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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

_(NASA-IM-10?b_Z) INTENSITY TO FRE,_U6NCY

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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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_ -,,leTRIPLE PULSE

lx3 3xl

COUPLER COUPLER

E

E

B E

B E

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Figure 2. - Schematic of expeflmental setup LED = light emitting diode,

VA = variable attenuator, RO = readout, PD = photodetector, SA =

signal analyzer.

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Lu

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0 .2 .4 .6 .8 1.0

RATIO a

Figure 3. - Theoretical and experimental plots of the "zero" frequency fo as function of the ratio a.
NationatAeronautics and Report Documentation Page

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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