Patent Publication Number: US-11640054-B2

Title: Multi-wavelength self-mixing interferometry

Description:
TECHNICAL FIELD 
     This disclosure relates generally to optics and in particular to optical sensing. 
     BACKGROUND INFORMATION 
     A variety of electrical and optical sensors have been developed to measure proximity and/or distance. Self-Mixing Interferometry sensors are optical sensors that may be used for measurements, for example. SMI sensors could benefit from increasing the accuracy of measurements, and in particular, the accuracy of total distance determinations could be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG.  1    illustrates an example head mounted device that includes a multi-wavelength self-mixed interferometer (SMI), in accordance with aspects of the disclosure. 
         FIG.  2    illustrates an example multi-wavelength SMI implementation that includes a plurality of SMI sensors, in accordance with aspects of the disclosure. 
         FIG.  3    illustrates an exploded view of an example SMI sensor, in accordance with aspects of the disclosure. 
         FIGS.  4 A- 4 C  illustrate a multi-wavelength SMI implementation including an SMI sensor having a swept-source laser as a light source, in accordance with aspects of the disclosure. 
         FIG.  5    illustrates a process of eye-tracking using a multi-wavelength SMI architecture, in accordance with aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of multi-wavelength self-mixing interferometry are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In aspects of this disclosure, visible light may be defined as having a wavelength range of approximately 380 nm-700 nm. Non-visible light may be defined as light having wavelengths that are outside the visible light range, such as ultraviolet light and infrared light. Infrared light has a wavelength range of approximately 700 nm-1 mm. Infrared light may include near-infrared light. In aspects of this disclosure, near-infrared light may be defined as having a wavelength range of approximately 700 nm-1.4 μm. 
     In some implementations of the disclosure, the term “near-eye” may be defined as including an element that is configured to be placed within 50 mm of an eye of a user while a near-eye device is being utilized. Therefore, a “near-eye optical element” or a “near-eye system” would include one or more elements configured to be placed within 50 mm of the eye of the user. 
     In aspects of this disclosure, the term “transparent” may be defined as having greater than 90% transmission of light. In some aspects, the term “transparent” may be defined as a material having greater than 90% transmission of visible light. 
     Self-Mixing Interferometry techniques include emitting coherent light from a light source (e.g. a laser) and generating a Self-Mixing Interferometer (SMI) signal in response to feedback light that is received by the optical cavity of the light source. Typically, an SMI sensor includes a light source (e.g. a laser) and a light sensor (e.g. a photodiode) optically coupled to receive an optical signal from the optical cavity of the light source. SMI sensors could benefit from increasing the accuracy of measurements, and in particular, the accuracy of total distance determinations could be improved. 
     Embodiments of the disclosure include multi-wavelength SMI systems and/or sensors to increase accuracy of distant measurements and/or velocity measurements of a target. In the particular context of head mounted devices, a multi-wavelength SMI system or sensor may improve the accuracy of absolute position measurements of an eye of a user with respect to the head mounted device and/or improve the accuracy of velocity measurements of the eye of the user with respect to the head mounted device. Using a multi-wavelength SMI architecture would provide the improved accuracy without the associated bulk of an Optical Coherence Tomography (OCT) system that scans a laser across a large wavelength range. These and other embodiments are described in more detail in connection with  FIGS.  1 - 5   . 
       FIG.  1    illustrates an example head mounted device  100  that includes a multi-wavelength self-mixed interferometer (SMI), in accordance with aspects of the disclosure. Head mounted device  100  includes frame  114  coupled to arms  111 A and  111 B. Lenses  121 A and  121 B are mounted to frame  114 . Lenses  121  may be prescription lenses matched to a particular wearer of head mounted device  100  or non-prescription lenses. The illustrated head mounted device  100  is configured to be worn on or about a head of a user of the head mounted device. 
     The frame  114  and arms  111  of the head mounted device  100  may include supporting hardware of head mounted device  100 . Head mounted device  100  may be considered “electronic glasses” since head mounted device includes electronics. Head mounted device  100  may include any of processing logic, wired and/or wireless data interface for sending and receiving data, graphic processors, and one or more memories for storing data and computer-executable instructions. In one embodiment, head mounted device  100  may be configured to receive wired power. In one embodiment, head mounted device  100  is configured to be powered by one or more batteries. In one embodiment, head mounted device  100  may be configured to receive wired data including video data via a wired communication channel. In one embodiment, head mounted device  100  is configured to receive wireless data including video data via a wireless communication channel. 
     Head mounted device  100  may be a head mounted display (HMD) when head mounted device  100  is configured with a near-eye display for presenting images to the eye of a user. In  FIG.  1   , each lens  121  includes a waveguide  150  to direct display light generated by a display  130  to an eyebox area for viewing by a wearer of head mounted device  100 . Display  130  may include an LCD, an organic light emitting diode (OLED) display, micro-LED display, quantum dot display, pico-projector, or liquid crystal on silicon (LCOS) display for directing display light to a wearer of head mounted device  100 . Near-eye display architectures that are different from the example implementation illustrated in  FIG.  1    may of course be used as alternatives, in some implementations. 
     Lenses  121  may appear transparent to a user to facilitate augmented reality or mixed reality where a user can view scene light from the environment around her while also receiving display light directed to her eye(s) by waveguide(s)  150 . Consequently, lenses  121  may be considered (or include) an optical combiner. In some embodiments, display light is only directed into one eye of the wearer of head mounted device  100 . In an embodiment, both displays  130 A and  130 B are included to direct display light into waveguides  150 A and  150 B, respectively. 
     The example head mounted device  100  of  FIG.  1    includes a camera  147 . Camera  147  may include a complementary metal-oxide semiconductor (CMOS) image sensor. An infrared filter that receives a narrow-band infrared wavelength may be placed over the image sensor so it is sensitive to the narrow-band infrared wavelength while rejecting visible light and wavelengths outside the narrow-band. Infrared illuminators (not illustrated) such as infrared LEDs or vertical-cavity surface-emitting lasers (VCSELs) that emit the narrow-band wavelength may be oriented to illuminate an eyebox area with the narrow-band infrared wavelength. In one embodiment, the infrared light is near-infrared light. In an implementation, the near-infrared light is centered around 850 nm. Camera  147  may be mounted on the inside of the temple of head mounted device  100 . The images of the eye captured by camera  147  may be used for eye-tracking purposes, or otherwise. Camera  147  may directly image the eye or image the eye by way of an optical combiner (not illustrated) included in lens  121 B that directs infrared light (reflected or scattered by the eye of the user) to camera  147 . 
     The example head mounted device  100  of  FIG.  1    includes an example in-field element  133  and an example out-of-field element  135 . In-field element  133  will be in a field of view (FOV) of a user of head mounted device  100  since element  133  is included in lens  121 B. Out-of-field element  135  will be out of the FOV of a user of head mounted device  100 . The multi-wavelength SMI sensors and systems of this disclosure may have an in-field element and/or an out-of-field element. While not specifically illustrated, SMI sensing hardware may also be associated with lens  121 A. Although camera  147  and elements  133  and  135  are illustrated on only one side of head mounted device  100 , they of course may be duplicated on the other side of head mounted device  100  (e.g. near lens  121 A) to facilitate imaging of both eyes of a wearer of head mounted device  100 . 
       FIG.  2    illustrates an example multi-wavelength SMI implementation  200  that includes a plurality of SMI sensors, in accordance with aspects of the disclosure. While SMI implementation  200  may be used in a variety of contexts, some example illustrations of the disclosure are illustrated in the context of near-eye sensing for eye-tracking of eye  280 . SMI implementation  200  illustrates that SMI sensors  240 A,  240 B, and  240 C (collectively referred to as SMI sensors  240 ) may be coupled with a transparent or semi-transparent optical element  221 . Optical element  221  may be included in lenses  121  of  FIG.  1   , for example. Optical element  221  may pass (transmit) scene light  295  from an environment of a user of a head mounted device  100  to eye  280 .  FIG.  2    also illustrates that in implementations where a head mounted device is also an HMD, optical element  221  may pass display light  296  to eye  280  to present images included in display light  296  to the eye  280  of a user of an HMD. All or a portion of SMI sensors  240  may be positioned to be out-of-field of eye  280  of a user of a head mounted device such as head mounted device  100 . In some implementations, all or a portion of SMI sensors  240  may be positioned to be within a FOV of eye  280  of a user of head mounted device  100 . 
     Each SMI sensor  240 A,  240 B, and  240 C includes a light source (e.g. a near-infrared laser) and a light sensor (e.g. a photodiode). Therefore, the first SMI sensor  240 A includes a first light source and a first light sensor, the second SMI sensor  240 B includes a second light source and a second light sensor, and third SMI sensor  240 C includes a third light source and a third light sensor. The light source may be an infrared light source emitting infrared light. The light source may be a near-infrared light source emitting near-infrared light. The light source emits coherent light. The light source may be a laser. The light sensor may be a photodiode. 
     Three SMI sensors emitting different wavelengths of light are illustrated in  FIG.  2   , although implementations of the disclosure may include two or more SMI sensors that emit different wavelengths of light. In the illustrated example, the first light source of first SMI sensor  240 A emits first coherent light  290  having a first wavelength. The second light source of second SMI sensor  240 B emits second coherent light  291  having a second wavelength that is different from the first wavelength. The third light source of third SMI sensor  240 C emits third coherent light  297  having a third wavelength different from the first wavelength and different from the second wavelength. By illuminating the eyebox with a plurality of wavelengths, the multi-wavelength SMI implementation  200  achieves increased accuracy of absolute position measurements of an eye  280  of a user with respect to the head mounted device and/or improves the accuracy of velocity measurements of the eye of the user with respect to the head mounted device. 
       FIG.  3    illustrates an exploded view of an example SMI sensor  340  that may be used as any of SMI sensors  240 , in accordance with aspects of the disclosure. The exploded view of SMI sensor  340  (indicated by diagonal dashed lines) shows that SMI sensor  340  includes a light source  310  and a light sensor  360 . The example light sensor  360  is illustrated as a photodiode but light sensor  360  may be an alternative photosensor. 
     Light source  310  includes a cavity  316  (e.g. an optical cavity, which may be a laser cavity) defined by two reflective elements (e.g. reflective surfaces  312  and  314 ). In some embodiments, the reflective elements are distributed Bragg reflectors. In some embodiments, the light source  310  may be a laser source, such as a vertical cavity surface emitting laser (VCSEL) or a vertical external cavity surface emitting laser (VECSEL). 
     Optical cavity  316  is used to generate coherent light  390  and light source  310  is positioned to output at least a portion of the coherent light  390  towards object  380 . Surface  312  is semi-reflective (e.g. surface  312  is a partially reflective and partially transmissive mirror). For example, the reflectance of the surface  314  is greater than the reflectance of the surface  312  (e.g. surface  314  has a reflectance of 100%, 99.99%, 99.9%, 99%, 98%, 97%, 96%, 95%, 90% or an interval between any two of the aforementioned values, and surface  312  has a reflectance of 99.99%, 99.9%, 99%, 98%, 97%, 96%, 95%, 90% or an interval between any two of the aforementioned values). In some configurations, surface  312  has a transmittance of at least 0.01%, 0.1%, 1%, or an interval between any two of the aforementioned values. Surface  312  reflects a portion of the light propagating toward the surface  312  within the cavity  316  back toward surface  314  and transmits a portion of the light propagating toward surface  312  within optical cavity  316  (e.g. surface  312  is configured to reflect at least a portion of the light generated inside the cavity  316  back into cavity  316  and to transmit at least a portion of the light generated inside the cavity  316 ). The transmitted light is emitted from light source  310  as coherent light  390 . SMI sensor  340  (and hence, light source  310  of the SMI sensor  340 ) is configured to (e.g. positioned to) receive, via surface  312 , at least a portion of the coherent light back from object  380  as feedback light  392 . Object  380  reflects or scatters a portion of the incident coherent light  390  back to surface  312  as feedback light  392 . Feedback light  392  may propagate along an optical path that is considered the reverse optical path of coherent light  390  that becomes incident onto a given target location. Feedback light  392  enters optical cavity  316  of light source  310  and interferes with the generation of the coherent light inside optical cavity  316 , leading to a modulation of the intensity of the generated coherent light. The intensity of the light may be defined as the radiant flux (power) received by a surface per unit area. 
     Modulated coherent light  394  (e.g. coherent light with modulated intensity) is output from the light source  310  (e.g. output from cavity  316 ) and at least a portion of the modulated coherent light  394  is received and detected by the light sensor  360 . Light sensor  360  is configured to generate one or more SMI signals  363  based on the detected intensity (e.g. modulated intensity) of the modulated coherent light  394 . Information regarding movement information of object  380  (e.g. movement of the pupil of an eye  280 ) can be determined by analyzing the modulated coherent light  394  or the one or more SMI signals  363  generated by light sensor  360 . SMI signal  363  may be generated in response to an electrical current generated by a photodiode of light sensor  360  in response to modulated coherent light  394 , for example. 
     This measurement technique is known as “self-mixing interferometry,” where coherent light (e.g. a laser beam) is reflected from a target (e.g. a target object such as an eye) back into the light source (e.g. the laser cavity) and the reflected light interferes with, and modulates, the coherent light generated inside the light source (e.g. modulates the power and/or intensity of the light generated by the light source). Position and/or movement information regarding the target can be determined from (e.g. based on, using) intensity or power measurements of the modulated coherent light. The self-mixing interferometry is also called “feedback interferometry,” “induced-modulation interferometry,” and “backscatter modulation interferometry.” 
     SMI sensor  340  (and hence, light source  310  of SMI sensor  340 ) is configured to (e.g. positioned to) output coherent light  390 , via surface  312 , towards an object  380 . In some contexts of the disclosure, object  380  may be an eye or an eyebox area of a user of a head mounted device. In some contexts of the disclosure, object  380  is a hand, fingers, or portion of a face. SMI sensor  340  (and hence, light source  310  of the SMI sensor  340 ) is also configured to (e.g. positioned to) receive, via surface  312 , at least a portion of the coherent light back from object  380  as feedback light  392 . Object  380  reflects or scatters a portion of the incident coherent light  390  back to surface  312  as feedback light  392 . Since the surface  314  is semi-reflective (e.g. a partially reflective and partially transmissive mirror), at least a portion of the coherent light generated inside the cavity  316  is output from the light source  310 , via the surface  314 , toward light sensor  360 . Light sensor  360  is configured to (e.g. positioned to) receive (e.g. detect) at least a portion of the modulated coherent light  394  output from the light source  310  via the surface  314 , and generate one or more SMI signals  363  based on the detected intensity (e.g. modulated intensity) of the modulated coherent light  394 . 
     Example system  300  of  FIG.  3    includes processing logic  350  configured to receive one or more SMI signals  363  from light sensor  360  of SMI sensor  340  via communication channel X 2 . In the example illustration of  FIG.  3   , processing logic  350  is configured to activate light source  310  by way of communication channel X 1 . Activating light source  310  may include turning light source  310  on and/or modulating light source  310  with an electrical current or electrical voltage, for example. 
     Referring back to  FIG.  2   , processing logic  250  is configured to receive SMI signals  263  from the SMI sensors  240 . In the particular example of  FIG.  2   , processing logic  250  is configured to receive first SMI signal  263 A via communication channel X 2  from first SMI sensor  240 A. First SMI signal  263 A is generated by a first light sensor of first SMI sensor  240 A in response to first feedback light  292  entering a first optical cavity of the first light source. First feedback light  292  is a portion of first coherent light  290  received back from an eyebox region, illustrated particularly as eyebox location  281  in the example illustration of  FIG.  2   . 
     Processing logic  250  is also configured to receive second SMI signal  263 B via communication channel X 4  from second SMI sensor  240 B. Second SMI signal  263 B is generated by a second light sensor of second SMI sensor  240 B in response to second feedback light  293  entering a second optical cavity of the second light source. Second feedback light  293  is a portion of second coherent light  291  received back from an eyebox region, illustrated particularly as eyebox location  282  in the example illustration of  FIG.  2   . 
     Processing logic  250  is further configured to receive third SMI signal  263 C via communication channel X 6  from third SMI sensor  240 C. Third SMI signal  263 C is generated by a third light sensor of third SMI sensor  240 C in response to third feedback light  299  entering a third optical cavity of the third light source. Third feedback light  299  is a portion of third coherent light  297  received back from an eyebox region, illustrated particularly as eyebox location  283  in the example illustration of  FIG.  2   . 
     In some implementations, a first light source of first SMI sensor  240 A is configured to illuminate a first eyebox location (e.g. eyebox location  281 ) with first coherent light and a second light source of second SMI sensor  240 B is configured to illuminate that same first eyebox location with second coherent light  291 . Of course, a third light source of third SMI sensor  240 C may also be configured to illuminate the first eyebox location with third coherent light  297 , in some implementations. In this implementation, SMI sensors  240 A,  240 B, and  240 C may be co-located or even consolidated into a same electronics package. 
     Processing logic  250  is configured to generate eye data  253  in response to at least a first SMI signal (e.g. signal  263 A) and a second SMI signal (e.g. signal  263 B). Of course, processing logic  250  may generate eye data  253  in response to third SMI signal  263 C and additional SMI signals in implementations that include more than three SMI sensors emitting different wavelengths of coherent light. Processing logic  250  may also generate eye data  253  in response to a first prior SMI signal generated by first SMI sensor  240 A and in response to a second prior SMI signal generated by second SMI sensor  240 B. For example, prior SMI signals may be used to determine a prior position of the eye and the prior SMI signal is compared to the subsequent SMI signal (e.g. SMI signal  263 ) to generate a velocity measurement. Eye data  253  may include eye distance data and/or eye velocity data. The eye distance data may be generated based on a depth of eye  280  from a first optical cavity of first SMI sensor  240 A and a second depth of eye  280  from the second optical cavity of second SMI sensor  240 B. In contexts where the measurement target is other than an eye (e.g. face, hand, and/or finger), data  253  may be target data and include distance data to the target and/or velocity data of the target. 
     Processing logic  250  may be configured to activate a first light source of first SMI sensor  240 A by way of communication channel X 1  to emit coherent light  290 . Processing logic  250  may also be configured to activate a second light source of second SMI sensor  240 B by way of communication channel X 3  to emit coherent light  291  and configured to activate a third light source of third SMI sensor  240 C by way of communication channel X 5  to emit coherent light  297 . 
     In operation, processing logic  250  may activate one or more of the light sources of SMI sensors  240  to illuminate eye  280  with coherent light having different wavelengths. The different wavelengths of coherent light may all be within a near-infrared band. The linewidth of the coherent light from the light sources of the SMI sensors  240  may be plus-or-minus two nm, in some implementations. In some implementations, first coherent light  290  is emitted simultaneously with second coherent light  291 . Third coherent light  297  may also be emitted simultaneously with first coherent light  290  and second coherent light  291 . 
       FIGS.  4 A- 4 C  illustrate a multi-wavelength SMI implementation  400  including an SMI sensor  440  having a swept-source laser as a light source, in accordance with aspects of the disclosure.  FIG.  4 A  illustrates SMI sensor  440  may be coupled with optical element  221 . SMI sensor  440  may be positioned to be out-of-field of eye  280  or in a FOV of eye  280 . Processing logic  450  is communicatively coupled to SMI sensor  440  by way of communication channel X 2 . Processing logic  450  is configured to receive SMI signals  463 A,  463 B, and  463 C by way of communication channel X 2 . 
     SMI sensor  440  includes a swept-source laser as its light source. Thus, the light source  310  of SMI sensor  340  would be replaced with a swept-source laser. The swept-source laser may be a near-infrared swept-source laser. The swept-source laser may be configured to execute a broad wavelength scan (e.g. 15 nm or 100 nm) at a relatively high sweep frequency (e.g. 10 kHz to 1 MHz) to provide improved accuracy for distance measurements. Swept-source lasers can be driven to adjust the wavelength of the laser light outputted by the laser and may also be referred to as “wavelength-swept lasers” or “tunable lasers” by those skilled in the art. Processing logic  450  is configured to control the swept-source laser to sweep through different wavelengths of coherent light via communication channel X 1 . Controlling the swept-source laser may include modulating the swept-source laser with an electrical current or electrical voltage to scan through different wavelengths of laser light, for example. 
       FIG.  4 A  illustrates that at a first time period, processing logic  450  is configured to drive the swept-source laser of SMI sensor  440  to emit first coherent light  490  having a first wavelength. The light sensor of SMI sensor  440  generates a first SMI signal  463 A in response to first feedback light  492  received back from the target (eye  280  is the example target in  FIGS.  4 A- 4 C ). First feedback light  492  is a portion of the first wavelength of the first coherent light  490  received back from the target. In the illustrated example, first coherent light  490  illuminates a first eyebox location  481 . Processing logic  450  is configured to receive and measure first SMI signal  463 A. 
       FIG.  4 B  illustrates that at a second time period, processing logic  450  is configured to drive the swept-source laser of SMI sensor  440  to emit second coherent light  491  having a second wavelength that is different from the first wavelength of first coherent light  490 . The light sensor of SMI sensor  440  generates a second SMI signal  463 B in response to second feedback light  493  received back from the target. Second feedback light  493  is a portion of the second wavelength of the second coherent light  491  received back from the target. In the illustrated example, second coherent light  491  illuminates first eyebox location  481 . Processing logic  450  is configured to receive and measure second SMI signal  463 B. 
       FIG.  4 C  illustrates that at a third time period, processing logic  450  is configured to drive the swept-source laser of SMI sensor  440  to emit third coherent light  497  having a third wavelength that is different from the first wavelength of first coherent light  490  and different from the second wavelength of second coherent light  491 . The light sensor of SMI sensor  440  generates a third SMI signal  463 C in response to third feedback light  499  received back from the target. Third feedback light  499  is a portion of the third wavelength of third coherent light  497  received back from the target. In the illustrated example, third coherent light  497  illuminates first eyebox location  481 . Processing logic  450  is configured to receive and measure third SMI signal  463 C. 
     Target data  453  may be generated by processing logic  450  in response to receiving two or more SMI signals  463 . In the illustrated implementations, target data  453  is generated in response to first SMI signal  463 A, second SMI signal  463 B, and third SMI signal  463 C. While only three time periods and three SMI signals  463  are illustrated in  FIGS.  4 A- 4 C , those skilled in the art appreciated that hundreds or thousands of SMI signals per second may be generated by SMI sensor  440  as the swept-source laser progresses through its scan. Thus, processing logic  450  may receive a plurality of SMI signals prior to generating target data  453  illustrated in  FIG.  4 C . Of course, target data  453  may be referred to as eye data  453  in the context of scanning eye  280 . 
       FIG.  5    illustrates a process  500  of eye-tracking using a multi-wavelength SMI architecture, in accordance with aspects of the disclosure. The order in which some or all of the process blocks appear in process  500  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. 
     In process block  505 , an eyebox region is illuminated with first coherent light having a first wavelength. 
     In process block  510 , the eyebox region is illuminated with second coherent light having a second wavelength that is different from the first wavelength. The first wavelength and the second wavelength may be different wavelengths of near-infrared light. 
     In process block  515 , a first SMI signal is generated in response to first feedback light. The first feedback light is a portion of the first coherent light received back from the eyebox region. 
     In process block  520 , a second SMI signal is generated in response to second feedback light. The second feedback light is a portion of the second coherent light received back from the eyebox region. 
     In process block  525 , eye data is generated in response to at least the first SMI signal and the second SMI signal. Process  500  may return to process block  505  to continue scanning the eyebox region. 
     In a multi-wavelength SMI implementation that includes a swept-source laser (e.g. implementation  400 ), the first coherent light in process  500  may be coherent light  490  and the first feedback light may be feedback light  492 . The first coherent light may be emitted by a swept-source laser of an SMI sensor (e.g. sensor  440 ) during a first time period and the first SMI signal (e.g. signal  463 A) is generated by a light sensor optically coupled to a swept-source optical cavity of the SMI sensor. The second coherent light in process  500  may be coherent light  491  and the second feedback light may be feedback light  493 . The second coherent light may be emitted by the swept-source laser during a second time period and the second SMI signal (e.g. signal  463 B) is generated by the light sensor optically coupled to the swept-source optical cavity of the SMI sensor. In this context, the eyebox region may be illuminated by the second coherent light subsequent to being illuminated by the first coherent light and the eye data is eye data  453 . The first coherent light and the second coherent light may illuminate the same eyebox location (e.g. eyebox location  481 ). If process  500  is implemented in the context of implementation  400 , process  500  may be executed partially or entirely by processing logic  450 . 
     In a multi-wavelength SMI implementation that includes multiple SMI sensors (e.g. implementation  200 ), the first coherent light in process  500  may be coherent light  290  and the first feedback light may be feedback light  292 . The first coherent light may be emitted by a first light source of a first optical cavity of a first SMI sensor (e.g. sensor  240 A) and the first SMI signal (e.g. signal  263 A) is generated by a first light sensor optically coupled to the first optical cavity of the first SMI sensor. The second coherent light in process  500  may be coherent light  291  and the second feedback light may be feedback light  293 . The second coherent light may be emitted by a second light source of a second optical cavity of a second SMI sensor and the second SMI signal is generated by a second light sensor optically coupled to the second optical cavity of the second SMI sensor. In this context, the first coherent light and the second coherent light may be emitted simultaneously. In some implementations, the first coherent light may illuminate a first eyebox location (e.g. eyebox location  281 ) and the second coherent light may illuminate a second eyebox location (e.g. eyebox location  282 ). If process  500  is implemented in the context of implementation  200  of  FIG.  2   , process  500  may be executed partially or entirely by processing logic  250 . 
     Embodiments of the invention may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, e.g., a virtual reality (VR), an augmented reality (AR), a mixed reality (MR), a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, e.g., create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a head-mounted display (HIVID) connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. 
     The term “processing logic” (e.g. processing logic  250 ,  350 , or  450 ) in this disclosure may include one or more processors, microprocessors, multi-core processors, Application-specific integrated circuits (ASIC), and/or Field Programmable Gate Arrays (FPGAs) to execute operations disclosed herein. In some embodiments, memories (not illustrated) are integrated into the processing logic to store instructions to execute operations and/or store data. Processing logic may also include analog or digital circuitry to perform the operations in accordance with embodiments of the disclosure. 
     A “memory” or “memories” described in this disclosure may include one or more volatile or non-volatile memory architectures. The “memory” or “memories” may be removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Example memory technologies may include RAM, ROM, EEPROM, flash memory, CD-ROM, digital versatile disks (DVD), high-definition multimedia/data storage disks, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information for access by a computing device. 
     Communication channels may include or be routed through one or more wired or wireless communication utilizing IEEE 802.11 protocols, BlueTooth, SPI (Serial Peripheral Interface), I 2 C (Inter-Integrated Circuit), USB (Universal Serial Port), CAN (Controller Area Network), cellular data protocols (e.g. 3G, 4G, LTE, 5G), optical communication networks, Internet Service Providers (ISPs), a peer-to-peer network, a Local Area Network (LAN), a Wide Area Network (WAN), a public network (e.g. “the Internet”), a private network, a satellite network, or otherwise. 
     A computing device may include a desktop computer, a laptop computer, a tablet, a phablet, a smartphone, a feature phone, a server computer, or otherwise. A server computer may be located remotely in a data center or be stored locally. 
     The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a tangible or non-transitory machine (e.g., computer) readable storage medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or otherwise. 
     A tangible non-transitory machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable storage medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.). 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.