A HIGHLY SENSITIVE TECHNIQUE OF DETECTING SUPERNARROW SPECTRAL LINES BASED ON THE FREQUENCY RESONANCES OF A DOUBLE-MODE LASER

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A HIGHLY SENSITIVE TECHNIQUE OF

DETECTING SUPERNARROW SPECTRAL LINES BASED ON THE FREQUENCY RESONANCES OF A

DOUBLE-MODE LASER

N. Basov, M. Gubin, V. Nikitin, A. Nikulchin, D. Tyurikov, V. Petrovskiy, E.

Protscenko

To cite this version:

N. Basov, M. Gubin, V. Nikitin, A. Nikulchin, D. Tyurikov, et al.. A HIGHLY SENSITIVE TECH-

NIQUE OF DETECTING SUPERNARROW SPECTRAL LINES BASED ON THE FREQUENCY

RESONANCES OF A DOUBLE-MODE LASER. Journal de Physique Colloques, 1981, 42 (C8),

pp.C8-89-C8-95. �10.1051/jphyscol:1981811�. �jpa-00221706�

JOURNAL DE PHYSIQUE

CoZZoque C8, suppZ6ment au n012, Tome 42, de'cernbre 1981 page C8-89

A HIGHLY S E N S I T I V E TECHNIQUE OF DETECTING SUPERNARROW SPECTRAL L I N E S BASED ON THE FREQUENCY RESONANCES OF A DOUBLE-MODE LASER

N.G. Basov, M.A. Gubin, V.V. N i k i t i n , A.V. N i k u l c h i n , D.A. Tyurikov, V.N. P e t r o v s k i y (*) and E.D. P r o t s c e n k o (*)

P. N . Lebedev PhysicaZ I n s t i t u t e , Moscow, 127924, U. S . S. R.

( +)

I n s t i t u t e of PhysicaZ Engineering, Moscow, 11 5409, U. S. S. R.

s t a t - The present work i n v e s t i g a t e s a highly s e n s i t i v e

&&3+ que of obtaining narrow resonances of s a t u r a t e d d i s p e r s i o n

detected by t h e mode beat s i g n a l of a double-mode l a s e r with a n i n t e r n a l absorption c e l l which a)lows t o r e g i s t e r weak absorption l i n e s with t h e 10-11 cm-1 H Z ' / ~ s e n s i t i v i t y determined by t h e n a t u r a l frequency f l u c t u a t i o n s of the l a s e r . The resonances of 2 r 2 W z HWHN have been separated in t h e 80 om c e l l with t h e l i g h t beam of t h e 30 mm i n diameter i n experiments on t h e He-ESe/CH4 double-mode l a s e r ( = 3.39 mkm).

beam sources have been applied.

s o far t h i s tehhnique has permitted t o o b t a b a very high spec- t r a l r e s o l u t i o n i n t h e o p t i a 1 r a e and t o separate Lamb-dips with t h e -factor of 1010 - 1031 i o r y h e t r a n s i t i o n s of CH

C a O s O

[ 4 - 8 . The d e t e c t i o n of' t h a t high 9-spectral l i n e s d e d n d s $he d e of atomic beams or molecular gases of low densitiea. The value of i n v e r t e d Lamb-dip (a-) i s known t o be proportional t o p3.Vl (where p-

is t h e pressure of t h e absorption gas and V, is t h e c a u s t i c volume i n t h e absorption medium) and t o decrease uickly with t h e drop of t h e

p. . ^{A s} a r e s u l t t h e value of d- has a l o r c o n t r a s t a s compared t o t h e :background i n t e n s i t y ' i n t h e detector. This i n t u r n enhances t h e d i f f i c u l t i e s aP d e t e c t i n g a s i g n a l among t h e noise due t o t h e back- ground i n t e n s i t y ( t h e d e t e c t o r shot noise, f l u c t u a t i o n s i n t h e l a s e r

i n t e n s i t y , eto.). ^A few modified techniques of r e g i s t r a t i o n were suggested t o eliminate t h e anf3.uence of t h e background i n t e n s i t y on t h e s e n s i t i v i t y of d e t e c t i n g Lamb-dips. I n p a r t i c u l a r r e g i s t e r i n g ^may be performed with t h e he1 of a change i n t h e polariziation s t a t e of t h e probe nave i n the c e l f * p o l a r i z a t i o n speotroscopyn t [ 81 ^{) or with}

a fluorescence s i g n a l from he upper s t a t e of t h e r e f e r e n c e t r a n s i t i o n [ I ] . The l a t t e r is e s p e c i a l l y e f f i c i e n t f a r beam experiments on atoms when multiple fluorescence on t h e a d j a c e n t t r a n s i t i o n can be soraetimes ensured. For molecular l i n e s multiple fluorescence is hardly real&- a b l e bedause of t h e complex s t r u c t u r e of molecular energy s t a t e s .

The e n l i s t e d resonanoe d e t e c t i o n techniques m y be c a l l e d 'ampli- tude* by analogy with radio-technique as t h e y all imply r e g i s t r a t i o n of t h e i n t e n s i t y ( o r amplitude) of t h e l i g h t wave. However, it is known t h a t t h e s a t u r a t i o n of a h inhomogeneously broadened iine can be detected by t h e change of t h e phase-frequency c h a r a c t e r i s t i c s of t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981811

C8- 90 JOURNAL DE PHYSIQUE

l a s e r r a d i a t i o n as t h e frequency a s well a s t h e phase of t h e l a s e r wave depends on t h e change of the r e f r a c t i v e index i n t h e absorption medium. With t h e inhomogeneously broadened d i s p e r s i o n l i n e s a t u r a t e d by the oppositevely d i r e c t e d waves r e f r a c t i v e index i n t h e c e n t r e of t h e absorption l i n e undergoes resonance changes which i n t h e e x t e r n a l c e l l can be detected by a phase s h u t of %he wave with a g r e a t accu- r a c y by means of a frequency modulated method suggested i n [ 9,10 ] .

The s a t u r a t i o n of t h e d i s p e r s i o a l i n e i n a l a s e r with a n i n t e r n a l c e l l i s deteated a s a nonlinear dependence of t h e l a s e r frequency on t h e tuning frequency of t h e *empty, c a v i t y t h e e f f e c t of a nonlinear frequency p u l l i n g t o t h e l i n e c e n t r e [11, $21

I n terms of Lambqs theory the frequency o f m a O U with a n i n t e r - n a l absorption c e l l a t ( W I - ^U-1-X- can be w r i t t e n a s

where is t h e empty c a v i t y resonance frequency, E: ^is t h e mode i n t e n s i t y , Wl is the emission frequency, W- is the c e n t r e of the absorption l i n e , GA a r e t h e addends describing t h e l i n e a r (non- s a t u r a t e d 1 d i s p e r s i o n l i n e s of t h e a c t i v e and absorbing media and t h e term p+ ^El describes t h e s a t u r a t f o n of t h e d i s p e r s i o n l i n e of t h e a c t i v e medium. The last t e r m i n $q.(l) i s a narrow resonance of a d i s p e r s i o n kind pointing t o t h e nonlinear u l l i s g e f f e c t of the l a s e r frequency t o t h o c e n t r e of t h e absorption fine: OC- i s the nonsa- t u r a t e d absorption f a c t a r per u n i t length, C is t h e l i g h t velocity,

e - ^{i s} t h e c e l l length, e, ^is t h e f u l l c a v i t y length, fi is Plank*s constant, & is t h e homogeneous width of t h e absorption l i n e and

dl2 is t h l 9 t r i x e l e m n t of t h e d i p o l e moment of t h e transition.The f a c t o r ² i s t h e parameter of t h e t r a n s i t i o n satu-

r a t ion, - ---

For convenience we s h a l l shorten t h e resonance dependence i n Eq. i n he r a d i a t i o n i n t e n s i t y t o t h e *amplitude resonance* t 1) t o t h e *frequency resonance* (FR) and t h e i n v e r t e d Lamp d i p ^(AR).

The FR i n a OBdL i t h a n i n t e r n a l absor t i o n c e l l has been tho- rou&ly i t s e a t i g a t e d rl3-I 5 1 . I n l'l4 1 5 q it was used t o s t a b i l i z e freqnaney of a He--l/CH4 O U . Pros rmq.(l ) t f o l l o r s t h a t t h e value of the E'fZ =S-CI!-& to) i n c o n t r a s t t o t h e BB is l i n e a r l y de- pends on b-'%nd i n a d d i 4 i o n a s it was shown i n [ I 4 1 t h e frequency s t a b i l i z a t i o n ^of t h e FB reduces s h i f t s of t h e s t a b i l i z e d frequency due t o t h e s t r a y amplitude modulation.

A fzequency adjusted laser-heterodyne d e t e c t i n g FR by t h e beat s i g n a l a t t h e frequency ^{&the+ =} /w, - ^Whet/ is needed t o observe FR i n a OML, I n t h i s case t h e n o i s e l e v e l is determined by a r e l a t i v e s t a b i l i t y of a beat frequency &/he+ . ^AS t h e t e c h n i c a l f r e uency f l u c t u a t i o n s of t h e _LOhet a r e not c o r r e l a t e d by those of k e l a s e r under i n v e s t i g a t i o n even a t t h e a b s o l u t e l y p e r f e c t hete- rodyne s t a b i l i z a t i o n t h e n o i s e l e v e l be high as the achived l a s e r passive $aser frequency s t a b i l i t y cannot be mare than

103 + 10 HZ.H~"/~ . This f o r b i d s a n e f f i c i e n t a p p l i c a t i o n of FR i n t h e OML.

2. This work i n v e s t i g a t e s a more e f f i c i e n t technique of d e t e c t i n g FB when they a r e separated by a beat s i g n a l of a double-mode l a s e r

$DML) with a n i n t e r n a l absorption c e l l 116, 171 . The physics of

he mechanism of t h e PR i n a DML is similar t o t h a t of a OhfL where

t h e second mode i s used a s a *heterodyne* t o d e t e c t a beat s i g n a l

which reduces t e c h n i c a l frequency noise t o s e v e r a l orders, This

method may be defined a s

double frequency i n t r a c a v i t y spectroscopy * .

Prom [ I 1 , ^{161 it} can be seen t h a t t h e frequency d i f f e r e n c e of t h e two modes in the l a s e r with a nonlinear absorption a t ( W, -dl "&

is represented a s

where t h e second and t h i r d terms show t h e influence of t h e l i n e a r and nonlinear p a r t s in t h e d i s p e r s i o n of t h e a c t i v e medium and t h e f a c t o r of 1q2n i s r e l a t e d t o t h e second mode parameters. ~ q . ( 2 ) does n o t t a k e i n t o account small nonq,esonance a d d i t i o n s of the absorbing medium. If t h e frequency d i f f e r e n c e of t h e empty c a v i t y s a t i s f i e s t h e condition

--. - - -

(where fl ^K U t - homogeneous and Doppler w i d a s of theymedium) it is not d k f i c u l t t o prove t h a t t h e influence of t e c h n i c a l f l u c - t u a t i o n s due t o t h e addends of t h e a c t i v e medium may be neglected.

In t h i s case the frequency noises of mode beats a r e r e l a t e d only t o t h e t e c h n i c a l f l u c t u a t i o n s of frequency d i f f e r e n c e in t h e empty ca- vity. A s t h e t e c h n i c a l f l u c t u a t i o n s of 3 1 and 2 2 occur i n a c o r r e l a t e d way t h e n t h e i r c o n t r i b u t i o n t o t h e noise i s a few orders of magnit de lower t h a n i n a Om with an e x t e r n a l heterodyne. A s i s sjmwn i n PI? J t h e contribution of other n o b e sources ( i n particu- l a r a d d i t i v e noise of t h e photodetector) is s m a l l and n a t u r a l f r e - uency f l u c t u a t i o n s i n t h e mode f r e uency become a dominant n o i s e

%actore Estimates made f o r t h e m ~ e h H t h e value of t h e PR d W wld ^H. a t methane ~ e s s u r e /L--I g%rj - 1) show that a t t h e spe t r a l d n s i t y of p/atural l a s e r f r e uency f l u c t u a t i o n s

(10-7 - ^{10-5 ) Br.He- 2} t h e proposed Pechnique allows t o n g i s t e r t h e absorp on c e f f i c i e n t s f o r t h e weak l i n e s with t h e s e n s i t i v i t y

of lo-$+ cP.az-412 .

neriment. - An experimental i n v e s t i g a t i o n of narrow l?R was

*in a He-Be l a s e r with a n i n t e r n a l methane cell. The experhen- t a l a r r a ement shown i n Fig.1 consisted of t h r e e l a s e r s : a D m with a tzesssooplc l i g h t beam expander ( t h e c o u s t i o d i a e t e r d-of 30 m, t h e c e l l e- ⁼ 0.8 m, t h e a v i t y length = 1.8 m), a r e f e r a m e DYL = 3 mtu, !- = 0.7 m, 8. ^{= 1.6 rn)} with a narrow

1 0 Hz) and a laser-heterodyne one-mode operat- from t h e r e f e r e n c e DML. To reduce t h e l i n e - width of t h e l a s e r s t h e f r e q ey of t h e referanee DML was s t a b i l i e -

t h e non-resolved F ?8 methane component ( p. s 3 m Torr, ' v ? 8 0 MIz) by a broadbead two loop @C system. The AR of t h e r e f e r e m e DML worked a s t h e d i s c r i m i n a t a ~ : of t h e *slow* loop, while t h e * f a s t * loop used the FR 1173 . The frequency bandwidth of t h e combined system was about 20 kHz, The aarrow r a d i a t i o n spectrum of t h e r e f erance Dm by means of t h e l a s e r frequency coupling systems was t r a n s m i t t e d v i a t h e laser-heterodyne t o t h e DXG with t e l e s c o p i c expander.

The s t a b l e two-mode operation i n both He-Ne/CH l a s e r s was en- sured by a pabry-perot c a v i t y with two A/4 ^phase pfbtes. The pro- p e r t i e s of such a c a v i t y were i n v e s t i g a t e d i n [ 19,201 . ^{[ 20,211}

showed that when t h e a l e 8 between t h e o p t i c a l axes of t h e p l a t e s

approximates a e r o or 8 2 , then a t h ⁼ 3.39 m h Mi2 t r a n s i t i o n

a s t a b l e generation of two modes of t h e same t r a n s v e r s e s t r u c t u r e

with t h e mutually orthogonal l i n e a r p o l a r i z a t i o n s occurs. The dis-

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Fig.1. - The schematic view of t h e f u l l experimental set-up.

f a n ~ e between nwdes (& 12 ) is a d j u s t e d by the angle 8 i n t h e ran- g e 0.2 BBHz and greater. 'Phis r e a d i l y permits t o f u l f i l t h e necessary requirement (3) f o r t h e i n v e s t i g a t e d l a s e r , a s KU+, /+ 100 MHz, The d i f f e r e n c e of t h i s scheme from the one i n 1 7 7 i s t h a t t h e a c t i v e medium of t h e DBlCt with t e l e s c o p i c beam expan h e r was i n t h e weak t r a n s v e r s e magnetic f i e l d and the methane c e l l design incorpo- r a t e d l i q u i d nitrogen cooling. The r e g i s t r a t i o n scheme of t h e FB and i t s d e r i v a t i v e s i n a D U is shown i n Fig.2.

Dou bee -Mode Laser

*di2 -10%

for f r n ~ r r

Fige2. - The r e g i s t r a t i o n scheme of t h e frequency resonances and i t s d e r i v a t i v e s i n a double-mode l a s e r . The double-mode ope- r a t i o n of t h e l a s e r may be possibly determined by t h e two phase p l a t e s i n s i d e t h e c a v i t y ( s e e the text).

I

phase sensitive

detector derivative

sign aes

- w w /I -

- .

f / v

converfor -

Fig.3 g i v e s t h e spectrograms of t h e f S r s t and second d e r i v a t i v e s of t h e PR f o r t h e f methane l i n e obtained a t t h e gas p e s s m e

p- ⁼ 60 mkTorr, 'T = 77OK, t h e s a t u r a t i o n parameter of t h e separa- t e d component of BlIHPS 3,

0.4 and r a d i a t i o n f r e uency d e v i a t i o n 1500 Hz peak-to-peak(modu3ation frequency 400 8 . 3 . With t h e s e para- meters t h e HIAlKBQ of t h e detected resonances was m i n i m u m an& about

2.2 &a ( t h e spectrograms was rocessed with t h e account of t h e doublet s p l i t of each ooslgonenf of t h e MHPS due t o t h e r e c o i l ef- f e c t 3,4] . The speotrograms i n F i g 0 4 p o u t t o t h e influence of

t h e s a t u r a t i o n parameter on t h e s p e c t r a l component resolution.

nrnunE

- m p ~ i

MOD. F H P -400 Hz INTE8RATION C O N S

- f

-

-

0 fo 20 KHZ

-

pig.3. - The s i g n a l s of t h e S i r s t and second d e r i v a t i v e s of t h e frequency resonancfs) i n t h e double-mode He-Ne/CH4 laser.

The MHPS of the P2 methane l i n e is resolved*

It should be s t r e s s e d t h a t c e l l s of considerably g r e a t e r length and t r a n s v e r s e dimensions a r e used t o o b t a i n M1 signals i n t h e O U with t h e s i m i l a r r e s o l u t i o n and signal-to-noise r a t i o . To ensure the low l e v e l of t h e s a t u r a t e d power %he DBBL was operated wLth a considerable d l f f erence between i n t e n s i t i e s of t h e f i r s t and second modes. The i n t e n s i t y d i f f e r nce i n t h e modes was provided by a t r a n s v e r s e magnetic f i e l d Tfi along t h e p o l a r f r a t i o n 0 1 t h e second

$heterodyne) mode. The c a l c u l a t i o n and experiments groved t h a t owing o t h e Zeeman l i n e s p l i t of t h e g a i n curve a t 6 and 77- components t h e f i r s t mode s a t u r a t i n g t h e c e n t r e of the absorption l i n e is redu- ced while t h e second mode increases. The t h e o r e t i c a l i n t e r p r e t a t i o n of t h e behavow of t h e mode i n t e n s i t i e s i n t h e magnetic f i e l d is gi- ven i n Fig.5.

v e r i f i e d t h i s dependence. Hence t h e r e l a -

l a s e r double-mode generation can be obtained. ~t

t h e same t d e t h e n a t u r a l noise of t h e f r e q u e n y a p in t e n s i t y inc-

r e a s e n e g l i g i b l y (approximate1.y two times f o r

700) which

JOURNAL DE PHYSIQUE

i-st DERIVATIVE

~ig.4. - The same s i g n a l s a s i n Fig.3 under d i f f e r e n t s a t u r a t i o n parameters of t h e first mode (4 ) coincident with t h e absorption l i n e centre.

boubee -Mode

I.B+

-' ^-z -i- -- ^--

WB'

d i s c h d r ~ e fube

K

/

/ o ~ ~ r a t i o n

point ---

Fig.5. - The t h e o r e t i c a l de endence of t h e mode i n t e n s i t i e s i n the

double-mode He-He %ser ( = 3,39 nx m) v i a t r a n s v e r s e

magnetic f i e l d . The d i r e c t i o n 09 t h e magnetic f i e l d

coincides with t h e p o l a r b a t i o n d i r e c t i o n of t h e first

(weak) mode saturating t h e methane l i n e centre.

may be accounted by the deep s a t u r a t i o n of t h e g a i n curve by a s t r o n g

Q e n e r a t i o n a t t h e frequency c l o s e t o t h a t of the heterodyne mode

U r 2 was chosen i n a range 1-5 MHz). I n t h e case of a OW, t h e ecrease i n the s a t u r a t i o n parameter r e q u i r e s a s a r u l e a n applica- t i o n of l a r g e t e l e s c o p f c systems.

-ionat- The developed technique of t h e double h e uewy i n t r a - cav y spec roecopy based on t h e frequency d e t e c t i o n of &a s i g n a l is &am t o be highlyr s e n s i t i v e and promising i n l a s e r spectroscopy aud i n p a r t i c u l a r f o r t h e d e t e c t i o n of narrow reference lines. The a p p l i c a t i o n of frequency resonances of t h e double-mode l a s e r a s a n o t i c a l freqBency discrimator ensures small t e c h n i c a l s h i f t s high sgort-lierrn and long-term s t a b 5 , l i t i e e and a n e f f i c i e n t reduction of t h e f i e l d broadenbing. The a p p l i c a t i o n of t h e above technique i n t h e He-Ne/m l a s e r provided o p t i c a l r e f e r e n c e l i n e s of -. 2.2

width when us& the c e l l of 0.8 a length, t h e beam diameter of 30 nnn and t h e time of 1 sec f o r t h e s i g n a l i n t e g r a t i o n .

This technique i s multi-purposed and might be extended t o other types of l a s e r s far providing s u p e r s e n s i t i v e l i n e a r a s well a s non- l i n e a r l a s e r spectroscopy.

/I/ BASOV N. , ^{LETOCHOV V.,} Proceedings of t h e URSI Conference on La- ser Baeasurement (war saw, September 1968).

/ y LETOCHOV V. , CHEB(X'AYl3V V. Nonlinear Laser s ectroscopy, sprin- ger-Verlag, B e r l i n , ~ e i d e l k r g , New York, 197f

/3/ HALL J. science, &g (1978) 147.

/4/ HALL J., e t al., Phys. Rev. Lett., 37 (1976) 1339.

/5/ CHEBCTAYEV V., Proc. of t h e 2nd Freq. Standards and l e t o l . Sym- positurn, J u l y 1976, Copper Mountain, USA (1976) 385.

/ q BARGER R., Apple Phys.Lett., l4- (1979) 850.

/7/ BORDE C . , e t a l . Proc. of t h e I V Intern. Conf. on t h e Laser Spectrocsopy, Rottach-Eggern, BBD June 1979.

/8/ WIEW C. , GSCH ^{TO, f} hys. Rev. Lett. , ³⁶ (1976) 1170.

/9/ BJORQLUND G., Optics L e t t e r s & (1980) 15.

/10/ RAY B . e t al., phys. B ~ V . ~ a t t . , (1980) 1251.

/11/ LAblB We, Pbys. 8 e r r r &(%64) A%9*

/I 2/ LETOCHOV V., J e t p h Lett. u 1 9 7 7 ) 593.

/13/ KOSCHELJAEVSW N. e t al., Quantum E l e c t r o n i c s ( s o v i e t ) ,

1 (1974) 516.

/14/ JlRmR G. e t a l e , 2.f. Naturforsch., (1975) 1128.

/15/ BAGBEV S. e t al., ~ g p l . ~ h ~ s . 9 a (1976) 231.

/16/ VASKOV V. e t al., Quantum ~ l e ~ t r ~ n i ~ s (soviet), N 2 (14) (1973) 107.

/17/ BASOV El. e t al., i n Proc. o f t t h e 11 Intern. Conf. on p r e c i s i o n Measur. and Pundam. Constants, Gaithersburg, 118aryland, USA, 8-1 2 J une '1981.

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