Self-mixing interferometric sensor module, electronic device and method of detecting movement

Abstract

A self-mixing interferometric sensor module (10) comprises at least two light emitters (20) of the same type, a detector unit (30) and an electronic processing unit (40). Each light emitter (20) is operable to emit coherent electromagnetic radiation from the sensor module (10); and performing a self-mixing interference (SMI), the self-mixing interference (SMI) being caused by reflection of the emitted electromagnetic radiation from an external object external to the sensor module. The detector units (30) are operable to generate output signals indicative of SMIs of the light emitters, respectively. The electronic processing unit (40) is operable to generate a difference signal from an output signal indicative of a movement of the external object (53).

Description

[0001] The present disclosure relates to a self-mixing interferometry sensor module, an electronic device, and a method of detecting movement.

[0002] Electronic devices (such as smart phones, watches and other wearable devices) include more and more sensors for sensing and / or monitoring physiological parameters of users. For example, such wearable electronic devices can measure or monitor parameters such as heart rate, blood oxygen saturation, blood pressure, sleep cycle, body temperature, etc.

[0003] The electronic device includes a sensor module that derives a user's physiological state (e.g., blood oxygen saturation, heart rate, etc.) by applying a stimulus (such as light) and detecting a response from the interaction of the stimulus with the body. For example, some sensor modules include a light emitting diode (LED) for emitting light (such as infrared light) into a blood vessel and detecting a response of the light after the light interacts with the blood. In this case, one or more conditions of the blood (e.g., flow rate, oxygen content, etc.) may affect the transmitted light, and the transmitted light can be used to derive physiological parameters such as blood oxygen saturation.

[0004] Self-mixing interference involves light waves emitted from a laser being reflected by an external object back into the resonant cavity of a light transmitter (such as a laser) and interfering with the light waves in the resonant cavity. Because the feedback light carries information about the external object, the output signal of the light transmitter can be modulated by the external object, so that the physical parameters of the target can be measured by using this property. The SMI interferometer, or self-mixing interferometer sensor (hereinafter referred to as SMI sensor) may require only one optical path, so it provides a simple and compact structure, combined with easy alignment, high sensitivity and convenient signal detection, so that self-mixing interferometer has been widely used to measure displacement, vibration, surface condition (topography), acceleration, small angles, etc.

[0005] However, in recent years, self-mixing interferometry has become useful for sensing and / or monitoring physiological parameters using wearable electronic devices. SMI sensor technology can measure the displacement of a target with a resolution of about 500nm or less (depending on the wavelength provided by the light emitter). The technology also allows the velocity vector of the target to be measured with high accuracy. This property makes it currently a hot topic in the field of vital sign monitoring, where it can (among other things) measure pulse micro-movements to measure heart rate, or measure the flow rate of blood particles in blood vessels close to the surface of the skin.

[0006] In many cases, the measurement accuracy or quality of these modules depends on light penetrating the skin and interacting with one or more blood vessels. Therefore, these sensors may be sensitive to noise (e.g., breathing movements, muscle contractions, electronic noise, etc.) and require robust circuits to detect and / or process the signals received from the user's body. Another significant challenge when implementing it in a wearable device (such as a smartwatch) is that any unwanted movement of the body part (wrist) or tissue relative to the SMI sensor (e.g., in a watch) will result in noise superimposed on the SMI output signal.

[0007] A method of processing the SMI output signal (which may include measured laser power) may involve a fast Fourier transform (FFT) to extract a dominant frequency from the output signal. (Other algorithms on the FFT may also be used to extract quantities proportional to the velocity vector of the movement. For example, the ratio of the moments of the FFT function also gives a quantity with a frequency dimension, and this quantity is proportional to the velocity vector of the movement of the target.) This dominant frequency (denoted as f) can then be related to the velocity vector v of the movement of the target as: f = 2*v / wavelength. For example, when there is movement of the entire hand (or the tissue itself in which the blood vessels are embedded), this adds an additional velocity vector component, and the dominant frequency is offset relative to the absence of movement. However, if the movement is random, it is impossible to distinguish the contribution of the pulsating (or blood flow) velocity vector from the contribution of the unwanted movement.

[0008] An object to be achieved is to provide a sensor module for an electronic device which overcomes the aforementioned limitations of existing solutions. A further object is to provide a wearable electronic device comprising such a sensor module and a method of detecting movement.

[0009] These objects are achieved by means of the subject matter of the independent claims. Further developments and embodiments are described in the dependent claims. Summary of the invention

[0010] The following relates to improved concepts in the field of optical sensing. One aspect relates to a self-mixing interferometry sensor module that can be used to detect the movement of an object external to the module. For example, the module allows sensing and / or monitoring one or more physiological parameters of a user. In addition, the sensor module can be integrated into a wearable electronic device to measure or monitor parameters such as heart rate, blood oxygen saturation, blood pressure, sleep cycle, body temperature, etc.

[0011] Another aspect involves using two substantially identical light emitters (e.g., VCSEL laser diodes) rigidly connected to each other and separated by a distance that can be optimized for differential measurement. For example, in the output signals provided by the light emitters, one signal component may be significantly different, but the motion-related artifact noise component may be significantly similar. The proposed concept can be extended to include arrays of light emitters.

[0012] In at least one embodiment, a self-mixing interferometry sensor module includes at least two light emitters, a detector unit, and an electronic processing unit of the same type.

[0013] Each light emitter is operable to emit coherent electromagnetic radiation out of the sensor module, for example towards an external object. Furthermore, the light emitters are configured to experience self-mixing interference SMI, which may be caused by reflection of the emitted electromagnetic radiation from an external object outside the sensor module.

[0014] The detector unit is operable to generate output signals, each output signal being indicative of the SMI of a light emitter, respectively.

[0015] The electronic processing unit is operable to generate a difference signal from the output signal generated by the detector unit. In fact, the difference signal is indicative of the movement of the external object. (In the following, the difference signal generated may not be a subtraction of the original SMI output signal, but a subtraction of a parameter extracted from the output signal, such as a velocity vector or a main frequency from an FFT).

[0016] For example, the light emitter is arranged to achieve self-mixing interference and typically includes a cavity resonator, and at least a portion of the light emitted by the light emitter can be reflected or backscattered from an external object outside the module into the cavity resonator. For example, the light emitter is implemented as a laser diode and includes a laser cavity. The light emitter is configured to emit coherent light from the sensor module, such as in the infrared (IR), visible or ultraviolet (UV) range of the electromagnetic spectrum. For example, the light emitter is configured to generate a continuous emission or to emit light in a pulsed manner, the latter potentially contributing to an overall reduction in power consumption.

[0017] When the aforementioned emitted light is injected back into the cavity, and the light is reflected away due to the movement of an object outside the module, the light emitter is configured to undergo self-mixing interference. SMI changes the properties (e.g., wavelength) of the light emitted within or from the laser cavity, for example, it causes modulation of the amplitude and / or frequency of the emitted light, thereby generating periodic fringes in the signal. More precisely, SMI modulates the optical power (usually observed by measuring the optical power or bias voltage). In turn, the modulation causes changes in the electronic properties of the light emitter. For example, the diode current and / or voltage are modulated in the same manner. Self-mixing interference does not occur in the light emitter when there is no target outside the module in the emission field of the light source to intercept and reflect the light of the light source. The term "same type" of light emitters indicates that the light emitters are substantially identical, such as two copies of the same design.

[0018] As discussed above, the SMI ultimately changes a property of the light emitter. The property is measured indirectly by means of a detector unit that generates an output signal in dependence on the property or the change in the property. For example, the output signal may be a measured current or voltage. Thus, the detector unit may include means (e.g., an active or passive circuit) for measuring the change as an electronic property.

[0019] In addition, the electronic processing unit receives the output signals and, for each pair of light emitters, generates a difference signal from the output signals, i.e., a difference based on the extracted parameters. On the one hand, the difference signal is a function of the movement of the external object. On the other hand, the process of obtaining the difference of the output signals inherently takes into account the noise. Since the light emitters are of the same type and are arranged together in the same module, it is reliably assumed that the emitters will be affected by noise in a similar manner. Therefore, the difference signal can have a lower noise impact while containing information about the movement of the individual output signals. In fact, the signal-to-noise ratio (abbreviated: SNR) can be increased.

[0020] The detected difference signal (i.e. output signal) or the detected change of the electronic property depends on the movement of the external object. The sensor module may include further circuits or the functionality of the electronic processing unit may be extended to derive the movement of the external object from the change of the difference signal, for example in terms of position, direction and / or speed. Based on the determined movement, the sensor module also includes means for generating a processed output signal, which output signal contains information on the determined movement.

[0021] For example, the movement is a movement perpendicular or parallel to the emission direction of the light emitter. The processed output signal comprises information about the movement, for example it comprises information about the speed, duration and / or direction of the movement and the distance of the object (e.g. measured in the emission direction of the light emitter).

[0022] The improved concept provides motion artefact reduction and SNR improvement by separating the signal and noise into different modes (differential mode and common mode, respectively). No separate motion sensor (such as an accelerometer) is required to help remove motion artifacts. Relatively simple design changes and hardware additions may be sufficient to integrate two or more light emitters in the sensor module, such as multiple identical VCSELs instead of one. Simple subtraction is sufficient to yield an increase in SNR, as opposed to the complex calculations seen in the prior art.

[0023] In at least one embodiment, the light emitter, the detector unit and the electronic processing unit form an integrated semiconductor device, such as a CMOS integrated circuit device, on a common substrate. In particular, the sensor module according to the improved concept may be free of any dedicated photodetector for sensing reflected or backscattered light. Additionally or alternatively, the sensor module comprises a sensor package into which the light emitter, the detector unit and the electronic processing unit or an integrated semiconductor device formed by the light emitter, the detector unit and the electronic processing unit is integrated.

[0024] Wearable electronic devices can use such self-mixing interferometry sensor modules as part of the following actions: detecting the displacement, distance, movement, speed or velocity vector of an object outside the module, such as a gesture at a certain distance from the sensor module or a movement relative to the input surface of the module, i.e., a so-called sliding or tapping movement. In the following, all parameters that may be measured in this way will be referred to as "movement". In particular, such self-mixing interferometry sensor modules can be used with a wide range of consumer electronic devices and other electronic devices. For example, the movement can be related to the displacement of a target in the human body to sense and / or monitor physiological parameters, such as measuring pulse micro-vibrations to measure heart rate, or measuring the flow rate of blood particles in blood vessels close to the skin surface.

[0025] In at least one embodiment, the light emitters have substantially the same configuration, including substantially the same emission wavelength. In addition to using the same type of light emitters (i.e., copies of the same design), reducing the effects of noise is further supported by configuring the light emitters in the same manner. Such a configuration may include a drive current so that the light emitters emit light with substantially the same output power or intensity. In addition, the light emitters may be set or driven to emit coherent electromagnetic radiation having substantially the same wavelength or wavelength range.

[0026] In at least one embodiment, the light emitters are rigidly coupled to each other. The term "rigid coupling" indicates that the distance between any two light emitters is substantially constant, for example, on the operating scale of the sensor module. In other words, the light emitters have a constant relationship in terms of their distance. This relationship can be constant under changing temperature conditions or under mechanical stresses acting on the sensor module. As a result, the light emitters have a determined or possibly identical emission direction. In addition, any pair of light emitters can be characterized so that under defined conditions (for example, conditions without any movement), their output signals allow the determination of a reference difference signal.

[0027] In at least one embodiment, the light emitters are arranged in pairs at a distance from each other. The distance is selected so that output signals indicative of the SMI of the light emitter pair differ by a predetermined amount.

[0028] In order to generate the difference signal, at least two light emitters are arranged in the sensor module. However, any desired number of light emitters may be integrated into the sensor module, such that any one of these light emitters may experience an SMI and a corresponding output signal indicative of the SMI is generated by the detector unit, respectively. The difference signal may be generated from any pair of output signals to produce a corresponding difference signal. Thus, the electronic processing unit is operable to generate the difference signal from the output signals in a pair-by-pair manner.

[0029] In the case of only two emitters or an array of light emitters, the distance between any pair of light emitters can account for the predetermined amount. However, the actual amount is highly dependent on the desired application and may be affected by experiment and design choice. When selecting a distance, it may be helpful to consider the following guidelines.

[0030] Consider a pair consisting of two light emitters. The two light emitters may be separated by a distance such that the signal components of their respective output signals are significantly different in both, but the motion artifact noise components are substantially similar. For example, the two output signals may differ by up to a factor of 10 in terms of signal strength. The following provides guidelines for determining the minimum and / or maximum distance between light emitters.

[0031] For example, consider an array of light emitters (e.g., VCSELs with the same epitaxial design and the same substrate wafer). The distance between individual lasers in the array can be of the order of 250 μm pitch. Individual spots on the target can be separated using optics or meta-optics. Therefore, the minimum distance can be determined by the typical manufacturing pitch. The maximum distance can be determined based on the desired illumination on the target side, for example, by comparing the laser beam hitting the capillaries with the laser beam hitting the outside of the capillaries. The larger radius of the capillaries depends on the aorta (e.g., about 2.5 mm). The distance between the light emitters can be within this range in order to separate the capillaries. Therefore, the lasers in the array can be separated between a few hundred μm and a few mm. Note that the laser spots can be further separated by optics and the distance from the sensor to the capillaries (e.g., about 5 mm). If, for simplicity, the laser beams must be parallel (the angle of incidence can be taken into account), the distance between the light emitters can be within the range of about a few mm, so that one emitter is on the capillaries and the other is outside the capillaries. That is, the other is, for example, displaced with respect to the latter.

[0032] As an additional guide, the distance between the optical transmitters (e.g., pairs) can be set so that the noise component in the corresponding output signal becomes common mode and the signal component becomes differential mode. Thus, the difference signal can significantly reduce motion artifacts while only slightly reducing the signal. The signal-to-noise ratio can be greatly improved.

[0033] For example, in a wrist-based wearable electronic device for heart rate monitoring, the separation would be calculated using the size of the blood vessels on the wrist and their separation. The positioning of the light emitters and their separation should provide a high probability that one of the two sensors is significantly closer to the signal source (i.e., a pulsating blood vessel). However, the distance should be close enough so that unwanted motion between the wrist and the sensor is substantially the same for both light emitters.

[0034] In at least one embodiment, the sensor module includes an array of light emitters. The electronic processing unit is operable to generate a difference signal from output signals of pairs of light emitters.

[0035] In at least one embodiment, the light emitters are arranged in parallel so that the light emitters have the same emission direction. In this way, the light emitters emit coherent light towards the same target, but with an offset distance.

[0036] In at least one embodiment, the light emitter includes a semiconductor laser diode and / or a resonant cavity light emitting device. These devices are characterized by coherent emission to generate SMI fringes.

[0037] A resonant cavity light emitting device can be considered as a semiconductor device similar to a light emitting diode, which is operable to emit light based on a resonant process. In this process, a resonant cavity light emitting device can directly convert electrical energy into light, such as when directly charged with an electric current to produce amplified spontaneous emission. However, instead of producing stimulated emission, only spontaneous emission can be caused, such as spontaneous emission perpendicular to the semiconductor surface being amplified.

[0038] In at least one embodiment, the light emitter includes a vertical cavity surface emitting laser VCSEL diode. VCSEL is an example of a resonant cavity light emitting device and is characterized by a beam emission perpendicular to the main extension plane of the VCSEL top surface. The VCSEL diode can be formed from a semiconductor layer on a substrate, wherein the semiconductor layer includes two distributed Bragg reflectors (DBRs) that surround the active region layer in the middle and thus form a cavity. VCSEL and its operating principles are well-known concepts and are not further described in detail in this disclosure. For example, the VCSEL diode is configured to have an emission wavelength of 940nm, 850nm or another natural wavelength. For example, the VCSEL diode can be configured to emit coherent laser light when forward biased.

[0039] In at least one embodiment, the electronic processing unit is operable to perform a Fast Fourier Transform (FFT) on the output signal to extract the dominant frequency, and then generate a difference signal based on the dominant frequency.

[0040] As discussed above, in SMI, a portion of the coherent light emitted by the optical transmitter can be reflected by external objects and eventually re-injected into the transmitter (e.g., a laser cavity). The movement of the target causes modulation of the detected output signal. For example, the power emitted by the optical transmitter is in fact modulated in both amplitude (AM) and frequency (FM), thereby generating fringes in the interferometric output signal. Therefore, the output signal is affected by frequency and may be a periodic function of the phase of the backscattered field. Therefore, the output signal can be processed using a (fast) Fourier transform to extract characteristic frequencies, such as the dominant frequency.

[0041] As a first step, a possible processing may involve a Fast Fourier Transform of the output signal. Then, as a result of the FFT, one or more dominant frequencies may be extracted. A difference signal may be generated from the FFT result, for example by subtracting the FFT intermediate signal as a whole. Alternatively, a difference frequency or a dominant frequency may be determined. Finally, the extracted difference signal may be correlated with a movement or velocity vector of the external target.

[0042] In at least one embodiment, the detector unit is operable to detect the junction voltage of the light emitter, respectively. Then, the output signal is generated according to the junction voltage, respectively. The junction voltage is a possible electronic property of the light emitter, which may change as a result of the SMI. For example, the detector unit includes one or more voltmeters for detecting (one or more) junction voltages.

[0043] In at least one embodiment, the detector unit is operable to respectively detect the optical power output of the optical transmitter. Then, the output signal is generated respectively according to the optical power output. Optical power is another possible property of the optical transmitter that can be changed as a result of SMI. For example, the detector unit includes one or more photodetectors (such as photodiodes) for detecting the optical power output.

[0044] In at least one embodiment, a wearable electronic device includes a self-mixing interferometry sensor module according to one of the aforementioned aspects. In addition, it includes a housing including the sensor module and a support surface. The support surface is to be arranged on the skin of a user. The housing is configured to position the light emitter at a distance from the skin of the user.

[0045] For example, a wearable electronic device is considered to be any device that is wearable based on its appearance and / or is designed to be worn on the body. Wearable electronic devices may require being worn to function, for example, may be conceptually linked to the wearer's body. Some wearable electronic devices may require a user interface to be always present and available, other devices may not require input (such as a wrist unit or chest strap for a heart rate monitor). Wearable electronic devices may include smart watches or fitness trackers and the like.

[0046] In at least one embodiment, the wearable electronic device includes a processing unit coupled to the sensor module. The processing unit is configured to receive the difference signal from the sensor module and determine the movement of an external object outside the sensor module. In addition, the processing unit generates a processed output signal, which includes information of the determined movement.

[0047] For example, the processing unit can be a central processing unit CPU of the wearable electronic device, or a system on a chip SOC dedicated to processing the output signal of the light emitter. The output signal contains information about the detected movement, such as the direction, distance and / or speed of the movement of the external object under the user's skin.

[0048] In at least one embodiment, the light emitter is arranged in the housing so that the emission direction of the light emitter is substantially perpendicular to the support surface. The wearable electronic device including the sensor module can rest on the skin with the support surface facing downward. In this configuration, the light emitters are separated by a certain distance from each other, but have substantially the same distance with respect to the skin (and the support surface for that matter). Therefore, differences in the signal strength of the output signal may be due to different reflections or scattering at external objects. For example, the shape of a blood vessel can be spherical or ellipsoidal. Therefore, the coherent light of one light emitter can be reflected or scattered from a different relative distance than the coherent light of an adjacent light emitter.

[0049] In at least one embodiment, the light emitter is arranged in the housing so that the emission direction of the light emitter is inclined with respect to the support surface. A wearable electronic device including a sensor module can rest on the skin with the support surface facing downward. In this configuration, the light emitters are separated by a certain distance from each other and are inclined with respect to the support surface. Therefore, the light emitters are designed to have different distances with respect to the skin (and the support surface for that matter). The difference in signal intensity of the output signal may be due to different reflections or scattering at the external object. However, due to the inclined arrangement, for the two light emitters, the optical path toward the object may be affected by the movement of the sensor module in the same way. This allows further improvement in spatial resolution, and the signal difference may be more significant, while the noise remains at the same common mode level.

[0050] Further embodiments of the electronic device will become apparent to a skilled reader in the art based on the aforementioned embodiments of the self-mixing interferometry sensor module, and vice versa.

[0051] Furthermore, a method for detecting movement is provided, the method comprising at least the following steps. One step comprises emitting coherent electromagnetic radiation from a sensor module by means of two light emitters of the same type. Another step comprises causing self-mixing interference SMI in the light emitters, the SMI being caused by reflection or scattering of the emitted electromagnetic radiation from an object outside the sensor module. Another step comprises generating output signals indicating the SMI of the light emitters, respectively. Another step comprises generating a difference signal from the output signals indicating movement of the external object.

[0052] These steps may be supplemented by further program steps, such as determining a movement of the external object from the difference signal and generating a processed output signal comprising information of the determined movement.

[0053] Further embodiments of the present method will become apparent to a skilled reader in the art based on the aforementioned embodiments of the self-mixing interferometry sensor module and wearable electronic device, and vice versa.

[0054] The following description of the accompanying drawings may also illustrate and explain various aspects of the self-mixing interferometry sensor module, wearable electronic device, and method of detecting movement. Functionally identical or having the same effect of self-mixing interferometry sensor components and parts are represented by the same reference numerals. Identical or effectively identical components and parts may be described only with respect to the drawing in which they first appear. Their description is not necessarily repeated in consecutive drawings.

[0055] In the attached picture:

[0056] Figure 1 An exemplary embodiment of a self-mixing interferometry sensor module for a wearable electronic device is shown,

[0057] Figure 2 Another exemplary embodiment of a self-mixing interferometry sensor module for a wearable electronic device is shown, and

[0058] Figure 3A , 3B Exemplary measurements performed using a self-mixing interferometry sensor module for a wearable electronic device are shown.

[0059] The proposed self-mixing interferometry sensor module can be used in various electronic devices, including wearable electronic devices such as smart watches or fitness trackers and the like. For example, the sensor module enables an electronic device (wearable or non-wearable) to derive a user's physiological state (e.g., blood oxygen saturation, heart rate, etc.) by applying light as a stimulus and detecting a response from the interaction of the stimulus with the body. In the following, two possible examples are disclosed, which can be considered as representatives of various possible applications. These examples relate to pulse monitoring and blood flow measurement.

[0060] Figure 1 An exemplary embodiment of a self-mixing interferometry sensor module for a wearable electronic device is shown. The accompanying drawings show a wearable electronic device (e.g., a smart watch) including a sensor module 10. The sensor module is integrated into the wearable electronic device and electrically connected to the wearable electronic device. In fact, the wearable electronic device includes a housing 50 having a support surface 51. The sensor module is placed or installed in the housing.

[0061] The sensor module 10 also includes two light emitters 20, a detector unit 30 and an electronic processing unit 40. The sensor module can be implemented as a sensor package, and the light emitter, the detector unit and the electronic processing unit, or an integrated semiconductor device formed by the light emitter, the detector unit and the electronic processing unit is integrated into the sensor package. For example, the detector unit and the electronic processing unit form an integrated semiconductor device on a common substrate, such as a CMOS integrated circuit device. The light emitter can be integrated into the integrated semiconductor device or electrically connected to the integrated semiconductor device as an external component.

[0062] The light emitters 20 are implemented as copies of the same type (i.e., the same design). In this embodiment, the two light emitters are vertical cavity surface emitting lasers or VCSEL diodes. VCSEL is an example of a resonant cavity light emitting device. The light emitter comprises a semiconductor layer with a distributed Bragg reflector (not shown), which surrounds the active region layer in the middle, thus forming a cavity 21. The VCSEL is characterized by a beam of coherent electromagnetic radiation emitted perpendicular to the main extension plane of the top surface 22 of the VCSEL. For example, the VCSEL diode is configured to have an emission wavelength in the infrared range, for example at 940nm or 850nm.

[0063] The sensor module 10 may include a laser driver integrated into an integrated semiconductor device as a means for driving the optical emitter. The number of optical emitters implemented may only be limited by the desired application. The two emitters discussed herein should be considered as examples rather than any limitation of the proposed concepts.

[0064] The light emitters 20 are arranged in the housing 50 and are parallel to each other so that the emission direction 23 of the light emitters is substantially perpendicular to the support surface 51. In addition, the housing includes apertures 52 arranged in the support surface in front of the light emitters, respectively. The light emitters 20 are rigidly coupled to the sensor module 10, and the light emitters 20 are separated by a certain distance. The distance is set so that the signal components of their corresponding output signals in the two light emitters can be significantly different. As a possible guide, the distance between the paired light emitters, for example, can be set so that in the corresponding output signals, the noise component becomes a common mode and the signal component becomes a differential mode.

[0065] The detector unit 30 is shown as a schematic building block. The detector unit includes a device (e.g., an active or passive circuit) for measuring the optical or electronic properties of the light emitter 20. For example, the detector unit includes an ammeter or a voltmeter for respectively detecting the junction voltage of the light emitter. The junction voltage is a possible electronic property of the light emitter and may change due to self-mixing interference. In addition or alternatively, the detector unit includes one or more photodetectors (such as photodiodes) for respectively detecting the optical power output of the light emitter. The optical power output is a possible optical property of the light emitter and may change due to self-mixing interference. In some embodiments, the photodetector can be integrated on the epitaxy of the light emitter 20.

[0066] The laser can have optics (not shown in the figure) of either integrated (metalens, etched in the substrate) or separate (physical individual lenses: micro-optics or lenses) kind, or an array of common lenses or micro-lenses. For example, in blood flow velocity measurements, focusing is required to obtain sufficient SMI signal intensity.

[0067] The electronic processing unit 40 constitutes a functional unit of the sensor module, which performs a number of (pre)processing steps. Its function will be discussed in further detail below. These steps include performing a Fast Fourier Transform on the output signal of the optical transmitter and subtracting the signal (e.g. the result of the FFT) to generate a difference signal. For example, the electronic processing unit comprises a microprocessor or an ASIC.

[0068] The wearable electronic device includes additional components (not shown) (such as a processing unit) for receiving the difference signal from the sensor module and determining the movement of the external object outside the sensor module. For example, the processing unit can be a central processing unit CPU, a microprocessor or a system on a chip SOC of the wearable electronic device, which is dedicated to processing the output signal of the light emitter 20. As will be discussed in further detail below, the output signal or the difference signal contains information about the detected movement, such as the direction, distance and / or speed of the movement of the external object located under the user's skin 53.

[0069] In operation, the wearable electronic device is placed on the user's skin 53 with the supporting surface 51 of the housing 50 facing downward. Thus, the aperture 52 in front of the light emitter 20 faces the user's skin 53 and provides a corresponding opening for irradiating the skin with the aid of the light emitter. The light emitter emits coherent light (e.g., in the infrared (IR), visible or ultraviolet (UV) range of the electromagnetic spectrum) from the sensor module and projects it toward the skin via the aperture. For example, the light emitter generates a continuous emission or emits light in a pulsed manner, wherein the latter potentially contributes to achieving an overall reduction in power consumption.

[0070] In this exemplary application (pulse monitoring), the skin 53 constitutes a moving external object or target. The output signal provided by the light emitter 20 indicates the displacement of the skin. The output signal or the difference signal can be used to retrieve the displacement, as outlined in the method for detecting movement below. Basically, due to self-mixing interference SMI, the output signal generated by the light emitter is affected by the movement of the skin (surface) along the optical path of the light emitter (as indicated by arrow 54 in the figure). This is caused by different placement or relative movement of the electronic device on the skin and generally results in a noise component. In this way, for each light emitter, the relative distance between the light emitter 20 and the blood vessel 60 may be different (see the cross-section 61 of the skin surface in the figure).

[0071] Coherent light emitted by the light emitter strikes the skin 53. A portion of the light is reflected off the skin surface and scattered back toward the light emitter 20. Eventually, the reflected light enters the sensor module via the aperture 52 and is injected back into the cavity 21 of the light emitter. In the laser cavity, the injected light causes interference with the coherent light being generated by the light emitter. As a result, the light emitter undergoes self-mixing interference, or SMI for short. In addition, SMI changes the lasing process in the cavity and causes changes in the optical or electronic properties (e.g. wavelength) of the light emitted in or from the laser cavity. For example, SMI causes modulation of the amplitude and / or frequency of the emitted light, thereby generating a periodic fringe signal. In turn, this modulation causes changes in the electronic properties of the light emitter. For example, the diode current and / or voltage is also modulated. The voltage or optical power output can be detected by means of the detector unit 30, which generates a corresponding output signal.

[0072] The output signals generated by the light emitter 20 and ultimately by the detector unit 30 inherently include information about the movement of the skin 53 as an external object. The SMI induced in the light emitter is sensitive to the position and its relative changes. These affect how the emitted light and the back-reflected light interfere.

[0073] Due to the different relative distances between the skin 53 and the first of the two light emitters and between the skin 53 and the second of the two light emitters (indicated by dashed lines 55 in the figure), the output signals of the two light emitters 20 have different signal components due to the pulse caused by the moving blood vessels. This can be considered as a heart rate signal generated by the pulse.

[0074] The distance between the reflecting / scattering surface (i.e., skin 53) and the light emitter 20 is a parameter that affects interference. In an ideal case, the two light emitters have the same distance with respect to the skin and thus the blood vessels. In this case, the difference in the signal components in the light emitters provides a measure of the pulse and heart rate.

[0075] However, unwanted movement of a body part (wrist), skin or tissue relative to the sensor module causes noise to be superimposed on the SMI output signal. This is indicated by arrow 54 in the figure. However, the displacement caused by such movement can be assumed to affect the output signals of the two light emitters in the same way, that is, it is common mode for the two light emitters. However, as discussed above with respect to the distance between the light emitters 20, this noise component can be set in the corresponding output signal so that the noise component can become common mode and the signal component becomes differential mode. Therefore, generating a difference signal from the output signal significantly reduces motion artifacts while only slightly reducing the signal. The signal-to-noise ratio can be significantly improved.

[0076] The process of generating the difference signal is performed by means of the electronic processing unit 40. The electronic processing unit 40 receives the output signals from the optical transmitters detected by the detector unit 30. These output signals are subjected to FFT to produce intermediate signals. These intermediate signals are subtracted from each other in whole or in part (e.g., only the main frequency) to produce the difference signal. Because the noise is common mode for the two optical transmitters by placing the optical transmitters in the distance described above, they tend to cancel in the difference signal. At the same time, the actual signal component is not reduced or only slightly reduced.

[0077] The processing unit then receives the difference signal from the sensor module and determines the movement of the skin. The processing unit generates a processed output signal to derive the heart rate, the processed output signal comprising information of the determined movement.

[0078] Figure 2 Another exemplary embodiment of a self-mixing interferometry sensor module for a wearable electronic device is shown. The figure shows a wearable electronic device (e.g., a smart watch) comprising a sensor module 10 similar to Figure 1 Sensor module. Different from Figure 1 In an embodiment, the light emitters are separated by a certain distance from each other and are arranged to have a common tilt angle with respect to the support surface. Thus, the light emitters have different distances with respect to the skin (and the support surface for that matter) by design. For example, this arrangement is suitable for blood flow measurement.

[0079] In operation, the wearable electronic device including the sensor module 10 rests on the skin 53 with the support surface facing downwards. Differences in the signal strength of the output signal may be caused by different reflections or scattering at the blood vessels as external objects. Due to the inclined arrangement, the optical path can be extended or shortened depending on the position of the blood vessels 60. Therefore, the output signal of the light emitter differs depending on the position of the mobile blood vessel. Recording the output signal according to time allows the blood flow to be determined.

[0080] Movement of the sensor module 10 including the rigidly coupled light emitters 20 about the skin 53 affects both light emitters in a similar or identical manner. Therefore, the noise caused by such movement becomes common mode noise, similar to Figure 1 Example. Using essentially the same method and program steps, the electronic processing unit 40 receives the output signals from the light emitter 20 detected by the detector unit 30. These output signals undergo FFT to produce intermediate signals. These intermediate signals are subtracted from each other in whole or in part (e.g., only the main frequency) to produce a difference signal. Because the noise is common mode for the two light emitters, they tend to cancel in the difference signal. At the same time, the actual signal components are not reduced or only slightly reduced. This allows further improvement of the spatial resolution, and the signal difference may be more significant when the noise is kept at the same common mode level.

[0081] Figure 3A , 3B An exemplary measurement using a self-mixing interferometry sensor module for a wearable electronic device is shown. The figure shows SMI fringe signals (see Figure (a), output intensity versus time) and their Fourier spectra (see Figure (b) spectral intensity versus frequency). For example, Figure 3A The data depicted in has been recorded by the first light emitter 20, and Figure 3B The data depicted in has been recorded by the second light emitter 20, the first light emitter 20 and the second light emitter 20 being separated from each other by a certain distance.

[0082] Figure 3A The figure in (b) is dominated by noise motion, which is manifested by a peak frequency at 7kHz (see the dotted line in figure (b)). This peak is proportional to the noise motion velocity vector. Figure 3B The graph in (a) shows the contribution of both signal and noise motion, which is manifested as a peak frequency at 10 kHz (see the dashed line in (b)). The difference signal can be determined based on the peak difference (e.g., the magnitude of the peak), which in this example is equal to 3 kHz. This difference is proportional to the motion velocity vector and reduces the contribution of noise.

[0083] Although this specification contains many details, these details should not be interpreted as limitations on the scope of the present invention or the scope of the content that can be claimed, but as descriptions of features specific to a particular embodiment of the present invention. Certain features described in the context of a separate embodiment in this specification may also be combined and implemented in a single embodiment. On the contrary, the various features described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination. In addition, although features may be described above as working in certain combinations, and even initially claimed as such, one or more features from the claimed combination may be deleted from the combination in some cases, and the claimed combination may point to a sub-combination or a variation of a sub-combination.

[0084] Similarly, although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in a sequential order, or that all illustrated operations be performed, to achieve desired results. In certain circumstances, multitasking and parallel processing may be advantageous.

[0085] This patent application claims priority to U.S. patent application 63 / 344,873, the disclosure of which is incorporated herein by reference.

[0086] refer to

[0087] 10 sensor modules

[0088] 20 light emitters

[0089] 21 Cavity

[0090] 22 Top surface

[0091] 23 Emission direction

[0092] 30 detector units

[0093] 40 electronic processing units

[0094] 50 Shell

[0095] 51 Support Surfaces 52 Pores

[0096] 53 Skin (external object) 54 Arrows

[0097] 55 dotted line

[0098] 60 blood vessels

[0099] 61 cross section

Claims

1. A self-mixing interferometry sensor module (10) comprising at least two light emitters (20) of the same type, a detector unit (30) and an electronic processing unit (40), wherein each light emitter (20) is operable to: - emitting coherent electromagnetic radiation from the sensor module (10); as well as - undergoing self-mixing interference SMI caused by reflection of the emitted electromagnetic radiation from an external object outside the sensor module; wherein the detector unit (30) is operable to: - generating output signals indicative of said SMI of said optical transmitters, respectively; and The electronic processing unit (40) is operable to: - generating a difference signal indicative of the movement of the external object (53) from the output signal.

2. The sensor module according to claim 1, wherein the light emitters (20) have the same configuration, including the same emission wavelength.

3. The sensor module according to claim 1 or 2, wherein the light emitters (20) are rigidly coupled to each other.

4. The sensor module according to any one of claims 1 to 3, wherein - the light emitters (20) are arranged in pairs, at a certain distance from each other, and - said distance is selected such that said output signals indicative of said SMI of said pair of light emitters differ by a predetermined amount.

5. A sensor module according to any one of claims 1 to 4, comprising an array of said light emitters (20), wherein said electronic processing unit (40) is operable to generate a difference signal from said output signals of pairs of said light emitters (20).

6. The sensor module according to any one of claims 1 to 5, wherein the light emitter (20) comprises: - semiconductor laser diodes, and / or -Resonant cavity light emitting device.

7. The sensor module according to one of claims 1 to 6, wherein the light emitter (20) comprises a Vertical Cavity Surface Emitting Laser (VCSEL) diode.

8. The sensor module according to any one of claims 1 to 7, wherein the electronic processing unit (40) is operable to: - performing a Fast Fourier Transform on the output signal to extract the dominant frequencies, and - generating said difference signal according to said main frequency.

9. The sensor module according to one of claims 1 to 8, wherein the light emitters (20) are arranged in parallel such that the light emitters (20) have the same emission direction (23).

10. The sensor module according to any one of claims 1 to 9, wherein the detector unit (30) is operable to: - respectively detecting the junction voltage of the light emitter (20), and - generating the output signals respectively according to the junction voltages.

11. The sensor module according to any one of claims 1 to 10, wherein the detector unit (30) is operable to: - respectively detecting the optical power output of the optical transmitter (20), and - generating said output signals respectively according to said optical power outputs.

12. A wearable electronic device comprising: - A self-mixing interferometry sensor module (10) according to any one of claims 1 to 11, and - a housing (50) comprising the sensor module (10) and a supporting surface (51) to be arranged on the skin (53) of a user, wherein the housing (50) is configured to position the light emitter (20) at a distance from the skin (53) of the user.

13. The sensor module according to one of claims 1 to 9, wherein the light emitter (20) is arranged in the housing (50) so that the emission direction (23) of the light emitter (20) is substantially vertical or perpendicular to the supporting surface (51).

14. The sensor module according to one of claims 1 to 9, wherein the light emitter (20) is arranged in the housing (50) such that the emission direction (23) of the light emitter (20) is inclined with respect to the support surface (51).

15. A method for detecting movement, comprising the steps of: - emitting coherent electromagnetic radiation from the sensor module (10) by means of two light emitters (20) of the same type, - causing a self-mixing interference SMI in the optical transmitter (20), the self-mixing interference SMI being caused by reflection or scattering of the emitted electromagnetic radiation from an object (53) outside the sensor module (10), - generating an output signal indicative of said SMI of said optical transmitter (20), respectively, and - generating a difference signal from said output signal indicative of movement of said external object (53).

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