The Most Sensitive Optical Receivers Yet for for Laser-Beam Based Space Communications

The Most Sensitive Optical Receivers Yet for for Laser-Beam Based Space Communications

An illustration of the New Concept experimental setup. Photo credit: Yen Strandqvist / Chalmers University of Technology

Communications in space require the most sensitive receivers possible for maximum range while also requiring high bit rate operations. A novel concept for laser beam based communication using a nearly silent optical preamplifier in the receiver has recently been demonstrated by researchers from Chalmers University of Technology, Sweden.

Published in a new article in the journal Nature: light science & applicationsA team of researchers describes a free-space optical transmission system based on an optical amplifier that, in principle, does not generate excessive noise – in contrast to all other pre-existing optical amplifiers called phase-sensitive amplifiers (PSAs).

The researchers’ new concept shows an unprecedented receiver sensitivity of only one photons per information bit at a data rate of 10 gigabits per second.

“Our results show the feasibility of this new approach to extending the range and data rate in long-distance communication links. It therefore also has the promise to help overcome today’s data return bottleneck in space missions that space agencies around the world are suffering from today, ”says Professor Peter Andrekson, head of the research group and author of the articles with PhD Ravikiran Kakarla and the lead researcher Jochen Schröder at the Institute for Microtechnology and Nanosciences at Chalmers University of Technology.

Comparison of laser and radio beam footprints

A comparison of the laser and radio beam footprints. Photo credit: Yen Strandqvist / Chalmers University of Technology

A substantial increase in the range and the information rate for future high-speed links will have a major impact on technologies such as communication between satellites, space missions and earth surveillance with light detection and distance (lidar). Systems for such high speed data links are increasingly using optical laser beams instead of radio frequency beams. A main reason for this is that the loss of power in propagating the beam at wavelengths of light is much less because the beam divergence is reduced.

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Peter Andrekson

“This approach essentially leads to the best possible sensitivity of any pre-amplified optical receiver and also outperforms all other current state-of-the-art receiver technologies,” says Peter Andrekson, Professor of Photonics at Chalmers University of Technology. Photo credit: Henrik Sandsjö / Chalmers University of Technology

However, light rays also experience great losses over great distances. For example, a laser beam sent from the earth to the moon – approximately 400,000 kilometers – with an aperture size of 10 cm will experience a power loss of approximately 80 dB, which means that only 1 part in 100 million will be left. Since the power that can be transmitted is limited, it is crucial to have receivers who can recover the information sent with the least amount of power received. This sensitivity is quantified as the minimum number of photons per bit of information required to restore the data without errors.

In Chalmers’ new concept, information is encoded onto a signal wave that, together with a pump wave of different frequency, creates a conjugate wave (known as idle) in a non-linear medium. These three waves are hurled together into free space. At the point of reception, after detecting the light in an optical fiber, the PSA amplifies the signal using a regenerated pump wave. The amplified signal is then recorded in a conventional receiver.

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“This approach basically leads to the best possible sensitivity of a pre-amplified optical receiver and also surpasses all other current receiver technologies based on the latest state of the art,” says Peter Andrekson.

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 can easily be scaled to much higher data rates if required. It also works at room temperature, which means that it can be implemented in room terminals and not just on the floor.

The theoretical sensitivity limits of this approach are also discussed in the work and compared with other existing methods, with the conclusion that the new approach is essentially the best possible for a very wide range of data rates.

Reference: “A photon-per-bit receiver with almost noise-free phase-sensitive amplification” by Ravikiran Kakarla, Jochen Schröder and Peter A. Andrekson, September 2, 2020, Light: Science & Applications.
DOI: 10.1038 / s41377-020-00389-2

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