) If testing shows variances from expected losses troubleshoot the problems and correct them.

7.1 Continuity Testing

Perform continuity testing of optical fibers using a visual fiber tracer, visual fault locator, or

OLTS power meter and source. Trace the fiber from end to end through any interconnections to

ensure that the path is properly installed, and that polarization and routing are correct and

documented.

7.2 Insertion Loss

Insertion loss refers to the optical loss of the installed fibers when measured with a test source

and power meter (OLTS). Test multimode cables using TIA/EIA 526-14, and single mode

cables using TIA/EIA 526-7 (single mode).

a) Test multimode fiber at 850 and 1300 nm, and single mode fiber at 1310 and 1550 nm,

unless otherwise required by other standards or customer requirements.

b) Test reference test cables to verify quality and clean them often.

c) Cabling intended for use with high speed systems using laser sources may be tested with

appropriate laser sources to ensure that tests verify performance with that type of source.

7.3 OTDR Testing

The optical time domain reflectometer (OTDR) uses optical radar-like techniques to create a

picture of a fiber in an installed fiber optic cable. The picture, called a signature or trace,

contains data on the length of the fiber, loss in fiber segments, connectors, splices and loss

caused by stress during installation.

OTDRs are used to verify the quality of the installation or for troubleshooting. However, OTDR

testing shall not be used to determine cable loss.

OTDRs have limited distance resolution and may show confusing artifacts when testing short

cables typical of premises applications. If OTDR testing of premises cables is desired,

experienced personnel should evaluate the appropriateness of the tests.

OTDR testing should only performed by trained personnel, using certified equipment designed

for the purpose. The technicians performing the tests should be trained not only in operation of

the OTDR equipment, but also in the interpretation of OTDR traces.

8. Administration, Management, and Documentation

Documentation of the fiber optic cable plant is an integral part of the design, installation and

maintenance process for the fiber optic network. Documenting the installation properly will

facilitate installation, allow better planning for upgrading, simplify testing and future moves, adds

and changes.

Documentation of the fiber optic cable plant should follow ANSI/TIA/EIA-606, Administration

Standard for the Telecommunications Infrastructure of Commercial Buildings.

Fiber optic cables, especially those used for backbone cables, may contain many fibers that

connect a number of different links going to several different locations with interconnections at

patch panels or splice closures. The fiber optic cable plant should be documented as to the

exact path that every fiber in each cable follows, including intermediate connections and every

connector type.

Documentation should also include insertion loss data and optional OTDR traces.

Testing and Splicing of OFC

Contents

1. Equipment for Measuring Fibers

2. Events on Fibers

3. Important Parameters

4. Common Tasks

5. Practical Hints from OTDR Experts

6. Automatic Trace Analysis

7. Agilent Technologies’ OTDRs

8. Tables

1. Equipment for Measuring Fibers

In today’s world, the demand for optical networks is growing faster and faster. The networks are

becoming bigger, more powerful and more reliable. This requires more operators, installers and

maintenance contractors to provide information on the networks faster and with higher accuracy

than ever before.

1.1 Optical Time Domain Reflectometer

The Optical Time Domain Reflectometer (OTDR) is the preferred instrument for characterizing

optical fibers. With an OTDR you can evaluate the characteristic properties of a single fiber or a

complete link. In particular, you can see losses, faults, and the distances between Events at a

glance.

Agilent Technologies' OTDRs check the quality of fiber optic links by measuring backscatter.

Standards organizations, for example, the International Telecommunication Union (ITU), accept

backscatter measurements as a valid means for analyzing a fiber's attenuation. Backscatter is

also the only fiber optic measurement method that detects splices within an installed link. It can

also be used to measure the optical length of a fiber. Thus, the OTDR is a valuable tool for

anyone who manufactures, installs, or maintains optical fibers.

The OTDR functions by looking for “Events” in a fiber, for example, irregularities or splices. This

makes it an invaluable quality control tool for anyone who manufactures, installs, or maintains

fiber optic cables. The OTDR pinpoints these irregularities in the fiber, measures the distance to

them, the attenuation between them, the loss due to them, and the homogeneity of the

attenuation.

It is an especially valuable tool for the field. You can use it to regularly check if the link meets

the specifications. In order to document the quality and to store it for maintenance purposes it is

necessary to measure the optical length, the total loss, the losses of all splices and

connectors—including their return losses.

1.2 Laser Safety

If you look into a laser beam then your eye may focus the light onto a very small spot on your

retina. Depending on the energy absorbed by the retina, the eye may be damaged temporarily

or permanently.

The wavelengths used in today's fiber optic communication links are invisible. This makes even

small optical powers more dangerous than bright visible light. Because you cannot see it, you

may look much longer into a laser beam.

National and international organizations define standards for a safe operation of fiber optic light

sources.

All Agilent OTDRs meet the safety requirements of the most common standards. In the United

States this is 21 CFR class 1, and in Europe it is IEC 825 class 3A. Products that are

conforming to these standards are considered safe except if viewed with an optical tool (for

example, a microscope). Nevertheless you should not look directly at the output or into any fiber

end whenever a laser might be switched on.

WA R N I N G: Switch the OTDR off before you start to clean its connectors! Or at least disable

the laser.

WA R N I N G: INVISIBLE LASER RADIATION!

DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS. CLASS

3A LASER PRODUCT

2. Events on Fiber

An Event on a fiber is anything that causes loss or reflections other than normal scattering of the

fiber material itself. This applies to all kinds of connections as well as damages such as

bendings, cracks or breaks.

An OTDR trace displays the result of a measurement graphically on the screen. The vertical

axis is the power axis and the horizontal one is the distance axis. This section shows you drafts

of typical traces for the most common Events.

2.1 Single Fibers

A single fiber yields the following trace. You see the slightly decreasing power level

(attenuation) and strong reflections at the beginning and end of the fiber:

2.2 Whole Links

The trace of a whole link, for example, between two cities, may look like this. Besides the

normal attenuation you see Events and noise after the end of the link:

2.3 Beginning of a Fiber

If you are using a normal straight connector, the beginning of a fiber always shows a strong

reflection at the front connector:

2.4 Fiber End or Break

In most cases you see a strong reflection at the end of the fiber before the trace drops down to

noise level:

If the fiber is interrupted or broken, this is called a break. Breaks are non-reflective Events. The

trace drops down to noise level:

2.5 Connector or Mechanical Splice

Connectors within a link cause both reflection and loss:

A mechanical splice has a similar signature to a connector. Usually it has lower loss and

reflection values.

2.6 Fusion Splice

A fusion splice is a non-reflective Event, only loss can be detected. Modern fusion splices are so

good, they may be nearly invisible:

In the case of a bad splice, you may see some reflectance. Some splices appear as gainers as

if the power level increases. This is due to different backscatter coefficients in the fiber before

and after the splice:

If you see a gainer in a measurement taken in one direction, measure it from the other end of

the fiber. You will see a loss at this point in the fiber. The difference between the gainer and loss

(the "averaged loss value") shows the real loss at this point. This is why we recommend that

you take a 2-way aver- aging measurement of the fiber.

2.7 Bends and Macrobending

Bends in a fiber cause loss, but they are non-reflective Events:

To distinguish bends from splices, look at the installation and maintenance records. In the case

of macro bending, the loss is at an unknown location, Splices are at a documented, well-known

distance. If you measure at a higher wavelength, macro endings show a higher loss. We

therefore recommend that you make multi-wavelength measurements, so you can distinguish

between bending and splices.

2.8 Cracks

A crack refers to as a partially damaged fiber that causes relec- tion and loss:

The reflectance and loss may change when the cable is moved.

2.9 Patchcords

Patchcords are used to connect the OTDR to the fiber under test. The initial reflection is not

covering the beginning of the fiber. This allows better examination of the first connector:

3. Important Parameters

This section covers the definitions of the most important parameters used when characterizing

fibers.

3.1 Fiber Intrinsic Parameters

If you need more detailed information about your particular fiber, ask your fiber center.

• The Refractive Index

An OTDR calculates the distances to Events by measuring the time elapsed between

transmission of the light and reception of the reflection. This can be, for example, the rising

edge of the reflection of the front panel connector, or the reflection from a connector. The

distance displayed and the time measured is linked by the refractive index (sometimes

called group index). This means that changing the refractive index causes a change of the

computed distance.

How an OTDR measures a distance:

Definition of the refractive index:

Refractive index = (speed of light in vacuum) /(speed of a light pulse in a fiber)

Displayed distance on the OTDR:

distance = measured time x (speed of light in vacuum) / refractive index

The refractive index depends on the used fiber material and needs to be provided by the

fiber or cable manufacturer.

It is important to understand the refractive index of the fiber you are measuring. The error

due to this value not being known exactly is usually greater than any inaccuracies within the

instrument.

• The Scatter Coefficient

An OTDR receives not only signals from Events, but also from the fiber itself. As light travels

along a fiber, it is attenuated by Rayleigh scattering. This is caused by small changes of the

index of refraction of the glass. Some of the light is scattered directly back to the OTDR. This

effect is called backscatter. The scatter coefficient is a measure for how much light is scattered

back in the fiber. This affects the value of return loss and reflectance measurements.

The scatter coefficient is calculated as the ratio of the optical pulse power (not energy) at the

OTDR output to the backscatter power at the near end of the fiber. This ratio is expressed in dB

and is inversely proportional to the pulse width, because the optical pulse power is independent

of the pulse width.

A typical value is approximately 50 dB for 1 µs pulse width, depending on the wavelength

and the type of fiber.

3.2 Measurement Parameters

• The Pulse Width

One of the key parameters for good measurement results is the width of the light pulse

emitted into the fiber. It determines the distance resolution, which is very important to

separate Events clearly. The shorter the pulse, the better the distance resolution. A short

pulse, however, means that the dynamic range is smaller and the trace might be noisy.

If you want to measure long distances, you need a high dynamic range, so the pulse should

be long. Longer pulses, however, average the fiber over a wider section, which means lower

resolution.

Depending on the specific purpose of your measurement, you need a trade-off between

high-resolution and high dynamic range. Thus, choose a short pulse width if you want to

mea- sure the loss of splices or connectors that are close together. But choose a long pulse

width if you want to detect a break far away.

Short pulse width

It has high resolution but more noise. Decrease the pulse width in order to shorten the dead

zones and to separate close Events clearly.

Short Pulses for Better Resolution

Long pulse width

It has high dynamic range but long dead zones. Increase the pulse width in order to reduce the

noise and to detect Events far away.

Long Pulses for Large Dynamic Range

Typical values 5 ns / 10 ns / 30 ns / 100 ns / 300 ns / 1 µs (short links),

100 ns / 300 ns / 1 µs / 3 µs / 10 µs (long fiber links)

3.3 The Optimization Mode

A normal OTDR makes a trade-off between resolution and noise. The better the resolution, the

more the noise. This is because any hardware has a limited bandwidth. If the band- width is

narrow, you have less noise but also a poor resolution and a long recovery time after a strong

reflection. A wide band- width, however, can follow the received signal much faster—but the

circuit also produces more noise.

Agilent OTDRs have three different receiver paths in each module. Besides the Standard Mode,

one has narrower band- width and is optimized for the best Dynamic Range. The other has a

wider bandwidth for a good Resolution. You select a path by selecting the Optimization Mode

during the setup.

When optimizing for Dynamic Range, the OTDR uses long pulses and the trace has much less

noise. Thus, you can mea- sure the fiber even from great distances. But due to the narrower

bandwidth the receiver rounds the edges more than when optimizing for Resolution. It also

needs longer time to recover from connector reflections.

3.4 The Measurement Span

An OTDR measures a specified number of sampling points (max 15710). The measurement

span determines where these sampling points are distributed along the fiber. Hence, it defines

both the distance of a measurement and the sampling resolution. This resolution is the distance

between two adjacent measurement points.

Markers can only be set at sampling points. In order to place markers more accurately, you can

try varying the measurement span to yield sampling points closer to an Event.

The table below shows how sampling point distance and the measurement span are related:

3.5 Performance Parameters

• Dynamic Range

The dynamic range is one of the most important characteristics of an OTDR. It specifies the

maximum power loss between the beginning of the backscatter and the noise peaks.

If the device under test has a higher loss, the far end disappears in the noise. If it has less,

the end appears clearly above the noise and you can detect the break.

Please keep in mind that a trace is disturbed close to the noise level. For example, you need

the trace at least 6 dB above the noise in order to measure a 0.1 dB splice, and you need

approximately 3 dB to detect a break. This is why the dynamic range of the OTDR should be

at least 3 to 6 dB greater than your total system loss.

Like the deadzone, the dynamic range depends on the setup. The main influences are the

pulse width, the optimization mode and the wavelength. So any specification of dynamic

range must list the setup conditions.

The dynamic range can be given relative to the noise peaks or to the signal to noise ratio

(SNR) = 1. Using the noise peaks here is more appropriate. If the dynamic range is given as

SNR = 1, then subtract 2.2 dB to calculate the peak range.

• The Attenuation Deadzone

The deadzone is that part of an OTDR trace where a strong reflection covers measurement

data. This happens because a strong signal saturates the receiver and it takes some time

for it to recover. The attenuation deadzone describes the distance from the leading edge of

a reflective Event until it returns to the fiber’s backscatter level.

It is easy to determine the point where the leading edge starts but it is difficult to say when

recovery ends. So many companies place a +/– 0.5 dB margin around the backscatter after

the reflection. The deadzone ends at the point where the back- scatter stays within this

tolerance band.

In order to detect a splice or a break on the fiber, you need to examine the backscatter.

Events in the deadzone might be undetected, because the backscatter cannot be displayed.

The size of the attenuation deadzone depends strongly on the instrument's setup.

• The Event Deadzone

The Event deadzone is the minimum distance that you need between two Events of the

same type in order to see them separately.

For example, if you have two connectors two meters apart, you see a reflection with two

peaks and a drop between them. The drop indicates that there are really two reflections

from two different Events. If the Events are too close, then you would not see a drop and

you could not separate them.

The Event deadzone depends strongly on the instrument’s setup

• Averaging Time

The OTDR sends light pulses repetitively into the fiber. The results of each pulse are

averaged. This reduces the random noise of the receiver:

A longer averaging time increases the dynamic range by decreasing the noise floor of the

OTDR. The best improvements for the trace are achieved within the first three minutes:

4. Common Task

This section introduces the most common tasks that occur when measuring fibers and links.

The exact procedures to per- form the tasks are found in the manuals of your device or soft-

ware.

4.1 Cleaning a Fiber

To achieve accurate and repeatable measurements, all the connectors in your setup must be

clean. You can understand this requirement easily if you compare the diameter of a typical dust

particle with that of the core of a fiber. The dust is 10 to 100 µm across while single-mode fibers

have a 9 µm core. If you darken only 5% of the area where the light passes a connection, then

your insertion loss increases by 0.22 dB.

If you have doubts that the measurement result is correct, or if the measurement cannot be

repeated, then clean your connectors. In most cases a dirty adapter is the reason for such

errors. Thus, remove the connector interface and clean the instrument's connector, clean the

patchcord's connectors and clean the connectors on your fiber under test.

For cleaning the connectors, the following standard equipment is recommended:

• Dust and shutter caps

All cables come with covers to protect the cable ends from damage or contamination. Keep the

caps on the equipment at all times, except when your optical device is in use.

Be careful when replacing dust caps after use. Do not press the bottom of the cap onto the fiber

too hard, as any dust in the cap can scratch or pollute your fiber surface.

• Isopropyl alcohol

Only apply alcohol used for medical purposes. Never use any other solvent or alcohol with

additives, because they might damage your fiber.

After solving dust and dirt, remove the alcohol and dust with a soft swab or tissue.

• Cotton swabs

Use natural cotton swabs instead of foam swabs. Be care- ful when cleaning the fiber. Avoid

too much pressure, because it may scratch the fiber’s surface. Only use fresh clean swabs

and do not reuse them.

• Soft tissues

Cellulose tissues are very absorbent and softer than cotton tissues. Thus, they do not

scratch the surface unless you press too hard. Use care when cleaning the fiber and do not

reuse a tissue.

• Pipe cleaner

Pipe cleaners can be used to clean connector interfaces. Again, make sure you use a new,

fresh and soft cleaner and be careful to not scratch the device.

• Compressed air

The compressed air must be dry and free of dust, water and oil.

First spray into the air, as the initial stream of compressed air could contain some

condensation or propellant. Always hold the air can upright to keep propellant from escaping

and contaminating your device.

WA R N I N G: Disable the laser or switch the instrument off before you start to clean connectors!

4.2 Connecting the Instrument to a Fiber

Depending on the application, there are three major ways of connecting the fiber under test to the OTDR.

• Direct Connection

Agilent offers user exchangeable connector interfaces. If your fiber or cable has one of these connectors, then you can plug it in directly:

• Patchcord (Connector at Both Ends)

This is the recommended way if you want to measure a link in a system, especially if the terminal connector of the link is mounted in a rack:

• Pigtail with a Bare End

If the fiber under test has no connector at all, then use a bare fiber pigtail and an inexpensive mechanical splice. This pro- vides a good connection and repeatable measurement results:

4.3 The OTDR Display

All OTDRs display the measured fiber or link as a trace on the screen. The horizontal axis is the distance from the OTDR. The vertical axis is the relative power of the reflection of the emitted light pulse. The shape of the trace allows conclusions on the condition of the fiber and the included devices, such as connectors and splices.

In order to examine the trace in detail, you need to modify the trace view. The OTDR provides functions to change the scales of both axes, to zoom into parts of the trace, and to shift the trace along the axes.

The ranges in which you can display the trace are, for example, vertically between 0.2 dB/Div and 5 dB/Div and horizontally from full measurement to roughly 100 times larger.

Furthermore, you can set two markers A and B anywhere into the trace and make use of the zoom functions Around Marker A, Around Marker B, and Between Marke rs .

You need to be familiar with these functions, because they are most commonly used when working with an OTDR. Most of the tasks in the following sections are based on them.

4.4 Zooming into Traces

After the measurement is finished, the OTDR display presents an overview of the complete measurement. The vertical scale and the vertical offset are fixed:

Use the zoom functions around marker A or B to view particular regions in detail. The horizontal scale now is zoomed approximately to the factor of 10:

You can now move the marker position in this view gradually. The display, however, will still show the marker in the center. As a result, the trace seems to move to the left or right:

The scales for the complete trace of a 60 km link may be 6 km/Div and 5 dB/Div. This allows coarse positioning of a marker:

In the zoomed view, the scales may be 200 m/Div and 0.2 dB/Div. This allows much finer positioning of a marker:

In a fiber or cable production, you may need to test the uniformity of the attenuation. Position marker A at the beginning and marker B at least 500 to 2,000 m beyond marker A. Zoom the view between the markers to examine the attenuation. Additionally, you can move both markers parallel along the trace to view adjacent fiber parts:

4.5 Placing Markers Correctly

The position of an Event is always where the trace leaves the backscatter level. The exact locations of all Events are auto- matically determined and listed in the Event table.

For the position of a connector or another reflective Event, this is just at the beginning of the rising edge of the reflection:

The position of a non-reflective Event is just at the last back- scatter point before the trace bends downwards:

The location of a break is found at the beginning of the falling edge:

In order to measure the distance between two Events, place marker A before the first one and marker B before the second, as described on the previous page:

In order to measure the fiber’s attenuation between two Events, place marker A after the first Event, but place marker B before the second one:

Make sure that there are no Events between markers A and B, so the part of the trace between them is a straight line.

NOT E: Make sure that you use the correct refractive index in the setup; otherwise the distance values will be wrong!

4.6 Determining the Total Loss of a Link

Make a measurement of the whole link. Place marker A at the beginning and marker B at the end of the backscatter. Then zoom around marker A and position it precisely after the reflection of the first connector:

Now go to marker B and place it immediately before the end reflection:

Finally, go back to the full view and check whether or not the two markers are really placed correctly. Depending on your device, select the Loss function to display the total loss on the screen:

4.7 Determining the 2-Point Attenuation of a Fiber

Use the same procedure as for the measurement of the total loss (See “Determining the Total Loss of a Link” on page 44.). But instead of selecting the Loss function, choose 2-Point Attenuation.

The 2-point attenuation is the loss between markers A and B divided by the distance between the markers:

Because this function is only a division of the power difference by the distance it always gives reasonable results, even if there are connectors or splices between the markers.

4.8 Determining the Attenuation of a Fiber

The straight line between splices and connectors is the back- scatter of the fiber. In order to measure the attenuation of it precisely, place marker A after the first Event (to the left) and marker B before the second Event (to the right). Then select the Attenuation (LSA) function:

The LSA line causes severe errors if you include Events between the markers. So avoid this when using LSA.

Also, do not use the 2-point attenuation to measure a noisy fiber. The noise peaks may decrease the accuracy.

4.9 Determining the Loss of a Splice (Analyze Inser tion Loss)

Place Marker A at the splice and zoom the view around it. Select the Analyze Insertion Loss function. Four additional markers appear, that you can move on the trace. Place all four level-markers at the backscatter on the left-hand and on the right-hand side in order to approximate the fiber as closely as possible:

Keep the level-markers 2 and 3 close to the splice as shown above, and make the line segments between 1 and 2 and between 3 and 4 as long as possible. However, keep the lines strictly on the backscatter, even if it is noisy.

Make sure that the lines between the level-markers (the LSA line) follow a straight part of the trace. The LSA should not cover any part of the trace containing an Event:

4.10 Determining the Loss of a Connector

This measurement is very similar to the splice loss measurement, so it uses the same loss function. Place marker A at the connector and zoom around it. Start the Insertion Loss function. Four level-markers appear. Place all four level-markers at the backscatter at the left and at the right of the connector:

The same rules as for the splice measurement also apply to the level-markers here. Keep the lines strictly on the backscatter, even if it is noisy. In any case, avoid the region where the trace is rounded. This causes incorrect results:

4.11 Determining the Reflectance of a Connector

Place marker A at the beginning of the connector reflection and zoom around it. Make sure you can see both the backscatter and the top of the peak. If necessary, adjust the vertical zooming and the offset.

Activate the Reflectance function. Three level-markers appear. Move the first two markers to an average backscatter level (not on a noise peak) in front of the reflection. Confirm the position

and then move level-marker 3 to the peak of the reflection. The OTDR computes and displays the result in the readout field:

5. Practical Hints from OTDR Experts

This section contains practical hints and tricks collected from experienced people who use

OTDRs in factories, during installation and for maintaining telecommunication networks.

5.1 Know the Link to be tested

Before you start to characterize a fiber optic link, look at the installation plan. Make sure you have the right module and accessories. Determine the wavelength to be used.

Determine whether you are measuring this link for the first time, or whether you are comparing the measurement with an older one.

If you are comparing with an older measurement, you only need to load the previous trace as the reference in the com- pare mode. The OTDR will do the setup automatically and you only have to start the new measurement.

5.2 Clean the Connectors

A dirty connector makes measurements unreliable, very noisy, or even impossible. It may also damage the OTDR. Further- more, watch out for index–matching oil. Some types dissolve the adhesives inside connectors.

5.3 Is the Connector or the Patch Cord Damaged?

Be sure that the connector is clean. And check whether the patch cord, the module, and the fiber under test are single-mode or multimode. To test the patch cord, activate the laser in the CW mode and measure the power at the end of the patch cord with a power meter, for example, an Agilent E6006A. This should display between 0 and - 4 dBm for most single-mode modules and wavelengths.

5.4 Instrument Settings

If you use the OTDR regularly for similar links, then optimize the setup for these applications and store it in one of the four user definable settings. Use a meaningful name for it (for example, INTER STATE, CITY LINK, FEEDER, TRUNK, and so on).

5.5 Recommended Setup Parameters

Set the measurement span slightly longer than the length of the link. For example, if your link is 56.3 km long, choose 60 km. For distances greater than approximately 15 km, make your first measurement in longhaul mode, otherwise use shorthaul. Begin with a 1 ms pulse for spans greater than 10 km, and 100 ns below that span. Set the refractive index according to your information about the link. If the index is not known, use 1.4580 as this is a typical value.

5.6 Noisy Traces

If the trace is very noisy, increase the number of averages. If you already averaged more than 100 times, then increase the pulsewidth. Try to average over a longer time.

5.7 Real time Mode

Activate the instrument’s Real-time Mode, if you want to adjust the settings during a measurement. In this mode the instrument averages for 0.3 seconds only, thus, you get three display updates per second. This mode allows to change any setup parameter without the need to stop the measurement.

This is in contrast to the continuous average mode with one update per second. In this mode, it is required to stop a measurement explicitly before you can modify parameters. This avoids that you erase a trace averaged over a long time by accident.

You use Realtime mode to check your connection, the quality of splices, and whether a fiber is connected. Start in Automatic mode, then switch to Realtime mode and select the most suit- able parameters.

5.8 Very Long Deadzone

If the deadzone is too long to separate the Events of interest, reduce the pulsewidth. If you are in Optimize Dynamic mode, first try repeating the measurement in Optimize Resolution mode, before you reduce the pulsewidth.

5.9 What to Do if No Trace is Visible

In case you lost the trace when zooming into it, return to the full view.

If you see only noise instead of a trace, then either the measurement span is far too long, or the start position is beyond the end of the fiber. Check both values in the setup. Also check the connection to the fiber.

5.10 Adjust the Refractive Index

You can measure the refractive index if you know the exact physical length of the fiber under test. Start the measurement with refractive index 1.5000. Place a marker at the end of the fiber. Then select the Refractive Index function and adjust it until the displayed marker position is equal to the known fiber length. Now the effective refractive index is displayed.

5.11 The Exact One-Way Loss

The OTDR’s loss measurements are based on the backscatter effect in the fiber. Because this effect changes in different fibers, the loss accuracy may not meet your requirements. In order to measure the link’s loss more precisely, the single-mode modules provide a CW mode. This mode simply switches the laser on.

Measure the power (given in dBm) with a power meter (for example, the Agilent E6006A) at the end of a short patchcord. The absolute value of the power varies from one source mod- ule to another but the power for a particular module remains very stable over hours. Then connect the link to the patchcord and measure the power at the far end. The difference between the two results is the one-way loss of the fiber.

5.12 Bending Loss

In 1550 nm single-mode, fibers are very sensitive to macro bending as for example, a tight bend or local pressure on the cable. It happens sometimes that you see a bending loss clearly at this wavelength but not at all at 1310 nm. Hence, characterize your link at both wavelengths.

5.13 Before You Save a Trace

After your measurement is completed you should enter identification data before you save a trace on a disk or memory card. For this purpose, the OTDRs provide the Trace Information window, accessible from the File menu.

Use this feature to store the cable ID, the fiber ID, the origin and the termination location, and the fiber operator. The used OTDR and modules as well as the date and time of the measurement are saved automatically with the file.

This will help a lot if you need the trace later for comparison purposes or for further analysis on a personal computer.

5. Automatic Trace Analysis

Many links consist of several sections which are connected or spliced together. A good quality control after installation is the measurement of all losses on the link in order to verify that the splices, connectors, etc. meet their specification. However, doing this manually is a time consuming process.

5.1 Seeking Events above a Threshold

The Agilent OTDRs accelerate this task with a built-in trace analysis function: Scan Trace seeks Events on the trace from the beginning to the end. If an Event exceeds a given threshold (for

example, 0.05 dB) then the OTDR lists it in a table. The table contains the Event’s position, its loss and return loss (if it is reflective), and the fiber attenuation between the Events.

After a trace has been scanned automatically, the OTDR keeps the Event table together with the trace and the setup. This means that the table is also saved when you store the trace in a binary or in an ASCII file. By reading the ASCII file into a PC you can use this information to compute statistics.

For noisy sections of the fiber the OTDRs increase the threshold in order to reduce the sensitivity to noise peaks. However, it is still often very hard to decide whether something is a real Event or a distortion due to the noise. So it is important to look at the Events closely. If necessary, remove any reported Event that is just a noise peak. Or add any Event that was assumed to be noise.