Standoff Raman measurement with COTS components
Julia H. Rentz* , Craig R. Schwarze, Robert M. Vaillancourt, Michael Hercher
OPTRA, Inc., 461 Boston Street, Topsfield, MA, USA 01983
ABSTRACT
We present our work towards developing a compact reflector telescope (CRT) for short-range (1 to 50 m) standoff
Raman LIDAR applications, including a standoff Raman measurement employing our telescope with a commercial off
the shelf (COTS) laser, spectrometer, and Raman edge filter. This development effort was funded through an SBIR
contract from the Department of Energy. The application of this technology is standoff assessment of chemical spills.
The CRT system includes a small Galilean telescope to deliver the excitation beam to the surface under investigation; the
benefit of the delivery optics is a smaller laser spot at the target and significantly enhanced throughput relative to
systems which rely on the divergence of the excitation laser beam. The CRT itself is a 10-inch Cassegrain optimized for
this short standoff range with motor-driven focus adjustment. We executed a Raman measurement of acetone at a
standoff of 2 m using a Midwest Laser 325 nm helium cadmium laser, an Ocean Optics USB2000 grating spectrometer
(with uncooled CCD), and an Omega edge filter. We present the results overlayed with published reference spectra. To
the best of our knowledge, this is the first reported standoff Raman measurement performed with an uncooled CCD
detector.
Keywords: Raman, standoff, spectroscopy, reflector telescope
1.0 INTRODUCTION
1.1 Description of the opportunity
Standoff Raman spectroscopy with a UV source offers an exciting new tool for the detection and identification of
organic molecules. These molecules are generally detected using infrared spectroscopy, with its attendant problems of
low detector sensitivity, thermal backgrounds, and difficult optical materials. Raman spectroscopy allows equivalent
spectroscopic measurements to be made in the solar-blind UV, where background light levels are negligible and, because
of the use of an active source, measurements can be made remotely, with standoffs of meters or even tens of meters.
This stand-off capability is especially attractive in the detection and identification of hazardous chemicals , ranging from
industrial spills to chemical and biological warfare agents. There is also a forensic application for the identification of
organic compounds at a crime scene where the ability to obtain accurate data without intruding upon the crime scene is
seen as highly desirable.
The original solicitation for this program was derived from work at Brookhaven National Laboratory on a portable
Remote Raman Spectrometer for identifying chemical spills (the Miniature Raman LIDAR System or MRLS). By
making such a measurement remo tely, personnel are not directly exposed to hazardous chemicals.
The original MRLS assembled at Brookhaven was comprised of the following components:
• A laser-diode pumped, repetitively Q-switched, frequency quadrupled, Nd:YAG laser source;
• A 6″ diameter Newtonian collection telescope which imaged the laser-illuminated target surface onto the
entrance slit of the spectrometer;
• A blocking filter ahead of the spectrometer to block reflected laser light at 0.265µm;
• A small UV grating spectrometer;
• A thermoelectrically-cooled, amplified detector array.
The Brookhaven team successfully made remote Raman measurements of acetonitrile and published these results. (1) In
process, however, an opportunity was identified to further reduce the size and cost of the system while improving the
*
[email protected];
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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
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radiometric efficiency to a point of no longer requiring a cooled or intensified CCD at the focal plane of the grating
spectrometer. This improvement opportunity resulted in the SBIR solicitation to which OPTRA successfully responded.
1.2 Our technical approach
Our task was to decrease the overall size and cost of the MRLS collection optics and at the same time increase the
overall radiometric efficiency. We employed three separate tactics in order to do this (listed below).
Improved Collection Optics
We replaced the 6″ Newtonian telescope with a 10″ Cassegrain system that was specially designed to provide good onaxis imagery for target distances in the range of 2 meters to 50 meters. The Cassegrain design provided a much more
compact system (relative to the Newtonian), while the increased aperture diameter provided a roughly 2.7× increase in
collected light. The telescope had a spherical primary mirror and an aspheric secondary that obstructed only 10% of the
primary aperture area. Both telescope systems (the CRT and the laser beam telescope) were servo controlled via linear
motors to translate one optic relative to the other and a rotary encoder to provide position feedback. The goal was to
create an automated system which imaged the laser onto the target and the target onto the end of the collection fiber (i.e.
the three are conjugate).
Laser Beam Telescope
Diffraction causes a nominally collimated laser beam of diameter D to diverge with an angle θ ≈ λ/D, where λ is the
wavelength of the light. The role of the laser beam telescope in to expand the diameter of the laser beam so that it can be
focused to a small spot on the target surface. The goal is to keep the laser spot on the target smaller than the image
(projected by the collection telescope) of the fiber bundle.
Fiber Optic Image Transformer
A fundamental issue with coupling the light focused by a telescope into a grating spectrometer through a fiber optic
cable is the discrepancy between the shape of the telescope blur spot (a circle) and that of the entrance slit of the
spectrometer (a very thin rectangle). Our response to this issue is our fiber optic image transformer. This is a bundle of
91 fibers, each with a 50 µm core diameter and a 3 µm cladding diameter, which are arranged in roughly a circle at the
telescope end of the bundle (≈ 750 µm in diameter), and as a linear array at the spectrometer end (≈ 6 mm long and
effectively 50 µm wide).
As a result of these improvements to the system’s radiometric efficiency, we expected to see a 40× improvement in
signal level at the detectors. We were not able to make a direct comparison with the original system, but (as described in
a later section) we were able to record Raman spectra with good SNR from an acetone sample at a range of 2 meters,
using a low power CW helium cadmium (HeCd) laser source (325.0 nm) and a miniature Ocean Optics UV spectrometer
with a standard linear uncooled silicon detector array with no intensifier.
2.0 DESIGN
2.1 System Requirements
The following tables provide the Phase II system and functional requirements and the results we obtained from
measuring the performance of the hardware.
CRT Performance Requirements Results
Specification
Target Requirement
Result
Operating wavelength
220 - 400 nm
220 – 800+ n m
Operating range
2 – 50 m
2 – 50+ m
Range accuracy
0.76 m
≤1m
Range resolution
0.072 m
≤ 0.1 m
Aperture diameter
25.4 cm
≥ 25 cm
Primary mirror obscuration
9%
≤ 10 %
Optical throughput (efficiency)
91 %
≥ 80 %
Point spread function at image plane
100 microns
≤ 250 microns
Effective f-number
f/4
f/4
Position command response time
2 seconds
≤ 10 seconds
CRT Functional Requirements Results
Specification
Result
Provide a Graphical User Interface (GUI) to LabVIEW based GUI controls system operation. User can
select instrument modes
select between three operating modes: initialization, range
command, and jog command.
Provide a ready condition for Raman
System provides a trigger signal to initiate spectrometer data
spectra data collection
collection
Provide a real-time video image
A video camera aligned to system optic axis provides realtime visible images to the control monitor
2.2 Optical design
The following three sections detail the optical designs of the CRT, the Galilean laser beam telescope, and the fiber optic
image transformer respectively.
2.2.1 The compact reflector telescope
Figure 1 shows the optical layout of the CRT. The primary mirror is spherical and the secondary aspheric; both mirrors
are diamond turned aluminum with polished nickel plating to enhance UV reflectivity. The telescope was focused by
moving the primary with respect to the secondary along a sleeve bearing via motor control.
Figure 1: Compact
reflector telescope
This telescope is
designed to efficiently
collect light from
targets at distances in
the range 2 m to 50 m
or greater. The design
challenge was to
achieve good on-axis
image quality over
this range of target
distances while
retaining the
simplicity of a 2mirror optical system.
2.2.2 The Galilean telescope
Figure 2 shows the Galilean telescope we used to control the divergence of the excitation laser. The laser beam
telescope was a simple Galilean design based on off-the-shelf positive and negative fused silica singlets. The beam
telescope was focused by moving the negative lens relative to the positive lens. A Galilean configuration was chosen to
avoid a beam focus within the telescope; at high laser powers this might have produced an air breakdown (spark) and
thereby reduced the energy delivered to the target.
Figure 2: Laser Beam
Telescope
The laser beam telescope
serves the function of
bringing the laser beam to
a small focal spot on the
target surface. This
maximizes the
radiometric efficiency of
the complete system.
The focus of the beam
telescope is synchronized
with that of the reflector
collection telescope so
that both are focused at
the same range
Figure 3 shows the integrated CRT system with Galilean (laser beam) delivery telescope and other COTS components
(to be described in section 3.1).
Figure
ofof
Complete
System
Figure3 3:Photograph
Photograph
complete
system
This photograph shows all of the principal system component except for the target, which is 2 meters to the right. Light that is RamanThis photograph
shows
all
of
the
principle
system
components
except
for
the
target
whichisis
two meters
to the right.
Light that is
scattered by the target (a 10mm cuvette cell made of fused silica and containing a few ml of acetone)
collected
by the primary
mirror and
Raman scattered
theprimary
target and
(a 10
mm cuvette
fused
and containing
a of
few
ofbundle
acetone)
is collected
by the
then imageby
by the
secondary
mirrors made
onto theof
end
of thesilica
fiber bundle.
The other end
theml
fiber
is fastened
to the input
slit of theand
Ocean
Optics
miniature
primary mirror
then
imaged
by spectrometer.
the secondary mirror to the end of the fiber. The other end of the fiber is coupled to the input
slit of the Ocean Optics miniature spectrometer.
2.2.3 Fiber optic image transformer
The purpose of the fiber optic image transformer was to convert the circular image of the Raman scattered light formed
by the CRT to a thin rectangular shape approximating the entrance slit of the grating spectrometer. Our design was
optimized for the MRLS, but the concept is intended for any grating spectrometer. Our concept employs a fiber optic
bundle which is circularly arranged on the input end and linearly arranged on the output end. We were able to
successfully breadboard our concept during the program but are still in the process of negotiating some radiometric
issues relating to the cladding thickness. The tradeoff is having enough cladding to maintain mode confinement within
the core but not too much so that the overall fill factor is low. While these issues presently limit the radiometric
efficiency of our image transformer, the concept has the potential to significantly improve the coupling efficiency
between any telescope and grating spectrometer. Our prototype fiber optic image transformer employed 50 µm core
(multimode) fiber with 3 µm cladding and an additional 5 µm protective layer for a total fiber diameter of 66 µm. This
design assumes a 50 µm entrance slit width of the grating spectrometer.
Figure 4 below shows photographs of the fiber optic image transformer prototype. This image transformer has 91 fibers
for an input diameter of ≈ 750 µm, and an output slit that is 6 mm long × 50 µm wide. Ideally this fiber-optic ‘slit’ will
replace the actual spectrometer slit (rather than having to be precisely aligned with it).
Figure 4a: FiberOptic Image
Transformer
The completed 91-fiber
image-transformer; the
diameter of the larger
cylinder at each end is
0.5″.
Figure 4b: Input end of Image-Transformer
For this photograph, a dispersed spectrum of white
light was imaged across the length of the output end
of the image-transformer. Because the central fibers
at the input end of the image-transformer have been
mapped to the center of the output end, the blue and
red light ends up at the outer portions of the input
end as shown here (the green and yellow light has
saturated the camera’s CCD array and shows up as
white). The reticle lines in the photograph are 25
µm apart.
Figure 4c: Output end
of Image-Transformer
At the output end of the
image-transformer the
fibers are arranged in a
linear array; in this
photograph the input end
is illuminated with white
light.
2.3 Motion control
The figure below shows a block diagram of the motion control system including connections between the different
components.
Figure 5: Motion control block diagram
Cassegrain Telescope
Primary Mirror
Assembly
Motor &
Encoder
Power
Supplies
Servo
Amplifiers
Motion
Controller
Command
and
Control
Personal Computer
Motor &
Encoder
Eye Lens
Assembly
Galilean Telescope
Figure 5 is a block
diagram of the motion
control system. Each
telescope system
employs a small linear
motor and appropriate
bearing mount to adjust
the position of one optic
relative to the other to
control the focus. Each
telescope also employs
a rotary encoder to
provide position
feedback for each
servo. The servo
software is resident to
the PC and focus adjust
is commanded by the
operator through the
Graphical User
Interface.
The speed, accuracy, and repeatability requirements were achieved by using a position based closed loop servo control
system.
2.4
GUI
The primary function of the CRT GUI software is to provide control and status for the Galilean focusing telescope and
Cassegrain collection telescope based on user requests. Additionally, one display is provided depicting the spatial
collection point for the CRT MRLS and another display from the grating spectrometer for lab and field-testing purposes.
These basic functions flowdown to require the CRT software to perform the following:
• Power-up and initialization of the system
• Provide a user-friendly Graphical User Interface (GUI) for user control of the system. These inputs include:
o Focus control selection (manual or automatic)
o Manual focus control keystrokes (range control)
o Jog control with velocity selection
o Display scaling options
• Accept estimated sample range from the user
• Convert range data to Cassegrain and Galilean motion control motor commands via a pre-calibrated range-tomotor command look-up-table (LUT)
• Output motor commands to the Galilean focus control laser and the Cassegrain collection telescope subsystems
• Accept position status from the Galilean focus control laser and the Cassegrain collection telescope subsystems
• Process Galilean and Cassegrain position status to determine when system in “In Focus”
• Accept video image data from the Image subsystem
• Accept spectrometer data from the Spectrometer subsystem
• Provide status feedback Graphical User Interface which may include:
o Focus Control status (OK, not OK)
o Image data
o Spectrometer data
Figure 6 details the graphical user interface developed for the CRT program as part of the GUI software development
effort. The GUI incorporates options for Galilean and Cassegrain focus control, displays a zoomed image of the sample
whose Raman spectra will be collected, displays an in-focus indicator when the system has achieved focus at the
requested range, and a separate pop-up screen used during lab-testing of the CRT system for displaying and saving
collected Raman spectra.
Figure 6: CRT graphical user interface
Figure 6 shows the CRT GUI (left) and Ocean Optics GUI (right). The CRT
GUI shows a digital zoom image we used to view the target and the focus
control for both telescopes. The Ocean Optics GUI displays the actual Ramanshifted spectra.
3.0 TESTING AND RESULTS
3.1 Experimental set-up
In order to fully demonstrate the capabilities of the CRT, we elected to acquire the requisite instrumentation to carry out
an actual remote Raman measurement of a sample of acetone. The following table details the equipment purchased for
this measurement.
Equipment
UV Laser
Manufacturer
Mid-West Laser
Deuterium
Lamp
Edge Filter
Avantes
Description
325 / 442 nm Dual Wavelength Helium-Cadmium Laser
Series 2056, P = 7 mW at 325 nm
DH-2000 Deuterium / Halogen Lamp source
Omega Optical
330 nm cut-on (OD6 < 330 nm) edge filter
Spectrometer
Ocean Optics
Cuvette
Ocean Optics
USB2000 FO spectrometer with grating 7, 25 µm slit, and
UV detector upgrade
1 cm Quartz UV cuvette
The following figure shows the experimental set-up.
Figure 7: Remote Raman measurement set-up
Figure 7 shows the experimental set-up for our remote Raman measurement. We employed a series of fold mirrors to steer the
HeCd laser through the Galilean telescope and up to the optical axis of the CRT. The final fold mirror is positioned directly in
front of the secondary CRT mirror so not to contribute to the central obscuration. We focused the spot inside a cuvette of
acetone positioned 2 m from the system and efficiently collected the scattered light with the CRT. Prior to launching into the
fiber optic cable, we passed the collected converging light through a Raman edge filter which blocks the Rayleigh (elastically)
scattered wavelength and passes only the Raman shifted wavelengths. Note that we did not use the fiber optic image
transformer for this measurement. The fiber optic cable is coupled to the grating spectrometer which measures the Raman
shifted spectra.
3.2 Radiometry
In the process of selecting the COTS components depicted in figure 7 we completed a radiometric analysis projecting the
SNR of our measurement. The Raman signal at a given shifted wavelength is given by (1)
dσ
A
S=N ⋅l ⋅
⋅ I ⋅ ε system ⋅ γ CCD ⋅ o2 ⋅ ∆t [ = ] Counts
dΩ
R
Each quantity is detailed below.
N: Analyte number density
This value is equal to 8.2×1021 molecules/cm3 for acetone at room temperature
l : Pathlength through the sample (cm)
We set this value to 10 µm.
dσ
: Differential Raman cross section
dΩ
We scaled the published value of 3.9×10-30 cm2 /(molecule⋅ster) for the 782 cm-1 acetone band measured with a 488 nm
excitation wavelength by a factor of (488/325)4 ; the result is an estimated differential cross section of 2×10-29
cm2 /(molecule⋅ster) for this measurement. (3)
I: Excitation photons per second
This value is equal to P/(hν) where P is the laser power at 325 nm (= 7 mW), h is Planck’s constant (= 6.626×10-34 J⋅s),
and ν is the frequency of the laser (= 9.231×1014 Hz). I is equal to 1.144×1016 photons/s in this system.
εsystem: Radiometric efficiency of system
This projected value incorporated all of the following individual radiometric efficiencies.
εgalilean : efficiency of Galilean telescope (measured), 0.88
εfold mirrors: efficiency of fold mirrors (measured), 0.97
εcuvette: efficiency of quartz cuvette and sample (calculated), 0.86
εcassegrain : efficiency of Cassegrain telescope (measured), 0.91
εedge filter: efficiency of Raman edge filter (measured by vendor)), 0.80
εcouple in : efficiency of coupling a 657 µm image into a 200 µm fiber (calculated), 0.08
εcouple out : efficiency of coupling the output of a 200 µm fiber into a 25 µm slit (calculated), 0.16
εspectrometer: efficiency of spectrometer (measured by vendor), 0.406
εsystem: 0.002
γ CCD : Sensitivity of detector (including quantum efficiency)
Ocean optics published this value as 0.0116 counts/photon.
Ao : Area of CRT
The telescope area is 506.7 cm2 . The central obscuration is accounted for in the radiometric efficiency.
R: Range to target
We positioned the acetone sample 2 m from the CRT.
∆t: Integration time
The quantity of interest for this measurement is the SNR. Wu et. al. (1) recommend a SNR of at least 10 for this type of
remote Raman measurement. Note that they also recommend a spectral resolution of 22 cm-1 ; we project a 23.5 cm-1
spectral resolution for our system (based on the slit width and grating pitch). SNR is determined by dividing S by the
product (N⋅√∆f); N is the noise of the CCD in counts/√Hz and ∆f is the measurement bandwidth which is equal to 1/∆t.
The following figure shows our projected SNR as a function of integration time using a typical uncooled CCD noise
value of 5 counts/√Hz.
Figure 8: Projected SNR vs integration time
80
unitless
64
48
SNR ( ∆t )
32
16
0
0
10
20
30
40
50
60
∆t
seconds
Figure 8 shows the projected SNR for our measurement as a function of integration time.
Our goal is at least 10.
This figure shows that, first of all, we would be able to successfully make the Raman measurement with adequate SNR
based on the equipment we had selected. Second of all, this figure shows that we will be required to integrate for
approximately 20 seconds to achieve the recommended SNR. We elected to integrate for 30 seconds for the actual
measurement.
3.3 Results
Using the equipment described in the previous two sections, we successfully conducted the remote Raman measurement.
The following figure shows our measured acetone Raman spectra overlayed with reference spectra we obtained from the
Galactic Spectral Server. (4)
Figure Acetone
9: Acetone
Raman
spectrum
Raman
Spectra
Reference
Referencespectrum
spectra
Measured
Measuredspectrum
spectral
Signal (a.u.)
0.06
0.04
0.02
0.00
800
1000
1200
1400
1600
1800
-1
Raman Shift (cm )
Figure 9 shows our remotely measured Raman spectrum of acetone overlayed with a
reference spectrum. Our data clearly shows all of the reference features with
reduced spectral resolution (as expected).
The data was smoothed with a simple filter and normalized using the measured spectrometer response to the deuterium
lamp. We also corrected for some residual fluorescence detected in the background. In general, our measured spectra
show a good match and overlays quite well with the reference spectra.
4.0 CONCLUSION
During this Phase II R&D effort OPTRA successfully developed a compact spectrometer system suitable for remote
Raman or fluorescence spectroscopy. The key components of the system that led to this success are the following:
• A Galilean laser beam shaping telescope;
• A Compact 10-inch Cassegrain collection telescope;
• A spot to slit converting optical collection fiber assembly;
• A high-speed, high-accuracy range autofocus;
• GUI control software;
• And a lightweight and portable mechanical package.
The completed system fully met the Phase II system requirements. Continuing commercial prospects are extremely good
as evidenced by the successful measurement of Raman spectra of acetone using a low-cost commercial off-the-shelf laser
and spectrometer.
ACKNOWLEDGMENTS
This research was conducted under an SBIR Phase II contract funded by the U.S. Department of Energy. We would like
to thank our Technical Monitor, Arthur Sedlacek, Brookhaven National Laboratory, Environmental Sciences
Department. We would also like to thank John DiBenedetto, Department of Energy, Special Technologies Laboratory,
Santa Barbara, for his help with the fiber optic image transformer.
REFERENCES
1. Ray, Mark D. Sedlacek , Arthur J. III, “Mini-Raman Lidar System for Stand-off, In Situ Interrogation of Surface
Contaminants,” Proceedings of the SPIE, 1998
2. Wu, M., Ray, M., Hang, K., Ruckman, M.W., Harder, D., Sedlacek A.J. III, “Stand-off Detection of Chemicals by
UV Raman Spectroscopy,” Appl. Spec. No. 54, Vol. 6., 2000.
3. Nestor, James R., Lippincott, Ellis R., “The Effect of The Internal Field on Raman Scattering Cross Sections,” J. of
Ram. Spec., 1, 1973.
4. World Wide Web <http://spectra.galactic.com/SpectraOnline/Default_ie.htm>