Lasers operating in the transmission window of tissue at wavelengths between 700 and 800 nm are needed in numerous medical and biomedical applications, including photodynamic therapy and fluorescence microscopy. However, the performance of diode lasers in this spectral range is limited by the lack of appropriate compound semiconductors. Here, we review our recent research on 750 nm VECSELs. Two approaches to reaching the 750 nm wavelength will be discussed. The first approach relies on intra-cavity frequency doubling a wafer-fused 1500 nm VECSEL. The VECSEL gain chip comprises a GaAs-based DBR and an InP-based gain section, which allows for optical pumping with low-cost commercial diodes at 980 nm. With this scheme we have achieved watt-level output powers and tuning of the laser wavelength over a 40 nm band at around 750 nm. The second approach is direct emission at 750 nm using the AlGaAs/GaAs material system. In this approach visible wavelengths are required for optical pumping. However, the consequent higher costs compared to pumping at 980 nm are mitigated by the more compact laser setup and prospects of doubling the frequency to the ultraviolet range.
Esa J. Saarinen, Sanna Ranta, Jari Lyytikäinen, Antti Saarela, Alexei Sirbu, Vladimir Iakovlev, Eli Kapon, and Mircea Guina, "Advances in 750 nm VECSELs (Conference Presentation)," Proc. SPIE 10087, Vertical External Cavity Surface Emitting Lasers (VECSELs) VII, 100870I (Presented at SPIE LASE: January 31, 2017; Published: 21 April 2017); https://doi.org/10.1117/12.2252008.5387787328001.
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