A new and practical approach to the low-noise amplification of weakened optical signals has been unveiled by researchers in Sweden. Drawing from the principles of four-wave mixing, Rasmus Larsson and colleagues at Chalmers University of Technology believe their approach could have promising implications for laser-based communication systems in space.
Until recently, space-based communication systems have largely relied on radio waves to transmit signals. Increasingly, however, these systems are being replaced with optical laser beams. The shorter wavelengths of these signals offer numerous advantages over radio waves. These include higher data transmission rates; lower power requirements; and lower risks of interception.
However, when transmitted across the vast distances of space, even a tightly focused laser beam will spread out significantly by the time its light reaches its destination. This will weaken severely the signal’s strength.
To deal with this loss, receivers must be extremely sensitive to incoming signals. This involves the preamplification of the signal above the level of electronic noise in the receiver. But conventional optical amplifiers are far too noisy to achieve practical space-based communications.
Phase-sensitive amplification
In a 2021 study, Larsson’s team showed how these weak signals can, in theory, be amplified with zero noise using a phase-sensitive optical parametric amplifier (PSA). However, this approach did not solve the problem entirely.
“The PSA should be the ideal preamplifier for optical receivers,” Larsson explains. “However, we don’t see them in practice due to their complex implementation requirements, where several synchronized optical waves of different frequencies are needed to facilitate the amplification.” These cumbersome requirements place significant demands on both transmitter and receiver, which limits their use in space-based communications.
To simplify preamplification, Larsson’s team used four-wave mixing. Here, the interaction between light at three different wavelengths within a nonlinear medium produces light at a fourth wavelength.
In this case, a weakened transmitted signal is mixed with two strong “pump” waves that are generated within the receiver. When the phases of the signal and pump are synchronized inside a doped optical fibre, light at the fourth wavelength interferes constructively with the signal. This boosts the amplitude of the signal without sacrificing low-noise performance.
Auxiliary waves
“This allows us to generate all required auxiliary waves in the receiver, with the transmitter only having to generate the signal wave,” Larsson describes. “This is contrary to the case before where most, if not all waves were generated in the transmitter. The synchronization of the waves further uses the same specific lossless approach we demonstrated in 2021.”
The team says that this new approach offers a practical route to noiseless amplification within an optical receiver. “After optimizing the system, we were able to demonstrate the low-noise performance and a receiver sensitivity of 0.9 photons per bit,” Larsson explains. This amount of light is the minimum needed to reliably decode each bit of data and Larsson adds, “This is the lowest sensitivity achieved to date for any coherent modulation format.”
This unprecedented sensitivity enabled the team to establish optical communication links between a PSA-amplified receiver and a conventional, single-wave transmitter. With a clear route to noiseless preamplification through some further improvements, the researchers are now hopeful that their approach could open up new possibilities across a wide array of applications – especially for laser-based communications in space.
“In this rapidly emerging topic, the PSA we have demonstrated can facilitate much higher data rates than the bandwidth-limited single photon detection technology currently considered.”
This ability would make the team’s PSA ideally suited for communication links between space-based transmitters and ground-based receivers. In turn, astronomers could finally break the notorious “science return bottleneck”. This would remove many current restrictions on the speed and quantity of data that can be transmitted by satellites, probes, and telescopes scattered across the solar system.
The research is described in Optica.
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