Fiber optic temperature sensing

Fiber optic waveguides exhibiting a blackout phenomenon can be used for temperature sensing. A temperature sensing waveguide can be used in such applications as maintaining a material within a selected temperature range, freeze protection viscosity control of liquids in pipelines, leak detection of cryogenic fluids, fire detection, application of heat-recoverable materials, battery charging, and fluid level detection. Novel waveguides exhibiting blackout at selected temperatures for use in these applications are described.

CROSS-REFERENCE 
This application is related to copending and co-assigned application Ser. 
No. 136,076, filed Mar. 31, 1980 by Raymond Clarke, entitled "THERMOSTATIC 
FIBER OPTIC WAVEGUIDES", which is incorporated herein by this reference. 
BACKGROUND OF THE INVENTION 
The present invention is directed to methods for using fiber optic 
waveguides for temperature monitoring. 
The use of fiber optics for transmitting information has recently received 
a great deal of attention because of the light weight, security, safety, 
and electrical isolation that can be obtained with a fiber optic system. 
It has also been proposed to use optical fibers for noninterferometric 
measurement of temperatures in a paper by Gottlieb et al presented at the 
Electro-Optics conference in Anaheim, Calif. in October, 1978. Gottlieb et 
al proposed that the loss of light to the cladding of a waveguide depends 
upon the temperature of the waveguide. In U.S. Pat. No. 4,151,747 issued 
to Gottlieb et al, there are described fiber optic temperature monitoring 
systems. 
There are many applications in which it would be advantageous to use a 
fiber optic system for temperature measurement and monitoring, for which 
no system has been available. For example, when monitoring the temperature 
of flammable fluids, it would be preferred to use a non-electrical system 
to avoid the explosion hazard associated with electrical temperature 
monitoring systems. In some applications, such as monitoring the 
temperatures of electrical generators, the high RF noise produced 
interferes with conventional remote sensing methods. The use of fiber 
optic systems for monitoring temperatures in these applications andother 
applications has been hampered by the non-availability of suitable 
waveguides which have light transmission properties that vary 
substantially at useful temperatures. 
In view of the foregoing, there is a need for systems for monitoring 
temperatures using fiber optic waveguides. 
SUMMARY 
The present invention is directed to methods and systems using fiber optic 
waveguides for temperature sensing. The waveguides used comprise a core 
and a cladding disposed on and around the exterior surface of the core, 
where at least a portion of the waveguide exhibits blackout at a selected 
blackout temperature. By "blackout", there is meant that on one side of 
the blackout temperature or temperature range, the waveguide transmits 
light, but on the opposie side of the blackout temperature or temperature 
range, substantially no light is transmitted through the waveguide. This 
blackout phenomenon occurs when the index of refraction of the core and 
the index of refraction of the cladding become about equal. The blackout 
can also be the result of crystallization of the cladding, crystallization 
causing a change in the refractive index of the cladding and/or light 
scattering. 
Novel waveguides described herein which are the subject of the 
aforementioned application Ser. No. 136,076 by Clarke, provide the 
opportunity to use fiber optic systems in applications never heretofore 
thought possible. It is now possible to tailor make waveguides so that 
they blackout at selected temperatures. Among the applications for these 
waveguides are methods and systems for maintaining a material within a 
selected temperature range. In such an application, at least part of a 
waveguide is placed in thermal communication with the material so that the 
temperature of said part of the waveguide is responsive to the temprature 
of the material. The waveguide is chosen so that said part of the 
waveguide exhibits blackout at a selected blackout temperature at about 
the bottom or at about the top of the selected temperatue range. Light is 
directed at one end of the waveguide and the intensity of light 
transmitted by said part of the waveguide is monitored. The onset of a 
substantial change in the intensity of light transmitted by said part 
indicates that the material is at a temperature near the top or bottom of 
the selected temperature range. When the substantial change in the 
intensity of the light transmitted by said part of the waveguide occurs, 
the temperature of the material is adjusted so that it is maintained 
within the selected temperature range. 
Other systems and methods which can use these novel waveguides include 
systems and methods for preventing a material from undergoing a change in 
phase, such as preventing liquids from freezing; systems and methods for 
preventing the viscosity of a liquid in a pipeline from increasing above a 
selected value; systems and methods for detecting fire; systems and 
methods for regulating the charging of batteries; and systems and methods 
for applying an article containing a heat-activatable material to a 
substrate, such as a heat-recoverable tubular sleeve containing a 
heat-activatable adhesive to a pipe. In this last application, the 
waveguide is placed in thermal communication with the article so that the 
temperature of the waveguide is responsive to the temperature of the 
heat-activatable material. The waveguide is selected so that its blackout 
temperature is no less than the temperature at which the heat activatable 
material is activated. Light is directed at one end of the waveguide and 
the intensity of light transmitted by the waveguide is monitored. The 
heat-recoverable material is heated at least until the intensity of light 
transmitted by the waveguide has undergone a substantial change. 
DRAWINGS 
These and other features, aspects, and advantages of the present invention 
will become better understood with reference to the appended claims, the 
following description, and accompanying drawings, where:

DESCRIPTION 
A. Introduction and Definitions 
The present invention is directed to the use of fiber optic waveguides as 
temperature sensors. Use is made of the principle that waveguides can be 
prepared so that at a selected temperature or within a selected 
temperature range, the waveguide can exhibit blackout, i.e., the waveguide 
transmits substantially no light. Blackout is detected with a monitor in 
that it is determined that the light transmission property of the 
waveguide has undergone a substantial change. As used herein, the term 
"substantial change" in light transmission property refers to a decrease 
or increase of at least 3 db (decibels), and preferably at least about 5 
db, amounts that can be detected with state-of-the-art monitors. For 
example, a substantial change can be a change in attenuation from 5 db up 
to 8 db, from 20 db up to 25 db, 10 db down to 7 db, or 25 db down to 20 
db 
The 'blackout temperature" is the temperature or temperature range where 
blackout occurs. It is characterized by a substantial change in 
attenuation over a very small temperature change, and generally over a 
temperature change of 3.degree. C. or less. In other words, preferably a 
plot of attenuation vs. temperature has a positive slope of at least about 
1 db/(1.degree. C.) or a negative slope of about -1 db/(1.degree. C.) or 
less, at the blackout temperature. 
As used herein, the term "sensing element" refers to a waveguide or a 
portion of a waveguide that exhibits blackout at one or more selected 
temperatures or temperature ranges. 
To determine if a substantial change in the light transmission property of 
a waveguide has occurred, it is necessary to monitor the intensity of 
light transmitted by the waveguide. As used herein, the term "monitoring" 
the intensity of the light refers to monitoring light at either end of the 
waveguide using conventional monitoring equipment. For example, using an 
optical time domain reflectometer such as Model ODTR-103 sold by Orionics, 
Inc. of Albuquerque, N. Mex., it is possible to monitor for transmitted 
light at the same end of a waveguide at which light pulses are launched 
into the waveguide. 
In one version of the present invention, a waveguide can undergo a 
permanent change in its light transmission properties after its 
temperature is increased or lowered to a selected temperature. By the term 
"permanent" change, there is meant that the change in light transmission 
properties is irreversible. For example, a waveguide can be prepared that 
until it is heated to a temperature greater than about 100.degree. C., it 
is substantially incapable of transmitting light, but once it is heated to 
100.degree. C., it will transmit light, even if subsequently, the 
temperature of the waveguide is lowered to below 100.degree. C. 
B. Waveguides 
Waveguides consisting of a variety of materials have been developed in the 
prior art. For example, waveguides consisting of a glass fiber core and 
glass cladding, glass cladding and a liquid core, a polymeric fiber core 
and polymeric cladding, and a glass fiber core and polymeric cladding are 
known. U.S. patent application Ser. No. 964,506 filed by Ellis et al on 
Nov. 29, 1978, (now U.S. Pat. No. 4,290,668) is incorporated herein by 
reference, is directed to waveguides comprised of a quartz glass core and 
polymeric cladding of polydimethyl siloxane. U.S. Pat. No. 3,819,250 
issued to Kibler describes a waveguide comprising a quartz cladding and a 
liquid core which can be carbon tetrachloride. 
The effectiveness of the present invention relies upon the use of a 
waveguide where the refractive index of the core and the refractive index 
of the cladding change with temperature at different rates. For example, 
silica has a much lower coefficient of thermal expansion than polymers in 
general, and especially siloxanes. Because of this, the refractive index 
of a siloxane cladding changes much more rapidly with temperature than a 
silica core. Using the expression (1/P)(dP/dT)=-q(n).alpha.(Polymer 
Handbook, Immergut & Bandrup) it is possible to calculate the refractive 
indices of a core and a cladding vs. temperature. For example, FIG. 1 
presents the calculated change in refractive index vs. temperature for 
silica and two commercially-available siloxanes, Sylgard 184 and GE 670. 
Sylgard 184 is branched polydimethyl siloxane with some phenyl 
substitution available from Dow Corning. GE 670 is a branched polydimethyl 
siloxane available from General Electric. 
The waveguides used in the present invention include novel waveguides which 
use the invention of Raymond Clarke and which are the subject matter of 
the aforementioned application Ser. No. 136,076. 
In order for a waveguide to transmit light, it is necessary that the 
refractive index of the cladding be less than the refractive index of the 
core. When the refractive index of the core and the cladding are about 
equal, light is no longer contained by the cladding and a blackout occurs. 
FIG. 2 shows the blackout phenomenon for a number of commercially 
available siloxanes coated on a silica fiber. FIG. 2 presents attenuation 
in db vs. temperatue for GE 670, Sylgard 184, and GE 655 claddings on a 
silica core. The waveguides were about 100 meters long. The silica core 
was about 200 microns in diameter and the cladding was about 20 microns 
thick. GE 655 is branched polydimethyl siloxane with some phenyl 
substitution. The data presented in FIG. 2 were obtained by measuring the 
attenuation of the waveguide resulting from the waveguide being cooled at 
2.degree. C./minute. The blackout tempratures shown for the GE 670 and 
Sylgard 184 clad fibers correlate approximately with the crossover points 
in refractive index shown in FIG. 1. 
In practice, as the temperatue of one of the waveguides of FIG. 2 is 
decreased, a substantial change in attenuation is noted until eventually 
no light is transmitted. This phenomenon occurs even if only a very short 
portion of a long waveguide is cooled to the blackout temperature. For 
example, cooling a one centimeter length of a one kilometer long waveguide 
to the blackout temperature can be detected as a substantial increase in 
attenuation of transmitted light. 
The reverse of this phenomenon also occurs. As the GE 670/silica, Sylgard 
184/silica, and GE 655/silica waveguides are heated from a temperature of 
-100.degree. C. to a temperature greater than -40.degree. C., the amount 
of light transmitted by the waveguide increases. Initially, substantially 
no light is transmitted, until eventually light is transmitted by the 
waveguides. 
In some applications, it is desirable that the waveguide exhibit blackout 
as its temperature is raised. The attenuation of such a waveguide is shown 
in FIG. 3, where the waveguide exhibits blackout at about 50.degree. C. At 
temperatures less than about 50.degree. C., the waveguide transmits light. 
A waveguide with a cladding of silica, the exterior surface of which is 
coated with a material that will inhibit Light Propagation in the cladding 
i.e. a light absorptive material such as polymethylphenyl siloxane having 
an index of refraction of about 1.50 containing about 5% by weight of 
carbon black, and a core of polymethylphenyl siloxane with a refractive 
index of 1.47 at 23.degree. C. exhibits an attenuation vs. temperature 
curve similar to that shown in FIG. 3. A disadvantage with such a 
waveguide is that even at temperatures at which it transmits light, the 
amount of attenuation is substantially more than is obtained with a 
waveguide with a silica core because silica has much better optical 
properties than polymethylphenyl siloxane. However, this disadvantage is 
not important where only a short waveguide is required, or where a short 
sensing element is incorporated such as by splicing into a long waveguide, 
where the remainder of the waveguide has excellent light transmission 
properties. 
Another example of a waveguide that can exhibit blackout as its temperature 
is increased is the waveguide having a silica cladding and a liquid core 
described in the above-mentioned Kibler Pat. No. 3,819,250. With a liquid 
core of carbon tetrachloride, blackout can occur at a temperature of about 
25.degree. C., provided the exterior surface of the silica tube is coated 
with a light absorptive material which prevents light transmission within 
the silica. 
Another waveguide that exhibits blackout as its temperature is increased is 
one having a core of polymethylphenyl siloxane containing 12% phenyl by 
weight, and a cladding of a Kynar copolymer such as Kynar 7200 available 
from Pennwalt corporation, which is a copolymer of vinylidine fluoride and 
tetrafluoro ethylene. 
It is important to be able to control the temperature at which blackout 
occurs. For example, for heat tracing of a pipeline containing an aqueous 
fluid, it is desirable that blackout occur at a temperature slightly 
greater than freezing so that a heating element can be activated before 
the water freezes. 
Prior art waveguides exhibit blackout at temperatures of little practical 
significance. The present invention is directed to use of novel waveguides 
that exhibit blackout at temperatures of practical use. The novel 
waveguides exhibit blackout at selected blackout temperatures greater than 
about -20.degree. C. and less than 200.degree. C., and more preferably at 
temperatures greater than about 0.degree. C. and less than 100.degree. C., 
i.e. temperatures of commercial interest. Preferably the core of the novel 
waveguide is solid over the temperature range of desired waveguide use. 
One method to control the blackout temperature of a waveguide is to vary 
the refractive index of the cladding, the core, or both. For example, the 
waveguide comprising a silica core and GE 655 cladding was soaked in 
bromonapthalene which has a refractive index of 1.61. This raised the 
refractive index of the cladding, which had the effect of raising the 
blackout temperature. As shown in FIG. 2, the blackout temperature of this 
waveguide was raised to about 5.degree. C. Other additives and dopants can 
be added to a cladding to either raise or lower its refractive index, 
depending upon the blackout temperature desired. Preferably the dopant 
used is non-volatile so that it remains permanently in the cladding or 
core. Satisfactory dopants for siloxane claddings include monomeric high 
boiling materials which are compatible with the siloxane cladding. 
Examples of dopants which can be used to raise the refractive index of 
siloxane cladding are 2, 2-dimethyltetraphenylcyclotrisiloxane; 
1,1,1,5,5,5-hexamethyldiphenyltrisiloxane; hexaphenylcyclotrisiloxane; 
tetraphenylsilane; and diallyldiphenylsilane. The cladding can be 
irradiated to about 5 Mrads with an electron beam subsequent to imbibing 
in the dopant to permanently graft the dopant to the polymeric cladding. 
Other dopants can be introduced prior to the curing process. 
Other materials which may be used as dopants are low molecular weight 
chlorinated phenylsiloxanes and nitrile containing siloxanes. 
Monomeric high boiling materials such as neopentylglycolpolyadipate and 
paraffin oils which are not siloxanes can be used as dopants in small 
quantities, but they suffer from the fact that they are inadequately 
compatible with siloxane cladding and are expelled from the cladding with 
time. 
Another approach that can be used to provide a waveguide with a higher 
blackout temperature than the blackout tempratures obtained with 
conventional silica/polydimethyl siloxane claddings is the development of 
waveguides comprised of materials heretofore not used as cladding 
materials. Novel cladding materials developed include claddings comprising 
a polyalkylphenyl siloxane, where the alkyl portion of the siloxane 
contains no more than 10 carbon atoms, and preferably is a methyl group. 
The phenyl content is preferably at least 15% by weight; as the phenyl 
content of a polymethylphenyl siloxane increases, the refractive index of 
the siloxane increases. Table 1 presents the refractive index of 
polymethylphenyl siloxanes as a function of their phenyl content. The 
percent by weight phenyl is based upon the total weight of the siloxane. 
Also presented in Table 1 is the blackout temperature of a waveguide 
comprising a silica core and a polymethylphenyl siloxane cladding having 
the specified phenyl content. All phenyl contents referred to herein are 
determined by ultraviolet spectroscopy. 
TABLE 1 
______________________________________ 
% by Weight Ph 
Refractive Index 
Blackout Temp (.degree.C.) 
______________________________________ 
16.25 1.446 0.degree. C. 
17.0 1.448 5.degree. C. 
18.25 1.451 10.degree. C. 
19.0 1.453 15.degree. C. 
______________________________________ 
Cladding materials of different phenyl content can be prepared by blending 
methylphenyl siloxanes with different phenyl contents. However, in 
practice, it is found that blends which differ widely in phenyl content 
tend to be milky to opaque. Therefore, when blending methylphenyl 
siloxanes, preferably the siloxanes differ in refractive index by no more 
than about 0.02 and the viscosities of both siloxanes are in the range of 
from about 500 to about 10,000 cps as measured at 25.degree. C. 
It is also possible to cross-link methylphenyl siloxanes of different 
phenyl content. For example, by blending a methylphenyl siloxane having a 
viscosity of 2,000 cps and a phenyl content of 21% and having terminal 
vinyl content of 1 mole % with a second methylphenyl siloxane having a 
viscosity of 2,000 cps and a phenyl content of 16%, a cladding is produced 
which in combination with a silica core, provides any blackout temperature 
required in the range of 0.degree. to 15.degree. C. 
A polyalkylphenyl siloxane of the desired phenyl content can be prepared 
according to conventional polymerization techniques, where the starting 
materials include dialkyl chlorosilane, diphenyl chlorosilane, and 
alkylphenyl chlorosilane. In preparing the polyalkylphenyl siloxane, the 
alkyl groups can be the same or different. 
Another novel waveguide has a cladding made of a material that crystallizes 
as its temperature is lowered. FIG. 4 pesents the attenuation vs. 
temperature curve for a waveguide having a silica core of 200 microns, a 
first cladding of KE 103 having a thickness of 30 to 35 microns, and an 
outer cladding layer of Sylgard 184 having a thickness of 60 microns KE 
103 is a low molecular weight polydimethyl siloxane available from 
Shin-Etsu of Japan that crystallizes as its temperature is lowered. The 
outer layer of Sylgard is required because the KE 103 has poor mechanical 
properties. As shown by FIG. 4, the waveguide exhibits a large and sudden 
increase in attenuation at about -56.degree. C. as it temperature is 
decreased, and also exhibits a large and sudden decrease in attenuation at 
about -40.degree. C. as its temperature is increased, showing a hysteresis 
effect. This large and sudden change in attenuation occurs because KE 103 
is a linear, low molecular weight material and is able to crystallize. It 
has a differential scanning calorimeter melting point of -45.degree. C. 
when warmed from -120.degree. C. at 5.degree. C. per minute. Thus sudden 
and large changes in attenuation result from the KE 103 and are caused by 
the material changing from a crystalline to an amorphous material at its 
melting point, and by the material changing from an amorphous material to 
a crystalline material at its freezing point. 
At its crystallization temperature, a large increase in the refractive 
index of KE 103 occurs so that its refractive index is no longer less than 
the refractive index of the core. Thus blackout occurs. Also contributing 
to blackout is light scattering resulting from the crystallization. 
To insure that the crystallization occurs at a specified temperature, it is 
believed that a nucleating agent such as fumed silica can be used to 
prevent the freezing point from varying as a result of super cooling of 
the polymer liquid. 
As is evident from FIG. 4, an advantage of using a polymer that 
crystallizes as a cladding is that the blackout occurs over a very small 
temperature range. Thus, the waveguide can be used in applications where 
close control of the temprature of a material is essential. 
To be useful in waveguides, a material that exhibits this crystallization 
phenomenon preferably is sufficiently optically clear to be used as a 
cladding, and has a refractive index lower than that of silica. 
In addition to KE 103, copolymers of dimethylsiloxane and ehtylene oxide 
meet these requirements. The refractive index and crystalline melting 
point of the copolymer can be altered as required by varying the molar 
ration of the siloxane to the ethylene oxide and also by the chain length 
of the ethylene oxide block. A methd for making these copolymers is 
described in U.S. Pat. No. Re. 25,727 which is incorporated herein by this 
reference. Preferably the copolymer prepared has a refractive index less 
than that of silica (about 1.46 at 23.degree. C.) so that it can be used 
as a cladding for silica cores. Particularly suitable polyethylene oxide 
dimethylsiloxane copolymers for water freeze protection are those whose 
preparation is described in Examples 1 and 2 of the 25,727 patent. These 
copolymers have a freezing point of 1.degree. C. and refractive indices of 
1.4595 and 1.4555, respectively. These copolymers can be protected from 
absorbing moisture by a water resistant exterior cladding. 
Crosslinked polydialkyl siloxanes such as polydiethyl siloxane also exhibit 
this crystalline melting point phenomenon. Polyalkyl siloxanes for use as 
a cladding comprise the repeating unit: 
##STR1## 
where each R.sub.1 is independently selected from the group consisting of 
methyl, ethyl, and propyl groups; and 
where R.sub.2 is independently an alkyl group, and preferably a linear 
alkyl group, of at least 10 carbon atoms, and preferably no more than 
about 20 carbon atoms. 
These materials crystallize due to the presence of the long alkyl side 
groups. For example polymethylhexadecylsiloxane has a melting point of 
42.degree. C., a refractive index of 1.4524 (44.degree. C.), and a 
freezing point of 27.degree. C. Preferred materials are cross-linked 
polymethylalkyl siloxanes, i.e. R.sub.1 is a methyl group. 
The polymethylalkyl siloxanes can be prepared by reacting the alkene 
corresponding to the alkyl portion of the siloxane with polymethylhydrogen 
siloxane in the presence of chloroplatinic acid catalyst. From about 80 to 
about 95% of the hydrogens are reacted, and at least a portion of the 
remaining free hydrogens are cross-linked with cross-linking agents such 
as tetravinyl silane in the presence of chloroplatinic acid catalyst. 
Other polydialkyl siloxanes can be correspondingly prepared using 
polyethylhydrogen siloxane or polypropylhydrogen siloxane. 
The amount of substitution affects the crystalline melting point. For 
example, polymethyltetradecyl siloxane, prior to cross-linking, has a 
crystalline melting point of 7.degree. C. when 80% of the hydrogen is 
substituted with tetradecene, 12.degree. C. with 90% substitution, and 
14.degree. C. with 100% substitution. 
Waveguides consisting of a cladding of cross-linked polymethylpentadecyl 
siloxane on a glass core were prepared. When the glass core used was a 
silica cord, blackout occurred at about 5.degree. C. When the glass core 
used was made from sodiumborosilicate, blackout occured at about 
-1.degree. C. 
Preferably a wavguide used in the present invention has a core with a 
diameter of from about 100 to about 300 microns, and most preferably about 
200 microns. With cores of less than 100 microns, it is difficult to 
couple and connect the waveguide. Furthermore, with a larger core than 100 
microns, it is possible to transmit larger amounts of light for longer 
distances. However, at diameters much greater than 300 microns, the 
advantages obtained are insufficient to overcome the increased material 
costs and breakage caused by bending. 
Unless indicated otherwise, all refractive indices mentioned herein refer 
to the refractive index of a material measured at a temperature of 
25.degree. C. with sodium light 589 nm. However, the waveguides of the 
present invention are not limited to use with just visible light. They can 
be used with ultraviolet and infrared light. Thus the term "light" as used 
herin refers to visible light, ultraviolet light, and infrared light. 
The cladding can be applied to the core in situ, where the cladding is 
cross-linked directly on the core. In applying a cladding to an optical 
fiber, preferably the fiber is coated before moisture or other 
contaminants reach the fibre. Also, it is important to avoid scratching or 
otherwise abrading the fiber because this can drastically reduce the 
tensile strength of the fiber. With these problems in mind, it is prefered 
to apply a cladding with a low modulus applicator such as that described 
by A. C. Hart, Jr., and R. B. Albarino in "An Improved Fabrication 
Technique For Applying Coatings To Optical Fiber Wave Guides", Optical 
Fiber Transmission II Proceedings, Februrary 1977. Preferably the cladding 
material is applied to the fiber core as a liquid. 
In some applications it is desireable that the waveguide exhibits 
substantial change in its light transmission property at two temperatures. 
For example, when providing freeze protection, it is desirable that the 
waveguide used exhibits substantial attenuation of transmitted light at a 
temperature of about 5.degree. C., and then exhibit even more attenuation 
at about 1.degree. C. The 5.degree. C. breakpoint can be used as a signal 
for turning on a heater, and the 1.degree. C. breakpoint can be used as an 
emergency alarm. A waveguide suitable for such an application is shown in 
FIG. 5 and its attenuation vs. temperature curve is shown in FIG. 6. 
The waveguide of FIG. 5 comprises a core A, a first cladding B disposed on 
and around the exterior surface of the core, and a second cladding C 
disposed on and around the exterior surface of the first cladding. The 
refractive index of the first cladding B is less than the refractive index 
of the core A at temperatures greater than the first selected temperature 
T.sub.1, and is greater than or equal to the refractive index of the core 
A at temperatures less than T.sub.1. The refractive index of the second 
cladding C is less than the refractive index of the first cladding B at 
temperatures greater than the second selected temperature T.sub.2 and is 
greater than or equal to the refractive index of the first cladding B at 
temperatures less than T.sub.2. T.sub.1 is greater than T.sub.2. 
A waveguide of this construction has the attenuation vs. temperature curve 
as shown in FIG. 6. What occurs is that as the temperature of the 
waveguide is reduced to T.sub.1, which corresponds to about 10.degree. C. 
in FIG. 6, the refractive index of the first cladding becomes equal to the 
refractive index of the core. Thus, a portion of the transmitted light is 
absorbed by the cladding and the attenuation is increased. As the 
temperature of the waveguide is further decreased, the refractive index of 
the second cladding becomes equal to the refractive index of the core at 
T.sub.2, which corresponds to about 0.degree. C. in FIG. 6. At this point, 
blackout occurs. 
To be readily detectable, preferably the level of attenuation that occurs 
at temperatures less than T.sub.1 is at least 3 db greater than the level 
of attenuation at temperatures greater than T.sub.1. Also, preferably the 
level of attenuation that occurs at temperatures less than T.sub.2 is at 
least about 3 db greater than the level of attenuation at temperatures 
between T.sub.1 and T.sub.2. The amount of attenuation that occurs at 
temperatures less than T.sub.1 can be controlled by varying the thickness 
of the first cladding. The smaller the thickness, the less attenuation 
that occurs. Preferably the first cladding layer is thinner than the 
second cladding layer, and generally is on the order of about 5 microns 
thick vs. about 20 microns thick for the second cladding layer. 
An example of a waveguide that exhibits this two-step change in attenuation 
is one consisting of a silica core, a first cladding layer of cross-linked 
polymethylphenyl siloxane, and a second cladding of polydimethyl siloxane. 
For such a cladding T.sub.1 is 14.degree. C., and T.sub.2 is -52.degree. 
C. 
In some applications it is desirable that a single waveguide exhibit 
blackout at both ends of a selected temperature range of T.sub.3 to 
T.sub.4. The attenuation vs. temperature curve of such a waveguide is 
shown in FIG. 7, where light is transmitted without substantial 
attenuation between about 10.degree. C. to about 80.degree. C., but at 
about 10.degree. C. and 80.degree. C., blackout occurs. A waveguide with 
this performance characteristic can have the construction shown in FIG. 5, 
where it comprises a core A of high loss material, a light transmissive 
layer B disposed on and around the exterior surface of the core, and an 
exterior cladding C disposed on and around the exterior surface of the 
light transmissive layer B. The core A is a poorer transmitter of light at 
temperatures lower than or equal to T.sub.3 than is the light transmissive 
layer. The refractive index of the light transmissive layer B is greater 
than the refractive indices of both the core A and the exterior cladding C 
only at temperatures within the selected temperature range of T.sub.3 to 
T.sub.4, T.sub.3 being less than T.sub.4. At temperatures less than 
T.sub.3, the refractive index of the core A is greater than or equal to 
the refractive index of the light transmissive layer B, so that light is 
no longer contained by the core A in the light transmissive layer B 
because the core A is made of a high loss material, light passing into the 
core is absorbed and blackout occuring. At temperatures greater than 
T.sub.4, the refractive index of the exterior cladding C is greater than 
or equal to the refractive index of the light transmissive layer B. 
Because C is a less light transmissive material than B, there is an 
increase in attenuation at temperatures greater than T.sub.4. 
The change in refractive index vs. temperature for the components of a 
waveguide constructed in accordance with this version of the invention is 
shown in FIG. 8. These refractive index curves correspond to the 
attenuation curve shown in FIG. 7. As shown in FIG 8, the refractive index 
of the core A is less than the refractive index of the light transmissive 
layer B at temperatures greater than about 10.degree. C.(T.sub.3). At 
temperatures greater than about 80.degree. C.(T.sub.4), the refractive 
index of the exterior cladding C is greater than the refractive index of 
the light transmissive layer B. 
A waveguide having the attenuation vs. temperature curve shown in FIG. 7 
can comprise a core A made of polymethyltetradecyl siloxane, a light 
transmissive layer B of polymethylphenyl siloxane of 35% by weight phenyl 
content, and an outer cladding C of silica coated with polymethylphenyl 
siloxane of more than 50% by weight phenyl content, and containing 5% by 
weight of carbon black. 
Another waveguide having the attenuation vs. temperature curve of FIG. 7 
can be prepared where the refractive index of the core A is greater than 
or equal to the refractive index of the light transmissive layer B at 
temperatures greater than or equal to T.sub.4, and the refractive index of 
the exterior cladding C is greater than or equal to the refractive index 
of the light transmissive layer B at temperatures less than or equal to 
T.sub.3. The core is a poorer light transmitter at temperatures greater 
than or equal to T.sub.4 than is the light transmissive layer B. 
As described above, in some applications, it is desirable that once a 
waveguide undergoes a substantial change in its light transmission 
properties, that change be permanent and irreversible. An example of such 
a waveguide is one having a polyvinylidene fluoride core (available under 
the trade name Kynar from Pennwalt) or polymethylmethacrylate core, and a 
cladding of polydimethyl siloxane. The core is loaded with about 1% by 
weight of an antioxidant such as 2, 6 di-teriary butyl para-cresol. When 
the loaded core is irradiated with gamma rays to 5 Mrads, it becomes 
colored due to color centers forming from the antioxidant. Thus, due to 
the coloring, the amount of light transmission is substantially reduced. 
However, when the temperature of the core is raised up to about its 
melting point, the color centers are permanently eliminated. Thus, once a 
waveguide with the core having color centers is heated up to about the 
melting point of the core, the waveguide is permanently changed to one 
that can transmit light. 
As noted above, only a portion of a waveguide needs to exhibit blackout as 
a result of a temperature change. Thus a waveguide can have portions which 
exhibit blackout at a selected temperature or within a selected 
temperature range, where the portions are separated by a portion that does 
not exhibit blackout at the selected temperature or within the selected 
temperature range. This is particularly useful when the sensing element 
exhibits relatively poor transmission properties even when it is operating 
in its mode of transmitting light. 
In addition, a single waveguide can include a plurality (two or more) 
sensing elements which exhibit blackout at different temperatures. For 
example, one sensing element can be activated at about 0.degree. C. as 
cooled and another sensing element can be activated at about 100.degree. 
C. as heated. With such a waveguide, light can be transmitted only from 
about 0 to about 100.degree. C. Such a waveguide can be used as part of a 
system for keeping water liquid. 
A method of making such a waveguide with different temperature responsive 
sections is to remove a portion of the cladding from the conventional 
waveguide and replace the removed portion of the cladding with cladding 
that results in the waveguide having a temperature responsive sensing 
element. For example, a two centimeter length of the cladding can be 
removed from a waveguide comprising silica core and a GE670 cladding (a 
branched polydimethylsiloxane). The cladding can be removed with wire 
strippers, followed by removal of any residue with tetramethylguanidine, 
followed by a rinse with toluene and then isopropanol. The waveguide is 
maintained in a fixed position so that bare core can be surrounded with 
uncured cladding which can be cured in position. The cladding can be a 
methylphenylsiloxane whose refractive index controls the blackout 
temperature, or a methylalkylsiloxane in which the blackout temperature 
depends on the crystalline melting point of the cladding. 
Another method for preparing a waveguide having a short sensing element 
therein is to dope the cladding of a waveguide at selected locations with 
a dopant that alters the refractive index of the cladding. 
Rather than curing a replacement cladding in situ to replace a cladding 
that has been stripped from the waveguide, the new cladding can be placed 
inside a heat shrinkable sleeve. The heat shrinkable sleeve can be placed 
in position over an area of the waveguide where the cladding has been 
removed and then heated, thereby shrinking the sleeve. The coating on the 
inside of the sleeve can then provide a cladding having refractive index 
properties that provide the waveguide with a useful sensing element. 
Another method for producing a single waveguide having one or more sensing 
elements along its length, where the sensing elements can exhibit a 
substantial change in light transmission properties at different 
temperatures, is to pass the core through two applicators which are in 
tandem. By using a starve-feed system to each of the applicators, 
different claddings having different refractive index characteristics can 
be applied to different lengths of the core. 
The following examples present waveguides useful in the present invention. 
EXAMPLE 1 
The Example shows how a waveguide having a blackout temperature of about 
0.degree. C. for use in freeze protection can be prepared. 
A nine meter length of waveguide comprising a 200 micron fused silica core 
and a cladding of about 30 microns thick of polydimethylsiloxane available 
under the trade name GE670 was prepared. A two centimeter length of the 
cladding was mechanically stripped. Any residue present was removed with 
tetramethylguanidine and rinsed with toluene and isopropanol. The 
waveguide was held in a fixed position and the uncoated fiber was 
surrounded with a methylphenylsiloxane solution. The solution consisted of 
21.84% of a methylphenylsiloxane containing 15.5% phenyl by weight, 58.16% 
of a methylphenylsiloxane containing 20.5% phenyl by weight, and 20% of a 
methylphenylsiloxane containing 7% phenyl by weight. The refractive index 
of the solution before curing was 1.4466 and after curing in situ in the 
presence of a chloroplatinaic acid catalyst, the refractive index was 
1.4498. The thickness of the new cladding was about 1/4 inch. The 
attenuation vs. temperature curve for the waveguide is presented in FIG. 
9. 
EXAMPLE 2 
Using the same waveguide originally used for Example 1, a two centimeter 
length of the polydimethylsiloxane cladding was replaced with a siloxane 
composition containing 43% by weight phenyl and having a refractive index 
of 1.513 before curing. The waveguide did not transmit light at room 
temperature. However, as shown in FIG. 10, at temperatures above 
160.degree. C., the waveguide did transmit light. 
EXAMPLE 3 
This example demonstrates preparation of a waveguide that cannot transmit 
light at ambient temperature, but when raised to an elevated temperature, 
irreversibly changes so that it can transmit light, even after its 
temperature is reduced to ambient temperature. 
A waveguide was prepared having a core of polymethylmethacrylate having a 
diameter of 0.013 inch. The cladding was polydimethylsiloxane having a 
thickness of about 50 microns. A second waveguide was prepared, differing 
from the first waveguide in that it contained 1% by weight of Irganox 
1010, an antioxidant available from Ciba Geigy. When light from a helium 
neon laser was directed through each of the waveguides, the first 
waveguide transmitted light along the length of 22 inches and the second 
waveguide transmitted light satisfactorily along a length of 28 inches as 
detected visually be the experimenter. 
Both fibers were irradiated with a high energy electron beam to 10 Mrads. 
The first fiber still allowed light to transmit an amount of 60% of the 
previous length. However, the second fiber would not allow any light to be 
transmitted. The second fiber was heated to 80.degree. C. for two hours 
and was then able to transmit light in an amount of 50% of its original 
transmission properties, even after its temperature was reduced to ambient 
temperature. 
C. Applications 
A wide variety of applications are available for using the waveguides 
described above. For example, according to the present invention, a 
material can be maintained within a selected temperature range. This is 
accomplished by placing at least part of a waveguide in thermal 
communication with the material so that the temperature of the part of the 
waveguide is responsive to the temperature of the material. The waveguide 
is selected so that it exhibits blackout at a temperature at about the top 
and/or at about the bottom of the selected temperature range. Light is 
directed at one end of the waveguide and the intensity of light 
transmitted by the part of the waveguide in thermal communication with the 
material is monitored. At the onset of a substantial change in the 
intensity of light transmitted by said part of the waveguide, the 
temperature of the material is adjusted so that it is within the selected 
range. 
Such a temperature control system can be used for many applications. For 
example, it can be used for over-temperature protection or for 
under-temperature protection of motors. In over-temperature protection, 
when the temperature of the motor is higher than a selected temperature, 
the motor is automatically shut off. In such an application, the 
temperature range that is desired is all temperatures less than the 
temperature at which the motor is to be shut off. Under-temperature 
protection can be used to prevent a motor from being started up if the 
temperature is too cool for the oil to properly lubricate the motor. In 
such an application, the temperature range desired is all temperatures 
greater than a selected temperature. 
Such a system can also be used for over temperature protection in aircraft, 
motor vehicles, process equipment, and the like. It can be used in 
automobiles for anti-freeze protection with an alarm system in case any 
part of a car radiator system is below a pre-determined temperature. It 
can be used in dams and water systems to determine when freezing is taking 
place. It can be used as part of a waterbed heater control system by using 
a waveguide having a blackout temperature of about 90.degree. F. It can be 
used during charging of batteries to prevent over heating of the battery 
or to prevent charging of the battery when it is too cold for safe 
charging. For example, a temperature control system can be provided for a 
nickel cadmium battery because such batteries can explode if charged at 
temperatures less than about 0.degree. C. 
For many applications, it is desirable to use a waveguide with a blackout 
temperature of at least -20.degree. C. because there are only limited 
applications at temperatures lower than -20.degree. C. For example, a 
waveguide that has a blackout temperature of -20.degree. C. can be used 
for detecting leaks of liquefied natural gas. Also, freeze protection of 
aqueous solutions requires a waveguide with a blackout temperature at a 
temperature higher than -20.degree. C. 
This concept can better be understood with reference to FIG. 11. In FIG. 11 
it is assumed that it is desired to keep a material within a temperature 
range T.sub.1 to T.sub.2. To keep the material at a temperature no higher 
than T.sub.2, waveguide A or waveguide B can be used, where both of these 
waveguides have a blackout temperature at about T.sub.2. Waveguide A 
transmits light without substantial attenuation at temperatures greater 
than T.sub.2, and waveguide B transmits light without substantial 
attenuation at temperatures less than T.sub.2. When waveguide A is used, 
if the monitor used detects a substantial increase in the intensity of 
light transmitted, this indicates that the material is heating to a 
temperature greater than T.sub.2, and it is necessary to cool the 
material. When waveguide B is used, if the monitor detects a substantial 
decrease in the light transmitted this indicates that the material is 
heating to a temperature greater than T.sub.2, and it is necessary to cool 
the material. 
To maintain the material at a temperature higher than T.sub.1, waveguide C 
or waveguide D can be used, both of which have a blackout temperature at 
about T.sub.1. Waveguide C transmits light without substantial attenuation 
at temperatures less than T.sub.1 and waveguide D transmits light at 
temperatures greater than T.sub.1. When waveguide C is used if the monitor 
determines that there is a substantial increase in the light transmitted, 
this indicates that the material is cooling to a temperature less than 
T.sub.1, and it is necessary to heat the material. When waveguide D is 
used, if the monitor detects a substantial decrease in the light being 
transmitted, this indicates that the temperature of the material is 
cooling to a temperature less than T.sub.1, and it is necessary to heat 
the material. 
By the terms "at about the top of the selected temperature range" and "at 
about the bottom of the selected temperature range", there are meant 
temperatures which provide sufficient time to adjust the temperature of 
the material to maintain it within the desired temperature range. For 
example, when keeping water from freezing, a waveguide with a blackout 
temperature of about 5.degree. C. can be used. 
This technique could be used for preventing materials from changing phase, 
such as preventing water from freezing or boiling, or preventing a solid 
from melting or sublimating, or preventing a gas from condensing. For 
example, to prevent water from freezing, a waveguide can be placed in 
thermal communication with the water, where the waveguide exhibits 
blackout at a temperature slightly above the freezing temperature of 
water. By "slightly above" there is meant a temperature which provides 
sufficient time to heat the water to prevent it from freezing. For 
example, a waveguide that transmits light at temperatures above about 
5.degree. C., and has a blackout temperature at about 5.degree. C. is 
suitable. Light is directed at one end of the waveguide and the intensity 
of light transmitted by the waveguide is monitored. At the onset of a 
substantial change in the intensity of the light transmitted by the 
waveguide, the water is heated to prevent it from freezing. 
According to the present invention, waveguide temperature sensing systems 
can be used for preventing the viscosity of a liquid from changing beyond 
a selected value. For example, when pumping petroleum products in a 
pipeline, or pumping petroleum from a well, it is important to maintain 
the petroleum at a sufficiently high temperature that it can easily be 
pumped. For this purpose, conduits such as pipelines in cold environments 
are provided with a heating element, such as steam tracing, electrical 
resistant heaters, or strip heaters comprising a conductive polymer. 
According to the present invention, a waveguide that exhibits blackout at 
a temperature corresponding to the temperature at which the viscosity of 
the liquid increases above a selected value is placed in thermal 
communication with the liquid. This can be effected by placing the 
waveguide longitudinally or spirally along the exterior of the pipeline or 
longitudinally or spirally within the pipeline. The advantage of using 
fiber optic systems is that even if a waveguide breaks, there is no danger 
of an explosion and no fire hazard associated with the waveguide. This is 
unlike an electrical powered temperature sensing system. Light is directed 
at one end of the waveguide and the intensity of light transmitted by the 
waveguide is monitored. At the onset of a substantial change in the 
intensity of light transmitted, the liquid is heated to lower its 
viscosity. 
The present invention can also be used for detecting leaks of fluids out of 
a container. In general, the waveguides can be used for detecting leakage 
of a fluid from a container where the fluid is at a temperature other than 
ambient temperature, i.e., lower than ambient temperature or higher than 
ambient temperature. For example, a waveguide having the attenuation vs. 
temperature curve of the Ge 655 clad waveguide shown in FIG. 2 can be 
placed adjacent a container of liquid nitrogen or liquified natural gas. 
Light can be directed at tone end of the waveguide and the other end of 
the waveguide can be monitored. The presence of blackout indicates that 
leakage of the liquid nitrogen or LNG is occurring. 
In some applications, it is possible to detect leakage of a fluid that is 
at ambient temperature. In such an application, the waveguide is 
maintained at a temperature other than ambient temperature by a cooling or 
heating jacket or the like. Upon leakage of the fluid, the temperature of 
the jacket and waveguide changes towards ambient temperature. The 
waveguide can be selected so that as its temperature approaches ambient 
temperature, its light transmission properties undergo a substantial 
change. 
Use of waveguides is not limited to detecting leakage of a fluid from a 
container; they can also be used as part of monitoring systems in 
pipelines and the like to determine where the fluid is flowing. For 
example, for a tank having a plurality of valves, a waveguide can be 
provided on the downstream side of each valve, so that when the valve is 
opened, a signal is generated indicating that fluid is passing through the 
valve. 
Another application for waveguides of the present invention is for 
detecting the level of a fluid (i.e., liquid or gas) in a container, the 
fluid having a temperature different from ambient temperature. This is 
effected by selecting a waveguide, at least a portion of which exhibits 
blackout as the temperature of said portion is changed from ambient 
temperature to the temperature of the fluid, or a waveguide that begins to 
transmit light as the temperature of said portion is changed from ambient 
temperature to the temperature of the fluid. That portion of the waveguide 
is placed into the container, light is directed at one end of the 
waveguide, and the intensity of light transmitted by the portion is 
monitored. The onset of a substantial change in the intensity of light 
transmitted by the portion indicates that the portion is in thermal 
communication with the fluid. Fluids can be measured in tanks, cargo 
holds, deep wells, and the like. 
FIG. 12 shows an article 10 that makes use of the present invention. The 
article comprises an outer tubular sleeve 12, which can be made of a 
heat-recoverable material, a heat activatable insert 16, and a waveguide 
17 that has a blackout temperature at about the activation temperature of 
the heat-activatable insert 16. A common problem in using such articles is 
that the craftsmen in the field can be uncertain as to whether the entire 
heat-activatable material has been activated. This can be a serious 
problem when the heat-activatable material is a meltable material such as 
solder or an adhesive such as a heat-activatable adhesive or mastic which 
requires heat to perform its bonding and sealing functions. In applying 
the article 10, the craftsman should heat the entire article so that the 
entire heat-activatable insert is activated. However, because the insert 
is within the sleeve 12, a craftsman is unable to determine whether or not 
this occurs. However, with the article of FIG. 12, this problem is 
remedied. 
The waveguide 17 is one that undergoes a change in its light transmission 
characteristics once it reaches a temperature at which the 
heat-recoverable material has recovered and the insert has been activated. 
Preferably the change is permanent and irreversible because with long 
articles 10, after one end of it has been heated to a desired temperature, 
that end can cool down to below the required temperature while the 
remainder of the article 10 is being heated. If a waveguide that did not 
undergo a permanent change were used, an operator might needlessly reheat 
the first end. 
The heat-activatable material can be fusable material such as solder or 
polymeric material, an adhesive that requires heat for activation, and the 
like. The sleeve 12 can be made of a heat-recoverable or a heat-expandable 
material. It can be a polymeric material made from metal or other 
materials. Furthermore, it need not be heat recoverable at all, but only 
serve to hold a heat-activatable material in location until the 
activatable material has been activated, and then the sleeve is ready for 
removal. The sleeve need not be continuous in cross-section, but can have 
a slot along its length. Likewise, the sleeve need not be circular in 
cross-section, but can have different shapes. Exemplary of articles for 
which the present invention can be used are those described in U.S. Pat. 
Nos. 3,243,211; 3,297,819; 3,305,625; 3,312,772; 3,316,343; 3,324,230; 
3,382,121; 3,415,287; 3,525,799; 3,539,411; 3,610,291; 3,770,556; 
3,847,721; 3,852,517; 3,946,143; 3,957,382; 3,975,039; 3,988,399; 
3,990,661; 3,995,964; 4,016,356; 4,045,604; 4,092,193; 4,126,759 and 
4,179,320, all of which are incorporated herein by this reference. 
Although the present invention has been described in considerable detail 
with reference to certain versions thereof, other versions are possible. 
For example, waveguides can be used on the leading edge of airplanes to 
detect icing. They can be used in cable trays in nuclear power plants to 
check for overheating. Therefore the spirit and scope of the appended 
claims should not necessarily be limited to the description of the 
preferred versions contained herein.