Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: MAY 2016

Loss Mechanisms at Joints:

Fresnel Reflection

i. Optical Loss encountered at the interfaces even when two fiber ends are smooth, perpendicular to fiber axes and perfectly aligned.

ii. A small proportion of light may be reflected back into transmitting fiber causing attenuation at the joint.

iii. Light that is reflected back into the source fiber is lost. This reflection loss, called Fresnel reflection, occurs at every fiber interface.

iv. Fresnel reflection is caused by a step change in the refractive index that occurs at the fiber joint.

v. Can be reduced to a very low level using index matching fluid in the gap between jointed fibers.

$R = (\dfrac{n_1- n}{n_1+n})^2$

R is the fraction of the incident light reflected at the fiber

$n_1:$ R.I. of core

n: R.I. of interfacing medium ( = 1 for air) Loss in decibel due to FR at single interface

$Loss_{Fres} = -10 \log_{10}(1 - r)$

Index matching gel reduces the step change in the refractive index at the fiber interface, reducing Fresnel reflection.

Deviation in Geometrical & Optical Parameters:

All light from one fiber is not transmitted to another fiber; Because of mismatch of mechanical dimension Three major cases:

a) Core mismatch

b) NA mismatch

c) Index Profile

Intrinsic Losses:

Minimized using fibers manufactured with lowest tolerance i.e.(same fiber)

Losses due to:

$\rightarrow$ Fresnel Reflection

$\rightarrow$ Deviation in Geometrical & Optical parameters

Intrinsic coupling losses are limited by reducing fiber mismatches between the connected fibers.

Extrinsic Losses:

Extrinsic coupling losses are caused by jointing techniques. Fiber-to-fiber connection loss is increased by the following sources of intrinsic and extrinsic coupling loss:

$\rightarrow$ Reflection losses

$\rightarrow$ Fiber separation

$\rightarrow$ Lateral misalignment

$\rightarrow$ Angular misalignment

$\rightarrow$ Core and cladding diameter mismatch

$\rightarrow$ Numerical aperture (NA) mismatch

$\rightarrow$ Refractive index profile difference

$\rightarrow$ Poor fiber end preparation

$\rightarrow$ Losses due to some imperfection in splicing

$\rightarrow$ Caused by Misalignment

Three possible types of misalignment at joint:

(a) Longitudinal misalignment

(b) Lateral misalignment;

(c) Angular misalignment