Abstract: Micro machined defects can be introduced into a semiconductor laser to improve spectral characteristics of the F-P laser.Based on ray trace method, an expression of the output spectrum from a micro machined F-P semiconductor laser including two defects has been deduced.With the aid of the expression,the difference ΔN between the nominal threshold carrier density Nth and actual carrier density N of the laser can be determined, and the side mode suppression can be specified.