Losses in Fiber Optics

Losses in Fiber Optics

Attenuation, dispersion-intermodel, Intramodel, bend loss-micro macro scattering losses-Linear, Non linear, Absorption

Link Budget, Power Budget Block diagram and working of OTDR

Attenuation

Attenuation means loss of light energy as the light pulse travels from one end of the cable to the other.

It is also called as signal loss or fiber loss. It also decides the the number of repeaters required

between transmitter and receiver. Attenuation is directly proportional to the length of the

cable.

Attenuation Attenuation is defined as the ratio of optical output

power to the input power in the fiber of length L. α= 10log10 Pi/Po [in db/km]

where, Pi= Input Power

Po= Output Power, α is attenuation constant

The various losses in the cable are due to Absorption Scattering Dispersion Bending

Bending losses

The loss which exists when an optical fiber undergoes bending is called bending losses.

There are two types of bending

i) Macroscopic bending

Bending in which complete fiber undergoes bends which causes certain modes not to be reflected and therefore causes loss to the cladding.

ii) Microscopic Bending

Either the core or cladding undergoes slight bends at its surface. It causes light to be reflected at angles when there is no further reflection.

ISO 9001 : 2008 certified

Macroscopic Bending

Microscopic Bending

Absorption Loss

Absorption of light energy due to heating of ion impurities results in dimming of light at the end of the fiber.

Two types:

1. Intrinsic Absorption

2. Extrinsic Absorption

Intrinsic Absorption: Caused by the interaction with one or more

components of the glass Occurs when photon interacts with an electron in the

valence band & excites it to a higher energy level near the UV region.

Extrinsic Absorption: Also called impurity absorption. Results from the presence of transition metal ions like

iron, chromium, cobalt, copper & from OH ions i.e. from water.

Dispersion Loss

As an optical signal travels along the fiber, it becomes increasingly distorted.

This distortion is a sequence of intermodal and intramodal dispersion.

Two types:

1. Intermodal Dispersion

2. Intramodal Dispersion

Intermodal Dispersion: Pulse broadening due to intermodal dispersion results

from the propagation delay differences between modes within a multimode fiber.

Intramodal Dispersion: It is the pulse spreading that occurs within a single

mode. Material Dispersion Waveguide Dispersion

1) Material Dispersion: Also known as spectral dispersion or chromatic

dispersion. Results because of variation due to Refractive Index

of core as a function of wavelength, because of which pulse spreading occurs even when different wavelengths follow the same path.

2) Waveguide Dispersion: Whenever any optical signal is passed through the

optical fiber, practically 80% of optical power is confined to core & rest 20% optical power into cladding.

Scattering Losses

It occurs due to microscopic variations in the material density, compositional fluctuations, structural in homogeneities and manufacturing defects.

Linear Scattering Rayleigh Scattering losses Mie Scattering Losses Waveguide Scattering Losses

Non-linear Scattering Stimulated Brillouin Scattering(SBS) Stimulated Raman Scattering(SRS)

i) Linear Scattering

a) Rayleigh Scattering Losses: These losses are due to microscopic variation in the

material of the fiber. Unequal distribution of molecular densities or atomic

densities leads to Rayleigh Scattering losses Glass is made up of several acids like SiO2, P2O5,etc.

compositions, fluctuations can occur because of these several oxides which rise to Rayleigh scattering losses

b) Mie Scattering Losses: These losses results from the compositional

fluctuations & structural inhomogenerics & defects created during fiber fabrications, causes the light to scatter outside the fiber.

c) Waveguide Scattering Losses: It is a result of variation in the core diameter,

imperfections of the core cladding interface, change in RI of either core or cladding.

ii) Non-linear Scattering

a) SBS Scattering: Stimulated Brillouin Scattering(SBS) may be regarded

as the modulation of light through thermal molecular vibrations within the fiber.

Pb =4.4x10-3d2λ2α dB v watts

where, λ= operating wavelength μm

d= fiber core diameter μm

v = source bandwidth in GHz

b) SRS Scattering: Stimulated Raman Scattering is similar to SBS except

that high frequency optical phonon rather than acoustic phonon is generated in scattering processes.

Pb =5.9x10-2d2λα dB watts

Phonon:

Collective excitation in a periodic arrangement of atoms or molecules in solid.

Optical Time Domain Reflectometer

What is OTDR?

It is a trouble shooting device to find faults, splices and bends in fiber optic cable.

It is used to measure time and intensity of light reflected on an optical fiber.

It can detect light loss and pinpoint trouble areas making repair easy.

OTDR test can be anywhere along the length of fiber from ten seconds to three minutes

Principle of Operation

OTDR emits a high-power pulse that hits the fiber and bounces back.

What comes back is measured, factoring in time and distance, and results in “trouble spots” which can be targeted for repair.

The more quickly trouble areas are identified and addressed the less fiber optic network will suffer from data transfer problems.

Block Diagram

Pulsed Laser

Photo Detector

APD

Integrator Log Amplifier

Chart Recorder

Coupler Fiber

Working

A light pulsed is launched into the fiber in forward direction from an injection laser using a coupler or beam splitter.

Beam splitter or coupler makes possible to couple the optical excitation power impulse into the tested fiber and to deviate the backscattered power to the optical receiver.

The backscattered light is detected using an Avalanche Photodiode receiver.

Output of photodiode receiver drives an integrator.

Integrator improves SNR by giving an arithmetic average over a number of measurements taken at one point.

This signal is fed to Logarithmic amplifier and average measurements for successive points within the fiber are plotted as a Chart Recorder.

Overall link length can be determined from the time difference between reflection from the fiber input and output end faces.

The below fig shows the possible backscatter plot for the fiber under test.

ISO 9001 : 2008 certified

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ACS Important Questions 6. Losses in fiber optics (M=8)

1. With a neat diagram explain working of OTDR. (Twice) 2. Explain in brief the two fiber band losses. 3. Define: a) Reflection b) Diffraction c) Absorption 4. Explain Dispersion with help of light theory. 5. Explain the losses due to scattering in FOC & state applications. 6- What is dispersion? Explain inter model dispersion. 7. ) An optical fiber communication is to be designed to operate over an 8 KM length without using repeaters. The use times of the chosen component are source (LED) 8ns. Fiber inter model 5ns KM –1(Pulse broadening) intramodel 1ns KM –1.Detector (p-I-n photodiode) 6 ns b) The following parameters are established for a long haul single – mode optical fiber operating at a wavelength of 1.3 um. Mean power launched from the laser Transmitter –3 dbm Cable fiber loss 0.4 db KM –1 Splices loss 0.1 db KM –1 Connector losses at the Transmitter & Receiver When operating at 35 Mbits-1 (BER 10-9) –55 dbm When operating at 400 Mbits-1 (BER 10-9) –44 dbm Required safety margin 7db Estimate i) The maximum possible link length without repeaters when operating at 35 Mbits-1 (BER 10-9). It may be assumed that there is no dispersion equalization penalty at this bit rate. ii) The maximum possible link length without repeaters when operating at 400 Mbits-1 (BER 10-9). & assuming no dispersion equalization penalty. 8. Describe attenuation in optical fiber