A method for characterizing tunable semiconductor laser diodes in which the laser is stimulated in a way that discloses the optical properties and tuning current dependency of the individual sections of the laser, separately for each section, and independently of the other sections. A section of the laser is current modulated in order to excite a continuum of modes related to the spectral response of other sections. This process is observed by viewing the overall spectral response at an integration time significantly longer than the modulation time. The spectral positions of the modes and their dependence on the tuning current, are used to determine the tuning characteristic of that particular section. This method substantially reduces the time required for characterization of such lasers in comparison with prior art methods.