To detect guided ultrasonic waves within large structures, one important tool is a fiber Bragg grating (FBG) sensor, a type of optical fiber-based sensor. When passing through areas of damage in a structure, these waves provide information on the type and extent of damage in the structure. The sensors offer multiple advantages for use in extreme environments, including high-temperature applications or corrosive environments such as salt water. However, since their sensitivity to ultrasonic waves is actually quite low, in order to obtain accurate information on damage, it’s often necessary to amplify the measured signal.

However, researchers Kara Peters and Jee Myung Kim, professors of mechanical and aerospace engineering at North Carolina State University in Raleigh, noticed over the course of multiple experiments that the optical fiber could actually capture ultrasonic waves and transmit them. 

“This is counterintuitive, as the optical fibers are designed to transmit optical signals,” Peters said. “This means that the ultrasonic wave can be measured at any point along the optical fiber. Therefore, the [FBG] sensor does not have to be in the same environment, which can make its sensitivity to the ultrasonic wave much higher.”

 

Transferring ultrasonic waves

For this research, Peters and Kim wanted to take the concept one step further and actually transfer ultrasonic waves from one fiber to another, analogous to an optical fiber coupler for light waves.

Optical fiber couplers require extremely high-precision manufacturing as the light wavelengths are in the order of nanometers, compared to millimeters for ultrasonic wavelengths. Unlike the light waves that are confined to the core region, these waves are spread out in the optical fiber. 

“So, we can transfer waves simply by bonding a small segment of the two optical fibers together,” Peters said. “Our previous work demonstrated such coupling experimentally—this study will model the phenomenon.”

The team wanted to develop a model to predict the role of geometrical parameters on the coupling behavior of the acoustic coupler. 

“We first developed a finite element [FE] model of the acoustic coupler and validated it with the experimental data from previous researchers, based on varying the input fiber diameter,” Kim said. “Then the FE model was used to generate parameter sweeps of other coupler geometrical parameters to compare the theoretical models—spring and damper frictional contact models and coupled mode theory. Having theoretical models of acoustic couplers could provide insights into the critical geometrical parameters to optimize coupling.”

 

A simulation study

Comparing the FE model to the three theoretical models— spring and damper frictional contact models and coupled mode theory—revealed that the FE model and the spring model both replicated the experimental results. As a result, the FE model can be used as a surrogate for further experiments that will enable the design of acoustic couplers for FBG sensor applications.

“Since physically fabricating couplers with specific geometrical parameters can be challenging, the use of the FE model could significantly speed up the design process,” Kim explained. “There is also a benefit to having the spring model for rapid simulations of the coupler behavior.”

This work’s goal is to develop an FE model that describes coupler behavior as a function of the fiber and coupler properties for future coupler optimization. One of the biggest research challenges was independently measuring the ultrasonic waves propagating through the optical fibers. To understand how the ultrasonic (acoustic) coupler works, the team had to measure how these waves were converted from fiber to fiber at the coupler location. 

“We have telecommunications equipment to measure light waves, but we had to develop a micro-laser Doppler vibrometry technique to measure the ultrasonic waves,” Peters said.  

It was a big surprise that a simple, manually produced ultrasonic coupler, produced by simply adhesively bonding the two optical fibers together, could produce sufficient coupling. 

“This allows users to quickly add sensors to a sensor network already installed on a structure,” Peters said. “It also allows users to quickly connect and disconnect sensors by removing the bond and bonding the sensor at new location.”

The team was also surprised by how well the theoretical models predicted the system’s coupling behavior. Although modifications are still needed to account for standing waves that develop within the coupler, the theoretical simulations based on spring-damper systems are computationally efficient and can be used for initial coupler designs. To the researchers’ knowledge, these are the first models to be used for this design process.

 

For mechanical engineers

One event that highlighted the importance of structural health monitoring for high-performance vehicles, as well as the ability of acoustic emission testing to identify potential structural failure well before it occurs, was the Oceangate Titan submersible implosion. 

“These acoustic emission signals are one example of the type of ultrasonic waves we are trying to detect and process,” Peters said. “One of the challenges of applying such sensor networks is integrating them into structures without degrading the performance of the structure."

Peters and Kim are exploring technologies to build high densities of sensor networks with minimum perturbation to the structure itself. The use of multiple signal collection techniques and distributed sensing along a single optical fiber is one solution, with the hope of developing couplers to connect these fiber networks and to understand how the coupler design affects the measured waves.

Most engineers only use optical fibers to transmit light waves. “Our innovation is in using them to transmit multiple types of waves, thereby allowing us to perform unique signal processing of the signals,” Peters said. The theoretical models will allow the team to design methods to interpret these signals for structural health monitoring applications.

 

Future plans

Over the next few years, Peter’s research group plans to expand the idea of signal processing for ultrasonic waves using optical fibers beyond simply amplifying the waveform and transferring from one fiber to another. Of the multiple possibilities the team could explore, one is ultrasonic wave signal processing before they’re measured with the FBG. Peters also plans to look more generally at the concept of multi-modal signal collection and processing in optical fibers. 

“The ultimate goal is to create sensor networks for structural health monitoring that can produce high resolution signal collection and processing to give rapid feedback on the state of a structure, even in tough environmental conditions,” she added.


Mark Crawford is a technology writer in Corrales, N.M.