High resolution photonic tactile sensor

Abstract

Disclosed herein is a photonic integrated tactile sensing device capable of detecting the amplitude of a force exerted on one or more mechanical structures in an array of such structures defined on a surface of the device. The device uses a plurality of sensing elements based on the operating principle of the Mach-Zehnder Interferometer wherein a variation in the intensity of light transmitted by an optical waveguide is a function of an amplitude of a force exerted on the sensing unit. A force applied to a micropillar causes a deformation of a portion of a waveguide, causing the variation in the intensity of transmitted light.

Description

Attorney Docket: 8350.2024-210WO HIGH RESOLUTION PHOTONIC TACTILE SENSOR Related Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 636,565, filed April 19, 2024, the contents of which are incorporated herein in their entirety. Background

[0002] The growing development of intelligent robotic manipulators has raised awareness of the need for tactile sensors that provide rich information about contact surfaces for manipulation tasks in dynamic environments. An ideal robotic tactile sensor needs to have a small form factor, be flexible enough to conform to the curved surfaces of manipulators and possess high force sensitivity and spatial resolution to capture rich details about the contact surface. The sensor needs to be manufactured using a scalable fabrication process so that a large array of sensors can be realized with a high density of sensing units.

[0003] The tactile sensor also needs to be designed based on specific needs of the targeted applications. For example, everyday manipulation tasks call for detection of forces over a larger dynamic range, perhaps as large as 9.8N but with forgiving force resolution requirements of minimally 0.01 N. In addition, the sensor needs to cover a larger surface area over the manipulator endAttorney Docket: 8350.2024-210WO with an ideal spatial resolution of 1 mm. As another example, medical applications such as vitreoretinal microsurgery require tactile sensing over a much smaller dynamic range of 7.5 mN with a focus on high-precision force readings over a small tip.

[0004] Robotic tactile sensors have been implemented with different sensing modalities that include capacitance-based, resistance-based, or optical-based sensing. Among these modalities, optical sensing has been the subject of recent attention due to the high sensitivity of optical readout methods. Free- space optics methods have been used to detect deformations of a polymer membrane tactile sensor. These vision-based sensors work based on reflection imaging of deformed surfaces using a camera system. Gelsight, Inc. is a Waltham, Massachusetts based company that produces a state-of-the-art design for free-space optical tactile sensors. While Gelsight sensors provide micron-level high spatial resolution and accuracy, the free-space optical read- out requires an unflexible, bulky imaging system. To address this issue, optical fibers have been used as a tactile sensing mechanism to measure contact forces through the deformation of the fibers.

[0005] In one instance, a three-axis force tactile sensor with three intersected waveguides for one sensing element that exhibited a measuring capability of up to 3N with an average 0.089 N error was demonstrated. However, this design only included three fibers and was hard to scale up for multipoint tactile sensing over a larger area. A ring-core-based interferometer opticalAttorney Docket: 8350.2024-210WO sensor was demonstrated with a tapered two-layer cylindrical structure of single-mode fiber cladding and core. A type of interferometer-based ring- core fiber sensor for transversal force sensing with a sensitivity of 4.109 nm and a linearity of 0.9824 was also reported. However, the ring-core sensor design concerns single sensing units and requires manual assembly, which is not massively scalable for multi-unit arrays that satisfy tactile sensing needs.

[0006] Fabry-Perot-based optical sensors have been demonstrated with a measuring range of 6 mN and a sensitivity of 30–40 nm. Grating-based optical sensors detect tactile contacts when a normal force deforms the grating and changes the conditions of optical mode coupling. A grating-based 3×3 tactile sensor array that can detect normal force distributions based on the resonance peak shift of the grating was reported. However, the peak shift observation requires delicate and bulky spectrometers that are too large for small appliances such as robot fingers.

[0007] Therefore, given the state-of-the-art and the known prior art devices, it would be desirable to provide a device architecture that features a design and fabrication pipeline allowing customizable sensitivity and dynamic range as well as scalable array designs.Attorney Docket: 8350.2024-210WO Summary of the Invention

[0008] To address the issues identified above, disclosed herein is a novel device that uses integrated photonics to combine the optical circuit elements of lasers, waveguides, optical devices, and detectors onto a single chip. Its production uses scalable methods such as microfabrication that enable the design of microscale optical structures.

[0009] Using integrated photonic waveguides, ultra-compact, highly sensitive tactile sensors can be realized with total thicknesses of less than a few millimeters. The novel device of the present invention is referred to herein as a Photonic Integrated Tactile Sensor (PITS) device. The PITS device is based on a high- resolution sensing array consisting of near-millimeter independent sensing units and demonstrates multipoint sensing capabilities. It utilizes novel microscale asymmetric multimodal sensing units implemented in the Parylene photonics platform and in total measures less than 1 mm thick, excluding a non-functional Si-chip base. Parylene photonics is a flexible integrated photonic platform composed of Parylene C (a poly(para-xylylene) polymer) as the waveguide core and polydimethylsiloxane (PDMS) as the cladding. These waveguides are not only flexible but also confine and guide light effectively.Attorney Docket: 8350.2024-210WO Brief Description of the Drawings

[0010] By way of example, specific exemplary embodiments of the disclosed system and method will now be described, with reference to the accompanying drawings, in which:

[0011] FIG.1 is a schematic rendering of a single sensing unit.

[0012] FIG.2A is a side cross sectional view of a portion of a PITS device. FIG.2B is a cross-sectional view of the waveguide from FIG.2A.

[0013] FIG.3 is a schematic diagram showing the mechanics of the sensing unit.

[0014] FIG.4 is a schematic depiction of a deformed sensing unit.

[0015] FIG.5A shows the fabrication process of the of the waveguide layer. FIG.5B shows the fabrication of the micropillar layer and the joining of the two layers.

[0016] FIG.6 is a schematic diagram showing the arrangement of the micropillars with the micropillar layer.

[0017] FIG.7 is a schematic depiction of a completed PITS device.

[0018] FIG.8 is a graph showing the tradeoff among dynamic range, sensitivity and limit of detection of a sensing unit. Detailed Description

[0019] The novel Photonic Integrated Tactile Sensor disclosed herein utilizes sensing units that are based on the operating principles of the Mach-Zehnder Interferometer (MZI). The MZI is an optical device with two separatingAttorney Docket: 8350.2024-210WO branches split from its input and rejoined at its output. The device detects signals due to a degree of phase mismatch between its branches caused by an external stimulus, manifested as a change in the intensity of light detected at the output port of the sensor. The present invention uses a version of the MZI, referred to herein as an “asymmetrical MZI”. The asymmetrical MZI is different from traditional MZIs in that traditional MZIs require one branch to be stimulated by an input stimulus and the other branch to be isolated from the input stimulus, which can make the sensor bulky for force-sensing applications. In the asymmetrical design disclosed herein, this isolation is unnecessary because of the difference in the dimensions of the branches of the MZI. Herein, an asymmetrical MZI is referred to as a “sensing unit”.

[0020] The sensing units in the PITS device disclosed herein is the mechanism that transduces input-force-related mechanical deformation into waveguide optical signals in the form of light intensity modulation. Under indentation forces, the asymmetric branches of the sensing unit undergo different deformation levels that accumulate into phase mismatch changes, which results in interferometry condition changes that are observed as optical intensity changes at the device output due to interference of the light waves between the branches. This change in light intensity can be translated to the amplitude of the input force on the surface. The different bending levels of the branches affect the phase mismatch in two major ways: 1. different elongations of the originally equal-length branches that lead to optical pathAttorney Docket: 8350.2024-210WO differences; and 2. different effective refractive index (^^^^^) changes within the two asymmetric branches as a result of different bending deformation levels. However, the primary contributing factor is the different mechanical elongations of the branches. The effective index (^^^^^) change difference due to different branch bending levels can therefore be neglected.

[0021] This design is highly scalable using the planar fabrication process for Parylene photonics. The invention is explained herein in the context of a 4×4 array of identical sensing units. At the system level, the PITS device utilizes one input and a 1×16 array of outputs that can be detected using, for example, a single CCD camera. A four-layer Parylene waveguide Y-branch divider splits optical power equally into the 4×4 array. The PITS device has a two-layer vertical structure with a force transmitting structure, for example, a layer defining a 4x4 array of micropillars, align-bonded on top of the Parylene sensing unit array. The force transmitting structure is a mechanical device that distributes surface force locally to the sensing units and enables them to sense forces independently with reasonably low crosstalk. Although the invention is explained in the context of an array of micropillars acting as the force transmitting structure, any structure capable of transmitting force from a source of the force to the sensing units can be used. For example, an array or a single instance of any shaped structure (e.g., a micropillar of any cross- sectional shape, a cylinder, a plinth, a frustum, a pyramid, a cone, a half- sphere, a sphere etc.) defined on a top surface of a layer of polymer andAttorney Docket: 8350.2024-210WO positioned over the split portion of one of the waveguides such that the force applied to a shaped structure is transmitted to both branches of at least one of the optical waveguides. Additionally, a surface film (composed of, for example, PDMS or any other material capable of being formed into a film) of any thickness could also be used as the force transmitting structure. When the force is distributed over several shaped structures, differences in the deformation of the split branches of multiple sensing units allows the localization of the applied force within the array.

[0022] A single, exemplary sensing unit was designed to have a sensitivity of 0.28% intensity change per mN over a dynamic range of 0.08 N with <0.01 N force resolution, while having a signal-to-noise ratio (SNR) of 7.59 and an average hysteresis of 7.7% over 10 repeating indent cycles. The PITS device demonstrates the ability to detect off-center contacts, two-point contacts and predict basic contact shapes and locations when subject to a sequence of increasing indentations.

[0023] The PITS device is capable of designs beyond the embodiment illustrating the invention herein. The Parylene C photonic platform is highly customizable for both the sensor array and sensing unit. The array of the PITS device can be designed with different spatial resolutions and scaled to different numbers of sensing units to create arbitrary sensing areas as large as multiple ^^^^ଶ(although it is not necessary that the array of sensing units be arranged in a square pattern). The sensing units can be designed with different levels ofAttorney Docket: 8350.2024-210WO sensitivity and dynamic ranges by tuning its branch sizes and PDMS substrate stiffness. The 4×4 sensing array layout discussed herein also has a modular design and multiple arrays can be tiled together to make a larger sensing area without major changes in the circuit design.

[0024] One exemplary embodiment of a design of the sensing unit is shown schematically in FIG.1. The inner contour is composed of curves with bending radius ^^ெ^ூ = 800 ^^^^, smooth enough to prevent waveguidereflection loss due to bends. The outer contour is composed of concentric curves with those in the inner contour and has larger curvature radii of^^ெ^ூ + ^^ and ^^ெ^ூ + ^^ where ^^, ^^ are the respective primary andsecondary branch widths. The sensing unit has a total length of 300 ^^^^ and its split branches measure lengths of 169 ^^^^ with an opening of ^^ = 7 ^^^^ inthe middle.

[0025] A cross-section of a portion of a PITS device (showing only one sensing unit) is shown in FIGS.2(A,B). FIG.2B shows an exemplary split waveguide having a height of 7 ^^^^, wherein the primary branch is 12 ^^^^ in width and the secondary branch is 10.5 ^^^^ in width. The waveguides are composed of Parylene C. As shown in FIG.2A, the MTZ sensing unit is sandwiched between a 300 ^^^^ substrate and an approximate 380 ^^^^ micropillar layer composed of a 180^^^^ PDMS top film over a 200 ^^^^ tall PDMS micropillar. A layer of adhesive PDMS bonds the micropillar layer with the sensing unit layer. The PDMS and Parylene C materials act as optical materials, wherein the ParyleneAttorney Docket: 8350.2024-210WO C waveguides, with higher refractive indexes of 1.64, are embedded in the PDMS substrate that serves as cladding with lower refractive indexes of 1.42. The Parylene C and PDMS also act as mechanical materials, where the PDMS body is exposed to external applied forces and acts as a damping material to transduce small surface deformations into much smaller deformations of the sensitive sensing unit to produce the modulated light intensity signals.

[0026] It should be noted that, although preferred embodiments of the device use Parylene C and PDMS as the waveguide and cladding materials, respectively, any materials could be used, as long as the waveguide material is capable of transmitting light and the refractive index of the waveguide material is larger than the refractive index of the cladding material. For example, the waveguides could be constructed of polyurethane or silicon dioxide. The cladding material may selected based on the desired mechanical properties of the device. For example, a softer cladding material results in a more sensitive device, while a more rigid material results in a less sensitive device. The cladding may be, for example, a soft plastic, rubber, silicone, etc. In some embodiments, an extra layer of material may be added outside of the cladding to assist in the mechanical properties of the device.

[0027] During tactile sensing events, the indent force applied to a micropillar layer located at the sensor surface is translated to the local deformations of asymmetric sensing units within the photonic layer, which leads to outputAttorney Docket: 8350.2024-210WO light intensity modulation observed by an external camera pointing at the waveguide outputs.

[0028] The photonic part of the PITS device is centered on the asymmetric sensing unit. The detailed structure of the PITS device is shown regarding its photonic waveguide layer and micropillar layer. The asymmetric sensing unit design and its sensing mechanism will now be explained. The corresponding production method used for such a scenario is also explained.

[0029] The design of the on-chip asymmetric sensing unit is based on considerations of size and performance. The overall size of the asymmetric sensing unit determines the minimum spatial resolution of the PITS device, but smaller spatial resolution introduces more crosstalk and adds to the complexity to the sensor manufacturing and testing processes. As an illustration, a spatial resolution of near 1 ^^^^ was selected to imitate the highly sensitive parts of the human body, such as the fingertip. As a result, the overall surface area of the sensing unit was restricted to 330 ^^^^ × 31 ^^^^, with a narrow 7 ^^^^-wide branch opening, large enough to prevent the branches from touching during deformations. The asymmetric waveguide branch widths were selected to be 11.5 and 12 ^^^^ both with thicknesses of 6 ^^^^. These widths and thicknesses were chosen so the waveguide modes could be well-confined within the waveguide core, and the aspect ratio could be kept small while minimizing the overall size of the sensing unit.Attorney Docket: 8350.2024-210WO

[0030] With reference to FIG.3, the mechanism whereby the asymmetric sensing unit responds to external force stimuli based on the asymmetric deformation of its two branches under a common load is explained. It should be noted that, in this context, the term “asymmetric” refers to the two branches of the sensing unit being of different widths, however, the branches could differ in other dimensions. Further, the two branches, in the exemplary embodiments herein, are depicted as having rectangular cross-sectional shapes , but other cross-sectional shapes are also possible. For example, the two branches could have circular cross-sectional shapes and vary in diameter.

[0031] The two branches are preferably made of Parylene C with elastic modulus ^^, widths of ^^^(Index ^^ = 1,2 refers to the branches) and identical thicknesses of ℎ. Under external stress, the two branches undergo the same moment ^^, and the deformation of the sensing unit, as depicted in FIG.4, causes the same deviation ^^ from the neutral plane but with different moments of యinertia ^^^ = ^ೖ^ (^^ = 1,2) due to the different branch widths. This results indifferentstrains from the two branches ^^^ = − ெ^ (^^ = 1,2). Thedifference in strain |^^ − ^^ | leads toଶa difference in pathfrom the respective branches of the sensing unit. The asymmetric deformations of the branches also lead to the difference in optical mode effective index changes. These two factors contribute to the change in phaseAttorney Docket: 8350.2024-210WO difference between the two branches, resulting in the intensity change of the light transmitted by the sensing unit.

[0032] Because of the difference in widths between the branches, there is a phase difference between the two branches even prior to the deformation caused by the application of force. However, the deformation of the branches changes this phase difference. This deformation is denoted as ^^ and is a function of the time varying external force, expressed as ^^(^^) = ^^^(^^(^^)).Subsequently, the change of phase difference due to deformation for all optical modes, denoted ∆^^, is expressed as a function of the deformation ^^ in Eq. (1): ∆^^(^^) = ^^∆థ,ெ^ூ = 2^^^^ ∙ ൫^^^^^௧ ∙ ^^^^^௧ − ^^^^^^௧ ∙ ^^^^^^௧൯(1)

[0033] Here, ^^^ismodes. ^^^is the branch ^^ length, and ^^ is the wavelength of the laser. This change of phase difference ∆^^(^^) reveals itself as a change of output intensity, which iscos∆^^(^^). The output intensity is denoted as ^^ெ^ூ(^^) with the expression shown in Eq. (2): ^^ೌ^(2)Attorney Docket: 8350.2024-210WO

[0034] The index ^^ = 1,2, … , ^^^^௫ represents the number of light modes passingthrough the sensing unit, and the vector ^^^represents the optical power coupling coefficients from the fundamental mode of the input fiber to the modes within the branches. It is assumed that the power coefficients for the same order of modes in the left and right branches are identical and the amount of power coupling into the modes from the input waveguide’s higher order modes is neglected.

[0035] In summary, the sensing unit output intensity can be related to the input normal force ^^(^^) shown in Eq. (3) below. The change of this output intensity reflects the amount of force applied on the sensing unit given a certain deformation function ^^^(^^(^^)) dictated by the contact mechanics.^^ெ^ூ(^^) = ^^ூ,ெ^ூ ൬^^^∆థ,ெ^ூ൫^^^(^^(^^))൯^^(3)

[0036] The designsubstrate properties altogether influence the force sensing performance. A large difference in the widths of the two branches increases the sensitivity because a larger branch deformation difference takes place under external forces. A thinner or more rigid PDMS substrate makes the sensing unit harder to deform and increases the dynamic range of the unit. The sensing unit design also faces a tradeoff between its size and its radius of curvature^^ெ^ூ, which is defined as the bend radius of the curves that make up the branches.Attorney Docket: 8350.2024-210WO The size of the sensing unit can be reduced by using a lower ^^ெ^ூ, but when it reduces below 500 ^^^^, the Parylene C waveguide propagation loss increases by at least 10%. Such losses increase exponentially for a higher extent of bends. The sensing unit reported has a minimum ^^ெ^ூof 800 ^^^^ and measures a total length of 330 ^^^^. These parameters were intended to strike a balance between the optical power loss and compactness.

[0037] The array alone is insufficient for sensing touch events because input forces on arbitrary waveguide sections outside the sensing units would also contribute to signals under the influence of shear forces. As a solution, a separate force transmitting mechanism is introduced to distribute force from the source of the force to both branches of the asymmetric MZIs. Note that the applied force could be in the form of compression, tension or rotation, so the force transmitting mechanism may be a structure of any design appropriate for transmitting the particular type of applied force to the asymmetrical waveguides. Although any structure could be used for this purpose, in the exemplary embodiment disclosed herein, a micropillar layer is introduced to concentrate the external stress from the contact area onto the corresponding branches of the local sensing units, thereby constraining the contact conditions at the sites of the sensing units and localizing the applied force. The structure of the disclosed PITS device is made of a Parylene waveguide layer at the bottom and a PDMS micropillar layer at the top. The micropillar layer design is inspired by the human skin’s spinosum layer.Attorney Docket: 8350.2024-210WO

[0038] Utilizing the Parylene C waveguide microfabrication pipeline, multiple sensing units are packed in the PITS device, and the sensor maintains a thin profile despite having two combined layers. In one embodiment, the PITS device has an overall thickness of less than 1 mm, with its waveguide layer measuring 300 ^^^^ and its micropillar layer measuring 380 ^^^^, as shown in FIG.2. The cylindrical pillars from the micropillar layer are 200 ^^^^ tall, and their cross- sections are ellipses with radii of 200 ^^^^ and 110 ^^^^.

[0039] The design parameters of the micropillar layer prevent the micropillars from buckling under loads while minimizing the amount of mechanical crosstalk. The PITS device sits on top of a non-functional Si substrate, which serves as the bottom surface rather than a part of the sensor. The tilted 4×4 array of independent sensing units from the waveguide layer has spatial resolutions of 900 ^^^^ in the horizontal direction and 1250 ^^^^ in the longitudinal direction. The waveguide array requires low operational power despite the high channel count and its over 2 ^^^^ size. The input laser diode operates below its threshold current with <20 ^^^^ of power but provides enough power for the four-layer Y-branch Parylene waveguide power divider to distribute the input laser power into the 16 sensing units, with outputs observable by the external camera.

[0040] The waveguide layer and micropillar layer are fabricated in separate processes and flip-chip bonded together. A commercial single-mode optical fiber is used to connect to the waveguide array using optical glue. TheAttorney Docket: 8350.2024-210WO waveguide outputs form a horizontal 1×16 array that may be observed using a CMOS camera and a single-frame segmentation algorithm. The algorithm may be implemented in software and segregates each sensing unit output from a single image of all sensing unit outputs. The algorithm, in one embodiment, sums pixel brightness both row-wise and column-wise and is then able to detect peaks segregated by dark area, indicating each channel of PITS device. The peaks are then measured to determine intensity changes. The algorithm then translates the intensity changes into a profile of force exerted on the plurality of micropillars, including the magnitude of the force and on which the micropillars the force is being exerted.

[0041] The Parylene C-based PITS device can be further customized for in-plane designs because the Parylene C film is etched into patterns dictated by the computer-designed photomask during the fabrication process. A different array layout than the demonstrated tilted 4×4 array format as well as additional in-plane waveguide-based sensing components can be easily designed using this method.

[0042] The fabrication process of the device is shown in FIGS.5(A,B) and occurs in two separate processes, one for fabrication of the waveguide layer and one for fabrication of the micropillar layer.

[0043] The production process of the waveguide layer is shown in FIG.5A and involves the microfabrication of the parylene C and dicing of the waveguide array to produce the array. At step 1, a layer of PDMS is spin-coated onto a 4-Attorney Docket: 8350.2024-210WO inch silicon substrate and vacuum-cured (at, e.g., 110 °C) overnight. In one embodiment, the PDMS layer is 300 ^^^^ in thickness. In step 2, a layer of Parylene Cfilm is deposited using a chemical vapor deposition process. The thickness of the Parylene C layer should be the desired thickness of the waveguides, in one embodiment, 7 ^^^^. In step 3 the Parylene Cfilm is sputtered with a Cr hard mask (80 ^^^^) and spin-coated with photoresist (3^^^^). The photoresist was exposed for 62 ^^ using a 320 ^^^^ wavelength lightsource with 4.5 ^^^^ൗ ^^^^ଶ dosage and developed using a 1:4 volume ratio ofAZ 400K developer and de-ionized water. The exposed hard mask region is then etched using CR 1020 etchant. The exposed Parylene Cfilm is etched using RIE to form the Parylene C waveguide arrays. The remaining Cr hard mask is fully removed by CR 1020 to expose the etched Parylene waveguides. Each waveguide array is laser-cut through the PDMS and Silicon wafer outside of its boundaries into a 15 ^^^^ × 30 ^^^^ sized chip.

[0044] The production process of the micropillar layer is shown in FIG.5B. The patterned micropillar layer is a single piece of PDMS fabricated using a SU-8 micromold. In a first step of the process, SU8 (200 ^^^^) is spin-coated on a 4- inch silicon wafer. At step 2, the SU-8 was exposed with a SU-8 lithography tool for 45 ^^ and developed with SU-8 developer for 3 ℎ^^. In the third step, SU-8 wafer is silanized with 2 drops of Trichloro (1H,1H,2H,2H-perfluorooctyl) silane in a vacuum desiccator for 30 min. In the fourth step, a PDMS layerAttorney Docket: 8350.2024-210WO with a mixing ratio of 8:1 is cast on the SU-8 wafer (master mold) with a total weight of 3.5 grams over the 4-inch mold area and cured at room temperature for 2 days over a leveled surface. The resulting PDMS micropillar layer is cut along the boundary of its features from the cast PDMS layer, peeled off andflip-chip bonded with the previous laser-cut waveguide array chip.

[0045] The micropillars for each sensing units are offset from one another within the micropillar array, as shown in FIG.6, such as to primarily impinge on and deform only one sensing unit. Further, the waveguides are preferably arranged in a substantially parallel configuration and have their split portions offset from adjacent waveguides such as to minimize cross-forces exerted by micropillars coupled to other waveguides. In this manner, the exerted force may be localized using the differential in force exerted by each micropillar in the array.

[0046] The micropillar layer and the waveguide array chip obtained from prior steps were bonded using aflip-chip bonding process. A thin layer of PDMS with a mixing ratio of 1:10 using an equally weighted OS-20 as the thinning agent was applied to the waveguide layer surface over a leveled ground and half- cured for 30 min at room temperature before theflip-chip bonding process promoting adhesion between the layers.

[0047] An example of a complete PITS device formed on a single chip (minus the light sensors) is shown in FIG.7. The exemplary device has one input and aAttorney Docket: 8350.2024-210WO 1x16 array of outputs that can be detected using any optical detector, for example, a single CCD camera or one or more photodiodes (or any other device capable of being used as an optical detector). The light source is preferably a collimated light source, for example, a laser. Preferably, the frequency of the laser is any frequency between infrared and ultraviolet, although more preferably, the laser has a frequency in the visible range of the spectrum. The frequency of the laser may be dependent on the specific application of the PITS device and the construction of the waveguides. Generally, a smaller frequency of laser light results in a larger phase change under applied force, which may increase the sensitivity of the device but reduces the dynamic range. Ideally, the power of the laser would be minimized to the extent that the optical detector can still detect the light, even after losses in the waveguides and even after the intensity has been diminished by an applied force.

[0048] The device uses 16 sensing units, having the micropillars arranged in a 4x4 offset array. As should be realized, the array of sensing units may be larger or smaller than the 16 sensing units used in the exemplary embodiment disclosed herein. As would also be realized, multiple lasers and multiple cameras may be used to handle larger arrays.

[0049] The architecture of the PITS device enables a highly-customizable design pipeline with trade-offs in design goals of dynamic range, sensitivity and limit of detection (LoD) through simple parameter controls in the design andAttorney Docket: 8350.2024-210WO manufacturing process. The customizability of the PITS device caters to a wide range of tactile sensor applications, such as surgical manipulators that require highly-sensitive tactile feedback with forgiving dynamic range, or home robot manipulators that require a wide dynamic range with lesser requirement of perception sensitivity.

[0050] It has been empirically determined that the different elongation of the branches is the primary mechanism for the light intensity modulation of the sensing unit. The PITS device has a simple multi-stack thin-film-based sensor structure and operates under small elastic deformation of its PDMS and Parylene C materials.

[0051] The design parameters of the sensing unit have been identified as the widths of the split branches, their common length, thickness and the deformability of the substrate PDMS, represented as its thickness. The performance of the sensing unit is dependent on the metrics of dynamic range, sensitivity and LoD. The application of force is limited to normal forces onto the micropillar. There is no optimum presented for the design, which instead has trade-offs that should be tailored to the intended applications with different focuses on detection range, precision, etc.

[0052] Dynamic range: The sensing unit has a periodic transfer function due to its interferometry mechanism that exhibits signal changes from the cosine of phase mismatch across its branches. The dynamic range of the sensing unit is defined as the force that corresponds to the response at the first peak of itsAttorney Docket: 8350.2024-210WO periodic transfer function. In some embodiments, only the first periodic peak of the sensing unit is used for sensing purposes, although the sensing unit is still capable of qualitative force-sensing beyond the peak if signal processing methods are used.

[0053] Sensitivity: The sensitivity of the sensing unit represents the rate of the signal change at an applied input force, defined as ^^(^^) = ௗ ூ^^ೌ^^^(௧)ௗி(௧), where^^^^^^^^(^^) is the intensity change at time t under force ^^(^^).

[0054] LoD: The limit of detection is defined as the minimum force detectable with a level of confidence. A lower LoD number indicates a better ability of the sensor to read small forces. The waveguide-based sensing unit has its output signals observed by an external detector and the limiting factor is the precision pf the detector. It is assumed that the detector is capable of detecting 2% ^^^^ magnitude.

[0055] FIG.8 is a graph showing the tradeoffs among the dynamic range, sensitivity and LoD with an example comparing two sensing units with different levels of deformability from different substrate thicknesses. It is the deformation state of the sensing unit that affects its output intensity change status. The dynamic range is equivalent to the force that brought the sensing unit from the initial undeformed state ^^^to the deformed state ^^^. The more deformable sensing unit has a lower dynamic range ^^^^^because it reaches the deformation state with lesser forces. It has a higher sensitivity ^^^becauseAttorney Docket: 8350.2024-210WO its intensity changes more with the same amount of indent forces. Accordingly, its output ^^^^ magnitude will reach 2% with a smaller amount of force that reflects in its lower LoD. The dynamic range of the sensing unit is negatively correlated to its sensitivity. A sensing unit with higher sensitivity has better LoD, or namely a smaller LoD number.

[0056] The sensing unit design parameters are selected based on the considerations of their effects over the sensing unit performance metrics identified above and the ease of their customization in the sensing unit design and manufacturing process with reasonable tradeoffs. The deformability of the substrate PDMS and that of the Parylene C waveguide are effective factors in determining the performance of the sensing unit by affecting its mechanical deformations under external loads. The 2D design of the sensing unit also contributes greatly to the sensing unit performance with the most design customizability using computer-aided-design software and produced at the photolithography step of the microfabrication process.

[0057] PDMS substrate thickness: The PDMS substrate is a thin film, whose deformability is defined as Ӡ =௧ா where t is the substrate film thickness and E is the film’s compressive Young’s modulus. The adjustment of the substrate film thickness can be adjusted in the manufacturing process, but the substrate film compressive modulus can also be adjusted by altering the mixing ratio between the base and curing agent or changing the curingAttorney Docket: 8350.2024-210WO temperature, which adds extra flexibility in the design process. The PDMS substrate deformability has been considered only in the aspect of its thickness, although changes in its compressive modulus can produce equivalent effects. A more deformable PDMS substrate will result in more deformation of the sensing unit under the same applied load and lead to a greater amount of observed intensity change.

[0058] MZI asymmetry factor η: The level of asymmetry between the branches affects the sensing unit performance through changes in interferometry conditions, which is determined by the designed branch widths denoted a and b (a > b). The wider branch with width a is defined as the primary branch and the narrower branch with width b is defined as the secondary branch. The level of asymmetry is defined as an asymmetry factor η =^ି^^ (0 ≤ η <1). Given the same amount of applied force, a larger η will result in more elongation differences across the branches and greater intensity changes. The widths of the branches also affect the sensing unit performance in their influence on the center bending modulus, but are not modeled as a parameter because it is assumed that the design starts with a determined primary branch width and finds the suitable secondary branch width with relation to η.

[0059] MZI branch length: The branch length affects the interference conditions by the optical path lengths that determine the accumulated phase mismatch atAttorney Docket: 8350.2024-210WO the output. Longer branches will lead to a greater amount of phase change and increased intensity changes under the same applied load. The branch length also affects the bending modulus of the sensing unit due to the wider branches compared to the input and output waveguides, but this effect is assumed much smaller than the previous factor.

[0060] MZI thickness: The sensing unit thickness, determined by the thickness of the Parylene C film deposited on top of the PDMS substrate, affects the bending modulus of the branch under deformation and the resulting mechanical elongation of the branches. A thicker sensing unit will make the structure harder to deform under the same applied force and lead to less observed intensity change signals.

[0061] It should be noted that the top micropillar layer thickness does not affect the sensing unit’s performance metrics because it does not affect the amount of pressure applied to the sensing unit, which determines the deformation and response. Instead, it affects the force-displacement relation of the indent site that is equivalent to the softness of the touch. The geometry of the micropillar layer has more effects on the array than on an individual sensing units due to its importance of stress-concentration and inter-unit-stress crosstalk rejection.

[0062] The specific designs of the sensing units as well as the design of the overall PITS device described herein are intended to be exemplary in nature only. As would be realized by one of skill in the art, many variations on the specificAttorney Docket: 8350.2024-210WO designs presented herein can be made to achieve different objectives. For example, the array of devices can be made larger than the exemplary 4x4 array presented herein to accommodate a larger sensing area. Various design parameters of the sensing units, such as the substrate thickness, the level of asymmetry between the branches and the length of the branches, among others, and materials of construction can be varied to affect characteristics of the sensing units, for example, dynamic range, sensitivity and limit of detection.

Claims

Attorney Docket: 8350.2024-210WO Claims:

1. A device comprising: a plurality of optical waveguides encapsulated in a layer of polymer, each waveguide having a portion split into two branches of unequal dimensions; and a force transmitting structure for transmitting a compression, tension or rotation force applied to the device from a source of the force to both branches of at least one of the waveguides.

2. The device of claim 1 wherein the force transmiƫng structure comprises: one or more shaped structures defined on a top surface of the layer of polymer, each shaped structure positioned over the split portion of one of the waveguides such that a force applied to a shaped structure is transmitted to both branches of at least one of the waveguides.

3. The device of claim 1 wherein the force transmiƫng structure comprises: one or more layers of a film disposed on a top surface of the layer of polymer.Attorney Docket: 8350.2024-210WO 4. The device of claim 2 where the shaped structures are selected from a group consisƟng of a micropillar of any cross-secƟonal shape, a cylinder, a plinth, a frustum, a pyramid, a cone, a half-sphere and a sphere.

5. The device of claim 2 wherein a force exerted on a shaped structure results in deformaƟon of the split porƟon of at least one of the waveguides.

6. The device of claim 4 further comprising: one or more collimated light sources coupled to an input side of each waveguide.

7. The device of claim 6 wherein a deformaƟon of the split porƟon of any one of the plurality of waveguides causes a differenƟal in intensity of light transmiƩed by each branch of the waveguide and observable at an output side of the waveguide.

8. The device of claim 7 wherein changes in intensity of light observed at the output side of any waveguide are a funcƟon of the amplitude of the force exerted by a micropillar on the split porƟon of that waveguide.

9. The device of claim 2 wherein a plurality of the shaped structures are arranged in an array and further wherein force exerted on the array may be localized by determining aAttorney Docket: 8350.2024-210WO differenƟal in the forces exerted on each shaped structure.

10. The device of claim 8 further comprising: one or more optical detector coupled to the output sides of each waveguide.

11. The device of claim 10 wherein the one or more opƟcal detectors comprise one or more CCD cameras or one or more photodiodes sensing light transmiƩed by each waveguide.

12. The device of claim 10 further comprising: a processor; and software, executing on the processor and implementing an algorithm for segregating outputs from each of the sensing units from the single image of the optical detectors and detecting changes in intensity from each output.

13. The device of claim 12 wherein the soŌware algorithm further translates changes in intensity of one or more outputs of the sensing units into a profile of force exerted on the plurality of shaped structures.

14. The device of claim 2 wherein the layer of polymer is disposed on a silicon substrate.Attorney Docket: 8350.2024-210WO 15. The device of claim 2 wherein the plurality of waveguides is arranged in a parallel configuraƟon.

16. The device of claim 15 wherein the split porƟon of each waveguide is offset from the split porƟons of adjacent waveguides.

17. The device of claim 9 wherein the array of shaped structures is disposed in an offset arrangement such that each shaped structure exerts a force primarily on the split porƟon of one of the waveguides.

18. The device of claim 2 wherein the waveguides are encapsulated in PDMS and further wherein a force exerted on a shaped structure is transmiƩed through the PDMS to the split porƟon of a waveguide.

19. The device of claim 1 wherein each of the two branches of each waveguide has a rectangular cross-secƟonal shape.

20. The device of claim 1 wherein the two branches of each waveguide are of unequal widths.Attorney Docket: 8350.2024-210WO 21. The device of claim 1 wherein the two branches of each waveguide are of unequal heights.

22. The device of claim 1 wherein the waveguides are composed of Parylene C.

23. The device of claim 1 wherein the polymer is PDMS.

24. The device of claim 2 wherein the shaped structures are composed of PDMS.

25. A device comprising: afirst layer comprising a plurality of waveguides, each waveguide having a porƟon split into two branches of unequal dimensions; and a second layer disposed on thefirst layer and defining a plurality of shaped structures extending from a top surface thereof and arranged in an array configuraƟon, each shaped structure posiƟoned to exert a force on the split porƟon of one of the waveguides.

26. A device comprising: afirst layer comprising a plurality of waveguides, each waveguide having a porƟon split into two branches of unequal dimensions; andAttorney Docket: 8350.2024-210WO a second layer in the form of afilm disposed on thefirst layer and posiƟoned to transmit a force onto the split porƟons of the waveguides.

27. A method of determining the magnitude of an applied force comprising: determining a phase modulation of light transmitted through an optical waveguide having a split portion forming two asymmetrical branches, when the force is applied to both branches; wherein the phase modulation is expressed as a differential in intensity of the light transmitted through the waveguide; and translating the intensity differential into a profile of force exerted on the waveguide, including at least the magnitude of the force.

28. The method of claim 27 wherein the applied force is a compression, tension or rotaƟon force.

29. The method of claim 27 wherein the force is applied to a plurality of the waveguides arranged in a configuraƟon such that a source of the force can be localized by determining which of the waveguides responds to the applied force.Attorney Docket: 8350.2024-210WO 30. The method of claim 29 where in the plurality of waveguides are arranged in an offset array configuraƟon.

31. The method of claim 27 wherein the applied force is transmiƩed from a source of the force to the plurality of waveguides by a force transmiƫng structure.

32. The method of claim 31 wherein the force transmiƫng structure comprises one or more shaped structures defined on a top surface of the layer of polymer, each shaped structure posiƟoned over the split porƟon of one of the waveguides such that a force applied to a shaped structure is transmiƩed to both branches of at least one of the waveguides.

33. The method of claim 32 where the shaped structures are selected from a group consisƟng of a micropillar of any cross-secƟonal shape, a cylinder, a plinth, a frustum, a pyramid, a cone, a half-sphere and a sphere.

34. The method of claim 27 wherein the phase modulaƟon results from different mechanical deformaƟons of the asymmetrical branches due to the applied force.

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