Standoff Raman measurement with COTS components

Transcription

1 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 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 1.1 Description of the opportunity 1.0 INTRODUCTION 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 * jrentz@optra.com; phone ; fax Copyright 2003 Society of Photo-Op tical 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.

2 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.

3 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 nm nm Operating range 2 50 m m Range accuracy 1 m 0.76 m Range resolution 0.1 m m Aperture diameter 25 cm 25.4 cm Primary mirror obscuration 10 % 9 % Optical throughput (efficiency) 80 % 91 % Point spread function at image plane 250 microns 100 microns Effective f-number f/4 f/4 Position command response time 10 seconds 2 seconds Specification Provide a Graphical User Interface (GUI) to select instrument modes Provide a ready condition for Raman spectra data collection Provide a real-time video image CRT Functional Requirements Results Result LabVIEW based GUI controls system operation. User can select between three operating modes: initialization, range command, and jog command. System provides a trigger signal to initiate spectrometer data collection 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 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.

4 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 2- mirror optical system 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).

5 Figure 3 3: Photograph of of Complete complete System system This photograph shows all of the principal system component except for the target, which is 2 meters to the right. Light that is Ramanscattered by the target (a 10mm cuvette cell made of fused silica and containing a few ml of acetone) is collected by the primary mirror and then image by the primary and secondary mirrors onto the end of the fiber bundle. The other end of the fiber bundle is fastened to the input slit of the Ocean Optics miniature spectrometer. This photograph shows all of the principle system components except for the target which is two meters to the right. Light that is Raman scattered by the target (a 10 mm cuvette made of fused silica and containing a few ml of acetone) is collected by the primary mirror and then imaged by 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 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).

6 Figure 4a: Fiber- Optic 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 Power Supplies Cassegrain Telescope Primary Mirror Assembly Motor & Encoder Servo Amplifiers Motor & Encoder Eye Lens Assembly Galilean Telescope Motion Controller Command and Control Personal Computer 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.

7 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.

8 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 Manufacturer Description UV Laser Mid-West Laser 325 / 442 nm Dual Wavelength Helium-Cadmium Laser Series 2056, P = 7 mw at 325 nm Deuterium Avantes DH-2000 Deuterium / Halogen Lamp source Lamp Edge Filter Omega Optical 330 nm cut-on (OD6 < 330 nm) edge filter Spectrometer Ocean Optics USB2000 FO spectrometer with grating 7, 25 µm slit, and UV detector upgrade Cuvette Ocean Optics 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.

9 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 o S = N l I ε γ t [ = ] Counts system CCD 2 dω R Each quantity is detailed below. N: Analyte number density This value is equal to molecules/cm 3 for acetone at room temperature l : Pathlength through the sample (cm) We set this value to 10 µm. d σ : Differential Raman cross section dω We scaled the published value of cm 2 /(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 cm 2 /(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 (= J s), and ν is the frequency of the laser (= Hz). I is equal to photons/s in this system. e system : Radiometric efficiency of system This projected value incorporated all of the following individual radiometric efficiencies. ε galilean : efficiency of Galilean telescope (measured), 0.88 ε fold mirrors : efficiency of fold mirrors (measured), 0.97 ε cuvette : efficiency of quartz cuvette and sample (calculated), 0.86 ε cassegrain : efficiency of Cassegrain telescope (measured), 0.91 ε edge filter : efficiency of Raman edge filter (measured by vendor)), 0.80 ε couple in : efficiency of coupling a 657 µm image into a 200 µm fiber (calculated), 0.08 ε couple out : efficiency of coupling the output of a 200 µm fiber into a 25 µm slit (calculated), 0.16 ε spectrometer : efficiency of spectrometer (measured by vendor), ε system : g CCD : Sensitivity of detector (including quantum efficiency) Ocean optics published this value as counts/photon. A o : Area of CRT The telescope area is cm 2. The central obscuration is accounted for in the radiometric efficiency. R: Range to target We positioned the acetone sample 2 m from the CRT. Dt: 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.

10 80 Figure 8: Projected SNR vs integration time 64 unitless SNR( t) 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)

11 Figure 9: Acetone Raman Raman Spectra spectrum 0.06 Reference spectrum spectra Measured spectrum spectral Signal (a.u.) Raman Shift (cm -1 ) 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.

12 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, 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., Nestor, James R., Lippincott, Ellis R., The Effect of The Internal Field on Raman Scattering Cross Sections, J. of Ram. Spec., 1, World Wide Web <