Computer-controlled harmonic FM mode-locking of 40-GHz repetition-rate fiber laser

Abstract

Active FM harmonic mode-locking of a fiber laser by intracavity phase modulation allows obtaining of stable laser pulses with a high repetition rate. Driving the phase modulator by an external RF synthesizer has an advantage of constantly applying only one modulation frequency to the phase modulator. That greatly facilitates the generation of stable laser pulses with very small noise. However, this approach requires constant frequency tuning of the synthesizer to compensate for small temperature fluctuations causing changes in the fundamental frequency of the laser cavity. In order to control the modulation frequency we mixed the RF signal from the laser output detected by a fast photodiode with the signal from the synthesizer. The amplitude of the measured DC component of the mixed signal depends on the phase difference of the two signals. The phase difference varies approximately linear with the laser detuning near the mode locking resonance. We develop software that performs constant measurement of the mixed signal and tuning the modulation frequency in order to keep the DC component of the mixed signal at a preset value. The program performed approximately two auto-tuning steps per second. The presented method allows very simple and reliable obtaining of stable computer controlled harmonic mode-locking of a fiber laser at 40 GHz repetition rate frequency.