Ultrasound sensing and imaging based on whispering-gallery-mode (WGM) microresonators

Abstract

To provide whispering-gallery-mode optical resonator-based acoustic sensors, imaging systems that make use of the acoustic sensors, and methods of detecting ultrasound waves using the acoustic sensors.SOLUTION: An acoustic sensor comprises: a whispering-gallery-mode optical resonator; a coupling waveguide for optically coupling to the optical resonator; and a polymer enclosing a portion of the coupling waveguide and the optical resonator. The coupling waveguide is spaced from the optical resonator by a separation gap. The optical resonator and the coupling waveguide respectively have higher refractive indices than the refractive index of the polymer.SELECTED DRAWING: None

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 62 / 901,883, filed September 18, 2019, the entire disclosure of which is incorporated herein by reference.

[0002] (STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT) This invention was made with United States Government support under Grant Nos. W911NF1710189 and W911NF1210026 awarded by the National Institutes of Health. The United States Government has certain rights in this invention.

[0003] (Technical field) The present disclosure relates generally to systems and methods for ultrasound imaging using acoustic transducers that include whispering gallery mode resonators. [Background technology]

[0004] Ultrasound technology has gained increasing interest in various fields, especially in non-invasive measurement, remote sensing, and biomedical imaging. Ultrasound imaging is used in various settings to non-invasively image the internal structures of a patient by detecting ultrasonic pulses reflected from tissue boundaries within the patient's body. Ultrasound detectors used in ultrasound imaging applications generally have low noise equivalent power (NEP) and operate at high frequencies with wide bandwidths. Currently available piezoelectric ultrasound detectors generally meet these requirements. However, miniaturization of piezoelectric ultrasound detectors is accompanied by an increase in noise associated with ultrasound detection. Therefore, in ultrasound imaging applications where miniaturized ultrasound detectors are required, the use of piezoelectric ultrasound detectors is limited by noise.

[0005] Furthermore, when applying higher frequency sound waves to increase ultrasonic resolution, the penetration depth decreases due to increased acoustic attenuation. This trade-off between resolution and penetration depth has been a challenge for conventional piezoelectric ultrasonic sensors.

[0006] Photonic devices and optical pressure sensing techniques have attracted increasing interest because they are very promising in ultrasound detection, since photonic devices (e.g., diffraction gratings, etalons, etc.) can be fabricated at the microscale without sacrificing the performance or sensitivity of ultrasound detection. In photonic devices, the refractive index modulation and / or shape deformation caused by the strain induced by the acoustic wave are converted into changes in the intensity of the detected light or changes in the spectral characteristics of the device. In some existing photonic devices, optical resonators are used as highly sensitive ultrasound detectors. In these optical resonators, the incidence of ultrasound waves modulates the resonant frequency or transmitted light intensity. In general, the performance of an optical resonator is limited by its Q-factor (i.e., the higher the Q-factor, the lower the optical loss and the smaller the detectable resonance shift) as well as the acousto-optical and mechanical properties of the material forming the resonator.

[0007] Other objects and features will be in part apparent and in part pointed out hereinafter. Summary of the Invention [Means for solving the problem]

[0008] In one aspect, an acoustic sensor is disclosed that includes an optical resonator with a whispering gallery mode, a coupling waveguide for optically coupling to the optical resonator, and a polymer encapsulating a portion of the coupling waveguide and the optical resonator. The coupling waveguide is separated from the optical resonator by a separation gap. The optical resonator and the coupling waveguide each have a refractive index higher than the refractive index of the polymer.

[0009] In another aspect, an acoustic sensing system is disclosed that includes an optical resonator in a whispering gallery mode, a coupling waveguide for optically coupling to the optical resonator, a polymer encapsulating a portion of the coupling waveguide and the optical resonator, a light source coupled to a first end of the coupling waveguide and protruding from the low index polymer, and a photodetector coupled to a second end of the coupling waveguide and protruding from the low index polymer. The coupling waveguide has a first end and an opposite second end and is separated from the optical resonator by a separation gap. The optical resonator and the coupling waveguide each have a refractive index higher than the refractive index of the polymer.

[0010] In another aspect, a photoacoustic imaging device with an acoustic sensor is disclosed. The acoustic sensor includes a whispering gallery mode optical resonator, a coupling waveguide for optically coupling to the optical resonator, a polymer encasing a portion of the coupling waveguide and the optical resonator, a transducer light source coupled to a first end of the coupling waveguide and protruding from the low index polymer, and a transducer light detector coupled to a second end of the coupling waveguide and protruding from the low index polymer. The coupling waveguide has a first end and an opposite second end and is separated from the optical resonator by a separation gap. The optical resonator and the coupling waveguide each have a refractive index higher than the refractive index of the polymer.

[0011] In another aspect, a method of detecting ultrasound is provided that includes providing an acoustic sensor. The acoustic sensor includes a whispering gallery mode optical resonator, a coupling waveguide for optically coupling to the optical resonator, a polymer encapsulating a portion of the coupling waveguide and the optical resonator, a light source, and a photodetector. The coupling waveguide has a first end and an opposite second end and is separated from the optical resonator by a separation gap. The light source is coupled to the first end of the coupling waveguide and protrudes from the low index polymer. The photodetector is coupled to the second end of the coupling waveguide and protrudes from the low index polymer. The polymer forms a sample contact surface. The optical resonator and the coupling waveguide each have a refractive index higher than the refractive index of the polymer. The disclosed method further includes acoustically coupling the sample to the sample contact surface of the polymer and conducting ultrasonic waves emitted from within the sample through the polymer to a portion of the coupling waveguide and the optical resonator; introducing light generated by a light source into a first end of the coupling waveguide; detecting the light exiting the second end of the coupling waveguide with a photodetector and converting the detected light into a detector signal by encoding the amplitude of the light; and converting the detector signal into pressure using a predetermined calibration rule.

[0012] Other objects and features will be in part apparent and in part pointed out hereinafter. [Brief description of the drawings]

[0013] Those skilled in the art will appreciate that the drawings described below are for illustrative purposes only and are not intended to limit the scope of the present teachings in any way.

[0014] [Figure 1]FIG 1A is a graph summarizing the fiber mode index as a function of wavelength for optical fibers of various diameters in air. FIG 1B is a map showing the fiber mode distribution for the 1.6 μm optical fiber shown in FIG 1A. FIG 1C is a graph summarizing the fiber mode index as a function of wavelength for optical fibers of various diameters in a polymer with a refractive index (n) of 1.33. FIG 1D is a map showing the fiber mode distribution for the 1.6 μm optical fiber shown in FIG 1C. [Diagram 2] FIG. 2A is a graph summarizing the effective refractive index for light transmission at 883 nm in an optical fiber (nfiber) and a microtoroid resonator (mWGM,tr) encapsulated in a polymer with a refractive index (n) of 1.33. FIG. 2B is a map showing the fiber mode distribution of the optical fiber shown in FIG. 2A. FIG. 2C is a graph summarizing the effective refractive index for light transmission at 778 nm in an optical fiber (nfiber) and a microtoroid resonator (mWGM,tr) encapsulated in a polymer with a refractive index (n) of 1.33. FIG. 2D is a map showing the fiber mode distribution of the optical fiber shown in FIG. 2C. FIG. 2E is a graph summarizing the effective refractive index for light transmission at 709 nm in an optical fiber (nfiber) and a microtoroid resonator (mWGM,tr) encapsulated in a polymer with a refractive index (n) of 1.33. FIG. 2F is a map showing the fiber mode distribution of the optical fiber shown in FIG. 2E. [Diagram 3] 1 is an image of an optical resonator-based acoustic pressure sensor according to one aspect of the present disclosure. [Figure 4A] FIG. 4 is a cross-sectional schematic diagram of the optical resonator acoustic pressure sensor shown in FIG. 3, where the optical resonator acoustic pressure sensor is embedded within a casing formed of a low refractive index polymer material. [Figure 4B] FIG. 4B is a schematic side view of the embedded optical resonator acoustic sensor shown in FIG. 4A. [Diagram 5] FIG. 1 illustrates an acoustic imaging system with an optical resonator-based acoustic sensing probe according to one aspect of the present disclosure. [Figure 6]FIG. 1 shows the spectrum of transmitted light intensity through an optical resonator-based acoustic sensing probe as a function of transmitted light wavelength. [Figure 7] 1 is a graph illustrating an example of a signal generated by an optical resonator-based acoustic sensing probe in response to an ultrasonic pulse. [Figure 8] FIG. 1 is a block diagram illustrating a schematic diagram of a system according to one aspect of the present disclosure. [Figure 9] FIG. 1 is a block diagram illustrating a schematic of a computing device according to an aspect of the present disclosure. [Figure 10] FIG. 2 is a block diagram illustrating a schematic of a remote or user computing device according to an aspect of the present disclosure. [Figure 11] FIG. 1 is a block diagram illustrating a schematic diagram of a server system according to an aspect of the present disclosure. [Figure 12] 1A-1C are schematic diagrams illustrating a method for manufacturing a microtoroid resonator according to one aspect of the present disclosure. [Figure 13A] 1A-1C are schematic side views illustrating a method of assembling an acoustic sensor according to one aspect of the present disclosure. [Figure 13B] 1A-1C are schematic top views illustrating a method of assembling an acoustic sensor according to one aspect of the present disclosure. [Figure 14] 1 is a screenshot illustrating a display of a handheld device for obtaining transmission readings from an optoacoustic sensor according to one aspect of the disclosure. [Figure 15] FIG. 1 is a schematic diagram illustrating elements of an ultrasound imaging system with an optical resonator-based acoustic sensor according to an aspect of the present disclosure. [Figure 16A] 1 is a graph summarizing the sensitivity loading factor with respect to gap detuning from critical coupling for optical resonator-based acoustic sensors in air. [Figure 16B] 13 is a graph summarizing the sensitivity loading factor with respect to gap detuning from critical coupling for an optical resonator acoustic sensor encapsulated in an n=1.33 polymer. [Figure 17A]1 is a graph showing the penetration depth of an ultrasound sensor as a function of ultrasound frequency for various sensor enhancements made possible by incorporating an optical resonator-based acoustic sensor with various degrees of sensor enhancement in tissue samples representative of adipose or breast tissue. [Figure 17B] 1 is a graph showing the penetration depth of an ultrasound sensor as a function of ultrasound frequency for various sensor enhancements made possible by incorporating an optical resonator-based acoustic sensor with various degrees of sensor enhancement in a tissue sample representative of blood. [Figure 18A] Graph summarizing different existing imaging modalities (OCT / Optical Coherence Tomography, PAT / Photoacoustic Tomography, US / Ultrasound, MRI / Magnetic Resonance Imaging) mapped as a function of image resolution and penetration depth. [Figure 18B] 18B is a graph corresponding to that of FIG. 18A illustrating the effect of improved sensitivity provided by an optical resonator-based acoustic sensor in various imaging modalities according to one embodiment of the present disclosure. [Figure 19] 1 is an optical microscope image showing a single microtoroid sensor according to an embodiment of the present disclosure. [Figure 20A] 1 is a graph summarizing the signal measured by a polymer-encapsulated microtoroid pressure sensor in response to 5 MHz ultrasonic pulses delivered at various pressures. [Figure 20B] 1 is a graph summarizing the signal measured by a polymer-encapsulated microtoroid pressure sensor in response to 20 MHz ultrasonic pulses delivered at various pressures. [Figure 21] 1 is a graph summarizing the signal amplitude obtained using a polymer-encapsulated microtoroid pressure sensor as a function of frequency detuning used to evaluate the SNR of the polymer-encapsulated microtoroid pressure sensor. [Figure 22] 1 is a graph showing normalized signal amplitude obtained by a polymer-encapsulated microtoroid pressure sensor detecting ultrasonic pulses delivered at a range of frequencies. [Figure 23A]FIG. 13 is a schematic diagram illustrating the sign convention used to quantify the direction of an incident acoustic wave on the surface of a polymer-encapsulated microtoroid pressure sensor. [Figure 23B] 23B is a map of normalized signal intensity obtained by a polymer-encapsulated microtoroid pressure sensor detecting ultrasonic pulses delivered at the range of incident sound angles defined in FIG. 23A. [Figure 24] FIG. 1 is a schematic diagram of detection of an acoustic signal using a microtoroid pressure sensor according to one aspect of the present disclosure. [Diagram 25] 1 includes graphs showing wavelength detuning and transmission within a cavity during a wavelength upscan of the cavity. [Figure 26] FIG. 1 is a schematic diagram of an optoacoustic imaging system according to one aspect of the present disclosure.

[0015] Although the drawings show the currently described configuration, it should be understood that the present embodiment is not limited to the configuration shown, but is by way of illustration. Although various embodiments have been disclosed, further alternative embodiments of the present invention will become apparent to those skilled in the art from the following detailed description illustrating exemplary aspects of the present disclosure. As will be understood, the present invention can be modified in various ways without departing from the spirit and scope of the present invention. Thus, the drawings and detailed description should be regarded as illustrative in nature and not restrictive. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In various aspects, a pressure sensor is disclosed that includes an ultra-high quality whispering gallery mode (WGM) optical resonator encased in a low refractive index polymer. In various other aspects, the polymer-encased whispering gallery mode (WGM) optical resonator is included in a pressure detection system. In various other aspects, a medical imaging system, such as but not limited to an ultrasound imaging system, includes a whispering gallery mode (WGM) optical resonator as an acoustic sensor. The optical resonator can take advantage of the high sensitivity of circulating light to mechanical perturbations induced by incident acoustic waves compared to conventional hydrophones based on piezoelectric materials, and therefore can achieve higher sensitivity to acoustic signals. Additionally, the sub-millimeter footprint achievable using WGM microresonators (WGM optical resonators) is expected to (1) enable the formation of compact sensor arrays and (2) extend the acoustic response bandwidth to higher frequencies.

[0017] In various aspects, the polymer-encased WGM optical resonator has both a high optical quality factor (Q~107) and rich mechanical modes that respond to incoming acoustic signals, as described in more detail below. The polymer casing maintains the coupling of the fiber taper and the WGM optical resonator in a fixed configuration, thereby enabling robust optical actuation of the system without mechanical perturbations. The polymer casing further simplifies the optical packaging process during manufacturing, and such packaging simplifies the incorporation of the polymer-encased WGM optical resonator as a pressure sensor into the design of various devices, such as, but not limited to, ultrasound imaging systems.

[0018] I. OPTICAL RESONATOR-BASED ACOUSTIC SENSORS, SENSOR SYSTEMS, AND METHODS OF USE THEREOF

[0019] The details and various embodiments of the present invention may be better understood by referring to the drawings. Referring to FIG. 3, an optical resonator-based acoustic sensing system 100 is shown. In various aspects, the system 100 comprises a light source 101, such as, but not limited to, a tunable laser. The system 100 further comprises a WGM optical resonator 102 mounted on a substrate 113 and a coupling waveguide 104 for coupling laser energy into and out of a resonant mode of the WGM optical resonator 102. The system 100 further comprises an optical coupler 103 configured to direct the laser energy generated by the light source 101 into the coupling waveguide 104. Non-limiting examples of suitable light sources include semiconductor lasers (DFB or FP laser diodes), GaN or similar LED on-chip light sources, or on-chip WGM microlasers whose wavelengths can be finely tuned by temperature or drive current control. The laser signal 108 can be detected at the output port 110 of the coupled waveguide 104 using an optical receiver 106 (or optical detector) coupled to the end of the coupled waveguide 104 opposite the end where the optical coupler 103 is located.

[0020] In various aspects, both the light source 101 and the optical receiver 106 are connected to a computing device 112 (not shown). In various aspects, the computing device 112 is configured to control the operation of the light source 101 and to process the output from the optical receiver 106 to extract information regarding the optical transmission from the WGM optical resonator 102. In another aspect, as described in further detail below, the computing device 112 of the system 100 further includes a processor and a non-volatile computer readable memory (not shown).

[0021] 4A and 4B are schematic cross-sectional and side views, respectively, of a system 100 similar to that shown in FIG. 3, in which the WGM optical resonator 102 and a portion 114 of the coupling waveguide 104 are encapsulated in a low index polymer 116. In one embodiment, the low index polymer 116 maintains the coupling of the coupling waveguide 104 and the portion 114 of the WGM optical resonator 102 in a fixed configuration. In some embodiments, the fixed configuration includes a separation gap 118 that separates the coupling waveguide 104 from the WGM optical resonator 102. In one embodiment, the separation gap 118 is selected to provide critical coupling of laser energy directed through the coupling waveguide 104 to the WGM optical resonator 102. In another embodiment, the separation gap 118 is slightly offset from the critical coupling gap distance to move the operation of the WGM optical resonator 102 closer to the maximum loading factor associated with improved sensor sensitivity, as described in more detail below.

[0022] In various aspects, the selected value of the separation gap is influenced by any one or more of a number of factors, including, but not limited to, the dimensions and material of the WGM optical resonator, the dimensions and material of the coupling waveguide, the dimensions and material of the encapsulation polymer, the operating parameters of the WGM optical resonator-based pressure sensor, and any other relevant factors. A detailed discussion of the relationships of at least some of the above-mentioned factors is described in more detail below.

[0023] 4A and 4B, the low index polymer 116 is configured to efficiently transmit light between the coupling waveguide 104 and the WGM optical resonator 102, and to efficiently receive ultrasound waves from tissue imaged using an ultrasound imaging system, as described in more detail below. In various embodiments, the low index polymer 116 is applied in an uncured state onto the WGM optical resonator 102, the coupling waveguide 104, and the substrate 113, and then cured in situ using a curing method. Any known curing method may be used to cure the low index polymer 116, as long as it is compatible with the selected polymer material. Non-limiting examples of suitable curing methods include UV curing, moisture curing, and cross-linking curing. In some embodiments, the degree of curing may be adjusted to bring the acoustic impedance and / or refractive index of the low index polymer 116 to a level that allows for efficient operation of the system 100, as described in more detail below. As a non-limiting example, the curing method can produce a cured polymer characterized by a refractive index suitable for efficient optical transmission between the WGM optical resonator 102 and the coupling waveguide 104, as well as a mechanical index matched to the tissue being ultrasound imaged. Without being limited to a particular theory, the mechanical matching of the low refractive index polymer 116 to the tissue can facilitate efficient transmission of ultrasound pulses emanating from the tissue being imaged.

[0024] In another embodiment, the low index polymer 116 is covered with an additional membrane layer 120. In various embodiments, the membrane layer 120 is selected to seal the underlying low index polymer 116 and provide a barrier to prevent oxygen from contacting the low index polymer 116, thereby facilitating a curing process, such as a UV curing process, described in more detail below. In some embodiments, the membrane layer 120 is removed from the low index polymer 116 of the system 100 upon completion of the curing process. In another embodiment, the membrane layer 120 is retained on the exposed surface of the low index polymer 116. In these other embodiments, the material of the membrane layer 120 is selected to acoustically match the tissue to be imaged using an ultrasound imaging system, including the acoustic sensing system 100, as described in more detail below.

[0025] In various aspects, the membrane layer 120 is any suitable material capable of preventing oxygen from penetrating into the underlying layer of low refractive index polymer 116. Non-limiting examples of suitable membrane layer materials include a cover slip, a polymer layer, and any other suitable membrane material. Without being limited to any particular theory, the refractive index of the membrane layer does not affect the performance of the WGM optical resonator 102 and coupling waveguide 104 encapsulated in polymer as described herein. In some aspects, the membrane layer 120 is fabricated using a material that is acoustically compatible with the tissue being imaged, as described above.

[0026] In various embodiments, the membrane layer 120 may be preformed and applied over the low index polymer 116, or the membrane layer 120 may be applied in an uncured state over the low index polymer 116 and cured in place. As a non-limiting example, applying a preformed membrane layer over the low index polymer may facilitate removal of the membrane layer after curing of the low index polymer is complete as described above. As another non-limiting example, a membrane material that acoustically matches the tissue being imaged may be applied in place and cured to increase intimate contact between the membrane layer and the low index polymer, thereby facilitating efficient transmission of acoustic signals from the tissue to the sensor.

[0027] In various embodiments, the thickness of the membrane layer is in the range of about 0.1 mm to about 5 mm. In various other embodiments, the thickness of the membrane layer is in the range of about 0.1 mm to about 0.3 mm, about 0.2 mm to about 0.4 mm, about 0.3 mm to about 0.5 mm, about 0.5 mm to about 0.7 mm, about 0.6 mm to about 0.8 mm, about 0.7 mm to about 0.9 mm, about 0.8 mm to about 1.0 mm, about 0.9 mm to about 1.1 mm, about 1.0 mm to about 2 mm, about 1.5 mm to about 2.5 mm, about 2 mm to about 3 mm, about 2.5 mm to about 3.5 mm, about 3 mm to about 4 mm, about 3.5 mm to about 4.5 mm, and about 4 mm to about 5 mm.

[0028] Figure 4B is a side view of the system 100 shown in Figure 4A. As shown in Figure 4B, both ends of the coupling waveguide 104 adjacent to the portion 114 of the coupling waveguide 104 encapsulated with the low refractive index polymer 116 protrude from the low refractive index polymer 116. This allows the light source 101 to be coupled to the coupling waveguide 104 via the optical coupler 103 at one end of the coupling waveguide 104, and the photodetector 106 to be coupled to the coupling waveguide 104 at the other end of the coupling waveguide 104. The photodetector 106 is configured to detect the laser signal output from the output port 110 of the coupling waveguide 104 and to transmit a detector output signal 122 representative of the power of the detected laser signal.

[0029] In various aspects, as described in more detail below, the computing device 112 is further configured to operate the optical resonator-based acoustic sensing system 100 in a scanning mode to select an operating wavelength and to operate the system 100 locked at the operating wavelength to detect acoustic signals. In one aspect, as described in more detail below, the computing device 112 is configured to control and adjust the light source to scan the wavelength introduced into the coupling waveguide 104 and to execute a selection algorithm to analyze the transmission spectrum of the detector output signal, thereby deriving a detected degree of polarization and selecting a suitable degree of polarization from a plurality of degrees of polarization.

[0030] In various embodiments, the optical resonator can be characterized by a diameter in the range of about 50 μm to about 200 μm. In various other embodiments, the diameter of the resonator can be about 50 μm to about 60 μm, about 55 μm to about 65 μm, about 60 μm to about 70 μm, about 65 μm to about 75 μm, about 70 μm to about 80 μm, about 75 μm to about 85 μm, about 80 μm to about 90 μm, about 85 μm to about 95 μm, about 90 μm to about 100 μm, about 95 μm to about 105 μm, or about 100 μm to about 105 μm. μm, about 100 μm to about 120 μm, about 110 μm to about 130 μm, about 120 μm to about 140 μm, about 130 μm to about 150 μm, about 140 μm to about 160 μm, about 150 μm to about 170 μm, about 160 μm to about 180 μm, about 170 μm to about 190 μm, and about 180 μm to about 200 μm.

[0031] Without being limited to a particular theory, it is believed that the diameter of the optical resonator affects at least one of several factors related to the performance of an optical resonator-based pressure sensor, such as, but not limited to, the resonant wavelength and center frequency of the pressure sensor.

[0032] In various other embodiments, the coupling waveguide includes, but is not limited to, any suitable waveguide. In one embodiment, the coupling waveguide is a tapered optical fiber. In various embodiments, the minimum diameter of the tapered optical fiber is in the range of about 0.5 μm to about 5 μm. In various other embodiments, the minimum diameter of the tapered optical fiber is in the range of about 0.5 μm to about 0.7 μm, about 0.6 μm to about 0.8 μm, about 0.7 μm to about 0.9 μm, about 0.8 μm to about 1.0 μm, about 0.9 μm to about 1.1 μm, about 1 μm to about 2 μm, about 1.5 μm to about 2.5 μm, about 2 μm to about 3 μm, about 2.5 μm to about 3.5 μm, about 3 μm to about 4 μm, about 3.5 μm to about 4.5 μm, and about 4 μm to about 5 μm. Without being limited to any particular theory, it is believed that a small taper diameter can optimize the coupling of short optical wavelengths to the WGM optical resonators of the acoustic sensors of the present disclosure, as described herein.

[0033] In one embodiment, the coupling waveguide comprises a tapered optical fiber having a minimum diameter of at least 1.5 μm, a taper length of about 2 cm, and a fiber end diameter of about 125 μm. In various other embodiments, the coupling waveguide is constructed from any suitable material known in the art, including, but not limited to, fused silica material, low loss optical polymers, and any other suitable material.

[0034] In various embodiments, the performance of the WGM optical resonator-based acoustic sensor can be evaluated using any suitable existing analytical method, including but not limited to. In one embodiment, the ultrasonic amplitude measurements obtained by the toroidal resonator-based acoustic sensor can be analyzed using standard methods, such as those shown in FIG. 21, to evaluate the SNR and noise equivalent pressure. In another embodiment, the response amplitude measurements of the toroidal resonator-based acoustic sensor at a range of ultrasonic pressures can be analyzed using standard methods to measure the sensitivity of the sensor at various frequencies of ultrasonic waves. As a non-limiting example, the signal amplitude of a toroidal resonator-based acoustic sensor for 5 MHz and 20 MHz ultrasonic waves at various wave pressures are shown in FIG. 20A and FIG. 20B, respectively. In a further embodiment, the response amplitude measurements of the toroidal resonator-based acoustic sensor at a range of ultrasonic frequencies can be analyzed using standard methods to measure the bandwidth of the sensor, as shown in FIG. 22. The acoustic sensing performance of an acoustic sensor including a polymer-encapsulated WGM optical resonator as described herein is summarized in Table 1 below.

[0035] [Table 1]

[0036] As another non-limiting example, the acoustic sensing performance of an acoustic sensor as characterized in Table 1 is compared to the corresponding performance of a conventional piezoelectric transducer as shown in Table 2 below.

[0037] [Table 2]

[0038] With reference to FIG. 6, the transmission spectrum obtained from a high-Q WGM generally has a Lorentzian line shape. As shown in FIG. 24, the incident ultrasound can induce refractive index modulation and / or shape deformation (see the image on the left side of FIG. 24), which translates into a resonance shift in the transmission spectrum (see the graph on the right side of FIG. 24). When the operating wavelength is fixed, the variation of the resonance frequency is reflected as an oscillation of the transmitted optical power over time (see the output power signal on the right side of FIG. 24). In one embodiment, in order to maximize the amplitude of the output oscillation induced by the incident ultrasound, the operating wavelength is set to the wavelength corresponding to the point 2402 where the slope of the transmission spectrum is maximum.

[0039] It should be noted that only the thermally stable side is favored. Without being limited to a particular theory, the thermally stable side is either the short wavelength side or the long wavelength side depending on at least one factor including, but not limited to, the thermo-optic coefficient associated with the resonator material. If the thermo-optic coefficient of the resonator is dominated by a dielectric material characterized by a positive thermo-optic coefficient, such as, but not limited to, silica, silicon, silicon nitride, the short wavelength side (region 606 in FIG. 6) is the thermally stable side. If the thermo-optic coefficient of the resonator is dominated by a polymeric material characterized by a negative thermo-optic coefficient, the long wavelength side (region 608 in FIG. 6) is the thermally stable side.

[0040] As shown in FIG. 25, the thermally stable region in the transmission spectrum of a high-Q microresonator has been confirmed to have a triangular shape 2502 during the wavelength scanning process. As a non-limiting example, for a high-Q silica microresonator made from a dielectric material with a positive thermo-optic coefficient (dn / dT>0), when the wavelength of the pump laser approaches the resonance wavelength during the wavelength up-scanning process, the cavity starts to heat up, red-shifting the resonance wavelength and making the up-scan a tracking process between the resonance wavelength and the scanning pump wavelength, i.e., shifting both the resonance wavelength and the scanning pump wavelength in the same direction. Specifically, both the resonance wavelength and the pump wavelength increase linearly while the wavelength detuning between them decreases linearly. In this tracking process, the pump wavelength (shown as a dashed line in the top graph of FIG. 25) tracks the moving resonance wavelength (shown as a solid line in the top graph of FIG. 25) and the detuning between them falls within the bandwidth of the resonance. Thus, the resonator operates in a resonant state and the energy of the pump laser is coupled into the resonator. This tracking process continues until the pump wavelength catches up with the resonant wavelength, beyond which point the resonance condition is rapidly lost in the region 2504 of the moving resonant wavelength because the pump laser cannot push the resonant wavelength any further.

[0041] It should be noted that the thermally stable region is identified during the wavelength upscan (see FIG. 25) for resonator materials with positive thermo-optic coefficients as described above, and during the wavelength downscan (not shown) for resonator materials with negative thermo-optic coefficients. If the thermo-optic coefficient of the resonator is dominated by a dielectric material such as silica, silicon, silicon nitride, etc. with a thermo-optic coefficient >0, the wavelength upscan side (shorter wavelength side) is the thermally stable side. If the thermo-optic coefficient of the resonator is dominated by a polymer (thermo-optic coefficient <0), the wavelength upscan side (longer wavelength side) is the thermally stable side.

[0042] a. Arrangement of resonator and optical fiber

[0043] In various aspects, the dimensions and placement of the optical fiber and the optical resonator are determined according to one or more rules that incorporate various factors related to the performance of the sensor. In one aspect, a separation rule can be used to determine the gap that separates the tapered optical fiber and the periphery of the optical resonator. In another aspect, the dimensions and placement of the optical fiber and the optical resonator are influenced by several factors, as described below.

[0044] Without being limited to a particular theory, the dimensions of the fiber waveguide determine the effective index of the waveguide mode, and the gap between the fiber taper and the resonator determines the coupling strength. The coupling strength can alternatively be quantified as the coupling-induced loss. Ideally excited WGMs, also referred to herein as critical coupling, are characterized by a narrow and deep Lorentzian dip shape in the transmission spectrum of the coupled WGM and fiber taper. This critical coupling is achieved when (i) the effective index of the fiber waveguide mode matches the effective index of the WGM, and (ii) the coupling-induced loss is equal to the intrinsic loss inside the resonator.

[0045] b. Determining the ideal fiber taper

[0046] In various aspects, an ideal fiber taper for exciting optical modes in a whispering gallery mode (WGM) microtoroid resonator is determined using various criteria, which are described in more detail below.

[0047] In one aspect, an ideal fiber taper should satisfy the phase-matching condition with the target WGM, as shown below.

[0048]

number

[0049] In the above formula, n WGM、tr is the WGM microtoroid resonator (n WGM ) is the transformed effective refractive index of n fiberis determined by the diameter and refractive index of the fiber taper. WGM is determined by the dimensions and refractive index of the resonator and the refractive index of the surrounding medium. fiber and n WGM Both may be calculated numerically using any suitable method known in the art, such as, but not limited to, the Finite Element Method (FEM).

[0050] n fiber :

[0051] In one aspect, FEM modal analysis is applied to simulate the modal distribution and effective refractive index of fiber modes with various diameters and surrounding materials for various taper diameters in air (FIGS. 1A and 1B) and in a low index (n=1.3) polymer (FIGS. 1C and 1D), as shown in FIGS. 1A-1D.

[0052] n WGM :

[0053] In a further embodiment, FEM eigenfrequency analysis (2D rotationally symmetric model) is applied to simulate the mode distribution and eigenfrequencies of the WGM microtoroid resonator. WGM is derived using the following formula:

[0054]

number

[0055] In the above equation, m is the azimuthal mode number, c is the speed of light, and ω is the simulated natural frequency. e is the radial position of the “modal center of mass” and is expressed by the following formula:

[0056]

number

[0057] In the above equation, E is the amplitude of the electric field and n(r) is the refractive index.

[0058] n WGM、tr :

[0059] In another embodiment, n WGM、tr is calculated using the following formula:

[0060]

number

[0061] In the above formula, d sep is the distance between the "mode mass center" of the WGM and the center of the fiber.

[0062] When the coupling gap is very small, ie, the fiber taper almost touches the resonator, it can be expressed as:

[0063]

number

[0064] In the above formula, a and a f are the minor radii of the microtoroid and the fiber taper radius, respectively, and R p is the semimajor axis of the microtoroid resonator.

[0065] 2A, 2C, and 2E are graphs of effective refractive index n for varying fiber taper diameters at wavelengths of 883 nm, 778 nm, and 709 nm, respectively, generated using the equations and methods described above. WGM、tr and n fiber 2A, 2B, and 2C are graphs summarizing the fiber mode distribution of the optical fibers of FIGS. 2A, 2C, and 2E, respectively. FIGS. 2A, 2C, and 2E show that the ideal taper diameters for the polymer background are 1.41 μm, 1.31 μm, and 1.25 μm at wavelengths of 883 nm, 778 nm, and 709 nm, respectively.

[0066] c. Low refractive index polymer layer

[0067] In various embodiments, the WGM optical resonator and a portion of the coupling waveguide are encapsulated with a low refractive index polymer material. The polymer encasement provides various functions related to the fabrication, incorporation into various devices, such as ultrasound imaging systems, and use of the WGM optical resonator-based optical pressure sensor to detect pressure, as described herein. In some embodiments, the polymer encasement serves three functions: (i) protection of the coupling region, (ii) effective delivery of the input acoustic signal, and (iii) acting as a damping layer for the vibrating structure. The above multiple functions provided by the encasement polymer impose constraints on the selection of the encasement polymer material and polymer layer dimensions.

[0068] In one aspect, the encapsulation polymer material is selected to allow rapid curing and minimize changes in refractive index and density. In another aspect, the encapsulation polymer material is selected to be acoustically transparent to the input signal, including but not limited to ultrasound pulses emitted from tissue imaged using an ultrasound or photoacoustic imaging system. In another aspect, the acoustic signal delivered to the WGM optical resonator-based pressure sensor can be enhanced by modifying the dimensions of the encapsulation polymer layer. In this another aspect, the thickness of the polymer encapsulation layer can be set to (1 / 4+n / 2) times the acoustic central wavelength and / or the polymer encapsulation layer can be formed with a convex upper boundary to enhance acoustic focusing to the WGM optical resonator. In yet another aspect, the encapsulation polymer material is selected to enhance the effective damping of mechanical vibrations of the resonator structure in response to a received pressure pulse to eliminate the response tail to a pulsed acoustic input.

[0069] In one exemplary aspect, the encapsulating polymer material can be selected and engineered to enhance sensor sensitivity by forming an encapsulating polymer layer as described above, and to minimize response tails due to mechanical vibrations or multiple reflections by selecting a polymer with an appropriate level of acoustic attenuation.

[0070] In various aspects, the encapsulation polymer material can be a low index polymer, defined herein as a polymer having a refractive index less than that of the WGM optical resonator and the coupling waveguide. Without being limited to any particular theory, the inclusion of the encapsulation polymer material increases the coupling-induced mode broadening of the WGM optical resonator and the coupling waveguide compared to a comparable system in air. As a non-limiting example, the fiber mode distribution of an optical fiber encapsulated in a low index polymer (see FIG. 1D) is broadened compared to the fiber mode distribution of an optical fiber in air (see FIG. 1B). As a result, as described in more detail below, the refractive index of the selected encapsulation polymer can improve the sensitivity of the sensor performance to the precise positioning of the coupling fiber at the optimal separation gap distance. Without being limited to any particular theory, the refractive index contrast (n contrast =n resonator / n polymer ) affects the Lorentzian lineshape (FIG. 6) and load curve (FIG. 16B) of the WGM optical resonator, as will be explained in more detail below. In particular, n contrast As approaches 1, the Lorentzian lineshape and load curve of broadens and the Q factor decreases.

[0071] In various other embodiments, the acoustic impedance of the encapsulating polymer material can be matched to the acoustic impedance of the sample to be subjected to ultrasound imaging using an ultrasound imaging system incorporating a WGM optical resonator-based pressure sensor. As a non-limiting example, PDMS is a polymer that has a relatively low degree of acoustic mismatch (about 2x) with common biological tissues. In various other embodiments, the acoustic attenuation of the encapsulating polymer material can be selected to suppress the internal echo of ultrasound within the encapsulating polymer material while maintaining efficient transmission of ultrasound to the WGM optical resonator.

[0072] In various embodiments, any suitable optical polymer known in the art can be selected for use as the encapsulation polymer, including but not limited to, a UV-cured polymer or a water-cured polymer. Non-limiting examples of suitable encapsulation polymer materials include PDMS, PFOA, and non-PFOA fluoro(meth)acrylates.

[0073] d.Center frequency

[0074] In various embodiments, the center frequency of the acoustic response band of a WGM-based sensor is determined by the intrinsic mechanical mode of the resonator. As a non-limiting example, in a microsphere, the mechanical resonant frequency for the same order mechanical mode is inversely proportional to the diameter of the microsphere resonator, consistent with theoretical predictions based on the free-sphere model. As another non-limiting example, for a chip-based microdisk or microtoroid resonator, the mechanical resonant frequency is approximately inversely proportional to the length of the free-standing disk membrane, i.e., the undercut size.

[0075] e. Bond gap

[0076] In various aspects, the coupling gap of a WGM optical resonator-based pressure sensor is selected to enhance any one or more of at least several sensor performance parameters, such as, but not limited to, the critical coupling between the WGM optical resonator and the coupling waveguide, and the sensor sensitivity.

[0077] Without being limited to a particular theory, the bond-induced mode broadening κ c was found to be proportional to exp(-2γd), where γ is

[0078]

number

[0079] is the electric field attenuation coefficient outside the resonator body estimated by, and d is the width of the coupling gap. In an air background, n contrast =n resonator and for the polymer package, n contrast =n resonator / n polymer It is.

[0080] In some embodiments, the sensitivity of a WGM optical resonator-based pressure sensor is improved when the laser frequency is locked to the center of one side of the Lorentzian lineshape of the WGM optical resonator. Figure 16A shows a sensor without an encapsulant (air) and Figure 16B shows a sensor with a low index polymer encapsulant. When thermally locked, the short wavelength side is stable for materials with positive thermo-optic coefficients (dielectric materials), while the long wavelength side is stable for materials with negative thermo-optic coefficients (polymers). Analytical expression can be expressed as follows:

[0081]

number

[0082] In the above equation, Δω is the angular frequency detuning between the pump laser and the real-time resonance, and T is the observable transmission.

[0083] In another embodiment, the ideal operating detuning occurs at the maximum slope of the transmission spectrum so that the small signal indicated by the resonance shift is most amplified. In addition, the coupling induction κ c is related to the depth and width of the Lorentzian lineshape of the transmission. As a non-limiting example, Figure 6 is a graph of a transmission spectrum showing the depth 602, width 604, and maximum slope 606.

[0084] In one embodiment, the standard bonding criterion is defined as "critical bonding", where κ = κ c where the deepest lineshape is achieved by a direct indicator of the full extension of the transmitted power. If Δd is the gap width detuning from the critical coupling gap, then κ c=κ0exp(-2γΔd), so T´(Δω ideal The theoretical sensitivity, proportional to the Δd, can be expressed as a function of Δd:

[0085]

number

[0086] By normalizing the sensitivity at the critical coupling, a coupling induced load factor is defined.

[0087]

number

[0088] 16A and 16B are plots of the loading factor at 780 nm in air and in polymer (n=1.33), respectively.

[0089] f. How to make a microtoroid

[0090] In various embodiments, the microtoroid WGM optical resonator can be fabricated on a silicon wafer or any other suitable substrate using any suitable method known in the art. As a non-limiting example, FIG. 12 outlines a microtoroid fabrication method in one embodiment. With reference to FIG. 12, a silica layer is oxidized on a single crystal silicon substrate (step 1), and HDMS is deposited on the silica layer using any suitable method, such as but not limited to thermal evaporation and spin coating (step 2). In this embodiment, a UV mask is deposited on the photoresist followed by UV exposure (step 3), and the unmasked photoresist is removed during development (step 4). The portion of the silica layer exposed by the removal of the photoresist is removed using any suitable method (step 5). A portion of the silicon substrate is removed to form a support column using any suitable method (step 6), followed by a silica lip around the silica layer using any known method, such as but not limited to CO2 laser reflow to complete the microtoroid (step 7). An image of a microtoroid fabricated using the method described above is shown in FIG. 19.

[0091] In various aspects, the optical resonator may be provided in any suitable form, such as, but not limited to, a microtoroid, a microdisk, a microring, a microsphere, and any other suitable form. In various other aspects, the optical resonator may be constructed from any suitable material, such as, but not limited to, silicon, silica, lithium niobate, and any other suitable material.

[0092] g. Sensor packaging method

[0093] In various embodiments, WGM optical resonator-based acoustic sensors are characterized by having a sub-millimeter footprint, enabling the formation of compact sensor arrays. In some embodiments, the on-chip design of WGM optical resonator-based acoustic sensors can be used to build one- or two-dimensional sensor arrays with periods as small as 0.3 mm.

[0094] In one aspect, the WGM optical resonators described herein can be fabricated on commercially available silicon wafers using any suitable existing manufacturing method, including but not limited to, In another aspect, the cost of the sensor is significantly reduced due to the compatibility of suitable manufacturing methods with current semiconductor industry manufacturing systems and methods.

[0095] As a non-limiting example, as shown in Figures 13A and 13B, a microtoroid resonator 1302 is fabricated on a substrate 1312 having an optical mechanical spectrum and lateral support walls. A tapered optical fiber 1304 having an optimal diameter is fabricated, placed in an encapsulating polymer 1306, and coupled to the microtoroid 1302. In one embodiment, a fiber guide 1308 is fabricated on the substrate 1312 to facilitate positioning of the tapered optical fiber 1304 adjacent to the microtoroid 1302. In this embodiment, the substrate 1312 is coupled to an adjustable platform, such as, but not limited to, a microstage or micromanipulator configured to move the platform and couple the fiber guide 1308 and the microtoroid 1302 to the tapered optical fiber 1304.

[0096] In this embodiment, the microtoroid 1302 and fiber 1304 are covered with a UV-curable low refractive index polymer, as shown in Figures 13A and 13B. The polymer-covered microtoroid 1302 and fiber guide 1308 are then moved to adjust the optimum coupling gap in the polymer by adjusting the gap 1310 between the microtoroid 1302 and fiber 1304. In one embodiment, the optimum adjustment gap can be identified by iteratively acquiring transmission spectra for various gap distances using a computer device, as shown in Figure 14. Once the microtoroid 1302 and fiber 1304 are positioned at the desired gap as described above, the low refractive index polymer is exposed to direct UV light and the transmission spectrum is monitored in real time, as shown in Figure 14, until an indication of curing of the bonded region is detected. In one embodiment, an indication of curing of the bonded region includes a rapid shift in the optical resonance in the transmission spectrum. In another embodiment, the entire sensor device may be covered with a plastic film to create an inert environment for UV curing the polymer layer. In this inert environment, the entire encapsulating polymer layer can be cured in this manner.

[0097] In one embodiment, the optical resonator drive system shown in Figure 14 can be provided as a phone-sized system that can be used to evaluate device performance while positioning fibers and microtoroids, as well as to operate pressure sensors in various devices as described herein.

[0098] II. Imaging Systems

[0099] In various aspects, the optical resonator-based acoustic sensors described above simultaneously achieve high sensitivity and wide bandwidth at levels unprecedented in conventional acoustic sensors. In some aspects, at least one optical resonator-based acoustic sensor can function as an acoustic detector in a clinical imaging system, such as, but not limited to, an ultrasonic acoustic imaging system, a photothermal imaging system, and any other suitable clinical imaging system involving detection of acoustic signals.

[0100] a. Ultrasound imaging system

[0101] In one embodiment, as shown in FIG. 15, the optical resonator-based acoustic sensor is incorporated into an ultrasound imaging system 1500. In this embodiment, the optical resonator-based acoustic sensor 1502 replaces the existing piezoelectric transducer for detecting ultrasound waves in the probe array 1504, while retaining the piezoelectric transducer in the ultrasound probe head 1506 to generate US pulses that are scattered, reflected, or otherwise altered by the tissue being imaged. The optical resonator-based acoustic sensor 1502 provides improved sensitivity and broader bandwidth compared to conventional devices such as piezoelectric sensors, allowing sufficient information to be gleaned for image reconstruction even from high-frequency, strongly attenuated ultrasound signals. As a result, incorporating the optical resonator-based acoustic sensor 1502 helps overcome the trade-off between resolution (frequency determination) and penetration depth (penetration depth) of current clinical ultrasound systems.

[0102] In various aspects, the ultrasound imaging system generates ultrasound waves at any frequency, including but not limited to, greater than about 20 kHz. As described above, whispering gallery mode microresonators are configured to detect a wide range of acoustic frequencies. In some aspects, the ultrasound imaging system uses acoustic waves having frequencies ranging from about 20 kHz to about 200 MHz or more. In other aspects, the ultrasound imaging system uses acoustic waves having frequencies ranging from about 20 kHz to about 200 MHz or more, including but not limited to ultrasonic frequencies in the range of about 2 MHz to about 20 MHz used in existing ultrasound imaging systems, about 100 MHz to about 300 MHz, and any other suitable ultrasonic frequencies. The optical resonator-based ultrasound detectors of the present disclosure can significantly improve the resolution of ultrasound imaging by enabling the use of high frequency ultrasound for applications with deep penetration requirements.

[0103] 15, ultrasound signals are transmitted from a conventional transducer 1506 and echo signals are collected by optical resonator-based ultrasound sensors 1502 in a probe array 1504. A portable optical control module 1508 is used to drive the optical sensors 1502 in the probe array 1504 to detect ultrasound signals generated by the tissue being imaged in response to the ultrasound pulses generated by the conventional transducer 1506. The optical control module 1508 is operatively connected to each of the optical resonator-based ultrasound sensors 1502 in the probe array 1504. In addition, the optical control module 1508 is operatively connected to an imaging control and analysis system 1520 configured to operate the conventional transducer 1506, receive signals from the probe array 1504, and reconstruct an ultrasound image based on the signals received from the probe array 1504.

[0104] Referring again to FIG. 15, the light control module 1508 includes a light source 1516 and a light receiver 1514 operatively connected to the optical resonator-based ultrasonic sensor 1502 of the probe array 1504. The light source 1516 generates light that is directed to at least one coupling waveguide (not shown) used to couple the light to the optical resonator-based ultrasonic sensor 1502. The light receiver 1514 includes at least one photodetector (not shown) configured to detect light received from the at least one coupling waveguide modulated by the optical resonator-based ultrasonic sensor 1502. The light control module 1508 further includes a signal processing module 1512 configured to convert the output of the at least one photodetector of the optical receiver 1514 into an encoded electrical signal. The electrical signal generated by the signal processing module 1512 in response to the ultrasound signal detected from the tissue is communicated to the signal output 1510 of the probe array 1504 and communicated to the image control and analysis system 1520.

[0105] In various embodiments, the probe array 1504 includes at least one optical resonator-based ultrasonic sensor 1502. In some embodiments, the probe array 1504 includes a plurality of optical resonator-based ultrasonic sensors 1502 arranged in a predetermined array pattern, such as, but not limited to, a one-dimensional linear array pattern, a two-dimensional array pattern, and the like. The 2D array pattern includes, but is not limited to, any suitable 2D array pattern. Non-limiting examples of suitable 2D array patterns include a Cartesian grid pattern, a circular pattern such as a single circle of sensors or multiple concentric circles of sensors, and any combination thereof. In one embodiment, the probe array 1504 is a one-dimensional linear array including a plurality of sensors 1502, as shown in FIG. 15. All sensors of the probe array are encapsulated in a continuous low refractive index polymer layer, as described above.

[0106] In various aspects, the spacing between adjacent sensors 1502 in the sensor array 1504 may be uniform or may vary. The spacing between sensors 1502 in the probe array 1504 is selected based on at least one or more of several factors, including, but not limited to, the desired spatial resolution of the imaging system, the dimensions of the WGM optical resonators and coupling waveguides, the architecture of the sensor array, avoidance of crosstalk between adjacent sensors, and other related factors. In some aspects, the minimum spacing between adjacent sensors 1502 in the probe array 1504 is at least 5 μm to avoid crosstalk between the sensors 1502.

[0107] In various alternative embodiments, the sensors of the sensor array have dimensions that are essentially equal to one another. In alternative embodiments, at least some of the sensors 1502 may have different dimensions. Without being limited to a particular theory, different sensor sizes may provide a sensor array with increased sensitivity to acoustic signals over different frequency ranges.

[0108] In various alternative embodiments, each sensor 1502 of the probe array 1504 may be coupled to a separate coupling waveguide such that the number of sensors 1502 equals the number of coupling waveguides. In other alternative embodiments, at least a portion of the sensors may share a coupling to a shared coupling waveguide. In some alternative embodiments, the probe array may include one or more shared coupling waveguides shared by one or more portions of the sensors, as well as additional portions of the sensors each coupled to a separate coupling waveguide.

[0109] FIG. 5 is a schematic diagram illustrating the introduction of an ultrasonic pulse into tissue and the detection of an ultrasonic signal generated by the tissue using an optical resonator-based acoustic sensing system 100 in an ultrasound imaging system 500 similar to the system 1500 shown in FIG. 15. With reference to FIG. 5, at least one WGM microresonator 102 and a coupling waveguide 104 of the acoustic sensing system 100 may be acoustically coupled to the tissue 504 to be imaged. A conventional ultrasonic transducer 506 irradiates an ultrasonic pulse 508 toward the tissue 504, inducing an acoustic signal 502. In one aspect, the light coupled into the WGM microresonator 102 is generated using a light source 101 (laser) and directed through a polarization controller (not shown). The acoustic signal 502 interacts with the WGM microresonator 102 as described above and is detected in the form of a change in the laser signal detected by a photodetector 106 coupled to an output port 110 of the coupling waveguide 104. The output signal of the photodetector 106 is communicated to a computing device 112, which functions as a signal analyzer. In one embodiment, the computing device 112 converts the output signal of the photodetector 106 into an acoustic signal in the form of an optical resonance amplitude of the microresonator, as shown in Figure 7. In another embodiment, the computing device 112 converts the output signal of the photodetector 106 into an acoustic signal in the form of an acoustic signal obtained using a resonance wavelength identified from the spectral response of the microresonator 600, as shown in Figure 6.

[0110] In some embodiments, the ultrasound imaging system includes a single optical resonator-based acoustic sensing system, in which a focused ultrasound source is applied to transmit ultrasound waves and the single optical resonator-based acoustic sensing system collects echoes to determine the depth of the reflector (A-scan image). A B-scan image can be obtained by mechanically scanning the boundary source and the sensor.

[0111] In another embodiment, the ultrasound imaging system includes an array of optical resonator-based acoustic sensors, as shown in FIG. 15. In this embodiment, focusing and scanning of the ultrasound is achieved by a transducer phased array. In some embodiments, real-time B-scan images can be obtained by applying a suitable beamforming algorithm to the collected signals of the optical sensors. In another embodiment, tilted plane wave ultrasound can be transmitted by the transducer phased array to scan the transmission angle. The reflected signals can be collected by an optical resonator-based acoustic sensor array. With a suitable beamforming algorithm, ultra-fast acquisition of B-scan images can be achieved.

[0112] In various aspects, an array of optical resonator-based acoustic sensors includes at least two WGM optical resonators arranged in a spatial array, including, but not limited to, 2D arrays, such as, for example, linear, planar, or ring arrays, and 3D arrays, such as cylindrical or hemispherical arrays. In some aspects, the at least two WGM optical resonators in the array are each coupled to a separate coupling fiber, light source, and photodetector. In other aspects, the at least two WGM optical resonators in the array are coupled to the same coupling fiber, light source, and photodetector.

[0113] In other embodiments, at least two WGM optical resonator-based pressure sensor systems in the array are essentially identical in terms of dimensions, center frequency, operating wavelength, sensitivity, and any other relevant operating parameters. In yet other embodiments, at least two WGM optical resonator-based pressure sensor systems differ in one or more parameters, such as, but not limited to, resonator diameter, resonator material, separation gap distance, operating wavelength, encapsulation polymer material, coupling taper material, coupling taper diameter, and any other relevant parameters.

[0114] In another aspect, an optical resonator-based acoustic sensing system can be incorporated into an existing ultrasound imaging system to improve the quality of harmonic ultrasound imaging. Harmonic ultrasound imaging has high sensitivity and specificity due to high-frequency nonlinear ultrasound echoes from the target being imaged. However, the weak and broadband nature of the nonlinear signal poses difficult challenges to the ultrasound detector. An optical resonator-based acoustic sensor (1) can be used to efficiently collect nonlinear ultrasound echoes, and (2) can excite ultrasound at higher frequencies and shorter pulse durations (for higher resolution), thus enabling a sufficiently wide detection band.

[0115] b. Photoacoustic imaging system

[0116] In various aspects, the optical resonator-based acoustic sensor is incorporated into an ultrasound imaging system. Without being limited to any particular theory, at least one light pulse is introduced into the tissue to be imaged, and the energy of the incident photons is absorbed by structures in the tissue, re-emitting photoacoustic signals in the form of ultrasound waves. The emitted ultrasound waves are then detected by at least one ultrasound transducer, and the detected signals are used to reconstruct the photoacoustic image. In various aspects, the optical resonator-based acoustic sensor configured to detect the photoacoustic signals is compatible with any existing ultrasound imaging system, including, but not limited to, the following: photoacoustic microscopy (PAM) system, and photoacoustic computed tomography (PACT) system. Non-limiting examples of ultrasound systems compatible with the optical resonator-based acoustic sensor include photoacoustic microscopy (PAM) system, and photoacoustic computed tomography (PACT) system.

[0117] 26, an optoacoustic imaging system 2600 in one embodiment includes an optoacoustic (PA) light source 2602, including, but not limited to, at least one pulsed laser configured to generate a plurality of laser pulses 2604 that are introduced into tissue 2606 using at least one optical element of a PA optical module 2608. In one embodiment, each laser pulse 2604 generated by the PA light source 2602 is configured to induce a plurality of PA signals 2610 from a structure 2612 of a portion of tissue 2606 into which the laser pulse 2604 is introduced. As described above, the plurality of PA signals 2610 induced by a single laser pulse 2604 are detected by a transducer array 2614 and reconstructed into a PA image (not shown) using any suitable reconstruction method known in the art.

[0118] Referring again to FIG. 26, in one embodiment, the PA signal 2610 including a plurality of ultrasonic waves is collected by at least one optical resonator-based ultrasonic sensor 2616 in a transducer array 2614. Any suitable optical sensor described herein can be incorporated into the transducer array 2614 as the optical resonator-based ultrasonic sensor 2616, including but not limited to the optical resonator-based ultrasonic sensor 2616. As shown in FIG. 26, the photoacoustic imaging system 2600 further comprises at least one transducer light source 2618 and at least one transducer photodetector 2620 operably connected to the optical resonator-based ultrasonic sensor 2616 of the transducer array 2614. The light source 2618 generates light that is guided to at least one coupling waveguide 2622 used to couple the light to the optical resonator-based ultrasonic sensor 2616. The transducer photodetector 2620 includes at least one photodetector (not shown) configured to detect light received from at least one coupled waveguide 2622 modulated by the WGM optical resonator 2624 of the optical resonator-based ultrasonic sensor 2616.

[0119] The controller 2626 is used to drive the optical sensors 2616 in the transducer array 2614 to detect the PA signals 2610 generated by the tissue 2606 in response to the laser pulses 2604 generated by the PA light source 2602. The controller 2626 is operatively connected to each optical resonator-based ultrasonic sensor 2616 in the transducer array 2614 via the transducer light source 2618 and the transducer photodetector 2620. In some aspects, the controller includes a signal processing module (not shown) configured to convert the output of at least one transducer photodetector 2620 into an electrical signal encoding the detector output. In addition, the controller 2626 is operatively connected to a PA image analysis module 2628 configured to reconstruct an ultrasound image based on the signals received from the transducer array 2614. In some aspects, the controller 2626 is provided in the form of an optical control module similar to the portable drive system described above with reference to FIGS. 14 and 25.

[0120] In various aspects, each of the at least one pulsed laser of the PA light source 2602 can generate multiple laser pulses at a pulsed wavelength. The pulsed wavelength can be selected based on one or more factors, such as, but not limited to, enhanced penetration into the particular tissue being imaged by the pulsed wavelength, enhanced contrast of the structure of interest relative to surrounding structures that may be useful for unlabeled visualization of circulating tumor cells, and enhanced contrast of exogenous structures of interest that may be useful for SIP-PACT imaging of perfusion of contrast agents such as NIR dyes. In one aspect, a pulsed wavelength in the range of about 650 nm to about 1350 nm is selected to maximize light penetration through the entire body of the mammalian animal being imaged. This wavelength range is because it encompasses pulsed wavelengths that are less attenuated in the tissue of the mammalian animal compared to wavelengths that lie outside of this "optical window." In one particular aspect, a pulsed wavelength of about 1064 nm is selected to enable PA imaging of mammalian tissue using the PA imaging system 2600.

[0121] In various embodiments, the PA light source 2602 can generate laser pulses at a single wavelength, two (dual) wavelengths, or three or more (multi) wavelengths, as desired. In various embodiments, the multiple laser pulses can be generated at one or more wavelengths in the range of about 650 nm to about 1350 nm, thereby allowing maximum light penetration for whole-body imaging of an animal subject. Without being limited to any particular theory, this wavelength range is characterized by enhanced penetration into biological tissues. For example, it is conventionally known that this wavelength range corresponds to pulse wavelengths at which mammalian animal tissues least attenuate light.

[0122] In various alternative embodiments, the PA imaging system 2600 can use a single pulse wavelength selected to enhance penetration of the particular tissue being imaged and / or to enhance the contrast of the structure of interest relative to the surrounding structures. In another embodiment, the PA imaging system 2600 can use dual pulse wavelengths and / or multi-pulse wavelengths to enable functional imaging, including but not limited to, measurement of oxygen saturation in blood and other tissues. For example, a first pulse wavelength can be selected to enable maximum contrast for oxyhemoglobin and a second pulse wavelength can be selected to enable maximum contrast for deoxyhemoglobin or to enable maximum contrast for all hemoglobin. Dual / multi-pulse wavelengths can also be selected to enhance contrast of various structures of blood cells, CTCs, white blood cells, contrast agents (e.g., NIR dyes, etc.), or to enhance contrast of exogenous structures of interest (i.e., perfusion of contrast agents, such as NIR dyes). In various aspects, the PA imaging system 2600 includes a pulsed laser that generates laser pulses at a single pulse wavelength, including, but not limited to, a 720 nm laser such as the LS-2145-LT-150 Ti-sapphire (Ti-Sa) pulsed laser (Symphotic TII) with a repetition rate of 20 Hz and a pulse width of 12 ns, a 1064 nm laser such as the DLS9050 pulsed laser (Continuum) with a repetition rate of 50 Hz and a pulse width in the range of about 5 ns to about 9 ns, and any other suitable pulsed laser.

[0123] In various embodiments, the PA optics module 2608 of the PA imaging system 2600 includes one or more optical elements configured to direct multiple laser pulses generated by the PA light source 2602 to the tissue to be imaged 2606. In some embodiments, a focal region of the ultrasound transducer array 2614 coincides with at least a portion of the tissue to be imaged that is illuminated by the laser pulses 2604, and a PA signal 2610 induced by the multiple laser pulses 2604 is detected by the transducer array 2614 and used to reconstruct one or more PA images.

[0124] In various aspects, one or more optical elements (not shown) of the PA optics module 2608 are operatively connected to the PA light source 2602 to receive the plurality of laser pulses generated by the at least one pulsed laser. Additionally, the one or more optical elements are configured to perform various transformations of the plurality of laser pulses, including, but not limited to, changing the direction of travel of each laser pulse, redistributing the distribution of optical energy across the cross-sectional area of ​​each laser pulse into an essentially uniform spatial distribution of optical energy, changing the cross-sectional size and / or shape of each laser pulse, adjusting the optical intensity or fluence of each laser pulse, adjusting the relative time of arrival of two different laser pulses generated by two corresponding pulsed lasers, selectively transmitting or blocking transmission of laser pulses from one or more pulsed lasers, and any other suitable transformation of the plurality of laser pulses.

[0125] Non-limiting examples of suitable optical elements suitable for incorporation into the PA optics module 2608 of the PA imaging system 2600 include prisms, mirrors, diffusers, condensers, lenses, beam splitters, beam combiners, optical fibers, waveguides, and any other known optical elements suitable for modifying one or more characteristics of a laser pulse. Non-limiting examples of laser pulse characteristics that may be modified and / or modulated using one or more optical elements include cross-sectional shape, cross-sectional dimensions, direction of travel, wave speed, wavelength, polarization, intensity, phase, wavefront shape, superposition with other laser pulses, cross-sectional energy uniformity, pulse width, delay relative to other laser pulses in the pulse system, and other relevant characteristics of the laser pulse.

[0126] In one aspect, the diffuser can be configured to homogenize the profile of the laser pulse such that the energy intensity is uniformly distributed across the cross-sectional area of ​​the laser pulse. Non-limiting examples of suitable diffusers include various engineered diffusers, such as a ring diffuser. In one aspect, the diffuser can be a commercially available engineered diffuser, including but not limited to EDC-10-A-1r (RPC Photonics). Non-limiting examples of suitable condensers include various customized condensers, such as a customized ring condenser. Non-limiting examples of suitable prisms include triangular prisms, rhomboid prisms, and any other suitable prisms. Non-limiting examples of suitable lenses include convex lenses, concave lenses, cylindrical lenses, semi-cylinder lenses, compound lenses, and any other suitable lenses. In another aspect, the lenses can be commercially available lenses, including but not limited to AX-FS-1-140-0 conical lenses (Del Mar Photonics). Non-limiting examples of suitable mirrors include flat mirrors, convex mirrors, and concave mirrors.

[0127] In various aspects, the one or more optical elements may be further configured to enable an illumination approach selected depending on the area or tissue being imaged and / or the type of imaging being performed with PA imaging system 2600. In one aspect, the one or more optical elements may be configured to enable a top-illumination approach (see FIG. 26) or a side-illumination approach. The selection of the particular optical elements incorporated into PA imaging system 2600 will be influenced, at least in part, by the illumination approach used by PA imaging system 2600.

[0128] III. COMPUTER SYSTEMS AND COMPUTER DEVICES

[0129] FIG. 8 is a simplified block diagram of a computing device 800 for performing the methods described herein. As shown in FIG. 8, the computing device 800 is configured to perform at least some of the tasks associated with the methods of the present disclosure using the optical resonator-based pressure sensor system of the present disclosure. The computing device 800 comprises a computing device 802. In one embodiment, the computing device 802 is part of a server system 804 including a database server 806. The computing device 802 communicates with a database 808 via the database server 806. The computing device 802 is communicatively connected to an imaging system 810 and a user computing device 830 via a network 850. The network 850 may be any network that allows for local or wide area communication between devices. For example, the network 850 may be a network that allows for a communicative connection to the Internet through various interfaces, including but not limited to the Internet, a local area network (LAN), a wide area network (WAN), an integrated services digital network (ISDN), a dial-up connection, a digital subscriber line (DSL), a cellular connection, and a cable modem. The user computing device 830 may be any device capable of accessing the Internet, including but not limited to a desktop computer, a laptop computer, a personal digital assistant (PDA), a mobile phone, a smartphone, a tablet, a phablet, a wearable electronic device, a smart watch, or other web-based connectable or mobile device.

[0130] In another aspect, the computing device 802 is configured to perform a number of tasks associated with the operation of an optical resonator-based acoustic sensor and / or an imaging system incorporating an optical resonator-based acoustic sensor, including but not limited to the ultrasound (US) imaging system and photoacoustic (PA) imaging system described above. FIG. 9 illustrates a component configuration 900 of the computing device 902 including a database 910 along with other associated computer components. In some aspects, the computing device 902 is similar to the computing device 802 (shown in FIG. 8). A user 904 can access the components of the computing device 902. In some aspects, the database 910 is similar to the database 808 (shown in FIG. 8).

[0131] In one embodiment, the database 910 includes image data 918 and algorithm data 912. Non-limiting examples of suitable image data 918 include medical image data, such as, but not limited to, ultrasound image data or photoacoustic image data. Non-limiting examples of suitable algorithm data 912 include any values ​​of parameters that govern the operation of the WGM optical resonator-based acoustic sensor, ultrasound imaging system, and photoacoustic imaging system. Non-limiting examples of suitable algorithm data 912 include any values ​​of parameters that define algorithms associated with the methods of the present disclosure described herein, or any image reconstruction algorithms used to reconstruct ultrasound or photoacoustic images as described above.

[0132] Computing device 902 also includes a number of components that perform specific tasks. In an exemplary embodiment, computing device 902 includes a data storage device 930, an imaging component 940, an acoustic sensor component 950, and a communication component 960. Data storage device 930 is configured to store data received or generated by computing device 902, such as any data stored in database 910 or any output of any process performed by any component of computing device 902.

[0133] Communications component 960 is configured to enable communication between computing device 902 and other devices (e.g., user computing device 830 and imaging system 810 shown in FIG. 8) over a network, such as network 850 (shown in FIG. 8) or multiple network connections using a predefined network protocol, such as TCP / IP (Transmission Control Protocol / Internet Protocol).

[0134] 10 illustrates a configuration of a remote or user computing device 1002, such as user computing device 830 (shown in FIG. 8). The computing device 1002 includes a processor 1005 for executing instructions. In some aspects, executable instructions are stored in a memory area 1010. The processor 1005 may include one or more processing units (e.g., a multi-core configuration). The memory area 1010 may be any device that allows for the storage and retrieval of information, such as executable instructions and / or other data. The memory area 1010 may include one or more computer-readable media.

[0135] The computing device 1002 also includes at least one media output component 1015 for presenting information to the user 1001. The media output component 1015 may be any component capable of communicating information to the user 1001. In some aspects, the media output component 1015 may include an output adapter, such as a video adapter and / or an audio adapter. The output adapter is operably connected to the processor 1005. The output adapter is also operably connected to an output device, such as a display device (e.g., a liquid crystal display (LCD), an organic light emitting diode (OLED) display, a cathode ray tube (CRT), an "electronic ink" display), or an audio output device (e.g., speakers, headphones). In some aspects, the media output component 1015 may be configured to present an interactive user interface (e.g., a web browser or a client application) to the user 1001.

[0136] In some aspects, the computing device 1002 includes an input device 1020 for receiving input from the user 1001. The input device 1020 may include, for example, a keyboard, a pointing device, a mouse, a stylus, a touch-sensitive panel (e.g., a touchpad or touch screen), a camera, a gyroscope, an accelerometer, a position detector, and / or an audio input device. A single component, such as a touch screen, may function as both an output device for the media output component 1015 and as an input device 1020.

[0137] Computing device 1002 also includes a communications interface 1025 for communicatively coupling to a remote device, such as a wired or wireless network adapter or a wireless data transceiver for use with a cellular network (e.g., Global System for Mobile communications (GSM), 3G, 4G, or Bluetooth®) or other cellular data network (e.g., Worldwide Interoperability for Microwave Access (WIMAX)).

[0138] Stored in memory area 1010 are computer readable instructions for providing a user interface to user 1001, for example, via media output component 1015, and optionally receiving and processing input from input device 1020. The user interface may include a web browser and a client application, among other possibilities. The web browser allows user 1001 to view and interact with media and other information typically embedded in web pages or websites from a web server. The client application allows user 1001 to interact with server applications, for example, associated with a vendor or business.

[0139] 11 is a diagram illustrating an example of a configuration of a server system 1102. The server system 1102 includes, but is not limited to, a database server 806 and a computing device 802 (both shown in FIG. 8). In some aspects, the server system 1102 is similar to the server system 804 (shown in FIG. 8). The server system 1102 includes a processor 1105 for executing instructions. The instructions are stored, for example, in a storage device 1110. The processor 1105 may include one or more processing units (e.g., a multi-core configuration).

[0140] The processor 1105 is operatively connected to a communication interface 1115 such that the server system 1102 can communicate with a remote device, such as a user computing device 830 (shown in FIG. 8) or another server system 1102. For example, the communication interface 1115 can receive a request from a user computing device 830 over a network 850 (shown in FIG. 8).

[0141] The processor 1105 is operatively connected to the storage device 1110. The storage device 1110 may be any computer operating hardware suitable for storing and / or retrieving data. In some aspects, the storage device 1110 may be integrated with the server system 1102. For example, the server system 1102 may include one or more hard disk drives as the storage device 1110. In other aspects, the storage device 1110 may be external to the server system 1102 and may be accessed by multiple server systems 1102. For example, the storage device 1110 may include multiple storage units, such as hard disks or solid state disks in a redundant array of inexpensive disks (RAID) configuration. The storage device 1110 may include a storage area network (SAN) and / or a network attached storage (NAS) system.

[0142] In some aspects, the processor 1105 is operatively connected to the storage device 1110 via a storage interface 1120. The storage interface 1120 may be any component capable of providing the processor 1105 with access to the storage device 1110. The storage interface 1120 may include, for example, an Advanced Technology Attachment (ATA) adapter, a Serial ATA (SATA) adapter, a Small Computer System Interface (SCSI) adapter, a RAID controller, a SAN adapter, a network adapter, and / or any component that provides the processor 1105 with access to the storage device 1110.

[0143] Memory area 1010 (shown in FIG. 10) and storage device 1110 may include, but are not limited to, random access memory (RAM), such as dynamic RAM (DRAM) and static RAM (SRAM), read only memory (ROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and non-volatile RAM (NVRAM). The above memory types are merely examples and thus are not limiting of the types of memory that may be used to store computer programs.

[0144] The computer systems and computer-implemented methods described herein may include additional, fewer, or alternative operations and / or functions, including those described elsewhere herein. The computer systems include or are implemented via computer-executable instructions stored on a non-transitory computer-readable medium. The methods may be implemented via one or more local or remote processors, transceivers, servers, and / or sensors (e.g., processors, transceivers, servers, and / or sensors onboard a vehicle or mobile device or associated with a smart infrastructure or remote servers) and / or via computer-executable instructions stored on a non-transitory computer-readable medium.

[0145] As will be understood based on the description herein above, the above-described aspects of the disclosure can be implemented using computer programming or engineering techniques, including computer software, firmware, hardware, or any combination or subset thereof. Any such program having computer readable code means can be embodied or provided in one or more computer readable media, thereby producing a computer program product (i.e., an article of manufacture) according to the described aspects of the disclosure. The computer readable medium can be, for example, but not limited to, a fixed (hard) drive, a diskette, an optical disk, a magnetic tape, a semiconductor memory such as a read-only memory (ROM), and / or any transmission / reception medium, such as the Internet or other communications network or link. An article of manufacture including the computer code can be produced and / or used by executing the code directly from one medium, by copying the code from one medium to another medium, or by transmitting the code over a network.

[0146] These computer programs (also referred to as "programs", "software", "software applications", "applications", "code") contain machine instructions for a programmable processor and may be implemented by a high-level procedural and / or object-oriented programming language, and / or assembly / machine language. As used herein, the terms "machine-readable medium" and "computer-readable medium" refer to any computer program product, apparatus, and / or device (e.g., magnetic disks, optical disks, memory, programmable logic devices (PLDs)) used to provide machine instructions and / or data to a programmable processor, including machine-readable media that receive machine instructions as machine-readable signals. However, the terms "machine-readable medium" and "computer-readable medium" do not include transitory signals. The term "machine-readable signal" refers to any signal used to provide machine instructions and / or data to a programmable processor.

[0147] As used herein, a processor may include any programmable system, including systems that use microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuitry or processor capable of performing the functions described herein. The above examples are merely illustrative, and thus are not intended to limit in any way the definition and / or meaning of the term "processor."

[0148] As used herein, the terms "software" and "firmware" are used interchangeably and include any computer program stored in a memory for execution by a processor, such as, for example, RAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory. The above memory types are merely exemplary and thus are not limiting of the types of memory that may be used to store computer programs.

[0149] In one aspect, a computer program is provided, the program being embodied in a computer readable medium. In one aspect, the system runs on a single computer system without requiring connection to a server computer. In another aspect, the system runs in a Windows environment (Windows is a registered trademark of Microsoft Corporation, Redmond, Washington, USA). In yet another aspect, the system runs in a mainframe environment and a UNIX server environment (UNIX is a registered trademark of X / Open Company Limited, Reading, Berkshire, UK). The application is flexible and designed to run in a variety of environments without compromising its primary functionality.

[0150] In some embodiments, the system includes multiple components distributed among multiple computing devices. One or more of the components may be in the form of computer-executable instructions embodied in a computer-readable medium. The system and process are not limited to the specific embodiments described herein. Furthermore, each system and process component may be implemented separately and independently from other components and processes described herein. Each component and process may be used in combination with other assembly packages and processes. Aspects of the present invention may improve the functionality of a computer and / or computer system.

[0151] The definitions and methods set forth herein are provided to better define the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. Unless otherwise specified, terms should be understood according to conventional usage by those of ordinary skill in the relevant art.

[0152] In some embodiments, numbers expressing quantities of ingredients, properties, such as molecular weights, reaction conditions, and the like, used to describe and claim certain embodiments of the present disclosure should be understood in some instances to be modified by the term "about". In some embodiments, the term "about" is used to indicate that a value includes the standard deviation of the average for the device or method being employed to determine the value. In some embodiments, the numerical parameters set forth in this specification and the appended claims are approximations that can vary depending on the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed by taking into account the number of reported significant digits and applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the present disclosure are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as practicable. The numerical values ​​set forth in some embodiments of the present disclosure may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. The recitation of ranges of values ​​herein is merely intended to serve as a shorthand method of individually designating each individual value falling within the range. Unless otherwise specified herein, each individual value is incorporated into the specification as if it were individually set forth herein. A recitation of discrete values ​​is understood to include ranges between each value.

[0153] In some embodiments, the terms "a," "an," "the," and similar terms used in the context of describing particular embodiments (especially in the particular context of the claims below) can be construed to encompass both the singular and the plural, unless otherwise indicated. In some embodiments, the term "or" as used herein, including the claims, is used to mean "and / or," unless expressly indicated to refer to alternatives only or that the alternatives are mutually exclusive.

[0154] The terms "comprise," "have," and "include" are open-ended linking verbs. Any one or more of the variations or tenses of these verbs, such as "comprises," "comprising," "has," "having," "includes," and "including," are also open-ended. For example, any method that "comprises," "has," or "includes" one or more steps is not limited to having only those one or more steps, but may also include other unrecited steps. Similarly, any composition or device that "comprises," "has," or "includes" one or more features is not limited to having only those one or more features, but may also include other unrecited features.

[0155] All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. Any and all examples provided with respect to specific embodiments herein, or the use of exemplary language (e.g., "etc.") are intended merely to better illuminate the disclosure and do not impose limitations on the scope of the invention unless specifically recited in the claims. No language in this specification should be construed as indicating that any non-claimed element is essential to the practice of the disclosure.

[0156] Grouping of alternative elements or embodiments of the disclosure disclosed herein should not be construed as limiting. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements described herein. One or more members of a group may be included in or deleted from a group for reasons of convenience or patentability. When such inclusion or deletion is made, the specification is deemed to include the modified group and therefore fulfills all Markush group descriptions used in the appended claims.

[0157] All publications, patents, patent applications, and other documents cited in this application are incorporated herein by reference in their entirety for all purposes to the same extent as if each was specifically and individually indicated to be incorporated by reference in its entirety for all purposes. The citation of a document herein should not be construed as an admission that it is prior art to the present disclosure.

[0158] Although the present disclosure has been described in detail, it will be apparent that modifications, variations, and equivalents are possible without departing from the scope of the present disclosure as defined in the appended claims. Moreover, it should be understood that all examples in the present disclosure are provided as non-limiting examples.

Claims

1. 1. An acoustic sensor comprising: a whispering gallery mode optical resonator; a coupling waveguide for optically coupling to the optical resonator, the coupling waveguide being separated from the optical resonator by a separation gap; a polymer encapsulating a portion of the coupling waveguide and the optical resonator, the polymer filling the separation gap; the optical resonator and the coupling waveguide each have a refractive index higher than a refractive index of the polymer; The acoustic sensor, wherein the separation gap is sized such that with the polymer filling the separation gap, the acoustic sensor operates at a critical coupling between the optical resonator and the coupling waveguide or at a maximum load factor.

2. 2. The acoustic sensor according to claim 1, The optical resonator has a diameter in the range of 50 μm to 200 μm.

3. 2. The acoustic sensor according to claim 1, The acoustic sensor, wherein the separation gap has a distance in the range of 0.6 μm to 0.8 μm.

4. 2. The acoustic sensor according to claim 1, The acoustic sensor, wherein the separation gap is sized to provide a maximum load factor during operation of the acoustic sensor.

5. 2. The acoustic sensor according to claim 1, The acoustic sensor wherein the coupling waveguide is a tapered optical fiber having a minimum taper diameter of less than 1.5 μm.

6. 2. The acoustic sensor according to claim 1, The acoustic sensor is configured to encode pressure variations as variations in transmission amplitude through the optical resonator.

7. 2. The acoustic sensor according to claim 1, The acoustic sensor wherein the polymer is a UV curable polymer having a refractive index of 1.

33.

8. 1. An acoustic sensing system, comprising: a whispering gallery mode optical resonator; a coupling waveguide for optically coupling to the optical resonator, the coupling waveguide having a first end and an opposite second end and separated from the optical resonator by a separation gap; a polymer encapsulating a portion of the coupling waveguide and the optical resonator, the polymer filling the separation gap, the first end and the second end of the coupling waveguide protruding from the polymer; a light source coupled to the first end of the coupling waveguide; a photodetector coupled to the second end of the coupling waveguide; the optical resonator and the coupling waveguide each have a refractive index higher than a refractive index of the polymer; The acoustic sensing system, wherein the separation gap is sized such that with the polymer filling the separation gap, the acoustic sensing system operates at critical coupling between the optical resonator and the coupling waveguide or at a maximum load factor.

9. 9. The acoustic sensing system according to claim 8, a drive system including a computing device having a processor; the drive system is operatively connected to the light source and the light detector; The drive system comprises: obtaining a transmission spectrum by operating the light source over a range of wavelengths and receiving a number of detector signals from the photodetector, the detector signals encoding the transmission of light from the light source through a coupling fiber coupled to the optical resonator; selecting an operating wavelength for detecting pressure fluctuations based on the transmission spectrum; detecting pressure variations by operating the light source at the operating wavelength and receiving an additional number of detector signals from the light detector; The acoustic sensing system is configured to:

10. 10. The acoustic sensing system of claim 9, An acoustic sensing system configured to encode the pressure variations as variations in transmission amplitude through the optical resonator.

11. 1. An ultrasound imaging device comprising: Equipped with an acoustic sensor, The acoustic sensor includes: a whispering gallery mode optical resonator; a coupling waveguide for optically coupling to the optical resonator, the coupling waveguide having a first end and an opposite second end and separated from the optical resonator by a separation gap; a polymer encapsulating a portion of the coupling waveguide and the optical resonator, the polymer filling the separation gap, the first end and the second end of the coupling waveguide protruding from the polymer; a light source coupled to the first end of the coupling waveguide; a photodetector coupled to the second end of the coupling waveguide; the optical resonator and the coupling waveguide each have a refractive index higher than a refractive index of the polymer; An ultrasound imaging device, wherein the separation gap is sized such that with the polymer filling the separation gap, the acoustic sensor operates at critical coupling between the optical resonator and the coupling waveguide or at a maximum load factor.

12. 12. The ultrasound imaging device of claim 11, The ultrasonic imaging device, wherein the acoustic sensor is configured to encode pressure variations as variations in transmission amplitude through the optical resonator.

13. 12. The ultrasound imaging device of claim 11, An ultrasound imaging device, wherein the acoustic sensor is configured to detect ultrasound pulses generated within a region of interest in response to an excitatory ultrasound pulse directed at the region of interest by an ultrasound transducer.

14. 1. A photoacoustic imaging device, comprising: Equipped with an acoustic sensor, The acoustic sensor includes: a whispering gallery mode optical resonator; a coupling waveguide for optically coupling to the optical resonator, the coupling waveguide having a first end and an opposite second end and separated from the optical resonator by a separation gap; a polymer encapsulating a portion of the coupling waveguide and the optical resonator, the polymer filling the separation gap, the first end and the second end of the coupling waveguide protruding from the polymer; a transducer light source coupled to the first end of the coupling waveguide; a photodetector coupled to the second end of the coupling waveguide; the optical resonator and the coupling waveguide each have a refractive index higher than a refractive index of the polymer; An optoacoustic imaging device, wherein the separation gap is sized such that, with the polymer filling the separation gap, the acoustic sensor operates at critical coupling between the optical resonator and the coupling waveguide or at a maximum load factor.

15. 15. The photoacoustic imager of claim 14, An optoacoustic imaging device, wherein the acoustic sensor is configured to encode pressure variations as variations in transmission amplitude through the optical resonator.

16. 15. The photoacoustic imager of claim 14, An optoacoustic imaging device, wherein the acoustic sensor is configured to detect an optoacoustic signal generated within the region of interest in response to irradiation with a laser pulse generated by the transducer light source.

17. 1. A method for detecting ultrasound, comprising:

1. An acoustic sensor comprising: a whispering gallery mode optical resonator; a coupling waveguide for optically coupling to the optical resonator, the coupling waveguide having a first end and an opposite second end and separated from the optical resonator by a separation gap; a polymer encapsulating a portion of the coupling waveguide and the optical resonator, the polymer forming a sample contact surface and filling the separation gap, the first end and the second end of the coupling waveguide protruding from the polymer; a light source coupled to the first end of the coupling waveguide; a photodetector coupled to the second end of the coupling waveguide; providing an acoustic sensor, the optical resonator and the coupling waveguide each having a refractive index higher than the refractive index of the polymer, and the separation gap being sized such that with the polymer filling the separation gap, the acoustic sensor operates at a critical coupling between the optical resonator and the coupling waveguide or at a maximum load factor; acoustically coupling a sample to the sample contact surface of the polymer such that ultrasonic waves radiating from within the sample are conducted through the polymer to a portion of the coupling waveguide and to the optical resonator; introducing light generated by the light source into the first end of the coupling waveguide; detecting light exiting the second end of the coupling waveguide with the photodetector and converting the detected light into a detector signal by encoding the amplitude of the light; and converting the detector signal to pressure using a predetermined calibration rule.

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