A method of characterizing a tunable semiconductor laser diode by varying the laser tuning currents of a section, and measuring the output power transmitted by the laser through a wavelength discriminating device (WDD), in such a way that determines the currents needed to optimize the laser output within the passband of the WDD. A preferably employed method is to align the laser to the center of the passband, in order to maximize transmission. Since the laser alignment is performed relative to the spectral response curve of the WDD, there is no need to know the actual wavelength, either of the laser, or of the WDD. The WDD may be a multiplexer, a demultiplexer, an optical filter, or even a complete communications channel. The latter case thus constitutes a method of optimizing the performance of an optical communications channel by optimizing the wavelength of its laser source to the channel.