Laser sensor, electronic device with laser sensor and method for operating a laser sensor

Abstract

A self-mixing interferometric laser sensor (100) comprises a vertical cavity surface emitting laser, VCSEL, (10) configured to emit laser radiation (9) with a linear polarization (90) through an emission surface (14), a first photodetector (21) configured to monitor a first part (91) of the laser radiation (9) and to output a first signal (1), a first linear polarizer (23) arranged in front of the first photodetector (21) such that the first part (91) of the laser radiation (9) passes through the linear polarizer (23) before reaching the first photodetector (21), a second photodetector (22) configured to monitor a second part (92) of the laser radiation (9) and to output a second signal (2), a second linear polarizer (24) arranged in front of the second photodetector (22) such that the second part (92) of the laser radiation (9) passes through the second linear polarizer (24) before reaching the second photodetector (22), wherein the first linear polarizer (23) is oriented at a first angle (71) with respect to the linear polarization (90) of the VCSEL (10), wherein the second linear polarizer (24) is oriented at a second angle (72) with respect to the linear polarization (90) of the VCSEL (10), wherein the first angle (71) is substantially +45° and the second angle (72) is substantially -45°.

Description

[0001] Description

[0002] LASER SENSOR, ELECTRONIC DEVICE WITH LASER SENSOR AND METHOD FOR OPERATING A LASER SENSOR

[0003] This disclosure relates to a laser sensor . In particular, the laser sensor can be a sel f-mixing interference laser sensor . This disclosure further relates to an electronic device comprising such a laser sensor and to a method for operating the laser sensor .

[0004] This patent application claims the priority of the German patent application No . 10 2023 116 721 . 9 , the disclosure content of which is hereby included by reference .

[0005] BACKGROUND OF THE INVENTION

[0006] Optical sel f-mixing interference ( SMI ) laser sensors provide the possibility of measuring absolute and relative distances as well as vibrations and velocities of obj ects and thus enable an employment in a wide range of applications . SMI laser sensors rely on the interferometric ef fect of laser light , which is reflected or scattered back from an obj ect or a scene in the f ield-of-view of the sensor and re-enters the laser cavity, with the resonating radiation inside the cavity . As a result , the output properties of the laser, such as the output power and a j unction voltage , are influenced by the sel f-mixing interference . In particular, the resulting output power or frequency variations can contain traceable information on the movement or distance of the target obj ect with respect to the sensor . In order to measure absolute distance and speed by using SMI , it is necessary to modulate the frequency and, thus , the wavelength of the laser . State-of-the-art SMI laser sensors typically employ a semiconductor laser as light source . The ef fective cavity length in such device commonly depends on an applied current via the temperature of the laser device . In particular, the output frequency of semiconductor lasers basically instantaneously follows any current variations due to the simultaneously changed optical resonator length . Hence , by applying a defined modulation profile of the driving current , for example a periodic saw-tooth or triangular modulated current , the resulting di f ference in frequency between the resonating and the back-scattered light can be evaluated in a suitable evaluation unit and translated back to a desired position or velocity information, for instance . In particular, the sel f-mixing interference is detected and measured in the output power of the VCSEL by means of a photodetector that picks up a portion of the emitted light . From the generated photodetector signal data, the signal contribution due to the driving-current modulation, often signi ficantly larger than the SMI contribution, has to be subtracted in order to gain access to the useful SMI contribution of the photodetector signal . Consequently, it is necessary to do a demodulation, i . e . to subtract the modulation profile of the driving current . This subtraction is usually done by hardware or by a post-process . Typically, the subtraction is quite challenging as it can add noise on the hardware level or can require a huge amount of ADC ( analog-to-digital converter ) bits and a low gain ampli fication i f done by post-processing the data . Both ways , however, lead to a decrease of the signal-to-noise ratio ( SNR) . It is an obj ect of certain embodiments to provide a laser sensor and, in particular, an SMI laser sensor . Further obj ects of certain embodiments are to provide an electronic device with a laser sensor and a method for operating a laser sensor .

[0007] These obj ects are achieved with the subj ect-matter of the independent claims . Further developments and embodiments are described in dependent claims .

[0008] SUMMARY OF THE INVENTION

[0009] According to at least one embodiment , a laser sensor comprises a laser configured to emit laser radiation . In particular, the laser sensor is configured as a sel f-mixing interference ( SMI ) laser sensor . According to at least one further embodiment , in a method for operating a laser sensor a laser is operated to emit laser radiation . According to at least one further embodiment , an electronic device comprises a laser sensor . The features and embodiments disclosed in the following equally apply to the laser sensor, the electronic device and the method for operating the laser sensor .

[0010] Here and in the following, the terms "radiation" and " light" , as in " laser radiation" and " laser light" , are used synonymously for denoting the electromagnetic radiation that is emitted by the laser of the laser sensor . In particular, the laser can emit radiation with spectral components in the infrared, visible and / or ultraviolet wavelength range . Thus , the laser radiation can be or comprise infrared light , visible light or ultraviolet light . According to a further embodiment , the laser is a vertical cavity surface emitting laser (VCSEL ) . Accordingly, the laser sensor can comprise a VCSEL as light source that is configured to produce laser radiation and to emit the laser radiation through an emission surface . Particularly preferably, the laser of the laser sensor is configured to emit laser radiation with a linear polari zation . In other words , the laser is configured to emit , at least partly, linearly polari zed light . In particular, the laser radiation can be , at least partly, linearly polari zed along the die axis of the VCSEL . The laser radiation can have a linear polari zation characteri zed by a laser polari zation direction . Although in the following the laser of the laser sensor is described as a VCSEL, other laser designs emitting linearly- polari zed light are possible .

[0011] Preferably, the VCSEL is operated to emit frequency-modulated laser radiation . Preferably, the VCSEL is operated to emit a frequency-modulated continuous-wave ( EMCW) radiation . The frequency of the VCSEL can be modulated by applying a modulated driving current , i . e . a driving current having an AC ( alternating current ) component , for instance a saw tooth modulation or a triangular modulation, superimposing a DC ( direct current ) component . The modulation of the driving current typically leaves the polari zation of the emitted light unchanged as long as the VCSEL remains single mode .

[0012] According to a further embodiment , the laser sensor comprises two photodetectors . In particular, the laser sensor comprises a first photodetector configured to monitor a first part of the laser radiation of the VCSEL and to output a first signal , and a second photodetector configured to monitor a second part of the laser radiation of the VCSEL and to output a second signal.

[0013] According to a further embodiment, the laser sensor comprises two linear polarizers, wherein each of the linear polarizers is associated with one of the photodetectors. In particular, the laser sensor comprises a first linear polarizer is arranged in front of the first photodetector such that the first part of the laser radiation passes through the first linear polarizer before reaching the first photodetector, and a second linear polarizer is arranged in front of the second photodetector such that the second part of the laser radiation passes through the second linear polarizer before reaching the second photodetector. For example, the first linear polarizer is directly disposed on a photosensitive surface of the first photodetector and the second linear polarizer is directly disposed on or integrated in a photosensitive surface of the second photodetector.

[0014] Each of the first and second linear polarizer is oriented in such a way that an orientation of a passing polarization differs from the linear polarization of the laser radiation of the VCSEL, i.e. from the laser polarization direction, by an angle different from zero. In other words, the first linear polarizer is oriented at a first angle different from zero with respect to the linear polarization of the VCSEL, whereas the second linear polarizer is oriented at a second angle different from zero with respect to the linear polarization of the VCSEL. In particular, the second angle is different from the first angle. Preferably, the first angle is substantially +45° and the second angle is substantially -45°. Consequently, the first linear polarizer and the second linear polarizer are preferably arranged orthogonally to each other. That an angle "substantially is" a certain value can, in particular, mean that the angle deviates from that value by not more than + / -10° or, preferably, not more than + / -5° or, particularly preferably, not more than + / -1°. Consequently, each of the first and second linear polarizer is arranged in front of a photosensitive surface one of the first photodetector and the second photodetector, respectively, and has a passing polarization that is rotated with respect to the polarization direction of the radiation emitted by the VCSEL. In other words, the passing polarization of each of the first and second photodetector is rotated with respect to the linear polarization of the emitted radiation.

[0015] According to a further embodiment, the first and / or the second linear polarizer is chosen from a grating, a metasurface and / or an absorptive polarizer. Examples of absorptive polarizers include crystals that exhibit dichroism. Furthermore, absorptive polarizers can be formed from nanoparticles, such as silver nanoparticles, embedded in a thin transparent substrate as, for instance, glass. Further example for polarizers include Fresnel polarizers, birefringent polarizers, thin film polarizers, and wire-grid polarizers, for instance.

[0016] According to a further embodiment, the VCSEL has a vertical laser cavity and comprises a gain region that is sandwiched between two reflectors as, for instance, distributed Bragg reflectors (DBR) . The reflectors are configured to act as end mirrors and to feed current into the gain region. Typically, a first one of the reflectors can be highly reflective with a reflectivity in excess of 99.9%, while the second reflector can have a lower reflectivity for enabling out-coupling of light through an emission surface of the VCSEL . In an SMI laser sensor, this lower reflectivity reflector also serves as input port for the laser light that is received from the obj ect or scene such that the light can be reinj ected into the cavity via the emission surface . The cavity region comprises an active light generation region that is formed by an active laser medium, often also called a gain medium, as the source of optical gain required in a laser . The VCSEL can preferably mounted with the first reflector on a carrier . The terms "bottom" and "top" can preferably refer to that mounting orientation, so that the first reflector forms a bottom side of the VCSEL and the second reflector forms a top side of the VCSEL . A top side of the second reflector facing away from the cavity region acts as emission surface . The first and second reflector are designed for a speci fic emission wavelength or emission wavelength range of the

[0017] VCSEL .

[0018] Furthermore , it can be advantageous that VCSEL devices , due to their architecture , can emit , at least partly, linearly polari zed light while being operated in a single-mode domain . In order to increase the degree of polari zation of the emitted laser radiation, a third linear polari zer can be arranged on the emission surface of the VCSEL . The third polari zer can be , for instance , an optical grating arranged on or integrated in the emission surface . By using the third polari zer, the polari zation stability of the VCSEL can be increased over a larger current range .

[0019] As described above , the VCSEL is configured to undergo sel fmixing interference in its cavity . This means that light that is emitted by the VCSEL through the emission surface and reflected or scattered by an obj ect or a scene arranged distant to the laser sensor can re-enter the laser cavity via re-inj ection through the emission surface causing the sel fmixing interference . Sel f-mixing interference in turn causes an alteration, for example modulation, of the optical power in the laser cavity and thus of the laser output power . This ef fect of the sel f-mixing interference can be monitored via the above-mentioned first and second photodetector . Each of the first and second photodetector, for example comprising a photodiode that is photosensitive at the emission wavelength of the VCSEL, is arranged such that it detects a portion of the light emitted by the VCSEL, i . e . the first part and the second part of the laser radiation mentioned above . I f the VCSEL is characteri zed by two-sided emission as described further below, the first and second photodetector can be arranged facing a further emission surface of the VCSEL . Alternatively, a portion of the light emitted through the emission surface can be directed to the first photodetector and to the second photodetector for instance by a semitransparent element , i . e . a partly reflective and partly transparent element .

[0020] According to a further embodiment , the first photodetector and the second photodetector are each configured to detect changes in properties of the respective monitored part of the laser radiation, in particular in the light intensity, due to sel f-mixing interference . As mentioned before , light that is reflected from an obj ect or a scene and is reinj ected into the laser cavity can cause the buildup of sel f-mixing interference . To this end, the laser frequency is swept by means of current modulation such that standing waves of the light within the cavity and the reinj ected light can be formed . Meeting this condition signi ficantly influences the emitted output power of the VCSEL such that the presence and degree of sel f-mixing interference can be determined via monitoring the light intensity of the VCSEL . Thus , each of the first and second photodetector is configured to monitor alterations in the detected optical power and generate a signal comprising information about these alterations . The signals of the first and second photodetector can be provided to a calculation unit and / or an evaluation unit , comprised by the laser sensor or external , for instance for analysis and distance determination .

[0021] According to a further embodiment , the VCSEL is characteri zed by two-sided emission through the emission surface and a further emission surface opposite the emission surface . In this case , it can be advantageous i f the first photodetector and the second photodetector are arranged to capture laser radiation emitted through the further emission surface . A VCSEL with two-sided emission makes monitoring the output power via a second emission surface , for example through a bottom side of the VCSEL, particularly easy i f the first and second photodetector are arranged facing the further emission surface . Thus , preferably, the VCSEL is arranged over or on the first and second photodetector so that the further emission surface faces the first and second photodetector . For example , the VCSEL is arranged on a carrier, which includes the first and second photodetector on or within a top surface such that the bottom side of the VCSEL, i . e . the further emission side , is arranged above the first and second photodetector . In this case , output power fluctuations due to SMI can be easily monitored as SMI influences the emission of the VCSEL on both sides in the same manner .

[0022] According to a further embodiment , the laser sensor further comprises a semitransparent cover element arranged over the emission surface and configured for partly transmitting and partly reflecting the laser radiation . For instance , the cover element can have a reflectivity of equal to or less than 50% or equal to or less than 10% or equal to or less than 1 % and of equal to or greater than 0 . 01 % . For instance , the semitransparent cover element can be a window of a housing that is part of the laser sensor and that houses the VCSEL and the photodetectors . Alternatively, the cover element can be cover of an environment package , for instance a display glass of the electronic device . In particular, the first photodetector and the second photodetector are arranged to capture laser radiation reflected by the cover element . Thus , the first and second photodetector are arranged in a beam path of the laser radiation emitted from the emission surface and reflected by the cover element . In other words , the laser sensor can comprise the semitransparent cover element arranged distant from the emission surface and configured to transmit a portion of the emitted light to an obj ect or a scene , and reflect a remaining portion of the emitted light toward the first and second photodetectors . For instance , the first photodetector and the second photodetector are arranged laterally to the VCSEL on or in a common carrier .

[0023] According to a further embodiment , the laser sensor comprises a carrier, wherein the VCSEL is mounted on the carrier . Furthermore , as described above , preferably the first photodetector and the second photodetector are mounted on the carrier or are integrated into the carrier . In the latter case , preferably also the first linear polari zer and the second linear polari zer are integrated into the carrier . Alternatively, the first and second linear polari zer can be mounted on the carrier . The carrier together with the VCSEL, the photodetectors and the linear polarizers can form a package and, thus, a highly integrated and compact device, preferably with a volume of equal to or less than 1 mm3.

[0024] According to a further embodiment, the carrier comprises or is an application-specific integrated circuit (ASIC) that is configured to operate the VCSEL and / or the first and second photodetectors. The first and second photodetector can be integrated into the ASIC or can be discrete photodiode components, for instance based on Si, InGaAs, GaAs or any other material suitable for detecting the laser radiation.

[0025] According to a further embodiment, when operating the laser sensor a difference signal representing a difference of the first signal and the second signal is provided. Preferably, the laser sensor comprises a signal processing unit coupled to the first and second photodetector and configured to output a difference signal representing a difference of the first signal and the second signal. Preferably, the signal processing unit is a part of the carrier. Thus, the signal processing unit can be integrated in the carrier and can be, for instance, a part of the ASIC. Due to the driving current modulation and the SMI effects both the first and second signal can comprise a slow modulation component, caused by the driving-current modulation, superposed by a fast modulation component, caused by the SMI effects. Since the first and second linear polarizer are preferably oriented orthogonally to each other and at an angle of substantially +45° and -45°, respectively, to the laser polarization, slow modulation component of the first and second signal are in phase, whereas the fast modulation component of the first signal and the fast modulation component of the second signal are in anti-phase. By subtracting one of the first and signal form the other of the first and second signal the slow modulation component as well as environmental noise, caused, or instance, by power fluctuations, vibrations or other disturbances, is substantially removed so that, as a result, the difference signal mainly comprises the fast modulation component with an amplitude corresponding to the sum of the fast modulation component amplitudes of the first and second signal. Thus, compared to the usual SMI signal by using one photodetector and no linear polarizer over the one photodetector, the SNR can be significantly increased.

[0026] According to a further embodiment, the laser sensor further comprises an evaluation unit, preferably coupled to signal processing unit or integrated in the signal processing unit, to determine an absolute distance, a relative distance, and / or a velocity of an object distant to the laser sensor from the difference signal. Preferably, the evaluation unit is a part of the carrier and, thus, integrated in the carrier. The evaluation unit can be, for instance, a part of the ASIC. The difference signal generated by the signal processing unit carries information of the self— mixing interference formed in the laser cavity. The evaluation unit receives the difference signal and extracts said information, from which an absolute or relative distance, a velocity or a vibration of an object distant to the laser sensor can be determined. To this end, the evaluation unit, for example realized as an integrated circuit, is electrically coupled to the photodetector.

[0027] According to a further embodiment, in the electronic device comprising the laser sensor the laser sensor is configured to measure an absolute distance, a relative distance and / or a velocity of an object on or distant from the electronic device in the f ield-of-view of the laser sensor . For example , the laser sensor can be configured to be operated in one or more of the following applications : 4D coherent ranger, finger slider, optical microphone , eye tracking, proximity sensor, rotary encoder, vital sign sensor, loud speaker feedback, force sensor .

[0028] The electronic device can be a mobile or portable device including a smartphone , a tablet computer, a laptop computer or a wearable accessory such as a smart wristband, a smartwatch or an earphone device . Furthermore , the electronic device can be a part of an automotive application .

[0029] In particular, long range FMCW SMI sensors suf fer from a very weak SMI signal strength due the large distance range from centimeters to meters and a large FMCW triangle modulation which can saturates the ADC and prevent a good ampli fication of the useful SMI signal . Using a preferably about 45 ° with respect to the linearly polari zed solitary VCSEL rotated polari zer in front of a photodetector can drastically enhanced the readout of the SMI signal which can be particularly interesting for FMCW SMI sensor applications or when the SNR has to be huge and robust . Moreover, using two polari zers rotated at +45 ° and -45 ° with respect to the solitary VCSEL in front of two photodetectors can lead to two SMI signals in anti-phase which allow to remove the drivingcurrent modulation part as well as environmental noise in an FMCW SMI sensor signal . Thus , subtracting the signals of the first and second photodetector with their respective linear polari zer results in an ef fective common noise suppression . Alternatively to SMI FMCW applications , the laser sensor described herein can be used in any other SMI systems that uses a power readout via photodiodes . Consequently, the laser sensor described herein can have the following advantages over usual SMI sensors : higher stability of the SMI signal ; higher signal amplitude and thus higher SNR; longer distance range with higher resolution for distance and speed sensor applications ; lower noise level thanks to the di f ferential readout .

[0030] Further advantages , advantageous embodiments and further developments are revealed by the embodiments described below in connection with the figures .

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the figures :

[0033] Figures 1A and IB show schematic illustrations of an exemplary embodiment of a laser sensor and a method for operating the laser sensor ;

[0034] Figures 2A to 20 show graphs of an exemplary measurement with a laser sensor ;

[0035] Figures 3 to 5 show schematic illustrations of further exemplary embodiments of a laser sensor ;

[0036] Figure 6 shows a schematic illustration of an exemplary embodiment of an electronic device comprising a laser sensor .

[0037] DETAILED DESCRIPTION

[0038] The following description of figures may further illustrate and explain aspects of the laser sensor, the electronic device and the method for operating the laser sensor . In the embodiments and figures , identical , similar or identically acting elements are provided in each case with the same reference numerals . The elements illustrated and their si ze ratios to one another should not be regarded as being to scale , but rather individual elements , such as for example layers , components , devices and regions , may have been made exaggeratedly large to illustrate them better and / or to aid comprehension . 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 .

[0039] Figures 1A and IB show an embodiment of a laser sensor 100 configured as sel f-mixing interference laser sensor . Figure 1A additionally indicates polari zation directions , whereas in Figure IB components of the carrier 30 are shown . The following description equally applies to Figures 1A and IB .

[0040] The laser sensor 100 comprises a laser 10 that is embodied as a vertical-cavity surface-emitting laser (VCSEL ) emitting laser radiation 9 during operation . Thus , the following description simply refers to VCSEL 10 . However, laser designs other than a VCSEL are possible .

[0041] The VCSEL 10 is mounted on a carrier 30 . Preferably, the carrier comprises or is an electronic integrated-circuit device like an AS IC that is configured to operate the VCSEL 10 . Thus , the VCSEL 10 is mounted, for instance by soldering, to contact terminals of the carrier 30 .

[0042] The VCSEL 10 comprises a VCSEL layer structure that is formed by a cavity region 11 that is embedded between a first reflector 12 and a second reflector 13 forming end mirrors that can be , for instance , configured as distributed Bragg reflectors , thus forming a laser cavity in between . The cavity region 11 comprises an electrically pumped gain medium . The reflectors 12 , 13 are characteri zed by a reflectivity close to unity, however, in this embodiment allowing a two-sided emission of laser radiation 9 through an emission surface 14 and through a further emission surface 15 opposite the emission surface 14 . In particular, a forward emission part 90 is emitted from the top side of the laser sensor 100 .

[0043] For example , the reflectivity of the reflectors 12 , 13 are in the order of 99% at an emission wavelength of the VCSEL 10 , for example a NIR wavelength from 800 nm to 1000 nm . Alternatively, the laser sensor 100 and, thus , the VCSEL 10 can be configured to emit laser radiation with another wavelength . The reflectivity of the top side second reflector 13 facing away from the carrier 30 further allows reflected radiation 99 , reflected or scattered from an obj ect or a scene in the f ield-of-view of the laser sensor 100 , to be reinj ected into the cavity of the VCSEL 10 for generating sel f-mixing interference .

[0044] The operating current for current inj ection into the gain medium in the cavity region 11 is provided by a current source (not shown) that is coupled to the VCSEL 10 and that can be embedded in the carrier 30 as part of an integrated circuit . Alternatively, the current source can be an external device . Preferably, the VCSEL 10 is operated to emit frequency-modulated laser radiation 9 . Particularly preferably, the VCSEL 10 is operated to emit frequency- modulated continuous-wave ( FMCW) laser radiation 9 . For that , the current source is configured to apply a driving current composed of a DC contribution for enabling the lasing of the VCSEL 10 and an AC modulation for modulating the output frequency of the VCSEL 10 in order to scan through regimes , in which the conditions for constructive sel f-mixing interference are ful filled . For example , the modulation signal , for example a saw tooth or triangular signal , can be supplied to the current source by means of a function generator, which likewise can be a component integrated in the carrier 30 , or can be an external device .

[0045] The laser sensor 100 further comprises a first photodetector

[0046] 21 and a second photodetector 22 that are arranged on or within the carrier 30 . In the embodiment shown, the first and second photodetector 21 , 22 are integrated in the carrier 30 . Alternatively, the first and second photodetector 21 , 22 can be discrete devices that are mounted in cavities of the carrier 30 . Consequently, the first and second photodetector 21 , 22 can be integrated into the carrier 30 or can be discrete photodiode components and can be based, for instance , on Si , InGaAs , GaAs or any other material suitable for detecting the laser radiation 9 . In particular, the carrier 30 can be configured to operate the first and second photodetector 21 , 22 .

[0047] The first photodetector 21 and the second photodetector 22 are arranged to capture laser radiation 9 emitted through the further emission surface 15 . A VCSEL 10 with two-sided emission makes monitoring the output power via the second emission surface , i . e . through a bottom side of the VCSEL 10 , particularly easy i f the first and second photodetector 21 ,

[0048] 22 are arranged, as shown in Figures 1A and IB, facing the further emission surface 15 . Thus , preferably, the VCSEL 10 is arranged on or over the first and second photodetector 21 , 22 so that the further emission surface 15 faces the first and second photodetector 21 , 22 . Consequently, the VCSEL 10 is arranged on the carrier 30 , which includes the first and second photodetector 21 , 22 on or within a top surface such that the bottom side of the VCSEL 10 , i . e . the further emission side 15 , is arranged above the first and second photodetector 21 , 22 . This way, output power fluctuations due to SMI can be easily monitored as SMI influences the emission of the VCSEL on both sides in the same manner .

[0049] The first photodetector 21 and the second photodetector 22 are configured to receive light of the VCSEL 10 that is emitted through the further emission surface 15 . In particular, the first photodetector is configured to monitor a first part 91 of the laser radiation 9 of the VCSEL 10 and to output a first signal 1 , and the second photodetector 22 is configured to monitor a second part 92 of the laser radiation 9 of the VCSEL 10 and to output a second signal 2 . The first and second photodetector 21 , 22 can be reali zed as photodiodes , for instance . In other words , further emission surface 15 forming a second output port on the bottom side of the VCSEL 10 acts as a monitoring port , while emission surface 14 forming the first output port on the top side acts as a sensing port .

[0050] The laser sensor 100 further comprises a first linear polari zer 23 that is arranged in front of a photosensitive surface of the first photodetector 21 and a second linear polari zer 24 that is arranged in front of a photosensitive surface of the second photodetector 22 . In particular, the first linear polari zer 23 and the second linear polari zer 24 are configured and arranged such that radiation 9 emitted from the further emission surface 15 of the VCSEL 10 passes through the respective linear polari zer 23 , 24 before being detected by the first and second photodetector 21 , 22 . Consequently, the first part 91 of the laser radiation 9 passes through the first linear polarizer 23 before reaching the first photodetector 21, and the second part 92 of the laser radiation 9 passes through the second linear polarizer 24 before reaching the second photodetector 22.

[0051] Preferably, also the first linear polarizer 23 and the second linear polarizer 24 are integrated into the carrier 30. Alternatively, the first and second linear polarizer 23, 24 can be mounted on the carrier 30. Each of the first and second linear polarizer 23, 24 can be or comprise a grating or a metasurface or another polarizer configuration, for instance as mentioned above. The carrier 30 together with the VCSEL 10, the photodetectors 21, 22 and the linear polarizers 23, 24 can form a package and, thus, a highly integrated and compact device, preferably with a volume of equal to or less than 1 mm3.

[0052] Each of the first and second linear polarizer 23, 24 is oriented in such a way that an orientation of a passing polarization differs from the polarization direction of the linear polarization of the laser radiation 9 of the VCSEL 10 by an angle 71, 72 different from zero. In Figure 1A the polarization direction of the emitted laser radiation 9 is indicated by a polarization indicator 80, whereas the passing polarizations of the first and second linear polarizer 23, 24 are indicated by the polarization indicators 81, 82. The dashed lines in the polarization indicators 81, 82 indicate the polarization direction of the emitted laser radiation 9. The first linear polarizer 23, i.e. the passing polarization of the first linear polarizer 23, is oriented at a first angle 71 with respect to the linear polarization of the VCSEL 10, whereas the second linear polarizer 24, i.e. the passing polarization of the second linear polarizer 24, is oriented at a second angle 72 with respect to the linear polarization of the VCSEL 10.

[0053] In particular, the second angle 72 is different from the first angle 71. Preferably, the first angle 71 is substantially +45° and the second angle 72 is substantially -45°. Consequently, the first linear polarizer 23 and the second linear polarizer 24, i.e. the passing polarizations of the first and second linear polarizer 23, 24, are preferably arranged orthogonally to each other. Consequently, each of the first and second linear polarizer 23 ,24 is arranged in front of a photosensitive surface of one of the first photodetector 23 and the second photodetector 24, respectively, and has a passing polarization that is rotated with respect to a polarization of the radiation 9 emitted by the VCSEL 10.

[0054] The first photodetector 21 and the second photodetector 22 are each configured to detect changes in properties of the emitted laser radiation 9, in particular in the emitted light intensity, due to self-mixing interference. As aforementioned, light that is reflected from an object or a scene and is reinjected into the laser cavity can cause the buildup of self-mixing interference. To this end, the laser frequency is swept by means of current modulation such that standing waves of the light within the cavity and the reinjected light can be formed. Meeting this condition significantly influences the emitted output power of the VCSEL 10 such that the presence and degree of self-mixing interference can be determined via monitoring the output intensity of the VCSEL 10. Thus, each of the first and second photodetector 21 ,22 is configured to monitor alterations in the detected optical power and generate a signal 1 , 2 comprising information of these alterations . The signals 1 , 2 can be provided to a calculation unit and / or an evaluation unit as described below, for analysis and distance determination, for instance .

[0055] As shown in Figure IB, the laser sensor 100 can comprise a signal processing unit 40 coupled to each of the first and second photodetector 21 , 22 and configured to output a di f ference signal 3 representing a di f ference of the first signal 1 and the second signal 2 . As shown, preferably the signal processing unit 40 is a part of the carrier 30 . Thus , the signal processing unit 40 can be integrated in the carrier 30 and can be , for instance , a part of the AS IC forming the carrier 30 . Thus , when operating the laser sensor, the VCSEL 10 is operated to emit frequency-modulated laser radiation 9 . The first part 91 and the second part 92 of the laser radiation 9 are monitored by the first and second photodetectors 21 , 22 and a di f ference signal 3 representing a di f ference of the first signal 1 and the second signal 2 is provided .

[0056] Due to the driving-current modulation and the SMI ef fects both the first and second signal 1 , 2 comprise a slow modulation component , caused by the current modulation, superposed by a fast modulation component , caused by the SMI ef fects , as shown in Figure 2A. Figure 2A shows the signal strength S versus the time T of the first and second signal 1 , 2 of an exemplary measurement with a laser sensor 1 as explained before . For comparison, the signal 4 of a photodetector without a polari zer is also shown . This signal 4 is dominated by the slow modulation, whereas the SMI caused fast modulation component is barely discernible . As can be easily seen, analyzing the first and second part 91, 92 of the laser radiation 9 with the +45° polarized first photodetector 21 and the -45° polarized second photodetector 22, with respect to the polarization direction 90 of the VCSEL 10, increases the SMI related signal amplitude significantly. In the measurement example the increase in amplitude is on the order of 100. Furthermore, the modulation-related amplitude is decreased by a factor 2.

[0057] Since the first and second linear polarizer 23, 24 are preferably oriented orthogonally to each other and at an angle 71 ,72 of substantially +45° and -45°, respectively, with respect to the laser polarization, the fast modulation component of the first signal 1 and the fast modulation component of the second signal 2 both are in anti-phase. By subtracting one of the first signal 1 and second signal 2 from the other of the first signal 1 and the second signal 2, the slow modulation component as well as environmental noise, caused, for instance, by power fluctuations, vibrations or other disturbances, is removed so that, as a result, the difference signal 3 mainly comprises the fast modulation component with an amplitude corresponding to the sum of the fast modulation component amplitudes of the first and second signal 1, 2, as shown in Figure 2B. In comparison, the demodulated signal 5 of the comparison signal 4 of Figure 4 is also shown. It can be immediately understood that the SNR can be significantly increased by using the first and second photodetectors 21, 22 with the first and second linear polarizers 23, 24. This becomes also apparent form Figure 2C that shows, depending on the DC component C of the driving current the power spectral density (PSD) of the first signal 1, represented by graph 6, the PSD of the second signal 2, represented by graph 7, as well as the PSD of the comparison signal 4 of Figure 2A, represented by graph 8. Thus, using polarized readouts allows to enhance the signal amplitude, which can be 100 times more, and to decrease the noise level thanks to the differential readout, so that the overall SNR of the system can be significantly increased.

[0058] As indicated in Figure IB, the laser sensor 100 can further comprises an evaluation unit 50, preferably coupled to the signal processing unit 40 or integrated in the signal processing unit 40, to determine from the difference signal 3 for example an absolute distance, a relative distance and / or a velocity of an object distant to the laser sensor 100. As shown, the evaluation unit 50 is preferably a part of the carrier 30 and, thus, integrated in the carrier 30. Thus, the evaluation unit 50 can be, for instance, a part of the ASIC. The difference signal 3 generated by the signal processing unit 40 carries information of the self— mixing interference formed in the laser cavity. The evaluation unit 50 receives the difference signal 3 and extracts said information, from which an absolute or relative distance, a velocity or a vibration of an object distant to the laser sensor 100 can be determined .

[0059] In the following, modifications of the laser sensor 100 are shown. Thus, the following description mainly concentrates on the differences to the previous embodiment.

[0060] In order to increase the degree of polarization of the emitted laser radiation 9, a third linear polarizer 16 can be arranged on the emission surface 14 of the VCSEL 10 as shown in Figure 3. The third polarizer 16 can be an optical grating, for instance, that is arranged on or integrated in the emission surface 14. By using the third polarizer 16, the polari zation stability of the VCSEL 10 can be increased over a larger current range , since an optical grating arranged on or in the emission surface 14 can ensure a stable and / or predetermined polari zation of the emitted light 9 to an even higher degree and particular! ' on a larger current range , at which potentially higher orde ■ modes start to lase .

[0061] The VCSEL 10 of the embodiments described so far is characteri zed by two-sided emission through the emission surface 14 and the further emission surface 15 opposite the emission surface 14 . Figure 4 shows a further embodiment of the laser sensor 100 , wherein the laser sensor 100 further comprises a semitransparent cover element 60 arranged over the emission surface 14 and configured for partly transmitting and partly reflecting the laser radiation 9 . For instance , the cover element 60 can have a reflectivity of equal to or less than 50% or equal to or less than 10% or equal to or less than 1 % and of equal to or greater than 0 . 01 % . For instance , the semitransparent cover element 60 can be a window of a housing that is part of the laser sensor 100 and that houses the VCSEL 10 and the photodetectors 21 , 22 . Alternatively, the cover element 60 can be cover of an environment package , for instance a display glass of an electronic device the laser sensor 100 is built in . In particular, the first photodetector 21 and the second photodetector 22 are arranged to capture a first part 91 and a second part 92 , respectively, of the laser radiation 9 reflected by the cover element 60 . Thus , the first and second photodetector 21 , 22 are arranged in a beam path of the laser radiation 9 emitted from the emission surface 14 and reflected by the cover element 60 . In other words , the laser sensor 100 can comprise the semitransparent cover 60 element arranged distant from the emission surface 14 and configured to transmit a part 90 of the emitted light 9 to an obj ect or a scene , and reflect remaining portion comprising the first and second part 91 , 92 of the emitted light 9 toward the first and second photodetectors 21 , 22 . For instance , the first photodetector 21 and the second photodetector 22 are arranged laterally to the VCSEL 10 and can be , as in the previous embodiments , be integrated in the carrier 30 .

[0062] Alternatively, as shown in Figure 5 , the first photodetector 21 and the second photodetector 22 can also be mounted on the carrier .

[0063] Figure 6 shows an embodiment of an electronic device 1000 comprising a laser sensor 100 according to the previous embodiemnts . For example , the electronic device 1000 is a smart phone , as depicted . Alternatively, the electronic device 1000 can be any portable or mobile electronic device including tablet or laptop computers , augmented or virtual reality glasses , smartwatches or other wearable devices or dedicated distance sensors . Possible applications of the laser sensor 100 in such electronic devices 1000 include proximity sensors , ranging applications , eye tracking applications , loud speaker feedback applications , etc .

[0064] The embodiments of the laser sensor 100 , the electronic device 1000 and the method for operating laser sensor 100 disclosed herein have been discussed for the purpose of familiari zing the reader with novel aspects of the idea . Although preferred embodiments have been shown and described, changes , modi fications , equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims . It will be appreciated that the disclosure is not limited to the disclosed embodiments and to what has been particularly shown and described hereinabove . Rather, features recited in separate dependent claims or in the description may advantageously be combined . Furthermore , the scope of the disclosure includes those variations and modi fications , which will be apparent to those skilled in the art and fall within the scope of the appended claims . The term " comprising" , insofar it was used in the claims or in the description, does not exclude other elements or steps of a corresponding feature or procedure . In case that the terms " a" or " an" were used in conj unction with features , they do not exclude a plurality of such features . Moreover, any reference numerals in the claims should not be construed as limiting the scope .

[0065] Reference numerals

[0066] 1, 2, 3 signal

[0067] 4, 5 signal

[0068] 6, 7, 8 power spectral density

[0069] 9 laser radiation

[0070] 10 laser

[0071] 11 cavity region

[0072] 12, 13 reflector

[0073] 14 emission surface

[0074] 15 further emission surface

[0075] 16 linear polarizer

[0076] 21, 22 photodetector

[0077] 23, 24 linear polarizer

[0078] 30 carrier

[0079] 40 signal processing unit

[0080] 50 evaluation unit

[0081] 90 forward emission part

[0082] 91 first part

[0083] 92 second part

[0084] 99 reflected radiation

[0085] 100 laser sensor

[0086] 1000 electronic device

Claims

Patent claims1. A self-mixing interferometric laser sensor (100) , comprising a vertical cavity surface emitting laser, VCSEL, (10) configured to emit laser radiation (9) with a linear polarization (90) through an emission surface (14) , a first photodetector (21) configured to monitor a first part (91) of the laser radiation (9) and to output a first signal ( 1 ) , a first linear polarizer (23) arranged in front of the first photodetector (21) such that the first part (91) of the laser radiation (9) passes through the linear polarizer (23) before reaching the first photodetector (21) , a second photodetector (22) configured to monitor a second part (92) of the laser radiation (9) and to output a second signal (2) , a second linear polarizer (24) arranged in front of the second photodetector (22) such that the second part (92) of the laser radiation (9) passes through the second linear polarizer (24) before reaching the second photodetector (22) , wherein the first linear polarizer (23) is oriented at a first angle (71) with respect to the linear polarization (90) of the VCSEL (10) , wherein the second linear polarizer (24) is oriented at a second angle (72) with respect to the linear polarization (90) of the VCSEL (10) , wherein the first angle (71) is substantially +45° and the second angle (72) is substantially -45°.

2. The self-mixing interferometric laser sensor (100) according to claim 1, wherein the first linear polarizer (23) and / or the second linear polarizer (24) comprises a grating, a metasurface and / or an absorptive polarizer.

3. The self-mixing interferometric laser sensor (100) according to claim 1 or 2, wherein the first photodetector (21) and the second photodetector (22) each are configured to detect changes in properties of the monitored laser radiation (91, 92) , in particular in the monitored light intensity, due to self-mixing interference .

4. The self-mixing interferometric laser sensor (100) according to one of the preceding claims, wherein the VCSEL (10) is characterized by two-sided emission through the emission surface (14) and a further emission surface (15) opposite the emission surface (14) , and the first photodetector (21) and the second photodetector (22) are arranged to capture laser radiation (91, 92) emitted through the further emission surface ( 15) .

5. The self-mixing interferometric laser sensor (100) according to the preceding claim, wherein the VCSEL (10) is arranged above the first and second photodetector (21, 22) so that the further emission surface (15) faces the first and second photodetector (21, 22) .

6. The self-mixing interferometric laser sensor (100) according to one of the claims 1 to 3, further comprising a semitransparent cover element (60) arranged over the emission surface (14) and configuredfor partly transmitting and partly reflecting the laser radiation ( 9 ) , wherein the first photodetector (21) and the second photodetector (22) are arranged to capture laser radiation (91, 92) reflected by the cover element (60) .

7. The self-mixing interferometric laser sensor (100) according to the preceding claim, wherein the first photodetector (21) and the second photodetector (22) are arranged laterally to the VCSEL (10) .

8. The self-mixing interferometric laser sensor (100) according to one of the preceding claims, further comprising a carrier (30) , wherein the VCSEL (10) is mounted on the carrier (30) .

9. The self-mixing interferometric laser sensor (100) according to claim 8, wherein the first photodetector (21) and the second photodetector (22) are mounted on the carrier (30) .

10. The self-mixing interferometric laser sensor (100) according to claim 8, wherein the first photodetector (21) and the second photodetector (22) are integrated into the carrier (30) .

11. The self-mixing interferometric laser sensor (100) according to claim 10, wherein the first linear polarizer (23) and the second linear polarizer (24) are integrated into the carrier (30) .

12. The self-mixing interferometric laser sensor (100) according to one of the claims 8 to 11, wherein thecarrier (30) comprises or is an application-specific integrated circuit, ASIC, that is configured to operate the VCSEL (10) and the first and second photodetector (21, 22) .

13. The self-mixing interferometric laser sensor (100) according to one of the preceding claims, further comprising a signal processing unit (40) coupled to the first and second photodetector (21, 22) and configured to output a difference signal (3) representing a difference of the first signal (1) and the second signal (2) .

14. The self-mixing interferometric laser sensor (100) according to claim 13 and one of the claims 8 to 12, wherein the signal processing unit (40) is a part of the carrier ( 30 ) .

15. The self-mixing interferometric laser sensor (100) according to claims 13 or 14, further comprising an evaluation unit (50) configured to determine an absolute distance, a relative distance, and / or a velocity of an object distant to the laser sensor (10) from the difference signal (3) .

16. The self-mixing interferometric laser sensor (100) according to claim 15, wherein the evaluation unit (50) is a part of the carrier (30) .

17. The self-mixing interferometric laser sensor (100) according to one of the preceding claims, further comprising a third linear polarizer (16) arranged on the emission surface (14) of the VCSEL (10) .

18. The self-mixing interferometric laser sensor (100) according to one of the preceding claims, wherein the laser sensor (100) has a volume of equal to or less than 1 mm3.

19. Electronic device (1000) , comprising a laser sensor (100) according to one of the claims 1 to 18.

20. A method for operating a laser sensor (100) according to one of the claims 1 to 18, wherein- the VCSEL (10) is operated to emit frequency-modulated laser radiation (9) ,- a difference signal (3) representing a difference of the first signal (1) and the second signal (2) is provided.

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