Optical based ultrasound sensing system

Abstract

An acoustic chip is disclosed. The acoustic chip may include: an optical waveguide, comprising: an optical core; and a cladding structure surrounding the optical core; and an optically absorbing layer attached to the cladding structure, wherein the cladding structure comprises at least two layers, a bottom cladding layer and a top cladding layer covering the optical core.

Description

OPTICAL BASED ULTRASOUND SENSING SYSTEMCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No 63 / 692,750, filed September 10, 2024, the contents of which are all incorporated herein by reference in their entirety.FIELD OF THE INVENTION

[0002] The present invention relates to the field of acoustic sensing and more specifically to ultrasound detection.BACKGROUND

[0003] Ultrasound-based technologies are abundant in everyday life. These technologies enable non-destructive measurements of products and supply medical information through sonography. The detection of ultrasound is conventionally performed by piezoelectric transducers - electro-mechanical devices that generate a voltage in response to pressure transients. While piezoelectric transducers have been the enabling technology in medical ultrasonography, they exhibit inherent limitations that hinder the development of new applications. Piezoelectric transducers are both opaque and vulnerable to electromagnetic interference. In addition, the sensitivity of piezoelectric transducers scales with size, decreasing resolution and making them incompatible with several medical applications. For example, the performance of piezoelectric technology is often inadequate in applications such as intravascular photoacoustic imaging where both miniaturization and high sensitivity are desired, and / or in magnetoacoustics, where immunity to electromagnetic interference (EMI) is needed.

[0004] Interferometric ultrasound detection is an alternative to piezoelectric technology, where the intensity of the interference between two optical signals, one of which emanating from the detector, is modulated by an impinging ultrasound acoustic wave. Interferometric detectors are largely immune to electromagnetic interference and may be produced on transparent substrates. However, optical interferometry has yet to reach sub-Pascal sensitivities, in particular for applicationssuch as optoacoustic imaging which is commonly achieved by large-area piezoelectric detectors (which also impose a resolution limit on these techniques).SUMMARY

[0005] Some aspects of the invention may be related to an acoustic chip, comprising: an optical waveguide, comprising: an optical core; and a cladding structure surrounding the optical core; and an optically absorbing layer attached to the cladding structure, wherein the cladding structure comprises at least two layers, a bottom cladding layer and a top cladding layer covering the optical core.

[0006] In some embodiments, the top cladding layer may have a thickness of at most half the acoustic wavelength of a used central frequency of acoustic waves. In some embodiments, the acoustic chip may further include a substrate holding the optical waveguide attached to the bottom cladding layer. In some embodiments, the optical core may include a material selected from the group consisting of silicon, silicon nitride, and chalcogenide glass.

[0007] In some embodiments, the top cladding layer may be made of a polymer with a higher photo-elastic coefficient and / or lower Young's modulus than the optical core and the bottom cladding layer. In some embodiments, the top cladding layer may be made of polydimethylsiloxane (PDMS). In some embodiments, the top cladding layer may be applied with an optically absorbent material selected from the group consisting of metal layers, metal nanoparticles, and carbon particles. In some embodiments, the substrate may be optically transparent to the excitation beam.

[0008] In some embodiments, the optical core may have a refractive index of at least 1.6. In some embodiments, the top cladding layer may have a thickness smaller than twice the axial imaging resolution of the sensor. In some embodiments, the optically absorbing layer may be one of: attached on to of the top cladding layer or attached below the bottom cladding layer. In some embodiments, the optically absorbing layer may include materials selected from the group consisting of carbon particles, metal nanoparticles, and elastomeric composites.

[0009] In some embodiments, the optically absorbing layer may have a thickness optimized to ensure that the penetration depth of the optical energy is considerably smaller than the smallest acoustic wavelength in the measurement. In some embodiments, the optically absorbing layer may be applied on the substrate.

[0010] In some embodiments, the optically absorbing layer may have a thickness of at most 1 pm. In some embodiments, the planar optical waveguide may include a resonator. In some embodiments, the bottom cladding layer may have a thickness of at most 5 pm and a width of at most 20 pm. In some embodiments, the may be bottom cladding layer comprises a transparent polymer. In some embodiments, the optical waveguide may be a planar optical waveguide. In some embodiments, the acoustic chip may be planar.

[0011] Some additional aspects of the invention may be related to an ultrasound system comprising: the chip according to any one of the embodiments disclosed herein; a first laser source configured to generate and direct an optoacoustic laser beam to the chip, for generating ultrasound waves; and an interrogation optical system configured to generate and direct an interrogation laser beam guided by the core; and to convert the interrogation beam into a detected acoustic signal.

[0012] In some embodiments, the interrogation optical system may include: a second laser source configured to generate the interrogation beam; a detector configured to detect the interrogation beam, exiting from the chip; and a processor configured to convert a signal received from the detector into the acoustic signal. In some embodiments, the waveguide of the chip may include a resonator and wherein the second laser source may be configured to generate the interrogation beam tuned to the wavelength of the resonator.

[0013] In some embodiments, the laser beam may be a temporally modulated beam. In some embodiments, the first laser source may have a pulse width of less than 1 psecond. In some embodiments, the first laser source may be configured to produce a variety of illumination patterns using a digital micromirror device (DMD). In some embodiments, the first laser source may be configured to generate high-intensity laser pulses with a pulse energy of between 10 pj to 1 J and a repetition rate of 1 Hz to 1 MHz. In some embodiments, the first laser source may be configured to project coded S -matrix illumination patterns onto the optically absorbing layer.

[0014] Some additional aspects of the invention may be related to an ultrasound (US) device comprising: a source for generating a spatially modulated optoacoustic beam; transparent substrate, transparent to the optoacoustic beam; and an optically absorbing layer, deposited on at least one surface of the transparent substrate, wherein the spatially modulated optoacoustic beam is directed towards the optically absorbing layer, and wherein the spatially modulated optoacoustic beam comprises an illumination pattern.

[0015] In some embodiments, the illumination pattern may be one of: a plurality of illumination pixels on a single line, a two-dimensional pattern of illumination pixels, two- dimensional pattern of illumination lines, three-dimensional pattern of illumination pixels and three- dimensional pattern of illumination lines. In some embodiments, the illumination pattern may include a binary structure comprising illuminated pixels and non-illuminated pixels. In some embodiments, the illumination pattern may be adjustable.

[0016] Some additional aspects of the invention may be related to a detection system comprising: the US device accoridng to any one of the embodiments disclosed herein, and a detector configured to detect US wave generated by the US device.BRIEF DISCRIPTION OF THE DRAWINGS

[0017] Exemplary embodiments are illustrated in referenced figures. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below.

[0018] Figs. 1 A, IB, and 1C are schematic illustrations showing cross-sections of acoustic chips, according to some embodiments of the invention;

[0019] Fig. ID is an illustration of a waveguide to be included in an acoustic sensor bent in an Archimedean-spiral according to some embodiments of the invention;

[0020] Fig. IE show simulations of the energy distribution of the guided optical mode of the interrogation beam for the case of (left) silicon core and (right) silicon nitride core according to some embodiments of the invention;

[0021] Fig. 2A is a schematic illustration showing a cross-section of an ultrasound system comprising an ultrasound generator and the acoustic chip according to some embodiments of the invention;

[0022] Fig. 2B is a block diagram of the ultrasound system of Fig. 2A according to some embodiments of the invention;

[0023] Fig. 3 is a schematic illustration and data acquisition schemes for the ultrasound system having 2D and 3D transmitter arrays according to some embodiments of the invention;

[0024] Fig. 4 includes illustrations and graphs of acoustic signals measured for a pointlike, line-like and plane-like ultrasound excitation geometries according to some embodiments of the invention;

[0025] Fig. 5 includes schemes, graphs, and measurements of the focusing capabilities of the 2D and 3D transmitter arrays obtained in post-processing according to some embodiments of the invention;

[0026] Fig. 6 shows an illustration and results of plane wave signals propagating at different angles obtained in post-processing according to some embodiments of the invention;

[0027] Fig. 7 shows an illustration and results of transmission matrix measurement in the presence of a scatterer according to some embodiments of the invention; and

[0028] Fig. 8 shows images of diffraction measurements obtained by subtracting the incident plane wave signal and scatterer reconstructions according to some embodiments of the invention.DETAILED DESCRIPTION OF APPLICATIONS

[0029] Some aspects of the present invention provide a pulse-echo ultrasound system comprising an acoustic sensor and an ultrasound generator. The optical detection is typically based on a photo-elastic effect, where stress or strain in optical components of the acoustic sensor leads to changes in the refractive index of the optical components. These changes are indicative of the acoustic waves impinging upon the acoustic sensor, thereby allowing optical-based detection of the acoustic waves.

[0030] Some aspects of the invention provide an integrated ultrasound system (e.g., pulse echo ultrasound system) that combines an acoustic chip with an interrogation optical system and / or an excitation optoacoustic system. This system leverages the advantages of all components to achieve high-resolution and high-sensitivity pulse-echo ultrasound, suitable for various applications, including medical imaging and non-destructive testing.Optoacoustic Ultrasound Excitation

[0031] Some aspects of the invention may be directed to an ultrasound system based on optoacoustic ultrasound excitation. The ultrasound system may include a layer of optically opaquematerial applied on a substrate and a laser source configured to generate and direct a pattern of temporally modulated (e.g., pulsating) optical laser beams for optoacoustic excitation on the layer.

[0032] Optoacoustic ultrasound excitation may involve illuminating an optical absorbing layer with a laser pulse to rapidly increase its temperature, leading to thermal expansion and excitation of a pressure wave. For an energy-deposition profile expressed by H(r, t), the equation governing the excitation and propagation of the pressure wave in a homogeneous medium is given by Eq (1).

[0033] where f represents the dimensionless Grueneisen parameter describing the conversion efficiency between absorbed optical power and pressure in the excited material, and c is the speed of sound.

[0034] Typically, the laser pulse width is shorter than the thermal and acoustic relaxation times, and the heating function can be approximated by H(r, t) = Hr(r)6(t), where 6(t) is the Dirac delta function. In that case, the acoustic signal p(r, t) can be calculated as follows by Eq. (2):

[0035] where ^4(t) represents the surface for which |r — r'| = ct.

[0036] When the illumination is performed on the optically absorbing layer, the spatial energy-deposition map Hr(r) can be expressed as Hr(r) = H(x, y)exp(— paz), whereais the absorption coefficient of the optically absorbing layer and z is the propagation direction of the optical pulse. Ifais sufficiently large such that the penetration depth is considerably smaller than the smallest acoustic wavelength in the measurement, one may use the following approximation: Hr(r) = H(x, y)8(z). This assumption is valid for carbon-based coatings since their light penetration depth is typically on the scale of 1 pm;. In comparison, the acoustic wavelength in high-resolution optoacoustic imaging is typically on the scale of 10 pm to 100 pm.

[0037] Since the optical illumination is spatially modulated by a DMD, H(x, y) may be modeled as a pixelated im

[0038] where the coefficients hfp q) G {0,1} indicate whether a certain pixel in a digital micromirror device (DMD) is off or on, Ax, Ayare the pixel spacing.

[0039] In some embodiments, the mathematical representation of the pulsating laser patterns of optical beam for optoacoustic excitation, was expressed by the binary coefficientsin equation (3). Denote x G RNxNtasthe matrix containing the measured acoustic signals for all possibilities of single-pixel excitation in Eq. (3) where N = P X Q and Ntis the number of time samples. Since Eq. (1) is linear, the detected acoustic signal to any excitation pattern given by Eq. (3) may be calculated by multiplying x by a matrix. Considering a set of N excitation patterns, the measured data y G RNxNt is given by equation (4), y = Wx + n,

[0040] where W G RNxNis the multiplexing matrix in which the coefficients hp qof all the projected patterns are stored, and n is an additive-noise term. Specifically, one can assume that n ~ N (0, o2I) is a randomly distributed Gaussian white noise with variance o2.

[0041] Assuming an invertible matrix W, the desired signal can be recovered as x = W-1y, which yields x = x + W-1n.

[0042] Recovering x from a multiplexed measurement may result in a signal-to-noise-ratio (SNR) enhancement in comparison to a direct measurement of x, assuming the same additive noise in both measurements. The SNR gain, G, is given by equation (6).G = N / tr W)-1].

[0043] While numerous possible constructions for W may be considered, in this work, the S-matrix was used, since it achieves an SNR gain of G « / V / 2, close to the theoretical limit for nonnegative matrices and Gaussian white noise.

[0044] In some embodiments, using the multiplexing approach for the optoacoustic signal is done when the laser power may be sufficiently high to illuminate the entire emitter area close to the coating’s damage threshold, or damage threshold of any other component through which the beam propagates to reach the coating. In that case, focusing cannot be used to increase the signal further, as it would damage the coating or other components in the imaging probe. Therefore, the directmeasurement of x would involve blocking most of the laser beam, leaving only the pixel of interest illuminated. Consequently, the measurement in y represents N / 2 times more optical power than in x, which is used to gain an SNR improvement of approximately VN / 2 in the demultiplexed signals x.

[0045] Two S-matrix construction algorithms are employed in this work: the twin-prime and the quadratic residue construction algorithms. The twin-prime algorithm utilizes a pair of prime numbers, P > 1 and Q such that Q = P + 2, to create an S-matrix of order N = P X Q. The proximity of the two prime numbers ensures that the elements can be arranged in an almost square array. Therefore, this algorithm is suitable for creating two-dimensional arrays. One-dimensional arrays (Q = 1) are constructed using the quadratic residue algorithm, which uses primes of the form P = 4m + 3, where m is an integer, to create an S-matrix of order N = P.Acoustic sensing

[0046] Some aspects of the invention may be directed to acoustic sensing. Acoustic sensing involves the detection of acoustic waves, such as ultrasound, by measuring the stress or strain they induce in an optical sensing element. The optical detection is typically based on the photo-elastic effect, where stress or strain in optical components of the acoustic sensor leads to changes in the refractive index of the optical components. These changes are indicative of the acoustic waves impinging upon the acoustic sensor, thereby allowing optical-based detection of the acoustic waves.

[0047] In some embodiments, an acoustic chip, for acoustic sensing, may include an optical waveguide (e.g., a planer optical waveguide) with a high refractive index core and a cladding structure. In some embodiments, the acoustic chip and / or the waveguide may be substantially planar. As used herein, substantially planar may be defined as a flat structure (e.g., nearly two-dimensional object ) that may be bent by not more than 10° with respect to a fully planar structure. The waveguide core may be made of materials such as silicon, silicon nitride, chalcogenide glass, and the like, which have high refractive indices (e.g., n > 1.6). The cladding structure typically includes a polymer, preferably an elastomer like PDMS, which has a higher photo-elastic coefficient and / or lower Young's modulus than the core and bottom cladding. This configuration enhances the sensitivity of the sensor by ensuring that a significant portion of the optical mode's energy resides within the polymer layer, where pressure modulations lead to higher refractive index changes.

[0048] The mathematical representation of the acoustic sensing process may be described using the following equations. Two connected metrics are frequently used to quantify the effect of mechanical perturbations on the optical properties of a waveguide. When the change is detected in the optical phase (<|)) in a waveguide of a given length due to uniform pressure (P), the phase sensitivity: S(|) = d<|) / dP, is used. For applications in which a resonator is used, the normalized sensitivity is more appropriate: SX = dlres / ( res dP), where Arcs is the resonance wavelength and dXres is the shift in wavelength due to the perturbation. S,_ may be calculated by using the following equation:

[0049] where neff is the refractive index of guided mode and szis the strain in the z direction. The relation between S and S^> is given by

[0050] where X is the incident light wavelength and L is the effective length of the sensor.

[0051] To calculate S due to a plane longitudinal acoustic wave that impinges on the chip perpendicularly, the below model was used in which sz, sx— >0. The calculation of Sz is performed by computing the change in neff of the guided mode due to deformation and change in the refractive index of the materials by the photo-elastic effect, given by the following equations:

[0052] where and C2are the photo-elastic constants and v is the Poisson ratio. It is noted that for Si and SiCh, the optical parameters were measured at X = 1550 nm, whereas for BCB, which may be used as the top-cladding polymer, their values were obtained at X =1536 nm. The calculation of SX via Eqs. (7) and (9) used a mode solver to find the perturbations to the effective refractive index. In accordance with some applications of the present invention, COMSOL Multiphysics was used and the analysis was conducted for waveguide 20 and for waveguide with the silica over-cladding and without the BCB over-cladding, for both the TE the TM modes. For the wavelength A = 1540 nm, the values obtained for the TM and TE modes were neff= 1.78 and neff=2.46, respectively, for the silica over-cladding and neff = 1.84 and neff = 2.47, respectively, for the BCB over-cladding.

[0053] Reference is now made to Figs. 1A, IB, and 1C which are schematic illustrations showing cross-sections of acoustic chips included in according to some embodiments of the invention. An acoustic chip 100a, 100b or 100c may include an optical waveguide 20, the optical waveguide comprising: an optical core 22, a cladding structure 30 surrounding optical core 22. Optical core 22 may have a high (e.g., n>=1.6) refractive index. In some embodiments, optical waveguide 20 may be a planar waveguide. For example, the refractive indices may be 1.6, 1.8, 2, 2.5, 3, 3.5, 4, and any value or range in between. In some embodiments, optical core 22 may include Si, SiN, chalcogenide glass, and the like. Optical core 22 may be surrounded by a cladding structure 30 with at least two layers (bottom cladding and top cladding). In some embodiments, core 22 may have a maximum height of 1 pm, for example, 0.9 pm, 0.8 pm, 0.7 pm, 0.5 pm, 0.4 pm, 0.1 pm, 0.05 pm, 0.01 pm and any value or range in between. In some embodiments, core 22 may have a maximum width of 10 pm, for example, 9 pm, 8, pm, 5 pm, 3, pm, 1 pm, 0.5 pm or any value or range in between.

[0054] In some embodiments, cladding structure 30 may include a top cladding layer 32 and a bottom cladding layer 34 surrounding optical core 22. In one embodiment, optical core 22 may be attached to one surface of bottom cladding layer 34 and covered by top cladding layer 32, as illustrated in Fig. 1A. In another embodiments, core 22 may be located inside bottom cladding layer 34 that may in turn be covered by top cladding layer 32. Additionally, any intermediate configuration of the two is within the scope of the invention, core 22 may be partially embedded in bottom layer 34 and covered by top layer 32, as illustrated in Fig. 1C. In another configuration, core 22 may be fully embedded in bottom layer 34, such that only one surface of core 22 is covered by top layer 34.

[0055] In some embodiments, top cladding 32 may be made out of a polymer, for example, an elastomer (e.g. PDMS) with a higher photo-elastic coefficient and / or lower Young modulus than that of the core and bottom cladding 34. Sensing is performed by an interrogation beam, discussed with respect to Fig. 2B, that travels inside core 20 in the mode’s volume that overlaps with the polymer of the cladding structure (i.e. the top cladding). In some embodiments, >5% or even >10% of the mode’s energy should reside within the polymer layer since pressure modulations there lead to a higher modulation in the refractive index, as shown in Fig. IE.

[0056] In some embodiments, bottom cladding layer 34 may have a thickness of at most 10 gm and at least 0.5 gm. For example, bottom cladding layer 34a or 34b may have a thickness of at most 4.5 pm, 4 pm, 3 pm, 2 pm, 1 pm, 0.8 pm, 0.7 pm, 0.5 pm, 0.3 pm, 0.1 pm and any value or range in between. Fig. IE shows the energy distribution of the guided optical mode of the interrogation beam for the case of (left) silicon core and (right) silicon nitride core. In both cases, more than 10% of the mode’s energy is located inside the top cladding and participates in the sensing.

[0057] In some embodiments, chip lOOa / lOOb / lOOc may be included by a substrate 10. Substrate 10 may be optically transparent to the optical beam for the optoacoustic excitation.

[0058] In some embodiment, top cladding 32 may be applied with optically absorbent layer 40, for generating acoustic waves.

[0059] In some embodiments, optically absorbing layer 40 is designed to efficiently absorb optical energy from a laser source and convert it into acoustic waves via rapid thermal expansion. This optically absorbing layer is typically coated on a substrate that is optically transparent to the optical beam for the optoacoustic excitation, ensuring that the maximum amount of optical energy reaches the optically absorbing layer.

[0060] In some embodiments, an additional thin layer 36, illustrated in Fig. IB, may be included in cladding structure 30. Layer 36 may be included in any one of devices lOOa / lOOb / lOOc and may be used, for example, to enhance adhesion between optically absorbing layer 40 and cladding structure 30. Layer 36, may be optically transparent in the working wavelengths of devices lOOa / lOOb / lOOc.

[0061] Optically absorbing layer 40, used for optoacoustic ultrasound excitation, may be composed of materials with high optical absorption coefficients and / or favorable thermal conduction and / or expansion properties. Examples of such materials include carbon particles, metal nanoparticles, and elastomeric composites. These materials are chosen for their ability to absorb laser pulses and generate wideband ultrasound signals with high efficiency. The thickness of the optically absorbing layer is optimized to ensure that the penetration depth of the optical energy is considerably smaller than the smallest acoustic wavelength in the measurement, typically on the scale of 1 pm. This ensures that the generated acoustic waves have a high frequency and broad bandwidth, suitable for high-resolution imaging applications.

[0062] In a nonlimiting example, optically absorbing layer 40 may be a thin (~1 pm) layer of black ink spin-coated on a 1 mm microscope slide for 20 s at 2000 RPM. This coating was shown to produce acoustic bandwidths exceeding 100 MHz when illuminated with a focused optical with a diameter of approximately 3 pm. The dimensions of the transmitter were determined by the size of the illumination. Each element in the two-dimensional array configuration consists of a 3 x 6 pixel array, corresponding to a square transmitter element with a side length of 64.8 pm. The transmitter array elements are arranged in a rectangular pattern, resulting in a total of 151 x 149 transmitters, covering an area of 9.7 x 9.6 mm2. The number of transmitters adheres to the twin primes construction algorithm constraint, and cannot be adjusted without altering the transmitter's dimensions. The onedimensional array is constructed using the quadratic residue algorithm, consisting of 179 line transmitters, each with a width of 64.8 pm.

[0063] An additional optional design for the location of optically absorbing layer 40 is given in Fig. 1C, where optically absorbing layer 40 is applied below bottom cladding 34. This structure may be suitable, for example, when waveguide transfer to the transparent substrate is done. This layer structure is achieved by coating one of sides of substrate 10 prior to the bonding process. This layer structure is also applicable to both cladding configurations presented in Figs. 1 A, and IB.

[0064] In some embodiments, when two ultrasound waves may be propagated in chip 100a, 100b or 100c, one travelling outwards and the other travelling inwards, i.e. back towards the chip (see, Fig. 2A). The inward propagating wave may be then reflected by the interface between top and bottom cladding 32 and 34 layers due their strong difference in Young moduli or acoustic impendences, for example, bottom cladding layer 34 may have a Young modulus (or acoustic impedance) at least 4 times higher than that of the top cladding layer 32. The reflected acoustic wave then interferes with the initial outward-propagating wave. To avoid destructive interference, it may be required that the thickness of the polymer top-cladding 32 to be smaller than half the acoustic wavelength corresponding to the central frequency of the system. Alternatively, it may be required that top-cladding width 32 may be smaller than twice the axial imaging resolution of the sensor. Under this condition, the interference is either partially or fully constructive, thus increasing the magnitude of the transmitted acoustic wave. In some embodiments, the top cladding layer is sufficiently flat, i.e. variations in its width are smaller than its average width, that no geometric beam steering or focusing occurs due to its shape.

[0065] As used herein, the generated acoustic waves may be used for sensing and / or image formation.

[0066] In some embodiments, detection may be performed with a single chip or multiple chips. Each chip may or may not include a resonator to increase light localization and sensitivity without adding to the physical length of the waveguide. Resonator types may include micro-rings, Fabry-Perots, or pi-phase-shifted Bragg grating. If no resonator is used, the waveguide may include bends to increase the sensing length while maintaining a short physical length. Fig. ID shows an example of a waveguide bent in an Archimedean-spiral increase the sensing length. If a resonator is used, the refractive-index modulation may be detected by monitoring the wavelength shifts of the resonance by tuning a continuous-wave (CW) laser to the region of the resonance wavelength and monitoring the intensity or phase modulation at the output of the resonator. If no resonator is used, the phase modulation at the output of the sensing arm may be detected by interfering it with a reference beam from the same CW laser.

[0067] Reference is now made to Figs. 2A, and 2B which are a schematic illustration showing a cross-section of an ultrasound system and a block diagram of the ultrasound system according to some embodiments of the invention. An ultrasound system 200 may be a pulse-echo ultrasound system, and may include acoustic chip lOOa / lOOb / lOOc, a first laser source 50 and interrogation optical system 60. Chip 100a / l 00b.100c may include optically absorbing layer 40 of optoacoustic polymer In some embodiments, first laser source 50 configured to generate and direct at least one optical beam 55 (i.e., laser beam for optoacoustic excitation) to the chip lOOa / lOOb / lOOc. Laser beam(s) 55 may be directed towards layer(s) 40 of chip lOOa / lOOb / lOOc.

[0068] In some embodiments, first laser source 50 may generate high-energy laser pulses, typically with a pulse width of less than 100 nanoseconds, to ensure efficient thermal expansion and subsequent pressure wave 45 excitation in the optically absorbing layer. First laser source 50 may be configured to produce a variety of illumination patterns (e.g., a spot beam, a line beam, a ID pattern, a 2D pattern (e.g., as shown in Fig. 3(a)), etc.), which may be spatially modulated to achieve desired acoustic wave characteristics. For example, the patterns may be generated using a spatial light modulator (SLM) such as a digital micromirror device (DMD) or a physical mask that blocks a portion of beam 55 according to the pattern fabricated on the mask. The DMD allows for precise control over the spatial distribution of the laser pulses, enabling the creation of complex illuminationpatterns (shown in Fig. 3) that can be tailored to specific applications. Alternatively, the laser source may be focused to a line- or point-like shape and scanned over the device. Spot and line beams may be produced by focusing systems containing lenes and / or mirrors.

[0069] In a nonlimiting example, first laser source 50 may generate a 1 ns pulsed laser beam 55 (Waveguard-D, Optogama) with a wavelength of 1064 nm, pulse energy of 120 pJ, and repetition rate of 1kHz is used together with a DMD (DLP4500NIR, Texas Instruments) to project coded S-matrix illumination patterns onto optically absorbing layer 40 that includes an ink-based light-absorbing coating, creating spatially modulated ultrasound pulses via the optoacoustic effect. The laser beam is expanded to cover the DMD screen, and the image is magnified using an optical system consisting of two lenses. The magnification M determines the pixel size on the light-absorbing coating plane, which, in turn, determines the size of each acoustic source. The DMD consists of 912 *1140 pixels with a spatial resolution of 10.8 x 5.4 pm2 and a maximum refresh rate of 4220 Hz. Consequently, a magnification of M=2 was chosen to cover an area of 19.71 x 12.32 mm2 in the acoustic source plane. The second part is the ultrasound detection system (e.g., acoustic sensor 100). It consists of a single-element silicon-photonics acoustic detector ( SPADE) mounted on a three-axis scan system and submerged in water. Please note that in contrast to the stationary surface used for ultrasound excitation, the size and location of the detection surface can be readily modified in the scanning protocol, making the system configurable.

[0070] In some embodiments, interrogation optical system 60 may be configured to generate and guide an interrogation laser beam 65 by core 20; and to convert the interrogation beam into an acoustic signal. In some embodiments, interrogation optical system may include a second laser source 62 configured to generate interrogation beam 65; a detector 64 configured to detect interrogation beam 65, exiting from chip lOOa / lOOb / lOOc; and a processor 66 (illustrated in Fig. 2B) configured to convert a signal received from detector 64 into the acoustic signal.

[0071] In some embodiments, second laser source 62, may be any laser source. In some embodiments, when lOOa / lOOb / lOOc comprises a resonator core 22, the wavelength of second source 62 should overlap with the optical resonance. For example, second laser source 62 may be a CW laser tuned to the wavelength of the resonator.

[0072] In some embodiments, detector 64 may be any device configured to detect interrogation beam 65, exiting from chip lOOa / lOOb / lOOc, for example, detector 64 may be aphotodetector connected directly to chip lOOa / lOOb / lOOc. In another example, detector 64 may include a device configured to detect phase modulation at the output of the sensing arm by interfering it with a reference beam.

[0073] In operation, laser source 50 may generate and direct the laser beam onto the optically absorbing layer 40, causing it to generate ultrasound waves 45 through rapid thermal expansion. Ultrasound waves 45 may propagate through acoustic chip lOOa / lOOb as illustrated in Fig. 2A and are detected by the acoustic chip lOOa / lOOb. The optical detection is based on the photoelastic effect, where stress or strain inside the polymer cladding of the acoustic chip leads to changes in the refractive index, allowing for the detection of the acoustic waves.

[0074] Some embodiments of the invention may be directed to an ultrasound (US) device for generating US waves using an optical beam for optoacoustic excitation, according to any one of the embodiments discussed herein above. A specific configuration of such a device is shown in Fig. 3 and some examples for the operation of such a device are given in the Examples section herein below. A device 300 may include a source 310 for generating a spatially modulated optoacoustic beam 315;a transparent substrate 320, transparent to the optoacoustic beam; an optically absorbing layer 330, deposited on at least one surface 323 of transparent substrate 320. Transparent substrate 320 may be substantially the same as substrate 10 and optically absorbing layer 330 may be substantially the same as layer 40 of chips lOOa / lOOb / lOOc, as discussed herein above.

[0075] In some embodiments, spatially modulated optoacoustic beam 315 is directed towards the optically absorbing layer, as shown in Fig. 3, for example, using any number of directing optical elements such as lenses, mirrors, etc.

[0076] In some embodiments, spatially modulated optoacoustic beam 135 comprises an illumination pattern. For example, the illumination pattern may one of: a plurality of illumination pixels on a single line, a two-dimensional pattern of illumination pixels (as shown in Fig. 3(b)), two- dimensional pattern of illumination lines (as shown in Fig. 3(c)) In some embodiments, the illumination pattern may comprise a binary structure comprising illuminated pixels and nonilluminated pixels.

[0077] In some embodiments, US device 300 may be included in a detection system that may further include a detector configured to detect US wave generated by the US device.

[0078] Device 300 although comprising only some of the components of chips lOOa / lOOb / lOOc was used in an experiments providing data and scientific base for the invention.Examples

[0079] The number of devices 300 in the measurement is determined by the scan positions of SPADE. For the one-dimensional laser transmitter array, the chip was scanned over 2,000 positions, spaced 6 pm apart and spanning 12 mm. The signals were averaged ten times, which resulted in an acquisition time of 60 mm for the entire transmission matrix. For the two-dimensional laser transmitter array, the chip was stationary, and the same averaging was used, leading to a measurement duration of 3.7 min for all the transmitter patterns.

[0080] The SPADE used the experiments has a bandwidth of 110 MHz, limited by the system's electronics, a noise equivalent pressure (NEP) of 6.5 mPa / ^Hz, and an effective length of 74 pm, which was determined by the light localization of the resonant mode. In both configurations, the receiver was located 3.5 mm from the transmitter array.Signal Characterization

[0081] Reference is now made to Fig. 3 which shows system 300, discussed above, and data acquisition schemes for one- and two-dimensional transmitter arrays, (a) A nanosecond pulsed laser illuminates a DMD, creating illumination patterns that are magnified using two lenses, denoted as LI and L2, and projected onto a light-absorbing optically absorbing layer submerged in water, resulting in the excitation of ultrasound signals. The signals are measured using an acoustic detector, SPADE, mounted on a three-axis stage, (b) Data acquisition process for a two-dimensional array configuration. A stationary detector, positioned against the center of the array, measures ultrasound signals coded with an S-matrix. The measured signals are then demultiplexed using an inverse S- matrix to create an equivalent measurement of the detector response for each array element, (c) Transmission matrix acquisition process for one-dimensional transmitter and receiver arrays. The detector measures the response to each coded signal along the scanning line, and the resulting measurements are demultiplexed to obtain the complete transmission matrix.

[0082] The acoustic source characteristics in the time and frequency domains for the two array configurations are shown in Fig. 4. The single-element source measurement configuration isdescribed in Fig. 4(a), the detector was positioned against the center of the array, and only the middle element was active. The signal shown in Fig. 4(b) is bipolar and anti-symmetric, with 40 MHz central frequency, and a bandwidth of 60 MHz. Fig. 4(c)-(d) show a similar measurement for the line transmitter. In this case, the signal is bipolar, with 30 MHz central frequency, and a bandwidth of 70 MHz. In contrast to the single-element source, the line transmitter signal is not anti-symmetric, and its leading edge is sharper than its trailing edge. Additionally, a plane wave signal is illustrated in Fig. 4(e)-(f). The signal is unipolar, with a frequency response effectively starting at DC and a bandwidth of 55 MHz. The qualitative features of the three types of acoustic signals in Fig. 4 agree with the theory of spherical, cylindrical, and plane optoacoustic sources, respectively.

[0083] The NEP of the detector, the measurement bandwidth, and the calculated SNR were used to estimate the peak-to-peak pressure of the signals presented in Fig. 4. The estimated pressure values, recorded 3.5 mm from the transmitter array in the direct measurement (i.e., without multiplexing), are 60 Pa for the single-element source, 420 Pa for the line source, and 2.1 kPa for the plane wave signal. After applying the multiplexing scheme, the equivalent pressure values are 4.4 kPa for the single-element source and 3.2 kPa for the line source. These values correspond to SNR gains of 75 and 7.8 for the single-element and line source signals, respectively, and are in full agreement with the expected values indicated by Eq. (6).Beam Focusing and Steering

[0084] The transmission matrix of the system, which holds information on the response of each transmitter-receiver pair, was obtained by demultiplexing the data. The matrix information is further processed into different bases, such as a focused basis and a plane wave basis.

[0085] Beam focusing is achieved by summing the signals from all emitters with appropriate time delays in post-processing, as illustrated in Error! Reference source not found, for each array configuration. Error! Reference source not found.(a) depicts the three-dimensional focusing scheme of the two-dimensional transmitter array. The sinogram shown in Error! Reference source not found.(b) is part of the complete measured sinogram before applying beamforming, illustrating the behavior of a scanned single-element source. The result shown in Error! Reference source not found.(c) is an optoacoustic reconstruction of the source using the filtered back-projection algorithm

[0058] , which can be interpreted as the focusing of the transmitted signals at the detector. The focal dimensions are 82 x 123 pm2 in the XY plane, with an axial width 18 pm.

[0086] Error! Reference source not found.(d) demonstrates focusing using a onedimensional receiver array. In this example, the focusing is done in reception by activating a single transmitting line element and measuring the acoustic signals at every detector position, shown in Error! Reference source not found. (e). The result illustrated in Error! Reference source not found.(f) is a two-dimensional image of the source using the delay-and-sum algorithm, which has a lateral length of 79 pm and an axial width of 18 pm.

[0087] Beam steering is demonstrated by synthetically creating plane waves propagating at different angles a with respect to the X axis. This is achieved by introducing a time delay TI CXto each transmitting element positioned at xLin a post-processing stage. Qwhere x0is the position of a reference transmitter.

[0088] Error! Reference source not found.(a) demonstrates plane waves propagating at angles a = —20°, 0°, 10° by introducing delays to the transmission matrix signals using Eq. (). Error! Reference source not found.(b)-(d) show the synthetic plane wave signals. The wavefront arrives at each receiver with a linear time delay, indicating the tilting angle of the plane wave.Diffraction Measurements of a Scatterer

[0089] The system is demonstrated in a 2D configuration to capture the complete transmission matrix in the presence of a cylindrical scatterer. Specifically, a nylon thread with a diameter of 400 pm was placed between the transmission and the reception surfaces, as illustrated in Fig. 7(a). Utilizing the transmission matrix, the forward-scattered waves to any incident wave may be obtained in post-processing. For example, Fig. 7(b) shows the response to a plane wave propagating at an angle a=0, computed using the transmission matrix and Eq. (10). This response includes several distinct features: 1) the unscattered plane wave, 2) a diffracted wave arriving before the unscattered wave due to the higher speed of sound of nylon, 3) diffracted waves from the edges of the scatterer synchronized with the unscattered wave, and 4) diffraction from the edges of the plane wave.

[0090] Fig. 7(c) presents the acoustic waveforms captured at x=0 and x=2 mm, showing a time difference of 115 ns between the two signals. Accordingly, the speed of sound inside the thread, as estimated from the measurement, is 2600.5 m / s, consistent with the previously published value of2620 m / s. In addition, Fig. 7(c) shows a broadening of the signal that propagated through the thread, indicating an attenuation of high-frequency ultrasound in nylon.

[0091] To further validate the results, the transmission matrix was simulated using the k- Wave toolbox. The simulation assumed the nylon has a speed of sound of 2620 m / s and a frequency - dependent attenuation of 0.1 dBVMHzA2 cm. Fig. 8(a) and (c) show the simulated and measured responses to a plane wave with a=0 after subtraction of the unscattered plane wave, demonstrating an excellent agreement between theory and experiment. The simulated and measured transmission matrices were further used to produce a cross-sectional image of the scatterer by using the delay-and- sum algorithm for a synthetic aperture setup, where each pixel in the resulting image is obtained by focusing the signals both in transmission and reception. In both reconstructions [Fig. 8(b) and (d)], the top and bottom boundaries of the scatterers are clearly visible, whereas the side boundaries are smeared.

[0092] To highlight the advantages of the all-optical platform compared to conventional piezoelectric technology, the transmission matrix measurement was reiterated, substituting a piezoelectric transducer over the SPADE. To optimize compatibility with the scatterer geometry, a cylindrically focused transducer was employed (V319, Olympus), aligning its unfocused axis with the direction of the thread's length. The transducer has a diameter of 12.7 mm, a focal length of 25.76 mm, a central frequency of 15 MHz, a bandwidth of 15 MHz, and an acceptance angle of ±7A°. These parameters are common to piezoelectric technology. The transducer was focused 3.5 mm from the source and scanned laterally to produce a set of virtual detectors at the same positions over which SPADE was scanned. The delay due to the additional propagation of 25.76 mm from the focus to the transducer's surface was subtracted from the measurement to produce the acoustic pressure at the focus.

[0093] The waveform measured by the piezoelectric transducer is shown in Fig. 8(e), demonstrating only a partial capture of the complex diffraction pattern due to the limited angular response of the transducer, as well as signal smearing and distortion due to the transducer's limited bandwidth. The reconstruction, also performed using the delay-and-sum algorithm is shown in Fig. 8(f). As expected, the low-signal fidelity in Fig. 8(e) led to considerable distortions in the reconstruction, significantly deviating from the optimal reconstruction of Fig. 8(b).

[0094] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.

Claims

CLAIMS1. An acoustic chip, comprising: an optical waveguide, comprising: an optical core; and a cladding structure surrounding the optical core; and optically absorbing layer attached to the cladding structure, wherein the cladding structure comprises at least two layers, a bottom cladding layer and a top cladding layer covering the optical core.

2. The acoustic chip of claim 1 , wherein the top cladding layer has a thickness of at most half the acoustic wavelength of a used central frequency of acoustic waves.

3. The acoustic chip of claim 1 or claim 2, further comprising a substrate holding the optical waveguide attached to the bottom cladding layer.

4. The acoustic chip of any one of claims 1 to 3, wherein the optical core comprises a material selected from the group consisting of silicon, silicon nitride, and chalcogenide glass.

5. The acoustic chip of any one of claims 1 to 4, wherein the top cladding layer is made of a polymer with a higher photo-elastic coefficient and / or lower Young's modulus than the optical core and the bottom cladding layer.

6. The acoustic chip of any one of claim 1 to 5, wherein the top cladding layer is made of polydimethylsiloxane (PDMS).

7. The acoustic chip of any one of claim 1 to 6, wherein the top cladding layer is applied with an optically absorbent material selected from the group consisting of metal layers, metal nanoparticles, and carbon particles.

8. The acoustic sensor of any one of claims 3 to 6, wherein the substrate is optically transparent to the excitation beam.

9. 7 The acoustic chip of any one of claim 1 to 8, wherein the optical core has a refractive index of at least 1.6.

10. The acoustic chip of any one of claim 1 to 9, wherein the top cladding layer has a thickness smaller than twice the axial imaging resolution of the sensor.

11. The acoustic chip of any one of claim 1 to 10, wherein the optically absorbing layer is one of: attached to of the top cladding layer or attached below the bottom cladding layer.

12. The acoustic chip of any one of claim 1 to 11, wherein the optically absorbing layer comprises materials selected from the group consisting of carbon particles, metal nanoparticles, and elastomeric composites.

13. The acoustic chip of any one of claim 1 to 12, wherein the optically absorbing layer has a thickness optimized to ensure that the penetration depth of the optical energy is considerably smaller than the smallest acoustic wavelength in the measurement.

14. The acoustic chip of any one of claims 2 to 13, wherein the optically absorbing layer is applied on the substrate.

15. The acoustic chip of any one of claim 1 to 14, wherein the optically absorbing layer has a thickness of at most 1 pm.

16. The acoustic chip of any one of claim 1 to 15, wherein the optical waveguide comprises a resonator.

17. The acoustic chip of any one of claim 1 to 16, wherein the bottom cladding layer has a thickness of at most 10 pm.

18. The acoustic chip of any one of claim 1 to 17, wherein the is bottom cladding layer comprises a transparent polymer.

19. The acoustic chip of any one of claim 1 to 18, wherein the optical waveguide is a planar optical waveguide.

20. The acoustic chip of any one of claim 1 to 18, being planar.

21. An ultrasound system comprising: the chip according to any one of claims 1 to 20; a first laser source configured to generate and direct an optoacoustic laser beam to the chip, for generating ultrasound waves; and an interrogation optical system configured to generate and direct an interrogation laser beam guided by the core ; and to convert the interrogation beam into a detected acoustic signal.

22. The ultrasound system of claim 21, wherein the interrogation optical system comprises: a second laser source configured to generate the interrogation beam; a detector configured to detect the interrogation beam, exiting from the chip; anda processor configured to convert a signal received from the detector into the acoustic signal.

23. The ultrasound system of claim 22, wherein the waveguide of the chip comprises a resonator and wherein the second laser source is configured generate the interrogation beam tuned to the wavelength of the resonator.

24. The ultrasound system of any one of claims 21 to 23, wherein the laser beam is a temporally modulated beam.

25. The ultrasound system of any one of claims 21 to 24, wherein the first laser source has a pulse width of less than 1 psecond.

26. The ultrasound system of any one of claims 21 to 25, wherein the first laser source is configured to produce a variety of illumination patterns using a digital micromirror device (DMD).

27. The ultrasound system of any one of claims 21 to 26, wherein the first laser source is configured to generate high-intensity laser pulses with a pulse energy of between 10 pj to 1 J and a repetition rate of 1 Hz to 1 MHz.

28. The ultrasound system of any one of claims 18 to 24, wherein the first laser source is configured to project coded S -matrix illumination patterns onto the optically absorbing layer.

29. An ultrasound (US) device comprising: a source for generating a spatially modulated optoacoustic beam; transparent substrate, transparent to the optoacoustic beam; and an optically absorbing layer, deposited on at least one surface of the transparent substrate, wherein the spatially modulated optoacoustic beam is directed towards the optically absorbing layer, wherein the spatially modulated optoacoustic beam comprises an illumination pattern.

30. The US device 27, wherein the illumination pattern comprises one of: a plurality of illumination pixels on a single line, a two-dimensional pattern of illumination pixels, two- dimensional pattern of illumination lines, three-dimensional pattern of illumination pixels and three-dimensional pattern of illumination lines.

31. The US device of claim 28, wherein the illumination pattern comprises a binary structure comprising illuminated pixels and non-illuminated pixels.

32. The US device of claim 28, wherein the illumination pattern is adjustable.

33. A detection system comprising: the US device accoridng to any one of claims 29 to 32; and a detector configured to detect US wave generated by the US device.

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