Discrete frequency sweeping of a semiconductor laser by temperature change and electrical feedback

In a constant interval multifrequency measurement to evaluate optical device characteristics, considering that a constant time is required for the measurement at each frequency, a sweeping method that maintains the frequency during the measurement is desirable. Although this stepwise frequency sweeping can be achieved by changing the injection current of the semiconductor laser, the possible sweep frequency range is much wider for the temperature change than for the current change. In this paper, noting this wideband sweeping range by the temperature change, two frequency sweeping methods are proposed. In these methods, the frequency changes stepwise for the temperature change proportional to the time that is easy to realize, and are verified and compared experimentally. In this method, a part of the laser light is incident on the Fabry-Perot resonator with the temperature change, and the transmitted light signal or its differential signal is fed back into the injection current. As a result, the feedback signal corresponding to the resonant characteristics controls the shift of the resonance frequency by the temperature change in the neighborhood of the resonance frequency, realizing the frequency sweep that changes stepwise with the spacing equal to the free spectral range of the Fabry-Perot resonator.