Abstract:
The present disclosure is directed to automatic gain switching circuits for implementation with photodetectors that include a switchable storage network including a storage element. The switchable storage network, such as one or more capacitors, is configured and arranged to respond to a photocurrent from the photodetector and provide an increased storage for the circuit at a predetermined photocurrent. The storage elements can include one or more capacitors that can be coupled to integration capacitors of the photodetector. The switchable networks can include flux sensing switches such as MOSFETS that can activate at a desired or predetermined photocurrent level. Related methods of providing multiple gain values for a photodetector circuit, as well as focal plane arrays and imaging systems with automatic gain shifting are also described.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    Not applicable. 
       BACKGROUND 
       [0002]    Optical detectors commonly use arrays of photodiodes in which each photodiode (or a row or column of such photodiodes in the array) is/are coupled to capacitors as a way to convert the charge produced by the respective photodiodes into voltages corresponding to the photons received by the respective photodiodes. These photodiode arrays are often referred to a charge-coupled devices or “CCDs”. 
         [0003]    Because of the very large dynamic range of photon fluxes than can be encountered over various lighting conditions (e.g., twilight to midday sun), as used in imaging sensors CCD arrays are often subject to and expected to perform well over a photon fluxes differing by five or more orders or magnitude (logs). For example, at low lighting levels such as would be encountered at dusk or in a dimly lit room, a typical photon flux incident on a CCD array of a digital camera would be many orders of magnitude less than for the other end of the optical dynamic range, such as would be encountered under lighting conditions at midday in cloudless weather. Similar dynamic ranges for photon flux levels occur for optical sensors operating in non-visible wavelengths as well, e.g., ultraviolet (“UV”) and infrared (“IR”). 
         [0004]    Saturation occurs for CCD arrays when an integration capacitor connected to the photodiode reaches full charge while the photocurrent is still increasing; any additional photocurrent is not accumulated in the integration capacitor, which can lead to a star pattern or other saturation effects such as so-called pixel blooming. Active Pixel Sensors (“APS”) and CMOS Image Sensors have also had the same or similar limiting dynamic range issues. 
         [0005]    To address such saturation issues while under extreme flux environments, previous attempts at gain adjustment have been made. For example, traditionally the problem has been solved by using a two-channel approach. For scanners, a brute force method of custom low-gain and high gain-channels have been produced. For staring arrays, two channels have been created in the unit cell; one that integrates for a target integration time for low flux levels (corresponding to high-gain), and one that integrates for a significantly lower integration time for higher flux levels (low-gain). 
         [0006]    Both of such traditional solutions for staring arrays and scanned arrays (scanners) require additional unit cell real estate and significant down stream signal processing. In these conventional solutions or techniques, both high-gain and low-gain channels are digitized and compared. Based on the output levels of both channels, a decision is made as to which channel to use. Then, a switch is activated to switch to the desired channel. 
         [0007]    While prior art techniques have proven useful for their respective intended purposes, they can present difficulties or limitations with respect to complexity and cost. What is needed therefore are techniques that address optical sensor saturation problems while at the same time providing relatively simple circuit designs with commensurate costs. 
       SUMMARY 
       [0008]    The present disclosures provides methods, techniques, systems, and apparatus that address the limitations noted previously for prior art techniques. Automatic gain shifting (or switching, e.g., from one gain value or function to another) can be provided by utilizing a switch to selectively add or subtract an individual storage block or network of such storage blocks to a photodetector. Such aspects of the present disclosure can be applicable to MWIR as well as the entire EO spectrum, including but not limited to the UV, SWIR, MWIR, LWIR, and VLWIR. 
         [0009]    One aspect of the present disclosure includes a photodetector and a switchable storage network including a storage element, in which the switchable storage network is configured and arranged to respond to a photocurrent from the photodetector and provide an increased storage for the circuit at a predetermined photocurrent. The storage elements can include one or more capacitors that can be coupled to integration capacitors of the photodetector. The switchable networks can include flux sensing switches such as MOSFETS that can activate at a desired or predetermined photocurrent level. 
         [0010]    Further aspects of the present disclosure are directed to related methods, focal plane arrays and imaging systems. 
         [0011]    Other features and advantages of the present disclosure will be understood upon reading and understanding the detailed description of exemplary embodiments, described herein, in conjunction with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    Aspects of the disclosure may be more fully understood from the following description when read together with the accompanying drawings, which are to be regarded as illustrative in nature, and not as limiting. The drawings are not necessarily to scale, emphasis instead being placed on the principles of the disclosure. In the drawings: 
           [0013]      FIG. 1  depicts a circuit diagram, in accordance with an embodiment of the present disclosure; 
           [0014]      FIG. 2  depicts a graph of output voltage vs. photocurrent for of a circuit in accordance with an embodiment of the present disclosure; 
           [0015]      FIG. 3  depicts a diagrammatic view of a focal plane array with automatic gain switching features, in accordance with an exemplary embodiment of the present disclosure; 
           [0016]      FIG. 4  depicts a diagrammatic view of a generic optical system with a focal plane array with automatic gain switching, in accordance with exemplary embodiments of the present disclosure; and 
           [0017]      FIG. 5  is a box diagram representing a method in accordance with an embodiment of the present disclosure. 
       
    
    
       [0018]    One skilled in the art will appreciate that the embodiments depicted in the drawings are illustrative and that variations of those shown, as well as other embodiments described herein, may be envisioned and practiced within the scope of the present disclosure. 
       DETAILED DESCRIPTION 
       [0019]    Embodiments of the present disclosure are directed to devices, apparatus, systems and methods providing automatic gain switching for optical sensors or photodetectors. Such switching can be provided by utilizing a transistor, e.g., a MOSFET, as a switch to switch in or out one or more additional storage blocks, e.g., capacitors, for the optical sensor. 
         [0020]    Embodiments of the present disclosure can provide an electronics circuit solution for electro-optical applications requiring very large instantaneous dynamic range while preserving sensitivity (maintaining high signal-to-noise ratio) at low flux levels. For example, for Medium Wave Infrared (MWIR) remote sensing, detecting many orders of magnitude of irradiance (photon flux) within the focal plane array (FPA) is desired. As was note previously, this has historically been a challenging problem due to FPA unit cell (pixel) constraints. 
         [0021]    The present disclosure provides techniques utilizing a general purpose circuit that can be implemented in a form to provide very large instantaneous dynamic range for optical sensors, e.g., at the FPA unit cell level. Circuits according to the present disclosure can be used for or implemented with monolithic or hybrid types of FPAs. Circuits of the present disclosure can be implemented in various configurations, and can be used with any suitable type of preamplifier, as described in further detail below. Additionally, the circuits of the present disclosure can be utilized with or for any suitable clamp and/or sample and hold circuits used for FPAs. 
         [0022]      FIG. 1  depicts a circuit diagram of a circuit  100 , in accordance with an exemplary embodiment of the present disclosure, including a photodetector section, e.g., a photodetector unit cell of an FPA,  110  and a switched storage block or storage network section  120 . Photodetector section  110  can include a photodiode  112 . Photodiode  112  can be (but is not necessarily) connected to an integration capacitor  114  and reset switch  116 . Switched storage network  120  can include a switch  122 , e.g., n-MOSFET, and a storage block  126 , e.g., a second capacitor or capacitor network. Circuit  100  can include preamplifier section, denoted by  111  and can include optional additional preamplifier elements as denoted by circuit section  115  with optional representative capacitive transimpedance amplifier (“CTIA”) architecture shown. 
         [0023]    In operation of circuit  100 , switch  122  functions as a flux sensing switch. As a photon flux  1  (with photon energy, hv, indicated) impinges upon photodiode  112 , a corresponding photocurrent  113  is produced. The photocurrent  113  accumulates in integration capacitor  114 . The preamplifier circuit  111  is configured such that, at low flux levels, a small integration capacitor  114  is used for high Signal-to-Noise Ratio (low noise). At higher flux levels, the flux sensing switch  122  activates (e.g., turns off) and an additional storage block/element (e.g., second capacitor  126 ) is automatically switched in to (i) alter the gain (e.g., charge over capacitance) of the circuit  100 , and (ii) map the rest of the desired dynamic range for the optical sensor  112 . If desired, the circuit  100  can be implemented with additional switches and capacitors forming one or more additional switched storage network  120  so that the circuit  100  operates to switch in more capacitance as needed for operation over a desired dynamic range. 
         [0024]    In exemplary embodiments, switch  122  is a MOSFET, e.g., an n-MOSFET. The bulk of the MOSFET is connected to the substrate. The movement of the bulk-to-source potential is advantageously used to trigger the switching (either on or off) of the transistor and thereby connect or disconnect the additional storage elements as needed for the flux conditions present on the photosensor, e.g., photodiode  112 . A MOSFET used as switch  122  can thus provide automatic switching and connection to the additional storage element(s) based on a changing differential between the output voltage  117  of the circuit and the bulk-to-source voltage: Δ(V OUT −V BS ), indicated in  FIG. 1  by V OUT    117 -V WELL    124 . 
         [0025]    With continued reference to  FIG. 1 , for exemplary embodiments including a CTIA preamplifier configuration, as shown in the additional preamplifier elements circuit section  115 , a network of one or more integration capacitors can be automatically switched in and out depending on the incoming flux level (which produces a corresponding photocurrent in the photodiode or photodiodes). The automatic switching mechanism is a switch (transistor) placed between the capacitor feedback node and the CTIA output. The bulk of the transistor is connected to substrate, and as the output of the CTIA integrates downward, the Bulk to Source potential (V BS ) of the switch increases. While the V BS , of the switch increases, the threshold voltage of the switch increases. Eventually, due to the movement V BS , the switch  122  will alter state, e.g., turn off. 
         [0026]    Further illustrating the general applicability of circuits of the present disclosure to different optical sensor preamplifier designs, in embodiments where preamplifier  115  is configured as a source follower with detector  112  (as a source follower per detector, or “SFD”), at the beginning of frame/line integration, the SFD will be high gain mode set by integration capacitor  114 . At a particular flux level, determined by V GAINBIAS  and the semiconductor process, transistor  122  will turn on (as opposed to off in the CTIA previously described) as a result of the difference in the bulk-to-source potential and V OUT . The SFD will then be in low gain mode set by capacitors  114  and  126 . 
         [0027]    With continued reference to  FIG. 1 , exemplary embodiments of circuit  100  can be implemented on a substrate utilizing a deep sub-micron process, e.g., 0.35 micron for IR detectors, and a 0.18 micron process for visible detectors, such as made commercially available by JAZZ Semiconductor. As described in further detail for  FIG. 3 , infra, exemplary embodiments include an array of unit cells of detectors and switchable storage circuits implemented on a suitable substrate. 
         [0028]      FIG. 2  depicts a graph  200  of output voltage vs. photocurrent for of a circuit in accordance with an embodiment of the present disclosure. As shown, at low flux levels, higher gain is provided, as indicated by steeper slope S 1 . This corresponds to the use of a small integration capacitor (capacitance) used for high SNR and low noise. At higher photon flux levels, the flux sensing switch (e.g., as formed by MOSFET shown in  FIG. 1 ) changes state (e.g., turns off) and additional capacitance is automatically switched in to the circuit to map the rest of the dynamic range. 
         [0029]    In  FIG. 2 , slopes S 2  and S 3  correspond to the switching in of additional capacitors (of desired capacitance) to handle higher optical flux levels.  FIG. 2  also indicates transition points T 1  and T 2  between slopes S 1 -S 3 . Transition points T 1  and T 2 , corresponding to when the transition or shift between different gain regimes can be selected, e.g., by adjusting the V GAINBIAS    128  to MOSFET  122  in  FIG. 1 . 
         [0030]    As described previously, a switch (e.g., switching transistor) and storage (capacitive) network, e.g., circuit portion  120  in  FIG. 1 , can be implemented in many different types of configurations and with many different suitable types of preamplifier sections to provide large dynamic gain to optical detectors, e.g., FPAs. Certain non-exhaustive examples of suitable direct injection (“DI”) configuration preamplifier sections/circuits, in which embodiments of the present disclosure can be implemented with or adapted to, are disclosed in U.S. Pat. No. 4,093,872 and U.S. Pat. No. 5,382,977; the entire contents of both of which are incorporated herein by reference. As used herein, the term “DI” is also intended to refer to suitable feedback-enhanced direct injection (“FEDI”) circuits such as those disclosed in U.S. Pat. No. 6,133,596, the entire contents of which are incorporated herein by reference. Certain non-exhaustive suitable source follower (“SF”) configurations in which embodiments of the present disclosure can be implemented with or adapted to are disclosed in U.S. Pat. No. 4,445,117 and U.S. Pat. No. 5,083,016; the entire contents of both of which are incorporated herein by reference. Certain non-exhaustive examples of suitable CTIA configurations for use with or adaptation for embodiments of the present disclosure are disclosed in U.S. Pat. No. 4,978,872, the entire content of which is incorporated herein by reference. Further suitable preamplifier circuit configurations useful for implementation with circuits of the present disclosure include those disclosed in Dakin, et al,  Handbook of Optoelectronics , Taylor &amp; Francis, Inc., Vol. 1 (2006) (see, e.g., pages 112-114); the entire contents of which are incorporated herein by reference. 
         [0031]      FIG. 3  depicts a diagrammatic view of a focal plane array  300  with automatic gain switching features, in accordance with an exemplary embodiment of the present disclosure. As shown, FPA  300  can include a desired number (M×N) of unit cells  302  including photodetectors and automatic gain switching, e.g., circuit sections  110  and  120  of shown and previously described for  FIG. 1 . FPAs according to the present disclosure can be implemented with any suitable optical systems. The FPA can include suitable readout integrated circuitry, or “ROIC,” and can be either of a monolithic or hybrid design. 
         [0032]      FIG. 4  depicts a diagrammatic view of a generic optical system  400  with a focal plane array with automatic gain switching, in accordance with exemplary embodiments of the present disclosure. System  400  includes FPA  402 , configured and arranged at the focal plane of lens  404 . One or more additional lens  406  can be implemented with lens  404  as part of an optical system having desired optical performance characteristics, e.g., focal length, field of view  408  (“FOV”) size, operational wavelength(s), lens material, etc. In exemplary embodiments, optical system  400  can be implemented as an electrooptic imager operational at or over a desired wavelength range, e.g., near infrared (“NIR”) or MWIR, etc. 
         [0033]      FIG. 5  is a box diagram representing a method  500  in accordance with an embodiment of the present disclosure. A first capacitor can be charged with a photocurrent from a photodetector, as described at  502 . A capacitor output voltage can be outputted based on the charge of the first capacitor, as described at  504 . A differential voltage between the capacitor output voltage and a bulk-to-source voltage can be utilized to switch a second capacitor to a parallel connection with the first capacitor, as described at  506 . 
         [0034]    Continuing with the description of method  500 , the gain of the photodetector can be shifted with the second capacitor, as described at  508 . The method  500  can be repeated for multiple photodetectors in a FPA, as described at  510 , such as FPA  300  shown and described for  FIG. 3 . 
         [0035]    Advantages: thus, embodiments of the present disclosure/invention, can provide a compact solution to saturation and the need to accommodate large optical flux dynamic ranges. Embodiments of the present invention do not require downstream signal processing. Hence, they can be more compact, lower power, and ease system implementation and integration. 
         [0036]    Accordingly, compared to the existing technologies, embodiments of the present disclosure can provide the advantage of automatically providing large dynamic ranges for optical sensors. Techniques and apparatus of the present disclosure can be much simpler and easier to implement in integrated circuits than prior art techniques. Systems according to the present disclosure can be compact and do not require downstream signal processing Systems of the present disclosure, which can be disposable, can be relatively inexpensive. 
         [0037]    While certain embodiments have been described herein, it will be understood by one skilled in the art that the methods, systems, and apparatus of the present disclosure may be embodied in other specific forms without departing from the spirit thereof. For example, while storage elements/blocks have been described in the context or one or more capacitors specifically, others may be used within the scope of the present disclosure. For example, a storage element could alternatively be implemented as a register or a series of MOSFETs. 
         [0038]    Accordingly, the embodiments described herein are to be considered in all respects as illustrative of the present disclosure and not restrictive.