Precision optical spectral lineshape measurements on semiconductor lasers using Michelson and Fabry-Perot interferometers are presented. Measurements ranging from 30 MHz up to 100 GHz give the spectral behavior above and below threshold even for very small emitted optical powers. Variations of the linewidth above and below threshold as a function of injected current are 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 semiconductor lasers above threshold when submitted to operating system conditions such as modulation and optical feedback. Under modulation, line frequency and signal form are modified because of chirping effects. A simple method for chirp evaluation at low frequencies based on lineshape evaluation is proposed. Optical feedback effects are studied as a function of the feedback coefficient C showing various regimes differing for single-mode and multimode laser diodes