FINE DUST SENSORS BASED ON SELF-MIXING SPLIT-BEAM INTERFEROMETRY SENSORS

Abstract

Fine dust sensor (300), comprising: a self-mixing interferometry sensor (302) configured to generate a self-mixing interference signal; a set of one or more optical elements (304) positioned to receive an optical emission (306) from the self-mixing interferometry sensor (302), split the optical emission (306) into multiple beams (308), and direct each beam of the multiple beams (308) in a different direction; and switching logic configured to: extract (902) unsigned Doppler frequency shifts from the self-mixing interference signal; estimate (904) a particle velocity using relative orientations of the multiple beams (308) and the unsigned Doppler frequency shifts; estimate (906), using the particle velocity, an airflow volume through respective measurement ranges of the multiple beams;Counting (908) a number of particles passing through the respective measuring ranges over a period of time; and estimating (910) a fine dust concentration using the number of particles and the airflow volume.;

Description

CROSS REFERENCE TO RELATED APPLICATIONSThis patent application according to Patent Cooperation Treaty claims priority to U.S. Non-Provisional Patent Application US 2020 / 0 319 082 A1, filed March 27, 2020, entitled "Fine Dust Sensors Based on Self-Mixed Interferometry Sensors" ("Particulate Matter Sensors Based on Split Beam Self-Mixing Interferometry Sensors") and U.S. Provisional Patent Application US 2019 / 62 830 085 A1, filed April 5, 2019, entitled "Fine Dust Sensors Based on Self-Mixed Interferometry Sensors" ("Particulate Matter Sensors Based on Split Beam Self-Mixing Interferometry Sensors"), The contents of which are incorporated herein by reference as if fully disclosed herein.REGIONThe described embodiments relate generally to particulate matter sensors. In particular, the described embodiments relate to fine dust sensors based on self-mixing split beam interferometry sensors.BACKGROUNDSelf-mixing interferometry is an optical sensor technology that can be used for fine dust detection and air quality monitoring. In self-mixing interferometry, coherent or partially coherent electromagnetic radiation emitted from a stimulated emission-based electromagnetic radiation source (e.g., a laser) may be fed back into the optical resonator of the electromagnetic radiation source by reflection / back scattering of particulate material (e.g., small solid or liquid particles such as particles contained in air pollution (smog), ash, dust, pollen, water vapor, and so forth). Such feedback induces a measurable phase sensitive change (e.g., a Doppler frequency shift) in the electric field and carrier distribution of the optical resonator.The document WO 2018 / 104 154 A1 describes a laser sensor module for detecting the particle density.The document WO 2018 / 104 153 A1 describes a laser sensor module which is designed to emit first and second measurement beams at different angles.The document WO 2017 / 198 699 A1 describes an optical particle sensor.US 2012 / 0 016 615 A1 describes a method for adjusting a self-mixing laser sensor system for measuring the speed of a vehicle.SUMMARYThe invention is defined in the independent claims. Advantageous embodiments are defined in the dependent claims.Embodiments of the systems, apparatus, methods, and apparatus described in the present disclosure are directed to particulate matter sensors, and more particularly, to particulate matter sensors based on self-mixing split beam interferometry sensors. A self-mixing interferometry sensor is defined herein as a sensor configured to generate electromagnetic radiation within an optical resonator, emit the electromagnetic radiation from the optical resonator, receive reflection or backscatter of the electromagnetic radiation back into the optical resonator, mix the generated and reflected / backscattered electromagnetic radiation within the optical resonator itself, and generate an output indicative of self-mixing. The generated, emitted and received electromagnetic radiation may be coherent or partially coherent. In some examples, the electromagnetic radiation emitted by a self-mixing interferometry sensor may be generated by an electromagnetic radiation source such as a vertical cavity surface emitting laser (VCSEL), a vertical external cavity surface emitting laser (VECSEL), a quantum dot laser (QDL), a quantum cascade laser (QCL), or a light emitting diode (LED) (e.g., an organic LED (OLED)), a resonator LED (RC LED), a micro LED (mLED), a superluminescent LED (SLED), or an edge emitting LED, and so forth. The generated, emitted, and received electromagnetic radiation may include, for example, visible or invisible light (e.g., green light, infrared (IR) light, ultraviolet (UV) light, etc.). The output of a self-mixing interferometry sensor may include a photocurrent generated by a photodetector (e.g., a photodiode) integrated into or positioned below, above, or next to the electromagnetic radiation source of the sensor. Alternatively or additionally, the output of a self-mixing interferometry sensor may include measurement of a quiescent current or pn-junction of the self-mixing interferometry sensor.As described herein, a single self-mixing interferometry sensor may be used to determine one or more of the existence of particulate matter within a set of measurement ranges, a velocity of particles passing through the measurement ranges, an air flow through the measurement ranges, a particle concentration within the measurement ranges, an air quality within the measurement ranges, and so forth.In a first aspect, the present disclosure describes a particulate matter sensor. The particulate matter sensor may include a self-mixing interferometry sensor and a set of one or more optical elements. The set of one or more optical elements may be positioned to receive an optical emission of the self-mixing interferometry sensor, split the optical emission into a plurality of beams, and direct each beam of the plurality of beams in a different direction. The self-mixing interferometry sensor may be configured to generate particle velocity information for particles passing through the respective measurement regions of the plurality of beams.In another aspect, the present disclosure describes a sensor that includes an electromagnetic radiation source and a splitter. The electromagnetic radiation source may comprise a resonant optical resonator. The splitter may be configured to split an optical emission of the electromagnetic radiation source into a set of multiple beams and receive reflections or backscatters of the multiple beams and direct the received reflections or backscatters into the optical resonator.In yet another aspect of the disclosure, a method for detecting particulate matter is described. The method may include splitting an optical emission received from a self-mixing interferometry sensor into a plurality of beams; directing each beam of the plurality of beams in a different direction; and outputting, from the self-mixing interferometry sensor, particle velocity information for particles passing through the respective measurement regions of the plurality of beams.In addition to the exemplary aspects and embodiments described above, other aspects and embodiments will become apparent by reference to the drawings and by studying the following description.BRIEF DESCRIPTION OF THE DRAWINGSThe disclosure will be readily understood from the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and wherein: FIGS. 1A and 1B show a first example of a device that may include a particulate matter sensor; FIGS. 2A and 2B show a second example of an apparatus that may include a particulate matter sensor; FIG. 3 is an example block diagram of a particulate matter sensor; FIG. 4 shows an example elevation of a particulate matter sensor; FIG. 5 is a plan view of the particulate matter sensor; FIGS. 6A-8B show respective plan views ("A" views) and elevational views ("B" views) of the beams shown in FIGS. 4 and 5 ; FIG. 9 shows an example method for estimating particulate matter concentration, the method may be performed by circuitry associated with a self-mixing interferometry sensor; FIG. 10 shows an example method for detecting particulate matter; and FIG. 11 shows an example electrical block diagram of an electronic device.The use of shading in the appended figures is generally provided to clarify the boundaries between adjacent elements and also to improve the readability of the figures. Therefore, neither the presence nor absence of hatchings or shading is intended to indicate or indicate a preference or requirement for particular materials, material properties, element proportions, element dimensions, commonities of similarly depicted elements, or any other characteristics, attributes, or property for any element depicted in the appended figures.In addition, it should be understood that the (either relative or absolute) proportions and dimensions of the various features and elements (and their clusters and groupings) as well as the boundaries, separations, and positional relationships depicted therebetween are provided in the accompanying drawings merely to facilitate understanding of the various embodiments described herein and thus are not necessarily shown or depicted to scale and do not present a preference or requirement for a depicted embodiment with the exclusion of embodiments described with reference thereto.DETAILED DESCRIPTIONReference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to a preferred embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the described embodiments as defined by the appended claims.The following description relates to detecting particulate matter. In estimating the particle concentration using self-mixing interferometry, the accurate particle concentration estimation depends on an accurate three-dimensional (3D) reconstruction of the particle velocity (e.g., reconstruction of a particle velocity vector). An intrinsic limit for self-mixing interferometry, however, is that when a particle passes through a beam of electromagnetic radiation near the focus of the beam, only the absolute particle velocity of a particle in the beam direction can be measured, while the velocity of the particle perpendicular to the beam is lost. Thus, the conventional approach to estimating the particle concentration using self-mixing interferometry is to use three self-mixing interferometry sensors oriented in three different directions. However, the use of multiple self-mixing interferometry sensors increases the cost, complexity and power consumption of a particulate matter sensor as well as the size of the particulate matter sensor.As described herein, a single self-mixing interferometry sensor may be used to determine one or more of the existence of particulate matter within a set of measurement ranges, a velocity of particles passing through the measurement ranges, an air flow through the measurement ranges, a particle concentration within the measurement ranges, an air quality within the measurement ranges, and so forth. A single self-mixing interferometry sensor may be used to detect particulate matter by splitting an optical emission of the self-mixing interferometry sensor into multiple beams having different directions. The optical emission may be split into a plurality of beams using a set of one or more optical elements, wherein the optical element(s) may include one or more diffractive optical elements (or holographic elements or periodic sub-wavelength elements or aperiodic sub-wavelength elements) and one or more beam shaping optical elements (e.g. one or more focusing, tilting and / or collimating optical elements). Particle velocity information obtained from a self-mixing interference signal (sometimes also referred to as a self-mixing interferometry signal) generated by the self-mixing interferometry sensor in combination with information regarding beam geometry may then be used to estimate particle velocity, particulate matter concentration, and other parameters. In some cases, the detected particulate matter may include particulate matter on the order of PM 10 (particulate matter having a diameter of less than 10 microns (μm)) or PM 2.5 (particulate matter having a diameter of less than 2.5 μm).These and other embodiments and advantages will be discussed with reference to FIGS. 1A-11. However, it will be readily apparent to those skilled in the art that the detailed description given herein with respect to these figures is for explanatory purposes only and is not to be construed as limiting.Directional terminology such as "top," "bottom," "top / r / s," "bottom / r / s'", "front," "back," "above," "below," "above," "below," "left," "right," etc. will be used with reference to the orientation of some of the components in some of the figures described below. Since components of various embodiments may be positioned in a number of different orientations, directional terminology is used for purposes of illustration only and is in no way limiting. Directional terminology is to be broadly construed and therefore should not be construed as excluding a different orientation of components. The use of alternative terminology, such as "or", is intended to indicate various combinations of the alternative elements. For example, A or B is intended to include A, or B, or A and B.FIGS. 1A and 1B show a first example of a device 100 that may include a particulate matter sensor. The device dimensions and form factor, including the ratio of the length of its long sides to the length of its short sides, suggest that the device 100 is a mobile phone (e.g., a smartphone). However, the dimensions and form factor of the device are arbitrarily chosen, and the device 100 could alternatively be any portable electronic device including, for example, a mobile phone, tablet computer, portable computer, portable music player, health monitoring device, portable terminal, car navigation system, robot navigation system, or other portable or mobile device. The apparatus 100 could also be a device that is semi-permanent in a single location (or installed). FIG. 1A shows a front isometric view of the device 100, and FIG. 1B shows a rear isometric view of the device 100. The device 100 may include a housing 102 at least partially surrounding a display 104. The housing 102 may include or support a front cover 106 or a rear cover 108. The front cover 106 may be positioned over the display 104 and provide a window through which the display 104 may be displayed. In some embodiments, the display 104 may be attached (or abuts) to the housing 102 and / or the front cover 106. In alternative embodiments of the device 100, the display 104 may not be included and / or the housing 102 may have an alternative configuration.The display 104 may include one or more light emitting elements including, for example, an LED, OLED, liquid crystal display (LCD), electroluminescent display (EL), or other type of display element. In some embodiments, the display 104 may include or be associated with one or more touch and / or force sensors configured to sense a touch and / or force applied to a surface of the front cover 106.The various components of the housing 102 may be formed from the same or different materials. For example, the sidewall 118 may be formed using one or more metals (e.g., stainless steel), polymers (e.g., plastics), ceramics, or composites (e.g., carbon fiber). In some cases, the sidewall 118 may be a multi-piece sidewall that includes a set of antennas. The antennas may form structural components of the sidewall 118. The antennas may be structurally coupled (to each other or to other components) and electrically isolated (from each other or from other components) by one or more non-conductive segments of the sidewall 118. For example, the front cover 106 may be formed using one or more of glass, a crystal (e.g., sapphire), or a transparent polymer (e.g., plastic), which allows a user to view the display 104 through the front cover 106. In some cases, a portion of the front cover 106 (e.g., a perimeter portion of the front cover 106) may be coated with an opaque ink to conceal components contained within the housing 102. The back cover 108 may be formed of the same material(s) used to form the side wall 118 or the front cover 106. In some cases, the back cover 108 may be part of a monolithic element that also forms the sidewall 118 (or, in cases where the sidewall 118 is a multi-piece sidewall, those portions of the sidewall 118 that are non-conductive). In still other embodiments, all outer components of the housing 102 may be formed from a transparent material, and components within the device 100 may or may not be occluded by an opaque ink or opaque structure within the housing 102.The front cover 106 may be mounted to the side wall 118 to cover an opening defined by the side wall 118 (i.e., an opening into an interior volume in which various electronic components of the device 100, including the display 104, may be positioned). The front cover 106 may be mounted to the sidewall 118 using fasteners, adhesives, seals, gaskets, or other components.A display stack or device stack (hereinafter referred to as a "stack") including the display 104 may be attached (or abut) to an inner surface of the front cover 106 and extend into the inner volume of the device 100. In some cases, the stack may include a touch sensor (e.g., a grid of capacitive, resistive, strain-based, ultrasonic, or other types of touch sensing elements) or other layers of optical, mechanical, electrical, or other types of components. In some cases, the touch sensor (or a portion of a touch sensor system) may be configured to sense a touch applied to an outer surface of the front cover 106 (e.g., to a display surface of the device 100).In some cases, a force sensor (or a portion of a force sensor system) may be positioned within the interior volume below and / or to the side of the display 104 (and in some cases within the device stack). The force sensor (or force sensor system) may be triggered in response to the touch sensor sensing one or more touches on the front cover 106 (or a location or locations of one or more touches on the front cover 106), and may determine an amount of force associated with each touch or an amount of force associated with the collection of touches as a whole.As shown primarily in FIG. 1A, the apparatus 100 may include various other components. For example, the front of the device 100 may include one or more front facing cameras 110, speakers 112, microphones, or other components 114 (e.g., audio, imaging, and / or sensing components) configured to transmit or receive signals to / from the device 100. In some cases, a front facing camera 110, alone or in combination with other sensors, may be configured to operate as a biometric or face detection sensor. The device 100 may also include various input devices, including a mechanical or virtual button 116 accessible from the front surface (or display surface) of the device 100. In some cases, the front-side camera 110, the virtual button 116, and / or other sensors of the device 100 may be integrated into a display stack of the display 104 and moved under the display 104.The device 100 may also include buttons or other input devices positioned along the sidewall 118 and / or on a rear surface of the device 100. For example, a volume key or multifunction key 120 may be positioned along the sidewall 118 and, in some cases, extend through an opening in the sidewall 118. The sidewall 118 may include one or more ports 122 that allow air, but not liquids, to flow into and out of the device 100. In some embodiments, one or more sensors may be positioned in or near the port(s) 122. For example, an ambient pressure sensor, an ambient temperature sensor, an internal / external differential pressure sensor, a gas sensor, a particulate matter sensor, or an air quality sensor may be positioned in or near a port 122.In some embodiments, the rear surface of the device 100 may include a rear facing camera 124 or other optical sensor (see FIG. 1B ). A flash or light source 126 may also be positioned along the back of the device 100 (e.g., proximate to the rear facing camera). In some cases, the rear surface of the device 100 may include multiple rear facing cameras.The camera(s), microphone(s), pressure sensor(s), temperature sensor(s), biometric sensor(s), key(s), proximity sensor(s), touch sensor(s), force sensor(s), particulate matter or air quality sensor(s), etc. of the device 100 may form portions of various sensor systems.FIGS. 2A and 2B show a second example of a device 200 that may include a particulate matter sensor. The dimensions and form factor of the device and the inclusion of a band 204 suggest that the device 200 is an electronic watch. However, the device 200 could alternatively be any portable electronic device. FIG. 2A shows a front isometric view of the device 200, and FIG. 2B shows a rear isometric view of the device 200. The device 200 may include a body 202 (e.g., a watch body) and a strap 204. Watch body 202 may include an input or selection device, such as crown 214 or button 216. The strap 204 may be used to attach the body 202 to a body part (e.g., an arm, wrist, leg, ankle, or waist) of a user. The body 202 may include a housing 206 that at least partially surrounds a display 208. The housing 206 may include or support a front cover 210 (FIG. 2A ) or a rear cover 212 (FIG. 2B ). The front cover 210 may be positioned over the display 208 and provide a window through which the display 208 may be viewed. In some embodiments, the display 208 may be attached (or abuts) to the housing 206 and / or the front cover 210. In alternative embodiments of the device 200, the display 208 may not be included and / or the housing 206 may have an alternative configuration.The housing 206 may be similar to the housing 102 described with reference to FIGS. 1A-1B in some cases, and the display 208 may be similar to the display 104 described with reference to FIGS. 1A-1B in some cases.The device 200 may include various sensor systems and, in some embodiments, may include some or all of the sensor systems included in the device 100 described with reference to FIGS. 1A-1B. In some embodiments, the device 200 may include a port 218 (or set of ports) on a side of the housing 206 (or otherwise), and an ambient pressure sensor, an ambient temperature sensor, an internal / external differential pressure sensor, a gas sensor, a particulate matter sensor, or an air quality sensor may be positioned in or near the port(s) 218.In some cases, the back surface (or skin-side surface) of the device 200 may include a flat or raised area 220 that includes one or more skin-side sensors. For example, the region 220 may include a heart rate monitor, a breathing rate monitor, or a blood pressure monitor. Region 220 may also include an off-wrist detector or other sensor.FIG. 3 shows an example block diagram of a particulate matter sensor 300. The particulate matter sensor 300 may include a self-mixing interferometry sensor 302 and a set of one or more optical elements 304 (hereafter referred to as a set of optical elements 304). The set of optical elements 304 may be positioned to receive an optical emission 306 of the self-mixing interferometry sensor 302. The set of optical elements 304 may split the optical emission 306 into a set of multiple beams 308- 1, 308- 2, 308- 3 and direct each beam of the multiple beams 308- 1, 308- 2, 308- 3 in a different direction. The self-mixing interferometry sensor 302 may be configured to generate particle velocity information 310 for particles 312 passing through respective measurement regions 314- 1, 314- 2, 314- 3 of the plurality of beams 308- 1, 308- 2, 308- 3.The self-mixing interferometry sensor 302 may include an electromagnetic radiation source that emits the optical emission 306. The optical emission 306 may include a beam of coherent or partially coherent electromagnetic radiation. In some embodiments, the electromagnetic radiation source may include a VCSEL, a VECSEL, a QDL, a QCL, or an LED (e.g., an OLED, an RC LED, a mLED, a SLED, or an edge emitting LED).In some embodiments, the self-mixing interferometry sensor 302 may include an electromagnetic radiation source integrated with a photodetector. In some embodiments, the self-mixing interferometry sensor 302 may include an electromagnetic radiation source stacked on, below, or near (e.g., adjacent to) a photodetector. In embodiments where a photodetector is positioned proximate to an electromagnetic radiation source, the set of optical elements 304 may redirect received reflections or backscatters of the plurality of beams 308- 1, 308- 2, 308- 3 toward the photodetector. An output current of the photodetector (e.g., a photocurrent, I PD) may provide the particle velocity information 310. Alternatively, the particle velocity information 310 may be obtained by monitoring a junction voltage of the electromagnetic radiation source when the electromagnetic radiation source is operated at a constant current or by monitoring a current of the electromagnetic radiation source when the electromagnetic radiation source is operated at a constant voltage.The output current of the photodetector, or the junction voltage of the electromagnetic radiation source, or the current of the electromagnetic radiation source, may vary in response to coherent self-mixing of 1) electromagnetic radiation generated by the electromagnetic radiation source and 2) reflected or backscattered electromagnetic radiation received in an optical resonator of the electromagnetic radiation source and coherently mixed with the electromagnetic radiation generated by the electromagnetic radiation source.The set of optical elements 304 may include, for example, one or more diffractive optical elements (and in some cases a holographic element, a sub-wavelength periodic element, and / or a sub-wavelength aperiodic element), and one or more beam shaping optical elements (e.g., one or more focusing, tilting, and / or collimating optical elements). The set of optical elements 304 may split the optical emission of the self-mixing interferometry sensor 302 into two, three, four or more beams. Splitting the optical emission into more beams may improve the accuracy of the particle velocity information 310 generated by the self-mixing interferometry sensor 302 (e.g., when the beams are not perpendicular to each other), but at the expense of reducing the optical power per beam 308- 1, 308- 2, 308- 3 and reducing the sensitivity of the particulate matter sensor 300 (e.g., as a result of increasing the minimum particle size that may be detected by the particulate matter sensor 300). Conversely, splitting the optical emission 306 into fewer beams may increase the sensitivity of the particulate matter sensor 300 (e.g., as a result of the reduction in the minimum particle size that may be detected by the particulate matter sensor 300), but at the expense of reducing the accuracy of the particle velocity information 310 generated by the self-mixing interferometry sensor 302. As described herein, calculations for estimating the particle velocity, the air flow volume, and the particle concentration can be simplified by splitting the optical emission 310 of the self-mixing interferometry sensor 302 into three beams, and more specifically three beams forming an orthogonal base.In some cases, the set of optical elements 304 may focus each beam of the plurality of beams 308- 1, 308- 2, 308- 3 onto one of the respective measurement regions 314- 1, 314- 2, 314- 3. In some cases, the set of optical elements 304 may include one or more free-form optics, a micro-lens, a micro-lens per beam, a tilted total internal reflection (TIR) lens per beam, and so forth.In addition to splitting an optical emission of the self-mixing interferometry sensor 302 into multiple beams, the set of optical elements 304 may receive reflections or backscatters of the multiple beams 308- 1, 308- 2, 308- 3 and direct the received reflections or backscatters into an optical resonator of the electromagnetic radiation source of the self-mixing interferometry sensor. The set of optical elements 304 may therefore function as a transmitter and a splitter when optical transmissions pass the splitter in a first general direction, and function as a receiver when optical transmissions pass the splitter in a second general direction opposite the first general direction.FiIG. FIG. 4 shows an example elevation of a particulate matter sensor 400. The particulate matter sensor 400 is an example of the particulate matter sensor described with reference to FIG. 3, and may include a self-mixing interferometry sensor 402 and a set of one or more optical elements 404 (hereinafter referred to as a set of optical elements 404). The set of optical elements 404 may be positioned to receive an optical emission 406 of the self-mixing interferometry sensor 402. The set of optical elements 404 may split the optical emission 406 into a set of multiple beams 408- 1, 408- 2, 408- 3 and direct each beam of the multiple beams 408- 1, 408- 2, 408- 3 in a different direction. The self-mixing interferometry sensor 402 may be configured to generate particle velocity information 410 for particles 412 passing through respective measurement regions 414- 1, 414- 2, 414- 3 of the plurality of beams 408- 1, 408- 2, 408- 3.The self-mixing interferometry sensor 402 may include an electromagnetic radiation source 416, which in some cases may take the form of any of the electromagnetic radiation sources described with reference to FIG. 3. In some embodiments, the electromagnetic radiation source 416 may include a first (or lower) mirror 418- 1 and a second (or upper) mirror 418- 2 stacked (e.g., formed) on a semiconductor substrate. The first and second mirrors 418- 1, 418- 2 may have reflective surfaces facing each other to form (e.g., connect) an optical resonator 420 therebetween. The second mirror 418- 2 may be partially transmissive and may 1) allow a portion of the electromagnetic radiation generated by the electromagnetic radiation source 416 to exit the optical resonator 420 as the optical transmission 406, and 2) allow a portion of the redirected electromagnetic radiation to be redirected (e.g., reflected or scattered) by a particle 412 passing through one of the measurement regions 414- 1, 414- 2, 414- 3 to re-enter the electromagnetic radiation source 416 and coherently mix with electromagnetic radiation generated by the electromagnetic radiation source 416. In some embodiments, the transmittance of the second (or upper) mirror for the wavelength of electromagnetic radiation generated / received by the electromagnetic radiation source 416 may be about 0.5%, although higher or lower transmittances may be used. The first (or lower) mirror 418- 1 may also be partially transmissive to the wavelength of electromagnetic radiation generated / received by the self-mixing interferometry sensor 402, but in some embodiments may be less transmissive than the second mirror 418- 2.In some cases, the self-mixing interferometry sensor 402 may also include a photodetector 422 which, in some cases, may be integrated with the electromagnetic radiation source 416. The electromagnetic radiation source 416 may emit the optical emission 406, and an output current of the photodetector 422 or a junction voltage or current of the electromagnetic radiation source 416 may provide the particle velocity information 410 (e.g., a self-mixing interference signal including the particle velocity information 410).In some cases, the self-mixing interferometry sensor 402 may include an electromagnetic radiation source stacked on or below a photodetector. In these examples, the first mirror 418- 1 may be at least partially transmissive, and electromagnetic radiation that passes through the first mirror 418- 1 may be detected by the photodetector.In some embodiments, the set of optical elements 404 may include a diffractive optical element 424 that receives and splits the optical emission 406 into the multiple beams (e.g., three beams 408- 1, 408- 2, and 408- 3). Diffractive optical element 424 may in some cases be a three optical or physical aperture element that transmits portions of the electromagnetic radiation emitted from self-mixing interferometry sensor 402. The set of optical elements 404 may also include a set of one or more beamforming elements 426 that receives the three beams 408- 1, 408- 2, 408- 3 and focuses each of the beams onto a respective one of the measurement regions 414- 1, 414- 2, 414- 3. In some embodiments, the set of beamforming elements 426 may include a planoconcave lens whose concave surface faces a surface of the diffractive optical element 424. In some embodiments, the set of beamforming elements 426 may focus the plurality of beams 408- 1, 408- 2, 408- 3 onto overlapping or matching measurement regions 414- 1, 414- 2, 414- 3 (rather than into disjunct measurement regions 414- 1, 414- 2, 414- 3 as shown).In addition to splitting an optical emission of the self-mixing interferometry sensor 402 into multiple beams, the set of optical elements 404 may receive reflections or backscatters of the multiple beams 408- 1, 408- 2, 408- 3 and direct the received reflections or backscatters into an optical resonator of the electromagnetic radiation source of the self-mixing interferometry sensor. The set of optical elements 404 shown in FIG. 4 functions as a whole as a splitter.As shown in FIG. 4, particles 412 passing through the measurement regions 414- 1, 414- 2, 414- 3 may have a particle velocity including a particle velocity component in the x direction and a particle velocity component in the z direction. The particles 412 may also have a particle velocity component in the y-direction.FIG. 5 is a plan view of the particulate matter sensor 400. Although the beams 408- 1, 408- 2, 408- 3 may have different relationships to each other, computations performed by a processor to estimate the particle velocity and / or other parameters may be simplified if the beams 408- 1, 408- 2, 408- 3 form an orthogonal basis. The beams 408- 1, 408- 2, 408- 3 may form an orthogonal base, in some embodiments, when each beam 408- 1, 408- 2, 408- 3 is separated from other beams 408- 1, 408- 2, 408- 3 by 120 degrees (120°), in a plane perpendicular to an optical emission axis 500 described with reference to FIG. 4, and when each beam has a secondary axis 502- 1, 502- 2, or 502- 3 that deviates from the optical emission axis 500 by an angle θ, where cos 2( θ)=1 / 3. The angle θ may be configured as part of the optical design of the particulate matter sensor 400.In FIG. 5 and subsequent figures, beams 408- 1, 408- 2, 408- 3 are referred to as beam 1 408- 1, beam 2 408- 2, and beam 3 408- 3, respectively.FIGS. 6A-8B show respective plan views and elevations of each of beam 1, beam 2, and beam 3. Referring first to FIGS. 6A and 6B, FIG. 6A shows a plan view of beam 1 with respect to beam 2 and beam 3, and FIG. 6B shows an elevation of beam 1. in a plane perpendicular to axis 500 of optical emission 406, beam 1 is separated from beam 2 and beam 3 by 120°, respectively. Beam 1 has a secondary axis 502- 1 deviating from the optical emission axis 500 by an angle θ. In some cases, θ may satisfy the following condition: cos 2( θ)=1 / 3.A particle moving through the measurement region 414- 1 may include a particle velocity vector: where x̂, ŷ and ẑ are unit vectors in orthogonal x, y and z directions. However, the self-mixing interferometry sensor 402 is only allowed to measure a component of the velocity of the particle which is in the direction of a unit vector 1. The unit vector 1 shares an axis with ray 1 and can be defined as: with respect to an x / y / z orthogonal base.As a particle passes the measurement region 414- 1, the particle may generate a self-mixing interference signal (at the self-mixing interferometry sensor 402) that oscillates at the frequency of the Doppler frequency shift induced by the particle on the reflected or backscattered electromagnetic radiation. The Doppler frequency shift may be associated with a Doppler frequency f 1 where the Doppler frequency may be proportional to the velocity of the particle towards the beam 1 and is defined as follows:FIG. 7A shows a top view of a beam 2 with respect to beam 1 and beam 3, and FIG. 7B shows an elevation of beam 2. in a plane perpendicular to the axis 500 of the optical emission 406 described with reference to FIG. 4, beam 2 is separated from beam 1 and beam 3 by 120°, respectively. Beam 2 has a secondary axis 502- 2 deviating from the optical emission axis 500 by an angle θ. In some cases, θ may satisfy the following condition: cos 2( θ)=1 / 3.A particle moving through measurement region 414- 2 may also include the particle velocity vector: however, self-mixing interferometry sensor 402 may only measure a component of the velocity of the particle that is in the direction of a unit vector 2. The unit vector 2 shares an axis with ray 2 and may be defined as: with respect to an x / y / z orthogonal base.As a particle passes the measurement region 414- 2, the particle may generate a self-mixing interference signal (at the self-mixing interferometry sensor 402) that oscillates at the frequency of the Doppler frequency shift induced by the particle on the reflected or backscattered electromagnetic radiation. The Doppler frequency shift may be associated with a Doppler frequency f 2 where the Doppler frequency may be proportional to the velocity of the particle towards the beam 2 and is defined as follows:FIG. 8A shows a top view of a beam 3 with respect to beam 1 and beam 2, and FIG. 8B shows an elevation of beam 3. Beam 3 has a secondary axis 502- 3 deviating from the optical emission axis 500 by an angle θ. In some cases, θ may satisfy the following condition: cos 2( θ)=1 / 3.A particle moving through the measurement region 414- 3 may include a particle velocity vector: where x̂, ŷ and ẑ are unit vectors in orthogonal x, y and z directions. However, the self-mixing interferometry sensor 402 is only allowed to measure a component of the velocity of the particle which is in the direction of a unit vector 3. The unit vector 3 shares an axis with ray 3 and can be defined as: with respect to an x / y / z orthogonal base.As a particle passes the measurement region 414- 3, the particle may generate a self-mixing interference signal (at the self-mixing interferometry sensor 402) that oscillates at the frequency of the Doppler frequency shift induced by the particle on the reflected or backscattered electromagnetic radiation. The Doppler frequency shift may be associated with a Doppler frequency f 3 where the Doppler frequency may be proportional to the velocity of the particle towards the beam 3 and is defined as follows:Different particles may pass through the different measurement regions 414- 1, 414- 2, 414- 3 described with reference to FIGS. 4-8B. However, the self-mixing interference signal provided by the self-mixing interferometry sensor 402 may include particle velocity information for particles passing through each of the measurement regions 414- 1, 414- 2, 414- 3. In most cases, particles will pass through the various measurement regions 414- 1, 414- 2, 414- 3 at different times, and thus the particle velocity information for different particles is temporally distinguishable (although the particular beam and the particular measurement region through which a particle passed to generate the particle velocity information may not be identifiable). The particle velocity information may take the form of unsigned Doppler frequency shifts (e.g., |f 1|, | f 2|, and |f 3|) included in the self-mixing interference signal. Thus, from the self-mixing interference signal, only particle speed information (i.e., particle speed magnitude) and not particle speed information (i.e., speed and moving direction) can be extracted.As discussed with reference to FIGS. 6A-8B, the particle velocities (v x, v y and v z) of particles 412 passing through the measurement regions 414- 1, 414- 2 and 414- 3 are related to the angle 0, the wavelength λ of the optical emission 406, and the Doppler frequencies f 1, f 2 and f 3 as follows:Solving the above equations for the particle velocities v x, v y and v z yields the equations:Substituting the above equations into the particle velocity equation yields:If θ is chosen such that cos 2( θ)=1 / 3, the above equation simplifies to: and the particle velocity can be estimated using unsigned Doppler frequency shifts |f 1|, | f 2| and |f 3| without knowing the signs of the Doppler frequency shifts and without knowing which Doppler frequency shift experiences which of the multiple beams 408- 1, 408- 2, 408- 3 (i.e., |f 1|, | f 2| and |f 3| are interchangeable in the above equation, like f 1, f 2 and f 3).The particle velocity can be estimated even when cos 2( θ)≠ 1 / 3, but the accuracy of estimation improves (and the error decreases) as cos 2( θ)=1 / 3 approaches. If cos 2( θ)≠ 1 / 3, the particle velocity can still be estimated with sufficient accuracy by setting limits for the particle flow direction.FIG. 9 shows an example method 900 for estimating particulate matter concentration, where the method 900 may be performed by circuitry (e.g., an ASIC or a processor) associated with a self-mixing interferometry sensor. The circuitry may receive as input digitized samples of a self-mixing interference signal generated by the self-mixing interferometry sensor, or in some cases may receive, sample and digitize the self-mixing interference signal. The self-mixing interferometry sensor may include any of the self-mixing interferometry sensors described with reference to FIGS. 3-8B in some embodiments. Alternatively, the self-mixing interferometry sensor may be another type of self-mixing interferometry sensor.At block 902, the method 900 may include performing a frequency domain analysis (e.g., a fast Fourier transform (FFT)) or a time-frequency domain analysis (e.g., a continuous wavelet transform) to extract a set of unsigned Doppler frequency shifts (e.g., |f 1|, | f 2| and |f 3|) from the self-mixing interference signal.At block 904, the method 900 may include estimating a particle velocity using relative orientations of the plurality of beams and the unsigned Doppler frequency shifts. It should be noted that the estimated particle velocity is actually the velocity of a hypothetical particle, assuming that the hypothetical particle behaves similarly to the particles passing through the different measurement regions, each associated with different beams split from an optical emission of the self-mixing interferometry sensor.At block 906, the method 900 may include estimating, using the particle velocity, an airflow volume through the measurement regions (e.g., how much air a collective volume of the three measurement regions has passed within a given time (e.g., 100 microliters (μL) of air in one second (s)).At block 908, the method 900 may include counting a number of particles passing through the measurement regions over a period of time (e.g., 30 particles in a second). Particles can be counted by counting the number of time domain interference (or Doppler frequency shifts) in the self-mixing interference signal over a period of time.At block 910, the method 900 may include estimating a particulate concentration (e.g., a particulate concentration in air) using the number of particulates counted at block 908 and the airflow volume estimated at block 906. In some embodiments, the circuitry may take a fixed particle size and a fixed mass density to convert from a number of particles within a volume to a particle mass or density within a volume.In some cases, circuitry performing method 900 or other circuitry may be configured to detect a presence of particulate matter (instead of or in addition to particulate matter concentration). In some cases, the presence (or absence) of particulate matter may be detected using particulate velocity information (e.g., the Doppler frequency shifts) extracted from the self-mixing interference signal.FIG. 10 shows an example method 1000 for detecting particulate matter. The method 1000 may be performed by a self-mixing interferometry sensor and circuitry (e.g., an ASIC or a processor) associated with the self-mixing interferometry sensor. The self-mixing interferometry sensor may provide as input to the circuitry digitized samples of a self-mixing interference signal generated by the self-mixing interferometry sensor. Alternatively, the circuitry may receive the self-mixing interference signal and sample and digitize the self-mixing interference signal. The self-mixing interferometry sensor may include any of the self-mixing interferometry sensors described with reference to FIGS. 3-8B in some embodiments. Alternatively, the self-mixing interferometry sensor may be another type of self-mixing interferometry sensor. The circuitry may in some cases be or include some or all of the circuitry performing the method 900 described with reference to FIG. 9.At block 1002, the method 1000 may include splitting an optical emission received from a self-mixing interferometry sensor into multiple beams. In some embodiments, the operation(s) in block 1002 may be performed by the set of optical elements described with reference to any of FIGS. 3-8B.At block 1004, the method 1000 may include directing each beam of the plurality of beams in a different direction. Optionally, the operation(s) in block 1004 may include focusing each beam of the plurality of beams on one of the respective measurement regions. In some embodiments, the operation(s) in block 1004 may be performed by the set of optical elements described with reference to any of FIGS. 3-8B.At block 1006, the method 1000 may include outputting, from the self-mixing interferometry sensor, particle velocity information for particles passing through the plurality of beams (e.g., through respective measurement ranges of the plurality of beams). In some embodiments, the particle velocity information may include a set of unsigned Doppler frequency shifts (e.g., |f 1|, | f 2| and |f 3|) included in a self-mixing interference signal.At block 1008, the method 1000 may optionally include estimating a particulate matter concentration using the particulate velocity information. The estimation may be performed by the circuitry associated with the self-mixing interferometry sensor.Optionally, the method 1000 may include the operations of the method 900 described with reference to FIG. 9.FIG. 11 shows an example electrical block diagram of an electronic device 1100, where the electronic device may take the form of the device described with reference to FIGS. 1A-1B or 2A-2B, and / or include a particulate matter sensor, such as with reference to any of FIGS. 3-8B, in some cases. The electronic device 1100 may include a display 1102 (e.g., a light emitting display), a processor 1104, a power source 1106, a memory 1108 or storage device, a sensor system 1110, and an input / output (I / O) mechanism 1112 (e.g., an input / output device, an input / output port, or a haptic input / output interface). The processor 1104 may control some or all of the operations of the electronic device 1100. The processor 1104 may communicate, either directly or indirectly, with some or all of the components of the electronic device 1100. For example, a system bus or other communication mechanism 1114 may provide communication between the display 1102, the processor 1104, the power source 1106, the memory 1108, the sensor system 1110, and the I / O mechanism 1112.The processor 1104 may be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions, whether such data or instructions are in the form of software or firmware or otherwise encoded. For example, processor 1104 may include a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term "processor" is intended to include a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements.Note that the components of the electronic device 1100 may be controlled by a plurality of processors. For example, selected components of the electronic device 1100 (e.g., a sensor 1110) may be controlled by a first processor, and other components of the electronic device 1100 (e.g., the display 1102) may be controlled by a second processor, where the first and second processors may or may not be in communication with each other.The power source 1106 may be implemented with any device capable of providing power to the electronic device 1100. For example, the power source 1106 may include one or more batteries or rechargeable batteries. Additionally or alternatively, the power source 1106 may include a power outlet or power cord that connects the electronic device 1100 to another power source, such as a power outlet.The memory 1108 may store electronic data that may be used by the electronic device 1100. For example, storage 1108 may store electrical data or content such as audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, and data structures or databases. The memory 1108 may include any type of memory. For example only, the memory 1108 may include random access memory, read only memory, flash memory, removable memory, other types of memory elements, or combinations of such types of memory.The electronic device 1100 may also include one or more sensors 1110 disposed almost anywhere on the electronic device 1100. In some cases, the sensor systems 1110 may be arranged as described with reference to FIGS. 1A-1B or 2A-2B. The sensor system(s) 1110 / may be configured to sense one or more types of parameters, such as, but not limited to, light; touch; force; heat; motion; relative motion; biometric data (e.g., biological parameters) of a user; particulate matter concentration; air quality; proximity; position; connection; etc. By way of example, the sensor system(s) 1110 may include a thermal sensor, a position sensor, a light or optical sensor, an accelerometer, a pressure transducer, a gyroscope, a magnetometer, a health monitoring sensor, a particulate matter sensor, and so forth. Additionally, the one or more sensor systems 1110 may utilize any suitable sensing technology including, but not limited to, capacitive, ultrasonic, resistive, optical, acoustic, piezoelectric, or thermal sensing technology.I / O mechanism 1112 may transmit data to or receive data from a user or other electronic device. The I / O device 1112 may include a display 1102, a touch sensing input surface, a crown, one or more buttons (e.g., a "home page" button of a graphical user interface), one or more cameras (including an under display camera), one or more microphones or speakers, one or more ports such as a microphone port, and / or a keyboard. Additionally or alternatively, the I / O mechanism 1112 may transmit electronic signals over a communication interface, such as a wireless, wired, and / or optical communication interface. Examples of wireless and wired communication interfaces include, but are not limited to, cellular and Wi-Fi communication interfaces.The foregoing description uses specific nomenclature for purposes of explanation to provide a thorough understanding of the described embodiments. However, it will be apparent to those skilled in the art that, upon reading this specification, the specific details are not needed to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are provided herein for purposes of illustration and description. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to those skilled in the art that after reading this specification, many modifications and variations are possible in light of the above teachings.

Claims

A particulate matter sensor (300) comprising: a self-mixing interferometry sensor (302) configured to generate a self-mixing interference signal; a set of one or more optical elements (304) positioned to receive an optical emission (306) of the self-mixing interferometry sensor (302), split the optical emission (306) into a plurality of beams (308), and direct each beam of the plurality of beams (308) in a different direction; and circuitry configured to: extract (902) unsigned Doppler frequency shifts from the self-mixing interference signal; estimate (904) a particle velocity using relative orientations of the plurality of beams (308) and the unsigned Doppler frequency shifts; estimating (906), using the particle velocity, an air flow volume through respective measurement regions of the plurality of beams; counting (908) a number of particles passing through the respective measurement regions over a period of time; and estimating (910) a particulate concentration using the number of particles and the air flow volume.The particulate matter sensor (300) of claim 1, wherein the circuitry is further configured to: perform frequency domain analysis to extract the unsigned Doppler frequency shifts from the self-mixing interference signal.The particulate matter sensor (300) of claim 1, wherein the circuitry is further configured to: perform a time-frequency range analysis to extract the unsigned Doppler frequency shifts from the self-mixing interference signal.The particulate matter sensor (300) of claim 1, wherein the set of one or more optical elements (304) focuses each beam of the plurality of beams (308) onto one of the respective measurement regions.The particulate matter sensor (300) of claim 1, wherein the circuitry is further configured to detect a presence of particulate matter using the particulate velocity.The particulate matter sensor (300) of claim 1, wherein the plurality of beams (308) consist of three beams.The particulate matter sensor (300) of claim 1, wherein the self-mixing interferometry sensor (302) comprises an electromagnetic radiation source integrated with a photodetector.The particulate matter sensor (300) of claim 1, wherein: the self-mixing interferometry sensor (302) comprises an electromagnetic radiation source and a photodetector; the electromagnetic radiation source has an optical resonator bounded by first and second mirrors, the first mirror and the second mirror each being at least partially transmissive to a wavelength of electromagnetic radiation; and the electromagnetic radiation source is stacked on the photodetector.The particulate matter sensor (300) of claim 1, wherein: the set of one or more optical elements (304) is a first set of one or more optical elements; and the particulate matter sensor (300) further comprises a second set of one or more optical elements configured to receive the set of multiple beams and redirect the set of multiple beams toward a set of overlapping or matching measurement ranges.A method for detecting particulate matter, comprising: splitting an optical emission (306) received from a self-mixing interferometry sensor (302) into a plurality of beams (308); directing each beam of the plurality of beams (308) in a different direction; and outputting a self-mixing interference signal from the self-mixing interferometry sensor (302); extracting (902) unsigned Doppler frequency shifts from the self-mixing interference signal; estimating (904) a particle velocity using relative orientations of the plurality of beams and the unsigned Doppler frequency shifts; estimating (906), using the particle velocity, an airflow volume through respective measurement regions of the plurality of beams (308); counting (908) a number of particles passing the respective measurement regions over a period of time; and estimating (910) a particulate matter concentration using the number of particles and the air flow volume.The method of claim 10, further comprising: performing a frequency domain analysis to extract the unsigned Doppler frequency shifts from the self-mixing interference signal.The method of claim 10, further comprising: focusing each beam of the plurality of beams (308) onto one of the respective measurement areas.The method of claim 10, further comprising: performing a time-frequency domain analysis to extract the unsigned Doppler frequency shifts from the self-mixing interference signal.The method of claim 10, further comprising: receiving the set of multiple beams (308); and redirecting the set of multiple beams toward a set of overlapping or matching measurement ranges.

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