Nonlinear Mirror Mode-Locked Laser for Suppressing Q-Switching Instability
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 mode-locking(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 mode-locked 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.
mode-lockingcrystal waveguideQ-switchingmode-lockingcontinuous-wave mode-lockingnonlinear mirror mode-lockinghigh power
万方数据
维普
