Self-mixing interferometry sensor, readout and control method and electronic device

Abstract

A self-mixing interferometry sensor having a stacked structure, comprising in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

Description

[0001] SELF-MIXING INTERFEROMETRY SENSOR, READOUT AND

[0002] CONTROL METHOD AND ELECTRONIC DEVICE

[0003] TECHNICAL FIELD

[0004] The present disclosure generally pertains to a self-mixing interferometry sensor, a readout and control method for a self-mixing interferometry sensor and an electronic device.

[0005] TECHNICAL BACKGROUND

[0006] Generally, three-dimensional (“3D”) imaging and sensing sensors are known which can be based on many different techniques such as time-of-flight, triangulation, pattern projection, interferometry etc.

[0007] In particular, self-mixing interferometry (“SMI”) exploits active laser illumination to measure the distance by exploiting the interference that establishes in a laser when the generated light is fed back into the cavity, and measuring the beating of this interference. The SMI technique can measure the distance while having a very high rejection of ambient / background light and at the same time requiring the use of a single device performing simultaneously the receiver and emitter function.

[0008] Unfortunately, the SMI signal (for example, in a case of triangular current / wavelength modulation) may be very weak and of difficult extraction, as it contains the distance information as interference fringes having a specific frequency depending on the distance.

[0009] Signal processing such as filtering in the frequency domain, e.g., by means of FFT (“Fast- Fourier Transform”) may be feasible for single point devices, however, for multipixel architectures it may be, in some cases, too area and power consuming in integrated solutions and non-parallelizable.

[0010] Moreover, disturbances appearing as tones in the spectrum may be mistaken as the main frequency which may lead to errors in the measurement.

[0011] Although there exist techniques for self-mixing interferometry sensors, it is generally desirable to improve the existing techniques.

[0012] SUMMARY

[0013] According to a first aspect, the disclosure provides a self-mixing interferometry sensor having a stacked structure, comprising in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0014] According to a second aspect, the disclosure provides a self-mixing interferometry sensor, comprising: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0015] According to a third aspect, the disclosure provides a readout and control method for a selfmixing interferometry sensor including a plurality of self-mixing interferometry devices, comprising: generating a drive current signal for each or each group of self-mixing interferometry devices; applying the drive current signal; reading out a self-mixing interferometry signal; receiving a reference signal having a constant reference frequency; comparing the reference signal with the self-mixing interferometry signal to generate a frequency difference signal; and controlling, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0016] According to a fourth aspect, the disclosure provides an electronic device, comprising: a self-mixing interferometry sensor having a stacked structure, including in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0017] Further aspects are set forth in the dependent claims, the drawings and the following description.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments are explained by way of example with respect to the accompanying drawings, in which:

[0020] Fig. 1 schematically illustrates an embodiment of a self-mixing interferometry device;

[0021] Fig. 2 schematically illustrates expected effects of noise;

[0022] Fig. 3 schematically illustrates in Fig. 3 A two embodiments of a self-mixing interferometry sensor and in Fig. 3B an embodiment of a functional concept of a self-mixing interferometry sensor; Fig. 4 schematically illustrates in Fig. 4A an embodiment of a self-mixing interferometry device, in Fig. 4B an embodiment of a self-mixing interferometry sensor with pixel-parallel architecture and in Fig. 4C an embodiment of a self-mixing interferometry sensor with row-parallel architecture;

[0023] Fig. 5 schematically illustrates in a block diagram an embodiment of a self-mixing interferometry sensor;

[0024] Fig. 6 schematically illustrates an embodiment of drive current signal wrapping;

[0025] Fig. 7 schematically illustrates in a block diagram an embodiment of a self-mixing interferometry sensor;

[0026] Fig. 8 schematically illustrates an embodiment of a linear search and an embodiment of a binary search;

[0027] Fig. 9 schematically illustrates in a block diagram an embodiment of a self-mixing interferometry sensor;

[0028] Fig. 10 schematically illustrates in a flow diagram an embodiment of a readout and control method for a self-mixing interferometry sensor;

[0029] Fig. 11 schematically illustrates in a flow diagram an embodiment of a readout and control method for a self-mixing interferometry sensor;

[0030] Fig. 12 schematically illustrates in a flow diagram an embodiment of a readout and control method for a self-mixing interferometry sensor; and

[0031] Fig. 13 schematically illustrates in a flow diagram an embodiment of a readout and control method for a self-mixing interferometry sensor.

[0032] DETAILED DESCRIPTION OF EMBODIMENTS

[0033] Before a detailed description of the embodiments under reference of Fig. 4 is given, general explanations are made.

[0034] As mentioned in the outset, generally, three-dimensional (“3D”) imaging and sensing sensors are known which can be based on many different techniques such as time-of-flight, triangulation, pattern projection, interferometry etc.

[0035] In particular, self-mixing interferometry (“SMI”) exploits active laser illumination while having a very high rejection of ambient / background light and at the same time requiring the use of a single device performing simultaneously the receiver and emitter function. Unfortunately, the SMI signal (for example, in a case of triangular current / wavelength modulation) may be very weak and difficult of extraction, as it contains the distance information as interference fringes having a specific frequency depending on the distance.

[0036] Signal processing such as filtering in the frequency domain, e.g., by means of FFT (“Fast- Fourier Transform”) may be feasible for single point devices, however, for multipixel architectures it may be, in some cases, too area and power consuming in integrated solutions and non-parallelizable.

[0037] Moreover, disturbances appearing as tones in the spectrum may be mistaken as the main frequency which may lead to errors in the measurement.

[0038] It has been recognized that a closed loop readout and control of a SMI device may improve the existing techniques by exploiting the possibility to manipulate the driving current of the laser to optimally filter the output to improve a signal-to-noise (“SNR”) ratio and, thus, an accuracy of the distance measurement.

[0039] In particular, it has been recognized to provide a control circuit for the illuminator such that the control circuit drives the fringe frequency towards a fixed reference value, thereby enabling optimization of the readout circuit to optimally filter the output.

[0040] Moreover, it has been recognized to provide a frequency hopping technique that allows to filter out band pure tones disturbances.

[0041] Hence, some embodiments pertain to a SMI sensor having a stacked structure, comprising in order from top to bottom: a plurality of SMI devices arranged in an array; and a readout and control circuit provided for each SMI device or for each group of SMI devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a SMI signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the SMI signal is locked to the reference frequency. Some embodiments pertain to a self-mixing interferometry sensor, including: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0042] Generally, SMI devices are known which typically includes at least a laser for voltage readout. The laser itself may be the detecting element for the SMI signal by detecting a voltage measurement across its terminals (voltage readout). Additionally, typically a photoelectric conversion element is included in a SMI device for light intensity readout (which may also be referred to as photodiode current readout). The photoelectric conversion element may be a photodiode for example.

[0043] For enhancing the general understanding of the present disclosure an embodiment of a SMI device 1 is discussed in the following under reference of Fig. 1 and 2.

[0044] The SMI device 1 includes a laser which includes an optical resonator 2, a top mirror 3a and a bottom mirror 3b. The SMI device 1 further includes a photodiode 4 for light intensity readout.

[0045] In other embodiments the photodiode 4 may not be present and the voltage readout is used for obtaining a SMI signal. In the voltage readout, in some embodiments, the electrical input is a current to the laser, while the output is a voltage from the laser terminals and the laser itself corresponds to the detector. The interference in the laser cavity is causing a variation in the intrinsic impedance of the laser. Since that is controlled with a current (input) the impedance variation causes the generation of a voltage which is proportional to the interference superimposed to the modulation voltage due to the input modulated current.

[0046] The optical resonator 2 includes a laser medium which is electrically or optically pumped, wherein, in the case of optical pumping, the pump light may be electrically controlled. Each of the top and bottom mirror 3 a and 3b have a high reflectivity with respect to the laser wavelengths of one or more laser modes which build up in the optical resonator 2.

[0047] A part of the laser light is coupled out via the top mirror 3a to measure a distance z to a target 5 which reflects a part of the incident laser light such that it is coupled back into the optical resonator 2 via the top mirror 3a.

[0048] This incoming light modulates both amplitude and beat frequency of the laser (the wavelength of the laser is not changed by this interference), thereby generating interferometric fringes depending on the phase of the back-scattered radiation.

[0049] Thus, the laser light coupled out via the bottom mirror 3b that is incident on the photodiode 4 is modulated accordingly, thereby generating a SMI signal SsMi(t).

[0050] As the phase depends on the distance z to the target 5, a frequency of the fringes of the SMI signal SsMi(t) are indicative for a displacement of the target 5 light (a fringe occurs for a phase shift of an entire period of 2n).

[0051] However, for fixed laser wavelengths, a moving target is required to observe the fringes.

[0052] It has been recognized that the phase of the back-scattered light further depends on the laser wavelength or emission wavelength such that changing the wavelength allows also to measure the distance to the target 5.

[0053] For the case of a triangular modulation of the wavelength - which is for example achieved with a VCSEL (“Vertical-Cavity Surface-Emitting Laser”) by modulating the drive current accordingly due to thermal effects - the distance z is given by:

[0054] NA2z=-

[0055] 2- AX’ wherein AX is the triangular wavelength modulation depth and N is the number of fringes within the respective modulation period.

[0056] The SMI technique typically has a very high rejection of background light, a simple coaxial optical arrangement and allows to perform measurements of the speed of the target.

[0057] However, as depicted in Fig. 2, which schematically illustrates expected effects of noise, some characteristic noise contributions exist in SMI systems.

[0058] A first noise contribution is driver noise that may result in a jitter of the SMI signal, a nonconstant inter-fringe time and an aperiodic waveform, as illustrated schematically in the upper graph. A second noise contribution is amplifier noise that may result in voltage / current noise such that fringes may be corrupted and more or less fringes may be counted than actually present, as illustrated schematically in the middle graph.

[0059] A third noise contribution is caused by disturbances (such as vibrations and interferences) that may result in a superimposed frequency such that mistakes in fringe counting may occur and a constant distance may be extracted due to the fixed tone disturbance.

[0060] These contributions and resulting effects on the SMI signal may limit the SNR and the overall performance for extracting the distance.

[0061] It has thus been recognized that an effective noise filtering and disturbance rejection is desirable.

[0062] Returning to the general explanations, typical known SMI devices use at the frontend low-noise voltage amplifiers (for voltage readout) or transimpedance amplifiers (for photodiode current readout).

[0063] The readout signal is then typically analyzed with a low-noise amplifying chain and typically a frequency domain analysis is performed after the low-noise signal amplification, since the amplified signal requires further post processing to extract the SMI relevant signal. Other post processing techniques may be used as well.

[0064] The low-noise amplifying chain may include amplification, filtering, fringe identification and counting either in the digital or analog domain or the mixed signal domain.

[0065] However, this may have challenging gain and noise requirements, in particular due to the wide bandwidth to cover short and long distances.

[0066] The frequency domain analysis may include amplification, digitization and FFT, filtering in the frequency domain and digital processing.

[0067] However, this may have challenging area and power requirements (speed, memory, processing) and may require a low noise wideband input channel, making this implementation hardly scalable to multi -channels architectures.

[0068] As mentioned above, it has thus been recognized that a closed loop readout and control of a SMI device may improve the existing techniques.

[0069] Hence, each readout and control circuit of the SMI sensor receives a reference signal having a constant reference frequency, compares the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, and controls a time derivative of the drive current signal, based on the generated frequency difference signal such that a frequency of the SMI signal is locked to the reference frequency.

[0070] For further enhancing the general understanding of the present disclosure two embodiments of a SMI sensor are discussed under reference of Fig. 3, which also apply to other embodiments of the present disclosure.

[0071] A SMI device 11 - used in a SMI sensor - has a laser, typically a VCSEL, a readout and control circuit for signal readout and driving of the laser, wherein the laser which works both as a light signal emitter and receiver at the same time.

[0072] Referring to Fig. 3A, on the left, there is schematically illustrated in top view a SMI sensor 10-1 which includes a plurality of SMI devices 11 arranged in an array.

[0073] For each of the SMI devices 11 an own readout and control circuit (not shown) is provided below the respective SMI device 11 such that the SMI sensor 10-1 has a stacked structure with pixel-wise readout and control circuit.

[0074] On the right, there is schematically illustrated in top view a SMI sensor 10-2 which includes a plurality of SMI devices 11 arranged in an array.

[0075] For each row of the SMI devices 11 an own readout and control circuit 12 is provided below and aside the respective row of SMI devices 11 such that the SMI sensor 10-2 has a stacked structure with row- wise readout and control circuit.

[0076] Referring to Fig. 3B, there is schematically illustrated an embodiment of a functional concept of the SMI sensors 10-1 and 10-2, which is discussed in the following.

[0077] The readout and control circuit 12 includes an internal control (Int Ctrl) which generates one or more internal control signals such as the reference signal, a clock signal, a mode selection signal, a measurement start / stop signal etc.

[0078] The SMI sensor includes an external control (Ext Ctrl) which generates one or more external control signals such as the reference signal, a frequency selection signal, a mode selection signal, a measurement start / stop signal etc.

[0079] Several parameters or reference signals can be provided either from on-chip generation or externally.

[0080] The readout and control circuit 12 includes a waveform generation and driver unit to generate, apply and control the drive current signal Idriver(t), which may be, for example, a triangular signal. The SMI device 11 generates the SMI signal SsMi(t) and the readout and control circuit 12 includes a signal amplification and processing unit.

[0081] The amplified and processed SsMi(t) is output (here denoted as output signal) externally and to the waveform generation and driver unit to form a control loop.

[0082] The driver and signal chain work together to optimize performance and the control loop can act both in the analog, digital or mixed signal domain.

[0083] The loop provides an analog or digital output representing the result of signal processing.

[0084] In order to lock the SMI frequency to the reference frequency, the waveform generation and driver unit determines a frequency difference and controls the time derivative of the drive current according to the determined frequency difference, since it has been recognized that the time derivative of the drive current signal can be used to adjust the frequency of the SMI signal (which may also be referred to as SMI frequency).

[0085] Once the SMI frequency is locked to the reference frequency, the distance to a target can be determined based on the absolute value of the time derivative of the drive current signal, as will be discussed in the following.

[0086] This allows to simplify the signal readout and processing chain, since, for example, no complex fringe detection and counting or frequency domain analysis is required anymore. The signal readout chain can be optimized for a single frequency such that a narrowband band-pass filter and amplification can be used which improves the SNR.

[0087] The operating principle will be discussed under assumption of a triangular drive current signal, however, the drive current signal is not limited to the special case of a triangular modulation.

[0088] With a linear wavelength modulation of AX around A and target distance z, the number of fringes is given by:

[0089] 2-AX

[0090] N = z — 5-.

[0091] X2

[0092] With triangle modulation of period Tmod, the SMI frequency fSMIis given by 2-times the fringes (up and down):

[0093] 2N 4-AX fSMI -z’ '

[0094] 1mod A. 1mod

[0095] If the loop keeps the frequency constant and equal to the reference frequency fref, this forces a wavelength variation of: iA» i1modPA = — - fr

[0096] 4z

[0097] With | A |= !< / ' I AImod| reflecting the connection between modulating the wavelength by modulating the drive current, the required modulation frequency and current modulation depth jointly result in:

[0098] More generally, this results in:

[0099] Thus, locking a fixed SMI frequency frefcan be achieved with variable AImod(current modulation depth) and fmod(modulation frequency), more generally a current time derivative |dlmod / dt|. The control loop and / or external controls can therefore work on |dlmod / dt| to lock the loop and extract the distance. In locked conditions, |dlmod / dt| is expected to be constant and therefore the general case tends to the triangular modulation:

[0100] Hence, when the SMI frequency matches the reference frequency, the distance can be determined solely based on the time derivative of the current drive signal |lmod | (k is a known scaling constant which may be determined experimentally or by simulation, X is the known center wavelength and fref is the known reference frequency). The signal readout chain may thus be optimized for a single frequency.

[0101] The locking may require some iterations such that the drive current is regularly adjusted to find and keep the frequency locking. As will be discussed later below, the convergence of the SMI frequency towards the reference frequency can be accelerated by a linear search or successive approximation techniques (e.g., binary search) which allow to find an initial estimate of the locking current derivative.

[0102] Returning to the general explanations, in some embodiments, the drive current signal is a triangular signal, as discussed.

[0103] As mentioned above, the readout and control circuit compares the reference signal with the selfmixing interferometry signal to generate a frequency difference signal. The frequency comparison allows to detect whether locking is already achieved and, if not, to determine the direction (and amount in some embodiments) in which the time derivative of the drive current signal needs to be adjusted, based on the frequency difference signal.

[0104] In some embodiments, the readout and control circuit includes a frequency comparator for comparing the reference signal with the SMI signal to generate the frequency difference signal.

[0105] The frequency comparator, in some embodiments, exploits both amplitude and frequency of the (amplified and filtered) SMI signal to provide the frequency difference signal as output (helping fast convergence towards reference frequency).

[0106] In some embodiments, the readout and control circuit is configured to store one or more samples of the SMI signal in their time order (e.g., together with a sample number), for example, to determine a signal part with a maximum amplitude for associating this signal part with a time derivative of the drive current signal that generated the maximum amplitude.

[0107] The readout and control circuit may be implemented such that the readout and control loop is implemented in the analog domain or in the mixed signal domain or in a combination of both approaches.

[0108] Referring to the analog domain:

[0109] In the analog domain, the time derivative of the drive current signal is controlled in accordance with a continuous and continuously varying signal. Moreover, the read out SMI signal and signal processing of the SMI signal is in the analog domain.

[0110] In the analog domain, the generated frequency difference signal may either be used as it is for controlling the time derivative of the drive current signal or analog signal processing may be applied on it for controlling the time derivative of the drive current signal.

[0111] Hence, in some embodiments, the readout and control circuit is further configured to apply analog signal processing on the generated frequency difference signal, wherein the time derivative of the drive current signal is controlled in the analog domain according to the signal processed frequency difference signal.

[0112] The analog signal processing may include signal amplification and filtering. The analog signal processing may include signal processing according to a PI (“Proportional-Integral”) or PID (“Proportional -Integral -Derivative”) signal control.

[0113] In some embodiments, the readout and control circuit is further configured to output the frequency difference signal or the signal processed frequency difference signal. As discussed above, this output is used for determining the time derivative of the drive current signal for determining the target distance z.

[0114] In the analog domain, the readout and control circuit performs a tracking operation in which it tracks the frequency difference signal: if the frequency of the SMI signal slightly changes, the time derivative of the drive current signal is finely adjusted.

[0115] However, if the lock points are far away, the readout and control circuit may be slow in reaching it. Hence, a linear search (which may also be referred to as linear sweep) or a binary search, for example, may be performed to find an initial estimate of the locking point.

[0116] Thus, in some embodiments, the readout and control circuit is further configured to perform a linear search and / or a successive approximation (“SAR”) operation.

[0117] In some embodiments, the readout and control circuit is further configured to determine an initial estimate for the derivative of the drive current signal that locks the frequency of the SMI signal to the reference frequency by performing a linear search, wherein linear search includes performing a modulation sweep of the drive current signal and determining a part of the SMI signal that has a maximum signal amplitude.

[0118] In some embodiments, the frequency comparator may be insensitive to phase, differently from PLL (“Phase-Locked Loop”) phase frequency comparators.

[0119] This may result in increased robustness, since driver-induced SMI signal jitter may not jeopardize the frequency locking operation.

[0120] In some embodiments, the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the selfmixing interferometry signal to the reference frequency by successively adjusting the time derivative of the drive current signal in discrete amounts depending on the signal processed frequency difference signal.

[0121] For performing the linear search or the binary search (and / or other SAR techniques) to determine the initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency, the readout and control circuit includes digital electronic elements to generate a first digital value indicating the sign of the frequency difference signal, wherein the time derivative of the drive current signal is then controlled in the digital domain according to the first digital value.

[0122] In such embodiments in which the linear search or the binary search or other SAR techniques are performed for determining the initial estimate of the for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency, the readout and control circuit is further configured to generate a second digital value representing the time derivative of the drive current signal by sampling the frequency difference signal and determining the time derivative of the drive current signal that reduces the corresponding frequency difference. The second digital value may be based on the first digital value, a predetermined digital value representing a discrete amount of change of the time derivative of the drive current signal and the previous second digital value.

[0123] The digital electronic elements may further provide a set of discrete values for the time derivative of the drive current signal for performing the binary search.

[0124] In the binary search, the readout and control circuit performs a predetermined number of iterations, wherein the discrete amount decreases each iteration such that the locking point is successively reached. The discrete amount decreases until a lower boundary is reached which may depend on the digital resolution.

[0125] In some embodiments, the readout and control circuit includes a band-pass filter configured to pass frequencies of the SMI that are within a frequency range centered around the reference frequency.

[0126] In some embodiments, the band-pass filter is tunable, and wherein the readout and control circuit is further configured to successively decrease the bandwidth of the band-pass filter when determining the initial estimate.

[0127] The band-pass filter may be tunable with respect to the center frequency of the frequency range that is passed or with respect to the width of the frequency range (bandwidth) or both.

[0128] This allows the band-pass filter to be tuned over time, in particular to adapt the bandwidth over time, for example, a large bandwidth is used at start and then the bandwidth is narrowed.

[0129] As discussed above, the determination of the initial estimate of the time derivative of the drive current signal requires an opening of the loop, forcing the analog controller to provide either the linear sweep of the derivative control or a discrete set of values for the time derivative of the drive current signal for the binary search. After the search loop, the loop is set to the operating point according to the determined initial estimate, closed again and taken back to continuous analog operation again.

[0130] Referring to the mixed signal domain:

[0131] In the mixed signal domain, the time derivative of the drive current signal is adjusted in discrete amounts. The time derivative of the drive current signal is then controlled in the digital domain in accordance with a first digital value indicating the sign of the frequency difference signal. The read out SMI signal and signal processing of the SMI signal is in the analog domain however.

[0132] Hence, in some embodiments, the readout and control circuit controls the time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency by successively adjusting time derivative of the drive current signal in discrete amounts according to a generated first digital value indicating the sign of the frequency difference signal.

[0133] In some embodiments, the readout and control circuit is further configured to output a second digital value representing the time derivative of the drive current signal. The second digital value is based on sampling the frequency difference signal and determining the time derivative of the drive current signal that reduces the corresponding frequency difference. The second digital value may be based on the first digital value, a predetermined digital value representing a discrete amount of change of the time derivative of the drive current signal and the previous second digital value. As discussed above, this digital output is then used for determining the target distance z.

[0134] In the mixed signal domain, the readout and control circuit may include a digital control circuit for implementing a counter, e.g. for the linear search, or a SAR.

[0135] Also in the mixed signal domain the readout and control circuit performs a tracking operation in which it tracks the frequency difference signal: if the frequency of the SMI signal slightly changes, the time derivative of the drive current signal is finely adjusted.

[0136] However, as also discussed for the analog domain, if the lock points are far away, the readout and control circuit may be slow in reaching it. Hence, a linear search (which may also be referred to as linear sweep) or a binary search, for example, may be performed to quickly find an initial estimate of the locking point.

[0137] However, in contrast to the analog domain, in the mixed signal domain, the readout and control circuit intrinsically work according to the linear or binary search. The readout and control circuit may switch between linear search and binary search in response to a control signal.

[0138] It is thus envisaged that an initial estimate of the time derivative of the drive current signal may also be performed in the mixed signal domain.

[0139] However, the linear search or the binary search is performed with coarser discrete amounts than in the tracking operation. Thus, in some embodiments, the readout and control circuit is further configured to perform a linear search or a SAR operation.

[0140] In some embodiments, the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the selfmixing interferometry signal to the reference frequency by successively adjusting the time derivative of the drive current signal in discrete amounts depending on the signal processed frequency difference signal.

[0141] For performing the linear search or the binary search, the readout and control circuit includes digital electronic elements to generate a first digital value indicating the sign of the frequency difference signal, wherein the time derivative of the drive current signal is then controlled in the digital domain according to the first digital value. The digital electronic elements may further provide a set of discrete values for the time derivative of the drive current signal for performing the binary search.

[0142] In the binary search, the readout and control circuit performs a predetermined number of iterations, wherein the discrete amount decreases each iteration such that the locking point is successively reached. The discrete amount decreases until a lower boundary is reached which may depend on the digital resolution.

[0143] In some embodiments, the readout and control circuit includes a band-pass filter configured to pass frequencies of the SMI that are within a frequency range centered around the reference frequency.

[0144] In some embodiments, the band-pass filter is tunable, and wherein the readout and control circuit is further configured to successively decrease the bandwidth of the band-pass filter when determining the initial estimate.

[0145] The band-pass filter may be tunable with respect to the center frequency of the frequency range that is passed or with respect to the width of the frequency range (bandwidth) or both.

[0146] This allows the band-pass filter to be tuned over time, in particular to adapt the bandwidth over time, for example, a large bandwidth is used at start and then the bandwidth is narrowed.

[0147] Accordingly, the loop may also be opened in the mixed signal domain, the counter or SAR are forced to perform the search with coarser steps in order to speed up convergence, then starting from the optimal point going back to the closed loop operation.

[0148] A hybrid operation is further envisaged, using both SAR and counter: first the SAR performs a (binary / generalized) search and, thus, pre-determining the most significant bits of the output code (second digital value), then the counter intervenes performing a finer tracking (less significant bits).

[0149] Referring back to both analog domain and mixed signal domain implementations:

[0150] The constant input frequency can be itself frequency modulated (e.g., between two values) implementing a “frequency hopping”, similarly to the commonly used voltage chopping.

[0151] Thus, in some embodiments, the band-pass filter is tunable and the readout and control circuit is further configured to scale the reference frequency by a factor M and to scale a center frequency of the band-pass filter and the derivative of the drive current signal accordingly.

[0152] In some embodiments, the readout and control circuit is further configured to receive a frequency selection signal and to select, based on the received frequency selection signal, between the reference signal and a scaled reference signal having the scaled reference frequency.

[0153] This may allow to increase robustness against disturbances which act as constant frequencies such that the readout and control circuit can lock, e.g., at two different frequencies which enables detection of disturbances and their rejection.

[0154] In some embodiments, readout and control circuit is further configured to control an amplitude of the drive current signal such that the amplitude is within predetermined boundaries.

[0155] In some embodiments, the readout and control circuit is further configured to output the SMI signal.

[0156] In some embodiments, each SMI device includes: a Vertical-Cavity Surface-Emitting Laser, VCSEL, configured to emit light according to the applied drive current signal, an on-chip lens arranged on a top surface of the VCSEL and configured to collimate outgoing light and to couple incoming light into the VCSEL via the top surface, a photodiode arranged on a bottom surface of the VCSEL and configured to generate the SMI signal according to an amount of light that is coupled out of the VCSEL via the bottom surface.

[0157] In some embodiments, a readout and control circuit is provided for each SMI device, and wherein the drive current signal is applied via through-chip connectivity.

[0158] In some embodiments, a readout and control circuit is provided for each group of SMI devices, the group being a row of the array, and wherein the drive current signal is applied via row-wise wire-bonding connectivity. Some embodiments pertain to a (corresponding) readout and control method for a SMI sensor including a plurality of SMI devices, including: generating a drive current signal for each or each group of SMI devices; applying the drive current signal; reading out a SMI; receiving a reference signal having a constant reference frequency; comparing the reference signal with the self-mixing interferometry signal to generate a frequency difference signal; and controlling, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the SMI signal is locked to the reference frequency. The readout and control method may be performed by the readout and control circuit and / or the SMI sensor as described herein.

[0159] Some embodiments pertain to an electronic device, including: a SMI sensor having a stacked structure, including in order from top to bottom: a plurality of SMI arranged in an array; and a readout and control circuit provided for each SMI device or for each group of SMI devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a SMI signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the SMI signal is locked to the reference frequency.

[0160] The electronic device may be a camera, mobile device (e.g., a smartphone, a tablet, a laptop etc.), a virtual reality device, a gaming device / console, a navigation device, a 3D reconstruction device, a vibrometer, an optical microphone etc.

[0161] The methods as described herein are also implemented in some embodiments as a computer program causing a computer and / or a processor to perform the method, when being carried out on the computer and / or processor. In some embodiments, also a non-transitory computer- readable recording medium is provided that stores therein a computer program product, which, when executed by a processor, such as the processor described above, causes the methods described herein to be performed.

[0162] Returning to Fig. 4, there is schematically illustrated in Fig. 4A an embodiment of a SMI device 11, in Fig. 4B an embodiment of a SMI sensor with pixel -parallel architecture and in Fig. 4C an embodiment of a SMI sensor with row-parallel architecture, which are discussed in the following.

[0163] The SMI devices 11 work in light intensity readout mode, however, voltage readout mode is also envisaged as an alternative embodiment herein.

[0164] Referring to Fig. 4A, the SMI device 11 includes an on-chip lens 20, a VCSEL 21 and a photodiode 22.

[0165] The VCSEL 21 emits light according to an applied drive current signal.

[0166] The on-chip lens 20 is arranged on a top surface of the VCSEL 21 and collimates outgoing light and couples incoming light into the VCSEL 21 via the top surface.

[0167] The photodiode 22 is arranged on a bottom surface of the VCSEL 21 and configured to generate the SMI signal according to an amount of light that is coupled out of the VCSEL 21 via the bottom surface.

[0168] Referring to Fig. 4B, the SMI sensor has the stacked structures 10-1 of Fig. 3 (pixel-parallel architecture), as schematically illustrated on the right for the sake of illustration only for three SMI devices 11.

[0169] The readout and control circuit 12 is connected to the VCSEL 21 via a first through-chip wire 23-1 via which the drive current signal is applied.

[0170] In other embodiments, the readout and control circuit 12 is connected to the VCSEL 21 via flip- chip wiring which is a front-to-front connection with backside VCSEL emission instead of a front-to-back connection.

[0171] The readout and control circuit 12 is connected to the photodiode 22 via a second through-chip wire 23-2 via which the SMI signal is readout.

[0172] Referring to Fig. 4C, the SMI sensor has the stacked structures 10-2 of Fig. 3 (row-parallel architecture). The readout and control circuit 12 may however be provided for each group of SMI devices on a separate chip without being stacked. The readout and control circuit 12 is connected to the VCSEL 21 via a first wire-bond (or multiple) 24-1 via which the drive current signal is applied, so a single VCSEL can be driven.

[0173] The readout and control circuit 12 is connected to the photodiode 22 via a second wire-bond 24- 2 via which the SMI signal of the driven VCSEL is readout.

[0174] As schematically illustrated for the sake of illustration for three SMI devices 11, there is one readout and control circuit 12 for each row (group of SMI devices).

[0175] Fig. 5 schematically illustrates in a block diagram an embodiment of a SMI sensor 30, which is discussed in the following.

[0176] The SMI sensor 30 includes a frequency comparator (“FC”) 31, a first amplifier 32, a controlled ramp generator (“CRG”) 33, a SMI device 11, a pre-amplifier 34, a band-pass filter (“BPF”) 35 and a second amplifier 36.

[0177] The CRG 33 generates a drive current signal Idriver(t) and applies it to the SMI device 11 which generates a SMI signal SsMi(t).

[0178] The SMI signal is pre-amplified by the pre-amplifier 34, filtered by the BPF 35, amplified by the second amplifier 36 and output to the FC 31 and externally.

[0179] The FC 31 receives a reference signal fref(t) which has a constant reference frequency and a filtered and amplified SMI signal fsMi(t).

[0180] The FC 31 compares the input signals and outputs a frequency difference signal Af(t) which is amplified by the first amplifier 32. The amplification typically minimizes the frequency difference while having a non-zero control to the CRG (the error is minimized, if loop gain is high enough).

[0181] The amplified frequency difference signal (as an embodiment of an analog signal processed frequency difference signal) is output as it indicates the time derivative of the drive current signal such that a distance to a target can be determined based on it, as discussed under reference of Fig. 3.

[0182] The amplified frequency difference signal is received by the CRG 33 which adjusts the derivative of the drive current signal according to the (amplified) frequency difference signal such that the frequency of the SMI signal is locked to the reference frequency. Here, it is assumed that the drive current signal is a triangular signal. Generally, the CRG 33 controls an amplitude (which may also be referred to as current modulation depth) of the drive current signal such that the amplitude is within predetermined boundaries, as depicted in Fig. 6.

[0183] Fig. 6 schematically illustrates an embodiment of drive current signal wrapping, which is discussed in the following.

[0184] As shown in the left graph, the absolute value of the derivative of the drive current signal converges to a fixed value when the frequency locking is reached.

[0185] However, as shown in the right graph, the amplitude of the drive current signal may increase continuously which may result in a very high current potentially causing damage to the SMI device 11.

[0186] Therefore, the CRG 33 wraps the drive current signal once predetermined boundaries are reached.

[0187] Fig. 7 schematically illustrates in a block diagram an embodiment of a SMI sensor 40, which is discussed in the following.

[0188] The SMI sensor 40 includes a frequency comparator (“FC”) 41, a digital control (“DC”) unit 42, a controlled ramp generator (“CRG”) 43, a SMI device 11, a pre-amplifier 44, a band-pass filter (“BPF”) 45 and a second amplifier 46.

[0189] The SMI device 11, the pre-amplifier 44 and the second amplifier 46 have basically the same functions as in Fig. 5 the SMI device 11, the pre-amplifier 34 and the second amplifier 36, respectively, and are thus not discussed in detail to avoid unnecessary repetition.

[0190] The basic difference between the SMI sensor 30 of Fig. 5 and the SMI sensor of Fig. 7 is that the first amplifier 32 is replaced with the DC (“DC”) unit 42.

[0191] In this embodiment, the DC unit 42 provides an N bit digital value as digital output instead of an analog signal such as the amplified version of the frequency difference signal of Fig. 5.

[0192] The DC unit 42 further receives a clock signal CLK to sample the frequency difference signal.

[0193] The DC unit 42, based on the input frequency difference, which is sampled on discrete instants by the clock signal CLK, determines at each sample the new current derivative to be provided in order to decrease the frequency difference.

[0194] This is the digital value that is output by the DC unit 42 in this embodiment. While the digital output is representing the imposed current time derivative, it may be thought as the digital code needed by a DAC to provide at the output the quantification of the current derivative.

[0195] The CRG 43 is able to interpret the digital value that is output by the DC unit 42 and adjust the time derivative of the drive current signal accordingly.

[0196] The DC unit 42 can exploit the measured frequency difference to instruct with discrete adjustments the CRG 43 to provide the current time derivative (the discrete amount may be variable, for example but not limited to, in the case of a binary search for an initial estimate as discussed further below).

[0197] In such cases the locking frequency is successively approached. This may be desirable, since the continuous control, as in Fig. 5, may cause a high fluctuation of the derivative of the drive current signal and a slow convergence.

[0198] Moreover, the BPF 45 may be tunable such that the frequency range (e.g., bandwidth) is narrowed (in discrete amounts) in each iteration until the frequency locking is reached (e.g., FC 41 outputs zero). At that point, the DC unit 42 can output a digital representation of the current time derivative, thus of the measured distance as stated previously.

[0199] Each of the SMI sensors 30 and 40 may perform an initialization procedure before a measurement to obtain an initial estimate for the derivative of the drive current signal that locks the SMI frequency to the reference frequency.

[0200] The initialization procedure may include a linear search.

[0201] The SMI sensor 40 may further perform a binary search.

[0202] Furthermore, linear and binary search can be combined to optimize convergence and tracking.

[0203] The initialization procedure may be initiated by a mode selection signal which indicates whether an initialization procedure is to be performed or whether a measurement with fine tuning and tracking is to be performed. The measurement is then started / stopped, for example, in response to a measurement start / stop signal.

[0204] Fig. 8 schematically illustrates an embodiment of a linear search and an embodiment of a binary search, which are discussed in the following.

[0205] Instead of waiting for a PLL-like system to lock autonomously (which may be a slow process), other techniques may be used to speed up the process and increase the time available to refine the measurement. In the linear search, as depicted on the left side, the CRG 33 or 43 performs a modulation sweep of the drive current signal. In other words, the CRG 33 generates a drive current signal with a plurality of parts in which each part has a different slope (derivative).

[0206] The FC 31 or 41 stores one or more samples of the resulting SMI signal in their time order (e.g., together with a sample number) and determines a part of the SMI signal that has a maximum signal amplitude, since the BPF 35 or 45 is narrowband and has its maximum transmission for the SMI frequency that corresponds to the reference frequency.

[0207] The frequency of the part of the SMI signal that has the maximum signal amplitude determines the initial estimate of the derivative of the drive current signal.

[0208] The FC 31 or 41 outputs the time information of the maximum to the CRG 33 or 43, respectively, such that it knows which slope to use for the subsequent measurement.

[0209] In the binary search, as depicted on the right side, the range is successively split in two and for each derivative it is checked whether the frequency difference signal is positive or negative or zero. After N cycles the loop will fine set and proceed with measurement or do directly the measurement. The binary search may also be performed with overlapping ranges, i.e. dividing the range in two parts but observing a larger part than half of the range, in order to give the possibility to recover from mistakes (wrong decisions) of the FC 31 or 41 that can occur in noisy conditions.

[0210] In more detail, the CRG 43 selects a first slope (derivative of the drive current signal) in the middle of the maximum possible slope and the minimum possible slope. If the DC unit 42 outputs that the frequency difference signal is positive, e.g., the SMI signal is higher than the reference frequency, the CRG 43 selects in the next iteration a lower slope that is in the middle of the first slope and the minimum possible slope. This is repeated for N cycles such that the locking frequency is successively reached.

[0211] The time derivative of the drive current signal after N cycles can either be the initial estimate that is refined with an incremental and tracking phase, or the final measurement is the quantization error is satisfactory.

[0212] Fig. 9 schematically illustrates in a block diagram an embodiment of a self-mixing interferometry sensor 50, which is discussed in the following.

[0213] The SMI sensor 50 may be based on any of the SMI sensors 30 and 40 which further include a multiplexer 58. The FC 51 and the CRG 53 have basically the same functions as in Fig. 5 the FC 31 the CRG 33 or as in Fig. 7 the FC 41 and the CRG 43, respectively, and are thus not discussed in detail to avoid unnecessary repetition.

[0214] The BPF 53 is tunable at least with respect to the center frequency. The BPF 53 may be tunable with respect to the center frequency of the bandwidth that is passed or with respect to the bandwidth or both.

[0215] The center frequency may be tuned in the case of frequency hopping.

[0216] The bandwidth may be tunable for increasing the SNR, while the SMI signal frequency gets closer and approximates better the reference frequency.

[0217] The reference signal is scaled by a known factor M (which may be an Integer for simplicity but not necessarily) to avoid loss of frequency lock and keep continuous operation.

[0218] The scaled reference signal may be selected by the multiplexer 58 based on a received frequency selection signal FSEL.

[0219] The driver current slope has to be scaled accordingly which can be achieved, e.g., in CMOS (“Complementary Metal-Oxide Semiconductor”) by scaling current mirrors.

[0220] Moreover, the BPF 53 scales its center frequency accordingly which can be achieved, e.g., in CMOS by using switched capacitor filters).

[0221] The output distance is proportional to and, thus, does not change.

[0222] Pure (fixed) tone interference are attenuated and rejected, or possibly detected by output change.

[0223] Fig. 10 schematically illustrates in a flow diagram an embodiment of a readout and control method 100 for a SMI sensor, which is discussed in the following.

[0224] The method 100 may be performed by the readout and control circuit and / or the SMI sensor as described herein.

[0225] At 101, a drive current signal is generated for each or each group of SMI devices, as discussed herein.

[0226] At 102, an amplitude of the drive current signal is controlled such that the amplitude is within predetermined boundaries, as discussed herein. At 103, the drive current signal is applied, as discussed herein.

[0227] At 104, a SMI signal is read out, as discussed herein.

[0228] At 105, by a band-pass filter, frequencies of the SMI signal that are within a frequency range centered around the reference frequency are passed, as discussed herein.

[0229] At 106, a reference signal having a constant reference frequency is received, as discussed herein.

[0230] At 107, the reference signal is compared with the SMI signal to generate a frequency difference signal.

[0231] At 108, a time derivative of the drive current signal is controlled, based on the frequency difference signal, such that a frequency of the SMI signal is locked to the reference frequency.

[0232] Typically, the method 100 according to 101 to 108 is performed in a loop and repeated a plurality of times.

[0233] Fig. 11 schematically illustrates in a flow diagram an embodiment of a readout and control method 200 for a SMI sensor, which is discussed in the following.

[0234] The method 200 may be performed by the readout and control circuit and / or the SMI sensor as described herein.

[0235] At 201, an initial estimate for the derivative of the drive current signal is determined that locks the frequency of the SMI signal to the reference frequency by successively adjusting the derivative of the drive current signal in discrete amounts depending on the frequency difference signal and successively decreasing the frequency range of the band-pass filter, wherein the bandpass filter is tunable, as discussed herein.

[0236] At 202, 101 to 108 of Fig. 10 are performed.

[0237] Fig. 12 schematically illustrates in a flow diagram an embodiment of a readout and control method 300 for a SMI sensor, which is discussed in the following.

[0238] The method 300 may be performed by the readout and control circuit and / or the SMI sensor as described herein.

[0239] At 301, an initial estimate for the derivative of the drive current signal that locks the frequency of the SMI signal to the reference frequency by performing a linear search, as discussed herein.

[0240] At 302, a modulation sweep of the drive current signal is performed and a part of the SMI signal is determined that has a maximum signal amplitude, wherein the frequency of the part of the SMI signal that has the maximum signal amplitude determines the initial estimate of the derivative of the drive current signal, as discussed herein.

[0241] At 303, 101 to 108 of Fig. 10 are performed.

[0242] Fig. 13 schematically illustrates in a flow diagram an embodiment of a readout and control method 400 for a SMI sensor, which is discussed in the following.

[0243] The method 400 may be performed by the readout and control circuit and / or the SMI sensor as described herein.

[0244] At 401, 101 to 108 of Fig. 10 are performed.

[0245] At 402, the reference frequency is scaled by a factor M and to scale a center frequency of the band-pass filter and the derivative of the drive current signal accordingly, as discussed herein.

[0246] At 403, a frequency selection signal is received, as discussed herein.

[0247] At 404, based on the received frequency selection signal, it is selected between the reference signal and a scaled reference signal having the scaled reference frequency, as discussed herein.

[0248] The method 400 may start again at 401 and may be repeated several times.

[0249] Generally, any of or all of 402 to 404 may be performed between any of 101 to 108 of 401.

[0250] Returning to the general explanations and summarizing some aspects of some embodiments:

[0251] The SMI sensor uses a closed loop readout between sensing and driving which enables the optimization of the sensitive readout chain by fixing the frequency of operation for maximizing the SNR.

[0252] The SMI sensor has an integrated driving and readout which allows to use shorter loops which improves the tight connection, thereby allowing reduced wiring and enabling compact and low noise range sensing systems.

[0253] The SMI sensor may use digital direct conversion with which digital controlled loops may be provided to directly convert the relevant to digital, e.g., with incremental or successive approximation techniques.

[0254] The SMI sensor may use techniques to achieve fast search of center distance, for example, by utilizing successive approximation or sweep techniques for the speed up of the locking process.

[0255] The SMI sensor may reject spurs / disturbances through frequency hopping which enables the switching (without loss of lock) between two or more reference frequencies in order to detect and / or reject pure tone disturbances. SNR optimization: Reduced amplifier specifications because of single frequency of operation.

[0256] Disturbances rejection: Closed loop control enables frequency hopping, enabling detection and / or rejection of disturbances.

[0257] Direct readout and digital conversion: Common ADC (“Analog-to-Digital Converter”) techniques can be integrated into the loop to provide simultaneous readout and conversion to digital.

[0258] It should be recognized that the embodiments describe methods with an exemplary ordering of method steps. The specific ordering of method steps is however given for illustrative purposes only and should not be construed as binding.

[0259] All units and entities described in this specification and claimed in the appended claims can, if not stated otherwise, be implemented as integrated circuit logic, for example on a chip, and functionality provided by such units and entities can, if not stated otherwise, be implemented by software.

[0260] In so far as the embodiments of the disclosure described above are implemented, at least in part, using software-controlled data processing apparatus, it will be appreciated that a computer program providing such software control and a transmission, storage or other medium by which such a computer program is provided are envisaged as aspects of the present disclosure.

[0261] Note that the present technology can also be configured as described below.

[0262] (1) A self-mixing interferometry sensor having a stacked structure, including in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0263] (2) A self-mixing interferometry sensor, including: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0264] (3) The self-mixing interferometry sensor of (1) or (2), wherein the readout and control circuit includes a band-pass filter configured to pass frequencies of the self-mixing interferometry signal that are within a bandwidth centered around the reference frequency.

[0265] (4) The self-mixing interferometry sensor of anyone of (1) to (3), wherein the readout and control circuit is further configured to apply analog signal processing on the generated frequency difference signal, wherein the time derivative of the drive current signal is controlled in the analog domain according to the signal processed frequency difference signal.

[0266] (5) The self-mixing interferometry sensor of (4), wherein the readout and control circuit is further configured to output the signal processed frequency difference signal.

[0267] (6) The self-mixing interferometry sensor of anyone of (3) to (5), wherein the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by successively adjusting the time derivative of the drive current signal in discrete amounts depending on the signal processed frequency difference signal. (7) The self-mixing interferometry sensor of (6), wherein the band-pass filter is tunable, and wherein the readout and control circuit is further configured to successively decrease the bandwidth of the band-pass filter when determining the initial estimate.

[0268] (8) The self-mixing interferometry sensor of anyone of (3) to (7), wherein the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by performing a linear search, wherein the linear search includes performing a modulation sweep of the drive current signal and determining a part of the self-mixing interferometry signal that has a maximum signal amplitude.

[0269] (9) The self-mixing interferometry sensor of anyone of (1) to (8), wherein the readout and control circuit is further configured to successively adjust the time derivative of the drive current signal in discrete amounts depending on the frequency difference signal.

[0270] (10) The self-mixing interferometry sensor of (9), wherein the readout and control circuit is further configured to generate a first digital value indicating the sign of the frequency difference signal, wherein the time derivative of the drive current signal is controlled in the digital domain according to the first digital value.

[0271] (11) The self-mixing interferometry sensor of (9) or (10), wherein the readout and control circuit is further configured to output a second digital value representing the time derivative of the drive current signal.

[0272] (12) The self-mixing interferometry sensor of anyone of (3) to (11), wherein the band-pass filter is tunable, and wherein the readout and control circuit is further configured to scale the reference frequency by a factor M and to scale a center frequency of the band-pass filter and the time derivative of the drive current signal accordingly.

[0273] (13) The self-mixing interferometry sensor of (12), wherein the readout and control circuit is further configured to receive a frequency selection signal and to select, based on the received frequency selection signal, between the reference signal and a scaled reference signal having the scaled reference frequency.

[0274] (14) The self-mixing interferometry sensor of anyone of (1) to (13), wherein readout and control circuit is further configured to control an amplitude of the drive current signal such that the amplitude is within predetermined boundaries.

[0275] (15) The self-mixing interferometry sensor of anyone of (1) to (14), wherein the readout and control circuit is further configured to output the self-mixing interferometry signal. (16) The self-mixing interferometry sensor of anyone of (1) to (15), wherein each self-mixing interferometry device includes: a Vertical-Cavity Surface-Emitting Laser, VCSEL, configured to emit light according to the applied drive current signal, an on-chip lens arranged on a top surface of the VCSEL and configured to collimate outgoing light and to couple incoming light into the VCSEL via the top surface, a photodiode arranged on a bottom surface of the VCSEL and configured to generate the self-mixing interferometry signal according to an amount of light that is coupled out of the VCSEL via the bottom surface.

[0276] (17) The self-mixing interferometry sensor of anyone of (1) to (16), wherein a readout and control circuit is provided for each self-mixing interferometry device, and wherein the drive current signal is applied via through-chip or flip-chip connectivity.

[0277] (18) The self-mixing interferometry sensor of anyone of (1) to (16), wherein a readout and control circuit is provided for each group of self-mixing interferometry devices, the group being a row of the array, and wherein the drive current signal is applied via row-wise through-chip, wire-bonding or flip-chip connectivity.

[0278] (19) A readout and control method for a self-mixing interferometry sensor including a plurality of self-mixing interferometry devices, comprising: generating a drive current signal for each or each group of self-mixing interferometry devices; applying the drive current signal; reading out a self-mixing interferometry signal; receiving a reference signal having a constant reference frequency; comparing the reference signal with the self-mixing interferometry signal to generate a frequency difference signal; and controlling, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

[0279] (20) The readout and control method of (19), further including passing, by a band-pass filter, frequencies of the self-mixing interferometry signal that are within a bandwidth centered around the reference frequency.

[0280] (21) The readout and control method of (19) or (20), further including applying analog signal processing on the generated frequency difference signal, wherein the time derivative of the drive current signal is controlled in the analog domain according to the signal processed frequency difference signal.

[0281] (22) The readout and control method of (21), further including outputting the signal processed frequency difference signal.

[0282] (23) The readout and control method of anyone of (20) to (22), further including determining an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by successively adjusting the time derivative of the drive current signal in discrete amounts depending on the signal processed frequency difference signal.

[0283] (24) The readout and control method of (23), wherein the band-pass filter is tunable, and further including successively decreasing the bandwidth of the band-pass filter when determining the initial estimate.

[0284] (25) The readout and control method of anyone of (20) to (24), further including determining an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by performing a linear search, wherein the linear search includes performing a modulation sweep of the drive current signal and determining a part of the self-mixing interferometry signal that has a maximum signal amplitude.

[0285] (26) The readout and control method of anyone of (19) to (25), further including successively adjusting the time derivative of the drive current signal in discrete amounts depending on the frequency difference signal.

[0286] (27) The readout and control method of (26), further including generating a first digital value indicating the sign of the frequency difference signal, wherein the time derivative of the drive current signal is controlled in the digital domain according to the first digital value.

[0287] (28) The readout and control method of (26) or (27), further including outputting a second digital value representing the time derivative of the drive current signal.

[0288] (29) The readout and control method of anyone of (20) to (28), wherein the band-pass filter is tunable, and further including scaling the reference frequency by a factor M and to scale a center frequency of the band-pass filter and the derivative of the drive current signal accordingly.

[0289] (30) The readout and control method of (29), further including: receiving a frequency selection signal; and selecting, based on the received frequency selection signal, between the reference signal and a scaled reference signal having the scaled reference frequency. (31) The readout and control method of anyone of (19) to (30), further including: controlling an amplitude of the drive current signal such that the amplitude is within predetermined boundaries.

[0290] (32) The readout and control method of anyone of (19) to (31), further including outputting the self-mixing interferometry signal.

[0291] (33) An electronic device, including: a self-mixing interferometry sensor according to anyone of (1) to (18).

Claims

CLAIMS1. A self-mixing interferometry sensor having a stacked structure, comprising in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

2. A self-mixing interferometry sensor, including: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

3. The self-mixing interferometry sensor of claim 1 or 2, wherein the readout and control circuit includes a band-pass filter configured to pass frequencies of the self-mixing interferometry signal that are within a bandwidth centered around the reference frequency.

4. The self-mixing interferometry sensor of claim 1, wherein the readout and control circuit is further configured to apply analog signal processing on the generated frequency difference signal, wherein the time derivative of the drive current signal is controlled in the analog domain according to the signal processed frequency difference signal.

5. The self-mixing interferometry sensor of claim 4, wherein the readout and control circuit is further configured to output the signal processed frequency difference signal.

6. The self-mixing interferometry sensor of claim 3, wherein the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by successively adjusting the time derivative of the drive current signal in discrete amounts depending on the signal processed frequency difference signal.

7. The self-mixing interferometry sensor of claim 6, wherein the band-pass filter is tunable, and wherein the readout and control circuit is further configured to successively decrease the bandwidth of the band-pass filter when determining the initial estimate.

8. The self-mixing interferometry sensor of claim 3, wherein the readout and control circuit is further configured to determine an initial estimate for the time derivative of the drive current signal that locks the frequency of the self-mixing interferometry signal to the reference frequency by performing a linear search, wherein the linear search includes performing a modulation sweep of the drive current signal and determining a part of the self-mixing interferometry signal that has a maximum signal amplitude.

9. The self-mixing interferometry sensor of claim 1, wherein the readout and control circuit is further configured to successively adjust the time derivative of the drive current signal in discrete amounts depending on the frequency difference signal.

10. The self-mixing interferometry sensor of claim 9, wherein the readout and control circuit is further configured to generate a first digital value indicating the sign of the frequency difference signal, wherein the time derivative of the drive current signal is controlled in the digital domain according to the first digital value.

11. The self-mixing interferometry sensor of claim 9, wherein the readout and control circuit is further configured to output a second digital value representing the time derivative of the drive current signal.

12. The self-mixing interferometry sensor of claim 3, wherein the band-pass filter is tunable, wherein the readout and control circuit is further configured to scale the reference frequency by afactor M and to scale a center frequency of the band-pass filter and the time derivative of the drive current signal accordingly.

13. The self-mixing interferometry sensor of claim 12, wherein the readout and control circuit is further configured to receive a frequency selection signal and to select, based on the received frequency selection signal, between the reference signal and a scaled reference signal having the scaled reference frequency.

14. The self-mixing interferometry sensor of claim 1, wherein readout and control circuit is further configured to control an amplitude of the drive current signal such that the amplitude is within predetermined boundaries.

15. The self-mixing interferometry sensor of claim 1, wherein the readout and control circuit is further configured to output the self-mixing interferometry signal.

16. The self-mixing interferometry sensor of claim 1, wherein each self-mixing interferometry device includes: a Vertical-Cavity Surface-Emitting Laser, VCSEL, configured to emit light according to the applied drive current signal, an on-chip lens arranged on a top surface of the VCSEL and configured to collimate outgoing light and to couple incoming light into the VCSEL via the top surface, a photodiode arranged on a bottom surface of the VCSEL and configured to generate the self-mixing interferometry signal according to an amount of light that is coupled out of the VCSEL via the bottom surface.

17. The self-mixing interferometry sensor of claim 1, wherein a readout and control circuit is provided for each self-mixing interferometry device, and wherein the drive current signal is applied via through-chip connectivity.

18. The self-mixing interferometry sensor of claim 1, wherein a readout and control circuit is provided for each group of self-mixing interferometry devices, the group being a row of the array, and wherein the drive current signal is applied via row-wise wire-bonding connectivity.

19. A readout and control method for a self-mixing interferometry sensor including a plurality of self-mixing interferometry devices, comprising: generating a drive current signal for each or each group of self-mixing interferometry devices; applying the drive current signal; reading out a self-mixing interferometry signal;receiving a reference signal having a constant reference frequency; comparing the reference signal with the self-mixing interferometry signal to generate a frequency difference signal; and controlling, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

20. An electronic device, comprising: a self-mixing interferometry sensor having a stacked structure, including in order from top to bottom: a plurality of self-mixing interferometry devices arranged in an array; and a readout and control circuit provided for each self-mixing interferometry device or for each group of self-mixing interferometry devices, wherein each readout and control circuit is configured to: generate a drive current signal, apply the drive current signal, read out a self-mixing interferometry signal, receive a reference signal having a constant reference frequency, compare the reference signal with the self-mixing interferometry signal to generate a frequency difference signal, control, based on the generated frequency difference signal, a time derivative of the drive current signal such that a frequency of the self-mixing interferometry signal is locked to the reference frequency.

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