820 Volume 51, Number 6, 1997 APPLIED SPECTROSCOPY0003-7028 / 97 / 5106-0820$2.00 / 0q 1997 Society for Applied Spectroscopy

Generation of 1.30- to 1.55-m m Tunable Radiation fromFirst Stokes Raman Shifting in Hydrogen

KENNETH W. ANIOLEK, DAVID L. MILLER, NICHOLAS P. CERNANSKY, andKEVIN G. OWENS*Department of Mechanical Engineering and Mechanics (K.W.A., D.L.M., N.P.C.) and Department of Chemistry (K.G.O.), DrexelUniversity, Philadelphia, Pennsylvania 19104-2284

Raman shifting to the near-infrared, when possible, provides a sim-ple and economical alternative to the optical parametric oscillator(OPO) or difference frequency mixing approach. We report theproduction of 1.30- to 1.55- m m radiation from ® rst Stokes Ramanshifting in a single-pass, open (no capillary waveguide), hydrogen-® lled Raman cell (constructed in-house) pumped with a Nd:YAG/dye laser combination operating near 900 nm. A maximum of 10mJ (19% ef® ciency) of ® rst Stokes energy was measured for thehighest cell pressure (490 psia) and input pulse energy (53 mJ). Thequality of the ® rst Stokes output is similar to the dye laser outputas indicated by polarization, shot-to-shot energy ¯ uctuation, beamdiameter , and linewidth. A characterization of the Stokes and anti-Stokes output was also conducted including one-dimensional spatialintensity pro® les and output line dependence on input pulse energyand cell pressure. A large ® rst Stokes conversion ef® ciency has beenattributed to little production of higher-order Stokes and anti-Stokes lines.

Index Headings: Raman shifting; Stimulated Raman scattering;Stimulated anti-Stokes Raman scattering; Near-infrared spectros-copy.

INTRODUCTION

The near-infrared region from 1.30 to 1.55 m m is ex-perimentally interesting, in part because of a richness ofpolyatomic vibrational overtone bands occurring there,including CO2 (1.43 m m), H2O (1.38 m m), C2H2 (1.54m m), CH4 (1.33 m m), etc.1 Of particular interest to thepresent study, however, is the 2A²(000)± 2A 9 (000) elec-tronic band (1.43 m m) of the hydroperoxy (HO2) radical,an important intermediate occurring in combustion en-vironments. Two potential absorption-based techniquesfor detecting polyatomic species in both combustion androom-temperature systems include degenerate four-wavemixing (DFWM)2 and cavity ringdown laser absorptionspectroscopy (CRLAS).3 The most common DFWM ge-ometry consists of a relatively weak probe beam crossingtwo counterpropagating pump beams at a small angle. Ifthe wavelength coincides with a molecular transition, agrating is formed in the beam overlap region,4 resultingin the scattering of the backward pump beam into a signalbeam. The spatial and temporal selectivity of DFWM, aswell as the generation of a coherent signal beam, makesit especially attractive in high background-noise combus-tion environments where molecular concentrations varyboth in space (¯ at ¯ ames) and time (static reactors). InCRLAS, a relatively new technique, a pulse of light isinjected into an optical resonator formed by two highlyre¯ ective mirrors. The intensity of the trapped light de-

Received 12 April 1996; accepted 30 October 1996.* Author to whom correspondence should be sent.

cays exponentially with time, with the time constant be-ing a function of the mirror re¯ ectivities, cavity dimen-sions, and concentration of any absorbing species present.CRLAS is highly desirable when sensitivity is critical anda path-integrated technique is acceptable. Typical pulsedlaser energies required range from 0.1 to 8.0 mJ forDFWM5,6 and approximately 1 mJ for CRLAS.7

A major obstacle in making a near-infrared measure-ment is producing near-infrared radiation with suf® cientlaser pulse energy. Dye lasers are ef® cient in the visiblepart of the spectrum; however, current laser dyes extendto only approximately 1.0 m m. Optical parametric oscil-lators (OPOs) have become an increasingly popular ap-proach, but narrow-bandwidth OPOs can be prohibitivelyexpensive in an age of shrinking laboratory budgets. TheOPO also replaces the conventional dye laser, which isstill the most common method of producing tunable light.When already present in the laboratory (as the case forthe present study), a more desirable approach would beto extend the dye laser output further into the infrared.Difference frequency mixing is one such method, but itrequires expensive crystals and a delicate alignment andwavelength tracking procedure. A ® nal possibility is red-shifting by stimulated Raman scattering (SRS) in a pres-surized-gas cell. The SRS approach is an economicallyef® cient alternative, with the gas cell easily manufacturedin-house and only common optical materials required.

Raman shifting theory is well established;8 hence onlya brief summary will be presented here. First Stokes (S1)photons are generated in the Raman medium by the in-teraction of a pump photon with a molecule in its vibra-tional ground state. The process, referred to as SRS,leaves the molecule in the ® rst vibrationally excited stateand produces an S1 photon with an energy equal to thepump photon energy minus the difference in energies be-tween the two states. In standard notation, this process issummarized by v S 5 v P 2 v R, where v S is the S1 photonenergy given in cm2 1, v P is the pump photon energy, andv R is the difference in energy between the n 5 0 and n5 1 vibrational levels (4155 cm2 1 for hydrogen and 2916cm2 1 for methane, the most widely used Raman shiftingmedia).9,10 As the S1 intensity builds up, it can begin toact as the pump source and produce second Stokes (S2)photons (with v P 5 S1 photon energy and v S 5 S2 pho-ton energy in the above equation). This process can con-tinue to produce higher-order Stokes output. Stimulatedanti-Stokes Raman scattering (SARS, also called four-wave mixing) is responsible for production of higher-en-ergy radiation (anti-Stokes lines). Four-wave mixing, re-quiring phase-matching conditions to be met, can also be

APPLIED SPECTROSCOPY 821

FIG. 1. Schematic diagram of the experimental arrangement. F1 and F2, focusing lenses; P, prism; F, fundamental beam; S, Stokes beam; AS,anti-Stokes beam.

responsible for the production of second- and higher-or-der Stokes lines. This phenomenon is responsible for thering-shaped spatial intensity pro® le often observed withanti-Stokes lines, since at higher pressures the phase-matching condition is satis® ed for wave vectors with non-zero angles along the focusing axis.11,12

Because of greater conversion ef® ciencies at high laserpulse input energies, most SRS investigations have beenmade with nontunable pump lasers.10± 13 Of the remainingstudies using tunable pump sources, Frey and Pradere,14,15

and later De Martino et al.,16 produced tunable infraredradiation with a powerful ruby pumped dye laser as theRaman pumping source. A continuous tunability was ob-tained in the 1- to 2-m m region via SRS in hydrogen withthe dye laser operating at 400 MW. For less energeticpumping sources, Rabinowitz et al.17 reported generationof tunable radiation in the 1- to 12-m m region (via gen-eration of multiple Stokes lines) with a Nd:YAG/dye la-ser combination (6± 9 MW) pumping a multipass Ramancell. Radiation at 1.62 m m was obtained by S2 productionin hydrogen. Mannik and Brown18 also used a Nd:YAGpumped dye laser, but used a Raman cell equipped witha capillary waveguide to obtain a maximum of 5.5 mJ of1.57-m m radiation. Both of the above studies used visiblelight as the pump source; hence the need for S2 conver-sion to obtain radiation in the 1.35- to 1.55-m m wave-length region. The S2 conversion ef® ciency is generallysmaller than that for S1, thus limiting maximum near-infrared laser pulse energy.

In this paper, we demonstrate the feasibility and sim-plicity of generating suf® cient radiation for DFWM,CRLAS, or other absorption methods in the 1.30- to 1.55-m m region by S1 production in a single-pass, open (nocapillary waveguide), hydrogen-® lled Raman cellpumped with a Nd:YAG/dye laser combination operatingnear 900 nm. The intensity and stability of the S1 line,as well as the ® rst two anti-Stokes lines, were measuredand the dependence on input laser pulse energy and hy-drogen pressure was investigated. One-dimensional beamintensity pro® les were also measured to spatially char-

acterize the Raman-shifted output beams. Finally, to dem-onstrate the potential usefulness of the S1 output forspectroscopic measurements and approximate the S1 out-put linewidth, a CO2 absorption spectrum in the near-infrared was measured by using the Raman-shifted radi-ation.

EXPERIMENTAL

A schematic diagram of the experimental apparatus isshown in Fig. 1. The second harmonic (532 nm) of a Nd:YAG laser (Continuum NY81) operating at 10 Hz (5± 10ns) served as the dye laser pump source. A Lambda Phy-sik LPD 3002E dye laser was used to produce tunablelinearly polarized light with a spectral linewidth of 0.3cm2 1 and a beam diameter of ; 3 mm. All laser dyes usedwere purchased from Exciton and dissolved in HPLC-grade Fisher Scienti® c and Aldrich solvents. A 0.7- to1.3-m m IR detection card (Kodak 133 4093) was used toaid in dye laser alignment, with a similar 0.8- to 1.6-m mcard (Kodak 814 9718) for locating the S1 beam.

The body of the Raman cell was constructed in-houseof 304 stainless steel material. A 39-cm-long tube(1.9-cm diameter) contained ¯ anged exit ports to whichtwo end caps were mounted (see Fig. 1). The end capswere machined at a Brewster’s angle to minimize beamre¯ ection losses. An additional stainless steel ¯ ange wasattached to the end caps to hold a 5-cm-diameter quartzfused-silica window. The caps resulted in an overall celllength of 52 cm for all experiments. The windows andend caps were sealed to the cell with O-rings to containthe pressurized gas. The ¯ anged design of the Raman cellallows for the possibility of easily extending the lengthby insertion of additional tubes. In a procedure to removeany oil remaining from machining, the cell was thor-oughly washed with hexane and rinsed with methanol.

Two ¼-in. Swagelok ports were attached to the cell forperiphery equipment. A vacuum pressure transducer(Leybold In® con 0± 10 Torr CM100-G10A) and a high-pressure gauge (Danton 0-500 psig) were attached to one

822 Volume 51, Number 6, 1997

TABLE I. Four potential methods for Raman shifting the dye laseroutput to 1.43 m m.

Meth-od

Laserdye

Cellgas

Funda-mentalwave-length(nm)

1stStokeswave-length(nm)

2ndStokeswave-length(nm)

1234

LDS 821ÐDCMLDS 867

CH4

CH4

H2

H2

7801010653897

10101430897

1430

1430245014303520

TABLE II. Optimized dye concentrations and solvents for produc-tion of the fundamental beam.

MethodLaserdye

Oscillatorconcentrationand solvent

Main ampli® erconcentrationand solvent

1 LDS 821 189.5 mg/L inmethanol

72.8 mg/L in meth-anol

3 DCM 270 mg/L in meth-anol

90 mg/L in metha-nol

4 LDS 867 150 mg/L in 55%methanol, 45%DMSO

79.8 mg/L in 55%methanol, 45%DMSO

FIG. 2. Energy output and tuning range of three potential laser dyesfor Raman shifting into the near-infrared. Nd:YAG second-harmonicpower is 1.7 W.

port to monitor cell pressure. The other port allowed forpumping down the cell with a vacuum pump (LeyboldTrivac D25BCS) to remove all air and back-® lling with99.9995% pure hydrogen (BOC Gases).

The output from the dye laser passed through an 51-cmplano-convex focusing lens (633 to 1064-nm AR coated)and re-collimated after the Raman cell with an identical-focal-length lens with a 1064- to 1550-nm broad-bandcoating to minimize re¯ ections of the S1 beam. A SchottSF10 prism was used to spatially separate the frequency-shifted radiation from the fundamental beam exiting theRaman cell. The power of the fundamental beam enteringthe Raman cell was measured with a calorimeter detector(Scientech Astral AC2501). The shot-to-shot energy ofthe Stokes and anti-Stokes beams was measured after theprism with a pyroelectric joulemeter (Scientech VectorP25). The detector response was displayed on an indi-cator (Scientech Vector D200) with a capability for sta-tistical analysis. To minimize the possibility of detectordamage, we removed the collimating lens for all energy-determining experiments.

The collimating lens was then reinserted to measurethe one-dimensional spatial intensity pro® les. A 100-m mpinhole, mounted in an adjustable XY translator (Oriel15380), was positioned in front of a high-speed photo-diode (Thorlabs germanium PIN diode DET3-GE for in-frared and silicon PIN diode DET2-SI for visible radia-tion). The pinhole position relative to the photodiode was® rst adjusted to optimize the measured intensity. The pin-hole/photodiode combination was then mounted an a sin-gle-axis translation stage. This single-axis stage was ad-justed in 100-m m increments to obtain the spatial beampro® les.

RESULTS AND DISCUSSION

Method Selection. The ® rst issue which had to be re-solved was choosing the optimum method of Ramanshifting to the near-infrared. Table I shows four possibil-ities of producing radiation at 1.43 m m [the band originfor the HO2

2A²(000)± 2A 9 (000) electronic band]. The ® rsttwo methods involve frequency shifting in methane witha 2916-cm2 1 shift, while the last two methods employ the4155-cm2 1 shift in hydrogen.

Method 2 (S1 in methane) was immediately ruled outbecause of the lack of laser dyes at 1010 nm. Of the threeremaining possibilities, the best choice was not immedi-ately apparent. With everything else equal, production ofnear-infrared radiation through S1 (method 4) is alwayspreferable over S2 (methods 1 and 3), since S2 produc-tion is a higher-order process, using S1 as the pumping

source, resulting in smaller (by an order of magnitude)output pulse energy. However, laser dyes are more ef® -cient in the visible range of the spectrum (favoring meth-od 3). Another complicating factor is the steady-state gaincoef® cient, g , which is several factors larger for hydrogenthan for methane.13

To settle this issue, we purchased three candidate laserdyes (LDS 821, DCM, LDS 867) and optimized dye con-centrations at the desired fundamental wavelengths (780,653, and 897 nm, respectively). The resulting dye solu-tions are shown in Table II. To shift the LDS 867 dyecurve to the red and improve performance at 897 nm, weused a methanol/dimethyl sulfoxide (DMSO) solution.Final dye laser tuning curves are shown in Fig. 2.

The LDS 821 laser dye was found to yield unsatisfac-tory intensity at 780 nm. Though it would be possible toblue-shift the dye curve, it still would not be as ef® cientas DCM (method 3). The larger g for hydrogen also fa-vors method 3 over method 1. A ® nal drawback is agrating-order change in the Lambda Physik dye laser be-tween 794 and 795 nm, making long-range wavelengthscanning dif® cult in this region. Therefore, in comparingthe two S2 methods, method 3 is clearly preferable overmethod 1.

Of the remaining two choices, DCM produced 40 mJat 653 nm and LDS 867 produced 31 mJ at 897 nm.Therefore, the visible dye was found to produce 1.3 timesmore pulse energy than the infrared dye. However, a fac-tor of 1.3 was not expected to overcome the larger S1conversion ef® ciency, which is typically of an order of

APPLIED SPECTROSCOPY 823

FIG. 3. Characterization of the 1st Stokes (S1) beam. (a) Dependenceof S1 energy on input energy and hydrogen pressure; and (b) simulta-neous measurement of shot-to-shot energy ¯ uctuation as indicated bythe coef® cient of variation [(standard deviation/mean) 3 100]. Datawere averaged for 300 pulses with a focusing length of 30 cm into thecell. Fundamental wavelength: 896 nm; S1 wavelength: 1427 nm.

magnitude. Also of concern with method 3 is the possi-bility of a serious degradation of the spatial beam qualityresulting from the four-wave mixing process. Therefore,production of 1.30- to 1.55-m m radiation by S1 Ramanshifting in hydrogen (method 4) was chosen as the bestapproach in terms of output pulse energy and beam qual-ity.

Stokes and Anti-Stokes Dependence on Pressureand Input Laser Pulse Energy. It was found that a fo-cusing length of 30 cm into the cell resulted in the bestconversion ef® ciency for the S1 line. This line, and sub-sequently the anti-Stokes lines, was found to have a cen-tral core of relatively intense energy surrounded by a con-centric circular area of lower intensity (e.g., see Fig. 7b).Only the central core would be useful in an absorptionexperiment, so the surrounding diffuse intensity was re-moved by an aperture for all energy-measuring experi-ments (data presented in Figs. 3± 6).

Figure 3a shows S1 output energy (hereafter, the term`̀ energy’ ’ will implicitly refer to `̀ laser pulse energy’’ asopposed to `̀ photon energy’’ ) as a function of funda-mental beam energy (18± 53 mJ) and hydrogen pressure(120± 490 psia). The highest cell pressure and input en-ergy (limited by a maximum pumping energy into thedye laser of 300 mJ) was found to produce 10 mJ (19%ef® ciency) of energy at 1.43 m m, with a polarization

identical to the dye laser output. The output is seen to behighly dependent on input energy and cell pressure witha threshold existing below which no conversion was ob-served. For a given fundamental energy, the curves areseen to rise quickly at lower pressures and level off atgreater pressures, indicating that this region is somewhatpressure-saturated. This plateau at higher pressures alsoindicates little if any S2 conversion, since signi® cant S2production typically results in depletion of the S1 line.We did not have any means of visualizing the 3.5-m mradiation and thus were unable to con® rm this possibility.We were, however, able to place the P25 joulemeter inthe approximate location of the S2 line and observed onlya slight increase in energy above the background at thislocation.

Figure 3b shows the energy stability (shot-to-shot en-ergy ¯ uctuation) of the S1 beam, as measured by thecoef® cient of variation, V (average-normalized standarddeviation expressed in percent). The results indicate that,for the best shot-to-shot stability, high cell pressures andinput energies are required to approach the stability ofthe dye laser output (VF ; 2.2). This ® gure also gives abetter picture of the energy/pressure threshold. The steeppart of the curves in Fig. 3a correspond to the most un-stable S1 output, with curves in the two ® gures beingalmost mirror-opposites of one another. Near the thresh-old, the nonlinear SRS gain greatly accentuates the shot-to-shot ¯ uctuation in the frequency-shifted energy, whichmust have a coef® cient of variation at least as large asthat of the fundamental beam (VS1 $ VF). At higher cellpressures, the SRS process becomes saturated, which ef-fectively dampens out any large energy ¯ uctuations (VS1

; VF).To provide a more complete description of the Raman

cell output, we made similar measurements for the ® rstanti-Stokes (AS1) line. Figure 4a shows AS1 dependenceon input energy and cell pressure. For a cell pressure of150 psia and input energy of 53 mJ, a maximum AS1energy of 1.4 mJ (2.6% ef® ciency) was measured. Allcurves are seen to have a threshold region similar to S1,a rapid increase in output energy with increasing pres-sure, followed by a slight leveling-off and decrease athigher pressures. This decrease in AS1 central core in-tensity at higher pressures is likely due to two effects:the onset of second anti-Stokes production, which is aminor effect as indicated by the much smaller energy ofthe second anti-Stokes line (Fig. 5a); and an increase inthe angular beam spread at higher pressures, as indicatedby the subsequent beam pro® le measurements. Figure 4bshows the shot-to-shot energy ¯ uctuation of the AS1lines. The results are qualitatively similar to the S1 re-sults, but with lower threshold pressures and slightlyhigher overall instabilities.

Figure 5a shows the dependence of second anti-Stokes(AS2) output on input energy and cell pressure. The max-imum AS2 energy output was 0.12 mJ, observed for afundamental energy of 54 mJ (0.22% ef® ciency) and acell pressure of 150 psia. The shapes of the curves aresimilar to the AS1 case, which again is most likely dueto greater angular dispersion at higher pressures. Beamstability, shown in Fig. 5b, is diminished in comparisonto the AS1 line.

We also observed a third anti-Stokes line, correspond-

824 Volume 51, Number 6, 1997

FIG. 4. Characterization of the ® rst anti-Stokes (AS1) beam. (a) De-pendence of AS1 energy on input energy and hydrogen pressure; and(b) simultaneous measurement of the shot-to-shot energy ¯ uctuation asindicated by the coef® cient of variation [(standard deviation/mean) 3100]. Data were averaged for 300 pulses with a focusing length of 30cm into the cell. Fundamental wavelength: 896 nm; AS1 wavelength:653 nm.

FIG. 5. Characterization of the second anti-Stokes (AS2) beam. (a)Dependence of AS2 energy on input energy and hydrogen pressure; and(b) simultaneous measurement of the shot-to-shot energy ¯ uctuation asindicated by the coef® cient of variation [(standard deviation/mean) 3100]. Data were averaged for 300 pulses with a focusing length of 30cm into the cell. Fundamental wavelength: 896 nm; AS2 wavelength:514 nm.

FIG. 6. Tuning range and shot-to-shot energy ¯ uctuation of the ® rstStokes (S1) beam in the near infrared. Nd:YAG power: 1.9 W; dye laserenergy at 896 nm: 34 mJ; Raman cell hydrogen pressure: 490 psia.

ing to 423 nm, but it was found to be too low in intensityfor any quantitative measurements.

Near-Infrared Tuning Range. The complete tuningrange and stability of the near-infrared radiation areshown in Fig. 6. The usable region (for best energy sta-bility) is seen to extend from 1.32 to 1.52 m m. The tuningrange can be extended by approximately 20 nm to shorterwavelengths by using a 100% methanol/LDS 867 dyesolution and, likewise, into the infrared by switching to100% DMSO. Another laser dye, LDS 925 (with a rangefrom 890 to 980 m m),19 can possibly be used to furtherextend the infrared tuning range to approximately 1.41±1.65 m m.

Horizontal Beam Pro® les. The S1 horizontal beampro® le measurements for cell pressures of 150 and 490psia are shown in Figs. 7a and 7b, respectively. A beamdiameter [full width at half-maximum (FWHM)] of 2.0mm at 490 psia is slightly larger than a diameter of 1.8mm for a cell pressure of 150 psia. An increasing beamdiameter, as well as the growth of `̀ wings’’ with increas-ing pressure, clearly indicates an increasing degree of S1saturation. A further increase in pressure will tend tostimulate the growth of the less-intense outer portion ofthe beam. However, complete saturation has not yet oc-curred, since both beam diameters are smaller than the

fundamental beam diameter (3.0 mm). This is only a mi-nor concern, since the `̀ wings’’ can be cleaned up withthe aid of an aperture.

The beam pro® le measurements for AS1 and AS2 foridentical pressures are shown in Figs. 7c± 7f. Little, if any,saturation occurs for the lower pressure, as indicated by

APPLIED SPECTROSCOPY 825

FIG. 7. One-dimensional beam intensity pro® les for two different hydrogen pressures in the Raman cell: 150 psia in left column and 490 psia inright column. (a and b) ® rst Stokes (S1); (c and d) ® rst anti-Stokes (AS1); and (e and f), second anti-Stokes (AS2) line. Nd:YAG power: 2.8 W;dye laser energy at 896 nm: 50 mJ.

FIG. 8. Absorption spectrum of the 3v3 CO2 vibrational overtone bandoriginating at 6,976 cm2 1. Pathlength: 268 cm; CO2 pressure: 1 atm;input energy at 6996 cm2 1: 4.2 mJ.

the relatively narrow beam diameter (approximately 0.8mm for both lines). As pressure increases, a ring of in-creasing intensity begins to form around the central core(as clearly shown in Fig. 7f). As discussed previously,this well-known effect is caused by a phase mismatchoccurring along the focusing axis with increasing molec-ular density. Moriwaki et al.12 have shown that the ringdiameter will increase with increasing pressure and thecentral core will eventually disappear. They also used asimple wave-vector model to show that the scattering an-gle will increase with increasing anti-Stokes order, whichis also indicated by our data [a smaller ring diameter forAS1 (Fig. 7d) than for AS2 (Fig. 7f)].

CO2 Absorption Spectrum. As a simple demonstra-tion of beam quality, an absorption spectrum of the3v3CO2 vibrational overtone band was measured with theuse of the S1 output and is shown in Fig. 8. The spectrum

826 Volume 51, Number 6, 1997

was recorded by passing 4.2 mJ of collimated near-infra-red radiation through a 268-cm-long static cell ® lled with1 atm of carbon dioxide. A germanium photodiode mea-sured a portion of the output radiation. A large pulseenergy and hydrogen pressure (490 psia) were used tomaximize the Raman shifting stability.

The Raman-shifted output linewidth will be of partic-ular importance for most applications. Generally, the Ra-man linewidth is a function of the cell pressure, temper-ature, and composition of the shifting medium. Differentbroadening processes dominate at different conditions,8

with collisional broadening dominating in this study, thuscomplicating any general theory and limiting the appli-cability of ab initio calculations. Bischel and Dyer20 de-rived an empirical formula for the FWHM Raman line-width as a function of cell temperature and H2 density.For room temperature, this relationship can be expressed as

D v 5 11.2/p 1 1.58p (1)

where p is the cell pressure in atmospheres, and D v isthe linewidth in cm2 1 3 103.21 This equation gives a Ra-man linewidth for this study (0.05 cm2 1 at 490 psia)which is smaller than the dye laser output (0.3 cm2 1). So,for our setup, the S1 linewidth is determined by the dyelaser, not the Raman shifting process. The average mea-sured linewidth of the spectrum (0.4 cm2 1) is only slight-ly larger than the dye laser output, possibly indicatingpower broadening in the absorbing medium.

CONCLUSION

We have shown that production of near-infrared radi-ation between 1.30 and 1.55 m m from ® rst Stokes Ramanshifting provides a relatively simple and inexpensive al-ternative to the OPO or difference frequency mixing ap-proach. The cell can be constructed in-house and doesnot require a waveguide or multipass mirrors to achievemodest conversion ef® ciencies (up to 19%) and S1 en-ergy (maximum of 10 mJ). The optical materials are alsoinexpensive, and the supporting periphery equipment re-quired should be relatively common to most laboratories.

We have also characterized the Stokes and anti-Stokesoutput as a function of cell pressure and input energy. Alarge S1 conversion ef® ciency is likely due to little sub-sequent depletion to S2. The quality of the S1 output issimilar to the dye laser output, as indicated by polariza-tion, shot-to-shot energy ¯ uctuation, and beam diameter.The onset of `̀ wings’’ in the S1 beam pro® le is indicativeof a small amount of saturation in the area of maximumpump intensity. The anti-Stokes rings result from an axial

phase-mismatch at higher pressuresÐ consistent with pre-vious investigations.

Finally, a near-infrared absorption spectrum of CO2

was easily obtained with the experimental setup. The res-olution of this spectrum indicates that the Stokes-shiftedlaser light is of suf® cient quality for use in meaningfulspectroscopic work. We believe that the present investi-gation demonstrates that Raman shifting provides near-infrared light with suf® cient energy and beam quality forspectroscopic measurements.

ACKNOWLEDGMENTS

This research was supported by the National Science Foundation(Grant No. CTS-9213932) and the U.S. Army Research Of® ce (ContractNumbers DAAH04-93-G-0145 and DAAH04-93-G-0042). We alsothank the reviewers for their useful comments and suggestions.

1. G. Herzberg, Molecular Spectra and Molecular Structure. II. Infra-red and Raman Spectra of Polyatomic Molecules (D. Van Nostrand,New York, 1945), pp. 272± 309.

2. R. L. Abrams, J. F. Lam, R. C. Lind, D. G. Steel, and P. F. Liao,in Optical Phase Conjugation, R. A. Fisher, Ed. (Academic Press,New York, 1983), Chap. 8, pp. 211± 284.

3. J. J. Scherer, J. B. Paul, A. O’ Keefe, and R. J. Saykally, in Advancesin Metal and Semiconductor Clusters, Vol. 3, M. A. Duncan, Ed.(JAI Press, Greenwich, Connecticut, 1995), pp. 149 ± 180.

4. H. J. Eichler, P. GuÈ nter, and D. W. Pohl, Laser-Induced DynamicGratings (Springer-Verlag, Berlin, 1986), pp. 114± 119.

5. B. A. Mann, S. V. O’ Leary, A. G. Astill, and D. A. Greenhalgh,Appl. Phys. B 54, 271 (1992).

6. R. L. Vander Wal, B. E. Holmes, J. B. Jeffries, P. M. Danehy, R.L. Farrow, and D. J. Rakestraw, Chem. Phys. Lett. 191, 251 (1992).

7. J. J. Scherer, D. Voelkel, D. J. Rakestraw, J. B. Paul, C. P. Collier,R. J. Saykally, and A. O’ Keefe, Chem. Phys. Lett. 245, 273 (1995).

8. J. C. White, in Tunable Lasers, L. F. Mollenauer and J. C. White,Eds. (Springer-Verlag, Berlin, 1987), Chap. 4, pp. 115± 207.

9. D. W. Trainor, H. A. Hyman, and R. M. Heinrichs, IEEE J. Quan-tum Electron. QE-18, 1929 (1982).

10. K. Sentrayan, L. Major, A. Michael, and V. Kushawaha, Appl. Phys.B 55, 311 (1992).

11. S. Wada, H. Moriwaki, A. Nakamura, and H. Tashiro, Opt. Lett.20, 848 (1995).

12. H. Moriwaki, S. Wada, H. Tashiro, K. Toyoda, A. Kasai, and A.Nakamura, J. Appl. Phys. 74, 2175 (1993).

13. G. B. Jarvis, S. Mathew, and J. E. Kenny, Appl. Opt. 33, 4938(1994).

14. R. Frey and F. Pradere, Opt. Commun. 12, 98 (1974).15. R. Frey and F. Pradere, Infrared Phys. 16, 117 (1976).16. A. De Martino, R. Frey, and F. Pradere, IEEE J. Quantum Electron.

QE-16, 1184 (1980).17. P. Rabinowitz, B. N. Perry, and N. Levinos, IEEE J. Quantum Elec-

tron. QE-22, 797 (1986).18. L. Mannik and S. K. Brown, Opt. Commun. 57, 360 (1986).19. Laser Dyes (Exciton, Dayton, Ohio, 1992), p. 25.20. W. K. Bischel and M. J. Dyer, Phys. Rev. A 33, 3113 (1986).21. Z. Chu, U. N. Singh, and T. D. Wilkerson, Appl. Opt. 30, 4350

(1991).