TABLE OF CONTENTS The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The principles of lasers were understood at the end of 1950's [Schawlow et al. 1958]. The first working laser was built by Maiman in 1960, and used a ruby crystal optically pumped by a flash lamp. The Nobel Prize for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the laser principle was awarded in 1964 to N.G. Basov, A.M. Prokhorov, and C.H, Townes.
The use of carrier injection across a p-n junction for stimulated emission from semiconductors was suggested as early as 1961. The stimulation emission itself was observed in a GaAs p-n junction one year later [e.g. Holonyak et al. 1962], and then the first scmieonductor lasers were fabricated. The light emitted from a laser can be a continuous beam of low or medium power, or it can consist of short bursts of intense light delivering millions of watts.
This Chapter will first review the fundamental mechanisms of a laser. It will then describe the first laser, which was realized using a ruby crystal and subsequently focus on more sophisticated semiconductor lasers.
