40 Gbit/s Optical Free Space Transmission experiment using QPSK modulation format

Advantages of optical links like small, lightweight and power efficient terminals are practical for high data rate services of disaster preparedness and environmental research. In this paper, we demonstrate experimental results of 40-Gbit/s optical free space transmission using single-polarization quadrature phase shift keying (SP-QPSK) modulation format and digital coherent detection. The digital coherent detection enabled a high sensitivity and a tolerance to transmission impairments, which have attractive features for free space transmission system. We developed a 50-Gbit/s SP-QPSK transmitter and offline-receiver with the optical antenna system. SP-QPSK optical modulation signal with a line rate of 50-Gbit/s including 20% FEC is employed for high receiver sensitivity. A cascade by EDFAs consisting of a low noise pre-EDFA and an optical level controlled EDFA is developed to compensate for level fluctuation without degrading receiver sensitivity. Maximal ratio combining algorithms and carrier phase estimation algorithms are used at the offline-receiver for QPSK signal detection. We succeeded 4-meter indoor free space transmission having same performance as that with fiber connection using the developed system. The optical received power was -42 dBm at bit error rate of 10(-3). While for outdoor 50-meter transmission, we confirmed the received bit error rate larger than FEC limit.