Thermal polarized light source

Abstract

Devices, systems and methods for producing a reliable and stable light source in the thermal infrared region are described. The described sources can be produced at low cost, with a small footprint and low power consumption. One example polarized light source includes a wire-grid element having a plurality of wires positioned parallel to each other to form a grid that can receive an electrical current from an electrical circuit. The polarized light source also includes an unpolarized illumination source, such as a blackbody source, operable at one or more wavelengths in an infrared range that spans between 3 to 14 microns. The wire-gird element is positioned to receive illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the first and the second conductive elements.

Description

THERMAL POLARIZED LIGHT SOURCECROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to the provisional application with serial number 63 / 637,177 titled “A POLARIZED LIGHT SOURCE FOR THE THERMAL INFRARED USING COTS WIRE-GRID POLARIZERS,” filed April 22, 2024. The entire content of the above noted provisional application is incorporated by reference as part of the disclosure of this document.TECHNICAL FIELD

[0002] The disclosed technology relates to polarized light sources that operate in infrared region.BACKGROUND

[0003] Thermal infrared polarimetry has emerged as a powerful tool in various fields, offering insights that traditional thermal imaging cannot provide. Despite its potential, the calibration of polarimetric instruments in the infrared (IR), and especially the long-wave infrared (LWIR), range remains a significant hurdle, primarily due to the lack of reliable polarized thermal sources.SUMMARY

[0004] The disclosed embodiments, among other features and benefits, introduce a device to produce a controlled, polarized IR emission. The disclosed technology can be implemented in various embodiments to produce light sources that are robust to environmental fluctuations, and can be used for generating a linearly polarized signal in the LWIR or thermal IR wavebands for use as a polarimetric calibration target. The disclosed configurations are characterized by having a small size, for print and power consumption (sometimes abbreviated as SWaP for low size, weight, and power), making the disclosed devices suitable for portable IR polarimeters and in-flight calibration of space-based systems, among other applications and uses.

[0005] One example polarized light source includes a wire-grid element that includes a plurality of wires positioned parallel to each other to form a grid, a first conductive element and a second conducive element configured to receive an electrical current from an electrical circuit. Each wire of the plurality of wires is electrically coupled at a first end to the first conductive element and electrically coupled at a second end to the second conductive element. The polarized light source also includes an unpolarized illumination source operable at one or more wavelengths in an infrared range that spans between 3 to 14 microns. The wire-gird element is positioned to receive illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the first and the second conductive elements.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1A illustrates a diagram of a measurement system associated with a wire-grid polarizer in accordance with an example embodiment

[0007] FIG. 1 B illustrates a system configured to measure the polarized signal transmitted by a linear wire-grid polarizer in accordance with an example embodiment.

[0008] FIG. 2 illustrates a wire-grid polarizer configuration with two bonded copper pads for supply of an electric current in accordance with an example embodiment.

[0009] FIG. 3 illustrates a wire-grid polarizer bonded to a heated graphite substrate in accordance with an example embodiment.

[0010] FIG. 4A illustrates multiple wire-grid elements that can be positioned within a single field of view of an imaging system in accordance with an example embodiment.

[0011] FIG. 4B illustrates a thermal image of the targets of FIG. 4A, where the left target is vertically polarized, the center target is unpolarized graphite, and the right target is horizontally polarized.

[0012] FIG. 5A illustrates an example set of tungsten wires bonded between brass plates with silver-based electrically conductive epoxy to form a wire-grid element in accordance with an example embodiment.

[0013] FIG. 5B illustrates mounting of the wire-grid element of FIG. 4A with alligator clips for supply of an electric current.

[0014] FIG. 6 illustrates the offset value (left panel) and the slope per pixel (right panel) calculated from two-point non-uniformity correction in accordance with an example embodiment.

[0015] FIG. 7 illustrates example camera counts versus analyzer angle for different blackbody temperatures associated with intrinsic diattenuation of an uncooled microbolometer focal plane.

[0016] FIG. 8 illustrates example camera counts versus analyzer angle for different blackbody temperatures associated with polarimetric modulation curves with the linear polarizer in the optical path as shown in FIG. 1 B.

[0017] FIG. 9 illustrates example camera counts versus analyzer angle associated with the results of applying an electrical current through a wire-grid polarizer shown in FIG. 2.

[0018] FIG. 10 illustrates example camera counts versus analyzer angle for different heater element powers associated with a single heated target placed in the field of view of the Stokes imaging system.

[0019] FIG. 11 illustrates example camera counts versus analyzer angle associated with from the three-source target presented in FIG. 4.

[0020] FIGS. 12A and 12B illustrate the results of supplying electrical current to wire-grid polarizer with tungsten wires shown in FIG. 5, where FIG. 12A illustrates a thermal image of hot wires and FIG. 12B illustrates Malus surfaces created from averaging across and along the wires for each Stokes analyzer orientation.

[0021] FIG. 13 illustrates a system that can be used to control the operations of a wire-gird polarizer and calibration in accordance with an example embodiment.DETAILED DESCRIPTION

[0022] Calibrating thermal infrared (IR) polarimeters is challenging because of the need for complex and sizable systems. Furthermore, IR systems experience performance fluctuations in short time frames, underscoring the importance of frequentrecali bration. Existing compact calibration methods have inherent limitations; notably, combining an unpolarized source with a wire-grid polarizer may lead to the polarizer emitting unpolarized light upon heating.

[0023] Furthermore, the continued push for small SWaP instruments to fit within small confines, such as part of the payload of a small satellites (e g., CubeSat) imposes further limitations on the specification of calibration modules. The desire for small form factors has resulted in drastically increased interest in the use of uncooled microbolometers (UMBs) in polarimetric systems. These detectors are known to exhibit diattenuation. Generating a signal in the thermal IR or LWIR bands with well- characterized and stable polarization is critical to the proper calibration and validation of thermal polarimeters. In this patent documents, the degree and angle of linear polarization (DoLP and AoLP, respectively) are used as example polarization parameters to illustrate the characteristics of polarized thermal and LWIR radiation in the experiment and analyses described herein.

[0024] It should be noted that the terms thermal IR and LWIR are used in connection with various embodiments. While thermal IR is typically considered to span a range of about 3-14 pm while LWIR typically spans a range of about 8-14 pm, the embodiments of the present technology are operable and applicable to both (or either) spectral ranges.

[0025] In the description that follows, section headings are provided to facilitate the explanation of certain aspects and not to limit the discussion of the topics only to within those sections.Overview of Example Radiometric Calibration Requirements

[0026] Thermal imaging systems have calibration requirements unique from those in the visible to near infrared (NIR) wavebands. In particular, the individual pixel response can vary spatially across the focal plane as well as drift with time and operating temperature. Non-uniformity correction (NUC) is performed with two or more target temperatures, the two-temperature instance being referred to as a "two-point NUC," to account for differences in pixel response across the focal plane. Flat field correction (FFC) is a single-point correction used to adjust the offset of the pixel response resulting from focal plane temperature fluctuations. NUC is commonlyperformed in the lab setting, while FFC is performed in situ on a more frequent schedule. Both NUC and FFC require a large area, uniform source to illuminate the system. For two-point NUC, this source must be capable of presenting a high and low temperature signal to the optical system. The individual pixel count value is mapped as a linear function of the average detector count value to produce a slope and offset correction factor.Overview of Example Polarimetric Calibration Requirements

[0027] Polarimetric calibration in the LWIR spectrum requires detailed knowledge of the Stokes vector of the radiation source, which describes the state of polarization of light. For an effective calibration, the source must consistently exhibit high DoLP and a stable AoLP across the entire operational temperature range. Typically, a data reduction matrix that represents the polarimeter system is calculated through Mueller calculus or experimentally measured in lab and assumed to be unchanging. Considering that narcissistic radiation and environmental fluctuations can alter the data reduction matrix for LWIR polarimeters, a predetermined model presents limited fidelity over extended deployments. This necessitates a robust source that can maintain its polarization characteristics despite fluctuations in ambient temperatures and thermal radiation from surrounding objects. Narcissistic radiation, which is based on the Narcissus effect, is a source of noise in thermal imaging systems and occurs when the detector sees a reflection of itself, often resulting in a dark spot at the center of the image.In Situ Calibration Instrumentation and Techniques

[0028] In situ calibration is of interest for any thermal imaging system which is expected to operate in dynamic environments and for extended deployment periods, such as long-duration high-altitude balloon flights or satellite-based systems. Radiometric calibration can be achieved without the need for onboard calibration units. Vicarious calibration relies on data acquired experimentally or from ground-based telemetry stations to calculate the Top of Atmosphere radiance using radiative transfer algorithms. Cross-calibration compares observations of the same target from multiple systems. These two methods commonly use the scene radiance of large bodies of water as a source of uniform, known temperature. However, dynamics of Earthobservations and asynchronous data acquisitions limit the accuracy of such targets. FFC can be achieved with a "cold-sky view" in which the thermal signal from space is directed into the field of view of the system. The thermal signal from deep space is constant and uniform 2.7 Kelvin, well below the minimum resolvable temperature of most thermal detectors. None of these calibration methods provide any use for polarimetric calibration purposes.

[0029] Onboard calibration units have been developed for both balloon- and space-based systems. Phase change materials and passive heat pipes provide a low- cost and energy-efficient method of producing a stable blackbody (BB) target. Such materials are not conducive to use as a BB target with adjustable temperature. Thermoelectric coolers (TECs) present a convenient SWaP for satellite-based systems to produce a large area, uniform BB target with a wide range of set temperatures.Generating a Polarized Signal

[0030] A well-characterized broadband polarized signal with high DoLP can be generated using simple linear polarizers. Rotating these polarizing elements adjusts the AoLP. However, applying such techniques in the LWIR comes with added complications. Linear polarizers in the LWIR are mostly limited to wire-grid elements, which operate by reflection and transmission of orthogonal polarization states. Since all objects above zero Kelvin emit thermal radiation, the environmental radiation reflected into the signal path can reduce or even dominate the polarization properties of the signal of interest. Furthermore, the narcissistic radiation from the focal plane and other optical elements in the signal path degrades the quality of observations.

[0031] Laser sources provide a narrow-spectrum signal with high DoLP and limited spatial extent, unless bulky beam expanders are utilized. When combined with a rotating half-wave linear retarder, adjustable AoLP can also be achieved. LWIR lasers, which cover the 8-12 pm wavelengths nearly continuously, are readily available and, while expensive, the costs and SWaP are not prohibitive for use in field-deployable systems. However, expanding the beam to the diameters necessary for NUC and FFC would require not only expensive, heavy, and large optics but also optics with a small F-Number, inducing polarization aberrations. More fundamentally, the non-uniformGaussian illumination profile produced by typical beam expanders is not conducive to correcting non-uniformities in detector response.

[0032] Thermal emission from smooth surfaces typically shows high DoLP at low emission angles due to Fresnel interactions at the material's surface. Research has demonstrated polarized emissions from thin films and wires across visible to mid-wave IR bands. The AoLP and DoLP from thin wires correlate with wire diameter (or cross- sectional area); emissions from wires significantly smaller than the wavelength typically exhibit high DoLP with AoLP aligned with the wire's orientation. Conversely, for diameters comparable to or larger than the wavelength, DoLP decreases and AoLP becomes orthogonal to the wire. However, prior systems, among other shortcomings, are not shown to operate in the thermal IR or LWIR range, only describe a single wire, and are disposed in vacuum chambers. Hence, they do not provide a viable calibration source in the desired spectral region, and can be costly and difficult to fabricate and implement.

[0033] The disclosed embodiments overcome the shortcomings of the prior systems, and among other features and benefits, introduce a calibration target device that utilizes thin wires to produce a controlled, polarized IR emission. By applying an electrical current through small-scale (e.g., nano-scale or micron-scale) wires of a wiregrid polarizer, the device produces a consistent source of polarized thermal infrared light with a pronounced degree of linear polarization. According to some embodiments, a robust, reliable and low SWaP calibration source in the LWIR range (8-12 microns) can be produced by running a current through nano-scale wires in a wire-grid polarizer or using, for example, 20-micron free-space wires, which yields polarized emissions, with AoLP aligned either parallel or orthogonal to the wire orientation, respectively. For example, a well-characterized polarized source in the thermal infrared can be produced for the verification, validation, and in-flight calibration of airborne and space-based thermal polarimeters that is robust to the environmental fluctuations.Experimental Setup

[0034] The example experimental setup disclosed herein can be used to facilitate the understanding of the underlying concepts. FIG. 1 A illustrates a simplified diagram of a measurement system associated with a wire-grid polarizer in accordance with anexample embodiment. The configuration in FIG. 1A includes a blackbody (BB) source. BB sources are often used for calibration of IR instruments, and it provides a heated (or cooled) source with a temperature that can be set to a precise value. The wire-grid polarizer is placed in a path between the BB source and an IR detector device, which in the example configuration of FIG. 1 A is a FLIR Boson uncooled microbolometer. The top path (arrow originating from the BB source that reaches the IP detector) represent thermal radiation from the BB source that is linearly polarized after passing through the wire-grid polarizer. The bottom path (curved arrow) represents reflected or stray light that can reach the IR detector; the stray light details are further illustrated and explained in connection with FIG. 1 B.

[0035] FIG. 1 B shows an example experimental setup configured to measure the polarized signal transmitted by a linear wire-grid polarizer in accordance with an example embodiment. A simple rotating polarizer linear Stokes imaging system was used to measure the linear polarization properties of the various methods for generating a polarized signal. This system includes of a FLIR Boson UMB and a wire-grid polarizer (“Fixed Polarizer”) in a motorized rotation mount that allows the wire-grid polarizer to be rotated with respect to the optical axis (i.e., the line connecting the BB source and the IR detector). An extended area blackbody (BB) source was used to illuminate the system with a uniform signal. The BB source was used for the initial NUC and FFC of the UMB. This BB also served as a stable background for all measurements where transmission was present. The linear polarizer was rotated through 360° in 22.5° steps. The transmitted signal (arrow originating from the BB source that reaches the IP detector) is polarized out of the plane of the page. The presence of stray and / or reflected light that is inevitably present in real systems is simulated using a reflection reference, which can be rotated to cause the reflected signal to be directed to the IR detector. Notably, when using any wire-grid polarizing elements, the reflected signal must be well characterized. Therefore, a reference BB target was placed next to the UMB. Reflective elements were then tilted at 10° to reflect this reference target. For all subsequent measurements, the reflection reference target remained at +25°C. It should be noted that the reflected signal in FIG. 1 B is polarized within the plane of the page and the transmitted signal is polarized out of the plane of the image. As such, the polarizations of the reflected and transmitted signals in FIG. 1A and 1 B are reversed;these merely illustrate different example orientations of the wire-grid polarizer in the system.

[0036] Electrical Contacts: To test the feasibility of running electrical current through a wire-grid polarizer, two copper pads were bonded to an off-the shelf wire-grid, and specifically to the wire surface of the element with electrically conductive silverbased epoxy as shown in FIG. 2. It should be noted that the wire-grid element shown in FIG. 2 includes a substrate that accommodates the wires. The unit was placed in front of the transmission BB and tilted at 10° to reflect the reference BB target. The transmission BB was set to a static -5°C and reference BB remained at +25°C. Electrical current was supplied through the alligator clips. The wire-grid polarizer was observed under increasing power supplied across the wires. A bluish hue observed in the center of the wire-grid element was indicative of the epoxy solvent having wicked into the wire structure through capillary action. This artifact is due to the simple way this experimental unit was assembled, and can produce undesired affects that are not present when the wire-grid unit is more meticulously assembled. Experimental results are shown in FIG. 9 (discussed below).

[0037] Heating Element: Placing the wire-grid polarizer in direct contact with a heating element may be another way of producing linearly polarized radiation. This technique, while offering a compact form factor, it may not be feasible in all applications in-part due to increased noise and interference due to the thermal source, and the added power required for activating the heating element. To illustrate the operations of a heated wire-grid element, a small resistive heating element was bonded to a graphite disk which was then bonded directly to the substrate of the polarizer with the wire-grid side facing out as shown in FIG. 3. In particular, the panel on the left side illustrates the wire-grid element mounted in a three-point lens mount to limit contact points for thermal conduction. The middle panel in FIG. 3 illustrates a side view in conjunction with a ruler to provide a sense of scale. Notably, in the example device of FIG. 3, the graphite substrate is 6 mm thick, and is formed as a 12 mm diameter disk. The right panel of FIG. 3 illustrates a front view of the wire-grid element that is placed in the mount. The graphite provides a more stable and uniform temperature distribution than would be present with the heater bonded directly to the wire-grid substrate. The unit was tilted at10° to reflect the reference target. Experimental results are shown in FIG. 10 (discussed below).

[0038] For polarimetric calibration, multiple AoLP signals are required to fully reconstruct the linear Stokes parameters. The small form factor of a heated polarizer provides an opportunity to place multiple targets together in a single field of view (FOV). To this end, an additional target was fabricated and placed in the FOV of the Stokes imagers along with a bare graphite target for radiometric reference. An example design is shown in FIG. 4A illustrating an RGB image of three thermal targets with different polarization signatures that are all placed in the FOV of a rotating polarizer linear Stokes imaging system. FIG. 4B illustrates a thermal image of the targets of FIG. 4A, acquired with analyzer angle of zero degrees. The left target is vertically polarized, the center target is unpolarized graphite, and the right target is horizontally polarized. The analyzer polarizer for this image was oriented with the horizontal transmission axis. Experimental results are shown in FIG. 11 (discussed below).

[0039] Custom Electrified Wire-Grid Without a Substrate: To illustrate the operations of the wire-grid polarizer with an electrical current running therethrough, a custom wire-grid element was constructed without a substrate. In one example setup, to test 20 pm wires in free space, an 18 mm tall by 15 mm wide target was fabricated, as illustrated in FIGS. 5A and 5B. Notably, two brass pads were placed on a foam base allowing for the wire to be continuously wrapped with each subsequent pass laid parallel to the previous. Once several wraps were performed, silver-based epoxy was dispensed over the wires and a second brass pad was used to hold the wires in place and act as a simple electrical interface. This is shown in FIGS. 5A and 5B, where FIG. 5A illustrates a 20-pm diameter tungsten wires (14 wires) that are bonded between brass plates with silver-based electrically conductive epoxy. FIG. 5B illustrates mounting of the wire-grid element of FIG. 5A with alligator clips. Once the epoxy was cured, the excess wire was cut from the target. The tension and uniformity of resistance for each wire were not measured. While great care was taken to ensure the wires were parallel and uniformly spaced, this target was fabricated by hand and by eye. Therefore, manufacturing errors are present.Example Results

[0040] While the uncooled microbolometers (UMB) has factory-calibrated nonuniformity correction (NUC) to account for the particular lens installed on the Boson module, the introduction of the analyzer polarizer requires a new NUC. This new correction accounts for non-uniformities caused by the self-radiance of the polarizer as well as the intrinsic diattenuation of the focal plane but have no impact on the polarimetric modulation. The precision BB was used to illuminate the system with a uniform signal. The BB source did not fill the entire 34° FOV of the UMB. This was not a concern for the measurements as all test units are compact and only subtend a small region of the focal plane within the corrected region. The NUC was performed at BB temperatures of -5°C and +50°C. FIG. 6 illustrates the offset value (left panel) and the slope per pixel (right panel) calculated from two-point NUC.

[0041] Once the NUC and initial FFC were applied, the next task was to analyze the polarimetric response intrinsic to the UMB. This was done in similar fashion to the NUC with additional temperatures between the two NUC temperature points. When the BB is set to a low temperature, polarimetric modulation is observed from the narcissistic radiation reflecting off the analyzer polarizer and becoming polarized. As the BB temperature is increased, modulation reduces to near zero when in equilibrium with the focal plane temperature, 40°C. While a simple subtraction of the lowest temperature measurement would be appropriate to correct for this polarimetric response and remove the thermal pedestal of the measurement setup, it was decided to leave all data in a raw form, only applying the NUC and FFC.Linear Polarizer in Path

[0042] Placing a linear polarizer in the path of a BB target is the simplest method of producing a polarized signal. As outlined earlier, this presents unique challenges in the LWIR, particularly due to reflections from environmental radiation. To demonstrate this, a wire-grid polarizer was placed in front of the precision BB as depicted in FIG. 1 B. The polarizer was tilted at 10° from normal incidence to reflect the reference BB target. Data of the resulting signal was captured with transmission BB temperatures ranging from -5°C to +50°C as shown in FIG. 7. In particular, FIG. 7 illustrates intrinsic diattenuation of UMB focal plane with the analyzer removed; notably, camera countrepresents the number or camera elements that registered a detection (i.e., the strength of the detections) versus the orientation angle of the analyzer for different temperatures of the BB source. The plots in FIG. 7 show that polarimetric modulation is observed from the narcissistic radiation reflecting off the analyzer polarizer and becoming polarized. As the BB temperature is increased, modulation reduces to near zero when in equilibrium with the focal plane temperature, 40°C.

[0043] The images were averaged over 2000-pixel region of interest (ROI) centered on the transmitting region of the linear polarizer with resulting modulation curves presented in FIG. 8. The linear polarizer was oriented such that its transmitted signal was vertical. Results from this measurement are shown in FIG. 8, which illustrates that for low BB temperatures, the modulation is representative of the reflected, horizontally polarized signal; for high temperatures, the signal is aligned with the transmitted, vertically polarized signal. Modulation is minimized when the BB temperature is equal to that of the reflection reference target, around +25°C. In other words, when the precision black body is below the temperature of the reflection reference target, the observed signal is dominated by the reflected signal with AoLP oriented near zero. As the BB temperature is increased, the modulation amplitude decreases to zero when in equilibrium with the reflection reference target. As the BB temperature is increased past the equilibrium point, the modulation begins to increase and the AoLP flips to be oriented with the transmitted signal, near 90°.Electrified Polarizer Using Off-the-Shelf Wire-Grid

[0044] Running current through the nano-scale wires of a wire-grid polarizer produces a polarized signal aligned with the orientation of the wires. This would constructively combine with the modulation from the reflection reference BB. However, this was not fully observed when an off-the-shelf wire-grid with a substate (see FIG. 2) was electrified. Instead, the modulation was slightly reduced as the current was supplied. FIG. 9 illustrates the results of applying an electrical current through a wiregrid polarizer shown in FIG. 2. The mW numbers shown in FIG. 9 correspond to the electrical potential associated with the current through the wire-grid. FIG. 9 plots illustrate that the modulation is dominated by the reflected polarization signal. Observed reduction of polarimetric modulation when power is supplied is attributed to the heatingof the polarizer substrate and subsequent transmission through the wire-grid. The suboptimum performance is believed to be the result of heating the substrate which then radiates through the wire-grid, producing an orthogonal polarization state. These results are attributed to errors in attaching the copper pads to the polarizer. Capillary action caused the epoxy to wick into the wire structure, providing a conductive path to bypass the wires. A more meticulous assembly and more precise fabrication techniques, along with the selection of proper wire configurations (further discussed below) can be used to produce more optimal results.Heated Wire-Grid Polarizer

[0045] The resistive heaters used in this work are power-limited at 1.4 W output. The heater reached saturation and became current-limited with 20 V supplied. With no power supplied, the graphite is in equilibrium with the reflection reference and thus no polarization modulation is observed. Results for a single heated target (i.e. , the off-the- shelf wire-grid element with a substrate and a heating element shown in FIG. 3) placed in the FOV of the Stokes imaging system are presented in FIG. 10. The power values (in Watts) shown in FIG. 9 represent the power supplied to the heating element. In this configuration, the wire-grid element was not electrified. With no power applied to the heater, the graphite is in equilibrium with the reflection reference and thus no polarization modulation is observed. As more power is supplied, the DoLP increases and a constant offset is introduced.

[0046] An additional heated polarizer unit was introduced along with the bare graphite target. The two polarized sources were oriented such that their AoLP were orthogonal to maximize the contrast during a single measurement. The two targets behaved identically to the single target. The polarimetric modulation for the vertically polarized target was increased due to the diattenuation of the UMB having a strong response for this AoLP. In particular, FIG. 11 illustrates the results from the three- source target presented in FIG. 4. The vertically polarized target presented a slightly higher modulation amplitude compared to the horizontally polarized target, which is attributed to the intrinsic diattenuation of the UMB.Electrified 20 urn Wires

[0047] The below experiment uses 20 pm diameter wires that are expected to produce a LWIR polarized signal with the AoLP orthogonal to the wire orientation. Notably, the 20 pm wire diameter is comparable to the wavelengths of interest. In this experiment, an electrical current was passed through 20 pm tungsten wires (see FIG. 5), and a polarized signal with the AoLP oriented orthogonally was observed. The maximum modulation occurred with a 2-V potential across the wires. FIGS. 12A and 12B illustrate the results of supplying electrical current to 20 pm diameter tungsten wires shown in FIG. 5. Images presented are captured with 2 V potential applied to the wires. FIG. 12A illustrates a thermal image of hot wires. FIG. 12B illustrates Malus surface created from averaging across (top) and along (bottom) the wires for each Stokes analyzer orientation. The polarization is orthogonal to the wire orientation. The modulation amplitude was approximately 600 counts, significantly higher than the 40- count amplitude of the intrinsic diattenuation observed in the UMB. However, this measurement likely underestimates the actual modulation, as the sub-resolution diameter of the wires introduces unpolarized background radiation into the pixel FOV, thus reducing the signal modulation. Beyond the 2-V voltage, a hotspot developed on one of the wires, causing measurement saturation and eventual wire melting.

[0048] These results illustrate that supplying an electrical current to a simple wiregrid element produces a linearly polarized light in the thermal and / or LWIR ranges. Various factors influence the design and behavior of the wire-grid polarizer. These include the size (diameter or cross-sectional area) of the wire compared to the wavelength (or range of wavelengths) of interest, the density (or spacing) of the wiregrid, the material of the wires, and the amount of electrical current (or power) that is supplied to the wire-grid element. These factors enable tuning and configuration of the wire-grid linear polarizer for a specific application and a particular set of requirements. In some embodiments, the cross-sectional area (or the diameter) of the wires are selected to be within one order of magnitude of the IR wavelength (e.g., average wavelength) of interest. In some embodiments, the ratio of the wire diameter to the wavelength of interest is between 0.01 and 0.08. In some embodiments, a wire diameter to wavelength ratio less than 1 / (2TT) is advantageous. In some applications, the size of the wires are selected based on the resolution of the imaging system; forexample, the wires are selected to have diameter such that individual wires cannot be resolved when imaged by the IR detector. While the disclosed wire-grid polarizers are low SWaP (small size, weight, and power) compared to the existing LWIR calibration sources, in applications where size is more critical, a more densely packed wire-grid can be configured to minimize the spaces between the wires. Such a configuration also reduces the impact of stray background radiation, but may consume more power compared to lower density wire-grid element. In applications, where low power consumption is critical, the wire-gird may be configured based on materials that exhibit lower resistance, thus providing higher efficiencies due to a reduced ohmic loss. For high-power observations, a vacuum or inert gas environment will prevent oxidation. Alternatively, using wires made of noble metals like gold or silver might allow higher operational temperatures in an open-air environment.

[0049] The disclosed wire-grid polarizers are advantages over the prior systems because they are simple and low cost to produce, while at the same time are compact, low weight, and provide an efficient and reliable LWIR and / or thermal IR source operable in different environmental conditions. The disclosed wire-grid polarizers are thus suitable candidates for calibrating LW and thermal I PR instrumentation and imaging systems. The spatial extent of the wire-grid array can be customized to fit or correlate to the field of view of the imaging system that is being calibrated. For example, the wire-grid polarizer can have an area in the range 1 mm2to 10 cm2. In some embodiments, an extended-area BB source with multiple polarizers bonded to the surface can be implemented to provide the four polarization angles required for full linear Stokes imaging. Coupled with a second BB for reflection reference, the system provides a means to tune the DoLP and modulate the AoLP for each bonded element.

[0050] One aspect of the disclosed embodiments relates to a polarized light source that includes a wire-grid element that includes a plurality of wires that are positioned parallel to each other to form a grid, a first conductive element and a second conducive element configured to receive an electrical current from an electrical circuit. Each wire of the plurality of wires is electrically coupled at a first end to the first conductive element and electrically coupled at a second end to the second conductive element. The polarized light source further includes an unpolarized illumination source operable at one or more wavelengths in an infrared range that spans between 3 to 14microns. The wire-gird element is positioned to receive illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the first and the second conductive elements.

[0051] In one example embodiment, the unpolarized illumination source is a blackbody illumination source that can be set to operate at one or more blackbody temperatures. In another example embodiment, the plurality of wires includes 14 wires. In yet another example embodiment, a cross-sectional diameter, or a cross-sectional area, of each of the plurality of wires is within one order of magnitude of the one more wavelengths associated with the illumination received by the wire-grid element from the unpolarized illumination source. In still another example embodiment, a diameter of each of the plurality of wires is 20 microns, and each wire comprises tungsten.

[0052] According to another example embodiment, the electrical circuit is configured to produce an adjustable output potential. In one example embodiment, the output potential is 2 volts or less. In another example embodiment, the linearly polarized light is polarized orthogonal to an orientation of the plurality of wires of the wire-grid element. In still another example embodiment, the wire-grid element is a free-space wire-grid element that excludes a substrate. In yet another example embodiment, the plurality of wires of the wire-grid element are uniformly spaced.

[0053] In one example embodiment, the linearly polarized light is characterized by an angle of linear polarization (AoLP) that remains substantially invariant in a presence of stray light. In another example embodiment, the plurality of wires are made of gold or silver. In another example embodiment, a density or spacing of the plurality of wires is selected to mitigate an effect of stray radiation. In yet another example embodiment, a density or spacing of the plurality of wires is selected based on a specified power consumption requirement for the wire-grid element. In still another example embodiment, the wire-grid element is bounded to a surface of the illumination source.

[0054] In another example embodiment, the wire-grid element is a first wire-grid element, and the polarized light source includes one or more additional wire-grid elements coupled to the electrical circuit, where each of the one or more additional wiregrid elements comprises a set of parallel wires that are oriented at a different anglecompared to the first wire-grid element. In this example embodiment, each of the one or more additional wire-grid elements are positioned to receive the illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the one or more additional wire-grid elements, thus each of the wire-grid elements producing linearly polarized light at a different angle of linear polarization. In one example embodiment, the first and the one or more additional wire-grid elements consist of four wire-grid elements that produce four polarization angles to enable a full linear Stokes imaging.

[0055] In another example embodiment, the polarized light source includes a reference reflector positioned to reflect at least a portion of the illumination originating from the unpolarized illumination source, wherein an amount or direction of light reflected by the reference reflector is adjustable to control one or both of a degree of polarization (DoLP) and an angle of linear polarization (AoLP) associated with the wiregrid element. In still another example embodiment, the infrared range spans 8-14 microns. In yet another example embodiment, the polarized light source is configured as a calibration source in a high-altitude balloon, a satellite-based system, or a spaceborne vehicle.

[0056] Another aspect of the disclosed embodiments relates to a wire-grid element operable to generate a polarized light. The wire-grid element includes a plurality of wires that are positioned parallel to each other to form a grid, a first conductive element coupled to a first end of each of the plurality of wires, and a second conducive element coupled to a second end of each of the plurality of wires. The wire-grid element is configured to receive an electrical current through the first and the second conductive elements, and upon receiving the electrical current and receiving illumination from an unpolarized light source operable at one or more wavelengths in a thermal or long wave infrared spectra, the wire-gird element transmits linearly polarized light in the thermal or long wave infrared spectra.

[0057] Various operations disclosed herein can be implemented using a processor / controller is configured to include, or be couple to, a memory that stores processor executable code that causes the processor / controller carry out variouscomputations and processing of information. The processor / controller can further generate and transmit / receive suitable information to / from the various system components, as well as suitable input / output (IO) capabilities (e.g., wired or wireless) to transmit and receive commands and / or data. The processor / controller may receive the information associated with optical rays and material parameters, and further process that information to simulate or trace rays throughout an optical system. FIG. 13 illustrates a system that can be used to control the operations of a wire-gird polarizer and calibration in accordance with an example embodiment. The processor or controller 1302, for example, can be configured to control the illumination (e.g., BB) source and / or an IR detector (1304), to collect, receive and process polarimetric and radiometric information to enable the various computations, and to control the operations of an electrical circuit 1306 to supply electricity to the disclosed wire-grid elements; the processor can further be coupled to one or more movable stages 1308 to control rotations and movements of various components such as the wire-grid polarizer or the polarizer analyzers disclosed herein.

[0058] Various information and data processing operations described herein may be implemented in one embodiment by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer- readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Therefore, the computer-readable media that is described in the present application comprises non- transitory storage media. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

[0059] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document

Claims

CLAIMSI / We claim:1 . A polarized light source, comprising: a wire-grid element comprising a plurality of wires that are positioned parallel to each other to form a grid; a first conductive element and a second conducive element configured to receive an electrical current from an electrical circuit, wherein: each wire of the plurality of wires is electrically coupled at a first end to the first conductive element and electrically coupled at a second end to the second conductive element; an unpolarized illumination source operable at one or more wavelengths in an infrared range that spans between 3 to 14 microns, wherein: the wire-gird element is positioned to receive illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the first and the second conductive elements.

2. The polarized light source of claim 1 , wherein the unpolarized illumination source is a blackbody illumination source that can be set to operate at one or more blackbody temperatures.

3. The polarized light source of claim 1 , wherein the plurality of wires includes 14 wires.

4. The polarized light source of claim 1 , wherein a cross-sectional diameter, or a cross-sectional area, of each of the plurality of wires is within one order ofmagnitude of the one more wavelengths associated with the illumination received by the wire-grid element from the unpolarized illumination source.

5. The polarized light source of claim 1 , wherein a diameter of each of the plurality of wires is 20 microns, and each wire comprises tungsten.

6. The polarized light source of claim 1 , wherein the electrical circuit is configured to produce an adjustable output potential.

7. The polarized light source of claim 6, wherein the output potential is 2 volts or less.

8. The polarized light source of claim 1 , wherein the linearly polarized light is polarized orthogonal to an orientation of plurality of wires of the wire-grid element.

9. The polarized light source of claim 1 , wherein the wire-grid element is a free-space wire-grid element that excludes a substrate.

10. The polarized light source of claim 1 , wherein the plurality of wires of the wire-grid element are uniformly spaced.

11. The polarized light source of claim 1 , wherein the linearly polarized light is characterized by an angle of linear polarization (AoLP) that remains substantially invariant in a presence of stray light.

12. The polarized light source of claim 1 , wherein the plurality of wires are made of gold or silver.

13. The polarized light source of claim 1 , wherein a density or spacing of the plurality of wires is selected to mitigate an effect of stray radiation.

14. The polarized light source of claim 1 , wherein a density or spacing of the plurality of wires is selected based on a specified power consumption requirement for the wire-grid element.

15. The polarized light source of claim 1 , wherein the wire-grid element is bounded to a surface of the illumination source.

16. The polarized light source of claim 1 , wherein: the wire-grid element is a first wire-grid element; the polarized light source includes one or more additional wire-grid elements coupled to the electrical circuit, each of the one or more additional wiregrid elements comprising a set of parallel wires that are oriented at a different angle compared to the first wire-grid element; and each of the one or more additional wire-grid elements are positioned to receive the illumination from the unpolarized illumination source and transmit linearly polarized light at the one or more wavelengths in the infrared range when the electrical current is supplied to the one or more additional wire-grid elements, thus each of the wire-grid elements producing linearly polarized light at a different angle of linear polarization.

17. The polarized light source of claim 16, wherein the first and the one or more additional wire-grid elements consist of four wire-grid elements that produce four polarization angles to enable a full linear Stokes imaging.

18. The polarized light source of claim 16, comprising a reference reflector positioned to reflect at least a portion of the illumination originating from the unpolarized illumination source, wherein an amount or direction of light reflected by the reference reflector is adjustable to control one or both of a degree ofpolarization (DoLP) and an angle of linear polarization (AoLP) associated with the wire-grid element.

19. The polarized light source of claim 16, wherein the infrared range spans 8- 14 microns.

20. The polarized light source of claim 16, wherein the polarized light source is configured as a calibration source in a high-altitude balloon, a satellite-based system, or a spaceborne vehicle.21 . A wire-grid element operable to generate a polarized light, comprising: a plurality of wires that are positioned parallel to each other to form a grid; a first conductive element coupled to a first end of each of the plurality of wires; a second conducive element coupled to a second end of each of the plurality of wires, wherein the wire-grid element is configured to receive an electrical current through the first and the second conductive elements, and upon receiving the electrical current and receiving illumination from an unpolarized light source operable at one or more wavelengths in a thermal or long wave infrared spectra, the wire-gird element transmits linearly polarized light in the thermal or long wave infrared spectra.

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