at toxic waste sites during remediation, process monitoring, and remote detection of highly toxic materials. Index Headings: Raman; Raman spectroscopy; ...
rapid communications
Remote-Raman Spectroscopy at Intermediate Ranges Using Low-Power cw Lasers S. M. ANGEL,* THOMAS J. KULP, and THOMAS M. V E S S Lawrence Livermore National Laboratory, Environmental Sciences Division, L-524, Livermore, California 94550
A portable Raman system is described that has been developed for line-
of-site spectral measurements of remotely located samples at intermediate ranges. Raman spectra were measured at distances up to 20 m with the use of a 40-mm-diameter collection optic (f/500) and at 16.7 m with a 22-mm-diameter collection optic (f/750). In all cases, low-power cw lasers were used with powers ranging from 23 to 100 mW. The system consists of a small f / 4 image-corrected spectrograph with a liquid-nitrogen-cooled CCD detector and has been demonstrated with both an argonion laser, emitting at 488 nm, and an 809-nm diode laser. Applications of the system include monitoring of organic and inorganic compounds at toxic waste sites during remediation, process monitoring, and remote detection of highly toxic materials. Index Headings: Raman; Raman spectroscopy; Raman imaging; Remote Raman; Remote spectroscopy; Spectroscopy.
INTRODUCTION Recent advances in the performance of CCD detectors, small high-throughput spectrographs, high-performance holographic laser-line rejection filters, and small relatively high power air-cooled cw lasers have led to the development of portable, yet sensitive, Raman spectrometers. 1-7 Many such systems have recently been described for the use of fiber optics for remote applications including e n v i r o n m e n t a l monitoring and process control, s-17 Recently, the imaging capability of CCD detectors has been combined with high-quality image-corrected spectrographs to optically multiplex optical fiber Raman probes. 17.~s This technology is demonstrated here for remote Raman at intermediate ranges with the use of small imaging optics. It is shown that excellent sensitivity is achieved with this system with the use of//500 to//750 collection optics. Remote Raman systems that were capable of measuring air pollutants at ranges up to several kilometers have been described by many researchers. 19-33However, in all of these systems, high peak-power pulsed lasers were used for excitation, and telescopes, with diameters up to 36 in., were used for collection. The systems described were either transportable on large trucks or were not transported. Also, these remote Raman systems were developed for monitoring atmospheric gases and stack emissions with integration over very long sample volumes. Received 4 March 1992. * Author to whom correspondence should be sent.
Volume 46, Number 7, 1992
An ongoing goal of our group in the Environmental Sciences Division is to develop techniques to remotely monitor contaminant concentrations at environmental remediation and waste disposal sites. Many of these wastes are characterized as mixed wastes, containing radioactive components as well as organic and inorganic contaminants. In some cases, high levels of these contaminants are found in complex mixtures. Although fiber-optic Raman systems are suitable for many of these measurements, sometimes it is desirable to have the capability of doing relatively rapid survey scans over a wide area before and during remediation. We have found that, for these applications, it is possible to remotely measure the compounds of interest with similar instrumentation used in a line-of-site mode, rather than with fiber optics. The system described here was developed for measuring chemical concentrations on surfaces in either a solid or semi-solid phase (e.g., sludge) and for measuring hazardous gases at percent-level concentrations. The principal application of the system is for monitoring contaminants in storage tanks that have limited access. For this reason, it is desirable to keep the collection optics as small as possible to allow easy entry through a small port in the top of the tank, and to minimize the size and weight of the system so that it can be easily transported from area to area. EXPERIMENTAL The main components of the remote Raman system are shown schematically in Fig. 1. The Raman spectrometer used for this work was assembled in-house and consists of a 0.25-m [/4 image-corrected spectrograph (Chromex Model 250IS) with a liquid-nitrogen-cooled CCD detector (Princeton Instruments Model LN-EEV). Three different gratings were used for this work including a 300 gr/mm grating blazed at 1 #m, a 600 gr/mm ion-etched grating blazed at 500 nm, and a 1200 gr/mm grating blazed at 750 nm. The detector was binned both vertically and horizontally. The vertical stripe was binned to match the size of the image on the CCD. The horizontal bin was two pixels wide, in most cases, to match the size of the entrance slit image. Two different optical collection systems were used, differing only in the diameter and f/# of the primary collection optic. This system was either a 40-mm-di-
0003-7028/92/4607-108552.00/0
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ameter effective focal distance (EFD) f/1.2 low-dispersion camera lens (Canon), or a 22-mm-diameter EFD [/4 camera lens (Canon). In each case, the image from this lens was transferred to the {/4 spectrograph with the use of a 25-mm f/1 piano-convex transfer lens, to collimate the light from the collection lens, followed by a 25-mm [/4 piano-convex focusing lens, to focus the collected light onto the slit of the spectrograph. The last two lenses were not anti-reflection coated. Typical slit widths of 10 to 30 #m yielded spectral resolutions of ~ 10 cm -1 in the visible and ~7 cm -1 in the near-infrared. A Rayleigh rejection filter was used either in the collimated part of the beam of the transfer optics or in front of the primary collection optic. These filters were holographic rejection types (Physical Optics Corp. RHE 488 for visible and RHE 810 for NIR experiments). They were angle tuned for maximum Rayleigh rejection and best spectral transmission at low energies. The 488-nm line of an argon laser (Spectra-Physics Model 2000) was used for all visible experiments. Its power was approximately 100 mW at the sample for these experiments. A 488-nm dielectric-coated bandpass filter was used in the laser beam to remove laser plasma emission lines. A GaA1As diode laser (Spectra Diode Lasers SDL-5400 with Model 800 power supply and temperature controller) was used for some experiments with an 811-nm dielectric-coated bandpass filter, tilted for use at 809 nm, to remove laser background. The diode laser was tuned to 809 nm by maintaining its temperature at 13°C for all experiments. Flat mirrors were used to steer the laser beam and field-of-view of the spectrograph. The two were nearly collinear for all experiments. A 50-mm aluminum-coated steering mirror was used for all visible laser experiments, and a 100-mm gold-coated mirror was used for the diode experiments. A beam expander was used to focus the 1086
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C~-1 s h i f t FIG. 2. R a m a n spectra of K4[Fe(II)CN6] measured at a distance of 16.7 m (55 ft) using 100 m W of 488-nm excitation and a 22-mm-diameter collection optic. These spectra were measured using 50-#m slits and ~10 cm -1 resolution, with a 600 g r / m m grating. (A) One-second exposure; (B) 60-s exposure. Atmospheric N2 and 05 bands are shown.
argon laser to a spot of about 5 mm diameter at the sample, and an uncoated 0.2 NA 10 x microscope objective was used to focus the diode laser to a spot of about 2 mm diameter at the sample. RESULTS AND DISCUSSION The use of an image-corrected spectrograph made it possible to obtain sharp undistorted images on the detector, thus minimizing the number of pixels to be r e a d - which, in turn, minimizes the detector noise. The CCD detector was binned both horizontally and vertically to minimize both readout and dark noise in the detector. Vertical binning was selected so that only the narrow stripe of pixels illuminated by the collection optics was read out and all other pixels were ignored. For all experiments shown here a 30 to 40 pixel high stripe was used. For the CCD used in these experiments this corresponds to an image height on the detector of 0.66 to 0.88 mm, respectively. This is about 10 times larger than the size of the image on the slit because of the 1.3 x magnification of the spectrograph and because the detector was not precisely positioned at the focal plane of the spectrograph. Positioning of the detector at the focal plane is more critical with the fast (//4) image-corrected spectrograph used in this work because very small changes in the position of the detector lead to large changes in the spot size. Figure 2 demonstrates the sensitivity and range of the system. Spectra 2A and 2B show 1-second and 60-s exposures, respectively, of a solid sample of sodium ferrocyanide, Na4[Fe(II)CN6], at a distance of 16.7 m (55 ft). These spectra were measured with 100 mW of 488nm excitation at the sample with a 22-mm-diameter [/4 collection lens and the 600 gr/mm grating. This corresponds to//750 for the collection optic, allowing the holographic filter to be used in front of it in this essentially collimated collection beam. The slit width used to obtain these spectra was 30 #m, corresponding to a resolution
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of about 12 cm -1. The sloping background was caused by fluorescence. The signal-to-noise (S/N) ratio for the spectrum in Fig. 2B is about 250/1 at the 2055-cm -1 ferrocyanide peak, which is expected for a shot-noise-limited detector and the measured signal intensity. It will be shown below (see Fig. 9) that 20 counts of signal will give an S/N ratio of about 4/1 for this detector. Considering that the Raman signal should be inversely proportional to the square of the distance to the sample, it should be possible to measure a spectrum of this sample at a distance about 20 times further than that shown here, i.e., 335 m (f/ 15,000), with an S/N ratio equal to 4/1. This approach assumes minimal background from such sources as solar radiation. Correspondingly, it should be possible to measure ~0.1% concentrations of ferrocyanide in a few minutes at 16.7 m with the same S/N ratio. Also visible in this spectrum are the atmospheric gases 02 (1555 cm -I) and N2 (2331 cm-~). These bands are observed in most samples measured at this distance. This observation is not surprising, considering that there is a considerable overlap between the excitation beam and the field of view of the spectrograph near the sample. Figure 3 shows the 02 and N2 bands at higher resolution, again with the use of a 22-ram collection lens and 100 mW of 488-nm excitation at the sample at a distance of 16.7 m. In this case, the 1200 gr/mm grating was used with 10-#m slits. A longer exposure (10 rain) was used in part to compensate for the inefficiency of the grating at this wavelength. Although not shown in this figure, the 7-cm -1 resolution was such that lsO/~60 and 15N/~4N isotopic features could be measured in the rotationalvibrational branches. The pathlength was estimated to be between 0.5 and 1 m. Typical mixed waste tanks contain both solid and liquid wastes. Common solid precipitates include NO3- and ferrocyanide salts. Figure 4 shows the Rarnan spectrum of a mixture of 10% sodium ferrocyanide and 10% NaNO3 in a NaC1 matrix at 16.7 m under the same experimental conditions as those used to obtain the data shown in Fig.
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FIG. 4. R a m a n spectrum of a mixture containing 10% K,[Fe(II)CN,] and 10% NaNOa in a NaC1 matrix measured at a distance of 16.7 m (55 ft) using 100 m W of 488-nm excitation and a 22-mm-diameter collection optic. This spectrum was measured in 60 s using 50-um slits and ~10 cm -~ resolution, with a 600 g r / m m grating. Atmospheric Nz and 02 bands are observed at 2331 and 1555 cm -~, respectively.
2B. As expected, the ferrocyanide peak intensity is about 10% of that shown in Fig. 2B. The NO3- bands are at 718, 1057, and 1379 cm -1. For the applications described above, it is important that there is no overlap in the NO3- and ferrocyanide bands because they are likely to be found as mixtures. As above, bands due to atmospheric 02 and N2 are observed. Spectra of liquid samples were also measured. Figure 5 shows the Raman spectra of 3 M NO~- (A) and 3 M N O c (B) at 16.7 m under the same experimental conditions as those used to obtain the data shown in Fig. 2B. The samples were contained in 1-cm glass cuvettes. There is no overlap between the most intense NO3- and N O c bands, and very little in other parts of the spectrum. 5000
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APPLIED S P E C T R O S C O P Y
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The holographic laser rejection filter allows Raman spectra to be measured close to the laser line. This capability is demonstrated for this system by the spectrum of neat CC14 shown in Fig. 6. This spectrum was measured with the sample located at a distance of 16.7 m under the same experimental conditions as those used to obtain the spectrum shown in Fig. 2B. The 219-cm -1 CC14 band is observed, although its intensity is lower than expected. Other spectra were measured in which this band had a higher relative intensity and in which its intensity was highly dependent on the angle of the holographic filter. The atmospheric 02 band at 1555 cm -1 overlaps with the broad CC14 band in that region. An f/1.2 40-mm collection optic was used in some experiments with significantly worse results than those for the 22-mm optic. Figure 7 shows spectra of acetaminophen measured at a distance of 16.7 m with the use of the smaller collection optic (A) and measured at a distance of 20 m with the larger collection optic (B). Other experimental conditions were the same as those described for Fig. 2B. The larger collection aperture of the 40-mm collection optic should have more than compensated for the greater distance. Thus, the signal levels should be comparable or larger in B than in A. However, it is observed that the signal levels in A are about three times as large as those in B. The reason for this result is related to the transfer optics. The 40-mm f/1.2 lens almost completely fills the 25-mm collimating and focusing lenses. Both of these lenses are simple spherical piano-convex types. As a result, aberrations in these lenses produce a much larger spot at the slit of the spectrograph and, thus, lower throughput. The 22-mm f/4 collection lens underfills these lenses considerably, resulting in less aberration in the focused image at the slit and higher throughput. Figure 8 shows another Raman spectrum of acetaminophen measured at a distance of 6.4 m (//160) with 23 mW of 809-nm excitation from a diode laser. Diode lasers are of interest because they are small, rugged, and relVolume 46, Number 7, 1992
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FIG. 6. Raman spectrum of neat CC14 measured at a distance of 16.7 m (55 ft) using 100 mW of 488-nm excitation and a 22-ram-diameter collection optic. This spectrum was measured in 60 s using 50-pro slits and - 1 0 cm -~ resolution, with a 600 gr/mm grating. The sample pathlength was about 20 ram. Atmospheric N~ and O~ bands are observed at 2331 and 1555 cm -~, respectively.
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Fro. 7. Raman spectra of acetaminophen powder measured at a distance of 16.7 m (A) and 20 m (B) using 100 mW of 488-nm excitation and 60-s exposures. The spectrum in A was measured using a 22-ramdiameter collection optic, 50-pro slits, and - 1 0 cm -' resolution, with a 600 gr/mm grating. The spectrum in B was measured using a 40-ramdiameter collection optic, 10-pro slits, and 45 cm -~ resolution, with a 1200 gr/mm grating. Atmospheric N~ is observed at 2331 cm -~.
atively high power devices, and are available with N I R wavelengths that produce minimal sample luminescence. The spectrum in Fig. 8 was measured as described above, but with the 300 gr/mm grating and with a Au-coated beam-steering mirror. It is possible to obtain 150-mW diode lasers commercially, so that spectra of quality similar to the quality of those shown in the previous figures should be possible. The maximum range of this system will ultimately be limited by the noise in the detector. Detector noise consists of thermal noise and readout noise. Although all the spectra shown above were shot-noise limited, this would not be the case at extremely long ranges with similar exposure times. In cases where the Raman signal is comparable to the detector thermal or readout noise,
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detector binning can significantly increase the S/N ratio. This observation is illustrated by Fig. 9, which shows Raman spectra of acetaminophen with signal levels comparable to the detector noise (e.g., ~20 counts)--corresponding to a level expected at a distance of 335 m. In this case, the detector noise is predominantly readout noise. Figure 9 shows three spectra measured with different size stripes on the detector; in all cases, the Raman signal was completely covered by the stripe. As expected, the noise contribution from the detector increases in direct proportion to the square root of the number of binned pixels. Because the Raman signal is the same in each case, the overall S/N ratio decreases. A second motivation to use the smallest vertical stripe possible is to reduce noise contributions from stray light. Pixels were also binned horizontally in groups of two, corresponding to the size of the slit image on the detector. This step resulted in higher signal levels with no loss of spectral resolution. Figure 10 shows how effective this horizontal binning was in reducing noise. System Optimization. A general expression for the magnitude of the Raman return signal power per molecule, P(R), is given by:
P(R ) = PL T2(R )TMIrcrt2(R)
(1)
where
PL is the laser power, r(R) is the atmospheric transmittance, R is the target range, nt is the transmitter optical efficiency, nr is the receiver optical efficiency, a is the Raman cross section, and t2(R) is the collector solid angle. Note that differences in the optical and atmospheric transmittances at the laser and Stokes-shifted wave-
FIG. 10. The effect of horizontal binning the CCD on the S/N ratio. These are 60-s exposures using 54 mW of 488-nm excitation at a distance of 6.3 m. A 40-ram-diameter collection optic was used, with 20~m slits and a 1200 gr/mm grating. In both cases the pixels were binned in such a way that all signal was measured: (A) binning two horizontal pixels; (B) no horizontal binning.
lengths are ignored, and that the collector solid angle is given by
~2(R) = Ao/R 2
(2)
where A0 is the area of the receiver aperture. The remote Raman system used to obtain the data shown above was assembled with the use of available optical components and does not, therefore, represent a fully optimized instrument. The magnitude of the collected signal can be increased by optimizing the throughput of the optical components at the wavelengths used (i.e., ~/t and ~/r) and the performance characteristics of the detector (i.e., quantum efficiency), and by increasing the solid angle of collection of the system [i.e., ~(R)]. With regard to the first type of improvement, the transmission efficiency of the receiver optics can be increased. As mentioned above, it is likely that the use of the two 25-mm piano-convex transfer lenses produces a large amount of spherical aberration in the image formed at the spectrograph slit, especially when used in conjunction with the 40-mm ]:/1.2 collection lens. This approach may result in clipping of a portion of the return signal at the slit and a slight [ / # mismatch into the spectrograph. These problems can be eliminated by replacing the three lenses of the receiver optics with a single [/4 camera lens that is anti-reflection coated for the wavelengths of interest. This step would also eliminate the reflectance losses at the uncoated transfer optics, thus increasing the optical throughput by 16% or more. Concerning the performance characteristics of the detector, the use of a backthinned CCD could increase the quantum efficiency from about 30% (for the CCD used in these experiments) to as much as 60%, representing a 100% total improvement. Taken together, it appears that these changes could increase the total collection efficiency of the system by about 116% with no change in the size of the collection optics. Increasing the diameter of the collection optics leads to trade-offs in the imaging characteristics of the system. APPLIED SPECTROSCOPY
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FIG. 11. Schematicof a generic optical system for remote Raman showing ray traces from the sample to the entrance slit of the spectrograph. R = distancefromsample to collectionoptics,D = collection lens diameter,d~= laser spot diameter at the sample, d~= imagespot diameter at the slit, NA = numericalaperture, m = magnification. While it is desirable to maximize the diameter of the entrance aperture of the receiver to maximize the collection solid angle of the system, this step will also result in an increase in the magnification of the image at the slit aperture. If the size of the image becomes greater than the slit width, the signal reaching the detector will no longer increase quadratically with collection aperture diameter, but will increase linearly. The point at which this occurrence takes place will be determined by the ultimate spectral resolution (i.e., slit width) that is desired. It should be noted that there will be a transition between a quadratic and linear dependence of return signal on collection optic diameter at the point where the image on the slit becomes larger than the slit width. This consideration can be explained further by consulting Fig. 11. The ratio of the front and rear numerical apertures (NA' and NA) of the generalized receiver optics determines the magnification (m) of the laser spot image at the slit plane,
The receiver efficiency can be described as the product of two terms, ~Eov, where rl~ is the total transmittance of the receiver system and Eov is an overlap efficiency of the Raman-scattered return image on the slit, equal to the ratio of the slit area covered by the image to the area of the image. If the image diameter (di) is less than the slit width (d,), Eou is assumed to be equal to one. If the image is larger than the slit, Eov is obtained by integrating the area of the image that overlaps with the spectrograph slit. For simplicity, we assume a uniform image profile. In reality, the image will have a Gaussian or Airy profile. Thus,
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and m = di/dt,
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(5)
In the intermediate-range applications that this spectrometer is intended for, the laser can be focused at the target by zoom-focusing transmission optics. The diffraction-limited laser spot diameter made at the target is given by d~(R ) = R [4Xbrdto] '
(6)
where 2, is the laser wavelength and dto is the laser beam diameter before focusing. Inserting this expression into Eq. 5 yields a new expression for dl, d~ = (16D)~)/~rdzo.
(7)
This expression shows that the increase in laser spot size with range is counteracted by an increase in demagnification with range, and the net result is that the image size at the entrance slit is independent of range, R.
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Volume 46, Number 7, 1992
Eov = (16/Trdi2)
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This equation can be stated in terms of D, },, and d~o by substituting the expression for d~ from Eq. 7. The resulting expression can be used in Eq. 1 to calculate the dependence of the return signal on receiver diameter and slit size. The results of this calculation are shown in Fig. 12. Note that we plot P R 2 / K versus D for various slit widths, where K is defined as: K = (PLT"2yt~o(r~r)/4,
(9)
because these variables are independent of the aperture and the slit width. Figure 12 shows that the collected signal is quadratically dependent on the receiver aperture up to the point that the slit is overfilled (e.g., smaller collection diameters), after which it is linearly dependent on aperture (larger collection diameters). In the linear region, it is assumed that the detector array is larger than the image; i.e., the CCD can be vertically binned to add the signal in that dimension. From Fig. 12, it may be seen that there is significant room for improvement in the design of the collection optics of our system by, in addition to replacing the optics used with higher-quality elements, increasing the receiver aperture. The graph
also demonstrates the magnitude of the trade-off between signal intensity and resolution. FUTURE RESEARCH Soon it should be possible to increase the range and sensitivity of the system considerably by using a backthinned CCD with higher quantum efficiency and by replacing the three optical collection components with a single, high-quality, m u l t i e l e m e n t / / 4 collection optic. Further enhancements will include the use of dielectric mirrors for the beam steering and field-of-view optics. Other improvements will include use of the imaging capability of the image-corrected spectrograph to obtain remote Raman images. CONCLUSION A portable Raman system has been described to measure line-of-site spectra of remotely located samples at intermediate ranges. The system has been demonstrated at ranges up to 20 m and [ numbers//750 with the use of relatively low power cw lasers including both the 488nm line of an argon laser and an 809-nm diode laser line. ACKNOWLEDGMENTS The work at Lawrence Livermore National Laboratory (LLNL) was conducted under the auspices of the U.S. Department of Energy under Contract W-7405-ENG-48. The authors would like to express thanks to Gerald Goldstein of the Office of Health and Environmental Research (RPIS No. 003906) for supporting the research presented here.
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