Self-mixing interference laser sensor and method for manufacturing self-mixing interference laser sensor

Abstract

A self-mixing interference (SMI) laser sensor (1), comprising: a VCSEL (10) configured to emit laser radiation having a linear polarization; a photodetector (20) configured to monitor the laser radiation of the VCSEL (10); and a linear polarizer (30) arranged in front of the photodetector (20) such that the laser radiation passes through the linear polarizer (30) before reaching the photodetector (20). The orientation of the linear polarizer (30) by polarization differs from the linear polarization of the laser radiation of the VCSEL (10) by an angle (alpha) different from zero.

Description

[0001] The present disclosure relates to a self-mixing interferometer laser sensor, to an electronic device including such a laser sensor, and to a method for manufacturing a SMI laser sensor. Background Art

[0002] Optical self-mixing interferometry (SMI) sensors offer the possibility to measure absolute and relative distances of objects as well as vibrations and velocities, and thus enable adoption in a wide range of applications. This could mean that SMI sensors could be adopted in a wide range of applications. Among others, SMI laser sensors rely on the interference effect of laser light, which is reflected or scattered back from an object or scene in the sensor's field of view and re-enters the laser cavity, accompanied by resonant radiation within the cavity. As a result, the output properties of the laser, such as output power and junction voltage, are affected by the self-mixing interference. In particular, the resulting output power or frequency changes contain traceable information about the motion or distance of the target object relative to the sensor.

[0003] Prior art SMI laser sensors typically use semiconductor lasers as light sources, for example as their light sources. The effective cavity length in such devices usually depends on the current applied via the temperature of the laser device. In particular, the output frequency of the semiconductor laser essentially instantaneously follows any current changes caused by the optical resonant cavity length that changes at the same time. Thus, for example by applying a defined current shape (e.g. a periodic sawtooth or triangular current), the frequency difference generated between the resonant light and the backscattered light can be evaluated in a suitable evaluation unit and converted back into the desired position or velocity information.

[0004] Vertical Cavity Surface Emitting Lasers (VCSELs) are the most prominent choice in SMI sensors, and include a gain region sandwiched between two distributed Bragg reflectors (DBRs) that act as end mirrors and for feeding current into the gain region. Typically, one of the DBRs is highly reflective, with a reflectivity of over 99.9%, while the other DBR has a lower reflectivity for enabling outcoupling of light through the emitting surface of the VCSEL. In SMI sensors, the lower reflectivity DBR also serves as an input port for laser light received from an object or scene, so that light can be re-injected into the cavity via the emitting surface. The advantage of VCSELs is that their surface emitting properties render them suitable for mass production and testing at the wafer level, which opens up the possibility of a low-cost production process. Furthermore, the output power can be scaled to some extent via the area of ​​the emitting surface.

[0005] For actual SMI measurement, the frequency of the VCSEL is tuned via applying a drive current modulated as described above (e.g., a drive current with an AC component, such as a sawtooth modulation) on top of the DC component. The self-mixing interference is detected and measured via the output power of the VCSEL via a photodetector that picks up part of the emitted light. The signal contribution due to the modulation (which is typically significantly larger than the SMI contribution) must be subtracted from the generated optical signal data in order to obtain the useful SMI contribution of the optical signal.

[0006] Current methods of removing the modulation contribution are either via post-processing or via hardware demodulation. However, both approaches are very challenging, as hardware solutions typically introduce additional noise, while post-processing requires a large number of ADC bits and low-gain amplification. In summary, both approaches result in a significant degradation of the signal-to-noise ratio of the desired SMI signal.

[0007] Therefore, an object to be achieved is to provide a SMI laser sensor that effectively removes the modulation signal, which overcomes the limitations of prior art devices. A further object is to provide a method of manufacturing such a SMI laser sensor.

[0008] These objects are achieved by the subject-matter of the independent claims. Further developments and embodiments are described in the dependent claims. Summary of the invention

[0009] The present disclosure overcomes the above technical limitations by providing a SMI laser sensor that removes the contribution of the optical signal caused by the current modulation in the optical domain. Therein, use is made of the fact that, due to its architecture, the VCSEL device emits linearly polarized light when operated in the single-mode domain, which is first of all desirable for SMI measurements. In order to remove the modulation from the detected signal, a cross-polarizer is used, which has a passing polarization that is rotated relative to the linear polarization emitted by the VCSEL (for example, perpendicular to this polarization). In this way, the modulation can be effectively suppressed and prevented from reaching the photodetector, so that the detected signal consists only of the desired contribution due to self-mixing interference.

[0010] Specifically, the self-mixing interferometric laser sensor includes: a vertical cavity surface emitting laser VCSEL, which is configured to emit laser radiation with linear polarization through an emitting surface; a photodetector, which is configured to monitor the laser radiation of the VCSEL; and a linear polarizer, which is arranged in front of the photodetector so that the laser radiation passes through the linear polarizer before reaching the photodetector. The orientation of the through polarization of the linear polarizer differs from the linear polarization of the laser radiation of the VCSEL by an angle different from zero.

[0011] The VCSEL has a vertical laser cavity formed between the front side of the first DBR and the back side of the second DBR. In other words, the first DBR and the second DBR act as end mirrors, wherein the laser cavity is formed by a cavity region sandwiched between the two. In particular, the cavity region can be sandwiched between the first DBR and the second DBR. The cavity region includes an active light generating region formed by an active laser medium (also commonly referred to as a gain medium), which serves as an optical gain source required for the laser. The VCSEL emits electromagnetic radiation through a partially transmissive end mirror (e.g., through the second DBR), so that the top side of the second DBR facing away from the cavity region acts as an emitting surface. Among them, the first DBR and the second DBR are designed for a specific emission wavelength or emission wavelength range of the VCSEL.

[0012] The VCSEL is configured to experience self-mixing interference in its cavity. This means that light emitted by the VCSEL through the emitting surface and reflected or scattered by an object or scene away from the laser sensor can re-enter the laser cavity via re-injection through the emitting surface, thereby causing self-mixing interference. The self-mixing interference in turn causes a change (e.g., modulation) in the optical power in the laser cavity, and therefore causes a change in the laser output power. This effect of self-mixing interference can be monitored via the optional photodetector described above. Alternatively or in addition, SMI can be detected via electrical properties of the VCSEL, such as bias current or junction voltage.

[0013] A photodetector (e.g. comprising a photodiode sensitive at the emission wavelength of the VCSEL) is arranged so that it detects a portion of the light emitted by the VCSEL. If the VCSEL is characterized by double-sided emission, the photodetector can be arranged facing the other emission surface of the VCSEL. Alternatively, for example, a portion of the light emitted by the emission surface can be directed to the photodetector via a beam splitting element.

[0014] For example, a linear polarizer is arranged in front of the photosensitive surface of the photodetector and has a pass polarization that is rotated relative to the polarization of the light emitted by the VCSEL. In other words, the pass polarization is different from the linear polarization of the emitted light. In this way, the signal due to self-mixing interference passes through the polarizer, while the signal due to the current modulation can be partially or completely rejected. Therefore, the suppression of the modulation is achieved entirely in the optical domain without the need for further hardware or post-processing. For example, a linear polarizer is deployed directly on the photosensitive surface of the photodetector.

[0015] In some embodiments, the pass polarization of the linear polarizer is orthogonal to the linear polarization of the laser radiation of the VCSEL. As long as the VCSEL remains single-mode, the modulation of the drive current typically keeps the polarization of the emitted light unchanged. However, since the signal caused by SMI typically modifies the polarization of the emitted light, the linear polarizer can be set to have a pass polarization orthogonal to the polarization of the emitted light. This leads to the fact that the modulation is rejected, while the optical signal carrying the self-mixing interference characteristics can reach the photodetector and is thus detected.

[0016] In some embodiments, the linear polarizer is an absorptive polarizer. This type of polarizer is configured to absorb unwanted polarization states, such as those emitted by a VCSEL. Examples of such polarizers include crystals that exhibit dichroism (i.e., preferential absorption of light polarized in a particular direction). Modern absorptive polarizers are typically formed from nanoparticles (such as silver nanoparticles) embedded in a thin transparent substrate (e.g., glass). This structure is capable of achieving polarization ratios of up to 100,000:1 and absorption of the correctly polarized light as low as 1.5%. These types of polarizers are often used for applications in the near-infrared domain, such as fiber optic communications. For example, an absorptive polarizer in an SMI sensor can help suppress unwanted stray light from being reflected from the inside.

[0017] Alternatively, the linear polarizer is a beam splitting polarizer. This type of polarizer is configured to split an incident light beam into two beams of different linear polarizations. For example, a beam splitting polarizer is ideal and outputs fully polarized light with orthogonal polarizations. Alternatively, only one of the two output beams is fully polarized, while the other contains a mixture of polarization states. An ideal beam splitting polarizer can be employed in an SMI sensor to direct light directed to a photodetector but rejected by the polarizer to a scene or object at a determined distance in order to improve the overall SNR by enhancing self-mixing interference. For example, this may mean that a beam splitting polarizer can be employed to direct light directed to a photodetector to a scene or object at a determined distance. For example, examples of beam splitting polarizers include Fresnel polarizers, birefringent polarizers, thin film polarizers, and wire grid polarizers.

[0018] In some embodiments, the photodetector is configured to detect changes in the properties of the emitted laser radiation, in particular changes in the intensity of the emitted light, due to self-mixing interference. As previously described, light reflected from an object or scene and re-injected into the laser cavity can cause the accumulation of self-mixing interference. To this end, the laser frequency is swept by means of current modulation so that a standing wave of the intracavity light and the re-injected light can be formed. Meeting this condition significantly affects the emission output power of the VCSEL, so that the presence and extent of self-mixing interference can be easily determined by monitoring the output intensity of the VCSEL. Therefore, the photodetector is configured to monitor changes in the emitted optical power and generate an optical signal including information about these changes. For example, the optical signal can be provided to an evaluation unit included by or external to the SMI sensor for analysis and distance determination. Here, "external" can mean that the evaluation unit is not included by the SMI sensor. For example, the SMI sensor may not have an evaluation unit. This can mean that the evaluation unit is an external device.

[0019] In some embodiments, the VCSEL is characterized by double-sided emission through an emission surface and an additional emission surface opposite to the emission surface. In addition, a photodetector is arranged to capture laser radiation emitted through the additional emission surface. The VCSEL with double-sided emission makes it simple to monitor the output power via the second emission surface (e.g., through the back side of the VCSEL). For example, the VCSEL is arranged on a substrate body, and the substrate body includes a photodetector on or in the top surface, so that the back side of the VCSEL is arranged above the photodetector. In this way, since the SMI affects the emission on both sides of the VCSEL in the same way, the output power fluctuation caused by the SMI can be easily monitored.

[0020] In some embodiments, the SMI laser sensor further comprises a beam splitter, which is arranged on or away from the emission surface. The beam splitter is configured to transmit a portion of the emitted light to an object or scene and reflect the remainder of the emitted light toward a linear polarizer and a photodetector. In particular, in embodiments in which the VCSEL is characterized by single-sided emission, monitoring of the output power can be achieved by directing a small portion of the emitted light from a main beam path directed between the VCSEL and the scene or object to a monitoring beam path toward the photodetector by means of a non-polarizing beam splitter. For example, the beam splitter is configured to direct a small portion (e.g., less than 1% or 10%) of the emitted light intensity to a monitoring beam path toward the photodetector, while the remainder of the emitted light is transmitted and thus directed to the object or scene. For example, the beam splitter can be a beam splitter cube formed by two triangular prisms. The beam splitter does not alter the polarization of the emitted light.

[0021] In some embodiments, the SMI laser sensor further comprises a transparent cover disposed away from the emitting surface and configured to transmit a portion of the emitted light to the object or scene and reflect the remainder of the emitted light toward the linear polarizer and the photodetector. Instead of the beam splitter, the SMI laser sensor may comprise a cover glass that is nearly transmissive but reflects a small portion (e.g., less than 1% or 10%) of the emitted light onto the photodetector.

[0022] In some embodiments, the SMI laser sensor further comprises an evaluation unit, which is coupled to the photodetector and is configured to determine the absolute distance, relative distance and / or speed of an object far from the SMI laser sensor based on the photodetector signal. The optical signal generated by the photodetector carries information of the self-mixing interference formed in the laser cavity. The evaluation unit receives the optical signal and extracts the information, from which the absolute or relative distance, speed or vibration of the object far from the laser sensor can be determined. To this end, the evaluation unit (e.g. implemented as an integrated circuit) is electrically coupled to the photodetector.

[0023] In some embodiments, the SMI laser sensor further comprises an additional linear polarizer arranged in front of the photodetector, such that the laser radiation passes through the linear polarizer and the additional linear polarizer before reaching the photodetector. In order to suppress the modulation to an even higher degree, a stack of two or more linear polarizers may be arranged in front of the photodetector, such that the imperfections of the first polarizer are compensated. Ideally, the orientation of the through polarization of the additional linear polarizer is equal to the orientation of the through polarization of the linear polarizer. In other words, the through polarization of the linear polarizer and the additional linear polarizer are parallel to each other.

[0024] In some embodiments, the orientation of the through polarization of the linear polarizer is adjustable. For example, to account for polarization drift of laser light (e.g., due to aging or contamination of the emitting surface), the through polarization of the linear polarizer can be made adjustable so that the drift can be accounted for by keeping the through polarization of the polarizer orthogonal to the polarization of the emitted light.

[0025] In some embodiments, the SMI laser sensor further comprises an optical grating disposed on the emitting surface. The optical grating disposed on the emitting surface can ensure a stable and / or predetermined polarization of the emitted light to an even higher degree, and in particular in a larger current range in which potential higher order modes begin to emit laser light.

[0026] Furthermore, an electronic device comprising the SMI laser sensor according to one of the above embodiments is provided, wherein the SMI laser sensor is configured to measure an absolute distance, a relative distance and / or a speed of an object in the field of view of the SMI laser sensor that is far from the electronic device. The electronic device may be a mobile or portable device, including a smart phone, a tablet, a laptop or a wearable accessory, such as a smart wristband, a smart watch or a headphone device.

[0027] In addition, a method for manufacturing an SMI laser sensor is provided. The method includes providing a vertical cavity surface emitting laser VCSEL, which is configured to emit laser radiation having a linear polarization through an emitting surface. The method also includes arranging a photodetector for monitoring the laser radiation of the VCSEL, and arranging a linear polarizer in front of the photodetector so that the laser radiation passes through the linear polarizer before reaching the photodetector. Wherein, the orientation of the through polarization of the linear polarizer differs from the linear polarization of the laser radiation of the VCSEL by an angle different from zero.

[0028] For example, the pass polarization of the linear polarizer is oriented orthogonally to the linear polarization of the laser radiation of the VCSEL.

[0029] From the above described embodiments of the SMI laser sensor and the electronics, further embodiments of the method will become apparent to the person skilled in the art, and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The following description of the figures may further illustrate and explain various aspects of the SMI laser sensor and the method of manufacturing the SMI laser sensor. Functionally identical or effectively identical components and parts of the SMI laser sensor are designated by the same reference symbols. Identical or effectively identical components and parts may be described only with respect to the figure in which they first appear. Their description is not necessarily repeated in consecutive figures. DETAILED DESCRIPTION

[0031] In each figure:

[0032] Figures 1 to 3 A schematic diagram showing an exemplary embodiment of the proposed laser sensor;

[0033] Figure 4 The output polarization of the VCSEL used in the proposed laser sensor is shown;

[0034] Figures 5 to 9 illustrates the working principle of a linear polarizer according to the improved concept; and

[0035] Fig.10 is a schematic diagram of an electronic device including the proposed laser sensor.

[0036] Figure 1 A first exemplary embodiment of a self-mixing interference laser sensor 1 according to the improved concept is shown. The laser sensor 1 comprises a vertical cavity surface emitting laser 10. The VCSEL 10 comprises a VCSEL layer structure formed by a cavity region 11 embedded between two end mirrors (implemented as distributed Bragg reflectors 12), thereby forming a laser cavity therebetween. The cavity region 11 comprises an electrically pumped gain medium. The DBR 12 is characterized by a reflectivity close to unity, however, in this embodiment, light 2 is allowed to be emitted on both sides through an emission surface 13 and a further emission surface 14 opposite to the emission surface 13. For example, at the emission wavelength of the VCSEL 10 (e.g., at NIR wavelengths of 800nm ​​to 1000nm), the reflectivity of the DBR 12 is approximately 99%. The reflectivity of the top side DBR 12 facing away from the substrate 50 also allows reflected or scattered light 3 from an object or scene in the field of view of the laser sensor 1 to be re-injected into the cavity of the VCSEL 10 for generating self-mixing interference.

[0037] The operating current for current injection into the gain medium is provided by a current source (not shown) that is coupled to the VCSEL and embedded in the substrate body 50 as part of an integrated circuit, or it is external. The current source is configured to apply a drive current consisting of a DC contribution for enabling the VCSEL 10 to emit laser light and an AC modulation for modulating the output frequency of the VCSEL 10 so as to scan through a region that satisfies the constructive self-mixing interference condition. For example, a modulation signal (e.g., a sawtooth or triangular signal) can be supplied to the current source by means of a function generator, which can also be an integrated component or an external device.

[0038] The laser sensor 1 further comprises a photodetector 20, which in this embodiment is arranged on or within a substrate 50 and is configured to receive light of the VCSEL 10 emitted through the additional emission surface 14. The photodetector 20 can be implemented, for example, as a photodiode. In other words, the second output port on the back of the VCSEL acts as a monitoring port, while the output port on the front acts as a sensing port. The laser sensor 1 further comprises a linear polarizer 30, which is arranged in front of the photosensitive surface of the photodetector 20 on the optical path, so that all emitted light from the additional emission surface 14 of the VCSEL 10 passes through the linear polarizer 30 before being detected by the photodetector 20. The linear polarizer 30 can be an absorption polarizer, such as a Polaroid film or a polarizer implemented by means of silver nanoparticles embedded in a thin glass plate. Alternatively, the linear polarizer 30 can be a beam splitting polarizer, such as a polarizer based on Fresnel reflection or a thin film polarizer. The linear polarizer itself is a well-known concept and is not described in detail here.

[0039] At sufficiently low drive currents for ensuring single-mode operation, constituting an essential requirement for effective SMI detection, the emitted light of the VCSEL 10 in the laser fundamental mode is typically linearly polarized. This polarization remains unchanged even if the laser light is frequency modulated via a modulated component of the drive current. Therefore, the linear polarizer 30 may be characterized as primarily or exclusively transmitting light having a polarization angle orthogonal to the polarization angle of the emitted light 2. In other words, the linear polarizer has a passing polarization rotated by an angle α greater than 0° (e.g. 90°) relative to the polarization direction of the emitted light 2. In this way, the photodetector 20 primarily or exclusively receives light having an altered polarization direction relative to the emitted light 2, the emitted light 2 being specifically altered due to self-mixing interference. Therefore, the modulation of the laser frequency, which typically also manifests itself as a corresponding modulation of the output power, can be effectively removed in the optical domain. In some applications, a rotation angle of the linear polarizer of less than 80° may be preferred.

[0040] The SMI laser sensor 1 in this embodiment further comprises an evaluation unit 40, which is coupled to the photodetector 20 and is configured to receive an optical signal generated based on the detected light. The evaluation unit 40 is configured to determine a spectral signature of self-mixing interference in the optical signal, and from the spectral signature a measurement signal can be derived and outputted, which indicates the (relative or absolute) distance or velocity or vibration of the object from which the reflected light 3 is received back and re-injected into the cavity of the VCSEL 10.

[0041] The VCSEL 10 of the laser sensor 1 may further include an aperture layer, such as an oxide aperture layer, for defining an emission window of the VCSEL 10 and / or for limiting light above a cutoff angle from re-entering the cavity. In addition, the VCSEL 10 may include a metallization layer, such as for defining electrical contacts and terminals of the VCSEL 10. The VCSEL 10 may be arranged on a substrate body 50.

[0042] Figure 2A second exemplary embodiment of a laser sensor 1 is shown. Compared to the first embodiment, the main difference is that the VCSEL 10 in this embodiment is characterized by single-sided emission by facing the emission surface 13 away from the substrate 50. This is achieved by providing a bottom DBR 12 with a reflectivity close to unity (e.g. 99.9%). In order for the photodetector 20 to monitor the properties of the emitted light 2, a portion of the emitted light 2 is directed from the main beam path connecting the emission surface 2 and the object or scene to be measured to the monitoring beam path towards the photodetector 20 and the linear polarizer 30. The redirection of a portion of the light can be achieved by means of a non-polarizing beam splitter 15 with a fixed transmission and reflection ratio. For example, the beam splitter 15 is characterized by a transmission and reflection ratio of 90:10 or 99:1. However, other split ratios (such as 50:50) can also be easily achieved depending on the requirements of the application. The beam splitter 15 has the additional advantage that no reflected or scattered light 3 (potentially having a different polarization than the emitted light 2) can reach the photodetector 20.

[0043] In this case, the linear polarizer 30 is configured as a structured layer arranged on the photosensitive surface of the photodetector 20 , which structured layer realizes a Fresnel polarizer.

[0044] Figure 3 A third exemplary embodiment of a laser sensor 1 is shown. In contrast to the second embodiment, in this embodiment the laser sensor 1 further comprises a transparent cover 16, for example implemented as a cover glass. By "transparent" in this case is meant the emission wavelength of the VCSEL 10. For example, the transparent cover 16 is transparent in the NIR domain of the electromagnetic spectrum. The transparent cover 16 itself or via a coating on one of its surfaces can be configured to reflect a portion of the emitted light 2 towards the linear polarizer 30 and the photodetector 20. For example, the amount of reflected light is approximately 1% to 10% of the total emitted light 2. Also in this case, the reflected or scattered light 3 is prevented from reaching the linear polarizer 30 and the photodetector 20. An advantage of this embodiment is that the photodetector can be arranged next to the VCSEL 10 on a common substrate 50, which further comprises an integrated circuit for electrically connecting the VCSEL 10 to a current source and electrically connecting the photodetector 20 to an evaluation unit 40. The transparent cover 16 can further help protect the remaining components of the laser sensor 1.

[0045] To further ensure a clean output polarization of the VCSEL 10, i.e. suppress the emission of higher order modes, an optical grating 12a may be arranged on the emission surface 13 of the VCSEL 10. For example, the optical grating 12a is a polarization-dependent diffraction grating that matches the linear polarization of the fundamental optical mode of the VCSEL 10 by polarization. This may ensure single-mode operation of the VCSEL 10 also at a larger drive current.

[0046] Figure 4 LIV measurements of a VCSEL are shown, illustrating the polarization of the light emitted by the VCSEL. For this measurement, the VCSEL output is detected directly with the aid of a photodetector, where a single or a pair of polarizers are arranged on the incident side of the photodetector, where the polarization is either parallel (0°) or orthogonal (90°) to the polarization of the emitted light 2 (see legend). As can be seen, the intensity detected in the typical operating range of 0.5 mA to 2 mA for a conventional 940 nm emitting VCSEL with parallel configured polarizers indicates that the output of the VCSEL is indeed linearly polarized, where the slight difference with its total emission (top curve) is due to higher order modes, which, however, are strongly suppressed by several orders of magnitude at these operating currents, with reference to the signal obtained for the polarizers in an orthogonal configuration. Measurements with two polarizers confirm this result.

[0047] Figure 5 The optical signal obtained by the laser sensor 1 according to one of the above-described embodiments is shown. For this measurement, the laser sensor 1 is pointed at a test object, from which the emitted light 2 is reflected and re-injected into the laser cavity of the VCSEL 10, while the change of the VCSEL output power is monitored according to the improved concept via a photodetector 20, which has a linear polarizer with a variable polarization angle arranged in front. For example, this may mean that the emitted light 2 is reflected at or on the test object. The emitted light 2 can be re-injected into the laser cavity of the VCSEL 10. For example, this may also mean that the linear polarizer with a variable polarization angle is arranged in front of the photodetector or at the front end of the photodetector. In this example, the laser frequency is modulated at a frequency of 100 Hz via a triangular modulation of the drive current applied to the VCSEL 10. Each time the emission frequency meets the self-mixing interference condition of the re-injected light and the circulating light in the cavity, a fringe appears in the signal, which is however much smaller in terms of its amplitude than the modulation of the laser output power generated by the frequency modulation (referenced to the signal at 0 degrees, i.e., by polarization matching the emitted polarization). This may, for example, mean that gridlines appear in the signal. In particular, this may also mean that the amplitude of the gridlines may be much smaller than the modulation of the laser output power generated by the frequency modulation. When detuning by polarization to an angle of up to 90°, i.e. to an orthogonal polarization, the triangular spectral profile is strongly reduced, making the gridlines due to SMI more prominent in the obtained signal.

[0048] Figure 6 Essentially shown in a separate panel Figure 5, used to emphasize the increase in signal contrast of the grid lines due to self-mixing interference. As can be seen, if the linear polarizer 30 is cross-polarized, i.e. has a through polarization orthogonal to the polarization of the emitted light 2, then grid line counting for determining the absolute or relative distance to the test object can be easily performed. Therefore, no further hardware or algorithms are necessary to remove the modulation signal, as this is already effectively achieved completely in the optical domain by means of the linear cross polarizers.

[0049] exist Figure 7 middle, Figure 5 and Figure 6 The data are evaluated in terms of individual contributions to the total signal. When the pass polarization of the linear polarizer 30 is detuned from the output polarization of the VCSEL 10, both the DC component and the AC component are greatly reduced due to modulation. The signal due to SMI also decreases towards 90°, however the rate of reduction is much lower than that of the DC and AC components.

[0050] Therefore, if Figure 8 As shown, a significant enhancement of the contrast or ratio between the SMI signal and the triangular background can be achieved, in this case the ratio is increased by a factor of 20. This leads to the fact that with an optimal detuning of the through polarization by 90° a significantly enhanced signal-to-noise ratio can be achieved. Since in this configuration the DC component is also strongly reduced with the crossed polarizers, the shot noise limited SNR is not significantly affected even if the SMI amplitude of the signal is reduced. Fig. 9 Shows Figure 5 and Figure 6 Measure the SNR of SMI detection.

[0051] Fig.10 An embodiment of an electronic device 100 including an SMI laser sensor 1 according to the improved concept is shown. For example, as depicted, the electronic device 100 is a smart phone. Alternatively, the electronic device can be any portable or mobile electronic device, including a tablet or laptop, augmented reality or virtual reality glasses, a smart watch or other wearable device, or a dedicated distance sensor. Possible applications of the SMI laser sensor 1 in such an electronic device 100 include proximity sensors, distance measurement applications, eye tracking applications, speaker feedback applications, etc.

[0052] Embodiments of the SMI laser sensor 1, electronic device 100, and method of manufacturing the SMI laser sensor 1 disclosed herein have been discussed for the purpose of familiarizing the reader with the novel aspects of the concepts. While preferred embodiments have been shown and described, those skilled in the art may make changes, modifications, equivalents, and substitutions to the disclosed concepts without unnecessarily departing from the scope of the claims.

[0053] It will be appreciated that the present disclosure is not limited to the disclosed embodiments and what is particularly shown and described above. Instead, features detailed in separate dependent claims or the specification may be advantageously combined. Furthermore, the scope of the present disclosure includes those changes and modifications that will be apparent to those skilled in the art and that fall within the scope of the appended claims.

[0054] The term "comprising", as it is used in the claims or in the description, does not exclude corresponding features or other elements or steps of the procedure. Where the terms "a" or "an" are used in conjunction with features, they do not exclude a plurality of such features. Furthermore, any reference signs in the claims shall not be construed as limiting the scope.

[0055] This patent application claims the priority of German patent application 10 2022 121 114.2, the disclosure content of which is incorporated herein by reference.

[0056] refer to

[0057] 1 laser sensor; 2 emitted radiation; 3 reflected or scattered radiation; 10 vertical cavity surface emitting laser; 11 cavity region; 12 distributed Bragg reflector; 12a optical grating; 13, 14 emitting surface; 15 beam splitter; 16 transparent cover; 20 photodetector; 30 linear polarizer; 40 evaluation unit; 50 substrate.

Claims

1. A self-mixing interferometer (SMI) laser sensor (1), comprising: - a vertical cavity surface emitting laser (10) VCSEL configured to emit laser radiation having a linear polarization through an emitting surface (13); - a photodetector (20) configured to monitor the laser radiation of the VCSEL (10); as well as - a linear polarizer (30) arranged in front of the photodetector (20) such that the laser radiation passes through the linear polarizer (30) before reaching the photodetector (20); - wherein the polarization orientation of the linear polarizer (30) differs from the linear polarization of the laser radiation of the VCSEL (10) by an angle (α) different from zero.

2. The SMI laser sensor (1) according to claim 1, wherein the pass polarization of the linear polarizer (30) is orthogonal to the linear polarization of the laser radiation of the VCSEL (10).

3. The SMI laser sensor (1) according to claim 1 or claim 2, wherein the linear polarizer (30) is an absorbing polarizer.

4. The SMI laser sensor (1) according to claim 1 or claim 2, wherein the linear polarizer (30) is a beam splitting polarizer.

5. The SMI laser sensor (1) according to any one of claims 1 to 4, wherein the photodetector (20) is configured to detect changes in properties of the emitted laser radiation, in particular changes in the emitted light intensity, due to self-mixing interference.

6. The SMI laser sensor (1) according to any one of claims 1 to 5, wherein - the VCSEL (10) is characterized by double-sided emission through the emission surface (13) and a further emission surface (14) opposite to the emission surface (13); and - the photodetector (20) is arranged to capture laser radiation emitted by the further emitting surface (14).

7. The SMI laser sensor (1) according to any one of claims 1 to 5, further comprising a beam splitter (15), which is arranged on the emitting surface (13) or away from the emitting surface (13) and is configured to: - transmitting a portion of the emitted light (2) to an object or scene; and - reflecting the remaining part of the emitted light (2) towards the linear polarizer (30) and the photodetector (20).

8. The SMI laser sensor (1) according to any one of claims 1 to 5, further comprising a transparent cover (16), which is arranged away from the emitting surface (13) and is configured as follows: - transmitting a portion of the emitted light (2) to an object or scene; and - reflecting the remaining part of the emitted light towards the linear polarizer (30) and the photodetector (20).

9. The SMI laser sensor (1) according to any one of claims 1 to 8, further comprising an evaluation unit (40) coupled to the photodetector (20) and configured to determine an absolute distance, a relative distance and / or a speed of an object away from the SMI laser sensor (1) based on the photodetector signal.

10. The SMI laser sensor (1) according to any one of claims 1 to claim 9, further comprising an additional linear polarizer (31), which is arranged in front of the photodetector (20) so that the laser radiation passes through the linear polarizer (30) and the additional linear polarizer (31) before reaching the photodetector (20).

11. The SMI laser sensor (1) according to claim 10, wherein: The orientation of the passing polarization of the further linear polarizer (31) is equal to the orientation of the passing polarization of the linear polarizer (30).

12. The SMI laser sensor (1) according to any one of claims 1 to 11, wherein the orientation of the linear polarizer (30) by polarization is adjustable.

13. The SMI laser sensor (1) according to any one of claims 1 to 12, further comprising an optical grating (12a) arranged on the emitting surface (13).

14. An electronic device (100) comprising a SMI laser sensor (1) according to any one of claims 1 to 13, wherein the SMI laser sensor (1) is configured to measure an absolute distance, a relative distance and / or a speed of an object far away from the electronic device (100) in the field of view of the SMI laser sensor (1).

15. A method for manufacturing a SMI laser sensor (1), the method comprising: - providing a vertical cavity surface emitting laser (10) VCSEL, which is configured to emit laser radiation having a linear polarization through an emitting surface (13); - a photodetector (20) arranged to monitor the laser radiation of said VCSEL (10); as well as - arranging a linear polarizer (30) in front of the photodetector (20) so that the laser radiation passes through the linear polarizer (20) before reaching the photodetector (30); - wherein the polarization orientation of the linear polarizer (30) differs from the linear polarization of the laser radiation of the VCSEL (10) by an angle (α) different from zero.

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