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Frequency-modulation spectroscopy with blue diode lasers

Gustafsson, U; Somesfalean, Gabriel; Alnis, J; Svanberg, Sune

Published in:Applied Optics

DOI:10.1364/AO.39.003774

2000

Link to publication

Citation for published version (APA):Gustafsson, U., Somesfalean, G., Alnis, J., & Svanberg, S. (2000). Frequency-modulation spectroscopy withblue diode lasers. Applied Optics, 39(21), 3774-3780. https://doi.org/10.1364/AO.39.003774

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Frequency-modulation spectroscopy with bluediode lasers

Ulf Gustafsson, Gabriel Somesfalean, Janis Alnis, and Sune Svanberg

Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The techniqueis especially adaptable to diode lasers, which can be modulated easily, and has been used extensively inthe near-infrared and infrared spectral regions. The availability of blue diode lasers now means that theaccessible wavelength region can be increased. We successfully demonstrate wavelength-modulationspectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line ofpotassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated6p2 3P2 metastable level. Information on the modulation parameters is obtained with a fitting proce-dure. Experimental signal-to-noise ratios at different absorption levels are compared with theoreticalsignal-to-noise ratios and show good agreement. Detection sensitivities of 2 3 1026 and 5 3 1026 forwavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth aredemonstrated. © 2000 Optical Society of America

OCIS codes: 300.6260, 300.6380, 300.1030.

8

1. Introduction

Diode laser absorption spectroscopy based on fre-quency modulation ~FM! is a commonly used methodfor fast and ultrasensitive detection of minute con-centrations of gas.1–6 The interaction between amodulated laser field and an absorbing sample leadsto the generation of an absorption-related signal thatcan be detected at the applied modulation, at an over-tone, or at an intermediate frequency by use offrequency- and phase-sensitive electronics. Charac-teristically this process shifts the detection band to ahigh-frequency region, where the laser excess ~1yf !noise is avoided. Although a variety of FM methodshave been implemented, they actually represent lim-iting cases of the same technique. Depending on thenumber of modulation tones, on the choice of modu-lation frequency relative to the spectral width of theabsorbing feature, and on the detection frequency,the methods are referred to as wavelength-modulation spectroscopy ~WMS!,7 single-tone FM

The authors are with the Department of Physics, Lund Instituteof Technology, P.O. Box 118, SE-221 00 Lund, Sweden. J. Alnis ison leave from the Institute of Atomic Physics and Spectroscopy,University of Latvia, Rainis Boulevard 19, LV-1586 Riga, Latvia.The e-mail address for S. Svanberg is sune.svanberg@fysik.lth.se.

Received 28 October 1999; revised manuscript received 21 April2000.

0003-6935y00y213774-07$15.00y0© 2000 Optical Society of America

3774 APPLIED OPTICS y Vol. 39, No. 21 y 20 July 2000

spectroscopy ~STFMS!, or two-tone FM spectroscopy~TTFMS!.9

In WMS the modulation frequency is much smallerthan the half-width of the absorbing feature, whereasSTFMS and TTFMS are characterized by modulationfrequencies that are comparable to or larger than thehalf-width of the absorbing feature. WMS is usuallyperformed at kilohertz frequencies ~low-frequencyWMS!, with conventional lock-in amplifiers used forsignal detection. For maximum sensitivity a largeFM index, defined as the ratio between the maximumfrequency deviation and the modulation frequency, isrequired. The sensitivity can be significantly im-proved by use of megahertz modulation frequencies~high-frequency WMS! and harmonic detection atmoderate FM indices.10–12 Inasmuch as STFMSand TTFMS use high modulation frequencies, highsensitivity can be achieved with a low FM index.10–12

STFMS used for broad atmospheric-pressure lineswith linewidths of some gigahertz requires detectionelectronics with matching bandwidths. Such instru-ments are complicated, expensive, and not alwaysavailable in the wavelength region of interest.TTFMS circumvents this problem by using high FMof the laser at two closely spaced frequencies anddetecting the signal at the difference frequency, oftenin the megahertz range. Thus, in TTFMS it is pos-sible to use detection electronics of moderate band-width and still preserve high sensitivity at a low FMindex. In the high-FM techniques ~high-frequencyWMS, STFMS, and TTFMS!, the lock-in amplifier is

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replaced by discrete components. The use of FM athigh frequencies offers the possibility of quantum-noise-limited detection.13–17

Diode lasers are especially well suited for high-sensitivity absorption spectroscopy because one canboth tune their emission wavelengths over the wholeabsorption profile by changing the temperature of thelaser capsule and modulate their frequencies directlyby applying an ac current on the drive current. Re-cently the Nichia Corporation introduced blue cw GaNdiode lasers that emit near 400 nm.18 Gustafsson etal.19 recently demonstrated the usefulness of these la-sers for spectroscopy by performing experiments withpotassium vapor. No attempts at sensitive detectionwere made in these experiments, which employed di-rect absorption, laser-induced fluorescence, and opto-galvanic spectroscopy. Here we extend this study byusing the blue diode laser and perform WMS andTTFMS on potassium as well as on lead. We studythe second resonance line, 4s 2S1y2–5p 2P3y2, in potas-sium ~39K! at 404.8 nm. We also record the transitionin lead ~208Pb! at 405.8 nm, starting from the ther-

ally weakly populated 6p2 3P2 metastable level situ-ated 1.3 eV above the ground state. Absorptionsignals are measured at different temperatures andthus at different absorptions. By means of a fittingprocedure, information regarding the modulation pa-rameters is obtained. We examine the possibilities ofmaking highly sensitive absorption measurements inwhat is for diode lasers a new and interesting wave-length region and compare experimentally deducedand theoretically calculated signal-to-noise ratios~SNR’s!. To achieve high sensitivity for both WMSand TTFMS, we perform WMS in the high-frequencywavelength-modulation regime, applying a modula-tion frequency of 5 MHz and detecting the second har-monic at 10 MHz, whereas for TTFMS we usemodulation frequencies near 900 MHz, i.e., larger thanthe Doppler widths ~HWHM!, which are less than 800MHz for the transitions studied and detect beat signalsat the same frequency as for WMS.

2. Experiment

A schematic of the setup for WMS and TTFMS ex-periments is shown in Fig. 1. The blue diode laser~Nichia NLHV500! has a nominal wavelength of 404nm at 25 °C and a typical output power of 5 mW.The laser diode is placed in a thermoelectricallycooled mount and is current and temperature con-trolled by a precision diode laser driver ~Melles Griot06DLD103!. The diverging laser beam is collimatedby a molded glass aspheric lens ~Geltech C230TM-A!and is divided by a neutral-density filter. One partof the beam is transmitted through a small electri-cally heated oven into which either a 4-cm-long po-tassium cell or a 3.5-cm-long lead cell is placed. Theother part of the beam is directed through a low-finesse confocal Fabry–Perot etalon ~free spectralrange, 1.5 GHz! for frequency calibration of the ab-sorption spectrum. Both beams are focused on de-tectors that contain p–i–n photodiodes ~HamamatsuS-1190! and homemade transimpedance amplifiers.

Wavelength scanning is achieved by repetitive appli-cation of a rectangular current pulse ~Tektronix

G501! with a duration of 1–2 ms and a repetitionate of 100 Hz to the diode laser drive current, whichs biased below the threshold current. A heat-nduced constant-current wavelength shift is pro-uced during the rectangular current pulse if the riseime of the pulse is short compared with the growthate of the junction temperature. This wavelength-canning technique minimizes variations in laser in-ensity and spectral characteristics and ensures anlmost constant FM index during the scan.20,21 It

also permits simultaneous recording of the absorp-tion line and of the zero intensity level in direct ab-sorption. The free-running laser typically lases on afew modes separated by ;0.05 nm, as evidenced in aseparate test with a high-resolution spectrometer.However, judicious choices of temperature and drivecurrent ensure nearly single-mode operation.

The FM schemes for WMS and TTFMS are almostidentical; the only difference is in the method of gen-erating the modulation frequencies. In TTFMS wegenerate the two modulation frequencies by mixing a905-MHz signal ~Wavetek 2510A! and a 5.35-MHzignal ~Tektronix SG503! in a frequency mixer ~Mini-ircuits ZFM-4H!. For WMS, only the 5.35-MHzodulation frequency is used. The modulation cur-

ent, before it is superimposed upon the laser driveurrent, passes a variable attenuator, which allowshe radio-frequency ~rf ! power to the diode laser to bearied. We detect the TTFMS and WMS signals byixing ~Mini-Circuits ZFM-3! the amplified ~Mini-ircuits ZFL-1000LN! beat and second-harmonic sig-als ~at 10.7 MHz! from the detector with therequency-doubled ~Mini-Circuits FD-2! and appro-riately phase shifted ~Synergy PP-921! 5.35-MHzignal. We adjust the rf power for the highest pos-ible WMS and TTFMS signal amplitudes that can bechieved without the introduction of any significantodulation broadening. We split off ~Mini-Circuits

FRSC-2050! one part of the detector signal beforehe frequency mixer so we can observe the direct

Fig. 1. Experimental setup for direct absorption spectroscopy,WMS, and TTFMS of potassium and lead vapor in sealed-off cells.

20 July 2000 y Vol. 39, No. 21 y APPLIED OPTICS 3775

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absorption signal. Where appropriate, bandpass-,high-pass, and low-pass filters are inserted to preventthe modulation frequencies from reaching the demod-ulation mixer, the detection frequency signal fromreflecting back toward the laser, and the modulatedabsorption-related signals from influencing the directabsorption signals. The direct and demodulated sig-nals are amplified and low-pass filtered at 30 kHz inlow-noise preamplifiers ~Stanford SR560! and thenaveraged 256 times in a digital oscilloscope ~Tektro-nix TDS520B!. Thus the effective bandwidth of thedetection system is approximately 120 Hz. Finally,the recorded waveforms are transferred to a com-puter for processing and evaluation.

In WMS it is possible to record both the absorptionand the dispersion related to the probed medium.Here we consider only the absorption-related signalby making the appropriate choice of detection phase.We can do this because the in-phase ~0 or p! compo-nent of the signal corresponds to pure absorption,whereas the quadrature ~6py2! component corre-sponds to pure dispersion. In TTFMS a small partof the dispersion component will fall into the detec-tion angle, provided that the detection phase ~approx-imately 0 or p! is adjusted for optimum signalamplitude.22 However, for the transitions studied,the chosen modulation frequencies, and the precisionof the detection phase adjustment ~65%!, the disper-sion component is less than 0.1% of the absorptioncomponent and is thus negligible.

3. Measurements

Natural potassium consists of two isotopes, 39K ~93%!and 41K ~7%!, both with a nuclear spin of 3y2. Thetudied sealed-off potassium gas cell contains onlyhe isotope 39K. We note that the existence of a

nonzero nuclear spin for potassium gives rise to ahyperfine splitting of the 4s 2S1y2–5p 2P3y2 transition.However, because of the small magnetic moment ofthe potassium nucleus, the hyperfine splittings aresmall; for 39K the ground-state splitting is only 462

Hz and the upper-state splitting is ;2 orders ofmagnitude smaller.23 Thus the Doppler-broadenedpotassium line, with a half-width of ;800 MHz forthe temperatures used here, is not expected to showany structure.

Typical recorded spectra for direct absorption andTTFMS measurements at 90 °C in potassium areshown in Fig. 2, where the low-finesse etalon fringesare also displayed. The WMS line shape is almostidentical to that of TTFMS and is not shown. Be-cause a rectangular current pulse is used for wave-length scanning, the laser output power is almostconstant during the scan, the zero intensity level canbe seen in the direct absorption recording, and thefrequency sweep is quite nonlinear. This meansthat neither any laser output power rectification norany separate zero intensity recording is necessarywith direct absorption. It means, though, that thefrequency scale has to be linearized, and subse-quently this was done for all the spectra shown in theremaining part of this paper.

776 APPLIED OPTICS y Vol. 39, No. 21 y 20 July 2000

WMS and TTFMS line shapes for potassium vaporwere recorded from ;90 °C to room temperature. InFig. 3, linearized recordings at 60 and 30 °C, corre-sponding to peak absorptions of 1.6 3 1023 and 6.7 31025, respectively, are displayed. Interferencefringes, caused by reflections between the neutral-density filter and the detector surface, are clearlyvisible in the recording of the TTFMS line shape at30 °C. Although FM spectroscopy offers the possi-bility of quantum-noise-limited detection, the mini-mum achievable sensitivity is in practice often set by

Fig. 2. ~a! Direct absorption and ~b! TTFMS spectra of the 4s2S1y2–5p 2P3y2 line at 404.8 nm recorded with a rectangular cur-ent pulse. ~c! Corresponding recording of the Fabry–Perot etalonringes.

Fig. 3. Linearized ~a! WMS and ~b! TTFMS recordings for potas-sium at two cell temperatures.

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interference fringes that are seen as a periodicallyoscillating background and originate from spuriousreflections along the laser beam path. The interfer-ence effect can often be removed or largely reduced bycareful angling of all transmissive optics, and, as canbe seen from the WMS recording in Fig. 3 @upperurve ~a!#, a slight adjustment of the neutral-densitylter removes the interference fringes.It is necessary to perform experiments at consid-

rably higher temperatures if one wishes to observehe absorption in lead, because of the low vapor pres-ure of this metal. We studied the transition at05.8 nm, starting from the thermally weakly popu-ated 6p2 3P2 metastable level situated 1.3 eV abovehe ground state. The lead experiments were per-ormed at temperatures that ranged from 500 to00 °C, corresponding to ground-state atomic densi-ies of 2 3 1017 m23 to 7 3 1019 m23 and to Boltzmannactors of 2 3 1028 to 9 3 1027. These atomic den-

sities can be compared with those in the potassiumcell, which range from 3 3 1014 m23 ~20 °C! to 3 31017 m23 ~90 °C!. The lead gas cell contains a nat-ural mixture of lead isotopes, but the specific mea-surements were performed on the transitioncorresponding to the 208Pb isotope with a nuclear spinf 0. The isotope, which has no hyperfine splittingecause of the zero nuclear spin, has a Doppler widthf ;550 MHz.The linearized WMS and TTFMS recordings at

00 °C ~peak absorption, 2.0 3 1024! are shown inFig. 4. These recordings were also influenced by in-terference fringes that originated from a reflectionfrom the detector surface back into the diode lasercavity. Despite cautious angling of all optics as wellas of the diode laser and the detector, we were notable to suppress completely the interference fringesin the lead experiments. We explain the differencein the results of the experiment with potassium fromthat with lead as being due to different diode laserbehaviors at the two wavelengths because differentcurrent and temperature settings were used. An-other unwanted and limiting effect in the lead exper-iments is a diode laser mode jump close to thetransition on the low-frequency side. The frequency

Fig. 4. Linearized ~a! WMS and ~b! TTFMS recordings of the lead05.8-nm line. Inset, diagram of the structure of lead.

position of the mode jump varies in time, and thisvariation adds an irregular background to the re-corded line shapes.

4. Results and Discussion

Here we first determine the modulation parametersin WMS and TTFMS by means of a fitting procedureof the recorded line shapes. Then we use these mod-ulation parameters to calculate the theoretical SNR’sand compare them with the experimentally deducedSNR’s. Finally, we estimate the maximum achiev-able sensitivity of this blue diode laser–based spec-trometer. The evaluation was performed mainlywith the experimental data for potassium, because,as we noted above, the lead experiments were ham-pered by interference fringes and a diode laser modejump, in which case, the SNR was set by these effectsrather than by the fundamental noise sources in aFM spectrometer. The formalism that describes theFM theory, including calculations of WMS andTTFMS line shapes and SNR’s, can be found, e.g., inRefs. 22 and 24–26.

FM of diode lasers is always accompanied by resid-ual amplitude modulation ~RAM!, which is noticeablen WMS and TTFMS recordings as a line-shapesymmetry. This RAM, which is accounted for inhe FM theory by the AM index M and the AM–FMhase difference c, is an effect that is caused by theaser intensity not being exactly uniform over the FMange, and it is undesirable because it carries some ofhe low-frequency noise into the FM signal. The AMndex and the AM–FM phase difference are closelyonnected in theory and are difficult to determinendependently. A value of c 5 py2 for the AM–FMhase difference is generally considered to be a goodpproximation for diode lasers,22,27,28 and, in what

follows, we adopt this value.As we mentioned above, the studied transition in

potassium actually consists of several hyperfine com-ponents. These can be divided mainly into twoGaussians that have a half-width given by the Dopp-ler width, are separated by 462 MHz and have anintensity ratio of 5y3, which is equal to the statisticalweights of the two hyperfine ground-state levels ~F 52 and F 5 1!. The FM index, designated b in the FM

omenclature, and the AM index M were determinedy fits to the WMS and TTFMS line shapes recordedt different temperatures. The parameters ob-ained were b 5 230 and M 5 0.045 for WMS and b 5

1.0 and M 5 0.035 for TTFMS. With these param-eter values, two curves were calculated, and they andtheir sum are displayed in Fig. 5, together with thepotassium line experimentally recorded at 80 °C.

The AM–FM index ratio Myb is an intrinsic prop-erty of a diode laser and depends on the modulationfrequency and the bias current.29 Typically, the ra-tio scales with the modulation frequency, and for ourexperiments with equal bias current for WMS andTTFMS the ratio of the AM–FM index ratio forTTFMS and WMS was 179, which is in excellentagreement with the ratio of the modulation frequen-cies, 181.

20 July 2000 y Vol. 39, No. 21 y APPLIED OPTICS 3777

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It can be observed from the experimental data thatthe signals ~peak-to-peak value! in WMS are approx-mately 20% higher than those of TTFMS; see Fig.–5. This result agreed well with the theoreticallyalculated signal values that yielded a signal differ-nce for the temperatures employed of ;21% for po-assium and ;23% for lead. No modulationroadening is observed in either the WMS or theTFMS line shape. This result is consistent with

he experimentally applied rf powers, which we care-ully adjusted for the highest signal amplitudeschievable without introducing modulation broaden-ng. The presence of modulation broadening would

ake the fitting procedure more difficult and lessccurate. WMS and TTFMS line shapes that wereptimized only for maximum signal amplitudeshowed considerable modulation broadening.In a frequency-modulated system the power SNRP

can be expressed as25

where the terms in the two expressions are in thesame order and where the mean-squared noise cur-rents in the denominator ~from left to right! are re-lated to laser-induced detector shot noise, detectorand amplifier thermal noise, AM-induced ~RAM!noise, and laser source or excess noise and the term in

Fig. 5. Observed ~symbols! and calculated ~solid curves! lineshapes for ~a! WMS and ~b! TTFMS of the potassium transition.The modulation parameters are b 5 230 and M 5 0.045 for WMSand b 5 1.0 and M 5 0.035 for TTFMS.

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778 APPLIED OPTICS y Vol. 39, No. 21 y 20 July 2000

the numerator is the time-averaged mean-squareddetector current. e is the charge of the electron, h isthe detector quantum efficiency, h is Planck’s con-tant, n is the transition frequency, P0 is the laser

power, Q~a, w! is a signal that is due to absorption aand dispersion w, M is the AM index, N is the numberof modulating tones, Df is the detection bandwidth, kis Boltzmann’s constant, T is the absolute tempera-ture of the detector, RL is the resistance of the detec-tion system, R~M! is the RAM function, sp is thestandard deviation of the laser power within thenoise equivalent bandwidth of the detector system, fis the detection frequency, and sex is a system-dependent constant that is defined as the laser powerfluctuations at 1-Hz bandwidth at 1-Hz frequency.The value of the exponent b is typically 1.0 but canange from 0.8 to 1.5.

The absorption- and dispersion-related line-shapeunction Q~a, w!, which depends on M, b and c, is

calculated as the peak-to-peak value of the absorp-tion component of the WMS and TTFMS line shapesbecause the dispersion component vanishes as a re-sult of our choice of detection phase. The RAM func-tion describes the nonzero signal that is detectedeven in the absence of absorption and is due to theAM. The reason for employing second-harmonic de-tection instead of first-harmonic detection in WMS isthat the influence of RAM will be decreased. ForWMS and second-harmonic detection, R~M! 5 1y4 M2

cos~2c 1 p!; for TTFMS, R~M! 5 M2. The totalpower impinging upon the detector is P0 5 3 mW, andthe parameters for detector and detection electronicsin our setup are h 5 0.37 and RL 5 50 V. Fromdirect absorption measurements, a value of 1 3 1023

P0 for the parameter sp was deduced. The only ad-justable parameter in the SNR calculations is sex,and its value is estimated to be 4.5 3 1024 P0. Thesealues of sp and sex are approximately 1 order of

magnitude higher than typical values for diode lasersused for FM in the near infrared and the infrared.10,14

We attribute the difference to the multimode behav-ior of our blue diode laser.

The experimental SNR is calculated as the ratio ofthe line-shape peak-to-peak value to the noise rmsvalue. Inasmuch as we measure voltage and notpower, the experimental SNR is related to the theo-retical voltage SNRV, which is given by SNRV 5~SNRP!1y2. In Fig. 6 the experimental SNR’s for

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WMS and TTFMS are presented, together with thetheoretical SNR curves. Clearly, absorption signalsfor less than the smallest experimentally recordedabsorption of 2 3 1025 ~at ;20 °C! are detectable, butwith our current setup it is not possible to cool the gascell and record signals below room temperature.The sensitivity limits that can be deduced are 2 31026 for WMS and 5 3 1026 for TTFMS. In the leadexperiments we were able to record WMS andTTFMS signals down to a SNR of 1, which occurs attemperatures of ;500 °C and corresponds to an ab-sorption of 5 3 1025. Thus the interference fringesand the mode jump limit the detection sensitivity by;1 order of magnitude.

There are two reasons for the approximately two-times better SNR in WMS than in TTFMS. The firstis the higher signal amplitude in WMS, and the sec-ond is the higher RAM noise level in TTFMS, asexpected from the different RAM dependencies givenby the RAM function. All other noise sources havethe same amplitude in WMS and TTFMS. The ratioof the shot noise, the thermal noise, the laser excessnoise, the TTFMS RAM noise, and the WMS RAMnoise is 1:1.7:0.7:5.4:2.2. These numbers clearlyshow that TTFMS is essentially limited by RAMnoise, whereas the sensitivity in WMS is set by bothRAM and thermal noise. We also note that the laserexcess noise is below the shot noise and that the mainpurpose for employing FM is achieved. RAM noiseand, as a consequence of the technique, laser excessnoise can be greatly reduced by the use of balancedhomodyne detection.13,15,21 However, the powertransmitted through the absorbing medium is halvedand the thermal noise is doubled in this technique.Theoretical calculations of the SNR’s in balanced ho-modyne detection, with the same parameter valuesas above, show that the SNR’s for TTFMS are almostthe same but that the SNR’s for WMS are lower, aswe also verified experimentally. The results ob-

Fig. 6. Experimentally deduced SNR’s for WMS ~diamonds! andTFMS ~triangles! for potassium together with theoretical curvesalculated from Eq. ~1! and the parameter values mentioned in theext.

tained in the present study are broadly in line withthose obtained in previous comparisons of the sensi-tivity of various modulation schemes in which infra-red lead-salt diode lasers10 and near-infrared GaAlAsdiode lasers11 were used.

A possible extension of the present study would beto monitor NO2, which has strong absorption bandsin the wavelength region about 400 nm. At atmo-spheric pressure these transitions are much broaderthan for potassium and lead and might also partiallyoverlap. Such a study would require continuousscans of the order of 50 GHz, which at present are notfeasible with the diode laser used. One should alsonote that the multimode behavior of the diode laserdoes not influence the spectral appearance for freeatoms with isolated spectral features as in our case,because only one of the oscillating modes interactswith the atoms. However, for molecules with a mul-titude of close-lying lines, multimode behavior is, ofcourse, unacceptable. As the blue diode laser used isa prototype laser, we believe that later versions willbe better suited for spectroscopy, i.e., will operate ina single mode. One can use an external-feedbackcavity to ensure single-mode operation and largefrequency-tuning ranges, but doing so will limit thehigh-FM capabilities and thus the detection sensitiv-ity.

5. Conclusions

We performed FM spectroscopy on potassium andlead, using a blue diode laser. We made highly sen-sitive absorption measurements in this new and in-teresting diode-laser wavelength region, employingWMS and TTFMS. In fits to experimental data themodulation parameters for the blue diode laser usedin our experimental setup were determined. Wehave shown that experimentally deduced SNR’s arein good agreement with theoretically calculatedSNR’s. Additionally, various noise sources were ex-amined, and we have concluded that TTFMS is dom-inated by RAM noise, whereas WMS is limited byboth RAM and thermal noise. A minimum detect-able absorption of 2 3 1026 for WMS and 5 3 1026 forTTFMS in a 120-Hz detection bandwidth was ob-served. By sum-frequency mixing blue and red di-ode lasers, even shorter emission wavelengths can beachieved, further extending the possibilities for sen-sitive absorption spectroscopy of many atomic andmolecular transitions.30

This study was supported by the Swedish ResearchCouncil for Engineering Sciences ~TFR! and the Knutand Alice Wallenberg Foundation. J. Alnis thanksthe Swedish Institute for a stipend supporting hisstay in Sweden.

References1. P. Werle, R. Mucke, F. D. Amato, and T. Lancia, “Near-

infrared trace-gas sensor based on room-temperature diodelasers,” Appl. Phys. B 67, 307–315 ~1998!.

2. G. Modugno, C. Corsi, M. Gabrysch, and M. Inguscio, “Detec-tion of H2S at ppm level using a telecommunication diodelaser,” Opt. Commun. 145, 76–80 ~1998!.

20 July 2000 y Vol. 39, No. 21 y APPLIED OPTICS 3779

3. H. Riris, C. B. Carlisle, D. F. McMillen, and D. E. Cooper,

1

17. M. Gehrtz, G. C. Bjorklund, and E. A. Whittaker, “Quantum-

3

“Explosives detection with a frequency modulation spectrom-eter,” Appl. Opt. 35, 4694–4704 ~1996!.

4. J. A. Silver, D. J. Kane, and P. S. Greenberg, “Quantitativespecies measurements in microgravity flames with near-IRdiode lasers,” Appl. Opt. 34, 2787–2801 ~1995!.

5. P. Kauranen, H. M. Hertz, and S. Svanberg, “Tomographicimaging of fluid flows by the use of two-tone frequency-modulation spectroscopy,” Opt. Lett. 19, 1489–1491 ~1994!.

6. P. Kauranen, I. Harwigsson, and B. Jonsson, “Relative vaporpressure measurements using a frequency-modulated tunablediode laser, a tool for water activity determination in solu-tions,” J. Phys. Chem. 98, 1411–1415 ~1994!.

7. E. G. Moses and C. L. Tang, “High sensitivity laser wavelengthmodulation spectroscopy,” Opt. Lett. 1, 115–117 ~1977!.

8. G. C. Bjorklund, “Frequency-modulation spectroscopy: a newmethod for measuring weak absorptions and dispersions,” Opt.Lett. 5, 15–17 ~1980!.

9. G. R. Janik, C. B. Carlisle, and T. F. Gallagher, “Two-tonefrequency-modulation spectroscopy,” J. Opt. Soc. Am. B 3,1070–1074 ~1986!.

10. D. S. Bomse, A. C. Stanton, and J. A. Silver, “Frequency mod-ulation and wavelength modulation spectroscopies: compar-ison of experimental methods using a lead-salt diode laser,”Appl. Opt. 31, 718–731 ~1992!.

11. F. S. Pavone and M. Inguscio, “Frequency- and wavelength-modulation spectroscopies: comparison of experimentalmethods using and AlGaAs diode laser,” Appl. Phys. B 56,118–122 ~1993!.

12. D. E. Cooper, R. U. Martinelli, C. B. Carlisle, H. Riris, D. B.Bour, and R. J. Menna, “Measurement of 12CO2: 13CO2 ratiosfor medical diagnostics with 1.6-mm distributed-feedbacksemiconductor diode lasers,” Appl. Opt. 32, 6727–6731 ~1993!.

3. G. Modugno, C. Corsi, M. Gabrysch, F. Marin, and M. Inguscio,“Fundamental noise sources in a high-sensitivity two-tone fre-quency modulation spectrometer and detection of CO2 at 1.6mm and 2 mm,” Appl. Phys. B 67, 289–296 ~1998!.

14. C. B. Carlisle, D. E. Cooper, and H. Preier, “Quantum noise-limited FM spectroscopy with a lead-salt diode laser,” Appl.Opt. 28, 2567–2576 ~1989!.

15. C. B. Carlisle and D. E. Cooper, “Tunable-diode-laserfrequency-modulation spectroscopy using balanced homodynedetection,” Opt. Lett. 14, 1306–1308 ~1989!.

16. P. Werle, F. Slemr, M. Gehrtz, and C. Brauchle, “Quantum-limited FM-spectroscopy with a lead salt diode laser,” Appl.Phys. B 49, 99–108 ~1989!.

780 APPLIED OPTICS y Vol. 39, No. 21 y 20 July 2000

limited laser frequency-modulation spectroscopy,” J. Opt. Soc.Am. B 2, 1510–1526 ~1985!.

18. S. Nakamura and G. Fasol, The Blue Laser Diodes ~Springer-Verlag, Heidelberg, 1997!.

19. U. Gustafsson, J. Alnis, and S. Svanberg, “Atomic spectroscopywith violet laser diodes,” Am. J. Phys. ~to be published!.

20. P. Kauranen and V. G. Avetisov, “Determination of absorptionline parameters using two-tone frequency-modulation spec-troscopy with diode lasers,” Opt. Commun. 106, 213–217~1994!.

21. V. G. Avetisov and P. Kauranen, “High-resolution absorptionmeasurements using two-tone frequency-modulation spectros-copy with diode lasers,” Appl. Opt. 36, 4043–4054 ~1997!.

22. V. G. Avetisov and P. Kauranen, “Two-tone frequency-modulation spectroscopy for quantitative measurements ofgaseous species: theoretical, numerical, and experimental in-vestigation of line shapes,” Appl. Opt. 35, 4705–4723 ~1996!.

23. E. Arimondo, M. Inguscio, and P. Violino, “Experimental de-terminations of the hyperfine structure in the alkali atoms,”Rev. Mod. Phys. 49, 31–75 ~1977!.

24. D. E. Cooper and R. E. Warren, “Frequency modulation spec-troscopy with lead-salt diode lasers: a comparison of single-tone and two-tone techniques,” Appl. Opt. 26, 3726–3732~1987!.

25. J. A. Silver, “Frequency-modulation spectroscopy for trace spe-cies detection: theory and comparison among experimentalmethods,” Appl. Opt. 31, 707–717 ~1992!.

26. J. M. Supplee, E. A. Whittaker, and W. Lenth, “Theoreticaldescription of frequency modulation and wavelength modula-tion spectroscopy,” Appl. Opt. 33, 6294–6302 ~1993!.

27. W. Lenth, “High frequency heterodyne spectroscopy withcurrent-modulated diode lasers,” IEEE J. Quantum Electron.QE-20, 1045–1050 ~1984!.

28. S. Kobayashi, Y. Yamamoto, M. Ito, and T. Kimura, “Directfrequency modulation in AlGaAs semiconductor lasers,” IEEEJ. Quantum Electron. QE-18, 582–595 ~1982!.

29. M. Osinski and J. Buus, “Linewidth broadening factors insemiconductor lasers: an overview,” IEEE J. Quantum Elec-tron. QE-23, 9–29 ~1987!.

30. J. Alnis, U. Gustafsson, G. Somesfalean, and S. Svanberg,“Sum-frequency generation with a blue diode laser for mercuryspectroscopy at 254 nm,” Appl. Phys. Lett. 76, 1234–1236~2000!.