Method of characterising a tuneable laser and determining actual wavelength

A method of evaluating a tuneable laser and determining suitable laser operation points, wherein the laser includes one or more tuneable sections in which injected current can be varied, said method comprising the steps of:

• leading part of light emitted by the laser to an arrangement that includes a first, periodic filter that provides a first signal that varies periodically with wavelength, a first detector to measure the power of the light from the periodic filter and to deliver a corresponding first detector signal, and a second detector to measure the power of unfiltered laser light and to deliver a corresponding second detector signal;
• arranging the detectors relative to the periodic filter so that the detector signals contain information relating at least to the wavelength of the detected light from a number of wavelengths given by the periodic filter;
• arranging a second filter parallel with the periodic filter, wherein said second filter is a filter whose response varies monotonically with the wavelength;
• leading light transmitted through the second filter to a third detector which delivers a third detector signal corresponding to the power of the light and also the wavelength;
• sweeping currents through the tuneable sections of the laser to pass through different tuning current combinations;
• measuring the ratio between the first and second detector signals during said sweep;
• controlling the third detector to sort out a relevant wavelength when the ratio between the first and second detector signals lies within a predetermined range and therewith signifies that the emitted light lies within one of a number of wavelengths given by the periodic filter;
• and storing the tuning current combination together with the sorted wavelength concerned.