Actively mode-locked fiber laser at 2-μm for free-space data transmission with tunable repetition rate

Abstract

We experimentally demonstrate a repetition rate controllable actively mode-locked fiber laser for 2-μm free-space data transmission. The actively mode-locking can be produced by a Mach-Zehnder modulator (MZM) driven by an arbitrary waveform generator (AWG). A Lyot filter and a polarization dependent isolator (PD-ISO) are used to suppress the super-mode noise and the ambient noise. The signal-to-noise ratio (SNR) of the mode-locked pulse with repetition rate of about 690MHz can be increased from 42.64dB to 49.45dB. By inserting a high-precision variable optical delay line (VODL) in the cavity, the fundamental frequency of the cavity can be adjusted from 18.21MHz to 18.13MHz with a tuning accuracy of 73.52Hz. The RF spectra of the 330th order harmonic mode-locked pulses with continuously tunable repetition rate can be observed. In addition, the mode-locked pulse sequence can be driven by sine wave, triangular wave and square-wave, and the duration of actively mode-locked pulse can be tuned independently by changing the type of the driven signal.