WO2024110145A1

WO2024110145A1 - Sensor module for self-mixing interferometry

Info

Publication number: WO2024110145A1
Application number: PCT/EP2023/079872
Authority: WO
Inventors: Marcelli GRIMAU; Laurent Nevou; Svenja Mauthe; Jens Geiger
Original assignee: Ams International Ag
Priority date: 2022-11-25
Filing date: 2023-10-26
Publication date: 2024-05-30

Abstract

A self-mixing interferometry sensor module comprises semiconductor laser (20) modulated to emit electromagnetic radiation out of the sensor module (10), and operable to undergo self-mixing interference, SMI, caused by reflections of the emitted radiation from a target to be placed outside the sensor module (10). The sensor module further comprises a detector unit (30) and an application-specific integrated circuit (40). A first fraction of the radiation is emitted by a front output (21) and a second fraction is emitted by a rear output (22) of the laser, the first and second fraction being in opposite phase. The detector unit (30) is operable to detect the first fraction and the second fraction and generate respective output signals (01, 02). The application-specific integrated circuit (40) is operable to determine a difference signal (DS) from the generated output signals (01, 02) being indicative of the SMI of the laser (20).

Description

SENSOR MODULE FOR SELF-MIXING INTERFEROMETRY

This disclosure relates to a self-mixing interferometry sensor module, an electronic device and to a method of operating a self-mixing interferometry sensor module.

BACKGROUND OF THE DISCLOSURE

In recent years self-mixing interference, or SMI, has become available for sensing and monitoring distance and speed using mobile electronic devices, such as smartphones, watches, and other wearable devices. For example, SMI has successfully been applied to sensing and/or monitoring physiological parameters, range or a fingerprint of a user. Another application includes rotary encoders.

Self-mixing interference occurs when part of the light emitted from a coherent light source is retro-fed back into the coherent source cavity (e.g., a laser such as a verticalcavity surface-emitting laser, or VCSEL, or as a distributed feedback laser, or DFB) . In turn, the coherent light source cavity produces a change in carrier population and refractive index. That change can be observed in a threshold current or threshold voltage change as well as on the optical power emitted by the cavity.

In order to measure absolute distance and speed with SMI, the drive current of the laser source can be modulated to tune the wavelength of the laser itself (and therefore the phase) . In turn, one has to post-process the SMI data or provide dedicated hardware to subtract the modulation function, e.g. a triangle modulation, and have access to the useful SMI signal . However, removing the modulation proves to be challenging because as it may add noise on hardware level or can require a large amount of ADC bits and a low gain ampli fication i f done at a later stage by post-processing the data . Both paths (HW demodulation or data post processing) typically lead to lower signal-to-noise , SNR .

Thus , an obj ect to be achieved is to provide a sensor module for electronic devices that overcomes the aforementioned limitations and allows for sel f-mixing interferometry with a higher SNR . A further obj ect is to provide an electronic device comprising such a sensor module and an improved method of operating a sel f-mixing interferometry sensor module .

These obj ectives are achieved with the subj ect-matter of the independent claims . Further developments and embodiments are described in dependent claims .

SUMMARY OF THE DISCLOSURE

The following relates to an improved concept in the field of optical sensing . One aspect relates to using a semiconductor laser, which is arranged to emit a first fraction of modulated radiation by a front output and a second fraction by a rear output of the laser . Then, the SMI signals from the front output and the back output of the laser are in opposite phase . As a result , the di f ference of those two SMI signals , i . e . output signal generated from the first fraction and the second fraction of the emitted modulated light would largely or completely remove the modulation and multiply the amplitude of the SMI signal twofold . In at least one embodiment, a self-mixing interferometry sensor module comprises a semiconductor laser, a detector unit and an application-specific integrated circuit.

The semiconductor laser is modulated to emit electromagnetic radiation out of the sensor module, e.g. towards an external object to be placed outside the sensor module. In turn, the laser is operable to undergo self-mixing interference, SMI, caused by reflections of the emitted radiation, which return back from the external back inside the sensor module.

The detector unit is operable to generate output signals which are indicative of an optical power output of the light emitter due to SMI. For example, a first fraction of the radiation is emitted by a front output of the laser. A second fraction is emitted by a rear output of the laser. The first and second fraction are in opposite phase. The detector unit is operable to detect both the first fraction and the second fraction. In turn, the detector unit generates respective output signals, e.g. a first and a second output signal.

As discussed below, SMI eventually alters a property of the laser. This property is indirectly measured by means of the detector unit, which generates the output signals as a function of said property, or change of said property. The output signals may, in addition, be a measured current or voltage, for example. Thus, the detector unit may have means, e.g. active or passive circuitry, to measure said change as an electronic property.

The application-specific integrated circuit, ASIC, is operable to determine a difference signal from the generated output signals, e.g. a difference between the first and second output signals. The difference signal is indicative of the SMI of the laser.

For example, the ASIC receives the output signals, e.g. first and second output signals, from the detector unit. The ASIC may comprise a processing unit to receive the signal and perform signal processing on the received signals. The processing unit can be a central processing unit, CPU, microprocessor, or a system-on-a-chip, SOC, which is dedicated to process the output signals, i.e. generate the difference signal. The ASIC may comprise additional electronic components, such as ADCs, logic, amplifiers and/or driver circuits. For example, the ASIC may comprise a driver circuit, which is operable to provide a modulated driving current to modulate the emission of the semiconductor laser. Furthermore, the detector unit can, at least in parts, be integrated into the ASIC, or electrically connected thereto.

The semiconductor laser is arranged to enable self-mixing interference, and may comprise a cavity resonator, into which at least a fraction of the light emitted by the laser can be back-reflected, or back-scattered, from the external target outside the module. The laser may be implemented as a laser diode and comprises a laser cavity. The laser is configured to emit coherent light, e.g. in an infrared (IR) , visible (VIS) or ultraviolet (UV) range of the electromagnetic spectrum, out of the sensor module. The semiconductor laser is modulated with a modulation function, such as a triangle function, and generates a continuous modulated emission.

Upon the aforementioned back-injection of the emitted light into the cavity, the light is reflected off a distance of the external target and the laser undergoes self-mixing interference which includes information of the distance , and, potentially speed of the external target . For example , when the emitted electromagnetic field from the laser cavity is reflected back into the cavity and changes its phase due to target distance changes , it causes a further modulation in amplitude and/or frequency of the laser electromagnetic light field due to interference . The sel f-mixing interference , or SMI , generates periodic fringes in the signal of the solitary laser . More accurately, SMI modulates the optical power (which can be observed by measuring it in a photodetector, e . g . as a photocurrent of the detector unit , for example ) and the threshold laser gain (which can be detected monitoring the laser voltage or laser current ) . Another way to generate SMI is through the modulation of the laser emission wavelength, e . g . ramping the laser current periodically (via triangular function current ramp or changing the laser cavity via a MEMS mirror ) .

When no external target is present outside the module in the field of emission of the light source to intercept and reflect light of the latter, no sel f-mixing interference occurs within the laser .

The proposed concept relies on the observation that the sel fmixing output signals measured between the two laser facet outputs are in phase opposition . The di f ference signal results from the subtraction of the two outputs and allows for a balanced detection that improves the signal quality, and allows canceling of unwanted signals due to laser modulation and disturbances on laser supply and transimpedance ampli fier, for example . SMI signals can be acquired by measuring the modulation of the power by a photodiode with a reasonable signal-to-noise ratio ( SNR) even when the feedback is very weak or by amplifying voltage variation of the diode junction. It can be shown that the front and rear outputs of a laser diode are in phase opposition .

The proposed concept allows to remove the modulation of laser emission, e.g. triangular modulation carrier into an FMCW SMI sensor, by means of the difference signal. For example, the SMI optical power signal is readout from the front and from the back side of the laser. Those two output signals are in perfect anti-phase. As a result, doing the difference between those two signals lead to the subtraction of the modulation, e.g. triangular modulation, and multiply the amplitude of the SMI signal twofold. Hence, the signal can be strongly amplified into the analogue domain. The amplification chain can be strongly increased, typically 100 times. The amount of ADC bits for post-processing can be reduced by 4 to 6 bits or can be used more efficiently. No triangular modulation post processing of the data have to be made and no special hardware circuitry design to remove the modulation may be necessary that could else lead to additional noise. The proposed concept can be applied to any SMI ranger system that modulate the laser driving current.

In at least one embodiment, the detector unit is operable to detect a junction voltage of the laser. In turn, the output signals are generated as a function of said junction voltages, respectively. Junction voltage is one possible electronic property of the laser which change as a result of SMI. For example, the detector unit comprises one or more voltage meters to detect the junction voltage (s) . An addressable array allows for voltage readout. The junction voltage of the laser provides one measurement parameters which is indicative of the SMI induced in the laser.

In at least one embodiment, the detector unit is operable to detect an optical power output by the front output and the rear output, respectively. In turn, the output signals are generated as a function of said optical power outputs, respectively. Optical power is another possible property of the light emitters which may change as a result of SMI. For example, the detector unit comprises one or more photodetectors, such as a photodiode, or a photodiode array to detect optical power outputs. A power readout may rely on an array of light detectors, e.g. to get laser independent signals. The optical power output provides another measure of the SMI induced in the laser.

In at least one embodiment, the detector unit comprises at least two photodetectors to detect the electromagnetic radiation. At least one photodetector is integrated into the application-specific integrated circuit. Integration into the ASIC allows for a more compact design and wafer-level processing, e.g. CMOS technology.

In at least one embodiment, at least one photodetector is integrated into a layer sequence of the semiconductor laser. In addition, or alternatively, at least one photodetector is arranged outside the laser, i.e. is not integrated into a layer sequence of the laser. Integration may involve all or parts of the photodetector, providing a high degree of design freedom, e.g. to meet a desired footprint.

For example, some configurations may use an "internal" photodetector where the laser is grown epitaxially on top of the photodetector, or the photodetector is integrated in one ("bottom") of the laser DBR mirrors of a VCSEL, or other types. Those configurations can have good light-coupling efficiencies but can complicate the epitaxial semiconductor design and increase the growth process cost. External photodetectors can be configured side to side to the laser, e.g. VCSEL. They can be part of the same epitaxy, modifying the laser epitaxy top DBR by wafer processing or they can be implemented from other systems (e.g., comparably cheap silicon photodetectors) .

In at least one embodiment, at least one photodetector is arranged outside of the semiconductor laser and/or application-specific integrated circuit.

In at least one embodiment, the sensor module further comprises a housing with a transparent cover. The laser and photodetectors, or ASIC, are arranged behind the cover such that emission by the front output is directed to one photodetector via the cover. In addition, or alternatively, and the laser and photodetectors, or ASIC, are arranged behind the cover such that emission by the rear output is directed to another photodetector. Detection of the first and second fraction of emitted light can be facilitated by means of the cover, which effectively acts as a beam splitter or optical guide. The housing can be implemented by mold structure or as a CAN package, for example.

In at least one embodiment, the laser and photodetectors, or ASIC, are arranged behind the cover such that emission by the front output is directed directly to one photodetector. In addition, or alternatively. The laser and photodetectors, or ASIC, are arranged behind the cover such that emission by the rear output is directed directly to another photodetector . The term "directly" , may indicate that the space between the laser and a photodetector is free of optical elements , or optical elements which steer the emitted beams of laser light to the respective photodetector . The housing can be implemented by mold structure or as a CAN package , for example .

In at least one embodiment , the sensor module further comprises a housing with a wafer-level optic . The laser and photodetectors , or AS IC, are arranged behind the wafer-level optic such that emission by the front output is directed to one photodetector via the cover . In addition, or alternatively, the laser and photodetectors , or AS IC, are arranged behind the wafer-level optic such that emission by the rear output is directed to another photodetector . Detection of the first and second fraction of emitted light can be facilitated by means of the wafer-level optic, which ef fectively acts as a beam splitter or optical guide . The wafer-level optic can be implemented at the wafer-level , e . g . by means of CMOS integration technology, for example .

In at least one embodiment , the semiconductor laser comprises a semiconductor laser diode , a resonant-cavity light emitting device , a distributed feedback laser, edge emitting laser and/or a vertical cavity surface emitting laser, VCSEL, diode .

These devices feature coherent emission to generate SMI fringes . A resonant-cavity light emitting device can be considered a semiconductor device , which is operable to emit coherent light based on a resonance process . In this process , the resonant-cavity light emitting device may directly convert electrical energy into light , e . g . , when pumped directly with an electrical current to create ampli fied stimulated emission .

For example , the semiconductor laser comprises a vertical cavity surface emitting laser, VCSEL, diodes . VCSELs are an example of a resonant-cavity light emitting device and feature a beam emission that is perpendicular to a main extension plane of a top surface of the VCSEL . The VCSEL diode can be formed from semiconductor layers on a substrate , wherein the semiconductor layers comprise two distributed Bragg reflectors enclosing active region layers in between and thus forming a cavity . VCSELs and their principle of operation are a well-known concept and are not further detailed throughout this disclosure . For example , the VCSEL diode is configured to have an emission wavelength of 940 nm, 850 nm, or another wavelength .

In at least one embodiment , the laser, the detector unit and/or the application-speci fic integrated circuit form an integrated semiconductor device , such as a CMOS integrated circuit device , on a common substrate or carrier . In addition, or alternatively, the sensor module comprises the housing to form a sensor package into which the laser, detector unit and the AS IC, or the integrated semiconductor device formed by the laser, detector unit and the AS IC, are integrated .

In at least one embodiment , the housing comprises or is a sensor package .

Furthermore , an electronic device is suggested . The device comprises at least one sel f-mixing interferometry sensor module according to one or more of the aforementioned aspects. Furthermore, the electronic device comprises a host system. The sensor module is integrated, or embedded, into the host system. The host system comprises one of a mobile device, a smartphone, a wearable mobile device, robots, cars and smart devices such as watches, bracelets, glasses, etc.

In at least one embodiment, the electronic device further comprises a processing unit which is coupled to the sensor module. The processing unit is configured to receive the difference signal from the sensor module and derive a distance and/or speed of the target to be placed outside the sensor module. The processing unit may, completely, or in parts, be implemented as, or on, the ASIC of the sensor module. The processing unit can be a central processing unit, CPU, microprocessor, or a system-on-a-chip, SOC, which is dedicated to process the difference signal in SMI signal processing to deduce higher order parameters like distance of speed .

Furthermore, a method of operating a self-mixing interferometry sensor module is suggested. One step involves modulating a semiconductor laser to emit electromagnetic radiation out of the sensor module, and undergo self-mixing interference, SMI, caused by reflections of the emitted radiation from a target to be placed outside the sensor module, wherein a first fraction of the radiation is emitted by a front output and a second fraction is emitted by a rear output of the laser, the first and second fraction being in opposite phase. Another step involves detecting the first fraction and the second fraction and generate respective output signals. Another step determining a difference signal from the generated output signals being indicative of the SMI of the laser .

Further embodiments of the method become apparent to the skilled reader from the aforementioned embodiments of the sel f-mixing interferometry sensor module and of the electronic device , and vice-versa .

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of figures may further illustrate and explain aspects of the sel f-mixing interferometry sensor module , electronic device and the method of determining an optical power ratio for sel f-mixing interferometry .

Components and parts of the sel f-mixing interferometry sensor that are functionally identical or have an identical ef fect are denoted by identical reference symbols . Identical or ef fectively identical components and parts might be described only with respect to the figures where they occur first . Their description is not necessarily repeated in successive figures .

In the figures :

Figures 1 to 6 show various example embodiments of sel f-mixing interferometry sensor modules , and

Figures 7A and 7B show an example measured using an embodiment of a sel f-mixing interferometry sensor module .

In the following di f ferent various examples embodiments are illustrated . In these examples , sel f-mixing interferometry sensor module 10 comprises a semiconductor laser 20 , a detector unit 30 and an application-speci fic integrated circuit 40 , or AS IC . These components of the sensor module are arranged inside a housing, e . g . a molded sensor package or a can package , etc .

The semiconductor laser 20 has two facets , i . e . the front output 21 and rear output 22 . In the examples shown herein, the laser constitutes a component which is electrically connected to the AS IC 40 rather than being integrated into the AS IC . This should not be construed as limiting . Other examples can be thought of , where the laser is integrated into the AS IC . For the sake of the following examples , the laser is implemented as a VCSEL . VCSEL are an example of resonant-cavity light emitting device . The light emitters comprise semiconductor layers with distributed Bragg reflectors (not shown) which enclose active region layers in between and thus forming a cavity . The VCSELs feature a beam emission of coherent electromagnetic radiation that is perpendicular to a main extension plane of a top surface of the VCSEL . Other examples may include edge mitting lasers or distributed feedback laser, for example .

The semiconductor laser 20 is operable for two side emission . In other words , the laser during operation, a first fraction of coherent electromagnetic radiation is emitted by the front output 21 and a second fraction is emitted by the rear output 22 . The AS IC 40 may comprise an integrated laser driver embedded as a means to drive the laser 20 . The laser driver provides a modulated driving current . Hereinafter, the modulation is assumed to be triangular, but other modulation schemes are possible as well . As a consequence , the semiconductor laser is modulated to emit modulated electromagnetic radiation out of the sensor module, via the front output 21 or the rear output 22.

The detector unit 30 comprises means, e.g. active or passive circuitry, to measure an optical or electronic property of the laser 20. In the presented examples the detector unit 30 comprises two photodetectors 31, 32, such as a photodiode, which can be arranged differently inside the module, or housing. Furthermore, the photodetectors can be manufactured differently, e.g. as components external or internal to the ASIC 40. The photodetectors are arranged with respect to the semiconductor laser 20, i.e. a front output 21 and a rear output 22 of the laser, and, thus, are operable to generate output signals indicative of an optical power output of the laser. The optical power output is a possible optical property of the laser and may change as a result of selfmixing interference. In some embodiments, one or more photodetectors can be integrated in the epitaxy of the laser.

In addition, or alternatively, the detector unit 30 comprises current or voltage meters to detect a junction voltage of the laser. Junction voltage is another possible electronic property of the laser and may change as a result of selfmixing interference.

The ASIC 40 constitutes a functional unit of the sensor module, which conducts a number of (pre ) -processing steps and basically operates the sensor module. For example, the ASIC comprises an integrated circuit with a microprocessor. At least, the application-specific integrated circuit is operable to determine a difference signal from the output signals generated by the detector unit 30. As will be discussed further below, the difference signal is indicative of the SMI of the laser 20.

Further functionality can be added. For example, the ASIC 40 comprises the laser driver discussed above to drive the laser for modulated emission. Furthermore, the ASIC comprises active and/or passive circuitry, such as ADCs, amplifiers, logic and a data interface to facilitate control of the sensor module, e.g. when embedded in a larger electronic device .

The semiconductor laser 20 is arranged inside the module, or housing, to undergo self-mixing interference, SMI. SMI can be induced into the laser cavity by reflections of the emitted radiation back from an external target to be placed outside the sensor module. As the laser features two side emission via its facets, emission via the front output 21 and rear output 22 is affected by SMI. The detector unit 30 by way of photodetectors 31, 32 detects a first fraction emitted via the front output 21 and a second fraction emitted via the rear output 22; and generates respective output signals. In the examples shown below, the convention is as follows. First photodetector 31 is arranged with respect to the front output 21 to detect the first fraction of light emitted via the front output. Second photodetector 33 is arranged with respect to the rear output 22 to detect the second fraction of light emitted via the rear output.

The output signals feature the same modulation. However, it can be shown that the first and second fraction are in opposite phase, and, consequently so are the respective output signals generated by the detector unit. The ASIC 40 determines the difference signal from the generated output signals. As front vs back output signals, i.e. SMI power readout emission, are out of phase, the direct subtraction between front and back signals (first and second fraction) lead to a demodulated SMI signal. In the examples discussed herein the triangular modulation is effectively removed.

Figure 1 shows an example embodiment of a self-mixing interferometry sensor module. In this example, the two photodetectors 31, 32 are integrated into the ASIC 40. The photodetectors can be implemented as Silicon photodiodes, which can be integrated by means of CMOS technology. The laser 20 is arranged on the ASIC, e.g. via electrical bumps or vias. The front output 21 faces away from the ASIC 40, whereas the rear output 22 of the laser faces the ASIC. The second photodetector is arranged below the laser, i.e. below the rear output. A transparent cover glass 12 is arranged in the housing and used to reflect the front light or first fraction of emitted light towards the first photodetector 31.

Figure 2 shows another example embodiment of a self-mixing interferometry sensor module. In this example, the first photodetector 31 is integrated into the ASIC 40. The first photodetector can be implemented as a Silicon photodiode, which can be integrated by means of CMOS technology. The second photodetector is integrated into the laser, e.g. as part the epitaxy of the laser. For example, the second photodetector can be implemented as an InGaAS photodiode integrated into a VCSEL.

The laser 20 is arranged on the ASIC 40, e.g. via electrical bumps or vias. The front output 21 faces away from the ASIC

40, whereas the rear output 22 of the laser faces the second photodetector 32 and is stacked on the ASIC with the photodetector 32 in-between. The second photodetector 32 is arranged below the laser, i.e. below the rear output. A transparent cover glass 12 is arranged in the housing and used to reflect the front light or first fraction of emitted light towards the first photodetector 31.

Figure 3 shows another example embodiment of a self-mixing interferometry sensor module. In this example, the first photodetector 31 is integrated into the laser 30, e.g. as part the epitaxy of the laser. For example, the second photodetector can be implemented as an InGaAS photodiode integrated into a VCSEL. The first photodetector 31 is arranged on the laser, i.e. on top of front output 21. The second photodetector 32 is integrated into the ASIC 40. For example, the second photodetector can be implemented as a Silicon photodiode integrated into the ASIC by means of CMOS technology. The second photodetector should typically absorb 50% of the emitted light only. In this example no cover is necessary to guide light to the photodetectors.

Figure 4 shows another example embodiment of a self-mixing interferometry sensor module. In this example, both photodetectors 31, 32, are integrated into the laser, e.g. as part the epitaxy of the laser. For example, the photodetectors can be implemented as InGaAS photodiodes integrated into a VCSEL. The photodetectors are integrated into the laser so that the first photodetector 31 is on the front output 21 to detect the first fraction of light emitted via the front output. Second photodetector 33 is arranged below the laser with respect to the rear output 22 to detect the second fraction of light emitted via the rear output. The front integrated photodetector should typically absorb 50% of the emitted light only. In the architecture of Figures 1 to 4 , the front PD is obviously sensitive to the background light (BGL ) environment that lead to higher shot noise . Nevertheless , SMI power readout is much less sensitive to the BGL ( compared to dTOF) because the photodetector sees a permanent DC light from the laser that is stronger than the BGL .

Figures 5 and 6 show further example embodiments of sel fmixing interferometry sensor modules . These examples make use of wafer level optic 11 to guide the first and second fraction of emitted light to the first and second photodetector 31 , 32 , respectively . The drawings indicate that the photodetectors are both arranged in the AS IC 40 below the laser 20 . The wafer level optic 11 acts as a beam splitter which guides the first fraction of light to the first photodetector 31 and the second fraction of light to the second photodetector 32 . Typically, the AS IC is partly transparent so that laser light may reach the photodetectors .

The wafer level optic 11 may further shield the sensor module from background light (BGL ) environment to achieve lower shot noise .

Figures 7A and 7B show an example measured using an embodiment of a sel f-mixing interferometry sensor module . The graphs show representative output signals detected with the first and second photodetector . Figure 7B is a magni fication of the graphs shown in Figure 7A.

Graph G1 shows an output signal 01 generated from the first fraction of light emitted by the front output 21 of the laser 20 . Graph G2 shows an output signal 02 generated from the second fraction of light emitted by the rear output 22 of the laser 20. Both signals show triangular modulation due to a modulated laser current and a stable target.

Graph G3 shows the difference signal DS, which results of subtraction of the two output signals 01, 02. As a result of the example embodiments shown above the two output signals 01, 02, i.e. the front and the rear signals, show the same amplitude modulation, but phase opposition of SMI signals. The difference signal DS can be generated by a differential amplifier, and leaves no, or negligible trace of the modulating signal, while SMI fringes are evident in the drawing .

The proposed concept allows to remove the modulation of laser emission, e.g. triangular modulation carrier into an FMCW SMI sensor, by means of the difference signal. For example, the SMI optical power signal is readout from the front and from the back side of the laser. Those two output signals are in perfect anti-phase. As a result, doing the difference between those two signal lead to the subtraction of the modulation, e.g. triangular modulation, and multiply the amplitude of the SMI signal twofold. Hence, the signal can be strongly amplified into the analogue domain. The amplification chain can be strongly increased, typically 100 times. The amount of ADC bits for post-processing can be reduced by 4 to 6 bits or can be used more efficiently. No triangular modulation post processing of the data have to be made and no special hardware circuitry design to remove the modulation may be necessary that could else lead to additional noise. The proposed concept can be applied to any SMI ranger system that modulate the laser driving current. While this speci fication contains many speci fics , these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features speci fic to particular embodiments of the invention . Certain features that are described in this speci fication in the context of separate embodiments can also be implemented in combination in a single embodiment . Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination . Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination .

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results . In certain circumstances , multitasking and parallel processing may be advantageous .

Furthermore , as used herein, the term "comprising" does not exclude other elements . In addition, as used herein, the article "a" is intended to include one or more than one component or element , and is not limited to be construed as meaning only one .

This patent application claims the priority of German patent application 102022131234 . 8 , the disclosure content of which is hereby incorporated by reference . References

10 sensor module

11 wafer level optic 12 transparent cover

20 semiconductor laser

21 front output

22 rear output

30 detection unit 31 first photodetector

32 second photodetector

40 application-speci fic integrated circuit , AS IC

DS di f ference signal

G1 graph G2 graph

01 output signal

02 output signal

Claims

1. A self-mixing interferometry sensor module comprising:

- a semiconductor laser (20) modulated to emit electromagnetic radiation out of the sensor module (10) , and operable to undergo self-mixing interference, SMI, caused by reflections of the emitted radiation from a target to be placed outside the sensor module (10) ; further comprising a detector unit (30) and an application-specific integrated circuit (40) ; wherein:

- a first fraction of the radiation is emitted by a front output (21) and a second fraction is emitted by a rear output (22) of the laser, the first and second fraction being in opposite phase;

- the detector unit (30) is operable to detect the first fraction and the second fraction and generate respective output signals (01, 02) ; and

- the application-specific integrated circuit (40) is operable to determine a difference signal (DS) from the generated output signals (01, 02) being indicative of the SMI of the laser (20) ,

- the detector unit (30) comprising at least two photodetectors (31, 32) to detect the electromagnetic radiation, wherein one photodetector (31) is integrated in the application-specific integrated circuit (40) , and the other photodetector (32) is integrated in a layer sequence of the semiconductor laser (20) .

2. The sensor module according to claim 1, wherein the detector unit is operable to:

- detect a junction voltage of the laser (20) , and - generate the output signals (01, 02) as a function of said junction voltages.

3. The sensor module according to claim 1 or 2, wherein the detector unit (30) is operable to:

- detect an optical power output by the front output (21) and the rear output (22) , respectively, and

- generate the output signals (01, 02) as a function of said optical power outputs.

4. The sensor module according to claim one of claims 1 to 3, further comprising a housing with a transparent cover (12) , wherein the laser (20) and photodetectors (31, 32) are arranged behind the cover such that:

- emission by the front output (21) is directed to the one photodetector (31) via the cover, and

- emission by the rear output (22) is directed to the other photodetector (32) .

5. The sensor module according to claim 1 to 3, further comprising a housing with a wafer-level optic (11) , wherein the laser (20) and photodetectors (31, 32) are arranged behind the wafer-level optic (11) such that:

- emission by the front output (21) is directed to the one photodetector (31) via the cover (12) , and

6. The sensor module according to claim 1, wherein the semiconductor laser (20) comprises:

- a semiconductor laser diode, - a resonant-cavity light emitting device,

- a distributed feedback laser,

- an edge emitting laser, and/or

- a vertical cavity surface emitting laser, VCSEL, diode.

7. The sensor module according to one of claims 1 to 6, wherein the laser (20) , the detector unit (30) and/or the application-specific integrated circuit (40) form an integrated semiconductor device.

8. The sensor module according to one of claims 4 to 5, wherein the housing comprises or is a sensor package.

9. An electronic device, comprising at least one self-mixing interferometry sensor module according to one of claims 1 to 8, and a host system, wherein:

- the sensor module (10) is integrated, or embedded, into the host system, and

- the host system comprises one of a mobile device, a smartphone, a wearable mobile device, a robot, a vehicle, or smart device such as a watch, a bracelet, or glasses.

10. The device according to claim 9, further comprising a processing unit which is coupled to the sensor module, wherein the processing unit is configured to receive the difference signal from the sensor module and derive a distance and/or speed of the target to be placed outside the sensor module.

11. A method of operating a self-mixing interferometry sensor module, comprising the steps of: - modulating a semiconductor laser (20) to emit electromagnetic radiation out of the sensor module (10) , and undergo self-mixing interference, SMI, caused by reflections of the emitted radiation from a target to be placed outside the sensor module (10) , wherein a first fraction of the radiation is emitted by a front output

(21) and a second fraction is emitted by a rear output

(22) of the laser (20) , the first and second fraction being in opposite phase,

- detecting the first fraction and the second fraction and generate respective output signals, and

- determining, by an application-specific integrated circuit (40) , a difference signal from the generated output signals being indicative of the SMI of the laser, wherein

- a detector unit (30) for detecting the first fraction and the second fraction comprises at least two photodetectors (31, 32) , wherein one photodetector (31) is integrated in the application-specific integrated circuit (40) , and the other photodetector (32) is integrated in a layer sequence of the semiconductor laser (20) .