Patent Application
20150369670
1. Field of the Invention
This invention relates to imaging bolometers, in particular imaging bolometers that operate without cryogenic cooling to reduce cost, size, weight and power.
2. Description of the Related Art
Imaging bolometers, particularly those that operate without cryogenic cooling, are desired in many imaging applications to reduce cost, size, weight, and power. Such devices collect photons from different points in an outside scene (as imaged on the detector by forward optics), convert these photons to heat in the individual pixels, then sense that heat to measure the image signal. In a typical configuration, each bolometer pixel comprises a micro-machined (MEMS) structure, sometimes referred to as a “bridge”, with legs that physically and thermally isolate the photo-absorptive region of the pixel from an underlying thermal sink. Absorbed photons heat the isolated portion of the pixel. In the most common configurations, this heat is sensed electrically using thermo resistance. The pixel is made from a material with a significant change in electrical resistance vs. temperature, such as Vanadium Oxide (VOX). An electrical current is run through the pixel to sense the resistance, generally running through the same legs that isolate the absorptive structure from the thermal sink. While the performance of such devices has been improving markedly over time, this structure imposes certain inherent limitations.
Unlike with photo-electric (PE) detectors, where the charge created by incoming photons is electrically integrated in a capacitor within each pixel, integration in bolometer pixels is mechanical. Absorbed photons produce heat and that heat is integrated in the thermally isolated part of the pixel. The better isolated the pixel is from the thermal sink (the larger the resistance to heat flow through the supporting legs), the better the signal to noise ratio. However, this is inevitably coupled with slower response to changing inputs (an increase in the effective time constant of the device).
To some extent, PE detectors face the same trade: the longer the integration time, the better the signal to noise ratio (within the limits of the signal integration). But, this trade is much more limiting and constrained for bolometers.
For PE detectors, integration time can be changed electrically, as shown in
In typical infra-red (IR) PE detectors, all pixels are all sampled at the same time. The integration time is adjustable, but the defined integration interval is the same shape and ends at the same time for every pixel in the array. This is referred to as a “snap-shot” readout.
Hence, the temporal response of each PE pixel can be expressed as a flat finite-impulse-response (FIR) filter. Each pixel approximates a uniform-weighted integral of the incoming flux over a programmable and finite integration time that is no larger than the frame time . . . and which may be very much smaller.
“Integration time” has no exact meaning in a bolometer. The nearest equivalent is the detector time constant. It is common, although quite incorrect, to use these terms interchangeably. The time constant of a bolometer specifies how quickly the heat produced by an absorbed photon dissipates into the thermal sink, specifically, how quickly the relative temporal response 106 falls to 1/e 108, as shown in
This time constant is set by the physical structure of the bolometer, specifically by the ratio of the thermal capacity of the integrator to the thermal resistance of the supporting legs. This ratio, and hence the time constant, cannot be adjusted during use. Integration to improve SNR is purely mechanical, integrating heat within this structure, so the detector cannot adapt to changing scene conditions.
Typical bolometer pixels “integrate” continuously, so that the temporal response for each pixel is anchored to (ends at) the time at which that pixel is sampled: that is, the temporal response is measured backward from the readout time.
The temporal response of each bolometer pixel can be expressed as an exponentially decaying infinite-impulse-response (IIR) filter. Each pixel output approximates a fixed (non programmable) exponentially weighted integral of the incoming flux over an infinite integration time, which extends into previous image frames.
Another bolometer trade has to do with the duration of the sense current used to measure the temperature of each pixel. The heat absorbed by each pixel is measured as a thermally-induced change in resistance, by applying a sense current. But, passing a current through the resistance creates unintended heat, which then competes with the sensed signal. This unintended heat must be dissipated to maintain the temperature of the thermal sink. When the pixel resistance increases, the sense current adds even more unintended heat. In typical designs where the pixel resistance increases with temperature, this creates a positive feedback. Under some circumstances this can induce thermal run-away. Hence, it is advantageous to keep the sensing-current pulse short, and to measure the temperature only once per frame, relying entirely on thermal integration for SNR gain.
Some bolometers cancel common mode errors, such as thermal reference drift, by comparing the signals between pixels that are exposed to scene radiation to pixels that are not. In some cases, the scene radiation is masked temporally, using a shutter, so that the same pixels are compared across time. In others cases, some of the pixels are permanently masked, and comparisons are made between different pixels in the same frame.
Some bolometers narrow their spectral sensitivity by placing interferometric structures in front of the absorptive element (U.S. Pat. No. 5,584,117, U.S. Pat. No. 5,629,521 and U.S. Pat. No. 7,145,143 Wood). These structures act as adjustable narrow-band filters. But, unlike conventional interference filters, the spectral pass-band may be altered dynamically by adjusting the spacing between interferometric elements. As in other bolometers, the filtered light is absorbed and the resulting heat is electrically sensed as a change in resistance. The pixel temperature accumulation and measurement mechanism is completely conventional.
The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.
A digital bolometer architecture provides dynamic control of a simultaneous integration time for all pixels, with a temporal response that is more uniform than conventional bolometers and lacks frame cross-talk from decay tails, and which supports sub-frame measurement for on readout computational imaging. This is accomplished by replacing resistive pixel temperature sensing with continuous optical interferometric measurement and subsequent signal accumulation. Balanced reference sensors allow rejection of temperature differences across the thermal sink. The thermal time constant of the pixels is substantially reduced and the lost SNR is recovered by integration of the measured signals, using a programmable integration time.
In an embodiment, a digital bolometer comprises a plurality of pixels configured to detect an image of a scene. Each pixel comprises an absorptive element configured to absorb radiation (e.g. IR or LWIR) from the scene, a thermal isolator connecting said absorptive element to a thermal sink, and a thermally sensitive optical phase shifter configured to phase shift a coherent measurement beam based on the temperature of the absorptive element. An interferometric temperature measurement device is configured to optically route the coherent measurement beam to and from the thermally sensitive optical phase shifter in each said pixel, mix a coherent reference beam with the phase shifted coherent measurement beam from each said pixel to detect a relative phase, and to accumulate the relative phase for each said pixel over a specified integration time as a measure of photon radiation. Readout logic is configured to provide the accumulated values for each pixel to external components.
In an embodiment, the design is configured so that the thermal time constant is reduced to a fraction of a frame interval, and the lost SNR is recovered by integration of the measured signals. The combination of integrating multiple samples with a reduced thermal time constant produces a more uniformly weighted temporal response. This response falls rapidly toward zero outside the signal integration time, greatly reducing frame-to-frame cross talk. And, since accumulation is no longer a purely a mechanical process, it may be dynamically adjusted. This allows dynamic trades of SNR vs. temporal smear and dynamic range, and allows adapting to the scene dynamic range.
In an embodiment, the design is configured so that the pixel thermal time constant is reduced to <½ of the frame interval. In another embodiment it is reduced to < 1/10 of the frame interval.
In an embodiment, the absorptive element comprises a conventional bolometer absorptive material, such as VOX, that changes resistance with temperature. In another embodiment, the absorber comprises an absorptive material, such as micro textured silicon, that is not characterized by a change in resistance over the range of pixel operating temperatures that would make be required for a conventional bolometer.
In an embodiment, an integrated detector chip comprises at least those components other than the measurement and reference beam sources, the phase detectors, and the subsequent accumulators and readout.
In an embodiment, the reference beam passes through a reference phase shifter, aiding cancellation of changes in the thermal sink temperature. This reference phase shifter may be shared between all pixels, or groups of pixels, or separate for each pixel so that variations in temperature across the thermal sink are also cancelled.
In an embodiment, the reference beam may be created by placing a beam splitter in the measurement beam.
In an embodiment, a controllable modulator may be placed in the reference beam, so that the path may be modulated to guarantee some minimum phase cycle rate in order to minimize accumulated quantization error. In an alternative embodiment, this accomplished using a reference source operating at a slightly different frequency. In another alternative embodiment, these approaches are mixed.
In an embodiment the optics guiding the measurement beam through the pixel phase modulator and into the phase detector may be configured as integrated optical waveguides integral to the detector chip. In an alternative embodiment, the measurement beam may be routed via optics external to the detector chip.
In an embodiment the phase shifter comprises a waveguide manufactured from a material with a thermally sensitive index of refraction. In an alternative embodiment the phase shifter comprises an optical element, such as a ring resonator, with a structured configured to change phase delay in response to temperature or thermal expansion, possibly via shifting of a resonance peak.
In an embodiment the phase detector may provide a digital output, consisting of 2 or more bits, to the subsequence phase accumulation logic. In an alternative embodiment the phase measurement and accumulation may be analog.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
a and 6b are diagrams of different embodiments for phase accumulator resets and integration periods; and
a and 7b are plots of pixel relative gain versus photon arrival time illustrating the effect of adding signal integration over a full frame time or half the frame time, respectively.
Imaging bolometers are desired as a lower cost/size/weight alternative to cryo-cooled PE infra-red detectors. But, as previously indicated, this cost reduction comes with other undesirable differences.
The temporal response of each bolometer pixel can be expressed as an exponentially decaying infinite-impulse-response (IIR) filter. That is, each pixel output approximates a fixed (non programmable) exponentially weighted integral of the incoming flux over an infinite integration time. This integration time extends into previous image frames. Pixels across the frame are sampled at different times, which is known as a rolling readout.
These bolometer behaviors are well known. However, the consequences, of these behaviors in some applications are generally neither well understood nor well handled. Behaviors that have no significant effect when imaging a static scene from a fixed position (which is a typical condition for tests and demonstrations), can seriously degrade performance in more stressing conditions. This generally low level of understanding is exacerbated by historical momentum, because operational differences that have no effect in testing typical PE detectors, due to the well-constrained integration time, are critical for bolometers.
Since bolometer temporal response has infinite tails, an “integration interval” can only be defined by determining (explicitly or implicitly) some level at which the response tail will be ignored. The detector “time constant” defines this interval for a cutoff gain of 1/e relative to the peak.
It is tempting to suggest reducing the response tails (and the consequent ghosting) by reducing the pixel thermal time constant. But, this destroys SNR and exacerbates temporal sampling problems.
Another issue is the rapid exponential fall off of the bolometer pixel time response. For any reasonable cutoff level, the bolometer response exponential decay will discard a large fraction of the incoming signal as compared to a FIR PE detector with the same cutoff time. The bolometer also provides much less attenuation for temporal frequencies that are aliased at the frame sample rate.
In typical non-cryogenic applications, the thermo-resistive pixel temperature readout of existing bolometers leaves no flexibility to solve these problems. The reasons have already been discussed: sensing the pixel temperature multiple times per frame adds heat to the pixel, which competes with the signal and degrades SNR.
Another issue is the bolometer rolling readout that causes pixels at different positions in the array to be sampled at different times. As the time constant gets shorter, the reduction in temporal correlation between samples exacerbates aspects of this problem. But even with a typical bolometer time constant, the rolling readout can be a major problem. For one example, closing a high performance control loop around a tracked object using an imaging sensor requires a good understanding of the measurement latency. But, with a rolling readout the latency changes with the object position in the image.
All of these factors pose severe limitations for computational imaging. In a computational imager, each point in the scene image is reconstructed computationally, from combinations of the detector pixels, sometimes over multiple frames. In conventional cameras individual pixels correspond to individual areas in the scene, and the image temporal characteristics at the detector are determined by the scene and the stability of the platform. In contrast, computational imagers intentionally spread points from a scene across multiple sensor pixels, spread energy from multiple points in the scene across multiple pixels, shift the scene across the detector, temporally modulate the optical path. This causes the mapping from the scene to detector pixels to change rapidly with time, or otherwise disrupts the traditional imaging model. Detector pixels no longer correspond to points in the scene, and may change at rates far faster than the scene content. Dharmpal Takhar et. al, “A New Compressive Imaging Camera Architecture using Optical-Domain Compression”, Proceedings of Computational Imaging IV at SPIE Electronic Imaging, pp. 43-52, 2006 provides an extreme example, where a single pixel observes rapidly modulated combinations of points across the entire scene.
As one would imagine, such approaches quickly fall apart when frame pixels are not sampled at the same time or when there is frame-to-frame cross talk. Hence, conventional bolometers are generally unsuitable for computational imaging applications. This is problematic since both bolometers and computational imaging are being looked to for improving system cost/capability trades. Bolometers are currently used in some computational imaging demonstration systems, but this is made possible by accepting limitations that are not viable in many real systems.
As in the development of early PE focal plane arrays, most of these problems are currently masked by other deficiencies of current bolometers. As bolometer performance improves, and as they are applied to more stressing applications, these issues will surface again, providing increased motivation for the present invention.
To summarize, existing bolometer designs electrically measure pixel absorber temperatures via electrical resistance, restrict readout intervals to limit induced heat, integrate photons entirely mechanically as heat in a pixel absorptive element, have a photon response that decays exponentially vs. time, accept frame cross-talk (due to exponential decay tails) as limited by the SNR trade, control a fixed SNR vs. temporal response trade via the absorber thermal time constant, and have a rolling readout where the pixel sample time changes with position in a frame.
The present invention uses interferometric temperature measurement to measure the heat absorbed in each pixel as a measure of photon radiation. This provides an alternate approach to building a bolometer with capabilities similar to those of existing bolometer designs. The interferometric design also provides a foundation that opens a new design space to add unique capabilities that overcomes the above limitations of existing bolometer designs; providing capabilities that currently exist only in PE detectors.
Referring now to
Interferometric temperature measurement device 406 is configured to optically route the coherent measurement beam 414 to and from the thermally sensitive optical phase shifter 412 in each pixel. Phase detector and accumulation logic 413 mixes a coherent reference beam 415 with the phase shifted coherent measurement beam from each pixel to detect a relative phase and accumulates the relative phase for each pixel over a specified integration time as a measure of photon radiation. In some embodiments it is advantageous to use measurement and reference beam sources that are substantially outside the absorption band of the pixel absorptive element.
Readout logic 416 is configured to provide the accumulated values for each pixel to external components. The readout logic may be integrated on the same chip as the pixels and interferometric device or may be provided on a separate chip.
The digital bolometer may use existing bolometer absorber materials, such as VOX for long wave IR. However, it also opens the design space to allow other options. PE detectors require materials that absorb photons and convert them to moving charges, so that the photons may be measured via this current. The second constraint, to produce charges with adequate mobility, increases the challenge of producing adequate materials. Similarly, bolometers require materials that absorb photons as heat, and have a large change in electrical resistance vs. temperature, so this heat may be measured as an measure of the incoming photons. The present invention frees the absorber from the requirement for thermally sensitive resistance. Conventional materials such as VOX may be used, but also other absorbers, ones that do not have significant thermally varying resistance, or which are not electrically conductive . . . as long as photons are absorbed as heat. In particular, this decoupling of the design space may prove enabling for the use of sub-wavelength thickness absorbers . . . that would facilitate configurations with lower time constants. It may also enable the use of micro textured absorbers that might be unsuitable for PE detectors due to low electron mobility. As one example, the material selected for the phase-sensitive ring resonator, possibly silicon, might be micro textured to produce an IR absorber. This design freedom is not generally available when constraint for PE effects of thermo resistance must by simultaneously satisfied.
Interferometric measurement of the pixel temperatures provides the foundation for additional capabilities, capabilities that currently exist only in PE detectors. These capabilities may include ability to dynamically select between one or more of integration times (controlled via signal accumulation), substantially uniform integration weight vs. arrival time for integrated photons, negligible cross-talk (decay tails) between frames, simultaneous integration of pixels within a frame (snap-shot) and measurement of pixel values at greater than the frame rate for on-readout processing.
These capabilities are provided via a variety of alternate embodiments that build upon the foundation provided by the optical interferometric architecture. Specifically, the digital bolometer may be configured to implement one or more of the following: measure absorber temperature at much higher than the frame rate and integrate the measurements to improve frame SNR, integrate photons partially mechanically and partially as measured signals, flatten the overall temporal response through uniform signal integration, limit frame cross talk by limiting mechanical integration, independent of frame SNR, allowing dynamic trade of SNR vs. temporal response by controlling the length of the signal integration, and allow snap-shot readout, where all pixels are measured at the same time
Interferometric measurement of the pixel temperatures is the foundation that makes all of this possible. It does so partly by reducing the heat dissipated into each pixel while measuring its temperature. This allows measurements to be performed at sub-frame rates (even continuously). This in turn frees signal accumulation from being a purely mechanical process; the mechanical integration of heat may be supplement with integration of the measured signal to recover or boost SNR. This allows the thermal time constant to be reduced without sacrificing SNR: it allows the SNR lost through exponential decay in current bolometers to be recovered. It also enables advanced capabilities such as on-readout processing.
The combination of integrating multiple samples with a reduced thermal time constant produces a much more uniformly weighted temporal response. This response falls rapidly toward zero, greatly reducing frame-to-frame cross talk. And, since accumulation is no longer a purely a mechanical process, it may be dynamically adjusted. This allows dynamic trades of SNR vs. smear, and allows adapting to the scene dynamic range.
Interferometeric pixel temperature measurement also enables snap-shot integration by eliminating barriers to measuring all of the pixel temperatures at the same time. Snap-shot vs. rolling readout out becomes controllable via control of the signal integration.
Referring now to
Interferometric temperature measurement device 506 is configured to optically route the coherent measurement beam 514 to and from the thermally sensitive optical phase shifter 512 in each pixel, mix a coherent reference beam 514 with the phase shifted coherent measurement beam from each pixel to detect a relative phase, and to accumulate the relative phase for each pixel over a specified integration time as a measure of the photon radiation.
Readout logic 516 is configured to provide the accumulated values for each pixel to an external interface. The readout logic may be integrated on the same chip as the pixels and interferometric device, or may be provided on a separate chip. The readout is generally configured to sample and output a frame of pixels at a time, with a “frame interval” between the start of one frame and the next. In a given system this frame interval may be fixed, may change during operation, or may be indefinite as frames are sampled and output based on some external signal. Similarly, each frame may sample all or a defined subset of the detector pixels, and the subset may be changed as desired. This is often referred to as a “windowing” readout.
The thermal mass of the absorptive element 508 and thermal resistance of thermal isolator 510 set the thermal time constant of the pixel. Ordinarily the thermal time constant is set to a nominal value equal to the typical frame interval (it cannot adjust to follow changes in the frame interval). In the preferred embodiment, the resistance of the thermal isolator is substantially decreased, reducing the thermal time constant of the pixel integrator by a factor of 2 or more, typically more than 10 to a fraction of the frame rate as shown in
Interferometric temperature measurement device 506 receives coherent measurement beam 514 from a coherent source 518, which may or may not be part of the device. The coherent measurement beam 514 is routed via pixel guiding optics 520 (suitably an integrated waveguide) into optical phase shifter 512 that shifts the phase of the beam in accordance with the pixel temperature. In this embodiment, the phase shifter is implemented using a ring resonator where the resonance peak shifts with temperature.
The phase shifted measurement beam 522 is then routed via pixel guiding optics 520 to a phase detector 524 that mixes it with a coherent reference beam 526 to detect a relative phase 528. As shown, source 518 feeds a coherent beam to a beam splitter 530 that splits the beam to form the coherent reference beam 526 and the coherent measurement beam 514. As shown, the beam splitter is integrated into the waveguide with the pixel guiding optics. Since the reference beam does not pass through the phase shifter 512, the phase detector 524 measures the shift induced by the pixel temperature. In the preferred embodiment, the measured relative phase 528 is represented as a two-bit digital electrical signal, corresponding to the SIGN of the in-phase and quadrature (I/Q) components of the relative phase of the measurement and reference beams. This allows subsequent logic to distinguish between forward and backward phase shifts, allowing a correct integration of delta-phase over multiple cycles, extending the shifter dynamic range.
Accumulation logic 532 integrates the phase 528 over an integration time determined by an integration control input 534. This is used to derive a measure of the integrated pixel temperature, hence photonic radiation 509 over the integration time. The readout logic 516 passes these measures for each pixel in the current output frame to an external interface 536.
In some implementations, the configuration thus far discussed would suffer from severe quantization in the measured relative phase signal 528. This quantization error would be amplified over time when integrated in the accumulation logic 532. In order to avoid this, the signal is manipulated to assure that I/Q SIGN transitions occur with at least a predetermined frequency, so that more accurate phase may be inferred. This may be accomplished by inserting a controllable phase modulator 538 into the reference path, which inserts a known shift determined by a modulator control signal 540. This control signal may be fed to the accumulation logic so that its effects may be removed from the resulting integration while preserving the effective decorrelation of the quantization error so that it may be averaged out. In another embodiment, the minimum I/Q rate is assured by replacing the split reference beam at a slightly different frequency at the second coherent source insertion point at the input to the phase modulator 538. The difference frequency between the sources guarantees a minimum rate. These two approaches may be used individually, or in combination. Additionally, the embodiment includes a reference phase shifter 542 that corrects for thermal drift in the thermal sink 502. This causes the phase detector to more precisely measure the temperature difference between the pixel and it's immediate thermal connection to the thermal sink 502.
This approach cancels interference before the pixel temperature is converted to an electrical signal. A single measurement of relative phase is made from the combined measurement and reference beams: measurement beam and reference beam photons remain entangled at the quantum level until the measurement is made. Optical phase delay is compared, rather than electronic signals.
An alternative embodiment may modify the accumulation to provide for digital stabilization 544 which may be done via in-readout processing. Scene motion inputs 546 estimate the motion of the scene across the pixel grid, and are used to cause the same point in the image to sum into the same accumulator, even though that input does not always come from the same pixel. The mechanisms for accomplishing this are discussed in detail in U.S. Ser. No. 14/064,161 filed Oct. 27, 2013.
The integration control signal 534 may be configured to operate the detector in different ways. As shown in
In another embodiment the integration control signal 534 is configured as shown in
Unlike in a conventional bolometer, the overall temporal response of each pixel depends not only on the pixel thermal time constant, but also on the commanded integration time and the sample rate over that integration. For example,
a and 7b show the effect of adding subsequent signal integration over a full frame time or half the frame time, respectively. Note that the effective period of integration in the first case 700 is slightly longer than the frame time. Similarly, the effective integration time 702 in the second case is somewhat longer than the commanded accumulation. This is due to the thermal time constant. Hence, it is generally preferable to set a maximum integration time limit that is somewhat lower than the frame interval. Note that the temporal response 704 is remarkably uniform compared to a conventional bolometer, even at ½ frame time “integration” 706. This has the consequence of providing greater overall photon gain than any reasonable time constant for a purely mechanical integration.
It is generally desirable for the interval between pixel temperature samples over the integration time to be somewhat less than the thermal time constant. As the sample interval decreases, the “hash” shown in the temporal response 704, 706 flattens out. In some embodiments the effective sampling is continuous, accumulating phase transitions as they occur. Since the thermal time constant induces correlation between samples, there is a point of diminishing returns for improving SNR that depends on the relative contribution of different noise sources in any particular system.
In summary, this digital bolometer architecture provides dynamic control of integration time, with a temporal response that is more uniform than conventional bolometers and which lacks frame cross-talk from response decay tails. It also allows sub-frame measurement for computational imaging methods such as electronic stabilization. This is accomplished by replacing resistive pixel temperature sensing with continuous optical interferometric measurement. Balanced sensors may be included to allow rejection of temperature differences across the thermal sink. The thermal time constant of the pixels is substantially reduced and the lost SNR is recovered by integration of the measured signals, using a programmable integration time. Together, these features fix many of the problems that render bolometers impractical in dynamic and computational imaging applications, opening the trade space to enable a new generation of low cost high performance systems.
While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
