Fiber Optic Tutorials

In some local-area networks, a coherent source is not required, and one can employ a light-emitting diode (LED), a less expensive and longer-lasting optical source with a relatively wide optical spectrum. The basic structure of an LED is similar to that of semiconductor lasers in the sense that both employ an active layer sandwiched between two cladding layers and pumped using a forward-biased p-n junction. The main difference is that stimulations emission does not occur because a population inversion is not realized. Rather, radiative recombination of electron-hole pairs in the active layer generates light through spontaneous emission, some of which...

Semiconductor lasers oscillate in several longitudinal modes simultaneously because of a relatively small gain difference (~0.1 cm^-1) between two neighboring modes of the cavity. The resulting spectral width (2-4 nm) is acceptable for some applications but becomes a concern for many others. This tutorial is devoted to techniques that can be used to design semiconductor lasers such that they emit light predominantly in a single longitudinal mode.

The basic idea is to design the laser such that losses are different for different longitudinal modes of the cavity, in contrast with FP lasers whose losses are mode-independent. The following figure shows the gain and loss profiles schematically for such a laser.

Under normal conditions, all materials absorb light rather than emit it. The absorption process can be understood by referring the figure (a) below, where the energy levels E1 and E2 correspond to the ground state and the excited state of atoms of the absorbing medium. If the photon energy hν of the incident light of frequency ν is about the same as the energy difference Eg = E2 - E1, the photon is absorbed by the atom, which ends up in the excited state. Incident light is attenuated as a result of many such absorption events occurring inside the medium. 1. Spontaneous and Stimulated Emissions...