Whispering Gallery Mode Resonators for Sensing Applications

Abstract

Sensing devices and methods for fabricating the sensing devices are disclosed herein. In some variations, the sensing device can include at least one optical waveguide and at least one whispering gallery mode (WGM) resonator (1602a-c) configured to propagate a set of WGMs, and the WGM resonator (1602a-c) transmits a set of signals corresponding to the set of WGMs to the at least one optical waveguide (1601a-c). In some variations, a polymer structure can encapsulate the at least one WGM resonator and / or the at least one optical waveguide. Furthermore, in some variations, the WGM resonator can have one or more selectable modes with different bandwidths and sensitivities for sensing, which can enable the sensing device to be tailored to specific applications, for example, having certain bandwidth and / or sensitivity requirements.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 62 / 945,538, filed December 9, 2019, which is incorporated by reference in its entirety.

[0002] The present disclosure relates generally to the field of sensing platforms, and more particularly to methods and devices that use whispering gallery mode (WGM) resonators for ultrasonic sensing. [Background technology]

[0003] Sensing applications using ultrasonic sensing are widely favored due to the advantages it offers. For example, in the field of medical imaging, ultrasonic imaging is known to be an advantageous non-invasive form of imaging.

[0004] Conventional ultrasonic sensing uses piezoelectric materials such as lead zirconate titanate (PZT), polymer thick film (PTF) and polyvinylidene fluoride (PVDF). These materials have several drawbacks. For example, some of the challenges associated with using piezoelectric 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.

[0005] Thus, there is a need for new and improved devices and methods for a variety of sensing applications, including ultrasonic sensing. Summary of the Invention [Means for solving the problem]

[0006] In general, in some variations, the device may include at least one optical waveguide, one or more whispering gallery mode (WGM) resonators (e.g., a plurality of WGM resonators), and a polymer structure. Each WGM resonator may be configured to propagate a first set of WGMs. The polymer structure may encapsulate the at least one optical waveguide and at least one WGM resonator of the plurality of WGM resonators. The at least one WGM resonator may be optically coupled to the at least one optical waveguide such that the at least one WGM resonator can transmit a first set of signals corresponding to the first set of WGMs to the at least one optical waveguide.

[0007] In some variations, the polymer structure may include a backing region and / or a matching region. The backing region may be configured to attenuate residual ultrasonic echoes to prevent reverberation. The matching region may be configured to increase the bandwidth of the WGM frequency response of one or more WGM resonators. Additionally, the matching region may be further configured to improve ultrasonic transmission to one or more WGM resonators.

[0008] In some variations, the at least one optical waveguide may be embedded within the backing region. Alternatively, the at least one optical waveguide may be embedded within the matching region. The backing region and / or the matching region may have a refractive index lower than that of the at least one optical waveguide.

[0009] In some variations, the at least one WGM resonator may be embedded within a matching region of the polymer structure. The effective refractive index of the polymer structure may be lower than the refractive index of the at least one WGM resonator. In some variations, the polymer structure includes an ultrasound enhancing material. The effective acoustic impedance of the polymer structure may be matched to the effective acoustic impedance of the at least one WGM resonator.

[0010] In some variations, in response to receiving the ultrasonic echoes, the at least one WGM resonator may be configured to propagate a second set of WGMs. The at least one WGM resonator may transmit a second set of signals corresponding to the second set of WGMs to the at least one optical waveguide. The device may further include a photodetector communicatively coupled to the at least one optical waveguide. The at least one optical waveguide may be configured to propagate the first set of signals and the second set of signals to the photodetector. In some variations, a system including the device and a plurality of array elements (e.g., piezoelectric sensors, single crystal material sensors, piezoelectric micromachined ultrasonic transducers (PMUTs), and capacitive micromachined ultrasonic transducer sensors (CMUTs)) may be configured to generate a set of ultrasonic signals. The ultrasonic echoes may correspond to the set of ultrasonic signals.

[0011] In some variations, the at least one optical waveguide may include an optical fiber or an integrated photonic waveguide. In some variations, the at least one optical waveguide may be coupled to a light source. The light source may include a broadband light or a tunable laser source.

[0012] In some variations, the at least one WGM resonator may be a microsphere or microbubble resonator. In some variations, the at least one WGM resonator may include a hollow chamber (e.g., a microbubble resonator). The hollow chamber may include an ultrasound enhancing material.

[0013] In some variations, each WGM resonator of the plurality of WGM resonators can be optically coupled to at least one optical waveguide. In some variations, the at least one optical waveguide can include two or more optical waveguides (e.g., multiple optical waveguides). Each WGM resonator can be optically coupled to a respective optical waveguide.

[0014] In some variations, at least some of the WGM resonators may be grouped in a linear arrangement. The WGM resonators may be equally or unequally spaced apart. In other variations, one or more WGM resonators may be bundled together in a circular arrangement. In some variations, at least some of the WGM resonators of the one or more WGM resonators may be of equal size. Alternatively, at least some of the WGM resonators may be of unequal size.

[0015] In general, a method of ultrasound imaging includes receiving a first set of signals from at least one optical waveguide. The first set of signals may correspond to a first set of WGMs propagating in one or more WGM resonators (e.g., a plurality of WGM resonators). The method may also include receiving a second set of signals from the at least one optical waveguide, the second set of signals may correspond to a second set of WGMs propagating in the one or more WGM resonators in response to the one or more WGM resonators receiving a plurality of ultrasonic echoes. The method also includes detecting a difference set between the first set of signals and the second set of signals. The at least one optical waveguide and the plurality of WGM resonators may be polymer structures.

[0016] In some variations, the polymer structure may include a backing region and / or a matching region. The backing region may be configured to attenuate residual ultrasonic echoes to prevent reverberation. The matching region may be configured to increase the bandwidth of the WGM frequency response of one or more WGM resonators. In addition, the matching region may also improve ultrasonic transmission to one or more WGM resonators.

[0017] In some variations, the at least one optical waveguide may be in the backing region. Alternatively, the at least one optical waveguide may be in the matching region. The backing region and / or the matching region may have a refractive index lower than that of the at least one optical waveguide. In some variations, the one or more WGM resonators may be in the matching region of the polymer structure. The effective refractive index of the polymer structure may be lower than that of the one or more WGM resonators.

[0018] In some variations, the polymer structure may include an ultrasound enhancing material. The effective acoustic impedance of the polymer structure may match the acoustic impedance of the one or more WGM resonators. At least one WGM resonator of the plurality of WGM resonators may include at least one of a microsphere and a microbubble.

[0019] In some variations, each WGM resonator of the one or more WGM resonators can be optically coupled to at least one optical waveguide. In some variations, the multiple ultrasound echoes can correspond to multiple ultrasound signals.

[0020] In some variations, the method of ultrasound imaging may further include performing a synthetic aperture (SA) operation on the first set of signals and the second set of signals. An image of the object may be generated based at least in part on the SA operation. In some variations, the method of ultrasound imaging may further include performing a compressive sensing (CS) operation on the first set of signals and the second set of signals. An image of the object may be generated based at least in part on the CS operation.

[0021] In general, the device may include a WGM resonator configured to propagate a set of WGMs. The WGM resonator may include a curved surface. The curved surface may include a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator. The second mode may be at a higher frequency than the first mode. The second spherical segment may be coated with a polymer configured to damp the second mode. The device may further include an optical waveguide optically coupled to the WGM resonator.

[0022] In some variations, a system including the device may further include a plurality of array elements configured to generate a set of ultrasonic signals, and the WGM resonator may be configured to propagate the set of WGMs in response to receiving a plurality of ultrasonic echoes corresponding to the set of ultrasonic signals.

[0023] In some variations, the WGM resonator can be a microsphere or microbubble resonator. The first spherical segment can be uncoated by a polymer. The first mode can be the fundamental mode of the WGM resonator. In some variations, the second spherical segment can be a spherical cap of the WGM resonator. The base of the spherical cap can be a distance of 3 μm to 15 μm from the equatorial plane of the WGM resonator.

[0024] In some variations, the WGM resonator may include a stem side and a non-stem side that is of opposite polarity to the stem side. In some variations, the spherical cap may be on the stem side of the WGM resonator. Alternatively or additionally, the spherical cap may be on the non-stem side of the WGM resonator. In some variations, the second spherical segment may be coated with a polymer. In some variations, the second spherical segment may be encapsulated by a polymer. In some variations, the polymer may have a refractive index higher than that of the WGM resonator. In some variations, the polymer may include an optical adhesive.

[0025] In general, in some variations, a method for making a WGM resonator may include a WGM resonator including a curved surface. The curved surface may include a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator. The second mode may be at a different frequency than the first mode. The method may further include coating the second spherical segment with a polymer configured to damp the second mode. The first mode may be a fundamental mode of the WGM resonator.

[0026] In some variations, the polymer does not coat the first spherical segment. The second spherical segment can be a spherical cap of the WGM resonator. In some variations, the WGM resonator can include a stem side and a non-stem side that is of opposite polarity to the stem side. The spherical cap can be on the stem side of the WGM resonator. Alternatively or additionally, the spherical cap can be on the non-stem side of the WGM resonator. In some variations, coating the second spherical segment with a polymer coating includes coating the spherical cap with a polymer. In some variations, coating the second spherical segment with a polymer coating includes immersing the spherical cap in a solution including a polymer. The polymer can have a refractive index higher than that of the WGM resonator and / or the polymer can include an optical adhesive. In some variations, the WGM resonator can be a microsphere or microbubble resonator. The present invention provides, for example, the following items. (Item 1) at least one optical waveguide; a plurality of whispering gallery mode (WGM) resonators, each WGM resonator configured to propagate a first set of whispering gallery modes (WGMs); a polymer structure encapsulating the at least one optical waveguide and at least one WGM resonator of the plurality of WGM resonators; an apparatus, wherein the at least one WGM resonator is optically coupled to the at least one optical waveguide such that the at least one WGM resonator is configured to transmit a first set of signals corresponding to the first set of WGMs to the at least one optical waveguide. (Item 2) The polymer structure is a backing region configured to attenuate residual ultrasonic echoes to prevent reverberation; and a matching region configured to increase a bandwidth of a WGM frequency response of the plurality of WGM resonators. (Item 3) 3. The apparatus of claim 2, wherein the matching region is further configured to improve ultrasonic transmission to the plurality of WGM resonators. (Item 4) 4. The apparatus of claim 3, wherein at least one of the backing region and the matching region has a first refractive index that is lower than a second refractive index of the at least one optical waveguide. (Item 5) 3. The device of claim 2, wherein the at least one optical waveguide is embedded within the backing region of the polymer structure. (Item 6) 3. The device of claim 2, wherein the at least one optical waveguide is embedded within the matching region of the polymer structure. (Item 7) 3. The device of claim 2, wherein the at least one WGM resonator is embedded within the matching region of the polymer structure. (Item 8) Item 1 , wherein the effective refractive index of the polymer structure is lower than the refractive index of the at least one WGM resonator. (Item 9) 2. The device of claim 1, wherein the polymer structure comprises an ultrasound enhancing material. (Item 10) 2. The apparatus of claim 1, wherein the effective acoustic impedance of the polymer structure matches the acoustic impedance of the at least one WGM resonator. (Item 11) 2. The apparatus of claim 1, wherein the at least one WGM resonator comprises at least one of a microsphere and a microbubble. (Item 12) the at least one WGM resonator receiving a plurality of ultrasonic echoes; 2. The apparatus of claim 1, configured to: propagate a second set of WGMs in response to the plurality of ultrasonic echoes. (Item 13) Item 13. The apparatus of item 12, wherein the at least one WGM resonator is further configured to transmit a second set of signals corresponding to the second set of WGMs to the at least one optical waveguide. (Item 14) Item 14. The apparatus of item 13, further comprising a photodetector communicatively coupled to the at least one optical waveguide, the at least one optical waveguide configured to propagate the first set of signals and the second set of signals to the photodetector. (Item 15) Item 1 , wherein the at least one optical waveguide comprises an optical fiber or an integrated photonic waveguide. (Item 16) Item 1 , wherein the at least one optical waveguide is coupled to a light source. (Item 17) Item 17. The apparatus of item 16, wherein the light source comprises a broadband light source or a tunable laser source. (Item 18) Item 1 , the at least one WGM resonator comprising a hollow chamber. (Item 19) 20. The apparatus of claim 18, wherein the hollow chamber comprises an ultrasound enhancing material. (Item 20) Item 1 , the device of item 1 , wherein each WGM resonator of the plurality of WGM resonators is optically coupled to the at least one optical waveguide. (Item 21) 2. The device of claim 1, wherein the at least one optical waveguide comprises a plurality of optical waveguides, and each WGM resonator of the plurality of WGM resonators is optically coupled to a respective optical waveguide of the plurality of optical waveguides. (Item 22) Item 1 , wherein at least a portion of the WGM resonators are grouped in a linear arrangement. (Item 23) Item 23. The apparatus of item 22, wherein the WGM resonators in the linear arrangement are equally spaced apart. (Item 24) Item 23. The apparatus of item 22, wherein the WGM resonators in the linear arrangement are unequally spaced apart. (Item 25) Item 2. The apparatus of item 1, wherein the multiple WGM resonators are bundled in a circular arrangement. (Item 26) Item 1 , wherein at least some of the WGM resonators are of equal size. (Item 27) Item 1 , wherein at least some of the WGM resonators are unequal sizes. (Item 28) 13. A system comprising the apparatus of claim 12 and a plurality of array elements configured to generate a set of ultrasound signals, the plurality of ultrasound echoes corresponding to the set of ultrasound signals. (Item 29) 30. The system of claim 28, wherein the array elements include at least one array element selected from the group consisting of a piezoelectric sensor, a single crystal material sensor, a piezoelectric micromachined ultrasonic transducer (PMUT), and a capacitive micromachined ultrasonic transducer sensor (CMUT). (Item 30) receiving a first set of signals from at least one optical waveguide corresponding to a first set of whispering gallery mode (WGM) resonators propagating within a plurality of WGM resonators, the at least one optical waveguide and the plurality of WGM resonators being within a polymer structure; receiving, from the at least one optical waveguide, a second set of signals corresponding to a second set of WGMs propagating within the plurality of WGM resonators, the second set of WGMs propagating in response to the plurality of WGM resonators receiving a plurality of ultrasonic echoes; detecting a set of differences between the first set of signals and the second set of signals. (Item 31) The polymer structure is a backing region configured to attenuate residual ultrasonic echoes to prevent reverberation; and a matching region configured to increase a bandwidth of a WGM frequency response of the plurality of WGM resonators. (Item 32) Item 32. The method of item 31, wherein the matching region is further configured to improve ultrasonic transmission to the plurality of WGM resonators. (Item 33) Item 32. The method of item 31, wherein at least one of the backing region and the matching region has a first refractive index that is lower than a second refractive index of the at least one optical waveguide. (Item 34) Item 34. The method of item 33, wherein the at least one optical waveguide is within the backing region of the polymer structure. (Item 35) Item 34. The method of item 33, wherein the at least one optical waveguide is within the matching region of the polymer structure. (Item 36) Item 34. The method of item 33, wherein the plurality of WGM resonators are within the matching region of the polymer structure. (Item 37) Item 31. The method of item 30, wherein the effective refractive index of the polymer structure is lower than the refractive index of the plurality of WGM resonators. (Item 38) 31. The method of claim 30, wherein the polymer structure comprises an ultrasound enhancing material. (Item 39) 31. The method of claim 30, wherein an effective acoustic impedance of the polymer structure matches an acoustic impedance of the plurality of WGM resonators. (Item 40) Item 31. The method of item 30, wherein at least one WGM resonator of the plurality of WGM resonators comprises at least one of a microsphere and a microbubble. (Item 41) Item 31. The method of item 30, wherein each of the plurality of WGM resonators is optically coupled to the at least one optical waveguide. (Item 42) 31. The method of claim 30, wherein the plurality of ultrasound echoes corresponds to a plurality of ultrasound signals. (Item 43) 31. The method of claim 30, further comprising performing a synthetic aperture (SA) operation on the first set of signals and the second set of signals, and generating an image of an object based at least in part on the SA operation. (Item 44) 31. The method of claim 30, further comprising performing a compressive sensing (CS) operation on the first set of signals and the second set of signals, and generating an image of an object based at least in part on the CS operation. (Item 45) 1. A whispering gallery mode (WGM) resonator configured to propagate a set of WGMs, the WGM resonator including a curved surface; the curved surface includes a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator, the second mode being at a higher frequency than the first mode; the second spherical segment is coated with a polymer configured to damp the second mode. (Item 46) Item 46. The apparatus of item 45, wherein the WGM resonator is a microsphere or microbubble resonator. (Item 47) Item 46. The device of item 45, wherein the first spherical segment is not coated with the polymer. (Item 48) Item 46. The apparatus of item 45, wherein the first mode is a fundamental mode of the WGM resonator. (Item 49) Item 46. The apparatus of item 45, wherein the second spherical segment is a spherical cap of the WGM resonator. (Item 50) Item 50. The apparatus of item 49, wherein the base of the spherical cap is at a distance of about 3 μm to 15 μm from the equatorial plane of the WGM resonator. (Item 51) 50. The apparatus of claim 49, wherein the WGM resonator includes a stem side and a non-stem side that is of opposite polarity to the stem side. (Item 52) Item 52. The apparatus of item 51, wherein the spherical cap is on the stem side of the WGM resonator. (Item 53) Item 52. The apparatus of item 51, wherein the spherical cap is on the non-stem side of the WGM resonator. (Item 54) Item 46. The device of item 45, wherein the second spherical segment is coated with the polymer. (Item 55) Item 46. The device of item 45, wherein the second spherical segment is encapsulated by the polymer. (Item 56) Item 46. The device of item 45, wherein the polymer has a refractive index higher than the refractive index of the WGM resonator. (Item 57) Item 46. The method of item 45, wherein the polymer comprises an optical adhesive. (Item 58) Item 46. The apparatus of item 45, further comprising an optical waveguide optically coupled to the WGM resonator. (Item 59) 59. A system comprising the apparatus of item 58 and a plurality of array elements configured to generate a set of ultrasonic signals, wherein the WGM resonator is configured to propagate the set of WGMs in response to receiving a plurality of ultrasonic echoes corresponding to the set of ultrasonic signals. (Item 60) 1. A method of making a whispering gallery mode (WGM) resonator, the method comprising: providing a WGM resonator including a curved surface, the curved surface including a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator, the second mode being at a different frequency than the first mode; and coating the second spherical segment with a polymer configured to damp the second mode. (Item 61) Item 61. The method of item 60, wherein the WGM resonator is a microsphere or microbubble resonator. (Item 62) Item 61. The method of item 60, wherein the polymer does not cover the first spherical segment. (Item 63) Item 61. The method of item 60, wherein the first mode is a fundamental mode of the WGM resonator. (Item 64) Item 61. The method of item 60, wherein the second spherical segment is a spherical cap of the WGM resonator. (Item 65) Item 65. The method of item 64, wherein the WGM resonator comprises a stem side and a non-stem side that is of opposite polarity to the stem side. (Item 66) Item 66. The apparatus of item 65, wherein the spherical cap is on the stem side of the WGM resonator. (Item 67) Item 66. The apparatus of item 65, wherein the spherical cap is on the non-stem side of the WGM resonator. (Item 68) Item 65. The method of item 64, wherein covering the second spherical segment with the polymer comprises coating the spherical cap with the polymer. (Item 69) Item 65. The method of item 64, wherein coating the second spherical segment with the polymer comprises immersing the spherical cap in a solution containing the polymer. (Item 70) Item 61. The method of item 60, wherein the polymer has a refractive index higher than the refractive index of the WGM resonator. (Item 71) Item 61. The method of item 60, wherein the polymer comprises an optical adhesive. [Brief description of the drawings]

[0027] [Figure 1] 1 illustrates an exemplary variation of a whispering gallery mode (WGM) microsphere resonator. [Diagram 2] 1A-1D are schematic diagrams of exemplary variations of WGM microbubble resonators. [Diagram 3] 1A-1D are schematic diagrams of exemplary variations of different geometries of WGM microbubble resonators. [Figure 4] The quality factor of the WGM microbubble resonator is shown for different microbubble wall thicknesses. [Diagram 5] FIG. 1 is a schematic diagram of the mode distribution of an exemplary WGM microbubble resonator. [Figure 6A] 1 is a depiction of the field distribution of the fundamental mode in an exemplary variation of a WGM resonator. [Figure 6B] 1 is a depiction of the field distribution of higher order modes in an exemplary variation of a WGM resonator. [Figure 6C] 1 is a depiction of the field distribution of higher order modes in an exemplary variation of a WGM resonator. [Figure 7] FIG. 1 is a schematic diagram of an exemplary optical coupling between a spherical WGM microbubble resonator and an optical waveguide. [Figure 8] 1 illustrates exemplary variations of WGM resonators and waveguides in polymer structures. [Figure 9] 1 illustrates an exemplary variation of a WGM resonator disposed on a substrate and optically coupled to an optical waveguide. [Figure 10] 1 illustrates an exemplary variation of a WGM resonator disposed on a substrate and optically coupled to an optical waveguide. [Figure 11] 1 illustrates an exemplary variation of a WGM resonator optically coupled to a chip-scale optical waveguide disposed on a substrate. [Figure 12] 1 is a schematic diagram of an exemplary WGM resonator array. [Figure 13] 1 is a schematic diagram of an exemplary WGM resonator array. [Figure 14] 1 is a schematic diagram of an exemplary WGM resonator array. [Figure 15] 1 is a schematic diagram of an exemplary WGM resonator array. [Figure 16] 1 is a schematic diagram of an exemplary WGM resonator array. [Figure 17A] FIG. 2 is a side view of an exemplary WGM resonator array. [Figure 17B] FIG. 17B is a top view of the example WGM resonator array of FIG. [Figure 18A] 1A-1C are schematic diagrams of exemplary variations of WGM resonator arrays packaged in polymer structures. [Figure 18B] 1A-1C are schematic diagrams of exemplary variations of WGM resonator arrays packaged within polymer structures. [Figure 19] 1A-1C are schematic diagrams of example variations of packaged WGM resonator arrays. [Figure 20A]13 illustrates an exemplary variation of a method for fabricating a WGM microsphere resonator by forming a cleaved portion in at least one end of an optical fiber. [Figure 20B] 13 illustrates an exemplary variation of a method for fabricating a WGM microsphere resonator using an optical fiber having a cleaved portion. [Figure 21A] 13A-13C show exemplary variations of tapered optical fiber tips for use in fabricating WGM microsphere resonators of various sizes. [Figure 21B] 13 shows an exemplary variation of a WGM microsphere resonator fabricated using a tapered optical fiber tip. [Figure 21C] 13 shows an exemplary variation of a WGM microsphere resonator fabricated using a tapered optical fiber tip. [Figure 22] 1 illustrates an exemplary variation of a WGM microsphere resonator fabricated from a polymer fiber. [Figure 23A] 1A-1C are schematic diagrams of exemplary variations of methods for fabricating WGM microsphere resonators using a transfer device. [Figure 23B] 1A-1C are schematic diagrams of exemplary variations of methods for fabricating WGM microsphere resonators using a transfer device. [Figure 23C] 1A-1C are schematic diagrams of exemplary variations of methods for fabricating WGM microsphere resonators using a transfer device. [Figure 24A] 1A-1C are schematic diagrams of an exemplary variation of a method for fabricating a WGM microsphere resonator using a dip-coating process. [Figure 24B] 1A-1C are schematic diagrams of an exemplary variation of a method for fabricating a WGM microsphere resonator using a dip-coating process. [Figure 24C] 1A-1C are schematic diagrams of an exemplary variation of a method for fabricating a WGM microsphere resonator using a dip-coating process. [Diagram 25] 1A-1C are schematic diagrams of exemplary variations of the method for fabricating WGM microbubble resonators. [Figure 26] 13 illustrates an exemplary variation of a method for fabricating individual WGM resonators for an array of WGM resonators along a single fiber. [Figure 27] 1A-1D are schematic diagrams of exemplary variations of methods for fabricating WGM microbubble resonator arrays and coupling the WGM microbubble resonators to optical waveguides. [Figure 28] 1 illustrates an exemplary variation of a WGM microsphere resonator having a polymer coating. [Figure 29] 13 illustrates an exemplary variation of a method for damping higher order modes. [Diagram 30] 1 illustrates an exemplary variation of a WGM microsphere resonator having a polymer coating. [Diagram 31] 13 shows an exemplary variation of a method for immersing a WGM microsphere resonator in a polymer to dampen higher order modes. [Figure 32A] FIG. 1 is a schematic diagram of an exemplary method for encapsulating a WGM resonator and an optical waveguide. [Figure 32B] FIG. 1 is a schematic diagram of an exemplary method for encapsulating a WGM resonator and an optical waveguide. [Diagram 33] 1 is a schematic diagram of an exemplary method for sensing a set of ultrasonic echoes using a WGM resonator array. [Diagram 34] FIG. 1 is a schematic diagram of an exemplary method for measuring the optical response of a WGM resonator array. [Diagram 35] FIG. 1 is a schematic diagram of an exemplary method for measuring the optical response of a WGM resonator array. [Diagram 36] FIG. 1 illustrates the optical response of an exemplary WGM resonator array. [Figure 37] 1 is a schematic diagram of an exemplary ultrasound probe. [Figure 38] 1 is a schematic diagram of an exemplary ultrasound probe. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Non-limiting examples of various aspects and variations of the present invention are described herein and illustrated in the accompanying drawings.

[0029] Disclosed herein are systems for optical sensing platforms and methods of fabricating such devices. More specifically, described herein are devices including whispering gallery mode (WGM) resonators and methods of fabricating such devices.

[0030] Optical sensing platforms are often simple and sensitive. They can be used in a variety of different applications (e.g., biosensors, chemical sensors, mechanical sensors, etc.). However, conventional optical sensing platforms have some drawbacks. For example, some conventional optical sensing platforms are fragile and susceptible to physical damage. Therefore, conventional optical platforms may be packaged to be bulky in order to protect them from physical damage. This may result in, for example, some conventional optical sensing platforms being less portable.

[0031] Furthermore, conventional optical sensing platforms may be susceptible to interference from environmental influences. For example, ambient light in the environment in which the optical sensing platform is located may affect the operation of the optical sensing platform. This, in turn, may affect the accuracy of the optical sensing platform. In addition, the operating frequency of some conventional optical sensing platforms may be difficult to control. For example, in many sensing applications, an optical sensing platform that operates within a sparse frequency spectrum may be preferred. This allows corresponding modes to be traceable and identifiable, which may be advantageous in most sensing applications. However, some conventional optical sensing platforms may not have the ability to restrict operation to a sparse frequency spectrum. In other words, it may be difficult to attenuate non-essential frequencies in conventional optical sensing platforms.

[0032] Therefore, there is a need for a sophisticated optical sensing platform that is compact, portable, and well suited for sensing applications.

[0033] Apparatus for sensing applications The systems and devices disclosed herein include a whispering gallery mode (WGM) resonator optically coupled to an optical waveguide. When light is coupled into the WGM resonator (e.g., via the optical waveguide), the WGM resonator can propagate a set of WGMs along a surface of the WGM resonator. The WGM resonator can transmit an optical signal corresponding to the set of WGMs to the optical waveguide.

[0034] In some variations, the WGM resonator and the optically coupled optical waveguide can be encapsulated in a polymer structure. The polymer structure can protect the optically coupled WGM resonator and the optical waveguide from physical damage. In other words, the WGM resonator coupled to the optical waveguide can be packaged in a polymer structure such that the package is unbreakable and portable. The polymer structure can protect the WGM resonator from interference from environmental influences. In addition, the polymer structure can include materials that enhance the sensing capabilities of the WGM resonator. For example, the refractive index, Young's modulus, elastic-optic coefficient, and / or acoustic impedance of the polymer structure can enhance the sensing capabilities of the WGM resonator based on the sensing application.

[0035] In some variations, at least a portion of the WGM resonator may include a polymer coating or other structure to dampen the WGMs at one or more non-essential frequencies. In other words, a polymer coating on the surface of the WGM resonator can restrict operation of the WGM resonator to a sparse spectrum of frequencies such that the corresponding modes are identifiable and trackable.

[0036] In some variations, multiple WGM resonators can be arranged together as an array to increase the sensitivity of the device. For example, multiple WGM resonators can be bundled together to form a sensor array. As another example, multiple WGM resonators can be arranged linearly at a specific distance from each other (e.g., a regular or irregular array that can be linear, rectangular, etc.). The sensor array can be incorporated into a suitable sensing platform for ultrasound sensing, endoscopy, etc.

[0037] WGM resonator Whispering gallery mode (WGM) resonators are suitable for high sensitivity applications because they can highly confine and trap light within a volume. In some variations, the WGM resonator may include a closed loop (e.g., a closed concave surface, a closed circular path, etc.) of a transparent medium. This may allow some allowed frequencies of light to propagate continuously within the closed loop, allowing light energy of the allowed frequencies of light to accumulate within the closed loop. In other words, the WGM resonator may allow the propagation of whispering gallery modes (WGMs) that travel along the surface of the WGM resonator and correspond to the allowed frequencies of light. The WGMs circulate around the WGM resonator. Each mode from the WGM corresponds to a specific frequency of light.

[0038] In some variations, the WGM resonator may include acoustically and optically transparent materials to optimize the response of the WGM resonator to sound waves. In addition, in some variations, the WGM resonator may include materials with high elastic-optical coefficients, such as polymers and plastic materials. Thus, the WGM resonator is well suited for sensing applications such as ultrasound sensing (e.g., ultrasound scanning, endoscopy, etc.). The WGM resonator can directly measure ultrasound waves via the photoelastic effect and / or physical deformation of the WGM resonator in response to ultrasound waves (e.g., ultrasound echoes). For example, in the presence of ultrasound waves (or any pressure), the WGM moving within the WGM resonator may undergo a spectral shift caused by changes in refractive index and changes in the shape of the WGM resonator induced by ultrasound waves. These spectral changes can be easily monitored and analyzed to generate sensor signals that are useful and suitable for ultrasound sensing applications.

[0039] Although the above paragraphs have described the properties of WGM resonators that make them suitable for ultrasonic sensing, it should be readily apparent that the WGM resonators described herein may be used in any other type of sensing application. For example, some photonic filters, such as microwave filters, may include WGM resonators. Other exemplary applications include spectroscopy, analysis of chemical and / or biological agents, mechanical sensors, lasers, switches and modulators, combinations thereof, and the like.

[0040] As mentioned above, the WGM resonator may include a suitable optically and / or acoustically transparent material. For example, the WGM resonator may include an optically transparent material, such as, for example, glass, transparent polymer, silica glass, silicon nitride, titanium dioxide, and / or any other suitable optically transparent material. In some variations, the WGM resonator may include an ultrasound enhancing material, such as, for example, polyvinylidene fluoride, parylene, polystyrene, and / or the like. Additionally, the WGM resonator may include a material with a low Young's modulus (e.g., a material with a Young's modulus less than 2 GPa) to increase the mechanical deformation induced by ultrasound.

[0041] In some variations, the WGM resonator may include a substantially curved portion (e.g., a spherical portion, a toroid-shaped portion, a ring-shaped portion, etc.). The substantially curved portion may be supported by a stem portion. In some variations, the substantially curved portion and the stem portion of the WGM resonator may be formed from the same material.

[0042] In some variations, the substantially curved portion and the stem portion of the WGM resonator can be formed together. For example, the substantially curved portion may be formed by applying heat to a target portion on the fiber tip and / or capillary. Another portion of the fiber tip and / or capillary that is not subjected to heat may form the stem portion of the WGM resonator.

[0043] The shape of the WGM resonator (e.g., the shape of the substantially curved portion of the WGM resonator) can be any suitable shape. For example, the shape of the WGM resonator can be spherical (i.e., a solid sphere), bubble-shaped (i.e., a sphere with a cavity), cylindrical, elliptical, ring, disk, toroid, etc. Some non-limiting examples of WGM resonators include microbottle resonators, microbubble resonators, microcylinder resonators, microdisk resonators, microtoroid resonators, etc.

[0044] While the structure and function of the WGM resonators are shown and described with respect to microsphere and microbubble resonators, this is for illustrative purposes only. It should be readily apparent that the WGM resonator embodiments described herein may be extended to WGM resonators of other shapes, such as WGM microbottle resonators, WGM microcylinder resonators, WGM microdisk resonators, WGM microtoroid resonators, and / or the like. The apparatus and methods described herein may use one or more WGM resonators of any suitable shape.

[0045] WGM microsphere resonator 1 is an exemplary variation of a WGM microsphere resonator 102. The WGM microsphere resonator 102 can include a substantially curved portion 102a (e.g., a spherical portion). The substantially curved portion 102a can be supported by a stem portion 102b. In some variations, the substantially curved portion 102a of the WGM microsphere resonator 102 can be on the stem portion 102b of the WGM microsphere resonator 102.

[0046] As described above, the WGM microsphere resonator 102 can trap light of a particular frequency. The frequency of light can circulate within the substantially curved portion 102a of the WGM microsphere resonator 102, thereby allowing propagation of whispering gallery modes along the surface of the WGM microsphere resonator 102 (e.g., along the circumference of the substantially curved portion 102a). Each set of WGMs propagated by the WGM microsphere resonator 102 can be confined to one or more planes within the WGM microsphere resonator 102. For example, the set of WGMs corresponding to a fundamental frequency (i.e., the fundamental mode of the WGMs) may be propagated in an equatorial plane within the WGM microsphere resonator 102.

[0047] The substantially curved portion 102a of the WGM microsphere resonator 102 may have a radius R. The WGM propagated within the WGM microsphere resonator 102 may correspond to a set of resonant frequencies that depend on parameters including the radius R of the WGM microsphere resonator 102 and / or the effective thickness of the WGM microsphere resonator 102. Additionally or alternatively, the set of resonant frequencies may depend on parameters such as the refractive index of the material of the WGM microsphere resonator 102.

[0048] As described above, the WGM microsphere resonator 102 can directly measure the intensity of ultrasound through the photoelastic effect of the WGM microsphere resonator 102 and / or the physical deformation of the WGM microsphere resonator 102. The ultrasound can induce a change in the refractive index and / or cause a physical deformation of the WGM microsphere resonator 102. This can lead to a resonant frequency shift in the WGM, which can be proportional to the amount of refractive index change and / or physical deformation. For example, the physical deformation of the WGM microsphere resonator 102 can be a change to the radius R of the WGM microsphere resonator 102. The change in radius R can change the set of resonant frequencies of the WGM microsphere resonator. The resonant frequency shift follows the following equation:

number

number

[0049] Although the WGM microsphere resonator shown in FIG. 1 has a spherical shape, the substantially curved portion 102a can be any suitable shape. In general, the performance of the WGM resonator 102 can depend on its shape. For example, in general, a more spherical microsphere can have better performance in confining the WGM. Some suitable variations of the WGM microsphere resonator 102 can be elliptical (e.g., having some degree of eccentricity, such as about 0 to about 0.9).

[0050] The WGM microsphere resonator 102 can be made from an optical fiber including an optically transparent material, such as, for example, glass, transparent polymer, silica glass, or any other suitable material that is optically transparent at the operating wavelength of the WGM microsphere resonator 102. In some variations, the WGM microsphere resonator 102 can be fabricated by manipulating the end of a fiber made from an optically transparent material. For example, the end of the fiber (e.g., the fiber tip) can be melted by subjecting the end of the fiber to heat (e.g., using a CO2 laser, an arc discharger, a heating coil, or any other suitable heat source). The melted fiber tip becomes spherical due to surface tension. In other words, the WGM microsphere resonator 102 can be fabricated by subjecting the end of the fiber to a reflow process. Additionally or alternatively, a needle or syringe can be used to transfer the optically transparent material to the end of a tapered or non-tapered fiber tip. Once the droplet is transferred to the end of the tapered or non-tapered fiber tip, the droplet can be cured using a suitable curing process (e.g., using heat, moisture, ultraviolet (UV) light, etc.). The surface tension of the optically transparent material maintains the spherical shape, thereby forming the WGM microsphere resonator 102. Additionally or alternatively, the tip of the tapered or non-tapered fiber can be immersed in a pool of optically transparent material. As the tapered or non-tapered fiber is retracted, the surface tension of the optically transparent material forms a sphere. The spherical portion can be cured using a suitable curing process (e.g., using heat, moisture, ultraviolet (UV) light, etc.). Fabrication of the WGM microsphere resonator is described in further detail below.

[0051] The geometric dimensions of the WGM microsphere resonator 102, including its radius R, can be altered by drawing the end of the optically transparent fiber into a tapered shape. Additionally or alternatively, the radius R of the WGM microsphere resonator 102 can be manipulated by controlling the heating process. The operating frequency of the WGM microsphere resonator 102 can be set by applying a polymer coating to a portion of the surface of the WGM microsphere resonator 102 (described in more detail below). The change in frequency of the WGM propagating within the WGM microsphere resonator 102 in response to an acoustic wave (or pressure wave) can be measured for sensing applications (e.g., ultrasonic sensing).

[0052] WGM microbubble resonators 2 is a schematic diagram of an exemplary variation of a WGM microbubble resonator 202. The WGM microbubble resonator 202 may include a substantially curved portion 202a (e.g., a spherical portion with a hollow cavity). The substantially curved portion 202a of the WGM microbubble resonator 202 may include an outer microbubble surface 202a' having a radius indicated by R and an inner microbubble surface 202a'' having a radius indicated by r, thereby defining a resonator wall thickness equal to (Rr). The space inside the inner microbubble surface 202a'' may be hollow (e.g., a cavity) or may include a material, such as an ultrasound enhancing material, as described in more detail below.

[0053] As described above, the WGM microbubble resonator 202 can trap light of a particular frequency. The light frequency can circulate within the substantially curved portion 202a of the WGM microbubble resonator 202, thereby enabling propagation of whispering gallery modes along the surface of the WGM microbubble resonator 202 (e.g., along the circumference of the substantially curved portion 202a). Each set of WGMs propagated by the WGM microbubble resonator 202 can be confined to one or more planes within the WGM microbubble resonator 202. For example, a set of WGMs corresponding to a fundamental frequency (i.e., a fundamental mode of the WGMs) may be propagated in an equatorial plane within the WGM microbubble resonator 202.

[0054] The WGM propagating within the WGM microbubble resonator 202 may correspond to a set of resonant frequencies that depend on parameters including the radius R of the outer microbubble surface 202a', the radius r of the inner microbubble surface 202a'', and / or the wall thickness of the WGM microbubble resonator 202. Additionally or alternatively, the set of resonant frequencies may depend on parameters such as the refractive index of the material of the WGM microbubble resonator 202.

[0055] As described above, the WGM microbubble resonator 202 can directly measure the intensity of ultrasound through the photoelastic effect of the WGM microbubble resonator 202 and / or the physical deformation of the WGM microbubble resonator 202. The ultrasound can induce a change in refractive index and / or cause a physical deformation of the microbubble resonator 202. This can lead to a resonant frequency shift in the WGM, which can be proportional to the amount of refractive index change and / or physical deformation. For example, the physical deformation of the WGM microbubble resonator 202 can be a change in radius R of the outer microbubble 202a'. Due to the change in radius R, the set of resonant frequencies of the WGM microbubble resonator 202 can change. The resonant frequency shift follows the equation:

number

number

[0056] Although the WGM microbubble resonator 202 shown in FIG. 2 has a spherical shape, the substantially curved portion 202a can be any suitable shape. In general, the performance of the WGM resonator 202 can depend on its shape. For example, a more spherical microbubble can have better performance in trapping WGM. Some suitable variations of the WGM microbubble resonator 202 can be elliptical (e.g., with some degree of eccentricity, such as about 0 to about 0.5, 0 to about 0.3, about 0 to about 0.2, about 0 to about 0.1, etc.). FIG. 3 depicts exemplary variations of various geometric shapes of the WGM microbubble resonator. For example, the WGM microbubble resonator can be an elliptical microbubble (e.g., a more eccentric microbubble 302 or a less eccentric microbubble 302') or a spherical microbubble 302''. As mentioned above, the spherical WGM microbubble resonator 302'' may have better resonator performance compared to the elliptical WGM microbubble resonators 302 and 302'.

[0057] Referring back to FIG. 2, the WGM microbubble resonator 202 can be made from a capillary tube, such as capillary tube 201. The capillary tube 201 can be made from an optically transparent material, such as, for example, glass, transparent polymer, silicon nitride, titanium dioxide, or any other material that is suitably optically transparent at the operating wavelength of the WGM microbubble resonator 202. The resonator wall (e.g., the resonator wall shown with a wall thickness equal to (Rr)) may comprise a material, such as, for example, the same material as the capillary tube 201. The space inside the inner microbubble resonator surface 202a'' may be a hollow chamber or may be filled with an ultrasound enhancing material, such as, for example, polyvinylidene fluoride, parylene, polystyrene, etc. The ultrasound enhancing material inside the optical WGM microbubble resonator 202 can increase the sensitivity of the microbubble resonator. For example, the ultrasound-enhancing material may have a relatively high elastic-optical coefficient, such that in response to the WGM microbubble resonator 202 receiving a set of ultrasound echoes, the refractive index of the ultrasound-enhancing material changes more than the refractive index of the material of the resonator wall (e.g., when subjected to mechanical stress or strain induced by the set of ultrasound echoes).

[0058] In some variations, the WGM microbubble resonator 202 can be formed by manipulating a capillary tube (e.g., capillary tube 201). The capillary tube 201 can be fabricated from an optically transparent material, such as glass, transparent polymers, silicon nitride, titanium dioxide, and any other suitable optically transparent material. The capillary tube 201 can be pressurized and heated at a location on the capillary tube 201 where a substantially curved portion 202a of the WGM microbubble resonator 202 is desired. Heating of the capillary tube 201 can be accomplished using any suitable heat source, such as a CO2 laser, an arc discharger, a heating coil, etc., while keeping the pressure within the capillary tube 201 stable using a pressure source. Fabrication of the WGM microbubble resonator is described in more detail below.

[0059] In some variations, for example, the heat source may be manipulated (e.g., to control heat direction, focused energy, etc.) using a set of one or more lenses, beam splitters, mirrors, thermally conductive surfaces, etc. The geometric dimensions of the microbubble resonator, including the radius R of the outer microbubble 202a', the radius r of the inner microbubble 202a'', and the wall thickness as described with respect to FIG. 2, may depend on a controlled heating and / or pressurization process. For example, precise control of the fabrication process may produce a WGM microbubble resonator of a suitable shape (e.g., the elliptical WGM microbubble resonator 302 or 302' of FIG. 3 and / or the spherical WGM microbubble resonator 302'' of FIG. 3). In some variations, the capillary tube 201 may be further processed by partially or entirely filling the hollow chamber of the WGM microbubble resonator 202 with an ultrasound-enhancing material as described above. For example, the ultrasound-enhancing material may be injected into the WGM microbubble resonator 202 through an open end of the capillary tube 201.

[0060] The operating frequency of the WGM microbubble resonator 202 can be set by applying a polymer coating to a portion of the surface of the WGM microbubble resonator 202 (described in more detail below). The change in frequency of the WGM propagating within the WGM microbubble resonator 202 in response to an acoustic wave (or pressure wave) can be measured for sensing applications (e.g., ultrasonic sensing).

[0061] As noted above, the WGM microsphere resonator 102 and the WGM microbubble resonator 202 have been described in detail for illustrative purposes only. The following sections describe the properties of any suitable WGM resonator (e.g., a WGM microsphere resonator, a WGM microbubble resonator, a WGM microbottle resonator, a WGM microtoroid resonator, a WGM microdisk resonator, a WGM microring resonator, etc.).

[0062] Quality factor (Q value) The ability to capture allowed frequencies of light in a closed loop can be attributed to the high quality factor (Q) of WGM resonators, which makes them highly suitable for high sensitivity applications. The Q of a WGM resonator can be based on factors including the geometric parameters of the WGM resonator, the refractive index of the transparent medium, and / or the refractive index of the environment surrounding the WGM resonator. For example, spherical WGM resonators may have a high Q due to their geometric shape.

[0063] The sensitivity of ultrasonic sensing can be improved by increasing the Q factor of the WGM resonator. The Q factor can be controlled by the physical factors of the WGM resonator, such as the shape of the WGM resonator, the wall thickness Rr of the WGM microbubble resonator, and the radius R of the WGM microsphere resonator. For example, the shape of the WGM resonator affects the confinement of the optical field within the WGM resonator, which in turn affects the Q factor of the WGM resonator. The confinement of the optical field within the WGM resonator may depend on the optical loss of the optical field while propagating around the surrounding walls of the WGM resonator, among other factors. The optical loss of the optical field may depend on the geometric parameters, including the bending radius of the circumference, and the physical parameters, including the absorption coefficient of the material of the WGM resonator. Generally, stronger optical field confinement can help to enhance the response of the WGM resonator to the pressure induced by the received ultrasonic echo. Generally, WGM resonators with shapes such as spherical, elliptical, cylindrical, toroidal, ring-shaped, etc. may have better resonator performance. For example, in FIG. 3, spherical bubbles 302'' have lower optical loss than elliptical bubbles 302 and 302' and exhibit higher Q values ​​that are more suitable for high sensitivity applications.

[0064] Similarly, the thickness of the wall of the WGM resonator may also affect the Q factor. Figure 4 shows the change in the Q factor limited by radiation losses of the WGM microbubble resonator as the wall thickness of the WGM microbubble resonator is changed. Other losses such as material absorption losses are not considered in the simulation. Increasing the inner microbubble surface radius and / or decreasing the wall thickness generally decreases the Q factor. The Q factor may decrease from a particular radius depending on the geometric parameters and refractive index of the WGM microbubble resonator and its surrounding medium. Graph 401 shows the Q factor of a WGM microbubble resonator with an outer microbubble diameter of 10 μm and a refractive index of 2 surrounded by air with a refractive index of 1. As shown in 401, the Q factor decreases as the wall thickness decreases, and when the wall thickness is thinner than 0.06 μm, the WGM microbubble resonator can barely confine light. Graph 402 shows the Q factor of a similar WGM microbubble resonator with an outer microbubble diameter of 10 μm and a refractive index of 2 surrounded by a material with a refractive index of 1.33. As seen in 402, due to the high refractive index of the surrounding material, the Q factor begins to drop significantly when the wall thickness is close to 0.26 μm, and the WGM microbubble resonator is barely able to confine light when the wall thickness is less than 0.16 μm. It should be readily understood that graphs 401 and 402 include values ​​of Q factors for illustrative purposes only. For example, graphs 401 and 402 merely show one or more trends of Q factors for WGM microbubble resonators. The value of the Q factor itself may vary depending on various factors. However, the general trends of Q factors may be similar to those shown in graphs 401 and 402.

[0065] WGM mode As mentioned above, WGM resonators can capture light of specific frequencies. These frequencies can circulate along the surface of the WGM resonator, thereby enabling the WGM to propagate. Thus, when a WGM resonator is coupled with light (e.g., using an optical waveguide), multiple modes corresponding to light with different spatial distributions can be excited within the WGM resonator.

[0066] 5 is a schematic diagram of mode distributions in various WGM microbubble resonators. A WGM microbubble resonator may include an outer microbubble surface having a radius indicated by R (as described in more detail in FIG. 2) and an inner microbubble surface having a radius indicated by r. Light propagation through whispering gallery modes (WGMs) in a WGM microbubble resonator with an outer microbubble surface radius of 10 μm and an inner microbubble surface radius of 0 μm is shown in mode distribution 501. Light propagation through the WGM in a WGM microbubble resonator with an outer microbubble surface radius of 10 μm and an inner microbubble surface radius of 8 μm is shown in mode distribution 502. Light propagation through the WGM in a WGM microbubble resonator with an outer microbubble radius of 10 μm and an inner microbubble surface radius of 9.8 μm is shown in mode distribution 503. Light propagation through the WGM in a WGM microbubble resonator with an outer microbubble radius of 10 μm and an inner microbubble radius of 9.94 μm is shown in mode distribution 504. In all the mode distributions described above, the refractive index of the microbubble material is 2.

[0067] In mode distribution 501 and mode distribution 502, there is a small difference between a microbubble with an outer surface radius of 10 μm, a microbubble with an inner surface radius of 0 μm, and a microbubble with a wall thickness of 2 μm. However, for a WGM microbubble resonator with a wall thickness of 0.06 μm, the WGM microbubble resonator becomes lossy and cannot confine light within the wall thickness (see, e.g., mode distribution 504). Light propagation through the WGM in a WGM microbubble resonator with an outer microbubble surface radius of 10 μm and an inner microbubble surface radius of 9.9 μm is shown in mode distribution 505. Light propagation through the WGM in a packaged WGM microbubble resonator (e.g., a WGM microbubble resonator encapsulated in a polymer structure) surrounded by a polymer structure with a lower refractive index of 1.33 and an outer microbubble radius of 10 μm and an inner microbubble radius of 9.9 μm is shown in mode distribution 506. Packaged WGM microbubble resonators are lossy and cannot confine light within their wall thickness because the wall thickness of WGM microbubble resonators is very thin (i.e., 0.1 μm) and smaller than the wavelength of light.

[0068] As seen in FIG. 5, multiple modes of WGMs with different spatial distributions (also referred to herein as "field distributions") can be excited in the WGM resonator. The field distribution of each mode can be generally confined to one or more specific planes. In other words, when a set of WGMs corresponding to a particular frequency propagates along the surface of the WGM resonator, the field distribution of that set of WGMs can be generally confined to the latitudinal extent of one or more specific planes in the surface of the WGM resonator. For example, FIG. 6A is an exemplary variation of a depiction of the field distribution of a fundamental mode in a WGM resonator 602. The fundamental mode of the WGM corresponding to the fundamental frequency can propagate in plane 606a. If the substantially curved portion of the WGM resonator is spherical (e.g., a WGM microsphere resonator, a WGM microbubble resonator, etc.), the WGM can propagate in the equatorial plane. The fundamental mode can have a broader bandwidth response to ultrasonic sensing than higher order modes due to the thinner field distribution of the fundamental mode.

[0069] The field distribution of the higher order mode corresponding to a frequency other than the fundamental frequency (e.g., a frequency higher than the fundamental frequency) may be confined within a wider range extending radially or polarly (north and south) beyond the field distribution of the fundamental frequency. For example, FIG. 6B is a depiction of the field distribution of a first higher order mode in an exemplary variation of a WGM resonator 602′. As seen in FIG. 6B, the field distribution of the first higher order mode extends radially or polarly from the equatorial plane 606a. In other words, the field distribution of the first higher order mode extends between the planes 606b and 606c. The planes 606b and 606c may be equidistant from the equatorial plane 606a. The planes 606b and 606c may also be parallel to the equatorial plane 606a.

[0070] FIG. 6C is a depiction of the field distribution of a second higher order mode (higher than the first higher order mode described above with respect to FIG. 6B) in an exemplary variation of WGM resonator 602″. Similar to the field distribution of the higher order modes depicted in FIG. 6B, the field distribution of the higher order modes in FIG. 6C extends in a radial or polar direction from equatorial plane 606a. The field distribution in this scenario extends from plane 606f to plane 606g. In other words, the higher order field distribution may be confined to a latitude that spans between a) equatorial plane 606a, b) planes 606f, 606d, and 606b in one direction, and c) planes 606g, 606c, and 606e in the opposite polar direction. Planes 606f and 606g may be equidistant from equatorial plane 606a. Similarly, from the equatorial plane 606a, the planes 606d and 606e may be equidistant, and the planes 606b and 606c may be equidistant. As mentioned above, the north and south boundaries of the field distribution of the higher order mode depicted in FIG. 6C are the planes 606f and 606g, respectively. Compared to the field distribution of the higher order mode shown in FIG. 6B, the field distribution of the higher order mode in FIG. 6C extends further north and south from the equatorial plane 606a. For example, the planes 606f and 606g may be at a greater distance from the equatorial plane 606a, as opposed to the planes 606b and 606c. Thus, the higher order mode in FIG. 6C may have a higher mode volume than the higher order mode in FIG. 6B. The optical loss for the higher order mode in FIG. 6C may be greater than the optical loss for the higher order mode in FIG. 6B.

[0071] As mentioned above, in a WGM resonator that is spherical in shape, the field distribution of the fundamental mode may be generally confined to the equatorial plane, while the field distribution of the higher order modes may extend along radial and / or polar directions from the equatorial plane. WGM resonators exhibit dense spectral features due to their large number of azimuthal and radial modes. Dense spectra may be very attractive for applications such as cavity quantum electrodynamics. However, for most sensing applications, a sparse spectrum with traceable and identifiable modes may be preferred. Furthermore, lower order modes may have a higher Q factor compared to higher order modes. Furthermore, lower order modes may have a smaller mode volume compared to higher order modes.

[0072] The spectrum of modes may depend on the acoustic impedance of the WGM resonator, the refractive index of the WGM resonator, the geometric parameters of the WGM resonator (e.g., the radius of the WGM resonator, the effective thickness of the WGM resonator, the wall thickness of the WGM resonator, etc.), the acoustic impedance of the environment surrounding the WGM resonator, and / or the refractive index of the environment surrounding the WGM resonator. Thus, the acoustic impedance and / or refractive index of the material of the WGM resonator and / or its surrounding materials, as well as the size of the WGM resonator, can be tailored to tune the bandwidth of the resonant modes.

[0073] Additionally or alternatively, higher order modes can be selectively attenuated by disrupting the geometry of the WGM resonator and / or its periphery at selected points close to the electric field, thereby reducing the number of higher order modes present in the transmission spectrum. For example, in some variations, higher order modes can be selectively attenuated by applying a polymer coating to a portion of the WGM resonator (described in more detail below). For example, applying a polymer coating to a portion of the periphery of the WGM resonator can disrupt the continuity of the WGM resonator's geometric shape along the coated portion, thereby changing the operating mode of the WGM resonator. When a polymer coating is applied to a spherical segment, as described in more detail below, the coating can further change the effective "thickness" (along the polar direction) of the WGM resonator. In other words, the polymer coating can reduce the effective thickness of the WGM resonator, thereby attenuating at least some of the higher order modes. In this way, WGM resonators can be well suited for sensing applications, as described in more detail below.

[0074] Waveguide Coupling a WGM resonator with light can enable the sensing capabilities of the WGM resonator. Light can be coupled by simply shining light onto the WGM resonator. However, this can be an inefficient way to excite the WGM resonator. This is because phase matching between the resonating light and the incident light may be required to enable the sensing capabilities of the WGM resonator by coupling light. Therefore, a more efficient way to couple light to a WGM resonator is to use an optical waveguide with specific properties to achieve phase matching. An optical waveguide can provide a controllable and robust light that can utilize the sensing capabilities of the WGM resonator.

[0075] However, the sensitivity of a WGM resonator coupled to an optical waveguide can vary depending on the type of optical waveguide, the efficiency of the optical waveguide, the type of coupling between the optical waveguide and the WGM resonator, the geometry of the optical waveguide, the geometry of the WGM resonator, etc. Therefore, it can be important to consider these factors when coupling a WGM resonator with an optical waveguide.

[0076] As mentioned above, the sensitivity of a WGM resonator may depend on the efficiency of the optical waveguide to which it is coupled. The efficiency of an optical waveguide may depend on phase locking, optimal overlap between the modes of the WGM resonator and the optical waveguide, and criticality. For example, the efficiency of an optical waveguide may be based on the fraction of the total transmitted optical power that can be phase matched to the WGM resonator. An optical waveguide may be considered most efficient if it can couple the maximum amount of light into the WGM resonator.

[0077] Additionally or alternatively, the sensitivity of the WGM resonator may depend on the coupling between the optical waveguide and the WGM resonator. Optimal optical coupling between the optical waveguide and the WGM resonator may depend on multiple factors. Some non-limiting examples of these factors include the distance of the coupling point on the optical waveguide from the WGM resonator, the alignment of the coupling point on the optical waveguide to the WGM resonator, etc. With optimal coupling, whispering gallery mode (WGM) operation is achieved with a high Q value for a set of resonant frequencies corresponding to a set of WGMs.

[0078] In some variations, optical coupling between the WGM resonator and one or more optical waveguides may be achieved, at least in part, by placing the WGM resonator in close proximity to one or more optical waveguides. For example, the WGM resonator and optical waveguides may be positioned such that the optical waveguide is aligned with the maximum diameter of the WGM resonator. In one example, the WGM resonator may have an elliptical shape, and the maximum circular diameter of the WGM resonator may be aligned with one or more optical waveguides.

[0079] 7 is a schematic diagram of an exemplary optical coupling between a spherical WGM microbubble resonator 702 and an optical waveguide 701. Coupling point 703 is the point on optical waveguide 701 that has the closest distance from the spherical WGM microbubble resonator 702. To achieve good coupling between the optical waveguide 701 and the spherical WGM microbubble resonator 702, the optical waveguide 701 can be aligned such that coupling point 703 is at the closest distance from the spherical WGM microbubble resonator 702. For example, the optical waveguide 701 may be positioned such that coupling point 703 is at the closest distance from the spherical WGM microbubble resonator 702.

[0080] In some variations, the optical waveguide may be an optical fiber that receives light from a light source, an integrated photonic waveguide that receives light from an on-chip light source, an integrated photonic waveguide coupled to an optical fiber that propagates light from the light source, or any other suitable medium that propagates light from the light source from the WGM resonator to a coupling point. Some non-limiting examples of optical waveguides include optical fibers, integrated photonics waveguides, chip-scale waveguides, slab waveguides, prisms, angle-polished fiber couplers, tapered fiber couplers, bent fiber couplers, etc.

[0081] Polymer Structure In some variations, at least a portion of the optical waveguide and the WGM resonator can be embedded within a polymer structure, thereby encapsulating the optical waveguide and the WGM resonator. As mentioned above, the WGM resonator can be fragile and susceptible to physical damage. The polymer structure can protect the WGM resonator and the optical waveguide from physical damage. In other words, the polymer structure can package the WGM resonator and the optical waveguide together. In variations where the packaged WGM resonator with the optical waveguide is used for sensing applications such as ultrasonic sensing (e.g., ultrasonic imaging such as endoscopy), the effective refractive index of the polymer structure can be lower than the effective refractive index of the WGM resonator. This can enable the packaged WGM resonator to respond to a wide range of frequencies of WGM while having high ultrasonic attenuation. The high ultrasonic attenuation can prevent reverberation of ultrasonic echoes when the packaged WGM resonator with the optical waveguide is used for ultrasonic sensing applications.

[0082] The polymer structure may include 1) a backing region (also referred to herein as a "backing polymer") and / or an acoustic matching region (also referred to herein as a "matching polymer"), 2) an encapsulation region, and 3) in some variations, a substrate. For example, with reference to FIG. 18A, the polymer structure may include a matching region 1808, an encapsulation region 1814, a substrate 1806, and a backing region 1810.

[0083] The acoustic matching region 1808 can be configured to facilitate ultrasonic transmission. For example, the matching region 1808 may include a polymer material having a low refractive index to reduce impedance of transmission of ultrasonic echoes to the WGM resonator. The matching region 1808 may have a low refractive index compared to the refractive index of the WGM resonator and the refractive index of the one or more optical waveguides.

[0084] The backing region 1810 may include a damping material, such as a polymeric material having a low refractive index, configured to obtain a broad whispering gallery mode (WGM) frequency response while having high ultrasonic attenuation to prevent reverberation of ultrasonic echoes. In some variations, the acoustic impedance of the damping material of the backing region 1810 may be matched to the acoustic impedance of a matching polymer utilized to package the WGM resonators, for example.

[0085] As discussed in some variations, the polymer structure may include a substrate (e.g., substrate 1806). The substrate may include silicon, silica, quartz, plastic, and / or any other suitable material. Suitable materials that function as substrates may include damping materials that eliminate residual vibrations and minimize ultrasonic echoes. The substrate may be coated with one or more coating materials (e.g., "coating layers"). In some variations, the coating layers may include one or more materials that function as reflective surfaces to bounce ultrasonic waves back to the WGM resonator. In some variations, the coating layers may further include a backing polymer and / or a matching polymer. For example, the substrate may be coated with a coating material that includes a backing polymer and / or a matching polymer. In some variations, a layer of the backing polymer and / or a matching polymer may be deposited on the substrate.

[0086] The WGM resonator may be disposed on the substrate 1806 and encapsulated within the encapsulation region 1814. In other words, the encapsulation region 1814 may be sandwiched between the matching region 1808 and the backing region 1810 such that the WGM resonator may be encapsulated within the encapsulation region 1814.

[0087] In some variations, the encapsulation region 1814 may include an ultrasound-enhancing material, such as polyvinylidene fluoride, parylene, polystyrene, and / or the like. The ultrasound-enhancing material may increase the sensitivity of the WGM resonator. For example, the ultrasound-enhancing material may have a high elastic-optic coefficient such that in response to the WGM resonator receiving a set of ultrasound echoes, the refractive index of the ultrasound-enhancing material may change substantially compared to the refractive index of a non-ultrasound-enhancing material (e.g., upon being subjected to a mechanical stress or strain induced by a set of ultrasound echoes). This increases the response of the WGM resonator to ultrasound signals.

[0088] In an alternative arrangement, referring to FIG. 32A, the polymer structure may include a matching region 3208a, a backing region 3210a, and a substrate 3206a. In this arrangement, the matching region 3208a may function as an encapsulation region. In other words, the WGM resonator may be encapsulated within the matching region 3208a. Thus, the matching region 3208a may include an ultrasound enhancing material. The backing region 3210a may be adjacent to the matching region 3208a and sandwiched between the matching region 3208a and the substrate 3206a. In such a variation, a layer of backing polymer may be deposited on the substrate 3206a. Additionally, a layer of matching polymer may be deposited on the backing polymer on the substrate.

[0089] In some variations, the acoustic impedance of the polymer structure (e.g., the acoustic impedance of the matching region, the acoustic impedance of the backing region, and / or the acoustic impedance of the encapsulation region) may be matched to the acoustic impedance of the WGM resonator, thereby allowing a majority of the acoustic energy from the ultrasound signal to pass through the WGM resonator, which can increase the output signal.

[0090] In some variations, the optical waveguide and the WGM resonator may be embedded in a matching polymer. Alternatively, the optical waveguide may be embedded in a backing polymer, while the WGM resonator is embedded in a matching polymer. In some variations, the optical waveguide may be partially embedded in the backing polymer and partially embedded in the matching polymer. In some variations, the optical waveguide may be coupled with the WGM resonator such that the WGM resonator is on a substrate contained in the polymer structure, while the optical waveguide is above the substrate. In some variations, the optical waveguide may be coupled with the WGM resonator such that the WGM resonator and the optical waveguide are both in direct contact with the substrate contained in the polymer structure. Encapsulating the optical waveguide and the WGM resonator in the polymer structure is described in more detail below. It should be readily apparent that the WGM resonator may be optically coupled to more than one optical waveguide. Thus, the WGM resonator may be embedded in the polymer structure with each of the optical waveguides coupled thereto.

[0091] Thus, as described above, the WGM resonator and / or one or more optical waveguides can be encapsulated within a polymer structure (e.g., a polymer structure including a backing polymer and a matching polymer). In some variations, the sensing device and / or sensing apparatus can include at least one WGM resonator coupled to one or more optical waveguides packaged within a polymer structure as described above. In other words, the sensing device and / or sensing apparatus can include a single WGM resonator coupled to a single optical waveguide packaged within a polymer structure. Alternatively, the sensing device and / or sensing apparatus can include a single WGM resonator coupled to two or more optical waveguides packaged within a polymer structure. In yet another variation, the sensing device and / or sensing apparatus can include an array of WGM resonators (described in more detail below) coupled to a single optical waveguide packaged within a polymer structure (e.g., each WGM resonator in the array of WGM resonators is coupled to the same optical waveguide). In yet another variation, the sensing device and / or sensing apparatus may include an array of WGM resonators coupled to two or more optical waveguides packaged within a polymer structure (e.g., at least some of the WGM resonators in the array of WGM resonators may be coupled to different optical waveguides).

[0092] The polymer structure can, for example, i) provide protection for the WGM resonator and one or more optical waveguides, thereby improving portability of the sensing device and / or sensing apparatus, ii) improve the response of the WGM resonator to incident ultrasonic echoes sensed by the sensing device and / or sensing apparatus, and / or iii) increase the operating bandwidth of the sensing device and / or sensing apparatus. In some variations, the polymer structure also helps to maintain optical coupling properties between the WGM resonator and one or more optical waveguides, such as by maintaining a consistent distance between the WGM resonator and one or more optical waveguides.

[0093] In one variation, a polymer structure including a matching region can broaden the operating bandwidth of the sensing device and / or sensing apparatus, as described below. Generally, in ultrasonic sensing applications, the operating frequency of the input light can be tuned to the resonance of the WGM resonator centered on the resonant frequency. The incident ultrasonic echo changes the resonant frequency of the WGM resonator by modulating the refractive index of the material of the WGM resonator or by deforming the WGM resonator, generating a resonance shift. If the linewidth of the resonance is too narrow, the resonance shift can be much larger than the linewidth of the resonance, and the resonance shift may be outside the dynamic range of the sensing device and / or sensing apparatus. However, the matching region can broaden the linewidth of the more optical resonance modes to better capture larger resonance shifts. For example, evanescent waves extending from the WGM resonator to the matching region may experience extra losses, leading to a decrease in the photon lifetime reflected as linewidth broadening of the optical resonance mode. Due to this linewidth broadening, the sensing device and / or sensing apparatus can tolerate a wider range of resonance frequency shifts induced by the incident ultrasonic signal. In other words, the matching polymer encapsulating the WGM resonator can broaden the operating bandwidth of the sensing device and / or sensing apparatus to improve the dynamic range of the sensing device and / or sensing apparatus, and can prevent the resonance shift from falling outside the dynamic range of the sensing device and / or sensing apparatus. Thus, the matching layer can improve the dynamic sensing range of the sensing device and / or sensing apparatus.

[0094] Exemplary Packaging FIG. 8 shows an exemplary variation of a WGM resonator 802 and an optical waveguide 801 in a polymer structure 804. As seen in FIG. 8, both the WGM resonator 802 and the optical waveguide 801 are embedded in the polymer structure. In one variation, the optical waveguide 801 may be embedded in the polymer structure 804, and then the WGM resonator 802 may be embedded in the polymer structure 804 such that the optical waveguide 801 is optically coupled to the WGM resonator 802. Alternatively, the WGM resonator 802 may be embedded in the polymer structure 804, and then the optical waveguide 801 may be embedded in the polymer structure 804 such that the optical waveguide 801 is optically coupled to the WGM resonator 802. In another alternative variation, the optical waveguide 801 and the WGM resonator 802 may be optically coupled. Following optical coupling, the coupled optical waveguide 801 and WGM resonator 802 may be embedded within a polymer structure 804. In this manner, the polymer structure 804 encapsulates the WGM resonator 802 and the optical waveguide 801. In one variation, the WGM resonator 802 may be a WGM microsphere resonator and the optical waveguide 801 may be a fiber taper.

[0095] As described above, the polymer structure may include a matching region, a backing region, a substrate, and an encapsulation region. In one variation, the WGM resonator may be disposed on a substrate such that the WGM resonator is in direct contact with the substrate. The WGM resonator with the substrate may be sandwiched between the matching region and the backing region. In some variations, the space between the matching region and the substrate may be an encapsulation region that encapsulates the WGM resonator.

[0096] FIG. 9 shows an exemplary variation of a WGM resonator 902 disposed on a substrate 906 and optically coupled to an optical waveguide 901. As seen in FIG. 9, the WGM resonator 902 is in direct contact with the substrate 906. For example, the WGM resonator 902 may be fixed (e.g., using an adhesive such as a polymer adhesive) on the substrate 906. Alternatively, the WGM resonator 902 may be embedded within the substrate 906. However, the optical waveguide 901 is coupled to the WGM resonator 902 such that the optical waveguide 901 is above the substrate 906 and is not in direct contact with the substrate 906. As described above, the substrate 906 with the WGM resonator 902 and the optical waveguide 901 may be sandwiched between a backing region and a matching region. In other words, the WGM resonator 902 and the optical waveguide 901 may be encapsulated in an encapsulation region that is between the backing region and the matching region. In one variation, an array of WGM resonators may be disposed on a substrate 906 in a manner similar to that of Figure 9. In such a variation, an optical waveguide, such as optical waveguide 901, may be coupled to two or more WGM resonators in the array of WGM resonators. In some variations, WGM resonator 902 may be a WGM microsphere resonator and optical waveguide 901 may be a fiber taper.

[0097] In contrast to FIG. 9, FIG. 10 shows a WGM resonator 1002 coupled with an optical waveguide 1001 such that both the WGM resonator 1002 and the optical waveguide 1001 are in direct contact with the substrate 1006. In other words, both the WGM resonator 1002 and the optical waveguide 1001 can be fixed (e.g., with an adhesive) onto the substrate 1006. Alternatively, both the WGM resonator 1002 and the optical waveguide 1001 can be embedded within the substrate 1006. The refractive index of the substrate 1006 can be lower than that of the optical waveguide 1001. Similar to FIG. 9, the substrate 1006, the WGM resonator 1002, and the optical waveguide 1001 can be sandwiched between a backing region and a matching region. In other words, the WGM resonator 1002 and the optical waveguide 1001 can be encapsulated in an encapsulation region that is between the backing region and the matching region. In some variations, an array of WGM resonators can be disposed on a substrate 1006 such that the WGM resonators and the optical waveguide are both in direct contact with the substrate 1006 (similar to FIG. 10). In such variations, an optical waveguide such as optical waveguide 1001 can be coupled to two or more WGM resonators in the array of WGM resonators. In one variation, WGM resonator 1002 can be a WGM microsphere resonator and optical waveguide 1001 can be a fiber taper.

[0098] FIG. 11 shows an exemplary variation of a WGM resonator 1102 coupled to a chip-scale optical waveguide 1101 such that the chip-scale optical waveguide 1101 is disposed on a substrate 1106. In this variation, the chip-scale optical waveguide 1101 may be in direct contact with the substrate 1106. The refractive index of the substrate 1106 may be lower than that of the chip-scale optical waveguide 1101. However, the WGM resonator 1102 may not be in direct contact with the substrate 1106, although it is optically coupled to the chip-scale waveguide 1101. For example, the chip-scale waveguide 1101 may be disposed on the substrate 1106 (e.g., fixed on the substrate 1106 using an adhesive or embedded within the substrate 1106). The WGM resonator 1102 may be disposed on the chip-scale waveguide 1101 without contacting the substrate 1106 (e.g., optically coupled to the chip-scale waveguide 1101). Similar to FIGS. 9 and 10, the substrate 1106, WGM resonator 1102, and optical waveguide 1101 may be sandwiched between a backing region and a matching region.

[0099] WGM resonator array In some variations, multiple WGM resonators can be packaged together in an array. Arrays of WGM resonators can be used in sensing devices and / or sensing apparatus. Arrays of WGM resonators can increase the sensitivity of the sensing device and / or sensing apparatus due to an increase in the sensing area (i.e., the area of ​​multiple WGM resonators relative to the area of ​​a single WGM resonator). In addition, each WGM resonator in an array of WGM resonators can cooperate such that they collectively respond to acoustic waves and / or pressure waves. This collective response can increase the strength of the output signal, thereby increasing the sensitivity of the sensing device and / or sensing apparatus.

[0100] The array of WGM resonators can be optically coupled to one or more optical waveguides. The polymer structure can encapsulate the optically coupled array of WGM resonators and optical waveguides. As described above, the polymer structure can include an encapsulation region that encapsulates the WGM resonators and / or optical waveguides, a matching region, a backing region, and optionally a substrate. In some variations, the matching region and / or the backing region can function as the encapsulation region. For example, the array of WGM resonators can be included in the matching region of the polymer structure. One or more optical waveguides can be positioned within the polymer structure such that each WGM resonator can be optically coupled to one or more optical waveguides. Each optical waveguide can be in the backing region, the matching region, or both. Additionally or alternatively, each WGM resonator in the array of WGM resonators and / or one or more optically coupled optical waveguides can be disposed on a substrate. The array of WGM resonators and the optical waveguide may be within an encapsulation region sandwiched between a matching region and a backing region.

[0101] As shown in Figures 12-17, the multiple WGM resonators can be arranged in various suitable types of arrays and in various suitable ways with respect to one or more optical waveguides. For example, Figure 12 is a schematic diagram of a WGM resonator array 1202 in which each individual WGM resonator is coupled to a unique respective optical waveguide. For example, the WGM resonators in the WGM resonator array 1202 may be arranged in parallel linear fashion on a substrate (substrate not shown in Figure 12). Each WGM resonator from the WGM resonator array 1202 may then be optically coupled with one or more separate optical waveguides 1201, such as by an alignment process (as described in Figure 7). In at least some variations, the WGM resonators in the array 1201 may be equidistant from one another. Additionally or alternatively, at least some of the WGM resonators in the array 1201 may be separated by different distances.

[0102] FIG. 13 is a schematic diagram of an exemplary WGM resonator array 1302. The WGM resonator array 1302 may be arranged in parallel linear fashion on a substrate 1306, similar to the arrangement shown in FIG. 12. In some variations, at least some of the WGM resonators in the array 1302 may be equidistant from each other. Additionally or alternatively, at least some of the WGM resonators in the array 1302 may be separated by different distances. However, in contrast to the arrangement shown in FIG. 12, the multiple WGM resonators in the array 1302 may all be optically coupled to a single optical waveguide 1301. Thus, signals from multiple WGM resonators may be coupled to and transmitted by a single optical waveguide.

[0103] FIG. 14 is a schematic diagram of an exemplary WGM resonator array 1402. The WGM resonator array 1402 may include WGM resonators having various diameters and propagating various whispering gallery modes (WGMs). In some variations, the WGM resonators in the array 1402 may not be the same size. In such variations, it may be difficult to optically couple each of them to a single optical waveguide, similar to the arrangement depicted in FIG. 13. To overcome this challenge, each WGM resonator in the WGM resonator array 1402 may be nonlinearly positioned on the substrate 1406 at various vertical distances from the major surface of the substrate 1406. As shown in FIG. 14, the WGM resonators in the center of the array 1402 are farther from the major surface of the substrate than the WGM resonators at the ends of the array 1402. The optical waveguide 1401 is positioned in alignment with each WGM resonator in the WGM resonator array 1402, and then after fixing the positions of the optical waveguide 1401 and the WGM resonator array 1402, the optical waveguide 1401 can be tightened so that it is optically coupled to all of the WGM resonators in the WGM resonator array 1402.

[0104] 15 is a schematic diagram of an exemplary WGM resonator array 1502. The WGM resonator array 1502 can be linearly arranged on a substrate 1506, such as a substrate 1506 having a low refractive index compared to the refractive index of the WGM resonator array 1502 and the optical waveguide 1501. The alignment procedure can become difficult as the number of WGM resonators in the WGM resonator array 1502 coupled with the optical waveguide 1501 increases. To alleviate this, the optical waveguide 1501 can first be embedded in or fixed on top of the substrate 1506. The WGM resonators can then be individually (e.g., one by one or in other suitable groupings) linearly arranged parallel to the substrate 1506 and the top of the optical waveguide 1501.

[0105] FIG. 16 is a schematic diagram of an exemplary WGM resonator array. The WGM resonator array 1602 may include multiple rows of WGM resonators to collectively form a resonator array. For example, in FIG. 16, the array 1602 includes row 1602a, row 1602b, and row 1602c of WGM resonators. The rows 1602a, 1602b, and 1602c of WGM resonators may be arranged such that the WGM resonators collectively form the array 1602. The array 1602 may be a rectangular array. However, it should be understood that the arrangement shown in FIG. 16 is merely exemplary and that the WGM resonator array in the rectangular array may have any suitable number of rows, columns, etc., and each row may have any suitable number of WGM resonators. Furthermore, the array may have any suitable form (e.g., a radial array, a hexagonal array, etc.).

[0106] In some variations, multiple optical waveguides (e.g., chip-scale optical waveguides) are arranged together on a substrate. For example, optical waveguides 1601a, 1601b, and 1601c are arranged on the substrate 1606. They may be fixed (e.g., using an adhesive) on the substrate 1606 at equal distances from each other in a linear manner. Alternatively, the optical waveguides 1601a, 1601b, and 1601c may be fixed on the substrate 1606 at different distances from each other. In some variations, the optical waveguides 1601a, 1601b, and 1601c may be embedded in the substrate 1606 at a predetermined distance from each other. Each optical waveguide may be optically coupled to a respective row of WGM resonators. For example, optical waveguide 1601a is optically coupled to the WGM resonators in row 1602a, optical waveguide 1601b is optically coupled to the WGM resonators in row 1602b, and optical waveguide 1601c is optically coupled to the WGM resonators in row 1602c. The WGM resonators in each row 1602a, 1602b, and 1602c can be individually (e.g., one by one or in other suitable groupings) linearly positioned on top of their respective optical waveguides 1601a, 1601b, and 1601c.

[0107] In some sensing applications, it may be advantageous for the sensing device and / or sensing apparatus to have a particular shape. For example, in applications such as endoscopy, it may be advantageous for the sensing device and / or sensing apparatus to have a circular or other compact cross-sectional shape. Similarly, a circular shaped sensing device and / or sensing apparatus may be best suited for use in a catheter. For example, an endoscope or catheter may include a thin tube or other member having a terminal sensing end. Thus, in some variations, multiple WGM resonators may be bundled together in a circular arrangement to form a WGM resonator array, such that the WGM resonator array forms the terminal sensing end of the sensing device and / or sensing apparatus.

[0108] FIG. 17A is a side view of an exemplary WGM resonator array 1702. As discussed in FIG. 1, some WGM resonators may include a substantially curved portion and a stem portion. In FIG. 17A, each of the WGM resonators in the WGM resonator array 1702 includes a substantially curved portion and a stem portion (e.g., the substantially curved portion is supported by the stem portion). For example, one WGM resonator in the array 1702 may include a substantially curved portion 1702a and a stem portion 1702b. Another WGM resonator in the array 1702 may include a substantially curved portion 1702a' and a stem portion 1702b'. The stem portions of the WGM resonators may be bundled together in a circular shape. That is, the stem portions of the WGM resonators may be bundled together such that collectively they may be in a circle. Alternatively, the stem portions of the WGM resonators may be bundled together such that collectively they may be in an ellipse. The stem portions of each WGM resonator may be parallel to one another. For example, stem portion 1702b and stem portion 1702b' are parallel to one another. The WGM resonator array 1702 may form a terminal sensing end of a sensing device and / or sensing apparatus. For example, the stem portions of the WGM resonators may be arranged together in a circular shape on a common plane or in an otherwise compact cross-sectional shape. Alternatively, the stem portions of the WGM resonators may be aligned on a common plane such that they form a circular shape or an otherwise compact cross-sectional shape. The substantially curved portions of the WGM resonators may collectively form a terminal end of a sensing device (e.g., a catheter or endoscope).

[0109] 17B shows a top view of an exemplary WGM resonator array 1702. As seen in FIG. 17B, the substantially curved portions of the WGM resonators collectively lie in a circle (which may be the end of a sensing device and / or sensing apparatus). An optical waveguide may be coupled to each of the WGM resonators in the array 1702. The bundled WGM resonators may be embedded in a polymer structure and then included in the sensing device and / or sensing apparatus.

[0110] It should be understood that any of the coupling arrangements described above with respect to Figures 12-17 can be combined in any suitable manner. In some variations, a WGM resonator array can include WGM resonators that can all be similar (e.g., similar size, similar thickness, similar material). Alternatively, a WGM resonator array can include WGM resonators that are different (e.g., different size, different thickness, different material) from at least some of the other WGM resonators in the array. The selection of similar or dissimilar WGM resonators can depend on the sensing application for which the WGM resonator array will be used. Similarly, the number of WGM resonators in the array can also vary depending on the sensing application.

[0111] Exemplary Packaged WGM Resonator Arrays After aligning the one or more optical waveguides with the WGM resonator array, the one or more optical waveguides and the WGM resonator array can be encapsulated in a polymer structure as described above. The polymer structure can help maintain the distance of the one or more optical waveguides from the WGM resonator array. The polymer structure can have a refractive index lower than that of the one or more optical waveguides and the WGM resonator array. In some variations, increasing the difference between the refractive index of the WGM resonators and the polymer structure can allow WGM resonators with smaller diameters to be utilized for sensing applications. Thus, the contrast between the refractive index of the WGM resonators in the array and the refractive index of the polymer structure can be increased to include WGM resonators with smaller diameters in the array. This can increase the operating bandwidth of the sensing device and / or sensing apparatus including the WGM resonator array.

[0112] FIG. 18A shows an exemplary variation of a WGM resonator array 1802 packaged in a polymer structure. The WGM resonators in the array 1802 can be disposed on a substrate 1806. For example, the WGM resonators can be fixed on the substrate 1806 such that each WGM resonator can be at an equal distance from the subsequent WGM resonators in the array 1802. Alternatively, the WGM resonators can be fixed on the substrate 1806 such that at least some WGM resonators can be at different distances from their subsequent WGM resonators. In yet another variation, the WGM resonators can be embedded in the substrate 1806 at a predetermined distance from each other. The WGM resonator array 1802 can be sandwiched between a backing region 1810 and a matching region 1808 along with the substrate 1806. The space between the matching region 1808 and the backing region 1810 that encapsulates the WGM resonator array 1802 on the substrate 1806 can form an encapsulation region 1814.

[0113] 18A, the WGM resonators in array 1802 can be arranged in ascending order of WGM resonator size. For example, WGM resonator 1802a is the smallest in array 1802 and WGM resonator 1802g is the largest in array 1802. The smallest WGM resonator 1802a is at one end of array 1802 and the largest WGM resonator 1802g is at the other end of array 1802.

[0114] In contrast, FIG. 18B illustrates an exemplary variation of a WGM array 1802′ packaged in a polymer structure such that the WGM resonators are arranged in no particular order by size. In other words, the WGM resonator array 1802′ may include WGM resonators of randomly distributed sizes (e.g., not arranged in ascending or descending order). For example, the largest WGM resonator 1802f′ is not at either end of the array 1802′. However, the WGM resonator 1802g′ located at one end of the array 1802′ is neither the largest in the array 1802′ nor the smallest in the array 1802′. Similar to FIG. 18A, the WGM resonator array 1802 may be packaged such that the WGM resonator array 1802' may be sandwiched between a matching region 1808' and a backing region 1810' together with a substrate 1806' (e.g., the WGM resonator array 1802' may be encapsulated within an encapsulation region 1814').

[0115] FIG. 19 is a schematic diagram of an exemplary variation of a packaged WGM resonator array 1902. The polymer structure may include a matching region 1908, a backing region 1910, and one or more coating layers 1912. In some variations, the coating layers 1912 and the matching region 1908 may include the same material. For example, the matching region 1908 and the coating layers 1912 may include multiple layers of suitable matching materials. Alternatively, in some variations, the material properties (e.g., acoustic impedance) within the polymer structure may be graduated (e.g., a graduated change in material properties from the matching region 1908 to the one or more coating layers 1912), which may maximize the transmission of acoustic waves to the WGM resonator array 1902 within the polymer structure and / or minimize undesired reflections of ultrasound waves. In other variations, the coating layers 1912 and the matching region 1908 may include different materials. For example, the matching region 1908 may include a polymer with a lower refractive index than the material of the WGM resonators and / or may include a material with low optical loss to better deliver light to the WGM resonators. The WGM resonator array 1902 may be embedded within the matching region 1908.

[0116] When used during ultrasonic sensing, a set of ultrasonic echoes 1901 can be transmitted through the matching polymer 1908 and / or other coating layers 1912 to reach the WGM resonator array 1902 with a transmitted intensity of a factor of:

number

number

number

number

number

[0117] Therefore, cos 2 Even when (kd)=0, d=λ / 4 may be desirable to obtain maximum transmission. Additionally, it is also desirable for the matching polymer 1908 to have a low refractive index so that the WGM resonator array 1902 can confine light to produce WGMs with high Q factors.

[0118] Fabrication of sensing device and / or sensing apparatus Some exemplary variations for fabricating the above-mentioned sensing device and / or sensing apparatus are described herein. As mentioned above, the operating bandwidth of the sensing device and / or sensing apparatus may depend on, among other factors, the size of the WGM resonator, the refractive index of the WGM resonator, and the acoustic impedance of the material used to fabricate the WGM resonator. Thus, the desired material for fabricating the WGM resonator can be selected based on the sensing application and the operating bandwidth of the application.

[0119] For example, for ultrasonic sensing, the material of the WGM resonator may be selected so that the acoustic impedance of the material matches the acoustic impedance of the polymer structure (e.g., the polymer structure including the backing region and the matching region). This can eliminate echoes in the sensing device and / or sensing apparatus, minimize the acoustic impedance mismatch between the WGM resonator and the polymer structure, and enhance the acoustic waves penetrating the WGM resonator. This, in turn, can increase the response of the WGM resonator through the elasto-optic effect and / or physical deformation of the WGM resonator. Furthermore, it may be desirable for the material to have a low Young's modulus to increase the mechanical deformation induced in the material by the ultrasonic waves. In addition, the material for fabricating the WGM resonator may be selected to increase the contrast between the refractive index of the material and that of the surrounding medium (e.g., the polymer structure). This can increase the operating bandwidth of the small diameter WGM resonator, thereby increasing the sensitivity of the sensing device and / or sensing apparatus. In some variations, the material for fabricating the WGM resonator may include an ultrasonic enhancement material, such as, for example, polyvinylidene fluoride, parylene, polystyrene, and / or the like. The ultrasound-enhancing material can increase the sensitivity of the WGM resonator. For example, the ultrasound-enhancing material may have a relatively high elasto-optic coefficient. Thus, when a set of ultrasound echoes is received, the refractive index of the ultrasound-enhancing material may change more than the refractive index of a non-enhancing material (e.g., upon being subjected to mechanical stress or strain induced by the set of ultrasound echoes). This can increase the response of the sensing device and / or sensing apparatus to the ultrasound signal.

[0120] In addition to selecting the material of the WGM resonator based on the sensing application, the size of the WGM resonator can be manipulated based on the sensing application. For example, for endoscopic applications, it may be desirable to have a smaller size WGM resonator (e.g., a WGM resonator having a size of less than 100 microns). The size of the WGM resonator can be manipulated by controlling the heating process. For example, the heating process may be precisely controlled to apply heat to a very specific and precise location. Alternatively or additionally, the size of the WGM resonator can be manipulated by controlling the amount of heat applied. Additionally or alternatively, the size of the fiber and / or capillary used to fabricate the WGM resonator can be manipulated to manipulate the size of the WGM resonator.

[0121] Fabrication of WGM resonators FIG. 20A illustrates an exemplary variation of a method for fabricating a WGM microsphere resonator by forming a cleaved portion at least one end of an optical fiber 2002′. In some variations, an optically transparent fiber, such as a silica fiber or a polymer fiber, may be used to fabricate a WGM microsphere resonator. In some variations, the optical fiber 2002′ may be glass, a transparent polymer (e.g., polydimethylsiloxane), silica glass, silicon nitride, titanium dioxide, and / or any other suitable optically transparent material. The optical fiber 2002′ may be selected, for example, based on its refractive index, acoustic properties, Young's modulus, and / or diameter. The sensing application in which the WGM microspheres may be used may be a factor considered when selecting the optical fiber 2002′. For example, the diameter of the optical fiber 2002′ may affect the size of the WGM microsphere resonator.

[0122] In some variations, once the optical fiber 2002' is selected, the cladding may be stripped (e.g., using a fiber coating stripper) from one end (e.g., end 2002b') of the optical fiber 2002', thereby exposing the inner core of the optical fiber 2002'. The one end 2002b' (e.g., the end from which the cladding was stripped) may be cleaved (e.g., using an optical fiber cleaver). In contrast to cleaving the optical fiber (e.g., using wire cutters), cleaving produces a smooth, uniform cut that can cause uniform reflow in subsequent processes. The optical fiber 2002' may be cleaned using methanol, ethanol, isopropyl alcohol, and the like. In one variation, the optical fiber 2002' may be cleaned after stripping the cladding. In another variation, the optical fiber 2002' may be cleaned after cleaving the optical fiber. In yet another variation, the optical fiber 2002' may be cleaned after both stripping the cladding and cleaving.

[0123] FIG. 20B illustrates an exemplary variation of a method for fabricating a WGM microsphere resonator 2002 using an optical fiber 2002′ having a cleaved end 2002b′. The WGM microsphere resonator 2002 can be fabricated from the optical fiber 2002′ using a reflow process. The cleaved optical fiber 2002′ can be subjected to heat at one end 2002b′ (e.g., the end of the cleaved optical fiber). A CO2 laser, an arc discharger, a heating coil, or any other suitable heat source can be used to heat the end of the optical fiber. In some variations, the heat source can be manipulated to control the amount of heat applied to the optical fiber. For example, the heat direction can be controlled using a set of one or more lenses, beam splitters, mirrors, thermally conductive surfaces, and the like. The end of the molten optical fiber forms a sphere 2002b due to the surface tension of the molten optical fiber. In some variations, the optical fiber can be a single mode fiber.

[0124] In some variations, one end of the cleaved optical fiber 2002' (e.g., the end of the cleaved optical fiber) can be placed in a fusion splicer to generate the spherical portion 2002b of the WGM microsphere resonator. As described above, some WGM resonators can include a stem portion. The portion of the cleaved optical fiber 2002' that is not subjected to heat can form the stem portion 2002ab of the WGM microsphere resonator. In some variations, the diameter of the spherical portion 2002b of the WGM microsphere resonator can be about 15 μm to about 500 μm. In some variations, the diameter of the WGM microsphere resonator can be 300 μm.

[0125] As mentioned above, the size of the WGM microsphere resonator can be changed by varying the diameter on the optical fiber. In one variation, one end of the optical fiber can be tapered to produce smaller WGM microsphere resonators. FIG. 21A shows an exemplary variation of a tapered optical fiber tip 2102 for use in producing WGM microsphere resonators of various sizes. One end of an optical fiber (e.g., an optical fiber comprising a material similar to that of the optical fiber of FIGS. 20A and 20B) can be tapered by gently stretching the end of the optical fiber while heating the end using a suitable heat source. The optical fiber becomes soft. Stretching the end of the optical fiber can make the optical fiber thinner over at least a portion of its length. In other words, stretching the end of the optical fiber can reduce the diameter of the optical fiber at least at the stretched end. In this manner, a tapered fiber tip 2102 can be produced. WGM microsphere resonators can be produced from the tapered fiber tip 2102 using a reflow process such as the method described in FIGS. 20A and 20B.

[0126] FIG. 21B shows an exemplary variation of a WGM microsphere resonator 2102′ fabricated using a tapered optical fiber tip 2012. In other words, the end of the tapered fiber tip 2102 can be cut and heated to generate a substantially curved portion (e.g., a spherical portion) of the WGM microsphere resonator 2102′. The WGM microsphere resonator 2102′ fabricated using a tapered fiber tip 2102 can have a smaller diameter. For example, the diameter of the substantially curved portion of the WGM microsphere resonator 2102′ can be 130 μm. To increase the size of the WGM microsphere resonator 2102′, a larger portion of the fiber tip 2102 may need to be heated. Heating more of the fiber tip 2102 increases the amount of material that can be reflowed into the substantially curved portion. FIG. 21C shows an exemplary variation of the method of a larger size WGM microsphere resonator 2102″ fabricated using a tapered optical fiber tip 2102. The WGM resonator 2102'' can be fabricated by increasing the amount of the portion of the fiber tip 2102 that is subjected to heat. In this example, the diameter of the substantially curved portion of the WGM microsphere resonator 2102'' is 180 μm.

[0127] As mentioned above, a WGM microsphere resonator can be fabricated from a silica fiber using the process disclosed in FIG. 20 and / or FIG. 21. The WGM microsphere resonators depicted in FIG. 20 and FIG. 21 are fabricated from a silica fiber. FIG. 22 shows an exemplary variation of a WGM microsphere resonator fabricated from a polymer fiber. For example, the polymer fiber may include polydimethylsiloxane. The WGM microsphere resonator of FIG. 22 can be fabricated using the process described in FIG. 23A-23C and / or FIG. 24A-24C described below.

[0128] 23A-23C are schematic diagrams of an exemplary variation of a method for fabricating a WGM microsphere resonator using a transfer device 2314. The method includes using the transfer device 2314 to transfer a material onto a fiber tip 2316. For example, the transfer device 2314 may be used to extract a suitable material for fabricating a WGM microsphere resonator. In some variations, the transfer device 2314 may be a needle. In some variations, the transfer device 2314 may be a syringe. In yet another variation, the transfer device 2314 may be a syringe with a needle on one end. In some variations, the material may be glass, a transparent polymer (e.g., polydimethylsiloxane), silica glass, silicon nitride, titanium dioxide, or any other suitable optically transparent material.

[0129] When the optically transparent material is extracted using the transfer device 2314, the optically transparent material may form a droplet 2302' on the transfer device 2314. The size of the WGM microsphere resonator may depend on the size of the droplet 2302'. The size of the droplet may be varied by changing the amount of pressure applied to the transfer device 2314, changing the size of the nozzle on the transfer device 2314, and / or changing the size of the needle on the transfer device 2314. In this manner, WGM microsphere resonators of various sizes may be fabricated in a controlled manner.

[0130] The extracted droplet 2302′ can be disposed on the end of the fiber tip 2316 (e.g., droplet 2302″). In some variations, the fiber tip 2316 can comprise the same material as the droplet 2302″. In some variations, the fiber tip 2316 can comprise a different optically transparent material than the droplet 2302″. In some variations, one end of the fiber tip 2316 (e.g., the end where the droplet is disposed) can be tapered. In some variations, the fiber tip may not be tapered. In some variations, the fiber tip 2316 may be a glass fiber tip. Once the droplet 2302″ is disposed on the fiber tip 2316, the transport device 2314 can be retracted to form a spherical portion 2302 that later forms the substantially curved portion of the WGM resonator. The spherical shape can be maintained by surface tension of the material (i.e., the material of the droplet). The spherical portion 2302 can be cured using heat, moisture, UV light, etc. to form a WGM microsphere resonator.

[0131] 24A-24C are schematic diagrams of exemplary variations of a method for fabricating a WGM microsphere resonator using a dip-coating process. In one variation, an optical fiber 2416 may be immersed in a pool 2418 of optically transparent material (as shown in FIG. 24B). For example, the pool 2418 may comprise glass, a transparent polymer (e.g., polydimethylsiloxane), silica glass, silicon nitride, titanium dioxide, or any other suitable optically transparent material. In some variations, the end of the optical fiber 2416 that is immersed in the pool 2418 may be tapered. In some variations, the optical fiber 2416 may not be tapered. The pool 2418 may comprise an optically transparent polymer. The optical fiber 2416 may be a silica fiber or glass.

[0132] When the optical fiber 2416 is retracted from the pool 2418 (as shown in FIG. 24C), the retracted end of the optical fiber 2416 may include optically transparent material from the pool 2418 such that the optically transparent material on the retracted end is in the shape of a sphere 2402. Surface tension of the optically transparent material may maintain the spherical shape. The spherical optically transparent material 2402 may be cured (e.g., using heat, moisture, UV light, etc.) to form a substantially curved portion of the WGM microsphere resonator. The portion of the optical fiber 2416 that is not immersed in the pool and not subjected to heat may form a stem portion of the WGM microsphere resonator. The size of the WGM microsphere resonator (e.g., diameter of the spherical portion) may be based on the diameter of the end of the optical fiber 2416 immersed in the pool 2418, the rate at which the optical fiber 2416 is retracted from the pool 2418, etc.

[0133] In another variation, a filament of optically transparent material can be extracted from the pool 2418. In such a variation, the filament can be heated using a suitable heat source (e.g., a furnace, a butane flame, a hydrogen flame, or a laser). The heat can melt the optically transparent material. The melted portion of the filament can be formed into a substantially curved portion (e.g., a spherical portion) of the WGM microsphere resonator using a reflow process (e.g., the reflow process illustrated in FIG. 20).

[0134] FIG. 25 is a schematic diagram of an exemplary variation of a method for fabricating a WGM microbubble resonator. The fabrication process may include sealing one end 2505B of the capillary tube 2501B. The seal on the end 2505B may be formed, for example, by heating and pinching off the open end of the capillary tube 2501B from a plug material (e.g., epoxy). Alternatively, the capillary tube 2501B may be molded with a closed end 2505B. The end opposite the sealed end 2505B may be connected to a pressure source (e.g., a pump or compressor) to control the gas flow in the capillary tube 2501B. Using the gas flow 2503B, a stable pressure may be maintained inside the capillary tube 2501B sealed at the end 2505B. The method may also include heating the capillary tube at one or more target microbubble locations using a set of resistive heating elements 2503B. Each resistive heating element 2503B can surround (e.g., encircle) the capillary 2501B at a respective target WGM microbubble resonator location, such that the set of resistive heating elements can precisely apply heat to a set of target WGM microbubble resonator locations. The manufacturing process for a set of WGM microbubble resonators can be repeated with precise control to consistently reproduce a set of WGM microbubble resonators having the desired geometric dimensions of the microbubble resonators 2502B, including outer microbubble radius R and inner microbubble radius r, and wall thickness, as described with respect to FIG. 2. With precise control of the manufacturing process, a set of WGM microbubble resonators can be manufactured to have a wall thickness that is smaller than the wavelength of the set of ultrasonic echoes that the set of WGM microbubble resonators are designed to sense.

[0135] It should be readily appreciated that the WGM resonators can be fabricated using any suitable optically transparent material using any one or combination of the methods described in Figures 20-25.

[0136] Fabrication of WGM resonator arrays As mentioned above, in some variations, the sensing device and / or sensing apparatus may include an array of WGM resonators. The WGM resonator array may be fabricated by fabricating each individual WGM resonator and aligning one or more optical waveguides to the WGM resonator such that the WGM resonator couples with the one or more optical waveguides. Each individual WGM resonator of the WGM resonator array may be fabricated simultaneously. Alternatively, at least some WGM resonators of the WGM resonator array may be fabricated at a different time (e.g., sequentially) than other WGM resonators of the WGM resonator array.

[0137] In some variations, the individual WGM resonators of a WGM resonator array can be fabricated along a single fiber and / or capillary. Referring back to FIG. 25, the exemplary method described above with respect to FIG. 25 can be used to fabricate multiple WGM microbubble resonators from a single capillary 2501B. These multiple WGM microbubble resonators can be fabricated simultaneously by using a set of resistive heating elements at precise locations at the same time. As described above with respect to FIG. 25, the size of each WGM microbubble resonator can be controlled in a precise manner. Thus, WGM resonator arrays with different sized WGM microbubble resonators can be fabricated from the same capillary 2501B. Similarly, WGM resonator arrays with the same sized WGM microbubble resonators can be fabricated from the same capillary 2501B.

[0138] FIG. 26 illustrates an exemplary variation of a method for fabricating individual WGM resonators for a WGM resonator array along a single optical fiber 2601. The optical fiber 2601 may include any suitable optically transparent material. Each WGM resonator of the WGM resonator array may be fabricated using a reflow process, such as using one or more of the methods described above with respect to FIGS. 20-22. Similar to that shown in FIG. 25, target locations on the optical fiber 2601 may be subjected to heat (simultaneously or sequentially). The reflow process produces substantially curved portions of the WGM resonators (e.g., portions 2602a-d).

[0139] After fabricating the substantially curved portion for each individual WGM resonator using the methods described above with respect to Figures 25 or 26, the optical fiber and / or capillary tube can be cut at specific locations to generate the individual WGM resonators. For example, the optical fiber and / or capillary tube can be cut such that each individual WGM resonator may include a substantially curved portion and a stem portion. Alternatively, the optical fiber and / or capillary tube may be cut such that each individual WGM resonator may include only a substantially curved portion.

[0140] In some variations, each of the multiple WGM resonators in the WGM resonator array can be individually fabricated using the methods described above with respect to Figures 23 and / or 24. For example, the multiple WGM resonators may be fabricated from multiple respective optical fibers and / or capillaries rather than collectively fabricated from a single optical fiber or capillary. Each of the fabricated WGM resonators can be arranged together to form a WGM resonator array (e.g., in any suitable arrangement such as those shown in Figures 12-19).

[0141] As described above, each WGM resonator in a WGM resonator array can be optically coupled to one or more waveguides. FIG. 27 is a schematic diagram of an exemplary variation of a method for fabricating a WGM microbubble resonator array and coupling the WGM microbubble resonators to an optical waveguide. A WGM microbubble resonator array 2702 can be fabricated from a single capillary tube and can include multiple WGM microbubble resonators in a linear arrangement. In other words, multiple WGM microbubble resonators can be formed in series along a single capillary tube (e.g., the method described in FIG. 25). For example, the capillary tube can be heated at multiple locations where it is desired to position the resonators in the WGM microbubble resonator array 2702. Additionally, multiple instances of the WGM microbubble resonator array 2702 can be combined (e.g., in parallel) to form a WGM microbubble resonator array. 27, each optical waveguide in a set of multiple optical waveguides 2701 can then be aligned to a respective WGM microbubble resonator in a WGM microbubble resonator array 2701 to optically couple the optical waveguide 2701 to the WGM microbubble resonator. Additionally or alternatively, two or more WGM microbubble resonators in a WGM microbubble resonator array 2702 can be aligned to a single optical waveguide (e.g., similar to those shown in FIGS. 13-15).

[0142] Damping of higher modes in WGM resonators. As mentioned above, multiple modes can be excited in a WGM resonator, starting from the fundamental mode confined to the equatorial plane in a WGM resonator including a spherical portion, to higher order modes extending in polar and radial directions in a WGM resonator including a spherical portion. Each mode with a different spatial distribution can have a different bandwidth and sensitivity. For example, the fundamental mode has a wider bandwidth than the higher order modes. Therefore, it can be advantageous to eliminate the higher order modes for sensing applications to increase the Q factor and sensitivity of the WGM resonator.

[0143] One method of damping higher-order modes may include applying a polymer coating to a substantially curved portion of the WGM resonator. FIG. 28 shows an exemplary variation of a WGM microsphere resonator with a polymer coating 2820. The WGM microsphere resonator may include a spherical portion 2802a and a stem portion 2802b. The polymer coating 2820 may be applied onto the spherical portion 2802a of the WGM microsphere resonator. The polymer coating 2820 may be applied onto the outer surface of the spherical portion 2802a of the WGM microsphere resonator. For example, the polymer coating 2820 may cover approximately half of the circumference of the spherical portion 2802a, thereby partially coating the WGM microsphere resonator. By partially coating the WGM microsphere, higher-order modes may be damped. In other words, the operating frequency of the WGM microsphere may be limited to the fundamental mode (a mode confined to the equatorial plane of the WGM microsphere).

[0144] The optical waveguide 2801 may be coupled to the WGM microsphere resonator. In some variations, the optical waveguide 2801 may be coupled to the WGM microsphere resonator prior to application of the polymer coating 2820. In some variations, the polymer coating 2820 may include an optically clear liquid photopolymer having a high viscosity. Additionally, the polymer coating 2820 may include adhesive properties. In some variations, the refractive index of the polymer coating may be higher than the refractive index of the WGM microsphere resonator. For example, the refractive index of the polymer coating may be higher than 1.5. In some variations, the polymer coating 2820 may include Norland Optical Adhesive 68 ("NOA68").

[0145] In some variations, the polymer coating 2820 may be applied to the highest / top point of the spherical portion 2802a (e.g., the zenith). In some variations, the polymer coating 2820 may be applied onto a very low height spherical cap located at the top of the spherical portion 2802a. For example, the polymer 2820 coating may be poured onto the highest point. The polymer coating 2820 may flow downward from the highest point to the lower region of the spherical portion 2802a, thereby coating the spherical cap of the spherical portion 2802a. Due to its high viscosity, the polymer coating 2820 takes time to flow towards the lower region of the spherical portion 2802a. Once the polymer coating 2820 has coated a suitable amount of the circumference of the spherical portion 2802a (e.g., a spherical cap with a base at a suitable distance from the equatorial plane), the polymer coating 2820 may be cured (e.g., using ultraviolet light). For example, the polymer coating 2820 may be cured when it covers approximately half of the circumference of the spherical portion 2802a. The time to cure the polymer coating 2820 may depend on the thickness of the polymer coating 2820, the intensity of the UV light used for curing, etc.

[0146] Consider a polymer coating covering a spherical cap of a WGM resonator. The base of the spherical cap can be a distance d from the equatorial plane. To confine the excitation of the WGMs in the resonator to the fundamental mode, the value of d can be from about 3 μm to about 15 μm in some variations.

[0147] FIG. 29 illustrates variations of methods for attenuating higher order modes. When a polymer coating 2920 is applied to cover a suitable amount of area on the circumference of the spherical portion 2902a of the WGM microsphere resonator, the control light 2922 can be transmitted through the stem portion 2902b of the WGM microsphere resonator to the spherical portion 2902a. The polymer coating 2920 can absorb power from the control light 2922. In some variations, the amount of power that the polymer coating 2920 absorbs from the control light 2922 can depend on the material included in the polymer coating 2920. For example, the polymer coating 2920 can include a material that can absorb a large amount of power from the control light 2922 (e.g., more than 50 percent, more than about 60 percent, more than about 70 percent, more than about 80 percent, or more than about 90 percent). This strong absorption of power from the control light 2922 can result in losses to higher order modes that overlap with the polymer coating 2920.

[0148] This can change the temperature of the polymer coating, thereby changing the resonant frequency of the WGM microsphere resonator. For example, the change in temperature can depend on the power of the control light 2922, the thermo-optic coefficient of the polymer coating 2920, and the material of the WGM microsphere resonator. The temperature change of the polymer coating 2920 can cause a shift in the resonant frequency of the WGM microsphere resonator due to the thermo-optic effect.

[0149] In some variations, the polymer coating 3020 may be applied to both the spherical portion 3002a and the stem portion 3002b of the WGM microsphere resonator, as shown in Figure 30. For example, the polymer coating 3020 may be applied to approximately half the circumference of the spherical portion 3002a and to at least a portion of the stem portion 3002b. In some variations, the WGM microsphere resonator may be immersed in a pool of polymer coating 3120, as shown in Figure 31. The effective thickness of the microsphere resonator can be controlled by immersing an appropriate area of ​​the stem portion 3120b in the pool of polymer coating 3120.

[0150] Encapsulation of coupled WGM resonators in polymer structures The fabricated WGM resonators and / or WGM resonator arrays and one or more optical waveguides can be encapsulated in a polymer structure. In one variation, the optical waveguide can be embedded in a polymer structure, and then the WGM resonator can be embedded in the polymer structure such that the optical waveguide is optically coupled to the WGM resonator. Alternatively, the WGM resonator can be embedded in a polymer structure, and then the optical waveguide can be embedded in the polymer structure such that the optical waveguide is optically coupled to the WGM resonator. In another alternative variation, the optical waveguide and the WGM resonator can be optically coupled. Following optical coupling, the coupled waveguide and WGM resonator can be embedded in a polymer structure.

[0151] 32A is a schematic diagram of an exemplary method of encapsulating a WGM resonator 3202a and / or an optical waveguide 3201a in a polymer structure to produce a packaged WGM resonator. The polymer structure may include a backing region 3210a and / or a matching region 3208a. In some variations, the backing region 3210a and / or the matching region 3208a may be layers deposited on a substrate 3206, as described in more detail below.

[0152] The method of packaging the WGM resonator 3202a may include cleaning the surface of the substrate 3206a, such as, for example, silicon, silica, quartz, plastic, or any other material suitable to serve as a substrate for a sensing device. A suitable material to serve as the substrate 3206a may include a damping material to eliminate residual vibrations and minimize ultrasonic echoes in the WGM resonator 3202a. Referring to FIG. 32A, the substrate 3206a may then be coated with a set of one or more coating materials including a backing polymer 3210a. The backing polymer 3210a may be a damping material such as a polymer material with a low refractive index configured to have high ultrasonic attenuation to prevent reverberation of the set of ultrasonic echoes that the WGM resonator 3202a is designed to sense while obtaining a broad whispering gallery mode (WGM) frequency response. In some variations, the acoustic impedance of the damping material of the backing polymer 3210a may be matched to the acoustic impedance of a layer of matching polymer 3208a utilized to package the WGM resonator 3202a, for example, as described further below.

[0153] As shown in FIG. 32A, one or more optical waveguides 3201a, such as optical fibers or integrated photonic waveguides, can be disposed within the backing polymer 3210a. The optical waveguide can be an optical fiber that receives light from a light source, an integrated photonic waveguide that receives light from an on-chip light source, an integrated photonic waveguide coupled to an optical fiber that propagates light from the light source, or any other suitable medium that propagates light from the light source from the WGM resonator to a coupling point. In some variations, the one or more optical waveguides 3201a can be disposed within the backing polymer 3210a prior to curing the backing polymer 3210a. By disposing the one or more optical waveguides 3201a on an uncured backing polymer, the one or more optical waveguides can be embedded within the backing polymer 3210a. After disposing the one or more optical waveguides 3201a, the backing polymer 3210a can be cured, such as by baking the backing polymer 3210a at a preset temperature.

[0154] The method of packaging the WGM resonator 3202a may further include placing the WGM resonator 3202a in proximity to one or more optical waveguides 3201a so as to optically couple the WGM resonator to at least one optical waveguide. For example, the WGM resonator 3202a and the optical waveguide 3201a may be positioned such that the optical waveguide is aligned with the maximum diameter of the WGM resonator. In one example, the WGM resonator 3202a may have an elliptical shape, and the maximum circular diameter of the WGM resonator 3202a may be aligned with one or more optical waveguides 3201a. It should be understood that although the method of packaging the WGM resonator is primarily described with the optical waveguide in front of the WGM resonator, in some variations the WGM resonator may be placed in front of the optical waveguide.

[0155] Referring to FIG. 32A, the method of packaging the WGM resonator 3202a may further include coating the WGM resonator 3202a, the backing polymer 3210a, and / or the substrate 3206a with a matching polymer 3208a. The matching polymer may be configured to facilitate ultrasonic transmission from a medium in contact with the ultrasonic sensor (embodied by the WGM resonator). For example, the layer of matching polymer 3208a may be a polymer material having a low refractive index to reduce impedance of transmission of a set of ultrasonic echoes to the WGM resonator 3202a. The matching polymer 3208a may have a low refractive index compared to the refractive index of the WGM resonator 3202a and the refractive index of one or more optical waveguides 3201a. The method of packaging the WGM resonator 3202a may include curing the layer of matching polymer.

[0156] Although the exemplary schematic of Figure 32A depicts the optical waveguide 3201a embedded in the backing polymer, in some variations the optical waveguide may be in the matching polymer. For example, as shown in Figure 32B, one or more optical waveguides 3201b may be placed on top of the backing polymer 3210b after the backing polymer 3210b is cured and before the WGM resonator 3202b, backing polymer 3210b, and / or substrate 3206b are coated with matching polymer 3208b, such that the optical waveguide 3201b is embedded in the matching polymer instead of the backing polymer. Furthermore, in some variations the optical waveguide may be partially embedded in the backing polymer and partially embedded in the matching polymer (e.g., about half of its cross-sectional area is in the backing polymer and about half of its cross-sectional area is in the matching polymer). Furthermore, it should be understood that in embodiments of devices having multiple optical waveguides in a polymer, some of the optical waveguides may be fully or partially embedded in a backing polymer, while other portions of the optical waveguides may be fully or partially embedded in a matching polymer in any suitable combination.

[0157] In some embodiments, the packaged WGM resonators 3202a, 3202b can be used as ultrasonic sensors in the form generally shown in Figures 32A and 32B. Alternatively, however, in some variations, the method of packaging the WGM resonators may include pre-coating the substrate with a release agent (e.g., polyvinyl alcohol, polyacrylic acid, polystyrene, fluorosilane, self-assembled monolayer, some types of photoresist such as OmniCoat, AZ series, etc.) before coating the substrate with a backing polymer so that the substrate can be released from the remainder of the packaged WGM resonator. Removal of the substrate may help, for example, to reduce the amount of reverberation of the set of ultrasonic echoes that the WGM resonators are designed to sense.

[0158] Sensing Applications WGM resonators and resonator arrays as described herein can be used in ultrasonic sensor devices, such as acousto-optic sensor devices. One or more elements in the acousto-optic sensor device can be configured to generate a set of ultrasonic signals. Some non-limiting examples of these elements can include piezoelectric sensors, single crystal material sensors, piezoelectric micromachined ultrasonic transducers (PMUTs), capacitive micromachined ultrasonic transducer sensors (CMUTs), and the like. The ultrasonic signals can induce changes in the refractive index and / or physical structure of the WGM resonators. Such changes can be measured as further described in the description accompanying FIGS. 33-38.

[0159] FIG. 33 is a schematic diagram of sensing ultrasonic echoes using an acousto-optical sensor device 3303. The acousto-optical sensor device 3303 includes a WGM resonator array. Although the WGM resonators are shown to be arranged linearly at a predetermined distance d from each other, it should be understood that in some variations, at least some of the resonators may be arranged at different distances from each other. The WGM resonator array can be configured to be optically coupled to one or more optical waveguides that propagate light from a light source. The light source can be 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-optical sensor device 3303. Light can be coupled to the WGM resonator array to propagate a first set of whispering gallery modes (WGMs) from the WGM resonator array around the surrounding walls of each WGM resonator. The propagation of the first set of WGMs generates a first set of optical signals corresponding to the resonant frequencies of the first set of WGMs.

[0160] In use, the acousto-optic sensor device 3303 can be configured to receive a set of ultrasonic echoes 3302 generated and / or reflected from the object 3301. The set of ultrasonic echoes can have different travel times (t1, t2, t3, etc.) to the individual WGM resonators. 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 can induce a set of changes to the radius and / or refractive index of the WGM resonator material and / or the refractive index of the ultrasound enhancing material in the WGM resonator. This set of changes can propagate a second set of WGMs from the WGM resonator array around the surrounding walls of each WGM resonator. The propagation of the second set of WGMs generates a second set of optical signals corresponding to the resonant frequencies of the second set of WGMs. The first set of optical signals and the second set of optical signals can be configured to propagate in one or more optical waveguides to an optical detector. The optical detector can be connected to the computing device for detecting a difference between the first set of optical signals and the second set of optical signals.

[0161] FIG. 34 depicts an exemplary measurement setup that may be used to process a set of signals from an acousto-optic sensor device 3405 sensing a set of ultrasonic echoes (also referred to herein as “ultrasound signals”). The measurement setup may include a broadband light source or tunable laser 3401 propagating light in an optical fiber 3402 and a fiber polarization controller 3403 that may be configured to control the polarization of the light. The measurement setup may further include an acousto-optic sensor device 3405 that may be configured to propagate a first set of optical signals based on the propagation of a first set of whispering gallery modes (WGMs) in a WGM resonator array included in the acousto-optic device 3405. The acousto-optic sensor device 3405 may be configured to receive a set of ultrasonic signals 3404 and propagate a second set of optical signals based on the propagation of a second set of WGMs propagating in the WGM resonator array. The acousto-optic sensor device 3405 may be further configured to transmit the first set of optical signals and / or the second set of optical signals to the optical fiber 3402. The measurement setup may further include a photodetector (also referred to herein as “optical detector”) 3406 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 3406 may be further configured to transmit the first set of electrical signals and / or the second set of electrical signals to an oscilloscope 3407 operatively connected to a computing device 3410 for processing and analyzing the first set of signals and / or the second set of signals. The oscilloscope 3407 may further be configured to transmit the set of analyzed signals to a function generator 3408. The function generator may be configured to generate and transmit a set of generated signals to the broadband light source or tunable laser 3401 to control the propagation of light in the optical fiber 3402.

[0162] FIG. 35 depicts an exemplary measurement setup that may be used to process a set of signals from an acousto-optic sensor device 3504 sensing a set of ultrasonic echoes. The measurement setup may include a laser 3501, 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 3504, to generate a laser light. The measurement setup may further include a beam splitter 3502, such as a 50:50 beam splitter, an 80:20 beam splitter, or any other beam splitter. 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 3503 to generate and transmit a modulated laser light. The measurement setup may be further configured to direct modulated laser light to the acousto-optic sensor device 3501 to generate a first set of signals corresponding to a first set of whispering gallery modes (WGMs). The acousto-optic sensor device 3501 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 3505 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 3506 operatively connected to the 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.

[0163] 36 is a schematic diagram of the optical response of an acousto-optic sensor device 3601. The acousto-optic sensor device 3601 can be configured to receive a set of ultrasonic echoes from an object 3602 positioned at any angle (e.g., as described in further detail with respect to FIG. 34 and / or FIG. 35) and detect the ultrasonic echoes using a set of WGM resonators. In one example, the acousto-optic sensor device 3601 can have a detection angle 3603 of about 160 degrees. In other words, the acousto-optic device 3601 has a wider detection angle than a conventional piezoelectric ultrasonic sensor.

[0164] Ultrasound probe In some embodiments, a WGM resonator array as described herein may be included in an ultrasonic probe (also referred to herein as an "ultrasound system"). The ultrasonic probe may include at least one optical waveguide for propagating light. The ultrasonic probe may further include a plurality of piezoelectric elements for generating a set of ultrasonic signals. The ultrasonic probe may further include a plurality of piezoelectric elements for receiving a set of ultrasonic echoes corresponding to the set of ultrasonic signals. The ultrasonic probe may further include a plurality of WGM resonators in the polymer structure, each WGM resonator configured to receive a plurality of ultrasonic echoes and propagate a set of whispering gallery modes (WGMs). The at least one WGM resonator is optically coupled to the at least one optical waveguide such that the WGM resonator is configured to transmit a first set of signals corresponding to the first set of WGMs to the at least one optical waveguide.

[0165] For example, FIG. 37 is a schematic diagram of an ultrasonic probe utilizing WGM resonators as described herein. FIG. 38 is a cross-sectional view of the probe depicted in FIG. 37. The ultrasonic probe may include a WGM resonator array 3701, a matching layer 3702, a piezoelectric crystal array 3703, an electrical connection array 3704, a set of optical fibers 3705, a backing material 3706, an acoustic insulator 3707, and a cable 3708 connected to ultrasonic data acquisition. The ultrasonic 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.

[0166] The ultrasonic probe may be configured to propagate a set of light from a light source to a set of WGM resonators in the WGM resonator array 3701 to generate a first set of optical signals corresponding to a first set of WGMs propagating in each WGM resonator in the WGM resonator array 3701. The piezoelectric crystal array 3703 may be configured to receive electrical signals from a control system via cables 3708 and electrical connection array 3704 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 WGM resonator array 3701 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 a set of optical fibers 3705 and / or optionally transmit the set of electrical signals via the electrical connection array 3704 to a cable 3708 connected to an ultrasonic data acquisition, control system, or display.

[0167] In some variations, an ultrasound probe such as that described above with respect to Figures 37 and 38 can be configured to repeat the scanning sequence while scanning across the field of view by using a phased array of piezoelectric crystal arrays. This results in a line-by-line image with the piezoelectric crystal array and one low-resolution image in the lateral direction for each WGM resonator array sensor. A high-resolution WGM resonator array sensor image can then be generated using known synthetic aperture (SA) algorithms.

[0168] Further, in some embodiments, one or more of the piezoelectric elements can 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 signal generated by the piezoelectric element may supplement or be combined with the signal transmitted by the WGM resonator in any suitable manner (e.g., to provide a multimodal sensor image).

[0169] Additionally or alternatively, in some embodiments, an ultrasound probe as described above with respect to Figures 37 and 38 can be configured to use different excitation patterns, such as using a piezoelectric crystal element or group of piezoelectric 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 a compressive sensing (CS) method. The general approach of the CS method 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.

[0170] 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 have been 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 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: At least one optical waveguide; a plurality of whispering gallery mode (WGM) resonators, each WGM resonator configured to propagate a first set of a plurality of whispering gallery modes (WGMs); a polymer structure including a plurality of regions, the plurality of regions including a region encapsulating the at least one optical waveguide and at least one WGM resonator of the plurality of WGM resonators; Including, the at least one WGM resonator is optically coupled to the at least one optical waveguide such that the at least one WGM resonator is configured to transmit a first set of a plurality of signals corresponding to the first set of a plurality of WGMs to the at least one optical waveguide; An apparatus, wherein an acoustic impedance of at least one of the plurality of regions is matched to an acoustic impedance of the encapsulated at least one WGM resonator.

2. The polymer structure is a backing region configured to attenuate residual ultrasonic echoes and thereby prevent reverberation; a matching region configured to increase a bandwidth of a WGM frequency response of the plurality of WGM resonators; The apparatus of claim 1 , comprising at least one of:

3. The apparatus of claim 2 , wherein the matching region is further configured to improve ultrasonic transmission to the plurality of WGM resonators.

4. The apparatus of claim 3 , wherein at least one of the backing region and the matching region has a first refractive index that is lower than a second refractive index of the at least one optical waveguide.

5. The device of claim 2 , wherein the at least one optical waveguide is embedded within the backing region of the polymer structure.

6. The device of claim 2 , wherein the at least one optical waveguide is embedded within the matching region of the polymer structure.

7. The apparatus of claim 2 , wherein the at least one WGM resonator is embedded within the matching region of the polymer structure.

8. The device of claim 1 , wherein the effective refractive index of the polymer structure is lower than the refractive index of the at least one WGM resonator.

9. The device of claim 1 , wherein the polymer structure comprises an ultrasound enhancing material.

10. The apparatus of claim 1 , wherein an effective acoustic impedance of the polymer structure matches an acoustic impedance of the at least one WGM resonator.

11. The apparatus of claim 1 , wherein the at least one WGM resonator comprises at least one of a microsphere and a microbubble.

12. The at least one WGM resonator comprises: receiving a plurality of ultrasonic echoes; propagating a second set of WGMs in response to the ultrasonic echoes; and The apparatus of claim 1 configured to:

13. 13. The apparatus of claim 12, wherein the at least one WGM resonator is further configured to transmit a second set of signals corresponding to the second set of WGMs to the at least one optical waveguide.

14. The apparatus of claim 13, further comprising a photodetector communicatively coupled to the at least one optical waveguide, the at least one optical waveguide configured to propagate the first set of multiple signals and the second set of multiple signals to the photodetector.

15. The device of claim 1 , wherein the at least one optical waveguide comprises an optical fiber or an integrated photonic waveguide.

16. The apparatus of claim 1 , wherein the at least one optical waveguide is coupled to a light source.

17. The apparatus of claim 16 , wherein the light source comprises a broadband light source or a tunable laser source.

18. The apparatus of claim 1 , wherein the at least one WGM resonator comprises a hollow chamber.

19. The apparatus of claim 18 , wherein the hollow chamber comprises an ultrasound enhancing material.

20. The apparatus of claim 1 , wherein each WGM resonator of the plurality of WGM resonators is optically coupled to the at least one optical waveguide.

21. 2. The apparatus of claim 1, wherein the at least one optical waveguide comprises a plurality of optical waveguides, each WGM resonator of the plurality of WGM resonators being optically coupled to a respective optical waveguide of the plurality of optical waveguides.

22. The apparatus of claim 1 , wherein at least a portion of the plurality of WGM resonators are grouped in a linear arrangement.

23. 23. The apparatus of claim 22, wherein the WGM resonators in the linear arrangement are equally spaced apart.

24. 23. The apparatus of claim 22, wherein the WGM resonators in the linear arrangement are unequally spaced apart.

25. The apparatus of claim 1 , wherein the plurality of WGM resonators are bundled in a circular arrangement.

26. The apparatus of claim 1 , wherein at least a portion of the plurality of WGM resonators are equal in size.

27. The apparatus of claim 1 , wherein at least some of the plurality of WGM resonators are unequal in size.

28. 13. A system comprising the apparatus of claim 12 and a plurality of array elements configured to generate a set of a plurality of ultrasound signals, the plurality of ultrasound echoes corresponding to the set of a plurality of ultrasound signals.

29. 30. The system of claim 28, wherein the plurality of array elements includes at least one array element selected from the group consisting of a piezoelectric sensor, a single crystal material sensor, a piezoelectric micromachined ultrasonic transducer (PMUT), and a capacitive micromachined ultrasonic transducer sensor (CMUT).

30. A method of ultrasound imaging, comprising: receiving a first set of signals corresponding to a first set of whispering gallery mode (WGM) resonators propagating within a plurality of WGMs from at least one optical waveguide, the at least one optical waveguide and the plurality of WGM resonators being within a polymer structure; receiving a second set of signals from the at least one optical waveguide corresponding to a second set of WGMs propagating within the WGM resonators, the second set of WGMs propagating in response to the WGM resonators receiving ultrasonic echoes; detecting a set of differences between the first set of signals and the second set of signals; A method comprising:

31. The polymer structure is a backing region configured to attenuate residual ultrasonic echoes and thereby prevent reverberation; a matching region configured to increase a bandwidth of a WGM frequency response of the plurality of WGM resonators; 31. The method of claim 30, comprising at least one of:

32. 32. The method of claim 31, wherein the matching region is further configured to improve ultrasonic transmission to the plurality of WGM resonators.

33. 32. The method of claim 31 , wherein at least one of the backing region and the matching region has a first refractive index that is lower than a second refractive index of the at least one optical waveguide.

34. The method of claim 33 , wherein the at least one optical waveguide is within the backing region of the polymer structure.

35. 34. The method of claim 33, wherein the at least one optical waveguide is within the matching region of the polymer structure.

36. The method of claim 33 , wherein the plurality of WGM resonators are within the matching region of the polymer structure.

37. The method of claim 30 , wherein the effective refractive index of the polymer structure is lower than the refractive index of the plurality of WGM resonators.

38. The method of claim 30 , wherein the polymer structure comprises an ultrasound enhancing material.

39. The method of claim 30 , wherein an effective acoustic impedance of the polymer structure matches an acoustic impedance of the plurality of WGM resonators.

40. 31. The method of claim 30, wherein at least one WGM resonator of the plurality of WGM resonators comprises at least one of a microsphere and a microbubble.

41. The method of claim 30 , wherein each of the plurality of WGM resonators is optically coupled to the at least one optical waveguide.

42. The method of claim 30 , wherein the plurality of ultrasound echoes corresponds to a plurality of ultrasound signals.

43. The method of claim 30, further comprising performing a synthetic aperture (SA) operation on the first set of multiple signals and the second set of multiple signals, and generating an image of an object based at least in part on the SA operation.

44. The method of claim 30, further comprising performing a compressive sensing (CS) operation on the first set of signals and the second set of signals, and generating an image of an object based at least in part on the CS operation.

45. An apparatus comprising: The apparatus includes a whispering gallery mode (WGM) resonator configured to propagate a set of WGMs, the WGM resonator including a curved surface; the curved surface includes a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator, the second mode being at a higher frequency than the first mode; The second spherical segment is coated with a polymer configured to damp the second mode.

46. 46. ​​The apparatus of claim 45, wherein the WGM resonator is a microsphere or microbubble resonator.

47. 46. ​​The device of claim 45, wherein the first spherical segment is not coated with the polymer.

48. 46. ​​The apparatus of claim 45, wherein the first mode is a fundamental mode of the WGM resonator.

49. 46. ​​The apparatus of claim 45, wherein the second spherical segment is a spherical cap of the WGM resonator.

50. 50. The apparatus of claim 49, wherein the base of the spherical cap is at a distance of about 3 μm to about 15 μm from the equatorial plane of the WGM resonator.

51. 50. The apparatus of claim 49, wherein the WGM resonator includes a stem side and a non-stem side that is of opposite polarity to the stem side.

52. 52. The apparatus of claim 51, wherein the spherical cap is on the stem side of the WGM resonator.

53. 52. The apparatus of claim 51 , wherein the spherical cap is on the non-stem side of the WGM resonator.

54. 46. ​​The device of claim 45, wherein the second spherical segment is coated with the polymer.

55. 46. ​​The device of claim 45, wherein the second spherical segment is encapsulated by the polymer.

56. 46. ​​The apparatus of claim 45, wherein the polymer has a refractive index higher than the refractive index of the WGM resonator.

57. 46. ​​The method of claim 45, wherein the polymer comprises an optical adhesive.

58. The apparatus of claim 45, further comprising an optical waveguide optically coupled to the WGM resonator.

59. 60. A system comprising the apparatus of claim 58 and a plurality of array elements configured to generate a set of a plurality of ultrasonic signals, wherein the WGM resonator is configured to propagate the set of a plurality of WGMs in response to receiving a plurality of ultrasonic echoes corresponding to the set of a plurality of ultrasonic signals.

60. 1. A method of making a whispering gallery mode (WGM) resonator, the method comprising: providing a WGM resonator including a curved surface, the curved surface including a first spherical segment corresponding to a first mode of the WGM resonator and a second spherical segment corresponding to a second mode of the WGM resonator, the second mode being at a different frequency than the first mode; coating the second spherical segment with a polymer configured to damp the second mode; A method comprising:

61. 61. The method of claim 60, wherein the WGM resonator is a microsphere or microbubble resonator.

62. 61. The method of claim 60, wherein the polymer does not coat the first spherical segment.

63. 61. The method of claim 60, wherein the first mode is a fundamental mode of the WGM resonator.

64. 61. The method of claim 60, wherein the second spherical segment is a spherical cap of the WGM resonator.

65. 65. The method of claim 64, wherein the WGM resonator includes a stem side and a non-stem side that is of opposite polarity to the stem side.

66. 66. The apparatus of claim 65, wherein the spherical cap is on the stem side of the WGM resonator.

67. 66. The apparatus of claim 65, wherein the spherical cap is on the non-stem side of the WGM resonator.

68. 65. The method of claim 64, wherein covering the second spherical segment with the polymer comprises coating the spherical cap with the polymer.

69. 65. The method of claim 64, wherein coating the second spherical segment with the polymer comprises immersing the spherical cap in a solution containing the polymer.

70. 61. The method of claim 60, wherein the polymer has a refractive index higher than the refractive index of the WGM resonator.

71. 61. The method of claim 60, wherein the polymer comprises an optical adhesive.

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