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266nm UV laser has a wide range of applications in various fields by its advantages in high single photon energy and
high resolution, which also has a development gradually moving in the direction of high power and high conversion
efficiency. In the process of high-power laser frequency doubling, BBO crystal inevitably absorbs part of fundamental
light power and frequency doubled light power, it induced the temperature rise along the direction of radiation in crystal
and destroyed the phase-matching conditions of BBO crystal that lead to phase mismatching. In order to improve
harmonic conversion efficiency as well as reduce the influence of output power and beam quality caused by phase
mismatching, in this paper we analyzed the process of phase mismatching, established the thermal-induced phase
mismatching model by using analytical expression of the nonlinear crystal temperature field equation which has been
given, and the three-dimensional phase mismatching distribution were obtained. There are three major contributions in
the paper. Firstly, the working process of the nonlinear crystal was analyzed, and the physical and mathematical models
of temperature distributions were established, and the BBO crystal three-dimensional temperature distributions were also
obtained. Secondly, a variety of factors that affect the temperature distributions within the BBO crystal were summarized.
For different 532nm waist radius and 532nm input power, they were numerical simulated use of MATLAB. Finally,
combined with the above analysis, the physical and mathematical models of phase mismatching caused by energy
absorption of BBO in forth harmonics generation were established, the phase mismatching distributions in the crystal
were simulated as well, especially the changes to phase mismatching distributions with different parameter were
analyzed. Combination of the multiplier theory, the influence of phase mismatching on frequency doubling conversion
efficiency was analyzed. The results indicate that the physical model which established in this paper can explain the
physical reasons in high-power laser frequency doubling system very well, such as the reduce in conversion efficiency
and output power and beam quality. All research results play instructive effect at the improvement of conversion
efficiency and the compensation for the phase mismatching for further research.
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