抑制调Q不稳定性的非线性镜锁模激光器

Nonlinear Mirror Mode⁃Locked Laser for Suppressing Q⁃Switching Instability

为了抑制高功率非线性镜锁模激光器的调Q不稳定性,提出了一种双晶体二次倍频非线性镜结构,通过加入第二块非线性晶体将二次谐波倍频为四次谐波来产生损耗,从而实现非线性反射率的提前饱和。该结构降低了非线性镜锁模器件的饱和峰值功率密度和调制深度,从而降低连续锁模阈值,得到稳定的连续锁模输出。通过理论分析这种非线性镜结构抑制调Q不稳定性的原理和表达,并使用大芯径晶体方波导作为增益介质,进一步搭建实验,验证了这一结构的可行性。最终实验获得了功率为26.5 W、重复频率为32.2 MHz、脉宽为0.95 ps的连续锁模脉冲输出。实验结果表明,这种高功率非线性镜锁模激光器结构简单、稳定可靠,并且该器件在高功率锁模激光器的发展中具有巨大潜力。

Objective High-power ultrashort-pulse lasers operating in the picosecond and femtosecond time domains have important applications in strong-field physics, biomedical imaging, optical frequency conversion, and nuclear laser fusion. The direct output of mode-locked lasers has a more promising signal-to-noise ratio, beam quality, and stability than the power amplification method for high-power ultrashort pulse generation. Commonly used mode-locking methods for high-power mode-locked lasers include semiconductor saturable absorber mirror mode-locking(SESAM), Kerr lens mode-locking(KLM), and non-linear mirror modelocking(NLM). In particular, the NLM mode-locking method, which utilizes a nonlinear crystal and an output coupling mirror, has demonstrated significant potential owing to its high stability, wide range of applicable wavelengths, and large modulation depth.However, achieving saturation in the NLM method requires a high power density, resulting in a peak power density within the cavity that is typically lower than that required for reflectivity saturation. The quality factor(Q) of a cavity in an NLM mode-locked laser increases with the peak power density within the cavity. This phenomenon results in Q-switching instability, making it challenging to attain stable continuous-wave mode-locking(CWML). This challenge can be addressed by introducing an early saturation tendency for the nonlinear reflectivity of NLM devices. This ensures a balance between gain and loss at the saturation point, ultimately achieving stable continuous-wave mode-locked pulses. Hence, the development of a straightforward, stable, and dependable method to achieve early saturation is important for achieving high-power mode-locked outputs.Methods In this paper, we present a novel NLM structure involving two crystals that undergo frequency doubling twice. This design introduces loss by incorporating a crystal that doubles the second harmonic, achieving early saturation of reflectivity to effectively suppress Q-switching instability and facilitate CWML. Based on the single-crystal NLM nonlinear reflectance expression,we derive an NLM reflectance expression for the two crystals undergoing frequency doubling twice. The materials and lengths of the two crystals are determined using a reflectance formula combined with their crystal properties. The calculation results show that the addition of a second frequency-doubling crystal introduces a rollover in the nonlinear reflectivity curve, thereby forming a new saturation point. This reduces the saturated peak power density and modulation depth of the NLM, theoretically proving the potential of suppressing Q-switching and mode-locking. The experimental validation involved a cavity comprising a semiconductor laser pump source, a large-core crystal waveguide, a 4f system, a dispersion compensation device, a polarizer, and an NLM device. In this experiment, the first frequency-doubling crystal was deployed and adjusted to the optimal state, which resulted in unstable Q-switched and mode-locked pulses. Subsequently, with the addition of a second frequency-doubling crystal and adjustments under the same conditions, the output signal transitioned from unstable Q-switching and mode-locking to stable CWML. This observation proves the effectiveness of our early saturation method employing a second frequency-doubling crystal to achieve CWML.Results and Discussions The output power of the laser varies with the pump power [Fig. 4(a)], with a maximum value of 26.5 W. In the mode-locking experiment, the laser spectral curve at the maximum output power [Fig. 4(b)] exhibits a center wavelength of 1030.1 nm and a spectral width of 1.3 nm. A signal diagram of the mode-locked pulses detected by the oscilloscope is shown in Fig. 4(c). The radio frequency spectrum, presented in Fig. 4(d), reveals a repetition frequency of 31.2 MHz and a signal-to-noise ratio of 46 dB, demonstrating excellent inter-pulse stability without side peaks. The autocorrelation curve in Fig. 4(e) exhibits a modelocked width of 0.95 ps. Fig. 4(f) depicts the measurements of the beam radius with a calculated beam quality factor of M2=1.05.Conclusions In this paper, we present a novel NLM crystal waveguide laser that effectively suppresses the Q-switching instability.The position of the saturation point can be tuned by adjusting the crystal thickness, thereby achieving a simple, stable, and reliable structure. We derive a nonlinear reflectivity expression and verify that the reflectivity produces a rollover to achieve early saturation.Stable continuous mode-locking can be experimentally obtained to verify the feasibility of this structure. A CWML output with an average power of 26.5 W, a pulse width of 0.95 ps, and a repetition frequency of 32.2 MHz is obtained. Future enhancements, such as utilizing a double-clad crystal waveguide and optimizing the equivalent transmittance, are expected to achieve higher-power CWML pulse outputs. This novel NLM device exhibits significant potential for the development of high-power mode-locked lasers.