UNIT – 2

Transmission Characteristics of optical Fibers

Syllabus: Introduction, Attenuation, Absorption, scattering lossess, bending losses, dispersion , Intramodal dispersion, Intermodal dispersion.

Transmission Characteristics of optical fibers

When suitability of Optical fibers for communication are investigated , the transmission characteristics of utmost interest are

1. Attenuation 2. Signal distortion

Attenuation plays major role in determining - the maximum transmission distance between a transmitter and

receiver or an inline amplifier.

The Signal distortion of the fiber - determines the bandwidth and hence volume of information that can

be transmitted in a given time.

Attenuation mechanisms in a fiber are1 Material absorption:

- due to impurities of glass such as iron manganese , copper etc . Can be reduced by containing these impurities during manufacturing

2. Linear and non linear scattering

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- associated with both fiber material and structural imperfection in optical waveguide

3. fiber bends and radiative losses of the optical energy – both microscopic and macroscopic bends

Attenuation Units:

As light travels along the fiber its power decreases exponentially with distance

Decibels refer to Logarithm with Base 10 and Nepers refer to logarithm with base e. (Conversion factor: See the box below)

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Attenuation in Bits per kilometer:

Let Pi be the input power and Po be the power. Therefore Pi/Po = 10(dB/10). In fiber optic communication the attenuation is usually expressed in Decibels per unit length (i.e. dB/ Km) following:

α dB L = 10 log 10 (Pi/Po )

where α dB is the signal attenuation per unit length in decibels and L is the fiber length.

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Example:

A signal of 10 mw is coupled to a 10 Km long fiber and a signal of 1 microwatt is detected at the end of the fiber. Find the loss in dB.

Solution:

Loss in dB = dB loss = 10 log10 (Pi/ Po)

= 10 log10 (10mw/ 10microwatt)

= 40 dB

This loss is for 10 kms. Hence loss per kilometer is 40dB/10 = 4 dB

Or attenuation is 4dB/km

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ABSORPTION

1. Absorption due to atomic defects in the glass composition

a. Atomic defects – missing molecules, high density clusters of atom groups, or oxygen defects in Glass

2. Extrinsic absorption by impurity atoms

3. intrinsic absorption by basic constituent atoms of the fiber material

When fibers are used in radiation environments (Space, nuclear plants, medical radiation therapies etc) the accumulated radiation doses damage the internal structures. These atomic defects absorb optical energy resulting in attenuation. The higher the radiation level larger is the attenuation. However, these attenuation centers (atomic defects) will relax or anneal out with time in the absence of radiation (Fig 3.1 K4 p- 92)

Figure 3.1 :

Dominant absorption factor in Glass –

presence of OH- ions dissolved in glass and

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other impurities like iron copper vanadium and chromium

Water impurity levels of less than a few parts per billion (ppb), are required if the attenuation is to be less than 20 dB/km (that was in early fibers).

The high levels of OH- in early fibers resulted in large absorption peaks at 725, 950, 1240, 1380 nm. Regions of low attenuation lie between these peaks.

The OH hydroxyl impurities are due to

usage of oxyhydrogen flame during manufacture of glass. Modern vapour phase method of manufacturing glass have reduced these impurities drastically improving the attenuation performance. SMF of 0.4 dB per km have been produced.

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Standard commercially available SMF have nominal attenuation of 0.4 dB/km at 1310 nm and less than 0.25 dB/km at 1550 nm. Further elimination of water ions diminishes the absorption peak around 1440 nm and thus open up another band for data transmission (E band). Optical fibers in this band are known by names such as “ low water peak” or “full spectrum fiber”.

Intrinsic Absorption

INTRINSIC ABORPTION is associated with the basic fiber material.

(eg: Pure SiO2 ) and

is the principal physical factor, that defines the transparency window of a material over a specified spectral region. Intrinsic absorption thus sets the fundamental lower limit on absorption for any particular material.

Intrinsic absorption results from electron absorption in the UV and from atomic vibration bands in NIR. (Near Infra Red)

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In NIR region above 1.2 μm the loss is predominantly determined by the presence of OH ions and the inherent IR absorption of the constituent material

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Scattering Losses

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STIMULATED RAMAN SCATTERING:

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DISPERSION affects both digital and analog transmission for digital transmission the dispersion mechanism within the fiber

cause broadening of the transmitted pulses as they travel along the channel

o each pulse broadens and overlaps with its neighbours eventually becoming indistinguishable at the receiver input. (this effect is known as Inter Symbol Interference).

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(ref: Fig 3.6 Senior book)

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Signal distortion in an optical fiber happens due to several reasons1.Intermodal delays (MODAL DISPERSION)2.Intramodal delays3.Polarisation mode dispersions

INTERMODAL DISPERSION:

Pulse broadening resulting from the polarization delay difference between modes within a MMF is known as Intermodal delay

- also known as intermodal delay or simply modal or mode dispersion- appears only in MMF- is due to different value of group velocity for each mode of the given

frequency- pulse width at the output is dependent on transmission times of

slowest and fastest modes- Step index fiber exhibit large amount of intermodal dispersion- Intermodal dispersion may be reduced by using GIF (near parabolic

profile)

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Fig 1IM

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Fig 2IM

In an MMF , the steeper the angle of propagation of Ray Congruence, the higher is the mode number and consequently the slower the axial group velocity

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INTRAMODAL DISPERSION

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(Or Chromatic Dispersion or Group Velocity Dispersion) Is the pulse spreading that takes place in a SMF Spreading of pulse due to finite spectral width

- The spectral width (or wavelength band) is characterized by root mean square (rms) spectral width σλ. - For LED depending on the device structure the spectral width is approximately 4 to 9 % of its central wavelength. For e.g., in figure 3.11 the spectral width is 36 nm and central wavelength is 850 nm. - Laser based diodes have narrower spectral width of 1-2 nm and 10-4 nm for single mode lasers.

Two main causes for Intramodal dispersion are as follows:

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(a) Material dispersion/Chromatic dispersion(a) Due to variations of RI of core material as a function of

wavelength(b) As group velocity is a function of wavelength, the different

wavelength components of the source travels at different speeds (even on the same path) resulting in pulse spreading

(b) Waveguide dispersion:(a) As core and cladding has different RI signals travels faster in cladding than in core resulting in dispersion.(b) Waveguide dispersion can be ignored in MMF but its effect is significant in SMF

POLARISATION MODE DISPERSION

In SMF light travels in two orthogonal modes.At the start of the fiber the two polarization states are aligned.

However, since fiber material is not perfectly uniform through out its length, each polarization mode will encounter a slightly different RI. As a result each mode will travel at a slightly different velocity. This causes pulse spreading.

If the resulting time difference in propagation times is denoted as ∆τpmd (between two orthogonal polarization), and ‘L’ the distance traveled by the pulse, then

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Unlike chromatic dispersion which is a relatively stable phenomenon, the PMD varies

randomly along a fiber. Due to this factor, the ∆τpmd cannot be used to compute the total PMD. A statistical estimation need to be done.

DISPERSION CALCULATION

The total chromatic dispersion in SMF consists mainly of material and waveguide dispersion. The resultant intra-modal and chromatic dispersion is represented by

where τ is the group delay. The dispersion is commonly expressed in ps/(nm.km). The broadening σ of an optical pulse over a fiber of length L is given by

where σλ is the half power spectral width of the optical source.

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Extra Material:

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