Passively Q-switched mode-locked Tm, Ho:LLF laser with a WS2 saturable absorber

Abstract

Using few-layer tungsten disulfide (WS2) doped polyvinyl alcohol as a saturable absorber for the initiation of the pulse generation, we experimentally demonstrate stable passively Q-switched mode-locked operations of Tm, Ho:LiLuF4 laser at 1895 nm for the first time. The laser is designed with an X-type four-mirror cavity and pumped by a Ti:sapphire laser operated at 785 nm, and its continuous operation is initiated when the absorbed pump power is 143 mW. When the absorbed pump power reaches 2.645 W, we obtain a maximum output power of 985 mW and a crystal slope efficiency of 39.8% by linear fitting. When the saturable absorber WS2 is inserted in the cavity, the threshold of the absorbed pump power is increased to 234 mW. With the increase of the pump power, Q-switch pulse sequence is first observed. When the absorbed pump power reaches 1.39 W, the stable operation of the Q-switched mode locked pulse is realized. A maximum average output power of 156 mW is achieved at an absorbed pump power of 2.6 W, which corresponds to a 25 kHz Q-switched repetition rate and a 300 μs-long pulse envelope. In this case, the modulation depth in Q-switching envelopes is close to 100%. After the passively Q-switched mode-locked is obtained stably, the mode-locked pulses inside the Q-switched pulse envelope have a repetition rate of 131.6 MHz, corresponding to a mode locked pulse energy of 1.19 nJ and a cavity length of 1.14 m. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we can assume that the duration of the pulse is approximately 1.25 times more than the rise time of the pulse. Then the width of the mode locked pulse is estimated to be about 878 ps. These experimental results show that WS2 is a promising broadband saturable absorption material for generating a 2 μm-wavelength mid-infrared solid-state laser pulse. By increasing the pump power and reducing the loss of WS2 material, it is possible to realize a continuous mode locking operation which has a narrower pulse duration. The mode-locked mid-infrared pulses are very stable and have a lot of potential applications such as ultrafast molecule spectroscopy, mid-IR pulse generation, laser radar, atmospheric environment monitoring, etc.