Achieving sub-MHz optical features resonator-free in solid-state

(Nanowerk Spotlight) Ultra-narrow optical features with a spectral bandwidth below 1 megahertz are highly sought-after for applications in ultrahigh-precision sensing, narrow-band filtering, and information storage for optical computing – but unfortunately they are extremely challenging to generate. Conventionally, high-Q resonators that have a minimum loss in optical power are used to generate such features in solid-state, but they require complex and costly fabrication processes, which limits their large-scale commercial production.
But now there is a novel way to achieve ultra-narrow optical features. And unlike conventional approaches, it is cost-effective, has a simple arrangement, and is loss-free, while offering on-demand tunability and ultrahigh sensitivity.
Writing in Nature Photonics ("Sub-megahertz spectral dip in a resonator-free twisted gain medium"), researchers at the University of Toronto have demonstrated a resonator-free approach to generate such ultra-narrow features using commercially available solid-state products.
"We use polarization-dependent gain in a twisted optical medium to realize sub-megahertz (0.72 MHz) spectral features," Neel Choksi, a PhD student in the Qian Group at UToronto's Department of Electrical & Computer Engineering, and the paper's first author, explains to Nanowerk. "As an example, we use Brillouin gain – a polarization-dependent gain in a commercial spun fiber, and realize the narrowest Brillouin feature ever reported! This effect has never been studied and reported in the literature, and this paper in Nature Photonics is its first demonstration."
Experimental and theoretical results showing a sub-megahertz spectral dip in the Brillouin gain spectrum of a spun fiber
Experimental and theoretical results showing a sub-megahertz spectral dip in the Brillouin gain spectrum of a spun fiber. (Reprinted with permission from Springer Nature Limited)
Choksi points out that this approach is not limited to Brillouin gain and spun fibers. It is also feasible to use other well-known polarization-dependent gain mechanisms such as Raman gain or parametric gain. And instead of spun fibers, alternatives such as helical waveguides or chiral waveguides could be used.
"The simplicity of our approach, its ultra-narrow bandwidth and on-demand tunability can have a broad range of potential applications from information storage to microwave photonics, and ultrahigh precision sensing," he notes.
In Brillouin scattering (named after French physicist León Nicolas Brillouin, who initially described this phenomenon in 1922), light, which consists of photons, interacts with elastic vibrations, which consist of phonons, at very high frequencies (tens of GHz) in a transparent medium. It is an unwanted effect in optical communication, but it has proven useful for sensing and spectroscopy applications.
"It is very challenging to narrow down the bandwidth of Brillouin gain, and one needs to engineer the spectrum in order to reduce the spectral gain bandwidth," says Choksi. "However, even with spectrum engineering, it has not been possible to reduce the spectral gain bandwidth below 1 megahertz. So one may ask – how do we achieve such sub-megahertz spectral features using Brillouin gain?"
"Spun fibers have also been known for quite a while, and they are used for developing commercial current sensors," he continues. "But such sharp spectral features have never been reported using spun fibers. So one may ask again – how do we achieve such features using spun fibers? The answer: because we use a combination of Brillouin gain and spun fibers."
Spun fibers have a special property: They have two polarization eigenmodes, which are sensitive to changes in frequency. When the researchers launch light with these polarizations, it does not experience any Brillouin gain, and it travels unaffected throughout the length of the fiber. But when they deviate the frequency of light slightly (<0.5 MHz), the light polarization deviates from the eigenmodes, and the light experiences a high Brillouin gain.
Thus, at this particular frequency of eigenmode, a sharp dip (sub-megahertz bandwidth) in the Brillouin gain occurs, whereas for all other frequencies, light experiences a high Brillouin gain.
"I worked on this project during my undergraduate thesis in Prof. Li Qian’s group," Choksi recounts how he got started on this research. "I learned about the research of my previous lab members. They had surprisingly observed a sharp spectral dip in the Brillouin gain spectrum of a spun fiber. Curiously, this dip has not been observed in any other fibers, and has not been reported anywhere in the literature. However, they weren't able to figure out the reason behind its occurrence. I realized that uncovering the reason behind its occurrence can have enormous implications in the fields of optical sensing and slow light. I was deeply fascinated by this problem, and so I decided to dedicate myself to theoretically investigating its occurrence during my Ph.D."
As the next step in his research, he is now working on the applications of this approach, more specifically, a proof-of-concept demonstration of fast light and current sensing applications.
The results from this research could be useful for developing ultrahigh precision optical sensors, narrow-band filtering, slow/fast light optical delays, and information storage applications. For instance, slow and fast light generated using this approach could be used in optical data centers to realize network delays or buffers. Furthermore, since this novel approach is fiber-based, there is no need to upgrade or change the existing architecture to achieve delays.
While it is relatively straightforward to implement in a lab environment, it will be more challenging to implement these techniques in commercial products. The development of stabilization techniques and precise calibration techniques is necessary in order to maintain the required ultrahigh sensitivity in a commercial environment.
"It is an exciting time to be working in photonics, especially in fiber optics and fiber optical sensing," Choksi concludes. "The growth in both of these fields has been unparalleled, owing to a sharp decrease in component prices and significant quality improvements. Moreover, it is expected that these fields will see phenomenal progress in the years to come, primarily driven by advancement in the Internet-of-Things (IoT) field and the increasing need for automation as well as predictive and preventive maintenance in industry. It is my hope that my work can potentially contribute to this progress, more specifically in the fields of sensing, and information storage."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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