INJECTION LASERS

An injection laser, also called a laser diode, is a special form of LED or IRED with a relatively large and flat p-n junction. The injection laser emits coherent electromagnetic (EM) waves, provided the applied current is sufficient. Coherent waves are all lined up and all have the same frequency, compared with the incoherent waves typical of most LEDs and lightproducing devices in general.
Figure 1 is a simplified diagram of a laser diode. The substrate is the material on which the component is built; it is like the foundation of a building. It also serves to carry away excess heat so that the device can carry fairly high current without being destroyed. There are mirrors at opposite ends of the piece of n-type material. One of the mirrors (the one labeled in the drawing) is partially reflective. The opposite mirror (not shown) is totally reflective. The coherent rays emerge from the end with the partially reflective mirror.

Fig. 1. Simplified cross-sectional drawing of an injection laser, also known as a laser diode.