Scientists at Discovery Semiconductors (Ewing, NJ), Science Applications International (Albuquerque, NM), the Air Force Research Laboratory (Kirtland AFB, NM), and Schafer (Albuquerque, NM) have created a satellite free-space optical communications system that transmits 10.7 Gbit/s at a 1.55 µm wavelength; with a commercial +37 dBm optical-booster amplifier, it would do so over a distance of several tens of thousands of kilometers. The receiver—the key development—is preamplified and has an optical-delay interferometer and a balanced photoreceiver.
The transmitter-receiver pair is based on return-to-zero differential phase-shift keying. The receiver has a sensitivity of 27 photons per bit at 10.7 GHz, less than 1 dB from the quantum limit of 22 photons/bit, resulting in a bit-error rate of 1 × 10-9. The photodetector is integrated with a clock-and-data recovery (CDR) unit to allow bit-error-rate measurements. To simulate Earth-to-satellite links, the system was tested with a “turbulence box,” containing heating elements and fans, which showed that even under heavy scintillation, the CDR locked onto the data stream, and also confirmed that the return-to-zero format was more robust than a non-return-to-zero format. Contact Christoph Wree at [email protected].