Filter design for dynamic frequency calibration of an external cavity diode tunable laser using frequency comb

In the present work, we propose a method to calibrate the instantaneous optical frequency of a tunable laser using frequency comb. The tunable laser is heterodyned with the equally spaced comb lines, and the heterodyne signal then passes through an electronic frequency selection unit. When the optical frequency of the tunable laser is in the vicinity of the comb lines, the output of the frequency selection unit delivers a peak. We analyzed the effect of the characteristics of the narrow bandpass filter (NBF) in the frequency selection unit. Simulated and experimental results show that the characteristic of the output peak is related to the normalized sweeping speed of the input tuning laser source. At small normalized tuning speed, the envelope of the filtered signal follows the amplitude-frequency response characteristic of the NBF. This shows that the filtered signal using Gaussian filter has broader peak than the one using Butterworth filter, due to the slower roll-off behavior in the transition band of Gaussian filters. At large sweeping speed, the envelope of the filtered signal deviates from the amplitude-frequency response character of the NBF. The peak intensity of the filtered signal is attenuated, and the bandwidth of full width at half maximum is broadened. Experiments were carried out to verify the simulated results. In the experiment, the instantaneous frequency of an external cavity laser diode was calibrated using the presented filtering method showing periodic non-linear tuning.