Weak optical signals are common in many science and technology applications. However, they are difficult to detect or deal with due to the incoherent noise inherent in any system. Ph.D. student Benjamin Crockett and his colleagues, under the direction of Professor José Azaña of the National Institute for Scientific Research (INRS), have demonstrated a technique for recovering weak optical signals dominated by noise. Their research was published in the journal Optical.
This achievement is relevant in various fields such as telecommunications, bioimaging and remote sensing. For example, there is considerable loss due to long distance propagation through optical fiber. In light-based biomedical imaging applications, there are limits to light output to avoid damage to living tissue. Similarly, in lidar (laser remote sensing) applications, high optical powers can pose a threat to the eyes of a passerby. In all these cases, the recovery of weak signals dominated by noise remains a major challenge.
Lasers have enabled considerable advances in many areas of research. Yet the problem of sensitivity and noise ultimately leads to an obstacle for further development.
The work, carried out in collaboration with the Montreal telecommunications company Fonex Data Systems Inc., has immediate potential practical interest for the industry.
An innovative approach
The most common approaches are not suitable for recovering a weak signal because they further attenuate the signal or inject more noise. The new method developed by the researchers uses passive amplification of the signal of interest over noise, allowing access to signals that could not be recovered otherwise. No other technique simultaneously increases the energy of the signal of interest while reducing its relative noise content.
To achieve this, the researchers exploit the Talbot self-imaging effect, which is observed when a periodic train of pulses propagates through a dispersive medium such as an optical fiber. It causes different frequency components of light (i.e. different “colors” of light) to travel at different speeds. The Talbot effect was previously used to “stack” or coherently combine consecutive pulses in the incoming train, leading to passive amplification of the peak power of the pulses.
Applications with arbitrary waveforms
In the works reported in OpticalINRS researchers have been able to extend this technique to achieve passive amplification of any given arbitrary signal, with no requirement for periodicity.
“We realized that we could come up with a particular arrangement so that the Talbot effect could be used to amplify aperiodic signals, i.e. virtually any of the signals found in these different practical fields.” says Professor Azaña.
Just as a physical lens can be used to focus a weak and large image into a narrower and more intense beam, the proposed system can be used to redistribute the energy of a weak signal into a series of high intensity pulses. This power focusing effect can be achieved without distorting the shape of the signal (i.e. the information carried) and without increasing the noise along the signal.
The researchers believe that this discovery opens many avenues for future lines of research. It might be possible to adapt this technique for 2D or 3D space images as well, a feat that could have important applications in astronomy research, photography or holography, among others. Additionally, while this demonstration focused on optical waveforms, such as those used in telecommunications, this technique could be adapted to different types of waves, including radio waves, microwaves, plasma, acoustic or still quantum.
Small chip provides a big boost in precision optics
Benjamin Crockett et al, Optical signal denoising by temporal passive amplification, Optical (2021). DOI: 10.1364/OPTICA.428727
Provided by the National Institute for Scientific Research – INRS
Quote: A Novel Amplifying Technique for Weak and Noisy Optical Signals (February 2, 2022) Retrieved February 2, 2022 from https://phys.org/news/2022-02-amplifying-technique-weak-noisy-optical.html
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