USE OF A CO2 LASER AS A LOCAL OSCILLATOR FOR A 10 MICRON HETERODYNE DETECTION

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Submitted on 1 Jan 1987

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USE OF A CO2 LASER AS A LOCAL OSCILLATOR FOR A 10 MICRON HETERODYNE DETECTION

A. Delahaigue, D. Courtois, C. Thiebeaux, H. Le Corre

To cite this version:

A. Delahaigue, D. Courtois, C. Thiebeaux, H. Le Corre. USE OF A CO2 LASER AS A LOCAL

OSCILLATOR FOR A 10 MICRON HETERODYNE DETECTION. Journal de Physique Colloques,

1987, 48 (C7), pp.C7-569-C7-571. �10.1051/jphyscol:19877135�. �jpa-00226955�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6men-t au n012, Tome 48, dkcembre 1987

USE OF A CO, LASER AS A LOCAL OSCILLATOR FOR A 10 MICRON HETERODYNE DETECTION

A. DELAHAIGUE, D. COURTOIS, C. THIEBEAUX and H. LE CORRE Laboratoire de Physique Mol&culaire,

CNRS UA-776,

Facult6 des Sciences de Reims, BP

3 4 7 , F - 5 1 0 6 2

Reims Cedex, France

A b s t r a c t : We d e s c r i b e i n t h i s p a p e r a laser h e t e r o d y n e spectrometer f o r t h e 10 m i c r o n r e g i o n ( I R ) . A cw C 0 2 l a s e r i s u s e d as a local o s c i l l a t o r . Labo- r a t o r y a n d atmospheric ozone s p e c t r a a r e p r e s e n t e d .

In t h e seventies, t h e h e t e r o d y n e d e t e c t i o n h a s been d e v e l o p p e d in t h e 10 m i c r o n r e g i o n ( r e f . 1 ) . T h i s h i g h r e s o l u t i o n spectroscopic t e c h n i q u e h a s been f i r s t u s e d f o r astronomical p u r p o s e s .

Usually, i n conventi:onal spectroscopy, t h e I R radiation, a f t e r o p t i c a l analysis, i s focused o n t h e detector. In t h i s case, t h e most i m p o r t a n t noise i s d u e t o t h e b a c k g r o u n d .

T h e laser h e t e r o d y n e s p e c t r o m e t r y i s a n a l t e r n a t e t e c h n i q u e w h i c h allows t o g a i n several o r d e r s o f m a g n i t u d e f o r t h e s i g n a l l n o i s e r a t i o . In t h i s technique, t h e I R r a d i a t i o n t o b e analysed i s superposed w i t h t h a t o f a

C 0 2

l a s e r (local o s c i l l a t o r ) ( r e f . 2 ) .

T h e t w o waves a r e focused o n a f a s t p h o t o d i o d e ( H g C d T e ) u s e d as a n o p t i c a l m i x e r . Within t h e d e t e c t o r b a n d w i d t h B, t h e I R s i g n a l i s t r a n s p o s e d in a r a d i o f r e q u e n c y ( R F ) signal. A l l t h e I R s i g n a l informations, w i t h i n a b a n d w i d t h ? B o n b o t h sides o f t h e I R local o s c i l l a t o r f r e q u e n c y , a r e i n c l u d e d i n t h i s R F signal.

T h e p o w e r s i g n a l l noise r a t i o a f t e r t h e o p t i c a l d e t e c t o r i s i n t r i n s i c a l l y low.

T h e RF signal i s t h e n a n a l y s e d w i t h a R F spectrometer w h i c h r e s o l u t i o n i s b = 5 MHz. T h i s spectrometer has been b u i l t i n t h e l a b o r a t o r y . It c o n s i s t s o f a 5 MHz f i l t e r a n d a second f r e q u e n c y c o n v e r s i o n . T h e l a t t e r uses a micro- c o m p u t e r c o n t r o l l e d Y l G f r e q u e n c y g e n e r a t o r ( r e f . 3 ) . A q u a d r a t i c R F detection, a n d a p o s t i n t e g r a t i o n w i t h time c o n s t a n t T i s t h e n achieved ( f i g u r e 1 ) .

F i g u r e 1 : laser h e t e r o d y n e spectrometer diagram.

thermal source

x

HgCdTe photodiode Optical

mixer

local oscillator C02 laser Absorber

( 03)

B

,

/ / /

Integration

b T

-

^{quadratic RF}

detection R F

filter

- -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877135

C7-570 JOURNAL DE PHYSIQUE

T h e f i n a l v o l t a g e s i g n a l l n o i s e r a t i o ( S 1 N ) P m u l t i p l i e d b y 6 . T h i s f a c t o r enhances t h e f i n a l S I N r a t i o .

I n o u r e x p e r i m e n t a l a r r a n g e m e n t we h a v e t h e f o l l o w i n g parameters:

B = 1 GHz, b = 5 MHz, T

=

5 s , N e r n s t t e m p e r a t u r e = 1300 K, A = 20 micron, 17 = d e t e c t o r e f f i c i e n c y = 0,2 a n d w i t h t h e s e values we o b t a i n : (S1N)P

=

0,16

( S 1 N ) V

=

800.

T h e f i g u r e 2 r e p r e s e n t s a l a b o r a t o r y ozone s p e c t r u m r e c o r d e d w i t h t h e p r e v i o u s c o n d i t i o n s . A d e g r a d a t i o n f a c t o r o f a b o u t '10 a p p e a r s a t low frequencies.

T h i s d e g r a d a t i o n f a c t o r increases w i t h t h e f r e q u e n c y d i f f e r e n c e . T h e reason i s t h e decrease o f t h e d e t e c t o r r e s p o n s i t i v i t y w i t h t h e f r e q u e n c y ( r e f . 4 ) .

T h e f i g u r e 3 r e p r e s e n t s a n atmospheric ozone s p e c t r u m r e c o r d e d i n m a r c h 1987 in RElMS (FRANCE) u s i n g a s u n t r a c k e r .

These f i r s t r e s u l t s p r o v e t h a t t h e h e t e r o d y n e l a s e r spectrometer i s a r a - t h e r g o o d i n s t r u m e n t f o r l a b o r a t o r y a n d atmospheric s t u d i e s .

I n t h e f u t u r e , we i n t e n d t o m o n i t o r t h e ozone q u a n t i t y in t h e atmosphere.

References

1

-

T.G. BLANEY

Space Science Reviews 17 (1975) pp 691 - 702.

2 - C . THIEBEAUX, A . DELAHAIGUE, D. COURTOIS a n d P. JOUVE, I n f r a r e d P h y s i c s 21 (1981) pp 41 - 44.

3 - D. COURTOIS, C . T H I E B E A U X a n d A . DELAHAIGUE,

I n t e r n a t i o n a l J o u r n a l o f i n f r a r e d a n d millimeter waves ( 5 - 2 ( 1 984) pp 185 - 197).

4 - C. THIEBEAUX,

These d e D o c t o r a t dlEtat ( 1987) U n i v . Reims ( F r a n c e ) .

I ^B

FDlSEl DF5 TO5 Dl91 1 1 SEPT 86

OZONE

100 ^A

A A A

A

A

-

- 3 k

A A

0

-

^{L1 .L}

m 4 C

g 2

PAIES ECARTS NOMERE? D'ONDE ^A TRANSITION (MHz (cm 1

F i g u r e 2 : L a b o r a t o r y ozone s p e c t r u m n e a r 9P20 C 0 2 laser l i n e

No Raie Ecart Nombres d'onde Transition 3 +425 MHz 1043.1774 cmml 4 3 (7-1-6)-(7-1-7) 4 -575 MHz 1043.1441 em-' 31 (15-7-9)-(16-8-8) 7 -920 MHz 1043.1326 cm-I 43+&!-$2 (30-7-24)-(29-7-23)

Figure 3 : Atmospheric ozone spectrum near 9P24 C 0 2 laser line