Method and system for remote optical detection

Abstract

Apparatus for detecting optical radiation from an environment, comprises a plurality of detection units. Each detection unit comprises an optical sensor configured to generate an electrical signal in response to optical radiation incident thereon, and a hollow and opaque tube coupled with the optical sensor. Each tube is configured to limit a respective individual field-of-view of a respective sensor to a different sector of the environment. The apparatus also comprises a controller coupled with each optical sensor for receiving a respective electrical signal therefrom.

Description

[0001] METHOD AND SYSTEM FOR REMOTE OPTICAL DETECTION

[0002] RELATED APPLICATION

[0003] This application claims the benefit of priority of Israel Patent Application No. 319192 filed on February 24, 2025, the contents of which are incorporated herein by reference in their entirety.

[0004] FIELD AND BACKGROUND OF THE INVENTION

[0005] The present invention, in some embodiments thereof, relates to remote detection and, more particularly, but not exclusively, to a method and system for remote optical detection.

[0006] Light detection and ranging (LIDAR) is a remote sensing technology that measures distances to targets and creates detailed spatial representations of environments. A LIDAR system consists of a light source, typically a laser that emits light at specific wavelengths in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum, and an optical receiver that detects the backscattered light from targets. The system operates by transmitting laser light toward objects, which scatter the light back to the receiver for analysis.

[0007] Examples where systems similar to LIDAR are used are referenced below.

[0008] An article titled "Laser Ranging at Few-photon Level by Photon-Number-Resolving Detection" by Bao et al., in Journal of Applied Optics Vol. 53 (2014) describes demonstrating laser ranging at a few-photon level using photon-number-resolving detectors.

[0009] An article titled "Low Intensity LiDAR using Compressed Sensing and a Photon Number Resolving Detector" by Sher et al., in Proceedings of the SPIE Vol. 10546 (2018) describes combining a high sensitivity photon number resolving diode with machine learning and a micro- mechanical digital mirror device to achieve 3D scanning.

[0010] An article titled "Enhancing LiDAR performance using threshold photon-number-resolving detection" by Wu et al., 2024, Optics Express, 32, 2:2574, discloses a detection scheme which combines noise suppression of threshold detection with signal amplification of photon-number-resolving detectors.

[0011] U.S. Published Application No. 2023228851 discloses an illumination system for scanned LIDAR, including a laser, an optic that receives the laser output and produces a fan-beam output, and a scanning system that scans the fan-beam output across a desired angular span.

[0012] U.S. Patent No. 8,817,271 discloses a fiber optic directional sensor including a hemispherical dome surface and a flat CCD. Optical fibers extend from the dome to the CCD, in a manner that different groups of 10-20 fibers correspond to a pixel of the CCD.U.S. Patent No. 4,183,664 discloses an optical sensor that provides a narrow field of view. The sensor includes a foraminous plate with many straight holes positioned in front of a photodetector, which integrates the radiant energy passing through the plate. The holes have non-reflective walls to reduce scattering.

[0013] SUMMARY OF THE INVENTION

[0014] According to some embodiments of the invention the present invention there is provided apparatus for detecting optical radiation from an environment. The apparatus comprises a plurality of detection units. Each detection unit comprises an optical sensor configured to generate an electrical signal in response to optical radiation incident thereon, and a hollow and opaque tube coupled with the optical sensor. Each tube limits a respective individual field-of-view of a respective sensor to a different sector of the environment. The apparatus also comprises a controller coupled with each optical sensor for receiving a respective electrical signal therefrom.

[0015] According to an aspect of some embodiments of the present invention there is provided a method of detecting optical radiation from an environment. The method comprises converting the optical radiation to electrical signals using a plurality of detection units. Each detection unit comprises an optical sensor configured to generate an electrical signal in response to optical radiation incident thereon, and a hollow and opaque tube coupled with the optical sensor, wherein each tube is configured to limit a respective individual field-of-view of a respective sensor to a different sector of the environment. The method also comprises processing the electrical signals, thereby detecting the optical radiation from the environment.

[0016] According to some embodiments of the invention the processing comprises estimating a direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors of one or more of the detection units.

[0017] According to some embodiments of the invention the method comprises estimating a gain of at least one sensor of the detection units.

[0018] According to some embodiments of the invention the method comprises estimating a background occupancy of at least one sensor of the detection units.

[0019] According to some embodiments of the invention the method comprises applying bias voltage to at least one sensor of the detection units.

[0020] According to some embodiments of the invention the method comprises selecting the bias voltage responsively to a respective electrical signal received from the at least one sensor.According to some embodiments of the invention the method comprises selecting the bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of the at least one sensor, and a change in voltage across the at least one sensor.

[0021] According to an aspect of some embodiments of the present invention there is provided a method of assembling apparatus for detecting optical radiation from an environment. The apparatus comprises a plurality of detection units. The method comprises providing a plurality of sensors each being capable of generating an electrical signal in response to optical radiation incident thereon, providing a plurality of hollow and opaque tubes, and coupling each tube to a different sensor or a different group of sensors so as to limit a respective individual field-of-view of the sensor or group of sensors to a different sector of the environment, thereby assembling the plurality of detection units, each comprises a respective tube and a respective sensor or group of sensors.

[0022] According to some embodiments of the invention the plurality of detection units is arranged as an array.

[0023] According to some embodiments of the invention at least two adjacent individual field-of-views are mutually exclusive.

[0024] According to some embodiments of the invention at least two adjacent individual field-of-views partially overlap.

[0025] According to some embodiments of the invention walls of each tube are configured to prevent guiding of optical radiation therethrough.

[0026] According to some embodiments of the invention the sensor comprises a photon number resolving detector.

[0027] According to some embodiments of the invention at least a portion of the tube is tapered toward the sensor.

[0028] According to some embodiments of the invention at least a portion of the tube is conical. According to some embodiments of the invention at least a portion of the tube is cylindrical.

[0029] According to some embodiments of the invention a cross section of the tube is circular along at least a portion of a length thereof. According to some embodiments of the invention a cross section of the tube is polygonal along at least a portion of a length thereof.

[0030] According to some embodiments of the invention at least one of the detection units comprises a lens within a respective tube or at an entrance aperture thereof.

[0031] According to some embodiments of the invention each sensor is coupled to a single tube.According to some embodiments of the invention at least one of the detection units comprises a single tube and a plurality of optical sensors coupled to the single tube.

[0032] According to some embodiments of the invention the controller comprises a circuit configured to estimate a direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors of one or more of the detection units.

[0033] According to some embodiments of the invention the controller comprises a circuit is configured to estimate a gain of a respective sensor.

[0034] According to some embodiments of the invention the controller comprises a circuit is configured to estimate a background occupancy of a respective sensor.

[0035] According to some embodiments of the invention the controller is configured to apply bias voltage to each sensor.

[0036] According to some embodiments of the invention the controller is configured to set the bias voltage applied to at least one sensor responsively to a respective electrical signal received from the at least one sensor.

[0037] According to some embodiments of the invention the controller is configured to set the bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of the at least one sensor, and a change in voltage across the at least one sensor.

[0038] According to some embodiments of the invention the apparatus is optically passive.

[0039] Unless otherwise defined, all technical and / or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and / or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

[0040] Implementation of the method and / or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and / or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system.

[0041] For example, hardware for performing selected tasks according to embodiments of the invention could be implemented as a chip or a circuit. As software, selected tasks according to embodiments of the invention could be implemented as a plurality of software instructions beingexecuted by a computer using any suitable operating system. In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of method and / or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and / or data and / or a non-volatile storage, for example, a magnetic hard-disk and / or removable media, for storing instructions and / or data. Optionally, a network connection is provided as well. A display and / or a user input device such as a keyboard or mouse are optionally provided as well.

[0042] BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0043] Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

[0044] In the drawings:

[0045] FIGs. 1A and IB are schematic perspective views illustration of a detection unit, according to some embodiments of the present invention;

[0046] FIG. 2 is a schematic cross-sectional view of the detection unit, according to some embodiments of the present invention;

[0047] FIG. 3 is a schematic perspective view illustration of apparatus for detecting optical radiation, according to some embodiments of the present invention;

[0048] FIG. 4 is a schematic drawing of electronic circuitry of a cell of a sensor, according to some embodiments of the present invention;

[0049] FIG. 5 is a flowchart diagram illustrating a method suitable for detecting optical radiation from an environment, according to some embodiments of the present invention;

[0050] FIG. 6 is a schematic block diagram of a method suitable for assembling apparatus for detecting optical radiation, according to some embodiments of the present invention; and

[0051] FIG. 7 is a schematic block diagram of a procedure suitable for detecting a flux of optical radiation impinging on an optical sensor, according to some embodiments of the present invention;DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

[0052] The present invention, in some embodiments thereof, relates to remote detection and, more particularly, but not exclusively, to method and system for remote optical object detection.

[0053] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and / or methods set forth in the following description and / or illustrated in the drawings and / or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

[0054] The inventors identified several challenges in the detection of sources of radiation in the environment. For example, the direction of an impinging laser beam is unknown a priori, which requires the detection system to accept radiation from a wide field-of-view (FOV) of interest, utilizing possibly a very large array of detectors and / or a cumbersome scanning mechanism. Additionally, the received signal may be very faint, comprising only a small number of photons. The divergence of a beam emitted by a distant source causes only a small fraction of the emitted flux of optical radiation to impinge on the receiving detector, posing a challenge to its sensitivity and requiring in many cases using a sensitive detector. Traditional applications of both active and passive detection systems require operating such systems either outdoors in full sun or indoors in a strong ambient lighting, which further challenges the detection of faint signals against a high-level of background illumination. Moreover, the background illumination may saturate the detectors.

[0055] Embodiments described herein address at least one of these challenges by providing a detection apparatus having a plurality of detection units, where each detection unit comprises an optical sensor and a hollow and opaque tube coupled with the optical sensor. Each tube limits the respective individual field-of-view of a respective sensor to a different sector of the environment. This provide the apparatus with a combined field-of-view that is larger than the field-of-view of each individual detection units. The Inventers found that this provide the apparatus with directional sensitivity, optionally and preferably without compromising on the sensitivity of the apparatus to weak optical signals. In some embodiments of the present invention the apparatus has a tolerance for high levels of ambient illumination. Preferably, the apparatus optionally and preferably is devoid of scanning or moving mechanical components. In some embodiments of the present invention, aside from the optical sensors, the detection apparatus comprises no optical elements.

[0056] In some embodiments of the present invention, the apparatus is optically passive. That is, the apparatus does not comprise a source of optical radiation and does not emit optical radiation into the environment. In these embodiments the apparatus operates exclusively by receiving optical radiationthat originates from external sources in the environment, which external sources can be any natural or artificial sources of optical radiation. These embodiments are useful in applications where it is desirable to avoid alerting a target to the presence of the detection apparatus, or where regulatory or operational constraints limit the emission of optical radiation.

[0057] FIGs. 1A and IB are schematic perspective views illustration of a single detection unit 100 of the apparatus, according to some embodiments of the present invention. Detection unit 100 comprises an optical sensor 102 configured to generate an electrical signal in response to optical radiation incident thereon, and a tube 104 coupled with optical sensor 102. A controller 105 is coupled with optical sensor 102 for receiving the electrical signal therefrom. Controller 105 optionally and preferably comprises a circuit configured for processing the received signal as further detailed hereinbelow. Controller 105 typically comprises a programmable processor, which is programmed in software and / or firmware to carry out the functions that are described herein. Alternatively or additionally, controller 105 can comprise hard-wired and / or programmable hardware logic circuits, which carry out at least some of the functions of the controller. Further alternatively or additionally, controller 105 can comprise suitable analog circuitry for implementing some of the required functionalities. Although controller 105 is shown, for the sake of simplicity, as a single, monolithic functional block, controller 105 may comprise a single chip or a set of two or more chips, with suitable interfaces for receiving and outputting the signals. Controller 105 may also be integrated with sensor 102, or disposed remotely, or externally, with adequate communication with sensor 102.

[0058] Tube 104 is configured to limit the individual field-of-view of sensor 102 to a specific sector of the environment. In some embodiments of the present invention at least a portion of tube 104 is tapered toward sensor 102 (see FIG. 1A), and in some embodiments of the present invention tube 104 is non-tapered (FIG. IB).

[0059] Sensor 102 is located within an exit aperture 106 of tube 104. An area Aout,i of exit aperture 106 of tube 104, together with an area Ain,i of an entrance aperture 108 and a length Li of tube 104 define a nominal solid angle Qnom,i defining the field-of-view of the sector of the environment that corresponds to detection unit 100. The nominal solid angle Qnom,i around an axis 112 of tube 104 is shown with dotted lines 110. An actual solid angle Qi from which detection unit 100 may receive radiation may be somewhat larger due to the finite areas Aout,i and Adet,i of exit aperture 106 and sensor 102.

[0060] The wall or walls of tube 104 can be fabricated of a material opaque to the received optical radiation. When several such detection units are used in an apparatus, the opaque material of the tubesabsorb the optical radiation and reduces or prevents optical crosstalk and leakage between the different sectors of the environment.

[0061] Tube 104 is preferably hollow. The interior of tube 104 can be filled with air. In any event, the refractive index of the medium filling the interior of tube 104 is less than the refractive index of the material forming the wall of tube 104. Preferably, the wall or walls of tube 104 is / are configured to prevent guiding of optical radiation therethrough. Thus, in these embodiments the wall of tube 104 is incapable of serving as a waveguide. That is, the walls are configured to prevent guiding by any mechanism, including both waveguides that guide light via total internal reflection and waveguides that guide light via other mechanisms such as photonic bandgap guidance or anti-resonant reflection.

[0062] Tube 104 is preferably elongated and may have any geometrical shape. In some embodiments of the present invention at least a portion of tube 104 is conical, in some embodiments of the present invention at least a portion of tube 104 is cylindrical, in some embodiments of the present invention a cross section of tube 104, at a plane perpendicular to its largest dimension, is circular along at least a portion of a length thereof, , in some embodiments of the present invention a cross section of tube 104, at a plane perpendicular to its largest dimension, is elliptical along at least a portion of a length thereof, and in some embodiments of the present invention a cross section of tube 104, at a plane perpendicular to its largest dimension, is polygonal along at least a portion of a length thereof. Polygonal cross-section is preferred from the standpoint of a filling factor and reduced optical losses. Representative examples of preferred shapes for tube 104, according to some embodiments of the present invention, include, without limitation, a frustum of a cone, a frustum of a pyramid, and a prism. The cross section of tube 104 may in some embodiments of the present invention be an arbitrary closed curve.

[0063] In some embodiments of the present invention sensor 102 comprises a photon number resolving detector (PNRD). A PNRD is an improved single-photon detector in which, instead of generating a binary detection signal, it distinguishes how many photons arrive in a defined temporal or spatial mode by outputting an estimate of the photon count per event or per time bin. Examples of types of PNRD suitable for the present embodiments include: Geiger-mode SPAD (Single-Photon Avalanche Diode) arrays, silicon photomultipliers (SiPM), MPPC (Multi-Cell Single Photon Multiplier), silicone avalanche photodiodes (Si-APD), InGaAs / InP avalanche photodiodes, and InGaAs / InP Geiger-mode SPAD arrays. Preferably, the PNRD is reverse-biased so as to operate in a Geiger mode, wherein a single photon impinging on the PNRD causes an avalanche of photoelectrons. The effective detection time of the PNRD is preferably less than 100 ns or less than 50 ns or less than 40 ns or less than 30 ns or less than 10 ns or less than 10 ns or less than 9 ns or less than 8 ns or lessthan 7 ns or less than 6 ns or less than 5 ns or less than 4 ns or less than 3 ns or less than 2 ns, e.g., about 1 ns.

[0064] FIG. 2 is a schematic cross-sectional view of detection unit 100. In the schematic illustration of FIG. 2, which is not to be considered as limiting, detection unit 100 also comprises an optical lens 202 mounted either at entrance aperture 108 of tube 104 or in another suitable location in tube 104. Use of lens 202 is advantageous since it increases the apparent collection aperture of apparatus detection unit 100, thus increasing the amount of collected light while still maintaining a small size and cost effectiveness.

[0065] FIG. 3 is a schematic perspective view illustration of apparatus 300 for detecting optical radiation, according to some embodiments of the present invention. Apparatus 300 comprises a plurality of detection units each can be similar to detection unit 100 described above with respect to FIGs. 1A and IB. Units 100 can be arranged as an array. Thus, apparatus 300 comprises an array 304 of sensors (individual sensors not shown in the figure), and an array of tubes 104. The sensors and tubes can be enclosed in an opto-mechanical enclosure 302. Controller 105 can be shared among the detection units, so that all electrical signals that are generated by the sensors in array 304 are received by controller 105.

[0066] Using a specific conical tube 104a as a non-limiting example, an entrance aperture 310a and an exit aperture 312a of the tube 104a define a solid angle Qa, which defines the sector of the environment from which the sensor aligned with tube 104a can receive optical radiation. The array of tubes 104 defines an array 314 of solid angles Q defining respective field-of-views corresponding to different sectors of the environment. The combination of these sectors defines a field-of-view of interest 316 from which apparatus 300 can receive optical radiation, with each sensor in array 304 resolving through its respective solid angle Q the direction from which the radiation was received.

[0067] Although the individual field-of-views of the detection units 100, as represented by the solid angles Q, are shown as mutually exclusive, e.g., separated from each other, the present embodiments also contemplate configurations in which two or more adjacent individual field-of-views partially overlap. This kind of partial overlap advantageously increases the directional resolution of apparatus 300, since the circuit of controller 105 may utilize, in addition to the signal received from a specific sector, the presence or absence of signals corresponding to radiation in adjacent, partially overlapping, field-of-views.

[0068] In some embodiments of the present invention, each sensor 102 is coupled to a single tube 104. In some embodiments of the present invention, each sensor 102 is coupled to a different tube104, such that there is a one-to-one correspondence between sensors 102 and tubes 104. This configuration is advantageous in that each tube 104 defines a unique sector of the environment for its respective sensor 102, providing a mapping between the electrical signal generated by each sensor 102 and the sector of the environment from which the radiation was received.

[0069] In some embodiments of the present invention, at least one of detection units 100 comprises a single tube 104 and a plurality of optical sensors 102 coupled to that single tube 104. In these embodiments, the plurality of sensors 102 within a single tube 104 may each be configured to generate an electrical signal responsive to received optical radiation having a different wavelength range. This configuration allows apparatus 300 to simultaneously detect optical radiation in multiple spectral bands within the same sector of the environment. The wavelength ranges may span, for example, the ultraviolet, visible, near-infrared, short-wave infrared, mid-wave infrared, or long-wave infrared portions of the electromagnetic spectrum, or any combination thereof. Each sensor 102 within the tube may be selected or configured to be sensitive to its respective wavelength range, for example by appropriate choice of detector material, by the use of optical filters, or by any other suitable means.

[0070] Referring now to the operation of controller 105, in some embodiments of the present invention the circuit of controller 105 is configured to estimate a direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors 102 of one or more of detection units 100. The direction estimation may be performed, for example, by identifying which sensor or sensors 102 generated a signal above a threshold, by comparing signal intensities across multiple sensors 102, and / or by interpolating between the individual field-of-views of adjacent detection units 100 in cases where partial overlap exists between adjacent field-of-views. The circuit of controller 105 may implement any suitable algorithm for direction estimation, including, without limitation, centroid computation, maximum likelihood estimation, matched filtering, or machine learning based inference.

[0071] In some embodiments of the present invention, the circuit of controller 105 is configured to estimate a gain of a respective sensor 102. The gain of a sensor 102, for example a PNRD operating in Geiger mode, may vary as a function of the applied bias voltage, the ambient temperature, the sensor's age, or other factors. Estimating the gain allows controller 105 to normalize the electrical signals received from different sensors 102 and to maintain a consistent response across the array of detection units 100. The gain estimation may be performed, for example, by analyzing the amplitude distribution of the electrical signals, by injecting a known calibration signal, or by any other suitable method. A procedure suitable for estimating the gain is described in the Examples section that follows.In some embodiments of the present invention, the circuit of controller 105 is configured to estimate a background occupancy of a respective sensor 102. Background occupancy refers to the ratio of sensor cells that are being discharged or have recovered, relative to the number of cells in the sensor. The background occupancy describes the level of saturation of the sensor. By estimating the background occupancy, controller 105 can adaptively adjust its detection thresholds or its bias voltage settings to compensate for varying background illumination conditions. A procedure suitable for estimating the background occupancy is described in the Examples section that follows.

[0072] In some embodiments of the present invention, controller 105 is configured to apply a bias voltage to each sensor 102. The bias voltage may be applied uniformly to all sensors 102 in array 304, or it may be set individually for each sensor 102. In some embodiments, controller 105 is configured to set the bias voltage responsively to a respective electrical signal received from that sensor 102, thus facilitating a closed-loop control. Such a control is advantageous since it allows controller 105 to adaptively optimize the operating point of each sensor 102 in response to changing conditions. For example, when the electrical signal from a particular sensor 102 indicates that the sensor is operating in a regime of high background occupancy or reduced gain, controller 105 may adjust the bias voltage applied to that sensor 102 accordingly. A preferred procedure for calculating the bias voltage is described in the Examples section that follows.

[0073] In some embodiments of the present invention, controller 105 sets the bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of sensor, and a change in voltage across the at least one sensor 102. Ambient temperature variations may affect the breakdown voltage of sensors such as SPADs or SiPMs, and therefore the effective gain of such sensors at a fixed bias voltage. By monitoring the ambient temperature or the temperature of sensor 102 directly, controller 105 may compensate for such variations by adjusting the bias voltage. A change in voltage across sensor 102 may indicate a change in the operating conditions of the sensor, such as a change in the photocurrent or in the quenching behavior of the sensor, and may serve as an additional input to the bias voltage control loop. With reference to FIGs. 1A, IB, and 2, the temperature of sensor 102 may be measured, for example, by a dedicated temperature sensor 204 co-located with sensor 102. The ambient temperature can be measured by a temperature sensor 206 spaced apart from sensor 102. Any of the temperature sensors 204 and 206 can be a thermistor, a thermocouple, or the like. The bias voltage may be set, for example, by a digital-to-analog converter, a programmable voltage regulator, or any other suitable voltage source under the control of controller 105.FIG. 4 is a schematic drawing of the electronic circuitry of a cell 500 of sensor 102. Cell 500 is particularly useful for the case in which sensor 102 comprises a SiPM, and is of the PNRD type. It is to be understood, however, that the present embodiments contemplate other types of optical sensors for use as sensor 102. The optical radiation impinging on cell 500 is represented by block arrow 516. Cell 500 comprises a SPAD 504. Sensor 102 can comprise multiple cells 500 coupled in parallel, driven jointly by bias voltage source 502 with a voltage VBIAS. An internal bias resistor 503 with a resistance of RBIAS can be coupled in series within the voltage source. Typically, the bias voltage across the junction of each SPAD is lower than VBIAS due to the voltage drop over the resistors in series with the diode. The current fed by voltage source 502 is shared by all cells 500, so that a voltage drop of the bias voltage may be felt also by a specific SPAD where no current flows. SPAD 504 is coupled in series with a quenching resistor 506, with a resistance of RQ. In FIG. 4, SPAD 504 is modeled as a parallel combination of a capacitor 508, representing a junction capacitance Cj, with a series combination of a switch 510 (S), a voltage source 512 providing a breakdown voltage VBD, and a series resistor 514 representing the resistance Rs of the entire SPAD during a discharge.

[0074] FIG. 5 is a flowchart diagram illustrating a method suitable for detecting optical radiation from an environment. It is to be understood that, unless otherwise defined, the operations described hereinbelow can be executed either contemporaneously or sequentially in many combinations or orders of execution. Specifically, the ordering of the flowchart diagrams is not to be considered as limiting. For example, two or more operations, appearing in the following description or in the flowchart diagrams in a particular order, can be executed in a different order (e.g., a reverse order) or substantially contemporaneously. Additionally, several operations described below are optional and may not be executed. The method begins at 520 and continues to 521 at which the optical radiation is converted to electrical signals using a plurality of detection units, such as, but not limited to, detection units 100. The method optionally and preferably proceeds to 522 at which the bias voltage to be applied to one or more of the sensors is selected responsively to the respective electrical signal received from the sensor as further detailed hereinabove, and to 523 at which the bias voltage is applied. The method proceeds to 524 at which the electrical signal is processed. The processing may include, estimation of the gain of one or more of the sensors, estimation of the background occupancy of one or more of the sensors, and / or estimation of the direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors of one or more of the detection units, as further detailed hereinabove. The method ends at 525.FIG. 6 is a schematic block diagram 600 of a method suitable for assembling apparatus for detecting optical radiation, such as, but not limited to, apparatus 300, according to some embodiments of the present invention. The method can also be used for detecting the direction of radiation.

[0075] At 602, controller 105, provided at 604, is coupled with a sensor such as, but not limited to, an ultra sensitive optical detector (USOD), provided at 606. USOD can be, for example, a PNRD. At 608, tube 308, provided at 610, is coupled with the respective sensor. At 612, tube 308 is oriented toward a desired sector Q of the field-of-view of interest. At 614, the oriented tube together with the coupled sensor are identified as a detection unit. At 616, N such detection units are assembled to an array, wherein controller 105 is coupled with each sensor the N detection units. At 618, the array of detection units receives optical radiation. At 620, controller 105 receives from each unit of the array an electrical signal indicating the intensity of optical radiation received by the respective unit. At 622, the controller infers from the received electrical signals the direction from which the optical radiation was received.

[0076] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

[0077] Various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below find experimental support in the following examples.

[0078] EXAMPLES

[0079] Reference is now made to the following examples, which together with the above descriptions illustrate some embodiments of the invention in a non limiting fashion.

[0080] Following is a description of procedures suitable for handling background of a high level of ambient illumination. In the following description, an SiPM is used as a specific case of a sensor, but these procedures are applicable to any kind of optical sensor, particularly PNRD.

[0081] In embodiments where the sensor of a detection unit such as detection unit 100 comprises a SiPM, the pronounced non-linearity of optical detectors of this kind is considered. The primary use of SiPM-based sensors is in the linear photon detection region, where single or few photons can be resolved. The term "linear photon detection region" is used herein to denote a region wherein eachreceived photon causes a single respective output electrical pulse emitted by the detector. When the number of photons increases and approaches the capacity of a discrete cell of the SiPM, such as cell 500, the detector becomes non-linear.

[0082] The non-linearity of the SiPM becomes more pronounced due to self-heating of one or more of the SPADs forming the SiPM as their breakdown voltages are dependent on the junction temperature of each diode. For each SPAD, an increase in the impinging flux of radiation due to ambient light causes an increase in the photocurrent of the diode, which in turn increases the power consumption and raises the temperature of the diode. Such a self-heating mechanism increases the breakdown voltage of the diode, reducing the overvoltage and the gain with it.

[0083] Overvoltage is defined as the difference between the bias voltage VBIAS and the breakdown voltage VBD, with a possible correction for a voltage drop across the series resistors in a cell, see EQ.

[0084] 10, below.

[0085] Additional non-linearity is caused by the voltage drop across bias resistance 503 in bias voltage source 502. The voltage drop across the serial resistance reduces the voltage drop across the diode, which in turn reduces its gain.

[0086] SiPM devices, therefore, become nonlinear with an increasing level of irradiance, affecting the ability to accurately read out an absolute level of high optical flux.

[0087] When an array of SiPMs is used for multiple sensors, such as the sensor array 304, it is advantageous to determine the absolute optical flux impinging on each SiPM for the purpose of comparing the levels of optical flux between different sections of the array. Since each section of the array potentially observes a different sector of the environment with different ambient light conditions, it may be difficulty to compare between sections of the array. In particular, it is advantage to obtain the absolute levels of the signals received from different sections when overlap of two or more adjacent sectors is implemented, so as to improve the accuracy of the interpolation between the signals from adjacent sectors.

[0088] Embodiment of the present invention can detect the absolute flux of optical radiation impinging on the detector under different light conditions. This allows comparing the optical flux received from different sectors of the environment. The procedure described below employs noise statistics and is based on an analysis of the shot-noise of the respective electrical output signal of each SiPM (including excess noise) in situ during the measurement. The procedure is advantageous since it does not require use of large look-up-tables or long calibration procedures. The procedure is described with reference to FIG. 7 which is a schematic block diagram 700 of a procedure suitable fordetecting a flux of optical radiation impinging on an optical sensor, according to some embodiments of the present invention.

[0089] Taking apparatus 300 as an example, the results for EQs. 1-8 below are computed by controller 105.

[0090] At 702, an overvoltage Vov of a specific SPAD (such as SPAD 504) is computed from a temperature (T) 704 and a bias voltage (VBIAS) 706 of the SPAD:

[0091] POP=VBIAS ~ VBD(T ’ (EQ. 1) where VBD(T) denotes the temperature-dependence of the breakdown voltage of the detector. The bias voltage VBIAS may be measured by a suitable voltmeter coupled to voltage source 502. T refers to the temperature of the junction of the SiPM. The value of T may optionally and preferably be determined as detailed in EQ. 11, hereinbelow.

[0092] At 708, the gain of the SiPM is estimated using for a gain input 710, either curves of gain vs. overvoltage from the specifications of the SiPM or its factory calibration.

[0093] At 712, a background occupancy level of the SiPM is estimated. The average DC background current of the SiPM depends linearly on its gain, but when the occupancy level approaches the number of cells of the SiPM (saturation), this linearity breaks down. For estimating the background occupancy level, both the average background DC current level and its variance are used, as will now be described.

[0094]

[0095] where q is the electron charge 1.6xl019C, G(V0^) is the SiPM gain for a bias voltage of Vov, and Pap(yov+Pxtik(Vov isa sumof after-pulse probability and crosstalk probability estimated from tables. Herein “after-pulse probability” and “crosstalk probability” refer to statistical probabilities that a discharged cell in the SiPM causes another cell to discharge, either through a delayed pulse or simultaneously with the primary discharges, respectively. The dependence of the above probabilities on Vov and are generally known, e.g., supplied by the manufacturer of the SiPM or obtained through a pre-calibration process. The parameter Nfl BGis defined asNfirednamely the number of “fired”

[0096] J’Ncell

[0097] cells divided by the number of cells, wherein the term “fired cell” refers to a cell that is discharged either by an impinging photon or due to thermal excitation (dark count) and is going through the discharge-quench-recover cycle. The parameter Nfl darkis the number of fired cells due to darkcounts divided by the number of cells, and the parameter fBWis the analog bandwidth (BW) for the root-mean-square (RMS) calculation.At 714, the signal is estimated from the background occupancy of the respective SiPM (operation 712) and from a signal mean 716, as detailed below.

[0098] The signal current is measured by subtracting the average background level from the average signal level:

[0099] > >

[0100] < > < >

[0101] <

[0102]

[0103] < > Using the approximations < Nap> « PAPand < Nxtk>« Pxtkand identifying:

[0104] <

[0105]

[0106] Var(J)

[0107] 2qfBw

[0108] (EQ. 4) one obtains:

[0109] <> < > < >

[0110]

[0111] From the above equations the parameter Nfl sIGcan be derived:

[0112]

[0113] where the photon detection efficiency (PDE) is also a function of the overvoltage and is given by either a factory calibration or by the specifications of the SiPM.

[0114] At 718, a normalized signal is computed from the estimated signal (operation 714) as:

[0115]

[0116] where Psigcomprises the final signal comparable between the multiple SiPMs of apparatus 300.Another procedure suitable for handling background of ambient illumination employs dynamic control of the bias voltage of the sensor. This procedure will now be described.

[0117] Due to the sensitivity of the sensor, a high level of ambient light may saturate the sensor. A fully saturated detector loses its sensitivity to an optical signal from a light source of interest. A strong photocurrent can also saturate one or more of the electronic circuits used for receiving and processing the signals of the, as these circuits are typically configured for small signals. Additionally, a strong photocurrent can damage the sensor due to the high bias voltage that may be required, as well as due to an increase of the junction temperature above a maximum permissible level.

[0118] The present embodiments solve the problem of saturation by employing a dynamic control of the bias voltage. The procedure comprises sensing the photocurrent and adjusting the bias voltage to have the photocurrent remain below a predetermined current limit. A dynamic control of bias voltage may be implemented in more than one way. In some embodiments of the present invention a gradual decrease of the bias voltage is applied via a closed feedback or feedforward loop, and in some embodiments of the present invention the bias voltage is calculated. The calculation of the bias voltage is preferred since the inventors found it to converge faster than a digital feedback loop, as the calculation can converge in a single iteration / clock count. To implement this approach a model-based algorithm was developed by the Inventors, as will now be explained.

[0119] When the background level of optical radiation 516 (see FIG. 4) changes, the current flowing through bias resistor 503 reduces the overvoltage Vov and thus the voltage across SPAD 504, causing changes in PDE. Taking apparatus 300 as an example, the results for EQs. 9-15 below are computed by controller 105.

[0120] The sum of the currents flowing through all SPADs 504 of the respective SiPM may be written as:

[0121]

[0122] The overvoltage Vov may be written as:

[0123] Vov=BIAS ~ IAPDX(+^S)—VBD- (EQ. 10) The junction temperature T of SPAD 504, due to self-heating, may be written as:

[0124] T = Ta+ 1APDx(EBD + EQV) x Rtherm, (EQ. 11) where Tais the ambient temperature as measured by the temperature sensor 206, and Rtherm is the thermal resistance of SPAD 504.

[0125] The breakdown voltage VBD depends on the temperature T:

[0126] VBD = VBD(T). (EQ. 12)Both gain G and PDE depend on the overvoltage Vov:

[0127] G = G(Vov), (EQ. 13) and

[0128] PDE = PDE(Vov. (EQ. 14) The average number of fired cells can be calculated using (NBG) and depends on PDE:

[0129] (1VBG) = {NBGKPDE). (EQ. 15) The present embodiments utilize the model described in EQs. 9-15 for bias control, by considering ambient temperature variation, self-heating and voltage drop due to resistive loads. The above equations can be solved numerically or iteratively for dynamic control of the bias voltage VBIAS across the cells of the respective SiPM, as well as to adjust the parameters for the statistical model that is used for detection and false alarm rejection.

[0130] Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

[0131] It is the intent of the applicant(s) that all publications, patents and patent applications referred to in this specification are to be incorporated in their entirety by reference into the specification, as if each individual publication, patent or patent application was specifically and individually noted when referenced that it is to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting. In addition, any priority document(s) of this application is / are hereby incorporated herein by reference in its / their entirety.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for detecting optical radiation from an environment, comprising:a plurality of detection units, each comprising an optical sensor configured to generate an electrical signal in response to optical radiation incident thereon, and a hollow and opaque tube coupled with said optical sensor; anda controller coupled with each optical sensor for receiving a respective electrical signal therefrom;wherein each tube is configured to limit a respective individual field-of-view of a respective sensor to a different sector of the environment.

2. The apparatus according to claim 1 , wherein said plurality of detection units is arranged as an array.

3. The apparatus according to any of claims 1 and 2, wherein at least two adjacent individual field-of-views are mutually exclusive.

4. The apparatus according to any of claims 1 and 2, wherein at least two adjacent individual field-of-views partially overlap.

5. The apparatus according to claim 1, wherein walls of each tube are configured to prevent guiding of optical radiation therethrough.

6. The apparatus according to any of claims 2-4, wherein walls of each tube are configured to prevent guiding of optical radiation therethrough.

7. The system according to claim 1, wherein said sensor comprises a photon number resolving detector.

8. The system according to any of claims 2-6, wherein said sensor comprises a photon number resolving detector.

9. The apparatus according to claim 1, wherein at least a portion of said tube is tapered toward said sensor.

10. The apparatus according to any of claims 2-8, wherein at least a portion of said tube is tapered toward said sensor.

11. The apparatus according to claim 1, wherein at least a portion of said tube is conical.

12. The apparatus according to any of claims 2-10, wherein at least a portion of said tube is conical.

13. The apparatus according to claim 1 , wherein at least a portion of said tube is cylindrical.

14. The apparatus according to any of claims 2-12, wherein at least a portion of said tube is cylindrical.

15. The apparatus according to claim 1, wherein a cross section of said tube is circular along at least a portion of a length thereof.

16. The apparatus according to any of claims 2-14, wherein a cross section of said tube is circular along at least a portion of a length thereof.

17. The apparatus according to claim 1, wherein a cross section of said tube is polygonal along at least a portion of a length thereof.

18. The apparatus according to any of claims 2-16, wherein a cross section of said tube is polygonal along at least a portion of a length thereof.

19. The apparatus according to claim 1, wherein at least one of said detection units comprises a lens within a respective tube or at an entrance aperture thereof.

20. The apparatus according to any of claims 2-18, wherein at least one of said detection units comprises a lens within a respective tube or at an entrance aperture thereof.

21. The apparatus according to claim 1, wherein each sensor is coupled to a single tube.

22. The apparatus according to any of claims 2-20, wherein each sensor is coupled to a single tube.

23. The apparatus according to claim 21, wherein each sensor is coupled to a different tube.

24. The apparatus according to claim 22, wherein each sensor is coupled to a different tube.

25. The apparatus according to claim 1, wherein at least one of said detection units comprises a single tube and a plurality of optical sensors coupled to said single tube.

26. The apparatus according to any of claims 2-24, wherein at least one of said detection units comprises a single tube and a plurality of optical sensors coupled to said single tube.

27. The apparatus according to claim 25, wherein each of said plurality of optical sensors is configured generate an electrical signal responsive to received optical radiation having a different wavelength range.

28. The apparatus according to claim 26, wherein each of said plurality of optical sensors is configured generate an electrical signal responsive to received optical radiation having a different wavelength range.

29. The apparatus according to claim 1, wherein said controller comprises a circuit configured to estimate a direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors of one or more of said detection units.

30. The apparatus according to any of claims 2-28, wherein said controller comprises a circuit configured to estimate a direction of the optical radiation within the environment based on apresence, absence, or intensity of electrical signals received from optical sensors of one or more of said detection units.

31. The apparatus according to claim 1, wherein said controller comprises a circuit is configured to estimate a gain of a respective sensor.

32. The apparatus according to any of claims 2-30, wherein said controller comprises a circuit is configured to estimate a gain of a respective sensor.

33. The apparatus according to claim 1, wherein said controller comprises a circuit is configured to estimate a background occupancy of a respective sensor.

34. The apparatus according to any of claims 2-32, wherein said controller comprises a circuit is configured to estimate a background occupancy of a respective sensor.

35. The apparatus according to claim 1, wherein said controller is configured to apply bias voltage to each sensor.

36. The apparatus according to any of claims 2-34, wherein said controller is configured to apply bias voltage to each sensor.

37. The apparatus according to claim 35, wherein said controller is configured to set said bias voltage applied to at least one sensor responsively to a respective electrical signal received from said at least one sensor.

38. The apparatus according to claim 36, wherein said controller is configured to set said bias voltage applied to at least one sensor responsively to a respective electrical signal received from said at least one sensor.

39. The apparatus according to claim 35, wherein the controller is configured to set said bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of said at least one sensor, and a change in voltage across said at least one sensor.

40. The apparatus according to any of claims 36-38, wherein the controller is configured to set said bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of said at least one sensor, and a change in voltage across said at least one sensor.

41. The apparatus according to claim 1, being optically passive.

42. The apparatus according to any of claims 2-40, being optically passive.

43. A method of detecting optical radiation from an environment, comprising: converting the optical radiation to electrical signals using a plurality of detection units, each comprising an optical sensor configured to generate an electrical signal in response to optical radiation incident thereon, and a hollow and opaque tube coupled with said optical sensor, wherein each tube is configured to limit a respective individual field-of-view of a respective sensor to a different sector of the environment; andprocessing said electrical signals, thereby detecting the optical radiation from the environment.

44. The method according to claim 43, wherein said processing comprises estimating a direction of the optical radiation within the environment based on a presence, absence, or intensity of electrical signals received from optical sensors of one or more of said detection units.

45. The method according to any of claims 43-44, comprising estimating a gain of at least one sensor of said detection units.

46. The method according to any of claims 43-45, comprising estimating a background occupancy of at least one sensor of said detection units.

47. The method according to any of claims 43-46, comprising applying bias voltage to at least one sensor of said detection units.

48. The method according to claim 47, comprising selecting said bias voltage responsively to a respective electrical signal received from said at least one sensor.

49. The method according to claim 45, comprising selecting said bias voltage according to at least one criterion selected from the group consisting of: a variation of an ambient temperature, a temperature of said at least one sensor, and a change in voltage across said at least one sensor.

50. The method according to any of claims 43-49, wherein said plurality of detection units is arranged as an array.

51. The method according to any of claims 43 and 50, wherein at least two adjacent individual field-of-views are mutually exclusive.

52. The method according to any of claims 43-51, wherein at least two adjacent individual field-of-views partially overlap.

53. The method according to any of claims 43-52, wherein walls of each tube are configured to prevent guiding of optical radiation therethrough.

54. The method according to any of claims 43-53, wherein said sensor comprises a photon number resolving detector.

55. The method according to any of claims 43-54, wherein at least a portion of said tube is tapered toward said sensor.

56. The method according to any of claims 43-55, wherein at least a portion of said tube is conical.

57. The method according to any of claims 43-56, wherein at least a portion of said tube is cylindrical.

58. The method according to any of claims 43-57, wherein a cross section of said tube is circular along at least a portion of a length thereof.

59. The method according to any of claims 43-58, wherein a cross section of said tube is polygonal along at least a portion of a length thereof.

60. The method according to any of claims 43-59, wherein at least one of said detection units comprises an optical lens between a respective tube and a respective sensor.

61. The method according to any of claims 43-60, wherein each sensor is coupled to a single tube.

62. The method according to claim 61, wherein each sensor is coupled to a different tube.

63. The method according to any of claims 43-61, wherein at least one of said detection units comprises a single tube and a plurality of optical sensors coupled to said single tube.

64. The method according to claim 62, wherein each of said plurality of optical sensors is configured generate an electrical signal responsive to received optical radiation having a different wavelength range.

65. The method according to any of claims 43-49, being optically passive.

66. A method of assembling apparatus for detecting optical radiation from an environment, the method comprising:providing a plurality of sensors each being capable of generating an electrical signal in response to optical radiation incident thereon;providing a plurality of hollow and opaque tubes; andcoupling each tube to a different sensor or a different group of sensors so as to limit a respective individual field-of-view of said sensor or group of sensors to a different sector of the environment, thereby assembling a plurality of detection units, each comprising a respective tube and a respective sensor or group of sensors, wherein the apparatus comprises said plurality of detection units.

67. The method according to claim 66, wherein at least two adjacent individual field-of-views partially overlap.

68. The method according to any of claims 66 and 67, wherein walls of each tube are configured to prevent guiding of optical radiation therethrough.

69. The method according to any of claims 66-68, wherein said sensor comprises a photon number resolving detector.

70. The method according to any of claims 66-69, wherein at least a portion of said tube is tapered toward said sensor.

71. The method according to any of claims 66-70, wherein at least a portion of said tube is conical.

72. The method according to any of claims 66-71, wherein at least a portion of said tube is cylindrical.

73. The method according to any of claims 66-72, wherein a cross section of said tube is circular along at least a portion of a length thereof.

74. The method according to any of claims 66-73, wherein a cross section of said tube is polygonal along at least a portion of a length thereof.

75. The method according to any of claims 66-74, wherein at least one of said detection units comprises a lens within a respective tube or at an entrance aperture thereof.

76. The method according to any of claims 66-75, wherein each sensor is coupled to a single tube.

77. The method according to claim 76, wherein each sensor is coupled to a different tube.

78. The method according to any of claims 66-77, wherein at least one of said detection units comprises a single tube and a plurality of optical sensors coupled to said single tube.

79. The method according to claim 78, wherein each of said plurality of optical sensors is configured generate an electrical signal responsive to received optical radiation having a different wavelength range.

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