Phase Sensitive Amplifiers for Free-Space Optical Communications

Sammanfattning: The demand for high data rate free-space communications is increasing due to the planned future space exploration missions. In the next few years there is a need to increase the speed by 100 times according to NASA, and this necessitates that transmission systems operate at higher carrier frequencies. Optical communication systems are capable of handling hundreds of Gigabits per second data with a single light carrier and are suitable for high data rate space communication links. The sensitivity of the receiver is one of the key factors to achieve such high speed communication. Phase sensitive parametric optical amplifier (PSA) can amplify optical signals ideally without degrading the signal to noise ratio. Employing these as pre-amplifiers in free-space receivers can thus improve the sensitivity significantly. In this thesis we investigate the prospects of implementing a PSA based receiver for free space links. We use a 10 GBd QPSK signal and a pump wave to generate the necessary idler wave at the transmitter. The three waves are sent through a free-space link where only loss is considered as the channel impairment. The transmitted pump power was much lower than the combined signal and idler wave powers which would otherwise impair the overall sensitivity. At the receiver, the received pump power is as low as -65 dBm whereas a combined signal and idler power of -50 dBm was needed to achieve a bit-error rate of $10^{-3}$. The received low power pump was recovered using injection locking and a phase locked loop setup. Key results show that, the sensitivity can be improved by 3 dB with respect to an low noise figure erbium doped fiber amplifier (EDFA) based receiver.