A novel sensing technique based on intensity-to-frequency conversion is explained and demonstrated. Experimental data are presented and a comparison with a theoretical model is made

Adamovsky, G.

Carr, T. W.

English

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i ''_ _ : :
NASA Technical Memorandum 102562 .....
!_,...::::::.._,:_::._inte=nsity,_to Frequency ConversionTechnique :
In Intensity Modulated-Fiber Optic .............
Sensing Systems
G. Adamovsky
Lewis Research Center
Cleveland, Ohio
and
T.W. Carr
Case Western Reserve University
Cleveland, Ohio ....
March 1990
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CONVERSIGN TZCHHIOUE IN IHTENSITY MODULATED
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INTENSITY TO FREQUENCY CONVERSION TECHNIQUE IN INTENSITY
MODULATED FIBER OPTIC SENSING SYSTEMS
G. Adamovsky
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135, U.S.A.
and
T.W. Carr*
Case Western Reserve University
Cleveland, Ohio 44106, U.S.A.
ABSTRACT
A novel sensing technique based on Intensity-to-frequency
conversion is explained and demonstrated. Experimental data are
presented and a comparison with a theoretical model Is made.
LO
I
Among different configurations of Intensity modulated fiber optic sensing
systems, those with the signal and reference channels separated In the tlme
domain are the most commonly used. Such a configuration has been already
dlscussed, l The configuration involves a short duration pulse sent towards a
sensing element through a delay llne. The resultant signal consists of two
pulses, signal and reference. The measurand affects the amplitude of the
signal pulse. The amplitude of the reference pulse remains unchanged. The
Information about the measurand is then retrieved from the relative amplitudes
of the pulses in the signal and reference channels. Several techniques have
been proposed to do this.2, 3 These techniques are based on measuring either
the relative amplitude of the pulses in the time domain directly or the
strength of the resultant signal at particular frequencies In the frequency
domain.
*Summer Intern at NASA Lewls Research Center. Present address:
Northwestern Unlverslty, Evanston, Illinois 60201, U.S.A.
The technique described in this paper employs a conversion from the
relative Intens|ty to a frequency value. To achieve the converslon, a slgnal
in the reference channel consists of two pulses of the same amplitude V and
duration t. The time delay between the first and second pulse in the
reference double pulse Is 26. The signal channel consists of a slngle pulse
of the same duration as the pulses in the reference channel. The signal pulse
Is delayed from the first reference pulse by 4. Thus the signal pulse occurs
In the mlddle of two reference pulses and forms a trlple pulse. Assume that
the pulses in this triple pulse have a rectangular shape and the trlple pulse
itself Is periodic with a period T. Then, uslng Fourier analysls 4 the
amplltude spectrum of such a triple pulse can be wrltten as:
iF(Wm) I = ___t.t sln(Wmt/2) [ 2 ]0.5
Wmt/2 2 + a + 4a cos(w m 4) + 2 cos(2w m 4)
where wm is the angular frequency, wm = 2_m/T, m Is an integer, and a Is
a ratio of the amplitude of the second (signal) pulse to the amplitude of any
other (reference) pulse In the trlple pulse. Analyzing the last term In
Eq. (l), the modulatlng factor S,
S = 2 + a + 4a cos(w m 4) + 2 cos(2w m 4) (21
it Is easy to show that the functlon IF(wm)l equals zero if the following
relatlonship between frequency fm = Wm/2X and the coefflclent a Is
satisfied:
Thus for any value of a from
function IF(wm)I goes to zero.
change in the value of the frequency at which the amplitude vanishes. Thls
frequency will be referred to as the "zero" frequency fo- Figure I shows
calculated plots of the modulating factor S described by Eq. (2) as a
(I)
a = -2 cos(2_ 6f m) (3)
0 £ a £ 2 there is a frequency at which the
A change In the value of a Is reflected as a
function of the frequency fm : Wm/2_ for three d|fferent values of a, a : I,
I/2, and O.
The experimental setup Is shown in F|g. 2. An 18 ns duration repetlt|ve
light pulse from a llght emitting diode (LED) enters a ] by 3 opt|cal coupler
and is sent into three flber ports. The repet|tion rate of the Inltla] light
pulse Is IO ps. The ports are connected to fibers of different lengths to
generate delays of approximately 36 and 72 ns for the second and the third
pulses with respect to the f|rst one. Another 3 by 1 optlca] coupler combines
the three pulses Into one triple pulse. To Imltate a change In the amplltude
of the second pulse, a variable attenuator (VA) with a readout (RO) is pIaced
in the port used to generate thls pulse. The tr|ple pulse is then rece|ved by
the photodetector (PD) and sent to the slgnal analyzer (SA). The position of
the "zero" frequency on the d|splayed spectrum Is observed and detected as a
function of the ratlo a, the relative intensity of the light pulse In the
signal channel.
To obtain the experimental data, readings from the attenuator were used
to calibrate the position of the "zero" frequency fo against the ratio a.
This was done in three steps. In the first step, measurements were made to
determine the relationship between a and the attenuator readlngs. The second
set of data taken was to determine the relationship between the attenuator
readings and the "zero" frequency. Finally, the relationship between a and
fo was established. Data was taken IO times at each datapolnt and their
average values were used later. During collection of the first set of
measurements the standard deviation from the average of a at each datapolnt
did not exceed O.O15. At the second stage the maximum deviation of the average
value of the "zero" frequency from the predicted one at each datapolnt did not
exceed I percent of the average value at that datapo|nt. The theoretical and
experimental data for the "zero" frequency tracking is presented in Fig. 3.
3
The technlque descr|bed is |nherently independent of the reference pulse
amplitude. A system based on thls technlque performs the Intensity to
frequency converslon and determlnes the relatlve Intensity of the light pulse
|n the slgnal channel wlthout actually measuring the |ntens|ty. Thus, a novel
senslng techn|que has been demonstrated. The experlmental results match
closely with that predlcted. The accuracy and repeatability of the
measurements could be improved by chooslng an appropr|ate repetitlon rate and
by improv|ng the "zero" frequency detection technique.
REFERENCES
1 SPILLMAN, N.B., and LORD, J.R." 'Self-referenclng multiplexing technlque for
f_ber-optic intensity sensors', O. L|ghtwave Techno1., 1987, LT-5,
pp. 865-869
2 ADAMOVSKY, G., and PILTCH, N.D." 'F|ber-optic thermometer using temperature
dependent absorption, broadband detectlon, and tlme domain referencing',
Appl. OPt., 1986, 25, pp. 4439-4443
3 ADAMOVSKY, G." 'Fiber-optlc d|splacement sensor w_th temporally separated
signal and reference channels', _pl. Opt., 1988, 277 pp. 1313-1315
4 SCHNARTZ, M." 'Informatlon, transmission, modulation, and nolse'
(McGraw-Hill, 1980), 3rd ed.
4
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O
3.0
2.4
1.8
1.2
.6
__ 0 a
-- O.5
"_ -- 1.0 /// "%
"-". _ \ / ",, / t / \
\',,\/,..-,,'..I//
- \;- /TA",,'\ ! /
6 12 18 24 30
FREQUENCY f, MHz
Figure 1. - Calculated plots of the modulating factor S described
by Eq. (2) for three values of a, a= 1, 1/2, and 0.
INITIAL PULSE
J-LI-U_ -,,le-
TRIPLE PULSE
lx3 3xl
COUPLER COUPLER
E
E
B E
B E
i
Figure 2. - Schematic of expeflmental setup LED = light emitting diode,
VA = variable attenuator, RO = readout, PD = photodetector, SA =
signal analyzer.
9.2
o 8.6
_ 8.0
Lu
_ 7.4
w
6.8
- _t,_,_ _OOE, .,t'
0_/_"
I I
0 .2 .4 .6 .8 1.0
RATIO a
Figure 3. - Theoretical and experimental plots of the "zero" fre-
quency fo as function of the ratio a.
Report Documentation PageNationatAeronautics and
Space Administration
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM- 102562
4. Title and Subtitle
Intensity to Frequency Conversion Technique in Intensity Modulated Fiber
Optic Sensing Systems
7. Author(s)
G. Adamovsky and T.W. Carr
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
5. Report Date
March 1990
6. Performing Organization Code
8. Performing Organization Report No.
E-5376
10, Work Unit No.
505-62-01
t 1. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
i15. Supplementary Notes
G. Adamovsky, NASA Lewis Research Center. T.W. Carr, Case Western Reserve University, Cleveland, Ohio
44106 and Summer Intern at NASA Lewis Research Center; present address: Northwestern University, Evanston,
Illinois 60201.
16. Abstract
A novel sensing technique based on intensity-to-frequency conversion is explained and demonstrated. Experimental
data are presented and a comparison with a theoretical model is made.
17. Key Words (Suggested by Author(s))
Fiber optics
Sensing
Referencing
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
19, Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 6
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