Realization of a Kerr-lens mode-locked vertical-external-cavity surface-emitting laser
Besides continuous wave (cw) operation, where light is emitted continuously over time, specially designed lasers can also generate short or even ultrashort pulses of light, the latter referred to as ultrafast lasers. So far, ultrafast laser systems have been used in different industrial and research...
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|Zusammenfassung:||Besides continuous wave (cw) operation, where light is emitted continuously over time, specially designed lasers can also generate short or even ultrashort pulses of light, the latter referred to as ultrafast lasers. So far, ultrafast laser systems have been used in different industrial and research areas such as biology, metrology or medicine. But these systems are subject to high costs and great complexity, limiting their use in new application areas that demand for low-cost and compact ultrafast laser sources, such as the optical clocking of microprocessors or free-space data communication. Semiconductor laserswould be ideally suited to meet this demand, however conventional semiconductor lasers are edge-emitters and their power cannot simply be scaled. The same is true for microcavity-based surfaceemitters. Moreover, the more powerful edge-emitters feature strongly asymmetric beam profiles, which makes them unsuitable for many ultrafast applications. Vertical-external-cavity surface-emitting lasers (VECSELs), also known as semiconductor disk lasers (SDLs), are powerful and very flexible coherent light sources. They can be considered as a hybrid system between ion-doped solid state lasers and conventional semiconductor lasers. SDLs combines the advantages of semiconductor gain, e.g. wavelength versatility, high gain cross sections, and simple fabrication, with the benefits of the ion-doped bulk lasers, such as a high-Q external cavity and excellent beam quality. Furthermore, due to the 1-D heat flow, resulting from the arrangement as a thin film laser, very efficient heat removal enables power scaling via the pump area as well as the mode size. SDLs have proved to be versatile lasers which allow for various emission schemes which on the one hand include remarkably high-power multimode or single-frequency cw operation, and on the other hand two-color as well as mode-locked emission. Mode-locked SDLs offer numerous advantages over their solid-state pendants, such as their low-complexity, compactness, cost-efficiency, and an extremely wide range of accessible emission wavelengths (from visible to mid-infrared, based on the employed material system) and repetition rates. This makes ultrafast SDLs very interesting for various applications that rely on a compact, cost-efficient and mass-producible laser technology. SDLs can be passively mode-locked using different mode-locking techniques. While previously saturable absorbers such as semiconductor saturable-absorber mirrors (SESAMs)- either external, or even internal, like in a mode-locked integrated external-cavity surface emitting laser (MIXSEL) - and recently novel-material-based carbon-nanotube or graphene saturable absorbers were employed. Up to date, the presented mode-locking techniques have led to a great enhancement in average powers, peak powers and repetition rates that can be achieved with passively mode-locked SDLs. However, the power-sensitive, complex and costly absorber mirrors, which have to be carefully designed for a certain wavelength range, naturally impose limitations on the device performance. Fortunately, on the other hand, a newmode-locking methodwas presented and discussed in recent years which is referred to as self-mode-locking (SML) or saturable-absorber-free operation of mode-locked SDLs. In this context, motivated by the demand for overcoming the aforementioned limitations, the goal of this thesis was to further exploit the potential of mode-locked SDLs. Particularly, focus on the SML or saturable-absorber-free operation technique, which is considered a promising technique for the realization of compact, robust and cost-efficient modelocked devices. In this thesis, experimental results of SML operation of SDLs in the subpicosecond regime will be presented. We show that the SML scheme is not only applicable to quantum-well-based SDLs, but also to quantum-dot-based devices. Moreover, harmonic mode-locking with sub-ps pulses is demonstrated at discrete power levels. Furthermore, to extend the applications of ultrafast SDLs, we realized an ultra-bright single-photon-source by optically exciting a deterministically integrated single quantum-dot microlens using a mode-locked SDL. The compact and stable laser system allows for overcoming the limited repetition rates of commercial mode-locked Ti:sapphire lasers and to excite the single quantum-dot microlens with a pulse repetition rate close to 500 MHz and a pulse width of 4.2 ps at a wavelength of 508 nm, utilizing second-harmonic generation in an external nonlinear crystal.|