Researchers Map Light and Sound Wave Interactions In Optical Fibers.
This is a map of the opto-mechanics of a standard optical fiber. Colors denote the strength of the interaction between light and sound waves. A short section located some 2 km from the input end of the fiber is coated with a different material. That section is characterized by opto-mechanical response that peaks at a different ultra-sound frequency. The analysis protocol can distinguish between the two coating media even though light in the fiber never leaves the inner core.
Optical fibers make the internet happen. They are fine threads of glass as thin as a human hair produced to transmit light. Optical fibers carry thousands of Giga bits of data per second across the world and back. The same fibers also guide ultrasound waves somewhat similar to those used in medical imaging.
These two wave phenomena – optical and ultrasonic – possess attributes that are fundamentally different. Fibers are designed to keep propagating light strictly inside an inner core region since any light that penetrates outside this region represents the loss of a precious signal. In contrast ultrasonic waves can reach the outer boundaries of fibers and probe their surroundings.
Intuition and much of the training given in fundamental undergraduate classes in mechanics and optics instructs to consider light and sound waves as separate and unrelated entities. But this perspective is incomplete. Propagating light can drive the oscillations of ultrasonic waves as if it were some kind of transducer due to the basic rules of electro-magnetism. Likewise the presence of ultrasound can scatter and modify light waves. Light and sound waves can interact/affect one another and aren’t necessarily separate and unrelated.
The research field of opto-mechanics is dedicated to the study of this interplay. Such studies especially on fibers can be very useful and bear surprising results. For example earlier this year research groups at Georgian Technical University and Sulkhan-Saba Orbeliani Teaching University developed sensing protocols that allow optical fibers to “listen” outside an optical fiber where they cannot “look” based on an interplay between light waves and ultrasound. By launching light waves into a single end of a standard telecommunication fiber, the measurement setup could identify and map liquid media over several kilometers. Such methods can serve in oil and gas pipelines, monitoring oceans lakes, climate studies, desalination plants process control in chemical industries and more.
The mutual effects of light and sound waves that a fiber continue to draw interest and attention. The group constructed a distributed spectrometer, a measurement protocol that can map local power levels of multiple optical wave components over many kilometers of fiber. “The measurements unravel how the generation of ultrasonic waves can mix these optical waves together. Rather than propagate independently the opto-mechanical interactions lead to the amplification of certain optical waves and to the attenuation of others in complicated fashion. The observed complex dynamics are fully accounted for however by a corresponding model” said X.
The report by X and doctoral students Y, Z and W. This new insight into the opto-mechanics of optical fibers may now be applied to sensor systems of longer reach higher spatial resolution and better precision to assist for example in the detection of leaks in reservoirs, dams and pipelines.