Question

Please define the active medium from angle of quantum theory. Why do we select the modified Hankel functions and Bessel functions within the cladding and core for a cylindrical fiber optic cable respectively?

Answer 1

The active laser medium (is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.

In order to fire a laser, the active gain medium must be in a nonthermal energy distribution known as a population inversion. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission,or with high-energy electron beams

There are certain limitations to ray model. The ray model does not predict correctly that even after total internal reflection there will be some field in the cladding Also it does not predict that rays can be launched at only discrete angles in an optical fiber. For an accurate and complete description of light propagation inside an optical fiber we have to go in for a more rigorous model, called the WAVE MODEL. Here we shall discuss the propagation of light inside an optical fiber, treating light as an electromagnetic wave. Inside a fiber core the optical energy gets guided i.e. the energy propagates along the axis of the core and the fields exponentially decay in the cladding away from the core-cladding interface. . In a practical fiber the cladding is surrounded by a protective layer. Generally, by the time the field reaches that layer, it dies down significantly so that the protecting layer does not affect the propagation of the wave significantly. That is, the whole propagation of light is governed by the core cladding interface and the interface between the cladding and other protecting layers does not affect the propagation. In other words, we can take the cladding to be of infinite size in our analysis, without incurring significant error.