Optical microcavity array devices for ultrasound sensing

Abstract

The present disclosure relates generally to the field of ultrasound, and more particularly to methods and devices that enable ultrasound sensing using an array of optical microresonators. The apparatus may include one or more optical fibers, one or more optical waveguides, and a plurality of resonator nodes arranged in an array of sensing locations. Each resonator node may include an optical coupling between the optical waveguide and an optical fiber having a set of resonant frequencies at the respective sensing location. Each resonator node may be further configured to communicate a set of signals corresponding to at least one shift in the set of resonant frequencies in the optical fiber at the respective sensing location.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 63 / 001,738, filed March 30, 2020, which is incorporated by reference in its entirety.

[0002] The present disclosure relates generally to the field of ultrasound, and more particularly to methods and devices that enable ultrasound sensing using arrays of optical microresonators. [Background technology]

[0003] Ultrasound sensing is used in a variety of industries, including medical imaging, due to several advantages. For example, ultrasound sensing utilizes ultrasound signals that have a significant penetration depth. Furthermore, ultrasound imaging is known to be an advantageous non-invasive form of imaging since it is based on non-ionizing radiation.

[0004] Conventional ultrasonic sensing uses piezoelectric materials such as lead zirconate titanate (PZT), polymer thick film (PTF) and polyvinylidene fluoride (PVDF). However, some of the challenges associated with using the piezoelectric properties of these materials include high operating voltage requirements, high electric field requirements (which can cause dielectric breakdown and failure), nonlinear response with high hysteresis, and limited detection angle. Thus, there is a need for new and improved devices and methods for ultrasonic sensing. Summary of the Invention [Means for solving the problem]

[0005] Generally, in some embodiments, a device may include one or more optical fibers, one or more optical waveguides, and a number of resonator nodes arranged in an array of sensing locations. Each resonator node may include an optical coupling between an optical waveguide and an optical fiber having a set of resonant frequencies at a respective sensing location. Each resonator node may be further configured to communicate a set of signals corresponding to a shift in the set of resonant frequencies in the optical fiber at the respective sensing location. In some embodiments, each optical fiber may have the same or substantially similar cross-sectional geometry and / or material uniformity such that the optical fibers may have the same or substantially similar set of resonant frequencies. By leveraging such uniform material properties (e.g., utilizing optical fibers produced in bulk), an ultrasonic sensing system including such a device may be more easily mass-produced in a cost-effective and consistent manner and have more consistent and predictable performance.

[0006] In some embodiments, the one or more optical fibers may be configured to receive a plurality of ultrasonic echoes. Further, the one or more optical fibers may be configured to experience a shift in a set of resonant frequencies in response to the plurality of ultrasonic echoes. In some embodiments, the one or more optical waveguides are configured to propagate a set of signals corresponding to at least one shift in the set of resonant frequencies to the optical detector.

[0007] In some embodiments, the one or more optical waveguides may include one or more tapered optical fibers or one or more integrated photonic waveguides (e.g., silicon photonic waveguides). The one or more tapered optical fibers may be in a polymer structure.

[0008] The one or more optical fibers may be arranged perpendicular to the one or more optical waveguides. For example, in some embodiments, the one or more optical fibers may be arranged in a straight line at a predetermined equidistant distance from each other and perpendicular to the one or more tapered optical fibers. Further, the one or more optical fibers and the one or more optical waveguides may be arranged in a rectangular grid.

[0009] In some embodiments, one or more optical waveguides may be coupled to a light source, which may propagate light within the one or more optical waveguides. For example, the light source may include a broadband light source or a tunable laser source.

[0010] The one or more optical fibers and the one or more optical waveguides may be arranged in any suitable manner that allows for optical coupling at each resonator node. For example, in some embodiments, the optical fibers may be in physical contact with the optical waveguides at the resonator nodes. Alternatively, in embodiments, there may be a short separation gap (e.g., a separation of about 1 μm or less) between the optical fibers and the optical waveguides at the resonator nodes. Furthermore, the optical waveguides and the optical fibers may be spaced apart in any suitable manner. For example, in some embodiments, the distance between the optical waveguides of the one or more optical waveguides may be at least about 20 times the wavelength of light from the light source. In some embodiments, the distance between the optical fibers of the one or more optical fibers may be less than about 3 times the wavelength of light.

[0011] Generally, in some embodiments, a method for ultrasonic sensing may include receiving, at one or more optical waveguides, a first set of signals corresponding to a first set of whispering gallery modes propagating along a circumference of one or more optical fibers, via a plurality (more than one) of resonator nodes. The method may further include receiving, at one or more optical waveguides, a second set of signals corresponding to a second set of whispering gallery modes propagating along a circumference of each optical fiber, via the plurality of resonator nodes. In some embodiments, the second set of whispering gallery modes may propagate in response to the one or more optical fibers receiving the plurality of ultrasonic echoes. The method may further include detecting a difference set between the first set of signals and the second set of signals. The method may further include calculating a magnitude of each ultrasonic echo at each resonator node based at least in part on the first set of signals, the second set of signals, and / or the difference set. The method may further include associating a magnitude of each ultrasonic echo with a sensing location of each resonator node. In some embodiments, the method may further include transmitting a plurality of ultrasonic signals using a plurality of piezoelectric elements. The method may further include receiving a plurality of ultrasonic echoes corresponding to the plurality of ultrasonic signals in one or more optical fibers, and the plurality of resonator nodes may be configured to perform a synthetic aperture (SA) operation or a compressed sensing (CS) operation.

[0012] In some embodiments, the one or more optical fibers may include multiple optical fibers having the same or substantially similar cross-sectional geometry and materials to have the same or substantially similar set of resonant frequencies. In some embodiments, the one or more optical fibers may be perpendicular to the one or more optical waveguides. In some embodiments, the one or more optical waveguides may include one or more tapered optical fibers and / or one or more integrated photonic waveguides, or another suitable waveguide that may be coupled to a light source to propagate light from the light source. In some embodiments, the one or more optical fibers and / or the one or more optical waveguides may be in a polymer structure.

[0013] The one or more optical fibers and the one or more optical waveguides may be arranged in any suitable manner that allows for optical coupling at each resonator node. For example, in some embodiments, the optical fibers may be in physical contact with the optical waveguides at the resonator nodes. Alternatively, in embodiments, there may be a short separation gap (e.g., a separation of about 1 μm or less) between the optical fibers and the optical waveguides at the resonator nodes. Furthermore, the optical waveguides and the optical fibers may be spaced apart in any suitable manner. For example, in some embodiments, the distance between the optical waveguides of the one or more optical waveguides may be at least about 20 times the wavelength of light from the light source. In some embodiments, the distance between the optical fibers of the one or more optical fibers may be less than about 3 times the wavelength of light.

[0014] Generally, in some embodiments, an apparatus may include one or more optical fibers and one or more optical waveguides optically coupled to the one or more optical fibers at a plurality of resonator nodes. A circumference of each optical fiber may be configured to propagate a first set of whispering gallery modes. In some embodiments, the one or more optical fibers communicate a first set of signals corresponding to the first set of whispering gallery modes to the one or more optical waveguides. The one or more optical waveguides may be configured to propagate the first set of signals to at least one optical detector.

[0015] In some embodiments, the one or more optical fibers may include multiple optical fibers having the same or substantially similar cross-sectional geometry and materials to have the same or substantially similar set of resonant frequencies. In some embodiments, the one or more optical fibers may be perpendicular to the one or more optical waveguides. In some embodiments, the one or more optical waveguides may include one or more tapered optical fibers and / or one or more integrated photonic waveguides, or another suitable waveguide that may be coupled to a light source to propagate light from the light source. In some embodiments, the one or more optical fibers and / or the one or more optical waveguides may be in a polymer structure.

[0016] The one or more optical fibers and the one or more optical waveguides may be arranged in any suitable manner that allows for optical coupling at each resonator node. For example, in some embodiments, the optical fibers may be in physical contact with the optical waveguides at the resonator nodes. Alternatively, in embodiments, there may be a short separation gap (e.g., a separation of about 1 μm or less) between the optical fibers and the optical waveguides at the resonator nodes. Furthermore, the optical waveguides and the optical fibers may be spaced apart in any suitable manner. For example, in some embodiments, the distance between the optical waveguides of the one or more optical waveguides may be at least about 20 times the wavelength of light from the light source. In some embodiments, the distance between the optical fibers of the one or more optical fibers may be less than about 3 times the wavelength of light.

[0017] In some embodiments, the one or more optical fibers are configured to receive the plurality of ultrasonic echoes and to propagate a second set of whispering gallery modes in response to the plurality of ultrasonic echoes. The one or more optical fibers are configured to communicate a second set of signals corresponding to the second set of whispering gallery modes to the one or more optical waveguides. In some embodiments, the one or more optical waveguides are configured to propagate the second set of signals to at least one optical detector.

[0018] In some embodiments, one or more optical fibers may be configured to communicate, through the multiple resonator nodes, to one or more optical waveguides, a set of signals corresponding to a difference between the first set of whispering gallery modes and the second set of whispering gallery modes, e.g., the difference between the first set of whispering gallery modes and the second set of whispering gallery modes comprises at least one of a shift in a resonance frequency of the optical fiber and / or a damping of a resonance of the one or more optical fibers. The present invention provides, for example, the following: (Item 1) An apparatus comprising: one or more optical fibers; one or more optical waveguides; a plurality of resonator nodes arranged at an array of sensing locations, each resonator node including an optical coupling between an optical waveguide and an optical fiber having a set of resonant frequencies at a respective sensing location; An apparatus, wherein each resonator node is configured to communicate a set of signals corresponding to at least one shift in said set of resonant frequencies induced by an ultrasonic echo in an optical fiber at a respective sensing location. (Item 2) Item 1, wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material. (Item 3) the one or more optical fibers receiving a plurality of ultrasonic echoes; and experiencing the at least one shift in the set of resonant frequencies in response to the plurality of ultrasonic echoes; 2. The apparatus of claim 1, wherein the one or more optical waveguides are configured to propagate a set of signals corresponding to the at least one shift in the set of resonant frequencies to a photodetector. (Item 4) Item 1 . The apparatus of item 1 , wherein the one or more optical waveguides comprise a tapered optical fiber or an integrated photonic waveguide. (Item 5) Item 1 , wherein the one or more optical fibers are within a polymer structure. (Item 6) Item 1 , wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure. (Item 7) Item 2. The device of item 1, wherein the one or more optical fibers are perpendicular to the one or more optical waveguides. (Item 8) 2. The apparatus of claim 1, wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are in physical contact. (Item 9) 2. The apparatus of claim 1, wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are offset by a separation gap. (Item 10) 2. The device of claim 1, wherein each of the one or more optical waveguides is coupled to a light source, such that the one or more optical waveguides propagate light from the light source. (Item 11) 10. The apparatus of claim 9, wherein the one or more optical waveguides comprise a plurality of optical fibers arranged in parallel, the plurality of optical fibers being spaced apart by a distance of at least about five times the wavelength of the light. (Item 12) 10. The apparatus of claim 9, wherein the one or more optical fibers comprise a plurality of optical fibers arranged in parallel, the plurality of optical fibers being spaced apart by a distance of at least less than about three times the wavelength of the light. (Item 13) 1. A method of ultrasonic sensing comprising: receiving, via a plurality of resonator nodes in one or more optical waveguides, a first set of signals corresponding to a first set of whispering gallery modes propagating along a circumference of the one or more optical fibers; receiving, in the one or more optical waveguides, through the plurality of resonator nodes, a second set of signals corresponding to a second set of whispering gallery modes propagating along a circumference of the one or more optical fibers, the second set of whispering gallery modes propagating in response to the one or more optical fibers receiving a plurality of ultrasonic echoes; detecting a set of differences between the first set of signals and the second set of signals. (Item 14) Item 14. The method of item 13, wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material. (Item 15) calculating a magnitude of each ultrasonic echo at each resonator node based at least in part on the set of differences between the first set of signals and the second set of signals; 14. The method of claim 13, further comprising associating the magnitude of each ultrasonic echo with a sensing location of each resonator node. (Item 16) Item 14. The method of item 13, wherein the one or more optical waveguides are configured to propagate at least one of the first set of signals and the second set of signals to a photodetector. (Item 17) transmitting a plurality of ultrasonic signals to a target; 14. The method of claim 13, further comprising receiving, with the one or more optical fibers, the plurality of ultrasonic echoes from the target corresponding to the plurality of ultrasonic signals, the one or more optical fibers being configured to perform a synthetic aperture (SA) operation or a compressed sensing (CS) operation. (Item 18) Item 14. The method of item 13, wherein the one or more optical waveguides comprise one or more tapered optical fibers or one or more integrated photonic waveguides. (Item 19) Item 14. The method of item 13, wherein the one or more optical fibers are within a polymer structure. (Item 20) 20. The method of claim 19, wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure. (Item 21) Item 14. The method of item 13, wherein each optical fiber is perpendicular to each optical waveguide. (Item 22) Item 14. The method of item 13, wherein the one or more optical waveguides are coupled to a light source, the light source propagating light within the one or more optical waveguides. (Item 23) Item 14. The method of item 13, wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are in physical contact. (Item 24) Item 14. The method of item 13, wherein in one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are offset by a separation gap. (Item 25) Item 14. The method of item 13, wherein the one or more optical waveguides are aligned with the one or more optical fibers to excite the first set of whispering gallery modes or the second set of whispering gallery modes in the one or more optical fibers. (Item 26) An apparatus comprising: one or more optical fibers, the circumference of each optical fiber being configured to propagate a set of whispering gallery modes; one or more optical waveguides, the one or more optical fibers optically coupled to the one or more optical fibers at a plurality of resonator nodes to communicate a set of signals corresponding to the set of whispering gallery modes to the one or more optical waveguides; the one or more optical waveguides are configured to propagate the set of signals to at least one optical detector. (Item 27) the one or more optical fibers receiving a plurality of ultrasonic echoes; propagating a second set of whispering gallery modes in response to the plurality of ultrasonic echoes; communicating a second set of signals corresponding to the second set of whispering gallery modes to the one or more optical waveguides; 27. The apparatus of claim 26, wherein the one or more optical waveguides are configured to propagate the second set of signals to the at least one optical detector. (Item 28) 28. The apparatus of claim 27, wherein the set of whispering gallery modes is a first set of whispering gallery modes, and the one or more optical fibers are configured to communicate a set of signals corresponding to a difference between the first set of whispering gallery modes and the second set of whispering gallery modes to the one or more optical waveguides through the plurality of resonator nodes. (Item 29) 30. The apparatus of claim 28, wherein the difference between the first set of whispering gallery modes and the second set of whispering modes is at least one of: at least one shift in a resonance frequency of the optical fiber and attenuation of a resonance of the one or more optical fibers. (Item 30) Item 27. The apparatus of item 26, wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material. (Item 31) Item 27. The apparatus of item 26, wherein the one or more optical waveguides comprise one or more tapered optical fibers or one or more integrated photonic waveguides. (Item 32) Item 27. The apparatus of item 26, wherein the one or more optical fibers are within a polymer structure. (Item 33) Item 33. The device of item 32, wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure. (Item 34) Item 33. The apparatus of item 32, wherein the one or more optical fibers are perpendicular to the one or more optical waveguides. (Item 35) Item 33. The apparatus of item 32, wherein each of the one or more optical waveguides is coupled to a light source, such that the one or more optical waveguides propagate light from the light source. [Brief description of the drawings]

[0019] [Figure 1A] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 1B] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 1C] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 1D] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 2A] 1 is a schematic illustration of an exemplary method for packaging an optical microresonator array. [Figure 2B] 1 is a schematic illustration of an exemplary method for packaging an optical microresonator array. [Figure 3A] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 3B] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 3C] 1 is a schematic illustration of an exemplary optical microresonator array. [Figure 4] 1 is a schematic illustration of an exemplary method for sensing a set of ultrasound echoes using an optical microresonator array device. [Diagram 5] 1 is a schematic illustration of an exemplary method for measuring the optical response of an optical microresonator array device. [Figure 6] 1 is a schematic illustration of an exemplary method for measuring the optical response of an optical microresonator array device. [Figure 7] 1 is a schematic illustration of an exemplary ultrasound probe. [Figure 8] 1 is a schematic illustration of an exemplary ultrasound probe. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Non-limiting examples of various aspects and variations of the present invention are described herein and illustrated in the accompanying drawings.

[0021] Exemplary optical microresonator arrays and methods for making the same are described herein. Furthermore, as described herein, such optical microresonator arrays can be combined to form optical microresonator arrays having high quality factors and various other beneficial characteristics, as described below, for use in applications such as ultrasound sensing and / or ultrasound imaging. Optical microcavity array

[0022] Described herein are optical microresonator arrays suitable for high sensitivity applications, including high sensitivity acousto-optic sensing systems. For example, as shown in FIG. 1A, an exemplary optical microresonator array 100A may include a set (e.g., a plurality) of optical fibers 103A, a set (e.g., a plurality) of optical waveguides 102A, and a plurality of resonator nodes 104A arranged in an array of sensing locations. Each resonator node 104A may comprise an optical coupling between the optical waveguides and the optical fibers at a respective sensing location. Each optical fiber may have a set of resonant frequencies, and each resonator node 104A may be further configured to communicate a set of signals corresponding to at least one shift in the set of resonant frequencies in the optical fiber at the respective sensing location. At each resonator node, a cross section of the optical fiber may function as an optical microresonator (e.g., a whispering gallery mode microresonator) configured to communicate a set of signals corresponding to at least one shift in the set of resonant frequencies of the optical fiber. These signals are associated with a respective sensing location for that resonator node.

[0023] Each optical microresonator of the optical microresonator array includes a closed loop of transparent medium that allows several allowed frequencies of light to propagate continuously within the closed loop and stores the optical energy of the allowed frequencies of light within the closed loop. In this manner, the optical microresonator moves around the periphery of the optical microresonator, allowing the propagation of whispering gallery modes (WGMs) corresponding to the allowed frequencies to circulate around the circumference of the optical microresonator. Each mode from the WGMs may correspond to the propagation of a frequency of light from the allowed frequencies of light, for example.

[0024] The optical microresonator arrays described herein have high sensitivity due at least in part to having a high quality factor, in that the optical microresonator array advantageously allows allowed frequencies of light to remain within the closed loop of the optical microresonator array for a long time. The allowed frequencies of light and the quality factor of the optical microresonator arrays described herein may be based at least in part on the geometric parameters of the optical microresonator array, the refractive index of the transparent medium, and the refractive index of the environment surrounding the optical microresonator array.

[0025] As further described herein, optical microresonator arrays can be configured to receive light, transmit light, and be practically useful (e.g., for ultrasound imaging or other sensing applications in acousto-optical systems). Acousto-optical systems based on optical microresonator arrays can directly measure ultrasound waves through the photoelastic effect and / or physical deformation of the optical microresonator array in response to ultrasound waves (e.g., ultrasound echoes). For example, in the presence of ultrasound waves (or any pressure waves), WGMs traveling through the optical microresonator array can undergo a set of one or more spectral shifts caused by changes in the refractive index and / or shape of the optical microresonator array. The spectral changes can be easily monitored and analyzed in spectral ranges and optical transmission intensity to and from the optical microresonator array. Furthermore, additional spatial and other information can be derived by monitoring and analyzing the WGMs shifting between multiple optical microresonator arrays.

[0026] In some embodiments, optical microresonator arrays may include optical fibers having the same or substantially similar cross-sectional characteristics (e.g., cross-sectional geometry and / or material properties) along their length and / or relative to each other, as described further herein. Thus, the availability of bulk-produced optical fibers may be leveraged to fabricate optical microresonator arrays in an efficient and highly reproducible manner suitable for mass production. Furthermore, using such similar bulk-produced optical fibers (and / or optical waveguides) in optical microresonator arrays may result in more consistent and reliable performance in ultrasound sensing. Thus, the optical microresonator arrays described herein offer several advantages over microresonator arrays formed from conventional techniques, where stand-alone microresonators must be painstakingly formed individually before being combined into an array, and further may exhibit inconsistent or unreliable performance as a result of variations between individual microresonators, and / or require complex micro- or nano-fabrication techniques to trim the resonant wavelengths of individual microresonators. Furthermore, for embodiments of optical microresonator arrays having optical fibers with identical or very close WGM resonance conditions and resonant frequencies, it may be possible to use a single-frequency laser or other single-frequency light source to simultaneously probe or excite all of the WGMs in the optical microresonator array, thereby simplifying the operation of the sensing microresonator array.

[0027] Alternatively, in some embodiments, at least some of the optical fibers may have varying and known cross-sectional characteristics (e.g., different radii, different material profiles) and therefore exhibit different resonant frequencies that are taken into account during analysis of the signals resulting from the shifting WGMs. Optical Fibers and Optical Waveguides

[0028] The set of optical fibers 103A in the optical microresonator array may be made of a material (e.g., glass, polymer, crystal, etc.) that is transparent to the frequency of light propagating inside the set of optical fibers. The set of optical fibers 103A may include, for example, one or more sets of single mode optical fibers, one or more sets of multimode optical fibers, one or more sets of graded index optical fibers, one or more step index optical fibers, one or more sets of polarization-maintaining optical fibers, and / or any optical fiber that is suitable for supporting whispering gallery modes at its circumference. In some embodiments, the set of optical fibers may include commercially available fibers that collectively have the same or significantly similar properties, for example, with respect to size, refractive index, and / or resonant wavelength. For example, the set of single mode optical fibers may have a highly uniform material refractive index of 1.5 and a diameter of 125 μm at any vertical cross section of the set of single mode optical fibers.

[0029] In some embodiments, optical fiber set 103A can be selected and / or etched to have a small radius. The smaller the radius of the optical fiber set, the greater the free spectral range of the set of resonant frequencies. As a result, the spectral density of the resonant modes supported by the radius of optical fiber set 103A is reduced, which can improve the dynamic range of the detection performance of optical microresonator array 100A.

[0030] In some embodiments, the set of optical waveguides 102A may include a set of integrated photonic waveguides made of glass, silicon, silicon nitride, and / or any material transparent to the frequency of light propagating inside the set of optical fibers. For example, the set of optical waveguides 102A may include a set of strip waveguides, slot waveguides, slab waveguides, strip-loaded slot waveguides, photonic crystal waveguides, and / or integrated photonic waveguides suitable for supporting the propagation of light over the length of an optical microresonator array.

[0031] Additionally or alternatively, the set of optical waveguides 102A may include a set of tapered optical fibers produced from optical fibers. For example, at least a portion of the set of optical waveguides 102A may be produced from a set of single mode optical fibers, multimode optical fibers, graded index optical fibers, step index optical fibers, polarization maintaining optical fibers, etc. Generally, in some embodiments, the set of tapered optical fibers may be produced by gradually stretching the optical fibers while being heated. In doing so, the optical fibers become thinner over a certain length. Additionally or alternatively, in some embodiments, at least a portion of the set of tapered fibers may be produced by etching (e.g., using wet etching) the cladding of the optical fibers in whole or in part.

[0032] In some embodiments, the optical fiber sets and / or tapered optical fiber sets may be etched to reduce their size. For example, the optical fiber sets and / or tapered optical fiber sets may be etched using a chemical solution (e.g., hydrochloric acid) to reduce their length and / or radius. Predetermined portions of the optical fiber sets and / or tapered optical fiber sets are exposed to the chemical solution such that only the portions exposed to the chemical solution are etched.

[0033] The system may further include one or more light sources. For example, the set of light guides 102A may be coupled to the set of one or more light sources to input light 101A from the set of light sources into the set of light guides 102A and optically coupled optical fibers 103A. The light then propagates azimuthally along each optical fiber to excite the WGMs of the resonator nodes, as further described herein. Additionally, the light may additionally couple out of the optical fibers 103A back into the light guides 102A.

[0034] The one or more light sources may include broadband light sources, tunable laser sources, optical frequency comb (OFC) laser sources using either digital modulation or Kerr four-wave mixing (FWM) methods, and / or any other light sources suitable for the operating frequency band of the optical microresonator array 100A. In some embodiments, the set of one or more light sources may include single frequency light sources configured to propagate light 101A having identical spectral characteristics into the set of optical waveguides 102A. In some embodiments, at least one light source may include a fiber laser source that launches the input light 101A directly into a set of optical fibers that may couple the input light 101A into the set of integrated photonic waveguides. In some embodiments, the set of light sources may include a set of chip-based laser sources that launch the set of input light 101A directly into a set of integrated photonic waveguides. The set of integrated photonic waveguides may then be configured to couple the set of input light into a set of tapered fibers. Additionally, in some embodiments, one or more light sources may be coupled into a slab or planar waveguide, as described in more detail below.

[0035] The optical fibers and optical waveguides may be arranged in any of a variety of suitable manners. For example, as shown in FIG. 1A, optical microresonator array 100A may include a tapered optical fiber (functioning as optical waveguide 102A) stacked on optical fiber 103A to form a plurality of resonator nodes 104A. Alternatively, at least a portion of optical fiber 103A may be stacked on top of the tapered optical fiber (waveguide 102A), as described in more detail below.

[0036] As another example, one or more optical fibers may be disposed on an integrated photonic waveguide platform, such as a silicon photonic platform, a silicon nitride platform, etc., as shown in Figures 1B-1D. Figure 1B is a schematic illustration of an exemplary integrated photonic optical waveguide array 100B. In some embodiments, the integrated photonic platform may include a substrate 107B (e.g., silicon, silica, silicon nitride, etc.), a buried oxide layer 106B, and other integrated photonic components. The optical waveguide array 100B may be coupled to a set of one or more light sources. In some embodiments, the set of light sources may be fiber-coupled light sources or other light sources that emit light 101B into a fiber that is aligned perpendicular to the on-chip grating coupler 102B. The grating coupler 102B in combination with other integrated photonic components (e.g., a multimode interference device 103B) may couple light into the optical waveguide array 100B having one or more optical waveguides 108B. 1B, input light from a single light source may be coupled into multiple light guides 108B via a splitting or branching pattern. In some embodiments, at least a portion of the light guide array 100B may be coated with an encapsulation layer 105B (e.g., spin coated with a matching polymer), while other portions of the light guide array 100B may remain uncoated with the encapsulation layer 105B (e.g., by selectively etching the encapsulation layer).

[0037] FIG. 1C is a schematic illustration of another exemplary optical microresonator array 100C in which a plurality of resonator nodes 104C are positioned at a set of sensing locations. The optical microresonator array 100C may include an optical waveguide array 102C having a plurality of optical waveguides 108C disposed on an integrated planar optical platform (similar to the optical waveguide array 100B described above with respect to FIG. 1B) and a set of optical fibers 103C optically coupled to the optical waveguide array 102C at the set of sensing locations to establish a plurality of resonator nodes 104C. Each resonator node may have a predetermined position relative to other resonator nodes to determine the location of an acoustic echo detected by the resonator node. Similar to that described above with respect to FIG. 1B, the optical waveguide array 102C may include integrated photonic waveguides fabricated on a substrate 105C. In some embodiments, the optical waveguide array may include multiple light sources that couple input light at the input waveguide 101C, each of which may in turn be connected to the multiple optical waveguides 108C (e.g., via a beam splitter or fiber coupler). It should be understood that the embodiment shown in FIG. 1C includes a single optical input for three optical waveguides, however, other embodiments may include any suitable ratio of optical inputs to the optical waveguides 108C (e.g., about 1:2, 1:3, 1:4, etc.). Each of the optical waveguides 108C in the optical waveguide array 102C may be perpendicular to a set of optical fibers 103C to generate multiple resonator nodes 104C. As described further herein, the multiple resonator nodes 104C may couple light into the set of optical fibers 103C to propagate through a set of WGMs and generate a set of optical signals into the optical waveguide array 102C. The optical waveguide array can be connected to a set of optical detectors and a multi-channel optical spectrum analyzer to characterize a set of optical signals for ultrasound sensing.

[0038] FIG. 1D is a schematic illustration of another optical microresonator array 100D in which a set of resonator nodes 104D are positioned at a set of sensing locations. The optical microresonator array 100D may include an optical waveguide array 102D having a plurality of optical waveguides 108D disposed on an integrated planar optical platform (similar to the optical waveguide array 100B described above with respect to FIG. 1B) and a set of optical fibers 103D optically coupled to the optical waveguide array 102D at the set of sensing locations to establish a plurality of resonator nodes 104D. Each resonator node may have a predetermined position relative to the other resonator nodes to determine the location of the acoustic echo detected by the resonator node. As shown in FIG. 1D, each optical waveguide 108D from the optical waveguide array 102D may be optically coupled to a respective optical input 101D (each of which may be coupled to a respective light source, for example, or at least some of the optical inputs may provide light from a common light source). In other words, the ratio of optical inputs to optical waveguides 108D may be 1:1. Each of the optical waveguides 108D in the optical waveguide array 102D may be perpendicular to the set of optical fibers 103D to generate a plurality of resonator nodes 104D. As described further herein, the plurality of resonator nodes 104D may couple light into the set of optical fibers 103D to propagate through the set of WGMs and generate a set of optical signals into the optical waveguide array 102D. The optical waveguide array may be connected to a set of optical detectors and a multi-channel optical spectrum analyzer to characterize the set of optical signals for ultrasound sensing. Resonator Node

[0039] The system may include a set of resonator nodes at various sensing locations where optical waveguides and optical fibers are optically coupled. For example, Figure 1A is a schematic illustration of an optical microresonator array 100A in which a resonator node 104A is positioned at a sensing location where an optical fiber 103A and an optical waveguide 102A are optically coupled. Each resonator node may have a predetermined position relative to other optical microresonators for determining the location of a detected acoustic echo.

[0040] In some embodiments, the input light 101A couples into a set of optical fibers 103A, which causes a set of whispering gallery modes (WGMs) to propagate around the inner circumference of the optical fibers 103A in the multiple resonator nodes 104A. Each resonator node may have an associated sensing coordinate. The multiple resonator nodes 104A may receive a set of ultrasonic echoes spatially distributed at various intensities across the multiple resonator nodes 104A of the optical microresonator array. In some embodiments, each resonator node may have a predefined position relative to other resonator nodes to determine the location of the acoustic echo detected by the resonator node. In some other cases, each resonator node may have a characteristic geometry and / or material refractive index associated with the resonator node that may distinguish the optical response of the resonator node from other resonator nodes. Sensing using the resonator nodes is described in more detail below.

[0041] The resonator nodes 104A may be arranged in various suitable types of arrays and in various suitable manners with respect to the set of optical fibers and the set of optical waveguides. In some embodiments, the set of optical fibers 103A may be perpendicular to the optical waveguides 102A to allow light from the optical waveguides 102A to circulate around a cross section of the optical fibers 130A. For example, the optical fibers may be arranged perpendicular to the optical waveguides using an alignment procedure. The alignment procedure may include propagating a first set of light having a first set of intensities to the set of optical waveguides 102A. The alignment procedure may further include detecting a second set of light having a second set of intensities. The alignment procedure may include adjusting a set of angles between the set of optical fibers 103A and the set of optical waveguides 102A (e.g., by using a high precision positioning system) to maximize the second set of intensities. In one example, adjusting the set of angles is to achieve a perpendicular angle between each optical fiber and the optical waveguide. In some cases, the alignment procedure may provide a gap between the optical fibers and the optical waveguides to achieve a coupling condition that results in the highest power coupled into the resonator node. In some other cases, the optical fibers may be in physical contact with the optical waveguides to improve the stability of the coupling condition. A right angle between the set of optical fibers 103A and the set of optical waveguides 102A may allow light to couple out of the set of optical fibers 103A and into the set of optical waveguides 102A, and vice versa. In some embodiments, the alignment procedure may include verifying perpendicularity between the set of optical fibers 103A and the set of optical waveguides 102A by verifying optical coupling between the optical fibers and the optical waveguides.

[0042] In some embodiments, the resonator nodes may be arranged in a lattice, such as a rectangular array, formed from optical fibers and optical waveguides arranged in a lattice. For example, the sets of waveguides may also be arranged parallel and equidistant to one another, and the optical fibers may be arranged parallel and equidistant to one another and perpendicular to the sets of waveguides. However, in some embodiments, the optical fibers may be irregularly spaced and / or the waveguides may be irregularly spaced.

[0043] In some embodiments, an optical microresonator array may include multiple optical waveguides and multiple optical fibers as shown in FIG. 1A, although it should be understood that multiple resonator nodes may also be formed from only a single optical waveguide or only a single optical fiber.

[0044] For example, as shown in FIG. 3A, in some embodiments, an optical microresonator array may include multiple resonator nodes formed by a single optical waveguide 302A optically coupled to multiple optical fibers 303A and 303A′ at multiple sensing locations. In some cases, a set of light having multiple different wavelengths may propagate within the single optical waveguide 302A. In some other cases, a single wavelength of light may propagate within the single optical waveguide 302A. In some other cases, a broadband light 301A may propagate within the single optical waveguide 302A. The single optical waveguide 302A may be perpendicularly and optically coupled to optical fibers 303A and 303A′ at multiple distinct sensing locations on the length of the optical waveguide, thereby forming first and second resonator nodes 304A and 304A′. Although two resonator nodes are illustrated in FIG. 3A, it should be understood that any suitable number of resonator nodes may be formed along a single optical waveguide by coupling any suitable number of optical fibers (e.g., three, four, five, or more) to the optical waveguide. An optical input 301A may be coupled and propagated around optical fibers 303A and 303A', respectively, to excite WGMs at resonator nodes 304A and 304A', respectively. An optional signal embodying the WGM and any shift of the WGM may then be coupled from the resonator node into an optical fiber and provided as output light 305A.

[0045] As another example, as shown in FIG. 3B, in some embodiments, an optical microresonator array may include multiple resonator nodes formed by multiple optical waveguides 302B and 302B' coupled to a single optical fiber 303B at multiple sensing locations. In some cases, a first input light 301B and / or a second input light 301B', each having a different wavelength, may propagate within each of the optical waveguides. In some other cases, a first single-wavelength light 301B may propagate within one optical waveguide (e.g., optical waveguide 302B) and a second single-wavelength light 301B' may propagate within another optical waveguide (e.g., optical waveguide 302B'). The optical waveguides 302B and 302B' may be perpendicular to and optically coupled to the single optical fiber 303B at multiple distinct sensing locations on the length of the optical fiber, thereby forming first and second resonator nodes 304B and 304B'. In some embodiments, optical waveguide 302B and optical waveguide 302B' may have identical radii and material uniformity profiles, and therefore exhibit identical WGM and resonant frequencies at sets of resonator nodes 304B and 304B. Although two resonator nodes are illustrated in FIG. 3B, it should be understood that any suitable number of resonator nodes may be formed along a single optical fiber by coupling any suitable number of optical waveguides (e.g., three, four, five, or more) to the optical fiber.

[0046] Optical inputs 301B and 301B' may be coupled to and propagated along optical fiber 303B to excite WGMs at resonator nodes 304B and 304B', respectively. Optional signals embodying the WGMs and any shifts of the WGMs may then be coupled from the resonator nodes into the optical fiber and provided as output light 305B and 305B'.

[0047] Sensing at Resonator Nodes For simplicity, sensing at a resonator node in an optical microresonator array is described below with respect to a single resonator node 304C shown in FIG. 3C. When an optical waveguide 301C is aligned and positioned to be optically coupled to a single optical fiber 303C, a resonator node 304C is formed at the sensing location. Multiple resonator nodes may be located at multiple sensing locations or coordinates throughout an optical microresonator array. For example, in some embodiments, a sensing location may be located at an intersection of an optical waveguide and an optical fiber.

[0048] The optical waveguide 302C may be located at the top of the optical fiber 303C or at the bottom of the optical fiber 303C (or laterally offset from the tangent by any suitable tangent or separation gap). The optical waveguide 302C may be optically coupled to the optical fiber 303C anywhere along the length of the single optical fiber 303C. The longitudinal axis of the optical waveguide 302C may be perpendicular to the longitudinal axis of the optical fiber 303C. The optical fiber 303C may be characterized by certain geometric and material characteristics, such as, for example, fiber radius, fiber surface roughness, and / or fiber material refractive index, each of which may affect the impedance, scattering losses, and / or absorption losses of the single optical fiber 303C that affect the set of signals provided by the resonator nodes.

[0049] Additionally, generally, the resonator node 304C has a characteristic set of resonant frequencies determined by the geometric and material properties of the features located at the sensing location. These geometric and material properties at the sensing location may be affected by the set of ultrasonic echoes received at the sensing location, thereby shifting the resonant frequencies and / or attenuating the resonant peaks or dips of the set of resonant frequencies of the resonator node 304C. Considering multiple resonator nodes across the optical microresonator array, the received ultrasonic echoes may affect the geometric and / or material properties of the multiple resonator nodes with different intensities at different sensing coordinates. Therefore, the ultrasonic echoes may shift the set of resonant frequencies and / or attenuate the resonant peaks / dips of the set of resonant frequencies of the multiple resonator nodes by different amounts indicating different intensities of the set of ultrasonic echoes.

[0050] During use in sensing, the optical waveguide 302C may receive and propagate light 301C from a light source. The light may be, for example, single wavelength light (e.g., a 532 nanometer laser), broadband light (e.g., an erbium-doped fiber amplifier), and / or multi-wavelength light (e.g., a frequency comb). The light may be coupled into the optical fiber 304C at the resonator node 304C to excite a set of WGMs that propagate azimuthally around the circumference of the optical fiber. The set of WGMs may result in a set of signals 305C in the optical waveguide 302C. The set of signals may include a set of resonant features that are characteristic of a single resonator node 304C, a single optical waveguide 302C, and / or a single optical fiber 303C.

[0051] The single resonator node 304C may then receive a set of ultrasonic echoes that mechanically vibrate the optical waveguide 302C, the optical fiber 303C, the resonator node 304C, and / or the materials inside and / or outside these components. Thus, the geometric features and / or material refractive index of the optical waveguide 302C, the optical fiber 303C, the resonator node 304C, and / or the materials inside and / or outside these components may experience a change associated with the change in the WGM relative to the resonant node. The change may be small or large compared to the default value of each geometric feature and / or material refractive index. Even a small change in each geometric feature and / or material refractive index may significantly affect the resonant feature and result in a detectable signal. In one illustrative example, the change in refractive index due to Δn=0.01 of the single optical fiber 303C may be two orders of magnitude smaller than the refractive index n=1.5 of the single optical fiber 303C. However, this change in refractive index may be sufficient to shift the resonant feature by a frequency amount comparable to the full width at half maximum of the resonant feature.

[0052] In other words, the set of one or more shifts in the resonance characteristics in response to receiving the ultrasonic echo may correspond to a shift in the propagation of the set of optical signals (e.g., related to the difference between a first set of WGMs and a second set of WGMs in response to the ultrasonic echo). The set of shifts in the resonance characteristics may be a plurality of shifts (i.e., a continuum of shifts, or shifts at each optical frequency). The optical signals may be transmitted to an optical detector or an optical spectrum analyzer and characterized. For example, a suitable computing device may be operatively coupled to the optical detector and / or the optical spectrum analyzer to detect the difference in the optical signals before and after receiving the ultrasonic echo. In some embodiments, the sensing signals may be read out from different optical fibers coupled by the same optical waveguide, and a timing mechanism may be implemented by synchronizing the optical readout time with a physical perturbation in the different optical fibers at the output end of the optical waveguide.

[0053] Furthermore, in some embodiments, if the different optical fibers have different cross-sectional properties (e.g., radii, materials) and therefore different resonant frequencies, light having different wavelengths can be transmitted along the same optical waveguide and used to probe WGMs in the different optical fibers. The optical signals read at the output end of the optical waveguide can be separated according to the transmitted wavelengths associated with each different optical fiber.

[0054] In one example, the optical microresonator array may include a set of resonator nodes having a resonant frequency of 193 THz and / or a resonant dip of 10% transmission corresponding to the resonant frequency. The optical microresonator array may have a first sensing coordinate of (x=1, y=1) of 0.03 W / cm 2 and 0.1 W / cm at the second sensing coordinates of (x=3, y=4). 2 For example, x may indicate the position of the optical fiber and y may indicate the position of the optical waveguide. A first resonator node located at a first sensing coordinate of (x=1, y=1) may receive an ultrasonic echo having an intensity of 0.03 W / cm 2 The resonant frequency of 193 THz may shift to 194 THz due to an ultrasonic echo having an intensity of 0.1 W / cm. Meanwhile, the second resonator node located at the second sensing coordinate of (x=3, y=4) may shift to 0.1 W / cm. 2 The resonant frequency of 193 THz may shift to 198 THz due to an ultrasonic echo having an intensity of 193 THz. As demonstrated in this example, the magnitude of the shift in the resonant frequency of each resonator node may be related to the magnitude of the shift in the intensity of the ultrasonic echo. By mapping the magnitude of the resonance shift to sense coordinates, ultrasonic imaging may be performed based on the ultrasonic echo.

[0055] The resonator nodes described herein have high sensitivity due at least in part to having a high quality factor, in that the resonator nodes advantageously allow allowed frequencies of light to remain within the closed loop of the optical microresonator for a long time. The allowed frequencies of light and quality factors of the optical microresonators described herein may be based at least in part on the geometric and material parameters (e.g., extinction coefficient, refractive index, defects, uniformity) of the set of optical fibers, the geometric parameters of the waveguides, the refractive index of the set of fibers and the set of waveguides, the refractive index of the environment surrounding the set of fibers and the set of waveguides, etc. Packaging of optical microcavity arrays

[0056] The performance of an optical microresonator array depends at least in part on the elastic and elastic-optical properties of the materials in the device. As shown in Figures 2A and 2B, the optical microresonator array can be packaged (e.g., in a polymer structure) to further enhance the capabilities of the optical microresonator array. Figure 2A is a schematic illustration of an exemplary method of packaging a set of optical fibers 203A and / or a set of optical waveguides 204A in a structure (e.g., in a polymer structure) to produce a packaged optical microresonator array. The structure can include a backing region 202A (e.g., a backing polymer) and / or a matching region 205A (e.g., a matching polymer). In some embodiments, the backing region 202A and / or the matching region 205A can be layers deposited (e.g., spin-coated) on a substrate, as described in more detail below. Additionally, the mechanical stability of the optical microresonator array may also be improved, at least in part, by packaging the optical microresonator array within a material structure, such as, for example, a soft material having a low refractive index (lower than the refractive index of at least the set of optical waveguides 204A and the set of optical fibers 203A).

[0057] The method of packaging the optical microresonator array may include cleaning the surface of a substrate 201A, such as, for example, silicon, silica, quartz, plastic, or any other material suitable to serve as a substrate for an acousto-optic sensor device. In some cases, the substrate may be selected to have a higher etch rate compared to other materials used for the optical microresonator array, so that the substrate may be etched away after packaging the optical microresonator array. Suitable materials to serve as a substrate may include a damping material to eliminate residual vibrations and minimize ultrasonic echoes within the sensor structure. Referring to FIG. 2A, the substrate 201A may then be coated with one or more coating materials, including a backing polymer 202A. The backing polymer 202A may be a damping material, such as a polymer material with a low refractive index, configured to obtain a broad whispering gallery mode (WGM) frequency response at the circumference of the set of optical fibers 203A, while having high ultrasonic attenuation to prevent reverberation of the set of ultrasonic echoes that the optical microresonator array is designed to sense. In some embodiments, the acoustic impedance of the backing polymer damping material may be matched to the acoustic impedance of a layer of matching polymer 205A utilized to package the optical resonator array, for example, as described further below.

[0058] As shown in FIG. 2A, a set of optical waveguides 204A, such as tapered optical fibers, may be disposed within the backing polymer 202A or the matching polymer 205A. As described above, the set of optical waveguides 204A may include a set of tapered optical fibers receiving a set of light from a fiber-coupled light source, a set of integrated photonic waveguides receiving a set of light from an on-chip light source, a set of integrated photonic waveguides coupled to a set of tapered optical fibers propagating a set of light from a fiber-coupled light source, or any other suitable medium for propagating a set of light from a light source to a set of resonator nodes that optically couple the set of optical fibers 203A and the set of optical waveguides 204A. In some embodiments, the set of optical fibers 203A may be disposed within the backing polymer 202A prior to curing the backing polymer 202A. Disposing the set of optical fibers 203A on an uncured backing polymer may result in the set of optical fibers being embedded within the backing polymer 202A. After placement of the set of optical waveguides 204A, the backing polymer may be hardened, such as by baking the backing polymer at a preset temperature.

[0059] The method of packaging an optical microresonator array may further include disposing set of optical waveguides 204A proximate set of optical fibers 203A so as to optically couple the set of optical fibers to the set of optical waveguides. For example, set of optical fibers 203A and set of optical waveguides 204A may be positioned (e.g., by using a high precision positioning system) such that the set of optical waveguides is perpendicular to the optical set of optical fibers (e.g., as described above with respect to FIG. 1). Although the method of packaging an optical microresonator array has been primarily described in terms of disposing the optical fibers in front of the optical waveguides, it should be understood that in some embodiments the optical waveguides may be disposed in front of the optical fibers.

[0060] FIG. 2B illustrates an exemplary fabrication process for an optical microresonator array (such as the optical microresonator array 100C described with respect to FIG. 1C). As shown in FIG. 2B, the optical microresonator array can be packaged (e.g., in a polymer structure) based on an integrated photonic platform to further enhance the capabilities of the optical microresonator array. FIG. 2B is a schematic illustration of an exemplary method for packaging a set of optical fibers 203B and / or a set of integrated photonic waveguides 204B in a structure (e.g., in a polymer structure) to produce a packaged optical microresonator array. The structure can include a backing region 202B (e.g., a backing polymer) and / or a matching region 205B (e.g., a matching polymer). In some embodiments, the backing region 202B and / or the matching region 205B can be layers deposited (e.g., spin-coated) on a substrate, as described in more detail below.

[0061] A method for packaging an optical microresonator array may include cleaning (e.g., using a piranha solution) the surface of a substrate 201B, such as silicon, silica, quartz, plastic, or any other material suitable to serve as a substrate for an acousto-optic sensor device. In some cases, the substrate may be selected for a higher etch rate compared to other materials used for the optical microresonator array, so that the substrate may be etched away to release the optical microresonator array. A suitable material to serve as a substrate may include a damping material to eliminate residual vibrations and minimize ultrasonic echoes within the sensor structure. Referring to FIG. 2B, the substrate 201B may then be coated with one or more coating materials, including a backing polymer 202B. The backing polymer 202B may be a damping material, such as a polymer material with a low refractive index, configured to obtain a broad whispering gallery mode (WGM) frequency response at the circumference of the set of optical fibers 203B, while having high ultrasonic attenuation to prevent reverberation of the set of ultrasonic echoes that the optical microresonator array is designed to sense. In some embodiments, the acoustic impedance of the backing polymer damping material may be matched to the acoustic impedance of a layer of matching polymer 205B utilized to package the optical resonator array, for example, as described further below.

[0062] As shown in FIG. 2B, a set of optical waveguides 204B, for example, a set of such integrated photonic waveguides, may be fabricated on top of the substrate 201B. In some embodiments, the set of optical waveguides 204B may be fabricated using lithography techniques from a computer-aided design (CAD) pattern. The material of the set of optical waveguides 204B on the integrated photonic platform may include silicon, silicon oxide, silicon nitride, or any material suitable for reliably and efficiently guiding optical signals. The set of optical waveguides 204B on the integrated photonic platform may receive a set of light from an on-chip light source or a fiber-coupled light source. Furthermore, the set of optical waveguides 204B may be configured to couple to other sets of integrated photonic components (e.g., beam splitters, interferometers, multimode interference devices, etc.).

[0063] In one example, the set of light sources can be single-wavelength fiber-coupled light sources that emit single-wavelength light into a fiber that is aligned perpendicular to an on-chip grating coupler. The grating coupler changes the wave vector of the single-wavelength light and directs it into an integrated photonic waveguide. The integrated photonic waveguide propagates the single-wavelength light to a multimode interference device that splits the single-wavelength light into multiple integrated photonic waveguides. The multiple integrated photonic waveguides couple the single-wavelength light into a set of optical fibers 203B to propagate the set of WGMs within their circumferences and generate a set of optical signals into the multiple integrated photonic waveguides.

[0064] In some embodiments, the set of optical waveguides 204B may be deposited or coated with an encapsulation layer 206B, which may, for example, improve mechanical stability to the optical microresonator array. Generally, in some embodiments, the encapsulation layer 206B may be the same material as the backing polymer 202B or the matching polymer 205B. The method of packaging an optical microresonator array may further include disposing the set of optical fibers 203B proximate to the set of optical waveguides 204B so as to optically couple the set of optical fibers 203B to the set of optical waveguides 204B. For example, the set of optical fibers 203B and the set of optical waveguides 204B may be positioned (e.g., by using a high precision positioning system) such that the set of optical waveguides is aligned perpendicular to the set of optical fibers (e.g., as described above with respect to FIG. 1). Although the method of packaging an optical microresonator array has been primarily described with the placement of the optical waveguides in front of the optical fibers, it should be understood that in some embodiments, the optical fibers may be placed in front of the optical waveguides. EXAMPLES

[0065] Optical microresonator arrays such as those described herein may be used in ultrasonic sensor devices, such as acousto-optic sensor devices. FIG. 4 is a schematic illustration of sensing ultrasonic echoes using an acousto-optic sensor device 403. The acousto-optic sensor device 403 includes an optical microresonator array that includes a plurality of resonator nodes 410. Although the set of optical waveguides and the set of optical fibers of the optical microresonator array are shown as being linearly arranged at a predetermined distance from each other, it should be understood that in some embodiments, at least some of the optical waveguides and / or optical fibers may be arranged at different distances from each other. Generally, however, the set of optical waveguides and the set of optical fibers may be arranged in a rectangular grid.

[0066] A set of optical waveguides carrying light from one or more light sources may be optically coupled to a set of optical fibers at resonator nodes at various sensing locations, as described above. Each optical fiber and each optical waveguide in the optical microresonator array may be characterized by a particular cross-sectional shape, material uniformity, a particular refractive index of the material from which the optical fiber or optical waveguide is made, and the refractive index of the packaging material, as described in more detail above with respect to Figures 2A and 2B. The light source may be a single wavelength light source, a broadband light source, a tunable laser source, an optical frequency comb (OFC) laser source using either digital modulation or Kerr four-wave mixing (FWM) methods, or any other light source suitable for the operating frequency band of the acousto-optic sensor device 403. Light may be coupled into the optical microresonator array to propagate a first set of whispering gallery modes (WGMs) around the circumferential walls of the set of optical fibers at the set of resonator nodes 410 where the set of optical fibers and the set of optical waveguides are optically coupled (e.g., intersect). Propagation of the first set of WGMs results in the generation of a first set of optical signals corresponding to the first set of resonant frequencies of the WGMs. In some cases, the distance between the set of optical waveguides is greater than five wavelengths of light. In some other cases, the distance between the set of optical fibers is greater than three wavelengths of light.

[0067] In use, the acousto-optic sensor device 403 may be configured to receive a set of ultrasonic echoes 402 generated and / or reflected from an object 401. The set of ultrasonic echoes may have varying travel times (t1, t2, t3, etc.) to the individual resonator nodes. That is, ultrasonic echoes from the same object arrive at each sensor in the array at slightly different times. With the known distance between each sensor and these measured slight delays, the location of the object is better calculated (e.g., with better spatial resolution). The set of ultrasonic echoes may induce a series of changes in the geometry, material uniformity, refractive index of the material from which the optical fiber or optical waveguide is made, and / or refractive index of the packaging material. The series of changes may introduce changes in the WGMs, e.g., a second set of WGMs, propagating around the walls of the set of optical fibers at the set of resonator nodes of the optical microresonator array. The propagation of the second set of WGMs results in the generation of a second set of optical signals corresponding to a second set of resonant frequencies of the WGMs. The first set of optical signals and the second set of optical signals may be configured to propagate in a set of optical waveguides to a set of optical detectors and / or a set of optical spectrum analyzers. The optical detectors may be connected to a computing device to detect a difference between the first set of optical signals and the second set of optical signals. In some embodiments, the difference between the first set of optical signals and the second set of optical signals may allow for measurement of a change in resonant frequency or frequency of a set of resonator nodes. In some other embodiments, the difference between the first set of optical signals and the second set of optical signals may additionally or alternatively allow for measurement of a change in resonant amplitude of a resonant frequency of a set of resonator nodes.

[0068] In some embodiments, the acousto-optical sensor device 403 may be configured to receive a continuum of ultrasonic echoes 402 generated and / or reflected from the object 401. The continuum of ultrasonic echoes may induce a continuum of changes in the geometry, material uniformity, and / or the refractive index of the material from which the optical fiber or optical waveguide is made, and / or the refractive index of the packaging material. The continuum of changes may introduce a continuum of WGMs (e.g., a second set of WGMs, a third second set of WGMs, a fourth set of WGMs, etc.) propagating around the walls of the set of optical fibers at the set of resonator nodes of the optical microresonator array. The propagation of the continuum of WGMs results in the generation of a continuum of optical signals corresponding to the continuum of resonant frequencies of the WGMs. The continuum of optical signals may be configured to propagate within the set of optical waveguides to a set of optical detectors and / or a set of optical spectrum analyzers. The optical detectors may be connected to a computing device to detect a continuum of differences between each pair of optical signals from the continuum of optical signals. The continuum of differences may allow the measurement of a continuum of changes in the resonant amplitude of the resonant frequencies of the set of resonator nodes.

[0069] FIG. 5 illustrates an exemplary measurement setup that may be used to process a set of signals from an acousto-optic sensor device 505 that senses a set of ultrasonic echoes 504 (also referred to herein as "ultrasound signals"). The measurement setup may include a single wavelength light source, a broadband light source, or a tunable laser 501 that propagates a set of light in an optical fiber 502, a fiber polarization controller 503 that may be configured to control the polarization of the light. The measurement setup may further include an acousto-optic sensor device 505 that may be configured to generate a first set of optical signals based on the propagation of a first set of whispering gallery modes (WGMs) in a set of resonator nodes (described above with respect to FIG. 1A) contained within the acousto-optic device 505. The acousto-optic sensor device 505 may be configured to receive the set of ultrasonic signals 504 and generate a second set of optical signals based on the propagation of a second set of WGMs in the set of resonator nodes. The acousto-optic sensor device 505 may be further configured to transmit the first set of optical signals and / or the second set of optical signals to the optical fiber 502. The measurement setup may further include a photodetector (also referred to herein as “optical detector”) 506 that receives the first set of optical signals and / or the second set of optical signals and converts them into a first set of electrical signals and / or a second set of electrical signals. The photodetector 506 may be further configured to transmit the first set of electrical signals and / or the second set of electrical signals to an oscilloscope 507 operatively connected to a computing device 510 for processing and analyzing the first set of signals and / or the second set of signals. The oscilloscope 507 may be further configured to transmit the set of analyzed signals to a function generator 508. The function generator may be configured to generate and transmit a set of generated signals to a broadband light source or tunable laser 501 to control the propagation of light in the optical fiber 502.

[0070] In another exemplary measurement setup, the function generator 508 of Figure 5 may be replaced by a locking system, such as a Pound-Drever-Hall (PDH) locking system, to set the wavelength of the light source to the resonance slope of the WGM. The photodetector 506 will receive an oscillating optical signal in response to a set of ultrasonic echoes.

[0071] FIG. 6 illustrates an exemplary measurement setup that may be used to process a set of signals from an acousto-optic sensor device 604 that senses a set of ultrasonic echoes 604. The measurement setup may include a laser 601, such as an optical frequency comb (OFC) laser source generated using either a digital modulation method or a Kerr four-wave mixing (FWM) method, or any other laser suitable for the operating frequency band of the acousto-optic sensor device 604, to generate a laser light. The measurement setup may further include a beam splitter 602, such as a 50:50 beam splitter, an 80:20 beam splitter, or any other beam splitter suitable for the measurement setup. The beam splitter may be configured to split the laser light into a first laser light and a second laser light. The measurement setup may be configured to direct the first laser light to an electro-optic modulator 603 to generate and transmit a modulated laser light. The measurement setup may be further configured to direct the modulated laser light to an acousto-optic sensor device 604 to generate a first set of signals corresponding to a first set of whispering gallery modes (WGMs). The acousto-optic sensor device 604 may be configured to receive the set of ultrasonic signals and generate a second set of signals corresponding to a second set of WGMs. The measurement setup may be further configured to direct the first set of signals, the second set of signals, and / or the second laser light to a coherent receiver 605 to mix the second laser light with the first set of signals and / or the second set of signals to generate electronic frequency signals. The measurement setup may be configured to transmit the electronic frequency signals to a Fast Fourier Transform (FFT) module 606 operably connected to a computing device to process and analyze differences between the first set of signals and the second set of signals. In one example, the OFC laser may generate a set of comb beams, and each comb beam from the set of comb beams may generate a data point for detecting differences between the first set of signals and the second set of signals. In this example, one set of comb beams can generate a set of data points corresponding to the set of comb beams. In some embodiments, the use of an OFC laser source can advantageously reduce sensing times to sensing times on the order of milliseconds to microseconds.

[0072] In the above examples, the optical microresonator array was used for ultrasound detection, but in some embodiments, the optical microresonator array may be used for nanoparticle detection, biomolecule detection, etc. In some cases, exposing a set of resonator nodes to nanoparticles and / or biomolecules may change the environment surrounding the resonator nodes at the intersection of a set of optical fibers and a set of optical waveguides. The change in the environment may result in a change in whispering gallery modes (WGMs) propagating in the set of resonator nodes, generating a second WGM. The change between the first set of optical signals associated with the first WGM and the second set of optical signals associated with the second WGM may be detected by an optical detector and / or an optical spectrum analyzer.

[0073] In some embodiments, an optical microresonator array as described herein may be included in an ultrasound probe (also referred to herein as an "ultrasound system"). The ultrasound probe may include at least one optical waveguide for propagating light. The ultrasound probe may further include a plurality of piezoelectric elements for generating a set of ultrasound signals. The ultrasound probe may further include a plurality of piezoelectric elements for receiving a set of ultrasound echoes corresponding to the set of ultrasound signals. The ultrasound probe may further include a set of resonator nodes within the polymer structure. Each resonator node has a preassigned sensing location at an intersection of an optical waveguide and an optical fiber. Each resonator node is configured to receive a plurality of ultrasound echoes and propagate a set of whispering gallery modes (WGMs). The set of optical fibers is coupled to the set of optical waveguides at the set of resonator nodes such that the set of optical fibers is configured to communicate a first set of signals corresponding to the first set of WGMs to the set of optical waveguides.

[0074] For example, FIG. 7 is a schematic illustration of an ultrasound probe utilizing an optical microresonator array as described herein. FIG. 8 is a cross-sectional view of the probe illustrated in FIG. 7. The ultrasound probe may include an optical microresonator array 801, a matching layer 802, a piezoelectric crystal array 803, an electrical connection array 804, a set of optical fibers 805, a backing material 806, a sound insulation material 807, and a cable 808 connected to ultrasound data acquisition. The ultrasound probe may be coupled to a control system and a display to perform operational procedures as described in further detail herein. The backing material may include a damping material coupled to the back of the probe's packaging to eliminate residual vibrations of the operating environment, improve portability, and minimize echoes generated within the sensor structure.

[0075] The ultrasonic probe may be configured to propagate light from a light source to a set of optical microresonators array 801 to generate a first set of optical signals corresponding to a first set of WGMs propagating in each optical fiber at a resonator node of the optical microresonator array 801. The piezoelectric crystal array 803 may be configured to receive electrical signals from a control system via cable 808 and electrical connection array 804 to generate a set of ultrasonic signals directed toward the object such that a set of reflected ultrasonic echoes are generated in a direction toward the ultrasonic probe. The ultrasonic probe may be further configured to generate a second set of optical signals corresponding to a second set of WGMs in the optical resonator array 801 after receiving the set of ultrasonic echoes. The ultrasonic probe may optionally be configured to receive the set of ultrasonic echoes to generate a set of electrical signals. The ultrasonic probe may be further configured to transmit the first set of signals and / or the second set of signals via the set of optical fibers 805 and / or optionally transmit the set of electrical signals via the electrical connection array 804 to a cable 808 connected to an ultrasonic data acquisition, control system, or display.

[0076] In some embodiments, the ultrasound probe can be configured to repeat the motion sequence while scanning across the field of view by using a phased array of piezoelectric crystal arrays. By doing so, for each resonator node, a line-by-line image and one low-resolution image in the lateral direction are acquired using the piezoelectric crystal array. Known synthetic aperture (SA) algorithms can then be used to generate a high-resolution optical resonator array sensor image.

[0077] Further, in some embodiments, one or more of the piezoelectric elements may be configured to receive ultrasonic echoes corresponding to the set of transmitted ultrasonic signals and generate a sensor signal based on these received ultrasonic echoes. For example, the sensor signals generated by the piezoelectric elements may supplement or be combined with the signals communicated by the resonator nodes in any suitable manner (e.g., to provide a multimodal sensor image).

[0078] Additionally or alternatively, in some embodiments, the ultrasound probe may be configured to use different patterns of excitation, such as using a group of piezoelectric crystal elements or crystals to transmit an ultrasound signal, while using all the piezoelectric crystal elements in a piezoelectric crystal array to receive a set of ultrasound echoes, also known as compressive sensing (CS) techniques. The general approach of the CS technique is to form a linear model (also known as a forward model) that represents the process of signal acquisition from an image, and solve a linear equation to obtain an image.

[0079] In the foregoing description, for purposes of explanation, specific nomenclature was used to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required to practice the present invention. Thus, the foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously, many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to explain the principles of the invention and its practical application, thereby enabling those skilled in the art to utilize the invention and its various embodiments with various modifications as suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.

Claims

1. An apparatus, comprising: one or more optical fibers; one or more optical waveguides; a plurality of resonator nodes arranged in an array of a plurality of sensing locations, each resonator node including an optical coupling between an optical waveguide and an optical fiber, the one or more optical fibers having a set of resonant frequencies at a respective sensing location, and one or more optical inputs coupled to the one or more optical fibers propagate along a corresponding circumference of the one or more optical fibers by exciting a set of whispering gallery modes at the plurality of resonator nodes; Equipped with each resonator node configured to communicate a set of signals corresponding to at least one shift in the set of resonant frequencies induced by ultrasonic echoes in the optical fiber at each of the plurality of sensing locations.

2. The apparatus of claim 1 , wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material.

3. The one or more optical fibers receiving a plurality of ultrasonic echoes; experiencing the at least one shift in the set of resonant frequencies in response to the plurality of ultrasonic echoes; The device is configured to: The apparatus of claim 1 , wherein the one or more optical waveguides are configured to propagate a set of signals corresponding to the at least one shift in the set of resonant frequencies to a photodetector.

4. The device of claim 1 , wherein the one or more optical waveguides comprise tapered optical fiber or integrated photonic waveguides.

5. The device of claim 1 , wherein the one or more optical fibers are within a polymer structure.

6. The device of claim 1 , wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure.

7. The apparatus of claim 1 , wherein the one or more optical fibers are perpendicular to the one or more optical waveguides.

8. The apparatus of claim 1 , wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are in physical contact.

9. The apparatus of claim 1 , wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are offset by a separation gap.

10. The apparatus of claim 1 , wherein each of the one or more optical waveguides is coupled to a light source, the one or more optical waveguides transmitting light from the light source.

11. 10. The apparatus of claim 9, wherein the one or more optical waveguides comprise a plurality of optical fibers arranged in parallel, the optical fibers being spaced apart by a distance of at least about five times the wavelength of the light.

12. 10. The apparatus of claim 9, wherein the one or more optical fibers comprise a plurality of optical fibers arranged in parallel, the plurality of optical fibers being spaced apart by a distance at least less than about three times the wavelength of the light.

13. 1. A method of ultrasonic sensing comprising: providing one or more optical fibers and one or more optical waveguides, the one or more optical waveguides arranged to form a plurality of resonator nodes, each resonator node including an optical coupling between an optical waveguide and an optical fiber; excit[pi]ng a first set of whispering gallery modes via one or more optical inputs coupled to the one or more optical fibers, the first set propagating along corresponding circumferences of the one or more optical fibers; receiving, via the plurality of resonator nodes in the one or more optical waveguides, a first set of signals corresponding to the first set of whispering gallery modes propagating along a circumference of the one or more optical fibers; receiving, in the one or more optical waveguides, via the plurality of resonator nodes, a second set of signals corresponding to a second set of whispering gallery modes propagating around a circumference of the one or more optical fibers, the second set of whispering gallery modes propagating in response to the one or more optical fibers receiving a plurality of ultrasonic echoes; detecting a set of differences between said first set of signals and said second set of signals; The method includes:

14. The method of claim 13 , wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material.

15. The method according to claim 1, calculating a magnitude of each ultrasonic echo at each resonator node based at least in part on the set of differences between the first set of signals and the second set of signals; associating said magnitude of each ultrasonic echo with a sensing location of each resonator node; The method of claim 13 further comprising:

16. The method of claim 13 , wherein the one or more optical waveguides are configured to propagate at least one of the first set of signals and the second set of signals to a photodetector.

17. The method according to claim 1, transmitting a plurality of ultrasonic signals to a target; receiving, with the one or more optical fibers, the plurality of ultrasonic echoes from the target corresponding to the plurality of ultrasonic signals, the one or more optical fibers configured to perform a synthetic aperture (SA) operation or a compressed sensing (CS) operation; The method of claim 13 further comprising:

18. The method of claim 13 , wherein the one or more optical waveguides comprise one or more tapered optical fibers or one or more integrated photonic waveguides.

19. The method of claim 13 , wherein the one or more optical fibers are within a polymer structure.

20. The method of claim 19 , wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure.

21. The method of claim 13 , wherein each optical fiber is perpendicular to each optical waveguide.

22. The method of claim 13 , wherein the one or more light guides are coupled to a light source, the light source propagating light within the one or more light guides.

23. The method of claim 13 , wherein at one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are in physical contact.

24. The method of claim 13 , wherein in one or more of the plurality of resonator nodes, the optical waveguide and the optical fiber are offset by a separation gap.

25. 14. The method of claim 13, wherein the one or more optical waveguides are aligned with the one or more optical fibers to excite the first set of whispering gallery modes or the second set of whispering gallery modes in the one or more optical fibers.

26. An apparatus, comprising: one or more optical fibers, the circumference of each optical fiber being configured to propagate a set of whispering gallery modes; one or more optical waveguides optically coupled to the one or more optical fibers to form a plurality of resonator nodes, wherein one or more optical inputs coupled to the one or more optical fibers excite the set of whispering gallery modes in the plurality of resonator nodes, and the one or more optical fibers communicate a set of signals corresponding to the set of whispering gallery modes to the one or more optical waveguides; Equipped with the one or more optical waveguides are configured to propagate the set of signals to at least one optical detector.

27. The one or more optical fibers receiving a plurality of ultrasonic echoes; propagating a second set of whispering gallery modes in response to the plurality of ultrasonic echoes; communicating a second set of signals corresponding to the second set of whispering gallery modes to the one or more optical waveguides; The device is configured to:

27. The apparatus of claim 26, wherein the one or more optical waveguides are configured to propagate the second set of signals to the at least one optical detector.

28. 28. The apparatus of claim 27, wherein the set of whispering gallery modes is a first set of whispering gallery modes, and the one or more optical fibers are configured to communicate, through the plurality of resonator nodes to the one or more optical waveguides, a set of signals corresponding to a difference between the first set of whispering gallery modes and the second set of whispering gallery modes.

29. 30. The apparatus of claim 28, wherein the difference between the first set of whispering gallery modes and the second set of whispering modes is at least one of: at least one shift in a resonance frequency of the optical fiber and attenuation of a resonance of the one or more optical fibers.

30. 27. The apparatus of claim 26, wherein the one or more optical fibers comprise a plurality of optical fibers having the same or substantially similar cross-sectional geometry and material.

31. 27. The apparatus of claim 26, wherein the one or more optical waveguides comprise one or more tapered optical fibers or one or more integrated photonic waveguides.

32. 27. The apparatus of claim 26, wherein the one or more optical fibers are within a polymer structure.

33. The apparatus of claim 32 , wherein the one or more optical fibers and the one or more optical waveguides are within a polymer structure.

34. 33. The apparatus of claim 32, wherein the one or more optical fibers are perpendicular to the one or more optical waveguides.

35. 33. The apparatus of claim 32, wherein each of the one or more optical waveguides is coupled to a light source, the one or more optical waveguides transmitting light from the light source.

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