"Temporal Passive Amplification" Boosts Weak Optical Signals a Hundredfold — But Leaves the Noise

A researcher team at the Institut National de la Recherche Scientifiqu (INRS) has developed a new way to boost the power of an optical signal while leaving the noise behind — and say the approach is of "potential immediate practical interest" in the fields of telecommunications, bio-imaging, and remote sensing.

The biggest issue with boosting a weak signal, as anyone who has turned up the volume on a badly-tuned radio will know, is that along with a stronger signal you also receive stronger noise. This puts a lower limit on how weak a signal can get before it's useless — which, in turn puts a limit on things like the distance you can transmit along an optical cable.

"In long-haul telecommunication systems," the research team notes, "the considerable loss from long distance propagation through the optical fiber combined with the injected noise from multiple optical amplifiers causes data signals to become weak and noisy, such that dedicated regeneration stages are required, which are costly and energy hungry.

"In light-based biomedical imaging applications, the mere fact that a living tissue is imaged poses important limitations on the attainable light powers to avoid damaging the biological sample. Similarly, in LIDAR applications, high optical powers can pose a threat to the eyes of a passer-by. There are many application examples such as the ones mentioned here that are ultimately limited by the ability to detect weak, noisy temporal waveforms."

The team's solution: Passive amplification exploiting concepts traced back to the Talbot self-imaging effect, named for the scientist Henry Fox Talbot who first observed the effect in 1836. The team's twist: Lossless temporal sampling via Talbot processing, capable of boosting the gain of a signal more than a hundredfold — yet punching through "much stronger background noise."

In effect, the system works to redistribute the energy of a signal into higher-intensity pulses — focusing the power without distorting the shape of the signal nor boosting the noise levels. "We realized that we could find a particular arrangement so that the Talbot effect could be used to amplify aperiodic signals," José Azaña, INRS professor and co-author of the paper, explains, "which is to say, virtually any of the signals found in these different practical fields."

"We hope that this technique could be used by researchers around the world to access previously unattainable information contained in weak, noisy signals," adds Benjamin Crockett, first author of the study.

The team's work has been published in the journal Optica under open-access terms.