NASA Technical Memorandum 100893 NASA-TM- 100893 1988001 7260 Fiber Optic Sensors With Internal Referencing Grigory Adamovsky Lewis Research Center Cleveland, Ohio and Duncan J. Maitland IV Cleveland State University Cleveland, Ohio Prepared for the Conference on "Optical Testing and Metrology 11" sponsored by the Society of Photo-Optical Instrumentation Engineers Dearborn, Michigan, June 26-30, 1988 https://ntrs.nasa.gov/search.jsp?R=19880017260 2018-06-17T06:38:23+00:00Z
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NASA Technical Memorandum 100893 NASA-TM- 100893 1988001 7260
Fiber Optic Sensors With Internal Referencing
Grigory Adamovsky Lewis Research Center Cleveland, Ohio
and
Duncan J. Maitland IV Cleveland State University Cleveland, Ohio
Prepared for the Conference on "Optical Testing and Metrology 11" sponsored by the Society of Photo-Optical Instrumentation Engineers Dearborn, Michigan, June 26-30, 1988
G r i g o r y Adamovsky N a t i o n a l Aeronau t i cs and Space A d m i n i s t r a t i o n
Lewis Research Center Cleve land, Ohio 44135
and
Duncan J. M a i t l a n d I V Cleve land S t a t e U n i v e r s i t y
Department o f Eng ineer ing Technology Cleve land, Ohio 44115
SUMMARY
The main problem w i t h amp l i tude modu la t ing t ype sensors i s t h a t any v a r i a - t i o n i n the i n t e n s i t y of t he o p t i c a l s i g n a l which occurs throughout t he sens ing system i s i n t e r p r e t e d by t he pho tode tec to r as r e s u l t i n g f r om the sensor i t s e l f and i s r e f l e c t e d as an e r r o r i n t he sensed parameter. To account f o r these e r r o r s , a r e f e r e n c i n g technique w i t h t he s i g n a l and re fe rence channels sepa- r a t e d i n t he t ime domain over t he same f i b e r l i n k can be used.
C3 - e I Selec ted sens ing and s i g n a l p rocess ing techniques i n v o l v i n g t empo ra l l y
separated s i g n a l and r e f e r e n c i n g channels a re descr ibed . A t r a n s i t i o n f r om the t ime i n t o the f requency domain i s a l s o d iscussed. Exper imental da ta a re presented.
INTRODUCTION
Ampl i tude modu la t ing f i b e r o p t i c sensors have found a p p l i c a t i o n s due t o t h e i r s i m p l i c i t y and r e l a t i v e l y low c o s t . These sensors respond t o t he sensed parameter by changing the i n t e n s i t y o f t he o p t i c a l s i gna l i n t he system. How- ever a sens ing system based on t h i s p r i n c i p l e i s a l s o s u s c e p t i b l e t o t he i n t e n - s i t y v a r i a t i o n s o f t he o p t i c a l s i g n a l which occur throughout t he system. These v a r i a t i o n s cannot be d i s t i n g u i s h e d by a pho tode tec to r f r om i n t e n s i t y changes caused by t he sensor i t s e l f and c o n t r i b u t e t o an e r r o r i n t he sensed parameter. Because o f t h i s the sens ing system r e q u i r e s two channels, a r e fe rence and a s i g n a l , t o compensate f o r v a r i a t i o n s i n t he source i n t e n s i t y , d e t e c t o r sens i - t i v i t y , and t r ansm iss i on c h a r a c t e r i s t i c s o f t he f i b e r - o p t i c 1 i n k ( r e f . 1 ) .
Among d i f f e r e n t r e f e r e n c i n g techniques a v a i l a b l e , t he one w i t h channels separated i n the t ime domain i s a t t r a c t i v e because i t pe rm i t s the use o f t he same f i b e r l i n k f o r bo th channels. The technique i s based on gene ra t i ng a pu l se t r a i n us i ng a pu l se modulated l i g h t source and a f i b e r o p t i c l oop ( r e f s . 2 and 3 ) . The f i b e r o p t i c l oop w i t h a sensor i nco rpo ra ted i n i t forms a sensor head. Thus, t he sepa ra t i on o f t he channels occurs i n t e r n a l l y w i t h i n t he sensor head. To r e t r i e v e i n f o r m a t i o n about t he measured parameter, t h e amp1 i tudes o f pu lses i n t h e t r a i n must be compared. The most r e c e n t technique developed t o do t h i s i n v o l v e s s p e c t r a l a n a l y s i s o f t he cor responding e l e c t r i c a l s i gna l f r om t h e pho tode tec to r ( r e f . 4 ) . Th is t r a n s i t i o n i n t o the f requency domain p rov ides new o p p o r t u n i t i e s f o r f i b e r o p t i c sensors.
TIME-TO-FREQUENCY CONVERSION
The phenomenon o f t ime- to- f requency convers ion has been exp la i ned i n r e fe rence 4. I t can be performed by t a k i n g a F o u r i e r t r ans fo rm o f a p e r i o d i c pu l se t r a i n t h a t e x i t s a f i b e r o p t i c loop . Analyses o f d i f f e r e n t components o f t he f requency spectrum p r o v i d e i n f o r m a t i o n about t he r e l a t i v e ampl i tudes o f t h e pu lses i n the t r a i n . The sepa ra t i on o f these components i s determined by t he r e p e t i t i o n r a t e o f t he i n i t i a l pu l se .
I n t he exper imenta l setup shown i n f i g u r e 1 a narrow r e p e t i t i v e pu l se ( pu l se w i d t h a t h a l f power i s 5 nsec and r e p e t i t i o n r a t e i s 20 MHz) i s sen t i n t o a r e f l e c t i v e Fabry-Perot t ype f i b e r o p t i c loop which c o n s i s t s o f a 2.5 m l ong mult imode f i b e r p laced between two m i r r o r s , M I and M2. The m i r r o r M I i s a semiref lec t ing-semi t ransmi t t ing m i r r o r w i t h a r e f l e c t i o n c o e f f i c i e n t o f about 0.2 a t a wavelength of 830 nm. The m i r r o r M2 i s a r e f l e c t o r p o s i t i o n e d on a t r a n s l a t i o n s tage T t o p r o v i d e a l o n g i t u d i n a l sepa ra t i on Q between t h e f i b e r end and t he r e f l e c t i v e su r f ace o f the m i r r o r . An o p t i c a l system c o n s i s t - i n g o f a c o l l i m a t i n g l ens L and a 50150 cube b e a m s p l i t t e r BS p rov i des the c o u p l i n g o f t h e l i g h t i n t o t h e f i b e r loop and d i r e c t s t h e r e t u r n i n g pu l se t r a i n towards t he pho tode tec to r D v i a t h e f i b e r F.
The c o n f i g u r a t i o n i s chosen t o p r o v i d e secondary pu lses due t o r e f l e c t i o n f r om t h e m i r r o r M2 cen te red between p e r i o d i c i n i t i a l pu lses . The amp l i tudes o f t h e i n i t i a l and t he secondary pu l ses a re ad jus ted t o be approx imate ly t he same. F i gu re 2(a) d e p i c t s t he shape of t h e o p t i c a l s i g n a l t h a t e x i t s t he f i b e r l oop and f i g u r e 2(b) shows t he f requency spectrum o f t h i s s i g n a l . I f t h e en t rance i n t o t he f i b e r o p t i c l oop i s b locked, t he secondary pu l se would n o t be gener- a ted. Th i s corresponds t o a case when t he m i r r o r M2 i s moved away f r o m t h e f i b e r end by such a d i s t ance t h a t t he re i s p r a c t i c a l l y no l i g h t coupled back i n t o t h e l oop upon r e f l e c t i o n . I n t h i s case t he s i g n a l de tec ted by t h e photo- d e t e c t o r resembles t h e p e r i o d i c i n i t i a l pu lses . F igures 3(a) and 3 (b> show, r e s p e c t i v e l y , the o p t i c a l s i g n a l t h a t reaches t h e pho tode tec to r when t he en t rance t o t he loop i s b locked ( t h e i n i t i a l s i g n a l ) and i t s f requency spec- trum. I t can be seen from t h e f requency spec t ra t h a t t he fundamental component a t 20 MHz f o r the i n i t i a l s i g n a l ( f i g . 3 (b )> i s n o t p resen t i n t he spectrum o f t he r e s u l t a n t s i g n a l t h a t e x i t s t he loop ( f i g . 2 ( b ) > . A t t he same t ime t h e f requency component a t 40 MHz i s p resen t i n bo th f i g u r e s , be ing t he fundamental o f t he r e s u l t a n t s i gna l and t h e second harmonic o f t he i n i t i a l one. If t h e amp l i tudes o f t he i n i t i a l and t h e secondary pu lses d i f f e r , the magnitude o f t he component a t 20 MHz would v a r y f r o m z e r o f o r a case o f equal amp l i tudes t o a maximum f o r a case o f an absent secondary pu l se . The component a t 40 MHz can be used as a r e fe rence t o compensate f o r i n t e n s i t y v a r i a t i o n s o u t s i d e t h e f i b e r o p t i c loop . Th is component a l s o changes i t s magnitude w i t h a change i n t h e secondary pu l se ampl i tude. However, t h e magnitude o f t h e 40 MHz compQnent decreases w i t h a decrease i n t h e amp l i tude o f t he secondary pu lse . I t can be shown t h a t a r a t i o o f these magnitudes i s independent o f t he i n i t i a l pu l se amp l i tude and i s a f u n c t i o n o f t h e amp l i tude o f t he secondary pu l se ( r e f . 4 ) . Thus, a change i n t he amp l i tude o f t he secondary pu l se can be de tec ted by meas- u r i n g and comparing t he f requency components a t 20 MHz and 40 MHz.
One o f t h e ways t o change t h e amp l i tude o f t he secondary pu l se i s t o change t h e amount o f l i g h t coupled back i n t o t he f i b e r l oop upon r e f l e c t i o n f r om the m i r r o r M2. I n t h i s r espec t t h e technique cou ld be used f o r d i sp l ace - ment measurements . I n o r d e r t o do t h i s t he s i g n a l f r om the pho tode tec to r
(See f i g . 1) i s s p l i t i n t o two channels, each of which c o n s i s t s o f an app rop r i - a t e f i l t e r , an a m p l i f i e r and a RF d e t e c t o r . The RF d e t e c t o r conver ts t he f i l - t e r e d and amp1 i f i e d CW s i g n a l a t e i t h e r -20 MHz o r 40 MHz i n t o a DC ou tpu t . The magnitude of t he DC o u t p u t i s a l i n e a r f u n c t i o n of t he magnitude of t he co r re - sponding f requency component. The DC s i g n a l s emerging f r om the RF d e t e c t o r s have been measured and t he r a t i o o f these s i g n a l s has been ob ta i ned f o r d i f f e r - e n t d i s t ances between t he loop and t he m i r r o r M2.. The graph i n f i g u r e 4 repre - sents t he e x p e r i m e n t a l l y ob ta i ned r e l a t i o n s h i p between t he r a t i o of t he DC s i g n a l s V ~ ( ) M H ~ / V ~ ( ) M H ~ and the m i r r o r d isp lacement Q over a range o f d i sp l ace - ments f r om 0 t o 600 pm. Dur ing t he exper iment a s e n s i t i v i t y o f 1 pm has been observed.
IN IT IAL DOUBLE PULSE TECHNIQUE
The technique descr ibed i n t he above s e c t i o n o f t h i s a r t i c l e i n v o l v e s i n i - t i a l pu lses w i t h h i g h r e p e t i t i o n r a t e . The h i g h r e p e t i t i o n r a t e p u l s i n g o f l a s e r d iodes leads t o ove rhea t i ng and a f f e c t s t he s t a b i l i t y o f the e n t i r e sys- tem. The disadvantages o f t h i s approach w i l l have g r e a t e r impact i n sens ing systems w i t h s h o r t e r f i b e r o p t i c loops and w i t h co r responden t l y h i ghe r r e p e t i - t i o n r a t e i n i t i a l pu lses . To min im ize t h i s problem a novel technique i s pro- posed. The technique i s based on u s i n g an i n i t i a l double pu l se w i t h a lower r e p e t i t i o n r a t e and sending t h i s double pu l se i n t o t he f i b e r o p t i c loop descr ibed i n t he p rev i ous s e c t i o n . I f the de lay between t he pu lses i n t h e i n i - t i a l double pu l se i s t w i c e as l o n g as t he t r a n s i t t ime f o r a pu l se i n t he f i b e r o p t i c loop , a t r a i n o f f o u r pu lses r e s u l t s , e q u i d i s t a n t i n t ime.
I n t he exper imenta l setup w i t h 2.5 m l o n g Fabry-Perot type f i b e r o p t i c loop t h e t r a n s i t t ime f o r a pu l se i n t he l o o p i s about 25 nsec. The de lay between pu lses i n t h e i n i t i a l double pu l se and t he r e p e t i t i o n r a t e a re chosen t o be about 50 nsec and 1.82 MHz, r e s p e c t i v e l y . Thus, t he s i gna l emerging f r om the loop c o n s i s t s o f a p e r i o d i c t r a i n o f f o u r pu lses 25 nsec a p a r t w i t h t he f irst and t h i r d be ing t he i n i t i a l pu lses and t h e second and f o u r t h be ing t h e i r r e s p e c t i v e secondary pu lses . Th i s s i g n a l and i t s spectrum, as w e l l as t he s i g n a l and i t s spectrum ob ta i ned when t h e en t rance t o t he l oop i s b locked, a re shown i n f i g u r e s 5 and 6, r e s p e c t i v e l y . The spectrum i n t h i s case c o n s i s t s o f many components t h a t appear under t he f requency envelope o f the F o u r i e r t r ans fo rm due t o t he r e p e t i t i o n r a t e . A t t h e same t ime, two bands o f f requency components cen te red a t 20 MHz and 40 MHz can be e a s i l y recognized. I t should be a l s o no ted t h a t t h e behav io r o f these f requency bands i s analogous t o t h a t o f t he cor responding f requency components a t e i t h e r 20 MHz o r 40 MHz o f the p e r i o d i c i n i t i a l pu lses d iscussed above. The technique r e q u i r e d t o process t he r e s u l t a n t t r a i n o f f o u r pu lses i n t h e f requency domain would a l s o be analogous t o t h a t d iscussed above. The d i f f e r e n c e would be i n u s i n g broad bandpass f i l - t e r s i n each channel t o cap tu re severa l components i n t he cor responding band. However, t h i s may l ead t o decrease i n s e n s i t i v i t y .
CONCLUSION
Both c o n f i g u r a t i o n s descr ibed i n t h e paper a re based on t h e use o f a f i b e r o p t i c loop t o modulate t h e spectrum o f an i n i t i a l s i g n a l . I t has been shown t h a t h i g h r e s o l u t i o n can be ach ieved u s i n g h i g h r e p e t i t i o n r a t e pu lses , a f i b e r o p t i c l oop o f an a p p r o p r i a t e l eng th , and the' t ime- to- f requency convers ion. However, h i g h r e p e t i t i o n r a t e s may be avo ided by u s i n g t he i n i t i a l double pu l se technique presented. I n t h i s r espec t an inc rease i n s e n s i t i v i t y cou ld be ach ieved by i n c r e a s i n g t he number o f pu lses i n t he i n i t i a l s i g n a l .
Due t o t h e f a c t t h a t t ime- to- f requency convers ion i n v o l v e s sepa ra t i on and p rocess ing o f t he two f requency bands by t a k i n g t he r a t i o o f t he emerging s i g - n a l s , t he r e s u l t i s independent o f t he i n t e n s i t y l e v e l o f t h e i n i t i a l s i g n a l . Th is makes t he technique u s e f u l i n p r o p u l s i o n c o n t r o l systems and o t h e r a p p l i - c a t i o n s i n v o l v i n g harsh env i ronment .
ACKNOWLEDGMENTS
One o f t he au thors (D.J. Ma i t l and ) wishes t o acknowledge t he suppor t o f t he NASA Lewis Research Center (Grant NCC-3-58).
REFERENCES
1. G . Adamovsky, "Referenc ing i n F i b e r O p t i c Sensing Systems," i n O p t i c a l Techniques f o r Sensing and Measurement i n H o s t i l e Environments, C.H. G i l l e s p i e and R.A. Greenwel l , eds., Proc. SPIE 787, pp. 17-23 (1987).
2 . W.B. .Spi l lman, J r . and J.R. Lord, "Se l f -Re fe renc ing M u l t i p l e x i n g Technique f o r F'i b e r - o p t i c I n t e n s i t y Sensors," J. L ightwave Techno1 . LT-5, pp. 865-869 (1987).
3. G. Adamovsky, "Time Domain Referencing i n I n t e n s i t y Modu la t i on F i b e r O p t i c Sensing Systems," i n O p t i c a l Tes t i ng and Met ro loqy , C.P. Grover , ed., Proc. SPIE 661, pp. 145-151 (1986).
4 . G. Adamovsky, "Ampl i tude Spectrum Modu la t ion Technique f o r Analog Data Pro- cess ing i n F i b e r O p t i c Sensing System w i t h Temporal Separa t ion o f Chan- n e l s , " - i n F i b e r O p t i c and ~ a s e r sensors V, R.P, DePaula and E. Udd, eds., Proc. SPIE 838, pp. 264-270 (1987).
AMPLIFIER Q Q t t
DC OUT DC OUT
("20 MHZ) ("40 MHZ)
FIGURE 1.- EXPERIMENTAL SETUP.
FIGURE 2. - SIGNAL THAT REACHES PHOTODETECTOR AFTER EXITING LOOP ( a ) AND ITS SPECTRUM (b) .
FIGURE 3. - SIGNAL THAT REACHES PHOTODETECTOR WHEN ENTRANCE TO LOOP I S BLOCKED ( a 1 AND ITS SPECTRUM (b 1.
FIGURE 4. - EXPERIMENTALLY OBTAINED RELATION- SHIP BETWEEN THE RATIO V20 MHz/V40 MHz AND THE MIRROR I'l2 DISPLACEMENT OVER THE RANGE FROH 0 TO 600 PM.
FIGURE 5. - SIGNAL THAT REACHES PHOTODETECTOR AFTER EXITING LOOP ( a ) AND ITS SPECTRUM (b) I N SYSTEM WITH IN IT IAL DOUBLE PULSE.
FIGURE 6. - SIGNAL THAT REACHES PHOTODETECTOR WHEN ENTRANCE TO LOOP IS BLOCKED ( a ) AND ITS SPECTRUM (b) IN SYSTEM WITH INITIAL DOUBLE PULSE.
NASA FORM 1626 OCT 86 'For sale by the National Technical Information Service, Springfield, Virginia 22161
NnsA Nalmnal Aeronaul~cs and Space Adm~n~srralton
Report Documentation Page 1. Report No.
NASA TM- 100893
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
F i b e r O p t i c Sensors W i t h I n t e r n a l R e f e r e n c i n g .
7. Author(s)
G r i g o r y Adamovsky and Duncan J. M a i t l a n d I V
5. Report Date
6. Performing Organization Code
8. Performing Organization Report No.
E-4134
10. Work Unit No.
505-62-01 9. Performing Organization Name and Address
N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n Lewis Research C e n t e r C l e v e l a n d , O h i o 44135-3191
12. Sponsoring Agency Name and Address
N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n Wash ington, D.C. 20546-0001
11. Contract or Grant No.
13. Type of Report and Period Covered
T e c h n i c a l Memorandum 14. Sponsoring Agency Code
15. Supplementary Notes
Prepared f o r t h e Conference on "Opt ica l Tes t ing and Metrology 11" sponsored by t h e Soc ie ty o f Photo-Optical Ins t rumenta t ion Engineers, Dearborn, Michigan, June 26-30, 1988. Gr igory Adamovsky, NASA Lewis Research Center; Duncan J . Ma i t land I V , Cleveland S t a t e U n i v e r s i t y , Dept . o f Engineer ing Technology, C leve land , Ohio 44115 (work funded under NASA Grant NCC-3-5-58).
16. Abstract
The ma in p r o b l e m w i t h a m p l i t u d e m o d u l a t i n g t y p e sensors i s t h a t any v a r i a t i o n i n t h e i n t e n s i t y o f t h e o p t i c a l s i g n a l wh ich o c c u r s t h r o u g h o u t t h e s e n s i n g sys tem i s i n t e r p r e t e d by t h e p h o t o d e t e c t o r as r e s u l t i n g f r o m t h e sensor i t s e l f and i s r e f l e c t e d as an e r r o r i n t h e sensed pa ramete r . To accoun t f o r t h e s e e r r o r s , a r e f e r e n c i n g t e c h n i q u e w i t h t h e s i g n a l and r e f e r e n c e channe ls s e p a r a t e d i n t h e t i m e domain o v e r t h e same f i b e r l i n k can be used. S e l e c t e d s e n s i n g and s i g n a l p r o c e s s i n g t e c h n i q u e s i n v o l v i n g t e m p o r a l l y s e p a r a t e d s i g n a l and r e f e r e n c i n g chan- n e l s a r e d e s c r i b e d . A t r a n s i t i o n f r o m t h e t i m e i n t o t h e f r e q u e n c y domain i s a l s o d i s c u s s e d . E x p e r i m e n t a l d a t a a r e p r e s e n t e d .
17. Key Words (Suggested by Author@))
F i b e r o p t i c s Sensors R e f e r e n c i n g
18. Distribution Statement
U n c l a s s i f i e d - U n l i m i t e d S u b j e c t C a t e g o r y 35
