Patent Publication Number: US-2023152888-A1

Title: Light Scanner Having Closed-Loop Control Based on Optical Feedback

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case is a continuation-in-part of co-pending U.S. patent application Ser. No. 17/874,385, filed Jul. 27, 2022 (Attorney Docket: 3001-006US4), which is a continuation-in-part of U.S. patent application Ser. No. 17/143,048 (now U.S. Pat. No. 11,435,823), filed Jan. 6, 2021 (Attorney Docket: 3001-006US3), which is a continuation-in-part of U.S. Non-Provisional patent application Ser. No. 17/087,302 (now U.S. Pat. No. 11,379,035), filed Nov. 2, 2020 (Attorney Docket: 3001-006US2), which is a continuation of U.S. Non-Provisional patent application Ser. No. 15/876,148 (now U.S. Pat. No. 10,824,229), filed Jan. 20, 2018 (Attorney Docket: 3001-006US1), which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/448,577 filed Jan. 20, 2017, and U.S. Provisional Patent Application Ser. No. 63/227,160 filed Jul. 29, 2021, each of which is incorporated by reference as if set forth at length herein. 
     The present Specification also includes concepts disclosed in U.S. Pat. No. 10,213,105, issued Feb. 26, 2019 (Attorney Docket: 3001-004US1), U.S. Pat. No. 10,317,672, issued Jun. 11, 2019 (Attorney Docket: 3001-004US2), and U.S. Pat. No. 10,908,683, issued Feb. 2, 2021 (Attorney Docket: 3146-004US1), each of which is incorporated by reference as if set forth at length herein. If there are any contradictions or inconsistencies in language between this application and the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in the instant case should be interpreted to be consistent with the language in the instant Specification. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to object-tracking systems and, more particularly, to eye-tracking systems. 
     BACKGROUND 
     MEMS-based scanning elements are very small form-factor devices used to steer optical beams in one or more dimensions. They are attractive for use in many applications, such as LiDAR, object tracking, free-space optical switching, eye tracking and more. 
     Typically, a scanning element is driven “open loop,” where the positional response of the element to a given drive voltage is assumed based on an initial calibration routine in which the position of the element is measured for a set of drive voltages, thereby indicating a transfer function for the device. 
     Unfortunately, while open-loop driving of a scanning element is the simplest approach, a device&#39;s transfer function is affected by many factors, including fabrication variations, variations within manufacturing positional and angular tolerances, and system drift. Such factors can lead to deviations from its anticipated response and uncertainty in the actual response of a mirror to a given drive signal. 
     Furthermore, these issues are exacerbated when a mirror is electrothermally actuated. Such actuators can subject a mirror to large temperature excursions and mechanical stresses that are known to accelerate material-degradation processes such as creep, electromigration, chemical reactions, and desorption, which can contribute to long-term instabilities in the material properties of the mirror and its actuators, thereby inducing changes in its transfer function. Still further, package stresses can couple into mirror devices, and variations in package stresses can be caused by gradual changes in substrate temperatures. 
     In many applications, such as eye tracking, device or system drift inevitably leads to a decrease in accuracy over time. As a result, frequent recalibration is necessary to enable proper operation of conventional MEMS-based scanning elements. 
     A scanning element that can be driven in closed-loop fashion would mitigate some, if not all, of the above issues; however, it has proven difficult to reliably sense the instantaneous position of a MEMS-based scanning element in order to provide a feedback signal to enable a closed-loop drive scheme. 
     SUMMARY 
     The present disclosure is directed toward two-axis, resonant, MEMS-based light scanners having an integrated feedback signal that indicates the instantaneous position of its scanning element. Embodiments in accordance with the present disclosure are particularly well suited for use in object tracking systems that employ a Lissajous light-scan pattern for rapidly interrogating a scan region with high scan density. 
     An advance over the prior art is realized by monitoring the instantaneous position of a two-axis scanning element using an optical-feedback signal arising from the mirror itself and within the package in which the scanning element is contained. This positional information can be used for calibration of the scanning element and/or in a closed-loop element-control drive scheme used to control the scanning element during operation. 
     An illustrative embodiment in accordance with the present disclosure is an eye tracking system that includes a source module and detector module mounted on a conventional eyeglass frame such that the source module is configured to steer a light signal through a two-dimensional pattern in a scan region on an eye, while the detector module is configured to detect one or more reflections of the light signal from the scan region. The detector module provides a commensurate output signal to a processor for determination of the gaze vector of the eye. 
     The source module includes a package having a housing and a transparent lid, where the housing encloses a scanner chip comprising a MEMS-based resonant scanning mirror that is monolithically integrated with a plurality of photodiodes that function as monitor detectors. The scanner chip and housing are arranged such that, as the light signal is directed through the lid and steered through its two-dimensional pattern, Fresnel reflections of the light signal off the lid are received by the monitor detectors. The output signals of the monitor detectors are monitored as a function of time and are used to determine the instantaneous position of the mirror. 
     In some embodiments, the drive signals and corresponding scanning-element positions are used in a calibration routine for determining the transfer function of the scanner. 
     In some embodiments, changes in the transfer function of the scanner are detected as changes in the output signals of the photodiodes, enabling the drive signals provided to the scanner to be adjusted accordingly. 
     In some embodiments, the lid includes an optical element configured to redirect a portion of the scan signal toward the photodiodes. 
     An embodiment in accordance with the present disclosure is a system comprising: a source module comprising: a light scanner having a scanning element that is configured to receive a first light signal and steer (1) an interrogation signal in a first two-dimensional pattern over a scan region and (2) an optical-feedback signal in a second two-dimensional pattern over a first region, the interrogation signal including a first portion of the first light signal and the optical-feedback signal including a second portion of the first light signal, wherein the light scanner comprises a micromechanical system (MEMS) device that is a two-axis device having a first axis characterized by a first resonant frequency and a second axis characterized by a second resonant frequency; and a first package that contains the light scanner, the first package including a lid that is substantially transparent for the first light signal; a position monitor located in the first region, the position monitor configured to provide a feedback signal based on the optical-feedback signal; and a processor configured to estimate an instantaneous position of the scanning element based on the feedback signal. 
     Another embodiment in accordance with the present disclosure is a method comprising: receiving a first light signal at a light scanner that is a micromechanical system (MEMS) device having a scanning element, a first actuator, and a second actuator, the first actuator being configured to rotate the scanning element about the first axis and the second actuator configured to rotate the scanning element about a second axis, wherein the first axis has a first resonant frequency and the second axis has a second resonant frequency; driving the light scanner to steer (1) an interrogation signal in a first two-dimensional pattern over a scan region and (2) an optical-feedback signal in a second two-dimensional pattern over a first region, the interrogation signal including a first portion of the first light signal and the optical-feedback signal including a second portion of the first light signal; providing a feedback signal based on the optical-feedback signal, the feedback signal being provided by a position monitor located within the first region; and estimating an instantaneous position of the scanning element based on the feedback signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts a high-level schematic block diagram illustrating an exemplary eye tracking system according to aspects of the present disclosure. 
         FIG.  2    depicts a schematic drawing of an exemplary operational geometry for an eye-tracker system in accordance with the present disclosure. 
         FIG.  3    depicts a schematic drawing of a cross-sectional view of a first example of a source module in accordance with the present disclosure. 
         FIG.  4    depicts a more detailed schematic drawing of a plan view of a scanner chip in accordance with the present disclosure. 
         FIG.  5    depicts a schematic drawing of a cross-sectional view of a second example of a source module in accordance with the present disclosure. 
         FIG.  6    depicts a schematic drawing of a cross-sectional view of a second example of a source module in accordance with the present disclosure. 
         FIG.  7    depicts a schematic drawing of a first exemplary eye tracking system comprising an optical arrangement for system-level optical position sensing in accordance with the present disclosure. 
         FIG.  8    depicts a schematic drawing of a second exemplary eye tracking system comprising an optical arrangement for system-level optical position sensing in accordance with the present disclosure. 
         FIG.  9    depicts a schematic drawing of a source module for optically monitoring beam position using an interference pattern in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. 
     Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
     Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     The functions of the various elements shown in the Drawing, including any functional blocks that may be labeled as “processors”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. 
     Software modules, or simply modules which are implied to be software, may be represented herein as any combination of flowchart elements or other elements indicating performance of process steps and/or textual description. Such modules may be executed by hardware that is expressly or implicitly shown. 
     Unless otherwise explicitly specified herein, the figures comprising the drawing are not drawn to scale. 
       FIG.  1    depicts a high-level schematic block diagram illustrating an exemplary eye tracking system according to aspects of the present disclosure. As will be apparent to those skilled in the art by inspection of this figure, such illustrative systems constructed according to aspects of the present disclosure exhibit substantial improvements in size, cost, power consumption, bandwidth and precision as compared with prior art eye-tracking systems. 
     In the depicted example, eye-tracking system  100  includes source module  102 , detector module  104 , and processor  106 , among other elements (e.g., memory, power module, etc.). 
     Source module  102  includes light source  108 , scanner  110 , and position monitor  112 . 
     Light source  108  is a conventional light source configured for providing a suitable light signal to scanner  110 . In the depicted example, light source  108  provides a light signal having approximately 1 mW of optical power at 940 nm. The light signal has a Full-Width-at-Half-Maximum (FWHM) divergence of approximately 20 degrees, which is collimated and redirected toward scanner  110  by a reflective Off-Axis-Parabola (OAP) that is integrated in the package, as discussed below. After it reflects off the OAP, the light signal has a divergence of approximately 1-2 degrees. It should be noted, however, that light source  108  can be any of any variety known in the art without departing from the scope of the present disclosure. 
     Scanner  110  is a two-axis resonant MEMS-based scanning element that is configured to receive the light signal and scan it in a two-dimensional scan pattern over a region of an eye being tracked. As discussed in more detail below, in the depicted example, the scanning element of scanner  110  is a mirror; however, myriad scanning element can be used in a scanner without departing from the scope of the present disclosure. As discussed in more detail below, scanner  110  includes first and second rotation axes, each of which is characterized by a resonant frequency, and each of which is driven with a periodic signal whose drive frequency is close to the resonant frequency of its respective axis. Emphasizing simple design principles according to the present disclosure, scanner  110  is preferably a two-axis, resonant micro-electromechanical system (MEMS) device having two orthogonal rotation axes. 
     Position monitor  112  is an optical detector configuration for providing feedback signal  114 , which provides processor  106  with an indication of the instantaneous position of the mirror of scanner  110 . Position monitor is described in more detail below and with respect to  FIGS.  3  and  4   . 
     Detector module  104  includes one or more non-imaging photodetectors configured to receive a portion of the light signal that is reflected from the eye region and provide output signal  116  to processor  106 . In the depicted example, detector module  104  includes a single non-imaging, discrete photodetector. For the purposes of this disclosure, including the appended claims, a “discrete detector” is defined as an optoelectronic device having no more than four electrically independent detection regions on a single substrate, where each detection region is operative for providing one electrical signal whose magnitude is based on the intensity of light incident upon that detection region. Examples of discrete detectors include detectors having only one detection region, split detectors having two detection regions, four-quadrant detectors having four detection regions, and position-sensitive detectors. The definition of discrete detector explicitly excludes individual pixels, or groups of pixels, within array devices for collectively providing spatially correlated image information (i.e., imaging detectors), such as focal-plane arrays, image sensors, and the like. 
     In some embodiments, detector module  104  includes multiple non-imaging photodetectors that provide multiple output signals to the processor. Photodetector  104  can be any of any variety known in the art without departing from the scope of the present disclosure. Although the depicted example includes a photodiode module having a single photodiode, in some embodiments, multiple photodiodes are used to provide a richer data set for position of the eye. 
     As illustrated, processor  106  is communicatively coupled to source module  102  via a number of signal lines namely, pwr, drive signal  118  (which includes Vθ and Vϕ), Vpd, Vvcsel, gnd, and feedback signal  114 , which correspond to electrical power, a drive signal for the scanner, driving voltages for detector module  104  and light source  108 , ground signals, and a position-feedback signal for the mirror of scanner  110 , respectively. Note that for simplicity, individual elements comprising the module(s) are not specifically shown in this illustrative figure namely, the MEMS scanning device, photodetector, VCSEL etc. As we shall show and describe, source module and detector module are located and operated at a physical distance apart from one another. 
     Processor  106  is a controller/processor configured to drive the components of source module  102  with appropriate drive signals, receive feedback signal  114  from source module  102 , receive output signal  116  from detector module  104 , and generate an estimate of the gaze vector of an eye being monitored with system  100  based on output signal  116 , among other functions. 
     In some embodiments, processor  106  includes one or more components containing processing and control circuitry that can include hardware structured to execute functions in accordance with the present disclosure. In some embodiments, such circuitry can include machine-readable media for configuring the hardware to execute the functions described herein. Furthermore, the processing circuitry of processor  106  can be embodied as one or more circuitry components including, but not limited to, processing circuitry, network interfaces, peripheral devices, input devices, output devices, sensors, etc. In some embodiments, such processing circuitry can take the form of one or more analog circuits, electronic circuits (e.g., integrated circuits (IC), application-specific integrated-circuits (ASICs), discrete circuits, system on a chip (SOCs) circuits, etc.), telecommunication circuits, hybrid circuits, and any other type of “circuit,” or combinations thereof. In this regard, “processing circuitry” can include one or more of any type of component for accomplishing or facilitating achievement of operations in accordance with the present disclosure. For example, a circuit as described herein can include one or more transistors, logic gates (e.g., NAND, AND, NOR, OR, XOR, NOT, XNOR, etc.), resistors, multiplexers, registers, capacitors, inductors, diodes, wiring, and so on). 
     “Processing circuitry” can also include one or more processors and/or controllers communicably coupled to one or more memory or memory devices. In this regard, the one or more processors can execute instructions stored in the memory or can execute instructions otherwise accessible to the one or more processors. In some embodiments, the one or more processors can be embodied in various ways. The one or more processors can be constructed in a manner sufficient to perform at least the operations described herein. In some embodiments, the one or more processors can be shared by multiple circuits (e.g., circuit A and circuit B can comprise or otherwise share the same processor which, in some example embodiments, can execute instructions stored, or otherwise accessed, via different areas of memory). Alternatively, or additionally, the one or more processors can be structured to perform or otherwise execute certain operations independent of one or more co-processors. In other example embodiments, two or more processors can be coupled via a bus to enable independent, parallel, pipelined, or multi-threaded instruction execution. Each processor can be implemented as one or more general-purpose processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), digital signal processors (DSPs), or other suitable electronic data processing components structured to execute instructions provided by memory. The one or more processors can take the form of a single core processor, multi-core processor (e.g., a dual core processor, triple core processor, quad core processor, etc.), microprocessor, etc. In some embodiments, the one or more processors can be external to the apparatus, for example the one or more processors can be a remote processor (e.g., a cloud-based processor). Alternatively, or additionally, the one or more processors can be internal and/or local to the apparatus. In this regard, a given circuit or components thereof can be disposed locally (e.g., as part of a local server, a local computing system, etc.) or remotely (e.g., as part of a remote server such as a cloud-based server). To that end, processing circuitry in accordance with the present disclosure can include components that are distributed across one or more locations. 
     The present disclosure contemplates methods, systems and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure can be implemented using existing computer processors, or by a special purpose computer processor and/or controller for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general-purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
       FIG.  2    depicts a schematic drawing of an exemplary operational geometry for an eye-tracker system in accordance with the present disclosure. Some eye-tracker system geometries according to aspects of the present disclosure include a MEMS scanner that sweeps a beam of light (i.e., an interrogation signal) in a two-dimensional scan pattern over a region of an eye (typically the region containing the cornea) and a photodetector that receives light reflected from the region. 
     In the depicted example, source module  102  is mounted to temple  214  of eyeglass frame  212 , while detector module  104  is mounted to the frame  212  near bridge  216 . 
     Light source  108  emits light signal  218 , which comprises optical energy that is substantially collimated within source module  102 . At least a portion of light signal  218  is then steered by scanner  110  over scan region  206  as interrogation signal  202 . In the depicted example, scan region  206  includes cornea  208 ; however, in some embodiments, a scan region includes a different surface feature of the eye. 
     The scan pattern generated by source module  102  is based on drive signal  118 , which includes electrical signals Vθ and Vϕ. These electrical signals are provided by processor  106  to the rotation axes of scanner  110 . As discussed below in more detail, each axis of scanner  110  is characterized by a resonant frequency and driven with a periodic signal having a drive frequency that is based on that resonant frequency. Those skilled in the art will appreciate that periodic drive signals provided to each axis of scanner  110  gives rise to a scan pattern that is substantially a Lissajous curve (also known as a Lissajous figure), which is the graph of a system of parametric equations defined by x=A sin(at+δ); y=B sin(bt). 
     Optical energy of interrogation signal  202  is reflected from the surface of cornea  208  at a glancing angle (˜60 degrees to the normal) onto photodiode module  104  as reflected signal  210 . As eye  204  rotates, the intensity of reflected signal  210  changes as a function of the position of a unique point on cornea  208 , thereby enabling a system according to aspects of the present disclosure to track position of this point and, as a consequence, the position of the cornea. Notably, the surface area of the photodiode acts as a spatial filter to remove any high frequencies from the far-field pattern projected by the scanner. 
     Detector module  104  detects reflected signal  210  and provides corresponding output signal  116  to processor  106 , which generates an estimate of the gaze vector of the eye based on the output signal. 
     Typically, prior-art scanning mirrors are driven “open loop” when interrogating a scan region, wherein the positional response of the mirror to a given drive voltage is assumed based on a transfer function for the device determined during an initial calibration routine. However, as discussed above, over time this transfer function can change due to changes in the material parameters of the scanning mirror and/or its actuators, packaging issues, environmental influences, and the like. As a result, prior-art scanners require frequent recalibration to ensure their proper function. 
     It is an aspect of the present disclosure that a portion of the optical energy giving rise to interrogation signal  202  can be used in real time as an optical-feedback signal to track the instantaneous position of the scanning mirror. This enables more rapid and simpler calibration of the scanning mirror and/or appropriate compensation of drive signal  118 . In other words, the inclusion of a position monitor enables a closed-loop drive scheme that substantially ensures proper system performance. 
     Preferably, an optical-feedback signal used to track the position of the mirror of scanner  110  arises from within source module  102  itself; however, an optical-feedback signal can be generated anywhere within system  100  without departing from the scope of the present disclosure. 
       FIG.  3    depicts a schematic drawing of a cross-sectional view of a first example of a source module in accordance with the present disclosure. Source module  102  comprises package  302 , which encloses scanner chip  304  and collimating optic  306 . 
     Package  302  is a substantially environmentally sealed package that includes housing  308  and lid  310 . 
     Housing  308  is a conventional electronics package suitable for containing scanner chip  304  and enabling electrical connectivity the scanner chip and light source  108 . 
     Lid  310  is a substantially transparent plate that is sealed in conventional fashion to housing  308  to provide an environmentally protected environment for scanner chip  304 , light source  108 , and collimating optic  306 , each of which is enclosed therein. In the depicted example, lid  310  is a glass substrate that is epoxied to housing  308  to complete package  302  and fully enclose scanner chip  304  and light source  108 . 
       FIG.  4    depicts a more detailed schematic drawing of a plan view of a scanner chip in accordance with the present disclosure. Scanner chip  304  includes light source  108 , scanner  110 , and position monitor  112 . 
     Scanner  110  is an isothermally actuated, two-axis, gimbal-mounted scanning mirror that is configured to controllably rotate about two orthogonal axes. Scanner  110  includes mirror  402  and actuators  404 -θ and  404 -ϕ. 
     Position monitor  112  is an optical detector configuration that provides electrical feedback signal  114  to processor  106  based on the instantaneous position of the mirror of scanner  110 . 
     In the depicted example, scanner  110  and position monitor  112  are formed on substrate  312  such that they are monolithically integrated. Preferably, scanner  110  and position monitor  112  are fabricated via conventional CMOS fabrication techniques, as described in U.S. Patent Publication 20150047078, entitled “Scanning Probe Microscope Comprising an Isothermal Actuator,” published Feb. 12, 2015, and U.S. Patent Publication 20070001248, entitled “MEMS Device Having Compact Actuator,” published Jan. 4, 2007, each of which is incorporated herein by reference. It should be noted that, the use of CMOS fabrication to form the scanner and position monitor facilitates their monolithic integration. 
     Furthermore, in the depicted example, scanner chip  304  also includes light source  108 , which is joined to substrate  312  using conventional hybrid integration techniques. In some embodiments, at least one of light source  108  and position monitor  112  is located on a different substrate than scanner  110 . In some embodiments, light source  108 , scanner  110 , and position monitor  112  are separate chips that are mounted on a common substrate to form a multi-chip module. 
     Mirror  402  is a substantially flat first-reflector surface formed on a structurally rigid plate. Although the depicted scanner comprises a scanning element that is a plane mirror, in some embodiments, scanner  110  includes a different scanning element, such as a Fresnel zone plate, etc. As will be apparent to one skilled in the art, after reading this Specification, scanner  110  can include any suitable scanning element for steering a light signal without departing from the scope of the present disclosure. Examples of alternative optical elements suitable for use in accordance with the present disclosure include, without limitation, other diffractive elements (e.g., holograms, Dammann gratings, etc.), refractive elements, lenses, and the like. 
     Mirror  402  is operatively coupled with actuators  404 -θ and  404 -ϕ (referred to, collectively, as actuators  404 ), which are arranged orthogonally about the mirror to enable its rotation about orthogonal θ and ϕ axes, each of which is characterized by a characteristic resonant frequency. 
     Each of actuators  404 -θ and  404 -ϕ comprises a pair of actuator elements that are located on opposite sides of mirror  402 , where the actuator elements are opposed serpentine, silicon-based, thermal actuators that collectively define their respective rotation axes. 
     Actuator  404 -θ is configured to impart rotation about the θ axis in response to voltage Vθ of drive signal  118  from processor  106 , while actuator  404 -ϕ is configured to impart rotation about the ϕ axis in response to voltage Vϕ of drive signal  118  from processor  106 . 
     In the depicted example, mirror  402  is electrically grounded and resonant two-dimensional scanning of the mirror is realized by applying appropriate sinusoidal drive signals to actuators  404 -θ and  404 -ϕ. In response to drive signal  118 , scanner  110  steers the interrogation signal through a desired two-dimensional pattern on scan region  206 . In the depicted example, the desired pattern is a precessing Lissajous curve in response to each of Vθ and Vϕ being a periodic signal whose frequency is close to the resonant frequency of its respective axis. As discussed in the parent application, U.S. patent application Ser. No. 17/874,385, the use of a precessing Lissajous curve to interrogate scan region  206  affords embodiments in accordance with the present disclosure significant advantages over the prior art. 
     It should be noted that mirror  402  and actuators  404 -θ and  404 -ϕ are merely exemplary and a wide range of scanning elements and/or MEMS actuators can be used in scanner  110  without departing from the scope of the present disclosure. Examples of alternative scanning elements and rotary actuators suitable for use in accordance with the present disclosure are described in detail in, for example, U.S. Pat. Pub. Nos. 20180210547, 20150047078, 20160166146, and 20190204913, each of which is incorporated herein by reference. 
     For example, in some embodiments, at least one actuator of a scanner is not an isothermal actuator. In some embodiments, at least one of actuators  404 -θ and  404 -ϕ comprises an actuator other than a thermal actuator, such as an electrostatic comb-drive actuator, parallel lid electrostatic actuator, piezoelectric actuator, electromagnetic actuator, and the like. 
     Returning now to  FIG.  3   , collimating optic  306  is a reflective optical element disposed on inside surface  314  of lid  310 . Collimating optic  306  collimates light signal  218  and redirects it toward scanner  110  without significant loss. After it reflects off collimating optic  306 , light signal  218  has a divergence of approximately 1-2 degrees. In some embodiments, collimating optic  306  only partially reduces the divergence of light signal  218  (i.e., it “partially collimates” the light signal) and scanner  110  further collimates the light signal. In some embodiments, source module  102  does not include collimating optic  306 . 
     In the depicted example, collimating optic  306  is reflective OAP. In some embodiments, collimating optic  306  is a different optical element, such as a flat mirror, a metasurface, a diffractive element, and the like. 
     Scanner  110  receives the light signal  218  and redirects a portion of it through lid  310  as interrogation signal  202 , scanning it in the desired precessing Lissajous pattern to interrogate scan region  206 . At surface  314 , Fresnel reflections give rise to optical-feedback (OF) signal  316 , which comprises a second portion of light signal  218  that typically includes up to 5% of its optical energy. As interrogation signal  202  is scanned in a two-dimensional pattern over scan region  206 , OF signal  316  forms an analogous scan pattern over the portion of scanner chip  304  that includes position monitor  112 . 
     In some embodiments, as opposed to relying on only Fresnel reflections at surface  314 , an optical element is included in lid  310  to give rise to OF signal  316 . Examples of optical element suitable for generating OF signal  316  include, without limitation, flat mirrors, metasurfaces, diffractive elements, diffuse patterns, and the like. 
     Position monitor  112  includes monitor detectors  406 - 1  through  406 - 3  (referred to, collectively, as monitor detectors  406 ), each of which is a photodiode that includes a p-n junction formed in substrate  312  during CMOS fabrication of scanner chip  304 . However, in some embodiments, at least one of monitor detectors  406  is a discrete photodetector or photodiode chip that is joined to substrate  312  using hybrid integration technology. 
     Monitor detectors  406  are distributed on the surface of substrate  312  such that, as scanner  110  steers interrogation signal  202  through its two-dimensional pattern, OF signal  316  sweeps over monitor detectors  406 - 1  through  406 - 3  such that each monitor detector generates one or more output pulses at one or more times that depend on the unique position of that monitor detector and the instantaneous position of mirror  402  that resulted in the optical-feedback signal being incident upon it. 
     Monitor detectors  406 - 1  through  406 - 3  provide output signals  408 - 1  through  408 - 3 , respectively, to processor  106  as feedback signal  114 . 
     It should be noted that the relative positions of monitor detectors  406 - 1  through  406 - 3  and mirror  402  will remain fixed throughout the lifetime of source module  102  unless package  302  is damaged or deformed in some way. 
       FIG.  5    depicts a schematic drawing of a cross-sectional view of a second example of a source module in accordance with the present disclosure. Source module  500  comprises package  502 , which encloses scanner chip  504  and light source  506 . 
     Package  502  is analogous to package  302 ; however, package  502  includes lid  508 , which is configured to mount light source  506  on its inner surface  510 , as well as provide electrical connectivity to the light source via conductive traces (not shown). 
     Scanner chip  504  is analogous to scanner chip  304 ; however, scanner chip  504  includes only scanner  110  and position monitor  112 . 
     Light source  506  is analogous to light source  108 ; however, light source  506  includes an integrated lens that substantially collimates light signal  218  as it exits the light source. In some embodiments, light signal  218  is at least partially collimated by scanner  110 . 
       FIG.  6    depicts a schematic drawing of a cross-sectional view of a second example of a source module in accordance with the present disclosure. Source module  600  comprises package  502 , which encloses scanner chip  602  and light source  506 . 
     Scanner chip  602  is analogous to scanner chip  304 ; however, scanner chip  602  includes scanner  604 , whose scanning element comprises a diffraction grating that splits a portion of the optical energy of light signal  218  off as OF signal  608  at a known angle relative to the plane of the scanning element. In other words, the scanning element of scanner  604  is configured to distribute light signal  218  into a first portion as interrogation signal  606  and a second, smaller, portion as OF signal  608 . 
     In some embodiments, the scanning element of scanner  604  is a different element suitable for this purpose, such as a mirrored facet, a blazed grating, a diffractive optical element, a hologram, and the like. 
     Interrogation signal  202  is used to interrogate scan region  206 , as described above. 
     OF signal  608  is directed toward redirecting optic  606 , which redirects the optical-feedback signal toward the portion of substrate  312  on which position monitor  112  is disposed. 
     In some embodiments, one or more position monitors are positioned within an overall system (such as a VR headset) but outside the scanner-module package. In some such embodiments, the components are arranged such that there is a direct path between interrogation signal and the position monitor(s), thereby enabling interrogation-signal position to be monitored relative to the overall eye-tracking system instead of only to the scanner package. In some embodiments, this is achieved by over-scanning or by redirecting part of interrogation signal  202 . 
       FIG.  7    depicts a schematic drawing of a first exemplary eye tracking system comprising an optical arrangement for system-level optical position sensing in accordance with the present disclosure. System  700  is analogous to system  100  described above; however, system  700  includes a source module configured to generate an optical-feedback signal that is sensed by a position monitor located outside of the source-module package. 
     Source module  702  is analogous to source module  600 ; however, in source module  702 , OF signal  706  is directed through lid  508  such that its propagation path diverges from that of interrogation signal  202 . 
     Position monitor  704  is analogous to position monitor  112 ; however, position  704  is contained in a separate package to that of source module  702 . Position monitor  704  can be mounted to frame  212  at any location suitable for enabling the position monitor to detect OF signal  706  during the scan period of interrogation signal  202 . 
     As interrogation signal  202  is scanned about the area of scan region  206 , OF signal  706  is scanned over the region of system  700  that contains position monitor  704 . The position monitor then provides a commensurate output signal to processor  106 , as described above and with respect to  FIGS.  3  and  4   . 
     In some embodiments, more than one position monitor is included in system  700 . 
       FIG.  8    depicts a schematic drawing of a second exemplary eye tracking system comprising an optical arrangement for system-level optical position sensing in accordance with the present disclosure. System  800  is analogous to system  100 ; however, like system  700 , system  800  includes a position monitor located outside of its source-module package. System  800  comprises source module  102 , reflector  802 , lens  804 , and position monitors  704 A and  704 B, as well as additional components for enabling eye tracking (e.g., detector module  104 , processor  106 , etc.). 
     Reflector  802  is configured to redirect interrogation signal  202  through conventional refractive lens  804 , which directs the interrogation signal to scan region  206 . 
     At lens  804 , Fresnel reflections cause some of the light of interrogation signal  202  to be redirected back toward reflector  802  as OF signal  806 . 
     As interrogation signal  202  is scanned about the area of scan region  206 , OF signal  806  is scanned over the portion of system  800  containing position monitors  704 A and  704 B. The position monitors then provide commensurate output signals to processor  106 , as described above and with respect to  FIGS.  3  and  4   . 
       FIG.  9    depicts a schematic drawing of a source module for optically monitoring beam position using an interference pattern in accordance with the present disclosure. Source module  900  is analogous to source module  102  described above; however, source module  900  is configured to exploit self-mixing of interrogation signal  202  to generate an optical-monitoring signal indicative of the instantaneous position of mirror  402 . 
     As will be apparent to one skilled in the art, after reading this Specification, self-mixing can occur at several angles of scanning mirror  402  while the interrogation signal is scanned about the area of scan region  206 . 
     Such self-mixing cases an internal reflection back to light source  108 , which in the depicted example, is a laser diode. This reflection back to light source  108  gives rise to an interference pattern that changes its optical output characteristics. 
     These changes of the output characteristics of the light source can be measured electrically (or optically) at the laser driver used to drive light source  108 . An interferometry event can be designed to occur at specific spatial locations, as discussed, for example, by N. Sarkar, et al., in U.S. Pat. No. 9,267,962, issued Feb. 23, 2016, which is incorporated herein by reference. This information can be used to map the system&#39;s angle outputs based on input drive conditions. In practice, with known feedback angles that produce a measurable time domain signal, a scanning pattern (e.g., a Lissajous pattern, etc.) can be calibrated into mirror space. 
     As briefly discussed above, embodiments disclosed herein provide an optical-feedback signal that can be used for calibration of an object tracking system or as a feedback signal in a closed-loop drive scheme. In addition, the output of a position monitor in accordance with the present disclosure can be used to determine the natural resonant frequency of each axis of a two-axis, resonant scanner, thereby enabling identification of a preferred set of drive frequencies for each axis of the scanner and/or quasi-closed-loop operation of the scanner, wherein the resonant frequencies of its axes can be periodically updated to ensure proper drive frequencies are used. 
     It is to be understood that the disclosure teaches just some examples of embodiments in accordance with the invention and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.