A NEW ABSOLUTE FREQUENCY REFERENCE GRID IN THE 28 THz RANGE A. Clairon, A. Van Lerberghe, Ch. Bréant, Ch. Salomon, G. Camy, Ch. Bordé

To cite this version: A. Clairon, A. Van Lerberghe, Ch. Bréant, Ch. Salomon, G. Camy, et al.. A NEW ABSOLUTE FREQUENCY REFERENCE GRID IN THE 28 THz RANGE. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-127-C8-135. .

HAL Id: jpa-00221710 https://hal.archives-ouvertes.fr/jpa-00221710 Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

JOURNAL DE PHYSIQUE

Colloque C8, suppze'ment au n012, Tome 42, de'cembre 1981

A NEW ABSOLUTE FREQUENCY REFERENCE G R I D I N T H E 28 THz RANGE

page C8-127

(*I

A. Clairon, A. Van Lerberghe (**), Ch. Brdant (**), Ch. Salomon (**), G. Camy (**) and Ch. J. Bordd (+*I

Laboratoire Primaire du Temps e t des Fre'quences, Obsematoire de Paris, 75014 Paris, France f* *) Laboratoire de Physique des Lasers (+I, ViZ Zetaneuse, France

Universitg Paris-Nord,

93430

Abstract. - A new grid of frequency markers based on 12c1602 lasers locked to OsO saturation peaks is presented. This grid extends from the P(22) to the 4 R(26) CO laser lines of the 10.4 w band and covers 1.12 THz with a 1 kHz 2 accuracy for all the measured frequency differences. This set of measurements includes the lg20s04 line, which is in close coincidence with the P(14) I2cl60 line and whose frequency has been connected to the Cesium primary 2 standard with the same accuracy.

Introduction. -As described in our previous paper [ 11, an absolute frequency reference grid using lasers locked to saturation resonances of heavy molecules like OsO& has a number of advantages over the C02 marker grid obtained by thefluorescence technique of Freed and Javan [ 2 1 [ 3 1 [ 13 1 :

1) Narrower lines with good signal-to-noise ratio can be achieved. A linewidth of 1.25 kHz (HWHK) has recently been obtained for SF6 with the modified Villetaneuse spectrometer [ 4 1. 2) For Os04, hyperfine structures are either completely resolved (1890s04) or absent

(1900s0q,1 9 2 ~ s ~ 4 ) [ 51121. ~

3) For such a heavy molecule the recoil splitting (15 Hz) [ 61 , the second-order Doppler effect (3 Hz) and transit effects are very small. For vibrationrotation transitions of spherical tops the pressure shift is also very small. These features result in a very high capability for absolute frequency Furthermore, the absolute frequency of one of the Os04 line has accuracy already been measured with a 1 kHz accuracy [ 7 ] [ 8 1.

.

In this paper we present a grid of frequency markers which covers 1.12 THz from the P(22) to the R(26)

12c160 laser lines using the natural isotopic mixture 2 bands of 1 9 2 ~ s 0 and 189~s04between 940 and 980 c m ' 4 1231602 laser lines in the same spectral region.

of Os04. Fig. 1 shows the v and the position of the

Adjacent C02 lines and lines separated by as much as 339 GHz have beenconnected by direct harmonic mixing with an X-band or a millimeter-wave klystron in a point-contact diode. (*) Work supported in part by DRET, CNRS and BNM (+) Associ6 au C.N.R.S. n0282

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981815

JOURNAL DE PHYSIQUE

C02

P4

P24

940

LASER

950

R4

960

R26

970

980

WAVENUMBER, cm" FROM R.S.McDOWELL and coil. J. Chem. Phys. 62,1513 (1978)

Fig.1 :Absorption spectra of the vg band of Os04 in t h e 10.4 pm

region

Experimental set-up The modified Villetaneuse spectrometer is shown in Fig. 2 and is described in more detail in [ 4 ] . The two low-pressure highly stable C02 lasers are locked to narrow saturation peaks of Osmium tetroxide using third-derivative servo-loops. Their spectral purity is of the order of 1 kHz when the lasers are free-running and lOOHz when they are locked to saturation lines. In order to estimate systematic shifts, the two laser beams are transmitted through separate absorption cells. Great care has been taken to control the wavefront quality in order to avoid curvature shifts [ 9 ] . In both optical systems wavefronts are flat to a very small fraction of a fringe over the beam diameter. The standing waves are produced by high quality retroreflector corner cubes. These have been built with three A110 plane mirrors, optically contacted with an angular precision of the order of 1 to 2 seconds. The absorption lengths are respectively 6 and 51 third-derivative linewidths of the order of 60 kHz and optimum signal-to-noise ratio. These linewidths result modulation broadening. Typical pressures are Torr

meters giving peak-to-peak 20 kHz respectively at the from coll'sion, power and and 5x10 Torr respectively.

-t

I

18177 C E L L

R

WAVEGUIDE

Fig.2 : Simplified

FREOUENCY

CUBE

P: PARABOLA

C: C O R N E R

X 7

f $f as

schematic diagram of the

TELESCOPE

spectrometer

X-Y

PLOTTER

TO M I M

DlOUE

JOURNAL DE PHYSIQUE

C8-130

As an example we give in Fig.3 theOs04 third-derivative spectrum around the

I

C 0 2 L,INE CENTER

I 1 I

+9989. kHz

I I r.

a

t J

/ ,

'

I

16 7 7 2 . kHz t

4

0 ~ 0 4pressure :

6.6

Pa

Fig.3 : Typical Os04 third -derivative spectrum with a

low- pressure

C02

Laser.

CO R(22) line center. The symmetry of this third-derivative line in the 6 m long cell was carefully checked as a function of modulation index, pressure and intensity. Large variations of these parameters did not give any significant change of the beat frequency (< 30 Hz) between the two lasers locked independently to saturation peaks. On the other hand, the third-derivative line in the large absorption cell is not symmetric in the Torr pressure range for which the signal-to-noise ratio is optimum. This is due to a curvature shift associated with the 3 meter long converging-diverging part of the beam inside the cell. This section of the beam contributes to the observed signal with a large saturation parameter under these conditions. At very high pressure (P > 3x10-~ Torr) or at very high resolution (P < Torr) the line becomes symmetric. In order to investigate the sign and the amplitude of this systematic frequency

2 3 s h i f t we have c a r e f u l l y s t u d i e d t h e F'(39) A i A2 equal i n t e n s i t y doublet of 1 9 2 ~ s 0 4

i n coincidence with the P( 12) CO

2

l a s e r l i n e 110 1. F i r s t , t h e 457.15 kHz f 60 Hz (10)

s n l i t t i n g has been measured by s u c c e s s i v e l ~locking the same l a s e r on both comaon e n t s , t h e second l a s e r remaining locked t o another l i n e . Then by interchanging l a s e r s locked t o t h e two components of t h e doublet and by varying p r e s s u r e , i n t e n s i t y and modulation index, we have been a b l e t o g i v e an e s t i m a t e of t h e frequency s h i f t i n our o p e r a t i n g conditions. The l a s e r i s found t o be locked a t -600Hz (f 400 Hz) from t h e molecular resonance, with a s i g n i n agreement with t h e theoret i c a l p r e d i c t i o n of [ 9 1. A p o i n t c o n t a c t M I M diode [ w - ~ i r]e c e i v e s beams from t h e two s t a b i l i z e d CO 2 l a s e r s and power from a k l y s t r o n with a frequency corresponding t o t h e frequency d i f f e r e n c e between C02 l i n e s . To measure t h e frequency d i f f e r e n c e between Os04 l i n e s i n coincidence with a d j a c e n t C02 l i n e s , t h e X-band k l y s t r o n was m u l t i p l i e d by 4 t o 7 , phase-locked t o t h e b e a t note and d i r e c t l y counted by a microwave counter. I t s time base has been c a l i b r a t e d a g a i n s t a Cs frequency standard. S i m i l a r l y , a m i l l i m e t e r wave k l y s t r o n was used t o connect non-adjacent l i n e s . This allowed us t o reduce u n c e r t a i n t i e s , t o avoid e r r o r propagation and t o d e t e c t p o s s i b l e systematic e r r o r s . A schematic diagram of t h e frequency measurement set-up i s shown i n Fig. 4. The m i l l i m e t e r wave k l y s t r o n i s phase-locked t o the b e a t n o t e coming from t h e M I F diode. I t s frequency i s determined by phase-locking t h e X-band k l y s t r o n t o t h e m i l l i m e t e r wave one. Then t h e X-band k l y s t r o n frequency i s d i r e c t l y counted.

Fig.4 : Frequency

measurement set-up

IF, and IF2 are intermediate frequencies in the 27-41 MHz range

JOURNAL DE PHYSIQUE Results and discussion The grid of measured frequency differences is displayed in Fig. 5; the uncertainty (1 a) for an individual measurement is+0.9kHz.For the sake of conveniencewe chose on each low-pressure CO laser emission profile (k50MHz) the most intenseand isolated OsO saturation line? We did not find any coincidence within* 50MHzon the P(6) and ~ ( 2 4C02 lines. However, with a high-pressure waveguide C02 laser having more than 500 MHz tuning range we have shown [ 1 1 ] that it is possible to reach a large number of Os04 lines for each CO line. Many of these are fundamental lines 2 of the v3 band and are under investigation for their spectroscopic interest [12]. The internal consistency of our measurements can be checked by calculating frequency differences using different pathways through the OsO grid. As an example the P(4) P(14) distance measured by two possible ways differs4 only by 1.5 kHz, well inside the f 2.3 kHz (1 o) calculated uncertainty. The calculated uncertainty over the whole grid, R(26) to P(22), is only 3.3 kHz. The 0.9 kHz uncertainty of the individual measurements comes from the following error budget : - the statistical uncertainty was found to be 0.7 kHz (1 a) This includes : 1) exchange of the side-bands used for klystron stabilization, 2) large variations of the frequency modulation index of the CO 2 lasers, 3) OsO pressure changes, and 4) laser permutations and optical 4 adjustments - As discussed before a + 0.6 (f 0.4) kHz correction has been applied to the frequency of the laser locked to the Os04 saturation peaks in the large cell, to ke into account the curvature shift of the line. -A 2 uncertainty comes from possible drifts of the time base between calibrations against the Cesium clock. - A 0.2 kHz error is also added to include electronic offsets.

-

.

.

* *

We give in table 1 the measured frequency separations between the various OsO lines and the OsO peak which is in coincidence with the C02 P(14) laser line. 4 4 Using the absolute value of this line reported in [7] [ 8 ]

we give the absolute frequency of the Os04 lines determined from our work. In order to help the users of these frequency markers the last column of table 1 indicates the approximate frequency offsets from the corresponding C02 line centers [ 13 1. Conclusion These results have still to be considered as preliminary ones since the present uncertainty can be reduced by at least two orders of magnitude. The most important limitation comes from the asymmetry of the Os04 line in the large cell. This system was designed for high resolution and not for metrological applications. For this purpose an external telescope would be desirable in order to avoid any wavefront curvature in the interaction region. As pointed out in our previous paper this grid can be extended by using many other molecules, among which are Ru04 [ 141 from 900 to 940 cm-' and Xe04 [ 15 ] from 810 to 900 cm-l, and also by using N,O and isotopic species of GO, in conventional or waveguide lasers. These frequencyLstandards can be used to syn:hesize and measure accurate frequencies between 1 THz and 150 THz. At the present time these markers are already used to give an absolute frequency calibration of'spectra of various molecular species in the 10.4 ~ u nregion [ 1 1.

12

C " 0 , Lines

Fig. 5 : Measured frequency differences between

Os04

Saturation

in kiloHertz

lines.

C8-134

JOURNAL DE PHYSIQUE

A b s o l u t e f r e q u e n c y d i f f e r e n c e s o f t h e v a r i o u s Os04 r e s o n a n c e s c o n s i d e r e d i n t h i s work r e f e r e n c e d t o t h e measurement of t h e 0s04 l i n e r e p o r t e d i n [7,81, and frequency d i s t a n c e t o CO 2'

CO l a s e r 2

line

Measured f r e q u e n c y d i s t a n c e i n lcHz t o P(14) reference l i n e

A b s o l u t e f r e q u e n c y i n Frequency d i s t a n c e k H z ( r e f e r e n c e 1920s0 ) t o C02 i n MHz 4

-212 747 424.2(.9) -158 442 817.9(1.4) -104 906 981.6(1.4) -52

053 827.8(1.)

Absolute r e f e r e n c e +51

375 050.8(1.)

+ l o 1 953 433.3(1.4) + I 5 1 876 690.7(1.4) +249 435 486.0C.9) +458 353 737.9(2.3) +502 746 255.5(2.) +546 460 434.1(1.7) +589 380 509.4(1.4) +631 598 015.4(1.7) +673 070 096.4(2.) +713 791 977.3(2.3) +753 773 091.9(2.5) +792 956 884.9(2.7) 4 3 1 469 420.0(2.8) +906 137 1>;.9:3.)

Table 1

References t

[ 1]

CLAIRON A., VAN LERBERGHE A., SALOMON Ch., OUHAYOUN M. and BORDE Ch. J., Optics comm. Vol. 2, no 3 , p. 368 (1980)

.

[ 2 1 FREED C. and JAVAN A., Appl. Phys. Lett.

17 (1970)

53.

[ 3 1 FREED Ch., BRADLEY L.C. and OtDONNELL R.O. IEEE Journal of Quant. Elect. no 11, p. 1195 (1980). Vol. QE

16,

[ 41

t

t

SALOMON Ch., BREANT Ch., BORDE Ch.J.,

and BARGER R.L.,

this issue.

[ 51 KOMPANETS O.N., KUKUDZHANOV A.R., LETOKHOV V.S., MINOGIN V.G. and MIKHAILOV E.L., Sov. Phys. JETP, Vol. 9, no 1 (1975). [ 6 1 HALL J.L.,

BORDE Ch. J. and UEHARA K., Phys. Rev. Lett.,

37 (1976)

1339.

[ 7 1 DOMPTIN Ju. S., KOSHELJAEVSKY N.B., TATARENKOV V.M. and SHUMJATSKY P.S., JETP Letters, 30 (1979) 273.

DOMNIN Ju.S., KOSHELJAEVSKY N.B., TATARENKOV V.M., SHUMJATSKY ?.S., KOMPANETS O.N., KUKUDZHANOV A.R., LETOKHOV V.S. and MICHAILOV E.L., JETP Letters, 30 (1979) 269. [ 8 1 DOMNIN Ju. S., KOSHELJAEVSKY N.B., TATARENKOV V.N., SHUMJATSKY P. S., IEEE, Transactions on Instrumentation and Measurement (1980) Vol. IM no 4, CPEM, p. 264. t

[ 9 1 BORDE Ch. J., HALL S.L., KUNASZ C.V. and HUMMER D.G., Phys. Rev. A (1976)

14

236.

(101

SALOMON Ch. 3rd Cycle thesis University of Paris XI11 (1979).

[ 11 ]

VAN LERBERGHE A., AVRILLIER S., BORDE Ch. J., J. Quant. Electron. OE (1978)

9,

16

481.

I

[12]

BORDE Ch. J., OUHAYOUN,M., VAN LERBERGHE A., SALOMON Ch., AVRILLIER S., CANTRELL C.D. and BORDE J., in Laser Spectroscopy IV, eds Walther H. and Rothe K.W. (Springer Verlag 1979) p. 142.

1 1 3 1 PETERSEN F.R., Mc DONALD D.G., CUPP J.D. and DANIELSON B.L.,

in Laser Spectroscopy, eds Brewer R.G. and Mooradian A. (Plenum Press 1973) p. 555.

[ 14 1

Mc DOlJELL R. S., ASPREY L.B. , and HOSKINS L.C., J. Chem. Phys.,

[15]

Mc DOWELL R.S. and ASPREY L.B., J. Chem. Phys.,

57

(1972) 3062.

2

(1 972) 5712.