Influence Of Thermal Distortion On The Propagation Of Beams In High Energy Laser Channel

With the progress of laser technology, the output power of continuous wave high power chemical Laser is becoming higher and higher, and the time duration is becoming longer and longer. Thermal distortion would be more severe on the reflectors in inner channel of high energy laser system nowadays, the influence of thermal distortion would be more significant on the laser beam quality, and it has been a matter of the utmost concern in the design of high energy chemical laser system.In this paper, models of the thermal distortions of Si mirrors, non-uniformly heated by high energy laser beam, are established from basic principles of thermal and structural static mechanics. The distortions with different supporting structures are calculated with finite-element method. The results show that the supporting structures have great effect on the distortion of mirrors, and mirrors suffer lower distortion under radial-fixing structure compared to three-points supporting structure. So, for the sake of reducing thermal distortion, the supporting ways should be chosen carefully in the design of high energy laser system.Mirror distortion results on some given discrete points are calculated by ANSYS, but the distortion on other points may be important for the convenience of beam propagation calculation. So the distorted mirror surfaces are fitted with 65 orders Zernike polynomials.Based on the Kirchhoff diffraction integration equation and model of ray tracing on the distorted mirrors,the propagation of non-uniform DF laser beam in inner channel of high energy laser system is calculated, and intensity distribution and phase distribution on each mirror are also calculated. The results show that the maximal intensity on some points of some mirrors is more than twice the value of input laser, so the coating standard must be higher to account for the intensity peak; After 12 times of reflection in laser inner channel, the PV value of optical field is 1.85λ, this result is valuable to the design of adaptive optics in high energy laser system.