Influence of laser diode phase noise on optical heterodyne spatial tracking systems

Abstract

In optical intersatellite communication, a high-precision antenna directivity is required for stable communication with a sharp optical beam. The antenna directive accuracy is affected strongly by the performance of the spatial tracking system which constantly tracks the direction of arrival of the optical beam sent by the sending satellite. The tracking system using an optical heterodyne detection to detect the angle of arrival of the optical beam has an advantage over the direct detection system in that its sensitivity is high and it is tolerant to the background light. Recently, a number of systems using laser diodes as the source have been studied extensively. Although the detection efficiency has been studied in the case where the effect of the electrical noise in the tracking signal detection circuit and the electric field distributions of the signal light and the local oscillator light are taken into account, no study has been carried out on the effect of the phase noise of the laser diode (LD) on the tracking characteristics. In this paper, the electric characteristics are studied for the tracking signal detection circuit using the optical heterodyne/square detection. The effect of the spectral spread by the phase noise of the LD on the spatial tracking system is studied. The relationship is found between the spectral width of the intermediate frequency signal and the bandwidth of the intermediate frequency bandpass filter which minimizes the effect of the phase noise of the LD. Further, the characteristics are compared with those of the system using the direct detection method.