Communications in space demand the most sensitive receivers possible for maximum reach, while also requiring high-bit-rate operations. A concept for laser beam-based communications using an almost noiseless optical preamplifier in the receiver has been developed. The free-space optical transmission system relies on an optical amplifier that, in principle, does not add any excess noise, in contrast to all other pre-existing optical amplifiers referred to as phase-sensitive amplifiers (PSAs).
The concept demonstrates a receiver sensitivity of just one photon per information bit at a data rate of 10 gigabits per second. The approach could extend the reach and data rate in long-distance space communication links and could eliminate the data-return bottleneck that exists in deep space missions.
Substantially increasing the reach and information rate for future high-speed links will have big implications for inter-satellite communication and Earth monitoring with light detection and ranging (LiDAR). Systems for such high-speed data connections are increasingly using optical laser beams rather than radio-frequency beams. A key reason for this is that the loss of power as the beam propagates is substantially smaller at light wavelengths, since the beam divergence is reduced.
Over long distances, light beams also experience large loss; for example, a laser beam sent from the Earth to the Moon — around 400,000 kilometers — with a 10-cm aperture size will experience a loss of power of around 80 dB, meaning only 1 part in 100 million will remain. As the transmittable power is limited, it is of critical importance to have receivers that can recover the information sent with as low power received as possible. This sensitivity is quantified as the minimum number of photons per information bit necessary to recover the data without error.
In the new approach, information is encoded onto a signal wave, which along with a pump wave at different frequency, generates a conjugated wave (known as an idler) in a nonlinear medium. These three waves are launched together into the free space. At the receiving point, after capturing the light in an optical fiber, the PSA amplifies the signal using a regenerated pump wave. The amplified signal is then detected in a conventional receiver.
The system uses a simple modulation format encoded with a standard error correction code and a coherent receiver with digital signal processing for signal recovery. This method is straightforwardly scalable to much higher data rates if needed. It also operates at room temperature, meaning it can be implemented in space terminals and not only on the ground.