Physical basis and corresponding instruments for PAT performance testing of inter-satellite laser communication terminals

Abstract

The propagation of laser beam in inter-satellite laser communications belongs to the far-field diffraction, but in the optical test and verification of pointing, acquisition and tracking (PAT) function on the laboratory the beam from a terminal propagates within the near-field. In this paper, in terms of the Fresnel diffraction theory the inherent difference is found that in the far-field diffraction the optical tracking position error is resulted from both the mutual movement between two laser communication terminals and the tilting of the receiver terminal, but the tilting of the transmitter has no effect on the error position; and that in the near-field diffraction the position error is caused by the tilting of the transmitter or the receiver, but the mutual movement has no effect. It is furthermore found that the use of a beam scanner in the test in the near-field can simulate exactly the mutual movement of satellites in the far-field, and the trajectory formula for the beam scanning is deduced that is the same as the mutual angular trajectory from one satellite to another. Therefore a practical PAT test bed of a double-focus laser collimator, a beam scanner and a fine beam steering device is developed by us to test and verify the PAT function of inter-satellite laser communication terminals. The optical aperture is about φ440mm for this use. And a test bed for concurrent test and verification of both PAT function and communication performance is also demonstrated. The test bed consists of a conventional laser collimator, an optical scanner and a far-field beam transmission simulator, which is a combination of a Fourier-transform lens an a followed multiple-stage imaging amplifier. The details of configurations are given. It is clear that these test beds can be also used to test and verify the functions of laser radar, passive optical tracker, and so on.