Design and research of LD pulse pumped Er³⁺/Yb³⁺：Lu₂Si₂O₇ crystal 100 kHz laser

Objective At present, LD pumped Er³⁺/Yb³⁺ co-doped glass/crystal passively Q-switched microchip laser is widely used in laser ranging and lidar. With the increase of laser output repetition rate, glass has a serious thermal effect problem. The thermal conductivity of crystal is more than 10 times that of glass, which can achieve higher repetition rate. This article reports a 100 kHz human eye safe laser pumped by an LD pulse end face in Er³⁺/Yb³⁺: Lu₂Si₂O₇ crystal. By optimizing experimental parameters such as pump spot size, pump duty cycle, and initial transmittance of Q-switched crystals, a repetition rate of 100 kHz and a single pulse energy of 0.7 μJ were obtained. A 1 537 nm pulsed laser output with a pulse width of 240 ns and a beam quality of M²=1.61. Finally, the consistency between the output pulse frequency and the pump has been achieved, ensuring the stability of the output repetition frequency and effectively solving the problems of randomness, instability, and uncontrollability of the output repetition frequency.

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 were obtained by comparing the free oscillating output power at different OC transmittance. In order to achieve the repetition frequency of 100 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), 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 8% to 30%. And when the length of medium increased, the output power of free oscillation also decreased. According to the experimental result (Fig.3), 2-mm-thick LPS was selected for passive Q-switched experiment. In order to achieve the repetition frequency of 100 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.1-3). Finally, the laser output with repetition frequency of 100 kHz, single pulse energy of 0.7 μJ, pulse width of 240 ns and M²=1.61 was obtained when the pump beam diameter is 130 μm, the initial transmittance of Co²⁺: MgAl₂O₄ was 99% and transmittance of OC was 10%.

Conclusions LD pulse end-pumped passively Q-switched 1537 nm laser with Er³⁺/Yb³⁺: Lu₂Si₂O₇ crystal at 100 kHz was reported.In this experiment, the doping concentration of LPS crystal was optimized by the free oscillation experiment and the 2-mm-thick 0.5at.%Er³⁺/4.0at.%Yb³⁺: LPS was selected for passive Q switching experiment. Secondly, By optimizing the size of the pump spot and passively Q-switching the initial transmittance of the crystal, the output efficiency of the laser can be improved. At the same time, combining the characteristics of LPS crystals with longer upper level lifetimes, the pulse width is appropriately shortened, the pump duty cycle is optimized, and the output repetition rate stability is improved. Finally, in the case of using a 976 nm wavelength LD pulse laser, a laser pump repetition rate of 100 kHz was used, with a power of 2.7 W and a pulse width of 3 μs. Obtained an output repetition rate of 100 kHz and a single pulse energy of 0.7 μJ. A 1 537 nm laser output with a laser pulse width of 240 ns and a beam quality factor of M²=1.61. The consistency between the output and pump repetition rates has been achieved, ensuring the stability of the output repetition rate and effectively solving the problems of randomness, instability, and uncontrollability of the output repetition rate.