Not only image sensors are able to detect single photons with ps timing accuracy these days. Chalmers University of Technology, Sweden, develops a free-space optical transmission system relying on an optical amplifier that, in principle, does not add any excess noise – in contrast to all other preexisting optical amplifiers, referred to as phase-sensitive amplifiers (PSAs). The researchers’ new concept demonstrates an unprecedented receiver sensitivity of just one photon-per-information bit at a data rate of 10Gbps.
In the new concept from Chalmers, 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.
“This approach fundamentally results in the best possible sensitivity of any pre-amplified optical receiver and also outperforms the all other current state-of-the-art receiver technologies,” 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 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.
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