TABLE OF CONTENTS In a semiconductor laser, population inversion mechanism is realized through a very unique method: by injecting electrical current directly into a p-n junction. This method of achieving population inversion is very efficient when compared to the process in ruby lasers or gas lasers. The semiconductor laser itself is also very compact (a typical size of the active laser part is only 100 ?m 1000 ?m 100 ?m = one part in a hundred thousand cubic centimeters!).
Moreover, semiconductor lasers can be easily integrated with other types of semiconductor devices such as transistors or even large-scale integrated circuits, and the laser output can be easily modulated by controlling the junction current. It is no surprise that semiconductor lasers are now widely used for high speed optical processing and optical communication.
Another great advantage for these lasers is an inherent optical cavity. Most popular semiconductors (III V, II VI) have natural cleavage planes, which are the crystallographic planes along which the atomic bonds are weakest and therefore most easily broken. For zinc-blende crystals, cleavage parallel to (110) and (1 10) planes can produce atomically flat mirrors for use in a Fabry-Perot optical cavity. The reflectivity of the mirrors is limited by the refractive index of the semiconductor, and is given by:
| (18.40) |  |
where n is the refractive index of the semiconductor. A typical semiconductor has a refractive index of 3.4 in the mid-infrared region, which gives a natural mirror reflectivity of 29.8 %.
