Whispering gallery mode resonators for sensing applications

Abstract

Disclosed herein are sensing devices and methods of manufacturing the same. In some variations, the sensing device may include at least one optical waveguide and at least one whispering gallery mode (WGM) resonator (1602a-c), the WGM resonator configured to propagate a set of WGMs, wherein 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 may encapsulate the at least one WGM resonator and / or the at least one optical waveguide. Additionally, in some variations, the WGM resonator may have one or more selectable modes with different bandwidths and sensitivities for sensing, which may, for example, enable the sensing device to be tailored to specific applications with specific bandwidth and / or sensitivity requirements.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. patent application serial number 62 / 945,538, filed on December 9, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates generally to the field of sensing platforms, and in particular to methods and apparatus for ultrasonic sensing using whispering gallery mode (WGM) resonators. Background Art

[0004] Sensing applications using ultrasound sensing are gaining popularity due to the advantages it offers. For example, in the field of medical imaging, ultrasound imaging is known to be an advantageous non-invasive imaging modality.

[0005] Conventional ultrasonic sensing uses piezoelectric materials such as lead zirconate titanate (PZT), polymer thick film (PTF), and polyvinylidene fluoride (PVDF). These materials have several disadvantages. For example, some of the challenges associated with using piezoelectric materials include high operating voltage requirements, high electric field requirements (which can lead to breakdown and failure), nonlinear response with high hysteresis, and limited detection angles.

[0006] Therefore, there is a need for new and improved apparatus and methods for various sensing applications, including ultrasonic sensing. Summary of the Invention

[0007] In general, in some variations, a 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 from 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.

[0008] 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 additionally configured to improve ultrasonic transmission to the one or more WGM resonators.

[0009] In some variations, at least one optical waveguide may be embedded in the backing region. Alternatively, at least one optical waveguide may be embedded in the matching region. The backing region and / or the matching region may have a lower refractive index than the at least one optical waveguide.

[0010] In some variations, at least one WGM resonator may be embedded in a matching region of a 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 enhancement material. The effective acoustic impedance of the polymer structure may match the effective acoustic impedance of the at least one WGM resonator.

[0011] In some variations, in response to receiving an ultrasonic echo, 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 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 comprising the device and a plurality of array elements (e.g., a piezoelectric sensor, a single crystal material sensor, a piezoelectric micromachined ultrasonic transducer (PMUT), and a capacitive micromachined ultrasonic transducer sensor (CMUT)) may be configured to generate a set of ultrasonic signals. The ultrasonic echo may correspond to the set of ultrasonic signals.

[0012] In some variations, the at least one optical waveguide may comprise 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 comprise a broadband light source or a tunable laser source.

[0013] In some variations, at least one WGM resonator may be a microsphere resonator or a microbubble resonator. In some variations, at least one WGM resonator may comprise a hollow chamber (eg, a microbubble resonator). The hollow chamber may comprise an ultrasound enhancing material.

[0014] In some variations, each WGM resonator in the plurality of WGM resonators may be optically coupled to at least one optical waveguide. In some variations, the at least one optical waveguide may include more than one optical waveguide (e.g., a plurality of optical waveguides). Each WGM resonator may be optically coupled to a respective optical waveguide.

[0015] In some variations, at least a portion of the WGM resonators may be grouped in a linear arrangement. The WGM resonators may be equally spaced or unequally spaced. In other variations, one or more WGM resonators may be bundled together in a circular arrangement. In some variations, at least some 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.

[0016] 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 ultrasound echoes. The method also includes detecting a set of differences 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.

[0017] 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 further improve ultrasonic transmission to the one or more WGM resonators.

[0018] In some variations, at least one optical waveguide may be in a backing region. Alternatively, at least one optical waveguide may be in a matching region. The refractive index of the backing region and / or the matching region may be lower than the refractive index of the at least one optical waveguide. In some variations, 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 the refractive index of the one or more WGM resonators.

[0019] 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 in the plurality of WGM resonators may include at least one of microspheres and microbubbles.

[0020] In some variations, each of the one or more WGM resonators may be optically coupled to at least one optical waveguide.In some variations, the plurality of ultrasound echoes may correspond to a plurality of ultrasound signals.

[0021] In some variations, the ultrasound imaging method 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 ultrasound imaging method may further include performing a compressed 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.

[0022] In general, a 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 have a higher frequency than the first mode. The second spherical segment may be covered by a polymer configured to attenuate the second mode. The device may further include an optical waveguide optically coupled to the WGM resonator.

[0023] In some variations, a system including the apparatus may further include a plurality of array elements configured to generate a set of ultrasound signals. The WGM resonator may be configured to propagate the set of WGMs in response to receiving a plurality of ultrasound echoes corresponding to the set of ultrasound signals.

[0024] In some variations, the WGM resonator may be a microsphere or microbubble resonator. The first spherical segment may not be covered by a polymer. The first mode may be the fundamental mode of the WGM resonator. In some variations, the second spherical segment may be a spherical cap of the WGM resonator. The distance between the base of the spherical cap and the equatorial plane of the WGM resonator may be between 3 μm and 15 μm.

[0025] In some variations, a WGM resonator may include a rod side and a non-rod side of opposite polarity to the rod side. In some variations, the spherical cap may be on the rod side of the WGM resonator. Alternatively or additionally, the spherical cap may be on the non-rod side of the WGM resonator. In some variations, the second spherical segment may be coated with a polymer.

[0026] In some variations, the second sphere segment may be encapsulated by a polymer. In some variations, the polymer may have a higher refractive index than the WGM resonator. In some variations, the polymer may include an optical adhesive.

[0027] In general, in some variations, a method for fabricating a WGM resonator may include a WGM resonator comprising 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 frequency of the second mode may be different from the frequency of the first mode. The method may further include covering the second spherical segment with a polymer configured to attenuate the second mode. The first mode may be a fundamental mode of the WGM resonator.

[0028] In some variations, the polymer does not cover the first spherical segment. The second spherical segment may be a spherical cap of the WGM resonator. In some variations, the WGM resonator may include a rod side and a non-rod side of opposite polarity to the rod side. The spherical cap may be on the rod side of the WGM resonator. Alternatively or additionally, the spherical cap may be on the non-rod side of the WGM resonator. In some variations, covering the second spherical segment with a polymer coating includes coating the spherical cap with a polymer. In some variations, covering the second spherical segment with a polymer coating includes immersing the spherical cap in a solution comprising a polymer. The refractive index of the polymer may be higher than the refractive index of the WGM resonator, and / or the polymer may include an optical adhesive. In some variations, the WGM resonator may be a microsphere or microbubble resonator. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 An exemplary variation of a whispering gallery mode (WGM) microsphere resonator is illustrated.

[0030] Figure 2 is a schematic depiction of an exemplary variation of a WGM microbubble resonator.

[0031] Figure 3 is a schematic depiction of exemplary variations of various geometries of WGM microbubble resonators.

[0032] Figure 4 Plotted are the quality factors of WGM microbubble resonators for varying microbubble wall thickness.

[0033] Figure 5 is a schematic depiction of the mode distribution of an exemplary WGM microbubble resonator.

[0034] Figure 6A is a depiction of the field distribution of the fundamental mode in an exemplary variation of a WGM resonator.

[0035] Figure 6B is a depiction of the field distribution of higher-order modes in an exemplary variation of a WGM resonator.

[0036] Figure 6C is a depiction of the field distribution of higher-order modes in an exemplary variation of a WGM resonator.

[0037] Figure 7 is a schematic depiction of an exemplary optical coupling between a spherical WGM microbubble resonator and an optical waveguide.

[0038] Figure 8 Illustrated are exemplary variations of WGM resonators and waveguides in polymer structures.

[0039] Figure 9 An exemplary variation of a WGM resonator placed on a substrate and optically coupled to an optical waveguide is illustrated.

[0040] Figure 10 An exemplary variation of a WGM resonator placed on a substrate and optically coupled to an optical waveguide is illustrated.

[0041] Figure 11 An exemplary variation of a WGM resonator optically coupled to a chip-scale optical waveguide placed on a substrate is illustrated.

[0042] Figure 12 is a schematic depiction of an exemplary WGM resonator array.

[0043] Figure 13 is a schematic depiction of an exemplary WGM resonator array.

[0044] Figure 14 is a schematic depiction of an exemplary WGM resonator array.

[0045] Figure 15 is a schematic depiction of an exemplary WGM resonator array.

[0046] Figure 16 is a schematic depiction of an exemplary WGM resonator array.

[0047] Figure 17A is a side view of an exemplary WGM resonator array.

[0048] Figure 17B yes Figure 17A A top view of an exemplary WGM resonator array in FIG.

[0049] Figure 18A is a schematic depiction of an exemplary variation of a WGM resonator array packaged in a polymer structure.

[0050] Figure 18B is a schematic depiction of an exemplary variation of a WGM resonator array packaged in a polymer structure.

[0051] Figure 19 is a schematic depiction of an exemplary variation of a packaged WGM resonator array.

[0052] Figure 20A An exemplary variation of a method of fabricating a WGM microsphere resonator by forming a cleaved portion on at least one end of an optical fiber is illustrated.

[0053] Figure 20B An exemplary variation of a method for fabricating a WGM microsphere resonator using an optical fiber having a cleaved portion is illustrated.

[0054] Figure 21A Illustrated are exemplary variations of tapered fiber tips for fabricating WGM microsphere resonators of different sizes.

[0055] Figure 21B An exemplary variation of a WGM microsphere resonator fabricated using a tapered fiber tip is illustrated.

[0056] Figure 21C An exemplary variation of a WGM microsphere resonator fabricated using a tapered fiber tip is illustrated.

[0057] Figure 22 Illustrated are exemplary variations of WGM microsphere resonators fabricated from polymer fibers.

[0058] Figures 23A-23C is a schematic description of an exemplary variation of a method for fabricating WGM microsphere resonators using a transfer apparatus.

[0059] Figures 24A-24C is a schematic description of an exemplary variation of a method for fabricating WGM microsphere resonators using a dip-coating process.

[0060] Figure 25 is a schematic description of an exemplary variation of a method for fabricating a WGM microbubble resonator.

[0061] Figure 26 Illustrated are exemplary variations of methods for fabricating a single WGM resonator for an array of WGM resonators along a single fiber.

[0062] Figure 27 is a schematic description of an exemplary variation of a method of fabricating an array of WGM microbubble resonators and coupling the WGM microbubble resonators to an optical waveguide.

[0063] Figure 28 An exemplary variation of a WGM microsphere resonator with a polymer coating is illustrated.

[0064] Figure 29 Exemplary variations of methods of attenuating higher-order modes are illustrated.

[0065] Figure 30 An exemplary variation of a WGM microsphere resonator with a polymer coating is illustrated.

[0066] Figure 31 An exemplary variation of the method of immersing WGM microsphere resonators in polymer to attenuate higher-order modes is illustrated.

[0067] Figure 32A is a schematic depiction of an exemplary method of packaging a WGM resonator and an optical waveguide.

[0068] Figure 32B is a schematic depiction of an exemplary method of packaging a WGM resonator and an optical waveguide.

[0069] Figure 33is a schematic description of an exemplary method for sensing a set of ultrasound echoes using a WGM resonator array.

[0070] Figure 34 is a schematic description of an exemplary method for measuring the optical response of a WGM resonator array.

[0071] Figure 35 is a schematic description of an exemplary method for measuring the optical response of a WGM resonator array.

[0072] Figure 36 is a graph of the optical response of an exemplary WGM resonator array.

[0073] Figure 37 is a schematic depiction of an exemplary ultrasound probe.

[0074] Figure 38 is a schematic depiction of an exemplary ultrasound probe. DETAILED DESCRIPTION

[0075] Non-limiting examples of various aspects and variations of the invention are described herein and illustrated in the accompanying drawings.

[0076] Disclosed herein are systems for optical sensing platforms and methods of making such devices. More particularly, described herein are devices comprising whispering gallery mode (WGM) resonators and methods of making such devices.

[0077] Optical sensing platforms are often simple and highly sensitive. They can be used in a variety of applications (e.g., biosensors, chemical sensors, mechanical sensors, etc.). However, conventional optical sensing platforms have several disadvantages. For example, some conventional optical sensing platforms are fragile and susceptible to physical damage. Therefore, to protect conventional optical sensing platforms from physical damage, they may be packaged in a bulky manner. This can, for example, reduce the portability of some conventional optical sensing platforms.

[0078] In addition, 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. Additionally, the operating frequency of some conventional optical sensing platforms may be difficult to control. For example, for many sensing applications, an optical sensing platform that operates within a sparse spectrum may be preferred. This makes the corresponding pattern traceable and identifiable, which is advantageous for most sensing applications. However, some conventional optical sensing platforms may not be able to restrict operation to a sparse spectrum range. In other words, attenuating non-essential frequencies in conventional optical sensing platforms may be difficult.

[0079] Therefore, there is an unmet need for complex optical sensing platforms that are compact, portable, and well-suited for sensing applications.

[0080] Devices for sensing applications

[0081] The systems and devices disclosed herein include a whispering gallery mode (WGM) resonator optically coupled to an optical waveguide. When optically coupled to the WGM resonator (e.g., via the optical waveguide), the WGM resonator can propagate a group of WGMs along a surface of the WGM resonator. The WGM resonator can transmit optical signals corresponding to the group of WGMs to the optical waveguide.

[0082] In some variations, the WGM resonator and the optically coupled optical waveguide may be encapsulated in a polymer structure. The polymer structure may protect the optically coupled WGM resonator and the optical waveguide from physical damage. In other words, the WGM resonator coupled to the optical waveguide may be packaged in the polymer structure, making the package resistant to breakage and portable. The polymer structure may protect the WGM resonator from interference caused by environmental influences. Additionally, the polymer structure may include a material that enhances the sensing capabilities of the WGM resonator. For example, the refractive index, Young's modulus, elasto-optical coefficient, and / or acoustic impedance of the polymer structure may enhance the sensing capabilities of the WGM resonator depending on the sensing application.

[0083] In some variations, at least a portion of a WGM resonator may include a polymer coating or other structure to attenuate WGMs at one or more non-fundamental frequencies. In other words, the polymer coating on the surface of the WGM resonator may restrict the operation of the WGM resonator to a sparse spectral range, thereby allowing the corresponding modes to be identified and tracked.

[0084] In some variations, to increase device sensitivity, multiple WGM resonators can be arranged together as an array. 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 distance from one another (e.g., in a regular or irregular array, such as a linear array, a rectangular array, or the like). The sensor array can be integrated into a suitable sensing platform for applications such as ultrasound sensing, endoscopy, and the like.

[0085] WGM resonator

[0086] Whispering gallery mode (WGM) resonators are suitable for high-sensitivity applications because they can trap light in a highly confined volume. In some variations, a WGM resonator can include a closed loop of transparent medium (e.g., a closed concave surface, a closed circular path, etc.). This allows light of certain permitted frequencies to propagate continuously within the closed loop and stores the optical energy of the light of the permitted frequencies in the closed loop. In other words, a WGM resonator can allow whispering gallery modes (WGMs) to propagate, which propagate on the surface of the WGM resonator and correspond to light of the permitted frequencies. The WGMs circulate around the circumference of the WGM resonator. Each mode of the WGM corresponds to a specific light frequency.

[0087] In some variations, WGM resonators may comprise acoustically and optically transparent materials to optimize the WGM resonator's response to acoustic waves. Additionally, in some variations, WGM resonators may comprise materials with high elasto-optical coefficients, such as polymers and plastics. Consequently, WGM resonators are well-suited for sensing applications, such as ultrasound sensing (e.g., ultrasound scanning, endoscopy, etc.). WGM resonators can directly measure ultrasound waves through photoelastic effects 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), a WGM traveling within a WGM resonator may experience spectral shifts caused by changes in refractive index, as well as changes in the WGM resonator's shape caused by the ultrasound waves. These spectral changes can be easily monitored and analyzed to generate sensor signals useful and compatible with ultrasound sensing applications.

[0088] While the above paragraphs describe WGM resonator properties that make them suitable for ultrasound sensing, it is clear that the WGM resonators described herein can be used in any other type of sensing application. For example, some photonic filters, such as microwave filters, can include WGM resonators. Other example applications include spectroscopy, analysis of chemical and / or biological agents, mechanical sensors, lasers, switches and modulators, combinations thereof, and the like.

[0089] As described above, the WGM resonator may comprise a suitably optically and / or acoustically transparent material. For example, the WGM resonator may comprise an optically transparent material such as glass, a transparent polymer, silica glass, silicon nitride, titanium dioxide, and / or any other suitable optically transparent material. In some variations, the WGM resonator may comprise an ultrasound-enhancing material such as polyvinylidene fluoride, parylene, polystyrene, and the like. Additionally, the WGM resonator may comprise a material having a low Young's modulus (e.g., a material having a Young's modulus of less than 2 GPa) to increase mechanical deformation induced by ultrasound.

[0090] In some variations, a WGM resonator may include a substantially curved portion (e.g., a spherical portion, a domed portion, a toroidal portion, etc.). The substantially curved portion may be supported by a rod portion. In some variations, the substantially curved portion and the rod portion of the WGM resonator may be formed from the same material.

[0091] In some variations, the substantially curved portion and the rod portion of the WGM resonator can be formed together. For example, the substantially curved portion can be formed by applying heat to a targeted portion of the fiber tip and / or capillary. Another portion of the fiber tip and / or capillary that is not heated can form the rod portion of the WGM resonator.

[0092] The shape of a WGM resonator (e.g., the shape of a substantially curved portion of the WGM resonator) can be any suitable shape. For example, the shape of a WGM resonator can be spherical (i.e., a solid sphere), bubble-shaped (i.e., a sphere with a cavity), cylindrical, elliptical, toroidal, disk-shaped, dome-shaped, etc. WGM resonators include micro-bottle resonators, micro-bubble resonators, micro-cylinder resonators, micro-disk resonators, micro-dome-shaped resonators, etc.

[0093] Although the structure and function of the WGM resonator are shown and described with respect to microsphere and microbubble resonators, this is for illustrative purposes only. It will be apparent that the aspects of the WGM resonator described herein can be extended to other shapes of WGM resonators, such as WGM microbottle resonators, WGM microcylinder resonators, WGM microdisk resonators, WGM microprotrusion resonators, and the like. The devices and methods described herein can utilize one or more WGM resonators of any suitable shape.

[0094] WGM microsphere resonator

[0095] Figure 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 supported on top of the stem portion 102b of the WGM microsphere resonator 102.

[0096] As described above, the WGM microsphere resonator 102 can capture certain frequencies of light. The frequencies of light can circulate within the substantially curved portion 102a of the WGM microsphere resonator 102, thereby permitting whispering gallery modes to propagate along the surface of the WGM microsphere resonator 102 (e.g., along the circumference of the substantially curved portion 102a). Each group 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, a group of WGMs corresponding to a fundamental frequency (i.e., a fundamental mode of the WGMs) can propagate within the equatorial plane of the WGM microsphere resonator 102.

[0097] The substantially curved portion 102a of the WGM microsphere resonator 102 can have a radius R. The WGM propagating within the WGM microsphere resonator 102 can 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 can depend on parameters such as the refractive index of the material of the WGM microsphere resonator 102.

[0098] As described above, the WGM microsphere resonator 102 can directly measure the intensity of ultrasound waves through the photoelastic effect of the WGM microsphere resonator 102 and / or physical deformation of the WGM microsphere resonator 102. Ultrasonic waves can cause a change in the refractive index and / or physical deformation of the WGM microsphere resonator 102. This can result in a resonant frequency shift in the WGM, which can be proportional to the amount of the refractive index change and / or physical deformation. For example, the physical deformation of the WGM microsphere resonator 102 can be a change in 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 can be controlled by the following equation:

[0099]

[0100] as well as

[0101]

[0102] where n eff is the effective refractive index of the optical mode and is determined by the portion of the light field in the optically transparent material and the refractive indices of the resonator and the surrounding environment, Δn eff is the effective refractive index change of the WGM in the microsphere resonator 102 , λ is the operating frequency, Δλ is the resonant frequency shift, and ΔR is the change in radius R of the WGM microsphere resonator 102 .

[0103] although Figure 1The WGM microsphere resonator shown in FIG has a spherical shape, but the substantially curved portion 102a can have any suitable shape. In general, the performance of the WGM resonator 102 can depend on its shape. For example, generally more spherical microspheres can have better performance in confining WGMs. Some suitable variations of the WGM microsphere resonator 102 can be elliptical (e.g., with a certain degree of eccentricity, such as between about 0 and about 0.9).

[0104] The WGM microsphere resonator 102 can be made from an optical fiber comprising an optically transparent material, such as glass, a transparent polymer, silica glass, or any other suitable optically transparent material 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 of 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). Due to surface tension, the melted end of the fiber forms a spherical shape. 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, the optically transparent material can be transferred to the end of a tapered or non-tapered fiber tip using a needle or syringe. Once the droplet has been 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. When the tapered or non-tapered fiber is retracted, the surface tension of the optically transparent material forms a spherical shape. The spherical portion can be cured using a suitable curing process (e.g., using heat, moisture, ultraviolet (UV) light, etc.). The fabrication of the WGM microsphere resonator will be described in further detail below.

[0105] The geometric dimensions of the WGM microsphere resonator 102, including its radius R, can be varied by tapering the end of an optically transparent fiber. Additionally or alternatively, the radius R of the WGM microsphere resonator 102 can be manipulated by controlling a 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 further detail below). Changes in the WGM frequency in response to acoustic waves (or pressure waves) propagating within the WGM microsphere resonator 102 can be measured for sensing applications (e.g., ultrasound sensing).

[0106] WGM microbubble resonator

[0107] Figure 2is a schematic depiction 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 represented by R and an inner microbubble surface 202a" having a radius represented by r, thereby defining a resonator wall thickness equivalent to (Rr). The space inside the inner microbubble surface 202a" may be hollow (e.g., a cavity) or include a material such as an ultrasound enhancement material, as described in further detail below.

[0108] As described above, the WGM microbubble resonator 202 can capture certain frequencies of light. The frequencies of light can circulate within the substantially curved portion 202a of the WGM microbubble resonator 202, thereby permitting whispering gallery modes to propagate along the surface of the WGM microbubble resonator 202 (e.g., along the circumference of the substantially curved portion 202a). Each group 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 group of WGMs corresponding to a fundamental frequency (i.e., a fundamental mode of the WGMs) can propagate within the equatorial plane of the WGM microbubble resonator 202.

[0109] The WGM propagating within the WGM microbubble resonator 202 may correspond to a set of resonant frequencies that depends 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.

[0110] As described above, the WGM microbubble resonator 202 can directly measure the intensity of ultrasound waves through the photoelastic effect of the WGM microbubble resonator 202 and / or physical deformation of the WGM microbubble resonator 202. Ultrasonic waves can cause a change in the refractive index and / or physical deformation of the microbubble resonator 202. This can result in a resonant frequency shift in the WGM, which can be proportional to the amount of the change in refractive index and / or physical deformation. For example, the physical deformation of the WGM microbubble resonator 202 can be a change in the radius R of the outer microbubble 202a′. The change in radius R can change the set of resonant frequencies of the WGM microbubble resonator 202. The resonant frequency shift can be controlled by the following equation:

[0111]

[0112] as well as

[0113]

[0114] where n effis the effective refractive index of the optical mode, determined by the portion of the light field in the optically transparent material and the refractive indices of the resonator and the surrounding environment, Δn eff is the effective refractive index change of the WGM in the microbubble resonator 202, λ is the operating frequency, Δλ is the resonant frequency shift, and ΔR is the change in radius R of the outer microbubble 202a'.

[0115] although Figure 2 The WGM microbubble resonator 202 shown in FIG has a spherical shape, but the substantially curved portion 202a can have 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 confining the WGM. Some suitable variations of the WGM microbubble resonator 202 can be elliptical (e.g., having a degree of eccentricity, such as between about 0 and about 0.5, between 0 and about 0.3, between about 0 and about 0.2, between about 0 and about 0.1, etc.). Figure 3 Depicted are exemplary variations of various geometries of WGM microbubble resonators. For example, the WGM microbubble resonator can be an elliptical microbubble (e.g., the more eccentric microbubble 302 or the less eccentric microbubble 302′) or a spherical microbubble 302″. As described above, the spherical WGM microbubble resonator 302″ can have better resonator performance than the elliptical WGM microbubble resonators 302 and 302′.

[0116] Return Reference Figure 2 The WGM microbubble resonator 202 can be made of a capillary, such as capillary 201. Capillary 201 can be made of an optically transparent material, such as glass, a 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 walls (e.g., indicated by a wall thickness equivalent to (Rr)) can include, for example, the same material as the capillary 201. The space inside the inner microbubble resonator surface 202a″ can be a hollow chamber or can be filled with an ultrasound-enhancing material, such as 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 can have a relatively high elasto-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 resonator wall material (e.g., upon receiving mechanical stress or strain caused by the set of ultrasound echoes).

[0117] In some variations, the WGM microbubble resonator 202 can be formed by manipulating a capillary (e.g., capillary 201). The capillary 201 can be made of an optically transparent material, such as glass, a transparent polymer, silicon nitride, titanium dioxide, and any other suitable optically transparent material. The capillary 201 can be pressurized and heated at the location on the capillary 201 where the substantially curved portion 202a of the WGM microbubble resonator 202 is desired. Heating the capillary 201 can be performed using a CO2 laser, an arc discharger, a heating coil, or any other suitable heat source, while maintaining a stable pressure within the capillary 201 using a pressure source. The fabrication of the WGM microbubble resonator will be described in further detail below.

[0118] In some variations, the heat source may be manipulated (e.g., to control the direction of the heat, focus the energy, etc.), such as by utilizing 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 radius r of the inner microbubble 202a", are shown in FIG. Figure 2 The wall thickness can be determined by a controlled heating and / or pressing process. For example, precise control of the manufacturing process can result in the production of WGM microbubble resonators with suitable shapes (e.g., Figure 3 The elliptical WGM microbubble resonator 302 or 302' and / or Figure 3 In some variations, the capillary 201 may be further treated by partially or completely filling the hollow chamber of the WGM microbubble resonator 202 with an ultrasound enhancing material, such as those described above. For example, the ultrasound enhancing material may be injected into the WGM microbubble resonator 202 through the open end of the capillary 201.

[0119] 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 further detail below). Changes in the frequency of the WGM in response to sound waves (or pressure waves) propagating within the WGM microbubble resonator 202 can be measured for sensing applications (e.g., ultrasound sensing).

[0120] As described above, the WGM microsphere resonator 102 and the WGM microbubble resonator 202 are described in detail for illustrative purposes only. The following section describes the properties of any suitable WGM resonator (e.g., a WGM microsphere resonator, a WGM microbubble resonator, a WGM microbottle resonator, a WGM microdot resonator, a WGM microdisk resonator, a WGM microring resonator, etc.).

[0121] Quality factor (Q factor)

[0122] The ability to capture light of the permitted frequency in a closed loop may be attributed to the high quality factor (Q factor) of the WGM resonator, which makes the WGM resonator well-suited for high-sensitivity applications. The Q factor of the 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, a spherical WGM resonator can have a high Q factor due to its geometry.

[0123] The sensitivity of ultrasound sensing can be improved by increasing the Q factor of the WGM resonator. The Q factor can in turn be controlled by physical factors of the WGM resonator, such as the shape of the WGM resonator, the wall thickness Rr of the WGM microbubble resonator, the radius R of the WGM microsphere resonator, etc. For example, the shape of the WGM resonator can affect the confinement of the light field within the WGM resonator, which in turn affects the Q factor of the WGM resonator. The confinement of the light field in the WGM resonator can depend on, among other factors, the optical loss of the light field as it propagates around the circumferential wall of the WGM resonator. The optical loss of the light field can depend on geometric parameters including the circumferential bending radius and physical parameters including the absorption coefficient of the WGM resonator material. In general, stronger light field confinement can help enhance the response of the WGM resonator to the pressure caused by the received ultrasound echo. In general, WGM resonators with shapes such as spherical, elliptical, cylindrical, domed, annular, etc. can have better resonator performance. For example, in Figure 3 , the spherical bulb 302" has lower optical loss than the elliptical bulbs 302 and 302' and exhibits a higher Q factor that is more suitable for highly sensitive applications.

[0124] Likewise, the wall thickness of the WGM resonator can also affect the Q factor. Figure 4The graph illustrates the change in the Q factor of a WGM microbubble resonator, limited by radiation losses, as the wall thickness of the WGM microbubble resonator varies. Other losses, such as material absorption losses, are not considered in the simulation. The Q factor generally decreases by increasing the internal microbubble surface radius and / or decreasing the wall thickness. Depending on the geometry and refractive index of the WGM microbubble resonator and its surrounding medium, the Q factor can decrease starting from a certain radius. Graph 401 illustrates the Q factor of a WGM microbubble resonator with an outer microbubble diameter of 10 μm, a refractive index of 2, and a refractive index of 1 for the surrounding air. 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 becomes almost unable to confine light. Graph 402 illustrates the Q factor of a similar WGM microbubble resonator with an outer microbubble diameter of 10 μm, a refractive index of 2, and a refractive index of 1.33 for the surrounding air. Due to the higher refractive index of the surrounding material, as shown in 402, the Q factor begins to drop significantly as the wall thickness approaches 0.26 μm. When the wall thickness is thinner than 0.16 μm, the WGM microbubble resonator becomes almost unable to confine light. It should be readily understood that graphs 401 and 402 include Q factor values ​​for illustrative purposes only. For example, graphs 401 and 402 merely illustrate one or more trends in the Q factor of a WGM microbubble resonator. The value of the Q factor itself can vary depending on various factors. However, the overall trend of the Q factor may be similar to the trends shown in graphs 401 and 402.

[0125] WGM Mode

[0126] As described above, WGM resonators can trap light of certain frequencies. Light of these frequencies can circulate along the surface of the WGM resonator, thereby allowing the WGM to propagate. Therefore, when the WGM resonator is coupled with light (e.g., using an optical waveguide), multiple modes corresponding to light with different spatial distributions can be excited in the WGM resonator.

[0127] Figure 5 is a schematic description of the mode distribution in various WGM microbubble resonators. The WGM microbubble resonator may include an outer microbubble surface with a radius represented by R and an inner microbubble surface with a radius represented by r (e.g. Figure 2). Mode distribution 501 shows the propagation of light via a whispering gallery mode (WGM) in a WGM microbubble resonator with an outer microbubble surface radius of 10 μm and an inner microbubble surface radius of 0 μm. Mode distribution 502 shows the propagation of light via a WGM in a WGM microbubble resonator with an outer microbubble surface radius of 10 μm and an inner microbubble surface radius of 8 μm. Mode distribution 503 shows the propagation of light via a WGM in a WGM microbubble resonator with an outer microbubble radius of 10 μm and an inner microbubble radius of 9.8 μm. Mode distribution 504 shows the propagation of light via a WGM in a WGM microbubble resonator with an outer microbubble radius of 10 μm and an inner microbubble radius of 9.94 μm. In all of the above mode distributions, the refractive index of the microbubble material is 2.

[0128] According to mode distribution 501 and mode distribution 502, there is a slight difference between a microbubble with an outer microbubble surface radius of 10 μm and an inner microbubble 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 in the wall thickness (e.g., see mode distribution 504). Mode distribution 505 shows light propagation via 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. Mode distribution 506 shows light propagation via WGM in a packaged WGM microbubble resonator (e.g., a WGM microbubble resonator encapsulated in a polymer structure), which is 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. The packaged WGM microbubble resonator becomes lossy and cannot confine light in the wall thickness. This is because the wall thickness of the WGM microbubble resonator is extremely small (ie, 0.1 μm), which is smaller than the wavelength of light.

[0129] like Figure 5 As shown, multiple modes of WGMs with different spatial distributions (also referred to herein as "field distributions") can be excited in a WGM resonator. The field distribution of each mode can generally be confined to one or more specific planes. In other words, as a group of WGMs corresponding to a specific frequency propagates along the surface of the WGM resonator, the field distribution of the group of WGMs can generally be confined to the latitude of one or more specific planes within the surface of the WGM resonator. For example, Figure 6A6 is an exemplary variation of the description of the field distribution of the fundamental mode in WGM resonator 602. The fundamental mode of the WGM corresponding to the fundamental frequency can propagate within 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 within the equatorial plane. Because the field distribution of the fundamental mode is thinner, the fundamental mode can have a wider ultrasound sensing bandwidth response than higher-order modes.

[0130] The field distribution of higher-order modes corresponding to frequencies other than the fundamental frequency (e.g., frequencies higher than the fundamental frequency) may be confined to a wider range extending in the radial or poloidal directions (north and south) beyond the field distribution of the fundamental frequency. Figure 6B is a depiction of the field distribution of the first higher-order mode in an exemplary variation of the WGM resonator 602′. Figure 6B As shown, the field distribution of the first higher-order mode extends from the equatorial plane 606a in a radial or poloidal direction. In other words, the field distribution of the first higher-order mode extends between plane 606b and plane 606c. Plane 606b and plane 606c can be equidistant from the equatorial plane 606a. Plane 606b and plane 606c can also be parallel to the equatorial plane 606a.

[0131] Figure 6C is the second higher order mode in the exemplary variation of the WGM resonator 602″ (higher than the one above with respect to Figure 6B The field distribution of the first higher-order mode (higher order) is depicted. Figure 6B The field distribution of the higher-order modes depicted in , Figure 6C The field distribution of the higher order modes in extends from the equatorial plane 606a in the radial or poloidal direction. The field distribution in this case extends from plane 606f to plane 606g. In other words, the higher order field distribution can be confined to the following latitudes: a) the equatorial plane 606a; b) planes 606f, 606d, and 606b in one direction; and c) planes 606g, 606c, and 606e in polar opposite directions. Plane 606f and plane 606g can be equidistant from the equatorial plane 606a. Similarly, plane 606d and plane 606e can be equidistant from the equatorial plane 606a, and plane 606b and plane 606c can be equidistant from the equatorial plane. As described above, Figure 6C The north and south boundaries of the field distribution of the high-order mode depicted in are plane 606f and plane 606g, respectively. Figure 6B The field distribution of the higher order modes shown is compared to Figure 6C The field distribution of the higher order modes in extends further north and south from the equatorial plane 606a. For example, plane 606f and plane 606g can be at a greater distance from the equatorial plane 606a than plane 606b and plane 606c. Therefore, Figure 6CThe higher order modes in Figure 6B Higher mode volumes for higher order modes in the Figure 6C The optical loss of the higher-order modes in Figure 6B The optical loss of higher-order modes in

[0132] As described above, in a spherical WGM resonator, the field distribution of the fundamental mode can generally be confined to the equatorial plane, while the field distribution of the higher-order modes can extend from the equatorial plane in radial and / or poloidal directions. Due to its numerous azimuthal and radial modes, WGM resonators exhibit dense spectral features. Dense spectra can be very attractive for applications such as cavity quantum electrodynamics. However, in most sensing applications, sparse spectra with traceable and identifiable modes may be preferred. In addition, lower-order modes can have higher Q factors than higher-order modes. Additionally, lower-order modes can have smaller mode volumes than higher-order modes.

[0133] The spectrum of a mode may depend on the acoustic impedance of the WGM resonator, the refractive index of the WGM resonator, 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 used for the WGM resonator and / or its surrounding material, as well as the size of the WGM resonator, may be customized to adjust the bandwidth of the resonant mode.

[0134] Additionally or alternatively, higher-order modes can be selectively attenuated by disrupting the geometry of the WGM resonator and / or its surroundings at selected points near 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 further detail below). For example, applying a polymer coating to a portion of the circumference of the WGM resonator can disrupt the continuity of the WGM resonator's geometry along the coated portion, thereby changing the operating mode of the WGM resonator. When the polymer coating is applied to a spherical segment, as described in further detail below, the coating can further change the effective "thickness" (in 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 manner, WGM resonators can be well-suited for sensing applications. This will be described in further detail below.

[0135] waveguide

[0136] Coupling a WGM resonator with light enables the WGM resonator's sensing capabilities. Light can be coupled simply by shining light onto the WGM resonator. However, this can be an inefficient way to excite a WGM resonator. This is because phase matching between the resonating light and the incident light may be required to achieve the WGM resonator's sensing capabilities through coupled light. Therefore, a more efficient method for optical coupling to a WGM resonator is to use an optical waveguide with specialized properties to achieve phase matching. The optical waveguide can provide controllable and robust light that can exploit the WGM resonator's sensing capabilities.

[0137] However, the sensitivity of a WGM resonator coupled to an optical waveguide may 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 shape of the optical waveguide, the shape of the WGM resonator, etc. Therefore, it may be important to consider these factors when coupling a WGM resonator to an optical waveguide.

[0138] As described above, the sensitivity of a WGM resonator can depend on the efficiency of the optical waveguide to which it is coupled. The efficiency of the optical waveguide, in turn, can depend on phase synchronization, optimal overlap between the WGM resonator and optical waveguide modes, and criticality. For example, the efficiency of an optical waveguide can be based on the portion of the total transmitted optical power that can be phase-matched to the WGM resonator. An optical waveguide that can couple the greatest amount of light into a WGM resonator can be considered the most efficient.

[0139] Additionally or alternatively, the sensitivity of the WGM resonator can depend on the coupling between the optical waveguide and the WGM resonator. Optimal optical coupling between the optical waveguide and the WGM resonator can depend on many factors. Non-limiting examples of these factors include the distance between the coupling point on the optical waveguide and the WGM resonator, the alignment of the coupling point on the optical waveguide with the WGM resonator, and the like. Optimal coupling enables whispering gallery mode (WGM) operation, which has a high Q factor corresponding to a set of resonant frequencies for a set of WGMs.

[0140] In some variations, optical coupling between a WGM resonator and one or more optical waveguides may be achieved at least in part by positioning the WGM resonator proximate to the one or more optical waveguides. For example, the WGM resonator and the optical waveguides may be positioned such that the optical waveguides are aligned with a 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 the one or more optical waveguides.

[0141] Figure 77 is a schematic depiction of an exemplary optical coupling between a spherical WGM microbubble resonator 702 and an optical waveguide 701. A coupling point 703 is the point on the optical waveguide 701 that is closest to 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 the coupling point 703 is closest to the spherical WGM microbubble resonator 702. For example, the optical waveguide 701 can be positioned such that the coupling point 703 is closest to the spherical WGM microbubble resonator 702.

[0142] In some variations, the optical waveguide may be an optical fiber receiving light from a light source, an integrated photonic waveguide receiving light from an on-chip light source, an integrated photonic waveguide coupled to an optical fiber propagating light from a light source, or any other suitable medium for propagating light from the light source to a coupling point from a WGM resonator. Some non-limiting examples of optical waveguides include optical fibers, integrated photonic waveguides, chip-scale waveguides, slab waveguides, prisms, angle-polished fiber couplers, tapered fiber couplers, bent fiber couplers, and the like.

[0143] polymer structure

[0144] In some variations, at least a portion of the optical waveguide and WGM resonator may be embedded in a polymer structure, thereby encapsulating the optical waveguide and WGM resonator. As described above, WGM resonators can be fragile and susceptible to physical damage. The polymer structure can protect the WGM resonator and optical waveguide from physical damage. In other words, the polymer structure can package the WGM resonator and optical waveguide together. In variations where the packaged WGM resonator with the optical waveguide is used for sensing applications such as ultrasound sensing (e.g., ultrasound 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 allow the packaged WGM resonator to respond to a wide range of WGM frequencies while having high ultrasonic attenuation. When the packaged WGM resonator with the optical waveguide is used in ultrasound sensing applications, the high ultrasonic attenuation can prevent reverberation of the ultrasonic echo.

[0145] 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, referring to Figure 18A , the polymer structure may include a matching region 1808 , an encapsulation region 1814 , a substrate 1806 , and a backing region 1810 .

[0146] The acoustic matching region 1808 can be configured to facilitate ultrasound transmission. For example, the matching region 1808 can include a polymer material having a low refractive index to reduce the transmission impedance of ultrasound echoes to the WGM resonator. The matching region 1808 can have a lower refractive index than the refractive index of the WGM resonator and the refractive index of the one or more optical waveguides.

[0147] Backing region 1810 may include a damping material, such as a polymer material having a low refractive index, configured to achieve a wide 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 backing region 1810 may be matched, for example, to the acoustic impedance of a matching polymer used to package the WGM resonator.

[0148] As discussed in some variations, the polymer structure may include a substrate (e.g., substrate 1806). The substrate may include silicon, silicon dioxide, quartz, plastic, and / or any other suitable material. Suitable materials for use as the substrate may include damping materials to eliminate residual vibrations and minimize ultrasonic echoes. The substrate may be coated with one or more coating materials (e.g., a "coating"). In some variations, the coating may include one or more materials that act as a reflective surface to reflect ultrasonic waves back to the WGM resonator. In some variations, the coating may additionally include a backing polymer and / or a matching polymer. For example, the substrate may be coated with a coating material including a backing polymer and / or a matching polymer. In some variations, the backing polymer layer and / or the matching polymer layer may be deposited on the substrate.

[0149] The WGM resonator may be placed 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.

[0150] In some variations, packaging region 1814 may include an ultrasound-enhancing material, such as polyvinylidene fluoride, parylene, polystyrene, 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 elasto-optical 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 significantly compared to the refractive index of a non-ultrasound-enhanced material (e.g., upon receiving mechanical stress or strain caused by the set of ultrasound echoes). This may increase the WGM resonator's response to ultrasound signals.

[0151] In an alternative arrangement, reference Figure 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 serve 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 ultrasonic enhancement material. The backing region 3210a may abut the matching region 3208a and be sandwiched between the matching region 3208a and the substrate 3206a. In such variations, the backing polymer layer may be deposited on the substrate 3206a. Alternatively, the matching polymer layer may be deposited on the backing polymer on the substrate.

[0152] 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) can match 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. This can increase the output signal.

[0153] 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 to the WGM resonator such that the WGM resonator is on a substrate included in the polymer structure, while the optical waveguide is above the substrate. In some variations, the optical waveguide may be coupled to the WGM resonator such that both the WGM resonator and the optical waveguide are in direct contact with the substrate included in the polymer structure. Encapsulating the optical waveguide and WGM resonator in a polymer structure is explained in further detail below. It will be apparent that a WGM resonator may be optically coupled to more than one optical waveguide. Thus, the WGM resonator, along with each coupled optical waveguide, may be embedded in a polymer structure.

[0154] Thus, as described above, a WGM resonator and / or one or more optical waveguides may be encapsulated in a polymer structure (e.g., a polymer structure comprising a backing polymer and a matching polymer). In some variations, a sensing device and / or sensing apparatus may include at least one WGM resonator coupled to one or more optical waveguides encapsulated in a polymer structure as described above. In other words, a sensing device and / or sensing apparatus may include a single WGM resonator coupled to a single optical waveguide encapsulated in a polymer structure. Alternatively, a sensing device and / or sensing apparatus may include a single WGM resonator coupled to more than one optical waveguide encapsulated in a polymer structure. In yet another variation, a sensing device and / or sensing apparatus may include an array of WGM resonators (described in further detail below) coupled to a single optical waveguide encapsulated in 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, a sensing device and / or sensing apparatus may include an array of WGM resonators coupled to more than one optical waveguide encapsulated in a polymer structure (eg, at least some of the WGM resonators in the array of WGM resonators may be coupled to different optical waveguides).

[0155] The polymer structure may provide, for example: i) protection of the WGM resonator and one or more optical waveguides, which may, for example, improve the portability of the sensing device and / or sensing apparatus; ii) enhanced response of the WGM resonator to incoming ultrasound echoes to be sensed by the sensing device and / or sensing apparatus; and / or iii) widen the operating bandwidth of the sensing device and / or sensing apparatus. In some variations, the polymer structure may also help maintain optical coupling properties between the WGM resonator and the one or more optical waveguides, such as by maintaining a consistent distance between the WGM resonator and the one or more optical waveguides.

[0156] In one variation, a polymer structure including a matching region can broaden the operating bandwidth of a sensing device and / or sensing apparatus, as described below. Generally speaking, in ultrasound sensing applications, the operating frequency of the input light can be tuned to the resonance of a WGM resonator centered around a resonant frequency. By modulating the refractive index of the WGM resonator material or deforming the WGM resonator, an incoming ultrasound echo shifts the resonant frequency of the WGM resonator to produce a resonant shift. If the linewidth of the resonance is too narrow, the resonant shift can be much larger than the linewidth of the resonance, causing the resonant shift to fall outside the dynamic range of the sensing device and / or sensing apparatus. However, the matching region can broaden the linewidth of the optical resonant mode to better capture larger resonant shifts. For example, evanescent waves extending from the WGM resonator to the matching region can experience additional losses, resulting in a shortened photon lifetime, which is reflected as a broadened linewidth of the optical resonant mode. Due to this broadened linewidth, the sensing device and / or sensing apparatus can withstand a wider range of resonant frequency shifts caused by the incoming ultrasound signal. In other words, the matching polymer encapsulating the WGM resonator can broaden the operating bandwidth of the sensing device and / or sensing apparatus, thereby increasing the dynamic range of the sensing device and / or sensing apparatus, and can prevent the resonance from shifting beyond the dynamic range of the sensing device and / or sensing apparatus. Therefore, the matching layer can increase the dynamic sensing range of the sensing device and / or sensing apparatus.

[0157] Exemplary packaging

[0158] Figure 8 Exemplary variations of WGM resonators 802 and optical waveguides 801 in polymer structures 804 are shown. Figure 8 As shown, both the WGM resonator 802 and the optical waveguide 801 are embedded in a polymer structure. In one variation, the optical waveguide 801 can be embedded in a polymer structure 804, and then the WGM resonator 802 can 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 can be embedded in the polymer structure 804, and then the optical waveguide 801 can 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 can be optically coupled together. After optical coupling, the coupled optical waveguide 801 and WGM resonator 802 can be embedded in the polymer structure 804. In this way, 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.

[0159] 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 placed on the 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 the encapsulation region that encapsulates the WGM resonator.

[0160] Figure 9 An exemplary variation of a WGM resonator 902 is shown disposed on a substrate 906 and optically coupled to an optical waveguide 901. Figure 9 As shown, the WGM resonator 902 is in direct contact with the substrate 906. For example, the WGM resonator 902 can be fixed to the substrate 906 (e.g., using an adhesive such as a polymer adhesive). Alternatively, the WGM resonator 902 can be embedded in the substrate 906. However, the optical waveguide 901 is coupled to the WGM resonator 902 so 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 can be sandwiched between a backing region and a matching region. In other words, the WGM resonator 902 and the optical waveguide 901 can be packaged in a packaging region located between the backing region and the matching region. In one variation, the WGM resonator array can be arranged in a manner similar to Figure 9 906. In such variations, an optical waveguide such as optical waveguide 901 may be coupled to more than one WGM resonator in a WGM resonator array. In some variations, WGM resonator 902 may be a WGM microsphere resonator and optical waveguide 901 may be a fiber taper.

[0161] and Figure 9 on the contrary, Figure 10 The WGM resonator 1002 is shown coupled to the 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 to the substrate 1006 (e.g., using an adhesive). Alternatively, both the WGM resonator 1002 and the optical waveguide 1001 can be embedded in the substrate 1006. The refractive index of the substrate 1006 can be lower than the refractive index of the optical waveguide 1001. Figure 9 , the substrate 1006, the WGM resonators 1002, and the optical waveguide 1001 may be sandwiched between the backing region and the matching region. In other words, the WGM resonators 1002 and the optical waveguide 1001 may be encapsulated in the encapsulation region between the backing region and the matching region. In some variations, the WGM resonator array may be placed on the substrate 1006 such that both the WGM resonators and the optical waveguide are in direct contact with the substrate 1006 (similar to Figure 10In such variations, an optical waveguide such as optical waveguide 1001 may be coupled to more than one WGM resonator in a WGM resonator array. In one variation, WGM resonator 1002 may be a WGM microsphere resonator and optical waveguide 1001 may be a fiber taper.

[0162] Figure 11 An exemplary variation of a WGM resonator 1102 coupled to a chip-scale optical waveguide 1101 is illustrated, such that the chip-scale optical waveguide 1101 is placed 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 the refractive index of the chip-scale optical waveguide 1101. However, the WGM resonator 1102, although optically coupled to the chip-scale waveguide 1101, may not be in direct contact with the substrate 1106. For example, the chip-scale waveguide 1101 may be placed on the substrate 1106 (e.g., fixed to the substrate 1106 using an adhesive or embedded in the substrate 1106). The WGM resonator 1102 may be placed on the chip-scale waveguide 1101 (e.g., optically coupled to the chip-scale waveguide 1101) without contacting the substrate 1106. Similar to Figure 9 and Figure 10 , the substrate 1106, the WGM resonator 1102 and the optical waveguide 1101 may be sandwiched between the backing region and the matching region.

[0163] WGM resonator array

[0164] In some variations, multiple WGM resonators may be packaged together in an array. The WGM resonator array may be used in a sensing device and / or a sensing apparatus. Due to the increased sensing area (i.e., the area of ​​multiple WGM resonators versus the area of ​​a single WGM resonator), the WGM resonator array may increase the sensitivity of the sensing device and / or the sensing apparatus. Additionally, each WGM resonator in the WGM resonator array may be coordinated so that they collectively respond to acoustic and / or pressure waves. This collective response may increase the strength of the output signal, thereby increasing the sensitivity of the sensing device and / or the sensing apparatus.

[0165] The WGM resonator array can be optically coupled to one or more optical waveguides. A polymer structure can encapsulate the WGM resonators and the optically coupled array of optical waveguides. As described above, the polymer structure can include an encapsulation region for encapsulating the WGM resonators and / or optical waveguides, a matching region, a backing region, and an optional substrate. In some variations, the matching region and / or the backing region can serve as the encapsulation region. For example, the WGM resonator array can be included in the matching region of the polymer structure. One or more optical waveguides can be positioned in 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 WGM resonator array and / or the one or more optically coupled optical waveguides can be placed on a substrate. The WGM resonator array and the optical waveguides can be in the encapsulation region sandwiched between the matching region and the backing region.

[0166] like Figure 12-1 7, a plurality of WGM resonators may be arranged in various suitable arrays and in various suitable ways relative to one or more optical waveguides. For example, Figure 12 is a schematic depiction of a WGM resonator array 1202, wherein each WGM resonator is coupled to a unique corresponding optical waveguide. For example, the WGM resonators in the WGM resonator array 1202 may be arranged linearly in parallel on a substrate (the substrate is not in the substrate). Figure 12 Each WGM resonator from the WGM resonator array 1202 can then be aligned as shown in FIG. Figure 7 1201) are optically coupled to one or more different optical waveguides 1201. In at least some variations, the WGM resonators in array 1201 can be equidistant from one another. Additionally or alternatively, at least some of the WGM resonators in array 1201 can be separated by different distances.

[0167] Figure 13 is a schematic depiction of an exemplary WGM resonator array 1302. The WGM resonator array 1302 may be arranged linearly in parallel on a substrate 1306, similar to Figure 12 In some variations, at least some of the WGM resonators in array 1302 may be equidistant from one another. Additionally or alternatively, at least some of the WGM resonators in array 1302 may be separated by different distances. Figure 12 In contrast to the arrangement shown, multiple WGM resonators in the array 1302 can each be optically coupled to a single optical waveguide 1301. Thus, signals from multiple WGM resonators can be coupled to and communicated through a single optical waveguide.

[0168] Figure 14is a schematic depiction 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 be of different sizes. In such variations, it may be difficult to optically couple each of them to a similar Figure 13 To overcome this challenge, each WGM resonator in the WGM resonator array 1402 may be arranged nonlinearly on the substrate 1406 at a different vertical distance from the base plane of the substrate 1406. Figure 14 As shown, the WGM resonators in the middle of the array 1402 are further from the base plane of the substrate than the WGM resonators at the ends of the array 1402. The optical waveguide 1401 can be aligned with each WGM resonator in the WGM resonator array 1402 and then fastened, such that after the positions of the optical waveguide 1401 and the WGM resonator array 1402 are fixed, the optical waveguide 1401 is configured to be optically coupled to each WGM resonator in the WGM resonator array 1402.

[0169] Figure 15 1 is a schematic depiction of an exemplary WGM resonator array 1502. The WGM resonator array 1502 can be arranged linearly 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. As the number of WGM resonators in the WGM resonator array 1502 to be coupled with the optical waveguide 1501 increases, the alignment procedure can become challenging. To alleviate this, the optical waveguide 1501 can first be embedded or fixed on top of the substrate 1506. The WGM resonators can then be arranged individually (e.g., one by one, or in other suitable groupings) and linearly in parallel on top of the substrate 1506 and the optical waveguide 1501.

[0170] Figure 16 is a schematic depiction 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, Figure 16 , array 1602 includes rows 1602a, 1602b, and 1602c of WGM resonators. The rows 1602a, 1602b, and 1602c of WGM resonators may be arranged such that the WGM resonators collectively form array 1602. Array 1602 may be a rectangular array. However, it should be understood that Figure 16 The arrangement shown in is illustrative only, and the array of WGM resonators in a rectangular array may have any suitable number of rows, columns, etc., and each row may have any suitable number of WGM resonators. Additionally, the array may have any suitable form (e.g., a radial array, a hexagonal array, etc.).

[0171] 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 placed on substrate 1606. They can be fixed to substrate 1606 in a linear manner at equal distances from each other (e.g., using an adhesive). Alternatively, optical waveguides 1601a, 1601b, and 1601c can be fixed to substrate 1606 at different distances from each other. In some variations, optical waveguides 1601a, 1601b, and 1601c can be embedded in substrate 1606 at a predetermined distance from each other. Each optical waveguide can be optically coupled to a corresponding WGM resonator row. 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) and linearly arranged on top of their respective optical waveguides 1601a, 1601b, and 1601c.

[0172] For some sensing applications, it may be advantageous for the sensing device and / or sensing arrangement to have a particular shape. For example, for applications such as endoscopy, it may be advantageous for the sensing device and / or sensing arrangement to have a circular or other compact cross-sectional shape. Similarly, a circular sensing device and / or sensing arrangement may be best suited for use in a catheter. For example, an endoscope or catheter may include a thin tube or other component with a terminal sensing end. Thus, in some variations, a plurality of 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 arrangement.

[0173] Figure 17A is a side view of an exemplary WGM resonator array 1702. Figure 1 As discussed in , some WGM resonators may include a substantially curved portion and a rod portion. Figure 17AIn the WGM resonator array 1702, each WGM resonator includes a substantially curved portion and a rod portion (e.g., the substantially curved portion is supported by the rod portion). For example, one WGM resonator in the array 1702 may include a substantially curved portion 1702a and a rod portion 1702b. Another WGM resonator in the array 1702 may include a substantially curved portion 1702a' and a rod portion 1702b'. The rod portions of the WGM resonators may be bundled together in a circular shape. That is, the rod portions of the WGM resonators may be bundled together so that they may collectively lie within a circle. Alternatively, the rod portions of the WGM resonators may be bundled together so that they may collectively lie within an ellipse. The rod portions of each WGM resonator may be parallel to one another. For example, rod portion 1702b and rod portion 1702b' may be 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 rod portions of the WGM resonators may be arranged together in a circular shape or other compact cross-sectional shape on a common plane. Alternatively, the rod portions of the WGM resonators may be aligned on a common plane so that they form a circular or other compact cross-sectional shape.The substantially curved portions of the WGM resonators may collectively form the terminal end of a sensing device (eg, a catheter or endoscope).

[0174] Figure 17B A top view of an exemplary WGM resonator array 1702 is shown. Figure 17B As shown, the substantially curved portions of the WGM resonators are collectively within a circle (which may be the terminals of a sensing device and / or sensing apparatus). An optical waveguide may be coupled to each WGM resonator in array 1702. The bundled WGM resonators may be embedded in a polymer structure and then included in a sensing device and / or sensing apparatus.

[0175] It should be understood that the above Figure 12-1 Any of the coupling arrangements described herein may be combined in any suitable manner. In some variations, a WGM resonator array may include WGM resonators that may all be similar (e.g., similar size, similar thickness, similar material). Alternatively, a WGM resonator array may include WGM resonators that are different from at least some of the other WGM resonators in the array (e.g., different size, different thickness, different material). The choice of similar or different WGM resonators may depend on the sensing application for which the WGM resonator array is to be used. Similarly, the number of WGM resonators in the array may also vary depending on the sensing application.

[0176] Example packaged WGM resonator array

[0177] After aligning 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 between the one or more optical waveguides and the WGM resonator array. The polymer structure can have a lower refractive index than the one or more optical waveguides and the WGM resonator array. In some variations, increasing the difference between the refractive index of the WGM resonator and the polymer structure can enable WGM resonators with smaller diameters to be used in sensing applications. As a result, the contrast between the refractive index of the WGM resonators in the array and the refractive index of the polymer structure can be high, allowing WGM resonators with smaller diameters to be included in the array. This, in turn, can increase the operating bandwidth of a sensing device and / or sensing apparatus including the WGM resonator array.

[0178] Figure 18A An exemplary variation of a WGM resonator array 1802 packaged in a polymer structure is illustrated. The WGM resonators in array 1802 may be arranged on a substrate 1806. For example, the WGM resonators may be affixed to substrate 1806 such that each WGM resonator is equidistant from its subsequent WGM resonators in array 1802. Alternatively, the WGM resonators may be affixed to substrate 1806 such that at least some WGM resonators are not equidistant from their subsequent WGM resonators. In yet another variation, the WGM resonators may be embedded in substrate 1806 at a predetermined distance from one another. The WGM resonator array 1802, along with substrate 1806, may be sandwiched between a backing region 1810 and a matching region 1808. The space between matching region 1808 and the backing region 1810 of the WGM resonator array 1802 on the packaging substrate 1806 may form a packaging region 1814.

[0179] like Figure 18A As shown, the WGM resonators in array 1802 can be arranged such that the sizes of the WGM resonators are arranged in ascending order. 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 located at one end of array 1802, and the largest WGM resonator 1802g is located at the other end of array 1802.

[0180] In contrast, Figure 18BAn exemplary variation of a WGM array 1802' packaged in a polymer structure is illustrated such that the WGM resonators are arranged in a manner such that the sizes may not be in a particular order. 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' arranged at one end of the array 1802' is neither the largest nor the smallest in the array 1802'. Similar to Figure 18A The WGM resonator array 1802 may be packaged such that the WGM resonator array 1802' along with the substrate 1806' may be sandwiched between the matching region 1808' and the backing region 1810' (eg, the WGM resonator array 1802' may be packaged within the packaging region 1814').

[0181] Figure 19 1902 is a schematic depiction of an exemplary variation of a packaged WGM resonator array 1902. A polymer structure may include a matching region 1908, a backing region 1910, and one or more coatings 1912. In some variations, the coating 1912 and the matching region 1908 may comprise the same material. For example, the matching region 1908 and the coating 1912 may comprise multiple layers of a suitable matching material. Alternatively, in some variations, the material properties (e.g., acoustic impedance) in the polymer structure may change in a gradual manner (e.g., a gradual change in material properties from the matching region 1908 to the one or more coatings 1912), which may maximize the transmission of acoustic waves to the WGM resonator array 1902 in the polymer structure and / or minimize undesirable reflections of ultrasonic waves. In other variations, the coating 1912 and the matching region 1908 may comprise different materials. For example, the matching region 1908 may comprise a polymer having a lower refractive index than the material of the WGM resonators and / or may comprise a material having low optical loss to better transmit light to the WGM resonators. The WGM resonator array 1902 may be embedded in the matching region 1908 .

[0182] When used during ultrasound sensing, a set of ultrasound echoes 1901 may be transmitted through the matching polymer 1908 and / or other coating 1912 to reach the WGM resonator array 1902 with a transmission intensity coefficient of:

[0183]

[0184] Where Z1 represents the acoustic impedance of the environment above the matching polymer 1908 and / or other coating 1912, Z2 represents the acoustic impedance of the matching polymer 1908, Z3 represents the acoustic impedance of the WGM resonators in the array 1902, and d represents the thickness of the matching polymer 1908. When d = λ / 4 (where λ represents the wavelength of a set of ultrasonic echoes), the transmission intensity is unit. In the case of the matching polymer 1908, the thickness of the matching polymer 1908 may be desirably set to d=λ / 4, because:

[0185]

[0186] And for 0<Z1<Z2<Z3,

[0187]

[0188] Therefore, it is still expected that cos 2 (kd) = 0, in which case d = λ / 4 for maximum transmission. Furthermore, it is desirable that the matching polymer 1908 has a low refractive index so that the WGM resonator array 1902 can confine light to generate WGMs with a high Q factor.

[0189] Manufacturing of sensing equipment and / or sensing devices

[0190] Described herein are some exemplary variations for fabricating the aforementioned sensing devices and / or sensing apparatuses. As described above, the operating bandwidth of the sensing devices and / or sensing apparatuses can depend on 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. Therefore, the desired material used to fabricate the WGM resonator can be selected based on the sensing application and the application's operating bandwidth.

[0191] For example, in the context of ultrasonic sensing, the material used for the WGM resonator can be selected so that the material's acoustic impedance 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 penetration of acoustic waves into the WGM resonator. This, in turn, can increase the response of the WGM resonator through elasto-optical effects 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 ultrasonic waves. Additionally, the material used to fabricate the WGM resonator can be selected to increase the contrast between the material's refractive index and the refractive index of the surrounding medium (e.g., the polymer structure). This can enable WGM resonators with small diameters to increase their operating bandwidth, thereby increasing the sensitivity of the sensing device and / or sensing apparatus. In some variations, the material used to fabricate the WGM resonator can include an ultrasonically enhancing material, such as polyvinylidene fluoride, parylene, polystyrene, etc. Ultrasonically enhancing materials can increase the sensitivity of the WGM resonator. For example, an ultrasound-enhanced material may have a relatively high elasto-optical coefficient. Consequently, upon receiving a set of ultrasound echoes, the refractive index of the ultrasound-enhanced material may change more significantly than the refractive index of a non-enhanced material (e.g., upon receiving mechanical stress or strain caused by the set of ultrasound echoes). This may increase the response of the sensing device and / or sensing apparatus to the ultrasound signal.

[0192] In addition to selecting materials for WGM resonators based on the sensing application, the size of the WGM resonator can also be manipulated based on the sensing application. For example, for endoscopic applications, a WGM resonator with a smaller size (e.g., a WGM resonator with a size less than 100 microns) may be desirable. The size of the WGM resonator can be controlled by controlling the heating process. For example, the heating process can be precisely controlled to apply heat at very specific and precise locations. 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 fibers and / or capillaries used to fabricate the WGM resonator can be manipulated to manipulate the size of the WGM resonator.

[0193] Fabrication of WGM resonators

[0194] Figure 20AThe diagram illustrates exemplary variations of a method for fabricating a WGM microsphere resonator by forming a cleaved portion on at least one end of an optical fiber 2002'. In some variations, optically transparent fibers such as silica fibers or polymer fibers can be used to fabricate the WGM microsphere resonator. In some variations, optical fiber 2002' can be glass, a transparent polymer (e.g., polydimethylsiloxane), silica glass, silicon nitride, titanium dioxide, and / or any other suitable optically transparent material. Fiber 2002' can be selected based on, for example, its refractive index, acoustic properties, Young's modulus, and / or diameter. The sensing application for which the WGM microspheres can be used may be a factor to consider when selecting optical fiber 2002'. For example, the diameter of optical fiber 2002' can affect the size of the WGM microsphere resonator.

[0195] In some variations, once optical fiber 2002' is selected, the cladding may be stripped from one end of optical fiber 2002' (e.g., end 2002b') (e.g., using a fiber coating stripper), thereby exposing the inner core of optical fiber 2002'. One end 2002b' (e.g., the end from which the cladding was stripped) may be cleaved (e.g., using a fiber cleaver). Cleavage may produce a smooth, uniform cut compared to cutting the optical fiber (e.g., using a wire cutter), which may result in uniform reflow in subsequent processes. Optical fiber 2002' may be cleaned using methanol, ethanol, isopropyl alcohol, or the like. In one variation, optical fiber 2002' may be cleaned after the cladding is stripped. In another variation, optical fiber 2002' may be cleaned after the cleavage. In yet another variation, optical fiber 2002' may be cleaned after both the cladding is stripped and the cleavage is performed.

[0196] Figure 20B An exemplary variation of a method for fabricating a WGM microsphere resonator 2002 using an optical fiber 2002' having a cleaved end 2002b' is illustrated. The WGM microsphere resonator 2002 can be fabricated from the optical fiber 2002' using a reflow process. The cleaved optical fiber 2002' can be heated at one end 2002b' (e.g., one 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, heat-conducting surfaces, etc. Due to surface tension of the molten optical fiber, the molten end of the optical fiber forms a spherical shape 2002b. In some variations, the optical fiber can be a single-mode fiber.

[0197] In some variations, one end of a cleaved optical fiber 2002' (e.g., an end of a cleaved optical fiber) can be placed in a fusion splicer to generate a spherical component 2002b of a WGM microsphere resonator. As described above, some WGM resonators may include a rod portion. The portion of the cleaved optical fiber 2002' that has not been subjected to heat can form the rod portion 2002ab of the WGM microsphere resonator. In some variations, the diameter of the spherical component 2002b of the WGM microsphere resonator can be between approximately 15 μm and approximately 500 μm. In some variations, the diameter of the WGM microsphere resonator can be 300 μm.

[0198] As described above, the size of the WGM microsphere resonator can be varied by changing the diameter of the optical fiber. In one variation, to make smaller WGM microsphere resonators, one end of the optical fiber can be tapered. Figure 21A Exemplary variations of tapered fiber tips 2102 for fabricating WGM microsphere resonators of varying sizes are shown. One end of the fiber (e.g., containing a Figure 20A and 20B An optical fiber (made of a material such as a fiber in a fiber) can be tapered by gently stretching the end of the fiber while heating the end using a suitable heat source. The fiber softens. Stretching the end of the fiber can thin the fiber over at least some of its length. In other words, stretching the end of the fiber can reduce the diameter of the fiber at least at the stretched end. In this way, a tapered fiber tip 2102 can be produced. WGM microsphere resonators can be used, for example, Figure 20A and 20B The reflow process described in the method is made by the tapered fiber tip 2102.

[0199] Figure 21B An exemplary variation of a WGM microsphere resonator 2102' fabricated using a tapered fiber tip 2012 is illustrated. In other words, the end of the tapered fiber tip 2102 can be cleaved and heated to create a substantially curved portion (e.g., a spherical portion) of the WGM microsphere resonator 2102'. A 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 reflow into the substantially curved portion. Figure 21C An exemplary variation of a method for fabricating a larger sized WGM microsphere resonator 2102" using a tapered fiber tip 2102 is illustrated. The WGM resonator 2102" can be fabricated by increasing the amount of 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.

[0200] As described above, WGM microsphere resonators can be fabricated from silica fibers using the processes disclosed in Figures 20 and / or 21. The WGM microsphere resonators depicted in Figures 20 and 21 are fabricated from silica fibers. Figure 22 An exemplary variation of a WGM microsphere resonator fabricated from polymer fibers is illustrated. For example, the polymer fibers may comprise polydimethylsiloxane. Figure 22 The WGM microsphere resonator in the Figures 23A-23C and / or Figures 24A-24C The process described in is used to manufacture.

[0201] Figures 23A-23C FIG2 is a schematic depiction 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 material onto a fiber tip 2316. For example, the transfer device 2314 can be used to extract a suitable material for fabricating a WGM microsphere resonator. In some variations, the transfer device 2314 can be a needle. In some variations, the transfer device 2314 can be a syringe. In yet another variation, the transfer device 2314 can be a syringe with a needle at one end. In some variations, the material can be glass, a transparent polymer (e.g., polydimethylsiloxane), silica glass, silicon nitride, titanium dioxide, or any other suitable optically transparent material.

[0202] When the optically transparent material is extracted using the transfer device 2314, the optically transparent material may form droplets 2302' on the transfer device 2314. The size of the WGM microsphere resonator may depend on the size of the droplets 2302'. The droplet size 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 varying sizes may be manufactured in a controlled manner.

[0203] The extracted droplet 2302' may be placed at the end of a fiber tip 2316 (e.g., droplet 2302"). In some variations, the fiber tip 2316 may comprise the same material as the droplet 2302". In some variations, the fiber tip 2316 may comprise an optically transparent material different from the droplet 2302". In some variations, one end of the fiber tip 2316 (e.g., the end where the droplet is placed) may be tapered. In some variations, the fiber tip may be non-tapered. In some variations, the fiber tip 2316 may be a glass fiber tip. Once the droplet 2302" is placed on the fiber tip 2316, the transfer device 2314 may be retracted to form a spherical 2302 portion, which then forms a substantially curved portion of the WGM resonator. The surface tension of the material (i.e., the material of the droplet) may maintain the spherical shape. The spherical portion 2302 may be cured using heat, moisture, ultraviolet light, etc. to form the WGM microsphere resonator.

[0204] Figures 24A-24C FIG. 2 is a schematic depiction of an exemplary variation 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 (e.g., Figure 24B ). For example, reservoir 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 optical fiber 2416 immersed in reservoir 2418 may be tapered. In some variations, optical fiber 2416 may be non-tapered. Reservoir 2418 may comprise an optically transparent polymer. Optical fiber 2416 may be silica fiber or glass.

[0205] When the optical fiber 2416 is retracted from the pool 2418 (e.g. Figure 24C 2418 ), the retracted end of optical fiber 2416 may include optically transparent material from pool 2418, such that the optically transparent material on the retracted end is in the shape of sphere 2402. Surface tension of the optically transparent material may maintain the spherical shape. Spherical optically transparent material 2402 may be cured (e.g., using heat, moisture, ultraviolet light, etc.) to form a substantially curved portion of a WGM microsphere resonator. The portion of optical fiber 2416 not immersed in the pool and not exposed to heat may form the rod portion of the WGM microsphere resonator. The size of the WGM microsphere resonator (e.g., the diameter of the spherical portion) may be based on the diameter of the end of optical fiber 2416 immersed in pool 2418, the speed at which optical fiber 2416 is retracted from pool 2418, and the like.

[0206] In another variation, a filament of optically transparent material can be extracted from the pool 2418. In such variations, 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 cause the optically transparent material to melt. 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 described in FIG. 20 ).

[0207] Figure 25 is a schematic depiction of an exemplary variation of a method for fabricating a WGM microbubble resonator. The fabrication process may include sealing one end 2505B of a capillary tube 2501B. The seal on end 2505B may be formed from a plug material (e.g., epoxy), for example, by heating and pinching off the open end of the capillary tube 2501B. 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 airflow within the capillary tube 2501B. A gas flow 2503B may be used to maintain a stable pressure within the capillary tube 2501B, which is sealed at 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 2502B. Each resistive heating element 2502B can surround (e.g., wrap around) the capillary 2501B at a corresponding target WGM microbubble resonator location, so that the set of resistive heating elements can precisely apply heat to a set of target WGM microbubble resonator locations. The manufacturing process of the WGM microbubble resonator group can be repeated with precise controllability to consistently reproduce the set of WGM microbubble resonators having the desired geometric dimensions of the microbubble resonator 2502B, including the radius R of the outer microbubble and the radius r of the inner microbubble, as well as the radius of the WGM microbubble resonator 2502B. Figure 2 The precise controllability of the manufacturing process can be used to manufacture the set of WGM microbubble resonators so that their wall thickness is less than the wavelength of a set of ultrasonic echoes that the set of WGM microbubble resonators are designed to sense.

[0208] It should be readily understood that WGM resonators can be fabricated using any suitable optically transparent material using any one or a combination of the methods described in Figures 20-25.

[0209] Fabrication of WGM resonator arrays

[0210] As described above, in some variations, a sensing device and / or sensing apparatus may include a WGM resonator array. The WGM resonator array may be fabricated by fabricating each individual WGM resonator and aligning one or more optical waveguides with 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 of the WGM resonators of the WGM resonator array may be fabricated at a different time (e.g., sequentially) than some other WGM resonators of the WGM resonator array.

[0211] In some variations, individual WGM resonators for a WGM resonator array may be fabricated along a single fiber and / or capillary. Figure 25 , using the above Figure 25 By using a plurality of resistive heating elements at precise locations simultaneously, these multiple WGM microbubble resonators can be fabricated simultaneously. Figure 25 As discussed, the size of each WGM microbubble resonator can be precisely controlled. Thus, a WGM resonator array having WGM microbubble resonators of different sizes can be fabricated from the same capillary 2501B. Similarly, a WGM resonator array having WGM microbubble resonators of the same size can be fabricated from the same capillary 2501B.

[0212] Figure 26 An exemplary variation of a method for fabricating a single WGM resonator for a WGM resonator array along a single optical fiber 2601 is illustrated. The optical fiber 2601 may comprise any suitable optically transparent material. Each WGM resonator for the WGM resonator array may be fabricated using a reflow process, such as one or more of the methods described above with respect to Figures 20-22. Figure 25 As shown, target locations on the optical fiber 2601 may be subjected to heat (simultaneously or one by one). The reflow process generates substantially curved portions of the WGM resonator (eg, portions 2602a-2602d).

[0213] In the use of Figure 25 or Figure 26 After the described method creates a substantially curved portion for each individual WGM resonator, the optical fiber and / or capillary can be cleaved at specific locations to produce each individual WGM resonator. For example, the optical fiber and / or capillary can be cleaved so that each individual WGM resonator includes both the substantially curved portion and the rod portion. Alternatively, the optical fiber and / or capillary can be cleaved so that each individual WGM resonator includes only the substantially curved portion.

[0214] In some variations, each of the plurality of WGM resonators in a WGM resonator array may be individually fabricated using the methods discussed above with respect to FIG. 23 and / or FIG. 24 . For example, rather than being collectively fabricated from a single optical fiber or capillary, the plurality of WGM resonators may be fabricated from a plurality of corresponding optical fibers and / or capillaries. Each fabricated WGM resonator may be arranged together to form a WGM resonator array (e.g., any suitable arrangement, such as Figure 12-19 arrangement shown in ).

[0215] As described above, each WGM resonator in the WGM resonator array may be optically coupled to one or more waveguides. Figure 27 2 is a schematic depiction of an exemplary variation of a method for fabricating a WGM microbubble resonator array and coupling the WGM microbubble resonator to an optical waveguide. The WGM microbubble resonator array 2702 can be fabricated from a single capillary and include a plurality of WGM microbubble resonators arranged in a linear fashion. In other words, a plurality of WGM microbubble resonators can be formed in series along a single capillary (e.g., Figure 25 For example, the capillary tube may be heated at multiple locations where it is desired to position resonators in the WGM microbubble resonator array 2702. Additionally, multiple instances of the WGM microbubble resonator array 2702 may be combined (e.g., in parallel) to form a WGM microbubble resonator array. Figure 27 As shown, each optical waveguide in a set of a plurality of optical waveguides 2701 can then be aligned to a corresponding WGM microbubble resonator in the 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 the WGM microbubble resonator array 2702 can be aligned to a single optical waveguide (e.g., similar to Figure 13-15 shown in ).

[0216] Attenuation of high-order modes in WGM resonators

[0217] As described above, multiple modes can be excited in a WGM resonator, starting with the fundamental mode, which is confined to the equatorial plane in a WGM resonator including a spherical portion, and ending with higher-order modes, which extend 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 higher-order modes. Therefore, it can be advantageous to eliminate higher-order modes for sensing applications to increase the Q factor and sensitivity of the WGM resonator.

[0218] One method of attenuating higher order modes may include applying a polymer coating on the substantially curved portion of the WGM resonator. Figure 28An exemplary variation of a WGM microsphere resonator having a polymer coating 2820 is illustrated. The WGM microsphere resonator may include a spherical portion 2802a and a rod portion 2802b. The polymer coating 2820 may be applied to the spherical portion 2802a of the WGM microsphere resonator. The polymer coating 2820 may be applied to 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. Partially coating the WGM microsphere may attenuate higher-order modes. In other words, the operating frequency of the WGM microsphere may be limited to the fundamental mode (the mode is confined to the equatorial plane of the WGM microsphere).

[0219] Optical waveguide 2801 may be coupled to the WGM microsphere resonator. In some variations, optical waveguide 2801 may be coupled to the WGM microsphere resonator before applying polymer coating 2820. In some variations, polymer coating 2820 may comprise an optically transparent liquid photopolymer having a high viscosity. Additionally, 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 greater than 1.5. In some variations, polymer coating 2820 may comprise Norland Optical Adhesive 68 ("NOA68").

[0220] In some variations, the polymer coating 2820 may be applied at the highest point / apex (e.g., the zenith) of the spherical portion 2802a. In some variations, the polymer coating 2820 may be applied to a spherical cap located at the top of the spherical portion 2802a, which has a relatively small height. For example, the polymer coating 2820 may be poured at the highest point. The polymer coating 2820 may flow downward from the highest point to the lower region of the spherical portion 2802a, thereby covering the spherical cap of the spherical portion 2802a. Due to its high viscosity, the polymer coating 2820 requires time to flow to the lower region of the spherical portion 2802a. When the polymer coating 2820 covers an appropriate amount of the circumference of the spherical portion 2802a (e.g., a spherical cap with a base that is an appropriate distance from the equatorial plane), the polymer coating 2820 may be cured (e.g., using ultraviolet light). For example, when the polymer coating 2820 covers almost half of the circumference of the spherical portion 2802a, the polymer coating 2820 may be cured. The time to cure the polymer coating 2820 may depend on the thickness of the polymer coating 2820, the intensity of the ultraviolet light used for curing, and the like.

[0221] Consider a polymer coating covering a spherical cap of a WGM resonator. The base of the spherical cap may be at a distance d from the equatorial plane. To confine the excitation of the WGM in the resonator to the fundamental mode, in some variations, the value of d may be between about 3 μm and about 15 μm.

[0222] Figure 29 An exemplary variation of a method for attenuating higher-order modes is illustrated. Once a polymer coating 2920 is applied so that it covers an appropriate number of regions around the circumference of the spherical portion 2902a of the WGM microsphere resonator, control light 2922 can be transmitted to the spherical portion 2902a via the rod portion 2902b of the WGM microsphere resonator. The polymer coating 2920 can absorb energy from the control light 2922. In some variations, the amount of power absorbed by the polymer coating 2920 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 absorbs a significant amount of power from the control light 2922 (e.g., greater than 50%, greater than approximately 60%, greater than approximately 70%, greater than approximately 80%, or greater than approximately 90%). This strong absorption of power from the control light 2922 can introduce losses into higher-order modes that overlap with the polymer coating 2920.

[0223] This may change the temperature of the polymer coating, thereby changing the resonant frequency of the WGM microsphere resonator. For example, the temperature change may 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. Due to the thermo-optic effect, the temperature change of the polymer coating 2920 may cause the resonant frequency of the WGM microsphere resonator to shift.

[0224] In some variations, the polymer coating 3020 may be applied to Figure 30 The WGM microsphere resonator shown has both a spherical portion 3002a and a rod portion 3002b. For example, the polymer coating 3020 may be applied to approximately half of the circumference of the spherical portion 3002a and at least some portion of the rod portion 3002b. In some variations, the WGM microsphere resonator may be immersed in a pool of polymer coating 3120, such as Figure 31 The effective thickness of the microsphere resonator can be controlled by immersing appropriate areas of the rod portion 3120b in a pool of polymer coating 3120.

[0225] Encapsulating coupled WGM resonators in polymer structures

[0226] The fabricated WGM resonator and / or WGM resonator array and one or more optical waveguides can be encapsulated in a polymer structure. In one variation, the optical waveguide can be embedded in the polymer structure, and then the WGM resonator can be embedded in the polymer structure, such that the optical waveguide and the WGM resonator are optically coupled. Alternatively, the WGM resonator can be embedded in the polymer structure, and then the optical waveguide can be embedded in the polymer structure, such that the optical waveguide and the WGM resonator are optically coupled. In another alternative variation, the optical waveguide and the WGM resonator can be optically coupled together. After optical coupling, the coupled waveguide and WGM resonator can be embedded in the polymer structure.

[0227] Figure 32A 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 a layer deposited on a substrate 3206, as described in further detail below.

[0228] The method of packaging the WGM resonator 3202a may include cleaning the surface of a substrate 3206a, such as silicon, silicon dioxide, quartz, plastic, or any other material suitable for use as a substrate for a sensing device. Suitable materials for the substrate 3206a may include damping materials to eliminate residual vibrations and minimize ultrasonic echoes within the WGM resonator 3202a. Figure 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 having a low refractive index, configured to achieve a wide whispering gallery mode (WGM) frequency response while having high ultrasonic attenuation to prevent reverberation of the set of ultrasonic echoes that the WGM resonator 3202a is designed to sense. In some variations, the acoustic impedance of the damping material of the backing polymer 3210a may be matched, for example, to the acoustic impedance of a matching polymer layer 3208a used to encapsulate the WGM resonator 3202a, as further described below.

[0229] like Figure 32AAs shown, one or more optical waveguides 3201a, such as optical fibers or integrated photonic waveguides, may be placed in a backing polymer 3210a. 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 a light source, or any other suitable medium for propagating light from the light source to a coupling point from a WGM resonator. In some variations, the one or more optical waveguides 3201a may be placed in the backing polymer 3210a before curing the backing polymer 3210a. Placing the one or more optical waveguides 3201a on an uncured backing polymer may cause the one or more optical waveguides to become embedded in the backing polymer 3210a. After placement of the one or more optical waveguides 3201a, the backing polymer 3210a may be cured, such as by baking the backing polymer 3210a at a predetermined temperature.

[0230] The method of packaging a WGM resonator 3202a may further include placing the WGM resonator 3202a proximate one or more optical waveguides 3201a to optically couple the WGM resonator to the 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 largest diameter of the WGM resonator. In one example, the WGM resonator 3202a may have an elliptical shape, and the largest circular diameter of the WGM resonator 3202a may be aligned with the one or more optical waveguides 3201a. Although the method of packaging a WGM resonator is primarily described as placing the optical waveguide before the WGM resonator, it should be understood that in some variations, the WGM resonator may be placed before the optical waveguide.

[0231] refer to Figure 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 contacted by the ultrasonic transducer (embodying the WGM resonator). For example, the matching polymer layer 3208a may be a polymer material having a low refractive index to reduce the transmission impedance of a set of ultrasonic echoes to the WGM resonator 3202a. The matching polymer 3208a may have a lower refractive index than the refractive index of the WGM resonator 3202a and the refractive index of the one or more optical waveguides 3201a. The method of packaging the WGM resonator 3202a may include curing the matching polymer layer.

[0232] although Figure 32A The exemplary schematic diagram of FIG depicts an optical waveguide 3201a embedded in a backing polymer, but in some variations, the optical waveguide may be in a matching polymer. Figure 32BAs shown, after curing the backing polymer 3210b and before the WGM resonator 3202b, the backing polymer 3210b, and / or the substrate 3206b are coated with the matching polymer 3208b, one or more optical waveguides 3201b can be placed on top of the backing polymer 3210b such that the optical waveguides 3201b are embedded in the matching polymer rather than the backing polymer. Additionally, in some variations, the optical waveguides can be partially embedded in the backing polymer and partially embedded in the matching polymer (e.g., approximately half of their cross-sectional area is in the backing polymer and approximately half of their cross-sectional area is in the matching polymer). Still further, it should be understood that in device embodiments having multiple polymer optical waveguides, a portion of the optical waveguides can be fully or partially embedded in the backing polymer, while another portion of the optical waveguides can be fully or partially embedded in the matching polymer in any suitable combination.

[0233] In some embodiments, the packaged WGM resonators 3202a, 3202b may be used as Figure 32A and 32B Alternatively, however, in some variations, the method of packaging a WGM resonator may include pre-coating the substrate with a release agent (e.g., polyvinyl alcohol, polyacrylic acid, polystyrene, fluorosilane, self-assembled monolayer, OmniCoat, several types of photoresists such as the AZ series, etc.) before coating the substrate with the backing polymer, so that the substrate can be demolded from the rest of the packaged WGM resonator. Removing the substrate can, for example, help reduce the amount of reverberation of a set of ultrasonic echoes that the WGM resonator is designed to sense.

[0234] Sensing Applications

[0235] WGM resonators and resonator arrays, such as those 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 such elements can include piezoelectric sensors, single crystal material sensors, piezoelectric micromachined ultrasonic transducers (PMUTs), capacitive micromachined ultrasonic transducer sensors (CMUTs), etc. The ultrasonic signal can cause a change in the refractive index and / or physical structure of the WGM resonator. Such changes can be as shown in the attached Figures 33-38 Measured as otherwise described in the description of .

[0236] Figure 33FIG3 is a schematic illustration of sensing ultrasonic echoes using an acousto-optic sensor device 3303. The acousto-optic sensor device 3303 includes an array of WGM resonators. Although the WGM resonators are shown as being arranged linearly at a predetermined distance d from one another, it should be understood that in some variations, at least some of the resonators may be arranged at different distances from one another. The WGM resonator array can be configured to be 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 digital modulation or Kerr four-wave mixing (FWM), or any other light source suitable for the operating frequency band of the acousto-optic sensor device 3303. Light can be coupled to the WGM resonator array to propagate a first set of whispering gallery modes (WGMs) around the circumferential walls of each WGM resonator in the WGM resonator array. The propagation of the first set of WGMs results in the generation of a first set of optical signals corresponding to a first set of resonant frequencies of the WGMs.

[0237] In use, the acousto-optic sensor device 3303 can be configured to receive a set of ultrasonic echoes 3302 generated and / or reflected from an object 3301. The set of ultrasonic echoes can have different propagation times (t1, t2, t3, etc.) for each WGM resonator. That is, ultrasonic echoes from the same object will arrive at each sensor in the array at slightly different times. The known distance between each sensor and the slight delay in these measurements allow for better calculation of the object's position (e.g., with better spatial resolution). The set of ultrasonic echoes can induce a set of changes in the radius and / or refractive index of the WGM resonator material and / or the refractive index of the ultrasonic enhancement material in the WGM resonator. These changes can propagate from the WGM resonator array around the circumferential wall of each WGM resonator to a second set of WGMs. The propagation of the second set of WGMs results in the generation of a second set of optical signals corresponding to a second set of resonant frequencies of the WGMs. The first and second sets of optical signals can be configured to propagate in one or more optical waveguides to a photodetector. The photodetector can be connected to a computing device to detect the difference between the first and second sets of optical signals.

[0238] Figure 34An exemplary measurement setup is depicted that can be used to process a set of signals from an acousto-optic sensor device 3405 sensing a set of ultrasound echoes (also referred to herein as "ultrasound signals"). The measurement setup can include a broadband light source or tunable laser 3401 that propagates light in an optical fiber 3402, and a fiber polarization controller 3403 that can be configured to control the polarization of the light. The measurement setup can also include the acousto-optic sensor device 3405, which can be configured to propagate a first set of optical signals based on a first set of whispering gallery modes (WGMs) propagating in an array of WGM resonators included in the acousto-optic sensor device 3405. The acousto-optic sensor device 3405 can be configured to receive a set of ultrasound signals 3404 and propagate a second set of optical signals based on a second set of WGMs propagating in the array of WGM resonators. The acousto-optic sensor device 3405 can also be 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 a "photodetector") 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 further be configured to transmit the first set of electrical signals and / or the second set of electrical signals to an oscilloscope 3407, which may be operably connected to a computer 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 a set of analyzed signals to a function generator 3408. The function generator may be configured to generate a set of generated signals and transmit them to the broadband light source or tunable laser 3401 to control the propagation of light in the optical fiber 3402.

[0239] Figure 35An exemplary measurement setup is depicted that can be used to process a set of signals from an acousto-optic sensor device 3504 sensing a set of ultrasonic echoes. The measurement setup can include a laser 3501, such as an optical frequency comb (OFC) laser source generated using digital modulation or Kerr four-wave mixing (FWM) methods, or any other laser suitable for the operating frequency band of the acousto-optic sensor device 3504, to generate laser light. The measurement setup can also 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 can be configured to split the laser light into a first laser light and a second laser light. The measurement setup can be configured to direct the first laser light to an electro-optic modulator 3503 to generate and transmit modulated laser light. The measurement setup can also be configured to direct the modulated laser light to the acousto-optic sensor device 3504 to generate a first set of signals corresponding to a first set of whispering gallery modes (WGMs). The acousto-optic sensor device 3504 can 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 can further be 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 and generate an electronic frequency signal. The measurement setup can be configured to transmit the electronic frequency signal to a fast Fourier transform (FFT) module 3506, which is operably connected to a computing device to process and analyze the difference between the first set of signals and the second set of signals. In one example, an OFC laser can generate a set of comb beams, and each comb beam from the set of comb beams can generate a data point to detect the difference between the first set of signals and the second set of signals. In this example, the 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 time from milliseconds to sensing time on the order of microseconds.

[0240] Figure 36 is a schematic depiction of the optical response of the acousto-optic sensor device 3601. The acousto-optic sensor device 3601 can be configured to receive a set of ultrasound echoes from an object 3602 positioned at any angle and detect the ultrasound echoes using a set of WGM resonators (e.g., as described with respect to FIG. Figure 34 and / or Figure 35 (described in further detail elsewhere). In one example, the acousto-optic sensor device 3601 can have a detection angle 3603 of approximately 160 degrees. In other words, the acousto-optic device 3601 has a wider detection angle than conventional piezoelectric ultrasonic sensors.

[0241] Ultrasound probe

[0242] In some embodiments, a WGM resonator array as described herein may be included in an ultrasound probe (also referred to herein as an "ultrasound system"). The ultrasound probe may include at least one optical waveguide to propagate light. The ultrasound probe may also include a plurality of piezoelectric elements to generate a set of ultrasound signals. The ultrasound probe may also include a plurality of piezoelectric elements to receive a set of ultrasound echoes corresponding to the set of ultrasound signals. The ultrasound probe may also include a plurality of WGM resonators in a polymer structure, each WGM resonator configured to receive a plurality of ultrasound echoes and propagate a set of whispering gallery modes (WGMs). At least one WGM resonator is optically coupled to at least one optical waveguide such that the WGM resonator is configured to transmit a first set of signals corresponding to a first set of WGMs to the at least one optical waveguide.

[0243] For example, Figure 37 is a schematic depiction of an ultrasound probe using a WGM resonator as described herein. Figure 38 yes Figure 37 37. A cross-sectional view of the probe is shown. The ultrasound 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 an ultrasound data acquisition. The ultrasound probe may be coupled to a control system and display to perform operating procedures as described in further detail herein. The backing material may include a damping material bonded to the back of the probe packaging to eliminate residual vibrations from the operating environment, improve portability, and minimize echoes generated within the sensor structure.

[0244] The ultrasound probe can be configured to transmit a set of light from a light source to the set of WGM resonator arrays 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 can be configured to receive electrical signals from a control system via a cable 3708 and an electrical connection array 3704 to generate a set of ultrasonic signals directed toward the subject, thereby generating a set of reflected ultrasonic echoes in a direction toward the ultrasound probe. The ultrasound probe can also be 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 ultrasound probe can optionally be configured to receive the set of ultrasonic echoes to generate a set of electrical signals. The ultrasound probe can also be configured to transmit the first set of signals and / or the second set of signals via the set of optical fibers 3705 and / or optionally transmit the electrical signals via the electrical connection array 3704 to a cable 3708 connected to an ultrasound data acquisition, control system, or display.

[0245] In some variations, an ultrasound probe, such as that described above with respect to Figure 37 and 38As described, the operating procedure can be configured to repeat while scanning the field of view using a phased array of piezoelectric crystal arrays. Doing so generates a line-by-line image and a lateral low-resolution image for each WGM resonator array sensor using the piezoelectric crystal array. A known synthetic aperture (SA) algorithm can then be used to generate a high-resolution WGM resonator array sensor image.

[0246] Additionally, 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 sensor signals based on these received ultrasonic echoes. For example, the sensor signals generated by the piezoelectric elements can be supplemented or combined with the signals transmitted by the WGM resonator in any suitable manner (e.g., to provide a multimodal sensor image).

[0247] Additionally or alternatively, in some embodiments, an ultrasound probe, as described above with respect to Figure 37 and 38 The described ultrasound probe can be configured to use different excitation modes, for example, using a piezoelectric crystal element or an array of piezoelectric crystals to transmit ultrasound signals while simultaneously using all piezoelectric crystal elements in a piezoelectric crystal array to receive a set of ultrasound echoes, also known as a compressed sensing (CS) method. The CS method generally forms a linear model (also known as a forward model) representing the process of acquiring signals from an image and solves the linear equation to obtain the image.

[0248] For the purpose of explanation, the foregoing description uses specific terms to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that specific details are not required to practice the present invention. Therefore, the foregoing descriptions of specific embodiments of the present invention are presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed; obviously, many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described to explain the principles of the present invention and its practical application, so that others skilled in the art can utilize the present invention and various embodiments with various modifications suitable for the particular use under consideration. It is expected that the appended claims and their equivalents will define the scope of the present invention.

Claims

1. A device for sensing applications, comprising: at least one optical waveguide; a plurality of whispering gallery mode resonators, each whispering gallery mode resonator configured to propagate a first set of whispering gallery modes; and a polymer structure encapsulating the at least one optical waveguide and at least one whispering gallery mode resonator of the plurality of whispering gallery mode resonators, the polymer structure comprising a first layer that attenuates higher-order modes in the at least one whispering gallery mode resonator and a second layer comprising a material having an acoustic impedance that matches the acoustic impedance of the at least one whispering gallery mode resonator, The at least one whispering gallery mode resonator is optically coupled to the at least one optical waveguide such that the at least one whispering gallery mode resonator is configured to transmit a first set of signals corresponding to the first set of whispering gallery modes to the at least one optical waveguide.

2. The device of claim 1 , wherein the polymer structure comprises at least one of: a backing region configured to attenuate residual ultrasonic echoes to prevent reverberation, and A matching region is configured to increase a bandwidth of a whispering gallery mode frequency response of the plurality of whispering gallery mode resonators. 3 . The apparatus of claim 2 , wherein the matching region is further configured to improve ultrasonic transmission to the plurality of whispering gallery mode resonators.

4. The device of claim 3, wherein a first refractive index of at least one of the backing region and the matching region 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 in the backing region of the polymer structure.

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

7. The device of claim 2, wherein the at least one whispering gallery mode resonator is embedded in 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 whispering gallery mode resonator.

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

10. The device of claim 1, wherein the at least one whispering gallery mode resonator comprises at least one of microspheres and microbubbles.

11. The apparatus of claim 1 , wherein the at least one whispering gallery mode resonator is configured as: receiving a plurality of ultrasound echoes; and In response to the plurality of ultrasonic echoes, a second set of whispering gallery modes is propagated. 12 . The apparatus of claim 11 , wherein the at least one whispering gallery mode resonator is further configured to transmit a second set of signals corresponding to the second set of whispering gallery modes to the at least one optical waveguide.

13. The apparatus of claim 12, further comprising a photodetector communicatively coupled to the at least one optical waveguide, wherein the at least one optical waveguide is configured to propagate the first set of signals and the second set of signals to the photodetector.

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

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

16. The device of claim 15, wherein the light source comprises a broadband light source or a tunable laser source.

17. The device of claim 1, wherein the at least one whispering gallery mode resonator comprises a hollow chamber.

18. The device of claim 17, wherein the hollow chamber comprises an ultrasonically enhanced material.

19. The apparatus of claim 1, each of the plurality of whispering gallery mode resonators being optically coupled to the at least one optical waveguide.

20. The apparatus of claim 1, wherein the at least one optical waveguide comprises a plurality of optical waveguides, and each whispering gallery mode resonator of the plurality of whispering gallery mode resonators is optically coupled to a corresponding optical waveguide of the plurality of optical waveguides.

21. The apparatus of claim 1, wherein at least a portion of the whispering gallery mode resonators are grouped in a linear arrangement.

22. The apparatus of claim 21, wherein the whispering gallery mode resonators in the linear arrangement are equally spaced apart.

23. The apparatus of claim 21, wherein the whispering gallery mode resonators in the linear arrangement are not equally spaced apart.

24. The apparatus of claim 1, wherein the plurality of whispering gallery mode resonators are bundled in a circular arrangement.

25. The apparatus of claim 1, wherein at least a portion of the whispering gallery mode resonators are equal in size.

26. The apparatus of claim 1, wherein at least a portion of the whispering gallery mode resonators are of unequal size.

27. A system for sensing applications, comprising the apparatus of claim 11 and a plurality of array elements configured to generate a set of ultrasound signals, wherein the plurality of ultrasound echoes correspond to the set of ultrasound signals.

28. The system of claim 27, wherein the array elements comprise at least array elements selected from the group consisting of: piezoelectric sensors, single crystal material sensors, piezoelectric micromachined ultrasound transducers, and capacitive micromachined ultrasound transducer sensors.

29. A method of ultrasound imaging, comprising: receiving a first set of signals corresponding to a first set of whispering gallery modes propagating in a plurality of whispering gallery mode resonators from at least one optical waveguide, wherein the at least one optical waveguide and the plurality of whispering gallery mode resonators are in a polymer structure comprising a first layer that attenuates higher-order modes in the at least one whispering gallery mode resonator and a second layer comprising a material having an acoustic impedance matched to the acoustic impedance of the at least one whispering gallery mode resonator; receiving, from the at least one optical waveguide, a second set of signals corresponding to a second set of whispering gallery modes propagating in the plurality of whispering gallery mode resonators, the second set of whispering gallery modes propagating in response to the plurality of whispering gallery mode resonators receiving a plurality of ultrasonic echoes; and A set of differences between the first set of signals and the second set of signals is detected.

30. The method of claim 29, wherein the polymer structure comprises at least one: a backing region configured to attenuate residual ultrasonic echoes to prevent reverberation, and A matching region is configured to increase a bandwidth of a whispering gallery mode frequency response of the plurality of whispering gallery mode resonators.

31. The method of claim 30, wherein the matching region is further configured to improve ultrasonic transmission to the plurality of whispering gallery mode resonators.

32. The method of claim 30, wherein a first refractive index of at least one of the backing region and the matching region is lower than a second refractive index of the at least one optical waveguide.

33. The method of claim 32, wherein the at least one optical waveguide is located in the backing region of the polymer structure.

34. The method of claim 32, wherein the at least one optical waveguide is located in the matching region of the polymer structure.

35. The method of claim 32, wherein the plurality of whispering gallery mode resonators are located in the matching region of the polymer structure.

36. The method of claim 29, wherein the polymer structure has an effective refractive index lower than a refractive index of the plurality of whispering gallery mode resonators.

37. The method of claim 29, wherein the polymer structure comprises an ultrasonically enhanced material.

38. The method of claim 29, wherein at least one whispering gallery mode resonator of the plurality of whispering gallery mode resonators comprises at least one of microspheres and microbubbles.

39. The method of claim 29, wherein each of the plurality of whispering gallery mode resonators is optically coupled to the at least one optical waveguide.

40. The method of claim 29, wherein the plurality of ultrasound echoes corresponds to a plurality of ultrasound signals.

41. The method of claim 29, further comprising: A synthetic aperture operation is performed on the first set of signals and the second set of signals, and an image of a subject is generated based at least in part on the synthetic aperture operation.

42. The method of claim 29, further comprising: A compressed sensing operation is performed on the first set of signals and the second set of signals, and an image of an object is generated based at least in part on the compressed sensing operation.

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