Optical sensor element, thermal image sensor, and method of detecting thermal radiation

Abstract

An optical sensor element (1) for sensing thermal radiation comprises a light emitter (10) having a cavity (11), the light emitter (10) being configured to emit coherent electromagnetic radiation through an emission surface (12) and to undergo self-mixing interference (SMI) caused by reflected electromagnetic radiation reinjected into the cavity (11). A micro-optical mechanical transducer (20) is arranged remote from the emission surface (12), the transducer (20) being configured to undergo mechanical deflection as a function of thermal radiation absorbed by the transducer (20) and to reflect electromagnetic radiation emitted by the light emitter (10) back into the cavity (11) to generate the SMI. The detection unit (30) is configured to detect a degree of the generated SMI, determine a deflection of the transducer (20) as a function of the detected degree, and generate an output signal indicative of the determined deflection.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 342,769, filed May 17, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to an optical sensor element for detecting thermal radiation, a thermal image sensor comprising a plurality of such optical sensor elements, and a method of detecting thermal radiation. Background Art

[0004] Thermal imaging (also commonly referred to as infrared thermography) is a technique for generating optical images based on photons in the long-wavelength infrared (LWIR) region (typically between 9 μm and 14 μm). This is due to the blackbody radiation law, according to which all objects with a temperature above absolute zero emit infrared radiation, where the amount of radiation emitted by an object increases with temperature. Thermal imaging thus enables visualization of an environment with or without visible illumination. For example, applications of thermal imaging include thermal mapping, medical imaging, building diagnostics, and night vision. Among other things, a thermal imaging camera converts energy from infrared wavelengths into visible light for display.

[0005] Conventional thermal imaging cameras are large and expensive. In order to also realize thermal imaging technology solutions for portable electronic devices (such as smart phones, wearable devices and laptop computers), integrated technology solutions are necessary, because in addition to price, space limitations usually constitute the biggest challenge in these devices. The ubiquitous silicon-based CMOS image sensors (CIS) often used in mobile devices can only image in the visible light (400-700nm) and part of the near infrared (700-1000nm) spectral range. Therefore, silicon cannot be used as an absorbing material in the thermal or LWIR spectral range. Therefore, conventional methods for thermal cameras for these applications, such as microbolometer arrays, use highly sensitive micro-optical mechanical MOM transducers, such as bimetallic cantilevers, in order to detect thermal radiation. The deflection of these cantilevers is usually read out in a piezoelectric, resistive or capacitive manner. However, these readout schemes experience problems such as lack of thermal isolation and Johnson noise.

[0006] State-of-the-art optical techniques are able to detect the thermal expansion of a bimetallic cantilever with a resolution of sub-angstrom, limited only by the thermal vibration noise. However, existing optical readout methods utilize deflection measurements using light from an LED with a diffractive optical element or from a laser, a technique also known from atomic force microscopy, where the light is reflected from the cantilever and directed to the CIS. The thermal image is then calculated by measuring the deflection of the light spot as the temperature changes using a multi-channel photodetector. However, this indirect measurement may lead to accuracy issues and increases the software burden of tracking and calculating the position of the light spot. Furthermore, such camera systems are not compact and are sensitive to external mechanical vibrations and alignment issues.

[0007] Therefore, an object to be achieved is to provide an optical sensor element for detecting thermal radiation having a high sensitivity and a compact structure. Another object is to provide a thermal image sensor comprising a plurality of such optical sensor elements, and a method of detecting thermal radiation.

[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 simple, compact optical sensor element for thermal imaging based on optical interferometry, wherein thermal radiation is converted into an optical interferometry signal directly read out by means of a detection unit. Wherein, the optical sensor element according to the improved concept utilizes a highly sensitive micro-optical mechanical transducer to detect thermal radiation. The deflection of the transducer due to the absorbed thermal radiation is optically detected using interferometry detection, which is based on the self-mixing interference measurement SMI in the cavity of the readout light emitter. Wherein, the degree of the SMI signal is proportional to the thermal energy incident on the MOM transducer and absorbed by the MOM transducer, resulting in significantly enhanced measurement accuracy compared to the prior art solutions. In addition, due to the reduction of mechanical elements, the system is much less sensitive to vibration and alignment. Specifically, the present disclosure utilizes a light emitter for both emission and detection. Therefore, the present disclosure combines the well-known concept of using a bimetallic cantilever as a transducer for temperature detection with, for example, an optical deflection from a transducer, so that a thermal image can be directly read out using a commercial light emitter for optical readout of self-mixing of optical interferometry.

[0010] In an embodiment, an optical sensor element for sensing thermal radiation comprises a light emitter having a cavity, wherein the light emitter is configured to emit coherent electromagnetic radiation through an emission surface. The light emitter is also configured to undergo self-mixing interference SMI, which is caused by reflected electromagnetic radiation re-injected into the cavity. The optical sensor element also includes a micro-optical mechanical transducer arranged away from the emission surface, wherein the transducer is configured to undergo mechanical deflection according to thermal radiation incident on the transducer and absorbed by the transducer. The transducer is also configured to reflect the electromagnetic radiation emitted by the light source back toward the emission surface of the light emitter so that it re-enters the cavity to generate the SMI. In other words, the reflected electromagnetic radiation is re-injected into the cavity. In addition, the detection unit of the optical sensor element is configured to detect the degree of the generated SMI, determine the deflection of the transducer according to the detected degree, and generate an output signal indicating the determined deflection.

[0011] A light emitter (e.g., a laser such as a vertical cavity surface emitting laser (VCSEL)) has a laser cavity and emits electromagnetic radiation through a partially transmissive end mirror (e.g., a Bragg mirror) arranged on the top side of the laser cavity. Wherein, the term "top side" refers to the side of the laser that is away from the substrate body on which the light emitter is arranged. Specifically, the light emitter can be arranged on a substrate (e.g., a CMOS silicon die) so that the bottom side of the light emitter is parallel to the substrate and faces the substrate. The substrate may include laser contacts for electrically connecting the laser to a laser driver. The substrate may further include an integrated circuit for controlling the emission of the light emitter and a circuit for reading out the SMI signal. For example, such an integrated circuit includes passive and active circuits for determining the degree of SMI, such as a transimpedance amplifier.

[0012] The transducer is arranged on a side of the light emitter opposite to the substrate, i.e. the transducer is arranged away from the emitting surface of the light emitter, so that the deflection of the transducer due to the absorption of thermal radiation changes the gap between the light emitter and the transducer. In other words, the deflection of the transducer changes the optical path length of the optical mode, wherein the path length is formed by the cavity and the gap. For example, the transducer is part of a MEMS die, which is bonded to the substrate, for example via a spacer. The transducer is configured to undergo a deflection when absorbing thermal photons, i.e. photons in the LWIR range. This can be achieved by means of a transducer that is formed in such a way that it has regions with different thermal expansion coefficients. Therefore, for any given energy absorption, these different regions undergo different expansions, generating stresses or strains in the transducer, so that the transducer shows a deflection, for example due to deformation, displacement or bending.

[0013] The transducer has a reflective surface, such as a bottom surface of the transducer, which faces the emitting surface and can therefore receive electromagnetic radiation emitted by the light emitter through the emitting surface. Wherein, at least a portion of the electromagnetic radiation received from the light emitter is reflected off the reflective surface and directed back to the emitting surface. At least a portion of the reflected electromagnetic radiation is coupled back into the cavity via 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 cavity, and therefore causes a change (e.g., modulation) in the output power of the light emitter through the emitting surface. In other words, the transducer converts thermal energy into mechanical deflection. In addition, the occurrence of self-mixing interference converts the mechanical deflection into an optical interference signal.

[0014] For example, a detection unit implemented by means of an integrated circuit detects the interferometric signal by means of monitoring the electrical characteristics of the light emitter or by monitoring the optical output power of the light emitter. The interferometric signal carries information about the degree of self-mixing interference, which in turn carries information about the precise deflection of the transducer in the emission direction of the light emitter (i.e., along the optical path). Thus, the detection unit can generate an output signal indicating the instantaneous position of the transducer in terms of its deflection, which in turn indicates the amount of thermal radiation absorbed by the transducer. Thus, the detection unit converts the optical interference signal into an electrical output signal carrying information about the degree of interference.

[0015] In an embodiment, the micro-optical mechanical transducer includes a bimorph or bimetallic layer structure formed of a first material and a second material having different thermal expansion coefficients. For example, the transducer is a bimorph or bimetallic strip, which includes two strips of different materials that expand at different rates as the temperature changes. Since the absorption of thermal photons causes heating of the transducer, the different expansions force the originally inherently flat strip to bend away from its resting position if heated in a first direction, and to bend to a second direction opposite to the first direction if cooled below its initial temperature. Specifically, the materials with the respective higher thermal expansion coefficients are located on the outside of the bending curvature when the strip is heated, and on the inside when cooled. It should be noted that in this article, the terms "bimorph" and "bimetallic" are used to emphasize the use of a transducer formed of two different materials with different thermal expansion characteristics (behavior) to convert thermal radiation into mechanical deformation. In particular, the term bimetallic does not necessarily require that both the first material and the second material are metals.

[0016] In an embodiment, the first material comprises silicon and the second material is a metal. For example, the first material is intrinsic silicon or silicon nitride and the second material is a metal such as gold. Silicon is characterized by a linear coefficient of 2.56*10 at 20°C. -6 K -1 , while the linear coefficient of gold at the same temperature is 14*10 -6 K-1 , and is therefore approximately 6 times larger. A similarly significant difference between the two materials is observed for the volume coefficient, which is 9*10 -6 K -1 , the volume coefficient of gold is 42*10 -6 K -1 Alternative metals that have significantly different thermal expansion coefficients than silicon include silver, copper, aluminum, and brass. Thus, a transducer may be formed from silicon as a first material and a metal as a second material such that a significant deflection of the transducer is achieved upon absorption of thermal photons in both materials.

[0017] In an embodiment, the first material forms a strip and the second material is arranged on the top and bottom sides of the strip. In order to achieve deflection along the optical path, the transducer can be formed by a first silicon layer coated with a second material (e.g., gold), wherein the first layer is parallel to the emitting surface and faces the emitting surface, and the second layer faces away from the emitting surface. For example, the side of the transducer facing the light emitter is formed of silicon or silicon nitride, while the side facing away from the light emitter is formed of metal, and vice versa. The bimetallic layered structure used to form the transducer is also referred to as a bimetallic bimorph structure.

[0018] In an embodiment, the micro-optical mechanical transducer is a cantilever. For example, the transducer is a rigid structural element extending horizontally, i.e. parallel to the emitting surface and perpendicular to the optical path defined by the emission direction of the electromagnetic radiation from the light emitter, and supported only at one end. For example, the transducer is a MEMS structure. Deflection of the cantilever causes the cantilever to bend towards or away from the emitting surface, depending on whether the cantilever is heated or cooled, and whether the side facing the emitting surface is formed of a material having a corresponding high or low thermal expansion coefficient.

[0019] In an embodiment, the micro-optical mechanical transducer is a double clamped beam. As an alternative to the transducer being a single-sided clamped cantilever as described above, the transducer can be a MEMS beam clamped at both ends to a support structure. Deflection of the beam causes the center of mass of the beam to bend toward or away from the emitting surface, depending on whether the cantilever is heated or cooled, and whether the side facing the emitting surface is formed of a material having a corresponding high or low coefficient of thermal expansion.

[0020] In an embodiment, the light emitter is a vertical cavity surface emitting laser VCSEL. The VCSEL diode is characterized by a beam emission perpendicular to the main extension plane of the top surface of the VCSEL. The VCSEL diode can be formed by a semiconductor layer on a substrate, wherein the semiconductor layer includes two distributed Bragg reflectors (DBRs) that surround an active region layer therebetween and thus form a cavity. VCSELs and their operating principles are well-known concepts and are not further described in detail throughout this disclosure. For example, the VCSEL diode is configured to have an emission wavelength of 940nm, 850nm, or another natural wavelength. For example, the VCSEL diode can be configured to emit coherent laser light when forward biased. Suitable alternative light emitters include semiconductor lasers, such as edge emitters, quantum cascades, and quantum dot lasers, which have suitable optical elements, such as lenses, grating couplers, etc., for coupling light into and out of such laser devices.

[0021] In an embodiment, the detection unit for detecting the degree of the generated SMI is configured to measure an electrical characteristic of the optical transmitter, in particular a junction voltage or a bias current. The quantity affected by the self-mixing interference in addition to the wavelength (and the optical power, because it is related to the emission frequency) is usually the laser junction voltage. Therein, it should be noted that both the output frequency and the junction voltage therefore show a correlation with the deflection of the transducer. Therefore, the measurement of the junction voltage (e.g. the junction voltage of a VCSEL) provides a convenient way to determine the deflection of the transducer, because it is characterized by the junction voltage being directly and substantially without delay transmitted to the optical transmitter (e.g. a VCSEL diode) via the SMI. Alternatively, for example, the modulation of the bias current can be detected as a changed electrical characteristic of the optical transmitter.

[0022] In an embodiment, the optical sensor element further comprises a photodetector. Wherein the light emitter is further configured to emit coherent electromagnetic radiation through a further emission surface in addition to the emission surface, the photodetector is configured to detect the electromagnetic radiation emitted through the further emission surface, and the detection unit for detecting the degree of the generated SMI is configured to measure the amount of electromagnetic radiation detected by the photodetector. The photodetector may be designed such that it is sensitive at the wavelength of the coherent electromagnetic radiation emitted by the light emitter, for example having its peak sensitivity.

[0023] As an alternative or in addition to the readout of the electrical characteristic, the optical sensor element may include a photodetector for detecting the optical output power of the light emitter. For example, the light emitter is a VCSEL having a double-sided emission through a first emission surface and a second emission surface opposite to the first emission surface. The transducer is arranged to be spaced apart from the first emission surface of the laser (e.g., the top emission surface facing the transducer), and the photosensitive element is arranged at the second emission surface of the laser (e.g., the bottom side of the cavity facing the substrate) or spaced apart from the second emission surface of the laser. Thus, the photodetector can detect the characteristics of self-mixing interference caused by the modulation of the output optical power through the second emission surface. In other words, the photodetector is arranged on the monitoring output of the cavity.

[0024] In an embodiment, the optical sensor element further comprises a lens element which is arranged away from the transducer and opposite to the light emitter and is configured to direct the thermal radiation onto a surface of the transducer. In order to direct the thermal radiation to the transducer so that optimal absorption is achieved, the lens element may focus the thermal radiation onto the transducer in a manner similar to, for example, an optical lens directing visible light onto an image sensor or onto pixels of an image sensor. The lens element may be formed of, for example, germanium (Ge), potassium bromide (KBr), zinc selenide (ZnSe) or sodium chloride (NaCl). The lens element is arranged between the thermal radiation source and a top surface of the transducer facing away from the light emitter.

[0025] In an embodiment, the lens element is a metalens. A metalens may be formed of a transparent material having nanostructures arranged on at least one side, the nanostructures being configured to focus light in a similar manner as compared to conventional lenses. Metalens are less bulky than conventional lenses because they do not require, for example, curved surfaces. As with conventional LWIR lenses, the same material selection may be used to implement a metalens as a lens element. A metalens may include a dielectric material or a semiconductor material, such as amorphous silicon, germanium, or a metal. A metalens may include structures like columns, slots, or holes, or H, U, V, + (plus sign), or cross-shaped structures.

[0026] In an embodiment, the optical sensor element further comprises a filter element arranged away from the transducer opposite the light emitter and characterized in that the passband comprises the long wavelength infrared (LWIR) portion of the electromagnetic spectrum. In order to limit unwanted radiation from reaching the transducer (where it may also be absorbed), the optical sensor element may further comprise a filter element arranged at a side of the transducer on which the thermal radiation is incident so as to block (e.g., reject or absorb) any unwanted light. The filter element may be a directional filter element that also blocks light of LWIR wavelengths impinging on the filter element at an angle of incidence greater than a maximum acceptance angle.

[0027] In an embodiment, the optical sensor element further comprises a further lens element arranged between the transducer and the emitting surface and configured to direct electromagnetic radiation from the light emitter onto the surface of the transducer and to re-inject reflected electromagnetic radiation into the cavity of the light emitter. Analogously to directing thermal radiation onto the transducer, the optical sensor element may likewise comprise a lens element for focusing light from the light emitter onto the transducer such that its deflection can be optimally detected.

[0028] The above object is further solved by a thermal image sensor comprising a plurality of pixels and a processing unit configured to generate a thermal image signal from an output signal of each pixel, wherein each pixel comprises an optical sensor element according to one of the aforementioned embodiments.

[0029] A thermal image sensor comprising a plurality of optical sensor elements allows thermal imaging, i.e. allows measuring the thermal radiation on a sensing surface and reconstructing an image from the outputs of the individual optical sensor elements. Specifically, a processing unit is configured to receive output signals from individual pixels and to generate image signals from the output signals in a similar manner compared to the reconstruction of conventional images. Since the optical sensor elements act as converters between thermal radiation and interference signals, the generated image signals may be referred to as thermal images, which are similar to those generated by means of conventional thermal camera systems, however, using a more compact and integrable image sensor.

[0030] In an embodiment, the processing unit (102) is further configured to divide the plurality of pixels (101) into pixel subgroups, enable sensor operation of monitoring pixels of at least one pixel subgroup while disabling the remaining pixels during an idle phase of the image sensor, and enable an active phase of the image sensor when a signal above a threshold is detected by means of the monitoring pixels, wherein sensor operation of all pixels of each pixel subgroup is enabled.

[0031] For example, the field of view of a thermal image sensor can be divided into multiple regions and mapped onto the sensor. In other words, a two-dimensional sensor array [M×N] can be divided into [m, n] regions. In each of these regions, all pixels except one or two so-called monitoring pixels are disabled. In this way, most of the pixels are disabled except for a few monitoring pixels mapped to key areas in the field of view. When any or all monitoring pixels in a region detect a certain signal, the operation of all pixels in the region or the entire pixel array can be enabled.

[0032] Thus, the resolution of the thermal image sensor can be dynamically adjusted by turning on or off certain optical sensor elements in the array that forms the image sensor. When the light emitter does not emit light, there is no self-mixing signal. This allows the image sensor to be used in the case where the camera device is activated when motion is detected in order to save power. This can allow specific applications such as occupancy monitoring with low power. For example, in occupancy monitoring and counting, there may be a situation where the imaging camera needs to be turned on only when the presence of a person is detected in the monitored room. When such presence is detected using only a few pixels of the image sensor, the camera resolution can be increased to full capacity, i.e. all pixels are enabled, in order to enable the counting process. Therefore, this feature enables low-power operation, which is beneficial for IOT and mobile applications. Another application can be used for thermal imaging with static contrast streaming operations to save bandwidth and power when operating.

[0033] Alternatively or additionally, the thermal image sensor may have a low power or idle mode in which only a small number of randomly selected pixels, groups of pixels, or groups of pixels in a region of interest are turned on or active during operation of the thermal image sensor, while the remaining pixels are turned off or inactive. For example, in the low power or idle mode, the number of active pixels is at most one tenth or one hundredth of the total number of pixels. Specifically, a pixel is active or turned on if the light emitter of the corresponding pixel emits electromagnetic radiation.

[0034] In addition, the thermal image sensor can have an imaging mode in which all or most pixels are active. For example, if an event or change in the output signal of one or more active pixels is detected during operation in a low power or idle mode, the thermal image sensor is awakened and operates in an imaging mode.

[0035] In an embodiment, the plurality of pixels form a one-dimensional array. For example, the thermal image sensor may be a 1D line scanner configured to capture thermal signals along one dimension. Alternatively, the plurality of pixels form a two-dimensional array for achieving two-dimensional imaging similar to that of conventional image sensors employed in modern image capture devices.

[0036] In an embodiment, the thermal image sensor further comprises a lens arrangement arranged away from the transducer of the pixel opposite the light emitter and configured to direct the thermal radiation onto a surface of the transducer.

[0037] In an embodiment, the lens arrangement is a microlens array. In a microlens array, each pixel may have its own lens element as described above, thus effectively forming a microlens array, since pixels are typically characterized by a footprint on the order of tens or hundreds of square micrometers. The microlens array may be implemented as a metalens array.

[0038] In an embodiment, the lens arrangement comprises a metalens. As described above, a smaller sensor height can be achieved with the aid of a metalens compared to a conventional lens. Wherein, the thermal image sensor may comprise a single metalens covering the pixels of the image sensor, or each pixel comprises a dedicated metalens element as described above.

[0039] Furthermore, an electronic device is provided, which comprises an optical sensor element or a thermal image sensor according to one of the above embodiments. 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.

[0040] In addition, a method for detecting thermal radiation is provided. The method comprises emitting coherent electromagnetic radiation by means of a light emitter through an emitting surface of the light emitter toward a micro-optical mechanical transducer arranged away from the emitting surface. The method also comprises re-injecting the electromagnetic radiation into the cavity of the light emitter by means of reflection from the transducer, and causing self-mixing interference SMI in the cavity caused by the re-injected electromagnetic radiation. The method also comprises detecting the degree of SMI, and determining the mechanical deflection of the transducer according to the detected degree. Wherein, the transducer is configured to undergo mechanical deflection according to the thermal radiation absorbed by the transducer.

[0041] Further embodiments of the method will become 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The following description of the drawings may further illustrate and explain aspects of the optical sensor element, the thermal image sensor and the method of detecting thermal radiation. Functionally identical components and parts of the optical sensor element or having the same effect are denoted by the same reference numerals. Identical or substantially identical components and parts may be described only with respect to the drawing in which they first appear. Their description is not necessarily repeated in consecutive drawings.

[0043] In the attached picture:

[0044] Figure 1 A first exemplary embodiment of an optical sensor element according to the improved concept is shown;

[0045] Figure 2 A second exemplary embodiment of an optical sensor element is shown;

[0046] Figure 3 An exemplary embodiment of a thermal image sensor including a plurality of optical sensor elements is shown;

[0047] Figure 4An exemplary array of optomechanical transducers employed in a thermal image sensor is shown;

[0048] FIG. 5 illustrates various exemplary embodiments of optomechanical transducers employed in optical sensor elements; and

[0049] Figure 6 A schematic diagram of an electronic device including a thermal image sensor is shown. DETAILED DESCRIPTION

[0050] Figure 1 A first exemplary embodiment of an optical sensor element 1 according to the improved concept is shown. The optical sensor element 1 includes a light emitter 10, which is arranged on an integrated circuit substrate 60 (e.g., a silicon chip including an integrated circuit) and is electrically connected to the circuit of the substrate 60. The electrical connection between the light emitter 10 and the contacts of the integrated circuit substrate 60 is achieved via a connection element 61, for example, a solder bump formed of a conductive material (such as AgSN, Cu or Au).

[0051] The light emitter 10 may be a vertical cavity surface emitting laser VCSEL, and includes an emitting surface 12, for example formed by a partially transmissive Bragg mirror relative to the emission wavelength of the VCSEL. The light emitter 10 also includes a cavity 11 arranged between the emitting surface 12 and the surface of the laser opposite to the emitting surface 12, wherein the cavity 11 is used as an optical resonator. The light emitter 10 is configured to emit coherent light in a vertical direction through the emitting surface 12, as shown in the figure. The light emitter 10 may be configured to emit light in the infrared IR, visible or ultraviolet UV domain of the electromagnetic spectrum. For example, the light emitter 10 is based on GaAs / AlGaAs material and emits light in the NIR range of 750-980nm, especially around 850nm. For example, VCSELs with alternative materials (such as indium phosphide) can be used to obtain other longer wavelengths such as 1.3μm, 1.55μm or more than 2μm. For readout using a photodetector, its sensitivity at the corresponding operating wavelength can be ensured by selecting an appropriate material for ensuring the corresponding sensitivity.

[0052] The optical sensor element 1 further comprises a micro-optical mechanical transducer 20, such as in this case a double-sided clamped beam, spaced from the emitting surface 12 of the light emitter 10. In other words, the transducer 20 is suspended above the emitting surface 12 with a gap formed between the transducer 20 and the light emitter 10. For example, the transducer 20 is clamped to a support structure 24 of a MEMS die, which is bonded to an integrated circuit substrate 60 via a spacer. The transducer 20 is formed by a bimetallic structure comprising a first layer 21 and a second layer 22 arranged on a top surface of the first layer 21. The first layer 21 and the second layer 22 are formed of different materials, wherein the materials differ at least in their coefficients of thermal expansion. For example, the first layer 21 is formed of the material of the support structure 24, such as silicon, and the second layer 22 is formed of a metal, such as gold. Typical gap heights are tens or hundreds of micrometers and depend on the spatial constraints of the intended application.

[0053] This results in the fact that, when the first layer 21 and the second layer 22 undergo different expansions due to their different thermal expansion coefficients, a deflection in the emission direction occurs upon absorption of thermal photons, i.e. photons in the LWIR range within the transducer 20. Thus, the main deflection direction of the transducer 20 is parallel to the emission direction of the light emitter 10, so that the deflection of the transducer 20 changes the gap distance between the transducer 20 and the emission surface 12 of the light emitter 10. Depending on whether the thermal expansion coefficient of the second layer 22 is greater or less than the thermal expansion coefficient of the first layer 21, the transducer 20 deflects towards or away from the light emitter 10 upon heating due to heat absorption, thereby reducing or increasing the gap between the transducer 20 and the light emitter 10.

[0054] The transducer 20 is at least partially reflective on the surface of the transducer 20 facing the light emitter 10, which means that light from the light emitter 10 impinging on the transducer 20 is reflected back towards the light emitter 10. The reflective properties of the surface can be achieved by making the surface of the transducer 20 itself reflective, or a mirror layer is arranged on the bottom side of the transducer 20 facing the light emitter 10. The reflective surface ensures that light from the light emitter 10 impinging on the reflective surface is guided back to the emitting surface 12 to re-inject the reflected light into the cavity 11.

[0055] Since the emitted light from the light emitter 10 is coherent, the reflected light re-injected into the cavity 11 through the emission surface 12 is superimposed with the light inside the cavity 11, depending on the phase shift introduced by the round trip to and from the transducer 20. This in turn leads to changes in the characteristics of the light emitted from the light emitter 10, including the output frequency, line width, threshold gain and therefore the output power. Therefore, the self-mixing interference that occurs leads to a change in the frequency (and optionally the amplitude) of the laser oscillation field inside the cavity 11. The deflection of the transducer 20 along the emission direction of the light emitter 10 leads to a change in the distance between the transducer 20 and the light emitter 10. Therein, even the smallest deflection is sufficient for a detectable change in the SMI inside the cavity 11.

[0056] The optical sensor element 1 further comprises a detection unit 30 which is electrically coupled to the optical emitter 10 such that electrical characteristics of the optical emitter 10 can be detected by means of the detection unit 30. For example, the detection unit 30 comprises components for monitoring and detecting the junction voltage (e.g. VCSEL junction voltage) of the optical emitter 10. In addition to the optical power of the optical emitter 10, the junction voltage is also affected by self-mixing and also shows changes when the transducer 20 is deflected. However, it should be noted that while the output power changes proportionally with the change in deflection, the junction voltage presents an inverse relationship. In other words, an increase in laser power coincides with a decrease in the laser junction voltage. Alternatively, the electronic control unit 20 may comprise components for monitoring and detecting changes in the bias current of the optical emitter 10, thereby showing similar changes due to the deflection of the transducer 20.

[0057] The detection unit 30 also includes components for analyzing the electrical characteristic and determining the deflection of the transducer 20 based on the detected change in the electrical characteristic and generating an output signal including information of the deflection. Thus, the deflection can be directly converted into the amount of thermal radiation absorbed by the transducer 20 in response to the incident thermal radiation.

[0058] The optical sensor element 1 in this embodiment further comprises a lens element 50 for directing incident thermal radiation onto the surface of the transducer 20. For example, the lens element 50 is formed of germanium (Ge), potassium bromide (KBr), zinc selenide (ZnSe) or sodium chloride (NaCl) and is transmissive at least for photons in the LWIR range. The lens element 50 is arranged between the source of thermal radiation and the top surface of the transducer 20 facing away from the light emitter 10. For example, the lens element 50 is configured to focus an incident parallel light beam onto a point or surface of the transducer 20 that experiences the maximum deflection when heated. An additional lens element 52 may be used to collimate the light emitted by the light emitter 10 and for focusing the reflected light back into the cavity 11. The further lens element 52 is arranged between the light emitter 10 and the transducer 20, for example, it is arranged on the emission surface 12.

[0059] The optical sensor element 1 in this embodiment further comprises an optical filter element 51 which is characterized by a passband in the LWIR range. In other words, the filter element 52, similar to an optical filter in the visible domain, is configured to transmit light in the LWIR range, while light outside this range is rejected. For example, unwanted light is absorbed or reflected. In addition, the transmission characteristics of the filter element 51 may be angle-dependent, so that only LWIR light impinging on the filter within a predetermined range of incident angles is transmitted, while stray light, for example from the sides, is rejected.

[0060] Figure 2 A second exemplary embodiment of an optical sensor element 1 according to an improved concept is shown. Figure 1 In contrast to the embodiment of FIG. 1 , in this embodiment, the transducer 20 is a cantilever, i.e., a single-sided clamping strip. Figure 1 The cantilever is a bimetallic structure, and due to this bimetallic structure, when heated or cooled from the initial temperature, the cantilever undergoes deflection. The deflection again changes the gap between the transducer 20 and the light emitter 10, so that the self-mixing interference signal in the cavity 11 is changed.

[0061] Another difference from the first embodiment is the readout mechanism, which in this case is optical rather than electrical. To this end, the optical sensor element 1 also includes a photodetector 40 arranged on or in the surface of the substrate 60, for example implemented as a silicon-based photodiode, because VCSELs typically emit in the visible light or NIR range, and silicon is photosensitive in the visible light or NIR range. Moreover, the light emitter 10 in this embodiment is configured to perform two-sided emission. In other words, the two ends of the cavity are defined by partially transmissive Bragg reflectors, so that light is emitted toward the transducer 20 through the emission surface 12 and toward the photodetector 40 through the other emission surface 13. As previously mentioned, the change in the distance between the transducer 20 and the light emitter 10 changes the self-mixing interference in the cavity, which in turn changes the output optical power of the light emitter, which in this case passes through both the emission surface 12 and the other emission surface 13. Therefore, the amount of light captured by the photodetector 40 within a specified integration time directly carries information about the degree of self-mixing interference, and therefore carries information about the deflection position of the transducer 20.

[0062] Thus, in this embodiment, the detection unit 30 is coupled to the photodetector 40 and is configured to determine the extent of the SMI and, therefore, the deflection of the transducer 20, from which the amount of absorbed thermal radiation can be determined, based on the photoelectric signal (e.g., photocurrent) generated by the photodetector 40. Similar to the further lens element 52 arranged on the emission surface 12, the optical sensor element 1 may include a further lens element 52 also between the further emission surface 13 and the photodetector 40 for directing (e.g., focusing) the light from the light emitter 10 onto the photodetector 40.

[0063] Without loss of generality, the features of the first and second embodiments (ie, transducer type and detection mechanism) may be interchanged, since the deflection of all types of bimetallic transducers 20 manifests itself as a corresponding degree of self-mixing interference, which may be read out electrically or optically.

[0064] Figure 3 An exemplary embodiment of a thermal image sensor 100 according to the improved concept is shown. The image sensor 100 comprises a one-dimensional or two-dimensional array of a plurality of pixels 101, depending on the application, wherein each of the pixels 101 comprises an optical sensor element 1. Figure 1 and Figure 2 In contrast to the first and second exemplary embodiments of the optical sensor element 100, in this embodiment, the lens arrangement 103 is shown as a single LWIR lens, which directs incident thermal radiation onto the pixels 101 of the image sensor 100. However, each pixel 101 may include its own lens element, for example forming a microlens array as the lens arrangement 103, for achieving the same purpose without limiting the functionality of the improved concept. Instead of an optical lens, the lens arrangement 103 may also be formed by one or more metalenses, for example forming a micrometalenses array. Likewise, the filter element 104 may be a single filter element across the image sensor 100, or each pixel 101 may have its own filter element as described above.

[0065] The thermal image sensor 100 further comprises a processing unit 102, which in this case is formed by an analog part 102a for receiving output signals from the optical sensor element 1, for example from each pixel 101, and for performing analog-to-digital conversion. A second element of the processing unit 102 is a digital part 102b, which comprises digital logic and interface circuits, for example for reconstructing a thermal image output by the processing unit 102 as a thermal image signal. The processing unit 102 may also comprise circuits for setting integration times, switching between readout modes, and other applications common to the operation of an image sensor.

[0066] Figure 4 It is shown that in Figure 31. An exemplary array 25 of micro-opto-mechanical transducers 20 employed in a thermal image sensor 100 of FIG. 1 is shown. The array 25 includes a support structure 24, such as a support frame formed of silicon, for supporting individual micro-opto-mechanical transducers 20, wherein each transducer 20 is part of an optical sensor element 1 as described throughout the present disclosure. Thus, the depicted array 25 is part of a thermal image sensor having a two-dimensional array of pixels 101. Alternatively, the optical sensor element 1 may include a one-dimensional array of pixels 101, such as for implementing a 1D line scan sensor array.

[0067] FIG5 shows various exemplary embodiments of an optomechanical transducer 20 employed in an optical sensor element 1 according to an improved concept. FIG5a shows a top view and a schematic cross-sectional view of a bimorph or bimetallic cantilever formed of three layers, wherein a first layer 21 is formed as a silicon strip extending away from a support structure 24, which is also made of silicon, wherein the strip is coated on its top and bottom sides with a second layer 22, for example formed of gold. Gold is a suitable material, since it has a significantly different coefficient of thermal expansion than silicon and can be deposited in a simple manner using existing manufacturing techniques. Alternatively, only the top or bottom side of the silicon strip, i.e. the first layer 21, can be coated with the second layer 22.

[0068] FIG5b shows a top view and a schematic cross-sectional view of a bimetallic double-sided clamped beam, which also forms a bimorph structure by arranging a second metal layer 22 on the bottom surface and / or top surface of the beam formed by a first layer 21 (e.g., silicon). Finally, FIG5c shows a top view and a schematic cross-sectional view of a bimetallic triangular cantilever, in which both ends of the triangular structure are clamped to the same support structure. Similar to ordinary cantilevers and double clamped beams, the triangular cantilever can also be coated with metal on both sides to form a bimetallic structure. These shapes show different exemplary shapes of transducers 20 employed in optical sensor elements 1 according to the improved concept, and can differ in their response (i.e., deflection) relative to absorbed thermal radiation, and can be designed according to the requirements of the actual application.

[0069] Figure 6 It is shown that including, for example, Figure 3Schematic diagram of an electronic device 200 of an embodiment of a thermal image sensor 100. For example, the electronic device 200 is a smart phone, as depicted. 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 thermal camera device. The thermal image sensor 100 can allow thermal imaging and thus enable applications such as generating thermal maps of buildings and environments or measuring surface temperatures at a certain resolution. Alternatively, a single optical sensor element 1 can be used as a thermal detector for measuring temperature. Alternatively, one or more optical sensor elements 1 can be incorporated into a conventional image sensor as additional thermal pixels for extending the sensitivity range to the LWIR domain.

[0070] Embodiments of the optical sensor element 1, image sensor 100, and method of detecting thermal radiation disclosed herein have been discussed in order to familiarize the reader with the novel aspects of the concept. Although preferred embodiments have been shown and described, those skilled in the art may make changes, modifications, equivalents, and substitutions to the disclosed concept without unnecessarily departing from the scope of the claims.

[0071] It should be understood that the present disclosure is not limited to the disclosed embodiments and the embodiments that have been specifically shown and described above. On the contrary, the features described in separate dependent claims or specifications can be advantageously combined. In addition, the scope of the present disclosure includes those changes and modifications that are obvious to those skilled in the art and fall within the scope of the appended claims.

[0072] The term "comprising" used in the claims or in the description 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.

[0073] Reference numerals

[0074] 1Sensor element

[0075] 10 optical transmitters

[0076] 11 chambers

[0077] 12, 13 Emission surface

[0078] 20 transducers

[0079] 21First floor

[0080] 22 Second Floor

[0081] 24 Support structure

[0082] 25 Array

[0083] 30 detection units

[0084] 40 Photodetector

[0085] 50, 52 lens elements

[0086] 51 filter element

[0087] 60 substrates

[0088] 61 Connecting elements

[0089] 100 Thermal Image Sensors

[0090] 101 Pixels

[0091] 102 processing units

[0092] 102a Analog Section

[0093] 102b digital part

[0094] 103 lens device

[0095] 104 filter device

[0096] 200 electronic equipment

Claims

1. An optical sensor element (1) for sensing thermal radiation, comprising: - a light emitter (10) having a cavity (11), the light emitter (10) being configured to emit coherent electromagnetic radiation through an emitting surface (12) and to undergo self-mixing interference (SMI) caused by reflected electromagnetic radiation re-injected into the cavity (11); - a micro-optomechanical transducer (20) arranged remote from the emission surface (12), the transducer (20) being configured to undergo a mechanical deflection as a function of thermal radiation absorbed by the transducer (20) and to reflect electromagnetic radiation emitted by the optical emitter (10) back into the cavity (11) to generate the SMI; and - a detection unit (30) configured to: - detect the extent of SMI generated; - determining the deflection of the transducer (20) based on the detected degree; and - generating an output signal indicative of the determined deflection.

2. An optical sensor element (1) as claimed in claim 1, wherein the micro-optomechanical transducer (20) comprises a bimorph or bimetallic layer structure formed by a first layer (21) of a first material and a second layer (22) of a second material, the first material and the second material having different thermal expansion coefficients.

3. The optical sensor element (1) as claimed in claim 2, wherein the first layer (21) comprises silicon and the second layer (22) comprises metal.

4. The optical sensor element (1) as claimed in claim 2 or 3, wherein the first layer (21) forms a strip and the second layer (22) is arranged on the top side and the bottom side of the strip.

5. The optical sensor element (1) according to any one of claims 1 to 4, wherein the micro-optomechanical transducer (20) is a cantilever or a doubly clamped beam.

6. The optical sensor element (1) according to any one of claims 1 to 5, wherein the light emitter (10) is a Vertical Cavity Surface Emitting Laser (VCSEL).

7. The optical sensor element (1) according to any one of claims 1 to 6, wherein the detection unit (30) for detecting the extent of the generated SMI is configured to measure electrical properties of the light emitter (10), in particular a junction voltage or a bias current.

8. The optical sensor element (1) according to any one of claims 1 to 7, further comprising a photodetector (40); wherein - the light emitter (10) is further configured to emit the coherent electromagnetic radiation through a further emitting surface (13) in addition to the emitting surface (12); - the photodetector (40) is configured to detect electromagnetic radiation emitted by the further emitting surface (13); and - the detection unit (30) for detecting the extent of the generated SMI is configured to measure the amount of electromagnetic radiation detected by the photodetector (40).

9. The optical sensor element (1) according to any one of claims 1 to 8, further comprising a lens element (50), which is arranged away from the transducer (20) opposite to the light emitter (10) and is configured to guide the thermal radiation onto the surface of the transducer (20).

10. The optical sensor element (1) as claimed in claim 9, wherein the lens element (50) is a metalens.

11. The optical sensor element (1) according to any one of claims 1 to 10, further comprising a filter element (51), the filter element (51) being arranged away from the transducer (20) opposite the light emitter (10), and characterized in that The passband includes the long wavelength infrared LWIR portion of the electromagnetic spectrum.

12. The optical sensor element (1) as claimed in any one of claims 1 to 11, further comprising another lens element (52), which is arranged between the transducer (20) and the emitting surface (12) and is configured to guide electromagnetic radiation from the light emitter (10) onto the surface of the transducer (20) and to re-inject reflected electromagnetic radiation into the cavity (11) of the light emitter (10).

13. A thermal image sensor (100), comprising: - a plurality of pixels (101), wherein each pixel (101) comprises an optical sensor element (1) as claimed in any one of claims 1 to 12; as well as - a processing unit (102) configured to generate a thermal image signal from the output signal of each of said pixels (101).

14. The thermal image sensor according to claim 13, wherein the plurality of pixels (101) form a one-dimensional array or a two-dimensional array.

15. The thermal image sensor according to claim 13 or 14, further comprising a lens arrangement (103), wherein the lens arrangement (103) is arranged away from the transducer (20) of the pixel (101) opposite to the light emitter (10) and is configured to direct the thermal radiation onto a surface of the transducer (20).

16. The thermal image sensor (100) of claim 15, wherein the lens arrangement (103) is a microlens array.

17. The thermal image sensor (100) of claim 15 or 16, wherein the lens arrangement (103) comprises a metalens.

18. The thermal image sensor (100) according to any one of claims 13 to 17, wherein the processing unit (102) is further configured to: - dividing the plurality of pixels (101) into pixel subgroups; - enabling sensor operation of monitoring pixels of at least one sub-group of pixels while disabling the remaining pixels during an idle phase of the image sensor; and - upon detection of a signal above a threshold by means of the monitoring pixel, enabling an active phase of the image sensor, wherein sensor operation of all pixels of each subgroup of pixels is enabled.

19. An electronic device (200) comprising the thermal image sensor (100) according to any one of claims 14 to 18 or the optical sensor element according to any one of claims 1 to 13.

20. A method for detecting thermal radiation, the method comprising: - by means of a light emitter (10), emitting coherent electromagnetic radiation through an emission surface (12) of the light emitter towards a micro-optomechanical transducer (20) arranged remote from the emission surface (12); - re-injecting the electromagnetic radiation into the cavity (11) of the light emitter (10) by means of reflection from the transducer (20); - causing self-mixing interference (SMI) in the cavity (11) caused by the re-injected electromagnetic radiation; - detecting the extent of said SMI; as well as - determining the mechanical deflection of the transducer (20) based on the detected degree; - wherein the transducer (20) is configured to undergo the mechanical deflection as a function of thermal radiation absorbed by the transducer (20).

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