Periodically locked continuous-wavecavity ringdown spectroscopy

Nicola J. van Leeuwen, Jan C. Diettrich, and Andrew C. Wilson

We demonstrate a simple periodically locked cw cavity ringdown spectroscopy technique that enables avery large number of ringdown events to be rapidly acquired. An external cavity diode laser is lockedto a high-finesse cavity, and as many as 16,000 ringdown events per second are obtained by periodicallyswitching off the light entering the high-finesse cavity. Following each ringdown event, the light to thecavity is switched back on and cavity lock is rapidly reacquired. Limited only by our relatively modestdigitization rate, we obtained a minimum detectable absorption loss of 4.7 � 10�9 cm�1, but we show thatfaster digitization could provide a sensitivity of 5.9 � 10�10 cm�1 Hz�1�2. © 2003 Optical Society ofAmerica

OCIS codes: 020.0020, 120.0120, 140.2020, 300.0300.

1. Introduction

Cavity ringdown spectroscopy �CRDS� is a techniquefor measuring weak absorption of light by a sample ofgas. The primary advantage of this technique isthat it is insensitive to fluctuations in laser intensity.The technique stems from work done by Anderson etal.1 to measure the reflectivity of high-reflectancemirrors. The reflectivity was determined by form-ing a high-finesse cavity and measuring the decaytime of light from the cavity. In CRDS, the additionof a gas to the high-finesse cavity leads to more rapiddecay when the excited-cavity mode matches the fre-quency of an absorption feature of the gas. Thissimple technique provides a sensitive and absoluteabsorption measurement and forms the basis of animportant aspect of laser spectroscopy.2–4 Nowwidely adopted, the basic technique of CRDS hasevolved to encompass a broad range of schemes5 andhas been refined to provide remarkable sensitivity.

The mirror-testing ringdown technique used byAnderson et al. employed a laser and a Pockels cell toswitch off the light entering the cavity; however, thattechnique suffered from the drawback of unpredict-able matching between the laser’s frequency and the

The authors are with the Department of Physics, University ofOtago, P.O. Box 56, Dunedin, New Zealand. N. J. van Leeuwen’se-mail address is nicolav@physics.otago.ac.nz.

Received 21 October 2002; revised manuscript received 4 March2003.

0003-6935�03�183670-08$15.00�0© 2003 Optical Society of America

3670 APPLIED OPTICS � Vol. 42, No. 18 � 20 June 2003

frequency of a cavity mode. To avoid this problem,O’Keefe and Deacon6 developed a technique that em-ployed a pulsed laser, eliminating the need for anoptical switch and bypassing the mode-matchingproblem. From this, the field of pulsed CRDSemerged.3,5

In 1997, single-mode cw lasers were reintroducedinto CRDS by Romanani et al.7 for high-resolutionspectroscopy. The key advantage of using a cw laseris efficient coupling of narrow-bandwidth light into asingle mode of the cavity, resulting in high spectralresolution and a large signal-to-noise ratio. Anacousto-optic modulator �AOM� was placed in front ofthe cavity as an optical switch, turning off the lightfrom a ring-dye laser upon sufficient buildup of lightin the cavity. In addition, to overcome thefrequency-matching problem encountered by Ander-son et al., Romanini et al. used a piezoelectric trans-ducer �PZT� to modulate the cavity length, makingthe frequency of one of the cavity modes oscillateabout the laser frequency. This provided regularringdown events and a noise-equivalent absorption of10�9 cm�1.

The cw method used by Romanini et al. has provedto be very popular, with numerous researchers usingthis or very similar techniques. Some examples in-clude the following. Schulz and Simpson8 used anexternal-cavity diode laser �ECDL� at 665 nm tostudy water vapor and achieved a sensitivity of 2 �10�9 cm�1. Hippler and Quack9 used an ECDL cen-tered at 1300 nm to monitor pulsed supersonic jets ofnitrous oxide and methane. Metsala et al.10 used atitanium sapphire laser to study acetylene to a sen-

sitivity of 9 � 10�9 cm�1. Paldus et al.11 used a cwquantum-cascade laser to detect ammonia andachieved a noise-equivalent absorbance of 3.4 � 10�9

cm�1 Hz�1�2. Kosterev et al.12 used a cw quantum-cascade laser to detect nitric oxide with a single-shotsensitivity of 2.2 � 10�8 cm�1. Totschnig et al.13

used two distributed-feedback lasers to measure mul-tiple gas species �methanol and isopropanol� to a sen-sitivity of 2.4 � 10�9 cm�1. Murtz et al.14 used afrequency-stabilization method to achieve a sensitiv-ity of 3 � 10�8 cm�1, using a CO2 laser to detectethylene near 10 �m.

In addition to these, other specialized cw CRDSschemes with different advantages have been devel-oped. In 1999, Hahn et al.15 demonstrated a ring-down spectrometer that was similar in function tothat developed by Romanini et al. but did not requirean AOM. Upon detection of sufficient light in thecavity, Hahn et al. triggered a transistor circuit thatvery quickly reduced the voltage to a PZT that con-trolled cavity length. This “jumped” the cavity outof resonance with the light, allowing the light in thecavity to decay. The detection sensitivity achievedwith this method was 3 � 10�9 cm�1, which approx-imately matches the sensitivity of the method used byRomanini et al.

Another cw CRDS scheme avoids the need for ei-ther an AOM or a fast switch. The basis of thistechnique is that one simply sweeps the cavity reso-nance across the laser frequency. However, duringringdown the Doppler-shifted intracavity light inter-feres with the light incident on the cavity, producingan oscillation on the cavity ringdown signal.16,17

This oscillation is related to the speed at which thecavity length is swept and is larger at a low-sweeprate.18 He and Orr19,20 used a fast sweep rate so thatthe oscillation was limited primarily to the beginningof the ringdown, and curve fitting to the tail provideda sensitivity of 3 � 10�9 cm�1.

Other cw systems have been developed in whichthe laser frequency and a cavity resonance are lockedtogether—a cavity-locked cw CRDS. This ensuresthat a single transverse mode of the cavity is excitedso that shot-to-shot variation in the decay constant isreduced and the dynamic range of the ringdown sig-nal is maximized. The most widely adopted schemefor frequency locking a narrow-linewidth laser to ahigh-finesse cavity is the Pound–Drever–Hall �PDH�method,21 since this method provides a very widelocking range and broadband stabilization. With alinear cavity, the PDH method uses polarization op-tics to select the reflection from the cavity to producean error signal for locking.22 Paldus et al.23 were thefirst to demonstrate cavity-locked cw CRDS. Theyused a three-mirror ring cavity and two orthogonalcomponents of polarized light—one to lock the laserto the cavity �by the PDH method� and the other toproduce a ringdown signal �with an AOM�. Thismeans that the cavity and the laser remain lockedwhile cavity ringdown measurements are performed,resulting in a high repetition rate limited only to theringdown �and ringup� time of the cavity. Although

the sensitivity obtained with this method was ini-tially no better than free-running �i.e., unlocked� cwCRDS schemes, future reduction of the detectionnoise led to significant improvements. Taking ad-vantage of the increased ringdown repetition rate,the sensitivity of this scheme was improved by Lev-enson et al.,24 who implemented heterodyne detectionto demonstrate a 31-dB signal-to-noise ratio, and bySpence et al.,25 who implemented an analog detectionscheme to demonstrate a long-term sensitivity of8.8 � 10�12 cm�1 Hz�1�2.

Another cavity-locked cw CRDS technique was de-veloped by Bucher et al.26 to study Doppler-free ab-sorption of ethylene. Light from a CO2 laser,independently locked to a stabilized reference cavity,was passed through an electro-optic modulator�EOM� to generate tunable frequency sidebands.The sidebands were then separated from the carrierby use of a polarizing beam splitter, and the PDHmethod was used to lock a high-finesse cavity toone of the sidebands. Although sidebands wereswitched off by the EOM to produce ringdown events,the short-term stability provided by the referencecavity was such that the lock to the high-finesse cav-ity was essentially maintained. This produced anoise-equivalent absorption of 4.1 � 10�9 cm�1

Hz�1�2 and the large intracavity intensity needed forsaturation of the transition.

Extending this idea, Fox et al.22 recently developeda cavity-locked cw CRDS method in which ringdownsignals were obtained by rapidly and repetitively un-locking and relocking an ECDL to the cavity.22 Un-locking is achieved by opening the servo loops thatlock the laser to the cavity. Once unlocked, ring-down is obtained by rapidly shifting the laser’s fre-quency to a stable off-resonant value by switching thediode injection current. This offers the enhancedsignal-to-noise ratio and ringdown repetition rate ofthe scheme by Paldus et al.23 but without the need touse a ring cavity to separate the processes of cavitylocking and ringdown generation.

Here we describe an alternative method for therepetitive cavity-locked cw CRDS schemes of Bucheret al. and Fox et al. In our method an ECDL islocked to a linear cavity by the PDH method, andringdown is obtained by periodically switching thelight entering the cavity with an AOM �in the tradi-tional manner of unlocked cw CRDS�. After eachringdown, the light to the cavity is switched back onand cavity lock is rapidly reacquired. This has theadvantages of the scheme by Fox et al. but eliminatesthe possibility of exciting higher-order transversemodes of the cavity and producing feedback to theECDL. In addition, there is no need to switch off thefrequency sidebands used for cavity locking and nopossibility of residual transmission through the cav-ity. In the following sections details of the experi-mental setup are given, the operation of our simpleperiodic cavity-locking scheme is described, and anabsorption profile of an overtone line of acetylenenear 788 nm is presented to demonstrate the method.

20 June 2003 � Vol. 42, No. 18 � APPLIED OPTICS 3671

2. Experimental Details

Figure 1 shows a diagram of our experimental setupin which the key components are the ECDL, theAOM, and the high-finesse ringdown cavity. Whenswitched on, the AOM deflects the laser beam so thatit enters the ringdown cavity. The ECDL is lockedto the ringdown cavity by the PDH method.21 Whilethe AOM is switched off, cavity ringdown occurs, andthe laser’s frequency does not drift out of the PDHlock range, so that, when the AOM is switched backon, the laser rapidly recovers lock. By periodicallyswitching the AOM off for a short period, repetitivecw cavity-locked ringdown signals are produced.

A. Ringdown Cavity

The high-finesse ringdown cavity is formed with twoNewport ultralow-loss mirrors �R � 99.97%�—oneplane �10CM00SR.40F� and one spherical with a 1-mradius of curvature �10CV00SR.40F�—spaced 27.8cm apart to give a 539.683-MHz free spectral range�FSR�. The mirrors are placed in Newport Ultimaseries mirror mounts attached to the ends of threehalf-inch-diameter Invar rods to provide thermal sta-bility of the cavity length �1 in. � 2.54 cm�. Theempty-cavity ringdown time was measured to be 3.65�s, giving a cavity finesse of approximately 12,400and a cavity linewidth �FWHM� of 44 kHz. Thespherical mirror is the output mirror and is attachedto a low-voltage PZT �DynaOptic motion, CTC-HPSt-150�20-15�12 VS35�, allowing 0.08-�m�V scanningof the cavity length. The ringdown cavity is in astainless steel ultrahigh vacuum chamber and isaligned before the chamber is sealed and evacuated.Preliminary cavity alignment is performed using a

CCD camera to image the cavity output beam, fol-lowed by precise cavity alignment using a photodiode�PD2 in Fig. 1� �Thorlabs PDA155� to minimize trans-verse modes as the cavity is swept through a FSR.The vacuum chamber is evacuated to a base pressureof 10�7 mbars using a turbomolecular pump. Theempty-cavity decay time is checked, and the chamberis then filled with spectroscopic-grade acetylene tothe desired pressure using a precision leak valve.

To isolate the alignment of the ringdown cavityfrom any adjustments to the ECDL and AOM and toobtain a TEM00 mode for matching to the cavity, asingle-mode polarization-preserving optical fiber islocated between the AOM and the ringdown cavity.Mode matching is achieved with two lenses of focallength 100 mm placed between the fiber and the ring-down cavity. When mode matched, the unwantedhigher-order transverse modes are suppressed to be-low 3% of the TEM00 mode intensity. The AOM�Isomet 1206C�, with a switching time of 80 ns, isplaced in front of the optical fiber and aligned so thatfirst-order diffraction is coupled into the fiber. Toproduce ringdown signals, the AOM is switched off,ensuring that no light enters the fiber and thus theringdown cavity.

B. ECDL and Locking

The ECDL is based on a design by Arnold et al.27 thatuses a Sanyo DL-7140-201 75-mW diode laser oper-ating near 788 nm, which we antireflection coated tosuppress multimode operation.28 The laser ismounted in the Littrow configuration but with anintracavity beam splitter29 that provides an outputbeam that does not move when the EDCL is tuned by

Fig. 1. Experimental setup for periodically locked cw CRDS. ECDL, external-cavity diode laser; OI, optical isolators; AOM, acousto-optic-modulator; EOM, electro-optic-modulator; PBS, polarizing beam splitter; PD1, photodiode for PDH locking; PD2, photodiode forringdown cavity transmission; DBM, double-balanced mixer.

3672 APPLIED OPTICS � Vol. 42, No. 18 � 20 June 2003

adjusting the grating. Two 35-dB optical isolatorsare used to protect the ECDL from optical feedback�primarily from the fiber and the cavity� as shown inFig. 1.

The PDH method is used to lock the ECDL to theringdown cavity. An EOM imprints sidebands onthe laser’s frequency at �15 MHz. A PBS and aquarter-wave plate at the entrance to the ringdowncavity direct light reflected from the cavity to a fastphotodiode �PD1 in Fig. 1� �Newport 818-BB-21A�.After amplification, the signal from this photodiode isdemodulated with a double balanced mixer at 15MHz to produce a PDH error signal. The error sig-nal is passed to servo-locking circuitry with two feed-back mechanisms to stabilize the ECDL.30

Feedback to the injection current of the diode laserprovides high-speed but narrow-range frequency sta-bilization, and feedback to the PZT provides low-speed but wide-range frequency stabilization. Theservo-locking circuit is a proportional-integral type,much like that described by Fox et al.,22 and wasoptimized following their methods to provide a band-width of approximately 2.5 MHz.

C. Ringdown Data Acquisition

To produce periodic ringdown events a transistor-transistor logic trigger pulse with �typically� a 1-kHzrepetition rate is used to switch the AOM off for 25�s, producing a ringdown event at least 6 timeslonger than the empty-cavity decay time. While theAOM is off, the PDH error signal is at zero so that thelock is essentially on “hold,” and when the light isswitched back on a short time later, the lock isquickly reacquired. A detailed description of thismode of operation is given in the following section.

A high-speed amplified photodiode �PD2� detectsthe ringdown events in the cavity transmission, andthese are digitized by a 9-bit digital oscilloscope �Tek-tronics TDS-3054B�. The oscilloscope is set to aver-age 256 events �performed in 0.66 s� before theaverage is transferred to a computer via a general-purpose interface bus connection for curve fitting. Anonlinear least-squares fitting routine is used to findthe best-fit decay time of the average ringdown. Atypical empty-cavity result is shown in Fig. 2.

A molecular-absorption profile �of acetylene in thiscase� is acquired by slowly stepping the cavitylength—and hence the cavity-locked laserfrequency—over the desired frequency range. Ateach acquisition frequency a single average of 256events is obtained. To provide background cavitydecay times with no absorption, an identical fre-quency scan with the laser well detuned from anymolecular resonance is also recorded.

3. Results

A. Periodic-Locking Scheme

The key to our periodic-locking scheme is that theservo controller is able to recover lock despite theregular interruptions to the PDH error signal causedby periodic switching of the light to the cavity. This

scheme takes advantage of the broad capture range ofthe PDH locking method and the double-integratordesign of the servo-locking circuit.

The PDH error signal is produced by phase modu-lating the light to imprint FM sidebands onto thelight incident upon the cavity. Differential couplingof these sidebands into the ringdown cavity convertssome of this FM to AM, which is then detected in lightreflected from the ringdown cavity with a photodiode�PD1�. By demodulating this signal at the FM fre-quency, an error signal for locking is produced. Pro-vided there is no significant residual AM produced bythe FM process, which is the case for a carefullyaligned EOM, the expected error signal is centered onzero volts with a large locking range as shown in Fig.3. Unwanted sources of noise and offsets must beavoided since these can seriously reduce the lockingrange. For instance, we use low-offset op-amps andremove any residual offset. Our error signal is cen-tered on zero, and the locking range spans the full 30MHz.

When the AOM switches off the light to the ring-down cavity, both photodiodes detect the cavity de-cay. The PDH error signal �generated from PD1�during ringdown is zero because there are no FMsidebands on the light leaving the ringdown cavity.With a servo controller providing purely integralfeedback �i.e., no proportional�, a zeroed error signalduring cavity decay would result in a temporary“freeze” or hold in the frequency stabilization. How-ever, in our circuit, obtaining a good lock to the high-finesse cavity requires a large resistor placed acrossthe integrator that provides feedback to the ECDLinjection current. This resistor rolls off the integra-tor action at very low frequencies, so that with azeroed error signal the output of the injection currentfeedback integrator decays toward zero. One mightexpect this to shift the laser’s frequency out of the

Fig. 2. Typical empty-cavity ringdown, consisting of 256 individ-ual ringdown events averaged within the oscilloscope, a curve fit toexponential decay, and the residual from the fit. The data and thefit are indistinguishable on this scale

20 June 2003 � Vol. 42, No. 18 � APPLIED OPTICS 3673

PDH locking range, but for three reasons this doesnot happen. First, the decay toward zero is slowenough that even with the light off for 100 �s �aduration of �25 cavity decay times� the laser almostalways remains within the strong central feature ofthe PDH locking signal so that the lock is quicklyreacquired. Second, so long as the laser frequency isnot perturbed beyond the very broad PDH lockingrange ��15 MHz�, lock is reacquired. However, ifthe perturbation is large, there may be insufficienttime for the servo to recover lock before the nextringdown event, so occasionally one or more ring-down events may be missed. Third, the double-integrator design30 uses a slow feedback to the PZTthat maintains the output of the injection currentfeedback integrator close to zero, so that any fre-quency drift caused by the integrator decay �while thelight is off � is minimized. One might also expectthat the decay in the high-speed integrator output,which is the input to the PZT integrator in the designof Rovera et al., might shift the laser’s frequency outof the PDH locking range. This does not happenbecause, although the PZT tuning range is broad, itsresponse is too slow. In our case, the response timeof the PZT feedback is approximately 50 ms, com-pared with the 25-�s period during which the light isswitched off.

When the light is switched back on, an error signalis reestablished, and cavity lock is rapidly reacquired.An example of this behavior is shown in Fig. 4. Inthis case, the light is switched off for 100 �s to showclearly the decay in the integrator output, but such along-off period is not required for our periodic-lockingscheme.

It is important to note here that the presence of anysignificant residual AM would undermine ourperiodic-locking scheme. For example, if the FMsidebands are produced by current modulation of the

ECDL, residual AM will cause an offset at the outputof the DBM �i.e., on the error signal�. To obtain acavity lock, this voltage offset must be subtracted.However, when the light is off and the error signalzeroed, the subtracted voltage will be present at theintegrator input and cause the lock to be lost.

For periodic locking, we repeatedly switch the lightto the ringdown cavity on and off. A typical result isshown in Fig. 5. During each cycle, the light is offfor 25 �s ��6 ringdown times� and on for 37.5 �s �a16-kHz repetition rate�. The repetition rate is fun-damentally limited by the ringdown time, but in prac-

Fig. 3. Calculated PDH error signal showing the tight centrallocking region when the laser is on resonance with the ringdowncavity �zero detuning� and the broad locking capture range �out to�15 MHz�. Fig. 4. �a� Ringdown cavity transmission obtained from PD2 with

the laser locked �0–100 �s�, followed by a ringdown event �100–200�s� and the laser reacquiring cavity lock �shortly after 200 �s�.�b� The output of the high-speed integrator �providing feedback tothe ECDL injection current� decays toward zero volts after thelight is switched off, and lock is rapidly reacquired after the lightis switched on.

Fig. 5. Transmission of the ringdown cavity showing successiveringdown events produced with the periodic cavity lock operatingat a repetition rate of 16 kHz.

3674 APPLIED OPTICS � Vol. 42, No. 18 � 20 June 2003

tice the rate is limited by the time to reacquire cavitylock, and this depends on the level of ambient tech-nical noise. If the laser’s frequency is perturbed suf-ficiently far from the strong central feature of thePDH locking signal, then recovering lock can be rel-atively slow, so occasionally a ringdown event isskipped. This becomes worse at higher repetitionrates when the laser light is on for shorter periods.In our laboratory environment, with the setup on aNewport optical table with isolator legs, the maxi-mum repetition rate for a robust lock is approxi-mately 16 kHz. Also, if the fraction of time duringwhich the laser is locked is reduced, there is less timefor the PZT to center the output of the injection cur-rent integrator on zero volts, so the lock becomesmore fragile, and slower scanning is necessary toobtain a molecular-absorption spectrum.

B. Ringdown Sensitivity and Acetylene

In CRDS the absorption of a sample is given by

�1c� 1

1�

10� , (1)

where 0 is the empty-cavity decay time and 1 is thecavity decay time with the sample present. InChapter 5 of Ref. 3, Paldus et al. give the minimumdetectable absorption loss �MDAL� as that which pro-duces a decay time one standard deviation ��� fromthe background decay time. For CRDS this leads to

MDAL �1c� 1

0�

10 � �� . (2)

To measure the single-shot minimum detectableabsorption loss �MDALshot� of our periodically lockedcw CRDS setup, ringdown times from 13,000 individ-ual ringdown events were recorded over a time spanof 2 h. These were collected in 26 groups of 500, withthe acquisition of each group taking approximately40 s. The distribution of the recorded ringdowntimes is shown in Fig. 6. From this we determinethat the empty-cavity decay time is 0 � 3.65 �s witha standard deviation of � � 0.03 �s. Using thisdistribution of ringdown times we obtain a MDAL ofan individual ringdown of 7.5 � 10�8 cm�1, which iscomparable with typical cw cavity ringdown schemes.

Data transfer from the oscilloscope limits the de-tection rate of individual ringdown events to 13 Hz.To overcome this limitation, an average of 256 ring-down events is performed by the oscilloscope, andthese averages can be acquired at a rate of 1.5 Hz.This provides a MDAL ��MDALshot��N� of 4.7 �10�9 cm�1, for an average of N � 256 ringdownevents. Ultimately, with a ringdown repetition rateof 16 kHz �and 9-bit digitization�, this periodicallylocked ringdown scheme offers a sensitivity of 5.9 �10�10 cm�1 Hz�1�2. Full utilization of the 16-kHzrepetition rate would require implementation of afaster detection method, such as using a high-speedcomputer-based digitization card �with a large

amount of onboard memory� or the analog detectionscheme demonstrated by Spence et al.25

To demonstrate the performance of the periodicallylocked cavity ringdown technique, the cavity wasfilled with 1.19 � 0.01 mbar of spectroscopic-gradeacetylene. A scan of the R�4� line of the 1 � 3 3overtone band at 12 686.646 cm�1 �788.2303 nm�31

was recorded and is shown in Fig. 7, together withresiduals from a fit to a Voigt profile.

The scan consists of 2150 points, each an average of256 individual ringdown events, and was recordedover a period of 50 min. The scan was produced byrecording an average of 256 ringdown events andthen stepping the voltage of the cavity’s PZT by a setamount. The frequency shift associated with eachvoltage step was determined by measuring the cavi-

Fig. 6. Distribution of ringdown times of 13,000 individual ring-down events recorded over a 2-h period.

Fig. 7. Periodically locked cw cavity ringdown measurement ofthe R�4� line of the 1 � 3 3 overtone band of acetylene with theresidual from the fit to a Voigt profile. The reduced �2 for the fitis 0.73.

20 June 2003 � Vol. 42, No. 18 � APPLIED OPTICS 3675

ty’s FSR �using a radio frequency technique� andmeasuring the voltage required to shift a FSR withan empty cavity. The PZT response is slightly non-linear over the 4-GHz scan, so the frequency changeper step is not perfectly constant. We corrected forthis by measuring the voltage required to scan suc-cessive FSRs and rescaling the frequency axis of theplot.

Residuals from the Voigt fit to the absorption fea-ture have a standard deviation of 1.4 � 10�8 cm�1.This is due to a systematic difference between thedata and the Voigt fit and long-term variations in thedecay time caused by technical noise in the labora-tory. The latter can be reduced by performing afaster scan. With the periodic lock operating at 16kHz we are able to scan 4 GHz �the width of the abovescan� in approximately 20 s while maintaining cavitylock, but our slow digitization means we are unable tocollect sufficient data on this time scale. However,with the implementation of a faster detection system�as discussed above� the potential sensitivity of thescheme could be achieved.

Finally, when performing a frequency scan we ob-serve that the ringdown time of the empty cavity isnot constant across the scan, varying from 3.62 �s atthe start to 3.67 �s at the end. Although we areunsure of the origin of this effect, tests at a singlefrequency with different servo-loop gain settingsshow a link between the quality of lock and the decaytime of the cavity, and the quality of the lock changesacross the frequency scan. Because the ringdowntime is insensitive to intensity noise and detuningfrom resonance, the link between cavity ringdowntime and the quality of the laser lock might be asso-ciated with the spectral properties of the laser. Wehave not observed evidence of etalon effects, whichmight account for the behavior. The backgroundvariation will be the subject of future work, but toremove this from the acetylene absorption measure-ment we detuned the laser from resonance to char-acterize the background change and subtracted itfrom the absorption profile.

4. Conclusions

We have demonstrated a simple periodically lockedcw CRDS technique, that enables a very large num-ber of ringdown events to be rapidly acquired forimproved sensitivity over traditional cw CRDS. AnECDL is locked to a high-finesse cavity using thePDH method, and ringdown events are obtained byperiodically switching the light entering the cavitywith an AOM �in the traditional manner of cwCRDS�. After each ringdown, the light to the cavityis switched back on, and cavity lock is rapidly reac-quired. This means that the ringdown repetitionrate is limited solely to the ringdown time itself.Even with a relatively modest digitization rate, thetechnique allows rapid signal averaging, and we haveobtained a sensitivity of 4.7 � 10�9 cm�1. However,we have shown that with improved digitization thetechnique could provide a sensitivity of 5.9 � 10�10

cm�1 Hz�1�2. The technique has been used for sen-

sitive detection of a transition in the 1 � 3 3 over-tone band of acetylene at 12 676 cm�1.

This research is supported by the New ZealandFoundation for Research, Science and Technology�New Economy Research Fund contract UOOX0217�.We thank Richard W. Fox at the National Institutefor Standards and Technology in Boulder, Colorado,and Donald M. Warrington, Peter J. Manson, andHenrik G. Kjaergaard at the University of Otago forhelpful discussions.

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