Detectors and sensors
Degradation studies of a VECSEL gain structure using a stable green pump laser
June 25, 2015
Abstract: The degradation of the gain structure of a red-emitting Optically Pumped Semiconductor (OPS).
Vertical External-Cavity Surface-Emitting Laser (VECSEL) pumped by an opus 532 laser emitting at 532nm is presented. The exceptional, long-term power stability of the opus 532 allows accurate monitoring of the performance of the VECSEL over several tens of hours, thereby helping to better understand the observed degradation effects within the VECSEL material.
OPS-VECSELs are becoming increasingly important as power scalable semiconductor lasers with a wide range of applications in biophotonics, spectroscopy, projector technologies and lithography. Interestingly there are, until now, almost no published studies addressing the long-term stability and degradation mechanisms of VECSEL gain structures1.
The gain structure under investigation is fabricated by metal-organic vapour-phase epitaxy (MOVPE). The substrate material is GaAs on which a Distributed Bragg Reflector (DBR) is grown. On top of the DBR, an AlAs heatspreading layer and the active region consisting of GaInP quantum wells embedded in AlGaInP barriers is grown. In total, the active region consists of 20 quantum wells with packages of four quantum wells each forming a Resonant Periodic Gain structure (RPG).
To test the gain structure, it is configured as an active medium in a v-shaped cavity of an OPS-VECSEL (see figure 1 and 2). Laser Quantum’s frequency-doubled Nd:YVO4 opus 532 laser is focused on the gain structure and optically pumps the VECSEL, which emits continuous wave radiation at 665nm. The near-unity M2 beam quality of the opus 532, and its excellent beam pointing stability, makes it an excellent pump source in such applications.
During the OPS-VECSEL experiments, it became apparent that there could exist a degradation mechanism within the VECSEL gain structure causing power loss during operation at 665nm. For a better understanding of the processes occurring within the VECSEL gain structure, the output power of the VECSEL, while pumped by the opus 532, was monitored over a long period (see figure 3). The good long-term power stability of the opus 532 makes it a perfect candidate for such studies. Figure 3 shows there is an exponential degradation of the VECSEL operation, after 18.2 hours the laser power decreased by 50% of its initial value.
To verify that the degradation of the VECSEL gain structure was the root cause of the observed power loss, large-area photoluminescence measurements were performed. These measurements show dark elliptical areas similar in size and shape to the pump laser focus from which no photoluminescence emission in the expected spectral range could be observed. The deposited heat by the pump laser is quite large since the quantum defect between the VECSEL wavelength of 665nm and the pump wavelength of 532nm is about 20%. It is found that this is the reason for the observed degradation since pumping the VECSEL with a laser closer to the emission wavelength showed less pronounced degradation effects.
In conclusion, the opus 532 was shown to be the ideal pump laser for probing degradation effects in OPS-VESCEL lasers, due to its excellent beam quality and long-term power stability, resulting in the investigations of several parameters under identical pump conditions.
Acknowledgements: Hermann Kahle, Maren Jäger and Dr. Michael Jetter from the University of Stuttgart, Dr. Lawrie Gloster and Dr. Gregor Klatt from Laser Quantum
 Kahle et al., Degradation studies and pump optimization of optically pumped red-emitting AlGaInP-VECSELs (March 4, 2015). Proc. SPIE 9349
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