University of Nebraska-Lincoln | College of Engineering | Electrical & Computer Engineering

Multiplexed Acoustic Emission Sensors and Sensor Systems

Many damage-related structural changes, such as crack initiation, crack growth, and fiber breakage, are associated with the generation of ultrasonic waves with a frequency range of 10s kHz - 1 MHz. These ultrasonic waves are called acoustic emission (AE) and the detection of AE can be used as a powerful tool for structural condition monitoring. In practical applications, AE signals are often superimposed onto the much larger quasi-static strains from the temperature variations and/or structural deformations, which makes AE detection a challenging task. In this project, we are developing adaptive fiber-optic AE sensors that are only sensitive to the AE signals and not affected by the large quasi-static strains. We are also developing novel multiplexing method so that a single fiber can have several AE sensors for AE detection at multiple points.

The following video shows some preliminary test results on a fiber-optic AE sensor based on a fiber-ring laser with fiber Bragg gratings (FBGs) as the sensing element. In this test, the fiber-optic sensor was bonded on an aluminum cantilever beam and the low-frequency large dynamic strain was simulated by the beam vibration induced by an electromagnetic shaker. The AE signal was simulated by a PZT source that generated ultrasonic pulses with a center frequency of 200 kHz. A PZT sensor was as a reference. Both the PZT source and the PZT sensor were bonded to the stationary part of the aluminum plate on the table so that the beam vibration did not affect their operation.

It can be seen that the fiber-optic sensor (FBG sensor) was still able to pick up the simulated AE signal even when the sensor was experiencing large low-frequency strains.

Distributive Fiber-Optic Ultrasound Generation

Active ultrasonic testing uses ultrasonic sources and ultrasonic sensors to actively excite the structure and, in the meantime, measure its ultrasonic response for material characterization. Due to its active nature, active ultrasonic testing can provide direct, on-demand accessing on the material properties and conditions. In this project, we are developing distributed fiber-optic ultrasonic transducers that are capable of generating ultrasonic emissions at multiple, selected locations in a controllable way for ultrasonic testing.

A method we have conceived and demonstrated for such application is based on a smart light tapping technology that we invented and makes use of the "ghost mode" of a tilted fiber Bragg grating (TFBG). The "ghost mode" of a TFBG is a group of back-propagating low-order cladding modes coupled from the forward propagating core mode at wavelengths slightly shorter than the regular Bragg wavelength. If a series of TFBGs, each of which has a different Bragg wavelength from others, are arranged in such a way that the ghost mode wavelength of a down-stream TFBG is longer than the Bragg wavelength of any upper-stream TFBGs, the light at the ghost mode wavelength can propagate through all upper-stream TFBGs without significant loss and reach the TFBG where it will be coupled to the ghost mode. Because the ghost mode is a group of cladding modes, most of the light energy lies in the cladding region. When the cladding before a TFBG is partially removed and replaced with absorbing material, the laser power in the ghost mode then can be used for laser-ultrasond generation. The following animation shows how it works and you can also read a paper we published in Optics Express for more details.

Ultraminiature Fiber-Optic Microphone

Conventional microphones are bulky and not suitable in the environment with restricted and/or hard-to-access space. In addition, the presence of a bulky microphone may perturb the acoustic field, causing measurement errors. An example of such applications is the noise characterization in ear canals for the assessment of the performance of hearing protection devices. In this project, we have developed an ultra-miniature, highly sensitive, and broad-band fiber-optic microphone with an overall size comparable to the fiber diameter (125 μm). The key component of the microphone is an ultrathin (~100 nm) metal film attached to a Fabry-Perot cavity at the fiber tip. The following figures show the schematic and a picture of the microphone and the microphone system.

Ultraminiature Microphone Ultraminiature Microphone

You can also listen to a music clip recorded by this microphone through a PC sound card:

Fiber-Optic Environmental Sensors

Environmental sensors are sensors for detection of environmental conditions such as refractive index, humidity, pH values, and the presence of biochemical molecules. Optical environmental sensors typically are based on optical resonators or gratings with appropriate transducers that can convert the environmental parameters to the refractive index change of the sensor devices. Environmental sensing is achieved by measuring the changes in their spectral features in response to the refractive index change of the sensor devices. Temperature cross-sensitivity is a limiting factor to the performance of most of the sensors. In this project, we develop fiber-optic biochemical sensors that are insensitive to temperature or with temperature self-compensation capability, using microstructure optical fibers and high-Q phase-shifted fiber Bragg gratings.