IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 2, APRIL 1999 563

Toward an Accurate Frequency Standardat 1.5 m Based on the Acetylene

Overtone Band TransitionAtsushi Onae, Ken’ichiro Okumura, Jun Yoda, Ken’ichi Nakagawa, Atsushi Yamaguchi,

Motonobu Kourogi, Kazuhiro Imai, and Bambang Widiyatomoko

Abstract—We have made a frequency standard operating atthe 1.5�m region using the saturated absorption of thev1 + v3

band of 13C2H2. To enhance the moderate power from a laserdiode and to saturate a very weak molecular overtone transition,we used a Fabry–Perot (FP) cavity and inside of it we put anacetylene cell. In order to obtain a longer life time for the cell, wedesigned a new scheme consisting of a cavity with a ULE (ultra-low expansion) glass spacer and a baked and sealed-off cell withBrewster windows. We observed the saturated spectrum havinga 1 MHz spectral linewidth. The signal obtained using the samecell ten months later had the same signal-to-noise ratio (SNR)and spectral linewidth. We made two versions of the frequencystandard and observed a beat note between these two lasers fromwhich we derived the frequency stability, in terms of the squareroot of the Allan variance �(�) = 2:4 � 10�13 at an integrationtime � = 1000 s.

Index Terms—Acetylene molecule, Allan variance, frequencystandard, laser frequency stabilization, laser spectroscopy, opti-cal communications, overtone band transition, saturation spec-troscopy.

I. INTRODUCTION

T HE use of a Fabry–Perot (FP) cavity to enhance signalwas proposed by Kastler in the 1960’s [1]. The technique

was later extended to the high-resolution, sub-Doppler spec-troscopy in several wavelength regions [2]–[6]. One of themost interesting applications of this technique is a frequencystandard at the 1.5m region with vibrational–rotational tran-sitions of molecules such as CH and HCN. These moleculesprovide almost equally-spaced frequency markers in this nearinfrared region. Nakagawa and de Labachelerie in the Ohtsugroup demonstrated the frequency stabilization of an extendedcavity laser diode (ECLD) to these molecular transitions[7]. They measured the absolute frequencies referring to theRb two-photon transition (5S-5D) at 778 nm [8] with anuncertainty of 10 ( 100 kHz) [9], demonstrating the greatpotential of these system for a frequency standard [10].

Manuscript received July 2, 1998.A. Onae, K. Okumura, and J. Yoda are with the National Research

Laboratory of Metrology, Ibaraki 305-8563, Japan.K. Nakagawa and A. Yamaguchi are with the Tokyo Institute of Polytech-

nics, Kanagawa 243-02, Japan.M. Kourogi, K. Imai, and B. Widiyatomoko are with the Interdisciplinary

Graduate School of Science and Engineering, Tokyo Institute of Technology,Yokohama, Kanagawa 227, Japan.

Publisher Item Identifier S 0018-9456(99)02938-1.

Fig. 1. Experimental setup: ISO: isolator; PBS: polarization beam splitter;EOM: electrooptical modulator; FR: Faraday rotator; and DBM: Doublebalanced mixer.

Based on their results, we have been developing a frequencystandard for practical use. In this paper, we report the stabilityand reproducibility obtained by our frequency standard op-erating at 1.5 m using Doppler-free, CH molecular lines.We improved their system in several points, through whichwe confirmed better performance in frequency stability for thelong term than was achieved in previous work [7].

II. FREQUENCY STANDARD WITH

SEALED-OFF FP CAVITY CELL

Our experimental setup is shown in Fig. 1. Our work isbasically a follow-up to the scheme described in [7], namely,we used an ECLD for a light source with narrow linewidthand a FP cavity to enhance the signal of very weak transitions.The spectrometer was constructed on a lightweight honeycombbreadboard cm . The whole control systemoccupied a standard rack. In our ECLD, a quantum-well(QW) type laser diode was used with one facet antireflective(AR)-coated (>0.1%), and a holographic blazed grating (1200grooves/mm) was placed at the end of the extended lasercavity. The power provided by the ECLD was 5 mW in acircular beam and the linewidth of sub MHz was confirmedby observation of the beat note between two identical ECLD’s.

0018–9456/99$10.00 1999 IEEE

564 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 2, APRIL 1999

After adjusting the position of the grating and the LD chip, wecould tune the wavelength over 5 nm without mode hopping.To reduce acoustic noise, we covered the ECLD tightly by thelid, and the output hole was also covered by an AR-coatedwindow. Introducing a micrometer-replacement actuator, wecould tune the frequency coarsely without opening the ECLDbox. We used a piezoelectric transducer (PZT) at the gratingfor providing fine frequency tuning. After passing througha Faraday isolator (60 dB), the output beam was split bya variable beam splitter ( and PBS), and almost halfof the power was sent to a spectrometer with a FP cavitycell. An electro-optical modulator (EOM) was employed forphase modulation of the FM side-band technique [11] inorder to eliminate dc bias of the demodulation signal, whichis introduced if we used injection current modulation. Afteradjusting the beam parameters with a mode match lens, thebeam was introduced into a FP cavity cell in which was filledan isotope acetylene gas (4 Pa).

From the point of view of long term operation, a simple cell(with dielectric mirrors glued on both sides) is not suitable, dueto vacuum leakage from a PZT element or contamination fromthe glass surface, because we cannot bake in this case. Withsuch a cell, the saturated absorption signal used to disappearwithin one week or so. To make a more practical version, wedesigned a new scheme that uses a FP cavity spaced by a ULEglass holding a baked and completely sealed-off cell havingBrewster windows (see Fig. 1). Special care was taken for notlosing the finesse when making and adjusting the Brewsterwidows of the cell at a glassworks. The length and the diameterof the cell were 200 mm and 16 mm, respectively. The quartzwindows were welded to a fused quartz body. In the centralpart of the cell, we used Pyrex glass for the length (10 mm)of the sealed-off tip. The length of the cavity was 303 mm. Theobserved finesse of the cavity containing the cell was about150 while the finesse calculated from the nominal reflectivityof two mirrors (98.5%) was 208. The finesse decreased toabout 110 when molecular absorption (the P(10) line, 4 Pa)existed. Only 50% of the incident beam was coupled to thecavity due to the lack of fine adjustment of the laser beamparameters.

The reflected beam from the cavity was sent to a detector(D1) and the signal was demodulated. The laser frequency waslocked to one of the FP cavity modes using the FM side-bandtechnique. The correction signal was fed back to the injectioncurrent of the laser diode for fast frequency corrections witha 100 kHz bandwidth servo and simultaneously fed back toa PZT element of the grating for slow and large correctionswith a 2 kHz bandwidth servo. Sweeping a voltage applied toa PZT of the FP cavity and applying a modulation voltage,we could observe the first derivative signal of the saturatedspectrum of the acetylene molecule by lock-in detection ofthe transmitted light via another detector (D2). The Dopplerabsorption signal was about 40% for the P(10) line, and thecontrast of the saturation signal (the saturation dip/Dopplerabsorption) was 3%. The recorded Doppler free spectrum isshown in Fig. 2. The width of the spectrum (about 1 MHz) ismainly determined by the three almost equal contributions, dueto the laser frequency jitter, the transit time, and the collision

Fig. 2. Two derivative signals of the Doppler-free spectrum of the P(10) lineof 13C2H2 recorded on September 21, 1996 (the right) and July 11, 1997 (theleft), respectively. The bandwidth of the recorder is 3 Hz. Time constants ofa lock-in amplifier are 10 ms (September 21, 1996) and 1 ms (July 11, 1997).The sweep time is about 15 s for both.

broadening of 4 Pa gas. We repeated this measurement tenmonths later with the same setup. The recorded spectrum (atthe left of Fig. 2) was quite similar to the previous spectrum(at the right). There was no obvious degrading of the SNR andno noticeable broadening of the linewidth.

III. A LLAN VARIANCE MEASUREMENT

We made two sets of the spectrometers described above.Both frequencies of the ECLD’s were independently locked,referring to the saturated absorption signals using the usualproportional integral derivative (PID) control electronics. Aftershifting the frequency of one of the output beams withan acoustooptic modulator (AOM) device by 80 MHz, weoverlapped the two output beams and observed the beat notesignal of about 80 MHz on an RF spectrum analyzer. TheSNR of the beat was more than 40 dB in a 100 kHz resolutionbandwidth. The linewidth of the beat note was about 800 kHzdue to modulation on the laser frequencies used for the lock-in detection of the saturated absorption. We did not use asynchronizing technique to squeeze the linewidth of the beatnote. We recorded the beat frequencies with a 1 s gate timeusing a universal counter for 5000 s. Part of the recorded beatfrequencies are shown in Fig. 3. From this data we derivedthe square root of the Allan variance of the beat frequencyfluctuation, as shown in Fig. 4. In this experiment, both laserfrequencies were independently stabilized on the same P(16)lines. At the integration time s, the frequencystability reached to and the flicker floor was notobserved yet.

In the context of comparison with the previous work [7],the long term performance was improved. The first reason isthe new cell design described above. We consider the secondreason is the use of an EOM. The modulation using an EOMdid not introduce the residual AM that had been introducedby a modulation of the laser current in the previous work.As discussed in [7], we consider our modulation schemeimproves the long term stability suppressing the offset in thefirst feedback loop which may fluctuate with temperature. An-other improvement was that we used two function generatorsconnected by the phase lock option to generate two 5 MHz sine

ONAE et al.: TOWARD AN ACCURATE FREQUENCY STANDARD 565

Fig. 3. Beat frequencies of two lasers independently stabilized to the satu-rated spectrum of the P(16) line.

Fig. 4. Square root of the Allan variance versus the integration time.

waves with an arbitrary phase difference. In the previous work,a long BNC cable was used as a phase shifter. This might bethe third reason because our scheme suppresses the stray 5MHz radiation. On the other hand, the short term stability isworse in the present work. We attribute this to the relativelylow modulation frequency of the lock-in detection (312 Hz)and the poor coupling to the FP cavity cell.

From 14 data sets of the beat frequency measurements takenfor six days, we obtained 5.6 kHz as a mean value of thedeviation from 80 MHz (i.e., a difference frequency betweentwo standard lasers) and 3 kHz as a standard deviation. Theformer value gives us information concerning the frequencyreproducibility (device-to-device) and the latter is consideredas the lock-to-lock reproducibility.

IV. CONCLUSION

We observed the narrow resonance of the saturated absorp-tion of acetylene molecules with a newly-developed sealed-offFP cavity cell. We also estimated the frequency stabilityand the reproducibility of the frequency standard. Furtherimprovement can be expected on the frequency stability byincreasing the SNR of the saturated absorption signal throughmore efficient coupling of the laser beam to the FP cavity cell.In the next stage, we will pursue the metrological properties ofour frequency standard, such as pressure shift and power shift,using the beat detection system developed here. To make ourfrequency standard compact and transportable is another goal.

We are also making a frequency chain for these CH lines,referring to the two-photon transition in rubidium at 778 nm.We take full advantage of the wide span of a frequency combgenerator, and more direct measurements are now in progressthan in previous work [9], [10].

REFERENCES

[1] A. Kastler, “Atomes a l’interierur d’un interferometer Perot–Fabry,”Appl. Opt., vol. 1, pp. 17–24, 1962.

[2] P. Cerez, A. Brillet, C. N. Man-Pichot, and R. Felder, “He-Ne lasersstabilized by saturated absorption in iodine at 612 nm,”IEEE Trans.Instrum. Meas., vol. IM-29, pp. 352–354, 1980.

[3] L.-S. Ma and J. L. Hall, “Optical heterodyne spectroscopy enhanced byan external optical cavity: Toward improved working standards,”IEEEJ. Quantum Electron., vol. QE-26, pp. 2006–2012, 1990.

[4] M. de Labachelerie, K. Nakagawa, and M. Ohtsu, “Ultranarrow13C2H2

saturated-absorption lines at 1.5�m,” Opt. Lett., vol. 19, pp. 840–842,1994.

[5] K. Suzumura and H. Sasada, “Observation of saturation spectrum of the2v3 band of methane,”Jpn. J. Appl. Phys., vol. 34, pp. L1620–1621,1995.

[6] J. Ye, L. S. Ma, and J. L. Hall, “Ultrasensitive detections in atomic andmolecular physics: Demonstration in molecular overtone spectroscopy,”J. Opt. Soc. Am. B, vol. 15, pp. 6–15, 1998.

[7] M. de Labachelerie, K. Nakagawa, Y. Awaji, and M. Ohtsu, “Highfrequency stability laser at 1.5�m using Doppler-free molecular lines,”Opt. Lett., vol. 20, no. 6, pp. 572–574, 1995.

[8] F. Nez, F. Biraben, R. Felder, and Y. Millerioux, “Optical frequencydetermination of the hyperfine components of the5S1=2-5D3=2 two-photon transitions in rubidium,”Opt. Commun., vol. 102, pp. 432–438,1993.

[9] Y. Awaji, K. Nakagawa, M. de Labachelerie, and M. Ohtsu, “Opticalfrequency measurement of the H12C14N Lamb-dip-stabilized 1.5�mdiode laser,”Opt. Lett., vol. 20, pp. 2024–2026, 1995.

[10] K. Nakagawa, M. de Labachelerie, Y. Awaji, and M. Kourogi, “Accurateoptical frequency atlas of the 1.5�m bands of acetylene,”J. Opt. Soc.Am. B, vol. 13, pp. 2708–2714, 1996, and references therein.

[11] R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A.J. Munley, and H. Ward, “Laser phase and frequency stabilization usingan optical resonator,”Appl. Phys. B, vol. 31, pp. 97–105, 1983.

Atsushi Onae was born in Tokyo, Japan, in 1958.He received the B.S., M.S., and Ph.D. degrees fromthe University of Tokyo, Japan, in 1982, and 1985,and 1990, respectively.

He joined the National Research Laboratory ofMetrology (NRLM), Tsukuba, Japan, in 1988, wherehe has been working on laser spectroscopy and laserfrequency measurement. From 1993 to 1994, hestudied frequency chain at the Max-Planck-Institutefuer Quantenoptik, Muenchen, Germany, as a visit-ing scholar supported by the Science and Technol-

ogy Agency, Japan.Dr. Onae is a member of the Physical Society of Japan and the Japan

Society of Applied Physics.

Ken’ichiro Okumura was born in Osaka, Japan,on March 26, 1950. He received the B.S. andM.S. degrees in electrical engineering from KyotoInstitute of Technology, Kyoto, Japan, in 1974 and1976, respectively.

From 1976 to 1977, he joined the MatsushitaElectric Industrial Company Ltd. Japan. From 1978to 1989, he was with the National Research Labo-ratory of Metrology (NRLM). His research field arefrequency stabilization of laser and nonlinear optics.He recently developed an Rb 2 photon frequency

standard named “Dragon” at NRLM.

566 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 2, APRIL 1999

Jun Yoda, for photograph and biography, see this issue, p. 536.

Ken’ichi Nakagawa was born in Kanagawa, Japan, in 1961. He receivedthe B.S., M.S., and Ph.D. degrees in physics from Tokyo University, Tokyo,Japan, in 1984, 1986, 1989, respectively.

He joined the Tokyo Institute of Technology in 1989. From 1995 to 1998,he was with the Tokyo Institute of Polytechnics and became an AssociateProfessor in 1997. Since 1998, he has been an Associate Professor at theInstitute for Laser Science, University of Electrocommunications, Tokyo,Japan. His research interests are frequency stabilization of lasers, opticalfrequency standards, laser cooling of atoms, and atom optics.

Atsushi Yamaguchi, photograph and biography not available at the time ofpublication.

Motonobu Kourogi was born in Shiga, Japan, in1966. He received the B.S. degree from ShizuokaUniversity, Shizuoka, Japan, in 1988, and the M.S.and Ph.D. degrees from the Tokyo Institute ofTechnology, Tokyo, Japan, in 1990 and 1993, re-spectively.

Since 1993, he has been a Research Associatewith the Tokyo Institute of Technology. His currentresearch interest is frequency synthesis of semicon-ductor lasers.

Dr. Kourogi is a member of the Japan Society ofApplied Physics.

Kazuhiro Imai was born in Tokyo, Japan, in 1971.He received the B.S. and M.S. degrees from TokyoInstitute of Technology, Tokyo, Japan, in 1994 and1996, respectively. He is currently pursuing thePh.D. degree in applied electronics at the sameinstitute.

Mr. Imai is a student member of the Japan Societyof Applied Physics.

Bambang Widiyatmoko was born in Boyolali, In-donesia, in 1962. He graduated in physics fromGadjahmada University, Yogyakarta, Indonesia, in1987 and received the M.S. degree in applied elec-tronics from the Tokyo Institute of Technology,Tokyo, Japan, in 1995. He is currently pursuingthe Ph.D. degree in applied electronics at the sameinstitute.

Since 1987, he has been on the research staffof Research and Development for Applied Physics,Indonesia Institute of Science.

Mr. Widiyatmoko is a student member of the Japan Society of AppliedPhysics.