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Acousticto-Optic Sensors for Catheter Tracking

Acousticto-optic devices use acoustic waves in a medium to control light. They have numerous applications in optical filters, modulators and Q-switches.

This paper introduces an innovative acousto-optic sensor designed for tracking catheter position during interventional magnetic resonance imaging (MRI). The sensor consists of a miniature coil coupled to a piezoelectric transducer mechanically connected to an optical fiber with FBGs.

How do acousto-optic sensors work?

Acousto-optic sensors are widely used to detect and process laser beams used for various applications, systems, and processes. Additionally, these sensors may also be implemented into other technologies, such as laser communication or signal processing.

Understanding how an acousto-optic device operates requires understanding its physics. At its core, an acoustic wave interacts with light waves in a crystal to produce an acousto-optic coupling between two beams that can be accurately controlled via modulation signals.

Acoustic waves propagated across a crystal induce changes in density that alter its refractive index – this effect is similar to the Bragg Scattering effect observed when x-rays scatter from various materials.

Enhancing acoustic-optic coupling requires amplifying and shifting an acoustic signal sent to an AOM (Acousto-Optic Modulator). An AOM optimizes its response to this acoustic signal by controlling rise time; that is, how quickly acoustic waves traverse an input beam; a high quality AOM can achieve rise times as short as 150 nanoseconds.

One of the most widely used acousto-optic devices is a single-mode, broadband acousto-optic modulator. This device can be created from various materials depending on wavelength and laser parameters for optimal operation.

At visible or near-infrared wavelengths, acousto-optic modulators may be constructed from gallium phosphide, indium phosphide, tellurium dioxide and fused quartz materials. Such modulators are useful in laser applications as well as improving acousto-optic systems’ ability to process high frequency signals efficiently in real time.

Acousto-optic sensors are increasingly being employed for long-range sensing of acoustic events and space-based environmental sensing applications. Acousto-optic sensors offer several advantages over their traditional counterparts in this regard, including nanoscale architectures, flexible designs without reflection-based positioning requirements and multicolor light modulation capabilities.

Acousto-optic sensors find use across numerous industries, including aerospace and defense, space science and life sciences. Acousto-optic products have been employed to track objects in space, detect gravitational waves and perform Raman spectroscopy; in space communications they transfer large volumes of data between satellites while they have also been employed to detect atmospheric and seismological events.


Acousto-optic sensors are used for various applications, including nondestructive testing, structural health monitoring and biomedical diagnostics. Furthermore, they can create and measure ultrasonic waves used for medical applications like ultrasound imaging using acoustic waves.

Acousto-optical sensors can be integrated into various devices, including optical fibers or microfluidic systems, providing more sensitivity and mechanical stability with their use of advanced piezoelectric transducers that convert RF energy into acoustic waves.

Typical acousto-optic modulators use elastic optical media such as glass or crystal, coupled with an attached piezoelectric transducer that vibrates in response to an electric signal, creating changes in density which lead to periodic variations in refractive index and shifts of reflection wavelength.

Distributed Bragg Grating (FBG). The reflectivity spectrum of an FBG changes due to periodic refractive index variations in its fiber core, and mechanical strain-induced changes. This technique can provide double path acousto-optic spectroscopy with increased spectrum coverage by covering more angles with each pulse of information.

An acousto-optic sensor equipped with a coil antenna has been created for catheter tracking during interventional MRI, consisting of an RF receiver coil for receiving transmitted RF signals, piezoelectric transducer and FBG sensor with an overall resonance frequency tuned at Larmor frequency to maximize electro-mechanical coupling efficiency and increase sensitivity.

Prototypes of this sensor exhibit lower acousto-optic modulation signal to noise ratios than active markers, with SNR only 10dB lower for 100kHz bandwidths. This result is extremely promising and may even be further improved through thin film piezoelectric transducers.

Acousto-optic sensors have become an integral component of both medical and industrial applications as they offer cost-effective alternatives to expensive diagnostic and surgical procedures. Their usage is expected to rise exponentially in APAC due to rising investments into optics technologies and growing laser applications.


Acoustic-optic sensors rely on optical-acoustic interactions between light and sound waves, with FBGs (fiber Bragg gratings) embedded into optical fibers to modulate acoustic waves and modulate acoustic waves modulated through them. This approach makes acoustic-optic sensors highly portable, easy to fabricate, and suitable for multiple applications.

Sensors consist of four main components, namely the loop coil antenna for receiving RF signals; piezoelectric transducer to convert electrical signal into acoustic waves; FBG sensor embedded into an optical fiber for acousto-optic modulation; and backend optoelectronics (light source and photodetector) which convert acousto-optic signals back into electrical ones. Sensors have many uses including remote diagnostics and biomedical engineering applications as well as wireless communications and signal processing applications.

The acousto-optic signal is then transferred via optical fiber to a tunable filter device that filters light in order to produce a specific spectral image, which can then be processed using a computer and used for measurements.

Typically, optical-acoustic interfaces involve collimating input light with a collimator before filtering it through an AOTF before focusing it onto a spectrometer or camera. Non-collinear designs may also be possible and result in single beam with no cross-polarization for greater efficiency.

These non-collinear designs require that the input waveform match that of the output wavelength, which makes dispersion likely and can result in mismatch between beams and desired images.

Preventing this from happening requires polarizing the input light before acoustic-optic transmission, with its polarization determined by Stokes and Anti-Stokes scattering parameters; two beams will then form that move slightly during scanning with respect to their respective Rf frequencies.

Similar to Rayleigh backscattering, an acoustic wave may perturb fiber strain at an microscopic level and cause minor variations in its reflection signal; such variations can be detected using distributed acoustic sensing techniques that detect pico-strain signatures of vibroacoustic disturbances.


In this study, we present an acousto-optic modulation sensor for catheter tracking that includes loop coil to receive radiofrequency (RF) signal, piezoelectric transducer to convert electrical signals to acoustic waves and fiber Bragg grating sensor embedded within an optical fiber for acousto-optic modulation modulation.

Acoustic waves in the FBG region modulate the intensity of reflected light reflected off surfaces, which is then converted to an electrical signal by a photodetector and fed directly into the transceiver coil plug of an MRI scanner transceiver coil plug plug of an MRI scanner, thus eliminating RF heating risks posed by conductor transmission lines which pose major concerns in interventional MRI studies.

Utilizing a prototype of this sensor, we compared its output with that of a conventional active marker consisting of an identical coil connected to conducting transmission lines. Utilizing normalized frequency spectrum analysis, we determined that SNR of our prototype sensor was 57dB when compared with that of its conventional marker counterpart.

Gradient Echo (GRE) sequence was utilized to test the visibility of an acousto-optic marker in an MR setting, by taking images with different flip angles (90o and 15o) for analysis. Sensor locations and coil placement were adjusted slightly so as to enhance visibility within images.

The acousto-optic sensor demonstrated a Signal to Noise Ratio of 57dB at 90o flip angle and 150ms flip time – both parameters comparable to an active marker. Furthermore, this acousto-optic based sensor demonstrated high sensitivity when it came to detecting spin-echo RF transmit signals with peak amplitudes reaching 1.72V when compared with 50.7 V in conventional active markers.

This study marks an important advance in acousto-optic sensor development that could be utilized in medical applications like ultrasound imaging and MRI. Acoustic optical modulation techniques offer many advantages for these purposes due to their minimal complexity, energy harvesting potential and all-in-one mechanical design.

We propose a novel approach for acousto-optic sensors using racetrack photonic microcavities and surface acoustic waves (SAW). This technique reduces multiplycative scattering waves 10-100 times, which is essential for visualizing whole-body zebrafish 30 days post fertilization. Furthermore, photonic microcavities may be made out of single-crystal lithium niobate thin film for easy integration into functional devices.
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