Heterodyne acquisition and tracking in a free-space diode laser link

A laboratory model of an optical intersatellite link employing InGaAs DFB semiconductor lasers operating at a wavelength of 1.55 micron was designed and realized. Heterodyne sensing was used for both the spatial acquisition and the spatial tracking processes. The function of a quadrant detector was realized by splitting the superimposed beam at the top of a reflecting pyramid into four subbeams. The angular resolution achieved - without using a telescope - is less than 5 microrad at a detector field of view of 1 mrad. The transmitter laser can be moved within a transverse plane along circular tracks. A microcomputer controls the receiver operation. During the acquisition process spiral scanning of the area of uncertainty is performed. For each search position the local oscillator laser is swept until a beat signal at 700 MHz is detected. Acquisition times of typically less than 16 s for a 200-element uncertainty area and tracking accuracies better than +/- 50 microrad for any examined test condition were achieved.