PATENT DOCUMENT

Publication Number: US-11874110-B2
Application Number: US-202017118394-A
Country: US
Kind Code: B2

Title: Self-mixing interferometry device configured for non-reciprocal sensing

Abstract:
Methods and systems concerning non-reciprocal sensing paths for a self-mixing interferometry operation are disclosed herein. Optical components may be used to direct light transmit from a light source along an illumination path. The optical components may additionally return light to the light source after being reflected from a target and may direct the returned light along a collection path. The illumination path and the collection path may be at least partially non-reciprocal so that the transmitted light and the returned light follow along partially different paths. Once the received light is received within a cavity of the light source, a self-mixing interferometry operation may be performed and may be used to detect a property of the target in relation to the light source.

Claims:
What is claimed is: 
     
       1. An apparatus for performing non-reciprocal self-mixing interferometry, the apparatus comprising:
 a self-mixing interferometry sensor having a light source configured to emit a beam of light; and 
 a birefringent circulator positioned over an aperture of the light source and configured to direct the beam of light emitted by the light source along an illumination path, the birefringent circulator comprising:
 a Faraday rotator; and 
 a wave plate, wherein the Faraday rotator and the wave plate are configured to transform a first polarization of the beam of light into a second polarization as the beam of light travels along the illumination path, wherein: 
 
 the birefringent circulator directs a returned portion of the beam of light, received along a collection path different from the illumination path, back toward the light source. 
 
     
     
       2. The apparatus of  claim 1 , wherein the Faraday rotator and the wave plate are configured to keep a third polarization of the returned portion of the beam of light unchanged as the returned portion of the beam of light is directed back toward the light source. 
     
     
       3. The apparatus of  claim 1 , wherein the wave plate is a half-wave plate with an optical axis aligned to the first polarization. 
     
     
       4. The apparatus of  claim 1 , wherein a first angle of the beam of light, as the beam of light leaves the birefringent circulator along the illumination path, is different than a second angle of the returned portion of the beam of light, as the returned portion of the beam of light enters the birefringent circulator along the collection path. 
     
     
       5. The apparatus of  claim 1 , wherein:
 the self-mixing interferometry sensor emits the beam of light having p-polarization before the beam of light enters the birefringent circulator; and 
 the birefringent circulator is configured to cause the beam of light to have s-polarization after the beam of light exits the birefringent circulator. 
 
     
     
       6. The apparatus of  claim 5 , wherein:
 the birefringent circulator is configured to receive the returned portion of the beam of light having p-polarization as the returned portion of the beam of light enters the birefringent circulator; and 
 the birefringent circulator is configured to cause the returned portion of the beam of light to have p-polarization after the returned portion of the beam of light exits the birefringent circulator. 
 
     
     
       7. The apparatus of  claim 1 , wherein:
 the birefringent circulator has a first refractive index in a first direction of travel along the illumination path; 
 the birefringent circulator has a second refractive index in a second direction of travel along the collection path; and 
 the first refractive index is different from the second refractive index. 
 
     
     
       8. The apparatus of  claim 1 , wherein the birefringent circulator focuses the illumination path on a common detection space. 
     
     
       9. An electronic device for performing non-reciprocal self- mixing interferometry, the electronic device comprising:
 a self-mixing interferometry sensor configured to emit a beam of light; and 
 a birefringent circulator positioned in an illumination path of the emitted beam of light and in a collection path of a returned portion of the beam of light, the birefringent circulator configured to direct the beam of light emitted from the self-mixing interferometry sensor from a first aperture of the birefringent circulator, the birefringent circulator comprising:
 a Faraday rotator; and 
 a wave plate, wherein the Faraday rotator and the wave plate are configured to transform a first polarization of the beam of light into a second polarization as the beam of light travels along the illumination path, wherein: 
 
 the birefringent circulator directs the returned portion of the beam of light, received from a second aperture of the birefringent circulator, into the self-mixing interferometry sensor. 
 
     
     
       10. The electronic device of  claim 9 , wherein: the birefringent circulator is configured to direct the beam of light to a first location on a target; and the birefringent circulator is configured to receive the returned portion of the beam of light from a second location on the target. 
     
     
       11. The electronic device of  claim 9 , further comprising a processing unit configured to determine, from an output of the self-mixing interferometry sensor, a displacement or a movement of a sub-surface feature of an object. 
     
     
       12. The electronic device of  claim 11 , wherein the processing unit is configured to:
 apply a frequency domain analysis to the self-mixing interferometry signal; and 
 determine the displacement or the movement of the sub-surface feature of the object based on a change in at least one property of an operational parameter of the self-mixing interferometry signal, as determined by the frequency domain analysis. 
 
     
     
       13. The electronic device of  claim 9 , wherein the electronic device is at least one of an electronic watch or a mobile phone. 
     
     
       14. An electronic device, comprising:
 a self-mixing interferometry sensor, comprising:
 a light emitter configured to emit a beam of light; and 
 a photodetector configured to receive a returned portion of the beam of light; and 
 
 a set of one or more optical components positioned over an aperture of the light emitter and comprising a first birefringent wedge, a second birefringent wedge and a Faraday rotator positioned between the first birefringent wedge and the second birefringent wedge, the one or more optical components defining:
 a first optical path configured to propagate the emitted beam of light between a first receiving location and a first emission location; and 
 a second optical path configured to propagate the returned portion of the beam of light between a second receiving location and a second emission location, wherein the first optical path is different from the second optical path. 
 
 
     
     
       15. The electronic device of  claim 14 , wherein the set of one or more optical components further comprises a lens configured to collimate the beam of light as the beam of light enters the set of one or more optical components. 
     
     
       16. The electronic device of  claim 14 , wherein the set of one or more optical components further comprises a half-wave plate positioned between the Faraday rotator and the second birefringent wedge. 
     
     
       17. The electronic device of  claim 14 , wherein the set of one or more optical components is a passive, non-reciprocal meta-structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a nonprovisional of and claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 63/083,413, filed Sep. 25, 2020, the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     Embodiments described herein generally relate to structures and configurations of self-mixing interferometry (SMI) sensors that may be used to detect, measure, or determine an object location, speed, velocity, distance, motion, and/or displacement. In particular, described embodiments may relate to an SMI sensor configured to perform non-reciprocal sensing. 
     BACKGROUND 
     Electronic devices may be provided with various kinds of sensors for sensing an internal or external environment of the electronic device. Internal sensing may be used to detect, for example, a user input, whereas external sensing may be used to detect, for example, a property of the device&#39;s user (e.g., a user biometric). 
     SUMMARY 
     In some embodiments, an apparatus for performing non-reciprocal self-mixing interferometry may comprise a self-mixing interferometry sensor configured to emit a beam of light and a birefringent circulator positioned over an aperture of the light source. The birefringent circulator may be configured to direct the beam of light emitted from the self-mixing interferometry sensor along an illumination path and to direct a returned portion of the beam of light, received along a collection path different from the illumination path, back toward the light source. 
     A birefringent circulator may comprise a Faraday rotator and a wave plate. The Faraday rotator and the wave plate may be configured to transform a first polarization of the beam of light into a second polarization as the beam of light travels along the illumination path. The Faraday rotator and the wave plate may be further configured to keep a third polarization of the returned portion of the beam of light unchanged as the returned portion of the beam of light is directed back toward the light source. In some embodiments, the wave plate may be a half-wave plate with an optical axis aligned to the first polarization. 
     A first angle of the beam of light, as the beam of light leaves the birefringent circulator along the illumination path, may be different than a second angle of the returned portion of the beam of light, as the returned portion of the beam of light enters the birefringent circulator along the collection path. 
     In some embodiments, a first polarization of the beam of light before the beam of light enters the birefringent circulator may be p-polarization and a second polarization of the beam of light after the beam of light exits the birefringent circulator may be s-polarization. 
     In some embodiments, a third polarization of the returned portion of the beam of light before the returned portion of the beam of light enters the birefringent circulator may be p-polarization and a fourth polarization of the returned portion of the beam of light after the returned portion of the beam of light exits the birefringent circulator may be p-polarization. 
     In some embodiments, the birefringent circulator may have a first refractive index in a first direction of travel along the illumination path. The birefringent circulator may further have a second refractive index in a second direction of travel along the collection path. The first refractive index may be different from the second refractive index. In some embodiments, the illumination path and the collection path may be focused on a common detection space. 
     In some embodiments, an electronic device for performing non-reciprocal self-mixing interferometry may comprise a self-mixing interferometry sensor configured to emit a beam of light and a birefringent circulator positioned in an illumination path of the emitted beam of light and in a collection path of a returned portion of the beam of light. The birefringent circulator may be configured to direct the beam of light emitted from the self-mixing interferometry sensor from a first aperture of the birefringent circulator and to direct a returned portion of the beam of light, received from a second aperture of the birefringent circulator, into the self-mixing interferometry sensor. 
     In some embodiments, the beam of light directed from the first aperture may be configured to travel along a curved path underneath a target and the returned portion of the beam of light may exit the curved path from under the target. 
     In some embodiments, a processing unit may be configured to determine, from an output of the self-mixing interferometry sensor, a displacement or a movement of a sub-surface feature of an object. 
     An electronic device may further comprise a gesture recognizer configured to receive a self-mixing interferometry signal from the self-mixing interferometry sensor and determine a gesture of the user based on the received self-mixing interferometry signal. 
     The gesture recognizer may be further configured to apply a frequency domain analysis to the self-mixing interferometry signal and may determine the gesture of the user based on the change in at least one property of the operational parameter of the self-mixing interferometry signal, as determined by the frequency domain analysis. 
     The electronic device may be at least one of an electronic watch or a mobile phone. 
     In some embodiments, an electronic device may comprise a self-mixing interferometry sensor comprising a light emitter configured to emit a beam of light and a photodetector configured to receive a returned portion of the beam of light. The electronic device may further comprise a set of one or more optical components positioned over an aperture of the light emitter. The set of one or more optical components may define a first optical path configured to propagate the emitted beam of light between a first receiving location and a first emission location and a second optical path configured to propagate the returned portion of the beam of light between a second receiving location and a second emission location. The first optical path may be different from the second optical path. 
     In some embodiments, the set of one or more optical components may comprise a first birefringent wedge, a second birefringent wedge, and a Faraday rotator positioned between the first birefringent wedge and the second birefringent wedge. 
     The set of one or more optical components may further comprise a lens configured to collimate the beam of light as the beam of light enters the set of one or more optical components. The set of one or more optical components may further comprise a half-wave plate positioned between the Faraday rotator and the second birefringent wedge. In some embodiments, the set of one or more optical components may be a passive, non-reciprocal meta-structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to representative embodiments illustrated in the accompanying figures. It should be understood that the following descriptions are not intended to limit the embodiments to one or more preferred embodiments. To the contrary, they are 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. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. 
         FIG.  1 A  depicts a vertical cavity surface emitting laser (VCSEL) diode used to transmit a beam of light toward a target. 
         FIG.  1 B  depicts a VCSEL and an integrated photodetector used to emit a beam of light toward a target and to perform a self-mixing operation. 
         FIG.  1 C  depicts a VCSEL used to transmit a beam of light toward a target and an associated photodetector used to receive light reflected from the target, where the VCSEL and the photodetector are present at different locations. 
         FIG.  2 A  depicts a diagram of a self-mixing interferometry operation. 
         FIG.  2 B  depicts a block diagram of a circuit operable to implement analyses for determining inputs using self-mixing interferometry. 
         FIG.  3 A  depicts a self-mixing interferometry sensor and non-reciprocal paths for a transmitted beam of light and a received beam of light. 
         FIG.  3 B  depicts a self-mixing interferometry sensor and non-reciprocal paths for a transmitted beam of light and a received beam of light where the detected object is a periodic structure. 
         FIG.  3 C  depicts a self-mixing interferometry sensor and non-reciprocal paths for a transmitted beam of light and a received beam of light where the detected object is a number of particles. 
         FIG.  3 D  depicts a self-mixing interferometry sensor, including non-reciprocal paths for a transmitted beam of light and a received beam of light, and including a first location where the transmitted beam of light contacts a target and a second location where the received beam of light returns from the target. 
         FIG.  3 E  depicts a non-confocal sensing system including a self-mixing interferometry sensor and non-reciprocal paths for a transmitted beam of light and a received beam of light where the detected object is one of a number of particles. 
         FIG.  4 A  depicts a beam of light with a vertical polarity transmit from a self-mixing interferometry sensor, through a birefringent circulator, and toward a target. 
         FIG.  4 B  depicts a beam of light reflected from a target, through a birefringent circulator, and toward a self-mixing interferometry sensor, where the received beam of light has the same polarity as the polarity transmit from the self-mixing interferometry sensor in  FIG.  4 A . 
         FIG.  4 C  depicts a beam of light reflected from a target, through a birefringent circulator, and toward a self-mixing interferometry sensor, where the received beam of light has a polarity different from the polarity transmit from the self-mixing interferometry sensor in  FIG.  4 A . 
         FIG.  5 A  depicts a beam of light with a horizontal polarity transmit from a self-mixing interferometry sensor, through a birefringent circulator, and toward a target. 
         FIG.  5 B  depicts a beam of light reflected from a target, through a birefringent circulator, and toward a self-mixing interferometry sensor, where the received beam of light has a polarity different from the polarity transmit from the self-mixing interferometry sensor in  FIG.  5 A . 
         FIG.  5 C  depicts a beam of light reflected from a target, through a birefringent circulator, and toward a self-mixing interferometry sensor, where the received beam of light has the same polarity as the polarity transmit from the self-mixing interferometry sensor in  FIG.  5 A . 
         FIG.  6 A  depicts an optical structure for the non-reciprocal treatment of light. 
         FIG.  6 B  depicts an optical structure for the non-reciprocal treatment of light, where the transmitted light has a polarization different from a polarization of the light transmit in  FIG.  5 A . 
         FIG.  7    depicts an example embodiment of a gesture recognition system using non-reciprocal self-mixing interferometry. 
         FIG.  8 A  depicts an example non-reciprocal self-mixing interferometry sensor incorporated into a mobile phone. 
         FIG.  8 B  depicts an example non-reciprocal self-mixing interferometry sensor incorporated into an electronic watch. 
         FIG.  8 C  depicts an example non-reciprocal self-mixing interferometry sensor incorporated into an earbud. 
         FIG.  9    depicts a self-mixing interferometry sensor including a non-reciprocal path for preventing overlapping side lobe interference for an optical phase array. 
         FIG.  10    depicts a birefringent meta-structure as a non-reciprocal optical element. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof), and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     The following disclosure relates to self-mixing interferometry sensors such as may be used as, for example, touch, biometric, and/or input sensors. In particular, a self-mixing interferometry sensor may utilize and/or include a set of one or more optical components to create non-reciprocal light paths to transmit and receive light with respect to a light source (e.g., a laser light emitter). As used herein, a self-mixing interferometry sensor may include a light source and/or a photodetector. For example, self-mixing interferometry sensors may use vertical cavity surface emitting laser (VCSEL) diodes, as a non-limiting example of a light source, and associated resonance cavity photodetectors (RCPDs), as a non-limiting example of a photodetector. An electronic device may use a self-mixing interferometry sensor as part of detecting, for example, a presence, proximity, size, displacement, distance, motion, speed, or velocity of surface or subsurface features of a target. In accordance with the provided disclosure, a self-mixing interferometry sensor may additionally be used to measure physical properties of surface or subsurface features. For example, a self-mixing interferometry sensor may be used to measure surface or subsurface reflectivity; microstructure texture and/or roughness; a particle density and/or size; a polarization state; and so on. As used herein, the phrase “target” may be used to refer to any structure toward which a self-mixing interferometry sensor transmits light for the purpose of sensing one or more parameters of the targets. As used herein, targets may include various input surfaces or objects. Hereinafter, for convenience, all possible measured parameters will be referred to as “detected properties.” 
     Such self-mixing interferometry sensors detect the detected properties of a target by causing one or more light sources (e.g., a laser light emitter) to emit a beam of phase-coherent light (e.g., a laser light) toward the target by applying a forward bias and by receiving a returned portion of the emitted beam of phase-coherent light back into a cavity of the light source component of the self-mixing interferometry sensor. A beam of phase-coherent light emitted from the light source may be referred to as transmitted light or a transmitted beam of light. The transmitted light may be returned (e.g., reflected or scattered) from the target and may be received back into a cavity of the light source and/or the self-mixing interferometry sensor. As used herein, returned light may include light reflected and/or scattered from a target and may be referred to as a returned portion of a beam of light, a returned beam of light, returned light, received light, and/or a received beam of light. 
     The returned light received by the self-mixing interferometry sensor may result in a self-mixing interference operation in which a first optical frequency of emitted light may be different than a second optical frequency of returning light. Self-mixing interference between intra-cavity light and returning light may result in a beating radio frequency that is equal to the difference between the first and the second optical frequencies. The beating radio frequency may be measured and/or analyzed and may correspond to the detected properties of the target. 
     In some embodiments, the target may have strongly retro-reflective components and may result in a large, or entire, portion of the returned light being directed back to the self-mixing interferometry sensor. In these embodiments, the retro-reflective components may result in a strong self-mixing signal, with respect to noise from, for example, aggressor circuitry, and a high degree of accuracy for detected properties may be obtained. In some embodiments, a strength of a self-mixing signal may be configured to be below a threshold value, as a self-mixing signal that is above the threshold value may interfere with operations of an associated light source. 
     In some embodiments, the target may be reflective and/or otherwise not strongly retro-reflective. However, in instances where the target is not strongly retro-reflective, the reflected light may be scattered in a large number of directions and the light source of the self-mixing interferometry sensor may receive an insufficient portion of the returned light. In these situations, a self-mixing interference operation may result in readings that inaccurately or irregularly correspond to the detected properties of the target. For example, a returned beam of light may be reflected at an angle different than an angle of incidence for the transmitted beam of light, resulting in an insufficient portion of the returned beam of light returning to the self-mixing interferometry sensor. To ensure that portions of such returned beams of light do sufficiently return to the self-mixing interferometry sensor, non-reciprocal optics may be provided as discussed herein. 
     In some embodiments, an optical circulator may be provided between a light source and a target. The optical circulator may include optical elements such that at least a portion of a transmitted beam of light follows along a path different from at least a portion of a returned beam of light. In some examples, the respective beams of light may enter or exist at different locations on an optical circulator. In this way, returned light may be collected and directed back toward the light source even if reflected at an angle different than an angle of incidence for the transmitted beam of light. As used herein, the optical circulator may be referenced as an optical component. 
     In some embodiments, an optical circulator may be defined as a multi-port (e.g., more than two ports) non-reciprocal component configured to transmit beams of light to different ports based on properties of the beams of light (e.g., a polarization or a wavelength). In some embodiments, an optical circulator may be a birefringent circulator and may include at least one birefringent crystal. A birefringent circulator may be used to direct the transmitted beam of light of a certain polarization toward one location on the target and to collect the returned beam of light of the same polarization as originating from a second location on the target. For example, some types of targets may be partially transparent to certain forms of light. The light that penetrates the target may follow a multi-path scattering trajectory at or below a surface of the target. Light, including a mixture of light polarizations, may then scatter at or below a surface of the target at a departing position different than the initial incident position. Without an optical or birefringent circulator, the light, including various polarizations, scattered from the departing position may not be collected by the self-mixing interferometry sensor or may be overwhelmed by light reflected from undesired sensing locations. For example, if features below a surface of a target are desirably detected, light reflected from positions on the surface of the target may overwhelm the light reflected from below-surface features. In embodiments with an optical or birefringent circulator, the optical or birefringent circulator may assist in redirecting the returned light toward the self-mixing interferometry sensor or in rejecting light reflecting from positions on a surface of a target, in situations where below-surface features are desirably detected. 
     By using non-reciprocal light paths, self-mixing interferometry operations may be performed even in situations where the returned beam of light is not retro-reflectively directed back to the self-mixing interferometry sensor. In this way, self-mixing interferometry operations may be performed on, for example, partially transparent; irregular; rough; and/or reflective targets. In the event that the transmitted beam of light fully or partially penetrates the target, objects below the top surface of the target (e.g., sub-surface targets) may be detected due to a lack of interference between the transmitted and returned light paths. 
     These and other embodiments are discussed with reference to  FIGS.  1 A- 10   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
       FIG.  1 A  depicts a self-mixing interferometry system  100  that uses a VCSEL  102  configured to transmit a beam of light  106  toward a target  110 . In the embodiment depicted in  FIG.  1 A , the beam of light  106  may be emitted in a cone (e.g., the light expands as it extends past the VCSEL  102 ). In some embodiments, the beam of light  106  may be substantially collimated or focused via, for example, a lens. The VCSEL  102  may emit the beam of light  106  under a forward current bias. To impart the forward current bias I BIAS    104 , a bias voltage may be supplied to the VCSEL  102 . In this way, a laser light emission (e.g., a beam of light) from the VCSEL  102  may be induced. 
     The beam of light  106  may be reflected from the target  110  as a number of reflections  112 . The target  110  may be reflective and/or diffusively reflective and may return the reflections  112  along any angle and/or direction. In some examples, a portion of the reflections  112  may be redirected back into the VCSEL  102  while another portion may be reflected away from the VCSEL  102 . In alternative or additional embodiments, the target  110  may be retro-reflective, or have partially retro-reflective properties, and may direct a majority of the reflections  112  back toward the VCSEL  102 . As described herein, light returned back toward the VCSEL  102  may be referred to as returned light and/or returned beams of light and may include light reflected and/or scattered from the target  110  either as reflections  112  or otherwise. 
     Some, or all, of the reflections  112  may be received back into the VCSEL  102  and, in particular, into a lasing cavity of the VCSEL  102 . Once received into the lasing cavity, a self-mixing interferometry operation may occur and a property of the beam of light  106  may be altered. In some embodiments, an electrical property of the VCSEL  102  may vary in response to a self-mixing interferometry operation. For example, a voltage monitor  108  may detect changes in a junction voltage of the VCSEL  102  that correlate with a distance or motion of the target  110 , with respect to the VCSEL  102 . 
       FIG.  1 B  depicts a self-mixing interferometry system  120  that uses a VCSEL  122  configured to emit a beam of light  126  toward a target  130 . As discussed with respect to  FIG.  1 A , the VCSEL  122  may emit the beam of light  126  under a forward current bias I BIAS    124  and may have a bias voltage supplied to the VCSEL  122 . Further, reflections  132  may be reflected back toward the VCSEL  122  where a self-mixing interference operation may occur. 
     A photodetector  136  may be integrated with the VCSEL  122  and may be positioned beneath the VCSEL  122 . As depicted, the VCSEL  122  and the photodetector  136  may be stacked on top of one another. In some embodiments, the VCSEL  122  may have one or more fixed or switchable linearly polarized outputs and the photodetector  136  may have one or more fixed or switchable linear polarizers. 
     The photodetector  136  may detect an operational change in the VCSEL  122  due to a self-mixing interference operation. For example, the VCSEL  122  may emit a portion of the laser light downwards into the photodetector  136  in addition to the transmitted beam of light  126 . Any alterations in the light emitted by the VCSEL  122 , such as alterations resulting from a self-mixing interference operation, may additionally be input to the photodetector  136 . A current monitor  134  may measure a bias current I PD  of the photodetector  136  which may be used to detect properties of the target  130 . 
       FIG.  1 C  illustrates a self-mixing interferometry system  140  that uses a VCSEL  142  configured to emit a beam of light  146  toward a target  150 . An intervening glass layer  158  may be provided to permit some reflections  151  to be directed toward an associated photodetector  156  before the beam of light  146  interacts with the target  150 . The reflections  151  may be used by the photodetector  156  as an initial reference in detecting possible self-mixing interferometry operations. A current monitor  154  may measure a bias current I PD  of the photodetector  156  which may be used to measure detected properties of the target  150 . As discussed with respect to  FIG.  1 A , the VCSEL  142  may emit the beam of light  146  under a forward current bias I BIAS    144  and may have a bias voltage supplied to the VCSEL  142 . 
     Though a VCSEL is described with respect to  FIGS.  1 A- 1 C , any type of light source may be used in accordance with the provided disclosure. In some embodiments, the VCSEL may be configured to perform a self-mixing interference operation only if the received light matches a polarity of the transmitted light. If the received light contains a mixture of light with various polarities, only the portion of light which shares a polarity of the transmitted light may be used in a self-mixing operation. 
       FIG.  2 A  depicts a self-mixing interferometry operation  200  with respect to a light source (e.g., a VCSEL active area). The descriptions provided herein are intended only to describe certain aspects of self-mixing interferometry needed to understand the disclosed embodiments. Other aspects of self-mixing interferometry will be understood by one of ordinary skill in the art. 
     In  FIG.  2 A , the laser cavity  206  has a fixed length, which may be established at a time of manufacture and may be bounded by a mirror  202  and a partially-reflective mirror  204 , and may emit a transmitted laser light  210  (e.g., an emission P 0 ) toward a target  216 . The transmitted laser light  210  may travel through a feedback cavity  208  and to the target  216 . The distance between the partially-reflective mirror  204  and the target  216  may be variable depending on the position of the laser cavity  206  with respect to the target  216 . In the example depicted in  FIG.  2 A , the feedback cavity  208  has a length L. 
     Once the transmitted laser light  210  contacts the target  216 , the laser light may be reflected as an optical feedback  212  back toward the laser cavity  206 . The optical feedback  212  may pass through the partially-reflective mirror  204  and may re-enter the laser cavity  206  where a self-mixing interferometry process occurs. As the optical feedback  212  coherently interacts with the cavity light, a new transmitted combined laser light  214  may be created. The transmitted combined laser light  214  may have characteristics (e.g., a wavelength and/or power) that differ from the initial characteristics of the transmitted laser light  210 . By performing certain analyses on the transmitted combined laser light  214  with respect to the transmitted laser light  210 , detected properties of the target  216  with respect to the laser cavity  206  may be determined. 
     In some examples, the electromagnetic radiation/beams of light emitted by a self-mixing interferometry sensor may be generated by an electromagnetic radiation source such as a vertical-cavity surface-emitting laser (VCSEL); an edge-emitting laser; a vertical external-cavity surface-emitting laser (VECSEL); a quantum-dot laser (QDL); a quantum cascade laser (QCL); a light-emitting diode (LED) (e.g., an organic LED (OLED)); a resonant-cavity LED (RC-LED); a micro LED (mLED); a superluminescent LED (SLED); any solid state, fiber or other laser; and so on. 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; and so on). The output of a self-mixing interferometry sensor (i.e., the self-mixing interferometry signal) may include a photocurrent produced by a photodetector (e.g., a photodiode), which photodetector is integrated with, or positioned under, above, or next to, the sensor&#39;s electromagnetic radiation source. Alternatively or additionally, the output of a self-mixing interferometry sensor may include a measurement of the current or junction voltage of the self-mixing interferometry sensor&#39;s electromagnetic radiation source. 
       FIG.  2 B  depicts a block diagram of a system  250  that may implement a self-mixing interferometry operation and associated analyses. The system  250  may generate an initial digital signal and may process the digital signal to produce a triangle-modulated laser bias current  252  as an input to a bias current of a VCSEL  260 . In an illustrated example, an initial step signal may be produced by a digital generator to approximate a triangle function (e.g., the triangle-modulated laser bias current  252 ). The triangle-modulated laser bias current  252  may be used as an input to a digital-to-analog (DAC) converter  254 . The resulting voltage signal may then be filtered by a low-pass filter  256  to remove quantization noise. Alternatively, an analog signal generator may be used to generate an equivalent triangle voltage signal directly. The filtered voltage signal may then be input to a voltage-to-current converter  258  to produce the desired triangle-modulated laser bias current  252  in a form for input to the VCSEL  260 . 
     As described above, reflections from a target may cause a self-mixing interferometry operation in the VCSEL  260  that results in an altered operational parameter of the VCSEL  260 . This alteration may be measured or inferred, either from an operational parameter of the VCSEL  260  or from an operational parameter of an associated photodetector. These alterations may be measured to produce a signal  262  (e.g., a combination of the triangle-modulated laser bias current  252  and a self-mixing signal). The signal  262  may have been measured by a photodetector and may be a triangle wave combined with a smaller and higher frequency signal related to the changes in the interferometric parameter. 
     The signal  262  may be passed through a high-pass filter  264 , which may convert the major ascending and descending ramp components of the signal  262  to DC offsets. As the signal  262  may be a current signal from a photodetector, a trans-impedance amplifier  266  may produce a corresponding voltage output for further processing. 
     The voltage output may then be sampled and quantized by an analog-to-digital converter (ADC)  268 . Before immediately applying a digital fast Fourier transform (FFT) to the output of the ADC  268 , an equalization may be applied in order to clear remaining residue of the triangle signal received by the photodiode, thus isolating the interferometric signal. The initial digital signal values from the digital generator used to produce the triangle modulated laser bias current  252  may be used as an input to a digital high-pass filter  270  to produce a digital signal to correspond to the output of the ADC  268 . An adjustable gain may be applied by a digital variable gain circuit  272  to the output of the digital high-pass filter  270 . 
     The output of the digital variable gain circuit  272  may be used as one input to the digital equalizer and subtractor  274 . The other input to the digital equalizer and subtractor  274  may be the output of the ADC  268 . The two signals may be differenced and may be used as part of a feedback to adjust a gain provided by the digital variable gain circuit  272 . 
     Once a sufficient correlation is obtained by the feedback, an FFT and tone extraction circuit  276  may apply an FFT to the output of the digital equalizer and subtractor circuit  274 . From the FFT spectra obtained, a property of the target (e.g., an absolute distance and directional velocity) may be inferred as indicated by block  278 . During the FFT and tone extraction circuit  276  step, processing electronics associated with the self-mixing interferometry operation  200  may perform a frequency domain analysis on the signal received from the digital equalizer and subtractor  274 . The frequency domain analysis may isolate signals corresponding to a change in an operational parameter of the VCSEL  260  and may be used to measure real-world events (e.g., a gesture, a distance between a target and a VCSEL, a speed of the target or the VCSEL, and so on). 
     The system  250  depicted in  FIG.  2 B  is just one example of a potential system that implements a self-mixing interferometry operation and performs associated analyses. In additional or alternative embodiments, additional, fewer, or alternate circuits, converters, filters, currents, diodes, and so on, may be provided. For example, the laser bias current  252  provided as an input to a bias current of a VCSEL  260  is described above as a triangle-modulated current. In some embodiments, a laser bias current may be provided in a different waveform such as, for example, a sinusoidal, square, or sawtooth waveform, and so on. In this way, a VCSEL  260  may be driven by any number of waveforms and/or laser bias currents. As another example, a self-mixing signal may be analyzed in a way different than that shown in  FIG.  2 B . For example, a self-mixing signal may be interrogated and analyzed in a time domain. The above examples are merely provided as exemplary systems and any method or system for processing and/or analyzing self-mixing signals may be used in accordance with the provided disclosure. 
       FIG.  3 A  may include the same kind of self-mixing interferometry sensors discussed with respect to  FIGS.  1 A- 2 B  as discussed above.  FIG.  3 A  depicts a self-mixing interferometry system  300  where a self-mixing interferometry sensor  302  transmits a transmitted laser light  306  along an illumination path  306   a  and receives a received laser light  308  from a collection path  308   a . The illumination path  306   a  and the collection path  308   a  may have non-reciprocal portions, as depicted in  FIG.  3 A . 
     The non-reciprocal illumination path  306   a  and collection path  308   a  may be created by a set of one or more optical components, such as an optical circulator, birefringent optics, a birefringent optical circulator, and so on. As discussed herein, any type or collection of appropriate optics may be used to create a non-reciprocal path. In the depicted embodiment, birefringent optics  304  (e.g., a birefringent circulator) may be used to create non-reciprocal paths. In some examples, the illumination path  306   a  may end at the same location where the collection path  308   a  begins (e.g., the self-mixing interferometry system  300  is confocal). As used herein, the birefringent optics  304  may be positioned over an aperture of the self-mixing interferometry sensor  302  and may be used to direct emitted or received light. 
     The birefringent optics  304  may be formed of a material, or combination of materials, with a refractive index that depends on the polarization and/or propagation direction of light. In the example depicted in  FIG.  3 A , the birefringent optics  304  may have a first refractive index in the direction of travel of the transmitted laser light  306  and a second refractive index in the direction of travel of the received laser light  308 . As used herein, the direction of travel of the transmitted laser light  306  may be referred to as a first direction of travel and the direction of travel of the received laser light  308  may be referred to as a second direction of travel. In this way, the paths (e.g., the illumination path  306   a  and the collection path  308   a ) may differ for transmitted and received laser light. The birefringent optics  304  are described in more detail with respect to, for example,  FIGS.  4 A- 6 C . In some embodiments, the birefringent optics  304  may be referred to as a birefringent circulator and/or a birefringent lens. In some embodiments, the birefringent optics  304  may include various lenses to collimate or otherwise focus received or transmitted light. 
     In some embodiments, the birefringent optics  304  may be configured to positive optical power as a finite conjugated optical system with a first aperture on a side closest to a self-mixing interferometry sensor  302  and with a second aperture closest to the target  310 . 
     The self-mixing interferometry sensor  302  may initially transmit a beam of laser light  306 . Before reaching the birefringent optics  304 , the transmitted laser light  306  may follow a substantially straight path. Once the transmitted laser light  306  is received by the birefringent optics  304 , a first refractive index of the birefringent optics  304  (e.g., a refractive index in a first direction) may result in the refraction of the transmitted laser light  306  as the transmitted laser light  306  moves through the birefringent optics  304 . In  FIG.  3 A , the illumination path  306   a  of the transmitted laser light  306  is depicted substantially linearly with various interspersed angles. This depiction is intended for graphical simplicity and any illumination path  306   a  of the transmitted laser light  306  is possible. 
     The transmitted laser light  306  may enter the birefringent optics  304  at a first receiving location on the birefringent optics  304  and may travel along a first optical path within the birefringent optics  304 . The transmitted laser light  306  may then exit the birefringent optics  304  at a first emission location. 
     Once the transmitted laser light  306  exits the birefringent optics  304  (e.g., at a first aperture on the birefringent optics  304 ), the transmitted laser light  306  may be directed toward a point P 1  on a target  310 . As such, the illumination path  306   a  of the laser light and the collection path  308   a  of the reflected laser light may converge (or be commonly focused) at point P 1 . In some embodiments, the illumination path  306   a  and the collection path  308   a  may be focused at a common point that is behind or in front of the target  310  instead of a point P 1  positioned on the target. The point P 1  may be referred to as a common detection space and may be a location on, behind, or in front of the target  310 . In some embodiments, the common detection space may be a sensing location. 
     The target  310  may be at least partially reflective and may reflect the transmitted laser light  306  toward a direction at an angle different than the angle of incidence of the transmitted laser light  306 , so as to enter the birefringent optics  304  at a second aperture different than the first aperture. Surface features present on the target  310  (e.g., grooves or protrusions) may be irregularly spaced and/or may be presented at different angles. As such, the reflected light may be reflected in a direction away from the location where the transmitted laser light  306  exited the birefringent optics  304 . Therefore, the transmitted laser light  306  may enter the birefringent optics  304  at a first angle different than a second angle of a received laser light  308  as the received laser light  308  enters the birefringent optics  304 . 
     The reflected light may be referenced as returned laser light  308  and may travel along a collection path  308   a . The returned laser light  308  may travel toward the birefringent optics  304  and may enter the birefringent optics  304  at an aperture (e.g., a second aperture) different than the aperture where the transmitted laser light  306  exited the birefringent optics  304  (e.g., a first aperture). However, due to the nature of the birefringent optics  304 , a refractive index of the birefringent optics  304  along a direction of travel of the returned laser light  308  may be different than the refractive index in the opposite direction (e.g., as experienced by the transmitted laser light  306 ). Therefore, the birefringent optics  304  may guide the returned laser light  308  back toward the self-mixing interferometry sensor  302  and the received laser light  308  may undergo a self-mixing process when received by the self-mixing interferometry sensor  302 , as discussed herein. In this way, partially or completely non-reciprocal paths may be used in a self-mixing operation. 
     The returned laser light  308  may enter the birefringent optics  304  at a second receiving location on the birefringent optics  304  and may travel along a second optical path within the birefringent optics  304 . The transmitted laser light  306  may then exit the birefringent optics  304  at a second emission location. As depicted in  FIG.  3 A , the first optical path may be different from the second optical path. 
     Various applications may be used in accordance with the confocal example depicted in  FIG.  3 A . 
     For example, a surface roughness of the target  310  may be measured based on properties of the returned beam of light  308 . The surface roughness of the target  310  may be determined, for example, when the roughness of the target (e.g., root mean squared (RMS) roughness) is higher than a one-half wavelength value, equal to a one-half wavelength value, or less than a one-half wavelength value by utilizing a self-mixing specklegram. As a speckle topography (e.g., a roughness of an input surface) may result in light reflecting at a number of different angles, a self-mixing interferometry sensor with circulator optics, as described herein, may be used to measure a surface roughness even when the roughness is higher than a half-wavelength of light. In this way, a speckle sensitivity range may be increased. 
     In alternative or additional examples, a target may be a two-dimensional or three-dimensional periodic structure such as, for example, a diffractive grating. The system  320  depicted in  FIG.  3 B  illustrates an example of such an arrangement. The system  320  includes a self-mixing interferometry sensor  302  and birefringent optics  304  as discussed with reference to  FIG.  3 A . Likewise, the system  320  may include transmitted laser light  326  following an illumination path  326   a  and returned laser light  328  following a collection path  328   a . Aspects of the transmitted laser light  326  and the returned laser light  328  may be similar to the respective transmitted laser light and returned laser light discussed above. 
     A target  330  may have a periodic structure, as depicted. Such a periodic structure may have repeating features such as ridges, protrusions, and/or valleys. To measure certain detected properties of such a periodic structure, an angular difference between an illumination path  326   a  and a collection path  328   a  may be selected in accordance with repeating features of the periodic structure (e.g., a diffraction order of a diffractive grating). By selecting corresponding angular differences and periodic structures, non-reciprocal sensing paths may be used to obtain high fidelity self-mixing measurements of detected properties. In this way, a periodicity of the target may be measured. 
     The point P 2  at which the transmitted laser light  326  and the returned laser light  328  meet on the target  330  may be positioned at any location on the target  330 . In some embodiments, the point P 2  may be positioned on a slope of the target  330  and an angle of the illumination path  326   a  and the collection path  328   a  may correspond to the slope of the target  330 . In some embodiments, a location of the point P 2  may change as the target  330  moves relatively from the self-mixing interferometry sensor  302 . 
     In alternative or additional examples, a target, instead of being an object as depicted in  FIGS.  3 A and  3 B , may be a number of particles (e.g., smoke particles, red blood cells, and so on). For example,  FIG.  3 C  depicts a system  340  where a self-mixing interferometry sensor  302  and birefringent optics  304  operate to direct and receive non-reciprocal light paths toward a number of particles  350 . The number of particles  350  may include any particle or collection of particles, either uniform particles or mixed particles, and may include, for example, cells; smoke particles; dust; water particles; and so on. The number of particles  350  may further have any number of densities; physical properties; concentration; and so on. 
     As discussed, the system  340  includes a self-mixing interferometry sensor  302  and birefringent optics  304  as discussed with reference to  FIG.  3 A . Likewise, the system  340  may include transmitted laser light  346  following an illumination path  346   a  and returned laser light  348  following a collection path  348   a . Aspects of the transmitted laser light  346  and the returned laser light  348  may be similar to the respective transmitted laser light and returned laser light discussed above. 
     As the number of particles  350  may exist in a three-dimensional space, a first subset of the particles may exist at or pass through a point P 3  (e.g., a confocal point where the illumination path  346   a  and the collection path  348   a  meet) and a second subset of particles that do not exist at or pass through point P 3 . The first subset of particles may be considered “in-focus particles” and the second subset of particles may be considered “out-of-focus particles.” In self-mixing systems without non-reciprocal optics (e.g., systems completely overlapping illumination and collection paths), it may be difficult to differentiate between in-focus particles and out-of-focus particles as, for example, in-focus particles with a small size and low reflectivity may result in self-mixing signals similar to those produced from out-of-focus particles with a large size and high reflectivity. However, in the system depicted in  FIG.  3 C , since the illumination path  346   a  and the collection path  348   a  only overlap at point P 3  (e.g., the confocal point), in-focus particles (e.g., particles at and/or passing through point P 3 ) may be distinguishable from out-of-focus particles due to different directions and/or polarity of reflected light. 
     Light beams  356  and  358  depict the continuation of beams  346  and  348  in the event that beams  346  and  348  are not blocked by a particle of the number of particles  350 . While a portion of light beams  358  and  356  may return to the birefringent optics  304  after interacting with any number of particles (e.g., out-of-focus particles), the signal generated in response to out-of-focus particles may be distinguishable from signals generated in response to in-focus particles, as described above. 
       FIG.  3 D  may contain substantially similar structures as discussed with reference to  FIGS.  3 A- 3 C . Notably,  FIG.  3 D  depicts a non-confocal arrangement, where the illumination path  366   a  and the collection path  368   a  impact an object at different points, a first point P A  and a second point P B . Light may travel through a curved path  372  underneath a surface of an object before being emitted from the object at a different location.  FIG.  3 D  depicts a self-mixing interferometry system  360  where a target  370  may be partially transparent with respect to at least a portion of transmitted laser light  366  and may have a number sub-surface features below a surface of the target  370 . For example, the target  370  may be a portion of a human body such as a layer of skin. Objects below the target  370  may include blood vessels, tissue, and so on. In this embodiment, transmitted laser light  366  may be transmitted from the self-mixing interferometry sensor  302 , may be directed toward a point P A  and returned laser light  368  may be reflected from a point P B . The transmitted laser light  366  may follow an illumination path  366   a , may reach point P A , may travel within a portion underneath the target  370 , along a curved path  372  (e.g., a “banana” path), and may exit the target  370  at point P B . After leaving point P B , returned laser light  368  may follow a collection path  368   a  and may return through the birefringent optics  304  to the self-mixing interferometry sensor  302 . As used herein, P A  may be referred to as a first location and P B  may be referred to as a second location. In some embodiments, P A  and P B  may be underneath a surface of the target  370 , in front of the target  370 , or may be positioned along the target  370 . As used herein, the birefringent optics  304  may be positioned over an aperture of the self-mixing interferometry sensor  302  and may be used to direct emitted or received light. 
     In the self-mixing interferometry system  360 , a property of sub-surface features underneath the target  370  may be measured. For example, a displacement or movement of blood flowing through veins may be measured by self-mixing interferometry operations so that, for example, a user&#39;s heart rate; blood oxygenation level; blood pressure; and so on may be determined. In some examples, a user&#39;s body mass index (BMI) may be measured by modelling reflective/absorption properties of different kinds of cells (e.g., fat cells or muscle cells). In alternate or additional examples, bone and/or muscle movement may be detected by a self-mixing interferometry system, such as described herein, and may be used to determine a gesture of a user&#39;s hand, wrist, arm, and so on. In this way, signal saturation from surface reflection off of a user&#39;s skin may be eliminated or reduced. 
     In some embodiments, the self-mixing interferometry system  360  may measure a Doppler frequency shift beneath the target  370  (e.g., with blood or tissue beneath a user&#39;s skin). As the light travels along the curved path  372 , a property of the light may change. This changing property may be a changing frequency of light thereby resulting in a Doppler frequency shift. The returning light may carry this Doppler shift and the self-mixing interferometry sensor  302  may perform self-mixing analyses to measure sub-dermal measurements such as a blood flow rate (e.g., a blood flowmetry) By this arrangement, any fluid may be measured (e.g., a flowmetry of any fluid may be measured). For example, a water flow, a stomach acid flow, and the like may be measured either as a sub-dermal feature or as a surface measurement. 
     As an additional or alternative embodiment, a photoplethysmogram (PPG) may be measured as a sub-dermal measurement. A PPG may refer to an optically obtained plethysmogram that detects a blood volume change with a bed of tissue. By detecting blood volume changes, information about a user&#39;s cardiovascular system may be obtained. The PPG measurement may be obtained directly from blood volume or may be inferred from related features (e.g., skin) that absorbs and/or reflects different amounts of light depending on an amount of blood volume. 
       FIG.  3 E  depicts a system  380  for an additional non-confocal arrangement where transmitted laser light  386  impacts a particle from a number of particles  390  at a first point P c  and received laser light  388  is emitted from the particle at a second point P D . The system  380  may include a self-mixing interferometry sensor  302 , birefringent optics  304 , transmitted laser light  386  following an illumination path  386   a , and received laser light  388  following a collection path  388   a.    
     As discussed in the non-confocal system of  FIG.  3 D , a curved path  392  extending through an object may connect the illumination path  386   a  and the collection path  388   a . In the depicted example, the object may be a particle from a number of particles  390  and the curved path  392  may extend through a diameter of the particle. Based on a changing property of light as it travels through the curved path  392 , as discussed above, a property of the particle may be measured through self-mixing operations. For example, a diameter of the particle may be determined based on differences between the transmitted laser light  386  and the received laser light  388  as measured by the self-mixing interferometry sensor  302 . 
     Light beams  396  and  398  depict the continuation of beams  386  and  388  in the event that beams  386  and  388  are not blocked by a particle of the number of particles  390 . While a portion of light beams  396  and  398  may return to the birefringent optics  304  after interacting with any number of particles, the signal generated in response to out-of-focus particles may be distinguishable from signals generated in response to non-confocal detection, as described above. 
       FIG.  4 A  depicts a transmitted beam of light  406  emitted from a self-mixing interferometry sensor  402  with a vertical polarization  408   a  (e.g., a p-polarization). As used herein, the polarization  408   a  may be referenced as a first polarization. Though the initial polarity is described as a vertical polarization, any kind of polarization at any angle may be used in accordance with this disclosure. The self-mixing interferometry sensor  402  may be the same self-mixing interferometry sensors as discussed with reference to  FIGS.  1 A- 3 B  and the birefringent circulator  404  may likewise be the same type of optics as birefringent optics  304 , with respect to  FIGS.  3 A and  3 B . As used herein, the birefringent optics  404  may be positioned over an aperture of the self-mixing interferometry sensor  402  and may be used to direct emitted or received light. 
     In  FIG.  4 A , the transmitted beam of light  406  may move through the birefringent circulator  404 . The birefringent circulator  404  may include a first lens  412 , a first birefringent material  414 , a Faraday rotator  416 , a half-wave plate  418 , a second birefringent material  420 , and a second lens  422 . The included optics are merely exemplary and any optics or combination of optics may be used in accordance with the present disclosure. 
     The first lens  412  may collimate the transmitted beam of light  406  so that the beam length of the transmitted beam of light  406  is relatively consistent throughout the birefringent circulator  404 . The first lens  412  may also serve to focus a collimated beam toward the self-mixing interferometry sensor  402  when a beam of light travels in the opposite direction (see, e.g.,  FIG.  4 B ). The second lens  422  may focus a collimated beam to a particular point. In addition, the second lens  422  may collimate a beam when a beam is reflected off of a target  410 . The first lens  412  and the second lens  422  may be referred to as collimating lenses or merely lenses. Any suitable lens may be used for the first lens  412  and the second lens  422  such as, but not limited to, geometric and/or refractive lenses fabricated from bulk; flat optics (e.g., diffractive and meta-surface flat optics); and so on. 
     The first birefringent material  414  may be bonded, or otherwise in proximity, to the first lens  412  and may refract components of the transmitted beam of light  406 . As understood by one of ordinary skill in the art, birefringent materials may have different refractive properties depending on a polarization and/or propagation direction of light resulting in ordinary rays (o-rays) or extraordinary rays (e-rays). As such, the first birefringent material  414  may refract the transmitted beam of light  406  differently based on a polarization of the transmitted beam of light  406 . Additionally, a returned beam of light may be refracted differently than the transmitted beam of light  406 , such as discussed herein. Any material or combination of materials with birefringent properties may be used as the first birefringent material  414  and the second birefringent material  420 . The first birefringent material  414  and the second birefringent material  420  may be fabricated from bulk; liquid crystals; meta-surfaces or meta-structures; and so on. 
     A Faraday rotator  416  may be bonded, or otherwise in proximity, to the first birefringent material  414 . Though described as a Faraday rotator, any manner of rotator may be used in accordance with the disclosure. The Faraday rotator  416  may be a polarization rotator and may change a polarization of the transmitted beam of light  406  as the transmitted beam of light  406  moves through the Faraday rotator  416 . The Faraday rotator  416  may use a magnet (e.g., a permanent magnet) and/or electromagnetic mechanisms to affect a polarization of the transmitted beam of light  406 . The angle of polarization rotation may be calculated by the equation β=VBd, where β is the angle of rotation, V is the Verdet constant for the material of the Faraday rotator  416 , B is the magnetic flux density in the direction of propagation, and d is the length of the Faraday rotator  416 . 
     The Faraday rotator  416  may be a non-reciprocal optical element. For example, a beam of light transmit in one direction through the Faraday rotator  416  may be initially rotated 20 degrees clockwise. If the rotated beam is reflected and re-enters the Faraday rotator  416  from an opposite direction, the initial rotation may not be canceled (e.g., the beam of light is not rotated 20 degrees counter-clockwise) but may be doubled (e.g., the beam of light is additionally rotated 20 degrees clockwise, such that the complete rotation of the beam of light is 40 degrees clockwise with respect to the transmitted beam of light). This property will be discussed in further detail with respect to  FIGS.  6 A and  6 B . 
     A half-wave plate  418  may be bonded, or otherwise in proximity, to the Faraday rotator  416 . The half-wave plate  418  may be any type of wave plate and may affect a polarization of the transmitted beam of light  406 . For example, the half-wave plate  418  may have a fast axis of 67.5 degrees, and any fast axis or other optical property may be used in accordance with the provided disclosure. Additionally, other types of wave plates may be used including, but not limited to, a quarter-wave plate or a full-wave plate. Unlike the Faraday rotator  416 , the half-wave plate may be reciprocal and may cancel or revert an applied polarization when a beam of light returns through another side of the half-wave plate  418 . In some embodiments, the half-wave plate may have an optical axis between 60 degrees and 75 degrees and/or aligned to a polarization of the transmitted beam of light  406 . In some embodiments, the half-wave plate may have any possible optical axis or other optical property. 
     The second birefringent material  420  may be formed of the same material or combination of materials as the first birefringent material  414  or may be formed of another material or combination of materials. The second birefringent material  420  may direct the transmitted beam of light  406  along a particular illumination path  406   a  depending on a polarity of the transmitted beam of light  406 . A second lens  422  may additionally be provided to focus the transmitted beam of light  406  onto a target  410 . 
     A polarization change of the transmitted beam of light  406  as the transmitted beam of light  406  travels along the illumination path  406   a  will now be described in accordance with the self-mixing interferometry system  400 . When the transmitted beam of light  406  is initially emitted from the self-mixing interferometry sensor  402 , the transmitted beam of light  406  may have a p-polarization  408   a . This p-polarization  408   a  may remain consistent (e.g., as p-polarization  408   b ) as the transmitted beam of light  406  travels through the first birefringent material  414  in one propagation direction and when the p-polarization  408   a  is aligned with an ordinary axis of the first birefringent material  414 . That is, an ordinary ray of the transmitted beam of light  406  may travel through the first birefringent material  414 , without refraction, as the p-polarization  408   a  is aligned with the ordinary axis of the first birefringent material  414 . The Faraday rotator  416  and the half-wave plate  418  may act in combination to change the p-polarization  408   b  into an s-polarization  408   c . This s-polarization  408   c  may remain consistent and may come into contact with the target as s-polarization  408   d . The illumination path  406   a  may be altered from the path of the transmitted beam of light  406  through the first birefringent material  414  when the s-polarization  408   c  is aligned with an extraordinary axis of the second birefringent material  420 . That is, an extraordinary ray of the transmitted beam of light  406  may travel through the second birefringent material  420 , as refracted, as the s-polarization  408   c  is aligned with the extraordinary axis of the second birefringent material  420 . Described simply and with reference to  FIG.  4 A , an ordinary ray component of the transmitted beam of light  406  is depicted as traveling through the first birefringent component  414  and an extraordinary ray component of the transmitted beam of light  406  is depicted as traveling through the second birefringent component  420 . As used herein, the polarization  408   d  may be referred to as a second polarization. 
       FIG.  4 B  depicts the self-mixing interferometry system  400 , as described with respect to  FIG.  4 A , at an instance after the transmitted beam of light reflects off of the target  410  and returns to the self-mixing interferometry sensor  402  as a returned beam of light  405 . The returned beam of light  405  may travel along a collection path  405   a . The illumination path  406   a  as described with respect to  FIG.  4 A  is depicted in phantom to facilitate a comparison between the illumination path  406   a  and the collection path  405   a . Identically numbered components present in  FIG.  4 B  are identical to those described herein. 
     Once the transmitted beam of light  406  reflects from the target  410 , the returned beam of light may comprise light with multiple polarization states (e.g., s-polarization and p-polarization states) from single and multi-pass reflection and light scattering at or below a surface of the target  410 .  FIG.  4 B  depicts the component of the returned beam of light  405  with p-polarization  408   e  and  FIG.  4 C  depicts the component of the returned beam of light  407  with s-polarization  408   i.    
     In  FIG.  4 B , the returned beam of light  405  reflects from the target  410  with a p-polarization  408   e . As the returned beam of light  405  moves along the collection path  405   a , the polarization stays as p-polarization as it moves through the second birefringent material  420  (e.g., p-polarization  408   f ), the first birefringent material  414  (e.g., p-polarization  408   g ), and as it is received by the self-mixing interferometry sensor  402  (e.g., p-polarization  408   h ). Though the returned beam of light  405  has a p-polarization along each of these markers, the returned beam of light  405  does not necessarily remain at a p-polarization throughout the entirety of the collection path  405   a . The half-wave plate  418  may shift the polarization of the returned beam of light  405  to a certain degree, though this shift may be canceled by the Faraday rotator  416  (see, e.g.,  FIGS.  6 A and  6 B ). As depicted in  FIG.  4 B , an ordinary ray component of the returned beam of light  405  does not refract as it moves through the first birefringent material  414  and the second birefringent material  420  as the ordinary ray component is aligned with the ordinary axes of each of the first birefringent material  414  and the second birefringent material  420 . 
     In some embodiments, a portion of the collection path  405   a  between the self-mixing interferometry sensor  402  and the first lens  412  may fully overlap with a portion of the illumination path  406   a  between the self-mixing interferometry sensor  402  and the first lens  412 . In this way, the returned beam of light  405  may return to the self-mixing interferometry sensor  402  with coherent mode/polarization matching with respect to the transmitted beam of light  406 . 
     Both the p-polarization  408   f  and the p-polarization  408   g  may align with the ordinary axis of both the first birefringent material  414  and the second birefringent material  420 . There may therefore be a region at or below a surface of the target  410  where the returned beam of light  406  travels along the collection path  405   a . This region at or below the surface of the target  410  may be a region of interest and may be a region where properties of or associated with the target  410  are detected. 
     In  FIG.  4 C , the returned beam of light  407  reflects from the target  410  with an s-polarization  408   i . As the returned beam of light  407  moves along the collection path  407   a , the polarization stays as s-polarization as it moves through the second birefringent material  420  (e.g., s-polarization  408   j ), the first birefringent material  414  (e.g., s-polarization  408   k ), and as it is received by the self-mixing interferometry sensor  402  (e.g., s-polarization  408   l ). Though the returned beam of light  407  has an s-polarization along each of these markers, the returned beam of light  407  does not necessarily remain at an s-polarization throughout the entirety of the collection path  407   a . The half-wave plate  418  may shift the polarization of the returned beam of light  407  to a certain degree, though this shift may be canceled by the Faraday rotator  416  (see, e.g.,  FIGS.  6 A and  6 B ). 
     As depicted in  FIG.  4 C , an extraordinary ray component of the returned beam of light  407  refracts as it moves through the first birefringent material  414  and the second birefringent material  420  as the extraordinary ray component is not aligned with the ordinary axes of each of the first birefringent material  414  and the second birefringent material  420 . The extraordinary ray component of the returned beam of light  407 , therefore, may move through both the first birefringent material  414  and the second birefringent material  420  as depicted in  FIG.  4 C . 
     Here, it is noted that the self-mixing interferometry sensor  402  may, in some embodiments, only be able to perform self-mixing processes if the polarization of the returned light matches the polarization of the transmitted laser light. In this case, the s-polarized light depicted in  FIG.  4 C  may enter the self-mixing interferometry sensor  402  but may not affect any self-mixing interference process (since light with p-polarization was initially transmit). In additional or alternative embodiments, the s-polarized light may be prevented from entering the self-mixing interferometry sensor  402  by, for example, a linear p-polarizer integrated to the self-mixing interferometry sensor. In some embodiments, different birefringent circulators may be used such that received light has the same polarization as the transmitted light in every circumstance. For example, a polarization insensitive optical circulator may be used, as understood by a person of ordinary skill in the art. The polarization  408   i  may be referred to as a third polarization and the polarization  408   l  may be referred to as a fourth polarization. 
     The difference between the illumination path  406   a , the collection path  405   a , and the collection path  407   a  may depend on polarization values of light traveling on the paths and a direction of propagation of the light through the birefringent circulator. For example, p-polarization may interact with an ordinary axis of birefringent elements to travel along a certain path. Similarly, s-polarization may interact with an extraordinary axis of birefringent elements to travel along another, at least partially distinct, path. Due to the birefringent properties discussed herein, non-reciprocal optics may be formed. Ordinary ray components and extraordinary ray components may move through each of the birefringent elements depending on whether the ray is aligned with an ordinary axis or an extraordinary axis of the birefringent elements. 
     Though the illumination and collection paths are depicted with certain boundaries, it is noted that these boundaries are for graphical purposes only. Any illumination or collection path from or to a light source, through a birefringent circulator, and to or from a target may be used in accordance with the present disclosure. 
       FIG.  5 A  depicts a transmitted beam of light  506  emitted from a self-mixing interferometry sensor  502  with a horizontal polarization (e.g., an s-polarization). Similar reference numbers in  FIGS.  5 A- 5 C  may correspond to their counterparts in  FIGS.  4 A- 4 C  and the structures depicted in  FIGS.  5 A- 5 C  may be substantially similar to those depicted in  FIGS.  4 A- 4 C . In some embodiments, the self-mixing interferometry system  500  may include substantially similar components as the self-mixing interferometry system  400 . 
     In  FIG.  5 A , the transmitted beam of light  506  may move through the birefringent circulator  504 . The birefringent circulator  504  may include a first lens  512 , a first birefringent material  514 , a Faraday rotator  516 , a half-wave plate  518 , a second birefringent material  520 , and a second lens  522 . The included optics are merely exemplary and any optics or combination of optics may be used in accordance with the present disclosure to produce non-reciprocal light paths. 
     The first lens  512  may collimate the transmitted beam of light  506  so that the beam length of the transmitted beam of light  506  is relatively consistent throughout the birefringent circulator  504 . The first lens  512  may also serve to focus a collimated beam toward the self-mixing interferometry sensor  502  when traveling in the opposite direction. The second lens  522  may focus a collimated beam to a particular point on the target  510 . In addition, the second lens  522  may collimate a beam when the beam is reflected off of the target  510 . The first and second lenses may be referred to as collimating lenses or merely lenses. Any suitable lens may be used for the first lens  512  and the second lens  522  such as, but not limited to, geometric and/or refractive lenses fabricated from bulk; flat optics (e.g., diffractive and meta-surface flat optics); and so on. 
     The first birefringent material  514  may be bonded, or otherwise in proximity, to the first lens  512  and may refract the transmitted beam of light  506 . As understood by one of ordinary skill in the art, birefringent materials may have different refractive properties depending on a polarization and/or propagation direction of light. As such, the first birefringent material  514  may refract the transmitted beam of light  506  differently based on a polarization of the transmitted beam of light  506 . Additionally, a returned beam of light may be refracted differently than the transmitted beam of light  506 . Any material or combination of materials with birefringent properties may be used as the first birefringent material  514  and the second birefringent material  520 . The first birefringent material  514  and the second birefringent material  520  may be fabricated from bulk; liquid crystals; meta-surfaces or meta-structures; and so on. 
     A Faraday rotator  516  may be bonded, or otherwise in proximity, to the first birefringent material  514 . Though described as a Faraday rotator, any manner of rotator may be used in accordance with the disclosure. The Faraday rotator  516  may be a polarization rotator and may change a polarization of the transmitted beam of light  506  as the transmitted beam of light  506  moves through the Faraday rotator  516 . The Faraday rotator  516  may use a magnet (e.g., a permanent magnet) and/or electromagnetic energy to affect a polarization of the transmitted beam of light  506 . The angle of polarization rotation may be calculated by the equation β=VBd, where β is the angle of rotation, V is the Verdet constant for the material of the Faraday rotator  516 , B is the magnetic flux density in the direction of propagation, and d is the length of the Faraday rotator  516 . 
     The Faraday rotator  516  may be a non-reciprocal optical element. For example, a beam of light transmit in one direction through the Faraday rotator  516  may be initially rotated 20 degrees clockwise. If the rotated beam is reflected and re-enters the Faraday rotator  516  from the other direction, the initial rotation is not canceled (e.g., the beam of light is not rotated 20 degrees counterclockwise) but may be doubled (e.g., the beam of light is additionally rotated 20 degrees clockwise, such that the complete rotation of the beam of light is 40 degrees clockwise with respect to the transmitted beam of light). 
     A half-wave plate  518  may be bonded, or otherwise in proximity, to the Faraday rotator  516 . The half-wave plate  518  may be any type of wave plate and may affect a polarization of the transmitted beam of light  506 . For example, the half-wave plate  518  may have a fast axis of 67.5 degrees. Additionally, other types of wave plates may be used including, but not limited to, a quarter-wave plate or a full-wave plate. Unlike the Faraday rotator  516 , the half-wave plate may be reciprocal and may cancel or revert an applied polarization when a beam of light returns through another side of the half-wave plate  518 . In some embodiments, the half-wave plate may have an optical axis between 60 degrees and 75 degrees and/or aligned to a polarization of the transmitted beam of light  506 . In some embodiments, the half-wave plate may have any optical axis. 
     The second birefringent material  520  may be formed of the same material or combination of materials as the first birefringent material  514  or may be formed of another material or combination of materials. The second birefringent material  520  may direct the transmitted beam of light  506  along a particular illumination path  506   a  depending on a polarity of the transmitted beam of light  506 . A second lens  522  may additionally be provided to focus the transmitted beam of light  506  onto a target  510 . 
     A polarization change of the transmitted beam of light  506  as the transmitted beam of light  506  travels along the illumination path  506   a  will now be described in accordance with the self-mixing interferometry system  500 . When the transmitted beam of light  506  is initially emitted from the self-mixing interferometry sensor  502 , the transmitted beam of light  506  may have an s-polarization  508   a . As used herein, the polarization  508   a  may be referred to as a first polarization. 
     The s-polarization  508   a  may remain consistent (e.g., as s-polarization  508   b ) as the transmitted beam of light  506  travels through the first birefringent material  514  in one propagation direction and when the s-polarization  508   a  is aligned with an extraordinary axis of the first birefringent material  514 . That is, an extraordinary ray of the transmitted beam of light  506  may travel through the first birefringent material  514 , as refracted, as the s-polarization  508   a  is aligned with the extraordinary axis of the first birefringent material  514 . The Faraday rotator  516  and the half-wave plate  518  may act in combination to change the s-polarization  508   b  into a p-polarization  508   c . This p-polarization  508   c  may remain consistent and may come into contact with the target as p-polarization  508   d . In this way, the illumination path  506   a  may be altered from the path of the transmitted beam of light  506  through the first birefringent material  514  when the p-polarization  508   c  is aligned with an ordinary axis of the second birefringent material  520 . That is, an ordinary ray of the transmitted beam of light  506  may travel through the second birefringent material  520 , without refraction, as the p-polarization  508   c  is aligned with the ordinary axis of the second birefringent material  520 . Described simply and with reference to  FIG.  5 A , an extraordinary ray component of the transmitted beam of light  506  is depicted as traveling through the first birefringent component  514  and an ordinary ray component of the transmitted beam of light  506  is depicted as traveling through the second birefringent component  520 . As used herein, the polarization  508   d  may be referred to as a second polarization. 
       FIG.  5 B  depicts the self-mixing interferometry system  500  at an instance after the transmitted beam of light reflects off of the target  510  and returns to the self-mixing interferometry sensor  502  as a returned beam of light  507 . The returned beam of light  507  may travel along a collection path  507   a . The illumination path  506   a  as described with respect to  FIG.  5 A  is depicted in phantom. Identically numbered components present in  FIG.  5 B  are identical to those described herein. 
     Once the transmitted beam of light  506  reflects from the target  510 , the returned beam of light may comprise light with multiple polarization states from single and multi-pass reflection and light scattering at or below a surface of the target  510 .  FIG.  5 B  depicts the component of the returned beam of light  507  with p-polarization  508   e  and  FIG.  5 C  depicts the component of the returned beam of light  509  with s-polarization  508   i.    
     In  FIG.  5 B , the returned beam of light  507  reflects from the target  510  with a p-polarization  508   e . As the returned beam of light  507  moves along the collection path  507   a , the polarization stays as p-polarization as it moves through the second birefringent material  520  (e.g., p-polarization  508   f ), the first birefringent material  514  (e.g., p-polarization  508   g ), and as it is received by the self-mixing interferometry sensor  502  (e.g., p-polarization  508   h ). Though the returned beam of light  507  has a p-polarization along each of these markers, the returned beam of light  507  does not necessarily remain at a p-polarization throughout the entirety of the collected path  507   a . The half-wave plate  518  may shift the polarization of the returned beam of light  507  to a certain degree and this shift may be canceled by the Faraday rotator  516  (see, e.g.,  FIGS.  6 A and  6 B ). As depicted in  FIG.  5 B , an ordinary ray component of the returned beam of light  507  does not refract as it moves through the first birefringent material  514  and the second birefringent material  520  as the ordinary ray component is aligned with the ordinary axes of each of the first birefringent material  514  and the second birefringent material  520 . 
     The difference between the illumination path  506   a , the collection path  507   a , and the collection path  509   a  may depend on polarization values of light traveling on the paths and a direction of propagation of the light through the birefringent circulator. For example, p-polarization may interact with an ordinary axis of birefringent elements to travel along a certain path. Similarly, s-polarization may interact with an extraordinary axis of birefringent elements to travel along another, at least partially distinct, path. Due to the birefringent properties discussed herein, non-reciprocal optics may be formed. 
     In some embodiments, a portion of the collection path  507   a  between the self-mixing interferometry sensor  502  and the first lens  512  may fully overlap with a portion of the illumination path  506   a  between the self-mixing interferometry sensor  502  and the first lens  512 . In this way, the returned beam of light  507  may return to the self-mixing interferometry sensor  502  with coherent mode/polarization matching with respect to the transmitted beam of light  506 . 
     Both the s-polarization  508   k  and the s-polarization  508   j  may align with the extraordinary axis of both the first birefringent material  514  and the second birefringent material  520 . There may therefore be a region at or below a surface of the target  510  where the returned beam of light  506  travels along the collection path  507   a . This region at or below the surface of the target  510  may be a region of interest and may be a region where properties of or associated with the target  510  are detected. 
     In  FIG.  5 C , the returned beam of light  509  reflects from the target  510  with an s-polarization  508   i . As the returned beam of light  509  moves along the collection path  509   a , the polarization stays as s-polarization as it moves through the second birefringent material  520  (e.g., s-polarization  508   j ), the first birefringent material  514  (e.g., s-polarization  508   k ), and as it is received by the self-mixing interferometry sensor  502  (e.g., s-polarization  508   l ). Though the returned beam of light  509  has an s-polarization along each of these markers, the returned beam of light  509  does not necessarily remain at an s-polarization throughout the entirety of the collection path  509   a . The half-wave plate  518  may shift the polarization of the returned beam of light  507  to a certain degree and this shift may be canceled by the Faraday rotator  516  (see, e.g.,  FIGS.  6 A and  6 B ). Here, it is noted that the self-mixing interferometry sensor  502  may, in some embodiments, only be able to perform self-mixing processes if the polarization of the returned light is identical to the polarization of the transmitted light. In this case, the p-polarization light depicted in  FIG.  5 B  may enter the self-mixing interferometry sensor  502  but may not affect any self-mixing interference process (since light with s-polarization was initially transmit). In some embodiments, a filter may be provided to prevent p-polarized light from entering the self-mixing interferometry sensor  502 . The polarization  508   i  may be referred to as a third polarization and the polarization  508   l  may be referred to as a fourth polarization. 
     Though the illumination and collection paths are depicted with certain boundaries, it is noted that these boundaries are for graphical purposes only. Any illumination or collection path from or to a light source, through a birefringent circulator, and to or from a target may be used in accordance with the present disclosure. 
       FIGS.  4 A- 5 C  generally depict non-confocal illumination and collection paths (e.g., the illumination and collection paths are focused on different locations on the target  410 / 510 ). It is understood that this non-confocal arrangement is provided merely for explanatory purposes. Depending on a selection of optical properties of a birefringent circulator and/or self-mixing interferometry sensor, illumination and collection paths may be focused at the same location on a target in a confocal arrangement (e.g., see  FIG.  3 A ). 
       FIGS.  6 A and  6 B  depict an optical structure  600  for the non-reciprocal treatment of light polarization. The optical structure  600  depicted in  FIGS.  6 A and  6 B  may be equivalent, or largely similar to, the Faraday rotator, wave plates, and/or other forms of optics or optical circulators discussed with reference to  FIGS.  1 A- 5 C . As described herein, the optical structure  600  may be used in accordance with a non-reciprocal self-mixing interferometry system. In order to change a polarization of light, a Faraday rotator  602  may be provided along with a wave plate  604 . The Faraday rotator  602  and the wave plate  604  may be aligned so that a beam of light passes through each successively. 
     In the example depicted in  FIG.  6 A , a first beam of light  606  may initially have an s-polarization  610   a , though the polarization is not limited to such. As the first beam of light  606  travels through the Faraday rotator  602 , a polarization of the first beam of light  606  may be rotated in a clockwise or counterclockwise direction (e.g., a clockwise direction is depicted in  FIG.  6 A ). The polarization at this stage may be an angled polarization  610   b  between s-polarization and p-polarization states. The angled polarization  610   b  may be angled at any angle depending on properties of the Faraday rotator  602 . The beam of light  606  with the angled polarization  610   b  may then pass through the wave plate  604 . The wave plate  604  may further rotate the polarization of the beam of light  606  so that the beam of light  606  has a p-polarization  610   c  after passing through the wave plate  604 . In this way, a p-polarized beam of light may be converted to an s-polarized beam of light. 
     A beam of light  608  may propagate in a direction opposite to the direction of propagation of the beam of light  606 . When traveling in the opposite direction, the beam of light  608  may first pass through the wave plate  604  before passing through the Faraday rotator  602 . In this direction, the wave plate  604  may initially shift the polarization of an initial p-polarization  612   a  of the beam of light  608  into an angled polarization  612   b . Here, it is noted that the polarization of the beam of light  606  at polarization state  610   b  and the polarization of the beam of light  608  at polarization state  612   b  may be the same, though the two polarization states may be different in other embodiments. However, as the beam of light  608  travels through the Faraday rotator  602 , rather than transforming to an s-polarization state (e.g., state  610   a ) as originally present in the beam of light  606 , the beam of light  608  may transition to a p-polarization state  612   c . In this way, non-reciprocal optics  600  may affect polarization states differently depending on a direction of propagation for a beam of light. 
     Though the polarizations  610   b  and  612   b  are displayed at an approximate 45-degree angle, it should be understood that any polarization angle may be used in accordance with the provided disclosure. The degree of polarization rotation of the Faraday rotator  602  and/or the wave plate  604  may be any amount and any associated optical properties may be provided. 
     In the example depicted in  FIG.  6 B , a first beam of light  606  may initially have a p-polarization  610   d , though the polarization is not limited to such. As the first beam of light  606  travels through the Faraday rotator  602 , a polarization of the first beam of light  606  may be rotated in a clockwise or counterclockwise direction (e.g., a clockwise direction is depicted in  FIG.  6 B ). The polarization at this stage may be an angled polarization  610   e  between s-polarization and p-polarization states. The angled polarization  610   e  may be angled at any angle depending on properties of the Faraday rotator  602 . The beam of light  606  with the angled polarization  610   e  may then pass through the wave plate  604 . The wave plate  604  may further rotate the polarization of the beam of light  606  so that the beam of light  606  has an s-polarization  610   f  after passing through the wave plate  604 . 
     A beam of light  608  may propagate in a direction opposite to the direction of propagation of the beam of light  606 . Thus, the beam of light  608  may first pass through the wave plate  604  before passing through the Faraday rotator  602 . In this direction, the wave plate  604  may initially shift the polarization of an initial s-polarization  612   d  of the beam of light  608  into an angled polarization  612   e . Here, it is noted that the polarization of the beam of light  606  at polarization state  610   e  and the polarization of the beam of light  608  at polarization state  612   e  may be the same, though the two polarization states may be different in other embodiments. However, as the beam of light  608  travels through the Faraday rotator  602 , rather than transforming to a p-polarization state (e.g., state  610   d ) as originally present in the beam of light  606 , the beam of light  608  may transition to an s-polarization state  612   f . In this way, non-reciprocal optics  600  may affect polarization states differently depending on a direction of propagation. 
     In some embodiments, therefore, a beam of light transmit in one direction may shift to an orthogonal polarization while a beam of light transmit in an opposite direction may remain at the same polarization. 
     In the above examples, the Faraday rotator  602  may always rotate a beam of light in the same direction, regardless of a direction of propagation or incident polarization. For example, the Faraday rotator  602  may always rotate a polarization of a beam of light in a clockwise direction. This may result in a non-reciprocal environment where an initial polarization shift cannot be reversed simply by traveling back through the Faraday rotator  602  (e.g., the polarization shift is increased rather than reversed). The wave plate  604  may be a reciprocal optic and may rotate a polarization of a beam of light in a different direction depending on a direction of propagation of a beam of light. Therefore, the wave plate  604  may undo or reverse an initial polarization shift if the shifted beam of light returns through the wave plate  604 . Though depicted as a plate in  FIGS.  6 A and  6 B , the Faraday rotator  602  and the wave plate  604  may include other shapes and may be formed from one optical component or multiple optical components (e.g., a number of lenses, magnets, and materials). For example, the Faraday rotator  602  and the wave plate  604  may be shaped as a tube formed from different materials. 
       FIG.  7    depicts an example embodiment of a gesture recognition system  700  using a self-mixing interferometry (SMI) sensor  708  such as, for example, discussed with reference to SMI sensors and associated optics in  FIGS.  1 A- 6 B . As discussed herein, the self-mixing interferometry sensor  708  may be provided with non-reciprocal optics to transmit and receive beams of light along different optical paths. In the embodiment depicted here, the non-reciprocal optics may be incorporated into the self-mixing interferometry sensor  708 , though in alternate embodiments the non-reciprocal optics may be positioned within an opening of an electronic device  702 . 
     As discussed here, the self-mixing interferometry sensor  708  may emit and receive beams of light  712 . The beams of light  712  may be directed toward a target  710 . The target  710  may be a body part of a user such as a wrist or hand. The beams of light  712  may partially penetrate the target  710  so that sub-surface features (e.g., veins) are measured. 
     The electronic device  702  may additionally include a processing unit  704  and a gesture recognizer  706 . The gesture recognizer  706  may work in conjunction with the processing unit  704  to store self-mixing interferometry signals associated with a certain gesture. In some embodiments, the processing unit  704  may be integrated with the gesture recognizer  706 . 
     As described herein, the electronic device  702  may be operated in a training mode where a user makes a number of gestures in response to a displayed gesture indication graphic. As the user makes the indicated gesture, self-mixing interferometry signals may be stored within the gesture recognizer and may be associated with a particular gesture. A gesture recognized by the processing unit  704  may be accepted as a valid input command and may affect an operation of the electronic device  702 . In some embodiments, systems of a self-mixing interferometry system may detect changes in operational parameters of the SMI sensor  708 , may apply a frequency domain analysis to the operational parameter, and may map or otherwise determine the gesture of the user by detected properties of the operational parameter as determined by the frequency domain analysis. As a non-limiting example, an operational parameter may be a voltage or current change of the SMI sensor  708 . 
       FIGS.  8 A- 8 C  depict example electronic devices in which a self-mixing interferometry sensor may be incorporated. These example electronic devices are intended to be merely explanatory and are not intended to be exclusive. 
       FIG.  8 A  depicts a mobile phone  800   a  including a display  804   a , a self-mixing interferometry sensor  802   a , and a housing  806   a . The housing  806   a  may include an opening and may surround a portion of the display  804   a . The self-mixing interferometry sensor  802   a  may utilize sensed self-mixing signals to determine the presence or motion of a part of a user (e.g., a user&#39;s finger). For example, a user may swipe her finger across the self-mixing interferometry sensor  802   a  to turn the mobile phone  800   a  on or off. In some embodiments, a user may place her finger on top of the self-mixing interferometry sensor  802   a  where sub-dermal measurements (e.g., a heartbeat measurement) may be collected. 
     Any of the self-mixing interferometry sensors, and associated optics and/or circulators, discussed with reference to  FIGS.  1 A- 7    may be integrated with the mobile phone  800   a , for example, a self-mixing interferometry sensor designed to measure detected properties of sub-surface features underneath, for example, a user&#39;s skin and/or of features on or above a target. 
     For example, a self-mixing interferometry sensor  802   a  may be used as a biometric sensor and may be configured to detect a user&#39;s blood flow. In additional or alternative embodiments, a self-mixing interferometry sensor  802   a  may be used to measure a surface roughness of a target (e.g., a skin surface or an inanimate object such as a wall, table, or aluminum can) and/or a particle density or detected particles (e.g., a concentration of smoke in the air). A self-mixing interferometry sensor  802  may additionally or alternatively be capable of measuring detected properties of any provided object and may, for example, treat a particular object or speed of an object as an input (e.g., a swipe may turn on or off the mobile phone  800   a ) when the object is either in contact with the mobile phone  800   a  or not in contact with the mobile phone  800   a.    
       FIG.  8 B  depicts an electronic watch  800   b  including a display  804   b , a self-mixing interferometry sensor  802   b , a housing  806   b , a digital crown  810 , and a strap  808 . The strap may connect the electronic watch  800   b  with an arm of a user. The digital crown  810  may be rotatable and may be used to detect an input. In some embodiments, the digital crown  810  may be immovable. The self-mixing interferometry sensor  802   b  may utilize self-mixing interferometry to detect a presence or motion of an object to detect, for example, a swipe. In some embodiments, the self-mixing interferometry sensor  802   b  may be provided underneath the electronic watch  800   b  to capture measurements from a user&#39;s wrist while the electronic watch  800   b  is being worn. 
     Any of the self-mixing interferometry sensors, and associated optics and/or circulators, discussed with reference to  FIGS.  1 A- 7    may be integrated with the electronic watch  800   b , for example, a self-mixing interferometry sensor designed to detect the detected properties of sub-surface features underneath, for example, a user&#39;s skin and/or of features on or above a target. 
     In some embodiments, the self-mixing interferometry sensor  802   b  may be focused at a point beneath a user&#39;s skin and may be configured to detect, for example, a blood flow and/or a blood oxygenation level. In additional or alternative examples, a self-mixing interferometry sensor  802   b  may be directed to emit light to a point on a user&#39;s skin, the light may follow a “banana path” underneath a user&#39;s skin, and the light may return back out of the user&#39;s skin at another point before being returned to the self-mixing interferometry sensor  802   b . By performing self-mixing analyses on the returned light, properties of sub-dermal features may be obtained. In additional or alternative examples, a self-mixing interferometry sensor  802   b  may be used as a gesture sensor and may be able to detect a user&#39;s gesture. 
       FIG.  8 C  depicts an earbud  800   c  including a housing  806   c , a self-mixing interferometry sensor  802   c , and a speaker  812 . The earbud  800   c  may be designed to fit within a user&#39;s ear to provide a sound via the speaker  812 . The self-mixing interferometry sensor  802   c  may be provided as an input detector and may detect the speed of a finger to change a volume of sound output from the speaker  812  and/or an operation state of the earbud  800   c . In some embodiments, the self-mixing interferometry sensor  802   c  may capture information about a user&#39;s ear canal structure and other biometric features such as a user&#39;s breathing, swallowing, speaking, and so on. 
     Any of the self-mixing interferometry sensors, and associated optics and/or circulators, discussed with reference to  FIGS.  1 A- 7    may be integrated with the earbud  800   c , for example, a self-mixing interferometry sensor designed to detect the detected properties of sub-surface features underneath, for example, a user&#39;s skin and/or of features on or above a target. 
     A self-mixing interferometry sensor  802   c  integrated within an earbud  800   c  may be used, for example, to detect a skin movement and/or gesture of a user. By measuring a gesture, an input to the earbud  800   c  may be detected such as, for example, a volume control or an on/off operation. In some embodiments, a self-mixing interferometry sensor  802   c  may measure detected properties of a user&#39;s skin and may, for example, increase or decrease the volume when a user is in motion, as determined by a change in skin shape. 
       FIG.  9    depicts a self-mixing interferometry system  900  with beam scanning and/or shaping capabilities as a system for preventing side lobe interference. The system as depicted in  FIG.  9    may use any of the self-mixing interferometry systems or sensors, along with associated non-reciprocal optics, discussed with respect to  FIGS.  1 A- 8 C . 
     In systems without birefringent optics, a beam of light transmit to and/or returned at an optical phase array may result in an overlap between main lobes and side lobes emitted through the optical phase array. This may result in side lobe interference. 
     In the self-mixing interferometry system  900 , birefringent optics  904 , such as described herein, may be used so that main lobes of transmitted and returned light overlap (e.g., to meet at a point of interest) while side lobes do not overlap so as to prevent side lobe interference. In the depicted embodiment, a transmitted beam of light  906  is directed toward an optical phase array  910  as may be used for scanning and/or aiming beams of light. After passing through the optical phase array  910 , the transmitted beam of light  906  may be separated into a transmitted main lobe  906   a  and a transmitted side lobe  906   b . The transmitted side lobe  906   b  may be aimed away from the target of interest while the transmitted main lobe  906   a  may be directed toward the target of interest (e.g., a target where self-mixing interferometry is performed). Returned light may be reflected toward the optical phase array  910  as both a returned main lobe  908   a  and a returned side lobe  908   b . The returned main lobe  908   a  may overlap with the transmitted main lobe  906   a  and may include light reflected from the target of interest. The returned side lobe  908   b  may be light reflected from surfaces other than the target of interest. The returned main lobe  908   a  and the returned side lobe  908   b  may be filtered through the optical phase array  910  and may be directed back through the birefringent optics  904  and the self-mixing interferometry sensor  902 . In this way, side lobe interference from an unwanted target and between the transmitted side lobe  906   b  and the returned side lobe  908   b  may be eliminated or reduced. 
     In some embodiments, the self-mixing interferometry system  900  may be used in coherent spatial scanning operations. As used herein, coherent spatial scanning may refer to optical coherence tomography (OCT) and may be used to collect three-dimensional images of an object (e.g., a tissue or other physical object). The self-mixing interferometry system  900  may scan or pulse the self-mixing interferometry sensor  902  across an object, may collect reflected light, and may perform self-mixing operations to convert a received signal into an image. As understood by one of ordinary skill in the art, OCT may be used to perform three- or two-dimensional imaging analysis of sub-dermal features (e.g., sub-dermal organs). In some embodiments, the self-mixing interferometry system  900  may be used for far-field three-dimensional scanning and may be referenced as light detection and ranging (LIDAR). 
     Though the optical phase array  910  is depicted as one unit, any number of units is possible. For example, one optical phase array may be provided for the transmitted beam of light  906  and a second optical phase array may be provided for the returned beam of light  908 . 
       FIG.  10    depicts an example passive non-reciprocal element  1000 . The system as depicted may use any of the self-mixing interferometry systems or sensors, along with associated non-reciprocal optics, discussed with respect to  FIGS.  1 A- 9   . The passive non-reciprocal element  1000  may be a birefringent meta-structure and may include different orientations of chiral elements so that permanent magnets and/or biased electric or magnetic fields are not required (e.g., as may be required in a Faraday rotator). A first chiral element  1002  may polarize a wave in a first direction (e.g., clockwise). The first chiral element  1002  may reflect a wave propagating in a second direction opposite from the first direction. A second chiral element  1004  may be a mirror-image of the first chiral element  1002  so that a wave is polarized in a second direction while being reflected in the first direction. The first chiral element  1002  and the second chiral element  1004  may be formed in a grid-like pattern as depicted in  FIG.  10    and may function as a non-reciprocal meta-surface mimicking Faraday rotation, such as described herein. As such, the passive non-reciprocal element  1000  may replace the Faraday rotators as described with respect to the above described embodiments. 
     As described above, one aspect of the present technology is measuring surface or sub-surface property on surfaces or objects. As such, personal information data, including heart rate, BMI, blood oxygenation level, location-based data, data or records relating to a user&#39;s health or level of fitness (e.g., vital signs measurements, medication information, exercise information), or any other identifying or personal information may be measured. 
     The present disclosure recognizes that the use of such personal information data, in the present technology, can be used to the benefit of users. For example, the personal information data can be used to provide health recommendations and history that are tailored to and/or derived from the user. Further, other uses for personal information data that benefit the user are also contemplated by the present disclosure. For instance, fitness data may be used to provide insights into a user&#39;s general wellness, or may be used as positive feedback to individuals using technology to pursue wellness goals. 
     The present disclosure contemplates that the entities responsible for the collection, analysis, disclosure, transfer, storage, or other use of such personal information data will comply with well-established privacy policies and/or privacy practices. In particular, such entities implement and consistently use privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for ensuring personal information data is kept private and secure. Such policies should be easily accessible by users, and should be updated as the collection and/or use of data changes. Personal information from users should be collected for the legitimate and reasonable uses of the entity and should not be shared or sold outside of those legitimate uses. Further, such collection/sharing should occur after receiving the informed consent of the users. Additionally, such entities should consider taking any needed steps for safeguarding and securing access to such personal information data and ensuring that others with access to the personal information data adhere to their privacy policies and procedures. Further, such entities can subject themselves to evaluation by third parties to certify their adherence to widely accepted privacy policies and practices. In addition, policies and practices should be adapted for the particular types of personal information data being collected and/or accessed and adapted to applicable laws and standards, including jurisdiction-specific considerations. For instance, in the US, collection of or access to certain health data may be governed by federal and/or state laws, such as the Health Insurance Portability and Accountability Act (“HIPAA”); whereas health data in other countries may be subject to other regulations and policies and should be handled accordingly. Hence different privacy practices should be maintained for different personal data types in each country. 
     Despite the foregoing, the present disclosure also contemplates embodiments in which users selectively block the use of, or access to, personal information data. That is, the present disclosure contemplates that hardware and/or software elements can be provided to prevent or block access to such personal information data. For example, in the case of determining spatial parameters, the present technology can be configured to allow users to select to “opt in” or “opt out” of participation in the collection of personal information data during registration for services or anytime thereafter. In addition to providing “opt in” and “opt out” options, the present disclosure contemplates providing notifications relating to the access or use of personal information. For instance, a user may be notified upon downloading an app that their personal information data will be accessed and then reminded again just before personal information data is accessed by the app. 
     Moreover, it is the intent of the present disclosure that personal information data should be managed and handled in a way to minimize risks of unintentional or unauthorized access or use. Risk can be minimized by limiting the collection of data and deleting data once it is no longer needed. In addition, and when applicable, including in certain health related applications, data de-identification can be used to protect a user&#39;s privacy. De-identification may be facilitated, when appropriate, by removing specific identifiers (e.g., date of birth, etc.), controlling the amount or specificity of data stored (e.g., collecting location data at a city level rather than at an address level), controlling how data is stored (e.g., aggregating data across users), and/or other methods. 
     Therefore, although the present disclosure broadly covers use of personal information data to implement one or more various disclosed embodiments, the present disclosure also contemplates that the various embodiments can also be implemented without the need for accessing such personal information data. That is, the various embodiments of the present technology are not rendered inoperable due to the lack of all or a portion of such personal information data. For example, sleep patterns may be provided based on non-personal information data or a bare minimum amount of personal information, such as events or states of the device associated with a user, other non-personal information, or publicly available information. 
     Other examples and implementations are within the scope and spirit of the disclosure and appended claims. For example, self-mixing interferometers may be used on fitness monitors, in wearable electronics, and in other pressure sensing/measuring systems. Also, as used herein, including in the claims, “or” as used in a list of items prefaced by “at least one of” indicates a disjunctive list such that, for example, a list of “at least one of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Further, the term “exemplary” does not mean that the described example is preferred or better than other examples. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20201210
Publication Date: 20240116
Grant Date: 20240116
Priority Date: 20200925
Inventors: CHEN, TONG
CIHAN, AHMET FATIH
JIN, Mingzhou
Assignee: APPLE INC
CPC Classifications: [{"code": "G01B9/02092", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/093", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/017", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/093", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01B9/02092", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01B11/303", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01B9/02092", "inventive": true, "first": false, "tree": "[]"}, {"code": "A61B5/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01B9/02091", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/017", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0304", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/017", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/093", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 80822267