Self-mixing interferometry-based absolute distance measurement with distance reference

A device includes a first component, a second component having a reconfigurable distance from the first component, an optical element, an SMI sensor, and a processor. The optical element has a fixed relationship with respect to the first component, and has a known optical thickness between a first surface and a second surface of the optical element. The SMI sensor has a fixed relationship with respect to the second component, and has an electromagnetic radiation emission axis that intersects the first and second surfaces of the optical element. The processor is configured to identify disturbances in an SMI signal generated by the SMI sensor, relate the disturbances to the known optical thickness of the optical element, and to determine a distance between the first and second components using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

FIELD

The described embodiments generally relate to devices that include one or more self-mixing interferometry (SMI) sensors and, more particularly, to devices that use a distance reference when measuring absolute distance using an SMI sensor.

BACKGROUND

Sensor systems are included in many of today's electronic devices, including electronic devices such as smartphones, computers (e.g., tablet computers or laptop computers), wearable electronic devices (e.g., electronic watches or health monitors), game controllers, navigation systems (e.g., vehicle navigation systems or robot navigation systems), and so on. Sensor systems may variously sense the presence of objects, distances to objects or proximities of objects, movements of objects (e.g., whether objects are moving, or the speed, acceleration, or direction of movement of objects), and so on.

Given the wide range of sensor system applications, any new development in the configuration or operation of a sensor system can be useful. New developments that may be particularly useful are developments that reduce the cost, size, complexity, part count, or manufacture time of the sensor system, or developments that improve the sensitivity or speed of sensor system operation, or developments that enable new functionality or applications.

SUMMARY

Embodiments of the systems, devices, methods, and apparatus described in the present disclosure are directed to use a distance reference when measuring the absolute distance between two objects using an SMI sensor. The distance reference (e.g., an optical element, such as an optical film) may have a fixed relationship with one of the objects (or may be one of the objects), and the SMI sensor may have a fixed relationship with the other of the objects (or may be one of the objects).

An SMI sensor is defined herein as a sensor configured to generate electromagnetic radiation (e.g., light), emit the electromagnetic radiation from a resonant cavity (e.g., a resonant optical cavity), receive a reflection or backscatter of the electromagnetic radiation (e.g., electromagnetic radiation reflected or backscattered from a surface, or an object having a surface (collectively referred to herein as a surface)) back into the resonant cavity, coherently or partially coherently self-mix the generated and reflected/backscattered electromagnetic radiation within the resonant cavity, and produce an output indicative of the self-mixing (i.e., an SMI signal). The generated, emitted, and received electromagnetic radiation may be coherent or partially coherent. In some examples, the electromagnetic radiation emitted by an SMI sensor may be generated by an electromagnetic radiation source such as a vertical-cavity surface-emitting laser (VCSEL), a vertical external-cavity surface-emitting laser (VECSEL), a quantum-dot laser (QDL), a quantum cascade laser (QCL), or a light-emitting diode (LED) (e.g., an organic LED (OLED), a resonant-cavity LED (RC-LED), a micro LED (mLED), a superluminescent LED (SLED), or an edge-emitting LED), 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 an SMI sensor (i.e., an SMI 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's electromagnetic radiation source. Alternatively or additionally, the output of an SMI sensor may include a measurement of the current or junction voltage of the SMI sensor's electromagnetic radiation source. The recoupling of reflected electromagnetic radiation, back into the resonant cavity of an SMI sensor, produces disturbances in the SMI signal, which disturbances can be measured and analyzed to determine, for example, the displacement, velocity, and/or absolute distance of a target.

In a first aspect, the present disclosure describes a device. The device may include a frame, an optical element attached to the frame, an SMI sensor attached to the frame, and a processor. The optical element may have a known optical thickness between a first surface and a second surface of the optical element. The SMI sensor may be configured to emit a modulated beam of electromagnetic radiation toward the first surface of the optical element and generate an SMI signal containing disturbances caused by reflections or backscatters of the beam from the first surface and the second surface. The processor may be configured to relate the disturbances to the known optical thickness of the optical element, and to determine a distance between two objects using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

In another aspect of the disclosure, the present disclosure describes another device. The device includes a first component, a second component having a reconfigurable distance from the first component, an optical element, an SMI sensor, and a processor. The optical element may have a fixed relationship with respect to the first component, and may have a known optical thickness between a first surface and a second surface of the optical element. The SMI sensor may have a fixed relationship with respect to the second component, and may have an electromagnetic radiation emission axis that intersects the first and second surfaces of the optical element. The processor may be configured to identify disturbances in an SMI signal generated by the SMI sensor; relate the disturbances to the known optical thickness of the optical element; and determine a distance between the first and second components using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

In another aspect, the present disclosure describes a method of determining a distance between a first object and a second object. The method includes emitting a beam of electromagnetic radiation from an SMI sensor having a fixed relationship with respect to the first object; receiving, from the SMI sensor, an SMI signal containing disturbances caused by reflections or backscatters of the beam from first and second surfaces of an optical element having a fixed relationship with respect to the second object; relating the disturbances to a known optical thickness of the optical element; and determining a distance between the first object and the second object, using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

DETAILED DESCRIPTION

The following description relates to systems, devices, methods, and apparatus that use a distance reference when measuring the absolute distance between two objects using an SMI sensor.

Self-mixing interferometry is an optical sensing technology that can be used to measure the displacement of a target with sub-wavelength resolution. By relating displacement measurement to measurement times, a velocity of a target can also be measured. In addition, by inducing a known wavelength modulation (e.g., a triangular modulation) into a beam of electromagnetic radiation emitted by an SMI sensor, the absolute distance between two objects can also be measured (e.g., with 0.1 millimeter (mm) or better resolution).

In some cases, the absolute distance between an SMI sensor and an object may be measured by triangularly modulating a beam of electromagnetic radiation emitted by the SMI sensor (e.g., by triangularly modulating the current of the SMI sensor's emitter). In such cases, the absolute distance between the SMI sensor and object may be determined using the following equation:

Absolute⁢⁢Distance⁢=14⁢ftriangle⁢λ2Δ⁢λ⁢fpeak=C⁢fpeak
where λ is the wavelength of the electromagnetic radiation generated by the SMI sensor; ftriangieis the frequency of the triangular modulation; Δλ, is the wavelength modulation range induced by the wavelength modulation method (e.g., current modulation of the electromagnetic radiation source); and fpeakis the peak frequency of the SMI signal's FFT spectra.

Unfortunately, there are a number of error sources in the above absolute distance measurement. For example, Δλ, can vary by >15% due to temperature change and/or degradation of the SMI sensor's light source (e.g., laser) over the lifetime of the SMI sensor. A can vary by >1% due to temperature change and/or degradation of the SMI sensor's light source (e.g., laser) over the lifetime of the SMI sensor. ftrianglecan vary by <0.001% due to temperature change and/or other factors affecting the stability of electronic components over the lifetime of the SMI sensor. fpeakcan vary based on the signal-to-noise ratio (SNR) and FFT integration time.

As a result of the above error sources, SMI-based absolute distance measurement can be poor (>15% error). The main error source is the change in peak-to-peak wavelength modulation with changes in temperature and SMI sensor degradation over time. Some tracking applications may require more accuracy (e.g., ±50 micrometer (μm) over 10 mm). In such an application, error due to SMI-based error sources can be greater than 1.5 mm, and SMI-based absolute distance measurement may not be suitable. In other words, SMI-based absolute distance measurement may be associated with a measurement error in excess of 15% when the required accuracy is less than 0.5% error.

Described herein are systems, devices, methods, and apparatus that use a distance reference to enable SMI-based absolute distance measurement with better accuracy. The distance reference is an optical element (e.g., an optical film, substrate, or block of material) having a known optical thickness (i.e., a known physical thickness times a known refractive index, each of which is known with high precision). The material used to form the optical element should have a well-defined and low thermal expansion coefficient, and in some cases may be glass, sapphire, or silicon. The optical element may in some cases be manufactured with less accuracy, but measured with high accuracy before integration into a sensing system. Alternatively, the optical element may be manufactured with high accuracy.

When a beam of electromagnetic radiation is emitted toward the optical element (e.g., perpendicular to first and second surfaces separated by a known optical distance (or optical thickness)), the electromagnetic radiation will reflect or backscatter from the first and second surfaces at two distinct frequencies in an SMI signal's FFT spectra. The difference between these frequencies may be considered a reference frequency. The ratio of the known optical thickness of the optical element to the reference frequency provides a high accuracy estimate of the coefficient, C, in the above absolute distance measurement equation (e.g., a known optical thickness of 2 mm is related to a measured frequency difference of 22 kHz-20 kHz (or 2 kHz). In some embodiments, C may be measured during every measurement frame, providing high accuracy SMI-based absolute distance measurement without needing to know, measure, or estimate any of λ, Δλ, or ftriangle. In such an approach, the factors that limit the accuracy of the SMI-based absolute distance measurement are no longer wavelength-dependent, but are instead dependent on the material properties and calibration (e.g., optical thickness measurement) of the distance reference, and their stability under different environmental conditions and throughout the lifetime of the sensor system. Factors for selecting and calibrating a distance reference (e.g., a distance reference made of glass, sapphire, silicon, or other materials), with high accuracy, are well-known.

These and other techniques are described with reference toFIGS.1A-17. 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.

Directional terminology, such as “top”, “bottom”, “upper”, “lower”, “front”, “back”, “over”, “under”, “beneath”, “left”, “right”, etc. may be used with reference to the orientation of some of the components in some of the figures described below. Because components in various embodiments can be positioned in a number of different orientations, directional terminology is used for purposes of illustration only and is in no way limiting. The directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude components being oriented in different ways. The use of alternative terminology, such as “or”, is intended to indicate different combinations of the alternative elements. For example, A or B is intended to include, A, or B, or A and B.

FIGS.1A-1Cshow an example of a device100that includes an SMI-based absolute distance measurement system. The device's dimensions and form factor, including the ratio of the length of its long sides to the length of its short sides, suggest that the device100is a mobile phone (e.g., a smartphone). However, the device's dimensions and form factor are arbitrarily chosen, and the device100could alternatively be any portable electronic device including, for example, a mobile phone, tablet computer, portable computer, portable music player, health monitor device, portable terminal, vehicle navigation system, robot navigation system, gaming device, virtual reality (VR), augmented reality (AR), or mixed reality (MR) device, or other portable or mobile device. The device100could also be a device that is semi-permanently located (or installed) at a single location.FIG.1Ashows a front isometric view of the device100,FIG.1Bshows a rear isometric view of the device100, andFIG.1Cshows a cross-section of the device100along line1C-1C inFIG.1A. The device100may include a frame102that at least partially surrounds a display104. The frame102may include or support a front cover106or a rear cover108, and together with the front and/or rear covers106,108may define a housing for the device100. The front cover106may be positioned over the display104, and may provide a window through which the display104may be viewed. In some embodiments, the display104may be attached to (or abut) the frame102and/or the front cover106. In alternative embodiments of the device100, the display104may not be included and/or the frame102may have an alternative configuration.

The display104may include one or more light-emitting elements including, for example, an LED, OLED, liquid crystal display (LCD), electroluminescent (EL) display, or other type of display element. In some embodiments, the display104may include, or be associated with, one or more touch and/or force sensors that are configured to detect a touch and/or a force applied to a surface of the front cover106.

The various components of the frame102may be formed from the same or different materials. For example, a sidewall118of the frame102may be formed using one or more metals (e.g., stainless steel), polymers (e.g., plastics), ceramics, or composites (e.g., carbon fiber). In some cases, the sidewall118may be a multi-segment sidewall including a set of antennas. The antennas may form structural components of the sidewall118. The antennas may be structurally coupled (to one another or to other components) and electrically isolated (from each other or from other components) by one or more non-conductive segments of the sidewall118. The front cover106may be formed, for example, using one or more of glass, a crystal (e.g., sapphire), or a transparent polymer (e.g., plastic) that enables a user to view the display104through the front cover106. In some cases, a portion of the front cover106(e.g., a perimeter portion of the front cover106) may be coated with an opaque ink to obscure components included within the frame102. The rear cover108may be formed using the same material(s) that are used to form the sidewall118or the front cover106. Alternatively, the rear cover108may be formed using different materials. In some cases, the rear cover108may be part of a monolithic element that also forms the sidewall118(or in cases where the sidewall118is a multi-segment sidewall, those portions of the sidewall118that are non-conductive). In still other embodiments, all of the exterior components of the frame102may be formed from a transparent material, and components within the device100may or may not be obscured by an opaque ink or opaque structure within the frame102.

The front cover106may be mounted to the frame102and/or sidewall118to cover an opening defined by the frame102and/or sidewall118(i.e., an opening into an interior volume in which various electronic components of the device100, including the display104, may be positioned). The front cover106may be mounted to the sidewall118using fasteners, adhesives, seals, gaskets, or other components.

A display stack or device stack (hereafter referred to as a “stack”) including the display104may be attached (or abutted) to an interior surface of the front cover106and extend into the interior volume of the device100. In some cases, the stack may include a touch sensor (e.g., a grid of capacitive, resistive, strain-based, ultrasonic, or other type of touch sensing elements), or other layers of optical, mechanical, electrical, or other types of components. In some cases, the touch sensor (or part of a touch sensor system) may be configured to detect a touch applied to an outer surface of the front cover106(e.g., to a display surface of the device100).

In some cases, a force sensor (or part of a force sensor system) may be positioned within the interior volume below and/or to the side of the display104(and in some cases within the device stack). The force sensor (or force sensor system) may be triggered in response to the touch sensor detecting one or more touches on the front cover106(or a location or locations of one or more touches on the front cover106), and may determine an amount of force associated with each touch, or an amount of force associated with the collection of touches as a whole. Alternatively, the force sensor (or force sensor system) may sense force independently of input from the touch sensor (or touch sensor system), or the force sensor (or force sensor system) may itself be operated as a touch sensor (or touch input system). In some embodiments, the force sensor may determine forces applied to the front cover106using the SMI-based distance, velocity, and motion sensing techniques described herein.

As shown primarily inFIG.1A, the device100may include various other components. For example, the front of the device100may include one or more front-facing cameras110, speakers112, microphones, or other components114(e.g., audio, imaging, and/or sensing components) that are configured to transmit or receive signals to/from the device100. In some cases, a front-facing camera110, alone or in combination with other sensors, may be configured to operate as a bio-authentication or facial recognition sensor. The device100may also include various input devices, including a mechanical or virtual button116, which may be accessible from the front surface (or display surface) of the device100. In some cases, the front-facing camera110, virtual button116, and/or other sensors of the device100may be integrated with a display stack of the display104and moved under the display104.

The device100may also include buttons or other input devices positioned along the sidewall118and/or on a rear surface of the device100. For example, a volume button or multipurpose button120may be positioned along the sidewall118, and in some cases may extend through an aperture in the sidewall118. The sidewall118may include one or more ports122that allow air, but not liquids, to flow into and out of the device100. In some embodiments, one or more sensors may be positioned in or near the port(s)122. For example, an ambient pressure sensor, ambient temperature sensor, internal/external differential pressure sensor, gas sensor, particulate matter concentration sensor, or air quality sensor may be positioned in or near a port122.

In some embodiments, the rear surface of the device100may include a rear-facing camera124or other optical sensor (seeFIG.1B). A flash or light source126may also be positioned along the rear of the device100(e.g., near the rear-facing camera). In some cases, the rear surface of the device100may include multiple rear-facing cameras.

The camera(s), microphone(s), pressure sensor(s), temperature sensor(s), biometric sensor(s), button(s), proximity sensor(s), touch sensor(s), force sensor(s), particulate matter or air quality sensor(s), and so on of the device100may form parts of various sensor systems. In some cases, a sensor system may perform best with a wide FoV or increased granularity (i.e., with a sensing field that includes multiple sensors). In these cases, a sensor system may include an array of sensors (e.g., a 1D or 2D array of sensors). For example, a bio-authentication sensor system may include a 2D array of sensors that emit and receive electromagnetic radiation (e.g., IR electromagnetic radiation). As another example, a presence-sensing sensor system may include a 1D or 2D array of sensors that emit and receive electromagnetic radiation.

FIG.1Cshows an example cross-section of the device100. As shown, one or more SMI sensors128may be mounted within the device100(e.g., attached to the frame102of the device100). A first optional SMI sensor128-1may be oriented so that a beam of electromagnetic radiation130-1emitted by the SMI sensor128-1impinges on the front cover106, or on an optical element132(e.g., an optical film, substrate, or block of material) attached directly to an interior surface of the front cover106(e.g., to one side of a viewing surface of the display104), or attached indirectly to the interior surface of the front cover106(e.g., by virtue of being attached to a display104or device stack attached to the interior surface of the front cover106).

The SMI sensor128-1may have an electromagnetic radiation emission axis that is oriented perpendicular to the front cover106. A processor134(e.g., a microprocessor, application-specific integrated circuit (ASIC), microcontroller, or set of integrated and/or discrete circuits) coupled to the SMI sensor128-1may modulate a wavelength of the beam of electromagnetic radiation (to produce a modulated beam); analyze samples of an SMI signal generated by the SMI sensor128-1; and determine a distance between the SMI sensor128-1and the front cover106(or between the SMI sensor128-1and the optical element132). Assuming the rear cover108of the device100has a fixed relationship with respect to the SMI sensor128-1, and that the optical element132(if provided) has a fixed relationship with the front cover106, the processor134may also determine a distance between the rear cover108and the front cover106, between the rear cover108and the optical element132, and so on. The distance(s) is/are determined by characterizing disturbances in the SMI signal generated by the SMI sensor128-1, which disturbances are caused by reflections or backscatters of the beam of electromagnetic radiation130-1from first and second surfaces of the front cover106or optical element132(i.e., first and second surfaces that are oriented parallel to the viewing surface, or exterior surface, of the display104); relating the disturbances to a known optical thickness of the front cover106or optical element (i.e., an optical thickness between the first and second surfaces of the front cover106or optical element132); and determining the distance between the SMI sensor128-1and front cover106(or optical element132) using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element132, as described in more detail with reference toFIGS.2-5. In some cases, relating the disturbances to the known optical thickness may include relating the disturbances to a known physical thickness of the optical element132using a known refractive index of the optical element132and/or other information (i.e., because the optical thickness of the optical element132is related to the physical thickness of the optical element132times the refractive index of the optical element132). When a user presses on the front cover106, a displacement, velocity, or other movement parameter of the front cover106may also be determined by the processor134.

In some cases, the processor134may triangularly modulate a wavelength of the beam of electromagnetic radiation130-1(to produce a triangularly modulated beam) when determining the distance between the SMI sensor128-1and the front cover106(or optical element132), but sinusoidally modulate a wavelength of the beam of electromagnetic radiation130-1(to produce a sinusoidally modulated beam) when monitoring for a user's touch or press on the front cover106, or when otherwise monitoring the displacement, velocity, or other movement parameter of the front cover106. A displacement of the front cover106may indicate that a force is being applied to the front cover106, and an amount of the displacement may indicate an amount (or magnitude) of the force.

A second optional SMI sensor128-2may be oriented so that a beam of electromagnetic radiation130-2emitted by the SMI sensor128-2impinges on an optical element136(e.g., an optical film, substrate, or block of material) attached to the sidewall118. The SMI sensor128-2may have an electromagnetic radiation emission axis that is oriented perpendicular to the sidewall118. The processor134may modulate a wavelength of the beam of electromagnetic radiation130-2(to produce a modulated beam); analyze samples of an SMI signal generated by the SMI sensor128-2; and determine a distance between the SMI sensor128-2and the optical element136. The processor134may also determine a displacement, velocity, or other movement parameter of the sidewall118with respect to the SMI sensor128-2.

In some embodiments, second and third optional SMI sensors128-2,128-3may emit beams of electromagnetic radiation130-2,130-3toward optical elements136,138attached to opposing sidewall portions of the device100. In some cases, the processor134may analyze SMI signals generated by the second and/or third SMI sensors128-2,128-3to determine whether, or how hard, a user is gripping the device100.

In alternative embodiments, the device100may include more or fewer SMI sensors128. For example, the device may include a set of SMI sensors128, each of which emits a respective beam of electromagnetic radiation130toward the front cover106, but from a different x/y position below the front cover106. In another example, the device100may include one or more SMI sensors128that emit respective beams of electromagnetic radiation130toward the rear cover108, and/or the device100may include more or fewer SMI sensors128that emit respective beams of electromagnetic radiation130toward different sidewall portions of the device100.

In alternative embodiments, the positions of any of the SMI sensors and corresponding optical elements may be swapped.

FIG.2shows an example portion of a device200including an SMI sensor202and an optical element204. The SMI sensor202may be positioned at a nominally fixed distance (d) from the optical element204. In some embodiments, the nominally fixed distance may be maintained by a frame206to which both the SMI sensor202and optical element204are attached. The optical element204may in some cases be one of the optical elements described with reference toFIG.1C(e.g., an optical film, substrate, or block of material), or the front or rear cover described with reference toFIGS.1A-1C. The optical element204may be optionally attached to another element220, such as a cover glass, a lens, or a frame component. The element220may be transparent, translucent, or opaque to electromagnetic radiation emitted by the SMI sensor202.

The SMI sensor202may be configured to emit a modulated beam of electromagnetic radiation208toward a first surface210of the optical element204, and to generate an SMI signal containing disturbances caused by reflections or backscatters of the beam208from first and second surfaces210,212of the optical element204. The first and second surfaces210,212may be opposite surfaces of the optical element204, and an electromagnetic radiation emission axis214of the SMI sensor202may intersect the first and second surfaces210,212—preferably at a right angle, to simplify computations.

The device200may further include a processor216. The processor216may directly or indirectly receive samples of the SMI signal generated by the SMI sensor202, or may itself sample the SMI signal. The processor216may be configured to relate disturbances in the SMI signal (i.e., the disturbances caused by the reflections or backscatters of the beam208from the first and second surfaces210,212of the optical element204, which disturbances appear in the samples of the SMI signal) to a known optical thickness of the optical element204. In some cases, relating the disturbances to the known optical thickness may include relating the disturbances to a known physical thickness of the optical element204using a known refractive index of the optical element204and/or other information. The processor216may also be configured to determine a distance between two objects (e.g., a distance (d) between the SMI sensor202and the optical element204) using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element204. The objects may be the SMI sensor202and the optical element204; or the SMI sensor202and any other object in the path of the beam of electromagnetic radiation208(e.g., another object attached to the frame206, an object held by a user (e.g., a stylus), a body part a user (e.g., a finger or facial feature), dust, or any object internal or external to the device200which is in the path of the beam of electromagnetic radiation208; or a first object having a fixed relationship to the SMI sensor202and a second object in the path of the beam of electromagnetic radiation208; or any two objects in the path of the beam of electromagnetic radiation208.

In some cases, the processor216may be configured to identify the disturbances in the SMI signal and/or characterize (e.g., measure) one or more parameters of the disturbances. For example, the processor216may perform a fast Fourier transform (FFT) on the samples of the SMI signal, and identify 1) a first frequency component, and magnitude of the first frequency component, corresponding to a reflection or backscatter of the beam208from the first surface210, and 2) a second frequency component, and magnitude of the second frequency component, corresponding to a reflection or backscatter of the beam208from the second surface212.

In some cases, the processor216may be configured to modulate a wavelength of the beam of electromagnetic radiation208(to produce a modulated beam). For example, when generating the SMI signal containing the disturbances caused by reflections or backscatters of the beam208from the first and second surfaces210,212, which disturbances are related to the known optical thickness of the optical element204, the processor216may triangularly modulate a wavelength of the beam208(to produce a triangularly modulated beam). The triangular modulation may have a triangular modulation frequency of ftriangie. When determining a displacement, velocity, or other movement parameter of the optical element204, the processor216may triangularly, sinusoidally (to produce a sinusoidally modulated beam), or otherwise modulate a wavelength of the beam208. In some cases, the processor216may modulate a wavelength of the beam208by modulating the current or junction voltage of the SMI sensor202.

As previously mentioned, the processor216may in some cases perform an FFT on the samples of the SMI signal generated by the SMI sensor202. An FFT performed on an example set of samples of an SMI signal is shown inFIGS.3A and3B.FIG.3Ashows an FFT300performed on the samples corresponding to ascending sides of a triangularly modulated beam208, andFIG.3Bshows an FFT310performed on the samples corresponding to descending sides of the triangularly modulated beam208. For increased frequency bin resolution, multiple ascending (or descending) sides can be concatenated after appropriate windowing (Gaussian, Hanning, Blackman, etc.) and inputted to an FFT processor. To cancel out white-noise contributions, the FFT peak values (e.g., values at frequencies302,304, and312) of multiple ascending (or descending) sides can be averaged out. Improved resolution of the FFT peaks, beyond the FFT bin resolution, can be achieved using interpolation (e.g., linear, parabolic, Gaussian, etc.), using neighboring bin values.

fref=4⁢ftriangle⁢Δ⁢λλ2⁢dref
where drefis the known optical thickness of the optical element (or distance reference). The frequency shift308(Doppler shift) between FFT peaks302,312of ascending and descending sides is equal to:
2fdoppler=4vtarget/λ
where vtargetcorresponds to unintended (or intended) movement of the optical reference.

FIG.4shows an FFT400performed on an alternative set of samples corresponding to ascending sides of a triangularly modulated beam208. The FFT400differs from the FFT300in that it includes additional frequency components at frequencies f3, f4, and f5. When these frequency components are separated from each other by the frequency fref, the frequency components may correspond to electromagnetic radiation received by the SMI sensor after one or more additional reflections between the first and second surfaces of the optical element204. The additional reflections corresponding to the frequencies f3, f4, and f5, as well as the reflections corresponding to f1(i.e., a first reflection off the first surface of the optical element) and f2(i.e., a first reflection off the second surface of the optical element), are shown inFIG.5.FIG.5shows an optical element500having first and second opposing surfaces502,504off which a beam of electromagnetic radiation reflects. In some cases, a portion of the beam may also pass through both the first and second opposing surfaces502,504. In some cases, a coating or surface treatment may be applied to the first and/or second surface of the optical element500to control how much electromagnetic radiation reflects from or passes through the surface. When coatings or surface treatments are applied to the first and second surfaces502,504, the same or different coatings or surface treatments may be applied to each surface502,504. The material from which the optical element500is made may also be chosen to adjust reflection, absorption, and pass-through of electromagnetic radiation.

The additional frequency components (or peaks) shown inFIG.4, as caused by the additional reflections shown inFIG.5, can be used to determine an average (and in some cases more accurate) frequency spacing, fref, between the frequency components.

FIG.6shows another example of a device600that includes an SMI-based absolute distance measurement system. The SMI-based absolute distance measurement system may include an SMI sensor602that emits electromagnetic radiation toward an optical element604. By way of example, the device600may be a camera, and the two components are shown to be an image sensor606and a lens608(or set of lenses). In some cases, the optical element604may be an optical film, substrate, or block of material that is separate from the lens608. In other cases, the optical element604and lens608may be the same component, or the optical element604may be one of the lenses in a set of lenses.

The SMI sensor602may have a fixed relationship with respect to the image sensor606, and the optical element604may have a fixed relationship with respect to the lens608. Alternatively, the positions of the SMI sensor602and optical element604may be swapped.

The distance between the image sensor606and the lens608may be adjustable. In some embodiments, the distance may be adjusted automatically, such as, in response to a processor610that performs an auto-focus function and sends instructions to a motor612that moves the lens608with respect to the image sensor606. In these embodiments, the motor612may have a fixed relationship with respect to the image sensor606. Alternatively, the motor612may have a fixed relationship with respect to the lens608, and the processor610may send instructions to the motor612to move the image sensor606with respect to the lens608. Alternatively, the distance may be adjusted manually, such as by a user.

In the case of a processor610performing an auto-focus function, the processor610may use the SMI sensor602to measure a distance (e.g., a first distance) between the SMI sensor602and the optical element604. Alternatively, given the fixed relationships between the SMI sensor602and image sensor606, and between the optical element604and lens608, the processor610may also or alternatively determine a distance between the image sensor606and the lens608. The distance(s) is/are determined by characterizing disturbances in the SMI signal, which disturbances are caused by reflections or backscatters of a beam of electromagnetic radiation, emitted by the SMI sensor602, from first and second surfaces of the optical element604; relating the disturbances to a known optical thickness of the optical element604(i.e., an optical thickness between the first and second surfaces of the optical element604); and determining the distance between the SMI sensor602and optical element604using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element604, as described in more detail with reference toFIGS.7and3A-5(and other figures). In some cases, relating the disturbances to the known optical thickness may include relating the disturbances to a known physical thickness of the optical element604using a known refractive index of the optical element604and/or other information.

In some cases, the processor610may triangularly modulate a wavelength of the beam of electromagnetic radiation (to produce a triangularly modulated beam) when sampling an SMI signal to determine the distance between the SMI sensor602and the optical element604. Thereafter, an auto-focus operation may be performed and the lens608may be adjusted (e.g., moved) with respect to the image sensor606, thereby changing the distance between the lens608and the image sensor606, and also changing the distance between the SMI sensor602and the optical element604. In some embodiments, the processor610may sinusoidally modulate a wavelength of the beam of electromagnetic radiation (to produce a sinusoidally modulated beam) while moving the lens608with respect to the image sensor606, and may monitor the phase and/or count phase changes of the sinusoidally modulated beam (e.g., in radians) to determine how far the lens608has been moved (i.e., to determine a displacement of the lens608due to the adjustment). After the lens608has been moved to a new position, the distance (e.g., a second distance) between the SMI sensor602and optical element604may be measured (e.g., confirmed) while the processor610once again triangularly modulates the beam emitted by the SMI sensor602.

Other types of devices may also have first and second components that are movable with respect to one another, and may have an SMI sensor mounted in a fixed relationship with respect to one of the components, and an optical element mounted in a fixed relationship with respect to the other one of the components. For example, the two components may be a glasses/goggles frame and a lens, or two different lenses in a camera or glasses/goggles frame (e.g., a distance between the lenses of a glasses/goggles frame may be adjusted to match an interpupillary distance (IPD) of a user of the glasses/goggles), or two components of a machine (e.g., two components of a semiconductor pick-and-place machine, a surgical machine, and so on). In the case of a pair of glasses or goggles, an IPD sensor may sense an IPD of a user and trigger operation of a motor that adjusts the IPD of the pair of glasses or goggles to match the sensed IPD of the user.

FIG.7shows an example portion of a device700including an SMI sensor702and an optical element704. At some arbitrary point in time, the SMI sensor702may be positioned at a first distance (d1) from the optical element704. At a second point in time, the SMI sensor702may be positioned at a second distance (d2) from the optical element704. Thus, the distance between the SMI sensor702and the optical element704(or the distance between components to which the SMI sensor702and optical element704are respectively attached) is a reconfigurable distance. In between the first and second points in time, or at other times, the optical element704may be moved with respect to the SMI sensor702. In some cases, the optical element704may be moved with respect to the SMI sensor702by a motor706.

The optical element704may in some cases be one of the optical elements described with reference toFIG.6(e.g., an optical film, substrate, block of material, or lens).

The optical element704may have a fixed relationship with respect to one or more components of the device700, which component(s) may in some cases include a first component708(e.g., a first frame component, and/or a first lens, or a first lens holder). The SMI sensor702may have a fixed relationship with respect to one or more other components of the device700, which component(s) may in some cases include a second component710(e.g., a second frame component, and/or a second lens, or a second lens holder). By way of example, the motor706is shown to have a motor housing712attached to the second component710. A shaft attached to a pinion gear714may extend from the motor housing712, and the pinion gear714may engage a linear rack716attached to (or formed in) the first component708. Rotation of the pinion gear714by the motor706therefore adjusts the distance between the first component708and the second component710(and therefore, the distance between the SMI sensor702and the optical element704).

The SMI sensor702may be configured to emit a modulated beam of electromagnetic radiation718toward a first surface720of the optical element704, and to generate an SMI signal containing disturbances caused by reflections or backscatters of the beam718from first and second surfaces720,722of the optical element704. The first and second surfaces720,722may be opposite surfaces of the optical element704, and an electromagnetic radiation emission axis724of the SMI sensor702may intersect the first and second surfaces720,722—and preferably at a right angle, to simplify computations.

The device700may further include a processor726. The processor726may directly or indirectly receive samples of the SMI signal generated by the SMI sensor702, or may generate the samples of the SMI signal. The processor726may be configured to relate disturbances in the SMI signal (i.e., the disturbances caused by the reflections or backscatters of the beam718from the first and second surfaces720,722of the optical element704, which disturbances appear in the samples of the SMI signal) to a known optical thickness of the optical element704. In some cases, relating the disturbances to the known optical thickness may include relating the disturbances to a known physical thickness of the optical element704using a known refractive index of the optical element704and/or other information. The processor726may also be configured to determine a distance (e.g., d1or d2) between the first and second components708,710(or between the SMI sensor702and the optical element704, and so on) using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element704. For purposes of this description, determining the distance between the first and second components708,710is considered equivalent to determining the distance between the SMI sensor702and optical element704, or determining the distance between any combination thereof separated by a configurable distance.

In some cases, the processor726may be configured to identify the disturbances in the SMI signal and/or characterize (e.g., measure) one or more parameters of the disturbances. For example, the processor726may perform an FFT on the samples of the SMI signal, and identify 1) a first frequency component, and magnitude of the first frequency component, corresponding to a reflection or backscatter of the beam emitted by the SMI sensor702from the first surface720, and 2) a second frequency component, and magnitude of the second frequency component, corresponding to a reflection or backscatter of the beam from the second surface722.

In some cases, the processor726may be configured to modulate a wavelength of the beam of electromagnetic radiation (to produce a modulated beam). For example, when the first and second components708,710are stationary with respect to each other (or moving slow enough with respect to each other, within the measurement time, that their movement can be neglected), the processor726may triangularly modulate the electromagnetic radiation of the SMI sensor702while obtaining a set of samples of the SMI signal generated by the SMI sensor702(i.e., while sampling the SMI signal). The set of samples may be used by the processor726to identify the disturbances in the SMI signal. In some embodiments, the processor726may triangularly modulate a wavelength of the beam (to produce a triangularly modulated beam).

In some embodiments, the processor726may determine the first distance (d1) between the SMI sensor702and the optical element704(or a corresponding first distance between the first and second components708,710); adjust the distance between the SMI sensor702and the optical element704to the second distance (d2) (or equivalently, adjust the distance between the first and second components708,710); and then determine the second distance (or a corresponding second distance between the first and second components708,710). After obtaining a first set of samples of the SMI signal for the purpose of determining the first distance (d1), and before adjusting the distance between the SMI sensor702and the optical element704to the second distance (d2), the processor726may switch the modulation of the wavelength of the SMI sensor's beam of electromagnetic radiation to a sinusoidal modulation. The processor726may then adjust the distance between the first and second components708,710while the electromagnetic radiation is sinusoidally modulated, and may obtain a second set of samples of the SMI signal while the distance is being adjusted (e.g., the processor726may obtain the samples over the entire duration of the adjustment). The processor726may use the second set of samples to monitor the phase and/or count phase changes of the sinusoidally modulated beam (e.g., in radians), to determine a change in displacement between the first and second components708,710due to the adjustment. After the distance between the SMI sensor702and the optical element704has been adjusted to the second distance (d2), the processor726may switch the modulation of the wavelength of the SMI sensor's beam of electromagnetic radiation to the triangular modulation, and obtain a third set of samples of the SMI signal while the electromagnetic radiation is triangularly modulated. The third set of samples may be used to determine the second distance (d2), in a manner similar to how the first set of samples may be used to determine the first distance (d1).

In some cases, the first distance (d1), second distance (d2), and second set of samples may be used to determine a wavelength (λ) of the electromagnetic radiation emitted by the SMI sensor702. For example, a difference between the first and second distances may be determined (e.g., d2−d1), and a phase difference (or number of phases) of the sinusoidal modulation measured by the SMI sensor702during the adjustment of the distance between the SMI sensor702and optical element704may be correlated to the distance d2−d1to determine the wavelength.

FIG.8shows an example arrangement of an SMI sensor802and optical element804with respect to an object806. The SMI sensor802and optical element804may be mounted in or on a device, and the object806may be external to the device. Alternatively, the object806may be mounted in or on the device, but on an opposite side of the optical element804from the SMI sensor802(i.e., with the optical element804positioned between the SMI sensor802and object806). The SMI sensor802and optical element804may be mounted at fixed or adjustable positions with respect to each other. In some cases, the SMI sensor802and optical element804may be an SMI sensor and optical element described with reference toFIG.1C,2,6, or7.

A portion of a beam808of electromagnetic radiation emitted by the SMI sensor802may pass through the optical element804and impinge on, and reflect from, the object806. In addition to determining the distance between the SMI sensor802and the optical element804, a processor of the device may use an SMI signal generated by the SMI sensor802, in combination with a known optical thickness of the optical element804(and in some cases other information), to determine a distance to the object806.

In alternative embodiments, the object806may be positioned between the SMI sensor802and optical element804. In these cases, the object806needs to be at least partially transparent, to allow light to reach the optical element804.

FIG.9shows an example alternative arrangement of an SMI sensor902and optical element904with respect to an object906. In the arrangement shown inFIG.9, the SMI sensor902and optical element904are mounted within a module900having a partially reflective (e.g., partially mirrored) internal surface908that functions as a beam splitter. For example, a first portion of a beam910of electromagnetic radiation emitted by the SMI sensor902may be reflected within the module900, and onto a first surface912of the optical element904. A first sub-portion of the first portion of the beam910may reflect from the first surface912; reflect within the module900; and be received back into a resonant cavity of the SMI sensor902. A second sub-portion of the first portion of the beam910may reflect from a second surface914of the optical element904, opposite the first surface912; reflect within the module900; and be received back into the resonant cavity of the SMI sensor902. A second portion of the beam910may pass through a housing916of the module900; reflect from the object906, exterior to the module900; pass back through the housing916of the module900; and be received back into the resonant cavity of the SMI sensor902.

In addition to determining the distance between the SMI sensor902and the optical element904, a processor of a device including the module900may use an SMI signal generated by the SMI sensor902, in combination with a known optical thickness of the optical element904(and in some cases other information), to determine a distance to the object906.

FIG.10shows an example method1000of determining a distance between a first object and a second object. The method1000may be employed by the processor, or other components, described with reference toFIGS.1A-1C,2,6, and7.

At block1002, the method1000may include emitting a beam of electromagnetic radiation from an SMI sensor having a fixed relationship with respect to the first object.

At block1004, the method1000may include receiving, from the SMI sensor, an SMI signal containing disturbances caused by reflections or backscatters of the beam from first and second surfaces of an optical element having a fixed relationship with respect to the second object.

At block1006, the method1000may include relating the disturbances to a known optical thickness of the optical element.

At block1008, the method1000may include determining a distance between the first object and the second object, using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

FIG.11shows another example method1100of determining a distance between a first object and a second object. The method1100may also be employed by the processor, or other components, described with reference toFIGS.1A-1C,2,6, and7.

At block1102, the method1100may include emitting a beam of electromagnetic radiation from an SMI sensor having a fixed relationship with respect to the first object.

At block1104, the method1100may include triangularly modulating the wavelength of the beam of electromagnetic radiation. Although perfect triangular modulation is ideal, most of the time it is impossible to achieve a perfect triangular modulation. For example, due to nonlinearities in the laser, its supporting electronics, and/or other factors, deviations from an ideal triangular modulation can occur. However, if the deviation is small and controlled, and signal-to-noise ratio is sufficiently high, information can still be reliably extracted in the presence of such deviations. For purposes of this description, triangular modulation including such irregularities is still considered triangular modulation.

At block1106, the method1100may include receiving, from the SMI sensor, an SMI signal containing disturbances caused by reflections or backscatters of the beam from first and second surfaces of an optical element having a fixed relationship with respect to the second object.

At block1108, while the SMI signal is being received, and while the beam of electromagnetic radiation is triangularly modulated, the method1100may include obtaining a first set of samples of the SMI signal.

At block1110, the method1100may include identifying the disturbances within the first set of samples. In some cases, the disturbances may be identified, for example, by identifying waveform peaks that exceed a threshold value in the time domain or the frequency domain.

At block1112, the method1100may include relating the disturbances to a known optical thickness of the optical element.

At block1114, the method1100may include determining a distance between the first object and the second object, using the SMI signal and the relationship of the disturbances to the known optical thickness of the optical element.

The operations performed at blocks1102-1108, and in some cases the operations performed at all blocks of the method1100, may be performed while the first and second objects are stationary with respect to one another.

FIG.12shows a method1200that, in some cases, may be performed after the operations of the method1100. The method1200may be used to not only determine the distance between the first and second objects, but a wavelength of the electromagnetic radiation emitted by the SMI sensor. Determining (or measuring) the wavelength can enable a high resolution tracking of the target in displacement mode (such as the I/Q mode or method described herein). The method1200presumes that the distance determined at block1114of method1100is a first distance between the first and second objects.

At block1202, and after obtaining the first set of samples (at block1108of method1100), the method1200may include switching the modulation of the wavelength of the beam of electromagnetic radiation to a sinusoidal modulation.

At block1204, the method1200may include adjusting the distance between the first object and the second object, from the first distance to a second distance, while the beam of electromagnetic radiation is sinusoidally modulated.

At block1206, the method1200may include obtaining a second set of samples of the SMI signal while the distance between the first object and the second object is being adjusted from the first distance to the second distance.

At block1208, and after the distance between the first and second objects is adjusted to the second distance, the method1200may include switching the modulation of the wavelength of the beam of electromagnetic radiation to the triangular mediation.

At block1210, the method1200may include determining the second distance using the SMI signal and the known optical thickness. The second distance may be determined similarly to the first distance (e.g., using the method described with reference toFIG.11).

At block1212, the method1200may include determining a wavelength (λ) of the beam of electromagnetic radiation using the first distance, the second distance, and the second set of samples.

FIG.13Ashows a first example SMI sensor1300that may be used in one or more of the systems, devices, or methods described with reference toFIGS.1A-12. In this example, the SMI sensor1300may include a VCSEL1302with an integrated resonant cavity (or intra-cavity) photodetector (RCPD)1304.

FIG.13Bshows a second example SMI sensor1310that may be used in one or more of the systems, devices, or methods described with reference toFIGS.1A-12. In this example, the SMI sensor1310may include a VCSEL1312with an extrinsic on-chip RCPD1314. As an example, the RCPD1314may form a disc around the VCSEL1312.

FIG.13Cshows a third example SMI sensor1320that may be used in one or more of the systems, devices, or methods described with reference toFIGS.1A-12. In this example, the SMI sensor1320may include a VCSEL1322with an extrinsic off-chip photodetector1324.

FIG.13Dshows a fourth example SMI sensor1330that may be used in one or more of the systems, devices, or methods described with reference toFIGS.1A-12. In this example, the SMI sensor1330may include a dual-emitting VCSEL1332with an extrinsic off-chip photodetector1334. For example, the top emission may be emitted towards optics and/or another target and the bottom emission may be provided to the extrinsic off-chip photodetector1334.

FIG.14shows a triangular bias (i.e., triangular modulation) procedure1400for determining velocity and absolute distance of a surface (or object) using self-mixing interferometry. The procedure1400may be used by one or more of the systems, devices, or methods described with reference toFIGS.1A-12, to modulate an SMI sensor using a triangular waveform.

At an initial stage1402, an initial signal is generated, such as by a digital or analog signal generator. At stage1406-1, the generated initial signal is processed as needed to produce the triangle waveform modulation current1502that is applied to a VCSEL (seeFIG.15). Stage1406-1can be, as needed, operations of a DAC (such as when the initial signal is an output of a digital step generator), low-pass filtering (such as to remove quantization noise from the DAC), and voltage-to-current conversion.

The application of the modulation current1502to the VCSEL induces an SMI output1518(i.e., a change in an interferometric property of the VCSEL). It will be assumed for simplicity of discussion that the SMI output1518is from a photodetector, but in other embodiments it may be from another component.

At initial stage1404inFIG.14, the SMI output1518is received. At stage1406-2, initial processing of the SMI output1518is performed as needed. Stage1406-2may include high-pass filtering or digital subtraction.

At stage1408, a processor may equalize the received signals in order to match their peak-to-peak values, mean values, root-mean-square values, or any other characteristic values, if necessary. For example the SMI output1518may be a predominant triangle waveform component being matched to the modulation current1502, with a smaller and higher frequency component due to changes in the interferometric property. High-pass filtering may be applied to the SMI output1518to obtain the component signal related to the interferometric property. Also this stage may involve separating and/or subtracting the parts of the SMI output1518and the modulation current1502corresponding to the ascending and to the descending time intervals of the modulation current1502. This stage may include sampling the separated information.

At stages1410and1412, a separate fast Fourier transform (FFT) may be first performed on the parts of the processed SMI output1518corresponding to the ascending and to the descending time intervals. The two FFT spectra may be analyzed at stage1414.

At stage1416, the FFT spectra may be further processed, such as to remove artifacts and reduce noise. Such further processing can include peak detection and Gaussian fitting around the detected peak for increased frequency precision. From the processed FFT spectra data, information regarding the absolute distance can be obtained at stage1418.

FIG.15shows a block diagram of a system (e.g., part or all of the processor described with reference toFIGS.1A-12) that may implement the spectrum analysis described in the method described above with respect toFIG.14. In the exemplary system shown, the system includes generating an initial digital signal and processing it as needed to produce a modulation current1502as an input to the VCSEL1510. In an illustrative example, an initial step signal may be produced by a digital generator to approximate a triangle function. The digital output values of the digital generator are used in the digital-to-analog converter (DAC)1504. The resulting voltage signal may then be filtered by the low-pass filter1506to remove quantization noise. Alternatively, an analog signal generator based on an integrator can be used to generate an equivalent voltage signal directly. The filtered voltage signal then is an input to a voltage-to-current converter1508to produce the desired modulation current1502in a form for input to the VCSEL1510.

As described above, movement of a target can cause changes in an interferometric parameter, such as a parameter of the VCSEL1510or of a photodetector operating in the system. The changes can be measured to produce an SMI output1518. In the embodiment shown, it will be assumed the SMI output1518is measured by a photodetector. For the modulation current1502having the triangle waveform, the SMI output1518may be a triangle wave of a similar period combined with a smaller and higher frequency signal related to the interferometric property. In some cases, the SMI output1518may not be perfectly linear, even though the modulation current1502is linear. This may be a result of the bias current versus light output curve of the VCSEL1510being non-linear (e.g., due to non-idealities, such as self-heating effects).

The SMI output1518is first passed into the high-pass filter1520, which can effectively convert the major ascending and descending ramp components of the SMI output1518to DC offsets. As the SMI output1518may typically be a current, the transimpedance amplifier1522can produce a corresponding voltage output (with or without amplification) for further processing.

The voltage output can then be sampled and quantized by the ADC block1524. Before immediately applying a digital FFT to the output of the ADC block1524, it can be helpful to apply equalization. The initial digital signal values from the digital generator used to produce the modulation current1502are used as input to the digital high-pass filter1512to produce a digital signal to correlate with the output of the ADC block1524. An adjustable gain can be applied by the digital variable gain block1514to the output of the digital high-pass filter1512.

The output of the digital variable gain block1514is used as one input to the digital equalizer and subtractor block1516. The other input to the digital equalizer and subtractor block1516is the output of the ADC block1524. The two signals are differenced, and used as part of a feedback to adjust the gain provided by the digital variable gain block1514.

Equalization and subtraction may be used to clean up any remaining artifacts from the triangle that may be present in the SMI output1518. For example, if there is a slope error or nonlinearity in the SMI output1518, the digital high-pass filter1512may not fully eliminate the triangle and artifacts may remain. In such a situation, these artifacts may show up as low frequency components after the FFT and make the peak detection difficult for nearby objects. Applying equalization and subtraction may partially or fully remove these artifacts.

Once an optimal correlation is obtained by the feedback, an FFT, indicated by block1528, can then be applied to the components of the output of the ADC block1524corresponding to the rising and descending side of the triangle wave. From the FFT spectra obtained, absolute distance and/or directional velocity may be inferred using the detected peak frequencies on the rising and descending sides, as discussed above and indicated by block1526.

The method just described, and its variations, involve applying a spectrum analysis to an SMI output. However, it is understood that this is an example. In other implementations, alternate methods for determining absolute distances may be obtained directly from a time domain SMI output, without applying a spectrum analysis. Various configurations are possible and contemplated without departing from the scope of the present disclosure.

FIG.16shows a sinusoidal bias (i.e., sinusoidal modulation) procedure1600for determining displacement or movement of a surface (or object) using quadrature demodulation with self-mixing interferometry. The procedure1600may be used by one or more of the systems, devices, or methods described with reference toFIGS.1A-12, to modulate an SMI sensor using a sinusoidal waveform.

As explained in more detail below,FIG.16shows components which generate and apply a sinusoidally modulated bias current to a VCSEL. The sinusoidal bias current can generate in a photodetector1616an output current depending on the frequency of the sinusoidal bias and the displacement to the structural component of the device. In the circuit ofFIG.16, the photodetector's1616output current is digitally sampled and then multiplied with a first sinusoid at the frequency of the original sinusoidal modulation of the bias current, and a second sinusoid at double that original frequency. The two separate multiplied outputs are then each low-pass filtered and the phase of the interferometric parameter may be calculated. Thereafter the displacement is determined using at least the phase.

The DC voltage generator1602is used to generate a constant bias voltage. A sine wave generator1604may produce an approximately single frequency sinusoid signal, to be combined with constant voltage. As shown inFIG.16, the sine wave generator1604is a digital generator, though in other implementations it may produce an analog sine wave. The low-pass filter1606-1provides filtering of the output of the DC voltage generator1602to reduce undesired varying of the constant bias voltage. The bandpass filter1606-2can be used to reduce distortion and noise in the output of the sine wave generator1604to reduce noise, quantization or other distortions, or frequency components of its signal away from its intended modulation frequency, ωm.

The circuit adder1608combines the low-pass filtered constant bias voltage and the bandpass filtered sine wave to produce on link1609a combined voltage signal which, in the embodiment ofFIG.16, has the form V0+Vmsin(ωmt). This voltage signal is used as an input to the voltage-to-current converter1610to produce a current to drive the lasing action of the VCSEL1614. The current from the voltage-to-current converter1610on the line1613can have the form I0+Imsin(ωmt).

The VCSEL1614is thus driven to emit a laser light modulated as described above. Reflections of the modulated laser light may then be received back within the lasing cavity of VCSEL1614and cause self-mixing interference. The resulting emitted optical power of the VCSEL1614may be modified due to self-mixing interference, and this modification can be detected by the photodetector1616. As described above, in such cases the photocurrent output of the photodetector1616on the link1615can have the form: iPD=i0+imsin(ωmt)+γ cos(φ0+φmsin(ωmt)). As the I/Q components to be used in subsequent stages are based on just the third term, the first two terms can be removed or reduced by the differential transimpedance amplifier and anti-aliasing (DTIA/AA) filter1618. To do such a removal/reduction, a proportional or scaled value of the first two terms is produced by the voltage divider1612. The voltage divider1612can use as input the combined voltage signal on the link1609produced by the circuit adder1608. The output of the voltage divider1612on link1611can then have the form: α (qV0+Vmsin(ωmt)). The photodetector current and this output of the voltage divider1612can be the inputs to the DTIA/AA filter1618. The output of the DTIA/AA filter1618can then be, at least mostly, proportional to the third term of the photodetector current.

The output of the DTIA/AA filter1618may then be quantized for subsequent calculation by the ADC block1620. Further, the output of the ADC block1620may have a residual signal component proportional to the sine wave originally generated by the sine wave generator1604. To filter this residual signal component, the originally generated sine wave can be scaled (such as by the indicated factor of β) at multiplier block1624-3, and then subtracted from the output of ADC block1620at subtraction block1622. The filtered output on link1621may have the form: A+B sin(ωmt)+C cos(2ωmt)+D sin(3ωmt)+ . . . , from the Fourier expansion of the γ cos(φ0+φmsin(ωmt)) term discussed above. The filtered output can then be used for extraction of the I/Q components by mixing.

The digital sine wave originally generated by sine wave generator1604onto link1607is mixed (multiplied) by the multiplier block1624-1with the filtered output on link1621. This product is then low-pass filtered at block1628-1to obtain the Q component discussed above, possibly after scaling with a number that is related to the amount of frequency modulation of the laser light and distance to the target.

Also, the originally generated digital sine wave is used as input into the squaring/filtering block1626to produce a digital cosine wave at a frequency double that of the originally produced digital sine wave. The digital cosine wave is then mixed (multiplied) at the multiplier block1624-2with the filtered output of the ADC block1620on link1621. This product is then low-pass filtered at block1628-2to obtain the I component discussed above, possibly after scaling with a number that is related to the amount of frequency modulation of the laser light and distance to the target.

The Q and the I components are then used by the phase calculation component1630to obtain the phase from which the displacement of the target can be calculated, as discussed above.

One skilled in the art will appreciate that while the embodiment shown inFIG.16makes use of the digital form of the originally generated sine wave produced by sine wave generator1604onto link1607, in other embodiments the originally generated sine wave may be an analog signal and mixed with an analog output of the DTIA/AA filter1618. In other embodiments, the voltage divider1612may be a variable voltage divider. In still other embodiments, the voltage divider1612may be omitted and the DTIA/AA filter1618may be a single-ended DTIA/AA filter. In such embodiments, subtraction may be done only digitally at subtraction block1622. In yet other embodiments, the subtraction block1622may be omitted and no subtraction of the modulation current may be performed.

The circuit ofFIG.16can be adapted to implement the modified I/Q method described above that uses Q′ ∝Lowpass{IPD×sin(3ωmt)}. Some such circuit adaptations can include directly generating both mixing signals sin(2ωmt) and sin(3ωmt), and multiplying each with the output of the ADC block1620, and then applying respective low-pass filtering, such as by the blocks1628-1,1628-2. The differential TIA and anti-aliasing filter1618may then be replaced by a filter to remove or greatly reduce the entire component of IPDat the original modulation frequency ωm. One skilled in the art will recognize other circuit adaptations for implementing this modified I/Q method. For example, the signal sin(3ωmt) may be generated by multiplying link1607and the output of squaring/filtering block1626, and subsequently performing bandpass filtering to reject frequency components other than sin(3ωmt).

In additional and/or alternative embodiments, the I/Q time domain based methods just described may be used with the spectrum based methods of the first family of embodiments. The spectrum methods of the first family can be used at certain times to determine the absolute distance to the target, and provide a value of L0. Thereafter, during subsequent time intervals, any of the various I/Q methods just described may be used to determine ΔL.

In additional and/or alternative embodiments, the spectrum methods based on triangle wave modulation of a bias current of a VCSEL may be used as a guide for the I/Q time domain methods. The I/Q methods operate optimally in the case that J1(b)=J2(b), so that the I and Q components have the same amplitude. However, b depends on the distance L. An embodiment may apply a triangle wave modulation to the VCSEL's bias current to determine a distance to a point of interest. Then this distance is used to find the optimal peak-to-peak sinusoidal modulation of the bias current to use in an I/Q approach. Such a dual method approach may provide improved signal-to-noise ratio and displacement accuracy obtained from the I/Q method.

In some cases, the modulation of an SMI sensor's current or junction voltage may be warped (e.g., modulated with an irregular triangular or sinusoidal signal) so that a beam of electromagnetic radiation produced by the SMI sensor is appropriately modulated (e.g., triangularly or sinusoidally modulated). Warping the modulation of an SMI sensor's current or junction voltage may involve emitting a beam of electromagnetic radiation toward an object; rate-correcting an SMI signal generated by the SMI sensor based on the zero-crossings of the SMI signal; and optimizing a modulation waveform for the SMI sensor's current or junction voltage such that an expected SMI signal is produced.

FIG.17shows a sample electrical block diagram of an electronic device1700, which electronic device may in some cases be implemented as any of the devices described with reference toFIGS.1A-1C,2,6, and7. The electronic device1700may include an electronic display1702(e.g., a light-emitting display), a processor1704, a power source1706, a memory1708or storage device, a sensor system1710, or an input/output (I/O) mechanism1712(e.g., an input/output device, input/output port, or haptic input/output interface). The processor1704may control some or all of the operations of the electronic device1700. The processor1704may communicate, either directly or indirectly, with some or all of the other components of the electronic device1700. For example, a system bus or other communication mechanism1714can provide communication between the electronic display1702, the processor1704, the power source1706, the memory1708, the sensor system1710, and the I/O mechanism1712.

The processor1704may be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions, whether such data or instructions is in the form of software or firmware or otherwise encoded. For example, the processor1704may include a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), a controller, or a combination of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. In some cases, the processor1704may provide part or all of the processor described with reference to any ofFIGS.1A-12.

It should be noted that the components of the electronic device1700can be controlled by multiple processors. For example, select components of the electronic device1700(e.g., the sensor system1710) may be controlled by a first processor and other components of the electronic device1700(e.g., the electronic display1702) may be controlled by a second processor, where the first and second processors may or may not be in communication with each other.

The power source1706can be implemented with any device capable of providing energy to the electronic device1700. For example, the power source1706may include one or more batteries or rechargeable batteries. Additionally or alternatively, the power source1706may include a power connector or power cord that connects the electronic device1700to another power source, such as a wall outlet.

The memory1708may store electronic data that can be used by the electronic device1700. For example, the memory1708may store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, and data structures or databases. The memory1708may include any type of memory. By way of example only, the memory1708may include random access memory, read-only memory, Flash memory, removable memory, other types of storage elements, or combinations of such memory types.

The electronic device1700may also include one or more sensor systems1710positioned almost anywhere on the electronic device1700. In some cases, the sensor systems1710may include one or more SMI sensors, positioned as described with reference to any ofFIGS.1A-12. The sensor system(s)1710may be configured to sense one or more types of parameters, such as but not limited to, vibration; light; touch; force; heat; movement; relative motion; biometric data (e.g., biological parameters) of a user; air quality; proximity; position; connectedness; and so on. By way of example, the sensor system(s)1710may include an SMI sensor, a heat sensor, a position sensor, a light or optical sensor, an accelerometer, a pressure transducer, a gyroscope, a magnetometer, a health monitoring sensor, and an air quality sensor, and so on. Additionally, the one or more sensor systems1710may utilize any suitable sensing technology, including, but not limited to, interferometric, magnetic, capacitive, ultrasonic, resistive, optical, acoustic, piezoelectric, or thermal technologies.

The I/O mechanism1712may transmit or receive data from a user or another electronic device. The I/O mechanism1712may include the electronic display1702, a touch sensing input surface, a crown, one or more buttons (e.g., a graphical user interface “home” button), one or more cameras (including an under-display camera), one or more microphones or speakers, one or more ports such as a microphone port, and/or a keyboard. Additionally or alternatively, the I/O mechanism1712may transmit electronic signals via a communications interface, such as a wireless, wired, and/or optical communications interface. Examples of wireless and wired communications interfaces include, but are not limited to, cellular and Wi-Fi communications interfaces.

The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art, after reading this description, 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, after reading this description, that many modifications and variations are possible in view of the above teachings.