High-power VECSELs in the 800-900 nm wavelength region

Abstract

The Master’s thesis is part of the UVQuanT EU funded project. The purpose of UVQuanT is to provide new deep ultraviolet (DUV) laser systems for complex quantum platforms, including Vexlum as a provider of the lasers. Laser applications at wavelengths below 266 nm are significantly limited by the scarcity of suitable light sources and optical components. To be able to reach a spectral region as low as deep ultraviolet, building a laser in the 8XX nm region is required. There are various laser options in the 8XX nm region such as a titanium-sapphire laser, diode lasers, and frequency converted fiber lasers, but the mentioned technologies are often bulky, expensive, and require a lot of maintenance. State-of-the-art DUV lasers are typically based on titanium-sapphire lasers. These continuous-wave DUV sources suffer from low output power (<200 mW), poor beam quality, high complexity and cost, and rapid degradation. Thus, exploring new possibilities for developing an alternative laser source is both fascinating and promising.

Vertical-external-cavity surface-emitting laser (VECSEL) is a promising alternative to replace titanium-sapphire laser. Compared to titanium-sapphire lasers, VECSELs are considered inexpensive, compact and reliable. There are various methods to reach higher output powers with VECSEL such as improving the heat dissipation or reducing heat load. This thesis focuses on increasing the output power of 8XX nm VECSELs by utilizing in-well pumping instead of traditional barrier-pumping of the gain chip. In-well pumping is advantageous since it enables the use of inexpensive pump sources emitting near 800 nm with hundreds of watts available pump power. Barrier pumping, on the other hand, requires the use of red laser diodes, which are often limited in power capability. In-well pumping also reduces the heat load of the gain chip due to lower quantum defect experienced by the gain chip compared to barrier pumping scheme.

In the thesis, an 860 nm gain structure is characterized both in in-well as well as barrier pumping scheme. Comparison of the results gained with each pumping technique is described. The results show that even over 10 W output power and slope efficiency of 56% can be reached with in-well pumping of the 860 nm gain structure with an 808 nm pump laser source. Not to mention that the quantum efficiency is increased by 20% when switching from barrier pumping to in-well pumping. To demonstrate that the 8XX nm VECSELs are capable of high-power single-frequency operation, and to study the laser characteristics in this operation mode, an 840 nm gain material was barrier-pumped with red diode laser to reach a laser wavelength of 843.346 nm. This specific emission was examined since its frequency-doubled output (i.e. 421.673 nm) could be used to manipulate the energy states of rubidium (Rb) atoms.

In the future, especially interesting would be to examine lifetime in high-power operation mode. Additionally, developing a single-frequency in-well-pumped VECSEL would be of interest as it could enable a ten-watt-level laser system to be realized for quantum technology applications.