Instrumentation
Interferometry in the Seventies Peter R. Griffiths Chemistry Department Ohio University Athens, Ohio 45701
In a recent editorial in this journal, Dr. Laitinen (7) cited infrared spectroscopy to illustrate the seven stages in the life of an analytical instrument. The seventh stage was defined as a "period of senescence" in which "other methods of greater speed, economy, convenience, sensitivity, selectivity, etc., surpass the method under consideration." In the case of infrared spectroscopy, an eighth stage has emerged, which might perhaps be defined as the rejuvenation stage. While it is certainly true that newer instrumentation is surpassing the sensitivity and selectivity of conventional infrared spectrophotometers for many applications, the development of mid infrared Fourier transform spectroscopy has enabled measurements to be made at far greater speed and/or sensitivity than is possible with a grating monochromator.
portant of these innovations was the development of a computerized highresolution Fourier transform spectrometer by a group at Block Engineering in Cambridge, Mass. Shortly after the publication of Low's article, this spectrometer became commercially available, and this and other similar Fourier transform spectrometers are now being used for a multitude of different applications in many laboratories across the world. This article will discuss the reasons behind the development of the Michelson interferometer for mid infrared spectroscopy and describe a few of the more interesting applications for which they are being used. Some knowledge of the theory of infrared FTS will be assumed, and several descriptions of this theory have been published in this and other journals.
In one of the first articles written for this feature some five years ago, Low (2) gave a brief description of the theory of infrared Fourier transform spectroscopy (FTS) and discussed the instrumentation then available together with several applications which had been carried out with FTS. At the end of this paper, Low summarized developments in the field of FTS which were taking place at that time. Certainly the most im-
Early Interferometers The first two interferometers used for mid infrared FTS were both designed for astronomical spectroscopy even though they were conceptually very different. The first type was developed by a French group led by Pierre and Janine Connes (3) for the purpose of extremely high-resolution measurements of weak sources. This interferometer was rather sophisticated and (along with its successors) has
been used to obtain some of the highest resolution infrared spectra ever measured on any kind of infrared spectrometer (4). This type of instrument is quite specialized in nature and is overdesigned for normal chemical spectroscopy in which a resolution higher than 0.1 c m - 1 is rarely called for. The second type of interferometer was developed by Mertz (5), and has a much simpler design than Connes' interferometer. Its most important feature is its high scan speed; the moving mirror of Mertz' interferometer is translated at a sufficiently high velocity that mid infrared radiation is modulated at audio frequencies. The theory of this and other rapid-scanning interferometers has been described in several articles (2, 6, 7), the most recent of which is by Griffiths et al. (7). This interferometer yielded spectra of rather low resolution (20 c m - 1 ), so that it did not meet with the general acceptance of the chemical community, even though many spectra were measured which had hitherto been difficult to measure with a conventional grating monochromator. The signal-to-noise ratio (S/N) of interferograms measured with a rapidscanning interferometer can be increased by coherently adding interfer-
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1890 Michelson interferometer Courtesy of Division of Physical Sciences, Smithsonian Institution
ogram data points digitized at equal intervals of optical path difference. However, the constant frequency clock which was used on the Mertz interferometer meant that the inter ferogram was digitized a t equal intervals of time; only if the moving mirror of the interferometer was translated at an absolutely constant velocity would the interferogram be digitized a t equal increments of optical path difference. The drive system of these early interferometers was based on a relatively simple transducer (similar to those used on loudspeakers) with no means of accurately controlling the mirror velocity, so that coherent addition could only be maintained over rather short optical path differences. For higher resolution spectroscopy, where much larger optical path differences are required, the scan speed produced by this type of drive is not sufficiently constant to permit true signal averaging, where the S/N increases with the square root of the number of scans. Fortunately, several parallel developments in fields unrelated to FTS allowed the successful development of relatively inexpensive Michelson interferometers which would yield spectra of a sufficiently high resolution to satisfy all but the most demanding of chemical spectroscopists. Innovations in Interferometry Two problems had to be solved to allow high-resolution data to be measured. The first involved designing a drive of such precision that the moving mirror remains in precisely the same plane throughout the entire scan, with a tilt of less than half of the shortest wavelength in the spectrum being measured. This goal was initially achieved by the use of air bearing drives, although oil bearings have since been used in certain in646 A ·
struments. The latest commercially available high-resolution interferometer (made by EOCOM Corp., 19722 Jamboree Road, Irvine, Calif. 92664) achieves an exceptionally tilt-free drive by the use of two symmetrical air bearings, whose center of resistance is the point at which the drive force is applied, thus providing a dynamic mechanical drive stability. The optical throughput of an interferometer is given by the product of the solid angle of the beam and its cross-sectional area at a focus. The solid angle of the beam passing through the interferometer is determined solely by the resolution and the highest frequency in the spectrum, so that for any measurement in which this solid angle has been optimized, the optical throughput is determined by the area of the mirrors in the interferometer; the larger the area of the mirrors, the higher is the optical throughput, and hence the higher is the S/N of the spectrum which can be measured. In practice it is more difficult to drive large mirrors with little or no tilt than small ones. Thus, until recently the mirrors on interferometers designed to measure highresolution (0.1 cm" a ) spectra have been small, generally of 1-in. diameter. The drive system developed by EOCOM has enabled 0.1 cm" 1 resolution spectra to be measured with interferometer mirrors up to 3 in. in diameter, so that the optical throughput is almost 10 times greater than that of earlier interferometers. The second problem which had to be overcome was the development of a method of digitizing the interferogram at equal intervals of mirror travel rather than equal intervals of time. In some early Michelson interferometers designed for far infrared spectroscopy, a Moiré fringe device had been successfully used for this
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purpose, but this method was not found to be applicable to mid infrared spectroscopy by use of rapidscan interferometers. It was actually the recent availability of inexpensive helium-neon lasers which allowed the problem to be overcome. When monochromatic radiation, such as that from a He-Ne laser, is passed through a scanning Michelson interferometer, the beam is modulated sinusoidally, owing to successive conditions of constructive and destructive interference which occur at wavelength intervals of optical path difference. By monitoring a second (reference) interferogram from a He-Ne laser at the same time as the main (signal) interferogram is being measured, the signal interferogram can be digitized at equal intervals each time that the reference interferogram reaches a certain value. Since the reference interferogram is usually ac coupled, the signal interferogram is most easily digitized at every zero crossing of the reference interferogram. Although this technique allows the signal interferogram to be digitized at equal intervals of mirror travel, it does not ensure that the first data point is collected at the same mirror position for every scan during the signal averaging process. To overcome this problem, a beam of white visible light is also passed through the interferometer, which gives rise to an interferogram similar in form to the signal interferogram from a broadband infrared source, but modulated over a shorter optical path difference because of the shorter wavelength of visible light compared with infrared radiation. If data collection is initiated when the signal from this white light interferogram reaches a certain value, Vf, coherent addition of the signal interferogram can indeed be
1974 Model 496 interferometer Courtesy of Digilab Inc.. Cambridge, Mass.
achieved. Thus, in a modern Fourier transform spectrometer, three interferograms are actually measured: one from the infrared signal being studied, one from the monochromatic laser beam, and one from the white light reference beam. In some instruments, the same beamsplitter is used for all three beams, with small areas at the top and bottom of the signal beamsplitter being coated with different materials to allow the laser and white light beams to be modulated and reach their respective detectors (Figure 1). In others, a separate reference interferometer is used, the moving mirror of which is attached to (and therefore driven at the same speed as) the moving mirror of the signal interferometer (Figure 2) ; Figure 3 shows the three interferograms measured with this type of interferometer. That the position of the peak of the white light interferogram can be displaced from that of the signal interferogram means that data points can be collected on both sides of zero path difference of the signal interferogram, leading to a somewhat more simple phase correction routine in the computation of the spectrum. For interferograms measured with the type of interferometer shown in Figure 1, the peaks of the white light and signal interferograms both occur simultaneously. Detectors for FTS
One further factor was critical to the successful measurement of mid infrared interferograms by use of these interferometers. For incandescent sources of the type normally used in conventional mid infrared spectrophotometers, the S/N of the interferograms can be very high, and if the speed of the moving mirror is slow, the S/N may be greater than the dynamic range of the analog-to-
digital converter (ADC). Reduction of the S/N of the interferogram to the level where the noise can be sampled by the ADC may be achieved by increasing the speed of the moving mirror. However, if commonly used thermal detectors are used to measure these interferograms, the highest modulation frequencies in the signal are so high that the response time of the detector is too long to permit them to be measured. Only the development of sensitive pyroelectric bolometers with very short response times, and in particular the triglycine sulfate (TGS) detector, has permitted wide range mid infrared spectra to be easily measured. The TGS detector has approximately the same detectivity, D*, as the various thermal detectors commonly used in grating spectrophotometers. However, TGS does have several undesirable properties, in that it cannot be heated above its Curie temperature (40°C), and it reacts with atmospheric water vapor so that it must always be sealed behind an infrared transmitting window. Since no readily obtainable material transmits both mid infrared and far infrared radiation, different detectors have to be used for mid and far infrared spectroscopy. Other pyroelectric materials, in particular strontium barium niobate, have been developed which overcome these problems, but as yet their sensitivity is so much lower than TGS that they have not been used for spectroscopic measurements. Sensitive photoconductive detectors have been applied to FTS, but they must be used under conditions where the S/N of the corresponding interferogram measured with a TGS detector is very low, so that the dynamic range problems discussed earlier in this section are avoided. Al-
though the useful frequency range of photodetectors is always less than that of pyroelectric detectors, a combination of indium antimonide and mercury cadmium telluride detectors (both of which operate a t the temperature of liquid nitrogen) has been used to cover the mid infrared spectrum above 800 cm" 1 at a sensitivity which is at least 10 times greater than is able to be attained with a TGS detector. Data Systems
In addition to its optics, a modern Fourier transform spectrometer requires a data system for signal averaging, performing the fast Fourier transform (FFT) and all subsequent data processing. Once again relatively recent technical developments, this time in the field of minicomputers, have given an extremely powerful and versatile data handling capability to Fourier transform spectrometers. By adding a disc memory to a minicomputer with 4K words of core, interferograms with 32K data points have been signal averaged and transformed in just 3 min to yield complete mid infrared spectra at better than 0.5 cm" 1 resolution. For lower resolution spectra, comparable in resolution to those measured on a small grating spectrophotometer, the computation time is only a few seconds. All data systems designed for use with rapid-scanning interferometers possess the capability of signal averaging interferograms and performing the FFT; however, the amount of data manipulation which can be carried out on digital spectra already stored in the data system of the spectrometer certainly adds to the appeal of FTS to spectroscopists. By far the most versatile and popular data system for infrared FTS has been developed by Digilab, Inc. (237 Putnam
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Figure 1. Schematic representation of interferometer which has beamsplitter for signal, laser reference, and white light interferograms in same plane
Figure 2. Schematic representation of interferometer whose reference interferometers are not in same plane as signal interferometer
Figure 3. Typical interferograms from (A) signal interferometer, (B) white light interferometer, and (C) laser reference interferometer Zero path differences for signal and white light interferograms can o c c u r at different time for interferometer of type shown in Figure 2, but they must o c c u r simultaneously for interferometer of type shown in Figure 1
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Ave., Cambridge, Mass. 02139). Some features on this system are relatively routine, such as the capability of plotting spectra linear in absorbance or replotting them under ordinate or abscissa scale expansion; other rou tines are of a more specialized nature. The following paragraphs describe a few applications for which this data system has been used where the data handling capability is critical to the success of the experiment. One routine which has often been found to be extremely useful is the capability of storing a single-beam spectrum for later use as the refer ence for a ratio-recorded spectrum. With this feature, strong bands in the reference spectrum which would be difficult to compensate with a dou ble-beam spectrometer can often be completely eliminated from the spec trum. For example, when studies of dilute solutions are made with longpath cells, it is convenient to use the same cell to hold the sample solution that is used to hold the reference sol vent so that exactly the same path length is involved for each measure ment. In another application, when spectra of materials under high pres sure are measured by a cell with dia mond windows, the absorption bands of diamond are always seen in spectra measured on a double-beam spectro photometer. However, if the spec trum of the empty cell is measured and stored in the computer memory before the sample is prepared, the di amond bands can be eliminated from the transmittance spectrum of the sample. Sometimes it is not possible to compensate for absorption bands found in the reference spectrum in this manner. In cases of this type, programs have been written to allow these bands to be compensated digi tally. The spectrum of each material whose absorption bands are to be compensated is measured and stored linear in absorbance, as is the spec trum of the sample. The absorbance spectrum of each material to be com pensated is multiplied by a constant (greater or less than unity) chosen so that the bands are exactly compen sated when the reference spectrum is subtracted from the sample spec trum. The use of this program can be il lustrated by showing how bands owing to atmospheric H2O and CO2 can be compensated in long-path spectroscopic measurements of trace organic air pollutants before and after these programs were written. Mea surements of this type made by FTS have been described by Hanst et al. (8), who employed a 400-m foldedpath gas cell to hold the samples of contaminated and clean air. In his first study, Hanst measured the sam
ple spectrum first and then had to carefully adjust the partial pressure of H2O and CO2 in clean air so that their bands were precisely compen sated. Finding the correct partial pressure of H2O and CO2 to exactly compensate the bands in the sample often took a lot of time and effort; however, with the programs described previously, the same effect can be achieved much more quickly. The linear absorbance spectrum of the at mospheric species is measured (by taking the ratio of the spectrum of the cell filled with clean air and that of the evacuated cell) and stored. The linear absorbance spectrum of the contaminated sample is then mea sured in the same manner. The first spectrum may be multiplied by a constant chosen so that when the scaled reference spectrum is subtract ed from the sample spectrum, all trace of the bands owing to H2O and CO2 is removed, leaving only the bands owing to the contaminants. Many other applications of the scaled subtraction routines could also be listed. This type of program is not only useful for linear absorbance spectra, but it can also be used for infrared emission spectroscopy. The background radiation often has to be subtracted out from an emission spectrum to observe or quantify the spectral features of interest. The background spectrum can usually be measured before or after the mea surement of the source, but it is quite rare that this spectrum does not have to be scaled up or down before sub traction so that background peaks are precisely compensated. One of the best examples of the use of a data system for infrared FTS is seen in its application to the on-line identification of materials separated by gas chromatography (GC-IR) (7, 9). Effluent gas from the chromatograph is passed via a heated transfer line to a heated light-pipe gas cell. The signal from the GC detector is monitored, and when the monitor in dicates that a peak is passing through the light-pipe, interferograms are sig nal averaged until it is through. At this point the interferogram is stored in the disc memory of the data sys tem. Interferograms of each GC peak are measured in this way and stored in successive memory arrays; at the end of the chromatogram, they are sequentially recalled and each spec trum is computed and plotted. If just one peak in the chromatogram is of interest, it can be automatically trapped in the light-pipe when the sample is at its maximum concentra tion so that more extensive signal av eraging can be performed to increase the S/N above that of the "on-thefly" method. Programs have also been written for stopping the flow of car-
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Table I. High-Resolution FTS Instrumentation Manufac turer
Model no.
496 Digilab 296 Digilab EOCOM 7201 Barnes Engi neering 1 F3
Max resolu tion, cm - 1
Mirror diam, in.
0.1 0.5 0.06
1 2 Up to 3
0.5
1
rier gas through the GC column while the sample is trapped in the lightpipe (interrupted elution) so that no peaks are missed during signal aver aging of the trapped sample. Instrumentation There are now several commercial sources of Fourier transform spec trometers for mid infrared spectrosco py, and it is believed that this list will be increased in the near future. Manufacturers making interferome ters capable of measuring high-reso lution spectra (0.5 cm~ 1 or better) are listed in Table I. Of these companies, Digilab is the only one to supply a data system at the present time, although it is un derstood that EOCOM will shortly be able to supply one also. The scan speed of all of these instruments is such that it usually takes less than 1 sec to measure a low-resolution (8 cm - a ) spectrum and less than 10 sec to measure a high-resolution (0.5 cm" λ) spectrum. Two companies, Di gilab and Barnes Engineering (30 Commerce Road, Stamford, Conn. 06902), have modified versions of their instruments so that very rapid scan rates (up to 100 scans/sec) can be achieved for low-resolution spec troscopy. Another manufacturer, Willey Corp. (P.O. Box 670, Melbourne, Fla. 32901), has developed a low er resolution slow scanning inter ferometer for the purpose of measur ing the total and diffuse reflectance spectra of samples. This instrument uses a rather different method of measuring the interferograms, in which interferogram data points of light reflected from the sample, from a 100% reflector and from a 0% re flector (blackbody), are collected dur ing a single scan (Figure 4). The sam ple is held at the surface of an inte grating sphere, and the 100% reflector and the detector are held at other po sitions on the surface. For measure ments made by use of an integrating sphere, a large detector gives the best results since the signal is effectively averaged over the entire surface, and Willey (10) has found that spectra of the highest S/N are measured with a
Figure 4. Optical diagram for Willey Model 318 total reflectance spectrometer (A) Path for laser reference beam; since this is slow-scanning interferometer, no white light reference is needed. (B) Path for sample interferogram. (C) How radiation reaches 100% reflective reference. Between open (sample) quadrant of chopper and reflective (reference) quadrant are two black (0% reflective) quadrants
large ( 5 x 5 mm) mercury cadmium telluride detector. The latest spectra measured with this instrument show a great improvement over earlier spectra measured with a small TGS detector. This instrument yields spectra of high photometric accuracy, even when highly scattering samples are being measured. Applications of FTS In a recent article by Jakobsen and Griffiths (11), the theoretical advantages of FTS over grating spectroscopy have been quantitatively appraised. The main conclusion reached by these authors was that the principal benefit of FTS for chemical spectroscopy is derived through Fellgett's (multiplex) advantage, whereas Jac-
quinot's (throughput) advantage is almost negligible. Quantitatively, Fellgett's advantage is that the S/N of a spectrum measured by FTS is yfM times higher than the same spectrum measured on a scanning monochromator in the same time, where M is the number of resolution elements in the spectrum (i.e., the frequency range/resolution). In the light of this study, three types of measurements were cited in which FTS should show its largest advantage over dispersion spectroscopy: • Applications where a large number of resolution elements are to be measured, i.e., high-resolution spectroscopy over a wide spectral range • Applications where only a limited time is available for the measurement since, whereas Fellgett's advan-
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tage is given by V l ^ f o r the S/N of spectra measured in equal times, it takes M times longer to measure a spectrum at a certain S/N with a dispersive spectrometer than with an interferometer • Applications where, even though Fellgett's advantage may be relatively small, the signal that is being measured is so weak that an unacceptably long time is required to measure the spectrum with a conventional spectrometer. In this case, even a small time saving can be important. An examination of the more recent papers where spectra have been measured by FTS shows that the measurements generally fall into one or more of these three categories. For example, in the studies of trace levels of air pollutants made by Hanst et al. (8) which were mentioned earlier, spectra were measured at fairly high resolution to increase the width of the spectral "windows" between atmospheric absorption bands, to increase the specificity of the method, and to increase the absorbance of sharp features (Q branches) in the spectra of light molecules. GC-IR presents a perfect example of an application falling into the second category. In this case, the measurement time is often limited to less than 10 sec by the narrow half width of most GC peaks. In GC-IR the computation and plotting stages are performed after the completion of the chromatogram; and even though they are time-consuming operations, they neither affect the measurement time of the interferogram nor the S/N of the spectrum for any peak. The largest number of reported FTS measurements fall into the final category. For example, interferometry is certainly the method of choice of most far infrared spectroscopists; here, the number of resolution elements is rather small, but the energy from far infrared sources is very low. An increasingly large number of measurements of the infrared emission spectra of materials at fairly low temperature is being reported (12-14). Occasionally, these studies are made at fairly high resolution; this author believes that high-resolution infrared emission spectroscopy may well become the method of choice for monitoring pollutants emerging from smokestacks. A typical high-resolution emission spectrum of N2O is shown in Figure 5 which demonstrates the three main advantages of FTS—high resolution, short measurement time, and high sensitivity. Other examples of studies in which the energy is limited include infrared ultramicrosampling (15, 16), studies of adsorbed species on scattering substrates (17, 18), and studies of materials under high pressure by use of
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Figure 5. High-resolution emission spectrum of N 2 0 (A) Full-range spectrum of 10-cm path length of nitrous oxide at 160°C and pressure of 19 mm Hg, measured on Digilab FTS-14 spectrometer at 0.5 c m - 1 resolution; measurement time, 10 min. Base line of this spectrum is not at zero owing to low-level emission from hot KCI windows and (possibly) brass walls of cell; hence, absorption features of atmospheric water vapor (between 1900 and 1300 c m " ' ) and carbon dioxide (around 670 c m ~ 1 ) can also be observed in this spec trum. (B) Expanded plot of above spectrum between 1360 and 1200 c m - 1 to demonstrate resolu tion of this measurement
cells w i t h small d i a m o n d windows a n d diffuse reflectance spectroscopy.
of infrared spectroscopy as an a n a l y t ical t e c h n i q u e .
T h e Future of FTS
References
It is now generally a c c e p t e d t h a t Fourier transform s p e c t r o m e t e r s pro duce m u c h higher q u a l i t y infrared s p e c t r a t h a n c o n v e n t i o n a l g r a t i n g or prism spectrometers; indeed, the p r i n c i p a l factor working a g a i n s t their more general a c c e p t a n c e is t h e price tag. A l t h o u g h t h e first low-resolution c o m p u t e r i z e d m i d infrared Fourier transform s p e c t r o m e t e r for less t h a n $50,000 h a s j u s t been a n n o u n c e d b y Willey Corp., all i n s t r u m e n t s w i t h a resolution of 0.5 c m " 1 or b e t t e r sell for more t h a n $70,000. T h e n e x t two years are a l m o s t certainly going to see a r e d u c t i o n in t h e cost of Fourier t r a n s f o r m s p e c t r o m e t e r s designed for t h e a n a l y t i c a l c h e m i s t . (This t r e n d h a s come a b o u t in t h e l a s t few years for pulsed N M R s p e c t r o m e t e r s . ) W h e n these i n s t r u m e n t s b e c o m e c o m m e r c i a l l y a v a i l a b l e , infrared F T S will be used for r o u t i n e analysis in an increasing n u m b e r of l a b o r a t o r i e s .
(1) H. A. Laitinen, Anal. Chem., 45, 2305 (1973). (2) M. J. D. Low, ibid., 41 (6), 97A (1969). (3) J. Pinard, J. Phvs., 28 (C2), 136 (1967). (4) G. Guelachvili and J.-P. Maillard, Aspen Int. Conf. on Fourier Spectrosc. 1970, G. A. Variasse, A. T. Stair, and D. J. Baker, Eds., AFCRL-71-0019, ρ 151, 1970. (5) L. Mertz, Astron. J., 70, 548 (1965). (6) M. J. D. Low, Naturwissenschaften, 57,280(1970). (7) P. R. Griffiths, C. T. Foskett, and R. Curbelo, Appl. Spectrosc. Rev., 6, 31 (1972). (8) P. L. Hanst, A. S. Lefohn, and B. W. Gay, Appl. Spectrosc, 27,188 (1973). (9) K. L. Kizer, Amer. Lab., p 40 (June 1973). (10) R. R. Willey, personal communica tion, 1973. (11) R. J. Jakobsen and P. R. Griffiths. Appl. Spectrosc, in press (1974). (12) P. R. Griffiths, ibid., 26, 73 (1972). (13) J. B. Bates and G. E. Boyd, ibid., 27, 204(1973). (14) H. W. Prengle, C. A. Morgan, C.-S. Fang, L.-K. Huang, P. Campani, and W. W. Wu, Environ. Sci. Technol., 7, 417 (1973). (15) S. S. T. King, J. Agr. Food Chem., 21,526(1973). (16) F. Block and P. R. Griffiths, Appl. Spectrosc., 27,431 (1973). (17) M. J. D. Low, A. J. Goodsel, and N. Takezawa, Environ. Sci. Technol., 5, 1191 (1971). (18) M. J. D. Low, A. J. Goodsel, and N. Takezawa, ibid., 6, 268 (1972).
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U l t i m a t e l y , we m a y be sure t h a t s o m e t h i n g b e t t e r will replace F T S ; t h e d e v e l o p m e n t of t u n a b l e lasers a t t h e p r e s e n t t i m e suggests t h a t t h e y m i g h t be used to m e a s u r e h i g h - q u a l i ty a b s o r p t i o n s p e c t r a w i t h i n t h e n e x t five y e a r s . Right now, however, interferometry is causing t h e r e j u v e n a t i o n
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