High-Power Operation of Semiconductor Disk Lasers
The development of semiconductor disk lasers (SDLs), which are also known as vertical-external-cavity surface-emitting lasers (VECSELs), gives rise to semiconductor lasers with high multi-watt output power combined with diffraction-limited output beam-profile. Owing to a steady progress in the fiel...
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|Summary:||The development of semiconductor disk lasers (SDLs), which are also known as vertical-external-cavity surface-emitting lasers (VECSELs), gives rise to semiconductor lasers with high multi-watt output power combined with diffraction-limited output beam-profile.
Owing to a steady progress in the field of SDLs, they feature many advantages over conventional semiconductor (diode) lasers. For instance, high output powers can be achieved with a TEM00 beam profile, no p-n junctions are needed in an SDL device which reduces losses due to free-carrier absorption in doped regions, broad wavelength tuning (> 100 nm) is possible due to a broad gain bandwidth in semiconductors, and external-cavity configurations allow for different operation schemes, i. e., intra-cavity frequency conversion, wavelength-tunable single-frequency operation and mode-locking. This versatility is particularly beneficial with respect to applications. Up to now, mainly quantum-well (QW) based SDLs were used due to their strong yield. However, quantum-dots (QDs) based SDLs become increasingly popular, because they offer a number of advantages hardly achievable when using QWs, such as a reduced lasing threshold, a lower thermal sensitivity, and a higher differential gain. In addition, QDs are also applicable for a coverage of different spectral regions such as in the range of 1 to 1.3 µm, they can provide enhanced wavelength tunability and ultrafast carrier dynamics, which potentially will improve mode-locked operation with respect to shorter puls durations.
The work presented in this thesis was focussed on the development and testing of high-power semiconductor disk lasers based on novel quantum-dot structures, and the analysis of optical-scattering losses in SDLs in general. The QDs in the SDL chip structure were formed by molecular-beam-epitaxy growth of InGaAs/GaAs semiconductor materials using the Stranski-Krastanov growth method, and supplied by our cooperation partners for investigations on the performance optimization. The employment of QD materials allowed for the realization of SDLs in the infrared spectral region between 1 and 1.3 µm. Devices with emission wavelengths of 1040 and 1180 nm were subject of this work and QD-based SDLs were tested with respect to high-power operation in a linear cavity configuration.
The experiments were performed in order to achieve a maximum output power in the continuous-wave (CW) regime for the existing chips. Therefore, the cavity parameters, i. e., the cavity length, the pump-spot width, and the transmittance of the output-coupler (OC) mirror, were systematically varied in order to reach the best performance of the studied device. As a consequence of the optimization of the operation conditions, record-high CW output powers up to 8.4 and 7.2 W are obtained at temperatures around 2 °C for SDLs emitting at 1040 and 1180 nm, respectively. Besides, by rotating an additionally inserted birefringent filter inside the laser cavity, the laser became wavelength tunable over a relatively large range of 45 and 37 nm for SDLs emitting at 1040 and 1180 nm, respectively. Although the results presented in this thesis may have certainly contributed to the development of QD SDLs, more effort is needed to fully explore the advantages of QD based materials. This will include wider research concerning the thermal sensitivity and operational stability of QD based lasers. That would allow for a more accurate design of the devices, which lead to a more efficient operation.
To highlight the influence of optical-scattering losses on the SDL's performance, the thermal resistance of a reference low-surface-quality SDLs chip was analyzed. From experimental input-output characteristics based on thermal roll-over for different output-coupler transmittance values, the optical surface-scattering losses were identified when using an expanded model that takes into account non-heating losses in a device. In this study, we've learned that optical surface-scattering is a non-negligible component of loss in an SDL system, thus further contributing to an understanding of limitations to high-power operation. In conclusion, the best-quality chips -not only with respect to the structural quality inside the chip, but also to the surface quality- are required for the purpose of high-power operation.|