Abstract: In this paper,physical principle of laser is interpreted by measuring semiconductor gain spectrum.A measurement applied to physical experiment is designed ,the relationship between two-sides amplified spontaneous emission （ASE） and net gain is educed, semiconductor is chosen as gain medium, and experiment setup that is easy to adjust and measure is used.The light ampliation caused by gain larger than loss in the system is shown by gain spectrum, while, the change of population inversion is expressed by gain in different pump current density.

Key words: laser, amplified spontaneous emission, gain, population inversion, experiment teaching