Using Light to Create Sound

“A basic limitation of current chemical sensing is the need for an interrelationship between the light and the tested substance”, explains Prof. Avi Zadok of the Faculty of Engineering and the Nanotechnology Institute at Bar Ilan University, who headed the research team. “This limitation has impeded the progress of researchers in the field of optical fiber sensors, for many years. In our study, we have recruited the reciprocal relationship between light and sound, and used sound as our messenger to the outside world”.  Zadok’s research findings have been published in the journal of the Optical Society, Optica.

Optical fibers serve as an efficient and convenient means for chemical sensing due to their tiny size, their capacity for remote use (they can measure chemicals situated many kilometers away), and the ability to install them almost anywhere, including in unsafe environments such as oil wells, in which use of electricity is prohibited. However, existing optical fiber sensor technologies require direct contact between the light passing through the fiber and the tested substance; yet this requirement contradicts the fundamental purpose of optical fibers, which is to prevent any emission of light. Previous efforts to overcome this limitation and achieve emission of light entailed significant modifications of the fiber, such as perforating it or polishing it down to obtain an especially narrow diameter. “Although high levels of sensitivity can be obtained using these methods, it is difficult to produce such sensors, and the modifications can even affect the sensor’s stability”, says Zadok.

Therefore, instead of using light directly, Zadok and his research team devised a method that utilizes the light transmitted though the optical fiber to create sound waves, or acoustic oscillations, while taking advantage of the ‘stimulated Brillouin scattering’ phenomenon. Although this mechanism is currently used by commercial optical fiber sensors, such sensors retain both the light and the sound waves within the fiber. “The crucial stage was finding mechanical behaviors that can exit the fiber, and to take advantage of these behaviors”, explains the Head researcher. “We found that a forward-directed scattering mechanism can serve to acquire data regarding the occurrences taking place outside the fiber”. This novel approach uses optical waves that are intense enough to create acoustical oscillations that reach outside the fiber. These oscillations gradually fade at a rate dependent on the characteristics of the substance surrounding the fiber, thus providing an indirect method for sensing the chemical composition of the fiber’s environment. Because the light remains inside the fiber, this method does not require any modifications of the optical fiber, other than removal of its protective plastic cover.

The researchers demonstrated their method’s abilities in ethanol and ion-free water, measuring the acoustic oscillations that indicate the liquid’s viscosity and speed of the sounds waves advancing in the fluid. The results obtained were similar to known values, demonstrating a 1% level of accuracy. In addition, this method enabled distinction between water samples with different concentrations of salinity. In the future, this method can serve for monitoring water desalination, for various electrochemical processes such as gas chambers, and for measurement of changes in ion concentrations or dissolved salts used for industrial chemical processes. This novel method may also be suitable for detection of specific chemicals. The researchers believe that modifying the fiber’s external surface can attract and thereby attach a desired substance, and thus cause a measureable change in the acoustic waves. Although further research and development is needed, such an approach, if successful, can be useful for detection of explosive materials or disease-causing elements. “Although we will certainly examine additional applications in the future, our current study concentrated on solving a seemingly paradoxical problem: things cannot be both inside and outside the fiber at the same time”, says Zadok. “We have found a way to bypass this paradox”.