1052 OPTICS LETTERS / Vol. 23, No. 13 / July 1, 1998

Spectroscopic detection of methane by use of guided-wavediode-pumped difference-frequency generation

Konstantin P. Petrov, Andrew T. Ryan, Thomas L. Patterson, Lee Huang, Simon J. Field, and Douglas J. Bamford

Gemfire Corporation, Suite 600, 2471 East Bayshore Road, Palo Alto, California 94303

Received February 4, 1998

We report spectroscopic gas detection by the use of mid-infrared difference-frequency mixing of two diode lasersin a channel waveguide. The waveguide was fabricated by annealed proton exchange in periodically poledlithium niobate. We generated 3.43 3.73-mm tunable radiation in a single waveguide at room temperatureby mixing diode lasers near 780 and 1010 nm. High-resolution spectra of methane were obtained in 2 swith electronically controlled frequency scans of 45 GHz. The use of highly efficient waveguide frequencyconverters pumped by fiber-coupled diode lasers will permit construction of compact, solid-state, room-temperature mid-infrared sources for use in trace-gas detection. 1998 Optical Society of America

OCIS codes: 130.2790, 130.3730, 190.2620, 300.6260, 300.6390, 280.3420.

Compact laser-based sensors operating in the mid-infrared spectral region offer a potential benefit tomany trace-gas detection applications, including suchcost-sensitive ones as air-quality control in large build-ings, hospitals, and aircraft, monitoring of emissionsin mining and drilling, and combustion diagnostics.The availability of such instruments is tied to thatof a small solid-state room-temperature laser sourceoperating in the region from 2 to 10 mm. This wave-length region is favored in spectroscopic detectionbecause it contains fundamental absorption bandsof most atmospheric trace gases. There are severaldeveloped laser sources operating in this region: gaslasers, lead-salt diode lasers, and color-center lasers.Although these laser sources perform well in thelaboratory environment, their use in field applicationshas been precluded because of a number of practicaldrawbacks, such as discrete tunability, high power con-sumption, large size, fragility, or the need for cryogeniccooling. Development of new tunable mid-infraredsources is under way. One promising category ofsources is the InAsxSb1 x semiconductor laser, withemission wavelengths from 2.7 to 4.3 mm.1 Operationof quantum cascade lasers was demonstrated at wave-lengths above 3.4 mm.2 Although pulsed operationof these devices was obtained at room temperature,cw single-frequency operation still requires cryo-genic cooling. Further development in this areais expected to make the sources suitable for highlysensitive trace-gas detection. Nonlinear frequencydownconversion using quasi-phase matching (QPM)is another technique for generation of tunable mid-infrared light. The cw optical parametric oscillatordelivers widely tunable diffraction-limited infraredlight with output power in excess of 1 W.3 Althoughsingle-frequency operation of these devices was re-cently demonstrated,4 the method that we discussbelow has the advantages of simplicity and a broadertuning range.

An alternative to the optical parametric oscillator isa laser source based on difference-frequency genera-tion (DFG).5 Although DFG is characterized by lowconversion eff iciency, it allows relatively simple accessto mid-infrared wavelengths. Diode-pumped DFG has

0146-9592/98/131052-03$15.00/0

been proven to deliver output power, tunability, andspectral purity suff icient for trace-gas detection at theparts in 109 (ppb) level.6 Demonstration of guided-wave DFG by Lim et al. constituted an important ad-vance in this area.7 Because of conf inement of thelight in a waveguide, there is no trade-off betweenmode size and interaction length as there is in a bulkmaterial. This lack of trade-off was proved to result inhigh conversion efficiency.8 Another feature of a DFGwaveguide, often considered a drawback but in manycircumstances beneficial to spectroscopy, is that it sup-ports multiple modes at the pump wavelength slpd andthe signal wavelength slsd while supporting one modeat the idler wavelength slid. Because the effective in-dex of refraction, N , is different for each mode, thereare multiple sets of wavelengths that satisfy both theenergy-conservation equation, 1ylp 2 1yls 2 1yli ­ 0,and the momentum-conservation equation, Npylp 2

Nsyls 2 Niyli ­ 1yL, in a structure with a given QPMperiod L. Thus a relatively broad tuning range canbe obtained with a single waveguide. Finally, wave-guides can be integrated into the same substrate asthe diode lasers that pump them, leading to a compact,low-cost device.

Figure 1 shows a schematic diagram of a guided-wave DFG spectrometer. The pump laser was adiscrete semiconductor master oscillator power am-plifier that was tunable from 775 to 795 nm with500-mW output power (SDL, Inc., Model TC40). Thesignal laser was an external-grating-stabilized taperedsemiconductor oscillator (SDL, Inc., Model 8630) with250-mW output power at 1010 nm. The laser beamswere combined at a dichroic beam splitter and coupledinto a periodically poled lithium niobate (PPLN)waveguide by a 6.33 microscope objective. Thewaveguide was operated without active temperaturecontrol. A pump power of 135 mW and a signal powerof 190 mW were delivered to the waveguide input.Pump and signal powers measured at the output of thewaveguide were 40 and 60 mW, respectively. Idlerradiation emerging at the output of the waveguidewas collimated by an f ­ 5 mm CaF2 lens and sepa-rated from the pump and signal light by a 3 5-mmbandpass f ilter. An uncoated ZnSe wedge split a

1998 Optical Society of America

July 1, 1998 / Vol. 23, No. 13 / OPTICS LETTERS 1053

Fig. 1. Schematic diagram of a diode-pumped DFG spectrometer using a PPLN waveguide. APE, annealed protonexchange; Preamp., preamplif ier.

portion of the idler beam onto an InSb reference detec-tor. The transmitted idler beam was passed through a30-cm-long cell containing 1–20 kPa of methane atroom temperature and then focused onto a secondInSb detector by an off-axis parabolic mirror. Thequadrature outputs of the signal and reference lock-inamplifiers were recorded by a digital oscilloscope thatdivided the signal voltage by the reference voltage.The ratio was displayed as a function of time, synchro-nized with the frequency scan of the pump laser. Theidler power produced in the waveguide during spectro-scopic scans was typically 0.15 mW, although as muchas 1.0 mW could be obtained at the peak of the phase-matching curve by adjustment of the beam-deliveryoptics.

A set of 2-cm-long channel waveguides was fabri-cated in Z-cut PPLN by use of annealed proton ex-change. A commercial 75-mm-diameter, 0.5-mm-thickwafer of lithium niobate was patterned by meansof electric-f ield poling.9 QPM periods from 17.4 to20.1 mm were chosen based on calculations using themodel of Bortz and Fejer.10 We applied a chromemask pattern to the wafer to produce 6-mm wide wave-guide channels. The patterned wafer was then dicedand proton exchanged in pure benzoic acid for 24 hat 170 ±C. The wafer chips were annealed in air at340 ±C for 34 h. The phase-matching curve of an an-nealed waveguide is shown in Fig. 2. The spectrumcontains multiple peaks because of the multiple wave-guide modes at the pump and the signal wavelengths.For a given set of interaction wavelengths, the polingperiod required for guided-wave QPM is shorter thanit would be for an interaction in bulk crystal because ofthe increased dispersion in the waveguide structure.For example, the mixing of 786 and 1010 nm (idlerwavelength, 3.54 mm) in bulk lithium niobate at roomtemperature would require a QPM period of 21.3 mm,11

in contrast with the 18.6-mm QPM period that wasused in the waveguide. We were unable to determinewhich spatial modes interacted to produce the multiplepeaks in Fig. 2.

The phase-matching curve in Fig. 2 also demon-strates the cutoff wavelength of the waveguide, withthe appearance of a broad DFG peak beginning at3.57 mm. Based on the earlier investigation of para-metric f luorescence in PPLN waveguides by Baldiet al.,12 we attribute this broad peak to phase-matched

generation of an unguided idler wave, the Cerenkovidler conf iguration. In our experiment this radiationwas observed directly, in contrast with the earlier ob-servation of the spectrally inverted signal wave.

A wide range of wavelengths at which appreciableDFG power could be obtained in one multimode wave-guide has spectroscopic applications. For example, the3.43 3.73-mm wavelength region shown in Fig. 2 cov-ers a portion of the n3 band of methane, an importantgreenhouse gas. We acquired the spectra of methaneby use of the diode-pumped waveguide DFG source de-scribed above to prove the source’s usefulness for quan-titative gas detection. Fine frequency scans of up to45 GHz were performed by piezodriven rotation of adiffraction feedback grating in the pump laser masteroscillator. Figure 3 shows a spectrum of methane ata pressure of 1.33 kPa. We calibrated the frequencyaxis by matching the peak positions to their frequencyassignments in the HITRAN database.13 The rms dif-ference of 0.009 between the measured and the calcu-lated traces in Fig. 3 corresponds to a signal/noise ratioof 111, limited by the presence of interference fringesin the signal and the reference beam paths after thebeam splitter. Unlike power f luctuations and fringesintroduced before the beam splitter, these interference

Fig. 2. Idler power versus wavelength for a 6-mm channelwaveguide in PPLN with a QPM period of 18.6 mm. Thepump laser was tuned from 780 to 795 nm, and the signallaser was fixed at 1010 nm. Interaction of multiple spatialmodes in the waveguide is evident from the multiple peaksin the phase-matching curve.

1054 OPTICS LETTERS / Vol. 23, No. 13 / July 1, 1998

Fig. 3. Spectrum of methane at 1.33 kPa in a 30-cm-longcell. The circles are plots of the ratio of the signal andthe reference traces, both recorded as a 16-sweep average.Each sweep, performed at a 0.5-Hz rate, was detectedby a lock-in amplif ier with a 3-ms time constant, whichcorresponds to an equivalent noise bandwidth of 10 Hz inthe ratio trace. The solid curve is a spectrum calculatedwith the HITRAN database and convoluted with a Lorentzianprofile with a FWHM of 240 MHz.

fringes cannot be eliminated by the simple power ratiotechnique used in our experiment. Using optimizedoptics and detection electronics, a noise-equivalentmethane column density of 6.3 ppb m sHzd21/2 was mea-sured in an earlier DFG experiment using 3.4 mW ofoutput power.14 From the observed magnitude andwidth of absorption peaks in Fig. 3, we concluded thatthe idler linewidth was comparable with the Doppler-broadened linewidth of methane at 3.4 mm. Althoughthis linewidth is insuff icient for Doppler-limited spec-troscopy, it is adequate for trace-gas detection in am-bient air, in which pressure-broadened linewidths aretypically several gigahertz. Earlier research in DFGspectroscopy14 shows that linewidths below 50 MHzcan be obtained with proper selection of pump andsignal sources.

We have demonstrated what is to our knowledgethe f irst spectroscopic gas detection using mid-infrareddifference-frequency generation of two diode lasers in achannel waveguide. The waveguide was prepared byannealed proton exchange in PPLN with a QPM periodof 18.6 mm. The proton exchange was performedin benzoic acid at 170 ±C for 24 h. Annealing wasperformed in air at 340 ±C for 34 h. One 2-cm-longmultimode waveguide was pumped with 40 mW ofpower near 780 nm and 60 mW of power near 1010 nm,producing radiation that was tunable from 3.43 to3.73 mm with up to 1.0 mW of output power and anestimated linewidth of 240 MHz. Absorption spectraof methane were acquired in a 30-cm cell at a pressureof 1.33 kPa in 2 s.

These results demonstrate the feasibility of usingguided-wave diode-pumped DFG for trace-gas detec-tion. The usefulness of this technique can be en-hanced by improvement of the design of the waveguideto raise the conversion efficiency h calculated from

the expression Pi ­ hPpPsL2, where L is the wave-guide length. We estimated that h ­ 4% W21 cm22,based on measured refractive-index profiles of pla-nar waveguides prepared with the same recipe, andan overlap integral12 of the TM00 waveguide modes.We observed a much lower conversion efficiency of0.01% W21 cm22, primarily because of the distribu-tion of pump and signal power among multiple spatialmodes of the waveguide. We estimate that our wave-guide supports 22 modes at 780 nm and 10 modes at1010 nm. The use of a periodically segmented taperedinput section15 for excitation of only the fundamentalTM00 mode at the pump and signal wavelengths wasshown to enhance the conversion eff iciency by 2–3 or-ders of magnitude.8 With a projected conversion effi-ciency of 4% W21 cm22, DFG output power of ,100 mWneeded for trace-gas detection can be obtained by use ofdiode lasers in the 100-mW class.

The authors thank Kris Wolfe, Rosemary Lucero,Eric Darko, and Linda Whittelsey for their assistancein preparing the PPLN waveguide sample. This re-search was supported by NASA, the U.S. Departmentof Energy, and the U.S. Air Force.

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