optical parametric amplifier

26 March 1999

Chemical Physics Letters 302 Ž1999. 555–562

Cavity ringdown laser absorption spectroscopy with a 1 kHz mid-infrared periodically poled lithium niobate optical parametric generatorroptical parametric amplifier K.W. Aniolek a , P.E. Powers b, T.J. Kulp a

a,)

, B.A. Richman a , S.E. Bisson

a

Combustion Research Facility, Sandia National Laboratories, LiÕermore, CA 94550-0969, USA b Department of Physics, UniÕersity of Dayton, Dayton, OH 45469-2314, USA Received 21 October 1998; in final form 26 January 1999

Abstract A cavity ringdown spectrometer is described that employs a novel mid-infrared light source based on periodically poled lithium niobate. The source generates tunable light using the three-step process of optical parametric generation, spectral filtering, and optical parametric amplification. Its use allows for improvements over previous pulsed ringdown measurements including the ability to acquire data rapidly Žat 1 kHz. over broad spectral regions Žin principle, over the entire 2220–7690 cmy1 PPLN transparency window. with narrow linewidth Ž( 0.08 cmy1 .. Data are presented that demonstrate performance and support its eventual use in a trace gas sensor. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Recently, much attention has been given to the use of periodically poled lithium niobate ŽPPLN. crystals as an efficient non-linear material for the generation of tunable infrared ŽIR. light. In PPLN crystals the sign of the non-linear coefficient is periodically reversed along the axis of propagation, with a period designed to ensure that the parametric conversion process is phase-matched for a specific set of pump, signal, and idler frequencies w1,2x. This process, called quasi-phase matching ŽQPM., does not require birefringence for non-linear mixing. The major advantage of QPM over birefringent phase matching is an increased conversion efficiency due ) Corresponding author. Fax: q1 925 294 2595; e-mail: [email protected]

to the accessibility of the large d 33 component of the non-linear susceptibility tensor w2x, the absence of beam walkoff in the crystal, and the use of longer crystals. For these reasons, PPLN has found widespread use in IR light sources based on optical parametric oscillation w3,4x and difference frequency mixing w5,6x. We recently described w7x a novel type of nanosecond-pulsed laser source based on PPLN using three serial elements: an optical parametric generator ŽOPG., a spectral filter, and an optical parametric amplifier ŽOPA.. Because it does not require a resonator, an OPG emits light having a continuous bandwidth with no longitudinal mode structure. This ensures that light is always present for amplification at the filter exit. Thus, tuning over the entire bandwidth of the OPGrOPA system can be accomplished with a passive filtering element Žas opposed to one

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 1 3 9 - 6

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actively locked to a cavity mode. which simplifies the process. Moreover, as this Letter will demonstrate, the high PPLN conversion efficiency results in energies suitable for spectroscopic applications when operating with high repetition rate pump laser sources where output energies are typically small and below the optical parametric generation threshold for bulk crystals. These attributes allow for a greatly simplified tunable mid-IR light source with an enhanced data acquisition rate for possible use in a trace gas sensor. The results presented in this Letter will show that this new type of laser source is compatible with, and in fact extends the capabilities of, cavity ringdown laser absorption spectroscopy ŽCRLAS or CRDS.. CRLAS, a linear absorption-based technique, has received widespread attention due to the high sensitivity that can be achieved Ždetection of absorbances - 10y8 cmy1 . while requiring relatively simple experimental and data analysis procedures w8x. This technique determines gas-phase absorbances from the rate of decay of light from an optical cavity rather than the attenuation of a beam as it passes through a sample. The exponential decay time constant, called the ringdown time, is ideally only a function of cavity properties Žsuch as the mirror reflectivity and spacing. and any absorption occurring in the beam overlap region. The ringdown time is typically measured by fitting an exponential function to the data, from which the total per-pass cavity loss factor Ž G . can be calculated. The absorption due to an atomic or molecular transition is then determined simply by calculating the difference in G measured at a resonance and a nearby baseline frequency w8x. Since G is independent of the input laser energy, CRLAS is a self-normalizing technique. This factor, in addition to its use of a high spectral brightness ligt source, makes it orders-of-magnitude more sensitive than direct absorption or FTIR spectroscopy w8,9x. While pulsed CRLAS measurements have already been made in the mid-IR using both optical parametric oscillators w9–11x and Ramanshifted dye lasers w12–14x, the present laser source allows for a more compact spectrometer with significantly shorter data acquisition times. In this Letter, we characterize the performance of CRLAS when using one implementation of the spectrally filtered OPGrOPA approach described above.

That device employed a piezoelectrically tunable air-spaced etalon as the tuning element to achieve continuous tuning over 15 cmy1 at a spectral resolution of ( 0.08 cmy1 and a pulse repetition rate of 1 kHz. The center of the continuous tuning segment can be positioned at any frequency within the coating bandwidth of the etalon filter, thereby allowing continuous segmented tuning over the signal range of 6150–6780 cmy1 and the idler range of 3250– 2620 cmy1 . As will be described, this tuning range can, in principle, be extended to the full transparency of PPLN by selecting the appropriate crystal periodicities and tuning elementŽs.. Several criteria were identified to test the usefulness of this laser source applied to CRLAS. These include the performance of CRLAS with the given OPGrOPA spectral bandwidth and mid-IR pulse energy, the noise level in the measured cavity loss, the ability to cover a broad range of wavelengths simply and quickly, and the maximum achievable data acquisition rate. These issues were examined experimentally, and the results show the OPGrOPA system to be an attractive CRLAS light source.

2. Experimental setup A simplified diagram of the PPLN light source w7x and the CRLAS experimental arrangement is shown in Fig. 1. The pump laser consisted of an injectionseeded, 1 kHz repetition rate Q-switched Nd:YAG laser ŽContinuum HPO 1000. with a pulse duration ŽFWHM. of 15 ns. The amount of pump energy directed to the two PPLN stages Žtypically 600 mJ to the OPG and 300 mJ to the OPA. was adjusted using a polarizing beamsplitter in conjunction with a pair of 1r2 wave plates. Both PPLN crystals were 50 mm long by 0.5 mm thick, with a width of either 5 mm Žfor the OPG. or 20 mm Žfor the OPA.. Each was heated to 115–1508C Ž"0.18C. during the course of the measurements. The OPG crystal was poled at a single periodicity of 29.75 mm while the OPA stage used a ‘fan-out’ crystal w15x in which the grating period changes continuously from 29.7 to 30.1 mm across the 20 mm width. The crystals were chosen because of their availability; the optimum crystal configuration will be determined in the future.

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Fig. 1. Schematic of the experimental arrangement: OI, optical isolator; HWP, 1r2 wave plate; PBS, polarizing beam splitter; FP, Fabry–Perot air-spaced etalon; PZTs, piezoelectric transducers; LWP, long wave pass optical filter; p, pump beam; s, signal beam; i, idler beam.

As generated in the first crystal, the OPG signal output was both spectrally broad Žca. 15 cmy1 . and continuous Ži.e., containing no mode structure.. Its spectral width was filtered to 0.05–0.08 cmy1 ŽFWHM. with a high finesse Ž450 maximum. airspaced Fabry–Perot etalon to form a spectrally narrow seed for the OPA stage. The bandwidth was ascertained from low-pressure absorption spectra of methane in a multi-pass cell w7x, with the minimum bandwidth achievable being sensitive to the alignment of the Fabry–Perot etalon. Adjustment of the wavelength of the OPGrOPA output was performed on either a large or small scale. Large-scale wavelength adjustments were made by simultaneously changing the temperatures of both stages, thus changing the center wavelength of the QPM phasematching curve. Once the crystal temperatures were set, continuous wavelength scanning on a smaller scale Ži.e., within the phase matching bandwidth. was accomplished by ramping the length of the etalon air space with a variable-period Ž50 ms to 300 s. voltage ramp applied to piezoelectric transducers ŽPZT. attached to the back etalon mirror. Setting the Žvariable. free spectral range of the etalon to 25 cmy1 allowed for continuous scanning over ca. 15 cmy1 with negligible transmission of OPG light through either adjacent etalon transmission fringe. The spectrally narrowed signal beam was combined with the pump beam and served as the seeding source for the second ŽOPA. stage. When the second

stage was operated just below its optical parametric generation threshold Ži.e., negligible output light generated in the absence of the seed beam., 10–15 mJ of idler Žat 3.3 mm. was routinely available for injection into the cavity ringdown cell. A portion of the signal output was sent to a monochromator for coarse wavelength calibration. A more precise calibration was achieved by comparing experimental methane spectra to calculated spectra with assigned features. In the future, a monitor etalon in conjunction with a calibration gas cell will be used to calibrate and linearize frequency scanning. The ringdown cavity consisted of a pair of mirrors Žplano-concave with a 6 m radius of curvature. that were 99.98% reflective at 3.3 mm and separated by 46.5 cm. The mirrors were held by motorized optical mounts ŽNew Focus Picomotorse. and adjusted remotely via a driver. The mirrors and holders were completely contained inside a sealed stainless steel chamber to prevent any changes in alignment when evacuating or filling the cell. The chamber was 69 cm long with 1Y CaF2 planerparallel windows mounted on each end and evacuated down to 10 mTorr before filling with a sample. Light generated by the OPGrOPA source was mode-matched to the ringdown chamber using a 15 cm focal length lens. The light exiting the chamber was focused and spectrally filtered with a long wave pass filter Ž) 2.5 mm. onto a 1.0 mm diameter liquid nitrogen-cooled InSb detector ŽEG & G J10D.. The

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K.W. Aniolek et al.r Chemical Physics Letters 302 (1999) 555–562

photovoltaic output of the detector was amplified Ža voltage gain of 100. with a 150 MHz bandwidth video preamplifier ŽPacific 2A50. and digitized by a fast Ž80 Msrs. 12 bit ArD PCI bus oscilloscope card ŽGaGe Compuscope 8012A. for data analysis. A LabVIEWe program acquired and analyzed the ringdown data. Data were collected in two modes: slow-scan or fast-scan. In slow-scan mode, the etalon ramp speed was reduced to allow the program to average a user-specified number of ringdown decay events at each wavelength, fit the resulting trace to an exponential function, and calculate the ringdown time and per-pass cavity loss. The resulting loss value would be displayed versus the relative optical frequency, and the process repeated for the next data point. Averaging ringdown traces in this manner reduces the baseline noise and increases detection sensitivity at the expense the sweep rate. In fast-scan mode, no shot-averaging was performed and a different LabVIEW program was used that was optimized for speed. This program was written to first acquire all the ringdown traces for the entire scan and sequentially store them in a 2-dimensional array before fitting each decay and displaying the spectrum. In this manner the data collection efficiency Ži.e., the fraction of laser shots collected. approaches 100% and the program keeps pace with the 1 kHz repetition rate of the pump laser.

3. Results and discussion In this section, data are presented that demonstrate the utility of this cavity ringdown spectrometer for fast and sensitive absorption measurements at atmospheric pressure. The specific issues addressed here include: the suitability of the idler bandwidth for atmospheric pressure spectroscopy, the performance of CRLAS with ( 15 mJ of pulse energy, the resulting noise level in the measured cavity loss, the ability to cover a broad range of wavelengths simply and quickly, and the maximum achievable data acquisition rate. As mentioned previously, the bandwidth of the OPA signal output was determined to be 0.05–0.08 cmy1 . This bandwidth is more than adequate for measurement of the rotationally unresolvable IR bands of large molecules. It can also be used to

measure individual rovibrational transitions of smaller molecules at 760 Torr, provided convolution effects are taken into account. For example, the atmospheric pressure Voigt linewidths for the methane n 3 fundamental band near 3020 cmy1 are ca. 0.12 cmy1 ŽFWHM. in air Žhowever, the effective linewidth for many J Y ) 1 transitions is larger due to tetrahedral splitting w16x.. The resolution of narrower features, such as Doppler-broadened transitions at lower pressures, require that the OPGrOPA system use a filter with a narrower bandwidth. This can be achieved in a variety of ways. Possibilities include increasing the finesse or decreasing the free spectral range of the Fabry–Perot etalon. If an etalon with a smaller free spectral range is chosen, an acousto-optical tunable filter ŽAOTF. with a suitable bandwidth can be placed at the output to filter out unwanted etalon transmission fringes and preserve a similar scanning capability. In either case, the minimum spectral bandwidth attainable is currently limited by the etalon and the energy threshold for seeding the OPA stage, since the available seed energy will decrease with a reduction in the spectral bandwidth. An alternative method would be to seed the OPA stage with a narrowband Ž; 300 kHz. diode laser source, as has been previously demonstrated w17x. However, these diode lasers are less tunable Žseveral hundred wavenumbers., with the net effect of reducing bandwidth at the expense of the overall tuning range. The OPGrOPA output is also longitudinally modeless in nature. This allows the user to filter the output of the OPG and amplify in the OPA at any frequency in the QPM bandwidth without the need to consider mode overlaps. In addition to single frequency operation, one could use more complicated filtering schemes allowing one to sculpt the PPLN spectrum to suit a particular application. For example, broadband or dual wavelength spectroscopic experiments could be performed by simply adjusting the operating characteristics of the etalon Žfor the dual wavelength case, two adjacent etalon transmission fringes would be present in the output.. Another issue involves the level of sensitivity attainable with a maximum of 10–15 mJ of idler pulse energy available for injection into the ringdown cell. This energy is approximately an order of magnitude smaller than that used in previous CR-

K.W. Aniolek et al.r Chemical Physics Letters 302 (1999) 555–562

LAS investigations employing pulsed mid-IR light w9–14x. With 100–200 mJ of 3.3 mm light, a sensitivity of ca. 3 = 10y6 Žbased on peak-to-peak noise, equivalent to ca. 8 = 10y8 cmy1 for a 39 cm path length. is typical w9x when averaging 16–24 ringdown signals Ž1.6–2.4 s of data acquisition time for the 10 Hz pump laser used in Refs. w9–11x.. The energy per shot required in the mid-IR is expected to be higher than in the visible due to the relatively poor performance of detectors in that wavelength range. For this reason, shot energies of several hundred microJoules have been recommended w18x when operating in the mid-IR. In the present study, however, it was expected that the lower pulse energy could be compensated by the factor of 100 increase in repetition rate allowed by the OPGrOPA light source. To test these assumptions, the rms noise in the length-normalized per-pass cavity loss factor Ž sG . was measured at a single wavelength with the ringdown chamber evacuated. The dependence of sG on laser input energy and number of laser shots averaged Ž N . was investigated and is shown in Fig. 2a. It can be seen that similar sensitivities can be obtained with this spectrometer by simply increasing the number of laser shots averaged. An rms noise of 2 = 10y8 cmy1 Žequivalent to 8 = 10y8 cmy1 of peak-to-peak noise. is obtained for a cell input energy of 12.5 mJ by averaging 300 laser shots. Although the averaging has increased due to the reduced signal reaching the detector, the overall data acquisition time has in fact decreased to 300 ms compared to Ref. w9x due to the higher repetition rate laser used here. Input energies as low as 2.5 mJ have been used, but a higher level of shot averaging was required to maintain the same rms noise. The data in Fig. 2a deviates only slightly from the Ny1 r2 relationship expected for pure ‘white’ noise Žthermal, shot, etc.., indicating that little systematic noise is present in our data. However, oscillations have been observed in the cavity loss baseline Ž( 4 = 10y7 cmy1 . for certain system alignments, the source of which is currently being investigated. This phenomenon is avoided by careful alignment of the laser Že.g., ensuring the seed beam is coaxial with the pump in the OPA stage. and ringdown cell, and is not present in any of the data presented here. A minimum rms noise level of 1.3 = 10y8 cmy1 was

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Fig. 2. Fluctuations in the length-normalized per-pass cavity loss Ž G . of an evacuated cell. In Ža. the rms noise in G Ž sG . is shown for two different input energies Ž2.5 and 12.5 mJ. and various levels of shot averaging. The solid line represents the Ny1 r2 relationship expected for pure ‘white’ noise. A typical baseline is shown in Žb. for an input energy of 12.5 mJ and 800 laser shots averaged.

obtained for the highest input energy and 800–1000 shots averaged, and a typical baseline for these conditions is shown in Fig. 2b. This value compares well to most other pulsed CRLAS studies. Sensitivities on the order of 10y8 cmy1 are typical for static cells and mirror reflectivities 0 99.98% w9,19–21x, although higher levels have been reported w22x. To demonstrate the ability to combine short scan segments, an experimental methane spectrum was obtained in the vicinity of the n 3 Q-branch near 3020 cmy1 and is shown in Fig. 3. The spectrum was obtained by compiling four separate CRLAS scans Žlabeled as scans a1–a4. of 500 ppb methane in nitrogen at a total pressure of 760 Torr. Each 15 cmy1 scan was obtained by ramping the voltage to the etalon PZTs with a ramp speed that ensured a point density of ( 0.03 cmy1 Žso that spectral features contained a minimum of 10 data points. when averaging 300 laser shots. The idler energy ranged from 8 to 10 mJ during the course of each scan. Following collection of an individual continuous

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Fig. 3. Experimental Žsolid line. and theoretical Ždashed line. spectra in the vicinity of the CH 4 n 3 Q-branch. The experimental spectrum was obtained by compiling four continuous 15 cmy1 scans using different oven temperatures. A total of 300 shots were averaged for each data point. The theoretical curve was calculated by convolving a spectrum predicted by HITRAN for the given cell conditions with an appropriate etalon transmission function. The features marked with an asterisk Ž). have been attributed to ca. 10 ppm of water vapor in the sample. Cell conditions: 500 ppb methane in nitrogen at a total pressure of 760 Torr.

sweep, the oven temperatures were adjusted Žsee Table 1. to coarsely tune to the next wavelength region. Methane spectral features were identified by the USF HITRAN-PC Ž1996. code and were used to generate a wavelength calibration curve for each scan. Other features in the spectrum Žmarked with an asterisk in Fig. 3. were identified by HITRAN to be due to ca. 10 ppm of water vapor contamination in the sample. The theoretical curve in Fig. 3 was obtained by convolving a spectrum generated by HITRAN for the given cell conditions with an estimate of the OPGrOPA lineshape. An etalon transmission function was used as the estimate of the laser lineshape. For the data in Fig. 3, an etalon Table 1 Temperatures of the OPG and OPA crystals used to obtain the four scans shown in Fig. 3 Scan

a1 a2 a3 a4

Crystal temperature Ž8C. OPG

OPA

127 135 143 150

115 123 131 138

transmission function with a FWHM of 0.05 cmy1 resulted in the best fit. The rms baseline noise when averaging 300 shots is approximately 5 = 10y8 cmy1 near 3025 cmy1 , which implies a detection limit of 10 ppb Ž2 = 10y1 1 molrcmy3 . for the RŽ0. AŽ1. 1 transition and 3 ppb Ž7 = 10y1 0 molrcmy3 . for the largest Q-branch feature at room temperature and pressure. By scanning the wavelength piecewise, large spectral regions can be covered Žca. 60 cmy1 in Fig. 3. while maintaining narrow bandwidth operation, as evidenced by the comparison with the convolved HITRAN spectrum. In principle, the entire PPLN transparency window Ž2220–7690 cmy1 . could be scanned in this way. To approach this, however, some practical issues must be addressed. These include limitations in the operating range of the etalon, the temperature and period tuning of a given PPLN crystal, and the increased OPG bandwidth that occurs near degeneracy Žcausing possible overlap of the OPG output with multiple etalon fringes.. Also, the overall measurement is limited by the bandwidth of the CRLAS cavity mirrors. The tuning range issues can be addressed by using more capable or multiple elements. It is possible to envision a system employing stacked PPLN crystals and two etalons,

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Fig. 4. Experimental Žsolid lines. and theoretical Ždashed lines. spectra of the CH 4 n 3 Q-branch and RŽ0. transition Žinset. near 3020 cmy1 . The rate of data acquisition was approximately 1 kHz, allowing for a 15 cmy1 scan of the Q-branch in 1 s and a 0.9 cmy1 scan of the RŽ0. transition in 50 ms. The theoretical curve was calculated by convolving a spectrum predicted by HITRAN for the given cell conditions with an appropriate etalon transmission function. Cell conditions: 500 ppb methane in nitrogen at a total pressure of 760 Torr.

and additional ringdown mirror sets. The degeneracy issue can be alleviated by using a higher finesse etalon, or two serial filters Že.g., a thickrthin etalon pair or an AOTF with an etalon.. To examine the maximum rate of data acquisition that can be achieved, the voltage ramp generator was run faster and ringdown signals were collected using the fast-scan LabVIEW program without averaging shots. The previous cell conditions were maintained and the resulting experimental spectra near 3020 cmy1 are shown in Fig. 4. The entire CH 4 n 3 Q-branch was scanned in 1 s and the RŽ0. transition in 50 ms while maintaining a 0.02 cmy1 point density. Since no trace averaging was performed, the rms noise level in these scans increased to 3 = 10y7 cmy1 for an input energy of 10 mJ, which reduces the sensitivity to ca. 20 ppb for the most intense Q-branch feature at 760 Torr. However, the sensitivity improves to ca. 8 ppb when averaging 8 scans Žas shown in the inset to Fig. 4. while maintaining a 50 ms scan rate. A 1 kHz data point collection rate was verified Žto within 1%. by counting the number of data points collected in a single scan. The experimental data were reproducible to within an absorption of "2 = 10y8 cmy1 and compared favorably to the convolved HITRAN spectrum. The data in Fig. 4 demonstrates that this sensor is capable of monitoring spectral information, such as

rotational temperatures, on a 1 s timescale. Such information could be recorded on a sub-second timescale by scanning faster, if a decrease in the data point density can be tolerated, or by use of a pump laser with a faster repetition rate. Relative or absolute concentrations can be measured in 50 ms, since only an individual absorption feature is needed for calibration at constant temperature. In fact, changes in molecular concentrations on a millisecond timescale can be detected by adjusting the laser wavelength to coincide with an absorption maximum and subtracting from the cavity loss value a previously determined baseline value.

4. Conclusions In conclusion, we have demonstrated the applicability of a new high repetition rate tunable PPLN OPGrOPA mid-IR laser for atmospheric pressure spectroscopy using the cavity ringdown technique. CRLAS data have been acquired rapidly Žat 1 kHz. over broad spectral regions Žcontinuously over 15 cmy1 and over 60 cmy1 by periodically adjusting the crystal temperature., at narrow linewidth Ž( 0.08 cmy1 ., and with a sensitivity comparable to that of previous pulsed CRLAS studies.

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Our laser source employs a simple design that is easy to operate and avoids the complexities associated with controlling an oscillator cavity. Future work will focus on reducing the overall size of the laserrspectrometer. The availability of compact passively Q-switched Nd:YAG microchip lasers w23x should be a considerable aid in this effort. Other possibilities for size reduction include the use of a fiber etalon for frequency filtering and the use of a single crystal Žin a two-pass configuration. for both optical parametric generation and amplification. It should therefore be possible using this source to construct a fast, on-line, compact, and sensitive chemical sensor.

Acknowledgements The authors gratefully acknowledge Mark Missey ŽUniversity of Dayton. for supplying one of the PPLN crystals. This research is supported by the US Department of Energy Office of Nonproliferation and National Security, Office of Research and Development.

References w1x M.M. Fejer, G.A. Magel, D.H. Jundt, R.L. Byer, IEEE J. Quantum Electron. 28 Ž1992. 2631. w2x L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 Ž1995. 2102. w3x L.E. Myers, G.D. Miller, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, Opt. Lett. 20 Ž1995. 52.

w4x G.W. Baxter, Y. He, B.J. Orr, Appl. Phys. B 67 Ž1998. 753. w5x A. Balakrishnan, S. Sanders, S. DeMars, J. Webjorn, ¨ D.W. Nam, R.J. Lang, D.G. Mehuys, R.G. Waarts, D.F. Welch, Opt. Lett. 21 Ž1996. 952. w6x K.P. Petrov, R.F. Curl, F.K. Tittel, Appl. Phys. B 66 Ž1998. 531. w7x P.E. Powers, K.W. Aniolek, T.J. Kulp, B.A. Richman, S.E. Bisson, Opt. Lett. 23 Ž1998. 1886. w8x J.J. Scherer, J.B. Paul, A. O’Keefe, R.J. Saykally, Chem. Rev. 97 Ž1997. 25. w9x J.J. Scherer, D. Voelkel, D.J. Rakestraw, Appl. Phys. B 64 Ž1997. 699. w10x J.J. Scherer, D. Voelkel, D.J. Rakestraw, J.B. Paul, C.P. Collier, R.J. Saykally, A. O’Keefe, Chem. Phys. Lett. 245 Ž1995. 273. w11x J.J. Scherer, K.W. Aniolek, N.P. Cernansky, D.J. Rakestraw, J. Chem. Phys. 107 Ž1997. 6196. w12x J.B. Paul, C.P. Collier, R.J. Saykally, J.J. Scherer, A. O’Keefe, J. Phys. Chem. A 101 Ž1997. 5211. w13x J.B. Paul, R.A. Provencal, C. Chapo, A. Petterson, R.J. Saykally, J. Chem. Phys. 109 Ž1998. 10201. w14x J.B. Paul, R.A. Provencal, R.J. Saykally, J. Phys. Chem. A 102 Ž1998. 3279. w15x P.E. Powers, T.J. Kulp, S.E. Bisson, Opt. Lett. 23 Ž1998. 159. w16x K.T. Hecht, J. Mol. Spectrosc. 5 Ž1960. 390. w17x A. Zakel, M. Missey, V. Dominic, P.E. Powers, P.P. Yaney, T.D. Raymond, Paper WMM6 presented at the 1998 OSA Annual Meeting, Baltimore, MD, 1998. w18x J.B. Paul, R.J. Saykally, Anal. Chem. 69 Ž1997. 287A. w19x A. O’Keefe, D.A.G. Deacon, Rev. Sci. Instrum. 59 Ž1988. 2544. w20x A.J. Ramponi, F.P. Milanovich, T. Kan, D. Deacon, Appl. Opt. 27 Ž1988. 4606. w21x J. Xie, B.A. Paldus, E.H. Wahl, J. Martin, T.G. Owano, C.H. Kruger, J.S. Harris, R.N. Zare, Chem. Phys. Lett. 284 Ž1998. 387. w22x D. Romanini, K.K. Lehmann, J. Chem. Phys. 99 Ž1993. 6287. w23x J.J. Zayhowski, Opt. Lett. 22 Ž1997. 169.