Imaging System and Detection Method

Abstract

An imaging system includes a light emitter (LE), a detector array (DA), and a synchronization circuit (SC). The light emitter (LE) is arranged to emit light of a modulated intensity, wherein the intensity is monotonically modulated during the acquisition of frames (A, B, A'). The synchronization circuit (SC) is arranged to synchronize the acquisition with the light emitter (LE).

Description

[0001] The present invention relates to the fields of optical detection, ranging, lidar (LIDAR), and systems. In addition, the present invention relates to imaging systems. More specifically, the present invention relates to an imaging system having an integrated LIDAR function for distance measurement. In particular, the present invention relates to a high-resolution imager having distance measurement capabilities.

[0002] The prior art mainly processes optical ranging systems by the so-called time-of-flight (TOF) method, in which a light pulse is emitted and the time taken for the light pulse to propagate is calculated using a time-to-digital converter TDC.

[0003] Current state-of-the-art LIDAR systems can be divided into three categories:

[0004] (1) Scanning LIDAR systems having movable parts and continuous scanning of surfaces. So far, such systems are used, for example, for autonomous driving.

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

[0006] (3) LIDAR systems in which a CMOS imager is used in combination with a receiving modulator. So far, such systems are limited to handheld devices and cannot be miniaturized because discrete optical components are required.

[0007] Both FIG. 7 and FIG. 8 illustrate the working principles of the above prior art methods (1) and (2). Figure 1 A LIDAR system is shown, in which a light source (e.g., a laser) is emitting light pulses towards an object, and the light pulses are reflected at the object. FIG. 8 shows a LIDAR system, in which the back-propagating light pulses are detected by a suitable photodetector. The time difference between the emission of the light pulse and the reception of the light pulse is measured using a time-to-digital converter TDC.

[0008] In both cases, the light pulses are emitted by a light source (usually a laser) and deflected onto several objects, where they are reflected. Then, the back-propagating light pulses are collected by a suitable photodetector (e.g., an avalanche photodetector or SPAD). The time difference between the emission of the light pulse and the reception of the light pulse is measured or counted by a time-to-digital converter TDC. In such a system, usually a single laser beam or a so-called point cloud is emitted into the scene. Therefore, the resolution is limited to only a single or a few hundred pixels.

[0009] In a third method (3), the light pulses are not typically emitted by a single or multiple collimated laser beams. Instead, a light source or laser beam with a certain field of view (FOV) uniformly illuminates the scene in an order of magnitude of a few degrees. After reflection, the light pulses are collected with a CMOS imaging sensor or a sensor array. The image sensor itself is configured such that it has a modulator built on top of it to modulate the intensity or phase of the input signal. First, an unmodulated image Idc is recorded. Then, a modulated image Imod is acquired. The modulation is performed such that the modulator attenuates the signal at the start of the frame. At the end of the frame, the modulator allows full transparency and the signal is no longer attenuated.

[0010] At this point, after two frames are acquired, the distance image is calculated as a function of the modulated and unmodulated images. Although the system can work with a CMOS imaging sensor of more than 1M pixels, the integration of the optical path including a polarization beam splitter and a complex modulator remains a challenge.

[0011] US 8471895 relates to systems and methods for high-resolution three-dimensional imaging. This reference discloses a LIDAR system based on the above method (3). The modulator is integrated on top of the detector. The modulator is made of an electro-optic Pockels element, which is made of a crystal material such as lithium niobate. Although such a system is known to be functional, its cost is very high due to the price of the modulator material. In addition, the integration with a CMOS imaging sensor is challenging.

[0012] US 10218962 relates to systems and methods for high-resolution three-dimensional imaging. This reference describes a LIDAR system similar to the first reference above, except that two sensor arrays are employed instead of a single sensor array with consecutive frames. In addition, the light source can have a diffuser or any other device attached to it to reduce its coherence and thus reduce speckle.

[0013] US 2017 / 0248796 A1 relates to a 3D imaging system. This reference shows the use of orthogonal polarization states on top of adjacent pixels to reduce flicker and suppress other unwanted effects. Similarly, a modulation principle is employed on the receiving side. A polarization beam splitter is required in the optical path, making the system large and bulky.

[0014] The aim is to provide an imaging system and a detection method that allow for easier integration.

[0015] These aims are achieved by the subject matter of the independent claims. Further developments and embodiments are described in the dependent claims.

[0016] It should be understood that, unless otherwise specified, any feature associated with any one embodiment can be used alone, or in combination with other features described herein, or in combination with one or more features of any other embodiment, or in any combination with any other embodiment. In addition, equivalent parts and modifications not described below can also be adopted without departing from the scope of the imaging system and detection method defined by the appended claims.

[0017] The following relates to improved concepts in the fields of optical detection, ranging, lidar (LIDAR), and systems. One aspect lies in the modulation of the light source rather than the receiver. In addition, the beam path is simplified, enabling, for example, the integration of a polarization beam splitter PBS into a CMOS imaging sensor.

[0018] In at least one embodiment, the imaging system includes a light emitter configured to emit light of a modulated intensity. During an acquisition frame, the intensity is monotonically modulated. In addition, the imaging system includes a detector array and a synchronization circuit to synchronize the acquisition with the light emitter.

[0019] 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 surface-emitting lasers, such as vertical-cavity surface-emitting lasers or VCSELs, or edge-emitting lasers. VCSELs combine characteristics such as surface emission, which provides design flexibility in terms of addressable arrays, low threshold current, improved reliability, and the possibility of using wafer-level manufacturing processes. Another option for the light emitter is to use a light-emitting diode (LED). LEDs are also surface-emitting devices, but their limitation is that the radiation is incoherent and has typical Lambertian emission characteristics compared to a directed laser beam. For example, the light emitter includes or is connected to a modulation circuit configured to modulate the intensity of the light emitter. An example of the modulation circuit is a laser driver circuit.

[0020] Typically, a light source or optical transmitter has one or more characteristic emission wavelengths. For example, the emission wavelength of the optical transmitter is in the near-infrared (NIR). For example, other wavelengths can include visible light or ultraviolet light, infrared (IR) or far-infrared (FIR). In one embodiment, the wavelength of the optical transmitter can be a visible light wavelength. In another embodiment, the light emission can be in the infrared wavelength range invisible to the human eye. In a preferred embodiment, the emission wavelength can be 850 nm or 940 nm. The narrow wavelength bandwidth (especially in the case of over-temperature) allows for more effective filtering at the receiver end (e.g., at the detector array), thereby improving the signal-to-noise ratio (SNR). For example, VCSEL lasers support this. This type of laser allows for the emission of a vertically cylindrical beam, which makes it simpler to integrate into an imaging system. The use of one or more diffusers can make the emission of the laser uniform to irradiate the scene evenly.

[0021] The detector array includes one or more photodetectors, which are hereinafter referred to as pixels. Typically, the pixels are implemented as solid-state or semiconductor photodetectors arranged or integrated into the array. Examples include CCD or CMOS imaging sensors, single-photon avalanche diode arrays, SPADs or other types of avalanche diodes, APDs. These types of photodetectors are sensitive to light (such as NIR, VIS, and UV), which facilitates the use of narrow pulse widths in the emission by means of the optical transmitter.

[0022] The synchronization circuit is arranged to synchronize the emission of light by means of the optical transmitter 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 time frame for detection and the pulse emission. For example, the delay between the end of the pulse and the start of detection can be set according to the distance or distance range to be detected. The time required to complete one distance measurement cycle depends on the distance between the imaging system and the external target.

[0023] The detector array is arranged to acquire one or more consecutive images (hereinafter referred to as frames). To acquire a frame, the detector array can integrate the incident light using the pixels of the array during the duration of the exposure time. The acquisition of consecutive frames can be performed at a frame rate (the frame rate is expressed, for example, in frames per second). In this context, the term "modulation" refers to the change in light intensity defined during the acquisition of a reference frame. In other words, the optical transmitter rather than the detector is modulated, and light is emitted with a modulated intensity. This modulation is monotonic in the mathematical sense, i.e., the change in light intensity can be described by a monotonic function (e.g., a monotonic function of time). A function is called monotonic if and only if it does not increase at all or does not decrease at all. Usually, the modulation can increase or decrease monotonically. The duration of a single frame sets the time of the modulation intensity. The modulation can also be related to a single pulse as a reference, respectively.

[0024] During operation, a light emitter emits light, e.g., towards an external target. The emitted light ultimately hits the external target, is reflected or scattered at the target, and returns to the imaging system, where a detector array finally detects the returned light. Light emitted towards a nearer target in the scene may have a different modulation phase when hitting the nearer target compared to light illuminating a more distant target in the scene. Thus, distance is encoded in the signal generated by a pixel, or by the image created from the array. The distance to the point of reflection or scattering can be calculated based on the image, e.g., by based on the relative intensity of each pixel or based on the entire image.

[0025] This method is different from the common time-of-flight concept, which relies on measuring the time between the sent light and the received light (e.g., emitting a given light pulse and receiving that pulse).

[0026] Most state-of-the-art systems utilize emitting a single or multiple light pulses (i.e., laser beams) towards the target. This limits the number of pixels or points that can be acquired. This is especially true when comparing such TOF applications with modern state-of-the-art CMOS imaging sensors. Only a few prior art examples make full use of the pixel count of CMOS imaging sensors. At this time, these prior arts in turn have the following problems: The receivers are complex and expensive, mainly because the beam path includes a polarization beam splitter, two imaging sensors that are partially parallel, and an expensive lithium niobate modulator or an unreliable polymer modulator; both build a Pockels cell.

[0027] The improved concept reduces the need for a complex receiver architecture. Instead, the system complexity of the linear system is partially transferred from the receiver to the emitter, where modulated illumination of the scene as well as DC illumination of the scene are generated. The improved concept enables high-resolution LIDAR systems to be applicable to various applications, especially automotive and autonomous driving. The proposed technology provides an imaging system that only scales to a single laser beam deflection of a few hundred pixel point clouds.

[0028] In at least one embodiment, at least a detector array and a synchronization circuit are integrated into the same chip. For example, the chip includes a general integrated circuit, and at least the detector array and the synchronization circuit are integrated into the general integrated circuit. Typically, an optical transmitter and the general integrated circuit are disposed on a shared carrier or substrate and are in electrical contact with each other via the shared carrier or the substrate. However, in other embodiments, the optical transmitter can also be integrated into the general integrated circuit. Integration allows for a compact design, reduces the circuit board space requirements, and enables a lightweight system design in a limited space design. In addition, the complexity of the light beam path can be reduced. Implementing on the same chip (e.g., embedded in the same sensor package) allows for an imaging system called a flash system, which is capable of observing the entire field of view (FOV) in one go. Such an imaging system can emit short light pulses, for example, in the near-infrared (NIR) wavelength region. For example, a portion of the energy is returned and converted, optionally in intensity and ultimately in velocity, over a distance. By sampling multiple times, noise (detected light that is not a reflection of the emitted pulse) can be filtered out.

[0029] In at least one embodiment, the imaging system includes a modulation circuit. The modulation circuit is arranged to drive the emission of light by means of an optical transmitter. For example, the modulation circuit is configured as a laser driver and is operable to modulate the intensity of the optical transmitter. The emission of light can be pulsed such that due to the modulation, at least some of the pulses have an intensity profile that monotonically increases or decreases according to time (e.g., the duration of a given pulse). The modulation circuit can also be integrated into the same integrated circuit or chip. Due to the modulation, at least some of the pulses have an intensity profile that monotonically increases or decreases according to time.

[0030] For example, under the control of the modulation circuit or laser driver, the optical transmitter emits light pulses instead of a continuous wave. The modulation circuit can drive the optical transmitter to emit pulses of a constant (e.g., constant over time) intensity. Optionally, the modulation circuit can drive the optical transmitter to emit pulses with a modulated intensity. The intensity of a given pulse either monotonically increases due to the modulation. For a pulse intensity to be considered to monotonically increase according to time, the intensity of the pulse does not necessarily have to increase, it just must not decrease. However, the intensity of a given pulse can also monotonically decrease during the modulation. For a pulse intensity to be considered to monotonically decrease according to time, the intensity of the pulse does not necessarily have to decrease, but must not increase.

[0031] In at least one embodiment, the synchronization circuit is operable to control the delay between the time frame for detection and the light emission. The synchronization can control the delay between the time frame for detection and the pulse emission. For example, the delay between the end of the pulse and the start of the detection can be set according to the distance or range of distances to be detected.

[0032] In at least one embodiment, the detector array includes pixels having a polarization function. For example, an integrated polarization-sensitive photodiode can be used. A possible sensor is disclosed in EP 3261130 A1, which is incorporated herein by reference.

[0033] For example, the detector array can have on-chip polarizers respectively associated with the pixels. Such a structure can be integrated using CMOS technology. Alternatively, the polarizer can be placed on-chip using an air-gap nanowire grating coated with an anti-reflection material that suppresses scintillation and ghosting. Such an on-chip design reduces polarization crosstalk and improves the extinction ratio. Alternatively, the detector array can be supplemented with a layer of polarizers respectively disposed above the pixels. The layer of polarizers is not necessarily integrated with the detector array. For example, the layer of polarizers includes a plurality of polarizers disposed in the layer such that they respectively coincide with the pixels when mounted on top of the detector array.

[0034] The polarizer can be implemented by an 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, where the rest of the back-end lines are transparent. However, polarizers including semiconductor or dielectric materials can also be envisioned. Additionally, a high-contrast grating can be integrated in the CMOS process using, for example, polysilicon or other gate materials.

[0035] During the operation of the imaging system, a light emitter can be used to irradiate the scene. Typically, the scene includes multiple external targets, all of which can reflect or scatter light. The reflection or scattering of light by the targets usually produces polarized light, such as linearly polarized light in a plane perpendicular to the incident light. Polarization can also be affected by the material properties of the external targets (e.g., their surface properties). Thus, polarization is visible in the images acquired by the detector array. For example, by irradiating the scene, multiple external targets at different distances are typically acquired in the image. However, a given target may exhibit polarization similar or identical to its characteristics, e.g., its surface features may not change significantly on the irradiated surface. In contrast, other targets at different distances may exhibit different polarizations. Therefore, pixels having a polarization function can further improve the contrast and signal-to-noise ratio of the resulting image, making it easier to distinguish different external targets in the image.

[0036] In at least one embodiment, adjacent pixels have orthogonal polarization functions. Using adjacent pixels with different polarization functions allows for the detection of more linear angles of polarized light, such as 0° and 90° or 180° and 270°. For example, this can be achieved by comparing the rise and fall of the intensities transmitted between adjacent pixels.

[0037] In at least one embodiment, the units of four pixels each have four different polarization functions. For example, the units are supplemented with polarizers associated with pixels at four different angles, such as 0°, 45°, 90°, and 135°. Each unit of four pixels can be combined into a computing unit such that four sensor signals can be collected for a given computing unit. The sensor signals from the computing unit are correlated via differently oriented polarizers and allow the degree of polarization and the polarization direction to be calculated.

[0038] In at least one embodiment, the emission wavelength of the light emitter is greater than 800 nm and less than 10000 nm. In at least one embodiment, the emission wavelength of the light emitter is from 840 nm to 1610 nm. Detection in these spectral ranges is substantially infrared and results in robust emission and detection. In addition, the light in these spectral ranges is substantially invisible and is masked by human vision.

[0039] In at least some embodiments, the imaging system can be used to perform a LIDAR detection method, for example, based on the concepts discussed in US8471895.

[0040] In at least one embodiment, the imaging system further includes a processing unit, such as a microcontroller or a processor. The processing unit is arranged to control the light emitter via a modulation circuit, for example, to emit a first unmodulated light pulse in order to acquire a first image using the detector array. In addition, the light emitter is controlled via the modulation circuit to emit a second modulated light pulse in order to acquire a second image using the detector array. The processing unit is also set to process the first image and the second image to calculate a LIDAR image inferred from the ratio of the second image to the first image, and to determine the distance of an object in the LIDAR image.

[0041] Detection can involve this example sequence of operations.

[0042] 1) Emit a light pulse with a constant irradiance,

[0043] 2) Acquire a "constant image" as the first image, for example, during a first frame,

[0044] 3) Emit a modulated light pulse,

[0045] 4) Acquire a "modulated image" as the second image, for example, during a second frame,

[0046] 5) Calculate the LIDAR image by dividing the "modulated image" by the "constant image".

[0047] In at least one embodiment, the detection method irradiates a scene with a first light pulse and a second light pulse. The first light pulse is unmodulated to acquire a first image. The second light pulse is modulated to acquire a second image. For example, during acquisition of a frame, the intensity is monotonically modulated. The method can be performed using the imaging system described above.

[0048] In at least one embodiment, the order of the images can also be reversed. That is, the modulated image is acquired first, and then the unmodulated image is acquired.

[0049] In at least one embodiment, the distance of an object in the scene is determined based on the first image and the second image.

[0050] In at least one embodiment, a LIDAR image is inferred based on the ratio of the second image to the first image, and distance information of the scene is determined based on the LIDAR image.

[0051] The distance to one or more objects can be confirmed based on the relative intensity of the LIDAR image. For example, as the intensity of the light source linearly decreases, such that more distant items in the scene receive a higher intensity. Nearby objects receive a smaller intensity. These differences can be evident in the LIDAR image and provide a measurement of distance. No modulation of the detector is required.

[0052] In at least one embodiment, a vehicle (such as an automobile or other motor vehicle) includes an imaging system according to the improved concepts discussed below. An in-vehicle electronic device (such as an advanced driver assistance system ADAS) is embedded in the vehicle. The imaging system is configured to provide an output signal to the in-vehicle electronic device. Possible applications include automobiles (such as autonomous driving, collision avoidance, safety, and monitoring), as well as industrial, automation, and consumer electronics.

[0053] Further embodiments of the detection method can be readily obtained from the various embodiments and implementations of the imaging system and the vehicle, and vice versa.

[0054] The concepts presented above are described in more detail below with reference to the accompanying drawings that illustrate examples of the embodiments. In the embodiments and drawings presented below, similar or identical elements are provided with the same reference numerals. However, the elements shown in the drawings and their size relationships to each other should not be considered true scale; rather, individual elements (such as layers, components, and regions) may be enlarged for better illustration or better understanding.

[0055] Figure 1 An example of the imaging system is shown

[0056] Figure 2 An example embodiment of a detector array having a polarization function is shown

[0057] Figure 3 shows a cross-section of an example detector array with a high-contrast grating polarizer,

[0058] Figure 4 shows an example embodiment of a LIDAR detection method,

[0059] Figure 5 shows an example embodiment of a LIDAR detection method,

[0060] Figure 6 shows an example timing diagram of a light source,

[0061] Figure 7 shows an example embodiment of a prior art LIDAR detection method, and

[0062] Figure 8 shows another example embodiment of a prior art LIDAR detection method.

[0063] Figure 1 shows an example imaging system. The imaging system includes a light source LS, a detector array DA, and a synchronization circuit SC, which are disposed adjacent to and electrically coupled to a carrier CA. For example, the carrier includes a substrate for providing electrical connection and mechanical support. The detector array and the synchronization circuit are integrated in the same chip CH to form a general integrated circuit. Typically, the light source and the general integrated circuit are disposed on the carrier and are in electrical contact with each other via the carrier. The components of the imaging system are embedded in a sensor package (not shown). Additional components (such as a processing unit (e.g., a processor or a microprocessor) and an ADC that perform the detection method) are also disposed in the sensor package and may be integrated into the same integrated circuit.

[0064] The light source LS includes a light emitter, such as a surface-emitting laser, such as a vertical-cavity surface-emitting laser or a 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, e.g., greater than 800 nm and less than 10000 nm. LIDAR applications can rely on the emission wavelength range of the light emitter from 840 nm to 1610 nm, which results in robust emission and detection. This range can be provided by a VCSEL.

[0065] The detector array DA includes one or more photodetectors or pixels. The pixel array forms an imaging sensor. The pixels are polarization-sensitive. Adjacent pixels of the imaging sensor are polarization-sensitive, and each of the adjacent pixels has an orthogonal polarization state arranged in a checkerboard pattern. This will be discussed in more detail below.

[0066] The imaging system includes a modulation circuit (not shown) configured to modulate the emission intensity by means of an optical emitter. In the case of a laser (such as a VCSEL), the modulation circuit can be implemented as a laser driver circuit. The modulation circuit can also be integrated in a general integrated circuit provided in a sensor package. In another embodiment, the laser driver can be located externally, where synchronization occurs between the laser driver and a sensor ASIC including a detector array.

[0067] A synchronization circuit SC is provided in the same sensor package and can be integrated in the general integrated circuit. The synchronization circuit is arranged to synchronize the emission of light (such as constant pulses and modulated light pulses) by means of the optical emitter and / or by means of the detector array (e.g., as frame A and frame B).

[0068] Figure 2 An example embodiment of a detector array with a polarization function is shown. The detector array DA or imaging sensor includes pixels arranged in a pixel map as shown. The imaging sensor can be characterized in that adjacent pixels have different polarization states. The accompanying drawings respectively show a detector array with on-chip polarizers associated with the pixels. Such a structure can be integrated using CMOS technology. Adjacent pixels have orthogonal polarization functions, 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 3261130 A1, which is incorporated herein by reference.

[0069] Figure 3 A cross-section of an example detector array with a high-contrast grating polarizer is shown. This example corresponds to that of EP 3261130 A1 Figure 1 and is hereby incorporated by reference for convenience. The remaining embodiments of the detector array in EP 3261130 A1 are not excluded but are incorporated by reference.

[0070] Figure 3 The photodetectors, the detector array shown in, includes a substrate 1 of semiconductor material (such as it can be silicon). The photodetectors or pixels of the array are adapted to detect electromagnetic radiation, particularly to detect light in a specific wavelength range (such as NIR), and are provided in the substrate 1, for example, in a general integrated circuit. The detector array can include any conventional photodetector structure and is therefore only schematically represented by a sensor region 2 in the substrate 1 in Figure 3 . The sensor region 2 can extend continuously as a layer of the substrate 1 or can be divided into multiple parts according to the photodetector array.

[0071] The substrate 1 can be doped at least in the region adjacent to the sensor region 2 to obtain conductivity, and the sensor region 2 can be doped completely or in separate parts to obtain the opposite type of conductivity. 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 an appropriate voltage. This is just an example, and the photodetector array can include different structures.

[0072] In the substrate 1 outside the sensor region 2, contact regions 10 or a plurality of contact regions 10 having a conductivity higher than that of the adjacent semiconductor material can be provided, in particular by a higher doping concentration. Additional contact regions 20 or a plurality of additional contact regions 20 having a conductivity higher than that of the sensor region 2 can be provided in the substrate 1 adjacent to the sensor region 2 or a part of the sensor region 2. An electrical contact 11 can be applied to each contact region 10 and an additional electrical contact 21 can be applied to each additional contact region 20 for external electrical connection.

[0073] An insulating region 3 can be formed above the sensor region 2. The insulating 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 insulating region 3 includes a dielectric material (such as a field oxide). If the semiconductor material is silicon, a field oxide can be produced at the surface of the substrate 1 by local oxidation of silicon (LOCOS). As Figure 3 shown, as the volume of the material increases during oxidation, the field oxide protrudes from the plane of the substrate surface.

[0074] On the surface 13 of the insulating region 3 above the sensor region 2, grid elements 4 are arranged at a distance d from each other. For example, the grid elements 4 can be directly arranged on the surface 13 of the insulating 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 grid formed by the grid elements 4. The length 1 of the grid element 4 is perpendicular to its width w, as Figure 3 shown, where one of the grid elements 4 is shown with a hidden contour in dashed lines in the perspective view.

[0075] The grid elements 4 are transparent or at least partially transparent to the electromagnetic radiation to be detected, and have a refractive index for the relevant wavelength. For example, the grid elements 4 can include polysilicon, silicon nitride or niobium pentoxide. Using polysilicon for the grid elements 4 has the advantage that the grid elements 4 can be formed together with the formation of polysilicon electrodes, etc. in a CMOS process.

[0076] The refractive index of the insulating region 3 is lower than the refractive index of the grid element 4. The insulating region 3 is an example of a lower refractive index region recited in the claims.

[0077] The grid element 4 is covered by another region of lower refractive index. Figure 3 In the photodetector device of the present invention, the grid element 4 is covered by a dielectric layer 5, which comprises a refractive index lower than the refractive index of the grid element 4. In particular, the dielectric layer 5 can include, for example, borophosphosilicate glass (BPSG) or silicon dioxide, which is used to form a metal inter-dielectric layer for wiring in a CMOS process. The grid element 4 is thus embedded in a material of a lower refractive index and forms a high contrast grating polarizer.

[0078] An antireflection coating 7 may be applied to the grid element 4. The antireflection coating 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. In particular, the antireflection coating 7 may be provided 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.

[0079] The array of grid elements 4 forms a high contrast grating which can be compared to a resonator including a high quality factor. Figure 3 The high contrast grating constitutes a reflector by taking into account the vector components of the electric field vector of the grid element 4 and the plane of the cross section shown. 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 part of the further lower refractive index regions 5, 15 located between the grid elements 4. Therefore, the incident electromagnetic wave reaches the surfaces 13, 16 of the lower refractive index regions 3, 6, which form the bottom of the high contrast grating, wherein there is a phase shift between the part that has passed through the grid element 4 and the part that has propagated between the grid elements 4. The high contrast grating can be designed to phase shift 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.

[0080] 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.

[0081] The grating element 4 may include a constant width w, and the distance d between adjacent grating elements 4 may also be constant, such that a high-contrast grating forms a regular grid. The pitch p of such a grating, which defines the shortest period of the grid, is the sum of the width w of one grating element 4 and the distance d. For the application of the grating element array 4 as a high-contrast grating polarizer, the pitch p is generally less than the wavelength of the electromagnetic radiation in the material of the lower refractive index n low1 and / or less than the wavelength of the electromagnetic radiation in the material of another lower refractive index n low2 region or even less than the wavelength in the grating element 4. In the lower refractive index n low1 region, the wavelength λ0 of the electromagnetic radiation to be detected in vacuum becomes λ1 = λ0 / n low1 . In another region of the lower refractive index n low2 , the wavelength becomes λ2 = λ0 / n low2 . If n high is the refractive index of the grating element 4, then in the grating element 4, the wavelength λ0 becomes λ3 = λ0 / n high , λ3 < λ0 / n low1 , λ3 < λ0 / n low2 . This dimension represents the difference between the high-contrast grating used as a polarizer in the above photodetector device and a conventional diffraction grating.

[0082] The pitch p may be greater than one-quarter of the wavelength of the electromagnetic radiation in the grating element 4. If the wavelength of the electromagnetic radiation to be detected in vacuum is λ0, then p > λ3 / 4 = λ0 / (4n high ). This differentiates the high-contrast grating used as a polarizer in the above detector array from a deep sub-wavelength grating. The length l of the grating element 4 is optionally greater than the wavelength λ3 = λ0 / n of the electromagnetic radiation in the grating element 4 high .

[0083] Thus, the polarizer based on the high-contrast grating alleviates the disadvantages of the strict manufacturing tolerances and layer thickness control of the diffraction grating; for a deep sub-wavelength grating, very small structures are required, and thus very advanced lithography techniques are needed. The detector array with the high-contrast grating polarizer can be used for a wide range of applications. Other advantages include: an improved extinction coefficient for the polarization state to be excluded and an enhanced responsivity for the desired polarization state.

[0084] Figure 4An example embodiment of a detection method (e.g., a LIDAR detection method) is shown. A light source (e.g., a VCSEL laser) is modulated such that the light source emits modulated light pulses. For example, a modulation circuit may operate as a laser driver circuit, i.e., the circuit drives light emission by means of a VCSEL. The light emission is pulsed and the intensity of the pulses is modulated, i.e., at least some of the pulses have an intensity profile that monotonically increases or decreases over time. The modulated light pulses in the drawings are characterized by starting with a high intensity. However, the modulation circuit is also capable of driving the VCSEL to emit light pulses with a constant irradiance (i.e., a constant intensity profile). After being emitted, any light pulse propagates until it is reflected or scattered by one or more objects in the scene. The reflected or scattered light returns to the imaging system, where a detector array ultimately detects the returned light. Figure 5 The reverse propagation path is shown in.

[0085] Figure 5 An example embodiment of a detection method (e.g., a LIDAR detection method) is shown. A light pulse propagates until it is reflected or scattered by one or more objects in the scene. The reflected or scattered light returns to the imaging system, where a detector array ultimately detects the returned light. The light pulse maintains its modulation such that the detector array detects the modulated intensity that varies with distance.

[0086] The imaging system or LiDAR system, due to its being set in a compact sensor package, may also include dedicated optics and allows for a one-time observation of the entire field of view (FOV), and thus is referred to as a flash system. Such a flash system is generally applicable to medium to short ranges (0 - 100 meters); and by capturing the entire scene at once, it is also able to correctly detect multiple objects and objects with a high relative speed. A synchronization circuit controls the delay between the time frame for detection and the emission of light, e.g., the delay between the time frame for detection and the pulse emission. The delay between the end of the pulse and the start of detection can be set, for example, according to the distance or distance range to be detected.

[0087] The detection may involve the following example operation sequence:

[0088] 1) Emitting a light pulse with a constant irradiance,

[0089] 2) Acquiring a "constant image" as the first image, for example, during the first frame,

[0090] 3) Emitting a modulated light pulse,

[0091] 4) Acquiring a "modulated image" as the second image, for example, during the second frame,

[0092] 5) Calculating the LIDAR image by dividing the "modulated image" by the "constant image".

[0093] The following Figure 6 shows a timing diagram of the light source.

[0094] In the first frame A, the light source operates at a certain level to illuminate the scene without time modulation. 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 frame (i.e., frame B), the intensity of the light source is modulated, for example, monotonically, such as linearly decreased, so that the farther objects in the scene receive a higher intensity. The closer objects receive a smaller intensity.

[0095] Of course, the modulation can also be carried out in the other direction, but for the farther objects, considering the natural decrease of the optical signal intensity, it may be more suitable to operate at a higher intensity.

[0096] The emission of the pulses and the modulated pulses and / or the detection by means of the detector (e.g., frame A and frame B) are synchronized by means of a synchronization circuit. In fact, the light emitter, the detector array, and the synchronization circuit can all be arranged in the same sensor package, where at least the detector array and the synchronization circuit are integrated in the same integrated circuit. Additional components (such as a microprocessor and an ADC that perform the detection method) can also be arranged in the same sensor package and integrated into the same integrated circuit.

[0097] It should be noted that so far, the above examples are not exhaustive, and for those skilled in the art, similar embodiments can also be envisioned. In addition to having a monolithic integrated polarizer, the imaging sensor can also have a polarizer made of a plastic film. In addition to the proposed single-die method, multiple imaging sensors can also be employed. Furthermore, a system without a polarizer can also be envisioned. Possible applications include automotive (such as autonomous driving, collision prevention, safety, and monitoring), industrial and automation, and consumer electronics.

[0098] Description of the reference numerals

[0099] 1 Substrate

[0100] 2 Sensor area

[0101] 3 Insulating area

[0102] 4 Grid element

[0103] 5 Dielectric layer

[0104] 7 Anti-reflection coating

[0105] 8 pn junction

[0106] 10 Contact area

[0107] 11 Contact

[0108] 13 surfaces

[0109] 20 additional contact areas

[0110] 21 additional contacts

[0111] d distance

[0112] p pitch

[0113] w width

[0114] A, A' frames

[0115] B frame

[0116] CA carrier, substrate

[0117] CH chip

[0118] DA detector array

[0119] LE light emitter

[0120] PxH pixel with horizontal polarizer

[0121] PxV pixel with vertical polarizer

[0122] SC synchronization circuit

Claims

1. An imaging system, comprising: - A light emitter (LE) arranged to emit light of a modulated intensity, wherein the intensity is monotonically modulated during the acquisition of frames (A, B, A'), - A detector array (DA), and - A synchronization circuit (SC) for synchronizing the acquisition with the light emitter (LE), the synchronization circuit (SC) being operable to control the delay between the emission of light and the time frame for detection, - A modulation circuit arranged to drive the emission of light by means of the light emitter (LE), and / or - The emission of light is pulsed such that, due to the modulation, at least some pulses have an intensity profile that monotonically increases or decreases over time, - A processing unit, the processing unit being configured to: - Control the light emitter (LE) via the modulation circuit to emit a first unmodulated light pulse in order to acquire a first image using the detector array (DA), - Control the light emitter (LE) via the modulation circuit to emit a second modulated light pulse in order to acquire a second image using the detector array (DA), wherein the light pulse maintains its modulation such that the detector array detects the modulated intensity varying with distance, and the processing unit is further arranged to - Process the first image and the second image to calculate a LIDAR image inferred from the ratio of the second image to the first image, and determine the distance of an object in the LIDAR image.

2. The imaging system according to claim 1, wherein - At least the detector array (DA) and the synchronization circuit (SC) are integrated into the same chip (CH), and / or - The imaging includes a sensor package that encloses the detector array (DA), the synchronization circuit (SC), and the light emitter (LE) integrated into the same chip (CH).

3. The imaging system according to claim 1, wherein The emission of light is pulsed, and the synchronization circuit (SC) sets a delay between the end of the pulse and the start of the time frame for detection, respectively.

4. The imaging system according to one of claims 1 to 2, wherein, The detector array (DA) includes pixels having a polarization function.

5. The imaging system according to claim 4, wherein Adjacent pixels have orthogonal polarization functions.

6. The imaging system according to any one of claims 1 to 2, wherein - The detector array (DA) includes units of four pixels, and - The units have four different polarization functions, respectively.

7. The imaging system according to any one of claims 1 to 2, wherein - The emission wavelength of the light emitter (LE) is greater than 800 nm and less than 10,000 nm, and / or - The emission wavelength of the light emitter (LE) is from 840 nm to 1610 nm.

8. The imaging system according to one of claims 1 to 2, wherein, The light emitter (LE) includes at least one semiconductor laser diode.

9. The imaging system according to one of claims 1 to 2, wherein, The light emitter (LE) includes a surface-emitting laser, a vertical-cavity surface-emitting laser (VCSEL), or an edge-emitting laser.

10. A vehicle, comprising - The imaging system according to any one of claims 1 to 9 and - In-vehicle electronics embedded in the vehicle, wherein: - The imaging system is configured to provide an output signal to the in-vehicle electronics.

11. A detection method, wherein the following light pulses are used to irradiate a scene: - irradiating with a first unmodulated optical pulse to acquire a first image, and - irradiating with a second modulated optical pulse to acquire a second image, wherein the optical pulse maintains its modulation such that a detector array detects the modulated intensity varying with distance, Among them, determining the distance of an object in the scene based on the first image and the second image, wherein a LIDAR image is inferred from the ratio of the second image to the first image, and distance information of the scene is determined from the LIDAR image.

Images