SUPERNARROW RESONANCES IN METHANE ON E-LINE OF THE P(7) TRANSITION OF THE ν3 BAND AND THEIR APPLICATION IN OPTICAL FREQUENCY STANDARDS

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SUPERNARROW RESONANCES IN METHANE ON E-LINE OF THE P(7) TRANSITION OF THE ν3 BAND AND THEIR APPLICATION IN OPTICAL

FREQUENCY STANDARDS

S. Bagayev, V. Chebotayev, A. Dychkov, S. Maltsev

To cite this version:

S. Bagayev, V. Chebotayev, A. Dychkov, S. Maltsev. SUPERNARROW RESONANCES IN METHANE ON E-LINE OF THE P(7) TRANSITION OF THEν3 BAND AND THEIR APPLICA- TION IN OPTICAL FREQUENCY STANDARDS. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-21-C8-28. �10.1051/jphyscol:1981803�. �jpa-00221698�

JOURNAL DE PHYSIQUE

CoZZoque C8, supple'ment au n022, Tome 42, de'cembre 2981 page C8-21

SUPERNARROW RESONANCES I N METHANE ON E - L I N E OF THE ~ ( 7 ) T R A N S I T I O N OF THE v 3 BAND AND T H E I R A P P L I C A T I O N I N O P T I C A L FREQUENCY STANDARDS

S.N. Bagayev, V.P. Chebotayev, A.S. Dychkov and S.V. Maltsev I n s t i t u t e of Thermophysics, Academy of Sciences of the U. S. S. R., Siberian Branch, 630090 Novosibirsk, U.S. S. R.

Abstract.- This paper reports on the obtaining of supernarmw resonances in methane on the E-line of the P(7) transition of the $ band with a relative a d t h of about 1'~'0 and on the use of- such resonances for frequency stabilization of a He-Be laser at 3.39 p. The new results are presented of investigat- ions on production of lasere with a limiting narrow line, The prospects of the development of works on optical frequency standards are discussed.

The attainment of high values of laser frequency stability and reproducibility is closely connected with the obtaining of narrow optical resonances, Over the last years a considerable progress has been achieved in this direction owing to the development of new physical principles of obtaining supernarrow resonances based on a nonlinear interaction of optical fields with a gas. There are three main methods for obtaining nonlinear Doppler free resonances: the method of absorption saturation, of two-photon resonance and the method of separated optical fields. All these methods were proposed by Prof. Chebotayev and collaborators at different times /I/. The method of two-photon resonance and the method of separated optical fields are promising in obtaining resonances of 10-100 Hz wide.

However by present the best results on frequency stabili~ation have been achieved by the method of absorption saturation.

The most narrow saturated resonances with an absolute width leas than 1 kHz have been obtained in methane on the

pi2) line Of

the P(7) transition of the $ band ( A ⁼ 3.39 p) by using external /2/ and internal /3/ telescopic expanders of a light beam (TBE) that permits to increase the time of coherent interaction of particles with field, By using such resonances it has become pos- sible to resolve and study a magnetic hyperfine etructure (MHS) of

the p i 2 ) line in methane /2, 3/, to directly obaeme a recoil effect

/4, 3/, an anomalous Zeeman effect /3, 5/ and so on, The use of a TBE inside a cavity enabled us to obtain more intense resonances in - methane with a relative width of about and to use them for frequency stabilization, In /3/ we reported the first results on frequency stabilization of a He-lTe/CH laser with a TBE to indivi- dual components of the mRIS of the P!2fline in methane and on the

L

achievement of high values of long-term frequency stability and reproducibility at the level of I O " ~

-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981803

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A t t h e previous Symposium on Frequency Standards and Hetrology i n Copper Nountain we reported t h e p r i n c i p l e of construction of t h e modern o p t i c a l frequency standard t h a t u s e s supernarrow resonances

a s an o p t i c a l discriminator. Fig. 1 shows t h e schematic of t h i s standard. It c o n s i s t s of a frequency s t a b l e l a s e r 1 w i t h a narrow l i n e , a tunable l a s e r 2, a system f o r obtaining narrow resonances and a feedback system. The narrow l i n e of t h e tunable l a s e r 2 i s p r o d d e d by phase locking i t s frequency t o t h a t of t h e s t a b l e l a s e r 1. In this case t h e short-term frequency s t a b i l i t y of l a s e r 2 i s the same a s t h a t of l a s e r 1. The frequency of l a s e r 2 i s tuned by varying t h e frequency of a radio-frequency o s c i l l a t o r , The r a d i a t i o n from t h e l a s e r 2 i s used t o o b t a i n narrow reeonsnces. The smooth tuning of t h e frequency of t h e l a s e r 2 t o t h e resonance maximum and i t s keeping a r e performed with t h e a i d of an e l e c t r o n system of automatic frequency control. An important advantage of t h i s scheme i s t h e p o s s i b i l i t y t o simultaneously o b t a i n high values of short- and long-tem frequency s t a b i l i t y and r e p r o d u c i b i l i t y .

Two most important elements of t h e described standard may be distinguished, These a r e t h e l a s e r with a narrow r a d i a t i o n l i n e and t h e system f o r obtaining supernarrow r2sonances. I n v e s t i g a t i o n s and use of supernarrow resonances of 10-10' Hz r e q u i r e l a s e r s with a linewidth of about 1 Hz. The production of such l a s e r s i s of g r e a t independent i n t e r e s t . This pager w i l l t h e r e f o r e present t h e l a t e s t r e s u l t s obtained with a He-IPe/CH4 3.39 p l a s e r whose frequency i s s t a b i l i z e d t o an i n t e n s e resonance in methane on t h e p i 2 ) l i n e with a width of about 50 kHz. The scheme f o s s t a b i l i z i n g t h e l a s e r freq- uency w a s described i n d e t a i l i n / 6 / . The linewidth was recorded by means of d i r e c t record of zero frequency beatings i n a recorder.

Figure 2 ehowa t h e record of zero frequency beatings of two He-Be/CH4 l a s e r s i n a recorder. A t y p i c a l o s c i l l a t i o n period was about 0.2 s. It was due t o frequency s h i f t of t h e l a s e r s . The observed p i c t u r e i s phase b e a t i n g s of o p t i c a l o s c i l l a t o r s .

Res. Syst.

r, f.o

Pia.1: Schematic of t h e modern o p t i c a l frequency standard

Pig. 2: Record of zero frequency beatings of two independently stabilized He-Ne/CH4 lasers in a recorder

HR. 2: Beat spectrum of frequencies of two stabilized lasers

Figure 3 shows the beat spectrum of laser fre uencies obtained from a Fourier analysis of the seriea of records

02

The use of resonances with an absolute width of about 1 kHz in methane on the F ~ ( ~ ) line is of interest in obtaining the limiting values of frequency reproducibility of a He-X?e/CH4 laser of

JOURNAL DE PHYSIQUE

However i n t h i s case a highly s t a b l e t e l e s c o p i c l a s e r with a beam diameter =: 20 cm i s an unwieldy system. The obtaining of resonances of 5-10 kHz Mde i s of g r e a t i n t e r e s t i n t h e production of simpler He-I'?e l a s e r s with t h e same high frequency r e p r o d u c i b i l i t y t h a t may be used i n solving various p r a c t i c a l problems.

In this case a s i n g l e f r e e of hyperfine s t r u c t u r e E-line of t h e P(7) t r a n s i t i o n of t h e \) ., band i n methane may be very convenient

2

i n frequency s t a b i l i z a t i o n and spectroscopic s t u d i e s . The resonances on t h i s l i n e were first observed i n /7, 8 / . Detailed s t u d i e s of these resonances and t h e i r a p p l i c a t i o n f o r frequency s t a b i l i z a t i o n were made i n /9-11/.

Also we s h a l l r e p o r t h e r e t h e new r e s u l t s on obtaining of super- narrow resonances i n methane on t h e E-line with a r e l a t i v e width of

1 0 " ~

-

I n o r d e r t o o b t a i n and study supernarrow resonances on t h e E-line we used a He-Ne l a s e r with a three-mirror t e l e s c o p i c beam expander. Pig. 4 shows t h e schematic of t h i s l a s e r . The f i e l d d i s t - r i b u t i o n i n t h e c a v i t y i s marked with the d o t t e d l i n e . The beam dia- meter i n an absorption c e l l was 4 om. The E-line of methane i s ,-3 GHz away from t h e c e n t e r of an amplification l i n e of t h e He-Be

R2-C" R3=1250cm

He-Ne

- - - - - - -

-612cm-

Ma. 4: Schematic of l a s e r with three-mirror t e l e s c o p i c beam expander

Fig. 3: Record of t h e resonance i n methane on E-line of t h e P(7) t r a n s i t i o n of t h e \) 3 ^band.

Methane pressure is 100 pTorr

I =;o 10 0 10 20 *

K i Loherts detuning f r o m s t a b l e Laser

Laser 2

Fig. 6: Scheme f o r s t a b i l i z i n g t h e frequency of a He-Ne l a s e r t o a super- narrow resonance of

T e L e s c o p l c L a s e r 3 E-line i n methane

1-st Harmonic

rcl

l a s e r t o t h e red region, For l i n e coincidence we uaed a t r a n s v e r s e magnetic f i e l d of r 1800 Oe applied t o an amplifying tube.

Figure 5 shows t h e t y p i c a l record of a narrow resonance i n methane on t h e E-line at a pressure of 100 yTorr. The resonance h a l f - width i s = 5 kHz, i n t e n s i t y about 30 pW. Such resonances were used

a s a reference point f o r frequency s t a b i l i z a t i o n .

Figure 6 shows t h e scheme of setup f o r s t a b i l i z i n g t h e frequency of a He-Ne l a s e r with a three-mirror t e l e s c o p i c beam expander t o the resonance i n methane on t h e E-line of 5 kHz width. The short-term frequency s t a b i l i t y of t h e t e l e s c o p i c l a s e r was provided by means of frequency-phase o f f a e t lock of i t s frequency t o t h a t of t h e s t a b l e l a s e r 1 with a linewidth l e s s than 10 He. The frequency of t h e l a s e r with t h e TBE w a s tuned t o t h e maximum supernarrow resonance over zero of t h e s i g n a l of t h e f i r s t harmonic i n t h e power of t h e system of automatic frequency control. Frequency modulation of t h e l a s e r 3 was made by tuning t h e frequency of a radio-frequency o s c i l l a t o r with a s i g n a l of an audio-frequency o s c i l l a t o r . The modulation freq- uency was 130 Hz, frequency d e v i a t i o n amplitude 2 kHz. With t h e frequency d e v i a t i o n from t h e resonance c e n t e r an e r r o r s i g n a l wae f e d from t h e output of t h e system f o r s t a b i l i z a t i o n t o t h e radio- -frequency o s c i l l a t o r . This s i g n a l tuned t h e frequency of t h e

radio-frequency o s c i l l a t o r (and consequently, t h e frequency of l a s e r 3 ) so t h a t t h e generation frequency coincided with t h e resonance maximum.

A usable s t a b l e l a s e r was a He-Me l a s e r whose frequency was s t a b i l i z e d t o an i n t e n s e resonance i n methane on t h e E-line of about 100 kHz wide. The c a v i t y l e n g t h of t h e l a s e r was 200 cm, abs- o r p t i o n c e l l 150 cm. The l a s e r l i n e w i d t h w a s l e s s t h a n 70 HZ.

C8-26 JOURNAL DE PHYSIQUE

!Phe coincidence of an amplification line with the E-component of an absorption line in the used lasers was made by using a trans- verse magnetic field. In order to exclude an unwanted &+ component we placed additional absorption cells filled with CH3Br /lo/ at a pressure of 4 Torr inside the laser cavities. A gas in discharge tubes was excited with a transverse high-frequency discharge at the frequency of 100 MHz. With the averaging time of 10 s a relative frequency stability of the telescopic laser was 6 10 5. m e line- width of the telescopic laser was about 10 Hz. In order to investi- gate frequency reproducibility we performed measurements of shifts of a nonlinear resonance in methane on the E-line on pressure of an absorbing gas and field intensity in the cavity.

The first experiments on observation of a nonlinear dependence of resonance shift in methane on pressure were performed by us on the F2 (2) line in 1972 /12/. Very convenient for studying particle collisions is the E-line of methane free of hyperfine splitting.

Figure 7 shows the experimental dependence of resonance shift in methane in the E-line on pressure. It is seen that in the pres- sure range of about 1 uTorr the shift slope is about 30 Hz/mTorr.

With an increase of pressure the shift grows. The difference of the shift slopes in the range of higlt and low pressures is about 8.

The nonlinear nature of the shift is due to the influence of elastic scattering of excLted particles at mall angles when a Doppler shift is kv@ (kv is a Doppler width, 8 is a typical scattering angle) and commensurable with a homogeneous linewidth at particle scatter- ing /12/. 'Fhe theory of shift of a nonlinear resonance in molecular low-pressure gases is discussed in /13, 14/. A small magnitude of the collisional resonance shift in the range of low pressures favours the achievement of high values of frequency reproducibility of the 3.39 p He-He laser.

We have performed measurements of shifts of the frequency of a telescopic laser stabilized to a supernarrow resonance in methane against field intensity in the cavity. In the pressure range from 50 to 100pTorr with variation of the field density in the cavity

Fig. 7: Dependence of the resonance shift in methane on the E-line on pressure

Neon pressure,mTorr

by two times ( s a t u r a t i o n parameter was v a r i e d from 0.1 t o 0.2) t h e frequency s h i f t l a y i n t h e limits of 5 Hz. The r e s u l t s of meas- urements enabled u s t o e s t i m a t e a frequency r e p r o d u c i b i l i t y of t h e

t e l e s c o p i c l a s e r s t a b i l i z e d t o a resonance on t h e E-line a t t h e l e v e l of It should be noted t h a t i n this c a s e a high accuracy of t h e frequency of t h e t e l e s c o p i c l a e e r of about 1 0 " ~ i s provided.

The p r i n c i p a l f a c t o r t h a t r e s t r i c t s t h e frequency accuracy and r e p r o d u c i b i l i t y of l a s e r s i s a second-order Doppler e f f e c t , !!!he uee of heavy molecules, p a r t i c l e cooling, development of t h e methods of s e l e c t i o n over a b s o l u t e v e l o c i t i e s w i l l permit a considerable decre- a s e of t h e i n f l u e n c e of a second-order Doppler e f f e c t .

The achieved r o g r e s s i n o b t a i n i n g narrow o p t i c a l resonances and i n t h e productfon of l a s e r s with a U g h r a d i a t i o n frequency sta- b i l i t y of 1 0 " ~ - 1 0 " ~ open up r e a l p o s s i b i l i t i e s t o c a r r y our some p r e c i s i o n physical experiments. Recently we r e p o r t e d on t h e p o s s i b i - l i t y t o produce l a s e r d e t e c t o r s of g ~ a v i t a t i o n a l waves /15/. By u s i n g t h e method of phase l o c k it i s p o s s i b l e t o t r a n s f e r s high frequency s t a b i l i t y of t h e He-Ne/CH4 l a s e r t o t h e o t h e r ranges w i t h no l o s s e s i n accuracy /16/.

Owing t o production of highly s t a b l e l a s e r s t h e problem of c r e a t i o n of an o p t i c a l time standard has been r e c e n t l y solved /17/.

This system provides a comparison of t h e time u n i t , a second, with an o p t i c a l o s c i l l a t i o n period at 3.39 p and allows measurements of

4 .

a b s o l u t e f r e q u e n c i e s i n t h e range 0

-

10"~. Owing t o t h e p o s s i b i l i t y of producing o p t i c a l time stan- d a r d s an i n t e r e s t t o o b t a i n i n g of l i m i t i n g high v a l u e s of l a s e r f r e - quency s t a b i l i t y and r e p r o d u c i b i l i t y w i l l grow.

F u r t h e r progress i n i n c r e a s i n g frequency s t a b i l i t y and repro- d u c i b i l i t y of l a s e r s i s due t o t h e o b t a i n i n g of supernarrow reson- ances of 10-100 Hz wide i n separated o p t i c a l f i e l d s and resonancela of ions i n t r a p s .

I n conclusion t h e a u t h o r s would l i k e t o thank D r s . V.G.Goldort and A.E.Om f o r development and production of e l e c t r o n systems f o r high-frequency e x c i t a t i o n of a discharge and f o r frequency s t a b i l - i z a t i o n of J a s e r s a s w e l l a s f o r h e l p i n making experiments.

References

1. Chebotayev V.P., Proceedinge of t h e Sergio Porto Memorial Sympositm, Rio de J a n e i r o , Brasil, 29 June-3 July 7980, p. 105.

2. H a l l J.L,, Borde C., Phys.Rev.Lett. 30 (1973) 1101.

3. Chebotayev V.P., Proceedings of 2nd Frequency Standards and Metrology Symposium, Copper Mountain, USA, 1976;

Bagayev S.N., Chebotayev V.P., Goldort V.G., Dmitriyev A.K., Dychkov A.S. , and Vasilenko L.S., Appl.Phys, 12 (1977) 291.

4. Hall J.L., Borde C., Uehara K., Phys.Rev.Lett. 2 (1976) 1339.

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5. Bagayev S,l?., Belyayev M.V., Dmitriyev A.K.9 Chebotayev V.P., Zh.Eksp. i Teor.33.z. Pis'ma Red. (1980) 661.

6. Bagayev S.M., Dmitriyev A.K., Dychkov A.S., Chebotayev V.P., Zh.Ekep. i T e 0 r . H ~ .

a

7. Luntz A-Co, Brewer R.G., F o s t e r K.L., Phys.Rev.Lett. 2 (1969) 951

8. Luntz A.C., Brewer R.G., 3.Chem.Phys. 54 (1971 ) 3641

9. Hall J.L., Magyar J.A., i n : High-Resolution Laser Spectroscopy, ed. by K.Shimoda ( S p r i n g e r V e r l a g , B e r l i n , Heidelberg, New York, 1976) pp. 173-199.

10. B r i l l e t A,, Cerez P., Hajdukovic S., Hartman F., Optics Commun.

a

11. Koshelyayevsky N.B., Malyshev Yu.M., Ovchinnikov S.B., Rastorguyev Yu.G., Tatarenkov V.M., Kvantovaya Elektronika 6 (1979) 478,

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12. Bagayev S.N., Baklanov E.V., Chebotayev V.P., ZhoEkspr i Teor.

Fiz. Pistma Red. 16 (1972) 344.

13. Alekseyev V.A., Andreyeva T.A., Sobelman I.I., Zh.Eksp. i Teor.

Fiz. (1973) 813.

14. Baklanov E.V., Optika i Spektr. 2 (1975) 24.

15. Bagayev S.N., Chebotayev V.P., Dychkov A.S., Goldort V.G., Appl.Phys.25 (1981 ) 161.

16. Chebotayev V.P., Report a t t h e XIX General Assembly of URSI, Helsinki, Finland (1978).

17. Chebotayev V.P., Report a t t h e 3d Symposium on Frequency Standards and Metrology, Aussois, France, October 1981.