Imager with fast pulse-shape detection and position determination

Abstract

Disclosed herein are device and method embodiments for photodetectors and photodetection. For example, a multi-mode seeker includes an imager having multiple light-sensing elements, each light-sensing element configured to convert photons to photocurrent. Image readout circuitry is electrically interfaced to multiple light-sensing elements and configured to generate image information based on photocurrent of each light-sensing element accumulated over a period of time. Pulse-shape readout circuitry is electrically interfaced to the imager and configured to continually sense photocurrent produced by the multiple light-sensing elements of the imager and generate one or more signals based on the sensed photocurrent. The pulse-shape readout circuitry is configured to operate independently of the image readout circuitry.

Description

IMAGER WITH FAST PULSE-SHAPE DETECTION AND POSITION DETERMINATIONCROSS-REFERENCE AND CLAIM OF PRIORITY

[0001] This patent application claims priority to U.S. Provisional Patent Application No. 63 / 371,008 filed August 10, 2022, the disclosure of which is hereby incorporated into this document by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure generally relates to semiconductor photon detectors.BACKGROUND

[0003] A focal plane array is a light-sensitive detector array (e.g., 1 -dimensional (ID) or 2- dimensional (2D)) including multiple pixels (picture elements). The pixels are typically coupled (electrically connected) to a Read Out Integrated Circuit (ROIC). Photodetector arrays made, for example, from Indium Gallium Arsenide (InGaAs) alloys (e.g., InxGai.xAs alloys) have been used in telecommunications, spectroscopy, and imaging. Depending on the composition of the InGaAs alloy, the detector may detect photons in the range from 400 nm to 2700 nm. A common composition, for example, is an InGaAs alloy including, for example, 53% InAs and 47% GaAs (Ino.53Gao.47As) which is lattice-matched to Indium phosphide (InP), enabling detection of photons from 400 nm to 1700 nm. These detector arrays (either 1-D or 2-D) may be interfaced with an ROIC which converts accumulated photocurrent to a voltage through an amplification circuit associated with each pixel. This enables, for example, measurement of the amount of light that strikes each pixel in a given amount of time. Typical image readout speeds may be 30-500 frames per second for an entire array, for example. However, this readout speed does not provide sufficient time resolution to determine the structure of a laser pulse of a few nanoseconds in duration.

[0004] Many munition systems, for example, rely on a signal seeker system to hit the target. Silicon quad detectors placed on various type of munitions provide a faster response than focal- plane array imaging systems. In a semi-active laser (SAL) system, for example, the laser illuminating a target is not connected to the receiver. The quad detector receives the reflectedbeam illumination unfocused on the detector. The amount of light that falls into each quad section is measured and the relative amount of signal enables the relative position of the reflected laser (in the field of view of the optics) to be determined. The seeker may perform guidance using a variety of techniques including but not limited to global position system (GPS), SAL, or imaging using automatic target recognition (ATR) algorithms. Combining multiple techniques may be used to improve accuracy but also increases system cost. Combining current SAL detectors and imaging arrays would require two different detectors, which would require two sets of optics or would require beam splitters. Dual optical systems are required because current SAL systems require a defocused image while an imaging system requires a high-performance optical system to produce focused images with high resolution of the target. Multiple optical systems increase the size, weight, complexity, and cost of the solution. Optimally, a system that uses a single detector and optic to both generate an image and measure the pulse shape (over time) of the semi-active (e.g., ~ 5 nsec pulse width) laser would allow higher seeker performance at a lower size, weight, power, and cost. This document describes imaging systems which include fast pulse-shape detection and position determination, and / or other capabilities.SUMMARY

[0005] This disclosure general relates to a specialty imager that allows for detection of light with photodetector arrays which allow simultaneous imaging of a scene with semi -active laser detection within the same scene. InGaAs material with visible and short-wave infrared sensitivity is one example detector material, though the structure may be used with other semiconductor materials (e.g., InSb, HgCdTe, colloidal quantum dots and corresponding wavelengths).

[0006] In an embodiment, a multi-mode seeker is disclosed. The multi-mode seeker includes an imager having multiple light-sensing elements, each light-sensing element configured to convert photons to photocurrent. The multi-mode seeker further includes image-readout circuitry electrically interfaced to the multiple light-sensing elements and configured to generate image information based on photocurrent of each light-sensing element accumulated over a period of time. The multi-mode seeker further includes pulse-shape readout circuitry electrically interfaced to the imager and configured to continually sense photocurrent produced by the multiple lightsensing elements of the imager and generate one or more signals based on the sensed photocurrent.The pulse-shape readout circuitry is configured to operate independently of the image readout circuitry.

[0007] Implementations of the disclosure may include one or more of the following optional features. In some embodiments, the pulse-shape readout circuitry is electrically interfaced to the imager at multiple points (e.g. the corners of the detector array), each of the multiple points contributing to the total current of the imager at any given time. The multiple light-sensing elements may include an array of light-sensing elements, and the multiple points may include four points but could be as small as three but could be as large as 100. In some embodiments, the pulseshape readout circuitry is further configured to sense photocurrent at each of the multiple points and based on relative magnitudes of the sensed photocurrents from those multiple points, determine a position of a fast pulsed illuminated spot on the imager. The pulse-shape readout circuitry may be further configured to determine a time difference between a first laser pulse illuminating the imager at a first time and a second laser pulse laser pulse illuminating the imager at a second time. In some examples, the pulse-shape readout circuitry is further configured to determine a pulse-rate frequency based on the time difference, determine a pulse shape over time based on the sensed photocurrent, and discriminate between a desired laser pulse and a decoy laser pulse based on the pulse-rate frequency and / or the pulse shape over time. In some embodiments, the image readout circuitry is configured to generate images and the image information includes multiple images generated by the image readout circuitry at a frame rate. The frame rate may be an adjustable frame rate.

[0008] In some examples, the imager has a detector layer that includes an intrinsic or n-type material, the detector layer sandwiched between a cathode layer including a larger bandgap material and a cap layer including the larger bandgap material. The intrinsic or n-type material may include InxGai-xAs and the larger bandgap material includes InP or InAlAs. The multiple light-sensing elements may include multiple p-type pixel regions, each pixel region including doped InxGai-xAs. The doped InxGai.xAs may include InxGai.xAs doped with Zn or Be. The multimode seeker may further include multiple high-speed transimpedance amplifiers electrically connected to the cathode layer. The light-sensing elements may be electrically interfaced to individual anodes, and the image readout circuitry may be electrically interfaced to a common cathode. That was a common cathode structure. The opposite polarity can also be used withintrinsic or p-type material of InxGai-xAs material with larger bandgap material surrounding it. Each of the pixels would be n-type in nature and this would be a common anode structure.

[0009] In an embodiment, a method of correlating a reflected laser position to an image is disclosed. The method includes receiving, by a multi-mode seeker, light from a reflected laser. The method further includes monitoring two or more signals generated by pulse-shape readout circuitry of the multi-mode seeker to detect a relative difference between the two or more signals. The method further includes determining a position, on an imager of the multi-mode seeker, of light from the reflected laser based on the relative magnitude between the three or more signals relative to one another.

[0010] Implementations of the disclosure may include one or more of the following optional features. The method may further include receiving, by the multi-mode seeker, multiple reflected laser pulses, monitoring the one or more signals generated by the pulse-shape readout circuitry to detect the reflected pulses, and determining a time difference between a first reflected pulse and a second reflected pulse at a similar relative position on the imager. In some examples, the method further includes determining a pulse shape over time based on the sensed photocurrent, discriminating between a desired laser pulse and a decoy laser pulse based on the time difference and the pulse shape over time, and determining the position, on the imager, of the desired laser pulse. The method may further include providing the position of the desired laser pulse to a guidance system.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. l is a cross-sectional view of an example semiconductor structure.

[0012] FIG. 2 illustrates an example readout circuit.

[0013] FIG. 3 illustrates an example a laser impinging on the semiconductor structure.

[0014] FIG. 4 illustrates an example readout circuit output from a common photodiode terminal connection.DETAILED DESCRIPTION

[0015] Accurate placement of munitions is critical in the modern battlefield to neutralize heavy armored targets while minimizing collateral damage and preventing fratricide. There are severalmethods to guide a munition accurately with various seeker technologies. One of the more effective methods is using a semi-active laser (SAL). In one example SAL method, a laser of a specific wavelength, pulse frequency, and pulse shape is directed onto a target to designate it. The seeker on the munition detects the reflected laser pulse hitting the target and provides the laser spot location to the guidance system.

[0016] Current SAL systems typically use a silicon photodiode quad detector. The silicon detector may be made large in area and enables detection of 1.064 pm lasers, for example. An imaging optic may be placed in front of the imager with a field of view (FOV) from 20 to 40 degrees, for example. The imaging optic is defocused on the detector to spread the light across the quad sections of the detector. The relative amount of light in each quad allows the location of the laser in the FOV to be determined. The signal from each quad of the detector is buffered by a high speed transimpedance amplifier (TIA) with bandwidths approaching (up to) 0.5 GHz to resolve the laser pulse shape timing. The TIA output is measured to both determine the laser pulse shape (over time) as well as the pulse repetition frequency (PRF) of the laser. These two measurements are used in conjunction to differentiate the desired laser pulse from other laser pulses in the field of view (FOV). In other words, the PRF may serve as a coded identifier of the desired laser. PRFs are typically in the range of a few 10s of Hz.

[0017] Referring to FIG. 1, infrared imaging focal plane arrays (FPAs) may be made of two major components, the detector material 205 and the readout integrated circuit (ROIC) 240. The detector material 205 can detect radiation over a specific wavelength range based on the effective energy gap of the absorption material. For SAL applications, InssGa-nAs (InGaAs) 205 can be utilized as it may detect from 400 nm to 1700 nm allowing for detection of both 1064 nm and 1550 nm lasers with very high quantum efficiency and high speed. For InGaAs, the detector material is processed to form photodiode arrays. The n-type or intrinsic (i) InGaAs detector material may be sandwiched between two larger band gap materials of n-type, for example, InP, InAlAs, or similar. One n-type layer may form a common cathode terminal 202 of the photodiodes and the other acts as a cap (uppermost) layer 220. The lowermost layer 200 may be a substrate including n-type material such as InP or could be non-conductive or semi-insulating. The formation of the pixels may be performed by doping through the cap layer. A p-type dopant like zinc (Zn) or beryllium (Be) may be diffused or implanted through the cap layer to form the p-type absorption layer e.g.,225. The n-type material of the cap layer separates the pixels from one another, with the space between the dopants (e.g., 225) determining the pitch of the pixel array. The number of dopant placement points determines the size of the array. Each one of these placements forms a separate anode (p+ region) of the photodiode array. Each pixel or anode may be electrically connected (coupled) to the ROIC through an ohmic contact 235 placed on top of the anode and a conductor 230 which connects the ohmic contact of the detector pixel to the metal contact on the ROIC 240. A cathode connection between the ROIC and the photodetector array may be made through holes or trenches 250 with metal (ohmic) contacts 235 around the periphery of the array of detectors to electrically connect to the bottom n-type layer of the detector material. This metal 235 is then brought to the surface of the detector chip to allow it to connect to the ROIC, via electrical connection 231, using a similar method as used for the individual pixel anodes.

[0018] Referring to FIG. 2, the ROIC may be positioned behind the infrared detector material with each pixel (for example, 480, 481) on the photodiode array connected to each pixel on the ROIC (for example, 420, 421). This connection 490, 492 may be made using a variety of techniques, all which essentially form conductors between the detector anode and the corresponding ROIC detector interface pads. Traditionally indium bumps may be used to make the electrical connections between the ROIC and detector material. The same metal may be used between the cathode contacts 510, 511 using conductor 491 to electrically connect the cathode of the detector to the ROIC. Cathode contacts may electrically connect each cathode to a high-speed transimpedance amplifier 430, 431 for readout at 400, 401 at rates of up to 10 GHz or even higher.

[0019] Photons that have equal or more energy than the bandgap of the absorbing material, for example, InGaAs, are absorbed and turned into electron-hole pairs. Thus, the absorbing material generates photocurrent proportional to the irradiance across the detector array. The ROIC typically applies a uniform bias 470 to each photodiode and measures their individual currents (for example, 450, 451) using various readout circuit topologies, such as a capacitive transimpedance amplifier (CTIA), direct injection (DI), or source follower per detector (SFD). The holes (of the electronhole pairs) are captured by the anodes (for example, 480, 481) while the electrons are collected by the cathode 500, resulting in current flow. This individual photodiode currents may be accumulated (integrated) over a period of time (e.g., by holding open switches 420, 421) and then measured (for example, by closing switches 440, 441 and reading the signal at 450, 451) togenerate an image at a given frame rate. Frame rates between 10 to 1000 frame per second for full frames may be achieved, for example, depending on the size of the array, application, and available optical signal. Imaging frame rates may be fast enough to determine PRF codes but are not fast enough to measure the shape (the intensity over time) of an incoming laser pulse. Also, many lasers may be present simultaneously in the FOV of the imager and may require many frames at a high enough frame rate to identify the laser with the correct PRF code. Under low light conditions, longer integration times at lower frame rates may be required to collect sufficient signal to form an image with adequate contrast for object identification, reducing the ability to also measure PRF coding under these same illumination conditions. At high light levels the integration time may be much shorter than the frame rate thus there is “dead” time when the imager is not imaging allowing for a laser pulsed to be missed.

[0020] Referring to FIG. 3, a standard ROIC built to be used with InGaAs typically has a common cathode connection driven by a single voltage source to bias the detectors and supply the photodiode current to the entire array, for example. All electrons collected go to the common cathode, for example. In this embodiment, cathode connections may be separated into a finite number of connections, for example, at physically separate locations along the periphery of the photodiode array (for example, 120-123). A typical implementation, for example, would be four cathode connections at each comer of the photodiode array 110 equidistant from its center but could be fewer (for example, three) with no real limit on the upper end. The ROIC 100 will have a corresponding number of connections (for example, 491, FIG. 2) to the cathode, also kept separate from each other on the ROIC, e.g., each TIA may have a separate bias (for example, 460, 461, FIG. 2). The current collected by each of the cathode connections may be individually buffered by high speed transimpedance amplifiers (for example, 130-133). The TIAs may be outside the imaging array and will work asynchronously from the imaging array. The TIAs may have sufficient bandwidth to resolve the shape of the laser pulse photocurrent over time asynchronously to the imaging array frame rate. The amount of current flowing (for example, signal strength 610 versus time 600, FIG. 4) through a given cathode connection (for example, 120, 121, FIG. 3) may be dependent on the relative position of the laser illumination pulse in the photodiode array to that cathode connection (for example, traces 620-623, FIG. 4). The TIAs may measure the cathode connections currents (for example, 120-123, FIG. 3) in parallel, with a lasersignal 140 striking the array producing a large flux of photocurrent over a short pulse width that may be measured by each TIA, and the output may be provided separately from one another and the imaging array. The relative magnitudes of the signals to one another are correlated to determine the pulse location in the array in a similar manner as when using a quad detector. The location may then be further refined and confirmed by analysis of the subsequent image which integrates signal over time through the charge captured by the anodes of each pixel. The TIAs are always measuring current flow through the cathode versus time (600) allowing the timing of each pulse to be measured precisely. The time resolution of the integrated image signal is the frame rate of the produced images. Both of these measurements are done without defocusing the illumination and allows the imager to image both the scene and the laser pulses. The laser pulse position with this method can be determined even if there is no imaging data or if the laser pulse does not occur during the integration period.

[0021] With both SAL pulse and imagery data available, a seeker system can combine ATR with SAL seeker algorithms for improved overall seeker performance. The fast TIA monitoring the current on the cathodes ensures that the system may find the correct laser quickly in 2 to 3 pulses, minimizing the chances for jamming or spoofing while also ensuring the correct laser is being imaged. Free from the requirements of identifying PRF codes, the imaging system’s frame rate and integration time may be independently selected for optimal image signal to noise performance for the current scene illumination conditions.

[0022] A two-dimensional focal plane array may be made of many pixels (picture elements) which are connected to a readout integrated circuit (ROIC). Arrays of photodetectors made of light sensitive material may be attached to a readout integrated circuit (ROIC) to form a focal plane array (FPA). An example of such a material that has photosensitivity from visible through shortwave infrared wavelengths is InAsyPi.y / InxGai.xAs / InAsyPi-y / InP, typically called (InGaAs). Also HgCdTe, colloidal quantum dots, or InSb could be used as the detector material. The ROIC stores charge for each pixel and then reads out the information either serially or in a parallel fashion. A 256x256 array of pixels, for example, requires 65,536 pixels to be read out. To read out that many pixels may take a period of time which may easily support video frame rates (30- 500 frames per second) or even higher (> 1 kHz), but may not allow the measurement of the shape and width of an incoming laser pulse as required for semi-active laser detection, which requiressignal bandwidths > 100 MHz. Typically, semi-active laser detection and tracking is done with quad-photo detectors that may not only measure the pulse shape but also the directional position of the pulse asynchronously. However, these standard quad detectors cannot produce an image. Rather, they determine the direction of the pulse and whether its shape and frequency are correct.

[0023] An imaging array may generate an image of the scene allowing the position of the pulse, relative to other objects, to be determined. This allows for greater accuracy for munition targeting and advanced functions such as automatic target recognition (ATR). Video frame rates support longer integration times of the optical signal, allowing imaging of a scene under low light level conditions. Long integration times, however, are not compatible with high-speed laser pulse shape and repetition rate measurement. Short integration times used in higher light levels leaves time when the imager is not collecting signal and there is a probability the laser pulse could occur during this non-collection period causing the imager to miss the pulse.

[0024] To allow the same photodetector array to both generate an image at video frame rates and measure laser pulse timing characteristics at higher bandwidth, the array’s common photodiode terminal connections may be simultaneously monitored using high speed transimpedance amplifiers (TIAs). The photodetectors used in an FPA are commonly constructed as an array of photodiodes where one photodiode terminal is unique for each pixel location and the other is a common terminal for the entire array. The common terminal of an imaging array is typically connected by redundant contacts at the periphery of the active imaging area of the array and collectively driven by a single voltage source to supply the detector current for all pixel sites at the required bias voltage. In this embodiment, to add the ability to measure laser pulse characteristics, the common terminal connections to the array may be made only at distinct positions around the array. For example, the array may be configured with connections in the four corners of the array equidistant from the center, though other numbers and locations of connections may also be used. The relative amount of current conducted by each connection may be monitored over time using independent TIAs connected to each contact location. The relative current measured at each corner may allow the position of the pulse in the imaging array scene to be determined, similar to how the relative currents measured by the sections of a quad detector illuminated with a defocused optical system allow it to determine laser pulse location within the field of view of the optics. The current flow versus time may be converted to a voltage waveform by the TIA, allowing the pulseshape and frequency to be directly measured. This measurement may be performed asynchronously to the imaging operation of the photodetector array and in parallel in time without interfering with the imaging function. The position calculated from the current flowing through the various positions (for example in this case, the four corners) may be correlated with the image data by identifying image locations that are close to, or at, saturation. The photodetector array may be imaging freely at video frame rates, with a laser pulse within a scene measured and its location determined before the corresponding image frame is read out. This allows the equivalent functionality of a quad detector performing semi-active laser (SAL) detection to be supported with the same photodetectors simultaneously generating two-dimensional imagery.

[0025] While the preceding description discloses an embodiment having a common cathode, embodiments having a common anode are also within the scope of the disclosure.

[0026] The features and functions described above, as well as alternatives, may be combined into many other different systems or applications. Various alternatives, modifications, variations or improvements may be made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments. Accordingly, it will be intended to include all such alternatives, modifications, and variations within the spirit and scope of the appended claims.

Claims

What is claimed is:

1. A multi-mode seeker comprising: an imager comprising a plurality of light-sensing elements, each light-sensing element configured to convert photons to photocurrent; image readout circuitry electrically interfaced to the plurality of light-sensing elements and configured to generate image information based on photocurrent of each light-sensing element accumulated over a specific period of time; and pulse-shape readout circuitry electrically interfaced to the imager and configured to continually sense photocurrent produced by every light-sensing element of the plurality of lightsensing elements of the imager and generate one or more signals based on the sensed photocurrent, the pulse-shape readout circuitry configured to operate independently of the image readout circuitry.

2. The multi-mode seeker of claim 1, wherein the pulse-shape readout circuitry is electrically interfaced to the imager at three or more points, each of the three or more points contributing to the total current of the imager at any given time.

3. The multi-mode seeker of claim 2, wherein the pulse-shape readout circuitry is further configured to: sense photocurrent at each of the three or more points; and based on relative magnitudes of the sensed photocurrents, determine a position of a fast pulsed illuminated spot on the imager.

4. The multi-mode seeker of claim 1, wherein the pulse-shape readout circuitry is further configured to determine a time difference between a first laser pulse illuminating the imager at a first time and a second laser pulse laser pulse illuminating the imager at a second time.

5. The multi-mode seeker of claim 4, wherein the pulse-shape readout circuitry is further configured to: determine a pulse-rate frequency based on the time difference;determine a pulse shape over time based on the sensed photocurrent; and discriminate between a desired laser pulse and a decoy laser pulse based on the pulse-rate frequency and / or the pulse shape over time.

6. A method of correlating a reflected laser position to an image, the method comprising: receiving, by the multi-mode seeker of claim 1, light from a reflected laser; monitoring three or more signals generated by the pulse-shape readout circuitry to detect a relative difference between the three or more signals; and determining a position, on the imager, of light from the reflected laser based on the relative magnitude between the three or more signals relative to one another.

7. The method of claim 6, further comprising: receiving, by the multi-mode seeker of claim 1, a plurality of reflected laser pulses; monitoring the one or more signals generated by the pulse-shape readout circuitry to detect the reflected pulses; and determining a time difference between a first reflected pulse and a second reflected pulse at a similar relative position on the imager.

8. The method of claim 7, further comprising: determining a pulse shape over time based on the sensed photocurrent; discriminating between a desired laser pulse and a decoy laser pulse based on the time difference and the pulse shape over time; and determining the position, on the imager, of the desired laser pulse.

9. The method of claim 8, further comprising: identifying, based on the three or more signals, a position on the imager of the laser image; and using the plurality of light-sensing elements in the area of the position, confirm the position, on the imager, of the desired laser pulse within the plurality of light-sensing elements.

10. The method of claim 9, wherein identifying the position on the imager of the laser image comprises identifying light-sensing elements that have significantly higher signal levels that may be close to, or at, saturation for a given integration time.

11. The method of claim 8, further comprising providing the position of the desired laser pulse to a guidance system.

12. The multi-mode seeker of claim 1, wherein: the image readout circuitry is configured to generate images; and the image information comprises a plurality of images generated by the image readout circuitry at a frame rate.

13. The multi-mode seeker of claim 12, wherein the frame rate is an adjustable frame rate.

14. The multi-mode seeker of claim 1, wherein the imager comprises: a detector layer comprising an intrinsic or n-type material, the detector layer sandwiched between: a cathode layer comprising a larger bandgap material; and a cap layer comprising the larger bandgap material.

15. The multi-mode seeker of claim 14, wherein: the intrinsic or n-type material comprises InxGai-xAs; and the larger bandgap material comprises InP or InAlAs.

16. The multi-mode seeker of claim 14, wherein the plurality of light-sensing elements comprise a plurality of p-type pixel regions, each pixel region comprising doped InxGai-xAs.

17. The multi-mode seeker of claim 16, wherein the doped InxGai-xAs comprises InxGai-xAs doped with Zn or Be.

18. The multi-mode seeker of claim 14, further comprising a plurality of high-speed transimpedance amplifiers electrically connected to the cathode layer.

19. The multi-mode seeker of claim 1, wherein: the light-sensing elements are electrically interfaced to individual anodes; and the image readout circuitry is electrically interfaced to a common cathode.

20. The multi-mode seeker of claim 1, wherein: the light-sensing elements are electrically interfaced to individual cathodes; and the image readout circuitry is electrically interfaced to a common anode.

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