Answer to Question #181369 in Optics for Asif

Question

State the two major categories of fiber–fiber joint, indicating the differences between them. Briefly discuss the problem of Fresnel reflection at all types of optical fiber joint, and indicate how it may be avoided. A silica multimode step index fiber has a core refractive index of 1.46. Determine the optical loss in decibels due to Fresnel reflection at a fiber joint with: (a) a small air gap; (b) an index-matching epoxy which has a refractive index of 1.40. It may be assumed that the fiber axes and end faces are perfectly aligned at the join

Answer 1

The two types of fibre-fibre joints are-

1. Multimode fibre joints

2.Single-mode fibre joints

Multimode fibre joints:In these kind of fibre-fibre joints,We consider here some of the expressions used to calculate losses due to lateral and angular misalignment of optical fiber joints.

Single Mode fibre joints:The joint shape is step index or graded index, such joints have hight optical loss. we can minimize the loss by making it a multimode joint. Hence Multimode joints are better.

Fresnal loss is also one of the major consideration while study optical loss. Even when the two jointed fiber ends are smooth and perpendicular to the fiber axes, and the two fiber axes are perfectly aligned, a small proportion of the light may be reflected back into the transmitting fiber causing attenuation at the joint. 

Factor responsible for losses:

1.Different core diameters;

2.Different relative refractive index differences;

3.Different refractive index.

4.Fiber faults (core ellipticity, core concentricity, etc.).

Method to reduce losses:

1. Make sure to adapt the high-quality cables with same properties as much as possible.

2. Choose qualified connectors as much as possible

3.Make sure that the insertion loss should be lower than 0.3dB and the additional loss should be lower than 0.2dB etc.