U.S. patent application number 09/960174 was filed with the patent office on 2003-04-24 for accelerated method for increasing the photosensitivity of a glassy material.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Brennan, James F. III, Fahey, Maureen T., Novack, James C., Sloan, Diann A..
Application Number | 20030074925 09/960174 |
Document ID | / |
Family ID | 24468095 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030074925 |
Kind Code |
A1 |
Brennan, James F. III ; et
al. |
April 24, 2003 |
Accelerated method for increasing the photosensitivity of a glassy
material
Abstract
A method for rapidly increasing the photosensitivity of an
optical fiber comprising the step of providing an optical fiber
comprising a glassy material and a thermally-stable coating. The
thermally-stable coating has a thermally-stable exposure band,
wherein desired time/temperature exposure parameters fall within
the time/temperature thermal stability exposure band for the
coating. The optical fiber is exposed for the desired
time/temperature exposure to a hydrogen-containing atmosphere. The
desired temperature is more than 250.degree. C. and the desired
time exposure does not exceed one hour. The glassy material then
may be irradiated with actinic radiation, such that the refractive
index of the irradiated portion results in a normalized index
change of at least 10.sup.-5.
Inventors: |
Brennan, James F. III;
(Austin, TX) ; Sloan, Diann A.; (Austin, TX)
; Fahey, Maureen T.; (Austin, TX) ; Novack, James
C.; (Hudson, WI) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
24468095 |
Appl. No.: |
09/960174 |
Filed: |
September 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09960174 |
Sep 21, 2001 |
|
|
|
09616117 |
Jul 14, 2000 |
|
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6311524 |
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Current U.S.
Class: |
65/392 ; 65/32.1;
65/507 |
Current CPC
Class: |
C03C 2201/21 20130101;
C03C 2201/31 20130101; C03C 25/00 20130101; C03C 25/607 20130101;
G02B 6/02114 20130101; C03C 3/06 20130101; C03C 25/6226
20130101 |
Class at
Publication: |
65/392 ; 65/32.1;
65/507 |
International
Class: |
C03B 037/00; C03B
037/018 |
Claims
What is claimed is:
1. A method for producing an optical fiber having an optical
grating, comprising the steps of: providing an optical fiber having
a glass core and a thermally stable exterior coating; exposing
first and second portions of said optical fiber to a
hydrogen-containing atmosphere under exposure conditions comprising
(a) an exposure temperature of greater than 250.degree. C., and (b)
an exposure time of less than about 1 hour, thereby forming a
treated optical fiber wherein the coating is thermally stable
during and after the exposing step; and writing an optical grating
on the treated optical fiber without removing the coating such that
said first portion of the optical fiber contains the grating and
said second portion of the optical fiber is devoid of any grating,
wherein the exposure time is within the range of about 1 minute to
about 10 minutes.
2. The method of claim 1, wherein the exposure time is within the
range of about 1 minute to about 5 minutes.
3. The method of claim 1, wherein the optical fiber is rapidly
cooled after exposure to the hydrogen-containing atmosphere,
wherein the cooling includes placing the fiber on a cold thermally
conductive surface.
4. A method for improving the photosensitivity of an optical fiber,
comprising the steps of: providing an optical fiber comprising a
glass core and an exterior coating; and exposing the optical fiber
to a hydrogen-containing atmosphere under exposure conditions
comprising (a) a temperature of greater than 250.degree. C., and
(b) an exposure time of less than about 1 hour; wherein the coating
is thermally stable under the exposure conditions. wherein the
exposure time is within the range of about 1 minute to about 10
minutes.
5. The method of claim 4, wherein the exposure time is within the
range of about 1 minute to 5 minutes.
6. The method of claim 4, wherein the optical fiber is rapidly
cooled after exposure to the hydrogen-containing atmosphere by
placing the fiber on a cold thermally conductive surface.
7. A method for producing an optical fiber having increased
photosensitivity, comprising the steps of: providing an optical
fiber comprising a glass core and a thermally stable exterior
coating; exposing the optical fiber to hydrogen gas heated to a
temperature of greater than 250.degree. C. until the optical fiber
is essentially saturated with hydrogen, thereby forming a treated
optical fiber and wherein the coating is thermally stable under the
conditions at which it is exposed to the hydrogen gas; and rapidly
cooling the treated optical fiber, wherein the optical fiber is
exposed to the hydrogen gas for an exposure time within the range
of about 1 minute to about 10 minutes.
8. The method of claim 7, wherein the optical fiber is exposed to
the hydrogen gas for an exposure time within the range of about 1
minutes to about 5 mines.
9. The method of claim 7, wherein the optical fiber is rapidly
cooled after exposure to the hydrogen-containing atmosphere by
placing the fiber on a cold thermally conductive surface.
Description
[0001] The present application is a Continuation of
commonly-assigned, co-pending U.S. Ser. No. 09/616,117, entitled
"ACCELERATED METHOD FOR INCREASING THE PHOTOSENSITIVITY OF A GLASSY
MATERIAL", filed Jul. 14, 2000.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for increasing the
photosensitivity of optical fibers at rates several times faster
than those accomplished by prior methods. Specifically, the present
invention comprises a method for rapidly diffusing hydrogen or
deuterium into silica glasses to increase the photosensitivity of
these glassy materials, and in particular of optical fibers.
[0003] Optical fiber-based devices are vital components in today's
expanding high-volume optical communications infrastructure. Many
of these devices rely on fiber Bragg gratings (FBGs) to perform
light manipulation. An FBG is an optical fiber with periodic,
aperiodic or pseudo-periodic variations of the refractive index
along its length in the light guiding region of the waveguide. The
ability to produce these refractive index perturbations in a fiber
is necessary to manufacture FBGs and, hence, a number of optical
components, such as optical sensors, wavelength-selective filters,
and dispersion compensators.
[0004] Gratings are written in optical fiber usually via the
phenomenon of photosensitivity. Photosensitivity is defined as the
effect whereby the refractive index of the glass is changed by
actinic radiation-induced alterations of the glass structure. The
term "actinic radiation" includes visible light, UV, IR radiation
and other forms of radiation that induce refractive index changes
in the glass. A given glass is considered to be more photosensitive
than another when a larger refractive index change is induced in it
with the same delivered radiation dose.
[0005] The level of photosensitivity of a glass determines how
large an index change can be induced in it and therefore places
limits on grating devices that can be fabricated practically.
Photosensitivity also affects the speed that a desired refractive
index change can be induced in the glass with a given radiation
intensity. By increasing the photosensitivity of a glass, one can
induce larger index perturbations in it at a faster rate.
[0006] The intrinsic photosensitivity of silica-based glasses, the
main component of high-quality optical fibers, is not very high.
Typically index changes of only .about.10.sup.-5 are possible using
standard germanium doped fiber. However, it has been observed that
by loading the glass with molecular hydrogen before irradiating it
with actinic radiation, one can increase tremendously the
photosensitivity of the glass. Index changes as large as 10.sup.-2
have been demonstrated in hydrogenated silica optical fibers.
[0007] An early reference to an increase in photosensitivity due to
exposure to hydrogen may be found in D. McStay, "Photosensitivity
Changes in Ge-Doped Fibers Observed by Raman Spectroscopy", SPIE,
Vol. 1314, Fibre Optics 1990. A peak was observed for samples of a
Corning 1521 fiber reported to have been immersed in a hydrogen
bath at varying pressure, times and temperatures up to 150.degree.
C. An exemplary reported exposure consisted of a fiber treated at 1
atmosphere and 24.degree. C. for 3 days. The fibers exhibited a
weak photosensitive reaction.
[0008] In F. Ouellette et al., "Permanent Photoinduced
Birefringence in a Ge-doped Fiber", Applied Physics Letters, Vol.
58, p. 1813, 29 Apr. 1991, reports an attempt to increase
photosensitivity by hydrogen exposure at relatively high
temperatures. Fiber strands having a core doped with germanium were
put in a pressure chamber with 12 atm H.sub.2 and heated at
400.degree. C. for 4 hours. The total index change for the
hydrogen-treated fiber was estimated to be close to 10.sup.-5. G.
Meltz et a., SPIE, Volume 1516, International Workshop on
Photoinduced Self-Organization in Optical Fiber, Ma 10-11, 1991,
paper 1516-18 reports treating a doped germanosilicate preform rod
for 75 hours at 610.degree. C. in 1 atm H.sub.2. Such
high-temperature exposure later was found to cause high optical
loss in the fiber, usually rendering the fiber useless. U.S. Pat.
Nos. 5,235,659 and 5,287,427 discuss a method for exposing at least
a portion of a waveguide at a temperature of at most 250.degree. C.
to H.sub.2 (partial pressure greater than 1 atmosphere (14.7
p.s.i.), such that irradiation can result in a normalized index
change of at least 10.sup.-5. U.S. Pat. No. 5,500,031, a
continuation-in-part of the above-mentioned '659 patent, speaks of
a method of exposing the glass to hydrogen or deuterium at a
pressure in the range of 14-11,000 p.s.i. and at a temperature in
the range 21-150.degree. C. The parameters described in these
references are probably most typical for hydrogen-loading an
optical fiber The '031, '659 and '427 references point out problems
with hydrogen loading methods in which temperatures exceed
250.degree. C., or even 150.degree. C. In teaching away from such
references, the '659 Patent indicates that at high-temperatures
"typical polymer fiber coatings would be destroyed or severely
damaged" (column 1, lines 51-54). It further emphasizes the fact
that "the prior art high temperature sensitization treatment
frequently increases the optical loss in the fiber and/or may
weaken the fiber" (column 1, lines 54-56). Finally, the '659 patent
differentiates itself from the prior art by stating that a high
temperature treatment involves "a different physical mechanism"
than does a low-temperature treatment.
[0009] To achieve the desired level of hydrogen in fiber with
conventional hydrogenating methods (.about.1 ppm), one will
typically expose fiber to a hydrogen atmosphere for several days
and, in some cases, for several weeks. Exemplary exposures such as
600 hours (25 days), 21.degree. C., at 738 atm or 13 days,
21.degree. C. at 208 atm are reported as typical. Obviously, such
long exposures extend the time required to fabricate optical
devices that rely on photosensitive glass. Because of the long
duration needed for traditional fiber hydrogenation, several
pressure vessels are needed in a high-volume production environment
to increase throughput and avoid idle time. These vessels are
costly to install safely and increase the potential for serious
accidents, especially when multiple vessels with separate control
valves and gas supply cylinders are involved. Although installing
multiple vessels can increase production throughput, the
hydrogenation process hampers grating fabrication cycle time, thus
new product and specialty product development time can be
compromised severely.
[0010] The need exists for a more time-effective method for
increasing the photosensitivity of glassy materials.
SUMMARY OF THE INVENTION
[0011] Hydrogen loading with prior methods relied on exposure times
measured usually in the range of days or weeks. Even high
temperature exposures were believed to require loading times in the
range of several hours. Prior references further taught away from
the use of high temperatures, indicating a belief that high
temperature hydrogen treatments involved a different physical
mechanism than low temperature treatments.
[0012] The present invention comprises a method to increase rapidly
the photosensitivity of glassy material and an apparatus for
accomplishing the method. The present invention also comprises
articles obtained as a result of the application of the method.
[0013] The present invention relies on what is believed to be a
more accurate understanding of the effect of temperature on
hydrogen loading and on increased photosensitivity of glassy
materials. A novel aspect of the present invention is the
recognition that significant changes to the photosensitivity of a
glassy material may be achieved by a novel loading method
comprising a high temperature (greater than 250.degree. C.) very
rapid (exposure times of less than one hour) hydrogen exposure. The
discovery of such rapid loading method allows for the use of
suitable thermally-stable coatings by harmonizing the thermal
stability time/temperature band of the coatings with the parameters
of the rapid loading method.
[0014] In one embodiment of the method of the present invention, a
glassy material is provided and is protected by a selected
thermally stable coating. Once coated, the glassy material is
placed into an atmosphere containing H.sub.2 and/or D.sub.2 at a
temperature greater than 250.degree. C. for an exposure time of
less than one hour. Finally, portions of the glassy material are
exposed to actinic radiation, resulting in a refractive index
change greater than 10.sup.-5. In a preferred embodiment, the
hydrogen exposure process of the present invention increases
tremendously the photosensitivity of a glassy body, such as a
silica optical fiber (125 .mu.m diameter), in less than one (1)
minute.
SUMMARY OF THE INVENTION
[0015] Hydrogen loading with prior methods relied on exposure times
measured usually in the range of days or weeks. Even high
temperature exposures were believed to require loading times in the
range of several hours. Prior references further taught away from
the use of high temperatures, indicating a belief that high
temperature hydrogen treatments involved a different physical
mechanism than low temperature treatments.
[0016] The present invention comprises a method to increase rapidly
the photosensitivity of glassy material and an apparatus for
accomplishing the method. The present invention also comprises
articles obtained as a result of the application of the method.
[0017] The present invention relies on what is believed to be a
more accurate understanding of the effect of temperature on
hydrogen loading and on increased photosensitivity of glassy
materials. A novel aspect of the present invention is the
recognition that significant changes to the photosensitivity of a
glassy material may be achieved by a novel loading method
comprising a high temperature (greater than 250.degree. C.) very
rapid (exposure times of less than one hour) hydrogen exposure. The
discovery of such rapid loading method allows for the use of
suitable thermally-stable coatings by harmonizing the thermal
stability time/temperature band of the coatings with the parameters
of the rapid loading method.
[0018] In one embodiment of the method of the present invention, a
glassy material is provided and is protected by a selected
thermally stable coating. Once coated, the glassy material is
placed into an atmosphere containing H.sub.2 and/or D.sub.2 at a
temperature greater than 250.degree. C. for an exposure time of
less than one hour. Finally, portions of the glassy material are
exposed to actinic radiation, resulting in a refractive index
change greater than 10.sup.-5. In a preferred embodiment, the
hydrogen exposure process of the present invention increases
tremendously the photosensitivity of a glassy body, such as a
silica optical fiber (125 .mu.m diameter), in less than one (1)
minute.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a simplified schematic diagram of a first
embodiment of a hydrogen loading apparatus in accordance with the
present invention.
[0020] FIG. 2 is a flow diagram of the method for hydrogenating a
glassy material in accordance with the present invention.
[0021] FIG. 3 is a graph of the hydrogen diffusion time into
125-.mu.m-diameter silica optical fiber as a function of
temperature using the theoretical model disclosed in the present
invention.
[0022] FIG. 4 is a graph of hydrogen solubility in silica optical
fiber as a function of exposure temperature.
[0023] FIG. 5 is a graph of hydrogen solubility in silica optical
fiber as a function of exposure pressure.
[0024] FIG. 6 is a graph comparing hydrogenation time under
traditional methods with hydrogenation time using the method of the
present invention.
[0025] FIG. 7 is a graph showing the degradation curve of a polymer
material in an air atmosphere, versus the degradation of the same
material in a nitrogen atmosphere.
[0026] FIG. 8 is a graph showing a TGA thermogram of the various
fiber coating samples in an inert atmosphere.
[0027] FIG. 9 is a graph comparing fiber Bragg gratings written in
nonsensitized fiber and fiber sensitized with the methods of the
present invention.
[0028] FIG. 10 is a graphical comparison of a baseline grating.
[0029] FIG. 11 is a graphical comparison of the permanent optical
loss of the high-temperature loaded fiber compared to that loaded
at a low-temperature (60.degree. C.).
DETAILED DESCRIPTION OF THE INVENTION
[0030] FIG. 1 is a simplified schematic diagram of a first
embodiment of a hydrogen loading apparatus 10 in accordance with
the present invention. The hydrogen loading apparatus 10 includes a
pressure vessel 12 and a hydrogen source 14. In one exemplary
embodiment of the present invention, the vessel 12 is a
high-pressure gas chamber, capable of withstanding gas pressures as
large as 30,000 psi (.about.20,600 kilopascals). The apparatus 10
further includes a heater unit 16 and accompanying insulation 18
placed around the pressure vessel 12. A source of a purge gas 22,
such as N.sub.2, is provided to help evacuate the pressure
vessel.
[0031] FIG. 2 is a flow diagram outlining the steps of an
embodiment of the method of the present invention. The method for
rapidly hydrogenizing a glassy material of the present invention
includes the step of providing a glassy material, such as silicon
glass. In one preferred embodiment, the material is coated with a
thermally-stable material selected to have thermal stability
characteristics that harmonize with the planned
time/temperature/pressure exposure. The material is exposed to a
hydrogen atmosphere at a temperature in excess of 250.degree. C.
for a time of less than one hour. In an alternative embodiment, the
hydrogen atmosphere is pressurized above atmospheric pressure. The
material is then rapidly cooled such as with an ice bath.
[0032] Without wishing to be bound by theory, the present invention
relies on what is believed to be a more accurate understanding of
the effect of temperature on hydrogen loading and of its ability to
increase the photosensitivity of glassy materials. An aspect of the
present invention is the recognition that significant changes to
the photosensitivity of a glassy material may be achieved by a
novel loading method comprising a high temperature (greater than
250.degree. C.) very rapid (exposure times of less than one hour)
hydrogen exposure. The discovery of such rapid loading method
allows for extremely fast hydrogen loading using suitable thermally
stable coatings by harmonizing the thermal stability
time/temperature band of the coatings with the parameters of the
rapid loading method.
[0033] Descriptions follow of methods to select appropriate high
temperature tolerant coating material and achieve hydrogen
saturation and subsequent photosensitization of a glassy material.
The described methods combine known equations and models regarding
diffusion with new understandings and approximation models based on
experimental data.
[0034] Hydrogenation Process Parameter Determination
[0035] Based on a model described below, it has been found that one
may increase rapidly the photosensitivity of a glassy material,
such as a silica fiber, by exposing the fiber to a high temperature
hydrogen environment (>250.degree. C.). It further has been
found that the process may be further accelerated by exposing the
fiber at a very high hydrogen pressure (>10,000 psi).
[0036] FIG. 3 shows a graph of the hydrogen diffusion time into a
fiber for a 125 .mu.m diameter fiber at temperatures ranging from
20 to 300.degree. C. made with a theoretical model shown below. For
the purposes of the present invention, the term hydrogen will be
used to describe H.sub.2, HD, and D.sub.2 molecules. The
concentration of hydrogen in a fiber as a function of radius r and
time t has been calculated to be 1 C ( r , t ) = S [ 1 - n = 1
.infin. 2 J 0 ( n r r 0 ) n J 1 ( n ) - D ( n r 0 ) 2 t ] . ( 1
)
[0037] where C is the concentration of hydrogen in the fiber, D is
the diffusion coefficient, .alpha..sub.n is the n.sup.th root of
the zero-order Bessel function of the first kind, J.sub.0. J.sub.1
is the first-order Bessel function of the first kind. Here the
hydrogen concentration at saturation inside the fiber is taken as
S. For convenience, the diffusion time constant is defined as 2 t D
1 D ( r 0 1 ) 2 0.174 r 0 2 D ( 2 )
[0038] The temperature dependence of the diffusion coefficient is
described by an Arrhenius expression of the form
D=D.sub.0.multidot.exp(-E/R.multidot.T) (3)
[0039] where D.sub.0 is a constant, E is the activation energy, R
is the gas constant (R=1.986 cal/mol.multidot.K) and T is the
absolute temperature. By placing the values for D.sub.0 and E into
(1) and (3), one can calculate the hydrogen diffusion time into a
fiber of a given radius. D.sub.0=125.multidot.10.sup.-4 cm.sup.2/s
and E=9.046 Kcal/mole were the values used for the results graphed
in FIG. 3. It should be noted that the diffusion time in FIG. 3 is
plotted on a logarithmic scale, indicating that relatively small
changes in the fiber temperature alter the diffusion time
tremendously.
[0040] The solubility of hydrogen in glass can be described by an
Arrhenius expression of the form
S=P.multidot.S.sub.0.multidot.exp(E.sub.S/R.multidot.T) (4)
[0041] where P is the loading pressure, S.sub.0 is a constant, and
E.sub.S is the solubility activation energy.
S.sub.0=3.5.multidot.10.sup.-2 (a.u.) and E.sub.S=1.78.+-.0.21
(Kcal/mole). The hydrogen solubility in silica fiber is plotted in
FIG. 4. The curve is normalized to the solubility at 80.degree. C.
for convenience. As shown in FIG. 4, the hydrogen solubility is a
relatively weak function of loading temperature compared to the
diffusion time.
[0042] The H.sub.2 content in the optical fibers versus different
loading pressures is plotted in FIG. 5. A slight divergence of the
data from a linear fit is noticeable, which may be due to
measurement uncertainty or saturation effects (e.g., Langmuir model
hole filing). Linear regression on this data yields a squared
correlation coefficient r.sup.2=0.998, but a slight divergence of
the data from the linear fit is noticeable, which may be due to
measurement uncertainty or saturation effects. Practically
speaking, the hydrogen solubility in silica optical fibers
increases linearly with loading pressure for pressures up to at
least 28,000 psi.
[0043] By combining (1) and (4), the concentration of the hydrogen
in the fiber, [H.sub.2], as a function of temperature, time, and
pressure may be expressed as 3 [ H 2 ] P S 0 E s R T [ 1 - n = 1
.infin. 2 J 0 ( n r r 0 ) n J 1 ( n ) - D ( n r 0 ) 2 t ] ( 5 )
[0044] where the diffusion constant D is a strong function of
temperature as detailed in (3).
[0045] Analyzing the equations above, and since [H.sub.2] is
linearly proportional to loading pressure, the model of the present
invention then divides by the pressure, and, by neglecting the
radial-position dependence of the hydrogen concentration. Equation
(5) may be approximated as 4 [ H 2 ] P S 0 E s R T [ 1 - - t t D ]
( 6 )
[0046] Accordingly, the above model, which is supported by
experimental observations, shows that an increase in temperature of
even a few tens of degrees above the traditional loading parameters
would greatly increase the concentration of hydrogen in a glassy
material, while drastically reducing the required exposure time.
Using the method of the present invention, by raising the pressure
and temperature of the loading conditions to .about.20,000 p.s.i.
and 260.degree. C., one may achieve the same concentration of
hydrogen in the fiber in .about.60 seconds than that previously
achieved after an exposure of 30 hours at 60.degree. C. at a
typical pressure of 2,000 psi., a reduction of
.about.1.8.times.10.sup.5 percent in exposure time. Assuming a
direct relationship between the hydrogen concentration and an
increase of photosensitivity of the glassy material, proportionally
larger changes in photosensitivity of the glassy material would be
achieved by such an exposure. FIG. 6 illustrates the comparative
loading time for a glassy material placed in a hydrogen atmosphere
at 20.degree. C. versus the same sample place in an atmosphere at
300.degree. C. Please note that the time scales are logarithmic and
that the first graph uses hour units, while the second graph uses
minute time units. The measurements are in arbitrary units
(a.u.).
[0047] A possible concern arises that increasing the temperature
used during hydrogenation of the optical fiber decreases the
hydrogen diffusion time, but the hydrogen solubility in the fiber
also decreases with increased temperature. An embodiment of the
method of the present invention counters this decrease in
solubility by increasing the pressure of the hydrogen environment.
The exact combination of time, temperature and pressure required to
load a particular fiber depends on the characteristics of the
fiber, the coating, and the desired hydrogen concentration.
[0048] Fiber Coating Selection Process
[0049] Prior references taught expressly away from use of
high-temperature hydrogen loading, indicating that polymer fiber
coatings would be destroyed or at least severely damaged at
temperatures higher than 250.degree. C. However, those prior
references failed to recognize the possibility of accelerated
exposure yielding extremely fast loading times and the ability to
harmonize the time/temperature requirements of the loading process
with the thermal stability characteristics of specially selected
materials. The present invention uses thermogravimetric (TGA)
analysis tools, to recognize and identify high-temperature loading
band for certain thermally stable polymers. Since, under the
loading model of the present invention, the higher the loading
exposure temperature, the shorter the required exposure time, it is
possible to identify polymer coatings capable of maintaining
sufficient performance characteristics for the combined time,
temperature, and pressure loading conditions of the present method.
By determining the behavior of a material as it is heated with the
analysis tools described below, a person skilled in the art may
harmonize the time/temperature thermal stability parameters of a
proper coating with the appropriate combination of temperature,
pressure and time required to load rapidly an optical fiber with
hydrogen.
[0050] Typically, polymer materials are used to coat optical fibers
only to protect them mechanically. "The thermal stability of
polymers is a characteristic rather sensitive not only to their
structure and composition but, in line with other phenomena
associated with chemical processes, to the action of environmental
factors, especially the presence of chemically active components in
the environment such as oxygen, additives and fillers," Degradation
of Filled Polymers: High Temperature and Thermal-Oxidative
Processes, by M. T. Bryk, Ellis Horwood, 1991, relevant portions
incorporated herein by reference. In addition, materials present in
the local environment of the analysis that may act in a catalytic
fashion on the chemical reactions of the degradative process need
be considered. These materials may include the products of
degradation and/or the materials from which the equipment is
made.
[0051] The thermal stability and the thermal and thermal-oxidative
processes of polymers have been studied. In polymers, the reactions
that lead to degradation of the material depend on the chemical
structure of the polymer. Chain scission reactions resulting from
the formation of free radicals is a mechanism of degradation in
polyethylene and poly(methyl methacrylate) whereas degradation of
polyethylene terephthalate (PET) is by random chain scission
followed by formation of lower molecular weight species with
carboxyl and vinyl ester end groups. Polycarbonates eliminate
carbon monoxide and carbon dioxide as a result of random cleavage
of ester bonds. Polyimides can eliminate carbon monoxide via the
thermal decomposition of the imide ring, or eliminate carbon
dioxide via isomerization of the imide ring into an isoimide
structure. Depending on the chemical structure of the polymer,
crosslinking reactions may compete with the chain scission
reactions.
[0052] The processes of polymer degradation may differ appreciably,
however, the ultimate issue is to what extent can degradation occur
without loosing the necessary characteristics of the polymers
performance in the present application. Certainly degradation that
leads to rapid chain scission followed by volatilization of monomer
such as in poly(phthalaldehyde) would not be beneficial for some
short duration exposures of the polymer to elevated temperatures.
Other mechanisms that degrade the polymer more slowly clearly would
be able to survive short duration exposures. Several factors are
significant, such as temperature, time of exposure, the atmosphere
and the surface area/volume ratio.
[0053] The coating material selection process in the present
invention is based on an understanding of the interactions between
the coating materials and the presently disclosed short-time,
high-temperature hydrogenation process. Suitable materials may be
selected from a group comprising fiber coating materials, that is,
protective materials that may be disposed around a bare optical
fiber or waveguide. The term "thermally stable material" is defined
in the present invention as a material having sufficient thermal
stability to avoid meaningful thermal degradation under the
hydrogen loading conditions of the method of the present invention.
Preferred materials are selected as to not deform, drip, fuse or
volatize, all of which may result in the fiber being left bare or
adhering to an adjacent fiber, under the required time/temperature
exposure parameters. Preferably, the materials are selected such
that when subject to the loading time and temperature
parameters:
[0054] 1) no visual bubbles or cracks in the coating or
delaminations between the coating and the glass are apparent as
measured optically at magnifications of 50.times. or less; and
[0055] 2) no substantial thermal degradation is observed, as
measured by thermogravimetric analysis, where no substantial
thermal degradation is defined as total coating weight loss of less
than about 10%, more preferably less than about 5% for conditions
that mimic the heating, loading and cooling temperature profile in
an inert atmosphere such as N.sub.2.
[0056] In one embodiment, the coating will lose less than 2-wt. %
as measured by thermogravimetric analysis (TGA) in conditions
analogous to the process conditions. The coating material further
is selected to not impede significantly the hydrogen diffusion into
the fiber. Finally, the selected material desirably has a coating
process compatible with fiber manufacture, and either the ability
to allow writing of a grating through the coating or the ability to
be stripped cleanly before writing.
[0057] The thermal stability of various materials was analyzed and
related to the particular high-temperature hydrogen-loading
process. The goal of the photosensitization process is to introduce
hydrogen rapidly into the core of the optical fiber via diffusion.
The issue was first to identify appropriate thermally stable
materials and then to identify the time/temperature stability band
for those materials, balancing between the increasing diffusion
rate of hydrogen at higher temperatures and the increasing
degradation rate of the polymer.
[0058] The analysis of the present invention considered the effect
of thermogravimetric analysis of common polymers in air versus
nitrogen. FIG. 7 shows the degradation curve of a polymer material
in an air atmosphere, versus the degradation of the same material
in a nitrogen atmosphere. It was observed that the inert atmosphere
slows down the degradation process. And, in the case of
thermo-oxidative processes, the amount of surface area exposed for
a given weight of material is known to have an effect on the rate
of degradation of the material.
[0059] Certainly an atmosphere of entirely hydrogen gas is not an
oxidizing atmosphere, but it may not be completely inert to the
polymer at elevated temperatures. The key is to identify the
minimum time required to saturate the fiber with enough hydrogen to
yield a desired photosensitivity and to coat the fiber with a
material able to withstand that process without degrading.
[0060] The following example indicates an exemplary methodology to
conduct the TGA material testing.
[0061] Material Testing Examples
[0062] Tests were performed on various samples using a modulated
high-resolution TGA analyzer (TA Instruments, Model 2950), that
included a gas-switching accessory. The tests were conducted using
both nitrogen and air as a purge gas.
[0063] The materials that were examined are tabulated below and
will be referred to herein as such:
1 Sample Source Material Type Material 1 Commercial Acrylate
Material 2 Commercial Polylmide Material 3 3M internal Acrylate
Material 4 3M internal Polyimide Material 5 3M internal Acrylate
Material 6 3M internal Carbon
[0064] Their product numbers may identify the sample materials as
follows:
2 Manufacturer Product ID 1) Corning SMF-28 601559189816 2)
Spectran SMT A131OH DC0556XA1 3) 3M Optical Transport Systems
CS-96-0110 907111-34879 4) 3M Optical Transport Systems CS-97-5114
814640-206111 5) 3M Optical Transport Systems CS-98-3103
908911-35446 6) 3M Optical Transport Systems 3M Experimental carbon
material
[0065] The samples used in this analysis were, with only one
exception, coated optical fibers. The exception was a
plasma-deposited carbon material. Since the coating weight of this
diamond-like carbon (DLC), when coated to a 200 .ANG. thickness on
optical fiber, accounts for only .about.0.06 wt % of the fiber, the
standard TGA measurements were impractical, so the sample comprised
material collected as flakes/powder from the coating chamber. The
plasma deposited carbon sample has a surface area/volume ratio that
is likely to be different than that on the fiber. The sample
generally behaves as it would if it were a film on a fiber,
assuming the silica surface does not affect the DLC
degradation.
[0066] To prepare the fiber samples for analysis, each fiber was
cut into lengths of .about.3-4 mm such that the pieces could fit
into the TGA platinum pan used in the TGA, which has a diameter of
5 mm and is .about.2mm deep. Powder-free gloves were worn during
the handling of the fiber to avoid transferring oils and
contaminants from the skin to the sample. It is assumed that
cutting the fiber length into 3-4 mm pieces does not appreciably
affect the degradation process through the increases in surface
area resulting from the cut ends. In the case of the
plasma-deposited carbon material, the flakes were crushed such that
an adequate amount of material could be placed into the sample
pan.
[0067] The flow rates for both the nitrogen and the air purge were
44 cc/min for the furnace and 22 cc/min for the balance. A platinum
sample pan was used, and the instrument was placed in mode
TGA-1000C.
[0068] To determine the long-term effects of a given temperature on
coating material(s), an isothermal method was used where the
temperature was increased rapidly to a desired temperature and then
held for an extended duration. In the present examples the
temperature of interest was 260.degree. C. To determine the coating
mass percentage, the temperature of specimens was increased to
900.degree. C. at 20.degree. C./min. in air until the coating was
removed. While the time intervals for hydrogen loading in this
inventive process are quite small, the thermogravimetric analyses
were conducted over 800 minutes. Each heating method was used on
all the tested samples with both nitrogen and air purges. The data
sampling rate for the isothermal analyses was set at 10.0
sec./point (48000 sec..times.0.1 point/sec.=4800 points). The
temperature ramps were run at a data-sampling rate of 2.0
sec./point.
[0069] In some cases a small amount of material degradation was
observed upon ramping to temperature for the isothermal runs, which
is likely due to the loss of adsorbed volatile components, such as
surface adsorbed water. This is most apparent in the sample of
diamond like carbon (DLC) as it has a very high surface area. It is
also apparent in the polyimide sample which, other than this
phenomenon, show little change in mass during the run.
[0070] The resulting data contains time, temperature, and weight.
The data is used to calculate a Weight % based on the initial
weight of the sample. The thermogram is corrected for the glass
fiber component, if any, and then normalized such that various
coatings can be compared independent of coating weights used.
[0071] The Normalized Corrected Wt % data is fit to a four factor
exponential decay of the form:
y=ae.sup.-bt+ce.sup.-dt
[0072] that provides r-square values of 0.99 or greater except in
cases where the curve is not one of degradation and/or there is
very little, if any mass loss (e.g. the polyimide coated
fibers).
[0073] FIG. 8 shows a TGA thermogram of the various fiber coating
samples in an inert atmosphere. The thermogram work done in inert
atmospheres shows that there is little mass loss over the short
periods of time necessary to effect hydrogen loading at 260.degree.
C. As the function of the polymeric coating is merely to provide a
barrier to the surface of the glass element from mechanical
abrasions and water, it is well possible that even longer periods
of time would not affect this function for the materials used as
fiber optic coatings. It is important to note that the TGA
thermograms show the normalized wt % (corrected) vs. time at 260 C.
This representation removes the silica component and coating weight
from the analysis such that the materials are being compared
equally.
[0074] Those fibers with a coating of polyimide (FIG. 8: Material 2
and 4) or DLC (Material 6) show no significant indication of
degradation at 260.degree. C. in an inert atmosphere. These
thermally-stable coatings would be preferred coatings to protect a
fiber during a 250-260.degree. C. high-temperature hydrogen loading
process in accordance with the present invention.
[0075] The method of the present invention also allows rapid
hydrogen loading even with acrylate-based coatings (Material 1, 3,
and 5). By matching the rapid hydrogen loading parameters of the
method of the present invention with the time/temperature exposure
characteristics of each compound, rapid hydrogen loading at
elevated temperatures may be achieved without significant
degradation.
[0076] Other possible suitable materials may be found. For
instance, polymers containing carbocyclic and heterocyclic rings in
the chain generally have been found to have the desired
characteristics. Exemplary families of materials in these
categories include polyimides; aromatic polyamides, polyesters,
polysulfones and polyethers; polynorbornenes; polybenzoxazoles; and
polyparaphenylene derivatives. Other suitable polymers include some
thermally-stable polymers containing fluorine, polymers containing
boron, phosphorous, or silicon, including polysiloxanes and
polymetallosiloxanes, co-ordination polymers, and organometallic
polymers. Examples and descriptions of possibly suitable materials
may be found in Chapter 5 of High Temperature Resistant Polymers,
A. H. Frazer, Interscience Publishers/John Wiley and Sons, New
York, 1968, the relevant portions of which are hereby incorporated
by reference. Other references, the relevant portions of which also
are incorporated by reference, include Degradation and
Stabilization of Polymers, G. Gueskens, Ed., Applied Science
Publishers Ltd, London 1975, Chapter 3, "The Development of
Heat-Resistant Organic Polymers" by W. W. Wright and Thermally
Stable Polymers, P. E. Cassidy, Marcel Dekker Inc. New York,
1980.
[0077] Examples of Rapid Photosensitization Process
[0078] An example of the method of the present invention follows.
Referring again to FIG. 1, a glassy material, such as a spool 30 of
silica glass optical fiber 32 having a Ge and/or B-doped core
and/or cladding was provided. Such fibers may be readily obtained
from companies such as Corning, Inc. of Corning, N.Y. or Redfern
Photonics of Eveleigh, Australia. Methods for the manufacture,
doping and coating of optical fibers are well known to those
skilled in the art. A glassy material is defined as a material
having no long-range structural order and being sufficiently solid
and rigid enough not to exhibit flow on an observable time scale.
This photosensitization process applies to other material systems,
such as planar waveguides.
[0079] The optical fibers include a coating of a thermally stable
material, such as polyimide coated fiber available from Polymicro
Technologies, LLC (Phoenix, Ariz.). The coating helps protect the
bare optical fibers from surface damage.
[0080] The thermal stability of the coating is determined according
to the limiting temperature and exposure time conditions that give
rise to some undesired change, normally degradation, in the coating
that adversely influences its other properties. The parameters for
the selection of thermally stable materials in the present
invention have been defined above.
[0081] The coated optical fibers to be hydrogenated were placed
inside of the vessel 12 preheated to 200.degree. C. In the present
example, the optical fiber 32 was wound on a spool 30 and the spool
30 was placed into a pressure vessel 12. The vessel was then purged
with nitrogen and heated up to 260.degree. C. over 35 minutes. The
260.degree. C. vessel was filled with hydrogen up to 1900 psi, and
the fiber 30 was then exposed to a hydrogen-containing atmosphere
for 30 minutes. After 30 minutes, the fibers 30 were removed
quickly from the vessel 12 and cooled rapidly by placing them on a
cold thermally-conductive surface at 0.degree. C., such as a plate
of ice. Alternatively, the spools may be placed on a heat sink
having a substantially lower temperature than the loading
temperature. It has been found that rapid cooling helps minimize
the amount of hydrogen that desorbs from the fiber.
[0082] The exposure process was conducted three times for three
different samples under different conditions. The fibers were then
exposed to actinic radiation patterns of similar intensity and for
a similar amount of time to impress upon the fiber changes in the
refractive index to fabricate fiber Bragg gratings. The
photosensitivity of the fibers was determined by measuring the
grating transmission of the resulting gratings. The results are
summarized in Table 1 below.
3 TABLE 1 Condition 1 Condition 2 Condition 3 Preheat temperature
200.degree. C. 240.degree. C. 60.degree. C. Final temperature
260.degree. C. 300.degree. C. 60.degree. C. Heating time 35 minutes
40 minutes 0 minutes Hydrogen pressure 1900 psi 1850 psi 1730 psi
Time fibers in the vessel 30 minutes 20 minutes 30 minutes Max. FBG
attenuation 4.5 dB 1.7 dB 0.05 dB
[0083] A significant increase in photosensitivity was observed for
the fibers that were exposed to high-temperature hydrogen
environments (Conditions 1 and 2), as compared with fibers that
were not loaded with hydrogen, as shown in FIG. 9. The
high-temperature loading process increased the strengths of the
fiber Bragg gratings by 34 to 90 times. For comparison purposes,
fibers were exposed to hydrogen environments that are commonly used
(Condition 3) for durations similar to the ones used for this
inventive high-speed photosensitization process. As shown in FIG.
10, the fibers loaded at 60.degree. C. showed no significant
increase in photosensitivity when compared to untreated fibers.
[0084] The permanent optical loss of the high-temperature loaded
fiber compared to that loaded at a low-temperature (60.degree. C.)
is shown in FIG. 11. As shown, the optical loss does increase in
high-temperature loaded fibers, but for many applications, this
amount of loss is acceptable for producing commercial devices.
Furthermore, by increasing the pressure of the hydrogen atmosphere
in the high-temperature exposure process, one can reduce this
optical loss by reducing the duration of the hydrogen exposure
process.
[0085] The high-temperature, short time exposure method for
increasing the photosensitivity of a glassy material of the present
invention may be used to manufacture a number of optical devices
that manipulate light. In an exemplary application, optical fibers
are hydrogen loaded using the method described above. At least a
portion of such optical fibers is exposed to a pattern of actinic
radiation, to create index perturbation patterns, such as Bragg
gratings.
[0086] Those skilled in the art will appreciate that the present
invention may be used in the manufacture of a variety of optical
components. While the present invention has been described with a
reference to exemplary preferred embodiments, the invention may be
embodied in other specific forms without departing from the spirit
of the invention. Accordingly, it should be understood that the
embodiments described and illustrated herein are only exemplary and
should not be considered as limiting the scope of the present
invention. Other variations and modifications may be made in
accordance with the spirit and scope of the present invention.
* * * * *