Experimental realization of high-efficiency blue light at 426 nm by external frequency doubling resonator
Authors: Tian Long , Wang Qing-Wei , Yao Wen-Xiu , Li Qing-Hui , Wang Ya-Jun , Zheng Yao-Hui ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Second harmonic generation (SHG) is used to get continuous wave laser with a lot of applications, it is a major way to provide pump power for generating nonclassical states, especially for squeezed states and entanglement states. High-efficiency SHG resonant on atoms lines also provides laser sources for atomic entanglement generation, light-atom interaction and high-speed quantum memory. For the frequency-doubling process at 426 nm, the major challenge of increasing the conversion efficiency is the thermal effect caused by the absorption in crystal. The degradation of mode-match efficiency induced by the severely thermal effect limits the conversion efficiency of the second harmonic generator. Furthermore, the blue light induced infrared absorption (BLIIRA) in the nonlinear crystal intensifies the thermal effect, it makes the conversion efficiency of the frequency-doubling cavity and the stability of the output blue laser worse, and it is more serious at high input power. Based on the theoretical analysis of thermal lens, we find that the thermal lens should not be placed at the center of the crystal, the location of the equivalently thermals lens has a deviation from the center of the crystal. Follow the theoretical analysis of thermal lens, we design a ring cavity with a 10 mm-long periodically poled potassium titanyle phosphate (PPKTP) crystal to reduce the thermal lens effect induced mode-mismatch. The location of nonlinear crystal is adjusted precisely to reduce the mode-mismatch caused by the thermal lens under our theoretical analysis. Finally, we realized a high conversion efficiency blue laser at 426 nm with the conversion efficiency up to 83.1% with an output power of 428 mW after the adjustment of the crystal location, corresponding to our theoretical analysis well. The measured beam quality factors (<i>M</i><sup>2</sup> value) of the generated blue laser are <inline-formula><tex-math id="Z-20200217024354-1">\begin{document}$ M^2(x) = 1.05 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20200217024354-2">\begin{document}$ M^2(y) = 1.02 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.png"/></alternatives></inline-formula>, respectively. The measured power stability of Generated Blue laser in 15 mins is 1.25%. The output power of the SHG is strong enough to provide pump power for the generation of the continuous variable squeezed vacuum state at 852 nm and the long-term stability of the output blue laser is also measured to be fine. To the best of our knowledge, the conversion efficiency is the highest-reported one at this wavelength. We believe that such high-performance frequency doubling system is a fundamental building block for quantum information science based non-classical states.