Physical Studies of Optically Pumped Dimer Lasers - American

32 Physical Studies of Optically Pumped Dimer Lasers C . N . M A N - P I C H O T and A .

BRILLET

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch032

Laboratoire de l'Horloge Atomique, Equipe de Recherche du C N R S , Associée à l'Université Paris-Sud, Bât. 221 - Université Paris-Sud, 91405-Orsay, France

We report on saturated absorption experiments in Na , realized with a tunable and stabilized argon laser. These experiments provide both spectrosco pic and physical results, which help in understand ing the behavior of o p t i c a l l y pumped a l k a l i dimer lasers. We briefly describe a new double resonance experiment which enables us to study the gain l i n e shapes of the dimer laser and to demonstrate the backward-forward gain competition. 2

Since 1977, optical pumping of dimer molecules such as I , B i and Te and especially a l k a l i metal dimers L i , Na and K , has provided us with a new family of v i s i b l e and near infrared CW lasers (1). The usual pumping scheme is the following: dimer molecules are pumped by an argon or krypton laser from a low l y ing rovibrational level of the X ground electronic state (0) to one of the A or B state levels (1), which may result in a popula tion inversion between this A or B level and high lying X state levels (2). 2

2

2

2

2

These s y s t e m s a r e i n t e r e s t i n g f r o m d i f f e r e n t v i e w p o i n t s : - f o r the s p e c t r o s c o p i s t , because they a l l o w a very p r e c i s e d e t e r m i n a t i o n of the t r a n s i t i o n wavelengths, r e s u l t i n g i n i m p r o v e d m o l e c u l a r c o n s t a n t s , and a l s o b e c a u s e i t becomes p o s s i b l e t o s t u d y h i g h e r e l e c t r o n i c l e v e l s , connected w i t h l e v e l 1 or 2 ( 2 j . - f o r the l a s e r s c i e n t i s t , dimer l a s e r s are 3 l e v e l s y s t e m s i n t e r a c t i n g w i t h two n e a r l y r e s o n a n t l a s e r fields. They p r o v i d e an e x c e l l e n t o p p o r t u n i t y f o r t h e e x p e r i m e n t a l s t u d y o f a number o f r e l a t e d e f f e c t s , s u c h as t h e d y n a m i c S t a r k e f f e c t , t h e Raman e f f e c t , multiphoton e f f e c t s r e s u l t i n g in gain w i t h o u t p o p u l a t i o n i n v e r s i o n , g a i n a n i s o t r o p y , e t c . (1_). - these l a s e r s are i n t e r e s t i n g i n themselves because o f t h e i r l a r g e number o f l i n e s , h i g h e f f i c i e n c y and e a s e o f s i n g l e f r e q u e n c y o p e r a t i o n . In p a r t i c u l a r , 0097-6156/82/0179-0487$05.00/0 ©

1982

A m e r i c a n Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2

METAL

488

BONDING A N D INTERACTIONS

t h e y seem t o be w e l l s u i t e d f o r a p p l i c a t i o n s s u c h as o p t i c a l frequency (or wavelength) standards s i n c e the a c t i v e medium i s a q u i e t v a p o r , w h i c h s h o u l d r e s u l t i n a v e r y n a r r o w e m i s s i o n l i n e w i d t h (_3). In t h i s p a p e r , we r e p o r t on two e x p e r i m e n t s w h i c h e n a b l e d us t o i m p r o v e o u r u n d e r s t a n d i n g o f N a and L i lasers. In p a r t i c u l a r , we o b t a i n e d a s s i g n m e n t o f t h e l e v e l s i n v o l v e d i n t h e l a s e r c y c l e s , and e v a l u a t i o n o f t h e i r l i f e t i m e s , r e l a x a t i o n r a t e s and m e c h a n i s m s , and t r a n s i t i o n d i p o l e moments. We w i l l a l s o g i v e p r e l i m i n a r y r e s u l t s on t h e e x p e r i m e n t a l s t u d y o f g a i n l i n e s h a p e s , w h i c h w i l l u l t i m a t e l y be compared w i t h a t h e o r e t i c a l m o d e l . 7

2


Assignment

o f Na, and L i

9

2

L a s e r and Pump

Lines

In a f i r s t s t e p t o u n d e r s t a n d t h e p h y s i c s o f t h e s e d i m e r l a s e r s , we measured p r e c i s e l y ( 1 0 ~ A o r b e t t e r ) and s i m u l t a n e o u s l y t h e d i m e r l a s e r and t h e a r g o n pump l a s e r w a v e l e n g t h s , u s i n g o u r lambdameter ( 4 ) . T h i s l e d t o t h e unambiguous a s s i g n m e n t o f most l a s e r l i n e s , many o f them b e i n g o b s e r v e d f o r t & e f i r s t t i m e ( 5 ) . In p a r t i c u l a r , when pumping L i w i t h t h e 4966 A a r g o n l a s e r l i n g , we c o u l d o b s e r v e a p e c u l i a r l a s e r o s c i l l a t i o n a t 5049 and 5133 A , w h i c h can be u n d e r s t o o d o n l y as p u r e Raman l i n e s , f o r t h e f o l l o w ing reasons: i ) t h e e n e r g y d i f f e r e n c e (324 c m " ) between t h e pump l i n e and t h e 5049 l i n e i s t o o s m a l l t o a l l o w a p o p u l a t i o n i n v e r s i o n between l e v e l s (1) and ( 2 ) ; i i ) t h e r e was no m e a s u r a b l e a b s o r p t i o n o f t h e pump l a s e r ; i i i ) t h e most l i k e l y a s s i g n m e n t , t a k i n g i n t o account the Franck-Condon f a c t o r s , i s Q(10)3-3 f o r t h e pump l i n e and Q ( 1 0 ) 3 - 4 and Q ( 1 0 ) 3 - 5 f o r t h e e m i s s i o n l i n e s . This corresponds to a detuning of 1 cm" (or 6 Doppler widths) f o r b o t h t h e pump and l a s e r l i n e s . T h i s Raman g a i n i s i n t e r e s t i n g , s i n c e i t e x t e n d s t h e t u n i n g r a n g e o f o p t i c a l l y pumped l a s e r s , p r o v i d e d t h e pump l a s e r i t s e l f i s t u n a b l e and p o w e r f u l enough. The i d e n t i f i c a t i o n work has been c o m p l e t e d r e c e n t l y by s a t u rated absorption experiments. A l l t h e c o i n c i d e n c e s between t h e a r g o n l a s e r l i n e s and N a . and L i a b s o r p t i o n l i n e s have been mea s u r e d w i t h an a c c u r a c y o f 0 . 0 0 2 c m " and i d e n t i f i e d i n t e r m s o f the B ^ y - X ^ g systems u s i n g the s e t o f m o l e c u l a r c o n s t a n t s g i v e n by Kusch and H e s s e l (6) and H e s s e l and V i d a l (7), f o r N a and L i , respectively. The r e s u l t s a r e shown i n T a b l e I. We have i n c l u d e d some t r a n s i t i o n s w h i c h were n o t measured i n s a t u r a t e d a b s o r p t i o n because t h e i r l i n e c e n t e r i s out o f the argon l a s e r t u n i n g r a n g e , b u t w h i c h we c o u l d n e v e r t h e l e s s l o c a l i z e and i d e n t i f y by o b s e r v i n g t h e i r ( b r o a d e r ) l i n e a r a b s o r p t i o n and b e c a u s e t h e i r pumping r e s u l t s i n d i m e r l a s e r e m i s s i o n . 2

2

1

1

2

1

2

7

2

Saturated Absorption

Experiments

The s a t u r a t e d a b s o r p t i o n e x p e r i m e n t s were used n o t o n l y t h e i d e n t i f i c a t i o n work d e s c r i b e d a b o v e , b u t a l s o f o r t h e


for

32.

Optically

MAN-PICHOT AND BRILLET

Pumped

Dimer

Lasers

489

T a b l e I. C o i n c i d e n c e s between t r a n s i t i o n s i n t h e B n - X Z g S y s tems o f N a and L i and a r g o n l a s e r l i n e s . Dimer l a s e r e m i s s i o n has been o b s e r v e d f r o m a l l t h e u n d e r l i n e d t r a n s i t i o n s . The a c c u r a c y i s 5 x 10" * A f o r a l l r e s o l v e d l i n e s . A i s the d i f f e r ence i n A between measured vacuum w a v e l e n g t h and t h e c a l c u l a t e d w a v e l e n g t h f r o m t h e c o n s t a n t s o f R e f e r e n c e s 6 and 7. 1

1

u

7

2

2

1

Na

7 2

Measurement

Li

2

a

Measurement

Transition R(8)9-2 Q(5)9-2

.0042 .0015

A

0(43)28-7

.0008

4580.6456 4580.6600

R(37)9-1 fR(9)13-4 '\q(63)18-6 (0(36)12-3 lQ(45)17-6

.0012 .0043 .0090 .0025 .0017

-4728 4728.2258

R(31)8-3 P(62)8-l

.0192

4766.1824 4766.1871 4766.1920 4766.1965 4766.2142

0(98)9-0 0(90)25-9 P(48)17-7 P(13 10-3 P(28)6-0

.0046 .0077 .0070 .0095 .0071

4766.1753 4766.1757 4766.1957 4766.2115 4766.2258

R(44)8-3 R(60)8-2 0(24)4-1 P(63)14-3 P(62)9-2

.0040 .0040 .0048 .0138 .0022

4881.2054 4881.2136 4881.2173 4881.2225 4881.2351 4881.2433

R(41)10-6 R(T28T23-8 Q(74)16-9 P(99)15-7 R(8)25-16 Q(43)6-3

.0018 .0026 .0010 .0004 .0085 .0083

4881.2052 4881.2082 4881.2452

0(55)14-6 Q(36)12-7 R(30)2-l

.0021 .0176 .0036

4966.4517 4966.4557 4966.4709 4966.4857

P(29)8-7 P(44)7-6 R(13)19-15 R(84)10-7

.0088 .0095 .0070 .0070

4966.4576 4966.4775 4966.4979

Q(47)0-0 R(78)12-3 P(70)9-4

.0210 .0044 .0023

5146.7217 5146.7293 5146.7353 5146.7441 5146.7583 5146.7656

R(151)0-2 P(65)9-12 0(49)11-4 R(73)13-15 R(115)7-9 P(76)3-7

.0008 .0075 .0070 .0057 .0027 .0028

4580.6468


Transition

a A

4728.1682 4728.1820

b

4728.1985

t

D i f f e r e n c e between t h e measured w a v e l e n g t h and t h e w a v e l e n g t h k computed f r o m ( 6 ) . Unresolved t r a n s i t i o n s . Line c e n t e r f a l l s out o f t h e argon t u n i n g range.


490

METAL

BONDING

AND

INTERACTIONS


e v a l u a t i o n o f t h e l i n e w i d t h , t h e r e l a x a t i o n r a t e s and o t h e r p r o c e s s e s i n v o l v i n g t h e l e v e l s ( 1 ) , and o f t h e t r a n s i t i o n d i p o l e mo ments o f t h e ( 0 ) - ( l ) t r a n s i t i o n s . I t i s i m p o r t a n t t o know a l l t h e s e p a r a m e t e r s i n o r d e r t o c o n s t r u c t a d e t a i l e d t h e o r e t i c a l mo del f o r the dimer l a s e r . Experimental Techniques. A block diagram of the experimen t a l s e t - u p used f o r s a t u r a t e d a b s o r p t i o n e x p e r i m e n t s i s shown i n F i g u r e 1. The a r g o n l a s e r i s a c o m m e r c i a l 4W t u b e i n a home made cavity. T h i s c a v i t y i s made o f t h r e e I n v a r r o d s , d e c o u p l e d f r o m the tube i n o r d e r t o a v o i d v i b r a t i o n s . L i n e s e l e c t i o n i s made w i t h a p r i s m , and s i n g l e f r e q u e n c y o p e r a t i o n i s o b t a i n e d w i t h a M i c h e l son i n t e r f e r o m e t e r . The l a s e r can be f r e q u e n c y l o c k e d t o a s t a b l e F a b r y - P e r o t r e s o n a t o r w i t h a d o u b l e s e r v o - l o o p a c t i n g on a f a s t PZT f o r l i n e n a r r o w i n g and on a g a l v o - p l a t e f o r w i d e t u n a bility. T h i s r e s u l t s i n a l i n e w i d t h o f l e s s t h a n 10 KHz and a c o n t i n u o u s t u n a b i l i t y o f 6 GHz. The a r g o n l a s e r beam i s s p l i t i n t o a s t r o n g s a t u r a t i n g beam and two w e a k e r p r o b e beams, c o u n t e r p r o p o g a t i n g i n a r i n g shaped s a t u r a t e d a b s o r p t i o n i n t e r f e r o m e t e r c o n t a i n i n g t h e Na o r L i h e a t pipe. The s a t u r a t i n g beam i s a m p l i t u d e - m o d u l a t e d a t 10 KHz w i t h an a c o u s t o - o p t i c m o d u l a t o r , and t h e d i f f e r e n c e between t h e 2 p r o b e beams i s s y n c h r o n o u s l y d e t e c t e d . Beam g e o m e t r i e s a r e c a r e f u l l y matched and c o n t r o l l e d , s i n c e t h e knowledge o f t h e s e g e o metries i s necessary i n order to understand q u a n t i t a t i v e l y the power b r o a d e n i n g c u r v e s . W a v e l e n g t h c a l i b r a t i o n i s p r o v i d e d by the lambdameter. Results. By v a r y i n g t h e t e m p e r a t u r e and t h e p r e s s u r e i n s i d e t h e h e a t - p i p e , we c o u l d measure t h e b r o a d e n i n g r a t e s o f t h e t r a n s i t i o n s due t o M - M o r M - A c o l l i s i o n s (M = N a , L i ) , s i n c e we used a r g o n as t h e b u f f e r gas i n b o t h h e a t p i p e s ( F i g u r e 2 ) . In t h e w o r k i n g r a n g e o f 1-10 T o r r , t h e e f f e c t o f v e l o c i t y c h a n g i n g c o l l i s i o n s was p r e d o m i n a n t as we c o u l d v e r i f y b o t h f r o m t h e n o n l i n e a r p r e s s u r e b r o a d e n i n g r a t e above 10 T o r r and f r o m t h e shape o f t h e c o l l i s i o n b r o a d e n e d s i g n a l s ( L o r e n t z i a n c u r v e above a Gaussian background). The power b r o a d e n i n g c u r v e s a l l o w one t o d e t e r m i n e t h e s a turation intensity: 2

2

2

2

(where y i s t h e n a t u r a l l i n e w i d t h ) and t h e n t o e v a l u a t e t h e d i p o l e moment p o f t h e t r a n s i t i o n . The d e t e r m i n a t i o n o f 1$ i s n o t e x a c t l y s t r a i g h t f o r w a r d , s i n c e t h e l a s e r beams a r e G a u s s i a n beams and n o t p l a n e w a v e s . T h e r e i s no s i m p l e a n a l y t i c a l f o r m u l a g i v ing the saturated absorption lineshape a t high i n t e n s i t y i n a G a u s s i a n beam, b u t one can o b t a i n n u m e r i c a l r e s u l t s . Figure 3 shows a c o m p a r i s o n o f e x p e r i m e n t a l r e s u l t s w i t h t h e t h e o r e t i c a l model o f Salomon ( 8 ) . By e x t r a p o l a t i n g t h e b r o a d e n i n g c u r v e s t o z e r o power and



Optically

Pumped

Dimer

Lasers


32.


491


492

METAL

Figure 2.

BONDING A N D INTERACTIONS

Pressure broadening rates of the P(99)15-7 Na line. Key:(M) collision; and (•) Na -Ar collision. 2

Na -Na 2

2

Figure 3. Power broadening of P(99)15-7 Na line, fitted with the theory (8) (rate equation approximation, Gaussian beams). 5 = 7 corresponds to an Ar laser intensity of 54 mW'/mm . 2

2


32.


Optically

Pumped

Dimer

493

Lasers

z e r o p r e s s u r e , one c a n g e t a v a l u e o f t h e n a t u r a l l i n e w i d t h o f t h e t r a n s i t i o n w h i c h i n t h i s c a s e s h o u l d be t h e n a t u r a l l i n e width o f level (1). As an e x a m p l e , T a b l e I I shows a summary o f t h e r e s u l t s o b t a i n e d f o r t h e P ( 9 9 ) 1 5 - 7 t r a n s i t i o n i n N a and t h e Q ( 4 7 ) 0 - 0 t r a n sition inL i . 2

2

Table II. L i n e broadening parameters f o r the f o l l o w i n g B ^ - X ^ i transitions: P ( 9 9 ) o f t h e ( 1 5 - 7 ) band o f N a , ; and Q ( 4 7 ) o f t h e ( 0 - 0 ) band o f L i . 7

2

M =


Collisional Braodening R a t e (FWHM) (MHz/Torr)

.. ,, " " ., . 2~ 2

N

n

A

Natural L i n e w i d t h , y (MHz) Transition Dipole Moment, y (ampsecond-meter)

Li Q(47)0-0

Na, P(99)15-7

2

Transition

(9.1

2

25.0

18.6

19.4

30

18

28

± 4) x 1 0 "

3 0

( 2 . 3 ± 1) x 1 0 "

2 9

The c o l l i s i o n a l b r o a d e n i n g r a t e s a r e i n good agreement w i t h t h o s e o b s e r v e d i n N a by T s a i ( 9 ) . The n a t u r a l l i n e w i d t h s a r e l a r g e r t h a n what one w o u l d e x p e c t f r o m t h e l i f e t i m e measurements o f D e m t r o d e r e t al_. ( 1 0 ) . The a d d i t i o n a l b r o a d e n i n g (7 MHz f o r N a and 20 MHz f o r Li^T c o u l d r e s u l t f r o m an u n r e s o l v e d h y p e r f i n e s t r u c t u r e o r f r o m t h e Zeeman e f f e c t p r o d u c e d by t h e oven h e a t i n g wire. 2

2

Gain Lineshape

Measurements

We a r e p r e s e n t l y p e r f o r m i n g g a i n measurements i n N a . The experimental set-up i s close t o the saturated absorption set-up, e x c e p t t h a t t h e p r o b e beams a r e r e p l a c e d by two beams f r o m a t u n a b l e dye l a s e r p a r a l l e l t o t h e a r g o n beam, one o f them p r o p a g a t i n g i n t h e same d i r e c t i o n ( f o r w a r d ) and t h e o t h e r one i n t h e opposite d i r e c t i o n (backward). W i t h t h i s s e t - u p , we c a n measure t h e g a i n l i n e s h a p e s i n b o t h d i r e c t i o n s , when t h e dye l a s e r i s tuned across t h e ( l ) - ( 2 ) t r a n s i t i o n s . F i g u r e 4 shows an i n t e r e s t i n g example o f t h e s e l i n e s h a p e s , i n a c a s e where t h e a r g o n l a s e r i s c l o s e t o r e s o n a n c e on t h e ( 0 ) - ( l ) t r a n s i t i o n . As e x p e c t e d , t h e g a i n l i n e s h a p e i s b r o a d e r and w e a k e r i n t h e b a c k w a r d d i rection. F u r t h e r m o r e , we o b s e r v e an a d d i t i o n a l f e a t u r e : as a r e s u l t o f t h e c o m p e t i t i o n between f o r w a r d and backward w a v e s , t h e b a c k w a r d g a i n p r o f i l e shows a h o l e i n t h e p r e s e n c e o f f o r w a r d 2



494

METAL BONDING AND INTERACTIONS

F R E Q U E N C Y (GHz) Figure 4. Gain lineshapes on P(43) 10-20 in Na probed by dye laser at 5389 A showing the hole in backward gain resulting from forward-backward competition. Key: , backward; and , forward. 2


32.


Optically

Pumped

Dimer

Lasers

495

amplification. T h i s e f f e c t e x p l a i n s why one n e v e r c o u l d o b s e r v e b a c k w a r d l a s e r o s c i l l a t i o n , even t h o u g h t h e b a c k w a r d g a i n may be l a r g e r than the c a v i t y l o s s e s . F u r t h e r e x p e r i m e n t s a r e underway w i t h t h i s s e t - u p , i n o r d e r t o s t u d y t h e Raman g a i n l i n e s h a p e s and t h e i r s a t u r a t i o n b e h a v i o r , and t o compare t h e s e r e s u l t s w i t h a t h e o r e t i c a l m o d e l .

Literature Cited


1. 2.

Wellegehausen, B. IEEE J. of Quant. Elec. 1979, QE-15, 1108. Carlson, N. W.; Kowalski, F. V . ; Teets, R. E.; Schawlow, A. L. Opt. Commun. 1979, 29, 302. 3. Drullinger, R.; Brillet, A.; Dallarosa, J.; H a l l , J . L. J . Opt. Soc. Am. 1978, 68, 635. 4. Cachenaut, J.; Man, C.; Cerez, P . ; Brillet, A.; Stoeckel, F . ; Jourdan, A . ; Hartmann, F. Revue de Physique Appliquée, 1979, 14, 685. 5. Man-Pichot, C.; Brillet, A. IEEE J. of Quant. Elec. 1980, QE-16, 1103. 6. Kusch, P . ; Hessel M. M. J. Chem. Phys. 1978, 68, 2591. 7. Hessel, M. M . ; V i d a l , C. R. J. Chem. Phys. 1979, 70, 4439. 8. Salomon, C. Thèse de 3e Cycle, 1978, Paris X I I I . 9. T s a i , J . Thesis, 1980, Stanford. 10. Demtröder, W.; Stetzenbach, W.; Stock, M . ; Witt, J . J . Mol. Spectrosc. 1976, 61, 382. RECEIVED August 26,

1981.


Physical Studies of Optically Pumped Dimer Lasers - American

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