Imaging Open-Path Fourier Transform Infrared Spectrometer
For 3D Cloud Profiling
Julia Rentz Dupuis, David J. Mansur, Robert Vaillancourt, David Carlson, Thomas Evans, and
Elizabeth Schundler
OPTRA, Inc.
461 Boston St., Topsfield, MA 01983
phone: (978) 887-6600 fax: (978) 887-0022
[email protected]
www.optra.com
Lori Todd, Kathleen Mottus
University of North Carolina
121 Rosenau Hall, Chapel Hill, NC 27599
ABSTRACT
OPTRA is developing an imaging open-path Fourier transform infrared (I-OP-FTIR) spectrometer for 3D profiling of
chemical and biological agent simulant plumes released into test ranges and chambers. An array of I-OP-FTIR
instruments positioned around the perimeter of the test site, in concert with advanced spectroscopic algorithms, enables
real time tomographic reconstruction of the plume. The approach is intended as a referee measurement for test ranges
and chambers. This Small Business Technology Transfer (STTR) effort combines the instrumentation and spectroscopic
capabilities of OPTRA, Inc. with the computed tomographic expertise of the University of North Carolina, Chapel Hill.
In this paper, we detail the design and build of a prototype I-OP-FTIR instrument.
Key Words: FTIR spectrometer, cloud profiling, tomography
1
INTRODUCTION
Under a U.S. Army Small Business Technology Transfer (STTR) solicitation, a need was identified for a near real-time
3D plume concentration profiler system for chemical and biological agent simulants released into test ranges and
chambers. The technology developed under this opportunity will be considered as a candidate referee sensor for the test
range or chamber. The requirements of the application include the ability to generate time-dependent concentration
information with sensitivity better than that of the sensors under test at the range. The referee sensor must be responsive
to a number of different chemical and biological agent simulants, and it must be able to operate in the zero-temperature
contrast scenario between the plume and the background against which it is being viewed.
In response to this opportunity, OPTRA is developing an imaging open-path Fourier transform infrared (I-OP-FTIR)
spectrometer which is used with an array of plastic injection molded retroreflector arrays to simultaneously monitor an
array of open paths (OPs) for chemical content and concentration by infrared spectroscopy. Via the optical design
presented in this paper, from a single FTIR modulator the I-OP-FTIR emits an array of discrete OP beams to respective
retroreflector arrays which return the beams to the instrument for multiplexed detection and processing. Our present
design emits an array of eleven OP beams spread over a 30° fan (i.e. spaced at 3° increments). All beams operate over 7
to 14 μm (1428 to 714 cm-1) with a spectral resolution of 4 cm-1. Unlike previous work employing a single channel OPFTIR with a scanner1, the OPTRA approach simultaneously monitors an angularly resolved region thereby eliminating
errors due to the movement of the plume over the duration of the single channel OP-FTIR scan. Unlike previous work
using passive hyperspectral imaging2, the OPTRA approach is quantitative without the need for background, plume, and
air column radiance estimation; the OPTRA approach is also valid in the zero temperature contrast scenario where
passive IR spectrometers, radiometers, and imagers cannot detect the plume.
With a number of I-OP-FTIR systems strategically positioned around the test range or chamber (Figure 1) in concert
with spectroscopic multicomponent algorithms, an accurate tomographic reconstruction of the plume concentration can
Copyright 2009 Society of Photo-Optical Instrumentation Engineers.
This paper will be published in The Proceedings of Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing
X 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
-1material in this paper for a fee or for commercial purposes, or modification
of the content of the paper are prohibited.
be made. Under this STTR effort, OPTRA has teamed with University of North Carolina, Chapel Hill, which brings
tomographic expertise for this IR spectroscopic monitoring application.
Table 1 details the specifications for the I-OP-FTIR system. The initial application of this work will be a point sensor
test range with the associated requirements listed in Table 1. Future work may scale the range of our instrument to on
the order of 1 km test grids for standoff sensor test ranges.
Figure 1: System Concept
Table 1: System Performance Specifications
OPTRA’s
I-OP-FTIR
(x3)
OP IR
beams
plume
Plastic Retroreflector
Arrays
Test range
REQUIREMENT
Noise equivalent concentration
Maximum concentration
Concentration accuracy
Update rate
Test grid size
Grid spatial resolution
VALUE
< 0.1 mg/m3
20 mg/m3
± 10%
< 5 sec
100x100x10 m
2.5 m
Figure 1 (left) shows the system concept. Each I-OPFTIR emits an array of discrete IR beams which are
projected through the plume to remotely located
retroreflector arrays and back again through the plume to
the instrument for multiplexed detection and processing.
This approach allows for simultaneous interrogation of the
plume unlike scanner-based systems. This approach also
supports detection in the zero temperature contrast
scenario unlike passive IR spectrometers.
The following contains the details of our prototype I-OP-FTIR design and build.
2
DESCRIPTION OF TECHNICAL APPROACH
2.1 Multichannel Concept
Our approach is similar to a traditional monostatic OP-FTIR in that we employ a single IR source from which IR
radiation is coupled by a set of lenses and directed through a Michelson interferometer before being expanded and
collimated by a projector optic and directed to a remotely located retroreflector array. The retroreflector array then
returns the OP beam to the sensor for detection. However, in this case, we couple the radiation exiting the interferometer
into an IR fiber optic assembly composed of eleven IR fibers bundled at the input end and free at the output end. The
free/output ends of these fibers are then positioned along the focal plane of the projector optic. The result is eleven OP
beams whose divergence is determined by a throughput match between the fiber diameter and numerical aperture (NA)
with the projector aperture. The pointing of the OP beams is set by the off-axis location of each fiber relative to the
optical axis of the projector lens. Each beam returned by the respective retroreflector array is then focused by the
projector lens and folded onto a respective detector using an intensity beamsplitter.
2.2 Radiometric Projections
The radiometric model is based on a per-channel noise equivalent spectral radiance (NESR) calculation along with
projected signal to noise (SNR) and a noise equivalent concentration (NEC) for a short list of compounds of interest to
the sponsor. The radiometric model takes into account the off axis performance of the projector lens and associated
radiometric efficiency due to overfilling the retroreflector arrays which is a function of range. The model also takes into
account the performance of the retroreflector arrays as well as atmospheric attenuation. NESR is given by:
Ad
W
[ =]
(1)
NESR =
2
D * ⋅η ⋅ Θ ⋅ Δσ ⋅ Δt
cm ⋅ ster ⋅ cm −1
where each of these values are detailed in Table 2. Radiometric signal to noise (SNR) is given by
-2-
ε source ⋅ N(σ, Tsource )
[=] unitless
(2)
NESR
where N(σ,Tsource) is the Planck equation for IR source temperature, Tsource, and εsource is the source emissivity (unitless).
Noise equivalent concentration is given by
1
1 ⎞ mg
⎛
NEC = −
⋅ ln⎜1 −
(3)
⎟ [ =]
α ⋅ L ⎝ SNR ⎠ m 3
where α and L are the line strength (m2/mg) and pathlength (m) respectively.
SNR =
Figures 2a and 2b show the projected per-channel SNR and NEC for m-cresol which is the weakest absorber on the
simulant list as a function of range assuming 1 m retroreflector arrays are used. Note that the other simulants of interest
to the army are sulfur hexafluoride, acetic acid, triethyl phosphate, and methyl salicylate. Values for εsource and Tsource are
0.9 and 1450°C (1723 K), respectively. We assume an absorption line strength of 4×10-4 m2/mg for m-cresol. We
assume the plume fills half of the standoff range (i.e. pathlength = range owing to double pass).
Table 2: Radiometric Projection Variables
VARIABLE
-1
Δσ (cm )
dint (cm)
uint (rad)*
Ωint (ster)**
Θ (cm2ster)**
D* (cm√Hz/W)***
dtel (cm)
η (unitless)****
AD (cm2)***
Δt (s)
DESCRIPTION
Spectral resolution
Interferometer mirror diameter
Interferometer ½ angle
Interferometer solid angle
Etendue
Detectivity
Telescope aperture diameter
Radiometric efficiency
Detector area
Integration time
VALUE
4
1.27
.016
7.85×10-4
10×10-4
5×108
15
η(R)
7.85×10-3
2.5
Notes for Table 2:
*
This angle is set by the throughput of the fibers (900 μm core, 0.25 NA) matched to that of the
interferometer aperture diameter (1.27 cm) and is below the obliquity limit associated with 4 cm-1 spectral
resolution and the highest optical frequency of 1428 cm-1 (7 μm).
∗∗
Ωint = 2π(1-cos(uint)) and Θint = Aint·Ωint where Aint = 1.27 cm2.
Eleven 1 mm uncooled lithium tantalite (LiTaO3) single element detectors are used.
****
Radiometric efficiency is based on a surface and vignetting loss analysis of the I-OP-FTIR as well as an
analysis of the range dependent losses of the retroreflector arrays. Figure 2c shows the results of a model
predicting the retroreflector performance as a function of range taking into account the following:
***
1. Geometric efficiency (geo) – the efficiency associated with each point emitted at the sensor aperture returning
on itself with a diameter equal to twice the retroreflector element diameter of 1.9 cm.
2. Edge efficiency (edge) – the efficiency associated with edge and bevel losses (assumes .001” edges and
bevels)
3. Angular accuracy efficiency (ang. acc.) – the efficiency associated with the angular accuracy of the
retroreflector elements. Note that this effect dominates diffraction (at 10 μm) for the plastic injection molded
retroreflector arrays.
4. Fill efficiency – the efficiency associated with the geometry of the retroreflector elements which have
triangular facets and an associated 66% radiometric efficiency.
5. Atmospheric losses – the efficiency associated with water vapor and carbon dioxide absorption in the
atmosphere. This projection uses Hitran3 reference spectra and integrates over the 7 to 14 μm spectral range.
6. Overfill efficiency – the efficiency associated with overfilling the retroreflector array with the open path
beam.
-3-
Figure 2a: SNR vs. Range
Figure 2b: NEC vs. Range for M-Cresol
Figure 2c: Retroreflector Array Efficiency vs. Range
According to our radiometric model and based on our
pathlength assumptions, all but the two outside
channels (± 15°) meet the NEC requirement of 0.1
mg/m3 for m-cresol (the weakest absorber) over the full
range of standoffs between 0 and 100 m. The
requirement is fully met for standoffs up to 80 m and
for all of the other chemical agent simulants of interest
over the full range of standoffs between 0 and 100 m.
2.3 Optical Design
Figure 3a shows the optical layout for the prototype I-OP-FTIR. We show one arm of the interferometer where the
second arm is opto-mechanically identical. Radiation emitted by the IR source is coupled into the interferometer using a
doublet lens to form an image of the source on the interferometer mirrors. The radiation traverses and exits the
interferometer and is focused to a field point at the common end of the fiber optic bundle (also called the fiber optic
image transformer). Each fiber therefore samples radiation from every point on the interferometer mirrors over an
angular subtense which is below the obliquity limit associated with the spectral resolution and highest optical frequency
of this system. Each fiber then transmits its respective portion of the total radiation to a point along the curved focal
plane of the projection lens where it passes through an intensity beamsplitter prior to illuminating the projector lens. The
result is an array of quasi-collimated beams transmitted to the remotely located retroreflector arrays where the
divergence of each beam is dictated by the throughput match between the 900 μm, f/2 fibers and the 15 cm diameter
projector lens. In other words, the divergence is about dfiber·ufiber/dproj = (900 μm)·(0.223 rad)/(15 cm) = 1.3 mrad where
0.223 rad is the half angle associated with the f/2 fibers. The pointing of the beams (i.e. the ± 15° spread) is dictated by
the off-axis location of each fiber.
All of the lenses shown in Figure 3a are germanium with a broadband antireflective (AR) coating for 7-14 μm. The
secondary beamsplitters are also germanium with a 50% intensity beamsplitter coating for 45° incidence (with a
broadband AR coating on the other side). The interferometer mirrors are diamond turned aluminum, and the
interferometer beamsplitter/compensator pair are zinc selenide.
-4-
Figure 3a: Optical System Layout
IR Source
Source
Lenses
11x Secondary
Beamsplitters
Exit Lens
Interferometer
Mirror
Beamsplitter/
Compensator
Fiber Optic Image
Transformer
Projection Lens
The fiber optic image transformer (Figure 3b) is composed of eleven silver halide IR fibers which are bundled at one end
and free at the other. The image shows all fiber ends covered because exposure to ultraviolet (UV) light will degrade the
IR transmission. The fiber cores are 900 μm, and the NA is 0.25 (approximately f/2). The purpose of the image
transformer is to evenly distribute the modulated radiation exiting the interferometer into an array of (modulated) sources
spread along the curved focal plane of the projector lens. The off-axis location of each “source” relative to the optical
axis of the projector lens determines the pointing of each OP beam within the fan.
Figure 3b: Fiber Optic Image Transformer
Free Output End
Common Input End
Figure 3c: Spot Diagram at Retroreflector Arrays
12° off-axis
9° off-axis
1m
1m
15° off-axis
1m
The projector lens is a germanium asphere
designed for optimal off-axis collimation (±15˚) at
f/2 set by the fibers. Figure 3c shows the spot
sizes at a range of 100 m for on-axis and 3˚, 6˚, 9˚,
12˚, and 15˚ off-axis where the scale bar is 1 m.
We extracted radiometric efficiency predictions
used in the radiometric projections (Section 2.2)
from these spot diagrams.
2.4 Mechanical Design
1m
-5-
on-axis
1m
3° off-axis
6° off-axis
1m
Figure 4a is a solid model showing the
mechanical design of the I-OP-FTIR. Figures 4b
and 4c show close-ups of the interferometer and
mounting for the fiber optic image transformer
from two views.
Figure 4a: Solid Model of I-OP-FTIR
Sighting Scope
Cover
Projector
Lens Mount
Image
Transformer
Mount
DAQ Terminal
Board
Interferometer
Heat fins for IR
source
Figure 4a: Solid Model of I-OP-FTIR
Figure 4a is a solid model of the I-OP-FTIR mechanical system. The interferometer, fiber optic
image transformer mount, projector lens, detectors with amplifiers, and terminal board for the data
acquisition are all housed in the sensor head. The fiber optic bundle is not shown in this model.
Electronics to control the interferometer as well as the data acquisition are located on PCI boards
housed by a “lunchbox” PC. The lunchbox PC also contains the power supply for the IR source.
The mechanical design allows for adequate (passive) heat dissipation for the IR source.
-6-
Figure 4b: Solid Model – Interferometer View
Figure 4c: Solid Model – Image Transformer View
IR Source with
Heat Sink
Fiber Optic Image
Transformer Mount
Fiber Bundle
Launch Mount
11x LiTaO3 Detectors
Detector
Preamp
Boards (x2)
Moving Mirror
Interferometer
Fiber Bundle
Launch Mount
Encoder
Reference
Figure 4b: Solid Model – Interferometer View
Figure 4b shows a close-up of the interferometer which
employs an encoder-based reference system. The
encoder-based interferometer is a variant of our J-Series
modulator4 developed for the Joint Services
Lightweight Standoff Chemical Agent Detector
(JSLSCAD). The IR portion of the interferometer is
identical to the J-Series with diamond turned aluminum
mirrors and a zinc selenide beamsplitter/compensator
pair.
Figure 4b: Solid Model – Image Transformer View
Figure 4c shows a close-up of the image transformer
mount which holds the eleven free/output fiber ends,
eleven secondary beamsplitters, and eleven LiTaO3
detectors. The free fiber ends are plane ferrules which
can be adjusted in z within the mount (relative to the
projector lens) to set the collimation of each OP beam.
Not shown is an initialization detector opposite the center
LiTaO3 detector. The initialization detector samples the
interferogram from the first secondary beamsplitter
reflection and does not require the retroreflector to be in
place. This detector is used to both initialize the scan as
well as check the alignment of the interferometer.
2.5 System Build and Integration
Figure 5a: Integrated I-OP-FTIR – Front View
Figure 5a is a photograph of the partially-integrated IOP-FTIR system (without detectors) showing the
germanium projector lens and a laser sighting system
that aids in the alignment of the retroreflector arrays.
The laser sighting system rotates and snaps into eleven
fixed positions every 3° (corresponding to the pointing
of the eleven OP beams). The fiber optic image
transformer mount is shown in the background.
Laser Sight
Germanium
Projector Lens
Figure 5b shows a side view of the I-OP-FTIR. This
shows the relative position of the projector lens to the
fiber optic image transformer (operating at f/2). The
fiber optic image transformer is shown on the right
where the common/input end is held at the field point at
the exit port of the interferometer by a tip/tilt mount
with x, y, and z adjustment. The free/output ends are
held in their bores by set screws and ultimately a UV
epoxy.
The eleven secondary beamsplitters are
germanium and therefore seal off the free/output ends
from all visible and UV light which would otherwise
damage the fibers. The eleven LiTaO3 signal detectors
Fiber Optic Image
Transformer Mount
-7-
are mounted above the secondary beamsplitters, and the single initialization detector (not shown) is mounted below the
secondary beamsplitter opposite the center channel signal detector. This photo also shows the two detector preamplifier
boards and the IR source heat sink.
Figure 5b: Integrated I-OP-FTIR – Side View
Laser Sight
Germanium
Projector Lens
11x LiTaO3
Signal
Detectors
Fiber Optic Image
Transformer Mount
Fiber Optic
Image
Transformer
Detector
Preamp
Boards
IR
Source
Heat
Sink
Fiber Optic Image Transformer Launch Mount
Figure 5c is a close-up of the fiber optic image transformer and mount before the detectors are mounted. This figure
shows the curved profile of the mount which effectively allows us to negate the effects of field curvature of the projector
lens on the collimation of the OP beams.
Figure 5c: Fiber Optic Image Transformer Close-up
Fiber Optic Image
Transformer
Mount
Interferometer
11x LiTaO3
Signal
Detectors
IR Source
Heat Sink
Fiber Optic Image
Transformer
Fiber Optic Image
Transformer
Launch Mount
Figure 5d is a close up of the interferometer showing the encoder and moving and stationary mirrors. The lens
assemblies which image the source onto the interferometer mirrors and focus the interferometer output to a field point
are enclosed in aluminum mounts. The IR source is a miniature igniter manufactured by St. Gobain.5
-8-
Figure 5d: Interferometer Close-up
IR Source
Source
Imaging
Lenses
Encoder
Reference
Fiber Optic Image
Transformer
Launch Mount
Moving Mirror
Stationary Mirror
Exit Lens
The fiber optic image transformer was aligned as follows. With the IR source aligned to the interferometer and the
interferometer itself aligned, the common/input end of the fiber optic image transformer was aligned to the field point by
serially monitoring the peak-to-peak interferogram amplitude (on an oscilloscope) of the energy exiting each fiber and
adjusting the common end in x, y, z, θx, and θy until all were optimized. Note that the alignment of the common end is
re-optimized as a final step once all detectors are in place and we have the ability to simultaneously monitor all eleven
channels. The free/output fiber ends were then positioned for collimation relative to the projector lens using an IR
camera focused at infinity to look back through the projector lens at each “source”. The fiber ends were each in turn
adjusted in z to minimize the spot imaged by the IR camera. Figures 6a through 6k show these images for the eleven
channels. The image of the 900 μm fiber formed by the 150 mm focal length IR camera lens should be about dimage =
dfiber·FIR camera/Fproj = (900 μm)·(150 mm)/(360 mm) = 375 μm or about seven or eight, 50 μm IR camera pixels (the
camera format is 120×160 pixels). Following this step, a retroreflector array was then positioned in fronts of the I-OPFTIR, and each detector was brought into focus by adjusting each in x, y, and z to optimize the peak-to-peak
interferogram amplitude (Figure 7). The I-OP-FTIR was rotated to direct each beam in turn onto the retroreflector array
for each detector alignment. Note the interferogram shown in Figure 7 was acquired at a much higher mirror velocity
and associated modulation frequency than will be used in operation (to provide a constant signal); because of the detector
bandwidth, a much cleaner interferogram will result at the operating mirror velocity. The interferogram shown in Figure
7 is also acquired with open-loop scanning.
Once all eleven channels were aligned and the detectors wired to the data acquisition system, a final optimization was
made by adjusting the common end of the fiber optic image transformer while simultaneously monitoring all eleven
detector outputs.
-9-
Figure 6a: -15º OP Beam Figure 6b: -12º OP Beam Figure 6c: -9º OP Beam Figure 6d: -6º OP Beam
Figure 6e: -3º OP Beam Figure 6f: 0º OP Beam Figure 6g: 3º OP Beam Figure 6h: 6º OP Beam
Figure 6i: 9º OP Beam Figure 6j: 12º OP Beam Figure 6k: 15º OP Beam
Figure 7: Interferogram Used for Alignment
2.6 Next Steps
At this point in time the I-OP-FTIR is fully integrated and is awaiting test. System characterization will include SNR
measurements over a series of standoff ranges up to 100 m as well as spectral resolution using a 10.6 μm carbon dioxide
laser. The interferometer itself has already been previously characterized for sampling errors, etc. Following system
characterization, a library or background spectra will be acquired for the purpose of training the automated
multicomponent algorithms6 which will be applied to identify and quantify the chemical agent simulants of interest to the
customer as well as a short list of compounds we will test at OPTRA. We will then perform a short series of evaluation
to verify the multicomponent algorithms are working properly prior to shipping the instrument to University of North
Carolina for a full tomographic demonstration
- 10 -
3
SUMMARY AND FUTURE PLANS
In summary, we have presented system level, optical, and mechanical designs of the I-OP-FTIR system for 3D cloud
profiling of chemical agent simulant plumes at test ranges and chambers. We have completed the build and integration
of an I-OP-FTIR prototype and will begin testing shortly. Test plans include characterization of SNR at a series of
standoff ranges as well as spectral resolution. We will perform a series of spectroscopic multicomponent demonstrations
at OPTRA before sending the prototype to University of North Carolina for a full tomographic demonstration.
University of North Carolina will demonstrate the tomography using a small array of Teflon balloons they have
developed specifically for this purpose to serve as stationary plumes which can be filled to known concentrations with
various chemical agent simulants. The I-OP-FTIR prototype will be serially positioned around the balloons and spectra
will be collected at each position (simulating multiple instruments). The results along with sensor geometry will then be
used to tomographically reconstruct the stationary plumes for each of multiple compounds.
Future direction for this work may include extended standoff range operation as well as processing upgrades to handle
the real time requirements which were not addressed during the Phase II.
Acknowledgements
This research was conducted under a Small Business Technology Transfer Phase II contract funded by the U.S. Army.
The technical monitor is Dr. Alan Samuels, Edgewood Chemical Biological Center.
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