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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. �10.1051/jphyscol:1981815�. �jpa-00221710�
JOURNAL DE PHYSIQUE
Colloque C8, suppze'ment au n012, Tome 42, de'cembre 1981 page C8-127
A NEW ABSOLUTE FREQUENCY REFERENCE G R I D I N T H E 28 THz RANGE
(*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, 7501 4 Paris, France
f* *) Laboratoire de Physique des Lasers (+I, Universitg Paris-Nord, 93430 V i Z Zetaneuse, France
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 accuracy for all the measured frequency differences. This set of measurements 2 includes the lg20s04 line, which is in close coincidence with the P(14) I2cl60line 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 refe- rence 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 ) [ 5 ~ 1121.
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 vibration- rotation transitions of spherical tops the pressure shift is also very small.These features result in a very high capability for absolute frequency accuracy
.
Furthermore, the absolute frequency of one of the Os04 line has 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
of Os04. Fig. 1 shows the v bands of 1 9 2 ~ s 0 and 189~s04 between 940 and 980 c m ' 4
and the position of the 1231602 laser lines in the same spectral region.
Adjacent C02 lines and lines separated by as much as 339 GHz have beenconnec- ted 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
C 0 2 LASER
P 2 4 P4 R 4 R 2 6
940 950 960 970 980
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 retro- reflector corner cubes. These have been built with three A110 plane mirrors, opti- cally contacted with an angular precision of the order of 1 to 2 seconds.
The absorption lengths are respectively 6 and 51 meters giving peak-to-peak third-derivative linewidths of the order of 60 kHz and 20 kHz respectively at the optimum signal-to-noise ratio. These linewidths result from coll'sion, power and modulation broadening. Typical pressures are Torr and 5x10
-t
Torr respectively.C: CORNER CUBE P: PARABOLA TO MIM DlOUE WAVEGUIDE R FREOUENCY X-Y PLOTTER TELESCOPE X 7
f $ f
as 18177 CELL I Fig.2 : Simplified schematic diagram of the spectrometerC8-130 JOURNAL DE PHYSIQUE
As an example we give in Fig.3 theOs04 third-derivative spectrum around the
I
C 0 2 L,INE CENTERI
1 +9989. kHz
0 ~ 0 4 pressure :
6.6
PaI
Fig.3 : Typical Os04 third -derivative spectrum with
I I
a
t J
r . '
16 7 7 2 . kHz
4 t
a low- pressure C 0 2 Laser.
/ ,
I
CO R ( 2 2 ) 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 e q u a l i n t e n s i t y d o u b l e t of 1 9 2 ~ s 0 4 i n coincidence with t h e P( 12) CO 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)
2
s n l i t t i n g has been measured by s u c c e s s i v e l ~ l o c k i n g the same l a s e r on both comao- n e n t s , t h e second l a s e r remaining locked t o a n o t h e r l i n e . Then by i n t e r c h a n g i n g
l a s e r s locked t o t h e two components of t h e d o u b l e t and by v a r y i n g 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 o u r o p e r a t i n g c o n d i t i o n s . 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, w i t h a s i g n i n agreement with t h e theore- t 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 l a s e r s and power from a k l y s t r o n w i t h a frequency corresponding t o t h e frequency 2 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 n o t e and d i r e c t l y counted by a microwave c o u n t e r . I t s time base h a s been c a l i b r a t e d a g a i n s t a Cs frequency s t a n d a r d . 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 s y s t e m a t i c e r r o r s . A schematic diagram of t h e frequency measurement set-up i s shown i n F i g . 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 t h e 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 uncer- tainty (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 4 C02 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 of the v3 band and are under investigation for their spectroscopic interest [12]. 2
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 stabiliza- tion, 2) large variations of the frequency modulation index of the CO lasers, 3) OsO pressure changes, and 4) laser permutations and optical 2 adjustments
.
4-
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 off sets from the corresponding C02 line centers [ 13
1.
Conclusion
These results have still to be considered as preliminary ones since the pre- sent uncertainty can be reduced by at least two orders of magnitude. The most impor- tant 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 wave- front 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 n region [ 1 1.
12
C " 0 , Lines
Fig. 5 : Measured frequency differences in kiloHertz b e t w e e n O s 0 4 Saturation 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 o f t h e 0 s 0 4 l i n e r e p o r t e d i n [ 7 , 8 1 , and fre- quency d i s t a n c e t o CO 2 '
CO l a s e r Measured f r e q u e n c y d i s - 2
line t a n c e i n lcHz t o P ( 1 4 ) r e f e r e n c e l i n e
-212 747 424.2(.9) -158 442 8 1 7 . 9 ( 1 . 4 ) -104 906 981.6(1.4) -52 053 8 2 7 . 8 ( 1 . ) A b s o l u t e r e f e r e n c e +51 375 0 5 0 . 8 ( 1 . ) + l o 1 953 433.3(1.4) + I 5 1 8 7 6 690.7(1.4) +249 435 486.0C.9) +458 353 737.9(2.3) +502 746 2 5 5 . 5 ( 2 . ) +546 460 4 3 4 . 1 ( 1 . 7 ) +589 380 509.4(1.4) +631 598 015.4(1.7) +673 0 7 0 096.4(2.) +713 791 977.3(2.3) +753 773 0 9 1 . 9 ( 2 . 5 ) +792 956 8 8 4 . 9 ( 2 . 7 ) 4 3 1 469 420.0(2.8) +906 137 1>;.9:3.)
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 4 ) t o C02 i n MHz
T a b l e 1
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t
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