Laser emitted through the off-axis Cassegrain antenna

Cassegrain antenna is commonly used in Free Space Optic communication system. The double reflection structure determines the system has a shade problem. In this paper, the off-axis emission (semi-aperture lighting) method is applied to solve the shade problem caused by the secondary mirror. This method is simple both in structure an implement. ZEMAX software and Fourier optics are used to analyze the Cassegrain antenna theoretically. We can improve the performance of Cassegrain antenna under the guidance of the analytical results. Some experiment has been carried out. The results showed that the percentage of exit-pupil power effectively increased from 9% to 53%, the diameter of spot was not exceeding 200mm at the distance of 600m.It is significant for engineering applications.