Optical Communication Questions and Answers

Answer: D

The technique best suited for low cost fibers is the double crucible method.

In the double crucible method, molten core glass in an inner crucible and cladding glass in an outer crucible flow out together through concentric nozzles and are drawn directly into fibre. It is a continuous process that works with low-melting multicomponent (soda-lime or borosilicate) glasses and needs no preform, so it is cheap, though the fibres have higher loss.

VAD, OVD and MCVD are vapour deposition methods that build a high-purity silica preform; they give very low-loss telecommunication fibre but at higher cost.

Answer: A

The key names graded index fibre.

I could not find a text that states this property for any of the four fibres, so I cannot confirm the key; nothing I found shows it wrong either. What can be said is how the options differ. A single-mode fibre carries nearly all its light near the axis of a very thin core, so axis impurities affect it more than any other fibre, which makes option B the least likely. A plastic-clad silica fibre has a step-index core and is limited mainly by the cladding, not by the axis.

In a graded-index fibre the refractive index is highest at the axis and falls smoothly outward, so rays are bent back toward the axis rather than travelling through it, and the key reads this as making the axis region less critical. The explanation is the University's reading, not a confirmed textbook statement.

Answer: C

A single-mode fibre has a step change from core to cladding.

Standard single-mode fibre (such as ITU-T G.652) is a step-index fibre: a very thin core, about 8-10 micrometres across, with a uniform refractive index slightly higher than the uniform index of the cladding, so the index drops abruptly at the boundary. Because the core is so small, only one mode propagates, and intermodal dispersion does not arise, so there is no need for a graded profile.

A gradual, roughly parabolic decrease from the core axis is the graded-index profile used in multimode fibre to reduce modal dispersion. An index increasing from core to cladding would not guide light at all, since total internal reflection needs the core index to be higher.