Er³⁺/Yb³⁺: Lu₂Si₂O₇ crystal microchip laser pumped by LD at kHz

  Objective  The 1.5 μm laser which has an excellent transparency in atmosphere and is in the eye safety wavelength region, has been widely used in range finders, LiDAR, optical communication, medicine and other fields. LD end-pumped Er³⁺/Yb³⁺ co-doped glass/crystal laser is an effective way to obtain 1.5 μm wavelength output micro laser because it meets the requirements of small volume, peak power, low cost and high efficiency. When using laser for ranging, the higher the laser repetition frequency is, the greater the single pulse energy is, the narrower the pulse width is, the faster the measurement speed is, the higher the accuracy is, and the farther the distance is. However, due to the low thermal conductivity of Er³⁺/Yb³⁺: glass and the increase of laser output energy and repetition frequency, the gain medium faces the prominent thermal effect problem, which makes it easier to reach the damage threshold of dielectric coating and glass, affecting the lifetime of the laser. For Lu₂Si₂O₇ (LPS) crystal, its upper level fluorescence lifetime can be compared with that of glass, and its thermal conductivity is more than 10 times higher than that of the glass. It is an excellent gain medium for realizing 1.5 μm pulsed laser with large energy and high repetition frequency. At present, LPS crystal is mainly pumped continuously. Continuous pumping will cause heat accumulation inside the crystal and reduce the output energy and beam quality of laser output. In this paper, the pulse pumping mode and Er³⁺/Yb³⁺:LPS are used as the gain medium to achieve a 1.5 μm laser output with repetition frequency stabilized at 1 kHz and 10 kHz.

  Methods  In this study, the factors that affect the output of LD end pumped passively Q-switched laser include crystal doping concentration and length, pump beam diameter, initial transmittance of saturable absorber and output coupling mirror (OC) transmittance. Under the theoretical simulation, the general optimization range of the above parameters was obtained, and the optimal parameters were obtained through the experiment. The optimal doping concentration of Yb³⁺ and length of Lu₂Si₂O₇ crystal was obtained by comparing the free oscillating output power at different OC transmittance. In order to achieve the repetition frequency of 1 kHz and 10 kHz Q-switched pulse laser output, we used the control variable method to optimize the pump beam diameter, initial transmittance of saturable absorber and OC transmittance, and obtained the best experimental parameters by comparing the output frequency and single pulse energy.

  Results and Discussions   The results of free oscillating were shown (Fig.2(a)), the output power of 0.5at.%Er³⁺/4.0at.%Yb³⁺:LPS was always higher than 0.5at.%Er³⁺/5.0at.%Yb³⁺:LPS with the transmittance of the OCs changing from 4% to 30%. And when the length of medium increased, the output power of free oscillation also decreased. According to the experimental result, 2.85-mm-thick 0.5at.%Er³⁺/4.0at.%Yb³⁺:LPS was selected for passive Q-switched experiment. The slope efficiency of 2.85-mm-thick 0.5at.%Er³⁺/4.0at.%Yb³⁺:LPS was further studied. The optimal slope efficiency of 5.8% was obtained by optimizing the pump beam diameter and the transmittance of OCs (Fig.2(b)-(d)). In order to achieve the repetition frequency of 1 kHz and 10 kHz Q-switched pulse laser output, we compared the repetition frequency, energy, pulse width of three sets of control variable experiments, the results were shown (Tab.1-6). Finally, the laser output with repetition frequency of 1 kHz, single pulse energy of 35 μJ, pulse width of 7 ns, peak power of 5 kW and M²=1.33 was obtained when the pump beam diameter is 300 μm, the initial transmittance of Co²⁺:MgAl₂O₄ is 94.5% and transmittance of OC is 15%. And the laser output with repetition frequency of 10 kHz, single pulse energy of pulse energy of10 μJ, pulse width of 10 ns, peak power of 1 kW and M²=1.51 was obtained when the pump beam diameter is240 μm, initial transmittance of Co²⁺:MgAl₂O₄ is 98.6% and transmittance of OC is 10%.

  Conclusions  LD pulse end-pumped passively Q-switched 1537 nm laser with Er³⁺/Yb³⁺:Lu₂Si₂O₇ crystal at 1 kHz and 10 kHz was reported. In this experiment, the doping concentration of LPS crystal was optimized by the free oscillation experiment and the 2.85-mm-thick 0.5at.%Er³⁺/4.0at.%Yb³⁺:LPS was selected for passive Q-switching experiment. Secondly, the Q-switching experiment was conducted to optimize the pump beam diameter, the initial transmittance of Co²⁺:MgAl₂O₄ and the transmittance of the output coupling mirror. Finally, the laser output with repetition frequency of 1 kHz, single pulse energy of 35 μJ, pulse width of 7 ns, peak power of 5 kW and M²=1.33 and repetition frequency of 10 kHz, single pulse energy of 10 μJ, pulse width of 10 ns, peak power of 1 kW and M²=1.51 were realized. The results show that Er³⁺/Yb³⁺:Lu₂Si₂O₇ crystal is an excellent medium for 1.5 μm laser output with high repetition frequency.