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Chemical laser is one of the most widely used high power infrared sources. Thermal deformation of mirrors in a
resonator is a key factor which hinders the improvement of the beam quality in high power chemical lasers, and it has
been a matter of the utmost concern in the design of high power chemical laser systems. Water-cooled technique is one
of the most widely methods employed in cooling mirrors in high power laser systems. Several kinds of water-cooled
mirrors have been studied by researchers in the world. Research teams of the United States and Russia have done a lot of
work and their water cooled mirrors behave very well. In China, the study of micro-channel water cooled silicon mirror
has been carried out by Cheng Zuhai in detail and the research of water jet cooled mirror has been performed by Li Bin.
Both the two types of water cooled mirrors show excellent capabilities in alleviating the thermal deformation of mirrors.
However, the issues that how and to what extent the beam quality is improved when the water cooled mirrors are
involved have not been analyzed so far. In this paper, through numerical simulations, the beam qualities of linear
resonators with various types of water cooled mirrors are analyzed in detail, and the effects of the two types of water
cooled mirrors are compared. The thermal distortion of uncooled Si mirrors is characterized by Zernike polynomials and
the beam quality is characterized with Strehl ratio, β factor, and M2 factor, which degenerates as the beam oscillates in
the distorted resonator. The aberrance of mirrors non-uniformly heated by high energy laser mainly distributes on the
low-even-order Zernike polynomials. When the uncooled Si mirrors are replaced by water cooled mirrors, the distortion
decreases greatly and the beam quality is improved a lot. Although the performance of the water jet cooled mirror on
mitigating distortion is better than the micro-channel water cooled mirror, the resonator made up of two water jet cooled
mirror is not the best structure in improving beam quality. The resonator composed of an uncooled reflecting mirror and
a micro-channel water cooled reflecting mirror has the most prominent capability in improving the beam quality. The
research method of beam quality and some results in this paper are useful for the design of resonators with water cooled
mirrors and the evaluation of beam quality in high power chemical laser systems.
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