A passively Q-switched mode-locked all-solid-state Tm:CaYAIO₄ (Tm:CYA) laser with a MoS₂ saturable absorber is demonstrated for the first time. Employing a typical X-type cavity and output coupling mirror of 9%, when the laser diode pump power is higher than 8.5 W, the laser enters into a stable Q-switched mode-locked operation, further increasing the pump power into a dual-wavelength operating state; the corresponding typical center wavelengths are 1,863 and 1,877 nm. When the pump power reaches 20 W, the maximum output power is 1.15 W, the frequency of mode-locked pulse is 103.7 MHz, and the modulation depth of mode-locked pulses in the Q envelope is close to 100%. The results prove that MoS₂ saturable absorber is suitable for 2-μm high-power mode locking.

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.

A passively Q-switched mode-locked (QML) operation in Tm,Ho:CaYAlO₄ bulk laser was demonstrated experimentally by employing double-walled carbon nanotubes (DWCNTs) as a saturable absorber. The laser is pumped by a self-made wavelength tunable Ti:sapphire laser, and the pump threshold of Tm,Ho:CaYAlO₄ laser was measured at 677 mW using transmittance of 1.5% output coupler. A stable QML operation state was achieved when the absorption pumping power reached 1,958 mW. When the pumping power reached 2.6 W, the maximum output power was 64 mw with a central wavelength of 2,085 nm, the corresponding repetition frequency of mode-locked pulse was 98.04 MHz, and the modulation depth in Q-switching envelopes is close to 100%.

在Tm: LuAG全固态激光器中实现了以氧化石墨烯可饱和吸收体为锁模启动元件的瓦级被动调Q锁模运转. 本实验装置以可调谐掺钛蓝宝石激光器作为泵浦源, 测得Tm: LuAG固态激光器出光阈值最低为325 mW, 当吸收抽运功率达到3420 mW时, 进入稳定的调Q锁模运行状态. 当抽运功率达到8.1 W时, 对应的最大输出功率为1740 mW, 中心波长为2023 nm, 重复频率为104.2 MHz, 最大单脉冲能量为16.7 nJ, 调制深度接近100%.

We experimentally demonstrated a passively mode-locked picosecond Tm,Ho:LiLuF₄ laser with low threshold and high efficiency. The stable continuous-wave (CW) mode-locked operation with 12 ps pulse width is obtained by using a five-mirror cavity structure and semiconductor saturable absorber mirrors (SESAMs). The results indicate that the laser offers a mode-locked threshold power of 1.03 W and maximum mode-locked output power of 350 mW. The repetition rate of mode-locked pulse sequence is 98.04 MHz, corresponding a maximum single pulse energy of 3.51 nJ.