Investigating Self-Mode-Locking in Semiconductor Disk Lasers

半导体盘激光器（SDLs），通常也被称为垂直外腔面发射激光器（VECSEL），可以作为实现紧凑、鲁棒和具有成本效益的fs脉冲激光器的理想平台。到目前为止，可饱和吸收镜，这是嵌入在外部谐振器的设备，已被用来实现锁模与SDL。然而，最近获得的自锁模效应允许设计不含饱和吸收体的器件，其不太复杂并且提供更大的灵活性，因为饱和吸收体反射镜必须针对期望的操作波长单独设计。虽然自锁模SDL可以规避由基于饱和吸收体的器件自然设置的限制，但是自锁模背后的机制仍然不清楚并且不被理解。然而，自锁模已经成功地获得了不同的设备，这是基于量子阱或量子点增益介质。由于马尔堡的SDL研究在最近的工作中对这些成就做出了贡献，我们的目标是在本项目的范围内进行关于自锁模的重要研究，以揭示锁模的机制和非线性效应，并促进飞秒脉冲SDL向功能强大，成本效益高的应用器件的发展。应在运行的SDL活动区进行泵浦探测实验，以获得增益动态的时间分辨图像。这可以揭示增益饱和对强度依赖的折射率的影响，这导致自相位调制和自聚焦。对激光器芯片的进一步实验研究将提供对可能的克尔透镜效应的直接测量，该效应被认为起主要作用。如果芯片表现出这种强度依赖性透镜效应，我们可以得出关于有效非线性折射率的结论。关于光脉冲的相位信息的附加测量可以最终允许对锁模背后的主要机制的清楚识别以及对脉冲光源的性质的更好理解。为了在光谱学、多光子显微镜、材料加工等应用中成功使用自锁模SDLs，必须揭示导致自锁模器件中脉冲形成的非线性的性质。因此，可以实现关于脉冲持续时间、光谱通用性和性能的显著改进。我们预计，甚至峰值功率的数量级上的几千瓦和脉冲持续时间约为几百fs来达到作为优化的结果，这样的饱和吸收自由VECSEL。

Semiconductor disk lasers (SDLs), also often referred to as vertical-external-cavity surface-emitting lasers (VECSELs), can serve as an ideal platform for the realization of compact, robust and cost-efficient fs-pulsed lasers. Up to now, saturable-absorber mirrors, which are embedded in the external-resonator device, have been employed toachieve mode-locking with SDLs. However, the recently obtained self-mode-locking effect allows for the design of saturable-absorber-free devices, which are less complex and which provide more flexibility, since saturable-absorber mirrors have to be individually designed for a desired operation wavelength. Although self-mode-locked SDLs can circumvent restrictions naturally set by saturabe-absorber based devices, the mechanisms behind self-mode-locking still remain unclear and are not understood. Nevertheless, self-mode-locking has been successfully obtained for different devices, which are based on quantum-well or quantum-dot gain media. Owing to the fact that SDLresearch in Marburg contributed to these achievements with recent work, our aim is to perform important investigations regarding self-mode-locking within the scope of this project in order to uncover the mechanisms and nonlinear effects responsible for mode-locking, and to boost development of fs-pulsed SDLs towards powerful, costefficient devices for applications. Pump-probe experiments on a running SDL’s active region shall be performed in order to acquire a time-resolved picture of the gain dynamics. This may reveal the influence of gain saturation on an intensity-dependent refractive index, which causes self-phase modulation and self-focusing. Furtherexperimental studies on the laser chip shall provide a direct measurement of a possible Kerr-lensing effect, which is assumed to play a major role. If the chip exhibits such an intensity-dependent lensing, we can draw a conclusion on the effective nonlinear refractive index. Additional measurements regarding the phase information of the optical pulses may finally allow for a clear identification of the main mechanism behind mode-locking and a better understanding of the properties of the pulsed light source. For the successful employment of self-mode-locked SDLs in applications, such as spectroscopy, multi-photon microscopy, material processing and more, it is essential to reveal the nature of the nonlinearity which causes pulse formation in the self-mode-locked device. Thereby, significant improvements can be achieved with respect to pulse duration, spectral versatility and performance. We expect even peak powers on the order of several kilo watts and pulse durations around a few 100 fs to come into reach as a consequence of optimization of such saturable-absorber-free VECSELs.