An optical window structure for wearable devices and its fabrication method

Abstract

This application discloses an optical window structure for wearable devices and its fabrication method, which maintains the light transmittance and structural reliability of the optical window structure during long-term use, while reducing the device's power consumption. The optical window structure provided in this application includes: a bottom cover structure disposed on the wearing side of the wearable device, the bottom cover structure having a stepped hole structure; an optical window body, the optical window body being a thin sheet structure made of a light-transmitting rigid material, the optical window body being embedded within the stepped hole structure and abutting against the stepped support surface to form axial support, forming an annular gap with the annular limiting sidewall; and a sealing adhesive layer filling the annular gap to form a continuous sealing interface between the optical window body and the bottom cover structure; wherein, the optical window body has a preset transmittance within a preset operating wavelength range of the PPG sensor, so that the detection light can be transmitted through the optical window body to human tissue and back, forming a PPG detection optical path.

Description

Technical Field

[0001] This application relates to the field of wearable device technology, and in particular to an optical window structure for wearable devices and a method for fabricating the same. Background Technology

[0002] Wearable devices such as smart rings have become important carriers for daily health monitoring, and PPG sensors are the core components in these devices used to collect physiological characteristics such as heart rate and blood oxygenation. The operation of a PPG sensor relies on a light-emitting element emitting detection light of a specific wavelength. This light penetrates the optical window and human skin tissue, undergoes reflection or scattering, and is then received by the receiving element through the optical window. Currently, the optical windows of existing smart ring PPG sensors typically employ the following two conventional schemes:

[0003] The first type is the potting and light-transmitting solution, which uses transparent silicone or epoxy resin as a potting material in the sensor area, directly curing to form an optical window and waterproof layer. The polymer structure of the potting material in this structure is prone to aging, leading to yellowing or fogging and causing internal and interface scattering. This results in the long-term equivalent transmittance of this solution typically dropping to 90% or below, causing a decrease in the PPG signal-to-noise ratio. Furthermore, to maintain signal strength, the system must increase the driving current of the light-emitting element, significantly increasing power consumption. In addition, the potted sensor area cannot be disassembled, making repair difficult.

[0004] The second type is the plastic or polycarbonate (PC) film window solution. The initial light transmittance of the film material is within an acceptable range, but its surface physical hardness is low, making it prone to surface scratches during daily wear and friction. Furthermore, this type of polymer film material also exhibits material property degradation phenomena such as fogging and yellowing during long-term use, and thus cannot meet the requirements for long-term reliable use. Summary of the Invention

[0005] This application provides an optical window structure for wearable devices and a method for fabricating the same, which maintains the light transmittance and structural reliability of the optical window structure during long-term use, while reducing the power consumption of the device.

[0006] The first aspect of this application provides an optical window structure for wearable devices, comprising: A bottom cover structure is provided on the wearing side of the wearable device. The bottom cover structure has a stepped hole structure at the position corresponding to the PPG sensor. The stepped hole structure includes a stepped support surface and an annular limiting sidewall surrounding the outer periphery of the stepped support surface. An optical window body is a thin sheet structure made of a light-transmitting rigid material. The optical window body is embedded in the stepped hole structure and abuts against the stepped support surface to form axial support. An annular gap is formed between the outer peripheral edge of the optical window body and the annular limiting sidewall. A sealant layer is filled in the annular gap to form a continuous sealing interface between the optical window body and the bottom cover structure. The optical window body has a preset transmittance within the preset operating wavelength range of the PPG sensor, so that the detection light can be transmitted through the optical window body to human tissue and transmitted back, forming a PPG detection optical path.

[0007] Optionally, the light-transmitting rigid material is selected from any one of chemically strengthened glass, sapphire single crystal material, or transparent alumina ceramic; The thickness of the optical window body is no greater than 0.30 mm, and the Mohs hardness of the light-transmitting rigid material is no less than 6.

[0008] Optionally, when the light-transmitting hard material is a sapphire single crystal material, the C-axis direction of the sapphire single crystal material is perpendicular to the surface of the optical window body, and the thickness of the sapphire single crystal material is 0.15 mm to 0.30 mm.

[0009] Optionally, when the light-transmitting rigid material is chemically strengthened glass, the surface of the chemically strengthened glass has a compressive stress layer formed by ion exchange process, and the thickness of the chemically strengthened glass is 0.10 mm to 0.15 mm.

[0010] Optionally, both the inner and outer surfaces of the optical window body are provided with an anti-reflective coating layer; The preset operating wavelength range includes at least one target wavelength among 530nm, 660nm and 940nm. The anti-reflective coating layer is configured with a thickness for the preset operating wavelength range so that the preset transmittance of the optical window body at the target wavelength is not less than 95%.

[0011] Optionally, the anti-reflective coating layer is composed of alternating layers of low-refractive-index material and high-refractive-index material; The low-refractive-index material layer is magnesium fluoride, and the high-refractive-index material layer is titanium dioxide.

[0012] Optionally, a surface functional layer is also stacked on the outer surface of the optical window body outside the anti-reflective coating layer; The surface functional layer includes at least one of an oleophobic layer or an anti-fog layer.

[0013] Optionally, the sealant layer is a UV-curable adhesive ring or a hot melt adhesive ring that has been cured by ultraviolet light. The radial width of the annular gap is 0.2 mm to 0.5 mm.

[0014] Optionally, the bottom cover structure forms a receiving cavity in the inner region corresponding to the optical window body; There is an air gap between the inner surface of the optical window body and the PPG sensor light-emitting and receiving elements installed in the receiving cavity, which is not filled with solid potting medium.

[0015] A second aspect of this application provides a method for fabricating an optical window structure for wearable devices, comprising: Based on the spatial size constraints of the wearable device and the relative positional relationship of the PPG sensors, the setting area of ​​the optical window body is determined; Based on the operating wavelength range of the PPG sensor and the target signal-to-noise ratio requirement, the preset transmittance of the optical window body at each operating wavelength is determined, and a mapping relationship between transmittance and PPG signal intensity is established. Based on the preset transmittance and the spatial size constraints, the target thickness range of the optical window body is calculated, and a light-transmitting rigid material that meets the requirements of light transmission performance and structural strength is selected to prepare a light-transmitting rigid sheet. Based on the structural dimensions of the optical window body, a stepped hole for embedding the optical window body is constructed on the bottom cover structure, so that the optical window body is supported by the stepped surface of the stepped hole, and the size parameters of the annular gap formed between the outer peripheral surface of the optical window body and the inner sidewall of the stepped hole are determined. Based on the size parameters of the annular gap and the waterproof requirements, the material type of the sealant layer is determined, and the sealant layer is continuously filled into the annular gap to form a circumferential sealing interface between the optical window body and the bottom cover structure. Optionally, the preparation method further includes: Based on the preset transmittance, thickness parameters of the optical window body and the state of the circumferential sealing interface, the signal attenuation value of the detection light in the PPG detection optical path is calculated, and the calculated signal attenuation value is compared with the preset signal threshold. Based on the comparison result between the signal attenuation value and the preset signal threshold, it is determined whether the current optical window structure meets the PPG signal acquisition requirements, and if it does not meet the requirements, the preset transmittance, thickness parameters or the anti-reflection coating parameters are adjusted. When the PPG signal acquisition requirements are met, the light emission driving parameters of the PPG sensor are optimized and controlled according to the finalized optical window structure to reduce the driving current while meeting the target signal-to-noise ratio requirements.

[0016] As can be seen from the above technical solutions, this application has the following advantages: The stepped hole structure of the bottom cover provides precise axial support and radial limiting for the optical window body, ensuring stable installation and coaxiality with the PPG sensor optical path, thus guaranteeing the stability of the detection light signal transmission path. As a core light-transmitting component, the optical window body uses a thin sheet structure of light-transmitting rigid material to fit the miniaturized ring layout, while maintaining a preset transmittance within the preset operating wavelength range of the PPG sensor, ensuring efficient transmission and return of the detection light. A sealing layer forms a continuous sealing interface between the optical window body and the bottom cover structure, providing reliable protection for the internal PPG sensor. The optical window structure provided in this application enables the detection light to stably form the PPG detection optical path, ensuring the stability and accuracy of PPG signal acquisition. It also maintains the light transmittance and structural reliability of the optical window structure during long-term use, thereby reducing the driving current of the PPG sensor's light-emitting element and reducing device power consumption, enabling long-term stable health monitoring of wearable devices. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the overall structure of the optical window structure for wearable devices provided in this application; Figure 2 A schematic diagram of the optical window body and sealing layer in the optical window structure for wearable devices provided in this application; Figure 3 A schematic flowchart of an embodiment of the method for fabricating an optical window structure for wearable devices provided in this application. Detailed Implementation

[0019] This application provides an optical window structure for wearable devices and a method for fabricating the same, which maintains the light transmittance and structural reliability of the optical window structure during long-term use, while reducing the power consumption of the device.

[0020] In this application, the terms "upper", "lower", "left", "right", "front", "rear", "top", "bottom", "inner", "outer", "middle", "vertical", "horizontal", "lateral", "longitudinal" and other terms indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only used to describe the relative positional relationship between the components or parts and do not specifically limit the specific installation orientation of each component or part.

[0021] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0022] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0023] Furthermore, the structures, proportions, sizes, etc., drawn in the accompanying drawings of this application are only used to complement the content disclosed in the specification for those skilled in the art to understand and read, and are not intended to limit the conditions under which this application can be implemented. Therefore, they have no substantial technical significance. Any modification to the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects and purposes that this application can produce, should still fall within the scope of the technical content disclosed in this application.

[0024] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] The following is a detailed description of an embodiment of the optical window structure for wearable devices provided in this application. Please refer to [link / reference]. Figure 1 and Figure 2 The optical window structure includes: The bottom cover structure 1 is located on the wearing side of the wearable device. The bottom cover structure 1 has a stepped hole structure at the corresponding PPG sensor position. The stepped hole structure includes a stepped support surface and an annular limiting sidewall surrounding the outer periphery of the stepped support surface. The optical window body 2 is a thin sheet structure made of light-transmitting rigid material. The optical window body 2 is embedded in the stepped hole structure and abuts against the stepped support surface to form axial support. An annular gap is formed between the outer peripheral edge of the optical window body 2 and the annular limiting sidewall. The sealant layer 3 fills the annular gap and is used to form a continuous sealing interface between the optical window body 2 and the bottom cover structure 1. The optical window body 2 has a preset transmittance within the preset operating wavelength range of the PPG sensor, so that the detection light can be transmitted through the optical window body 2 to human tissue and transmitted back, forming a PPG detection optical path.

[0026] In this embodiment, the bottom cover structure 1 is disposed on the wearing side of the wearable device, serving as the mounting base for the entire optical window structure and the internal PPG sensor. Its core function is to provide a stable assembly carrier for each component. The bottom cover structure 1 is preferably made of titanium alloy or stainless steel. By creating a stepped hole structure on the bottom cover structure 1 at the position corresponding to the PPG sensor, the optical window body 2 can be accurately positioned and fixed. This stepped hole structure consists of a stepped support surface and an annular limiting sidewall surrounding its outer perimeter. The stepped support surface provides axial support for the optical window body 2, ensuring that the optical path of the optical window body 2 and the PPG sensor remain coaxially aligned, avoiding optical signal transmission deviation due to positional offset. The annular limiting sidewall provides radial limiting for the optical window body 2, preventing misalignment and shaking during assembly and use, while also reserving uniform filling space for the sealing adhesive layer 3.

[0027] In some specific embodiments, the bottom cover structure 1 has multiple stepped hole structures, each corresponding to a different PPG sensor light-emitting element or receiving element. Each stepped hole structure has an independent optical window body embedded within it, and these optical window bodies are separated by the solid walls of the bottom cover structure. This multi-window independent configuration scheme enables physical isolation between optical paths, effectively suppressing optical crosstalk between adjacent light sources and improving the channel isolation of multi-channel PPG signals.

[0028] In other specific embodiments, the bottom cover structure 1 has only a single large-area stepped hole structure, the area of ​​which is sufficient to simultaneously cover the optical path projection area of ​​the entire PPG sensor's light-emitting and receiving elements. An integrated optical window body is embedded within this stepped hole structure. This single-window solution simplifies the assembly process, reduces the number of sealing interfaces, and helps improve mass production yield.

[0029] The optical window body 2 is made of a thin sheet structure of light-transmitting rigid material. It is a key component for optical signal transmission in the PPG detection optical path. Its core function is to achieve efficient transmission of detection light, ensuring that the detection light emitted by the PPG sensor's light-emitting element can smoothly penetrate and act on human skin tissue. At the same time, it efficiently transmits the light signal carrying physiological characteristic information after reflection and scattering by human skin tissue back to the sensor receiving element, forming a complete PPG detection optical path. Compared with the potting materials, plastics, or PC films used in existing technologies, the light-transmitting rigid material has excellent wear resistance and anti-aging properties, which can effectively avoid problems such as surface scratches, yellowing, and fogging during long-term use. This ensures that the preset transmittance of the optical window body 2 is stable within the preset operating wavelength range of the PPG sensor, thereby reducing light signal attenuation and improving the signal-to-noise ratio of the PPG signal. At the same time, the thin sheet structure design can adapt to the miniaturized space constraints of wearable devices, balancing structural compactness and light transmission performance.

[0030] After the optical window body 2 is embedded in the stepped hole structure, an annular gap is formed between its outer peripheral edge and the annular limiting sidewall. This annular gap not only provides a continuous and uniform filling space for the sealing layer 3, ensuring that the sealing layer 3 can completely cover the mating interface between the optical window body 2 and the bottom cover structure 1, but also avoids hard contact between the optical window body 2 and the bottom cover structure 1, effectively relieving the structural stress generated during assembly, preventing the optical window body 2, made of hard materials, from cracking or being damaged due to compression, and ensuring the reliability of the structural assembly. The core function of the sealing layer 3, which fills the annular gap, is to form a continuous and dense sealing interface between the optical window body 2 and the bottom cover structure 1, achieving waterproof and dustproof protection. It can effectively prevent external impurities such as sweat, water vapor, and dust from entering the ring during daily wear, avoiding corrosion and damage to the core components of the PPG sensor, such as the light-emitting element and the receiving element, and ensuring the long-term stable operation of the sensor. At the same time, the sealing layer 3 only fills the edge annular gap and does not cover the optical path area of ​​the optical window body 2, so it will not interfere with the transmission of detection light, ensuring the integrity and efficiency of optical signal transmission.

[0031] The overall working principle of the optical window structure in this embodiment is as follows: The PPG sensor is installed in the inner area of ​​the bottom cover structure 1 corresponding to the optical window body 2. During operation, the sensor's light-emitting element emits detection light of a specific wavelength. This detection light first penetrates the optical window body 2 and is successfully transmitted to the human skin tissue. The detection light is reflected and scattered within the human skin tissue, carrying physiological characteristic information such as heart rate and blood oxygenation, and then penetrates the optical window body 2 again to be transmitted back to the receiving element of the PPG sensor, completing one PPG signal acquisition and forming a complete PPG detection optical path. During the entire signal acquisition process, the bottom cover structure 1 achieves precise positioning of the optical window body 2 through the stepped hole structure, ensuring optical path alignment and avoiding light signal deviation. The optical window body 2, with its high light transmittance and high wear resistance, reduces light signal attenuation and ensures signal quality. The sealing layer 3 provides reliable protection through a continuous sealing interface, preventing external impurities from affecting the sensor's operation.

[0032] In this embodiment, the stepped hole structure of the bottom cover structure 1 provides precise axial support and radial limiting for the optical window body 2, achieving stable installation of the optical window body 2 and ensuring its coaxiality with the PPG sensor optical path, thus guaranteeing the stability of the detection light signal transmission path. As a core light-transmitting component, the optical window body 2 uses a thin sheet structure of light-transmitting rigid material to adapt to the miniaturized layout of the ring, while maintaining a preset transmittance within the preset operating wavelength range of the PPG sensor, ensuring efficient transmission and return of the detection light. The sealing layer 3 forms a continuous sealing interface between the optical window body 2 and the bottom cover structure 1, providing reliable protection for the internal PPG sensor. The optical window structure provided in this embodiment enables the detection light to stably form the PPG detection optical path, ensuring the stability and accuracy of PPG signal acquisition. It also maintains the light transmittance and structural reliability of the optical window structure during long-term use, thereby reducing the driving current of the PPG sensor's light-emitting element and reducing device power consumption, enabling long-term stable health monitoring of the wearable device.

[0033] In some optional embodiments, the light-transmitting rigid material is selected from any one of chemically strengthened glass, sapphire single crystal material, or transparent alumina ceramic; The thickness of the optical window body 2 is no more than 0.30 mm, and the Mohs hardness of the light-transmitting rigid material is no less than 6.

[0034] In this optional embodiment, the light-transmitting rigid material can be any one of chemically strengthened glass, sapphire single crystal material, or transparent alumina ceramic. Such materials possess both excellent light transmission performance and structural strength, with a Mohs hardness preferably not less than 6. This effectively resists contact and friction during daily wear, preventing surface scratches, and slows down the degradation of material properties over long-term use, ensuring the stability of the light transmission performance of the optical window body 2. Considering the space constraints of miniaturization in wearable devices, the thickness of the optical window body 2 is preferably controlled to be no more than 0.30 mm. This satisfies structural assembly requirements, ensures its own structural strength, and further adapts to the overall size design of the ring, balancing light transmission performance and device portability. This ensures efficient penetration of detection light and reduces the attenuation of the light signal in the optical window body 2.

[0035] In some optional embodiments, when the light-transmitting rigid material is a sapphire single crystal material, the C-axis direction of the sapphire single crystal material is set perpendicular to the surface of the optical window body 2, and the thickness of the sapphire single crystal material is 0.15 mm to 0.30 mm.

[0036] In this optional embodiment, when sapphire single crystal is selected as the transparent hard material, the crystal orientation and thickness of the sapphire single crystal can be specifically designed to further optimize the light transmission performance and structural adaptability of the optical window body 2. Specifically, at the level of crystal optical properties, sapphire single crystal, as an anisotropic medium, inherently exhibits birefringence. During the optical transmission process of the PPG sensor, if light passes through the sapphire along a direction deviating from the C-axis (i.e., the optical principal axis), the birefringence effect will cause the incident light to decompose into ordinary and extraordinary light with different propagation speeds, thus producing a phase delay. This optical distortion will be significantly amplified in the detection of weak PPG signals, causing polarization-related optical loss and distortion of the returned waveform. Based on this, in this optional embodiment, the crystal C-axis of the sapphire single crystal material can be precisely controlled to be perpendicular to the surface of the optical window through a crystal orientation cutting process. Since light does not undergo birefringence when propagating along the C-axis, this crystal orientation configuration allows the perpendicularly incident and reflected PPG detection light to penetrate the optical window body 2 without phase difference, eliminating polarization crosstalk from the bottom layer of the material and ensuring high-fidelity transmission of physiological characteristic signals to the greatest extent.

[0037] Secondly, the thickness of the optical window body 2 made of sapphire single crystal material is controlled within the range of 0.15mm to 0.30mm. This thickness range can meet the miniaturization assembly requirements of wearable devices, adapt to the limited installation space inside the ring, and ensure that the optical window body 2 has sufficient structural strength to resist external impacts and friction during daily wear, avoiding problems such as cracking and damage. Specifically, based on the physical characteristics of sapphire material, which is high in hardness and high in brittleness, if the thickness of the optical window body 2 is less than 0.15mm, its bending section modulus will be too low during the manufacturing process of double-sided polishing of the wafer, laser cutting, and subsequent pressing and assembly with the stepped hole. This will make it easy for microcracks to propagate, leading to an exponential increase in the breakage rate, which cannot meet the requirements of mass production. In addition, a thickness of more than 0.15mm can ensure that the window can withstand the local point loads and external impacts generated during daily wear within the suspended span (corresponding to the bottom cover opening area). Therefore, the thickness of the optical window body 2 made of sapphire single crystal material is preferably 0.20mm.

[0038] In some alternative embodiments, when the light-transmitting rigid material is chemically strengthened glass, the surface of the chemically strengthened glass has a compressive stress layer formed by an ion exchange process, and the thickness of the chemically strengthened glass is 0.10 mm to 0.15 mm.

[0039] In this optional embodiment, when chemically strengthened glass is selected as the light-transmitting rigid material, untreated ultrathin ordinary glass exhibits extremely high intrinsic brittleness. Its surface microcracks are prone to propagation and overall fracture under external tensile stress. Therefore, an ion exchange process is introduced to modify and reinforce the glass, replacing smaller-radius ions in the glass surface network structure with larger-radius ions. This physical crowding effect based on ion volume differences constructs a high-density compressive stress layer on both the inner and outer surfaces of the glass. When the ring is subjected to friction from sharp objects, blunt force impacts, or uneven radial compression during daily wear, this compressive stress layer can preemptively offset and absorb the tensile stress converted from external loads, significantly inhibiting the initiation of surface micro-defects and crack propagation. Through this mechanism, the bending fatigue strength, mechanical impact resistance, and surface scratch resistance of the optical window body 2 made of chemically strengthened glass can be improved, ensuring its long-term reliability as the external mechanical seal boundary of the ring.

[0040] Secondly, the thickness of the optical window body 2 made of chemically strengthened glass is controlled within the range of 0.10mm to 0.15mm. Controlling the upper limit of the thickness to 0.15mm minimizes the physical isolation distance between the PPG sensor's photoelectric transceiver element and the human skin. A shorter physical optical path not only reduces the background absorption loss of the glass substrate for specific working wavelengths of light but also constrains the internal reflection and scattering divergence angles of light during transmission, thereby improving the light power entering the subcutaneous tissue and the signal-to-noise ratio of the returned signal. Simultaneously, due to the internal stress distribution and processing limits of chemically strengthened glass, the lower limit of the thickness needs to be controlled at 0.10mm. If the thickness is below 0.10mm, even if compressive stress is formed on the surface, the compensating central tensile stress zone corresponding to the glass core will be extremely fragile due to the excessively thin cross-section. This insufficient absolute stiffness will cause the optical window to easily undergo irreversible warping deformation or even breakage due to thermal stress or assembly stress during subsequent processes such as high-temperature deposition of the anti-reflective film, mechanical pressing of the stepped holes, and UV curing of the sealant.

[0041] In some optional embodiments, both the inner and outer surfaces of the optical window body 2 are provided with an anti-reflection coating layer; The preset working wavelength range includes at least one target wavelength among 530nm, 660nm and 940nm. The anti-reflective coating layer is configured with a thickness for the preset working wavelength range so that the preset transmittance of the optical window body 2 at the target wavelength is not less than 95%.

[0042] In this optional embodiment, when light enters from a low-refractive-index medium and exits from a high-refractive-index medium, Fresnel reflection inevitably occurs due to the abrupt change in refractive index on both sides of the interface. This background reflection causes significant energy attenuation of the light signal as it penetrates the optical window body 2. This embodiment addresses this by simultaneously depositing anti-reflection coatings on both the inner and outer optical interfaces of the optical window body 2, utilizing the principle of thin-film interference for optical compensation. Specifically, by controlling the refractive index of the coating material, a specific phase difference is created between the light waves reflected from the outer surface of the film and those reflected from the inner surface of the film, causing destructive interference between the two reflected light waves. This macroscopically weakens or even eliminates the reflected light at the interface, converting the originally reflected light energy back into transmitted light energy.

[0043] Secondly, the multi-dimensional physiological detection of PPG sensors is highly dependent on the light source of specific target wavelengths: 530nm green light is typically used for high-precision superficial capillary heart rate monitoring, while the combination of 660nm red light and 940nm infrared light is used to calculate the blood oxygen saturation of deep tissues based on the difference in absorption rates of the two wavelengths. In this embodiment, the anti-reflection coating layer has a precise thickness configuration for this preset working wavelength range. By optimizing the coating layer thickness to match the optical characteristics of the target wavelength, the reflection loss of the detection light on the surface of the optical window body 2 is minimized, ensuring that the preset transmittance of the optical window body 2 at each target wavelength is controlled at 95% or above, further improving the signal-to-noise ratio of the PPG signal and ensuring the accuracy and reliability of physiological feature acquisition. At the same time, in conjunction with the light-transmitting rigid material characteristics of the optical window body 2, a better light transmission system is formed, which is suitable for the long-term stable health monitoring needs of wearable devices.

[0044] In some optional embodiments, the anti-reflective coating layer is composed of alternating layers of low-refractive-index material and high-refractive-index material; The low-refractive-index material layer is made of magnesium fluoride, and the high-refractive-index material layer is made of titanium dioxide.

[0045] In this optional embodiment, a multilayer antireflective structure with alternating stacks of magnesium fluoride and titanium dioxide is employed. This fully utilizes the interference effect of materials with large refractive index differences, minimizing interface reflection loss in the PPG sensor's optical path. Specifically, titanium dioxide, as a high refractive index medium (approximately 2.4), possesses excellent transparency and extremely high surface hardness in the visible to near-infrared bands; while magnesium fluoride, as a low refractive index medium (approximately 1.38), not only has extremely low optical loss but also good mechanical stability. Alternating stacking of the two creates a significant refractive index contrast between adjacent physical films. This significant refractive index step allows the antireflective coating to achieve highly efficient antireflection across a specific wideband, covering 530nm, 660nm, and 940nm, with a relatively small number of stacked layers. This allows for control over the number of stacked layers and the overall physical thickness, meeting the stringent space constraints of wearable devices. It also fundamentally reduces the internal residual thermal stress and intrinsic stress accumulated during the vacuum deposition process of multilayer films, effectively preventing wrinkling, cracking, or peeling of multilayer films when subjected to long-term environmental high and low temperature alternation or mechanical micro-deformation, and ensuring the long-term reliability of the bonding force between the anti-reflective coating layer and the rigid window substrate.

[0046] This multilayered alternating structure utilizes the principle of destructive interference of multiple beams in physical optics. When the emitted beam or weak return signal light from the PPG sensor penetrates the coating layer, the light waves undergo microscopic reflection and transmission at each heterogeneous interface between magnesium fluoride and titanium dioxide. By precisely calibrating and controlling the optical thickness of each single-layer material during the manufacturing process, a specific phase difference is generated between the sub-wavelengths reflected from each interface within the film system. These multiple reflected light waves with specific phase differences undergo destructive interference in space, causing the electric vector amplitudes of the reflected light waves to cancel each other out, thereby greatly suppressing the overall reflectivity of the interface on a macroscopic scale. According to the law of conservation of energy, the reflected light energy suppressed by the interference effect is forcibly redistributed and superimposed into the transmitted beam, achieving constructive interference at the target wavelength, thus bringing the transmittance close to the material limit.

[0047] In some optional embodiments, a surface functional layer is further stacked on the outer surface of the optical window body 2 outside the anti-reflective coating layer. The surface functional layer includes at least one of an oleophobic layer or an anti-fog layer.

[0048] In this optional embodiment, the surface functional layer, as the outermost physical boundary that directly contacts the optical window with the external actual wearing environment, needs to address the interference of dynamic physiological secretions and environmental changes on static optical performance. The surface functional layer can be at least one of an oleophobic layer or an anti-fog layer. The oleophobic layer effectively repels sweat, oil, and other contaminants adhering to the outer surface of the optical window body 2 during daily wear, reducing the interference of residual dirt on the transmission of detection light, while also facilitating cleaning and maintaining the cleanliness of the optical window surface. The anti-fog layer prevents fogging on the window surface caused by changes in ambient temperature or the evaporation of human sweat, preventing fog from obstructing the light path and reducing light transmittance, ensuring stable transmission of the detection light signal. However, the surface functional layer should not affect the light transmission optimization effect of the anti-reflective coating layer, nor change the structural strength and light transmission performance of the optical window body 2; it only enhances the surface function to further ensure the stable operation of the optical window in complex daily wearing scenarios.

[0049] In some optional embodiments, the sealant layer 3 is a UV-curable adhesive ring or a hot melt adhesive ring that has been cured by ultraviolet light. The radial width of the annular gap is 0.2 mm to 0.5 mm.

[0050] In this optional embodiment, the sealant layer 3 is selected from UV-curable adhesive rings or hot-melt adhesive rings that have been cured by ultraviolet light. These sealant materials possess excellent sealing and compatibility, and can cure quickly to form a dense, continuous sealing interface, suitable for filling the annular gap. Simultaneously, the radial width of the annular gap is controlled within the range of 0.2mm to 0.5mm. This width range provides sufficient filling space for the sealant layer 3, ensuring that the sealant layer 3 completely fills the gap without any air bubbles, achieving reliable waterproof and dustproof protection. It also avoids excessive sealant usage due to an overly wide gap, which would cause stress after curing and affect structural stability, and avoids insufficient sealant filling and sealing leaks due to an overly narrow gap, thus balancing sealing reliability and structural assembly rationality.

[0051] In some alternative embodiments, the bottom cover structure 1 forms a receiving cavity in the inner region of the corresponding optical window body 2; There is an air gap between the inner surface of the optical window body 2 and the PPG sensor light-emitting and receiving elements installed in the receiving cavity, which is not filled with solid potting medium.

[0052] In this optional embodiment, considering that traditional micro PPG sensor encapsulation typically involves directly injection molding or potting transparent silicone and epoxy resin above the light-emitting element and photodetector element to achieve optical bridging and physical protection, these polymer potting media inevitably undergo polymer molecular chain breakage and photothermal aging during long-term operation, accompanied by continuous local heat dissipation from the light-emitting element and temperature and humidity fluctuations in the external environment. This material-level degradation macroscopically manifests as colloid yellowing, nonlinear decay of light transmittance, and even microscopic cracks and fogging scattering within the colloid or at the interface under thermal shock. Based on this, in this embodiment, a certain air gap is maintained between the inner surface of the optical window body 2 and the PPG sensor element within the cavity, and this air gap is not filled with any light-transmitting potting medium. This design ensures that the optical path between the optical window and the sensor element is not affected by the aging or yellowing of the potting material, thereby avoiding a decrease in light transmittance and attenuation of optical signals during long-term use. In addition, the presence of the air gap can also buffer mechanical stress under minor vibrations or thermal expansion conditions, further protecting the structural integrity of the optical window and the sensor element. Furthermore, the design without physical potting medium facilitates sensor disassembly and repair, balancing signal transmission performance with ease of equipment maintenance.

[0053] Please see Figure 3 , Figure 3 An embodiment of the method for fabricating an optical window structure for wearable devices provided in this application includes an assembly and shaping process in steps S301-S305, and a verification and optimization process in steps S306-S308. Specifically, the fabrication method includes: S301. Determine the setting area of ​​the optical window body based on the spatial size constraints of the wearable device and the relative positional relationship of the PPG sensor. S302. Based on the operating wavelength range of the PPG sensor and the target signal-to-noise ratio requirements, determine the preset transmittance of the optical window body at each operating wavelength, and establish the mapping relationship between transmittance and PPG signal strength. S303. Based on the preset transmittance and spatial size constraints, calculate the target thickness range of the optical window body, and select a light-transmitting rigid material that meets the requirements of light transmission performance and structural strength to prepare a light-transmitting rigid sheet. Furthermore, based on the refractive index parameters and interface reflection characteristics of the selected light-transmitting rigid material, anti-reflection coating parameters for reducing interface reflection loss can be determined, and a coating layer can be formed on at least one side surface of the optical window body according to the anti-reflection coating parameters. S304. Based on the structural dimensions of the optical window body, construct a stepped hole on the bottom cover structure for embedding the optical window body, so that the optical window body is supported by the stepped surface of the stepped hole, and determine the dimensional parameters of the annular gap formed between the outer peripheral surface of the optical window body and the inner sidewall of the stepped hole. S305. Based on the dimensional parameters of the annular gap and the waterproofing requirements, determine the material type of the sealant layer and continuously fill the annular gap with the sealant layer to form a circumferential sealing interface between the optical window body and the bottom cover structure. In this embodiment, steps S301 to S305 constitute the mechanical assembly and shaping process of the optical window structure. First, based on the spatial size constraints of the wearable device and the relative positional relationship of the photoplethysmography (PPG) sensor, the setting area of ​​the optical window body is determined. Since the usable space inside the wearable device is limited, the selection of the setting area must simultaneously meet the physical constraints of the external structure of the ring and the optical path space requirements for the internal sensor's light emission and reception. After determining the setting area, based on the operating wavelength range of the PPG sensor and the target signal-to-noise ratio (SNR) requirement, the preset transmittance of the optical window body at each operating wavelength is determined. The SNR refers to the ratio of the effective physiological characteristic signal power to the background noise power, which directly determines the accuracy of physiological characteristic measurements. Based on this, a mapping relationship between transmittance and PPG signal intensity is established to quantify the signal change caused by light penetrating the window interface. Subsequently, based on the predetermined preset transmittance and the aforementioned spatial size constraints, the target thickness range of the optical window body is calculated, and a light-transmitting rigid material that meets the requirements of light transmission performance and structural strength is selected to prepare a light-transmitting rigid sheet. Compared to the soft potting compounds commonly used in existing technologies, using a light-transmitting rigid material to prepare a thin sheet avoids surface scratches caused by long-term wear friction and optical path loss due to material aging and yellowing. After fabricating the optical window body, a stepped hole for embedding the optical window body is constructed on the bottom cover structure according to the structural dimensions of the optical window body. The stepped hole has a support plane formed by the height difference, allowing the optical window body to be supported by the stepped surface of the stepped hole, thereby obtaining directional physical support when the ring is subjected to external pressure, preventing the structure from collapsing and deforming inward. At the same time, the dimensional parameters of the annular gap formed between the outer peripheral surface of the optical window body and the inner sidewall of the stepped hole are determined. This annular gap provides a defined accommodating space for the subsequent sealing medium. Then, based on the determined annular gap dimensional parameters and the waterproof requirements of the device, the material type of the sealant layer is determined, and the sealant layer is continuously filled into the annular gap to form a circumferential sealing interface between the optical window body and the bottom cover structure. This circumferential sealing interface can effectively prevent the intrusion of external sweat or domestic water, overcoming the defects of traditional non-sealed or direct adhesive designs that are prone to water ingress and damage to internal circuits. In a preferred embodiment, during sealing, a certain air gap is maintained between the inner surface of the optical window body and the PPG sensor element in the receiving cavity, and this air gap is not filled with any light-transmitting potting medium.

[0054] S306. Based on the preset transmittance, thickness parameters and circumferential sealing interface of the optical window body, calculate the signal attenuation value of the detection light in the PPG detection optical path, and compare the calculated signal attenuation value with the preset signal threshold. S307. Based on the comparison result between the signal attenuation value and the preset signal threshold, determine whether the current optical window structure meets the PPG signal acquisition requirements, and adjust the preset transmittance, thickness parameters or anti-reflection coating parameters if they do not meet the requirements. S308. When the PPG signal acquisition requirements are met, the light emission driving parameters of the PPG sensor are optimized and controlled according to the finalized optical window structure to reduce the driving current while meeting the target signal-to-noise ratio requirements.

[0055] In this embodiment, steps S306 to S308 are the optical path verification and system optimization process of the optical window structure. Based on the preset transmittance, thickness parameters, and the state of the circumferential sealing interface of the optical window body, the signal attenuation value of the detection light in the PPG detection optical path is calculated. The signal attenuation value objectively reflects the unavoidable energy loss of light during penetration of the optical window body, the sealing interface, and reflection between human subcutaneous tissues. The calculated signal attenuation value is compared with a preset signal threshold, and the comparison result is used to determine whether the current optical window structure meets the PPG signal acquisition requirements. When the comparison result shows that the acquisition requirements are not met, it indicates that the light loss caused by the current structure is too large. At this time, the preset transmittance, thickness parameters, or anti-reflection coating parameters are adjusted. The anti-reflection coating parameters are used to characterize the film layer properties coated on the material surface to reduce light interface reflection loss. By adjusting the above parameters, the light path transmission obstacle is reduced until the overall structure meets the signal acquisition standard. When it is determined that the PPG signal acquisition requirements are met, the light emission driving parameters of the PPG sensor are optimized and controlled according to the finalized optical window structure. Because the finalized optical window body provides a clear and low-loss light transmission path, the system can reduce the driving current while meeting the target signal-to-noise ratio requirements. This control step combines physical optical path optimization with underlying electrical signal driving. Compared with the existing technology that requires passively increasing the emission current due to the decrease in the transmittance of the medium, this effectively reduces the sensor's operating power consumption and extends the overall battery life.

Claims

1. An optical window structure for wearable devices, characterized in that, The optical window structure includes: A bottom cover structure is provided on the wearing side of the wearable device. The bottom cover structure has a stepped hole structure at the position corresponding to the PPG sensor. The stepped hole structure includes a stepped support surface and an annular limiting sidewall surrounding the outer periphery of the stepped support surface. An optical window body is a thin sheet structure made of a light-transmitting rigid material. The optical window body is embedded in the stepped hole structure and abuts against the stepped support surface to form axial support. An annular gap is formed between the outer peripheral edge of the optical window body and the annular limiting sidewall. A sealant layer is filled in the annular gap to form a continuous sealing interface between the optical window body and the bottom cover structure. The optical window body has a preset transmittance within the preset operating wavelength range of the PPG sensor, so that the detection light can be transmitted through the optical window body to human tissue and transmitted back, forming a PPG detection optical path.

2. The optical window structure according to claim 1, characterized in that, The light-transmitting rigid material is selected from any one of chemically strengthened glass, sapphire single crystal material, or transparent alumina ceramic; The thickness of the optical window body is no greater than 0.30 mm, and the Mohs hardness of the light-transmitting rigid material is no less than 6.

3. The optical window structure according to claim 2, characterized in that, When the light-transmitting hard material is a sapphire single crystal material, the C-axis direction of the sapphire single crystal material is perpendicular to the surface of the optical window body, and the thickness of the sapphire single crystal material is 0.15mm to 0.30mm.

4. The optical window structure according to claim 2, characterized in that, When the light-transmitting rigid material is chemically strengthened glass, the surface of the chemically strengthened glass has a compressive stress layer formed by ion exchange process, and the thickness of the chemically strengthened glass is 0.10 mm to 0.15 mm.

5. The optical window structure according to claim 1, characterized in that, Both the inner and outer surfaces of the optical window body are provided with an anti-reflection coating layer. The preset operating wavelength range includes at least one target wavelength among 530nm, 660nm and 940nm. The anti-reflective coating layer is configured with a thickness for the preset operating wavelength range so that the preset transmittance of the optical window body at the target wavelength is not less than 95%.

6. The optical window structure according to claim 5, characterized in that, The anti-reflective coating layer is composed of alternating layers of low-refractive-index material and high-refractive-index material. The low-refractive-index material layer is magnesium fluoride, and the high-refractive-index material layer is titanium dioxide.

7. The optical window structure according to claim 5, characterized in that, A surface functional layer is also stacked on the outer surface of the optical window body outside the anti-reflective coating layer; The surface functional layer includes at least one of an oleophobic layer or an anti-fog layer.

8. The optical window structure according to claim 1, characterized in that, The sealant layer is a UV-cured adhesive ring or a hot melt adhesive ring that has been cured by ultraviolet light. The radial width of the annular gap is 0.2 mm to 0.5 mm.

9. The optical window structure according to any one of claims 1 to 8, characterized in that, The bottom cover structure forms a receiving cavity in the inner region corresponding to the optical window body; There is an air gap between the inner surface of the optical window body and the PPG sensor light-emitting and receiving elements installed in the receiving cavity, which is not filled with solid potting medium.

10. A method for fabricating an optical window structure for wearable devices, characterized in that, The preparation method includes: Based on the spatial size constraints of the wearable device and the relative positional relationship of the PPG sensors, the setting area of ​​the optical window body is determined; Based on the operating wavelength range of the PPG sensor and the target signal-to-noise ratio requirement, the preset transmittance of the optical window body at each operating wavelength is determined, and a mapping relationship between transmittance and PPG signal intensity is established. Based on the preset transmittance and the spatial size constraints, the target thickness range of the optical window body is calculated, and a light-transmitting rigid material that meets the requirements of light transmission performance and structural strength is selected to prepare a light-transmitting rigid sheet. Based on the structural dimensions of the optical window body, a stepped hole for embedding the optical window body is constructed on the bottom cover structure, so that the optical window body is supported by the stepped surface of the stepped hole, and the size parameters of the annular gap formed between the outer peripheral surface of the optical window body and the inner sidewall of the stepped hole are determined. Based on the dimensional parameters of the annular gap and the waterproofing requirements, the material type of the sealant layer is determined, and the sealant layer is continuously filled into the annular gap to form a circumferential sealing interface between the optical window body and the bottom cover structure.

11. The preparation method according to claim 10, characterized in that, The preparation method further includes: Based on the preset transmittance, thickness parameters of the optical window body and the state of the circumferential sealing interface, the signal attenuation value of the detection light in the PPG detection optical path is calculated, and the calculated signal attenuation value is compared with the preset signal threshold. Based on the comparison result between the signal attenuation value and the preset signal threshold, it is determined whether the current optical window structure meets the PPG signal acquisition requirements, and if it does not meet the requirements, the preset transmittance, thickness parameters or the anti-reflection coating parameters are adjusted. When the PPG signal acquisition requirements are met, the light emission driving parameters of the PPG sensor are optimized and controlled according to the finalized optical window structure to reduce the driving current while meeting the target signal-to-noise ratio requirements.

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