Development of spatial coherent optical receiver with a size of 100mm square for inter-satellite communication

The optical inter-satellite communication system with high-speed, small size and low power consumption is required because of increase of data capacity for observation satellites. Optical coherent technology has promising potential not only high sensitivity of communication systems but also high immunity against the back ground light for communication between satellites as well as between satellite and ground station. To realize a high-speed and small optical terminal, we have developed a coherent optical receiver that integrates optical angular sensor for spatial beam tracking in wavelength of 1.5 mu m. The angular sensor is segmented photodiodes and it can detects the direction of arrival beam from a counterpart satellite by calculating the center of gravity of each output. The coherent optical frontend is a free-space optical 90-degree hybrid. By inputting the output light from coherent optical frontend to the segmented photodiodes, both orthogonal detection of communication signals and angle detection are achieved. The integrated coherent receiver is a size of 100 cm(2). As a demonstration of coherent angle detection, we compared the results of simulation and actual measurement for angle detection using segmented photodiodes, and verified the validity of the design. We demonstrated orthogonal detection with this receiver by measuring the heterodyne beat output from the segmented photodiodes with cut-off frequency of 3.5 GHz, which corresponds to frequency difference between local and signal light. Applying this optical receiver to laser communication terminal, since it is not necessary to branch the received optical power to the capture and tracking sensor, the transmitted optical power required for communication can be reduced.