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How Do Acoustic-Optic Sensors Work and Applications?

Acoustic-optic sensors can be used to track vibrations, flutter and atmospheric turbulence by employing donors and acceptors to absorb light that enters.

Acoustic-optic sensors have been demonstrated to track a catheter during interventional MRI. The prototype sensor shows linear response to RF coil flip angle without altering any other parameters of MRI imaging.

How do acousto-optic sensors work?

The acousto-optic effect (or photoelastic effect) is a periodic variation in the refractive index of crystals and glass due to vibrational movements caused by changes in pressure, temperature or stress levels. A laser light can be scattered by these fluctuations into light beams with unique characteristics; such diffraction can be used to control transmitted power, shift wavelength or change spatial direction of laser beams.

An AOM (acousto-optic modulator) is a device used to vary the transmission power of laser beams based on their acousto-optic effect, for a variety of uses including optical communication, tunable filters and optical switches. AOMs come as both bulk devices or compact fiber-pigtailed versions – fiber-pigtailed AOMs may connect directly with photodetectors via fiber and laser sources while bulk devices may integrate other components like amplifiers into optical sensors or transponders if integrated as bulk devices or bulk devices – bulk devices offer many applications beyond optical communication or communication applications such as communication between laser sources and photodetectors via fiber.

To operate an acousto-optic sensor, laser light must first be collimated before entering a modulator, which will then be stimulated with an electrical drive signal to generate acoustic waves in its material and cause its refractive index to change periodically, thus diffracting laser beams with AOMs depending on their intensity.

There are various kinds of acousto-optic sensors, each offering different properties. Some AOMs are optimized to maximize diffraction into a first-order position while others can accommodate for multiple orders of diffraction. Furthermore, AOMs can be used to modify phase shifts that alter laser light wavelength.

Recently, acousto-optic sensors have been employed to track catheters during interventional MRI. These sensors consist of a loop coil antenna to receive the RF signal, a piezoelectric transducer to convert this acoustic energy to acoustic waves, and an FBG sensor embedded into an optical fiber for modulating its transmission; their prototype sensor was tested in a 1.5T system alongside active markers for visibility comparisons.

How do they work?

Acoustic wave interaction with light can produce optoelectronic devices such as optical filters, modulators and Q-switches. To produce these devices, the acousto-optic effect is utilized by tuning wavelength, amplitude and propagation direction of laser beam.

Acousto-optic sensors function by coupling an acoustic transducer to an optical waveguide using either piezoelectric or electrostatic coupling techniques, where an acoustic wave interacts with it to cause a shift in mode shape and hence refractive index changes, thus changing laser beam propagation direction – detectable by photodetectors.

Acoustic-optic sensors have become essential tools in tracking catheter position during interventional magnetic resonance imaging (MRI). Current positioning systems rely on long conductor cables that carry local MRI signals from coils directly to position sensors; this increases risk due to high magnetic fields in their vicinity and can result in RF induced heating near trackers. An acousto-optic optical fiber sensor has been created in order to minimize this risk and provide real-time monitoring of the position of catheters in real time.

The sensor employs a miniature coil antenna connected to a piezoelectric transducer. Signal from an MRI scanner is transmitted to this transducer, which then generates elastic waves in an optical fiber over an area including an FBG, thus altering its geometry and changing reflected light intensity; any changes are then monitored by photodetectors and converted into electrical signals for conversion into electrical signals.

This acousto-optic sensor is smaller and lighter than existing trackers, and can be combined with low power LEDs to enable in-body communications. Furthermore, it is noninvasive, meaning it can operate within different parts of the body without complex insertion methods requiring complex tracking methods from traditional trackers. Furthermore, hybrid communication links between graphene- and quantum dot (QD)-based devices allow hybrid communication links for optical in-body networks which facilitate remote biomarker monitoring as well as patient data transmission between hospitals.


Acoustic-optic sensors combine the interaction of sound waves with light signals in a crystal material. As an acoustic signal passes through it, its waves interfere with laser light as if passing over reflective surfaces; resulting in nonlinear phase shifts that enable photodetectors to recognize it as light signal detection and decoding.

The acousto-optic effect is most frequently used in fiber optic communications systems, where acousto-optic modulators control the direction and amplitude of light signals. In this application, it enables data transmission at high speeds by manipulating refractive index. Materials which possess this property include TeO2, quartz crystal and fused silica; additionally there are options such as lithium niobate/indium phosphide glasses as well as gallium niobium as options.

An acousto-optical sensor was recently developed as a prototype device to track catheter position during interventional MRI procedures. It utilizes an FBG-based acousto-optic modulator and piezoelectric transducer which converts RF drive signals into acoustic waves; with linear response to signal amplitude over a variety of flip angles. Furthermore, this sensor allows researchers to easily determine the orientation of the catheter.

This acousto-optical sensor can serve as an alternative to active markers that cause unwanted RF heating and distort image quality. Instead, this acousto-optical sensor detects signals from multiple coils at once so as to enable tracking without distortion or heat generation – perfect for tracking catheters over time without distortion or heating!

Future applications for this acousto-optical sensor could include in-body hybrid acoustic communication channels, visible light communications, optical wireless networks and microfluidic cell and particle monitoring and tracking. Furthermore, when combined with dielectric transmission lines it provides a compact solution for sensing RF signals without using any metal components – providing significant performance gains at reduced costs compared to current technologies.


Acoustic-optic sensors’ ability to transmit radio frequency (RF) signals over long distances without significant energy loss makes them attractive options for use in challenging settings and environments, including body monitoring, remote sensing, wireless body area sensor networks (WBASNs), microfluidics, visible light communication as well as data transfer between space vehicles and ground stations.

Acoustic waves propagating through the medium generate eigenmodes within a photonic band-pass filter (BPF), which interact with an RF transmission signal to create three-wave mixing processes that vary the cavity resonant frequency and transmissivity. Control signals are converted into surface acoustic waves (SAWs), which cause excitation of material atoms which alter their phase with respect to probe signal, leading to changes in transmission spectrum as well as increasing its resonant transparency.

An acousto-optic sensor can be designed to modulate the eigenmode of its BPF in response to an acoustic input by including a piezoelectric transducer at its proximal end. Acoustic input is converted to electrical output which, in turn, is converted to digital signal by photodetectors at both ends of optical fiber; and then fed back into an MRI scanner transmission coil plug as an RF transmit signal.

A proof-of-concept prototype for this sensor has been validated in a 1.5T MRI system using Gradient Echo (GRE) sequence and gel phantom to simulate human body. Results demonstrate that an acousto-optic sensor detects RF signals transmitted by a tacking coil with SNR greater than an active marker in an equivalent phantom and with linear response against flip angle. This sensor illustrates the potential of FBG-based acousto-optic sensors for tracking catheters during interventional MRI with minimal image distortion or RF heating, with minimal image distortion or heating caused by dielectric transmission lines based acousto-optic sensing in MRI. While more sensitive modifications need to be implemented before real time operation and accurate tracking are achieved, this approach to dielectric transmission line-based acousto-optic sensing in MRI could prove beneficial in terms of patient safety and procedural efficiency.
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