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

摘要

The authors designed and realized a laboratory model of an optical intersatellite link employing InGaAs DFB semiconductor lasers operating at a wavelength of $lambda $EQ 1.55 $mu@m. Heterodyne sensing was used for both the spatial acquisition and the spatial tracking processes. An intermediate frequency of IF $EQ 700 MHz was chosen. The definition of wavelength and intermediate frequency asked for the development of a new position detector concept. The function of a quadrant detector was realized by splitting the superimposed beam at the top of a reflecting pyramid into four subbeams. These beams are then detected by separate pin photodiodes. The angular resolution achieved, (without using a telescope) is less than 5 $mu@rad 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. Tracking is activated by: first, closing an analog intermediate frequency stabilization loop; second, a digital spatial tracking loop is closed by calculating the required antenna correction from the position sensor information, achieving an average tracking error of $POM 25 $mu@rad.