Imaging system and detection method

Abstract

In at least one embodiment, an imaging system includes a light emitter, a detector array, and a synchronization circuit. The detector array includes pixels with built-in modulation capabilities. The synchronization circuit is operable to synchronize acquisition performed by the detector array with emission by means of a light source.

Description

[0001] The present disclosure relates to an imaging system and a detection method.

[0002] The present disclosure relates to the field of light detection and ranging systems (abbreviated as LIDAR systems). In addition, the present disclosure relates to imaging systems. More specifically, the present disclosure relates to imaging systems with integrated LIDAR that can be used for distance measurement. In particular, the present disclosure relates to high-resolution imagers with distance measurement capabilities. The imaging system can be implemented as a LIDAR system, which uses an image sensor with intra-pixel modulation. The prior art mainly deals with optical ranging systems through the so-called time-of-flight (TOF) method, in which light pulses are emitted and the propagation time of the light pulses is calculated using a time-to-digital converter (TDC).

[0003] Current state-of-the-art LIDAR systems use three options:

[0004] (1) Scanning LIDAR systems with movable parts and sequential scanning of the surface. To date, such systems have been used, for example, for autonomous driving.

[0005] (2) LIDAR systems with solid-state lighting systems, where many points are projected onto a surface. Such systems can be made small and compact, and are in principle suitable for miniaturization and use in mobile devices.

[0006] (3) LIDAR systems, where a CMOS imager is used in conjunction with a receiving modulator. To date, such systems have been limited to handheld devices and cannot be miniaturized due to the need for discrete optical components.

[0007] Fig.12 and Fig.13 Both illustrate the working principles of the above-mentioned prior art methods (1) and (2). Fig.12 A LIDAR system is shown where a light source (eg, a laser) is emitting pulses of light towards an object, which are reflected by the object. Fig.13 A LIDAR system is shown in which the backward propagating light pulse is detected by a suitable photodetector.The time difference between the emission and reception of the light pulse can be measured with a time to digital converter (TDC).

[0008] In both cases, a light pulse is emitted by a light source (usually a laser adapted to emit light pulses of very short duration - typically picoseconds) and deflected onto several objects where it is reflected. The backward propagating light pulse is then collected by a suitable photodetector, such as an avalanche photodetector or SPAD, which is suitable for fast light pulse detection. The time difference between the emission and the reception of the light pulse is measured or counted by a time-to-digital converter (TDC). In such systems, usually a single laser beam or a so-called point cloud is emitted into the scene. The resolution is therefore limited to a single or a few hundred pixels.

[0009] In a third method (3), light pulses are also used, which are usually emitted neither by a single collimated laser nor by multiple collimated lasers. More precisely, a light source or laser beam with a specific field of view (FOV) of the order of a few degrees is used. After reflection, the light pulses are collected using a CMOS image sensor or a sensor array. The image sensor itself is configured so that it has a modulator built on top of it to modulate the intensity or phase of the input signal. First, the unmodulated image Idc is recorded. Then, the modulated image Imod is acquired. The modulation is performed so that at the beginning of the frame the modulator is attenuating the signal. At the end of the frame, the modulator allows complete transparency and the signal is no longer attenuated.

[0010] Now after acquiring these two frames, the range image is calculated as a function of the modulated image and the unmodulated image. Although the system can be used with CMOS image sensors with more than 1 M pixels, the integration of the optical path including polarization beam splitters and complex modulators remains a challenge.

[0011] Although some prior art systems appear to produce high resolution LIDAR systems, they can be difficult to integrate due to complex beam paths involving polarization beam splitters (PBS) and complex modulator architectures employing Pockels cells as electro-optic modulators.

[0012] One object is to provide an imaging system and a detection method that can be miniaturized while maintaining a high pixel count.

[0013] These objects are achieved by the subject-matter of the independent claims. Further developments and embodiments are described in the dependent claims.

[0014] It should be understood that, unless described as an alternative, any feature described with respect to any one embodiment may be used alone or in combination with other features described herein, and may also be used in combination with one or more features of any other embodiment, or in combination with any other embodiment. In addition, equivalents and modifications not described below may also be employed without departing from the scope of the imaging system and detection method as defined in the appended claims.

[0015] The following relates to improved concepts in the field of light detection and ranging (LIDAR) systems. Modulation of receiver-side sensitivity can be achieved by in-pixel modulation of a detector array. The improved concept suggests performing modulation in a pixel, rather than using a modulator on top of or attached to a receiving element. For example, a pixel can be an improved four-transistor (4T) pixel unit including a pinned photodiode. The detector array can be complementary to a transistor or several transistors attached to a photodiode. The one or several transistors can be operated as an element that modulates the effective responsivity of a photodiode by introducing a controlled leakage path. The photodiode can be complementary to an integrated polarizer. A polarizer can be considered as any array of horizontal or vertical lines, which can be made of metal or other dielectric or semiconductor materials. Adjacent photodiodes may have different polarizer orientations to detect different polarization states of light. There can be only one metal layer, optionally metal 1, with the rest of the back end of the production line being clear. However, polarizers including semiconductor materials or dielectric materials can also be considered. In addition, high contrast gratings can be integrated into a CMOS process using, for example, polysilicon or other gate materials.

[0016] In at least one embodiment, an imaging system includes a light emitter, a detector array, and a synchronization circuit. The detector array includes pixels with built-in modulation capabilities. The synchronization circuit is operable to synchronize acquisition performed by the detector array with emission by means of a light source.

[0017] For example, the light emitter includes a light source such as a light emitting diode or a semiconductor laser diode. One type of semiconductor laser diode includes a surface emitting laser (such as a vertical cavity surface emitting laser or VCSEL) or an edge emitting laser. VCSEL combines properties such as surface emission with design flexibility of addressable arrays, low threshold current, higher reliability, and the possibility of using wafer-level manufacturing processes. For example, the light emitter is connected to a laser driver circuit. Another option for the light emitter is to use a light emitting diode (LED). LED is also a surface emitting device, but its limitation is that the radiation is incoherent and has a typical Lambertian emission characteristic that is opposite to a directional laser beam.

[0018] Typically, a light source or light emitter has one or more characteristic emission wavelengths. For example, the emission wavelength of the light emitter is in the near infrared (NIR). For example, other wavelengths may include the visible band or ultraviolet, infrared (IR) or far infrared (FIR). In one embodiment, the wavelength of the light emitter may be a visible light wavelength. In another embodiment, the light emission may be in the infrared wavelength range that is invisible to the human eye. In a preferred embodiment, the emission wavelength can be 850nm or 940nm. Narrow wavelength bandwidths (especially within a temperature range) allow for more efficient filtering at the receiver side (e.g., at the detector array), thereby improving the signal-to-noise ratio (SNR). For example, VCSELs support this. This type of laser allows for the emission of a vertical cylindrical beam, which makes integration into an imaging system more straightforward. The use of one or more diffusers enables the emission of the laser to be uniform so as to evenly illuminate the scene.

[0019] The detector array includes one or more photodetectors, hereinafter referred to as pixels. Typically, pixels are implemented as solid-state or semiconductor photodetectors, which are arranged or integrated to form an array. Examples include CCD or CMOS image sensors, single photon avalanche diode (SPAD) arrays, or other types of avalanche diodes (APDs). These types of photodetectors are sensitive to light (such as visible light, NIR, VIS, and UV), which facilitates the use of narrow pulse widths in emission with the aid of light emitters. The detector array is arranged to capture one or more continuous images, hereinafter referred to as frames. In order to capture a frame, the detector array can use the pixels of the array to integrate the incident light over the duration of the exposure time. The acquisition of continuous frames can be performed at a frame rate, for example expressed in frames per second.

[0020] The pixel has a built-in modulation function. In this context, the term "modulation" refers to a limited change in the sensitivity or responsiveness of the pixel during the acquisition frame. In other words, the detection of the detector array is modulated, not the emission of light. In a mathematical sense, this modulation can be monotonic, that is, the change in sensitivity can be described by means of a monotonic function (e.g., a monotonic function of time). A function is called a monotonic function if and only if it does not increase at all, or does not decrease at all. Typically, modulation can increase or decrease monotonically. The duration of a single frame may be limited to the time when the sensitivity is modulated. However, the advantage of the proposed concept is that the modulation time can also be and is often faster than the duration of the frame in practice. This makes the proposed concept different from the common time of flight (ToF), which relies on the ability to measure the actual time of the emission pulse and the corresponding reflected pulse in a stopwatch manner (e.g., by means of a time-to-digital converter (TDC)).

[0021] The synchronization circuit is arranged to synchronize the emission of light by means of the light emitter on one side and the detection of light by means of the detector array on the other side. The synchronization circuit can control the delay between the light emission (e.g., pulse) and the time frame for detection. For example, the delay between the start of the light pulse and the start of detection can be set, for example, depending on the distance or distance range to be detected. The time taken to complete a distance measurement cycle depends on the distance between the imaging system and the external target.

[0022] During operation of the imaging system, a light emitter emits light, for example, toward a scene containing an external target or object. The emitted light eventually strikes the external target, is reflected or scattered at the target, and then returns to the imaging system, where the detector array eventually detects the return light. Light returning from closer targets in the scene encounters a different modulation phase in the receiver than light returning from targets farther away in the scene. Thus, the distance is encoded in the signal generated by the pixel, or in the signal generated by the image created from the array. The distance to the reflection or scattering point can be calculated from the image, for example, by evaluating the relative intensity based on each pixel or from the entire image. This approach is different from the common concept of time of flight, which relies on measuring the time between the emission and reception of light (e.g., the emission of a given light pulse and the reception of the pulse).

[0023] The imaging system according to the improved concept allows to reduce the need for complex receiver architecture. Instead, the system complexity is reduced by introducing intra-pixel modulation at the receiver. This intra-pixel modulation scheme can be manufactured in a very cost-effective manner in the CMOS image sensor process and does not require a Pockels cell made of lithium niobate type materials or polymer films that exhibit an electro-optical effect. This allows a solid-state LIDAR system to be built without moving parts or very complex or very expensive receivers. The LIDAR system can be compact and cost-effective, while having the resolution of a modern CMOS image sensor (e.g., several M pixels).

[0024] The beam path is simplified, making it possible to integrate a polarization beam splitter (PBS) into the CMOS image sensor. In fact, the complexity of the receiver can be greatly reduced, thereby reducing the need for complex Pockels cells and other types of expensive (lithium niobate) or unreliable (polymer) electro-optic modulators. In addition, the improved concept reduces the complexity of the received beam path by replacing elements such as polarization beam splitters with integrated polarization-sensitive photodiodes or photodetectors with integrated polarizers. The present disclosure incorporates by reference the disclosure of document EP3261130A1, in which polarization-sensitive photodiodes are described.

[0025] In at least one embodiment, at least the detector array and the synchronization circuit are integrated in the same chip. For example, the chip includes a shared integrated circuit that integrates at least the detector array and the synchronization circuit. Typically, the light emitter and the shared integrated circuit are arranged on a shared carrier or substrate and are in electrical contact with each other via the shared carrier or substrate. However, in other embodiments, the light emitter can also be integrated into the shared integrated circuit.

[0026] Integration allows for compact design, thereby reducing board space requirements, enabling low profile system design in space constrained designs. In addition, the complexity of the beam path can be reduced. Implementation on the same chip (e.g. embedded in the same sensor package) allows for imaging systems that can observe the complete field of view (FOV) at once, known as Flash systems. Such imaging systems can emit pulses of light, such as infrared light pulses. For example, a portion of the energy is returned and converted into distance and optionally velocity.

[0027] In at least one embodiment, the detector array includes pixels with polarization functionality. For example, integrated polarization-sensitive photodiodes can be used. The detector array can have on-chip polarizers associated with the pixels, respectively. Such structures can be integrated using CMOS technology. Alternatively, the polarizers can be placed on-chip using air-gap nanowire grids coated with anti-reflective materials that suppress flare and ghosting. This on-chip design reduces polarization crosstalk and improves the extinction ratio. Alternatively, the detector array can be complementary to a polarizer layer that is arranged above the pixels, respectively. The polarizer layer is not necessarily integrated with the detector array. For example, the polarizer layer includes a plurality of polarizers that are arranged in the layer so that they coincide with the corresponding pixels when mounted on top of the detector array. Possible embodiments of a detector array containing pixels with polarization functionality are disclosed in EP 3261130 A1, which is incorporated herein by reference.

[0028] During operation of the imaging system, a scene may be illuminated using a light emitter. Typically, the scene includes a plurality of external targets, each of which may reflect or scatter light. Reflection or scattering of light from a target typically produces polarized light, such as linearly polarized light in a plane perpendicular to the incident light. Polarization may be further affected by the material properties of the external targets, such as their surface characteristics. Polarization is therefore visible in an image captured by the detector array. For example, by illuminating a scene, multiple external targets with different distances are typically captured in an image. However, a given target may exhibit similar or identical polarizations because its properties (such as their surface features) may not change significantly on the illuminated surface. In contrast, other targets at different distances may exhibit different polarizations. Therefore, pixels with polarization functionality may also improve the contrast and signal-to-noise ratio of the resulting image, making it easier to distinguish different external targets in the image.

[0029] Light sources such as VCSELs can produce polarized light. Objects or external targets may change the polarized light or rotate it. Therefore, an array containing pixels with different polarizations can produce more information about the object and can improve the accuracy of distance measurements.

[0030] In at least one embodiment, adjacent pixels have orthogonal polarization functions. Using adjacent pixels with different polarization functions allows detection of polarized light at more linear angles, such as 0° and 90°, or 180° and 270°. For example, this can be achieved by comparing the rise and fall of the intensity transmitted between adjacent pixels. Adjacent pixels can be considered as a group in order to calculate the degree of polarization and / or polarization direction.

[0031] In at least one embodiment, the modulation function is implemented by a modulation element.For example, the modulation element controls the sensitivity and / or responsivity of the pixels of the detector array.

[0032] In at least one embodiment, the modulation element introduces a leakage current that is linear or non-linear with the applied voltage. The voltage can be regulated using known components such as a voltage regulator. This allows for controlled and repeatable leakage current.

[0033] In at least one embodiment, the leakage current flowing through the modulating element has a first value at the beginning of the frame and a second value at the end of the frame, wherein the first value is higher than the second value. Alternatively, the first value is lower than the second value. The rise or fall of the leakage current due to the modulation is encoded in the image acquired by the detector array. For example, the intensity in the image allows distance information to be determined from the resulting image.

[0034] In at least one embodiment, the leakage current flowing through the modulation element decreases monotonically from a first value to a second value during a frame.

[0035] In at least one embodiment, the modulation element is a transistor. Examples involve leakage controlled transistors.

[0036] In at least one embodiment, the emission wavelength of the light emitter is greater than 800nm ​​and less than 10000nm. In at least one embodiment, the emission wavelength of the light emitter is 840nm to 1610nm. Detection in these spectral ranges is in the infrared and results in robust emission and detection. In addition, light in these spectral ranges is substantially invisible and masked to human vision.

[0037] In at least one embodiment, according to the detection method, a scene is illuminated by first and second light pulses. A first image is acquired according to the first light pulse by a detector array having a constant sensitivity. A second image is acquired according to the second light pulse by a detector array having a sensitivity that increases over time.

[0038] In at least one embodiment, the distance of the scene object is determined based on the first and second images. For example, the distance of the scene object is inferred based on the ratio of the second image to the first image.

[0039] In at least one embodiment, the LIDAR image is inferred based on a ratio of the second image to the first image, and distance information for the scene is determined based on the LIDAR image. The distance to one or more objects can be determined based on relative intensities in the LIDAR image. For example, as the sensitivity of the detector is modulated, the distance and relative distance of the objects are encoded in the LIDAR image. For example, objects that are close appear to have different intensities than objects that are far away. These differences may be apparent in the LIDAR image and provide a measure of distance.

[0040] In at least one embodiment, the scene is illuminated by a light source and the first and second light pulses have the same duration and height.

[0041] In at least one embodiment, a vehicle such as an automobile or other motor vehicle includes an imaging system according to the improved concept discussed below. Onboard electronics (e.g., an advanced driver assistance system ADAS) are embedded in the vehicle. The imaging system is arranged to provide output signals to the onboard electronics. Possible applications include automotive (such as autonomous driving, collision avoidance, safety and surveillance), as well as industrial and automation and consumer electronics.

[0042] Other embodiments of the detection method may be readily derived from the various embodiments and examples of the imaging system and the vehicle, and vice versa.

[0043] In the following, the above-mentioned concept is described in more detail with reference to the accompanying drawings showing examples of embodiments. In the embodiments and drawings presented below, similar or identical elements may have the same reference numerals, respectively. However, the elements shown in the figures and their size relationships to each other should not be regarded as true proportions, but individual elements (such as layers, components and regions) may be enlarged to achieve better description or better understanding.

[0044] Figure 1 An example of an imaging system is shown.

[0045] Figure 2 An example embodiment of a detector array with polarization functionality is shown,

[0046] Figure 3 shows a cross section of an example detector array with a high contrast grating polarizer,

[0047] Figure 4 shows an example embodiment of a modulation element,

[0048] Figure 5Another exemplary embodiment of a modulation element is shown,

[0049] Figure 6 Another exemplary embodiment of a modulation element is shown,

[0050] Figure 7 Another exemplary embodiment of a modulation element is shown,

[0051] Figure 8 Another exemplary embodiment of a modulation element is shown,

[0052] Fig. 9 An example embodiment of a detection method is shown,

[0053] Fig.10 An example embodiment of a detection method is shown,

[0054] Fig.11 shows an example timing diagram for a light source,

[0055] Fig.12 An example embodiment of a prior art LIDAR detection method is shown, and

[0056] Fig.13 Another example embodiment of a prior art LIDAR detection method is shown.

[0057] Figure 1 An example imaging system is shown. The imaging system includes a light source LS, a detector array DA and a synchronization circuit SC, which are arranged adjacent to a carrier CA and electrically coupled to the carrier CA. For example, the carrier includes a substrate that provides electrical connection and mechanical support. The detector array and the synchronization circuit are integrated into the same chip CH that constitutes a common integrated circuit. Typically, the light source and the common integrated circuit are arranged on the carrier and are electrically in contact with each other via the carrier. The components of the imaging system are embedded in a sensor package (not shown). Other components such as a processing unit (e.g., a processor or microprocessor) and an ADC that performs the detection method are also arranged in the sensor package and can be integrated into the same integrated circuit.

[0058] The light source LS comprises a light emitter, such as a surface emitting laser, for example a vertical cavity surface emitting laser or VCSEL. The light emitter has one or more characteristic emission wavelengths. For example, the emission wavelength of the light emitter is in the near infrared (NIR) region, for example greater than 800nm ​​and less than 10000nm. LIDAR applications may rely on the emission wavelength range of the light emitter being 840nm to 1610nm, thereby achieving robust emission and detection. This range can be provided by VCSEL. Light sources such as VCSELs can produce polarized light. External targets or objects may change the polarized light or rotate it. Therefore, an array containing pixels with different polarizations can produce more information about an object and improve the accuracy of distance measurements.

[0059] The detector array DA comprises one or more photodetectors or pixels. The array of pixels forms an image sensor. The photodetector may be a photodiode, such as a pinned photodiode, a pin photodiode or other photodiode. The detector array comprises an array of detection elements, such as a photodiode array. The pixels are polarization sensitive. Adjacent pixels of the image sensor are polarization sensitive, each pixel having an orthogonal polarization state arranged in a checkerboard pattern. This will be discussed in more detail below. The synchronization circuit SC is arranged in the same sensor package and is in fact integrated in a common integrated circuit. The synchronization circuit SC is arranged to synchronize the emission of light by means of the light emitter and / or to synchronize the detection by means of the detector array, for example for frames A and B.

[0060] Figure 2 An example embodiment of a detector array with polarization functionality is shown. The detector array DA or image sensor comprises pixels arranged in a pixel map as shown. The image sensor can be characterized in that adjacent pixels have different polarization states. The accompanying drawings show a detector array with on-chip polarizers respectively associated with the pixels. Such structures can be integrated using CMOS technology. Adjacent pixels have orthogonal polarization functionality, for example a pixel PxH with horizontal polarization and a pixel PxV with vertical polarization. An embodiment of the detector array is disclosed in EP 3261130A1, which is incorporated herein by reference.

[0061] Figure 3 A cross section of an example detector array with a high contrast grating polarizer is shown. This example corresponds to EP 3261130 A1 Figure 1 , and is incorporated herein by reference. The remaining embodiments of the detector array in EP 3261130 A1 are not excluded but are incorporated by reference.

[0062] Figure 3 The photodetector arrangement shown in the figure, the detector array comprises a substrate 1 of semiconductor material, which substrate may be, for example, silicon. The photodetectors or pixels of the array are suitable for detecting electromagnetic radiation, in particular light in a specific wavelength range (such as NIR), and are arranged in the substrate 1, for example in a common integrated circuit. The detector array may comprise any conventional photodetector structure, and thus in Figure 3 1 is schematically represented by only the sensor region 2 in the substrate 1. The sensor region 2 may extend continuously as a layer of the substrate 1 or may be divided into a plurality of sections according to a photodetector array.

[0063] The substrate 1 may be conductively doped at least in a region adjacent to the sensor region 2, and the sensor region 2 may be fully doped or doped with opposite type conductivity in a separate section. If the substrate 1 has p-type conductivity, the sensor region 2 has n-type conductivity, and vice versa. Thus, a pn junction 8 or a plurality of pn junctions 8 are formed at the boundary of the sensor region 2 and can be operated as a photodiode or a photodiode array by applying a suitable voltage. This is just an example, and the photodetector array may include different structures.

[0064] A contact region 10 or a plurality of contact regions 10 having a higher electrical conductivity than the adjacent semiconductor material (in particular due to a higher doping concentration) can be arranged in the substrate 1 outside the sensor region 2. A further contact region 20 or a plurality of further contact regions 20 having a higher electrical conductivity than the sensor region 2 can be arranged in the substrate 1 adjacent to the sensor region 2 or a section of the sensor region 2. An electrical contact 11 can be applied to each contact region 10 and a further electrical contact 21 can be applied to each further contact region 20 for external electrical connection.

[0065] An isolation region 3 may be formed above the sensor region 2. The isolation region 3 is transparent or at least partially transparent to the electromagnetic radiation to be detected and has a refractive index for the relevant wavelength of interest. For example, the isolation region 3 comprises a dielectric material such as a field oxide. If the semiconductor material is silicon, the field oxide can be produced at the surface of the substrate 1 by local oxidation of silicon (LOCOS). As the volume of the material increases during the oxidation process, the field oxide protrudes from the plane of the substrate surface, such as Figure 3 shown.

[0066] The grid elements 4 are arranged at a distance d from each other on the surface 13 of the isolation region 3 above the sensor region 2. For example, the grid elements 4 can be arranged directly on the surface 13 of the isolation region 3. The grid elements 4 can have the same width w, and the distance d between any two adjacent grid elements 4 can be the same. The sum of the width w and the distance d is the pitch p, which is the minimum period of the regular lattice formed by the grid elements 4. The length l of the grid elements 4 (the length l is perpendicular to their width w) is in Figure 3 , one of the grid elements 4 is shown in a perspective view, the hidden contours of which are indicated by dashed lines.

[0067] The grid element 4 is transparent or at least partially transparent to the electromagnetic radiation to be detected and has a refractive index for the relevant wavelength. For example, the grid element 4 may include polysilicon, silicon nitride or niobium pentoxide. The advantage of using polysilicon for the grid element 4 is that the grid element 4 can be formed in a CMOS process together with the formation of polysilicon electrodes, etc. The refractive index of the isolation region 3 is lower than the refractive index of the grid element 4. The isolation region 3 is an example of a low refractive index region as described in the claims.

[0068] The grid element 4 is covered by another low refractive index region. Figure 3 In the photodetector device of the embodiment of the present invention, the grid element 4 is covered by a dielectric layer 5, the refractive index of which is lower than the refractive index of the grid element 4. The dielectric layer 5 may include, for example, borophosphosilicate glass (BPSG), or silicon dioxide used to form metal intermetallic dielectric layers for wiring in CMOS processes. Thus, the grid element 4 is embedded in a material with a lower refractive index and forms a high contrast grating polarizer.

[0069] An antireflection coating 7 may be applied on the grid element 4. It may be formed by removing the dielectric layer 5 above the grid element 4, depositing a material suitable for the antireflection coating 7, and filling the openings with the dielectric material of the dielectric layer 5. The antireflection coating 7 may be provided in particular to match the phase of the incident radiation with its propagation constant in the substrate 1. For example, if the substrate 1 comprises silicon, the refractive index of the antireflection coating 7 may be at least approximately the square root of the refractive index of silicon. For example, silicon nitride may be used for the antireflection coating 7.

[0070] The array of grid elements 4 forms a high contrast grating, which is comparable to a resonator including a high quality factor. Figure 3 The high contrast grating constitutes a reflector. Due to the difference between the refractive indices, the optical path length of the incident electromagnetic wave is different in the grid element 4 and in the section of the other region 5, 15 of low refractive index located between the grid elements 4. Therefore, the incident electromagnetic wave reaches the surfaces 13, 16 of the low refractive index regions 3, 6 forming the high contrast grating substrate, with a phase shift between the part passing through the grid element 4 and the part propagating between the grid elements 4. The high contrast grating can be designed to shift the phase of a specific wavelength by π or 180°, so that the parts in question cancel each other out. Therefore, the high contrast grating constitutes a reflector for a specific wavelength and polarization.

[0071] When the vector component of the electric field vector is transverse to the longitudinal extension of the grid element 4, the electromagnetic wave passes through the grid element 4 essentially undisturbed and is absorbed in the underlying substrate 1. Electron-hole pairs are thus generated in the semiconductor material. The charge carriers generated by the incident radiation generate a current, through which the radiation is detected. Optionally, a voltage is applied to the pn junction 8 in the opposite direction.

[0072] The grid elements 4 may comprise a constant width w, and the distance d between adjacent grid elements 4 may also be constant, so that the high contrast grating forms a regular array of dots. The pitch p of such a grating defining the shortest period of the array is the sum of the width w of one grid element 4 and the distance d. For applications of the array of grid elements 4 as a high contrast grating polarizer, the pitch p is typically smaller than the lower refractive index n. low1 The wavelength of the electromagnetic radiation in the material of the region and / or another lower refractive index n low2 The wavelength of the electromagnetic radiation in the region of n is smaller than or even smaller than the wavelength in the grid element 4. low1 In the region, the wavelength λ0 of the electromagnetic radiation to be detected in vacuum becomes λ1=λ0 / n low1 In the other lower refractive index n low2 In the region, the wavelength becomes λ2=λ0 / n low2 If n high is the refractive index of the grid element 4, then the wavelength λ0 in the grid element 4 becomes λ3 = λ0 / n high ,λ3<λ0 / n low1 ,λ3<λ0 / n low2 This magnitude represents the difference between the high contrast grating used as a polarizer in the above-mentioned photodetector device and a conventional diffraction grating.

[0073] The pitch p may be greater than a quarter wavelength of the electromagnetic radiation in the grid element 4. If the wavelength of the electromagnetic radiation to be detected is λ0 in a vacuum, then p>λ3 / 4=λ0 / (4n high ). This distinguishes high contrast gratings used as polarizers in the above detector arrays from deep sub-wavelength gratings. The length l of the grid element 4 is optionally greater than the wavelength λ3=λ0 / n of the electromagnetic radiation in the grid element 4 high .

[0074] Thus, high contrast grating based polarizers mitigate the drawbacks of diffraction gratings' tight manufacturing tolerances and layer thickness control; and the necessity of very small structures and hence very advanced lithography machines for deep sub-wavelength gratings. Detector arrays with high contrast grating polarizers can be used in a wide range of applications. Other advantages include improvements in the extinction coefficient for polarization states to be excluded and enhanced responsivity for desired polarization states.

[0075] Figure 4 An example embodiment of a modulation element is shown. The accompanying drawings show a circuit layout of a pixel architecture or a 4T pixel cell implemented as a detector array DA of an image sensor. The pixel Px is connected as a 4T pixel architecture. The 4T pixel architecture comprises a transfer transistor Tt with a transfer gate TG, a first reset transistor Tr1 connected to a first floating diffusion Fd1 and a source follower Sf, a column select Cs transistor providing an output terminal Out. The pixel Px is also connected to a modulation element ME, which comprises a leakage control element LC, a second reset transistor Tr2 and a second floating diffusion Fd2. Vdd represents a power supply rail.

[0076] The modulation element ME includes a leakage control element LC, which is responsible for rerouting a certain amount of charge per unit time to a location different from the floating diffusion. This rerouting can be controlled using the gate of the leakage control element LC. The floating diffusion of the 4T pixel cell stores the relevant charge information. This charge is intentionally rerouted, thereby reducing the responsivity. In fact, there is a leakage path introduced to the second floating diffusion Fd2, thereby reducing the responsivity of the photodetector Px. For example, Fig.11 As shown, the modulation will be monotonically performed during modulation frame B.

[0077] Figure 5 Another example embodiment of a modulation element is shown. The accompanying drawings show a circuit layout of a pixel architecture implemented as a detector array DA of an image sensor. A 4T pixel cell is Figure 4 The 4T pixel cell of FIG. 1 is a 4T pixel cell of FIG. 1 but modified. The pixel Px is connected to a modulation element ME, which includes a leakage control element LC but does not include a second reset transistor Tr2 or a second floating diffusion Fd2. Instead, the leakage control element LC is connected to a power rail Vdd.

[0078] During the acquisition frame, the leakage control element LC redirects the charge to the power rail (e.g. Vdd). In effect, the power rail introduces a leakage path, which reduces the responsivity of the photodetector. The leakage control can be monotonic, so that the frame starts with a reduced sensitivity and then increases monotonically. This concept enables objects far away from the imaging system to contribute more signal.

[0079] Figure 6 Another exemplary embodiment of a modulation element is shown. Figure 5 However, the leakage control element LC is connected to ground instead of the power rail Vdd.

[0080] In the above embodiments, the leakage control element is optionally linear. This means that the leakage current is proportional to the control voltage applied to the element, for example via its gate. Alternatively, the leakage control element can be non-linear. Thus, the sensitivity of the photodetector during a frame can rise non-linearly or linearly. In both cases, the change in sensitivity during a given frame is monotonic, i.e., monotonically increasing or decreasing.

[0081] Figure 7 Another exemplary embodiment of a modulation element is shown. Figure 4 The circuit layout of the pixel architecture. The 4T pixel unit is Figure 4 4T pixel unit. However, the modulation element ME is modified. The pixel Px is connected to the modulation element ME, which includes a leakage control element LC but does not include a second reset transistor Tr2 or a second floating diffusion Fd2. Instead, the leakage control element LC is connected to the power rail Vdd via a voltage source VS to apply a control voltage Vleak. During the acquisition frame, the voltage between the leakage control element and the power rail (VDD) can vary, for example, linearly or nonlinearly. During this frame, the leakage control element (i.e., MOSFET) operates in weak inversion mode and behaves like a resistor. By varying the voltage Vleak linearly over time, the leakage current Ileak also varies linearly over time. By varying the voltage Vleak nonlinearly over time, the leakage current Ileak also varies linearly over time.

[0082] Figure 8 Another example embodiment of a modulation element is shown. In this embodiment, there is no additional leakage control element, but rather the transfer gate Tg of the transfer transistor Tt of the 4T pixel cell operates as a leakage control element during the acquisition portion of the frame. During a frame, the transfer gate can remain slightly open, thereby introducing a time-dependent leakage path to the first floating diffusion Fd1, which is reset by the reset transistor Tr before the actual charge transfer. During readout, the transfer gate Tg operates conventionally as in a 4T pixel cell, i.e. it isolates the photodetector from the first floating diffusion Fd1 in order not to change, for example, the conversion gain of the readout circuit.

[0083] Fig. 9 An example embodiment of a detection method (e.g., a LIDAR detection method) is shown. A light source (e.g., a VCSEL laser, LED, laser, or flashlight) emits an unmodulated light pulse. The light pulse is characterized in that it is synchronized with a photodetector. After being emitted, the light pulse propagates until it is reflected or scattered at one or more objects in the scene. The reflected or scattered light returns to the imaging system, where the detector array ultimately detects the return light. The reverse propagation path is as follows Fig.10 shown.

[0084] Fig.10 An example embodiment of a detection method (e.g., a LIDAR detection method) is shown. The figure shows the reverse path of a LIDAR system. A light pulse emitted by a light emitter propagates until it is reflected or scattered at one or more objects in the scene. The reflected or scattered light returns to the imaging system, where a detector array ultimately detects the return light. During detection, the reflected or scattered light pulse is modulated within the photodetectors of the detector array. The modulation function is integrated into the pixel. The modulation affects the sensitivity or responsiveness of the pixel. For example, the sensitivity during a frame increases monotonically or decreases monotonically as a function of time. The modulated sensitivity in the figure starts from low sensitivity and increases to high sensitivity.

[0085] Imaging systems or LIDAR systems, which due to their arrangement, e.g. in a compact sensor package which may also include dedicated optics, allow the complete field of view (FOV) to be observed at once, are called flash systems. Flash is generally suitable for short to medium distances (0-100m); and by acquiring the complete scene at once, multiple objects and objects with high relative speeds can also be correctly detected. Synchronization circuits control the delay between light emission and detection time frames, e.g. between pulse emission and detection time frames. For example, depending on the distance or distance range to be detected, a delay between the end of a pulse and the start of detection can be set.

[0086] Detection may involve the following example sequence of operations:

[0087] 1) emit light pulses with constant irradiance,

[0088] 2) Acquire a "constant image", for example as the first image during the first frame,

[0089] 3) emitting another light pulse with constant irradiance,

[0090] 4) acquiring a "modulated image", for example as a second image during a second frame,

[0091] 5) Calculate the "LIDAR image" by dividing the "modulated image" by the "constant image".

[0092] Fig.11An example timing diagram of a light source is shown. The figure shows the leakage voltage, leakage current and sensitivity over time, respectively. During the first integration of the first frame A, a steady-state image or "constant image" is acquired. The purpose is to acquire a steady-state or DC image. Depending on the application, such an image can be grayscale or color. In the second integration of the second frame B, a "modulated image" is acquired. During modulation, a voltage (e.g., a leakage voltage Vleak) is applied to the leakage control element LC as described above. In the second frame (i.e., frame B), the sensitivity of the pixel is modulated, for example, monotonically, i.e., the sensitivity increases or decreases linearly or nonlinearly. The distance image or "LIDAR image" is then synthesized by dividing the modulated image from frame B by the unmodulated image from frame A, or by performing another mathematical calculation. In the "LIDAR image", objects farther away in the scene show different intensities than closer objects.

[0093] In one example, the light source generates a light pulse having a duration less than the duration of frame A and the duration of frame B. For example, the duration of the light pulse may be less than 2ns or less than 1ns or less than 0.5ns. The duration of frame B may be higher than 1ms or higher than 5ms or higher than 10ms or higher than 20ms. The photodetector measures (e.g., integrates) during frame B. The photodetector may detect light during the entire duration of frame B or during most of the duration of frame B. The light source generates a light pulse at the beginning of frame B or shortly after the beginning of frame B. The object reflects the light pulse. The point in time at which the reflected light pulse reaches the photodetector depends on the distance between the object and the photodetector. During frame B, the sensitivity of the photodetector increases over time. In the case of a short distance, when the reflected light pulse hits the photodetector, the sensitivity of the photodetector is low. In the case of a long distance, when the reflected light pulse hits the photodetector, the sensitivity of the photodetector is high. Therefore, the signal value generated by the photodetector at the end of frame B depends on the amount of light in the reflected light pulse and the point in time at which the reflected light pulse reaches the photodetector.

[0094] During frame A and frame A', the photodetector has a constant sensitivity. Therefore, the signal value generated by the photodetector at the end of frame A depends on the amount of light in the reflected light pulse, and is independent of the point in time when the reflected light pulse arrives at the photodetector. The distance of the object to the photodetector can be calculated by using the signal value generated by the photodetector at the end of frame B and the signal value generated by the photodetector at the end of frame A or frame A'.

[0095] The timing discussed above can also be applied to Figure 8In an embodiment, i.e. without an additional leakage control element, the transfer gate Tg of the transfer transistor Tt of the 4T pixel cell operates as a leakage control element in the portion of the frame acquisition. In this case, the leakage voltage corresponds to the control voltage to be applied at the transfer gate Tg of the transfer transistor Tt.

[0096] The pulse emission and the detection by means of the detector, whether with or without modulated sensitivity, for example as frame A or B, are synchronized by means of a synchronization circuit. In practice, the light emitter, the detector array and the synchronization circuit can all be arranged in the same sensor package, wherein at least the detector array and the synchronization circuit are integrated in the same integrated circuit. Further components, such as a microprocessor and an ADC executing the detection method, can also be arranged in the same sensor package and integrated into the same integrated circuit.

[0097] As mentioned above Figures 1 to 11 The embodiments shown in the Figures represent example embodiments of the improved imaging system and detection method, and therefore they do not constitute a complete list of all embodiments according to the improved imaging system and detection method. For example, the actual imaging system and detection method may differ from the embodiments shown in terms of circuit components, shapes, sizes, and materials.

[0098] In this embodiment, the leakage control element comprises a transistor having a relatively long gate length.

[0099] It should be noted that the above examples are not exhaustive. This approach can be applied from 3T pixel cells to high-end HDR 8-transistor pixel arrays. The examples given above are all derived from 4T pixel architectures. In addition, the polarization properties of the photodetector do not need to be implemented in low-cost applications where size and cost are critical.

[0100] Applications of the imaging system may include, for example,

[0101] Automobile: autonomous driving, collision prevention

[0102] Security and surveillance

[0103] Industry and Automation

[0104] Consumer electronics

[0105] This imaging system is used in LIDAR and TOF systems, but not from a point cloud perspective, which enables high-resolution LIDAR systems for a variety of applications, especially automotive, autonomous driving, robotics, drones and industrial applications.

[0106] The techniques described in this disclosure provide advantages over systems that use only a single laser beam deflection of a point cloud of hundreds of pixels.

[0107] The imaging system implements VCSEL power control that can be implemented in CMOS image sensor designs.

[0108] The detection method can be implemented by the above imaging system. The light source can be called a light emitter. The image sensor can be named a detector array.

[0109] Reference numerals

[0110] 1 Substrate

[0111] 2 Sensor area

[0112] 3 Isolation Area

[0113] 4 Grid components

[0114] 5 Dielectric layer

[0115] 7 Anti-reflective coating

[0116] 8pn junction

[0117] 10 Contact area

[0118] 11. Contact

[0119] 13 Surface

[0120] 20 Additional contact areas

[0121] 21 Another Contact

[0122] d Distance

[0123] p Pitch

[0124] w Width

[0125] A, A' frame

[0126] B-frames

[0127] CA carrier, substrate

[0128] CH Chip

[0129] Cs Column Selection

[0130] DA detector array

[0131] Fd1 First floating diffusion

[0132] Fd2 Second floating diffusion

[0133] LC Leakage Control Components

[0134] LE Optical Transmitters

[0135] ME Modulation Components

[0136] Px Pixel

[0137] PxH Pixel with horizontal polarizer

[0138] PxV Pixel with vertical polarizer

[0139] SC Synchronous Circuit

[0140] Sf Source Follower

[0141] Tr1 First reset transistor

[0142] Tr2 Second reset transistor

[0143] Tt transfer transistor

[0144] TG Transmission Gate

[0145] Vdd power rail

Claims

1. An imaging system, comprising: - Light Source (LS), - a detector array (DA) comprising pixels (Px) having a built-in modulation function that varies the sensitivity or responsivity of the pixels during an acquisition frame according to a monotonic function, and - a synchronization circuit (SC) which synchronizes the acquisition performed by the detector array (DA) with the light source (LS), wherein the light source is operable to emit: - a first light pulse to acquire a first image by means of a detector array (DA) with constant sensitivity, and - a second light pulse to acquire a second image by means of a detector array having a sensitivity that increases over time; Therein, the distance of the objects of the scene is inferred based on the ratio of the second image to the first image.

2. The imaging system according to claim 1, wherein - at least said detector array (DA) and said synchronization circuit (SC) are integrated in the same chip (CH), and / or - The imaging comprises a sensor package enclosing a light source (LS) and a detector array (DA) and synchronization circuits (SC) integrated in the same chip (CH).

3. The imaging system according to claim 1 or 2, wherein: The modulation function is implemented by a modulation element (ME).

4. The imaging system according to claim 3, wherein: The modulation element (ME) introduces a leakage current that is linear with the applied voltage.

5. The imaging system according to claim 1 or 2, wherein: The pixel (Px) has a polarization function.

6. The imaging system according to claim 5, wherein: Adjacent pixels (Px) have orthogonal polarization functions.

7. The imaging system according to claim 4, wherein: A leakage current flowing through the modulation element (ME) has a first value at the beginning of a frame and a second value at the end of a frame, wherein the first value is higher than the second value.

8. The imaging system according to claim 4, wherein: During frame B, the leakage current flowing through the modulation element (ME) decreases monotonically from a first value to a second value.

9. The imaging system of claim 4, wherein: The modulation element (ME) is a transistor.

10. The imaging system according to claim 1 or 2, wherein: The emission wavelength of the light source (LS) is greater than 800 nm and less than 10000 nm.

11. The imaging system according to claim 1 or 2, wherein: The emission wavelength of the light source (LS) is 840 nm to 1610 nm.

12. The imaging system of claim 9, wherein: The transistor is a leakage controlled transistor.

13. A vehicle having - an imaging system according to claim 1 or 2, and - an on-board electronic device embedded in said vehicle, wherein: - The imaging system is arranged to provide an output signal to the on-board electronics.

14. A detection method, wherein a scene is illuminated with a light source (LS) emitting the following light pulses: - a first light pulse in order to acquire a first image by means of a detector array (DA) with constant sensitivity, and - a second light pulse in order to acquire a second image by means of a detector array having a sensitivity that increases over time, and wherein, a detector array comprising pixels (Px) having a built-in modulation function that varies the sensitivity or responsivity of the pixels during an acquisition frame according to a monotonic function operable to detect light, and wherein acquisition performed by the detector array (DA) is synchronized with said light source (LS); The distance of the object in the scene is inferred based on the ratio of the second image to the first image.

15. The detection method according to claim 14, wherein: The scene is illuminated by a light source (LS), and the first light pulse and the second light pulse have the same duration and pulse height.

Images