Optical short wave infrared sensor with META-structure and wearable device comprising the same with function of determining a concentration of blood components

Abstract

According to an embodiment of the present disclosure, an wearable electronic device comprising a housing including a back cover; and an optical sensor for determining a concentration of blood components and disposed over the back cover, The optical sensor comprising; a ring-shaped optical waveguide including a reflective coating on a portion of an upper end surface of the waveguide; an optical channel surrounded by the optical wave guide and being transparent to a SWIR radiation; an emitter disposed over the optical channel and configured to emit the short wave infrared (SWIR) radiation into biological tissues through a first area of the optical channel; at least one photodetector disposed on other portion of the optical waveguide at a position space apart from the emitter; and a ring-shaped metalens disposed on the optical waveguide opposite from the photodetector and comprising a substrate with a meta-structure. The meta-structure comprises an ordered array of nano-pillars with a predetermined distribution of radii and is configured to provide a phase delay of the SWIR radiation passed through the biological tissues to incline it towards the at least one photodetector. The instruction the ring-shaped metalens and the ring-shaped optical waveguide are configured to collect and direct the SWIR radiation passed through the biological tissues into the at least one photodetector.

Description

OPTICAL SHORT WAVE INFRARED SENSOR WITH META-STRUCTURE AND WEARABLE DEVICE COMPRISING THE SAME WITH FUNCTION OF DETERMINING A CONCENTRATION OF BLOOD COMPONENTS

[0001] The present invention relates to optical short wave infrared (SWIR) radiation sensors having a meta-structure for biomedical applications. The optical SWIR sensors according to the present invention are intended for to be used in wearable devices with a function of determining blood components having a low concentration such as glucose and glycated hemoglobin.

[0002] The present invention is made taking into account the effect of propagation of a SWIR radiation within biological tissues and is aimed at ensuring of emission, transmission and collection of the SWIR radiation from an extended area of a skin in order to determine a concentration of glucose and glycated hemoglobin.

[0003] Due to the development of medicine, an average life expectancy in the world is gradually increasing. The increase in the average life expectancy is also facilitated by the growing interest of the population in monitoring and managing its health. Concentrations of various blood components are markers of human health and their timely control contributes to the prevention, timely detection of diseases and control of their course, as well as other disorders in a body function. Therefore, constant monitoring of blood parameters is necessary. According to the World Health Organization (WHO), about 537 million people in the world suffered from diabetes in 2021, which is 10% of the population aged 20 to 79 years. Every 12-15 years, the number of diabetic patients doubles on average. In present, diabetes mellitus ranks third among the causes of high disability and mortality of patients after cardiovascular and oncological diseases. More than 50% of people with diabetes mellitus are unaware of their condition, and in some countries, the percentage of unawareness is as high as 80%. Diabetes care and treatment have shown that more frequently monitoring of blood glucose facilitates the prevention of many of the long-term complications of diabetes mellitus. Patients with diabetes mellitus are recommended to self-monitor a blood sugar (glucose) level several times a day, at least before the main meals and before bedtime, as well as periodically after eating. Diabetes mellitus patients are forced to test a blood glucose level at home, because without this information it is difficult for them to adjust their food ration, physical activity, the use of insulin and other hypoglycemic drugs. Most often, a glucose level is measured by pricking a finger and extracting a drop of blood that is applied to a test strip consisting of chemicals that are sensitive to a glucose content in a blood sample. An optical meter (glucometer) is used to analyze a blood sample, which provides a numerical value of the glucose level.

[0004] Glucose is a simple carbohydrate, a universal source of energy for a body. Having entered inside a cell, glucose serves as a source of energy, undergoing the process of glycolysis. However, for metabolic pathologies, glucose can accumulate in blood and exert a toxic effect on the heart and blood vessels. Glucose that does not circulate in blood accumulates in skeletal muscle and liver cells in the form of glycogen. Glucose metabolism is regulated by two hormones, i.e., insulin and glucagon. Insulin is produced in pancreas in response to rising a blood glucose level and helps glucose from the blood flow to enter into cells that need energy. Due to insulin, there is normally no excess glucose in blood. Glucagon is also produced in pancreas, but it acts exactly contrary: it "tracks" that a glucose content in blood is not too low. Normally, the "glucose - insulin - glucagon" system is in a dynamic balance: after meal, the glucose level rises, and in response to this, insulin is produced. If the glucose content becomes too low due to hunger or the action of insulin, then glucagon is produced, which increases the glucose level by enhancing the catabolism of the liver deposited glycogen. Due to insufficient production of insulin or glucagon, or owing to other disorders, hyperglycemia, i.e. an elevated blood glucose content or, contrary, hypoglycemia, i.e. a lowered blood glucose concentration develops. Analysis of a glucose level allows diabetes mellitus and other metabolic disorders to be timely detected.

[0005] The normal blood glucose level is necessary for normal functioning a number of biological tissues, including the brain, which consumes about 60% of the blood contained glucose for people who follow a sedentary lifestyle and do not eat. The blood glucose level is lowest in the morning, before the first eating for a day, and rises by a few millimoles after eating for one hour or two hours. The normal blood glucose level in a fasting state for people who has no diabetes mellitus is between of 3.9 and 5.5 mmol / L (70 to 100 mg / dL). For diabetic persons, according to the American Diabetes Association, the target fasting blood glucose range is between of 3.9 and 7.2 mmol / L (70 to 130 mg / dL) and less than 10 mmol / L (180 mg / dL) two hours after eating. Despite significant differences in the intervals between meals or the occasional consumption of high-carbohydrate foods, a human's blood glucose level generally remains within the norm.

[0006] Hemoglobin (Hb) is the main respiratory pigment and the main component of red blood cells, performing important functions in a human body: transfer of inhaled oxygen from lungs to biological tissues and organs, and transfer of carbon dioxide from the tissues and organs to the lungs, where it is exhaled. Erythrocytes or red blood cells are highly specialized blood cells whose main function is to participate in the gas exchange due to the ability of erythrocytes to bind oxygen and carbon dioxide and transport them within a blood flow. Oxygen binding is provided due to the high content of hemoglobin in erythrocytes. Erythrocytes are involved in hemostasis, maintaining the acid-base balance, immune reactions and represent the largest population of blood cells.

[0007] Glycated (glycosylated) hemoglobin (reference designation: hemoglobin A1c, HbA1c) is a biochemical indicator of blood that reflects an average blood glucose content over a long-term period (two to three months), in contrast to a blood glucose measurement result, that provides an idea of a blood glucose level only at the time of test. Glycated hemoglobin reflects the percentage of blood hemoglobin irreversibly bound to glucose molecules. In other words, glycated hemoglobin is an integral indicator of glycemia over about two to three months. The higher the glycated hemoglobin level, the more often there was glycemia, that is, the elevated glucose level over the past three months and, accordingly, the greater risk of developing of diabetes mellitus complications. Thus, at present, the glycated hemoglobin level is a generally accepted indicator of the severity and degree of compensation for carbohydrate metabolism disorders.

[0008] In arterial blood, substantially all (93-95%) hemoglobin HbA is bound to oxygen, i.e. it is in an oxygenated form, and serves to transfer oxygen. The rest of HbA (5-7%) is glycated hemoglobin and there are at least three options thereof: HbA1a, HbA1b, HbA1c, but only the HbA1c option, which is hemoglobin A modified by covalent addition of glucose thereto prevails quantitatively (4-6% of total hemoglobin) and provides a closer correlation with the severity of diabetes mellitus.

[0009] HbA1c values being of between 4% and 5.7% are considered to be normal, and values between 5.7% and 6.4% signalize a predisposition to diabetes. In diabetes, the HbA1c level is 6.5% or higher, which indicates a greater risk of developing of retinopathy, nephropathy and other complications. The International Diabetes Federation recommends maintaining the HbA1c level below 6.5%, and the HbA1c value being above 8% means that diabetes is not well controlled and therapy should be altered.

[0010] Summarizing the above, the HbA1c level is proportional to the average blood glucose level over the past 2-3 months and this level is necessary to be constantly monitored to prevent the development of complications and to predict the development and phase of diabetes associated microvascular complications. It should also be noted that the HbA1c content is 4-6% of the total amount of blood hemoglobin, and therefore, very high sensitivity of the device used for a method for measuring the HbA1c is necessary. In addition, it should be taken into account in optical measurement methods that the absorption / reflection spectra of glucose, oxygenated, glycated and total hemoglobin overlap to each other in the visible and near infrared (VIS-NIR) range, and therefore they are difficult to be separated, while there are clearly pronounced peaks in the glucose and glycated hemoglobin absorption in the SWIR range, and therefore such components are easier to be detected.

[0011] The concentration of total hemoglobin exceeds the concentration of glucose by about 140 times, and exceeds the concentration of glycated hemoglobin by about 15-25 times. Accordingly, the appropriate signal quality (signal-to-noise ratio) of the optical sensor should be provided for effective detecting a level of glucose or glycated hemoglobin that have low concentrations relative to other blood components. Hence, a new type of an optical sensor having high sensitivity and being able to detect change in a concentration of components such as glucose and glycated hemoglobin is necessary to be developed.

[0012] In present, a plurality of personal medical devices have been developed that allow people to check their health without visiting hospitals. As an example, many diabetic patients often check their blood glucose level with a compact tester. Diabetic patients have to check their blood glucose level several times for a day to make sure that the insulin level is enough.

[0013] In present, a plurality of devices with built-in multi-wavelength optical sensors which operate in the VIS-NIR range, i.e. about 450-900 nm, and are designed to detect blood components with a high concentration such as hemoglobin, are known. At the same time, spherical lenses are traditionally used in an optical system of such devices. For example, a hemoglobin level can be determined by using a smartwatch with a built-in photoplethysmographic (PPG) sensor. At the same time, Fresnel lenses are most often used in prior art to operate the trajectory of photons in order to selectively detect a signal in the deep skin layers.

[0014] Most optoelectronic devices for non-invasively measuring blood components use a broadband-pulsed radiation source with subsequent analysis of radiation absorption at a specific wavelength by narrowband interference filters and separate photodetectors.

[0015] Invasive, minimally invasive and non-invasive methods are used to determine a blood glucose and glycated hemoglobin level.

[0016] Invasive methods are laboratory techniques that require the patient's blood to be sampled from a finger or a peripheral vein. Meanwhile, it should be noted that regular blood sampling might be painful and create psychological stress. In addition, since frequent blood samplings can cause a potential risk of infection to a person, frequent blood sugar testing is undesirable. Laboratory techniques require qualified personnel and expensive laboratory equipment to perform blood tests, and waste should be disposed as well. Moreover, laboratory techniques are not suitable for continuously monitoring a glucose level and patients cannot perform them self-acting.

[0017] Electrophoresis, as well as methods that use an electrochemical biosensor, an implanted microsystem relate to minimally invasive methods. In particular, in recent years, a method for continuously monitoring a blood glucose concentration (Continuous Glucose Monitoring, CGM) has appeared and is being actively used. The gist of the method consists in constant automatically measuring a blood glucose level by using a CGM system for several days according to indirect data acquired regarding an intracellular (interstitial) fluid.

[0018] Polarimetric analysis, Raman spectroscopy, optical coherence tomography, photoacoustic spectroscopy, as well as IR spectroscopy relate to non-invasive methods.

[0019] In the large-scale arsenal of modern diagnostic methods, non-invasive methods occupy a rather modest place yet, however, in the medicine of future, the role of non-invasive diagnostics will steadily increase, since non-invasive methods:

[0020] - exclude the introduction of pathogenic viruses and bacteria, foreign substances (xenobiotics) into a body;

[0021] - allow the radiation exposure to a body, for example, during performing X-ray, radioisotope and ultrasound study techniques to be eliminated;

[0022] - free a patient from a complex of painful and unpleasant sensations;

[0023] - non-invasive methods based on the use of sensory and signal transmitting devices allow the main blood parameters having a low concentration, for example, glucose and glycated hemoglobin to be monitored and remote alarm systems to be developed. The latter are used as a means of constantly monitoring of certain groups of patients (patients with the danger of sudden cardiac death, hypertension, diabetes mellitus, etc.) from a hospital or clinic, and facilitate provide timely assistance when critical conditions are identified.

[0024] In non-invasive methods of monitoring a glucose and glycated hemoglobin level, a signal transmission through the patient's blood, tissue fluid, eye fluid, saliva, urine, and sweat is studied. Optical measurement methods that are most suitable for to be used in wearable devices suppose the use of optical radiation to probe the patient's biological tissues and organs in order to acquire diagnostic information on the biochemical composition and morphological structure of a studied area of the patient's body tissues from the radiation reflected (scattered) or transmitted through the biological tissues. The magnitude of the signal recorded in this case depends on both the radiation absorption coefficient and the radiation scattering coefficient of the patient's biological tissues, including blood. The main contribution to absorption is defined by water(H2O), proteins, lipids, various forms of hemoglobin, skin melanin and glucose.

[0025] The radiation transmission at each wavelength is a function of the thickness, color, and structure of skin, bones, blood, and other biological tissues which the radiation passes through. Thus, a glucose concentration can be determined by analyzing the change in an electromagnetic signal by the wavelength, polarization or radiation intensity.

[0026] Modern optical methods for measuring blood components are based on the phenomenon of absorption of electromagnetic radiations by groups of atoms of the tested object, generally in the IR range. The absorption is associated with the excitation of molecular vibrations by quanta of the IR radiation. When molecules are irradiated with the IR radiation, only those quanta are absorbed, whose frequencies correspond to the frequencies of valence, deformation vibrations and vibrations of the molecules. Glucose produces one of the weakest electromagnetic radiation absorption signal in the IR range among most main components of biological tissues. In addition, as noted above, the absorption / reflection spectra of some blood components overlap to each other in the VIS-NIR radiation range, and therefore they are difficult to be separated.

[0027] The use of SWIR radiation is promising for measuring a blood glucose level. In the case of the IR radiation, glucose measurement is possible at a tissue depth of up to several centimeters, while the signal in the SWIR range is collected from a smaller tissue depth, since the penetration depth decreases with increasing the radiation wavelength.

[0028] The SWIR radiation is the publicly accepted name for the short wave IR radiation of electromagnetic waves in the range of 0.9-2.5μm. Unlike the mid- and long-IR radiation, which comes from objects themselves, the SWIR radiation is similar to the visible range in that photons are either reflected or absorbed by the objects, and this property provides the wide dynamic range required for high-resolution images. Due to that the SWIR radiation passes through glass, lenses / objective lenses and other optical components such as optical filters and windows designed for the SWIR range can be manufactured according to the same technologies as those used for visible range components, which reduces manufacturing costs and makes it possible to use the filters and windows within a single optical system.

[0029] One of the factors that define the advantage of operating in the SWIR range is caused by the clear difference in the radiation wavelengths at which the absorption peaks of the main biological components of the tissue such as glucose, water and fats, are located, the data on which are shown in Table 1.

[0030] Particulars of absorption spectra of various biological componentsBiological componentHighest absorption peaks, nmGlucose1408, 1536, 1688, 2261Water1450, 1787, 1934Fats2299, 2342

[0031] Thus, the absorption peaks of the SWIR radiation by glucose, water and fats can be clearly identified, and therefore a blood glucose level can be measured.

[0032] The main problems of using the SWIR radiation are:

[0033] - a low signal-to-noise ratio (SNR) at wavelengths of 1500-2500 nm or more;

[0034] - increasing of the SNR with the sensor size, which limits the sizes of a photodetector, so a single photodetector is necessary to be used (or as few photodetectors as possible);

[0035] - lack of a compact and large-scale production suitable optical SWIR sensor required for a wearable device and capable of collecting more signals from the required depth of biological tissues.

[0036] The present invention overcomes the above problems and allows blood components with a low concentration, such as glucose or glycated hemoglobin, to be measured using the SWIR radiation.

[0037] It should be noted that constant monitoring of a blood glucose level is necessary to prevent developing of complications and to predict the development and phase of diabetes associated microvascular complications. It has been proven that hyperglycemia contributes to the development and progression of micro-, macroangiopathy, polyneuropathy with damage, dysfunction and subsequent insufficiency of body vital organs and systems. Retinal vascular lesions with rapid loss of vision, glomerulosclerosis with developing of chronic renal failure, diabetic foot, arterial hypertension, acute / chronic coronary disease of heart, brain and their consequences (myocardial infarction, heart failure, stroke, atrophy of the cerebral cortex with cognitive decline and developing of dementia) - this is far from a complete list of diabetes mellitus associated diseases and syndromes.

[0038] The following target groups can be identified for which monitoring of a blood glucose / glycated hemoglobin level is necessary:

[0039] - healthy people for the purpose of preventive control;

[0040] - children with diabetes;

[0041] - diabetic patients with abnormal renal glucose threshold;

[0042] - patients with type I diabetes mellitus, insulin-dependent;

[0043] - pregnant women with type II diabetes;

[0044] - other people when food ration or other habits are altered.

[0045] People are also at a risk group who has:

[0046] - a high blood pressure;

[0047] - vision problems;

[0048] - disease of the cardiovascular system and / or stroke incident;

[0049] - kidney disease.

[0050] Hence, the blood glucose and glycated hemoglobin level is the important and significant indicator of a human's health and requires to be continuously monitored.

[0051] In present, a number of devices for non-invasively measuring a blood glucose level have been developed that reduce discomfort for patients. However, they are not always applicable for continuously monitoring and are not suitable for operating in the SWIR range. Moreover, there are no wearable devices on the market, such as smartwatches or fitness bracelets designed to determine a glucose and glycated hemoglobin level by a non-invasive manner, with which a user can easily carry out the measurements unassisted.

[0052] A patent US10,799,128 B2, "Optical vital signs sensor"(Koninklijke Philips N.V.), discloses an optical sensor configured to measure or determine vital signs and comprising a light source configured to generate a light beam having an angular range of incidence angles, and a photodetector configured to detect a light reflected within or from the user's skin. A light shaping unit is configured to shape the light beam from the light source before the light beam enters the skin by limiting the angular range of the incidence angle to less than 20°

[0053] The drawback of such a solution is the use of optical concentrators only at a transmit side, that is, only for entering radiation into the skin, resulting in a signal-to-noise ratio is very low. In order to increase the signal-to-noise ratio, it is better to use the concentrator at a side for receiving a signal. In addition, this solution uses a plurality of light sources, so the optical efficiency is low.

[0054] A patent application US2021 / 0381958 A1, "Spectroscopic Measurements With Parallel Array Detector" (Nueon Inc.), discloses optical components arranged to provide parallel measurements of a biological sample. The parallel sample measurements provide improved accuracy with lower detection limit and may comprise one or more of measurements by using Raman spectroscopy or infrared spectroscopy. The parallel measurements can be combined with a light source in various ways. The light source emits a radiation of one or more wavelengths corresponding to resonance frequencies of one or more molecules of the sample, such as wavelengths of ultraviolet light. The parallel array optical arrangement can be combined with light wavelengths corresponding to the resonance frequencies.

[0055] The drawback of such a solution is the use of a hollow reflector having significant thickness and width to provide collecting of light. Hence, such a solution cannot be used in a wearable device.

[0056] A patent application US2023 / 0204506 A1, "Multiple light paths architecture and obscuration methods for signal and perfusion index optimization" (Apple Inc.), discloses a photoplethysmographic (PPG) device built in a smartwatch that can include one or more light emitters and one or more photodetectors to generate multiple light paths for measuring a PPG signal and perfusion indices of a user. The multiple light paths between each pair of the light emitters and photodetectors can include different separation distances to measure both an accurate PPG signal and a perfusion index value for different users under different conditions of use.

[0057] The PPG device can further include one or more lenses and / or reflectors to increase the signal strength and / or to obscure the optical components and associated wiring from being visible to a user's eye.

[0058] The drawbacks of such a solution are: using of several photodetectors, using of a radiation with a wavelength being different from the SWIR range, the lack of any light collecting elements except for widely used Fresnel lens.

[0059] A patent US11,553,851 B2, "Method for detecting biometric information by using spatial light modulation, electronic device, and storage medium" (Samsung Electronics Co., Ltd.), discloses an electronic device in the form of a smartwatch, that may comprise at least one light-emitting element mounted on a support structure; a spatial light modulator (SLM) disposed between a transparent plate and an LED while being spaced apart from the light-emitting element; a photodetector mounted on the support structure; and a processing circuit comprising at least one electrical path electrically connected to the SLM, the processing circuit being operatively coupled to the photodetector and being configured to generate a photoplethysmogram (PPG) data using the light-emitting element.

[0060] The main purpose is to use the SLM and the SLM control circuit such that the output light beam from the at least one light-emitting element is output to the position of the blood vessel.

[0061] The drawbacks of such a solution are: the use of several photodetectors, a poor collected signal due to use of Fresnel lens, the inability to be used in the SWIR range, the complexity of the structure.

[0062] The advantages of estimating blood parameters, in particular a glucose and glycated hemoglobin level by using wearable devices are instant or continuous measurement, usability, capability for long-term monitoring. At the same time, estimating blood parameters by using modern wearable devices is not accurate enough, especially for the components having a low concentration.

[0063] Thus, the main problem of this technical field consists in the lack of an optical sensor to be built in a wearable device and allowing a blood parameter with a low concentration, such as glucose or glycated hemoglobin, to be monitored instantly or continuously.

[0064] Hence, there is demand for an optical sensor to be built in a wearable device and a method implemented by using the same for non-invasively measuring blood parameters with a low concentration of the studied component such as glucose or glycated hemoglobin with high accuracy. In other words, a device for non-invasive, instant or continuous monitoring of a glucose and glycated hemoglobin level is required.

[0065] Taking into account the above technical problems, the proposed invention will be described hereinafter as an example, but not as a limitation, with reference to a description and accompanying drawings.

[0066] This summary of the invention essence precedes the detailed description of specific exemplary embodiments to give a general idea of aspects of the claimed invention that will be further explained below, and is in no way intended to define or limit the scope of the present invention.

[0067] An object of the present invention is to provide an optical SWIR sensor to be built in a wearable device and being capable of providing the high accuracy of measuring blood components with a low concentration, for example, glucose or glycated hemoglobin. Meanwhile, using the optical SWIR sensor built in a wearable device render possible to significantly reduce a cost of measurement, eliminate the use of consumables, provide an easy and convenient measurement process, etc.

[0068] Possible products in which the optical sensor according to the present invention is used are wearable electronic devices such as smartwatches or smart bracelets, stationary diagnostic tools, household appliances and gadgets for personal medical monitoring, but are not limited to. The wearable electronic devices may be placed, for example, on a user's wrist, shoulder, hip, ankle, chest, neck, finger, etc.

[0069] Within the scope of the present invention, an extensive study of particulars of interaction of the SWIR radiation with human biological tissues, detection and processing of the SWIR radiation has been performed, and also an optical sensor operating in the SWIR range and designed to measure a glucose and glycated hemoglobin level has been developed.

[0070] The proposed optical SWIR sensor and the wearable device comprising the same for determining a concentration of blood components allow blood parameters with a low concentration of a studied component to be personal, instant or continuous non-invasively monitored.

[0071] The present invention is an optical sensor for determining a concentration of blood components, the optical sensor comprising: an emitter configured to emit a short wave infrared (SWIR) radiation, an optical channel being transparent to the SWIR radiation, a ring-shaped optical waveguide with a reflective coating on an upper end surface of the waveguide, at least one photodetector, a ring-shaped metalens comprising a substrate with a meta-structure, wherein the emitter is disposed over the optical channel and is configured to emit the SWIR radiation into biological tissues through the optical channel, the ring-shaped optical waveguide is disposed coaxially to the optical channel and has a reflective coating-free portion on its upper end surface, the at least one photodetector is disposed over the reflective coating-free portion of the upper end surface of the optical waveguide, the meta-structure comprises an ordered array of nano-pillars with a predetermined distribution of radii and is configured to provide a phase delay of the SWIR radiation passed through the biological tissues to incline it towards the at least one photodetector, the ring-shaped metalens and the ring-shaped optical waveguide are configured to collect and direct the SWIR radiation passed through the biological tissues to the at least one photodetector.

[0072] According to an embodiment, a wearable electronic device comprises a housing including a back cover.

[0073] According to an embodiment, a wearable electronic device comprises at least one processor disposed in the housing.

[0074] According to an embodiment, a wearable electronic device comprises a memory configured to store instructions and disposed in the housing.

[0075] According to an embodiment, a wearable electronic device comprises an optical sensor for determining a concentration of blood components and disposed on the back cover.

[0076] According to an embodiment, an optical sensor comprises a ring-shaped optical waveguide including a reflective coating on a portion of an upper end surface of the waveguide.

[0077] According to an embodiment, an optical sensor comprises an optical channel surrounded by the optical wave guide and being transparent to a SWIR radiation.

[0078] According to an embodiment, an optical sensor comprises an emitter disposed over the optical channel and configured to emit the short wave infrared (SWIR) radiation into biological tissues through a first area of the optical channel.

[0079] According to an embodiment, an optical sensor comprises at least one photodetector disposed on other portion of the optical waveguide at a position space apart from the emitter.

[0080] According to an embodiment, an optical sensor comprises a ring-shaped metalens disposed on the optical waveguide opposite the photodetector and comprising a substrate with a meta-structure.

[0081] According to an embodiment, the ring-shaped optical waveguide further has the reflective coating on side surfaces.

[0082] According to an embodiment, the ring-shaped metalens further comprises protective glass of the metalens and an air gap between the protective glass of the metalens and the meta-structure.

[0083] According to an embodiment, said reflective coating-free portion of the upper end surface of the optical waveguide corresponds to a receiving surface of the at least one photodetector.

[0084] According to an embodiment, the emitter is a LED or a laser diode configured to emit the radiation with a wavelength in the range of 0.9-2.5 μm.

[0085] According to an embodiment, the metalens consists of at least two symmetrical parts.

[0086] According to an embodiment, the substrate and the meta-structure are made from amorphous silicon (α-Si) and / or silicon dioxide (SiO2).

[0087] According to an embodiment, the width of a bottom part of the optical waveguide is defined by a size of an area of collecting the SWIR radiation passed through the biological tissues, and the width of an upper part of the optical waveguide is defined by the geometry of the at least one photodetector.

[0088] According to an embodiment, the optical sensor further comprises an antireflection coating on an input surface of the optical waveguide and / or between an output surface of the optical waveguide and the at least one photodetector.

[0089] According to an embodiment, the at least one photodetector is attached to the optical waveguide with an optical glue.

[0090] According to an embodiment, the optical glue is a Master Bond EP30, MasterSil 151, MB600, or UV15 epoxy resin-based adhesive.

[0091] According to an embodiment, the optical sensor further comprises an optical window disposed over the output surface of the optical waveguide and the upper end surface of the optical channel.

[0092] According to an embodiment, the optical channel is made in the form of a cylinder.

[0093] According to an embodiment, the optical channel further comprises sidewalls reflecting the SWIR radiation.

[0094] According to an embodiment, the optical waveguide is made from quartz glass, chalcogenide glass, zinc sulfide or silicon.

[0095] According to an embodiment, the at least one photodetector is made on the basis of indium gallium arsenide (InGaAs), mercury cadmium telluride (HgCdTe), or indium antimonide (InSb).

[0096] According to an embodiment, the ring-shaped metalens is disposed such that the nano-pillars are directed towards the input surface of the optical waveguide.

[0097] According to an embodiment, the ring-shaped metalens is disposed such that the nano-pillars are directed away from the input surface of the optical waveguide.

[0098] According to an embodiment, the blood components are glucose and glycated hemoglobin.

[0099] The proposed invention also relates to a wearable device with a function of determining a concentration of blood components, the wearable device comprising the optical sensor according to any one of the above embodiments.

[0100] According to an embodiment, the wearable device further comprises a housing, a battery, a memory device, an input and output device and a communication module accommodated in the housing, wherein the memory device is configured to store an information, the input and output device is configured to display the concentration of the blood component, the communication module is configured to communicate with a remote server and / or a cloud storage.

[0101] According to an embodiment, the wearable device is configured to be placed on a finger, wrist, forearm, chest, waist, hip, leg, or ankle.

[0102] According to an embodiment, the wearable device is a smart device, preferably a smartwatch or a fitness bracelet.

[0103] The proposed optical sensor and the wearable device comprising the same allow for instantaneously or continuously monitoring of concentrations of blood components such as glucose and glycated hemoglobin in a non-invasive manner with high accuracy, and are suitable for non-professional use.

[0104] Hereinafter features and advantages of the present invention will be explained in the following description with reference to the accompanying drawings. The provided particular exemplary embodiments of the claimed invention being considered along with the drawings are not intended to limit the scope of the invention. Additional embodiments, modifications, or equivalents of the present invention will be apparent to skilled persons based on study of the presented description, and all such embodiments, modifications, and equivalents are deemed to be included within the scope of the present invention.

[0105] The drawings are provided merely to assist in understanding the description and they should in no way be construed as limiting the scope of the invention.

[0106] FIG. 1 illustrates interpolated absorption coefficients of key components of biological tissues and absorption coefficients of hemoglobin (Hb), glucose and glycated hemoglobin (HbA1c) depending on a radiation wavelength.

[0107] FIG. 2 schematically illustrates a scale of electromagnetic radiation and the locality of near infrared (NIR) radiation and short wave infrared (SWIR) radiation ranges on this scale.

[0108] FIG. 3 illustrates a simulated graph showing a penetration depth of the SWIR radiation photons with a wavelength of 2.2 μm into human biological tissues.

[0109] FIG. 4 schematically illustrates an exemplary configuration of the optical sensor according to the present invention.

[0110] FIGS. 5 and 6a, 6b illustrate in details a first key particular (KP1) of the present invention.

[0111] FIG. 7 illustrates an advantage provided by KP1 over a prior art solution.

[0112] FIG. 8 illustrates a second key particular (KP2) of the present invention compared with a prior art solution.

[0113] FIGS. 9a, 9b illustrate an advantage provided by the present invention over a prior art solution.

[0114] FIGS. 10a, 10b, 10c, 10d illustrate embodiments of the optical sensor according to the present invention.

[0115] FIG. 11 illustrates embodiments of the optical sensor according to the present invention in which walls of the waveguide have different shapes and in which optical contact between the photodetector and the waveguide is implemented in various ways.

[0116] FIG. 12 illustrates results of simulating the operation of the optical sensor according to an embodiment of the present invention.

[0117] FIG. 13 illustrates graphs of the dependence of efficiency of the optical sensor on the number of photodetectors and the use of KP1 and KP2 according to the present invention.

[0118] FIG. 14a schematically illustrates options of exemplary displaying of 24-hour glucose level measurement results and variance of multi-month glucose measurement results on a screen of the wearable device according to the present invention in a health monitoring application.

[0119] FIG. 14b schematically illustrates a graph displayed on a screen of the wearable device in the health monitoring application and allowing the variance of the glucose level from the target value to be monitored.

[0120] Hereinafter, particulars and exemplary embodiments of the present invention will be described in details with reference to the drawings. Persons skilled in the art will be appreciated that various exemplary embodiments should in no way be construed as limiting the scope of the claimed invention, and that other tangible and technical means equivalent to or similar to those listed below may be used by skilled persons to make / perform various components of the optical sensor, wearable device, functions, method steps, etc., described below. The present detailed description is not intended to limit the scope of the claimed invention, which is defined only by the appended claims.

[0121] Firstly, differences in measurements of blood components in the VIS-NIR and SWIR ranges and advantages provided by the measurements in the SWIR range will be described.

[0122] Graphs of FIG.1 illustrate interpolated absorption coefficients of key components of the biological tissues such as melanin, oxyhemoglobin, deoxyhemoglobin, water, collagen, and absorption coefficients of hemoglobin (Hb), glucose and glycated hemoglobin (HbA1c) depending on a radiation wavelength. It may be seen from the graph on the right in FIG. 1 taken from the article by Kaiwen Guo et al., High SNR Glucose Monitoring using a SWIR Super-Continuum Light Source, https: / islam.engin.umich.edu / wp-content / uploads / sites / 57 / 2017 / 12 / glucose.pdf / 1 / and illustrating overtone (first overtone) and combinational (compound) bands, that glucose has clearly distinguishable absorption peaks in the SWIR range, which are not affected by the Hb and HbA1c absorptions. In addition, in the range from about 2000 nm to 2400 nm, the influence of such a highly absorbing background as water on glucose absorption is minimized since in this range the radiation absorption by water is minimal. Moreover, as can be seen from the graph of FIG. 1, glycated hemoglobin has a wide combinational absorption band with a maximum at a wavelength of about 2200 nm, whereas total hemoglobin and water have no pronounced absorption peaks in the range of 2100-2400 nm. This creates the basis for measuring glycated hemoglobin in the SWIR range as well, wherein the use of the SWIR radiation at wavelengths of about 1700 nm and 2200 nm is of the greatest interest. As noted above, measuring of glycated hemoglobin in the VIS-NIR range is hampered due to the overlap of absorption peaks of glycated and total hemoglobin.

[0123] Optical sensors are actively used in various fields of human activity, and their use depends on the radiation spectrum to be recorded, which may be divided into sub-bands, in a broad sense, these are the ranges of radiation visible to a human eye, ultraviolet (UV) radiation and infrared (IR) radiation. The IR radiation range, as illustrated in FIG. 2, is located between the visible and microwave regions of the electromagnetic spectrum and may be divided into 4 sub-bands: a Near Infrared (NIR) radiation with a wavelength of 0.75-1 μm, a Short Wave Infrared (SWIR) radiation with a wavelength of 1-2.5 μm, a Medium-Wave Infrared (MWIR) radiation with a wavelength of 3-5 μm and a Long-Wave Infrared (LWIR) radiation with a wavelength of 8-12 μm. As known, with increasing a wavelength of electromagnetic radiation, energy decreases, which causes some differences in applications of the NIR and SWIR radiations.

[0124] The NIR range is used in food testing, remote control, agriculture, and biomedical instrumentation applications. For example, pulse oximeters that measure blood oxygen saturation most often use a combination of LEDs emitting the red (700 nm) and IR (900 nm) radiation. Self-driving cars use an infrared light detecting and ranging (LIDAR) locator to map their surroundings. LIDAR infrared optics typically operate at wavelengths of 905 or 940 nm. Night vision goggles and electro-optical converters also operate in the NIR range. Remote control devices also operate in this range, typically using LEDs with a wavelength of 940 nm for communicating with controlled devices.

[0125] Silicon, which has a sensitivity in the range of 400-1100 nm, is most often used as an optical material for NIR detectors. Many of materials for the IR region, which also operate in the UV and visible band of the electromagnetic spectrum, operate in the NIR range. Glasses and ceramics such as BK7 (optical borosilicate crown glass), the UV and IR fused silica, quartz and sapphire are used for optical windows, lenses, and other components due to excellent transmission in this spectral range.

[0126] SWIR spectroscopy is used in food and drug testing, and SWIR cameras are used to examine printed circuit boards, to non-destructive test artworks, packaged goods, and to inspect counterfeit. More recently, SWIR cameras have been increasingly used in medical research, small animal imaging, and brain imaging. Most of the SWIR spectral range is inexpediently to be used for due to the intense absorption by water and carbon dioxide. However, this range provides operation windows for optical coupling at some wavelengths, as shown in FIG. 1, where low transmission loss and low dispersion are observed.

[0127] Since silicon essentially does not absorb the SWIR radiation, typical SWIR radiation receivers are based on materials such as, for example, germanium (Ge), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), mercury cadmium telluride (HgCdTe) and indium antimonide (InSb). As an example, germanium has sensitivity to a radiation in the range of 0.8 to 1.6 μm, InGaAs may operate in a wider range from 0.55 to 2.5 μm.

[0128] In addition to the previously mentioned materials for the NIR range such as IR fused silica, IR radiation transparent materials include zinc selenide (ZnSe), zinc sulfide (ZnS), calcium fluoride(CaF2), and magnesium fluoride (MgF2).All of them operate in the range from the visible spectrum band up to 8-10 μm. An example of a material for optical windows and lenses operating in the SWIR range may be Corning HPFS®7979 IR (fused quartz glass).

[0129] The main advantage of operating in the NIR range is that the NIR radiation penetrates deeper into a biological tissue compared to the SWIR radiation, so it may easily reach the dermis layer of the skin with capillaries and even large vessels such as arteries and may interact with blood components. At the same time, strong scattering of the NIR radiation by the surface skin layers, low probability of glucose detection, a smaller bandwidth that does not correlate well with a blood glucose level, since the glucose reflection peaks in this band overlap with the reflection peaks of other blood components and a biological tissue, relate to the disadvantages. Thus, the NIR range is not optimal for monitoring a blood glucose level by a non-invasive method.

[0130] Summarizing the above, the devices operating in the SWIR radiation range are promising for measuring a glucose and glycated hemoglobin level. Meanwhile, measurements are not hindered by high concentration of other blood components, such as hemoglobin and water. Such devices are a high-performance option for photosensitive applications, which uses advantages of the short wave infrared spectrum and has ultra-high sensitivity, resolution and stability.

[0131] The use of the SWIR radiation is a potential solution for developing an optical sensor for non-invasively monitoring of a glucose and glycated hemoglobin level. The main advantages of operating in the SWIR range (0.9-2.5 μm) are:

[0132] - less interference due to low radiation absorption by skin chromophores (hemoglobin, melanin);

[0133] - high probability of glucose and glycated hemoglobin detection, since there are absorption peaks that do not overlap with absorption peaks of other blood components;

[0134] - materials for a SWIR emitter provide a high level of power efficiency and thermal stability.

[0135] At the same time, a smaller penetration depth of the SWIR radiation compared to the NIR radiation and a higher electrical noise level of the SWIR detectors compared to the NIR detectors relate to the drawbacks. In addition, as noted earlier, a compact photodetector that may be built into a wearable device is needed, since the signal-to-noise ratio decreases with increasing its size.

[0136] FIG. 3 illustrates a simulated graph showing a penetration depth of the SWIR radiation with a wavelength of 2.2 μm into a human skin. In can be seen from FIG. 3 that in this case a signal is collected from the two upper skin layers, i.e., epidermis and dermis with capillaries from a depth of 0.02-3.00 mm, preferably 0.2-2.0 mm, which encompasses the range of the population-average total thickness of the two upper skin layers, in which the glucose content variates. The glucose content variates faster in dermis, because it contains more capillaries.

[0137] Thus, the following main problems may be identified when operating in the SWIR range:

[0138] 1. The SWIR radiation has a smaller penetration depth into a biological tissue compared to the NIR radiation, whereby signals are collected from a smaller depth, and therefore, a set of collected signals is less informative. When a radiation is emitted to a skin, a significant part thereof is reflected from the upper skin layers and returns to the photodetector, whereby this will be powerful in value, but a harmful signal that does not carry any information about the studied object, i.e., being weakly correlated with a glucose level.

[0139] 2. SWIR detectors have a higher electrical noise level compared to NIR detectors.

[0140] Hence, the signal quality (signal-to-noise ratio) may not be enough for effective detection of biological objects, in particular for determination of a glucose level.

[0141] The solution according to the present disclosure, which uses the SWIR radiation, provides selectivity of glucose detection in the skin depth and prevents the radiation reflected and scattered by the upper skin layers from entering directly the photodetector. In addition, since the SWIR detectors have a high noise level, the solution according to the present disclosure provides for collecting of more quantity of photons per photodetector with a small aperture, which is optimal for the SWIR detectors with a high noise level.

[0142] These and other particulars of the present disclosure will be described in more detail below.

[0143] FIG. 4 illustrates an example configuration of an optical sensor according to the present invention, which is configured to be built into a wearable device, e.g., a smartwatch. According to an embodiment, the wearable device may include the optical sensor disposed within the housing. According to an embodiment, the optical sensor has small sizes, for example, approximately 2 2 2 mm, and may include an emitter, a photodetector, an optical channel, a ring-shaped optical waveguide with a reflective coating, a ring-shaped metalens and, optionally, an optical window disposed over an output surface of the optical waveguide and / or over an upper end surface of the optical channel. The optical window may be, for example, a glass window through which the radiation emitted by the emitter is inputted into the optical channel, and the radiation output from the optical waveguide enters the photodetector. The emitter is configured to emit a SWIR radiation through the optical channel onto a user's skin (biological tissues). The optical channel is made from such a material and is configured such as to prevent, firstly, of the SWIR radiation from directly entering from the emitter to the photodetector, and, secondly, the SWIR radiation reflected and scattered from the upper skin layers from entering the photodetector. The optical channel is made from a material transparent to the SWIR radiation, and is essentially an optical waveguide through which the SWIR radiation emitted by the emitter is inputted into skin. The optical channel generally has the shape of a cylinder, but is not limited to. For example, ashape of the optical channel may be a polygonal prism, a truncated polygonal or circular cone, a parallelepiped, a cube, etc., wherein the SWIR radiation passes through the parallel faces (end faces) of said figures from the emitter to the skin. The optical channel may further comprise SWIR radiation absorbing or reflecting sidewalls made from a SWIR radiation absorbing or reflecting material. Thus, the optical channel may be the SWIR radiation absorbing or reflecting channel, i.e., it may provides the passage of the SWIR radiation from the emitter towards only the skin. In a preferred embodiment, the optical channel is reflective. In addition, the SWIR radiation reflecting sidewalls of the optical channel may perform the function of a reflective coating of the optical waveguide applied to the inner side surface of the waveguide. The optical channel may be surrounded by the optical waveguide. For example, the optical waveguide may have a ring-shape and the optical channel may be positioned at a center portion of the ring-shaped optical waveguide. The emitter may be dispose over the optical channel. The ring-shaped metalens may be dispose on the optical waveguide opposite the photodetector.

[0144] If the optical sensor according to the present invention is built into a wearable device, for example, a smartwatch having a wrist strap or bracelet for fastening to a wrist, when the user is wearing the smartwatch, the lower end surface of the optical sensor, and therefore that of the optical channel is being in a close contact with the user's skin through the glass window disposed on the back cover of the wearable device, thereby preventing the flare light of the photodetector. In the case where the optical channel has walls, a through hole, which the SWIR radiation passes through from the emitter towards the user's skin, is usually filled with a material that provides effective passage of the radiation therethrough and input of the SWIR radiation into the skin. The through hole (cavity) should be ideally filled with air, however, in order to prevent foreign substances, for example, dust, skin particles, etc., from entering the cavity, it is filled with a material transparent to the SWIR radiation, in particular a suitable polymer selected, for example, from polyolefins, but is not limited to. The walls of the optical channel may be made from a material absorbing the SWIR radiation such as polymers, i.e. polymethyl methacrylate (PMMA) or polycarbonate (PC), but are not limited to. In particular, PMMA may be modified, for example, dyed in a suitable way in order to absorb the SWIR radiation. In addition, as discussed earlier, the walls of the optical channel may be made from a material reflecting the SWIR radiation.

[0145] Thus, due to that the radiation passes through the optical channel directly into the skin, a power of a background signal from the upper skin layers decreases and a signal-to-noise ratio increases. It should be noted that biological tissues are relatively transparent to the NIR and SWIR radiation, which makes non-invasively measuring at a depth of a few millimeters being safely for human.

[0146] As known, a trajectory of radiation photons passing through the biological tissues resembles a parabola, which is sometimes described by the term "banana-shape" (a shape of banana). Therefore, in prior art solutions, in order to provide collecting a signal from a predetermined depth of biological tissues, a radiation wavelength is selected and a distance between the emitter and the photodetector is set taking into account the propagation of the photons within the biological tissues along the "banana-shape" trajectory. When the emitter and the photodetector are traditionally arranged on the same printed circuit board adjacent to each other, a region of detecting (collecting) a signal has an area of πR2, whereR is an approximate radius of the photodetector with a round-shaped sensitive element, i.e. the area of collecting a signal increases with the size of the photodetector itself, which results to decreasing the signal-to-noise ratio in the prior art solutions, and therefore to the difficulty of extraction of the informative part of the signal. The above signal is not sufficient to solve the problem of the present invention consisting in providing of measuring blood components with a low concentration.

[0147] Taking into account the particular of the propagation of a radiation in biological tissues along the "banana-shape" trajectory, the optical waveguide and metalens are configured in a ring (annular) shape in the present disclosure in order to input a greater number of the radiation photons, that have passed through biological tissues, into the waveguide through the metalens. In the case of the ring-shaped waveguides and metalens, an area of a region of collecting the radiation is π( ), where R1is the outer radiusand R2is the inner radiusof the input surface of the waveguide and metalens. The use of the ring-shaped metalens and the ring-shaped optical waveguide allows photons to be collected from a larger area than in the solutions currently used in the optical sensors of the wearable devices. The large ring-shaped detection region of the optical sensor according to the present invention provides increasing in the signal-to-noise ratio due to the larger area of collecting a radiation. This provides the effect of selectivity in detecting a radiation reflected and scattered by biological tissues at a required depth.

[0148] More particularly, in the structure of the optical sensor according to the present invention, the ring-shaped optical waveguide is arranged coaxially with respect to the optical channel, wherein the metalens is disposed in an end surface of the optical waveguide directed towards a skin, also referred to as an input or receiving surface, and the photodetector is disposed on an opposite end surface of the waveguide, also referred to as an output surface.

[0149] Summarizing the above, the optical sensor according to the present invention is configured such that the radiation is firstly inputted into the skin, reflected and scattered therein, and then the skin reflected and scattered radiation passes through the metalens and optical waveguide and enters the photodetector.

[0150] Thus, a first key particular (KP1) of the present invention is the use of the ring-shaped metalens in the optical sensor, which provides a special phase distribution of the radiation incident on it. The metalens generally has at least two symmetrical parts related to the photodetector and providing a counter (opposite) phase increase in the inclination of the radiation photons to the photodetector. However, in an embodiment, the metalens may be made as a single part.

[0151] The effects of KP1 are:

[0152] 1. Increasing the amount of the radiation entering the photodetector.

[0153] 2. Minimizing the thickness of the optical sensor due to the essentially zero thickness of the optical element, i.e. the metalens, wherein the thickness of the optical sensor is mainly defined by the optical waveguide thickness.

[0154] 3. The capability of varying the number of photodetectors (PD) used to select the optimal ratio between the SNR and the radiation collection efficiency, depending on the problems to be solved.

[0155] A second key particular (KP2) of the present invention is the use of the optical waveguide with a reflective coating.

[0156] The optical waveguide with a reflective coating prevents the radiation from propagating beyond an annular zone, which has a width along a bottom part defined by a width of the receiving surface of the waveguide through which the radiation transmitted through the skin enters the waveguide, and a width along an upper part defined by the geometry of the photodetector. The photodetector is disposed in a portion of the optical waveguide where there is no reflective coating.

[0157] Due to the combination of the optical waveguide and the metalens also referred here to as a lens with a meta-structure (meta-structured lens), rays propagating through the optical waveguide and transmitting of more radiation to the photodetector are provided.

[0158] The effects of KP2 are:

[0159] 1. Controlling a zone of collecting a radiation.

[0160] 2. Matching the zone of collecting a radiation with the photodetector geometry.

[0161] 3. Increasing the optical efficiency of collecting a radiation.

[0162] 4. Capability to ensure collecting a radiation with uniform intensity.

[0163] The optical SWIR sensor for biomedical applications according to the present invention provides the following technical effects:

[0164] - increasing a signal-to-noise ratio of a SWIR signal while maintaining a compact structure of the sensor;

[0165] - collecting and guiding the SWIR radiation passed through the skin to a photodetector with a small aperture (much smaller than the area of the detection region);

[0166] - capability to detect blood components with a low concentration.

[0167] The first key particular of the present invention consisting in the use of the ring-shaped metalens in the optical sensor, which provides a special phase distribution of the radiation, will be described with reference to FIG. 5.

[0168] At present, the technical field related to engineering and using of metamaterials is rapidly developing. Metamaterials are artificially structured materials used to control and manipulate light, sound, and many other physical phenomena. Their properties are defined both by the intrinsic properties of their constituent materials and by their geometric particulars.

[0169] The two-dimensional analogue of metamaterials are meta-surfaces, which are particularly well adapted for controlling light, since losses in them are generally lower than that in bulk metamaterials, and manufacturing is easier. The meta-surfaces reflect and / or refract light at different angles, depending on from which side and at which angle the light beam falls. The meta-surface used as a lens for light is referred to as a metalens. It has a small size, a flat shape, a thickness not exceeding 1-3 μm and includes nanostructured elements in the form of projections or depressions of various shapes, which are arranged in an ordered array. In the present invention, for simplicity of description, nano-pillars are described as the nanostructured elements, however, the present disclosure is not limited to, and nanostructured elements of other shapes may be used when the metalens is designed.

[0170] A plate or film of silicon dioxide (SiO2) or amorphous silicon (α-Si) with a thickness in the range from 50 nm to 1-3 μm serves as a substrate for the metalens. Periodically arranged nanostructured elements (nanoelements), or more specifically periodically ordered arrays of nanostructured elements are formed on or within the substrate layer. The nanostructured elements of meta-surface may be formed by etching, Atomic Layer Deposition (ALD), Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), and ion implantation, but are not limited to.

[0171] At the same time, the periodically ordered arrays of nanoelements are such that the size and distance between their constituent nanoelements are significantly smaller, usually by 1-3 orders, than the wavelength of the incident radiation. At the same time, the properties of metalens are caused not so much by the properties of its constituent nanoelements as by an artificially created periodic nanostructure or an ordered array of nanoelements, and are the result of the interaction of a radiation with the ordered array of nanoelements.

[0172] The interaction of light (radiation photons) with the nanostructure on the metalens surface generates a phase shift of the light wave and as a result, the light is refracted. Disturbance of optical reciprocity, i.e. "turning off" reflection in one of directions, may be useful for many practical applications, for example, in optical communication networks (see article by Guo X., Ding Y., Duan Y. et al., Nonreciprocal metasurface with space-time phase modulation), Light SciAppl, Volume 8, No. 123 (2019), https: / doi.org / 10.1038 / s41377-019-0225-z / 2 / ).

[0173] In the present invention, the optical sensor uses the ring-shaped lens with a meta-structure that provides a special phase distribution to incline radiation photons to the photodetector and to deliver more signals therein.

[0174] As shown in the enlarged side view of the metalens in FIG. 5, the metalens structure in one embodiment includes:

[0175] - a substrate (for example, made fromSiO2with a refractive index n~1.5) being in contact with the optical waveguide at one side;

[0176] - a meta-structure at the other side of the substrate, comprising a periodically ordered array of nanoelements, which are cylindrical elements (for example, nano-pillars etched on the substrate), with a predetermined distribution of radii, wherein the meta-structure is configured to provide a phase delay of the SWIR radiation reflected and scattered by biological tissues and passing through the meta-structure, due to the ordered array in which the radii of the nano-pillars are periodically varied;

[0177] - protective glass of the metalens (for example, quartz glass), also referred here to as meta-structure-protecting glass, being in contact with the skin during operation, to protect the substrate with nano-pillars, wherein an air gap is provided between the protective glass and the nano-pillars.

[0178] In the described exemplary structure of the metalens, the nano-pillars are directed towards the skin, as illustrated in FIG. 5, and the air gap is provided between the protective glass and the nano-pillars. However, the metalens is not limited to the above-described structure and, alternatively, may be configured such that the substrate is in contact with the protective glass, the nano-pillars are directed towards the photodetector, and the air gap is provided between the nano-pillars and the glass of the optical waveguide.

[0179] According to the present invention, the radius of the nano-pillars varies in the circular direction and is constant in the radial direction of the ring-shaped metalens.

[0180] As shown in a top view of the metalens in FIG. 5, the ring-shaped metalens has at least two symmetrical parts related to the photodetector, which provide a counter phase increase to incline rays towards the photodetector. The metalens in the illustrated embodiment consists of two ring-shaped dissymmetrical sectors or parts, i.e. part No. 1 extends from "6 to 12 o'clock," and part No. 2 - from "0 to 6 o'clock," as shown in FIG. 5. When viewed in the top view in FIG. 5, the emitter is disposed in the center of the structure, and the photodetector is disposed at the joint of the two parts of the metalens at the "6 o'clock" position.

[0181] Part No. 1 comprises an array of nano-pillars with a periodically increasing radius in the circular counterclockwise direction and provides the phase delay increase from 0 to 2πn (i.e., at each of the n periods of varying the radius of the nano-pillars, the phase delay increases by 2π) in the circular counterclockwise direction, so the rays are also inclined counterclockwise towards the photodetector. As an example, FIG. 5 shows an array of nano-pillars in which the radius from the first to the sixth row of nano-pillars increases in the circular direction, then a row with a minimum radius of nano-pillars is located again, and then 5 rows are located again, in each of which the radius of the nano-pillars increases relative to the previous row, etc.

[0182] Part No. 2 comprises nano-pillars with a periodically increasing radius in the circular clockwise direction and provides the phase delay increase from 0 to 2πn in the circular clockwise direction, so the rays are also inclined clockwise towards the photodetector.

[0183] In other words, parts Nos. 1 and 2 are dissymmetrical to each other relative to an imaginary line drawn in the radial direction of the metalens from "0 to 6 o'clock" in FIG. 5.

[0184] An exemplary mechanism of forming a phase delay by the ring-shaped metalens is shown with reference to FIG. 6A. A left graph of FIG. 6A shows a phase delay (shift) depending on the radius r1of thenano-pillar. With increasing the radius r1of the nano-pillar, the phase delay increases, wherein each subsequent nano-pillar, or more specifically a row of nano-pillars, contributes to the phase delay increase, thereby providing the phase delay of 2π at a distance of one period of arrangement of rows of nano-pillars from the smallest to the largest radius and the corresponding inclination of the light beam towards the photodetector. It should be noted that in the case of a 180°rotation of the metalens in the vertical direction, for example, a specular reflection, but when the incidence direction of radiation photons on the metalens is unchanged, the phase delay will also occur in the direction of increasing the nano-pillar radius, however, the light beam will be inclined in a different direction. Thus, when the metalens nano-pillars are positioned towards the skin or the photodetector, the radiation photons passing through the metalens will be inclined in different directions.

[0185] A right diagram of FIG. 6A shows inclination of the radiation by the metalens due to the phase delay, and more particularly, the results of the Floquet simulation of array of nanoelements for the meta-structure (the inclination for the first mode is shown). The radiation photons after having passed through the metalens, which includes nano-pillars, incline from their original propagation direction by about 30° However, the inclination angle is not limited to 30° and may be varied by appropriately selecting the meta-structure geometry, i.e. the layout and shape of the nano-pillars, the distance between the nano-pillars, etc. Floquet's theorem is essentially generalization of the theory of Fourier series for periodic functions. It allows a harmonic decomposition of any function to be acquired, values of which are periodically repeated with accuracy of up to an exponential factor. It is particularly this "periodic" function that describes fields in the vicinity of an infinite periodic lattice, the excitation of which has a uniform amplitude and a linearly phase-varying distribution.

[0186] As discussed earlier, the radii of the nano-pillars periodically vary in the circular direction of the metalens, thereby providing a phase delay of the radiation wave passing through the metalens, whereby the motion trajectory of the radiation photons is inclined. Thus, each subsequent group of nano-pillars or a metacell corresponding to the period of variation of their radius adds a phase delay of 2π. In other words, the first group of nano-pillars provides a phase delay from 0 to 2π 1, the first and second groups - from 0 to 2π 2, ... from the first to the nthgroup - from 0 to 2π n.

[0187] The meta-structure shown in FIG. 6A consists of periodically arranged metacells schematically illustrated in FIG. 6B, wherein, as an example, each metacell comprises six rows of nano-pillars, the radii of which vary only in the circular direction from the first row to the last row of nano-pillars in the metacell to provide a linear uniform phase delay from 0 to 2π in one metacell from a small to a large pillar due to the meta effect and thereby inclining the motion trajectory of the radiation photons, and do not vary in the radial direction, so meta effect with the inclination of the radiation photons in the radial direction is lack.

[0188] Calculation of the projection of the angles after passing through the meta-structure for the first mode is shown with reference to FIG. 6B:

[0189] - the general formula for the meta effect is:

[0190]

[0191] where is refractive index for air,

[0192] is refractive index for a substrate,

[0193] is a nano-pillar direction (1 or -1)

[0194] - if we move from air to the substrate:

[0195]

[0196]

[0197] Thus, by varying the geometry, size and location of the nano-pillars of the meta-structure, it is possible to provide the inclination of the radiation photons in the entire SWIR range.

[0198] The advantage provided by KP1 over the prior art solution will be explained with reference to FIG. 7. FIG. 7 schematically shows the passage paths of rays reflected and scattered by biological tissues in an optical sensor. It is conditionally shown that the rays are not refracted at a skin / glass interface, since skin and glass have approximately the same refractive indices.

[0199] A below diagram of FIG. 7 shows a common approach according to the prior art solution, which does not provide any focusing of the rays on the photodetector. With the common approach, rays No. 1 and No. 4 will return to the skin after reflecting from the upper side of the glass or will pass through the glass past the photodetector. Only rays No. 2 and No. 3 directed straightforwardly into the photodetector will enter it.

[0200] An above diagram of FIG. 7 shows the approach according to the present disclosure.

[0201] The metalens has two symmetrical parts related to the photodetector and provides a phase delay. The diagram shows that the phase delay increases towards the photodetector in each of the two symmetrical parts of the metalens, when viewed on the projection. Due to this, the metalens will incline the rays passing through it towards the photodetector. In the illustrated example, the metalens inclines all rays Nos. 1-4 towards the photodetector.

[0202] Thus, KP1 of the present invention provides the achievement of the following advantageous effects:

[0203] 1. The capability to collect and direct more radiation photons that have passed through biological tissues towards the photodetector with a small aperture.

[0204] 2. Substantially zero thickness of the optical element, i.e. the metalens.

[0205] 3. The capability to vary the number of the used photodetectors (PD) to select the optimal ratio between the SNR and the detection efficiency, depending on the problem to be solved.

[0206] The second key particular (KP2) of the present invention consisting in the use of the optical waveguide with a reflective coating will be described in details with reference to FIG. 8.

[0207] The optical waveguide of the optical sensor according to the present invention has the shape of a three-D ring, as shown in FIG. 8, the upper and lower end surfaces of which are flat and parallel to each other. A reflective coating is applied to the upper end surface of the waveguide. A reflective coating may also be applied to the outer and inner side surfaces of the waveguide. A reflective coating is not applied to the portion of the upper end surface of the waveguide intended for accommodating the photodetector in order to provide an opening for mounting the photodetector and to make optical contact of the waveguide with the receiving surface of the photodetector. One or more such reflective coating-free portions may be provided on the upper end surface of the waveguide to accommodate one or more photodetectors. In addition, an antireflection coating may be applied to the reflective coating-free portion of the upper end surface of the optical waveguide to improve optical contact between the optical waveguide and the photodetector. Further, the photodetector may be attached directly to the optical waveguide or via the antireflection coating by an optical adhesive. In addition, in one of embodiments of the present invention, an optical window disposed over the upper end surface of the optical waveguide and the upper end surface of the optical channel to protect these components may be provided. In addition, a glass protective window disposed over a printed circuit board with the emitter and photodetector for covering the entire upper surface of the optical sensor may be provided.

[0208] The receiving surface of the photodetector may be disposed within the waveguide, i.e., may be recessed into it, may be flush with or above the upper end surface of the waveguide. The thickness of the optical waveguide, i.e. the distance between its lower and upper end surfaces, is from about from 1 mm to 3-4 mm.

[0209] The lower end surface of the optical waveguide, on which there is no reflective coating, is in contact with the metalens. The radiation photons that have passed through the biological tissues of the user, namely, have reflected and scattered by the epidermis and dermis layers of the skin, are refracted when passing through the metalens and enter the optical waveguide, and then fall into the photodetector.

[0210] The shape of the optical waveguide is not limited to the one described above and may be a conical ring, etc. The cross section of the waveguide, taken in the radial direction, is a rectangle, a square, a trapezoid, etc. In other words, the shape and manufacturing material of the optical waveguide are selected such that the radiation photons entering it may focus on the receiving surface of the photodetector.

[0211] When a material for manufacturing the optical waveguide is selected, special requirements for physical and chemical properties and manufacturing technology associated with multiple heat treatments are taken into account. The optical waveguide according to the present disclosure may be made from alkali-free glass, for example, quartz glass, borosilicate glass, phosphatosilicate glass, various polymers, piezoceramics, lithium niobate, bismuth tantalate or silicate, and other materials. In a preferred embodiment, the optical waveguide is made from silicate (quartz) glass with a refractive index n=1.5. Other materials for the optical waveguide may be, in particular, zirconium oxide (n=2.2), silicon (n=3.48), various chalcogenides, for example, chalcogenide glass (n=2.8), ZnS (n=2.5).

[0212] The optical waveguide with a reflective coating according to the present disclosure prevents the radiation from scattering beyond the three-D annular zone of the waveguide, a width of the bottom part of which is defined by a required skin control region, i.e. an area of collecting the SWIR radiation scattered by biological tissues, which enters the optical waveguide through the metalens, and a width of the upper part is defined by the photodetector geometry.

[0213] An antireflection coating may be additionally applied to the input surface of the waveguide to improve the transmission of radiation photons. In addition, an antireflection coating may be applied to the output surface of the waveguide, which contacts directly or indirectly, for example, through an optical glue and / or an optical window, with the photodetector, to improve optical contact. Typically, the required thickness of the antireflection coating is from 1 / 2 wavelength to 2 wavelengths of a radiation and is selected depending on the application. The parameters of the antireflection coating are selected for a specific optical system and a material of the photodetector.

[0214] Diagrams of FIG. 8 illustrate the passage trajectories of radiation photons in a prior art optical sensor without using an optical waveguide and in the optical sensor according to the present disclosure. A left diagram of FIG. 8 illustrates that photons propagate through glass in arbitrary directions and only a small part enters the photodetector, for example, ray No. 2 enters directly the photodetector, while rays Nos. 1 and 3 do not enter it. In other words, most of the rays diverge from the photodetector, and therefore, the optical efficiency of such a sensor is low.

[0215] A right diagram of FIG. 8 illustrates the passage trajectories of radiation photons in the optical sensor according to the present invention. The optical waveguide and the presence of the reflective coating in it, in particular on the side surfaces, facilitate prevention of the radiation from scattering beyond the annular zone of the waveguide with the width of the bottom part defined by a size of an area of collecting the SWIR radiation scattered by biological tissues, i.e. limit the propagation of the radiation within the annular zone of the optical waveguide. For example, ray No. 2 enters directly the photodetector, as with the common approach. Unlike the common approach, rays No. 1 and No. 3 also enter the photodetector after being reflected from the walls (side surfaces) of the optical waveguide.

[0216] Thus, KP2 of the present invention provides the achievement of the following advantageous effects:

[0217] 1. Controlling a zone of collecting radiation photons.

[0218] 2. Matching the zone of collecting radiation photons with the photodetector geometry.

[0219] 3. Increasing the optical efficiency of the sensor.

[0220] FIG. 9A illustrates the propagation paths of radiation photons (rays) that did not enter directly the photodetector in the optical sensor in the presence of the reflective coating in the plane of location of the receiving surface of the photodetector for cases without and with the metalens.

[0221] In the case of the common approach without the metalens, all rays (Nos. 1-3) that have entered the optical waveguide from the skin will be reflected from the reflective coating and will return to the skin, as shown in the upper diagram of FIG. 9A. Only those rays will enter the photodetector that are directed straightforwardly into it.

[0222] As shown in the lower diagram of FIG. 9A, in the presence of the metalens and the optical waveguide (KP1+KP2), a part of rays will propagate within the glass of the waveguide due to reflection from the upper end surface of the optical waveguide, which is coated with a reflective coating, and from the ring-shaped metalens until it enters the photodetector. This effect of ray propagation occurs due to the initial inclination of rays by the metalens when the rays pass through the metalens from the skin.

[0223] As an example, it is schematically shown how the metalens with the ordered array of nano-pillars inclines rays Nos. 1-3 when they enter the optical sensor from the skin and pass through the metalens. After the rays being inclined towards the photodetector, 3 propagation paths of rays are possible depending on an angle of incidence on the reflective coating on the upper end surface of the optical waveguide and an angle of incidence on the metalens:

[0224] Condition 1: The rays return to skin through the metalens like ray No. 1.

[0225] Condition 2: The rays return to skin through the metalens, while changing the phase to the opposite, like ray No. 3.

[0226] Condition 3: The rays propagate within glass like ray No. 2 until they enter the photodetector.

[0227] The presence of the condition 3 allows the efficiency of the optical sensor according to the present invention to be increased.

[0228] Passage paths of rays Nos. 1-3 through the metalens will be explained with reference to FIG. 9B. FIG. 9B illustrates the reflectance and transmittance of a radiation by the meta-structure depending on an incidence angle of the radiation on the meta-structured lens.

[0229] More particularly, for ray No. 1 being incident on the meta-structure at an angle of about 10° the transmittance (by the meta-structure) is about 0.93, and the reflectance (by the meta-structure) is about 0.16. Hence, ray No. 1 will highly likely pass through the metalens back into the skin.

[0230] For ray No. 3 being incident on the meta-structure at an angle of about 36° the transmittance is about 0.93, and the reflectance is about 0.87. However, since ray No. 3 falls into another part of the meta-structure with the opposite direction of the phase delay, its reflectance is about 0.17, and an incidence angle is about -36° i.e. ray No. 3 goes in a different, negative direction compared to ray No. 1 after having passed through the metalens and passes back into the skin.

[0231] For ray No. 2 being incident on the meta-structure at an angle of about 19° the transmittance and the reflectance each is about 0.92. Hence, ray No. 2 highly likely will enter to the photodetector after several reflections.

[0232] Embodiments of Invention

[0233] FIGS. 10A-D and 11 illustrate embodiments of the present invention. The embodiments differ mainly in the number and location of the metalens sectors and the photodetectors, as well as the shape of the optical waveguide.

[0234] In a first embodiment illustrated in FIG. 10A, one, two or four photodetectors are used. In a preferred embodiment, the optical sensor includes one photodetector, and the metalens is divided into 2 identical annular sectors dissymmetrical relative to a vertical plane. In the case of two photodetectors being circumferentially apart from each other at 180° the metalens is divided into 4 annular sectors of the equal size. In the case of four photodetectors being apart from each other at 90° the metalens is divided into 8 annular sectors of the equal size. Thus, in this embodiment, the number of annular sectors of the metalens is twice as large as the number of photodetectors. The meta-structures of the annular sectors of the metalens are configured such that the phase delay in the two annular sectors adjacent to each of the photodetectors occurs in different directions, i.e. the phase delays in any two adjacent sectors are opposite in the circular direction. It should be noted that the exemplary embodiments of the present invention are not limited to the uniform spacing of the photodetectors relative to the circumference of the metalens. Accordingly, the annular sectors of the metalens do not necessarily have to be the same size.

[0235] This embodiment provides high optical efficiency.

[0236] In a second embodiment illustrated in FIG. 10A, which also uses one, two or four photodetectors, the meta-structures of the lens are made such that all annular sectors provide the same phase delay direction. Meanwhile, the number of photodetectors is equal to the number of annular sectors of the metalens.

[0237] This embodiment simplifies the process of manufacturing the optical sensor.

[0238] A third embodiment illustrated in FIG. 10B is characterized by a different arrangement of the optical sensor, namely, different positions of the metalens in the optical sensor.

[0239] In particular, in a structure illustrated in a left part of FIG. 10B, protective glass of the metalens is in contact with a user's skin, a meta-structure with nano-pillars directed upward towards the photodetector is disposed over the protective glass, then an optical waveguide (in FIG. 10B it is designated as waveguide glass) with a reflective coating on an upper end surface is disposed over the meta-structure through an air gap, and finally, a photodetector is disposed over the waveguide in a region where the reflective coating is not applied.

[0240] In a structure illustrated in a right part of FIG. 10B, an optical waveguide is in contact with a user's skin, protective glass of the metalens is disposed over it, over which, through an air gap, a meta-structure with nano-pillars directed downward away from the photodetector is disposed, and finally, a photodetector is disposed over the meta-structure in a region where a reflective coating is not applied.

[0241] This embodiment provides simplification of the process of manufacturing the optical sensor.

[0242] A fourth embodiment illustrated in a lower part of FIG. 10B is characterized by the presence of an antireflection coating, which is applied to both surfaces of the protective glass of the metalens. However, the present invention is not limited to this embodiment, and the antireflection coating may be applied only to one surface of the protective glass of the metalens, and also be applied to an input and / or output surface of the optical waveguide.

[0243] This embodiment provides higher optical efficiency.

[0244] A fifth embodiment illustrated in FIG. 10C is characterized by different annular region occupation with annular sectors of metalens, i.e. the metalens may not completely occupy the entire annular region of the optical sensor.

[0245] This embodiment provides increasing in the optical efficiency of the optical sensor in the case of a low efficiency of the metalens.

[0246] A sixth embodiment illustrated in FIG. 10C is characterized by the use of several photodetectors with different optical filters. As an example, FIG. 10C illustrates four photodetectors, each of which has a filter at one of wavelengths A, B, C, D nm. In addition, in this embodiment, several photodetectors may be used, each of which is designed to detect a radiation in a specific wavelength range.

[0247] This embodiment provides detection of a radiation of a specific wavelength.

[0248] A seventh embodiment illustrated in FIG. 10D is characterized by the use of the metalens with sectors optimized for different wavelengths. As an example, FIG. 10D illustrates that the metalens annular sectors adjacent to a particular photodetector are optimized for a wavelength that this photodetector has a filter for. Optimization for a specific wavelength may be carried out by modifying the geometry, radii of nano-pillars of the meta-structure, distances therebetween, etc.

[0249] This embodiment provides detection of a radiation of a specific wavelength with higher efficiency.

[0250] An eighth embodiment is characterized by the use of a laser radiation source or an array of sources with single or different wavelengths.

[0251] This embodiment provides high efficiency and allows the optical sensor to be used for a variety of applications in the case of different wavelengths.

[0252] A ninth embodiment is characterized by the use of metalens with nano-pillars of various shapes and types. As an example, in a cross-section (or in a top view), the nano-pillars may have the shape of a rectangle, a circle, a square, an ellipsoid. The nano-pillar shape is selected depending on a radiation wavelength to be used, the geometry of the optical sensor, etc.

[0253] This embodiment provides the use of the polarization properties of the radiation and the equalization of efficiency for different radiation wavelengths.

[0254] The first to ninth embodiments relate to various modifications of KP1, that is, to various implementations and arrangements of the metalens.

[0255] A tenth embodiment is characterized by the use of other types of diffractive elements such as a Fresnel lens, a holographic optical element (HOE), etc. in addition to the metalens.

[0256] FIG. 11 illustrates eleventh and twelfth embodiments related to various modifications of KP2, that is, to various implementations and arrangements of the optical waveguide.

[0257] The eleventh embodiment relates to a structure of the optical sensor in which walls of the waveguide have various shapes. At the same time, there may be implemented structures in which an outer wall and an inner wall of the waveguide do not vary their shape along the circular direction of the optical waveguide (schemes 1, 2 and 4, when viewed from top to bottom on the schemes of FIG. 11 on the left), and structures in which one of the walls of the waveguide or both walls (scheme 3 in FIG. 11 on the left) have a variable shape in the circular direction of the optical waveguide. As an example, in a cross-section, the optical waveguide without walls is a rectangle, a square, an isosceles or non-isosceles trapezoid, a pentagon with two parallel sides, etc.

[0258] The twelfth embodiment is characterized by a different implementation of optical contact between the photodetector and the optical waveguide. As an example, embodiments are illustrated in which a receiving surface of the photodetector is in optical contact with the optical waveguide through an air gap, via an optical glue, as well as an embodiment in which an antireflection coating is applied to glass of the optical waveguide. The above embodiments of the optical contact may be combined in various ways, for example, the antireflection coating and optical glue is possible to be shared.

[0259] The eleventh and twelfth embodiments provides high optical efficiency.

[0260] FIGS. 12 and 13 illustrate a model and simulation results of operation of a wearable device with the optical sensor built therein according to the present disclosure.

[0261] A configuration of the optical sensor was simulated in which a radiation is collected from a ring shape skin region with an inner radius of 0.73 mm and an outer radius of 1.27 mm. In other words, these radii define the inner and outer radii of the metalens and the input surface of the optical waveguide which the radiation passes through. The detection zone (an area of the receiving surface) of the photodetector was 0.25 mm2(0.5 mm 0.5 mm). A glass thickness (height) of the waveguide was varied from 0.3 mm to 1.2 mm.

[0262] FIG. 12 illustrates a cross-sectional view of an exemplary structure of the optical sensor according to one of embodiments of the invention. The optical sensor includes a SWIR emitter and a photodetector disposed on a single printed circuit board (PCB), a ring-shaped metalens, an optical waveguide with a reflective coating, an optical channel and, optionally, an optical window. Alternatively, when the optical sensor is built into a wearable device, a bottom surface of the optical sensor may be covered by protective glass that forms a part of a back cover of the wearable device. The optical window / protective glass is designed to contact with a user's skin when the wearable device operates, and is configured to transmit a radiation in the SWIR range. The optical channel is limited by the inner sidewall of the optical waveguide and is designed to direct the radiation into the user's skin at a certain angle and to prevent the radiation reflected from the surface and upper layers of the skin from entering directly the photodetector. In addition, the direct entering of the radiation reflected from the surface and upper layers of the skin into the photodetector is also limited due to the reflective walls of the optical waveguide.

[0263] A photodetector is attached over the optical waveguide in a reflective coating-free portion of its upper end surface by using an optical glue. The photodetector is attached to the end surface of the waveguide farthest from the protective glass that comes into contact with the skin during operation. An antireflection coating may be applied on said reflective coating-free portion of the upper end surface of the optical waveguide to provide effective optical contact, which prevents the reflection of photons from the photodetector and promotes their input into the photodetector. In addition, the optical glue, which is used to attach the photodetector to the optical waveguide, may act itself as the antireflection coating. At the same time, the refractive indices of the optical glue, waveguide and photodetector are to be matched to provide efficient input of the radiation into the photodetector. In addition, it is important to select the thickness of the optical glue properly to provide efficient input of the radiation into the photodetector. A thickness of the optical glue just as parameters of the antireflection coating are selected for a specific optical system and a material of the photodetector.

[0264] The following materials manufactured by Master Bond® may be used, for example, as the optical glue: Master Bond EP30, MasterSil 151, MB600, UV15, the transmission of which in the spectrum range of 500-2500 nm is almost 100%, but are not limited to. The preferred of these materials is Master Bond EP30 being a very low viscosity, two-component epoxy resin designed for high-performance bonding, coating, sealing, potting and encapsulation. EP30 possesses very good optical clarity along with superior transmission when compared to many other epoxy resins.

[0265] FIG. 12 also illustrates results obtained using Monte Carlo Simulation technology, in which a model of biological tissues of human skin was used, and after data on reflected and scattered radiation is acquired, RayTracing technology (for example, Comsol Modeling) was used to simulate the propagation of an electromagnetic radiation in various types of the optical sensors according to the present invention. The present inventors have performed many simulating experiments, including the selection of various structures of the metalens, a material of the optical waveguide, the use of an antireflection coating, an optical glue, etc. The conclusion about that the efficiency of the optical system may be changed by modifying various parameters of the structure of the optical sensor is confirmed.

[0266] In order to simulate operation of the wearable device with the built optical sensor having the different number of photodetectors, structures with one, two, four, five, six and eight photodetectors were used, some of the structures are illustrated as an example in FIG. 13.

[0267] A graph of FIG. 13 illustrates changing in optical efficiency of the optical sensor depending on the number of photodetectors used and the use of KP1 and KP2 according to the present disclosure. According to the experiments performed, the efficiency of the optical sensor that does not use KP1 and KP2 is the lowest, i.e. about 1.5% when one photodetector is used, while increasing linearly to about 11.3% when eight photodetectors are used. The efficiency of the optical sensor with one photodetector when using only KP1 is about 4%, when using only KP2 is about 8%, when using both KP1 and KP2 is the highest and is about 30%. For illustrative purposes, Table 2 below represents approximate values of the efficiencies of the optical sensor in different embodiments with different numbers of photodetectors and with or without using the key particulars of the present invention.

[0268] Optical sensor efficiency, %The number of FDsWithout KP1 and KP2With KP1With KP2With KP1 and KP2 (metalens efficiency is 80%)With KP1 and KP2 (metalens efficiency is 100%)11.54.08.027.232.022.57.015.034.040.533.810.522.042.150.045.214.130.047.255.456.617.838.149.657.868.421.145.250.058.9811.329.160.250.460.0

[0269] As may be seen from the graph of FIG. 13 and Table 2, the optical efficiency for the embodiments of the optical sensor without KP1 and KP2 or with using one of KP1 and KP2 increases essentially linearly with increasing in the number of photodetectors. At the same time, the optical efficiency of the optical sensor using both key particulars (KP1 and KP2) firstly increases linearly (up to the use of three or four photodetectors), and then reaches the "saturation" plateau.

[0270] It should be noted that eight sensors occupy about 60% of area of the output surface of the optical waveguide / metalens. Hence, further increasing in the number of photodetectors is impractical due to the lack of efficiency gains and the increase in the manufacturing cost of optical sensors.

[0271] The presence of the reflective surface of the optical waveguide in a combination with the presence of the metalens provides more efficient collection of a radiation by the receiving surface of the photodetector, which results to increasing in the efficiency of the optical sensor according to the present invention.

[0272] Thus, the optical sensor according to the present disclosure provides an improved quality of a received signal (an increased signal-to-noise ratio) and, therefore, the capability to detect blood components with a low concentration, for example, such a complex components to be measured as glucose or glycated hemoglobin. This is achieved due to the key particulars of the present disclosure consisting in the use of the meta-structured lens and the optical waveguide, which provide enhanced collection and transmission of radiation photons to the photodetector with a small aperture smaller than the detection region, which is necessary for biomedical sensors attached to the skin, due to a high noise level of the SWIR detectors. At the same time, the optical sensor according to the present invention has a miniature size and is suitable for to be used in small wearable devices.

[0273] The radiation sources used in the optical sensor according to the present invention are LEDs and semiconductor micro lasers. Their main advantages are a miniature size and low power consumption, high performance allowing them to be used in electronic circuits with high-speed microprocessors, a high electrical power into useful electromagnetic signal conversion factor. Among the wide variety of structures of semiconductor LEDs and lasers of the SWIR range, two main classes may be identified: injection and optically pumped ones. The used radiation source, for example, a semiconductor laser, generally comprises a multilayer semiconductor structure as a heterostructure with layers having specified thicknesses and bandgap energies, and a semiconductor electromagnetic pump source. The thickness of the semiconductor layers is selected on the basis of properties of the used semiconductor materials and the specified requirements for the spectrum and output power of the SWIR radiation. A LED or a semiconductor micro laser with a radiation wavelength of 0.9-2.5 μm is used as an electromagnetic pump source. Meanwhile, various optical and electronic components allowing the radiation wavelength to be shifted to the required region may be additionally used. InAsSb / InAsP, InAsSb / InAs, GaInSb heterostructure-based LEDs, Nd:YAG laser may be used as the radiation sources, but are not limited to. The use of a micro laser is reasonable in cases where it is necessary to obtain the maximally high electric power into light energy conversion factor, as well as when it is required to obtain a radiation source as close as possible to a point one (the technologically achievable size of the luminescence region for an LED pump source is 1 1 mm, and for a laser one is 0.1 0.1 mm). This is especially important when an infrared radiation source with optical components is necessary to be matched.

[0274] The manufacture of SWIR radiation receivers is an extremely complex process problem and has only recently been adopted. The photodetectors used in the optical sensor according to the present disclosure are made similarly to the silicon-based charge coupled device sensors or a complementary metal-oxide-semiconductor structure (CCD and CMOS sensors), they convert incident photons into electrons, so they are also often referred to as quantum detectors. As discussed above, germanium (Ge)-, indium gallium arsenide (InGaAs)-, indium gallium arsenide phosphide (InGaAsP)-, and mercury cadmium telluride (HgCdTe) (HCT)- or indium antimonide (InSb)-based photodetectors may be used to operate in the radiation range of 0.9-2.5 μm. For example, Ge is used in the range of 0.8-1.6 μm, whereas InGaAs is used in the range of 0.55-2.5 μm. Meanwhile, these photodetectors are sensitive to wavelengths of different ranges depending on the composition of the material whose chemical structure is variable and selected taking into account the detection tasks.

[0275] At the moment, the vast majority of scale-produced photodetectors are hybrid assemblies based on the phenomenon of internal photoelectric effect. So, the short wave IR radiation sensitive element is coupled (bonding process) with a CMOS sensor-based integrated reading circuit. Despite of the high values of quantum efficiency, low noise and power consumption levels, devices of this type have a number of drawbacks. First of all, this is the high cost of technological production, the limitation of the form factor associated with the technological limit of the bonding process, the non-uniformity of sensitivity and defectiveness of pixels, as well as the impossibility of implementing an active-pulse mode with a short strobe duration (less than 200 μs). The above listed drawbacks are absent in the case of building photodetector systems based on a hybrid photoelectronic device (HFP) with a photocathode heterostructure InP / InGaAs / InP and an electronically sensitive element, which are within the same vacuum volume. At the output of such a device, an electrical signal corresponding to the converted photon radiation is generated. This makes it possible to implement a scheme for accumulating and transmitting signals in digital form. In this case, the used elements of electronic optics to record the SWIR-, IR- and visible ranges are identical, which significantly reduces the cost of their manufacturing.

[0276] Thus, the InGaAs-based photodetectors are the most effective to be used due to their high quantum efficiency and a low dark current at a room temperature.

[0277] As an example, the InGaAs-based photodetector consists of multiple layers:

[0278] 1. An InP substrate, optionally with an antireflection coating.

[0279] 2. A photosensitive layer InGaAs grown by epitaxial methods on the InP substrate.

[0280] 3. Indium bumps.

[0281] 4. A reading integrated circuit.

[0282] The principle of operation is simple: photons scattered / reflected from objects of interest enter the photodetector through a lens, for which the same optics may be used as that in cameras operating in the visible or near-IR ranges, then the photons are absorbed in the indium-gallium arsenide layer, and as a result of the internal photoelectric effect, photoelectrons are generated, which are then transmitted, by means of the indium bumps, to the sensitive area of the silicon detector, i.e., the reading integrated circuit (RIC) organized according to the principle of a CCD or CMOS logic.

[0283] The photodetector in the present disclosure is, for example, a planar one-sided photovoltaic element of a circular shape, but is not limited to.

[0284] The technical result of the present invention is to provide an optical SWIR sensor for biomedical applications, that has a compact structure, provides an increased signal-to-noise ratio of the detected SWIR radiation, and therefore the capability to detect blood components having a low concentration, for example, glucose and glycated hemoglobin.

[0285] The optical SWIR sensor according to the present disclosure may be built into a wearable device for biomedical applications. In particular, one of functions of such a wearable device may be the capability to measure a glucose or glycated hemoglobin level. The wearable device according to the present disclosure may comprise a housing, a processor, for example, a microprocessor, a sensor module, a battery or a secondary cell, an input device and an output device, a display device, a memory device, a wireless communication module, etc., but is not limited to. The input device and the output device may be configured as a single input and output device. The sensor module may comprise an inertial motion sensor, a gyroscope, an accelerometer, an electrocardiogram (ECG) measuring sensor, an atmospheric pressure sensor, a humidity sensor, a Hall sensor, an ambient light sensor, a photoplethysmographic (PPG) sensor, a bio-impedance sensor, etc. In embodiments, the wearable device may be a smart device, in particular a smartwatch or a fitness bracelet, a medical wearable device for monitoring health status with the function of determining a level of glucose, glycated hemoglobin, and the like, but is not limited to. The wearable device according to the present invention may be an electronic device capable of bringing the optical SWIR sensor built therein into a contact with a user's skin. Contact with the user's skin is carried out by pressing the wearable device against the user's skin with a strap or a bracelet, with which the wearable device is attached to a part of the user's body. The wearable electronic device according to the present invention may be placed on the user's finger, wrist, forearm, chest, hip, ankle and may be configured to establish a wireless communication channel with external devices such as smartphones, wearable fitness bracelets, voice assistants, smart TVs, smartwatches, etc., or a server and to transfer data over a network or a cloud storage.

[0286] Industrial applicability

[0287] The number of people in the world suffering from diabetes mellitus is constantly growing. This disease can affect men, women and children of all ages in all countries. In order to prevent the risk to disease diabetes, it is necessary to follow a healthy diet, lose overweight, and physical exercise regularly. Patients who exercise regularly can reduce their risk to disease diabetes in half. Patients who lose at least 5% of their body weight can significantly reduce the risk of diabetes development.

[0288] As known, a blood glucose level is monitored by using two indicators: a glycated hemoglobin (HbA1c) level and an instant glucose level. HbA1c represents a long-term glucose indicator over last 1-3 months and it is not affected by short-term changes in a blood glucose level caused by eating, exercising, etc. at the day of testing. An instant glucose level is a short-term indicator of glucose at a particular time of testing, for example, a fasting or post-meal blood sugar level, and is prone to fluctuations due to eating and stress.

[0289] The wearable device with the built-in optical sensor according to the present invention is configured to instant or continuous non-invasive measure a blood glucose and glycated hemoglobin level, wherein a blood sugar may be measured both in a fasting state and after eating. Therefore, the wearable device according to the present invention may be used in proactively monitoring a glucose level and receiving feedback on user's treatment results.

[0290] Instantly glucose level measuring is possible even when the user puts on the wearable device according to the present invention, for example, a smartwatch for the first time. Continuously level measuring is possible when the smartwatch is constantly worn by the user. Meanwhile, the smartwatch equipped with the optical SWIR sensor according to the present invention provides an accurate estimation of both an instant glucose level and dynamics of a glucose level over a long period.

[0291] FIG. 14A schematically illustrates options of exemplary displaying of 24-hour glucose level measurement results and variances of multi-month glucose measurement results on a screen of the wearable device according to the invention in a health monitoring application. As illustrated in FIG. 14A, a user of the wearable device may see on the screen of the wearable device various data with glucose measurement results for different periods of time, including, for example, specific results of single measurements and average values for a day, for a month, for a year, etc. By using the health monitoring application, glucose may be monitored as follows:

[0292] - track an average blood glucose level,

[0293] - track how many peaks there are per day and a time of their occurrence,

[0294] - track whether blood glucose rises quickly or normally,

[0295] - determine whether a level is normal or abnormal, that is, whether the blood glucose level is within the norm or above the norm,

[0296] - track history of a glucose level within or outside the normal range.

[0297] Thus, by using the wearable device according to the present invention, it is possible to carry out preventive monitoring of a blood glucose level and to receive feedback on the treatment results.

[0298] FIG. 14B schematically illustrates a graph displayed on a screen of the wearable device in the health monitoring application and allowing variance of a glucose level from a target value to be monitored. With long-term glucose level monitoring, for example, over up to several months, variance from the target zone of the normal glucose level, as well as crossing of high and low HbAc level alert thresholds may be monitored. It is important to track any variances of said level from the target zone. Meanwhile, when monitoring a glucose level is carried out, the following situations are possible. When a high or low blood glucose level being beyond the target zone is determined, a user of the device is informed about the result obtained and about the preliminary diagnosis, for example, a state of predisposition to diabetes and general recommendations may be displayed on the screen: altering in diet, an increase in physical activity, outdoor walks or a recommendation to visit a doctor.

[0299] Meanwhile, short-term variances in a blood glucose level from the norm are not critical and may be corrected by a user of the device by using recommendations displayed on the display screen. In contrast, long-term variances in a blood glucose level from the norm may mean serious health problems and recommendations for obligatory visit to a doctor are displayed on the display screen. In addition, the data of the acquired glucose level monitoring results may be sent to a doctor directly from the measurement device.

[0300] A glycated hemoglobin level may be monitored in a similar manner.

[0301] The advantages of such monitoring are relatively low testing expenses, provision of a long-term period of continuous tracking of blood parameters that provides acquiring of reliable measurement data and establishing of diagnosis of a person more accurately based on the acquired data.

[0302] Advantageous effects of invention

[0303] The advantageous effects of the present invention are:

[0304] - providing the user with a blood condition estimation, in particular, an information on a blood glucose and glycated hemoglobin level;

[0305] - the capability of additionally usage to monitor other vital signs, such as heart rate;

[0306] - the capability for to be used in telemedicine.

[0307] Thus, as it is forecast, the optical sensor and the wearable device comprising the same according to the present invention, which is configured to non-invasive, continuous and / or on-demand determine a concentration of blood components, in particular glucose and glycated hemoglobin and is suitable for non-professional usage, will be in demand in the market.

[0308] The capability to determine a blood glucose / glycated hemoglobin level expands the functionality of the health monitoring application in a smartwatch, while providing additional parameters for a comprehensive analysis of the user's health status. The smartwatch may provide daily variations, current values, and a result of continuous estimation of glucose for a predetermined period.

[0309] Continuously monitoring of a glucose and / or glycated hemoglobin level provides an estimate of an average glucose level over a certain period of time, allows the direct relationship between poor monitoring and the development of complications to be determined, as well as the development and phase of diabetes associated microvascular complications to be forecast. Hence, preventive control of glycemia and feedback in the diabetes mellitus treatment may be provided. The determination of a glucose and / or glycated hemoglobin level by using the wearable device disclosed herein is performed in an easy-to-use manner for a user.

[0310] Thus, the technical result of the present invention is to provide a wearable device with an optical sensor, capable of automatically estimating a blood glucose or glycated hemoglobin level, while acquiring both an instant value and an average value for a certain period of time. At the same time, the capability of accurately estimating a blood glucose and glycated hemoglobin level by using the wearable device according to the present invention facilitates monitoring of a user's health status.

[0311] As used herein, the terms "SWIR emitter," "emitter," "radiation source," "LED / laser diode"; "photodetector," "radiation receiver"; "optical waveguide," "waveguide"; "metalens," "meta-structured lens," "lens with meta-structure"; "protective glass of metalens," "meta-structure protecting glass" can be used interchangeably.

[0312] It should be appreciated that various embodiments of the present disclosure and the terms used therein are not intended to limit the technological features set forth herein to particular embodiments and include various modifications, equivalents or replacements for a corresponding embodiment. It should also be understood that a singular form of a noun corresponding to an item might include one or more elements, unless the relevant context clearly indicates otherwise. As used herein, each of phrases such as "A or B," "at least one of A and B," "at least one of A or B," "A, B, or C," "at least one of A, B, and C," and "at least one of A, B, or C" may include any or all possible combinations of items listed together in the corresponding one of the phrases. As used herein, terms such as "upper / lower (bottom)," "horizontal / vertical," "right / left," "first / second, etc." may be used for ease of description or for distinguishing a corresponding component and / or its position relative to another one and do not limit the components in another aspect, for example, in spatial location, order, importance, or direction.

[0313] Although the invention has been described and illustrated with reference to different exemplary embodiments, it should be understood that the invention essence is not limited to these particular embodiments. In contrary, it is assumed that the invention essence includes all alternatives, modifications, and equivalents that may be included within the spirit and scope of the claims. It will also be understood that any of the embodiments described herein might be used in conjunction with any other embodiment(s) described herein. In addition, the invention includes all equivalents of the claimed invention, even if the claims will be modified in a process of consideration.

Claims

1.An wearable electronic device is comprising;a housing including a back cover; andan optical sensor for determining a concentration of blood components and disposed over the back cover,wherein the optical sensor comprising;a ring-shaped optical waveguide including a reflective coating on a portion of an upper end surface of the optical waveguide;an optical channel surrounded by the optical waveguide and being transparent to a SWIR radiation;an emitter disposed over the optical channel and configured to emit the short wave infrared (SWIR) radiation into biological tissues through a first area of the optical channel;at least one photodetector disposed on other portion of the optical waveguide at a position space apart from the emitter; and ,a ring-shaped metalens disposed on the optical waveguide opposite the photodetector and comprising a substrate with a meta-structure,wherein the meta-structure comprises an ordered array of nano-pillars with a predetermined distribution of radii and is configured to provide a phase delay of the SWIR radiation passed through the biological tissues to incline it towards the at least one photodetector,wherein the ring-shaped metalens and the ring-shaped optical waveguide are configured to collect and direct the SWIR radiation passed through the biological tissues into the at least one photodetector.2.The wearable electronic device according to claim 1, wherein the ring-shaped optical waveguide further has the reflective coating on side surfaces.3.The wearable electronic device according to claim 1 or 2, wherein the ring-shaped metalens further comprises protective glass of the metalens and an air gap between the protective glass of the metalens and the meta-structure.4.The wearable electronic deviceaccording to any one of claims 1-3, wherein the emitter is a LED or a laser diode configured to emit the radiation with a wavelength in the range of 0.9-2.5 μm.5.The wearable electronic device according to any one of claims 1-4, wherein the metalens consists of at least two symmetrical parts.6.The wearable electronic device according to any one of claims 1-5, wherein the substrate and the meta-structure are made from amorphous silicon (α-Si) and / or silicon dioxide (SiO2).7.The wearable electronic device according to any one of claims 1-6, wherein the width of a bottom part of the optical waveguide is defined by a size of an area of collecting the SWIR radiation passed through the biological tissues, and the width of an upper part of the optical waveguide is defined by the geometry of the at least one photodetector.8.The wearable electronic device according to any one of claims 1-7, further comprising an antireflection coating on an input surface of the optical waveguide and / or between an output surface of the optical waveguide and the at least one photodetector.9.The wearable electronic device according to any one of claims 1-8, wherein the at least one photodetector is attached to the optical waveguide with an optical glue.10.The wearable electronic device according to any one of claims 1-9, further comprising an optical window disposed over the output surface of the optical waveguide and the upper end surface of the optical channel.11.The wearable electronic device according to any one of claims 1-10, wherein the optical channel is made in the form of a cylinder.12.The wearable electronic device according to any one of claims 1-11, wherein the optical channel further comprises sidewalls reflecting the SWIR radiation.13.The optical sensor according to any one of claims 1-12, wherein the at least one photodetector is made on the basis of indium gallium arsenide (InGaAs), mercury cadmium telluride (HgCdTe), or indium antimonide (InSb).14.The optical sensor according to any one of claims 1-13, wherein the ring-shaped metalens is disposed such that the nano-pillars are directed towards the input surface of the optical waveguide.15.The optical sensor according to any one of claims 1-14, wherein the ring-shaped metalens is disposed such that the nano-pillars are directed away from the input surface of the optical waveguide.

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