Fiber optic sensors provide noise immunity and galvanic insulation at the measurement point. Interest in such sensors is increasing for these reasons. In the United States sales are expected to increase from 12 million dollars in 1981 to 180 million in 1991. Interferometric sensors based on single modus fibers deliver extremely high sensitivity, while sensors based on multi-modus fibers are more easily manufactured. The fiber optic sensors which are available today are based on point measurements. Development of fiber optic sensors in Norway is being carried out at the Central institute and has resulted in the development of medical manometers which are now undergoing clinical testing

Hansen, T. E.

English

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NbiSA TECHNICAL MEMORANDUM NASA TM-77357
J, ID 7I
1
GREAT PROSPECTS FOR FIBER OPTICS SENSORS
Thor-Erik Hansen
(I1AS & -Tlf-77357) GhEAZ PROSPECTS FUR FIFER
UPTICS SEIISORS (dationd! Aeronautics and
Space Advinistiatiou)	 11 F HC A02/mf A01
I.SCL 2OF
N84-11922
Uncias
G3/74 424bb
Translatio,: of "Store fremtidsutsikter, paa mange omraader",
Elektro (Nozway) Vol. 95, No. 2, Jan. 21, 1982, pp. 17 -18, 23
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C.
	 20546	 OCTOBER 1983
.6
V-
c
ORIGIN'A'L. FACC ^'i
OF P60R QUALITY
STANDARD TITLE PAGE
1.	 Report No.
NASA TM-77357
2. Government Accession No. T. Reclplent's Catalog No.
4. Tula and suliliUa
GREAT PROSPECTS FOR FIBER'OPTICS
L Report Doto
October	 1983
L PorbrnIngOrganisation CodeSENSORS
7.	 Audrar(s) L. PoslotWng Oryanfsetion Report No.
Thor-Erik Hansen
Central Institute, for Industrial	 llesearch 10, Work Unit No.
9.	 Perterstng Organisation Nava and Address	 11. Contract or Otani No.
Leo Kanner Associates -
Type of Report and PeriodCovare,tRedwood City,	 California	 94063	 12.
Translation
12.	 sponsoring Agency Nome and Address
National Aeronautics and Space Adminis — 14.	 Spen _ aring Agency Code	 I
tratior., Washington D.C.
	 20546
15.	 Supplementary Notes
Translation .of "Store fremtidsutsikter, paa mange omraader",
E]ektro
	 (Norway)	 Vol.	 95,	 No.	 2,	 Jan.	 21,	 1982,	 pp.	 17-18,23
tn. AD:trti^t Fiber optic sensors provide noise immunity and galvanic
insulation at the measurement point.	 Interest in sych sensors
is increasing for these reasons,
	
In the United States sales
are expected to increase from 12 million dollars in 1981 to
180 million	 in 1991.•	 Interferometric sensors based on single
modus fibers deliver extremely high sensitivity, while sensors
based on multi-modus.fibers are more easily manufactured.	 The
fiber optic sensors which are available today are based on
point measurements.
	
Development of'fiher optic sensors in Nor
way is beinq carried* out • at the Central !Dstitute and has re-
sulted in the development of medical manometers which are now
undergoing clinical
	 testing.
17. Key words (Selected by Author(s)) 1t.	 Distribution Stetament
Unclassified-Unlimited
19.	 Secwrtty Cloteil. (of this upon) 20.	 Severity Cloself. (of this page) 21. Me. of Pages 22.
Unclassified Unclassified 9
r)
110ui0 le ^l^^ /^:vPr tl
i	 1.0 to ti0.
1961 Mark"t	 S11 n111i0r
OF POOR QUAL17;
GREAT PROSPECTS FOR FIBER OPTIC SENSORS
Thor-Erik Hansen
Central Insti-L:ute for Industrial Research
Fiber optic sensors provide noise immunity and galvanic insula- /17 *
tion at the measurement point. Interest in such sensors is increas-
ing for these reasons. In the United States sales are expected to in-
crease from 12 million dollars in 1981 to 180 million in 1991. Inter-
feronletric sensors based onsingle-modus fibers deliver extremely high
sensitivity, while sensors based on multimodus fibers are more easily
manufactured. The fiber optic sensors which are availanle today are
based on point measurements. Development of fiber optic sensors in Nor-
way is being carried out at the Central Institute and has resulted in
the development of medical manometers which are now undergoing clinical
testing.
D' SO l aceaert
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f^•v •^Ow't
1991 Me-tet
	
S17S millior
Underseas
Space/Air/Missiles
KM Medical
Industrial
[le(.tric rower o-neration
Figure 1. Development of fiber optic sensor market.
*Numbers in the margin indicate pagination in the foreign text.
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Communication applications usually come to mind when fiber optics
is considered. However, in the last few years interest in applying the
advantages afforded by fiber optic signal transmission to the consLruc-
tion of various types of sensors has increased. The fact that fiber op-
tics provides immunity from electromagnetic noise and galvanic insulation
at the measurement point makes this iie? more attractive. Fiber optic
sensors can solve measuring problems in situations where conventional
sensors with electric connections are not practical. Situations in which
strong electromagnetic noise is encountered are found onboard ships and
airplanes and in the electrochemical processing industry. Fiber optic
0 
	
	 sensors can be used to measure high-voltage potentials in high-voltage
installations, and galvanic insulation allows measurements in situa-
tions where there is danger of fire and explosion. The use of fiber
t,
	 optics eliminates the dangers of patient exposure to dangerous voltages
1'.
	
in medical applications.
Stronq Increasc in the Market
A study carried out by Kessler Marketing Intelligance indicates
that the market in the United States is expected to increase from 12
i`	 million dollars in 1981 to almost 180 million in 1991 [1]. Development
i
in the fiber optics field has been dominated by military research and
' development programs for more than five years. For example, the United
States Navy has its FOSS (Fiber Optic Sensor System) program, which in-
cludes undersea acoustic detection systems, sensors for magnetic fields
and fiber optic gyros. Other military programs pertain to guidance sy-
stems for planes and missiles.
A not inconsiderable portion of the market in fiber optic sensors
is related to civilian applications. Such applications include medical
instruments and systems, control systems for electrical power suppliers
and industrial processes. In terms of dollars it is expected that these
applications will soon exceed the military activities. As of 1991 it is
expected that the industr i al applications will be dominant.
UW-
l 
	 J^
ORIW AL V S
OF POOR OUALITY
Fiber optic sensors for temperature, vibration, rate of revolution
(tachometers), fluid level and two-phase current measurements are comm-
ercially available at the present time. In addition the literature con-
tains descriptions of a number of different fiber optic sensors which
are in the developmental stage, but not all of these have been realized
experimentally.
Interferometric Sensors Yield Extremely High Sensitivity
Extremely small variations in the mechanical properties of an optic
fiber can result in considerable variation in the optic wavelength of
the light which propagates through the fiber. This is evident when we /18
consider that 1 cm of an optic fiber typically contains 10 5
 radians of
the optical carrier wave. A phase detector with a sensitivity of 1
radian therefore makes it possible to detect a variation of 1 part in
10 5
 for 1 cm of optic fiber. The sensitivity increases with the length
of the optic fiber, so that 1 part in 10 9
 should be easily detectable.
if we increase the sensitivity of the phase detector to, for example,
10 -4
 radians then the sensitivity for the fiber optic sensor would be
1 part in 1013.
XMITC n(.:
	
of R C1 -ON {1•r
Figure 2. Interferometric Hydrophone.
Light from a laser is split between the sig-
nal fiber and reference fibers.  The light
......ca ll,,(	 at the output of the two fibers is combinednA" i(aS,Y til(1
in a photodetector and emits a siqnal cor-
responding to the acoustic frequency.
Phase shifts in the optic fiber can be introduced ty a number of
external influences. This is especially true with regard to stress,
pressure and temperature. The most delicate sensor configuration oc-
curs when two "single modus" fibers are used as branches in a "Mach-
Zender interferometer". A hydrophone can he based on this principle [2].
In this case the signal fiber is a coil of "single modus" fiber which is
exposed to the acoustic field. The resulting variations in the optical
wavelength are detected by mixing the light from the output of the sig-
3
n
A
qmq
at
na]. fiber with light from a stable reference fiber. 	 An interaction
length of 25 cm gives a sensitivity of -135 dB relative to 1 u Pa [21
normalized to a bandwidth of 1 Hz. With an interaction length of 10 m
and a power level of 10 mW the predicted sensitivity is -180 dB. These
data are n.any orders of magnitude better than those provided by conven-
tional hydrophones.
A considerable effort has been carried out under the auspices of
the United States Navy's FOSS program to develop this type of fiber optic
h_drophone. Magnetic sensors based on this principle have been realized.
une night also mention a microscope for use in acoustooptic spectroscopy
and a sensor for measuring movement in the earth's crust. However, the
latter have not been realized experimentally.
The design of the "Mach-Zender" interferometer is simple in princi-
ple, but the alinement of the optical components is extremely critical.
The fundamental problem with interferometric sensors which operate at
relatively low frequencies is that the configuration is intrinsically
sensitive to vibrations and temperature variations which mask the signal
which one wishes to measure. For example the phase response for a temp-
erature change of 1 0 C is more than 10 6 times greater than the response
for a pressure change of 1 Pa [3]: One method of counteracting this eff-
ect is to place the signal Tiber and the reference fiber in the closest
possible physical contact, but no complete solution to the problem has
yet been devised. During a study trip to the United States in 1981 the
author received the impression that many researchers doubted whether a
practical interferometric sensor could be developed at all. One poss-
ible exception might be temperature sensors, such as those used to test
jet engines, but in this case simpler methods are also available.
Sensors Based on Multi Modus Fibers
A great deal of interest has been expressed in sensors based on
multi modus fibers because of the fundamental problems connected with
f
	 interferometric sensors. In this case the so-called "microbending"
rt
1
4
	
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f
	
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ORIGINAL PACE ,9i
OF POOR QUALITY
effect, which has been a problem in other applications and caused ex-
tra loss in optic fibers used for communications purposes, is used.
The principle involved is that deformations or inhomogeneities in the
fiber geometry create connections and energy transmissions between var-
ious propagation moth in the fiber. For example propagation modi can
be connected to modi which stream out from the fiber. The resulting
energy transfer represents a damping of the light which is transmitted
through the fiber. The effect is dependent upon the difference in the
refractive index between the fiber's core and th- insulation, and this
parameter can be rendered much less temperature dependent that the ref-
.ractive index for the core alone. However, the sensitivity of this
type of fiber optic sensor is lower than that of interferometric sen-
sors.
W,
.MIND
.p.0	 ••s.,n n.n
..nut _^L ASI M	 -- I "70 .	 Figure 3. Principle of Microbending Sensor.
Two corrugated plates provide periodic de-
formation of the fiber geometry.
If a fiber is exposed to periodic geometry deformation by squeezing
it between two corrugated plates [3] extremely effective coupling is ach-
ieved between modi where the difference in number of waves corresponds
to the periodicity determined by the corrugations. An indication of the
forces which are pressing the plates against one another can be obtain-
ed by measuring variations in the optic signal by means of a photodetec-
tor. Hydrophones which have equal or better sensitivity than convention-
al units have been made in this way.
Instead of using corrugated plates it is possible simply to allow
the effect which one wishes to measure, such as an acoustic field to
work d'-re-tly or. the fiber. This yields an extremely simple sensor con-
figuration. i'he detection principle is based upon the phase difference
between different modi which is caused by the external influence in
5
rc 12)
UkIUI),d L I
	 `:j
OF POOR QUALITY
question. Sensors for acoustic fields, currents and electrical currents /23
have been designed in this manner.	 These types of sensors are roughly
three times less sensitive than corresponding interferometric sensors,
but they are still sufficiently sensitive for many applications.
i^
J
.^UNE
Figure 4. Fiber optic pressure transducer developed by the Central Ins-
titute. The greatest diameter is roughly 2 mm, length 2 m. The measur-
ment range extends from -50 to 300 Hq. The sensitivity is better than
0.05 mm Hg. Potential frequency response up to 15 kHz.
Miniaturized Sensors for Point Measurements
The sensors which have been discussed up to this point use the op-
tic fiber itself as the sensitive element. However, ^ number of sensors
have been designed where the optic fibers are used only to conduct light
to and from the point of measurement. Here the light path is broken, and
the connection between the fibers which lead from fhe light source and
the fibers which lead to the photodetector is modulated by the force to
be measured. This provides possibilities for miniaturizing in many ca-
ses.
A number of configurations have been realized, and some of these
+ have resulted in commercial products. For example there are tempera-
ture measuring systems wherein optic fibers conduct light to and from
the measuring tip, which contains liquid crystals which change color
Iml
v	 ^ ^I
Uwith temperature variations. Thermometers have been marketed where an
opt i c fiber conducts heat radiation from the measuremen t_
 point to an
IR detector. Fiber optic systems forlevel measurements are constructed
by coupling light between two fibers using a prism. The connections
will depend upon whether the prism is in the fluid or in the air.
Level measuring systems of this type have been installed in atomic re-
actors, using metal-clad fibers which can tolerate up to 500° C. An
entirely analagous principle is employed	 for measurements in two-
phase current. With regard to commercially available fiber optic sen-
sors we should also mention tachometers where an optic fiber bundle is
used tc measure how rapidly black and white fields on the rotating
axle pass by the measurement pint. Sensors which use fiber bundlFs to
measure level and vibrations are also available.
Medical Manometers with Great User Potential
The development of fiber optic sensors in Norway has been carried
out by the Central T_nstitute for Industrial Research (SI). This work
has already been described in Elektro [4]. This development ! g as re-
sulted in medical manometers. The use of fiber optics means that it
is no longer necessary to connect electrical wires to the patient.
This is important for the safety of the patient, and satisfies the great
demands for galvanic insulation imposed by the medical equipment which
is now coming into use. The frequency response is excellent because
of the miniaturized membrane, which is 1.5 mm in diameter. These mano-
meters are now undergoing clinical testing. Up until now they have
been used in the heart/blood vessel system, in the duodenum and in the
bladder. In the near future they will be used for measurements in the
womb during childbirth. Fiber optic sensors are also used to measure
oxygen saturation in the blood. This unit will be combined with the
fiber optic manometer in a combination medical probe.
The principle involved in the medical manontoters is that the -on-
nection between the fibers which lead to the measurement point from the
light source and the fibers which lead to the photodetector from the
measuring point is modulated by the curvature of the membrane. In future
4
7	 1) 1'I
ndevelopmental work at the Central Institute we will attempt to use
the fiber itself as the sensitive element. we hope to make use of the
microbending effect described earlier. The goal here is to develop sen-
sors for process monitoring in, for example, situations where strong
electromagnetic noise is a problem.
s
REFERENCES
1. Electro-Optical Systems Design, June 1981.
2. J.	 H.	 Cole,	 R.	 L.	 Johnson and P.	 G.	 I3huta,
	 "Fiber-optic detection
of sound,"	 J.	 Acoust.	 Soc.	 Am.,	 Vol.	 62, pp.	 1136-1138, Nov-
ember 1977.
3. J.	 N.	 Fields,	 C.	 K.	 Asawa,	 0.	 G.	 Ramer,	 and M.	 K.	 Barnoski: "Fiber
optic pressure sensor," J.
	
Acoust.	 Soc. Am.,	 Vol.	 67,	 pp 816-
818,	 March	 1980.
4. T.	 E.	 Hansen,	 "Fiber optic senscrs: 	 New possibilities on the trans-
4
mitter front,"	 Elektro,	 Vol.	 10,	 1979.
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Great prospects for fiber optics sensors

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