Investigating Self-Mode-Locking in Semiconductor Disk Lasers

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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.