Passively Q-switched mode-locked low threshold Tm, Ho: LLF laser with an single walled carbon nanotubes saturable absorber

Abstract

Employing single walled carbon nanotubes (SWCNT) grown by the vertical growth method as a saturable absorber for the initiation of the pulse generation, and designing a low threshold resonant cavity, we demonstrate a stable passively Q-switched mode-locked (QML) Tm, Ho:LiLuF4 solid-state laser with low threshold for the first time. With wavelength tunable Ti:sapphire solid laser operating at 785 nm as a pumping source, continuous-wave (CW) absorbed pump thresholds of 52, 59 and 62 mW are obtained by using 1.5%, 3% and 5% output coupled mirrors respectively. In this case, the maximum output powers are 645, 828 and 940 mW respectively, whose corresponding slope deficiencies are 31.02%, 39.16% and 43.78%, respectively. When the SWCNT-SAs is inserted in the cavity, the cavity loss is further increased, so the laser threshold is improved. Employing the 1.5% output mirror, a laser threshold is obtained to be as low as 85 mW, but the maximum laser output power is only 70 mW, corresponding slope efficiency is 3.42%; employing the 3% output coupling mirror, the laser threshold is obtained to be as low as 99 mW, the maximum output power is 154 mW, and the corresponding slope efficiency is 8.47%; employing the 5% output mirror, owing to the loss in the cavity being too large, the QML operation cannot be achieved. The output power of the 3% output mirror is twice higher than that of the 1.5% output mirror, but the laser threshold difference is only 14 mW. With a comprehensive analysis, we use the 3% output mirror. In this case, a stable QML operation with a threshold of 250 mW is obtained. When the absorption pump power is 1.85 W, the maximum output power is 154 mW with a typical Q-switched pulse envelope width of 300 s, which is corresponding to a 178.6 MHz of the mode-locked frequency. The modulation depth in Q-switching envelope is close to 100%. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we could assume the duration of the pulse to be approximately 1.25 times more than the rise time of the pulse. So the width of the mode locked pulse is estimated to be about 663 ps. The results show that the SWCNT is a promising SA for QML solid-state laser with the 2 m wavelength. In the later stage, we increase the pump power, optimize the quality of the SWCNT material, and compensate for the dispersion in the cavity. It is expected to achieve a stable continuous mode-locking operation, and obtain a femtosecond mode-locked ultrashort pulse output. The mode-locked mid-infrared pulses have a lot of potential applications such as ultrafast molecule spectroscopy, the generation of mid-IR pulse, laser radar, atmospheric environment monitoring, etc.