How does a laser diode work?

Published on: Wed Feb 02 2011

I was looking at the Helium Neon laser at the LTC today, and wondering how a laser diode like the ones in a laser pointer works, since it must be so much smaller and I doubt it contains a tube of gas. What makes laser light special is that the light is a coherent stream of photons. There are various types of devices that are used to produce laser light. Some types of lasers are: ruby laser, chemical laser, dye laser, gas laser, and rare-earth solid-state laser. All of these lasers work by exciting atoms to a higher state, so more atoms are in the higher state than the lower one. This is called an inverted population of atoms. The reason the population needs to be inverted is so that emission of photons will be dominant, instead of absorption. The second important feature is that the higher state must be metastable, meaning the electrons remain excited for longer than usual so the transition to the lower state happens because of stimulated emission. Stimulated emission is the process of a photon of energy equal to an excited atom striking the excited atom, causing it to transition to the lower state and emit a second photon of the same frequency. This process gives two photons in exactly the same phase and moving in the same direction. Cd players and laser pointers use semiconductor diode lasers. These lasers work by layering two materials, one on top of the other.  One of the materials is an n-type semiconductor. An example is an arsenic-doped silicon crystal, where negatively charged electrons carry the current. The other material is a p-type semiconductor, which  is populated with net positive gaps. Overall through both n and p type semiconductors have no net charge. Because of the energy difference between the n and p layers there is an inverted population of atoms.  This means an electron can jump down causing a photon to be emitted, which in turn stimulates another electron to transition to a lower energy level and emit a photon, causing stimulated emission. The two materials form the laser diode. I'm not sure how the beam emerges, and will look for books on this at the LTC .