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LASER-GAS SPECTROPHONE MEASUREMENTS WITH A FIBER-OPTIC INTERFEROMETER
D. Leslie, R. Miles, A. Dandridge
To cite this version:
D. Leslie, R. Miles, A. Dandridge. LASER-GAS SPECTROPHONE MEASUREMENTS WITH A
FIBER-OPTIC INTERFEROMETER. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-537-C6-
540. �10.1051/jphyscol:1983688�. �jpa-00223247�
LASER-GAS SPECTROPHONE MEASUREMENTS WITH A FIBER-OPTIC INTERFEROMETER D.H. Leslie, R.O. Miles and A. Dandridge
Optical Sciences ~ i v i s i o n , Naval Research Laboratory, Washington, D. C. 20375, U. S . A.
Resume.- Un i n t e r f e r o m e t r e
a
f i b r e o p t i q u e e s t u t i l i s e comme c a p t e u r acous- t i q u e dans un spectrophonea
gaz. Une d e t e c t i v i t e de 1,6 x cm-"W/JFii a @ t & demontree.A b s t r a c t . - A f i b e r - o p t i c i n t e r f e r o m e t e r i s t h e a c o u s t i c sensor i n a gas spectrophone. A minimum d e t e c t a b l e a b s o r p t i o n c a p a b i l i t y o f
1.6 x c m - ' - ~ / f i has been demonstrated.
I n t r o d u c t i o n
We d e s c r i b e t h e performance o f a unique f i b e r - o p t i c spectrophone i n w h i c h t h e a c o u s t i c t r a n s d u c e r i s one arm o f an a l l - f i b e r i n t e r f e r o m e t e r . A minimum d e t e c t a b l e a b s o r p t i o n c a p a b i l i t y o f 1.6 x cm-I
-wh/Fif-
has been demonstrated, a l t h o u g h a background s i g n a l about 30dBV above t h i s l e v e l h i n d e r e d measurements. The d e v i c e d e s c r i b e d here i s c o n s i d e r a b l y more s e n s i t i v e t h a n t h e f i r s t f i b e r - o p t i c spectrophone p r e v i o u s l y r e p o r t e d ( I ) , and improvements i n background compensation and a d d i t i o n a l i n t e r f e r o m e t e r l e n g t h c o u l d i n c r e a s e t h e u l t i m a t e s e n s i t i v i t y even more. An a d d i t i o n a l f e a t u r e of t h i s sensor i s i t s l a r g e s e n s i t i v e area, m d e p o s s i b l e by t h e i nherent geometric f l e x i b i l i t y p r o v i d e d by t h e o p t i c a l f i b e r s . T h i s l a r g e a r e a f e a t u r e my prove u s e f u l f o r p h o t o a c o u s t i c measurements o f aerosol a b s o r p t i o n , where l a r g e sensor area i s advantageous. A t comparables e n s i t i v i t y t h e f i b e r o p t i c spectrophone o f f e r s geometric design f l e x i b i l i t y n o t a v a i l a b l e w i t h e i t h e r microphone o r l a s e r - b e a m - d e f l e c t i o n techniques.
D e v i c e C o n s t r u c t i o n and O p e r a t i o n
The s e n s i n g element was formed by w i n d i n g 20 meters o f single-mode f i b e r around a 2.54 cm d i a m e t e r h o l l o w core formed by wrapping a 3-mi 1 sheet o f a l u m i n i z e d n y l a r i n t o a c y l i n d e r . The n y l a r had been r u n t h r o u g h a computer c a r d punch t o f o r m 400 r e g u l a r l y spaced holes. T h i s loosly-wound
c o n f i g u r a t i o n proved t o have rmch b e t t e r h i g h - f requency response (>1 kHz) t h a n t h e p r e v i o u s d e v i c e ( 1 ) which had been bound t o g e t h e r w i t h
s i l i c o n - r u b b e r adhesive. The 10 cm l o n g by 2.5 cm diam c y l i n d e r was l o o s l y p l a c e d i n t o a 12 cm l o n g by 3 cm diam chamber w i t h i n a s p l i t brass c y l i n d e r 24 cm l o n g by 7 cm diameter. Two o p t i c a l e n t r a n c e channels 5 cm l o n g by 1.3 cm diam l e d i n t o and o u t o f t h e i n n e r chamber, as shown i n F i g
1.
The f i b e r ends were fed t h r o u g h s m a l l h o l e s bored i n t o t h e brass, t h e h o l e s f i l l e d w i t h epoxy, and t h e f i b e r ends f u s e d i n t o t h e i n t e r f e r o m e t e r . A gas i n l e t v a l v e a l l o w e d pumping and f i l l i n g o f t h e chamber. The d e v i c e was s e a l e d w i t h CaF2 windows and O-rings. I n a d d i t i o n t o t h e a c o u s t i c i s o l a t i o np r o v i d e d by t h e m s s i v e brass s h e l l , t h e e n t i r e device was operated w i t h i n a 1 ead and f o m-1 i n e d wooden box.
A 0.5 rn gas m i x i n g and pumping s t a t i o n was used t o mix room a i r w i t h
3
CHq i n m i x t u r e s r a n g i n g f r o m lOOppm t o 1 atm methane. Laser a b s o r p t i o n measurements were performed w i t h an unfocussed 3.39 um HeNe l a s e r which p r o v i d e d 10 ni4 o f average power a f t e r b e i n g modulated by an o p t i c a l chopper.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983688
JOURNAL DE PHYSIQUE
,FIBER-OPTIC COIL 3.3gprn HeNe
-1-
-LASER
0 . 6 3 2 8 p ~ N e
-
COUPLER - COUPLERLASER COMPENSATOR
-
DETECTORS SPECTRUM ANALYSER
FIGURE 1 Fiber Optic Spectrophone
FIGURE 2 Sample.Data. The Lower Trace Recorded With Beam Blocked.
" ' I ' " I
' " I 1
10-2 10-1
ABSORPTION COEFFICIENT a (cm-I)
FIGURE 3 Iteasured Photoacoustic Signal vs Absorption Coefficient.
t h e chopping frequency was t u n e d t o t h e lowest sub-chamber l o n g i t u d i n a l resonance a t about 1550 Hz. We observed a resonance Q
-
100. The resonance frequency was observed t o i n c r e a s e s l i g h t l y a t t h e r i c h e r methane mixtures, as expected f r o m t h e h i g h e r speed o f sound i n methane vs a i r .The f i b e r ends f r o m t h e c y l i n d e r were f u s e d i n t o one arm o f an a l l - f i b e r Mach-Zehnder i n t e r f e r o m e t e r . A s t a b l e , 1
mW,
s i n g l e mode, 0.6328 urn HeNe l a s e r was coupled t o t h e i n t e r f e r o m e t e r . I n t h e h i g h - g a i n a c o u s t i c sensor c o n f i g u r a t i on an a c t i v e feedback scheme p r o v i d s mi c r o r a d i an i n t e r f e r o m e t e r - s e n s i t i v i t y and a l a r g e dynamic range a t f r e q u e n c i e s f r o m 1 Hz t o 5 kHz ( r e f 3 and r e f e r e n c e s c o n t a i n e d t h e r i n ) . The measured n o i s e l e v e l i n t h ei n t e r f e r o m e t e r was 2 u r a d m a t 1 kHz. The i n t e r f e r o m e t e r response was u n i f o r m between 10 Hz and 5 kHz. A schematic o f t h e o p t i c s and e l e c t r o n i c s i s given i n F i g
2.
The two arms o f t h e i n t e r f e r o m e t e r a r e k e p t i nquadrature, t h e most s e n s i t i v e alignment, by a d j u s t i n g t h e phase o f t h e r e f e r e n c e arm u s i n g a v o l t a g e - c o n t r o l l e d p i e z o e l e c t r i c c y l i n d e r . T h i s a c t i v e homodyne d e t e c t i o n scheme i s run i n t h e h i g h g a i n mode, where t h e v o l t a g e a p p l i e d t o t h e PZ c y l i n d e r compensates n o t o n l y f o r low frequency t h e r m a l d r i f t s , b u t a l s o f o r s i g n a l s a t t h e m o d u l a t i o n frequency (3). T h i s compensated o u t p u t i s f e d i n t o an HP-3582 Spectrum Analyzer t u n e d t o t h e chopping frequency. A two-channel l o c k - i n a m p l i f i e r a l s o proved u s e f u l f o r m o n i t o r i n g t h e phase of t h e p h o t o a c o u s t i c s i g n a l . A sample o u t p u t f r o m t h e Spectrum A n a l y z e r i s g i v e n i n F i g
2.
R e s u l t s
Measurements were performed a t a range of c o n c e n t r a t i o n s and r e s u l t s were recorded as i n F i g
2.
These a r e p l o t t e d i n F i g3.
The i m p o r t a n t r e s u l t s f r o m t h e s e d a t a are:1 ) The observed n o i s e f l o o r was -110 dBV a t 0.73 Hz bandwidth.
T h i s i s shown on t h e l o w e s t t r a c e i n F i g
2.
2) F o r room a i r and f o r m i x t u r e s w i t h a b s o r p t i o n a <
l o w 3
cm-l a background s i g n a l o f -83 dBV was observed.3) F o r a > cm-l we observed S
-
a0S6l w i t h very h i g h c o r r e l a t i o n , f o r s i g n a l s f r o m -82 dBV t o -54 dBV.As now c o n f i g u r e d t h e background s i g n a l l i m i t s t h e s e n s i t i v i t y , c e r t a i n l y a common problem i n p h o t o a c o u s t i c measurements. We expect t h i s i s due t o s c a t t e r e d l i g h t b e i n g absorbed d i r e c t l y by t h e f i b e r c l a d d i n g , and t h e next s e r i e s o f measurements c o u l d employ a focussed l a s e r beam and/or b a f f l i n g , o r a compensation scheme t o s u b t r a c t o f f t h e background.
I f we e x t r a p o l a t e t h e l i m i t imposed n o t by t h e background b u t r a t h e r by t h e observed n o i s e f l o o r we o b t a i n an e x p e r i m e n t a l n o i s e l i m i t o f 1.6 x
lom5
c m - l m . These measurements were performed a t a very modest average power l e v e l o f 10 mW. Hence t h e minimum d e t e c t a b l e a b s o r p t i o n c a p a b i l i t y r e p o r t e d h e r e i s 1.6 x c m - l - ~ f l z . T h i s n o i s e - l i m i t e d c a p a b i l i t y approaches t h e s e n s i t i v i t y o f d i r e c t d e t e c t i o n i n photothermal i n t e r f e r o m e t e r t e c h n i q u e s ( 4 ) . We expect f i b e r o p t i c spectrophone performance t o be even b e t t e r i n l i q u i d s where t h e a c o u s t i c impedance match between f i b e r sensor and t h e l i q u i d i s much b e t t e r t h a n w i t h a i r .
Acknowledgements
We acknowledge t h e c a r e f u l work performed by Mr. F. T i d b a l l i n
assembling t h e spectrophone, and t h e a s s i s t a n c e o f Dr. A. Tveten and Dr. R.
M o e l l e r i n assembling t h e i n t e r f e r o m e t e r .
JOURNAL DE PHYSIQUE
References
1. D. H. Leslie, G. L. Trusty, A. Dandridge, & T. G. G i a1 l o r e n z i
,
"A F i ber-Optic Spectrophone", E l e c t r o n i c s L e t t e r s2,
581-582 (1981).2. L. B. Kreuzer, "Ultra-Low Gas Concentration I n f r a r e d Absorption Spectroscopy1', J. Applied Physics
42,
2934-43 (1971).3. T. G. G i a l l o r e n z i , et. a1
.,
"Optical F i b e r Sensor Technology", IEEE J.Quantum E l e c t r o n i c s QE-18, 626-665 (1982).
4. A. J. Campillo, S r P e t u c h o w s k i , C. C. Davis, & H-B. Lin, "Fabry-Perot Photothermal Trace Detection", Applied Physics L e t t e r s
