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Improving Sensitivity of Optical Receivers for Space Communication

Scientists from Changchun Institute of Optics, Fine Mechanics and Physics, develop a transmission system that reduces excess noise.

In space exploration maintaining communication is key for deep-space missions, inter-satellite data transfer, as well as earth monitoring. These space activities require high-speed data connectivity. Optical laser beams are commonly used in such systems over radio-frequency beams. This is attributed to the reduced loss of power as the beam is propagated substantially smaller at light wavelengths, and beam divergence is reduced.

Light beams however, have a disadvantage over long distances. Power is lost when a laser beam is sent from a distance of about 400,000 kilometres (earth to the moon) with a 10-centimetre aperture size will be about 80 decibels (i.e. 1 part in 100 million will remain). Due to the limited transmission power, there is a need for receivers to be able to recover as much information sent through the laser beam as possible. The sensitivity of a receiver in obtaining the information is calculated as the minimum number of photons per information bit needed to recover the data without error.

It has been well studied that power-efficient pulse position modulation formats along with nanowire-based photon-counting receivers being cooled to a certain temperature while being able to operate at speeds below one Gigabit per second. To achieve higher transmission rates, pre-amplified receivers together with advance signal generation and processing techniques through optical fibre communications should be considered.

A team of scientists led by Professor Peter A. Andrekson, recently published a study in Light Science & Application on their development of a free-space optical transmission system relying on an optical amplifier that does not, in principle, add any excess noise in contrast to all other known optical amplifiers, referred to as a phase-sensitive amplifier (PSA).

The study highlighted the concept of encoding information onto a signal wave, which together with a pump wave at different frequencies will be able to generate a conjugated wave in a non-linear medium. Overall, these three waves will be launched into space and at the receiver, light is first captured by an optical fibre, the PSA amplifies the signal and passively carry the data using a regenerated pump wave. The amplified signal is then detected in a conventional coherent receiver and results in the best possible sensitivity in any pre-amplified optical receiver.

Using this method, the team was able to show unprecedented error-free, “black-box” sensitivity of one photon-per-information-bit at a data rate of 10.5 Gigabits per second. With 10 Watts of transmitter power, this receiver would allow for a link loss of 100 decibels. The system uses a simple modulation format encoded with a standard forward error correction code and a coherent receiver with digital signal processing for signal recovery. When required in the future, this method has the potential to be scalable for higher data rates. It also operates at room temperature, allowing this to be implemented on the ground and in space terminals.

The scientists shared that the results of the paper, “show the viability of this new approach for extending the reach and data rate in long-haul space communication links. It therefore also has the promise to help break through the present-day science data return bottleneck in deep-space missions, which space agencies around the world are suffering from today.” [APBN]