Tunable Q-switched mode-locked Cr:LiSAF laser

Abstract

We have achieved stable passively Q-switched mode-locked (QML) operation in a Cr: LiSAF laser. In the experiments, a 10 mm long, %1.5 Cr-doped LiSAF crystal was pumped by two 150 mW single-mode diodes at 660 nm. An AlGaAs based saturable Bragg reflector (SBR) with a central wavelength of 850 nm and a modulation depth of 0.8% was used for initiating and sustaining QML operation. The QML laser was self-starting and robust against environmental fluctuations. In QML operation, the system produced sub-5-ps long mode-locked pulses at 150–200 MHz, inside 2–2.5 μs long Q-switched envelopes with 30–70 kHz repetition rate. The modulation depth of mode-locking stayed around 50%. We have also observed variation of mode-locked pulse parameters within the Q-switched envelope and also from Q-switched pulse to pulse. At an absorbed pump power level of around 280 mW, the QML pulses had an average power of 50 mW, whereas the estimated peak power of the mode-locked pulses were around 0.5 kW. Simple rotation of an intracavity birefringent tuning plate (BRF) enabled tuning of the central wavelength of the pulses between 835 and 865 nm. To our knowledge, this study is the first work focusing on QML operation in Cr: LiSAF lasers. The findings in this initial study is an initial attempt to understand the dynamics of QML operation in Cr:LiSAF. Further work will focus on reducing the mode-locked pulsewidths to sub-1-ps level and also on increasing the modulation depth of mode-locking for further peak power scaling.