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FTIR modulator for first responder applications James R. Engel* , Julia H. Rentz, David L. Carlson OPTRA, Inc., 461 Boston Street, Topsfield, MA, USA 01983 ABSTRACT This paper describes the evolution and capabilities of our Fourier transform infrared (FTIR) modulator in multiple configurations for various applications concerning Homeland Security. The heart of our system is the extremely compact and rugged Michelson interferometer we originally developed during our involvement with the JSLSCAD program. (1) Our “J-Series” modulator is capable of resolving the 7 to 14 µm spectral region to 4 or 16 cm-1 with a measured radiometric sensitivity of 2×10-9 W/(cm2 ster·cm-1 ). This system has successfully undergone rigorous testing for operation over a temperature range of -40 to +65ºC and vibration levels associated with a spectrum of military ground, air, and water vehicles. The following describes the design and characterization of our J-Series modulator as well as the subsequent evolutions of the instrument in the forms of an active open-path FTIR and an imaging (hyperspectral) FTIR. We present this system in passive and active configurations with cooled, uncooled, and imaging detectors. We also project sensitivity limits for each configuration and measurement for some common chemical agents as well as industrial compounds. Key Words: FTIR spectrometer, chemical agent detection, first response, industrial monitoring 1.0 INTRODUCTION 1.1 Description of the opportunity There exists a long family history of chemical agent detectors based on FTIR in this spectral region. The modulator described in this paper is the latest manifestation of FTS design concepts that have been applied to chemical agent detection for nearly three decades. The COIN system (Correlation Interferometer, designed by Engel et al at Block Engineering) was developed in the early 1970’s to demonstrate that ruggedized, low-power FTS systems were practical for use in demanding field environments. COIN also served as a platform for the development of some of the earliest detection algorithms based on processing in interferogram space using correlation techniques. This activity was separated in time by the development of chemical agent detection systems using circular variable filters before a return to FTS technology in the XM-21 (Honeywell Electro-Optics Center). The XM-21 system design was later type-classified as the M-21 (Intellitec, now part of General Dynamics). Engel and Carlson (previously at Block Engineering) were involved in these latter FTS efforts. A new era in the application of FTS technology to chemical agent detection was launched with OPTRA’s work, under subcontract to BAIRD Corporation, for the LSCAD (Lightweight Standoff Chemical Agent Detector). This program consisted of a major redesign of the modulator with an emphasis on a lightweight, low power design. The I LSCAD modulator design served as the basis of the Improved-LSCAD (work done by Block Engineering) and a series of FTS systems now offered by Block Engineering, including the basic modulator employed in the MCAD (Mobile Chemical Agent Detector) system. And with even more improvements developed by OPTRA, this modulator design is now incorporated into the JSLSCAD (Joint Standoff Chemical Agent Detector). While the JSLSCAD design effort counted the LSCAD/I LSCAD design as a solid baseline from which to build, two specific areas of improvement were identified at the onset: 1. 2. Increasing the retardation rate to accommodate more demanding search timelines Increasing the operation range of the instrument with respect to vibration and temperature The following table lists the system requirements against which OPTRA’s J-Series modulator was designed. * [email protected] Copyright 2003 Society of Photo-Optical Instrumentation Engineers. This paper will be published in The Proceedings of Optical Technologies for Industrial, Environmental, and Biological Sensing and is made available as an electronic preprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. Table 1: JSLSCAD system requirements Requirement Value Units Spectral Range Spectral Resolutions NESR Scan Rates Retardation Rate Field of view Entrance aperture Temperature Range 2.0 7 – 14 4 & 16 Better than LSCAD 18 & 72 5.2 1.5 2.54 -40 to +65 µm cm-1 W/(cm2ster·cm-1) igrams/second cm/s degrees (circular) cm C THE J-SERIES MODULATOR The J-Series modulator is a rugged Michelson interferometer employing a voice coil motor driven flexure assembly to translate the moving mirror and a laser diode reference system based on spatial fringes instead of the more traditional fringe counters. The flexure assembly was designed to specifically meet the environmental requirements of the system and translates the moving mirror with less than 15 asec of tilt over the 4 cm-1 stroke length. The reference system includes a tilted facet on the edge of the stationary mirror which results in spatial fringes as the two beams recombine; we sample the fringes with our OPTRA-patented Triphase Detector (TPD) which is a linear silicon array. The interferometer has a zinc selenide beamsplitter/compensator pair with a special low chromatic phase dispersion beamsplitter coating and visible/near IR (NIR) channels on the top and bottom for the laser reference as well as a white light reference for use during alignment. The 1.27 cm diameter moving and stationary mirrors are diamond-turned aluminum and are mounted to the flexure assembly and an adjustable ball mount respectively. The interferometer with no fore or exit optics measures a compact 1.765×1.825×2.725 inches. Figure 1 shows the J-Series modulator with all of the components labeled. The following sections detail the sub-system designs. Figure 1: J-Series Modulator Laser diode reference assembly Moving mirror / flexure assembly 1.825” 2.725” Reference detector assembly Stationary mirror assembly 1.765” Entrance aperture Exit aperture Vibration isolation mounts 2.1 Optical design The optical design of the J-Series in the passive configuration with the mechanically cooled mercury cadmium telluride (MCT) detector (i.e. the JSLSCAD configuration) is shown in figure 2. We employ a germanium (Ge) refractive telescope, exit lens, and field lens inside the detector assembly. The 1.5º full field on the 2.54 cm diameter entrance aperture is converted to a 3º full field inside the 1.27 cm diameter interferometer. Note that this is roughly half the field value permitted by the obliquity limit imposed by 4 cm-1 spectral resolution at 1428 cm-1 (i.e. the highest optical frequency). Figure 2: J-Series optical design Fold mirror Primary Primary telescope Telescope Lens lens Secondary Secondary telescope Telescope Lens lens Cold Cold Optics optics Exit ExitLens lens Scanning Moving Mirror mirror Compensator Beamsplitter/ compensator Stationary Stationary mirror Mirror 2.2 Figure 2 shows the optical layout of the J-Series modulator in passive configuration with the cooled MCT detector (i.e. the JSLSCAD configuration). We employ a Ge refractive telescope which converts a 1.5º field of view to a 3º field inside the interferometer. The exit optics assembly includes a Ge exit lens, an aluminum fold mirror, a Ge window, a field stop, and a Ge field lens immediately in front of the detector. Laser diode reference assembly The laser diode reference is a wavelength stabilized distributed feedback (DFB) source at 810 nm. The reference system is similar to a standard laser interferometer except for a tilted facet on the edge of the stationary mirror which induces an incidence angle in the beam traversing the stationary mirror arm (figure 3). The result is spatial fringes when the two beams recombine at the TPD. The TPD is a silicon array with every third element wired in parallel. The tilt angle and wavelength of the laser diode result in one fringe for every three elements; the three channels are thus exactly 120º apart in phase. We interpolate this phase measurement and yield an absolute position measurement of the optical path difference (OPD) with significantly higher resolution than is achievable by standard fringe-counting laser reference systems. This capability allows for considerable flexibility and configurability for both rapid scanning and step scanning applications. Figure 3: Laser diode reference assembly Triphase detector (with fold mirror) Moving mirror Laser diode module (with fold mirror) Stationary mirror with tilted facet VIS/NIR patch of BS/comp Figure 3 shows our laser diode reference system. We impose spatial fringes on the TPD as a result of the small tilt in the visible/NIR channel on the stationary mirror. As the moving mirror scans, the spatial fringes move across the TPD. We interpolate the phase of the fringes to yield a high resolution absolute position measurement which we use to servo control the moving mirror and clock the interferogram. 2.3 Moving mirror assembly The moving mirror assembly is the key to the environmental performance of the J-Series modulator. Significant design and empirical effort produced a moving mirror which is able to maintain modulation efficiency over the temperature and vibration level ranges imposed by our system requirements. The final design employs membrane flexures with constrained layer damping (CLD) driven by a voice coil motor to achieve the required 1.25 mm (physical) stroke length for 4 cm-1 resolution with less than 15 asec of dynamic tilt. Figure 4a is a photo of the motor assembly; 4b shows a typical tilt measurement (taken using an electronic autocollimator) and 4c the results of vibration testing demonstrating the utility of the CLD. Figure 4a: Moving mirror assembly Figure 4b: Dynamic tilt data 10 8 6 4 2 0 -2 -4 -6 -8 -10 -1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1 Motor Displacement (mm) Motor Response Curves FigureJSLSCAD 4c: J -Series motor response CLD vs No CLD 10 4.0 CLD No CLD Optical Displacement (µm ) 10 3.0 10 2.0 CLD 10 1.0 No CLD 10 0.0 10 -1.0 0 100 200 300 400 500 600 Frequency (Hz) 2.4 700 800 900 1000 Figure 4 summarizes the moving mirror assembly. Figure 4a is a photo of the flexure, diamondturned aluminum mirror, and housing which encloses the voice coil motor. The tabs on the top and bottom of the mirror are the two visible/NIR channels. Figure 4b shows typical data from a tilt measurement over the stroke length. Figure 4c shows the motor response as a function of frequency with and without the CLD. The servo controls the primary resonance at 50 Hz and the CLD is responsible for minimizing the secondary resonance at 350 Hz. Performance evaluation The following shows a typical NESR plot for search (16 cm-1 ) and confirm (4 cm-1 ) mode over the spectral range of the instrument. Also provided are LSCAD and ILSCAD values for the same. NESR SN005 Mode mode (4 co-added scans) Figure 5b: Confirm Confirm NESR NESR, SN003, Search Mode Figure 5a: Search mode NESR . LSCAD I-LSCAD JSLSCAD LSCAD I-LSCAD JSLSCAD 1.E-07 W/(cm2 ster cm-1) W/(ster cm2 cm-1) (4 coadded scans) 1.E-06 1.E-06 1.E-08 1.E-09 600 800 1000 1200 1400 1.E-07 1.E-08 1.E-09 600 1600 800 Optical Frequency (wavenumber) 1000 1200 1400 1600 Optical Frequency (wavenumber) Figures 6a and 6b show the clean (i.e. no spectral satellites) instrument function in search and confirm mode. CO2 Line, System 1, Confirm mode Mode Figure 6a: Confirm CO2 Line, System 2, Search Mode FigureFWHH 6a:= 15.08 Search mode cm-1 Instrument Function FWHH = 3.75 cm-1 Instrument Function 30 30 25 25 20 20 15 AU FWHH = 15.08 cm -1 10 10 5 5 0 0 -5 800 -5 850 900 950 1000 1050 FWHH = 3.75 cm -1 AU 15 1100 800 850 900 950 1000 1050 1100 Frequency (cm-1) Frequency (cm-1) 3.0 EVOLUTIONS OF THE J-SERIES This section is intended to describe some actual and theoretical evolutions of the J-Series modulator in a number of different configurations. 3.1 Passive standoff detection Passive standoff detection by definition is a single ended system that relies on a slight temperature contrast between the background and the agent we wish to detect. Figure 7 shows the set up. Figure 7: Passive chemical detection configuration Fore-Optics Exit-Optics with MCT or DLATGS T background Absorbing / Emitting Medium, Tchemical J-Series Modulator Figure 7 depicts the passive configuration. This instrument stares out over a spatial area of interest, continuously collecting IR radiation. If an agent or chemical plume enters into the FOV of the instrument with sufficient temperature contrast relative to the background, a positive detection can be made. The instrument in passive detection mode employs simple fore-optics composed of either of a refractive telescope (as shown) with germanium lenses or a reflective telescope. The exit optics module consists of an exit lens (also germanium) and either a mechanically cooled mercury cadmium telluride (MCT) detector or an uncooled deuterated Lalanine tri-glycine sulfate (DLATGS) pyroelectic detector, depending on the required sensitivity, expected concentration levels, temperature contrast, and integration time permitted. The DLATGS represents a 15x reduction in price relative to the MCT at the cost of a 35x reduction in sensitivity. The DLATGS will provide a broader spectral response (over the 400 to 4000 cm-1 range). The intended installation will likely dictate the most appropriate detector choice. 3.2 Active standoff detection Active standoff detection requires the introduction of an IR source module which is placed in front of the FTIR spectrometer as shown in figure 8a or integration into the modulator as shown in figure 8b. The former is called bistatic and the latter monostatic. The later also requires a large mirror or retroreflector array to return the open-path beam to the spectrometer for detection. While the passive measurements may be either molecular absorption or emission (depending on the relative temperatures of the compound and background), active measurements are always molecular absorption which rely on the large temperature difference between the compound and a hot (T ˜ 1273 K) IR source seen as background. Figure 8a: Bistatic active chemical detection configuration Exit-Optics with MCT or DLATGS IR-Source, Tsource Absorbing Medium, Tchemical J-Series Modulator Figure 8 b: Monostatic active chemical detection configuration IR-Source, T source Absorbing Medium, Tchemical Exit-Optics with MCT or DLATGS J-Series Modulator Figure 8a shows our system in bistatic active mode. We place an IR source with collimating optics (such as a parabolic reflector) in front of the instrument which imposes the needed temperature contrast between the chemical and the new “background”. Figure 8b shows our system in monostatic active mode where the IR source is modulated by the interferometer prior to being sent through the absorbing medium. The monostatic configuration requires a mirror or retroreflector array to return the open-path beam to the spectrometer for detection. In each case, the spectrum is measured in absorption and adds considerably more sensitivity to the measurement relative to passive mode. In active mode, our system may or may not require fore-optics, depending on the range of the open path; this configuration can also be used with a sampling chamber. Active configuration may include either the DLATGS or MCT detector but may require a special (> 16 bit) A to D to cover the dynamic range. OPTRA has realized this J-Series evolution under another development effort to design and build a low-cost open-path FTIR for industrial monitoring (figure 9a). (2) Our monostatic OP-FTIR operates over the same wavelength range with 8 cm-1 resolution; this system employs the uncooled DLATGS detector. This effort also produced a novel plastic injection molded retroreflector array (patent pending) to return the open-path beam (figure 9b). Figure 9a: OPTRA’s OP-FTIR spectrometer Figure 9 b: Plastic retroreflector array Figure 9a shows an evolution of the J-Series modulator in active mode. We designed and constructed our low-cost OP-FTIR spectrometer using the J-Series modulator as the base. The spectrometer module measures a compact 7.25×8.5×10.5”. Figure 9b shows the plastic injection molded retroreflector array we developed as part of this effort. 3.3 Hyperspectral passive standoff detection Our system in hyperspectral mode simultaneously provides two-dimensional spatial (image) information and spectral information (across the entire image). Figure 10 shows the sys tem in hyperspectral mode. Figure 10: Hyperspectral (passive) chemical detection Absorbing / Emitting Medium, Tchemical Exit-Optics with FPA Spectral Output X Y T background J-Series Modulator λ Hyperspectral Data Cube Figure 10 shows our system in hyperspectral configuration. For this mode we step scan the moving mirror, acquiring a frame from the FPA at each step across the stroke of the mirror. This alleviates the computational complexity of frames acquired while the mirror is in motion. The FFT is performed on each pixel, producing a spectrum of each spatial resolution element across the field of view of the image (i.e. the hyperspectral cube). This configuration is ideal for a scenario where we wish to monitor a large area but want the capability to spatially locate the area containing the chemical or biological agent or organic compound. A point of interest with respect to our system in its hyperspectral configuration stems from our spatial fringe reference and our high-resolution spatial control of the scanning mirror. This allows us to do two things. First of all, we can step scan the mirror through the stroke length, which means that the entire FPA is readout at the same retardation location or optical path difference (OPD). This may dramatically simplify computation, as no post processing is necessary to line the samples up within each interferogram. Second of all, our high spatial resolution on our servo control of the scanning mirror allows us to hold the mirror at zero OPD where we record a simple broadband IR image no different from an IR camera. This capability may be of significant use within detection and discrimination to simplify computation. Other hyperspectral systems based on Fabry-Perots or acousto-optic modulators can not do this. 3.4 Raman capability The final configuration of the FTIR modulator exploits the fact that there are two separate visible/NIR channels to the beamsplitter/compensator, only one of which is in use during normal operation (i.e. the laser reference channel). We propose that the other channel may be used to modulate Raman scattered light, the transform of which yields the spectra. While this concept has yet to be demonstrated, its inclusion is entirely feasible and has the potential to further increase the utility of the FTIR modulator system. 4.0 PROJECTED SENSITIVITIES Having described a number of configurations of the FTIR modulator, here we present the projected sensitivities of each configuration in the format of minimum detectable concentration pathlengths of a list of common chemical agents and a second list of industrial organic compounds. For each we project a noise equivalent spectral radiance which is calculated according to NESR = AD D * ⋅η ⋅ Θ ⋅ ∆σ ⋅ ∆t [ =] W cm ⋅ ster ⋅ cm −1 2 (1) A D is the detector area, D* is the detector detectivity, η ?is the radiometric efficiency, Θ is the etendue or throughput, ∆σ is the spectral resolution, and ∆t is the integration time. For all configurations, we take η to be 0.1, and Θ is dictated by the obliquity limit and the 1.27 cm interferometer mirrors. We project for spectral resolutions of 4 and 16 cm-1 , and ∆t is one second unless otherwise noted (for hyperspectral mode). We present the single channel configurations, active and passive, with both the cooled, MCT detector and the uncooled, DLATGS detectors of 2 mm in diameter in each case. For passive mode we assume a 3 K temperature contrast between the agent/compound and the background of 300 K; for active mode, we assume a 1273K IR source and room-temperature agent/compound. For active mode we also provide short and long range estimates, the former for which the obliquity limits the throughput, and the latter for which the pathlength limits the throughput. Table 2a gives the minimum detectable concentration pathlengths in the standard units of mg/m2 for seven common chemical agents; table 2b gives the same in standard units of ppm·m for seven common organic industrial compounds. The limits are based on a single line analysis using published reference spectra. (3, 4) In each configuration we also provide the calculated NESR in units of W/(cm2 ster·cm-1 ). The hyperspectral internal field of view (per pixel) is 0.11º. Table 2a: Chemical agent minimum detectable concentration pathlengths Tabun Sarin Soman GF VX D. Mustard Lewsite α: 9.30E-04 1.50E-03 1.12E-03 1.10E-03 4.00E-04 1.30E-04 5.60E-04 NESR CLmin CLmin CLmin CLmin CLmin CLmin Clmin mg/m2 mg/m2 mg/m2 mg/m2 mg/m2 mg/m2 mg/m2 7.95E-08 200.2 121.3 162.4 167.3 461.1 1810.9 289.0 1.98E-08 46.5 28.2 37.8 38.9 107.1 412.7 67.7 9.94E-10 2.3 1.4 1.9 1.9 5.3 20.2 3.3 2.48E-10 0.6 0.3 0.5 0.5 1.3 5.0 0.8 Configuration Passive DLATGS, 4 cm-1 DLATGS, 16 cm-1 MCT, 4 cm-1 MCT, 16 cm-1 Active, Short Range DLATGS, 4 cm-1 7.95E-08 0.14 DLATGS, 16 cm-1 1.98E-08 0.04 MCT, 4 cm-1 9.94E-10 1.8E-03 MCT, 16 cm-1 2.48E-10 4.5E-04 Active, Long Range DLATGS, 4 cm-1 9.59E-07 1.75 DLATGS, 16 cm-1 2.40E-07 0.44 MCT, 4 cm-1 1.20E-08 2.2E-02 MCT, 16 cm-1 3.00E-09 5.5E-03 Imaging, Per Pixel, 60s integration FPA, 16 cm-1 1.44E-06 544 Imaging, 1 Superpixel, 60s integration FPA SP, 4 6.38E-07 208 FPA SP, 16 cm-1 1.60E-07 49 0.09 0.02 1.2E-03 2.9E-04 0.12 0.03 1.5E-03 3.9E-04 0.12 0.03 1.6E-03 3.9E-04 0.34 0.08 4.3E-03 1.1E-03 0.86 0.21 1.1E-02 2.7E-03 0.34 0.09 4.3E-03 1.1E-03 1.12 0.28 1.4E-02 3.5E-03 1.50 0.37 1.9E-02 4.7E-03 1.50 0.37 1.9E-02 4.7E-03 4.12 1.03 5.1E-02 1.3E-02 10.32 2.58 1.3E-01 3.2E-02 4.16 1.04 5.2E-02 1.3E-02 328 440 454 1252 5187 768 126 29 169 39 174 41 479 112 1885 431 300 71 Table 2b: Organic compound minimum detectable concentration pathlengths MEK meth Cl toluene phenol xylenes formald TCE α: 4.00E-04 1.54E-03 3.38E-04 5.68E-04 2.90E-04 7.68E-04 1.27E-03 NESR CLmin CLmin CLmin CLmin CLmin CLmin CLmin ppm m ppm m ppm m ppm m ppm m ppm m ppm m 7.95E-08 552.4 106.0 487.8 396.8 558.1 532.1 128.0 1.98E-08 126.6 24.8 114.2 90.8 130.8 113.6 30.0 9.94E-10 6.2 1.2 5.6 4.4 6.4 5.5 1.5 2.48E-10 1.5 0.3 1.4 1.1 1.6 1.4 0.4 Configuration Passive DLATGS, 4 cm-1 DLATGS, 16 cm-1 MCT, 4 cm-1 MCT, 16 cm-1 Active, Short Range DLATGS, 4 cm-1 7.95E-08 0.29 DLATGS, 16 cm-1 1.98E-08 0.07 MCT, 4 cm-1 9.94E-10 3.6E-03 MCT, 16 cm-1 2.48E-10 9.0E-04 Active, Long Range DLATGS, 4 cm-1 9.59E-07 3.50 DLATGS, 16 cm-1 2.40E-07 0.87 MCT, 4 cm-1 1.20E-08 4.4E-02 MCT, 16 cm-1 3.00E-09 1.1E-02 Imaging, Per Pixel, 60s integration FPA, 16 cm-1 1.44E-06 1557 Imaging, 1 Superpixel, 60s integration FPA SP, 4 6.38E-07 575 FPA SP, 16 cm-1 1.60E-07 132 0.14 0.04 1.8E-03 4.4E-04 0.69 0.17 8.6E-03 2.2E-03 0.20 0.05 2.5E-03 6.3E-04 0.70 0.18 8.8E-03 2.2E-03 0.12 0.03 1.5E-03 3.7E-04 0.14 0.04 1.8E-03 4.5E-04 1.72 0.43 2.2E-02 5.4E-03 8.36 2.09 1.0E-01 2.6E-02 2.43 0.61 3.0E-02 7.6E-03 8.49 2.12 1.1E-01 2.7E-02 1.41 0.35 1.8E-02 4.4E-03 1.73 0.43 2.2E-02 5.4E-03 282 1299 1124 1482 1998 340 110 26 507 119 413 95 580 137 556 119 133 31 5.0 CONCLUSION In conclusion, OPTRA has developed an extraordinarily versatile FTIR modulator of which we are in the process of exploring configurations and applications. The rugged Michelson has demonstrated conservation of modulation efficiency across an extremely harsh range of temperature and vibration levels. We have demonstrated the capability of the system in both passive and active mode and look forward to demonstrating hyperspectral mode. In addition to the considerable operational capacity of this instrument, it also holds strong promise of low cost and portability. We feel that these factors will open up a considerable number of new applications for which FTIR spectroscopy has historically been dismissed as too complicated, large, expensive, and fragile. REFERENCES 1. Joint Services Lightweight Standoff Chemical Agent Detector (JSLSCAD). 2. United States Air Force SBIR Phase II Contract No. F4265001C 0185: Low-Cost OP-FTIR Spectrometer with NanoScale Reference for Industrial Monitoring 3. D.F. Flanigan, "Infrared Spectra and Absorption Coefficients for GA, GB, GD, VM, VX, and the G analog", EATM 321-6 (August 1966) 4. World Wide Web < http://www.epa.gov/ttnemc01/ftir/refnam.html#v> EPA - TTN EMC - Spectral Database Fourier Transform Infrared (FTIR) Reference Spectra