For free-space optical (FSO) communications systems, sensitive optical receivers are the key to
closing the link over long distances in inter-satellite transmission scenarios, or to overcome large
atmospheric attenuation in terrestrial FSO systems. We present a 10.7 Gb/s optical transmitterreceiver
pair operating at 1550-nm, based on return-to-zero, differential phase-shift keying (RZDPSK).
The receiver is pre-amplified and uses an optical delay interferometer and a balanced
photo-receiver. The outer dimensions, the weight, and power consumption are 44×44×18 cm3,
14.1 kg, and 35 W, respectively. This optical receiver is single-mode fiber coupled. At 10.7
Gb/s, a receiver sensitivity of 27 photons/bit was measured, which yields a bit error rate of 1e-9.
This is less than 1 dB from the quantum limit (22 photons/bit). Coupled with a commercial
optical booster amplifier having an output power of about +37 dBm, a link loss of more than 80
dB can be bridged. In an inter-satellite communications scenario, this corresponds to several
tens of thousands of kilometers. Additionally, high link losses can also be experienced in
terrestrial systems as the result of atmospheric scintillation. To study this effect, the transmitter
and receiver combination were tested with simulated turbulence (scintillation). A turbulence box
was used to emulate different levels of scintillation under which the pre-amplified RZ-DPSK
system was investigated. Results of these tests are presented.