Optical Fiber Concepts

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Transcript of Optical Fiber Concepts

  • Fiber Optics Basics

  • Principal Fiber Optic Transmissionthe electrical signal processing is according to international standards

    the conversion into the "optical frequency band" enables to use the advantages coming up with F.O. transmission

    electricalsignalprocessingElectricalTransmissionE / O -ConversionElectricalTransmissionO / E -ConversionelectricalsignalprocessingOpticalTransmissionFiber as transmission medium

  • Fiber PrinciplesA ray of light enters into the fiber at a small angle a.The capability (maximum acceptable value) of the fiber cable to receive light on its core is determined by its numerical aperture NA:

  • Fiber Principleswhere: a0: maximum angle of acceptance(i.e limit between reflection and refraction)n1: core refractive indexn2: cladding refractive index

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    The velocity at which light travels through a medium is determined by the refractive index of the medium. The refractive index (n) is a unit less number which represents the ratio of the velocity of light in a vacuum to the velocity of the light in the medium.n= c/vWhere n: Refractive Indexc: Speed of light in a vacuum (approximately 3 x 108 m/s)V: Speed of light in the transmission mediumTypical values of n lie between 1.45 and 1.55.

  • Light propagation. If a > a0: the ray is fully refracted and not captured by the core.

    If a < a0: the ray is reflected and remains in the core.

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  • Advantage of F.O. TransmissionEnormous bandwith-->Broadband servicesVery low attenuation-->Long repeater distanceNo crosstalk-->Immunity, good qualityResistance against-->Widely applicableenvironmentNO RFI, EMI-->High reliabilityLeight weight of fiber-->Airplane, Sea...SiO2-->No availability restrictions

  • FormulaPhase velocity c c = l x fl wavelengthf frequencyPhase velocity c0 c0 = 300 000 Km/sin vacuumRefractiv index ni ni = c0 : cini refractiv index in medium ic0 velocity in vacuumci velocity in medium i

    Snells law (refraction law)

    sin a1 / sin a2 = n2 / n1 = c1 / c2a1 angle of incident in medium 1a2 angle of transition medium 2n1 refractive index in medium 1n2 refractive index in medium 2 c1 velocity in medium 1c2 velocity in medium 2

  • Factors Causing AttenuationLight Absorption: Intrinsic absorptions (due to fiber material and molecular resonance) and extrinsic absorptions (due to impurities such as OH- ions at around 1240 nm and 1390 nm). In modern fibers, extrinsic factors are almost negligible.Rayleigh scattering: Scattering causes the light energy to be dispersed in all directions, with some of the light escaping the fiber core.Bending losses

    Micro bending: Caused by light escaping the core due to imperfections at the core/clad boundaryInternal angle of acceptance: The angle of incidence of the light energy at the core/cladding boundary exceeding the Numerical ApertureMacro bending: Bending of the fiber

    Forward light scattering (Raman Scattering) and backward scattering (Brillouin scattering) are two additionalscattering phenomena that can be seen in optical materials under high-power conditions.

  • Types of DispersionModal dispersion: when a very short pulse is injected into the fiber within the numerical aperture, all of the energy does not reach the end of the fiber at the same time. Different modes of oscillation carry energy down the fiber down different paths and thus travel further. Chromatic dispersion: the pulse sent down the fiber is usually composed of a small spectrum of wavelengths. This means they go through the fiber at different speeds.

    Chromatic dispersion is expressed in picoseconds per nanometer per kilometer: ps / (nm x km). This coefficient, at a given wavelength, represents the difference after one kilometer between the propagation time of two wavelengths which differ by a given number of nanometers. Chromatic dispersion is the dominant dispersion mechanism in single mode fibers. In single mode fibers there is a minimum or zero (chromatic) dispersion wavelength determined by fiber design and manufacture, and this wavelength is generally chosen to be near the operating wavelength of the system. Historically (in standard single mode fiber), this was near 1310 nm, but for newer systems, so-called dispersion shifted fibers are used with the zero dispersion wavelength moved closer to 1550 nm to take advantage of the lower fiber attenuation at that wavelength.

  • Fiber Types: Multimode Fiber

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    Step index multimode fibersStep-index fiber guides light rays through total reflection on the boundarybetween core and cladding. The refractive index is uniform in the core.

    Graded-index multimode fibersGraded-index (GI) fibers are obtained by giving to the core a non-uniformrefractive index, decreasing gradually from the central axis to the cladding.This index variation of the core forces the rays to progress in the fiber in asinusoidal manner.

  • Fiber Types: Single mode Fiber

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    The advantage of single mode fiber is its higher performance with respect to bandwidth and attenuation. The reduced core diameter limits the light to propagation of only one mode, eliminating modal dispersion completely. With proper components, a single mode fiber system can carry signals in excess of 10 GHz for over 100 km. The system carrying capacity may be further increased by injecting multiple signals of slightly differing wavelengths (Wavelength Division Multiplexing) into one fiber. The small core size generally requires more expensive light sources and alignment systems to achieve efficient coupling and splicing and connectorization is also somewhat complicated. Nonetheless, for high performance system or systems over a few kilometers, single mode fibers remain the best solution.

  • Transmission tests End-to-end optical link loss Rate of attenuation per unit lengthAttenuation contribution to splices, connectors, couplers (events)Length of fiber or distance to an eventLinearity of fiber loss per unit length (attenuation discontinuities)Reflectance or optical return loss

    These are main measurements implemented on optical fibers and optical fiber systems in order to qualify their use for information transmission purposes.

  • Attenuation of Different Fiber Components0.2 dB/km for single mode fiber loss at 1550 nm;0.35 dB/km for single mode fiber loss at 1310 nm;0.05 dB for a fusion splice0.1 dB for a mechanical splice;0.2 - 0.5 dB for a connector pair;3.5 dB for a 1 to 2 splitter (3 dB splitting loss plus 0.5 dB excess loss).

    Optical loss budgetAn optical loss budget lies within maximum and minimum values:The maximum value is defined as the ratio of the minimum optical power launched by the transmitter to the minimum which may be received by the receiver whist still maintaining communication;The minimum value is defined as the ratio of the maximum optical power launched by the transmitter to the maximum which may be received by the receiver whist still maintaining communication.

  • The OTDR depends on two types of optical phenomena: Rayleigh Backscattering and Fresnel Reflections:Rayleigh scattering is intrinsic to the fiber material itself and is present along the entire length of the fiber. Fresnel reflections are "point" events and occur only where the fiber comes in contact with air or another media such as at a mechanical connection/splice or joint.Principles of an OTDR

    An OTDR (Optical Time Domain Reflectometer) is a fiber optic tester characterizing fibers and optical networks. The aim of this instrument is to detect, locate and measure events at any location in the fiber link.The OTDRs ability to characterize a fiber is based on detecting small signals returned back to the OTDR in response to injection of a large signal, much like a "radar". In this regard, the OTDR depends on two types of optical phenomena: Rayleigh Backscattering and Fresnel Reflections.

  • Rayleigh scattering

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    When a pulse of light is sent down a fiber, some of the photons of light are scattered in random directions from microscopic particles. This effect, referred to as Rayleigh scattering, provides amplitude and temporal information along the length of cable. Some of the light is scattered back in the opposite direction of the pulse and is called the backscattered signal.

  • Fresnel reflection

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    Fresnel reflection is due to the light reflecting off a boundary of two optical transmissive materials, each having different index of refraction. This boundary can occur either at a joint (connector or mechanical splice), at an non-terminated fiber end, or at a break.The magnitude of the Fresnel reflection is dependent upon the incident power and the relative difference between the two indices of refraction. The amount of light reflected depends upon the boundary surface smoothness and the index difference.Reflected light from a boundary between a fiber and air has a theoretical value of -14 dB. This value can be over 4000 times more powerful than the level of the backscatter. This means that the OTDR detector must be able to process signals which can vary in power enormously.

  • OTDR block diagram

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    The OTDR injects light energy into the fiber through a laser diode and pulse generator. The returning light energy is separated from the injected signal using a coupler and fed to the photodiode. The optical signal is converted to an electrical value, amplified, sampled and then displayed on a screen.

  • OTDR ComponentsLaser diodes: Laser diodes are selected according to the wavelength of the test.Pulse generator with laser diode: A pulse generator controls a laser diode which sends powerful light pulses (from 10 mW