Study of the propagation properties of 432 m laser reflected by retro reflector array

Some recent researches showed that retro reflector array (RRA) has installation superiorities that single retro reflector (RR) does not possess in the far-infrared (FIR) polarimeter/interferometer (PI) system for Tokamak. As a key precondition for replacing RR with RRA, the reflected light from RRA must change to Gaussian profile to guarantee the detectors get stronger signals and the threshold distance to Gaussian profile should be less than the optical path between the first wall and detectors. In this paper, the propagation properties of the reflected beams from RR and RRAs of different combinations have been measured by a beam profiler. The experimental results show that the reflected beam profiles from RRA of two retro reflectors (RRs) evolve into Gaussian distributions beyond a certain reflected optical distance and the reflected beam profiles from RRA of three RRs have a similar tendency to be Gaussian distributions with increasing reflected optical distance. The theory predictions, deriving from the integral theorem of Helmholtz and Kirchhoff, provide the same evolutions with experiments, but the threshold distances to Gaussian profile existing distinctions. These results suggest that RRA could be applied in the FIR PI system for the future ITER.