The acquisition of the experimental data and the further data processing are discussed. The process is illustrated through the characterization of the actual gain spectrum of a multi-quantum-well FP laser diode. The method can be applied to various semiconductor lasers exhibiting FP resonances below threshold current. The data obtained from this method provides important information for laser designers to understand materials and optimize laser structures to meet specifications. The data can also serve as an input file for scientists to model or simulate laser behavior when essential material properties are not certain. The experimental method is one of the advanced tests in optoelectronics. Therefore, students and engineers in photonics and compound semiconductors will find the method useful.
