New breakthroughs in the research of mid-infrared light parametric laser conversion efficiency

Recently, Jiang Haihe, Research Fellow of the Medical Laser Technology Research Center of the Medical Physics and Technology Center of the Chinese Academy of Sciences and the Wu Haixin Research Group, a researcher of the Department of Crystal Materials of the Anhui Institute of Optics and Fine Mechanics, conducted a mid-infrared phosphonium-neon zinc optical parameter. Research on oscillators (ZGP-OPO) has made new breakthroughs in conversion efficiency. Research results have been published in Optics Express, 2017, 25(4):3373.

Coherently tunable mid-infrared lasers have important applications in laser spectroscopy, biomedical applications, environmental detection, and optoelectronic countermeasures. Optical parametric oscillator (OPO) is currently an important means to obtain coherently tunable mid-infrared laser output. The improvement of its conversion efficiency can not only obtain greater output energy, make the system work away from the critical state of damage, but also improve the system's Stability and safety reliability. However, it is limited by the influence of many factors such as beam quality, pulse width, and pulse energy of the pump laser, quality of the nonlinear crystal, and phase mismatch, reverse conversion, and walk-off effect during the parametric conversion process, resulting in high conversion efficiency. Pulsed mid-IR laser output has always been a challenge. It is well known that the optical parametric conversion efficiency is closely related to the pump beam diameter and its power density. In order to obtain a high pump power density, researchers usually have to focus smaller energy pumping light on non-linear crystals with small spots. It also limits the improvement of the conversion efficiency, and the focusing will inevitably cause phase mismatch, reverse conversion, and walk-off effects in the nonlinear conversion process, hindering the achievement of high conversion efficiency. For a long time, the optical-optical conversion efficiency of optical parameters has remained at 50-60%.

The use of autonomously grown mid-infrared nonlinear laser ZGP crystals, ZGP crystals with high damage threshold and high nonlinear coefficient, is the best crystal material in the mid-infrared 3 ~ 5 micron band. However, the crystal must be pumped with a laser having a wavelength of 2 μm or more. However, commercial lasers in this band cannot satisfy the requirement of high conversion efficiency, and it is not possible to obtain both shorter laser pulses and higher pump energy. Based on the accumulation of previous work, this study used a self-developed high-quality Q-switched YAG laser with a 2.09-micron wavelength as the pump source to achieve a laser output with a pulse energy of 32 mJ, a pulse width of 29.9 ns, and a beam quality of 1.29. Through a well-designed OPO system, under the unfocused large beam pump of 2.1mm and 3.1mm diameter, the influence of multiple factors such as phase mismatch, reverse conversion, and walk-off effect was suppressed, and pulse energy 19mJ was obtained. The tunable mid-infrared laser output in the ~5 micron band increases the light-to-optical conversion efficiency to 75.7% (83% slope efficiency), creating a new record for mid-infrared ZGP parametric conversion efficiency, converting the original parametric light to light. The efficiency record has increased by 30%.

The design of this OPO system takes into account various parameters such as the optimal size and power density of the pump beam, the optimal length of the nonlinear crystal, and the optimum coupling output rate, which ensures the high peak power density required for nonlinear conversion. At the same time, the pump beam was optimized to improve its spatial uniformity and reduce the divergence angle; at the same time, an OPO structure that suppresses the spatial walk-off effect and reverse conversion was designed, combined with high-quality ZGP crystals, making the The OPO system works in a state close to ideal conditions. The design of this OPO system balances well the contradiction between high peak power density and reverse conversion and crystal damage, thus achieving a breakthrough in high conversion efficiency.

The large-sized mid-infrared nonlinear laser ZGP crystals in this study have been banned and technically blocked by the United States and the European Union. This research has broken through the key technologies such as crystal stress caused by temperature rise fluctuations, and Bridgman direct growth in seed matching direction. Problems, and the use of component-compensated thermal annealing and high-energy electron irradiation, further reduce the near-infrared residual light absorption of the crystal. The self-prepared ZGP crystal elements have reached the leading level of similar foreign crystals in terms of crystal size, optical consistency, stability, laser damage threshold, and absorption coefficient (2 micron band absorption coefficient: <0.02/cm) and other core indicators. The research work provides key components.

This work was supported by the National Natural Science Foundation of China, the China Academy of Engineering Physics Joint Fund, and the Knowledge Innovation Project of the Chinese Academy of Sciences.

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