MEASUREMENT OF SPECTRAL CHARACTERISTICS OF SEMICONDUCTOR-LASER DIODES- EFFECT OF INJECTED CURRENT MODULATION AND OPTICAL FEEDBACK

We present precision optical spectral lineshape measurements on semico nductor lasers using Michelson and Fabry-Perot interferometers. Measur ements ranging from 30 MHz up to 100 GHz give the spectral behavior ab ove and below threshold even for very small emitted optical powers. Va riations of the linewidth above and below threshold as a function of i njected current have been measured and used to evaluate the linewidth enhancement factor and behavior at threshold. The lineshape can differ from the modified Schawlow-Townes almost Lorentzian form for semicond uctor lasers above threshold when submitted to operating system condit ions such as modulation and optical feedback. Under modulation, line f requency and signal form are modified because of chirping effects. We propose a simple method for chirp evaluation at low frequencies based on lineshape evaluation. Optical feedback effects are also studied as a function of the feedback coefficient C showing various regimes diffe ring for singlemode and multimode laser diodes.