In some ways, CMUT technology has advanced farther than that of PMUTs, yet both technologies likely require further research to improve their performances, sensitivities, and bandwidths Optical techniques are increasingly considered a promising alternative to sound detection in optoacoustic systems, with the potential to address limitations of piezoelectric and capacitive technologies. Interestingly, optical sound detection leads to optoacoustic systems in which sound is both generated and detected exclusively by optical components and may offer higher sensitivity and broader detection bandwidths with smaller form factors than piezoelectric transducers, PMUTs or CMUTs.
In this review, we examine progress with optical technology for sound detection employed in all-optical optoacoustic systems. Some related technologies have recently been reviewed for an expert audience 22 ; however, our goal in the present review is to bring these advances to a broader audience and present them in a comprehensive context with a broader range of reported applications. First, we classify different techniques according to their principles of operation, explain their technical specifications and discuss their major physical and operational characteristics, including their advantages and limitations.
Then, we profile the unique biomedical sensing and imaging applications made possible by advances in optoacoustic applications. Finally, we examine how these advances may lead to new applications in non-biomedical applications, such as in the emerging field of magnetoacoustics as well as in the field of non-destructive testing. The two optical methods commonly applied for detecting ultrasound waves are refractometry and interferometry. Refractometry-based detection uses the photoelastic principle, which states that acoustic waves interacting with a medium induce mechanical stress in that medium, causing the refractive index RI to change proportionally with the mechanical pressure The method uses a laser beam called an interrogating or probe beam to measure changes in the RI of a single medium or at the interface between two adjacent media in response to propagating acoustic waves Fig.
Changes in the intensity, deflection angle or phase of the probe beam are recorded at an optical detector, providing information about the ultrasound signals interrogated. Interferometric methods detect changes in optical interference patterns induced by ultrasound.
Ultrasound waves alter the interference condition by interacting directly with an optical beam, by causing vibrations of a reflector or by altering the resonance frequency of a resonator Fig. Perturbations in the interference pattern may be triggered by changes in the mean free path, the optical phase or the optical wavelength, depending on the interferometric configuration employed.
The resulting changes in intensity or frequency at the interferometer output are detected by a photodiode or a wavelength meter and reveal information about the ultrasound signals. The ultrasound wave is applied at a , b the beam path, a , c the reflector, or d the resonator. In the following, we discuss the principles of operation of refractometric and interferometric methods in all-optical optoacoustic detection and illustrate their major advantages and disadvantages for biomedical applications. The intensity of an optical beam incident on an interface between two media with different RI e.
Then, ultrasound measurement is achieved by recording the fluctuations in optical intensity using an optical detector such as a photodiode Fig. Different variations on the basic design in Fig. One design splits the probe beam into two polarized components and measures ultrasound-induced changes in intensity in each reflected component separately. This allows the calculation of the reflectance ratio of the two components, which reduces noise in the probe beam and thereby increases the signal-to-noise ratio SNR Another design creates a surface plasmon resonance at the interface so that ultrasound waves affect the probe beam reflectivity by altering the plasmon resonance condition.
In this approach, the bottom of the prism is coated with a metal—dielectric interface and a polarized probe beam is used to induce surface plasmons at the interface. When the resonance condition for the creation of plasmons at the dielectric interface is satisfied, pressure-induced changes at the interface modulate the light-plasmon coupling and thus modulate the intensity of the reflected probe beam 25 , On the other hand, intensity-sensitive methods show limited potential for optoacoustic imaging.
In optoacoustic microscopy, the optical beam path needs to be guided through a prism Fig. The poor sensitivity makes the technique inadequate for demanding optoacoustic microscopy and flow measurement applications. Nevertheless, an optoacoustic microscope has been designed in which the ultrasound signal is detected extremely close to the area of excitation, before the acoustic waves have propagated into the surrounding tissue, i.
Instead of detecting ultrasound based on changes in the intensity of a probe beam, which requires an interface between two media of different RI, ultrasound can additionally be detected based on the deflection of the probe beam crossing the acoustic field Fig. The acoustic waves alter the RI of the medium and interact with the electric field of the probe laser beam, deflecting it in proportion to the pressure gradient of the acoustic wave.
This deflection is detected using a position-sensitive detector such as a quadrant photodiode. The frequency bandwidth that can be detected is determined not only by the photodiode rise time, as in intensity-based methods, but also by the diameter of the probe beam, with smaller diameters able to detect wider bandwidths. Deflection-based methods involve an extremely small active sensing area, which allows the use of objectives with high numerical apertures such as for optoacoustic microscopy.
The strong potential of these methods for optoacoustic microscopy is evidenced by the success of acousto-optic beam deflectometry AOD. However, in AOD setups, the laser beam must be narrowly focused and guided through the medium close to the acoustic source. This requirement may limit implementation in tightly spaced optoacoustic microscopy and tomography setups that utilize optical guiding systems for sample illumination, further reducing the space available for an interrogation system. In addition, current tomographic image reconstruction algorithms do not take into account the interaction of the probe beam with multiple acoustic sources, which means AOD cannot yet be implemented in optoacoustic tomography.
Instead, AOD may more quickly find application in optical-resolution optoacoustic microscopy, where image resolution is determined by the optical resolution of the system 30 , Another all-optical approach for detecting ultrasound involves measuring ultrasound-induced shifts in the phase of a collimated probe beam In this approach, a highly collimated light beam is passed through an acoustic field. Due to the change in RI produced by the acoustic field in its propagation medium, some photons in the beam are scattered or deflected from the original path Eq.
This perturbed probe beam is then tightly focused through a spatial filter in the Fourier plane Fig. In this way, the camera reproduces an intensity map of the acoustic field 33 that can be converted into two- or three-dimensional images of the source of the ultrasound signals by standard tomographic reconstruction techniques. Optical phase-sensitive ultrasound detection methods are already widely used in characterizing ultrasound transducers and studying the effects of acoustic shock waves on aircraft, where the acoustic field intensity is hundreds of kPa.
Phase-sensitive systems can also detect ultrasound with an NEP as low as 5. With this sensitivity and bandwidth, phase-sensitive ultrasound detection can be implemented in optoacoustic tomography using various experimental approaches, such as Schlieren photography, phase contrast imaging, and shadowgraphy These optical approaches, which differ primarily in the filtering method applied in the Fourier plane, have been combined with light sheet excitation for optical sectioning Here, a two-dimensional ultrasound projection through depth known as a B-scan is imaged during each laser shot, and a three-dimensional image is obtained by moving the sample along the axis perpendicularly to the light sheet illumination.
Real-time three-dimensional optoacoustic tomography without the need for computational reconstruction has been achieved using an acoustic lens and all-optical phase-sensitive ultrasound detection In this method, the volumetric optoacoustic field is collected at one side of the acoustic lens and re-focused at the other side into a water tank, where the pressure field is imaged by the phase-sensitive system Fig. In this way, the acoustic field is decoupled from the sample, allowing easy optoacoustic detection without the interferences that would arise if the sample were positioned directly in the Schlieren beam 38 — These interferences arise due to optical absorption as well as scattering of the Schlieren beam in the sample and are the key challenges of phase-sensitive methods.
While acoustic lenses can address this issue, acoustic losses due to attenuation and inadequate lens materials need to be reduced. Contrary to single-element transducers, these phase-sensitive optical methods for ultrasound detection can capture the entire acoustic field in a single snapshot in which the acoustic signal is spatially encoded. Here, the acoustic bandwidth is determined by the optical resolution of the system rather than by the transducer bandwidth as is currently the case in optoacoustic tomography 7.
Methods of optical interferometry for sound detection require a sensing system and a read-out interrogation system. In the sensing system, physical characteristics of ultrasound waves e. In the read-out system, these altered characteristics are detected and converted into voltage signals.
Therefore, the NEP of an interferometric ultrasound detector is determined by the efficiency with which acoustic perturbations are converted into changes in light characteristics in the sensing system, as well as by the sensitivity of the read-out system for detecting those changes. Below, the two systems are discussed separately for simplicity, although certain sensing systems are usually used with certain read-out systems. The earliest interferometers, Michelson interferometers 42 , 43 MI and Mach—Zehnder interferometers 44 , 45 MZI , were first used to measure vibrational displacement in the s.
In the s, optical resonators 46 , 47 and optical fibers 48 — 50 emerged as more sensitive systems for detecting acoustic waves, opening the door to miniaturized portable devices. In the s and s, interferometers highly efficient at collecting scattered light 51 — 53 allowed non-contact detection of ultrasound from rough surfaces.
Since then, several non-contact approaches, among them Doppler interferometry DI , have given rise to an entire field of laser ultrasonics Non-contact approaches avoid the need for the sensing element to come into physical contact with the sample, making an acoustic coupling medium unnecessary. In MIs and MZIs, a laser beam is split into two optical paths, one of which is perturbed by the ultrasound wave and the other serves as a reference Fig.
The two beams are combined at the interferometer output and their interference is measured; ultrasound-induced changes in the optical path cause proportional changes in the intensity of the recombined beam. In MIs, ultrasound may interact with the beam itself or with a reflector that the beam strikes Fig.
In MIs and MZIs, an acoustic coupling medium must be used when ultrasound interacts with the beam path, but not when it interacts with a reflector, which can be the sample itself 55 — Two-beam interferometers are usually implemented in fiber-based 45 , 58 , 59 or free-beam 44 , 60 configurations. These results were obtained under laboratory conditions of minimal vibration and electromagnetic noise, so further work is needed to establish the robustness of this approach.
In contrast to two-beam interferometers, Doppler interferometers sense ultrasound by detecting Doppler shifts in light frequency Fig. In DI, the ultrasound waves interact with a reflector similar to some MI setups, but the reflected probe beam does not interfere with a reference beam; instead, Doppler shifts in the probe beam induced by pressure-induced oscillations of the reflector are recorded by a wavelength demodulator. DI offer advantages over MIs, particularly when the reflector is a rough surface.
The rough surface creates a distorted wavefront that interferes with itself, providing greater interferometric contrast than when a distorted wavefront combines with the nearly planar wavefront of the reference beam as in MIs. DI shows great potential for non-contact optoacoustic imaging, but the lack of suitable read-out systems poses a problem see next section. A promising alternative to two-beam interferometry and DI is ultrasound sensing based on optical resonators. These resonators confine the probe beam to a small volume, prolonging the interaction between the beam and the acoustic wave, thereby increasing the sensitivity of the beam to acoustic perturbations.
An optical resonator can detect ultrasound waves travelling either perpendicularly or parallel to the probe beam, depending on the geometry Fig. The use of micron-scale resonators enables miniaturization of the entire sensor system, because the acoustic perturbation affects only the light inside the resonator. The resonators in this approach trap the probe beam between two opposing flat mirrors or reflecting surfaces.
This simple resonator geometry serves as the basis for diverse sensor designs. A particularly successful design is slab geometry based on thin transparent foils 62 , 76 in which both sides of the foil are coated with a reflecting material. A focused probe beam interrogates this resonator at specific positions; scanning the probe beam over the foil effectively mimics a dense array of ultrasound transducers.
The density of this array can be even higher than that of piezoelectric transducers and does not require mechanical scanning, which is particularly advantageous in optoacoustic tomography. The simplistic resonator design allows the manufacture of enhanced resonators on the tip of optical fibers, e. Nevertheless, these resonators cannot achieve total light confinement, and the inevitable losses of light may make them unsuitable for applications demanding higher detector sensitivity, such as optoacoustic microscopy.
MRR Fig. The MRRs in that study were fabricated on a glass microscope slide, meaning that the detector was fully transparent and therefore ideal for microscopy in reflection mode. On the other hand, MRRs typically provide a broad detection bandwidth only at narrow opening angles 16 , which is not ideal for tomography.
While they can be used for optoacoustic tomography, they usually provide lower resolution and allow less straightforward image reconstruction than point-like detectors. One example of such waveguides is silicon-on-insulator waveguides 81 , 82 , for which sensors on the order of 0. The choice of read-out system for coupling to a sensing mechanism in an interferometric ultrasound detector can strongly condition the performance and potential applications of the detector.
For example, the cavity design, acoustic matching, and optical and electrical characteristics of the read-out system govern the sensitivity and bandwidth of resonator-based setups. DI has not been widely implemented for lack of fast, wavelength-sensitive read-out mechanisms. Most read-out systems rely on continuous-wave CW lasers. In the case of two-beam interferometers or optical resonators, the laser is usually tuned to a wavelength in which the optical spectrum of the interferometer or resonator is approximately linear, and the output power is monitored.
The acoustic wave generates stress and strain in the optical sensing element, altering the optical paths within the element and therefore the spectrum, which in turn leads to variations in the monitored power output Fig. The fidelity with which this system detects ultrasound depends only on the sensing element, while the noise levels and robustness of the measurements depend on the read-out system. When the sensing element is not acoustically matched to water, acoustic reverberation within the element may lead to signal distortion 84 , External disturbances and high-amplitude acoustic signals reduce sensitivity and limit the dynamic range at time t 3.
Detecting the spectral shift with an optical demodulator provides high detection sensitivity and dynamic range and allows interrogation of multiple resonators multiplexing. CW lasers are also used when sensing ultrasound with DI. Here, the demodulator Fig. The demodulator is relatively insensitive to low-frequency vibrations because the Doppler shift is proportional to the speed of the vibrating reflectors. The linear region of the interferometer resonance is tuned to the laser wavelength such that ultrasound-induced frequency shifts in the laser beam are translated to intensity variations that can be recorded.
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Despite its widespread use, CW interferometry has severe disadvantages as a read-out system in optoacoustic imaging. One is difficult scalability: it cannot simultaneously interrogate numerous sensors without scale-up of bulky or expensive components, most notably the interrogating laser itself. Generally, if each sensor operates at a different wavelength, the same number of lasers as sensors is needed to achieve simultaneous read-out. This need for hardware scale-up can be mitigated by tuning all read-out interferometers to the same wavelength as the laser 88 or by using frequency-modulation schemes that rely on the sinusoidal spectrum of the interferometers 89 — Another disadvantage of CW interferometry is its sensitivity to temperature drifts and vibrations, such as motion of large samples during in vivo imaging.
This sensitivity to disturbances can be significantly reduced using feedback-based stabilization under carefully controlled conditions 93 , 94 , but whether this approach works robustly under real-world operating conditions has not been demonstrated. Potentially more robust performance has been obtained using frequency-modulation techniques in two-beam interferometers, such as heterodyne detection 53 , 57 , Pulse interferometry, based on pulsed rather than CW lasers, may offer a more scalable and stable alternative to CW interferometry.
In pulse interferometry, optical pulses whose bandwidth is considerably wider than that of the resonator are sent to the resonator Fig. The resonator then acts as a spectral filter, such that light exiting it has its spectral shape. An optical demodulator at the output of the resonator detects wavelength shifts. Passive optical demodulators operate stably as long as the spectrum of the resonator is contained within that of the pulses When the bandwidth of the laser is sufficiently broad, numerous sensors can be interrogated with a single laser by using wavelength-division multiplexing similar to that used in fiber-based sensors of temperature and strain The advantages of optical over piezoelectric or capacitive ultrasound detection have already been demonstrated in optoacoustic imaging applications; however, they also create the possibility of new optoacoustic applications.
In particular, the miniaturization achieved by optical detection of ultrasound could facilitate specialized applications in different fields as summarized in the following. An attractive feature of all-optical sound detection in biomedical optoacoustic applications relates to the possibilities enabled by non-contact operation. DI and two-beam interferometry have been used for non-contact optoacoustic imaging 55 , 88 , 97 — 99 based on optical coherence technology 98 , or holographic techniques , , opening up possibilities for dual-modality imaging.
In these configurations, a probe beam is directed onto the sample surface, and vibrations of the tissue edge modulate the reflected light; in other words, the tissue—air interface serves as the reflector Fig. In this way, for example, a non-contact heterodyne MZI has been used to image blood vasculature in a chicken chorioallantoic membrane in vivo Non-contact optoacoustic imaging is nevertheless limited by the effects of the large acoustic impedance mismatch at the air-tissue interface and by inefficient light-collection from rough tissue—air interfaces, leading to low detection sensitivity This low sensitivity can be compensated to some degree by optically amplifying the initial interrogation beam or the reflected light 83 , 88 , 99 , Piezoelectric transducers typically interfere with light delivery to the sample imaged in optoacoustic tomography setups and necessitate designs that may compromise light delivery.
Optical ultrasound detection can be seamlessly integrated with illumination optics because detectors can be made transparent and insensitive to the illuminating light. Improvements in the sensitivity of refractometric sensing in general, and phase-sensitive detectors in particular, may support three-dimensional biomedical imaging orders of magnitude faster than existing optoacoustic tomography methods 34 , as recently demonstrated in a study of vasculature in mouse leg Fig.
They are transparent, which means they can be placed in contact with the sample, limiting acoustic attenuation and providing a nearly complete tomographic view of the first few millimeters of tissue. Optoacoustic systems based on planar optical resonators have been used for imaging mouse and chicken embryos Fig.
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Reproduced with permission from ref. Image size, 1. The fact that optical resonators are optically transparent and can be seamlessly integrated with other fiber-based optical imaging modalities makes them well-suited for generating hybrid imaging systems. In a different approach, an MZI has been used to build an all-optical hybrid system combining optoacoustic tomography and laser-ultrasound tomography for imaging zebrafish Fig. A major challenge for optical ultrasound sensors in optoacoustic tomography remains the parallelized read-out of multiple sensors without duplicating the read-out system.
Solutions to this problem may be possible using fiber-based MZIs 59 , Moreover, the broad bandwidth achieved by optical sound detection makes optical sound detectors a ubiquitous technology for different implementations of acoustic-resolution optoacoustic imaging, spanning from optoacoustic microscopy to optoacoustic macroscopy applications 1.
Therefore, the same detector could be employed for imaging at different scales, possibly extending the operating characteristics of an optoacoustic system from acoustic resolution microscopy to mesoscopy and macroscopy. Optical resolution optoacoustic microscopy utilizes focused light beams for tissue illumination. The imaging resolution achieved in this case obeys the laws of optical diffraction, not ultrasonic diffraction.
Similar to limitations seen in acoustic-resolution optoacoustic imaging, the use of microscope objectives directs the placement of ultrasound transducers away from and possibly at an angle to the volume illuminated. This geometrical arrangement establishes long ultrasound propagation paths, typically through a coupling medium such as water, reducing the effective sensitivity and bandwidth of the setup. In practice, this limits many optoacoustic microscopy setups to transmission-mode geometries, which cannot be used with thicker samples such as animals 3.
MRRs have been used to integrate confocal and fluorescence microscopy Fig. The broad detection bandwidth of MRRs can provide higher axial sampling resolution than piezo-based alternatives in optoacoustic microscopy , Interferometric ultrasound detection systems can be miniaturized to a much greater extent than their piezoelectric counterparts. The fact that optical resonators can be miniaturized and fabricated in optical fibers or coupled to optical fibers makes them highly attractive for minimally invasive optoacoustic imaging in medical endoscopy. The same approach may prove effective for optical detection of sound in optoacoustics, given the parallels between this field and laser ultrasonics.
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All-optical sound detection has been explored in magnetoacoustics and non-destructive testing, where systems are typically exposed to high levels of electromagnetic interference. This noise poses a problem when ultrasound is detected using piezoelectric transducers because the piezoelectric element itself or the signal cables act as antennas. Optical components to detect ultrasound, in contrast, are immune to such electromagnetic interference.
Magnetoacoustic imaging devices visualize features of a sample exposed to transient magnetic fields, which trigger the production of ultrasound waves that travel from the sample to a detector The ultrasound is generated via one of two mechanisms. In the first, a changing magnetic field induces eddy currents that generate internal pressure due to Lorentz forces; the sample can then be imaged based on electrical conductivity , In the second mechanism, magnetic energy deposition in the imaged object leads to thermal expansion such as in ultrasound generation in optoacoustics; in this case, the sample is imaged based on magnetic susceptibility , Fig.
The frequency-doubled signal S1 red differs from the recorded noise blue , confirming that the signal is not due to electromagnetic interference. Reproduced from ref. The small magnetic susceptibility of biological samples means that magnetoacoustics cannot generate sufficient contrast for imaging purposes However, adding magnetic suspensions such as Fe 3 O 4 nanoparticles to the sample generates contrast based on magnetic susceptibility Fig. Magnetoacoustic measurements can be rendered insensitive to electromagnetic interference using optical detection of ultrasound; this is particularly important when the magnetic excitation is a continuous wave, because time gating cannot be used in this case to reject electromagnetic noise picked up by piezoelectric transducers.
Even in the presence of strong contrast due to magnetic nanoparticles, CW magnetoacoustic sensing appears to be practical only with optical detection of ultrasound. Optical methods for ultrasound detection have also been investigated in non-destructive testing, such as for sensing applications in solid materials using two-beam mixing interferometry 55 or for sensing applications in the related field of laser ultrasonics. First introduced in the s, laser ultrasonics has become a well-established method for the analysis of material properties and detection of structural flaws, mainly in industrial applications 87 , Bamber, C.
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Ultrasound in Medicine and Biology , 5 , , Brady, John Kissling. Thesis, University of Illinois at Urbana-Champaign, Chan, R. Fish and W. O'Brien, Jr. Proceedings of the Frontiers of Engineering in Health Care , 68, Dines, A. Weyman, T. Franklin, J.
Cuddeback, N. Sanghvi, K. Avery, A. Baird and F. Circulation , 60 , , Duback, L. Frizzel and W. Proceedings of the Ultrasonic Symposium , , Biological Effects of Ultrasound. In Ultrasound Short Course Transacations , ed. Repacholi and D. Edwards, Charles Aaron.
Fish, W. Carle Selected Papers , 32 , , Goss, L. Frizzel and F. Ultrasonic Absorption and Attenuation in Mammalian Tissues. Goss, R. Johnston, V. Maynard, L. Nider, L. Frizzel, W. Maynard, J. Brady, L. Goss and W. Journal of the Acoustical Society of America , 65 , , Johnston, Ronald Lee. Kelly Fry, P. Kelly-Fry, N. Sanghvi, F. Fry and H. In Ultrasonic Tissue Characterization , ed. Printing Office, Washington DC, Lerner, Benjamin. Brady and F. Dunn and J. Fry, , Elsevier, Amsterdam, Dunn and W. Ultrasonic Absorption and Dispersion. Erdmann, N. Sanghvi, M. Gardner and F. In Ultrasound in Medicine , ed.
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Fry, K. Egenes and E. Ultrasonic Characterization of Infarcted Myocardium. Franklin, K. Egenes, N. Sanghvi, R. Reid, T. Oei and F. White and E. Egenes, J.
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Fallon, N. Sanghvi and F. Fry, and J. Barger Acoustical properties of the human skull. Journal of the Acoustical Society of America , 63 , , Goss, Stephen Anthony. Johnston, and F. Acoustic Letters , 1 , , Journal of the Acoustical Society of America , 64 , , Harper, E. Kelly Fry. Januzik, Scott Joseph. Kelly-Fry, P. Harper, G. Fry, G. Gardner and H. Kelly-Fry, H. Medicine and Biology , ed. Almost all these contents are hosted and accessed from respective sources. The responsibility for authenticity, relevance, completeness, accuracy, reliability and suitability of these contents rests with respective organization from where the contents are sourced and NDL India has no responsibility or liability for these.
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