IMAGING SYSTEM AND DETECTION METHOD

Abstract

An imaging system comprises a light emitter, a detector array and a synchronization circuit. The light emitter is arranged to emit light of modulated intensity, wherein the intensity is modulated monotonously during the acquisition of a frame. The synchronization circuit is arranged to synchronize the acquisition with the light emitter.

Claims

1. An imaging system comprising: a light emitter arranged to emit light of modulated intensity, wherein the intensity is modulated monotonously during the acquisition of a frame, a detector array, and a synchronization circuit to synchronize the acquisition with the light emitter.

2. The imaging system according to claim 1, wherein at least the detector array and the synchronization circuit are integrated into a same chip and/or the imaging comprises a sensor package, which encloses the detector array and the synchronization circuit integrated into the same chip as well as the light emitter.

3. The imaging system according to claim 1, further comprising: a modulating circuit arranged to drive emission of light by means of the light emitter, and/or emission of light is pulsed such that, due to modulation, at least some pulses have an intensity profile which is monotonously increasing or monotonously decreasing as a function of time.

4. The imaging system according to claim 1, wherein the synchronization circuit is operable to control a delay between emission of light and a time frame for detection.

5. The imaging system according to claim 4, wherein the emission of light is pulsed and the synchronization circuit sets a delay between an end of a pulse and a beginning of a time frame for detection, respectively.

6. The imaging system according to claim 1, wherein the detector array comprises pixels, which have a polarizing function.

7. The imaging system according to claim 1, wherein adjacent pixels have orthogonal polarization functions.

8. The imaging system according to claim 1, wherein the detector array comprises units of four pixels, and a unit has four different polarization functions, respectively.

9. The imaging system according to claim 1, wherein the emission wavelength of the light emitter is larger than 800 nm and smaller than 10.000 nm, and/or the emission wavelength of the light emitter is in between 840 nm and 1610 nm.

10. The imaging system according to claim 1, wherein the light emitter comprises at least one semiconductor laser diode such as a surface emitting laser, vertical-cavity surface-emitting laser, VCSEL, or edge emitter laser.

11. The imaging system according to claim 3, further comprising a processing unit which is arranged to: control the light emitter via the modulating circuit to emit a first, unmodulated light pulse in order to acquire a first image using the detector array, control the light emitter via the modulating circuit to emit a second, modulated light pulse in order to acquire a second image using the detector array, and the processing unit is further arranged to process the first and second image to calculate a LIDAR image inferred from a ratio of the second image to the first image and to determine a distance of objects in the LIDAR image.

12. A vehicle comprising, an imaging system according to claim 1, and board electronics embedded in the vehicle, wherein: the imaging system is arranged to provide an output signal to the board electronics.

13. A detection method where a scene is illuminated with: a first, unmodulated light pulse in order to acquire a first image, and a second, modulated light pulse in order to acquire a second image.

14. The detection method according to claim 13, wherein a distance of objects of the scene is determined depending on the first and second image.

15. The detection method according to claim 14, wherein a LIDAR image is inferred from a ratio of the second image to the first image and distance information of the scene is determined from the LIDAR image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG. 1 shows an example of the imaging system.

[0057] FIG. 2 shows an example embodiment of a detector array with polarization function,

[0058] FIG. 3 shows a cross section of an example detector array with a high-contrast grating polarizer,

[0059] FIG. 4 shows an example embodiment of a LIDAR detection method,

[0060] FIG. 5 shows an example embodiment of a LIDAR detection method,

[0061] FIG. 6 shows an example timing diagram of the light source,

[0062] FIG. 7 shows an example embodiment of a prior art LIDAR detection method, and

[0063] FIG. 8 shows another example embodiment of a prior art LIDAR detection method.

DETAILED DESCRIPTION

[0064] FIG. 1 shows an example imaging system. The imaging system comprises a light source LS, a detector array DA and a synchronization circuit SC, which are arranged contiguous with and electrically coupled to a carrier CA. For example, the carrier comprises a substrate to provide electrical connectivity and mechanical support. The detector array and the synchronization circuit are integrated into a same chip CH which constitutes a common integrated circuit. Typically, the light source and the common integrated circuit are arranged on and electrically contacted to each other via the carrier. The components of imaging system are embedded in a sensor package (not shown). Further components such as a processing unit, e.g., a processor or microprocessor, to execute the detection method and ADCs, etc. are also arranged in the sensor package and may be integrated into the same integrated circuit.

[0065] The light source LS comprises a light emitter such as a surface emitting laser, such as the vertical-cavity surface-emitting laser, or VCSEL. The light emitter has one or more characteristic emission wavelengths. For example, an emission wavelength of the light emitter lies in the near infrared, NIR, e.g., larger than 800 nm and smaller than 10.000 nm. LIDAR applications may rely on the range of emission wavelength of the light emitter is in between 840 nm and 1610 nm, which results in robust emission and detection. This range can be offered by the VCSEL.

[0066] The detector array DA comprises one or more photodetectors or pixels. The array of pixels forms an imaging sensor. The pixels are polarization sensitive. Adjacent pixels of the imaging sensor are polarization sensitive each having an orthogonal state of polarization arranged in a checker-board pattern. This will be discussed in more detail below.

[0067] The imaging system comprises a modulating circuit (not shown) which is arranged to modulate the intensity of emission by means of the light emitter. In case of a laser, such as the VCSEL, the modulating circuit can be implemented as a laser driver circuit. The modulating circuit may also be integrated in the common integrated circuit arranged in the sensor package. In another embodiment, the laser driver may be located externally with a synchronization in between the laser driver and the sensor ASIC comprising the detector array.

[0068] The synchronization circuit SC is arranged in the same sensor package, and, may be integrated in the common integrated circuit. The synchronization circuit is arranged to synchronize emission of light, e.g., constant pulses and modulated light pulses, by means of the light emitter and/or detection by means of the detector array, e.g., as frames A and B.

[0069] FIG. 2 shows an example embodiment of a detector array with polarization function. The detector array DA, or imaging sensor, comprises pixels, which are arranged in a pixel map as shown. The imaging sensor can be characterized in that adjacent pixels have different states of polarization. The drawings shows a detector array with on-chip polarizers associated with the pixels, respectively. Such a structure may be integrated using CMOS technology. Adjacent pixels have orthogonal polarization functions, e.g., pixels PxH with horizontal polarization and pixels PxV with vertical polarization. Embodiments of said detector array are disclosed in in EP 3261130 A1, which is hereby incorporated by reference.

[0070] FIG. 3 shows a cross section of an example detector array with a high-contrast grating polarizer. This example corresponds to FIG. 1 of EP 3261130 A1 and is cited here for easy reference. The remaining embodiments of detector arrays in EP 3261130 A1 are not excluded but rather incorporated by reference.

[0071] The photodetector device, detector array, shown in FIG. 3 comprises a substrate 1 of semiconductor material, which may be silicon, for instance. The photodetectors, or pixels, of the array are suitable for detecting electromagnetic radiation, especially light within a specified range of wavelengths, such as NIR, and are arranged in the substrate 1, e.g., in the common integrated circuit. The detector array may comprise any conventional photodetector structure and is therefore only schematically represented in FIG. 3 by a sensor region 2 in the substrate 1. The sensor region 2 may extend continuously as a layer of the substrate 1, or it may be divided into sections according to a photodetector array.

[0072] The substrate 1 may be doped for electric conductivity at least in a region adjacent to the sensor region 2, and the sensor region 2 may be doped, either entirely or in separate sections, for the opposite type of electric 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 is formed at the boundary of the sensor region 2 and can be operated as a photodiode or array of photodiodes by applying a suitable voltage. This is only an example, and the photodetector array may comprise different structures.

[0073] A contact region 10 or a plurality of contact regions 10 comprising an electric conductivity that is higher than the conductivity of the adjacent semiconductor material may be provided in the substrate 1 outside the sensor region 2, especially by a higher doping concentration. A further contact region 20 or a plurality of further contact regions 20 comprising an electric conductivity that is higher than the conductivity of the sensor region 2 may be arranged in the substrate 1 contiguous to the sensor region 2 or a section of the sensor region 2. An electric contact 11 can be applied on each contact region 10 and a further electric contact 21 can be applied on each further contact region 20 for external electric connections.

[0074] 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 that is to be detected and has a refractive index for the relevant wavelengths of interest. The isolation region 3 comprises a dielectric material like a field oxide, for instance. 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 oxidation, the field oxide protrudes from the plane of the substrate surface as shown in FIG. 3.

[0075] Grid elements 4 are arranged at a distance d from one another on the surface 13 of the isolation region 3 above the sensor region 2. For example, the grid elements 4 can be arranged immediately on the surface 13 of the isolation region 3. The grid elements 4 may have the same width w, and the distance d may be the same between any two adjacent grid elements 4. The sum of the width w and the distance d is the pitch p, which is a minimal period of the regular lattice formed by the grid elements 4. The length l of the grid elements 4, which is perpendicular to their width w, is indicated in FIG. 3 for one of the grid elements 4 in a perspective view showing the hidden contours by broken lines.

[0076] The grid elements 4 are transparent or at least partially transparent to the electromagnetic radiation that is to be detected and have a refractive index for the relevant wavelengths. The grid elements 4 may comprise polysilicon, silicon nitride or niobium pentoxide, for instance. The use of polysilicon for the grid elements 4 has the advantage that the grid elements 4 can be formed in a CMOS process together with the formation of polysilicon electrodes or the like.

[0077] The refractive index of the isolation region 3 is lower than the refractive index of the grid elements 4. The isolation region 3 is an example of the region of lower refractive index recited in the claims.

[0078] The grid elements 4 are covered by a further region of lower refractive index. In the photodetector device according to FIG. 3, the grid elements 4 are covered by a dielectric layer 5 comprising a refractive index that is lower than the refractive index of the grid elements 4. The dielectric layer 5 may especially comprise borophosphosilicate glass (BPSG), for instance, or silicon dioxide, which is employed in a CMOS process to form intermetal dielectric layers of the wiring. The grid elements 4 are thus embedded in material of lower refractive index and form a high-contrast grating polarizer.

[0079] An antireflective coating 7 may be applied on the grid elements 4. It may be formed by removing the dielectric layer 5 above the grid elements 4, depositing a material that is suitable for the antireflective coating 7, and filling the openings with the dielectric material of the dielectric layer 5. The antireflective coating 7 may especially be provided to match the phase of the incident radiation to its propagation constant in the substrate 1. For example, if the substrate 1 comprises silicon, the refractive index of the antireflective coating 7 may be at least approximately the square root of the refractive index of silicon. Silicon nitride may be used for the antireflective coating 7, for instance.

[0080] The array of grid elements 4 forms a high-contrast grating, which is comparable to a resonator comprising a high quality-factor. For the vector component of the electric field vector that is parallel to the longitudinal extension of the grid elements 4, i.e., perpendicular to the plane of the cross sections shown in FIG. 3, the high-contrast grating constitutes a reflector. Owing to the difference between the refractive indices, the optical path length of an incident electromagnetic wave is different in the grid elements 4 and in the sections of the further region of lower refractive index 5, 15 located between the grid elements 4. Hence an incident electromagnetic wave reaches the surface 13, 16 of the region of lower refractive index 3, 6, which forms the base of the high-contrast grating, with a phase shift between the portions that have passed a grid element 4 and the portions that have propagated between the grid elements 4. The high-contrast grating can be designed to make the phase shift n or 180° for a specified wavelength, so that the portions in question cancel each other. The high-contrast grating thus constitutes a reflector for a specified wavelength and polarization.

[0081] When the vector component of the electric field vector is transverse to the longitudinal extension of the grid elements 4, the electromagnetic wave passes the grid elements 4 essentially undisturbed and is absorbed within the substrate 1 underneath. Thus electron-hole pairs are generated in the semiconductor material. The charge carriers generated by the incident radiation produce an electric current, by which the radiation is detected. Optionally, a voltage is applied to the pn-junction 8 in the reverse direction.

[0082] 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 lattice. The pitch p of such a grating, which defines a shortest period of the lattice, is the sum of the width w of one grid element 4 and the distance d. For the application of the array of grid elements 4 as a high-contrast grating polarizer, the pitch p is typically smaller than the wavelength of the electromagnetic radiation in the material of the region of lower refractive index n.sub.low1 and/or in the further region of lower refractive index n.sub.low2 or even smaller than the wavelength in the grid elements 4. In the region of lower refractive index n.sub.low1 the wavelength λ.sub.0 in vacuum of the electromagnetic radiation to be detected becomes λ.sub.1=λ.sub.0/n.sub.low1. In the further region of lower refractive index n.sub.low2 the wavelength becomes λ.sub.2=λ.sub.0/n.sub.low2. If n.sub.high is the refractive index of the grid elements 4, the wavelength λ.sub.0 becomes λ.sub.3=λ.sub.0/n.sub.high in the grid elements 4, λ.sub.3<λ.sub.0/n.sub.low1, λ.sub.3<λ.sub.0/n.sub.low2 This dimension denotes a difference between the high-contrast grating used as a polarizer in the photodetector device described above and a conventional diffraction grating.

[0083] The pitch p may be larger than a quarter wavelength of the electromagnetic radiation in the grid elements 4. If the wavelength of the electromagnetic radiation to be detected is λ.sub.0 in vacuum, p>λ.sub.3/4=λ.sub.0/(4n.sub.high). This distinguishes the high-contrast grating used as a polarizer in the detector array described above from deep-subwavelength gratings. The length l of the grid elements 4 is optionally larger than the wavelength λ.sub.3=λ.sub.0/n.sub.high of the electromagnetic radiation in the grid elements 4.

[0084] Therefore, the high-contrast grating based polarizer alleviates drawbacks of tight fabrication tolerance and layer thickness control as for diffraction gratings; and the necessity of very small structures and thus very advanced lithograph as for deep sub-wavelength gratings. The detector array with high-contrast grating polarizer can be used for a broad range of applications. Further advantages include an improved extinction coefficient for states of polarization that are to be excluded and an enhanced responsivity for the desired state of polarization.

[0085] FIG. 4 shows an example embodiment of a detection method, e.g. a LIDAR detection method. The light source, e.g., the VCSEL laser, is modulated such that it emits a modulated light pulse. The modulation circuit is operable as a laser driver circuit, i.e., the circuit drives emission of light by means of the VCSEL, for example. Emission of light is pulsed and the pulses are modulated in intensity, i.e., at least some pulses have an intensity profile, which is monotonously increasing or monotonously decreasing as a function of time. The modulated light pulse in the drawing is characterized in that it starts at high intensity. However, the modulation circuit can also drive the VCSEL to emit light pulses with constant irradiance, i.e., a constant intensity profile. After being emitted either light pulse travels until it is reflected or scattered at one or more objects of the scenery.

[0086] The reflected or scattered light returns to the imaging system where the detector array eventually detects the returning light. The backward travelling path is shown in FIG. 5.

[0087] FIG. 5 shows an example embodiment of a detection method, e.g., a LIDAR detection method. The light pulse travels until it is reflected or scattered at one or more objects of the scenery. The reflected or scattered light returns to the imaging system where the detector array eventually detects the returning light. The light pulse keeps its modulation so that the detector array detects a modulated intensity as a function of distance.

[0088] The imaging system, or LiDAR system, due to its arrangement in a compact sensor package, which may also include dedicated optics, and allows for observing a complete field of view (FOV) at once, called a Flash system. Such a Flash system typically works well for short to mid-range (0-100 m); and by capturing a complete scene at once also several objects and objects with high relative speeds can be detected properly. The synchronization circuit controls a delay between emission of light and a time frame for detection, e.g., the delay between emission of the pulses and a time frame for detection. The delay between an end of the pulse and a beginning of detection may be set, e.g., depending on a distance or distance range to be detected.

[0089] The detection may involve this example sequence of operation: [0090] 1) Emission of a light pulse with constant irradiance, [0091] 2) Acquisition of the “constant image”, e.g., as first image during a first frame, [0092] 3) Emission of a modulated light pulse, [0093] 4) Acquisition of the “modulated image”, e.g., as second image during a second frame, [0094] 5) Calculation of the LIDAR image by division of “modulated image” by “constant image”.

[0095] FIG. 6 below shows the timing diagram of the light source. In a first frame A, the light source is operating at a certain level such to illuminate the scene without temporal modulation. The purpose is to acquire a steady-state or dc image. Such image may be grayscale or color depending on the application. In a second frame, frame B, the intensity of the light source is modulated, for example monotonically, e.g., linearly reduced such that items being further away in the scene receive a higher intensity. Close objects receive less intensity.

[0096] Of course, also modulation may proceed in the other direction, but it may be more amenable to work with a higher intensity for objects that a further away to account for the natural reduction of the optical signal strength.

[0097] Emission of pulses and modulated pulses and/or detection by means of the detector (as frames A and B, for example) is synchronized by means of a synchronization circuit. In fact, light emitter, detector array and synchronization circuit may all be arranged in a same sensor package, wherein at least the detector array and synchronization circuit are integrated in the same integrated circuit. Further components such as a microprocessor to execute the detection method and ADCs, etc. may also be arranged in a same sensor package and integrated into the same integrated circuit.

[0098] It should be noted that the examples above are by far non-exhaustive and for a person skilled in the art also similar embodiments could be envisioned. Instead of having monolithically integrated polarizers, the imaging sensor may also have polarizers made from a plastic film. Instead of the proposed single-die approach also several imaging sensors may be employed. Moreover, also a system without polarizers may be contemplated. Possible applications include automotive such as autonomous driving, collision prevention, security and surveillance, industry and automation and consumer electronics.