Laser sensor and method of manufacturing a laser sensor

Abstract

A self-mixing interferometry SMI laser sensor (1) comprises a vertical cavity surface emitting laser (10) (VCSEL) configured to emit laser radiation, the VCSEL comprising a first distributed Bragg reflector (11) (DBR), a second DBR (12), and a cavity region (13) comprising an active light generating region (13a), where the cavity region (13) is arranged in a layer structure between a front side of the first DBR (11) and a back side of the second DBR (12). Wherein at least one of the first and second DBRs (11, 12) comprises a first contrast region (14) and a second contrast region (15), the first contrast region (14) having a first refractive index contrast [Delta] n1 with respect to an emission wavelength of the VCSEL (10), and the second contrast region (15) having a second refractive index contrast [Delta] n2 / n greater than the first refractive index contrast [Delta] n1 / n.

Description

[0001] The present disclosure relates to self-mixing interferometry laser sensors, electronic devices including such laser sensors, and methods of manufacturing SMI laser sensors.

[0002] Optical self-mixing interferometry (SMI) sensors offer the possibility to measure absolute and relative distances as well as vibrations and velocities of objects, and are therefore being adopted in a wide range of applications. Among others, SMI laser sensors rely on the interferometric 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, where it resonates and radiates. As a result, the output properties of the laser, such as output power and junction voltage, are affected by self-mixing interference. In particular, the resulting output power or frequency changes contain traceable information about the movement or distance of the target object relative to the sensor.

[0003] The latest SMI laser sensors usually employ semiconductor lasers as light sources. The effective cavity length in such devices usually depends on the current applied via the temperature of the laser device. In particular, since the optical resonator length is changed at the same time, the output frequency of the semiconductor laser follows any current changes essentially instantaneously. Thus, by applying a defined current shape, such as a periodic sawtooth or triangular current, the resulting frequency difference between the resonant light and the backscattered light can be evaluated in a suitable evaluation unit and converted back into, for example, the desired position or velocity information.

[0004] The most prominent choice in SMI sensors is the vertical cavity surface emitting laser (VCSEL), which includes a gain region sandwiched between two distributed Bragg reflectors (DBRs) that act as end mirrors and are used to feed current into the gain region. Typically, one of the DBRs has a high reflectivity of more than 99.9% reflectivity, while the other has a lower reflectivity for coupling out the light through the emitting surface of the VCSEL. In SMI sensors, this lower reflectivity DBR also serves as the input port for the laser light received from the object or scene, so that the light can be re-injected into the cavity via the emitting surface. VCSELs have the advantage that their surface emitting properties make them suitable for mass production and testing at the wafer level, which opens the possibility of a low-cost production process. In addition, the output power can be scaled to a certain extent via the area of ​​the emitting surface.

[0005] However, currently used VCSELs are limited in the maximum current allowed before the laser stops its single-mode operation. This limits the maximum allowable modulation amplitude applied to the laser current, which in turn sets an upper limit on the maximum distance resolution that can be achieved by the SMI sensor. Specifically, at some specific currents, high-order transverse modes begin to generate lasing inside the VCSEL, causing the SMI signal to become unstable and difficult to read due to the superposition of multiple wavelengths.

[0006] The approach to suppressing transverse modes in VCSELs is to lengthen the cavity length so that the resonator is more stable at larger applied currents and modulation amplitudes. However, simply extending the cavity length comes with the disadvantage of increasing bandwidth, which supports additional longitudinal modes, so that multimode emission behavior is expected to occur again.

[0007] Therefore, an object to be achieved is to provide a SMI laser sensor with increased distance resolution and distance range, 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.

[0009] The present disclosure overcomes the above-mentioned technical limitations by providing an SMI laser sensor including a VCSEL in which at least one DBR includes a low contrast region that effectively acts as a cavity extension and a filter for maintaining a limited bandwidth. In this way, a VCSEL is provided that can be driven with significantly higher currents and current modulations while maintaining single-mode operation that is critical to self-mixing interferometry. By virtue of the ability to apply larger current modulations, a significant improvement in distance measurement resolution is achieved. In addition, the effectively extended cavity also results in an increase in the coherence length of the VCSEL, thereby also increasing the range of the SMI laser sensor over which distance can be determined.

[0010] In particular, the self-mixing interferometry laser sensor comprises a vertical cavity surface emitting laser (VCSEL) configured to emit laser radiation, wherein the VCSEL comprises a first distributed Bragg reflector (DBR), a second DBR and a cavity region comprising an active light generating region, wherein the cavity region is arranged in a layer structure between a front side of the first DBR and a rear side of the second DBR. At least one of the first DBR and the second DBR comprises a first contrast region and a second contrast region, the first contrast region consisting of a plurality of alternating first material layers, the first material having a first refractive index contrast Δn1 / n with respect to an emission wavelength of the VCSEL, and the second contrast region consisting of a plurality of alternating second material layers, the second material having a second refractive index contrast Δn2 / n greater than the first refractive index contrast Δn1 / n. The refractive index contrast is defined by the difference in the refractive index of the alternating material layers divided by their mean. In addition, the SMI laser sensor further comprises an evaluation unit configured to monitor the laser radiation of the VCSEL.

[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 and second DBRs act as end mirrors, wherein the laser cavity is formed by the cavity region sandwiched in between. The cavity region includes an active light generating region formed by an active laser medium (also commonly referred to as a gain medium), which acts as a source of optical gain required in 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. Therein, the first and second DBRs are designed for a specific emission wavelength or emission wavelength range of the VCSEL.

[0012] In some embodiments, the VCSEL may have double-sided emission, so that the first DBR is also partially transmissive. The laser may be arranged on a substrate (e.g., a CMOS silicon die) so that the bottom side of the VCSEL (i.e., the second emission surface in the case of double-sided emission) is parallel to and faces the substrate. The substrate may include laser contacts and a photosensitive element for electrically connecting the laser to a laser driver, the photosensitive element being arranged on the substrate surface or integrated into the substrate, for example, as an embedded photodetector in the case of double-sided emission. In such an embodiment, the photosensitive surface of the photosensitive element faces the back side of the first DBR (i.e., the bottom surface of the VCSEL), and can therefore receive electromagnetic radiation emitted by the laser through the bottom emission surface. The photosensitive element may be arranged away from the bottom emission surface, or it may be in contact with the latter.

[0013] 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 arranged 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 thereby causes a change in the laser output power. This effect of self-mixing interference can be monitored via the optional photodetector described above or via the electrical properties of the VCSEL (e.g., bias current or junction voltage).

[0014] At least one of the first and second DBRs has a first contrast region and a second contrast region. The two regions are characterized by a low contrast of refractive index in the first contrast region and a high contrast of refractive index in the second contrast region. The terms "high" and "low" here and throughout the present disclosure indicate that the low contrast and high contrast are different and the difference between the two is not just a tolerance. Here and throughout the present disclosure, the term "contrast" refers to the difference in refractive index of the alternating material layers forming the corresponding parts of the DBR. Therefore, the materials forming the alternating layers in the first contrast region have refractive indices that are closer to each other than the refractive indices of the materials forming the alternating layers in the second contrast region. Therefore, the first contrast region can be considered a low contrast region, while the second contrast region can be considered a high contrast region. The low contrast region and the high contrast region of the DBR effectively form a DBR arrangement that includes a low contrast DBR arranged adjacent to a high contrast DBR. Therefore, the first and / or second DBR including a low contrast region and a high contrast region can be considered a system consisting of two DBRs.

[0015] The evaluation unit comprises means for determining the degree of self-mixing interference in the laser cavity. For example, the evaluation unit measures the output power of the VCSEL optically via the second emission port by means of a photodetector or via electrical properties of the laser by monitoring a bias current or a junction voltage of the VCSEL.

[0016] In at least one embodiment, the first contrast region is arranged between the second contrast region and the cavity region. The low contrast region of the DBR is characterized by a lower reflectivity compared to the high contrast region. Wherein, a low contrast region sandwiched between the at least one side of the laser cavity and the high contrast region of the same DBR is arranged on at least one side of the laser cavity, effectively extending the length of the laser cavity, thereby improving the single-mode performance of the VCSEL. In this way, higher output powers can be achieved while still maintaining single-mode operation compared to VCSELs with shorter cavity lengths. In addition, increasing the effective cavity length leads to a reduction in the optical linewidth of the VCSEL and, therefore, an increase in the coherence length. This means that the SMI laser sensor can be operated at a greater distance from the scene or object. Compared to conventional cavity extensions implemented by means of transparent spacers or buffers, the low contrast region arranged between the high contrast region and the cavity region is characterized by a low bandwidth, and therefore effectively suppresses the establishment of additional longitudinal optical modes. Therefore, the low contrast DBR enhances single-mode operation by both extending the cavity length and reducing the bandwidth of the laser, thereby achieving significantly greater output power.

[0017] In some embodiments, the cavity region further comprises a cavity extension region arranged between the active light generating region and the first or second DBR. In addition to extending the cavity via the above-mentioned low contrast region of at least one of the first and second DBRs, further enhancement of the VCSEL single-mode operation can be achieved with the aid of a buffer zone or a transparent spacer. For example, the cavity region comprises a gain region arranged to be embedded in or adjacent to the transparent buffer zone.

[0018] In some embodiments, the first refractive index contrast Δn1 / n is less than 10%, in particular less than 6%. Similarly, the second refractive index contrast Δn2 / n can be greater than 10%, in particular greater than 16%. Wherein, the refractive index contrast is defined by the difference in the refractive index of the alternating material layers divided by their mean. The low contrast region is manufactured via alternating material layers, where the refractive index of the two materials at the emission wavelength differs by less than 0.3, or even less than 0.2, for an average refractive index of about 3.45, which means that the contrast is significantly lower than that of conventional DBRs used as end mirrors in VCSELs, which typically have a contrast of 17-20%, i.e., a difference in refractive index of 0.5 or more. Similarly, the high contrast region can be made of a material with a contrast of 10% or more. The exact first and second refractive index contrasts and their differences can depend on the desired specifications of the SMI laser sensor and therefore on the specific application. Specifically, a large contrast means high reflectivity, while a low contrast can help reduce the optical bandwidth, as described above.

[0019] In some embodiments, each of the first and second DBRs includes a corresponding first contrast region and a second contrast region. Designing both the first DBR and the second DBR as a DBR system including a low contrast region and a high contrast region also increases the effective cavity length while potentially reducing the bandwidth of the VCSEL, which in turn results in a larger bias current being applied without losing the single-mode operation of the VCSEL. Wherein, the low contrast region of each of the first DBR and the second DBR can be arranged between (e.g., sandwiched between) the cavity region and the high contrast region of the corresponding one of the first DBR and the second DBR. For example, the low contrast regions of the first DBR and the second DBR can have the same properties, i.e., formed by the same material, number of layers, and layer thickness. The properties of the high contrast regions of the first DBR and the second DBR can be different, for example, for achieving different reflectivity values. For double-sided emission, the high contrast regions of the first DBR and the second DBR can alternatively have the same properties.

[0020] In some embodiments, the first contrast region is formed by alternating layers of GaAs and AlGaAs, GaAs and InGaAs, or AlGaAs and InGaAs, in particular GaAs and AlGaAs with x≤0.5. X Ga 1-X Alternating layers of As are formed. The refractive index of GaAs at 940 nm is about 3.6. X Ga 1-X The aluminum content in the As alloy, the refractive index of aluminum gallium arsenide at 940nm is 3.0 to 3.5. Preferably, an AlGaAs alloy with x≤0.5 is selected. Similarly, depending on the In X Ga 1-X The indium content in the As alloy, InGaAs, has a refractive index between 3.5 and 3.7 at an optical wavelength of 940 nm. Thus, by selecting an appropriate alloy composition, a low contrast in the refractive index of the alternating material layers in the low contrast portion can be achieved.

[0021] In some embodiments, the second contrast region is formed by alternating layers of AlAs and one of the following: GaAs, AlGaAs, and InGaAs. The refractive index of aluminum arsenide at 940nm is about 3.0. Therefore, the high contrast portion of the DBR can be efficiently implemented using AlAs with any other compound and appropriate alloy composition. Alternatively, the high contrast region can be formed by AlGaAs as a material layer with a low refractive index. Other material selections are also possible, especially for emission wavelengths other than 940nm.

[0022] In some embodiments, the emission wavelength of the VCSEL is in the infrared domain, particularly in the NIR domain including 940 nm. In general, TOF distance measurement and mapping preferably operates at optical wavelengths that are not visible to the device user. The 940 nm wavelength located in the NIR portion of the electromagnetic spectrum has been demonstrated to be a suitable candidate wavelength because efficient and cost-effective optical components are available or can be manufactured at this wavelength.

[0023] In some embodiments, the VCSEL is characterized by single-mode operation for diode currents up to 3 mA, in particular up to 5 mA. Conventional VCSEL devices employed in currently available SMI laser sensors typically start multi-mode operation at a diode current of about 1.5 mA, thus limiting the maximum modulation amplitude in the distance measurement and, therefore, the resolution of the sensor. In contrast, the effective extension of the optical cavity combined with the low bandwidth introduced by means of at least one low-contrast region makes it possible to increase the maximum allowable bias current by at least a factor of two and to increase the window for single-mode emission to currents up to 5 mA and even higher. Here, the trade-offs have to be tailored to the application requirements in terms of the overall thickness of the VCSEL device and its optical performance. In particular, the ability to introduce a laser current modulation with an amplitude increased by a factor of 10 also results in an increase in the resolution of the distance measurement by the same factor, i.e., an order of magnitude.

[0024] In some embodiments, the evaluation unit comprises a photodetector configured to detect changes in properties of the emitted laser radiation due to self-mixing interference, in particular changes in the intensity of the emitted light. The detection of the SMI laser sensor can be configured to optically detect and monitor the SMI formed inside the laser cavity. As described above, SMI leads to modulation of the laser output power. Therefore, a photodetector can be used to monitor the output power of the VCSEL to determine the extent of the SMI formed in the cavity. To this end, an optical element (e.g., a beam splitter) acting as a pickup mirror on the emission side can be used to transfer a specific portion of the emitted light to the photodetector.

[0025] Furthermore, the VCSEL may be characterized by double-sided emission, and the photodetector is arranged on the rear side of the first DBR. As an alternative to the above process, the VCSEL may be characterized by double-sided emission, such that the photodetector can monitor the output power of the VCSEL via the second emission surface (e.g., via the bottom surface of the VCSEL).

[0026] In some embodiments, the evaluation unit is configured to detect changes in the junction voltage of the VCSEL due to self-mixing interference. Alternatively or in addition to the optical detection of the output power, the evaluation unit may include means for monitoring the junction voltage or bias current of the VCSEL, which is also modulated together with the formation of the SMI inside the cavity. Thus, monitoring of such electrical properties may also be used to determine the extent of the SMI and thus evaluate the distance measurement.

[0027] Furthermore, an electronic device is provided, the electronic device comprising the SMI laser sensor according to one of the above embodiments, wherein the SMI laser sensor is configured to measure an absolute distance, a relative distance and / or a speed of an object in a 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 computer, a laptop computer or a wearable accessory, such as a smart wristband, a smart watch or a headphone device.

[0028] In addition, a method for manufacturing an SMI laser sensor is provided. The method includes providing a vertical cavity surface emitting laser (VCSEL) configured to emit laser radiation. Providing the VCSEL includes: providing a first distributed Bragg reflector (DBR); arranging a cavity region on the front side of the first DBR, the cavity region including an active light generating region; and arranging a second DBR on the front side of the cavity region. The method also includes providing an evaluation unit configured to monitor the laser radiation of the VCSEL. Providing at least one of the first DBR and the second DBR includes: forming a first contrast region and a second contrast region, the first contrast region is formed by a plurality of alternating material layers having a first refractive index contrast Δn1 / n with respect to the emission wavelength of the VCSEL, and the second contrast region is formed by a plurality of alternating material layers having a second refractive index contrast Δn2 / n, the second refractive index contrast Δn2 / n being greater than the first refractive index contrast Δn1 / n.

[0029] For example, the method includes disposing a first contrast region between a second contrast region and the cavity region.

[0030] Further embodiments of the method will be apparent to a person skilled in the art from the aforementioned embodiments of the optical sensor element, the thermal image sensor and the electronic device, and vice versa.

[0031] The following description of the drawings may also illustrate and explain various aspects of the SMI laser sensor and the method of manufacturing the SMI laser sensor. Components and parts of the SMI laser sensor that are functionally identical or have the same effect are denoted by the same reference numerals. Components and parts that are identical or identical in effect may be described only with respect to the drawing in which they first appear. Their description need not be repeated in subsequent drawings.

[0032] In the attached figure:

[0033] Figure 1 is a schematic diagram of a first exemplary embodiment of the proposed laser sensor;

[0034] Figure 2 is a schematic diagram of a second exemplary embodiment of the proposed laser sensor;

[0035] Figure 3 illustrates the cavity properties of an exemplary embodiment of a VCSEL for the proposed laser sensor;

[0036] Figure 4 and 5 are schematic diagrams of second and third exemplary embodiments of a VCSEL;

[0037] Figure 6-9 The key properties of VCSELs according to the improved concept and prior art VCSELs are compared;

[0038] Fig.10 illustrates the reflectivity behavior of low contrast regions depending on the alloy composition and number of layer pairs of a VCSEL embodiment; and

[0039] Fig.11 is a schematic diagram of an electronic device including the proposed laser sensor.

[0040] Figure 1A first embodiment of a self-mixing interferometric laser sensor 1 according to the improved concept is shown. The laser sensor 1 comprises a first exemplary embodiment of a vertical cavity surface emitting laser 10. In this exemplary embodiment, the VCSEL 10 comprises a VCSEL layer structure formed by a cavity region 13, which is embedded between a first distributed Bragg reflector 11 and a second distributed Bragg reflector 12 to form a laser cavity. The cavity region 13 comprises an electrically pumped gain medium 13a and an optional cavity extension 13b, i.e. a transparent non-active medium. As depicted, the gain medium 13a may be embedded in the cavity extension 13b. Alternatively, the gain medium 13a may be arranged adjacent to the cavity extension 13b. The first DBR 11 is a stacked arrangement of a first contrast region 14 and a second contrast region 15, optionally separated by a spacer 16. The second contrast region 15 is highly reflective, preferably having a reflectivity of >99.5% at the emission wavelength of the VCSEL 10 (e.g. 940nm) for single-sided emission. The second DBR 12 is characterized by a smaller reflectivity allowing emission of the light 2. In other words, the top side of the second DBR 12 facing away from the cavity region 13 serves as the emission surface of the VCSEL 10. The first DBR 11 is p-doped, while the second DBR is n-doped, or vice versa, for allowing efficient feeding of current into the cavity region 13.

[0041] The first contrast region 14 is arranged between the second contrast region 15 and the cavity region 13 and is characterized by a refractive index contrast Δn1 / n that is less than the refractive index contrast Δn2 / n of the second contrast region 15. Therein, the refractive index contrast Δn / n is defined as the difference in refractive index of the alternating material layers forming the contrast region divided by their average. For example, the second contrast region 15 is formed by alternating layers of a first material having a refractive index of about 3.0 at 940 nm (e.g., AlAs) and a second material having a refractive index of about 3.6 at 940 nm (e.g., GaAs), resulting in a refractive index contrast Δn2 / n of about 18%. As a result, approximately 20 pairs are required to achieve a reflectivity of 99.5%. A number of 45 pairs will result in a reflectivity of 99.95%.

[0042] In contrast, the first contrast region 14 is composed of a first material having a refractive index of about 3.6 at 940 nm (eg, GaAs) and a second material having a refractive index of about 3.3 at 940 nm (eg, Al 0.5 Ga 0.5 As) are formed in alternating layers, resulting in a refractive index contrast Δn1 / n of about 8.7%. By reducing the Al X Ga 1-X As components can achieve even smaller contrast. Basically, AlX Ga 1-X The refractive index of As can be approximated by n = -0.6x + 3.6. Alternative material choices for the first and second contrast regions 14, 15 include various compositions of AlGaAs and InGaAs. Typical thicknesses of the individual layers forming the first and second contrast regions 14, 15 are about 4 times the optical emission wavelength of the VCSEL 10 divided by the corresponding refractive index. Both the first contrast region and the second contrast region are formed from a plurality of alternating layers, typically between 20 and 50 pairs of alternating material layers.

[0043] Thus, the first contrast region 14 can be considered as a low contrast DBR, which is arranged on the front surface of the high contrast DBR facing the cavity region 13. The high contrast DBR formed by the second contrast region 15 is characterized by a high reflectivity, while the first contrast region 14 is characterized by a relatively low reflectivity, but also by a lower bandwidth with respect to the emission wavelength of the VCSEL 10. In fact, the low contrast region acts as a cavity extension, increasing the length of the laser cavity and, therefore, the coherence length, resulting in that the SMI laser sensor can be operated over a greater distance. In addition, due to the cavity extension, the low contrast first region 14 achieves increased stability without additional longitudinal modes starting to generate laser light. This is due to the fact that the additional modes have higher losses due to the small bandwidth of the first contrast region 14 with low contrast. Therefore, compared to conventional devices, the VCSEL 10 can be operated at higher currents while maintaining single-mode operation, which is a necessary requirement for efficient SMI detection.

[0044] The operating current for current injection into the gain medium 13a is provided by a current source unit 22 coupled to the VCSEL. The laser sensor further comprises a control unit 21 coupled to the current source 22 for applying a modulation to the operating current, wherein the modulation of the operating current results in a frequency modulation of the emitted light. The operating principle of a SMI laser sensor for determining an absolute or relative distance is a well-known concept and is not described in further detail throughout this disclosure. However, it is to be noted that the performance of a SMI laser sensor is determined by its maximum operating current (and therefore the maximum range of the current modulation) while maintaining single-mode operation, and by the coherence length of the emitted light which indicates the maximum range of action of the laser sensor.

[0045] The laser sensor 1 further comprises an evaluation unit 20 which monitors the output intensity of the VCSEL, for example via monitoring the junction voltage of the VCSEL 10 which is modulated together with the output power, the output power being indicative of the degree of self-mixing interference. Therein, backscattered or reflected radiation 3 from the target object influences the wavelength of the emitted radiation 2, which translates into an influence on the output power and, in turn, into an influence on the junction voltage of the VCSEL 10. For monitoring the junction voltage, the evaluation unit 20 is coupled to a control unit 21 for communication purposes and is configured to determine the distance (relative or absolute) and / or the speed to the object or scene from which the emitted radiation 2 is reflected or backscattered. In Figures 6 to 9 A comparison between a conventional VCSEL employed in an SMI laser sensor and a VCSEL according to the improved concept is described in detail in .

[0046] The laser sensor 1 may further include an aperture layer 17, such as an oxide aperture layer, for defining an emission window of the VCSEL 10 and / or for limiting light above a cut-off angle from re-entering the cavity. In addition, the VCSEL 10 may include a metallization layer 18, for example, for defining electrical contacts and terminals of the VCSEL 10. The VCSEL 10 may be arranged on a substrate body 19. Since this proposed VCSEL structure allows for efficient suppression of high-order modes, the oxide aperture of the VCSEL may be increased in order to reduce the series resistance Rs and reduce the noise level in the SMI voltage readout result. In addition, the proposed VCSEL structure also allows for a narrower beam divergence, which may be beneficial in some applications.

[0047] Figure 2 A second embodiment of a self-mixing interference laser sensor 1 according to the improved concept is shown. Compared to the first embodiment, in this second embodiment, the readout of the self-mixing interference is performed in the optical domain. To this end, the laser sensor 1 further comprises a photosensitive element 23 (e.g. a photodiode), and the VCSEL 10 is configured to perform double-sided emission through both the first DBR 11 and the second DBR 12. The photodetector 23 can be embedded in the surface of the substrate body 19 or arranged on the surface of the substrate body 19 and coupled to the evaluation unit 20. Like the emission light 2 through the front side of the VCSEL 10, the emission light 4 through the back side also carries information about the self-mixing interference formed inside the laser cavity. In other words, when experiencing SMI, the back side emission light 4 is also modulated in terms of its output power, so that monitoring this emission gives information about the degree of SMI in the cavity.

[0048] To achieve double-sided emission, the second contrast region 15 of the first DBR 11 is characterized by a slightly lower reflectivity so that a portion of the light can leave the cavity through the first DBR 11 on the rear side of the VCSEL 10. As an alternative to the depicted arrangement, in the case where the VCSEL is characterized by single-sided emission, a pickup element in the optical path of the emitted light 2 can transfer some of the emitted light 2 to the photodetector 23.

[0049] Figure 3 The effect of the low contrast region formed by the first contrast region 14 is illustrated, which acts as a (further) cavity extension and bandwidth filter. As shown in the left panel, due to the elongated cavity, additional longitudinal modes may potentially be formed in addition to the target mode at 940nm as the cavity extension reduces the free spectral range of the VCSEL 10. In particular, in this example, two additional modes at 910nm and 970nm appear to be supported, which would begin to generate lasers at the threshold current if the cavity was extended using a conventional buffer as an extension. However, since the first contrast region 14 is a low contrast DBR, as depicted in the right panel, the additional modes are characterized by a lower total reflectivity compared to the target optical mode. Even though this difference seems extremely small, such a small difference in reflectance is sufficient to suppress the optical mode from generating lasers. Therefore, compared to conventional VCSELs, the VCSEL 10 according to the improved concept can be operated at significantly larger operating currents while maintaining single-mode emission.

[0050] Figure 4 FIG. 1 shows another exemplary embodiment of a VCSEL 10 used in an SMI laser sensor 1 according to an improved concept. Figure 1 Compared to the embodiment of the VCSEL 10 of FIG. 1 , in this embodiment, the first DBR 11 is a conventional high contrast DBR as an end mirror, and the second DBR 12 is characterized by a first contrast region 14 (e.g., a low contrast region) and a second contrast region 15 (e.g., a high contrast region). From a manufacturing point of view, an embodiment in which the top DBR has two different contrast regions may be beneficial.

[0051] Figure 5Another exemplary embodiment of a VCSEL 10 employed in an SMI laser sensor 1 according to an improved concept is shown. In this embodiment, both the first DBR 11 and the second DBR 12 include respective first contrast regions 14 and second contrast regions 15. Among them, the contrast region with a lower refractive index contrast (i.e., the first contrast region 14) is arranged in the respective DBRs 11, 12 facing the cavity region 13, while the second contrast region as an end mirror is arranged to face away from the cavity region 13. The embodiment in which both the first DBR 11 and the second DBR 12 have low contrast regions and high contrast regions also increases the cavity length, and therefore increases the coherence length, while the dual low contrast regions can also help suppress the establishment of unwanted optical modes due to the small bandwidth of these low contrast regions.

[0052] Figures 6 to 9 Key aspects of the 940 nm VCSEL 10 according to the improved concept are illustrated in the corresponding panel (b), while these aspects are compared with the corresponding characteristics (shown in the corresponding panel (a)) of an existing 940 nm VCSEL device with similar specifications but without any low contrast regions (i.e. with two conventional DBR end mirrors). Figure 6 The LIV measurements obtained at different temperatures are illustrated, where the optical output power and the junction voltage depend on the applied operating current. As can be discerned, both the conventional VCSEL and the VCSEL 10 according to the improved concept experience similar behavior in terms of these two quantities. Therefore, the additional low contrast area 14 does not significantly change the performance of the VCSEL 10.

[0053] Figure 7 The single mode behavior obtained with the aid of polarization output power measurement is illustrated to discern the contribution of the different modes to the total measured power. As can be seen, the VCSEL 10 according to the improved concept undergoes single mode operation at a significantly larger scale at currents exceeding 5 mA, whereas the conventional VCSEL starts its multimode behavior already at about 1.3 mA, i.e. the point where the additional mode comes within 10 dB of the target mode, above which SMI measurements cannot be reliably performed. Note that in the VCSEL of panel (b), polarization flipping could potentially occur, however, single mode operation remains stable, thus ensuring reliable SMI operation even at large (modulation) currents.

[0054] Figure 8Spectra are shown to illustrate the single-mode and multi-mode operating domains. For conventional VCSELs, higher-order longitudinal modes appear at an operating current of about 2.5 mA, making SMI measurements extremely difficult or even impossible. However, as shown for different temperatures, the VCSEL 10 according to the improved concept shows suppression of higher-order modes of more than 20 dB even at a current of 5 mA.

[0055] Fig. 9 Finally, the SMI signal and noise amplitudes between the laser sensor 1 employing a conventional VCSEL and the VCSEL 10 according to the improved concept are compared. Due to the occurrence of higher order modes in the conventional SMI laser sensor, the signal-to-noise ratio is already significantly reduced at an operating current of 1.5 mA, thus severely limiting the maximum modulation amplitude. However, the VCSEL 10 according to the improved concept, due to its multi-mode operation at an operating current exceeding 5 mA, still shows a significant SNR and thus enables long-distance high-resolution measurements due to the extended cavity and the ability to apply a larger modulation amplitude to the operating current.

[0056] Fig.10 Panel (a) shows the GaAs and Al X Ga 1-X The aforementioned reflectivity of the first contrast region 14 formed by the alternating layers of As shows that GaAs has a refractive index of 3.6 at 940 nm, and Al X Ga 1-X As has different refractive indices for different components and therefore. As can be seen, the lower the aluminum content, indicating a higher refractive index according to the approximate result n = -0.6x + 3.6, the smaller the refractive index contrast Δn1 / n of the first contrast region 14, and the lower the bandwidth. However, reducing the contrast will significantly reduce the reflectivity, which can be compensated by increasing the number of layer pairs so that a high reflectivity close to 1 can be achieved in the case of a small bandwidth. Panel (b) shows a DBR (e.g. Figure 1 The reflectance spectrum of the first DBR 11) is shown in FIG. 1 , which includes 30 pairs of AlAs / GaAs pairs in the second contrast region 15 and 30 pairs of AlAs / GaAs pairs forming the first contrast region 14 facing the cavity region 13. 0.5 Ga 0.5 As / GaAs pairs. Among them, the low bandwidth effectively suppresses the occurrence of lasing in higher-order optical modes.

[0057] Fig.11An 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 computer, augmented or virtual reality glasses, a smart watch or other wearable device or a dedicated distance sensor.

[0058] 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 concept. 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.

[0059] It will be appreciated that the present disclosure is not limited to the disclosed embodiments and the contents specifically shown and described above. On the contrary, the features described in separate dependent claims or in the specification may be advantageously combined. In addition, the scope of the present disclosure includes those variations and modifications that will be apparent to those skilled in the art and fall within the scope of the appended claims.

[0060] As far as the term "comprising" is used in the claims or the description, it does not exclude other elements or steps of the corresponding features or processes. Where the terms "a" or "an" are used in conjunction with a feature, they do not exclude a plurality of such features. Furthermore, any reference signs in the claims should not be construed as limiting the scope.

[0061] This patent application claims priority to U.S. Patent Application No. 17 / 892,389, the disclosure of which is incorporated herein by reference.

[0062] refer to

[0063] 1 Laser sensor

[0064] 2 Emitted Radiation

[0065] 3Reflected or scattered radiation

[0066] 4 Emitted Radiation

[0067] 10 Vertical Cavity Surface Emitting Lasers

[0068] 11, 12 Distributed Bragg reflector

[0069] 13 cavity area

[0070] 13 Active light generation area

[0071] 13b cavity extension area

[0072] 14, 15 Contrast Area

[0073] 16 Spacer

[0074] 17 Porous Layer

[0075] 18 metallization layers

[0076] 19 substrate body

[0077] 20 evaluation units

[0078] 21Control Unit

[0079] 22 Current source units

[0080] 23 Photodetector

Claims

1. A self-mixing interferometry (SMI) laser sensor (1), comprising: A vertical cavity surface emitting laser (VCSEL) (10) configured to emit laser radiation, the VCSEL comprising a first distributed Bragg reflector (DBR) (11), a second DBR (12) and a cavity region (13) comprising an active light generating region (13a), wherein the cavity region (13) is arranged in a layer structure between a front side of the first DBR (11) and a rear side of the second DBR (12); and an evaluation unit (20) configured to monitor the laser radiation of the VCSEL (10); At least one of the first and second DBRs (11, 12) comprises a first contrast region (14) and a second contrast region (15), wherein the first contrast region (14) is formed by a plurality of alternating layers of a first material, the first material having a first refractive index contrast Δn1 / n with respect to an emission wavelength of the VCSEL (10), and the second contrast region (15) is formed by a plurality of alternating layers of a second material, the second material having a second refractive index contrast Δn2 / n greater than the first refractive index contrast Δn1 / n.

2. The SMI laser sensor (1) according to claim 1, wherein the first contrast region (14) is arranged between the second contrast region (15) and the cavity region (13).

3. The SMI laser sensor (1) according to claim 1 or 2, wherein the cavity region (13) further comprises a cavity extension region (13b) arranged between the active light generating region (13a) and the first or second DBR (11, 12).

4. The SMI laser sensor (1) according to any one of claims 1 to 3, wherein the first refractive index contrast Δn1 / n is less than 10%, in particular less than 6%.

5. The SMI laser sensor (1) according to any one of claims 1 to 4, wherein the second refractive index contrast Δn2 / n is greater than 10%, in particular greater than 16%.

6. The SMI laser sensor (1) according to any one of claims 1 to 5, wherein each of the first and second DBRs (11, 12) comprises a respective first contrast region (14) and a second contrast region (15).

7. The SMI laser sensor (1) according to any one of claims 1 to 5, wherein the first contrast region (14) is formed by alternating layers of GaAs and AlGaAs, GaAs and InGaAs, or AlGaAs and InGaAs, in particular by alternating layers of GaAs and AlGaAs with x≤0.

5. X Ga 1-X Alternating layers of As are formed.

8. The SMI laser sensor (1) according to one of claims 1 to 7, wherein the second contrast region (15) is formed by alternating layers of AlAs and one of the following: GaAs, AlGaAs and InGaAs.

9. The SMI laser sensor (1) according to one of claims 1 to 8, wherein the emission wavelength of the VCSEL (10) is in the infrared domain, in particular in the NIR domain including 940 nm.

10. The SMI laser sensor (1) according to one of claims 1 to 9, wherein the VCSEL (10) is characterized by a single-mode operation for a diode current up to 3 mA, in particular up to 5 mA.

11. The SMI laser sensor (1) according to one of claims 1 to 10, further comprising a photodetector (23) which is coupled to the evaluation unit (20) and 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.

12. The SMI laser sensor (1) according to claim 11, wherein the VCSEL (10) is characterized by double-sided emission and the photodetector (23) is arranged on the back side of the first DBR (11).

13. The SMI laser sensor (1) according to one of claims 1 to 12, wherein the evaluation unit (20) is configured to detect changes in the junction voltage of the VCSEL (10) due to self-mixing interference.

14. An electronic device (100) comprising a SMI laser sensor (1) according to one of claims 1 to 13, wherein the SMI laser sensor (1) is configured to measure the absolute distance, relative distance and / or 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: A vertical cavity surface emitting laser (VCSEL) (10) is provided, the VCSEL (10) being configured to emit laser radiation, wherein providing the VCSEL (10) comprises: Providing a first distributed Bragg reflector DBR (11); Arranging a cavity region (13) on the front side of the first DBR (11), the cavity region (13) including an active light generating region (13a); and Arranging a second DBR (12) on the front side of the cavity region (13); and providing an evaluation unit (20) configured to monitor the laser radiation of the VCSEL (10); Wherein providing at least one of the first and second DBRs (11, 12) comprises forming a first contrast region (14) and a second contrast region (15), wherein the first contrast region (14) is formed by a plurality of alternating material layers having a first refractive index contrast Δn1 / n with respect to an emission wavelength of the VCSEL (10), and the second contrast region (15) is formed by a plurality of alternating material layers having a second refractive index contrast Δn2 / n, the second refractive index contrast Δn2 / n being greater than the first refractive index contrast Δn1 / n.

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