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Produced by the NASA Center for Aerospace Information (CASI)
• 1

NASA TECHNICAL MEMORANDUM NASA TM-77404

M"I us wo

FIFER OPTIC SENSORS

Joachim Hesse and W. Sohler

(N"A-Ti4-77404) FIBES GPTIC SENSORS N84-24346 d:.ational AeronduticS and Space Alminist-ration) 15 p BC A02/5F A01 CSC:L 20F

OncidS

G3/74 13203

Translation of "Faseroptische Sensoren",

Flektronik, Vol. 31, No. 12, June 18, 1982,

PP• 89-92.

NATIONAL AERONAUTICS AND SPACE ADMINISTi<ATION

WASHINGTON, D.C. 20546 MAY 1984

a%

ie
2. Fiber Optics Sensors with Multiwave Fibers

2,1 "External" fiber optics sensors

— — — — — — — — -- — — — — — — — —

The optical sensor itself is an "external" component which

is supplied with light through a glass fiber and which is "interrogated"

through a second fiber. In most cases, here the "external" optical

sensor varies the transmission in the supply fiber/signal fiber

pathway. A simple light barrier, the position of which is detected

with optical fibers, can thus be termed an optical sensor of

this type. It is utilized for position determination or for measurement of fill status.

A reflecting membrane which hooks up the pressure - dependent

light from the supply fiber with the signal fiber is a. further example

[2]. This kind of sensor, even with fiber bundles, still remains so

small that, installed in a catheter, it makes possible pressure /90

measurements within the living heart [3]. Fluid crystals [4]

or semi-conductors [5] between the two fibers serve for temperature

determination; thereby use is made of the fact that the arrangement

of the fluid crystal. and the spectral location of the band-edge

absorption of the semi-conductor are temperature dependent. Very

small and sensitive sensors have then alreadv been manufactured;

a possible area of application is hyperthermal cancer therapy [6].

A phosphor optically activated by UV light through the supply

fiber can also be termed an "external" optical sensor. Its temperature-dependent luminescent radiation is conducted out through the

signal fiber and analyzed to determine temperature [7]. A device with

this kind of a sensor is already commercially offered in the USA; the

alriost punctiform measuring with disappearing heat capacity makes

possible quite a variety of applications.

2.2 "Internal" fiber optics sensors

The actual optical sensor, in the case of this class, is a

specially treated segment of fiber between the supply and the signal

fiber. It can be a "naked" fiber section (consisting of the fiber

3
FIBER OPTIC SENSORS

Dr. Joachim Hesse and Dr. W. Sohler

CTtMAMA DV /89*

As Zlways, there exists a great demand for new and improved

sensors. The requirements set for them are above all reliability,

insensitivity to interference by electrical fields and a reasonable

price. One of the most recent developments in this direction is the

fiber optics sensor, a by-product of light guide technology. Meanwhile, ways have been found with which one can in this way measure

all kinds of physical values, in some cases with extraordinarily

high accuracy. This area has already become quite complex; this

paper i s intended to provide a survey of the field.

1. Introduction

The significant progress in the manufacture of low attenuation

glass fibers has made a decisive contribution to the development of

fiber optics sensors [1]. A fiber optics sensor is an optical

detector which is supplied with light through a glass fiber, and which

conducts an optical signal, suitably coded according to the

measured magnitude, to a detector, and thereby to signal processing

with displays or regulators, this being done through another fiber

(which under certain circumstances can be identical to the supply

fiber) , (Figure 1) .

In this way, one exploits, for one thing, the advantages of

optical signal transmission through glass fiber cables which are also 

more and more being put to work in commercial transmission routes.

k

Numbers in the margin indicate foreign pagination.

1
r

ORIGINAL PAGE 19

r OF POOR QUALITY

.j The insensitivity of optical transmission circuits to electromagnetic field interference and their isolation from electrical

potentials have here the greatest significance for fiber optics sensors.

In addition, the use of an optical sensor makes possible the direct

conversion of the measured magnitude into an optical signal without

a second electro-optical conversion as in the case of electronic

sensors with optical signal transmission. Moreover, (fiber) optical

sensors open up new capabilities for measurement; in many cases }

they can be more accurate, more rugged and cheaper than the usual

electronic sensors.

The term "fiber optics sensor" nevertheless does not describe

a completely unitary concept: The actual optical sensor (Figure 1)

can, for example, "externally" modulate, as a function of the measured

magnitude, the transmission in the supply fiber/signal fiber pathway;

but it can also be "internally" a part of a specially prepared

fiber. While most of these sensors are made with multiwave fibers

(multimode), there are also fiber optics measurement systems with

single-wave (monomode) fibers--in general of complicated design.

In the latter, the phase location or the polarization state is

changed by the effect of the measured magnitude on the fiber, while,

in the sensor with multimode fibers, the light amplitude (and

thereby the intensity) is usually a function of the measured amplitude.

These two types of fiber optics sensors will be presented below, using

some %xamples.

r-- - --- - - —

Measured =

.;y_a, Displays

magnitude Sensor ' _L_ ?'Regulators

Computer

' - ..-^, Lamp

Light supply

Figure 1. Schematic representation of a fiber optics sensor.

r'

r

2
2. Fiber Optics Sensors with Multiwave Fibers 

2.1 "External" fiber optics —

sensorsThe optical sensor itself is an "external" component which

is supplied with light through a glass fiber and which is "interrogated"

through a second fiber. In most cases, here the "external" optical

sensor varies the transmission in the supply fiber/signal fiber

pathway. A simple light barrier, the position of which is detected

with optical fibers, can thus be termed an optical sensor of

this type. It is utilized for position determination or for measurement of fill status.

A reflecting membrane which hooks up the pressure - dependent

light from the supply fiber with the signal fiber is a. further example

[2]. This kind of sensor, even with fiber bundles, still remains so

small that, installed in a catheter, it makes possible pressure /90

measurements within the living heart [3]. Fluid crystals [4]

or semi-conductors [S] between the two fibers serve for temperature

determination; thereby use is made of the fact that the arrangement

of the fluid crystal and the spectral location of the band-edge

absorption of the semi-conductor are temperature dependent. Very

small and sensitive sensors have then alreadv been manufactured;

a possible area of application is hyperthermal cancer therapy [6].

A phosphor optically activated by UV light through the supply

fiber can also be termed an "external" optical sensor. Its temperature-dependent luminescent radiation is conducted out through the 

signal fiber and analyzed to determine temperature [7]. A device with

this kind of a sensor is already commerciaily offered in the USA; the

almiost punctiform measuring with disappearing heat capacity makes

possible quite a variety of applications.

2.2 "Internal" fiber optics sensors

The actual optical sensor, in the case of this class, is a

specially treated segment of fiber between the supply and the signal

fiber. It can be a "naked" fiber section (consisting of the fiber

3
ORIGINAL PAGE 18

OF POOR QUALITY

core only), the aperture of which, and hence the transmission, is

dependent on the refraction index_ of the surrounding medium.

Such a sensor is suitable for the determination of the level of a

liquid [8] and/or for the measurement of its refraction index

(Figure 2) [9]. Another example is represented by a segment of

fiber which is deformed by an external force. Thus are created

curvature losses (light decoupled from the fiber) which are a

measure of the applied force (or pressure) [10]. Such a sensor

also makes possible indirectly very exact measurement of a displacement. In addition, there are fibers specially doped (for example

with ions of rare earths such as neodymium a.nd europium)whose

optically activated luminescence and transmission are a measure

of temperature [10].

r 

20

1 

10

y 5 Readine

C accuracy :2^

Pefract. index! `-

2 Oechsl e deg.

135 36 1.37 •3ei Refraction index

0 20 40 6C 80 100 120 °Oecnsie

Figure 2. Light stream of a fiber optics refractometer

as a function of refraction Windex or of sugar

content (Oechsle degree) of a solution.

r 4

r,
3. Fiber Optics Sensors with Monowave Fibers

This group of sensors, the sensitivity of which increases

proportional to the length of the measurement fiber,has in recent

times been further developed with particular intensity [111. In

this way measurement sensitivity has been enormously improved through

the employment of ever longer fibers, possibly due to their extremely

low attenuation up to < 1 dB/km. Many of these sensors (or sensor

systems) promise to become interesting with respect to their

performance and their price.

3.1 Mach-Zehnder and Michelsk n Interferometers

- - - - - - - - - - - - - - - - - - - - - -

With a fiber optics Mach-Zehnder interferometer (Figure 3),

temperature differences [121, changes in length [131 and pressure

differences [121 can be detected with particular sensitivity as a

phase difference between measurement and comparison fiber. For

example, a fiber optics hydrophone with long fibers attains such

a high sensitivity that one can measure "sea state zero", the frequency dependent background noise in underw,iter sonar technique [141.

Here it is of great interest that, on account of the flexibility

of quartz glass fiber, sensors and sensor systems with directional

characteristics can be built.

By means of a special "coating" of the fibers it is possible

not only to increase the sensitivity of pressure measurements [151;

but it also makes possible measurement of other magnitudes such

as magnetic field and current which do not directly affect the

phase position of the light in the quartz glass fiber (there is

an exception in the case of large magnetic fields and special sensor

arrangements due to Faraday effect; see also Paragraph 3.4). For

example, the measurement fiber can be coated with a magnetostrictive

material (e.g., nickel) [161 which when in a magnetic field exercises

forces on the fiber which then gives rise to changes in length and in

refractive index. The cot-responding phase change can be easily

measured. In experiments sensitivities of 10 -7 Oe/meter of fiber

bz 5 __ 
J

ORIGINAL PAGE 13

OF POOR QUALITY

Meas. magnitude

Detectors Differential

amplifier

Meas. fiber

Beam Comparison fiber --'

spIitter^ 

Phase compensation `^^^^ Low-pass

------fil^e^- -^

s, grai o- — -4Locx In - - - - - High-pass_

`NLVA 'filter

Figure 3. Design of a fiber optics Mach-Zehnder interferometer.

length have been attained [171. Through the known relationship

between magnetic field and current (e.g., in a coil), the current

strength can then be determined. The induced phase difference can

amount to up to 100 rad/A x meters of fiber length. Sensors have /91

been manufactured with 10 cm long fibers which show a linear relationship beween sensor signal and current strength in the range

between 0.5 pA and 5 mA [171.

A further possibility for measuring current strength consists

in the analysis of a corresponding change in temperature. For this,

the current is conducted directly to the measurement fiber by a

metal coating. The temperature change generated thereby again

varies the optical length of the measurement fiber,thus produces

a phase change proportional to the current strength. At 1 Hz

sensitivities were achieved of 100 rad/A x meters of fiber length and

at 100 Hz still 1 rad/A x meters fiber length [171.

All these measurement capabilities are also present with a fiber

optics Michelson interferometer which operates with reflective fiber

ends (Figure Q). In many cases with this a simpler, more compact sensor

is possible.

6

[

i

,
3.2 Sagnac_Interferomoter (fiber optics gyroscope)_

One of the most extensively developed fiber optics sensor

systems is a gy roscope which analyzes as a measurement value the angular-velocity-dependent phase difference between the oppositely

directed light waves in a fiber optics Sagnac interferometer

[34,35]. The sensitivity of the gyroscope can be immensely 

increased by the use of longer fiber lengths on account of the

extremely small attenuation in modern quartz glass fibers.

Today detection sensitivities have already been attained which

permit measurement of the rotation of the earth. Despite these

impressive results, we are still a long way from the theoretically

possible detection limit of the fiber optics gyroscope. Estimates

have shown that sensitivities of up to around 10 -30 /hr should be

possible [20]. For this reason, the fiber optics gyroscope will

certainly soon start competing with the expensive mechanical

gyroscope systems which are many times more costly.

3.3 Wave-guide resonators 

A special kind of interferometric measuring instrument is the

fiber resonator %Figure 6) [21]. It semi-integrates both arms of

the interferometer into itself. It is possible, through the choice

of the mirror reflectivity, to set in any desired condition between

"two beam" and "multibeam" interference. In this way an increase in

sensitivity can be achieved which roughly corresponds to the

acuteness of the resonator (to about 100).

3.4 Polarimetric sensors

Polarimetric sensors form a further class of fiber optics

sensors with monomode wave-guides. They can be also considered as

interferometers which unit both arms into a single fiber. The

measurements are of the relative phase differences between the

7
^E

1

1

0.5

OR►G1NAL PAGS 1,9

OF POOR QUALITY

orthagonally polarized components of the transmitted light. Thus,

the physical field to be measured must generate and/or modify a

birefringence in the fiber. Thereby, the sensitivity is normally

considerably smaller than with the classical interferometer; but

the measurement with a single fiber permits the design of m , :ch simpler

sensors, for example, for temperature [22] or pressure [23] measurement

Fotod ode x.ar

Piezo -

Quarz

Laser Reflecting

—

M. as, fiber end Beam surfaces

splatter

Mirror Objeahve 

Comparison fiber

700 850 V 1000

U

0 2 l pm 6

QA - y

Figure 4. Schematic representation of a fiber optics

Michelson interferometer x•.ith the experimental results of a rotation measurement.

Also, the rotation of the polarization plane of linearly

polarized light in a longitudinal magnetic field (Faraday effect)

can be used for measurement of the current. For this, a long

glass fiber is wound as a coil around a conductor carrying a

current. The rotation of the polarization plane is proportional to

the linear integral (^ H dr, and thereby to the current strength in /9

the enclosed conductor. Figure 7 s l-:nws the comparison of conventional and fiber optics transfor*, , ers [24]. Prototypes of this kind

of sensor have already been ins-lled in power plants for current

measurement on high tension lines 'no insulation problem) [25).

8
ORIGINAL PAGr 19

OF POOR QUAD"

Supply fiber Resonator reflector

1 au

4

3

i

2 NAU)

30 3S 00 °C

T --+ 

Figure 5. Fiber optics resc,nator (1 cm long) as

a temperature se;;sor.

9
ORIGINAL PAIGE I

OF POOR QUALITY

1 1

`

6153

'( S218

IIA

4210

3150

2165 0225 0,2W

^s 

050 0 091

01

, 5 r 559

t—

Figure 6. Comparison between conventional [1] and fiber

optics transformers with staged alteration

of current strength.

^. Outlook

A series of specialized components have been developed for the

realization of fiber optics sensors. To be mentioned here are .:lberfiber couplers [26], fiber polarizers [27], fiber phase shifters [28],

polarization stabilizer p [29] and birefringent fibers 1201. Alongside these of course are used the components already extensively

developed in optical signal technology such as semi-conductor

lasers, luminescent diodes, fiber jacks, wave length multiplexers,

filters and detectors. Also, the components of the integrated

optics, which preferably have been developed as monomode structural

elements, offer great potential for pirely opt-i cal or electrooptical signal processing. To be mentioned as an examp;c would be

an integrated polarization regulator which can transfor;*I;in

arbitrary polarization condition of the light entering into any

other polarization condition at emission [311. Pure integrated

optical sensors have also already been manufactured; an example

is the integrated optical spectrum analyzer for ver y, high resolution. spectroscopy [32].

With all of these coLiponents and with the multiplicity of

concepts for fiber optics sensors, a series of useful new measurement

devices can be expected in the foreseeable future. Fiber optics gyroscopes, hydrephones, transformers, and temperature sensor y already

stand on the threshold of industrial use. A wider palette of

sensors will. follow; their preferred employment will be in electromagnetically disturbed, chemically aggressive or explosive envirorLwn_s.

10

-4 .
at

ORIoript

r)F POOP nU^i.^'

This work was given as a paper at the NTG/GMR Specialist

Meeting "Sensors--Technology and Application" at Bad Nauheim,

9-11 March 1982 [33].

Prof. Dr. rer. nat. Joachim Hesse was

born in Dresden. He studied physics in

Goettingen and lectured in Frankfurt.

In 1965 he went to Rork at AEG Telefunken

and was finally Chief of Technology Development in the Semi-conductor Division. Since

1980 he has been Director of the new Fraunhofer Institute for Measurement Technique

in Freiburg.

Hobbies: Family, Music

Prof. Dr. rer. nat. Wolfgang Sohler was

born in Wangen/Allgaue, but grew up in

Marl/Westphalia. ::is professional posts

were University of. Munich (Physics major

and grad) Dortmund U. (research assistant and iectu.Ler) , Fraunhof(7!r Institute

for Measurement Technique, Freiburg

(Chief, Fiber Optics Branch) and since

early 1982 at Paderborn University

(Prof. of Applied Physics). Here his

main interest is integrated optics.

Hobbies: Family, skiing, hiking

11
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13