532-nm second harmonic generation with enhanced efficiency using subharmonic cavity modulation-based quasi-Q-switched-mode-locked pulses.

We report the results of our investigation of the second harmonic generation (SHG) green pulse generation with an active, quasi-Q-switched-mode-locked (QML) fiber laser based on subharmonic cavity modulation. First, we investigate the working principle of the quasi-QML pulse generation technique, based on subharmonic cavity modulation through analytical calculation. We show that the stable QML-like pulse generation by subharmonic cavity modulation is induced by phase-locked interference of a large number of subharmonic modulation-induced frequency components within a cavity. Next, we experimentally realize a 1064 nm quasi-QML fiber laser with an ytterbium (Yb)-doped fiber ring cavity incorporating an acousto-optic modulator. Finally, using the implemented quasi-QML laser, we experimentally show that the use of 1064 nm quasi-QML pulses could result in the increase of the SHG conversion efficiency in a MgO:PPLN, compared to the use of continuous mode-locked (ML) pulses. For our particular experimental configuration, we could readily achieve a noticeable SHG efficiency increase of 8% by using quasi-QML pulses with a subharmonic order of 80, compared to continuous ML pulses.