Patent Grant
7038789
The following description relates in general to the fields of vibrational spectroscopy, spectroradiometry, and Mueller matrix photopolarimetry. In particular, this invention relates to the fusion in a single integrated device of a panoramic chemical imaging sensor that monitors a surrounding infrared panorama for absorption or emission spectra characteristic of various chemical species in order to detect the presence of a chemical cloud, together with a photopolarimeter communicator to broadcast an encrypted signal of its findings modulated and directed by elements that are shared with the chemical imaging sensor.
A variety of research projects are involved in design and development of systems for rapid and reliable detection of chemical and biological warfare agents in the field. One such effort, initiated by the U.S. Army in support of the Department of Defense (DoD) Joint Service Wide Area Detection (JSWAD) program, is PANSPEC (short for panoramic-imaging spectroradiometer). Various aspects of PANSPEC systems and designs are discussed in U.S. Pat. No. 5,708,503 issued to Carrieri, on Jan. 13, 1998 (“'503 patent”), U.S. Pat. No. 6,060,710 issued to Carrieri, on May 19, 2000 (“'710 patent”) and U.S. Pat. No. 6,389,408 issued to Carrieri, et al., on May 14, 2002 (“'408 patent”). The '503, '710, and '408 patents are incorporated herein by reference as if fully set forth.
Spectral measurements of chemical clouds in the open environment require a sensor of adequate photosensitivity, spectral resolution, numerical aperture and throughput to produce a sufficiently large signal-to-noise ratio for performing useful radiometric detections of such targets. Moreover, for such a device to be of practical value, it must withstand and compensate for extremes in weather, and be sufficiently robust to operate reliably in the field under a variety of adverse conditions.
In accordance with one aspect, a chemical imaging sensor collects and collimates middle infrared (MIR) radiance over an ambient-environment panoramic field of view (FOV), processes that radiance interferometrically via ultrahigh-speed solid-state birefringent photoelastic modulators (PEM), spatially images the modulated radiance onto a focal-plane array of detector elements, records the measured modulated radiance (i.e., the interferograms) and transforms the interferograms into their corresponding spectra. In accordance with another aspect, the data is processed in parallel using a Fourier transform to yield spectral information. In accordance with another aspect, the chemical imaging sensor analyzes the spectra for the presence of infrared absorption/emission bands characteristic of one or more targeted analytes. In accordance with another aspect, the system performs the analysis via a neural network. In accordance with another aspect, the focal plane array detector provides a matrix of n2 detector elements matched to the size and spatial resolution of the imaged panorama. In accordance with another aspect, spectral information is obtained for each detector element individually, and data from the imaged field as a whole is processed to yield information concerning movement of analytes in the FOV.
In accordance with another aspect, data concerning the analyte is photopolarimetrically relayed into the sensor's panoramic object space via a beacon modulated by a binary Mueller matrix encryption scheme. In accordance with another aspect, the beacon includes an infrared laser that is directed back, at least in part, through system optics shared with the interferometer, to provide an omnidirectional broadcast of the data. In accordance with another aspect, the photopolarimetrically-relayed data includes identity, location, and heading information for analytes. As an active photonics communicator, the sensor utilizes the same PEM optics comprising its passive interferometer component, but the middle PEM is driven in a distinctly different manner such that the instrument becomes a photopolarimeter to an integrated infrared laser continuous-wave beam source. The active photonics communicator, with its advantage of no moving parts and full electro-optical control, modulates the sensor's active carrier beam with encoded binary Mueller matrix elements by driving the middle PEM such that it alternately presents ½ and ¼ waveplates to the active carrier beam. The modulated beam is then directed back through the system optics where it is projected out into the sensor's panoramic object space thus providing an omnidirectional communications capability for the identification and tracking of a chemical vapor threat detected by the interferometer at the focal plane array.
In accordance with another aspect, vignetting of images and stray light analyses are performed to improve image quality. In accordance with another aspect, compensation is provided to account for changes in ambient environment such as temperature, humidity and pressure. In one example, the compensation includes enclosing one or more elements of the system in an evacuated or partially evacuated chamber. In another example, the compensation includes providing a protective index-matched hydrophobic coating. In yet another example, the compensation includes modeling the fluctuations in environmental parameters and providing a compensation model.
a) shows a cross section of the shared interferometer/photopolarimeter optics configured as an interferometer;
b) shows a cross section of the shared interferometer/photopolarimeter optics configured as a photopolarimeter;
a shows a mapping of ˜106 random rays traced through the optical system from source to focal plane array.
b shows maps a mapping of the same ˜106 random rays as in
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention, as claimed, may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbered elements refer to like elements throughout. As will be appreciated by one of skill in the art, the present invention may be embodied in methods and devices. Accordingly, the present invention may take the form of an entirely hardware embodiment, or an embodiment combining software and hardware aspects.
System 100 includes a collector 101 to gather and direct light from a panorama within certain predefined ranges of azimuth and elevation. In the example of
The optical elements of the system 100 are housed within a protective chamber 105 which serves to isolate the sensor from dirt and dust and protect the optics from misalignment that may occur due to changes in pressure, temperature, humidity. The top portion of protective chamber 105 includes a hemispherical Germanium shell 102, which is optimized to minimize aberrations and facilitate the transmission of middle infrared radiance from the panorama. Germanium shell 102 may also include a thin protective index-matched hydrophobic coating 103 to protect system components from changes in ambient humidity.
Light (MIR) passing through Germanium shell 102 is reflected by primary convex hyperboloid mirror 110 to secondary concave hyperboloid mirror 112. Secondary concave hyperboloid mirror 112 reflects the light back in the direction of primary mirror 110 and through an axial opening 116 in primary mirror 110. Axial opening 116 may also include a system stop 118, such as an iris, to adjust the amount of light entering system 100. The light from axial opening 116 next passes through a collimator 124 which aligns the light rays in a direction parallel to vertical axis 114. Collimator 124 is a Schwarzchild collimator and includes a first spherical mirror 122 that reflects the light received from opening 116 to a second spherical mirror 126 that reflects collimated light back through an axial opening 128 of first spherical mirror 122. A correction lens may be included at axial opening 128.
The collimated light emerging from axial opening 128 proceeds downward in a direction parallel to the axis 114 to a series of optical elements that make up the shared interferometer/photopolarimeter optics 129. Interferometer/photopolarimeter 129 is comprised of a first linear polarizer 130 having successive surfaces P13-14, a first photo-elastic modulator PEM 136, having successive surfaces M17-18, a second PEM 142 having successive surfaces M19-20, a third PEM 148 having successive surfaces M21-22, and a second polarizer 154 having successive surfaces P21-22, which acts as a linear polarizer when the apparatus operates in the passive interferometer mode.
Interferometer/photopolarimeter 129 operates on the principle of stress- birefringence. In the embodiment of
All optics of interferometer/photopolarimeter 129 are antireflection-coated to yield MIR transmissions exceeding 0.95. In this example, PEMs 136, 142 and 148 are commercially available ZnSe modulators that produce spectra approximately 20 cm−1, (wavenumbers) in resolution, a generally adequate resolution to detect the fundamentally broad MIR vibration bands of relatively large nerve agent molecules. The birefringence quantity in the modulating crystals of the interferometer is directly proportional to resolution of its spectral content from the interferogram output. Hence, to measure detail in the more narrow structural bands of chemical warfare agent compounds of lesser molecular weight (i.e., the blood agents HCN, CNCI, . . . etc) either the interferometer must accommodate additional ZnSe PEMs for added birefringence, or crystalline materials with a stronger birefringence property (i.e., increased phase retardation when stressed) need to replace the ZnSe windows.
The crystal elements of PEMs 136, 142 and 148 are compressed-then-relaxed at their natural mechanical resonance frequency ω by the piezoelectric transducer elements T. This action causes an oscillating phase difference (due to relative optical path differences) in two mutually coherent sets of infrared electromagnetic field waves that are perpendicular to each other. One electric field component is made to vibrate along the crystal's “fast” axis (ordinary component, E0in) whereupon its phase character is constant on transmission through the crystal. The other, perpendicular, component lies along the crystal's “slow” axis (extraordinary component, Eeoin) and possesses an oscillating retardation or phase delay, δ in response to the mechanical compressions induced by the piezoelectric transducer elements T. Accordingly, δ=67 0 cos ωt, where δ0 is the maximum amplitude of phase retardation induced by the ZnSe crystal via transducer element T and ω is the natural mechanical resonance frequency of the crystal. These orthogonal components combine at the back surface of each crystal, sweeping a continuum of polarization states in the electric field vector at its exit boundary (modulating Stokes vector), and thereby producing a temporal (t) intensity output signal by the interferometer optical system in the functional form of If(ν,t:ωm,ωn,ω1)∝|Eeoin+E0in|2 called an interferogram. This interferogram data record is modulated at the MIR energy (ν) with fundamental and overtone frequencies (ωm,ωn,ω1) of the respective driven optics of PEMs 136, 142 and 148. This form of modulation is a manifestation of the piezoelectric tensor T operating on opposite ends of the PEM crystals called stress birefringence: a generator of the interferogram voltage waveform If. PEM interferometry is analogous to Michelson interferometry, however, in PEM interferometry, phase retardation is accomplished electro-optically via stress-birefringent crystals whereas Michelson interferometry requires an oscillating mirror or inertial spinning prism phase-shifter.
After the incident light has passed through the various elements of the shared interferometer/photopolarimeter optics 129 it is focused by imager 160 onto a focal plane array 162 of photo detectors having a surface FPA36. Imager 160 includes four lenses arranged in a doublet with a first lens 164 having successive surfaces L28-29, a second lens 170 having successive surfaces L30-31, a third lens 176 having successive surfaces L32-33, and a fourth lens 182 having successive surfaces L34-35. The focal plane array 162 is made from mercury- cadmium-tellurium (HgCdTe) pixel detector elements. The doublet lenses of imager 160 are made from zinc selenium germanium ZnSeGe optimized for minimum aberration and maximum resolution of a distant chemical cloud object imaged from panoramic object space.
a) and 2(b) show interferometer/photopolarimeter optics 129 in greater detail. In this example, PEMs 136, 142 and 148 include stress-birefringence ZnSe crystals driven at mechanical resonance frequency by piezoelectric elements and are positioned between linear polarizers 130 and 158 (vis-à-vis ZnSe or Ge plates angled at their Brewster angle). This optical array performs passive thermal interferometry via a triple polarization-modulation operation by the PEMs on throughput infrared radiance collected and collimated over the instrument field of view. Infrared spectra are measured simultaneously at each HgCdTe pixel element of an n×n square detector focal plane array 162 cooled by liquid nitrogen (the cooling temperature of liquid nitrogen enhances infrared photosensitivity of the pixel HgCdTe semiconductor material) and matched to the open environment imaged panorama. Each pixel element of the focal plane array 162 produces a time varying electrical signal. The signals output by focal plane array 162 are digitized by parallel analog to digital converter circuits 180, processed in parallel by a spectrum analyzer indicated at 182, and each of the processed spectra are respectively matched against spectral patterns for chemical contaminants of interest in a pattern recognition operation indicated at 184. In one embodiment the spectrum analyzer performs a discrete Fourier transform to yield spectra for each signal from the focal plane array. Thus, each electrical signal provided by the focal plane array 162 (i.e., the pixels in the image) yields “Stokes Parameters” which represent complete polarization information for a location in the panorama. The polarimetry information provided by system 100 is much more efficient than a point-by-point scan, as was required in some previous polarimetry techniques based on PEMs. Because elements of focal plane arrayl 62 subtend contiguous solid angles in the FOV, those localized pixels that yield fingerprint absorption or emission bands of a chemical vapor mass will also provide information on its volumetric location and movement such as the cloud's heading and speed of travel or dispersion. The system 100 provides n2 interferograms, one per pixel element of the focal plane array 162, that are acquired with a parallel architecture of analog-to-digital signal converter circuits 180.
These interferogram data records are simultaneously analyzed at 182 into infrared spectra and compared with known data by a pattern recognition operation 184 multiplexed to this architecture. The pattern recognition operation 184 performs pattern recognition of absorption or emission band moieties from one or a group of pixels as the chemical cloud intercepts, proceeds through, and exits the sensor FOV. In one embodiment the pattern recognition operation 184 comprises a neural network such as the neural network system described in the '408 patent, incorporated herein by reference.
Following an absorption/emission vibrational spectrum match by the pattern recognition operation 184 a signal such as an alarm 186 is provided to alert users of the presence of a chemical or biological hazard. Post-processing functions such as a spatial mapping of the chemical gas and instructions on dealing with toxic properties associated with the detected chemical may also be provided.
The imager 160 and focal plane array 162 illustrated in
Tolerance analysis (TA) evaluates fabrication and alignment errors expected to occur when shaping system optics, and the mounting of these optics as single and grouped elements. A major performance metric of the TA is peak Modulation Transfer Function (MTF) over the MIR wavelengths of 8–12 μm. In performing a TA, variances are placed on optimized parameters defining the geometry of system optics, and on irregularities in their decentration and indices of refraction. The fabrication parameters under test include: radius of curvature, thickness, and conic constant; while the alignment parameters include: lateral translation, air space, tilt, spin and roll. Thickness of back focus (i.e., location of the focal plane array, surface 36, Table 1) was the compensator surface used in this TA. This compensator surface parameter is a sort of ‘relief valve’ diagnostic of the TA, easing overly constraining tolerances on the critical optics most sensitive to change in the MTF metric. Thus, a small air gap between surface 35 of lens 178 and focal plane array 162 can be provided in the final assembly stage of manufacturing the sensor of
The TA initially proceeded along these lines of assigned variances on all single and paired optics: ±0.1% radius of curvature, ±0.001 inch on thickness and airspace, ±10.1% conic constant, ¼ wave surface error, ±0.001 inch vertical-horizontal decentration, ±0.001 inch back focal distance, and ±50 arc-second vertical-horizontal tilt. Under the above tolerances, a sensitivity test was performed for: a test wavelength of 10.6 μm; a MTF nominal spatial frequency test value of 40 line-pairs per millimeter; a sampling of conjugate image rays over a 128×128 grid of averaged tangential and sagittal rays launched from and over the EP; and a sum total of 20 Monte Carlo simulations. (A Monte Carlo simulation was used to estimate image performance by simultaneously perturbing all optics tagged for this TA.) The sensitivity resulted in an image performance of mean MTF=0.37±0.012 (standard deviation) for the following five worst offender optics: [30, TFRN, 0.025]; [32, TFRN, 1.000]; [2, TSTY, −0.025]; [2, TSTY, +0.025]; [30, TSTX, 0.100]; where the entries in brackets are the optic surface number of Table 1, the tolerance operand (TFRN=curvature in fringes, TSTY[X]=horizontal[vertical] tilt in degree), and the tolerance quantity, respectively. Tightest tolerances (of most concern to an optics fabricator) appeared to reside in curvature and tilt of the imager doublets (±1 fringe, ±0.100°), tilt of the collector Ge dome (±0.025 deg), and tilt of the collector-collimator conic mirrors (±0.025°), in that order. Clear panoramic imagery may be expected, provided that fabrication and mounting specifications are maintained or exceeded. Fabricating the system to less critical specifications on the remainder optics would then follow.
Operating the system 100 continuously in the open atmosphere introduces several challenges regarding the maintenance of good image performance. They include diurnal swings of refractive index in ZnSe and Ge optics and the expansion/contraction of optical materials (and their mounts) as environmental pressure and temperature states fluctuate. These phenomena can be modeled in a manner similar to the tolerance-sensitivity analysis mentioned above. However, if the inner volume from dome 102 to focal plane array 162 is placed in enclosure 105 that provides a stable atmosphere, partial vacuum or vacuum, then undesired image-degrading effects brought on by changing atmospheric conditions may be improved. Finally, a thin protective index-matched hydrophobic coating 103 on the outside surface of dome 102 will improve operation in an ambient environment of high relative humidity.
Various system deficiencies are not well modeled by the computer sequential ray- tracing (SRT) procedure used to design the system parameters of Table 1. For example, the SRT method ignores rays that undergo total internal reflection (TIR); where a ray totally reflects at a surface boundary separating more-dense and less-dense media, provided that ray is incident to the surface from the more-dense side and beyond a critical angle. Generally, the SRT method is invalid for tracking multiple ray-paths, as when a ray splits and undergoes multiple reflections at an interface whereby that surface, and possibly several other surfaces of the optical system, become illuminated in a nonsequential order. Moreover, SRT cannot be applied to the problem of diffuse scattering and blackbody (or colored body) emittance, as rays tend to liberate from an optically rough irradiation zone in a uniform Lambertian-like angular distribution. The image performance of an optical system is potentially compromised by such phenomena. Therefore, nonsequential ray paths traversing the system must be tracked for the benefit of analyzing stray light (i.e., the indirect and undesired scattering/emission of light) and ghost imaging (via TIR, scattering, and/or multiple reflections from PEM windows, polarizer plates, and lens surface components) at its focal plane array 162. Baffle structures to intercept stray light may be positioned in optical systems based on nonsequential ray-tracing (NSRT) analysis and identification of stray light source paths. Nonsequential ray-tracing analysis can also identify optical surfaces requiring a high transmission (antireflection) coating layer(s) that would effectively attenuate or eliminate ghost images at or in the neighborhood of the image plane.
A nonsequential ray-tracing model was used to globally track rays undergoing TIR, multiply reflections, and/or diffuse scattering in this model design.
In the simulation, 106 random rays were traced through the model system.
Ghost images appear primarily as multiple-path light reflections by one or several of the doublet lens surfaces 28–35, Table 1. With two-layered antireflection coatings placed on all imager lens surfaces, a ghost image analysis was conducted on the receiver nonsequential ray-tracing interferometer-imager-focal plane array optical group. A nearest ghost image of a real surface object (with pupil), with appreciable intensity, was found to be present at a location of −47.35 mm (left, on-axis) from the focal plane array 162. This is sufficiently removed from the focal plane array 162, and provides positive assurance that the system specified in Table 1 is removed of ghost imaging interference.
The photonics communication feature or active broadcast beacon of the system 100 enables rapid communication of sensor data to a variety of receivers. Information is optically transmitted to all points in the panorama that are within reasonable range in the following manner. A thin beam of collimated light exiting laser 190 is first expanded at telescopic lens system 193-194 to increase the diameter of the laser beam to that of the collimated radiant beam (directed by the Schwarzchid collimation optics to the FPA, in the spectroradiometer mode). That collimated beam is directed along the axis 60 to polarizer 154. The polarizer 154 linearly polarizes the collimated beam received along the axis 60 and directs it upward along the axis 114 to the PEM 148. As the collimated light passes through PEM 142 its amplitude and phase is changed by modulation provided by the controller 199 to PEM 142 so that the instrument as shown in
As a photopolarimeter, the end PEMs 148 and 136 (M21-22 and M17-18) modulate in exactly the same way as when the system operates in its passive interferometer mode, while the middle PEM optic =
9-20(λ/2|λ/4) is now driven into alternate half-waveplate
19-20(λ/2) and quarter-waveplate
9-20(λ/4) states. The particular photopolarimeter configuration of
in the laser beam exiting the dome 102. (This waveform is similar to the interferogram data records of the ambient MIR radiant field produced at the focal plane array 162, when the device operates as a passive spectroradiometer). Specifically, four exact orientations of the end paired-optics of
as depicted in Tables 3–6, and described as follows.
A number of different orientations of the axes of the optical elements of combined interferometer/photopolarimeter 129 are possible. The particular orientation will determine which elements of the Muller matrix are used for photopolarimeter communication. Four cases (A, B, C, and D) are examined in Tables 3, 4, 5 and 6. For example, in Table 3 (case A), orientations of the photopolarimeter optics of polarizer 130, PEM 136, PEM 148 and polarizer 154 are vertical, −45°, +45°, and vertical, respectively, relative to optical axis 114. In this configuration communication takes place over the M34 matrix element. In Table 4 orientations of those same elements are vertical, −45°, vertical, and −45°, enabling binary communications over the and
elements. Tables 5 and 6 likewise examine two other possible orientations.
In one embodiment, the laser 190 of system 100 is of the waveguide cavity design variety, producing a continuous-wave CO2 source beam operating on the (hot) P20 line at 10.6 μm wavelength, 10-50 watts beam power. This waveguide laser source was chosen because the technology is mature (high beam spatial mode stability and relatively short coherence length) and beam power is sufficiently large (the beam is made to spread over a rather large- volume FOV) so that long-range detection is made possible over a wide solid angle in panoramic object space. (Along with beam intensity, range at which the beacon light is detected depends on atmospheric transmission, infrared sensitivity of the detector element used in the remote receiver, gain of that detector's amplifiers and other electronics factors.) Other laser sources may also be employed. For example, an infrared laser based on solid-state diode technology, can replace the waveguide CO2 system with less bulk, cooling requirements, and power consumption. However, range of the beacon will be reduced because of reduced beam intensity available for transmission. Furthermore, quality of beam encryption may be compromised on account of the generally poor temporal and spatial coherence of laser diode light with possible spatial mode-hopping behavior.
The polarogram beacon passed back into the panorama is derived from the following photopolarimeter system matrix equation:
ψ=P13-14M17-18M21-22P24-25 (Eq. 1)
where subscripts represent optical surfaces as listed in Table 1, P and M are the Mueller matrices of polarizers 130 and 154, respectively, and Mueller matrix has elements of the PEM 142 (which acts as a switching waveplate)
19-20(λ/2, λ/4) given by:
where α is the azimuth angle orientation in the middle ZnSe crystal relative to its fast axis (see the above text).
The Mueller matrices =
19-20(λ/2, λ/4) are measured in a sequence of compressions by the piezoelectric tensor
19-20 of PEM 142 that enacts precise stress amplitudes in the ZnSe material causing, respectively, ½- and ¼-wave retardations in a laser beam whose electric field oscillates in a plane along the crystal's extraordinary axis. Pulse duration of the incident laser beam is set by a chopping action of shutter 192 as it is gated to those exact periods of ½- and ¼-wave retardations in
The gated beam pulse duration must be sufficiently long compared to the modulation periods of PEMs 148 and 136 (M21-22 and M17-18) so that electronic lock-in measurement of the Mueller matrix elements
can be resolved (by a digital or analog electronic acquisition system) from the polarogram waveform in real-time.
The system matrix ψ given by Equation 1 transforms the incident beam Stokes vector (si) preset by polarizer 154 (P24-25) into the transmitted beam Stokes vector (st) preset by polarizer 130 P13-14 accordingly:
st=ψsi. (Eq. 4)
The normalized intensity of the polarogram takes the following form when substituting Eq. 1 into Eq. 4, inserting known values of the matrix elements of M, P and s, utilizing a Bessel generator function to express modulating element terms in the Mueller matrices M17-18, and M21-22, and expanding, then factorizing, this resultant function.
If/I0=ψ1,1+I(ω1, ω2) (Eq. 5)
The normalized elements of can be expressed as ψi,j/ψ1,1, as the ψ1,1 element has no polarization dependence and is referred to as the dc component of the polarogram. I(ω1, (ω2) is the polarogram's Fourier expansion representing primary circular frequencies (ω1, ω2) of M17-18 and M21-22, respectively, and their overtones (nω1, mω2). It is referred to as the ac component of the polarogram. The amplitude coefficient of each frequency component of order two or less in I(ω1, ω2) is Cn,m, as these dominant harmonics are grouped in the first summation term to the right in Equation 6. These terms comprise a zero, product of two (zero and first order coefficients), and product of three (zero, first and second order coefficients) Bessel function factors whose arguments have a fixed retardation ω0=2.404 rad, and a factor that is one of eight Mueller matrix elements ψi,j of the waveplates
=
19-20(λ/2|λ/4). All higher harmonics beyond second order are grouped in the second summation term to the right in Equation 6. This equation, therefore, represents an infinite series expansion in terms of all primary and overtone modulation frequencies, namely, the sums and differences of all integral multiples of the frequencies of PEMs 136 and 148, which can range between 10–100 KHz for ZnSe, and are typically offset ˜2 KHz. Those frequencies in I(ω1, ω2) that are 2nd order (in Cn,m) and below are locked into as the dominant intensities of the Fourier components. In other words, the second summation of Equation 6 is dropped, since these harmonics monotonically reduce intensity of the Mueller matrix coefficients, tagged by their respective PEM overtones, below signal-to-noise levels of those HgCdTe pixel elements comprising the focal plane array 162. Determination of the Mueller elements of Equations 2 and 3 is thus equivalent to the measurement of those amplitudes of the corresponding frequency components of Equation 6 that lock into their respective matrix elements as shown in Tables 3-6. In one example, this measurement is accomplished with an electronic circuit of analog multipliers and filters. Digital signal processing may also be used in other examples. The circuit will accept sum and difference reference modulation frequencies, one at the driving frequency and the other at twice the driving frequency of M21-33 and M17-18. For example, to generate reference frequency (2ω2+ω1) necessary for lock-in measurement of the [3,4] element of
, Table 4, both 2ω2 and ω1 signals from the PEM controller 199 (cos 2ω2t and cos ω1t) are input to a multiplier circuit, which generates an output of ½{cos(2ω2−ω1)t+cos(2ω2+ω1)t}. A high-Q filter located at the output end of this multiplier circuit will subsequently transmit the desired overtone while suppressing the other.
A complete set of primary and overtone frequencies shown in Tables 3-6 is generated in this manner, from combinations of ω1, ω2, 2ω2, and 2ω2 outputs by the multiplier circuit. These reference frequency waveforms are input to their respective lock-in amplification (LIA) circuit channels, one per reference frequency with a total of eight LIA circuits. The other LIA circuit input is the amplified polarogram signal (Equation 6), as detected from the active laser carrier beam, via a remote receiver. The LIA multiplies this polarogram data record together with all individual reference frequencies (see above), producing a voltage output signal that is proportional to the amplitude of the polarogram, at that particular reference frequency, times a phase factor. This signal is proportional to the binary Mueller matrix elements as shown in Tables 3–6 (with proportionality constant Cn,m). It is these binary elemental signals from which decoding of the source beam encryption is established. In particular, it is the binary states of the fourth-quadrant elements
[−1,0],
[0,−1],
[0,1], and
[−1,0] that are of interest here for beam encoding purposes.
An optical receiver staring at, and within the FOV of the photopolarimeter system, is thereby capable of measuring this encryption if properly equipped with the appropriate telescope, infrared detector, amplifier, and phase-sensitive electronics as described above. For example, if the system optics [P24-25, M21-22|M17-18, P13-14] are orientated as [+45°, vertical | vertical, −45°] (Table 5) and a remote aircraft, lying within its FOV and staring at the beacon, locks into element [−1,0] and its compliment element
[0,−1].
After a focal plane array 162 pixel group reveals spectral identification of a noxious vapor target over timeframe τ, the system changes into its photonics communications mode and transmits a binary code in its laser carrier beam through the one or more Mueller matrix channels. For example in Table 5 (Case C) configuration, a communications would take place over the 2ω2−2ω1 channel (the measurement). A properly encrypted binary sequence gathered by a suitable receiver will decode data concerning the analyte and its movement. This data may include the heading of the vapor cloud by encoding those coordinates in pixels of the focal plane array 162 that produced the analyte spectrum—via the interferometer in the passive spectroscopy measurement timeframe τ. Physical properties of the cloud analyte such as molecular weight, nomenclature, toxicity, . . . etc, can follow in some encoded sequence of
bits. In addition, in the example in Table 5, a simultaneous conformation of these data bits would reside over the complimentary channel ω1+2ω2 (the
34 measurement) and serve, for instance, as a check for parity errors in the encryption.
The system of the present invention provides a simple lens configuration with good image performance for spectroscopic operation and in which practical tolerances of optics, vignetting, ghost imaging, and stray light were analyzed for the benefit of preserving that good image quality for continuous operations of long duration. To maintain good image performance as the sensor operates continually in an environment of changing pressure and temperature the sensor may be enclosed within a cylindrical sleeve, encompassing internal optics and structural supports, and placing the cylindrical volume under a vacuum, partial vacuum or stable pressure and equipped with temperature stabilizing elements to provide heating and cooling as necessary. As an active photonics communicator, the beacon facilitates an omnidirectional alert of conditions presented by a chemical vapor threat. The location and heading of that threat may be communicated into the environment over a wide-angle field-of- view via a carrier beam such as a laser, modulated with binary encrypted Mueller matrix-element encoding by a photopolarimeter comprising one or more photoelastic modulator elements.
A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims.
(a)Circular 5.25 mm max radius,
(b)Circular 4.28 mm max radius,
(c)17.018 × 20 mm2 rectangular half-widths,
(d)Interferometer Birefringent In,
(e)Interferometer Birefringent Out,
(f)17.018 × 36 mm2 rectangular half-widths,
(g)50 × 50 pixels2 focal-plane array,
(h)Brewester angle,
(i)2-layers antireflection coating,
(j)high reflectance thin aluminum layer
aChief ray polar angle θ in the X(horizontal)–Y(vertical) plane,
bnull decentration of entrance pupil along the X-axis,
cdecentration of entrance pupil along the +Y-axis,
dcompression of entrance pupil along the +X-axis, and
ecompression of entrance pupil along the +Y-axis.
for [½, ¼] waveplate state of optic
of FIG. 2b.
element is used for binary
aScalar element of the Mueller matrix with no polarization dependence.
bNot accessible for measurement given Case A.
for [½, ¼] waveplate states of optic
of FIG. 2b.
and
elements are used for binary encryption. Optical
aScalar element of the Mueller matrix with no poliarization dependence.
bNot accessible for measurement given Case B.
for [½, ¼] waveplate states of optic
of FIG. 2b.
,
,
and
elements are used for binary encryption. Optical
aScalar element of the Mueller matrix with no polarization dependence.
bNot accessible for measurement given Case C.
for [½, ¼] waveplate states of optic
of FIG. 2b.
and
elements
aScalar element of the Mueller matrix with no poliarization dependence.
bNot accessible for measurement given Case D.
The invention described herein may be manufactured, licensed, and used by or for the U.S. Government.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3977787 | Fletcher et al. | Aug 1976 | A |
| 5659391 | Carrieri | Aug 1997 | A |
| 5708503 | Carrieri | Jan 1998 | A |
| 6060710 | Carrieri et al. | May 2000 | A |
| 6389408 | Carrieri | May 2002 | B1 |
