Its microwave cavity length is 0.96m, and the dimensions are almost

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1 The Optcally Pumped Cs Frequency Standard at the NRLM S.Ohshma, Y.Nakadan and Y. Koga Natonal Research Laboratory of Metrology 1-1-4, Umezono, Tukuba-sh, barak 305 JAPAN ABSTRACT The optcally pumped Cs frequency standard s beng developed at the NRLM, and the constructon s almost fnshed. ts mcrowave cavty length s 0.96m, and the dmensons are almost the same as the conventonal type frequency standards at the NRLM. Lght sources for optcal pumpng and detecton, whose frequences are stablzed by means of saturated absorpton spectra of Cs, are also developed. Usng ths apparatus, we could observe Ramsey resonance sgnal and could confrm the two frequency pumpng effect. n addton, pumpng wth + a polarzed laser lght, we also confrmed that the magnetc feld could be measured usng Zeeman cols n the same way of conventonal type. 1. ntroducton Snce an optcally pumped Cs frequency standard usng laser dodes was proposed1>, basc experments were carred OUt2>.3>.4>.6> and frequency standards based on ths new method are beng realzed at several laboratores. At the Natonal Research Laboratory of Metrology ( NRLM >, the development of ths type of standard amng a prmary frequency standard was started n ts dmensons are made to be almost the same as the NRLM- and the NRLM- ( ths s also a newly developed conventonal type standard for the purpose of comparng both types. The other condtons, for example; the degree of vacuum, magnetc shelds, etc., are also almost the same. As the constructon was almost fnshed, some prelmnary experments were carred out to confrm the fundamental functons of ths standard and to fnd out the problems of the apparatus. Ths paper descrbes the constructon of the optcally pumped Cs frequency standard developed at the NRLM usng photographs and some results of the prelmnary experments.

2 2. Apparatus 2.1 Atomc Beam Tube Fgure 1 shows the optcally pumped frequency standard developed at the NRLM. The vacuum chamber s cylndrcal, and ts length s about 2.5m and dameter s about 400mmq5. Ths chamber can be separated nto one central chamber and two sde chambers. Each of the sde chambers contans a Cs oven and ts sldng mechansm to adjust the poston of the oven. Two onpumps are attached and 10-6Pa s kept. Four frequency stablzed laser 1 ght sources are mounted to the beam tube. There are eght wndows whch make t possble to put n the laser lght parallel and perpendcular to the C-feld drecton. ( C-feld drecton s perpendcular to the beam tube and parallel to the ground. 1 Then, the experments of two frequency laser optcal pumpng6' can be carred out wth any combnatons of laser lght polarzatons. The magnetc sheld conssts of three layer cylndrcal permalloy cases. Fgure 2 shows the resonator. The mcrowave cavty length s 0.96m, and a Helmholtz col for C-feld generaton and eght Zeeman cols for C-feld measurement are mounted on several alumnum plates fxed to four phosphor bronze bars. The compensaton cols to wden the unform C-feld regon are placed at the both ends. Two pars of photo-detectng system composed of photo-dodes and lght collectng mrrors are placed at the both sdes. 2.2 Frequency Stablzed Laser Lght Source A laser dode (LD) s mounted on a copper plate as shown n Fg.3. The laser temperature stablzaton system conssts of two stages to reduce the nfluence of the room temperature fluctuaton, and both of them are constructed by a thermstor, a Pelter cooler and a PD controller. The temperature fluctuaton of the copper plate whch the laser dode s mounted s shown n Fg.4. The spectral wdth of the laser dodes s 10-15MHz, then the hyperfne structure can be resolved but all the Zeeman sublevels are ncluded n the laser spectral dth.' Fgure 5 shows the frequency stablzed laser lght source whose frequency s locked to the saturated absorpton spectrum. The block dagram of ths lght source s shown n Fg.6. The lght put out from LD s lnearly polarzed and s splt n two beams by a polarzed beam spltter (PBSl). The ntensty rato of the two laser beams s adjusted by the rotaton of the A /2 wave plate. The reflected lght s strong C usually several mw ) and s used for optcal pumpng or detecton, and ts secton s about 2mmX 4mm. The transmtted 1 ght s weak ( about 100 W 1 and s used for laser frequency stablzaton. The saturated absorpton spectrum superposed on the lnear absorpton spectrum s observed by the photo-dode (PDl), and the lnear absorpton

3 spectrum s observed by the PD2. Subtractng the sgnal of the PD2 from that of the PD1, we can obtan almost only the saturated absorpton spectrum. We adopted ths methods' because t does not need a mechancal chopper and there s no mechancal vbraton. 3. Prelmnary Experments Condtons of ths experment are as follows. The Cs oven temperature s 120 C C-feld s 7.9A/m and the effectve secton of the Cs atomc beam s about 6mmX 4mm. ( The beam secton s the temporary one whch s not desgned for the optcally pumped standard. The degree of vacuum s 4X 10-6Pa and the room temperature s 23 C. Laser lght sources are used as follows. Lght source Reg on Frequency Polarzaton ncdentpower 0 Pumpng Pump ng Detecton F=4-F'=4 F=3-F7=3 F=4-F'=4 n CT -1.OmW -0.6mW -0. lmw Where, F=4- Fq=4 means the trans t on frequency between C2S1/z,F=41 and CP/,F'=, and a -polzaton s lnear polalzed lght whose ncdent drecton s parallel to the C-feld. The combnaton and 0 satsfes the condton of the two frequency optcal pumpng. Power s adjusted by neutral densty flters. Those flters functoned as optcal solators and the ncdent power s manly lmted by the nfluence of the reflecton lght from the wndows of the beam tube. Ths lmtaton s one of the problems whch have to be mproved. Fgure 7 s an example of the Rab-Ramsey spectra obtaned by sweepng the mcrowave frequency. We can recognze that almost all atoms gathered to the central Rab-Ramsey spectrum owng to the two frequency optcal pumpng. The spectra of mp=+l are remaned slghtly. Those spectra can be reduced by adjustng the crossng ponts of laser beams and Cs beam, whch s mportant because the pullng by the neghborng spectra can be reduced and the uncertantes connected wth C-feld can be also reduced by weakenng the C-feld strength. The center frequency of the Ramsey resonance n the central spectrum corresponds to the clock frequency, and Fg.8 shows ts enlarged spectrum. The SN rato of ths Ramsey spectrum s mproved n comparson wth the Ramsey spectrum by sngle frequency pumpng usng only the lght The wdth of the spectrum shown n Fg.8 s about llohz, whch s almost the same as the wdth of the spectrum by the sngle frequency optcal pumpng although the two frequency pumpng needs hgher power of laser. Ths means that the optcal pumpng s almost saturated. t s consdered that the remaned

4 nose n Fg.8 s manly caused by the laser frequency fluctuaton due to the reflected laser lght from the beam tube wndows. The estmaton of the C-feld strength s very mportant for a prmary frequency standard. n the conventonal type standard, Zeeman cols are used for the estmaton by measurng the Zeeman transton frequency of F=4:mp=-4 and -3. The measurement of that trans ton s possble bascally n the conventonal type, but we have to adopt other method n the optcally pumped standard. Then the method of the followng procedure s adopted. Pumpng by a a * polarzed laser lght stablzed to F=4- Ff=5 spectrum, all Cs atoms n the energy level of F=4 are optcally pumped to F=4:mp=4 sublevel. Then those atoms are perturbed by Zeeman cols and the transtons to the other Zeeman sublevels are occurred, and those transtons are observed by the detecton of the fluorescence lght excted by other + a polarzed laser lght also stablzed to F=4-F'=5 spectrum. The observed Zeeman spectrum contans manly the transton between F=4:mp=4 and mp=3. A1 though the other A mp=+ 1 transtons mght be contaned slghtly, those are neglgble for the C-feld estmaton. Fgure 9 s an example of the obtaned Zeeman spectra. The Zeeman transton can be observed wth good SN rato easly. The reason s that the lght reflected from the vacuum chamber wndows cannot return to the laser dodes n ths expermental condton and the frequency stablty of the laser dodes s not reduced. The a + polarzed lght s produced by placng a h /4 wave plate n front of the lght source, and the combnaton of the h /4 wave plate and PBSl becomes an solator. 4. Conclusons The constructon of the optcally pumped Cs frequency standard s almost fnshed, and we could carry out some prelmnary experments. Ramsey resonance and Zeeman transton were observed, and the prospect of the success of the development was obtaned. The laser power of about 1mW s almost suffcent for the two frequency optcal pumpng. The ntensty of the mp=f 1 Rab- Ramsey spectra s controllable by changng the crossng postons of the two laser beams and the Cs beam. The reflecton lghts from the wndows have to be reduced to mprove the laser frequency stablty and to decrease the stray 1 ght. Acknowledgment: The authors would lke to thank Dr. M. Mormura and Dr. T. Ohsh for ther encouragement and support.

5 References 1. J.L. Pcqu6, "Hyperfne optcal pumpng of a cesum atomc beam, and applcatons," Metrologa, vo1.13, pp , M. Ardt and J.L. Pcqu6, "A cesum beam atomc clock usng laser optcal pumpng. Prelmnary results." J. Phys. Lett. (Pars), vo1.41, pp.l379-l381, M. Ardt, "A caesum beam atomc clock wth laser optcal pumpng, as a potental frequency standard," Metrologa, vo1.18, pp.59-66, G. Avla, E. de Clercq, M. de Labachellere and P.Cerez, "Mcrowave Ramsey resonances from a laser dode optcally pumped cesum beam resonator," EEE Trans. nstrum. Meas., vol.m-34, pp , A. Derbyshre, R.E. Drullnger, M. Feldman, D.J. Glaze, D. Hllard, D.A. Howe, L.L. Lews and J.H. Shrley, "Optcally pumped Small Cesum Beam Standard : A Status Report," n Proc. 39th Ann. Freq. Control Symp., pp.18-21, L. L. Lews and M. Feldman, "Optcal pumpng by lasers n atomc frequency standard," n Proc. 35th Ann. Freq. Control Symp., pp , S. Ohshma, Y. Nakadan and Y. Koga, "Spectral wdth of saturated absorpton spectra of Cs wth a laser dode," EEE J. Quantum Electron. vol.qe-23, pp , T. Yabusak, H. Hor, M. Ktano and T. Ogawa, "Freqency Stablzaton of Dode Lasers usng Doppler-Free Atomc Spectra," n Proc. nt. Conf. LASERS '83.

6 Fg.1 : Optcally pumped Cs frequency standard developed at the NRLM. Fg.2 : Resonator of the Cs atomc beam tube.

7 laser - ' :!. :!, 1-1, Fg.3 : Laser dode and ts mount wth a temperature stablzaton system. 'k mk ?rsll..r 2L 1 Fg.4 : Temperature fluctuaton of the laser mount. ', :,! Fg.5 : Frequency stablzed Fg.6 : Schematc dagram of lght source. the lght source. 251

8 Fg.7 : Example of the Rab-Ramsey resonance spectra obtaned by two frequency optcal pumpng. ("0" means the clock trans ton frequency. Fg.8 : Example of the Ramsey resonance spectrum. Ths spectrum s observed n the same condton of Fg.7 except the sweep wdth of the frequency.

9 k kHz > 'Fg.9 : Example of the Zeeman resonance spectrum.