Linewidth enhancement factor simplified

The dependence of laser linewidth on the laser power without taking into considerations of the laser AM-FM coupling effects, namely the Schawlow-Townes linewidth, can be derived by taking the laser as a self-consistent feedback system. Without employing a full quantum statistical analysis of the semiconductor laser noises, we extend this intuitive approach to include the effects of the laser AM-FM coupling on the laser amplitude noise as well as the laser phase noise. Then by using Fourier transform arguments, we arrive at the laser linewidth and the linewidth enhancement factor which agree with the results from the authoritative quantum statistics theory. This fully classic presentation simplifies the derivation of the laser linewidth enhancement factor and is suitable for undergraduate teaching of laser electronics.