Note: Descriptions are shown in the official language in which they were submitted.
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SECTIONALLY PHOTOSENSITIVE OPTICAL WAVEGUIDES
BACKGROUND OF THE INVENTION
The present invention relates to photosensitive optical waveguides. In
particular, the present invention relates to optical fibers having selected
and
discrete longitudinal regions of high-photosensitivity.
Optical fibers and optical fiber devices are widely used in signal
transmission and handling applications. 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 (FBG's) to
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 FBG's and, hence, a
number of
optical components, such as optical sensors, wavelength-selective filters, and
15 dispersion compensators.
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
20 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.
The level of photosensitivity of a glass determines how laxge an index
change can be induced in it and therefore places limits on grating devices
that can
25 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.
The intrinsic photosensitivity of silica-based glasses, the main component
30 of high-quality optical fibers, is not very high. Typically index changes
of only
approximately 10-5 are possible using standard germanium doped fiber.
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However, it has been observed that by loading the glass with molecular
hydrogen before irradiating it with actinic radiation, one may increase
significantly
the photosensitivity of the glass. Exposing Ge-doped silica optical fibers to
hydrogen or deuterium atmospheres at certain temperatures and pressures
photosensitizes the fibers. Index changes as large as 10-2 have been
demonstrated
in hydrogenated silica optical fibers.
Prior references have emphasized upper limits on the temperature for such
hydrogen loading. For example, United States Patent Nos. 5,235,659 and
5,287,427 discuss a method for exposing least a portion of a waveguide at a
temperature of at most 250°C to H2 (partial pressure greater than 1
atmosphere
(14.7 psi or 9.65 x104 Pa), such that irradiation can result in a normalized
index
change of at least 10-5. United States Patent 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 psi (9.65 x104 Pa) Pa
to
11,000 psi (7.58 x107 Pa) and at a temperature in the range 21°C to
150°C. The
parameters described in these references are probably 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°C, or even
150°C. In teaching
away from higher temperatures, 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. For
example, United States Patent No. 5,235,659 explicitly indicates that
temperatures
of "at most 250°C" should be used.
It has been observed that at higher temperatures the polymer coating,
(usually an acrylate material), that protects the glass from harmful chemical
reactions in a normal environment will degrade or oxidize (burn). Coatings
that
have degraded or oxidized and lost their protective value need to be removed
and
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replaced, which can be a difficult and expensive process. Uncoated fiber is
fragile,
and requires great care during handling.
Most of the gratings written today by industry involve about 5 cm (2 inches
or less) of the length of a fiber, depending on the type of grating to be
written.
Traditionally, it has been taught to place an entire length of optical fiber
in a vessel
containing hydrogen or deuterium atmospheres at certain temperatures and
pressures. The grating manufacturing process usually entails a first process
of
placing a fiber spool in a hydrogen or deuterium containing vessel, placing
the
vessel in an oven and loading the entire fiber through the polymer coating.
1o To achieve the desired level of hydrogen in fiber with conventional
hydrogenating methods (aboutl ppm), one will typically expose fiber to a
hydrogen
atmosphere for several days and, in some cases, for several weeps. Exemplary
exposures such as 600 hours (25 days), 21°C, at 738 atm (10,800 psi or
7.48x107
Pa) or 13 days, 21°C at 208 atm (3060 psi or 2.11x107 Pa) 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.
Once the length of fiber has been hydrogen-loaded, the coating is stripped
(mechanically, chemically or by other means) from the area where the grating
is to
be written. A technician then uses a source of actinic radiation to write each
grating individually. The fibers are then annealed by again heating the fiber
to
reduce the degradation curve of the gratings. The portion of the fiber that
was
3o stripped is then recoated.
The traditional Bragg grating manufacturing processes are slow and do not
lend themselves to mass manufacturing. The traditional hydrogen loading
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techniques require that the entire length of fiber be subject to the hydrogen
loading
and heating cycles. The need to expose the entire fiber may result in optical
effects
on the fiber and places constraints on materials, such as fiber coatings, that
may be
used. One negative effect of hydrogen loading at higher temperatures is that
it may
increase the optical loss characteristics of an optical fiber. Furthermore,
high-
temperature heating cycles may deteriorate optical fiber coatings.
It would be desirable to have the ability to provide an optical fiber which
would allow the photosensitive writing of gratings, while reducing the
deleterious
effects of loading the entire fiber.
1o SUMMARY OF THE INVENTION
The present invention relates to a sectionally highly-photosensitive optical
waveguide having discrete and localized hydrogen-loaded areas. Such waveguides
provide significant advantages over traditionally hydrogen loaded long lengths
of
fiber. Advantages included reduced optical loss and flexibility in Bragg
grating
15 production.
An optical device according to the present invention includes a length of
waveguide and at least one discrete longitudinal section having increased
photosensitivity with respect to other portions of the waveguide. The
longitudinal
section may have a photosensitivity at least two-times greater than the rest
of the
20 length of waveguide. The waveguide may include a plurality of discrete
longitudinal sections, wherein at least one section has a different
photosensitivity
than other sections.
In particular embodiments, the waveguide is an optical fiber having a core
region and at least one cladding layer and the longitudinal section has
increased
25 photosensitivity along the core region and/or cladding layers.
The high-photosensitivity longitudinal sections may be identified by the
presence of hydroxyl band. The hydroxyl band has an absorption in the 1410 nm
absorption region. The waveguide may include a short wavelength absorption
band having a magnitude > 0.1''dB/cm at wavelengths less than or equal to 800
nm.
3o Neither the hydroxyl band nor the short wavelength band are present in the
waveguide outside of the at least one longitudinal section.
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The longitudinal sections may be separate from each other or adjacent. The
amount of hydrogen loading in each section may be the same or may be
different.
In one embodiment having a first and a second discrete longitudinal portions,
the
hydroxyl band in the first portion has a greater attenuation than the hydroxyl
band
in the second portion. In another embodiment having a plurality of discrete
longitudinal poutions, each portion has a different attenuation value in the
respective hydroxyl band. In yet another embodiment, the plurality of discrete
longitudinal portions are sequentially adjacent to each other and the
attenuation
values follow a predetermined function.
to An optical device in accordance with the present invention may then
include a length of optical fiber, a section of increased refractive index
along the
length of optical fiber, amd a hydroxyl absorption band along the grating. The
grating may be a Bragg grating. In one particular embodiment, the grating has
a
refractive index that changes by < 20 % due to annealing at 300 C for 10
minutes.
The present invention also contemplates a method for writing an optical
grating. In an exemplary embodiment, the method includes the step of providing
a
length of optical waveguide comprising at least one discrete longitudinal
section of
the optical waveguide. The section has a hydroxyl band with absorption in the
1410 nm absorption region and a short wavelength absorption band having a
2o magnitude > 0.1 dB/cm at wavelengths less than or equal to 800 nm. Neither
the
hydroxyl band nor the short wavelength band are present in the waveguide
outside
of the at least one longitudinal section. One or more of the at least one
longitudinal
sections are exposed to a patterned source of actinic radiation.
For waveguide having a plurality of discrete longitudinal sections where the
hydroxyl band in one section has a different attenuation than the hydroxyl
band in
another section, the step of exposing may include using the same writing
conditions in each section to create a plurality of gratings having different
center
wavelengths and percent reflectivity.
BRIEF DESCRIPTION OF THE DRAWINGS
3o Figure 1 is a schematic diagram of an optical waveguide in accordance with
the
present invention.
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Figure 2 is a graph of writing time vs. photosensitivity for an optical fiber
in
accordance with the present invention.
Figure 3 is a graph of anneal time vs. NICC (Normalized Integrated Coupling
Coefficient) for an optical fiber in accordance with the present invention.
Figure 4 is a side elevation view of a first embodiment of a hydrogen loading
apparatus in accordance with the present invention.
Figure 5 is a side elevation view of a second embodiment of a hydrogen loading
apparatus in accordance with the present invention.
Figure 6 is a schematic view of a coolant circulation system for the
embodiment of
l0 a hydrogen loading apparatus depicted in Figure 4 or 5.
Figure 7 is a side cross-sectional elevation view of a third embodiment of a
hydrogen loading apparatus in accordance with the present invention.
Figure 8 is a longitudinal cross-sectional view of a fourth embodiment of a
hydrogen loading apparatus in accordance with the present invention.
15 Figure 9 is a plan cross-sectional detail view of a heater block and fiber
of the
loading apparatus depicted in Figure 8.
Figure 10 is a cross-sectional elevation view of a fifth embodiment of a
loading
apparatus in accordance with the present invention in an open position.
Figure 11 is a cross-sectional elevation view of the apparatus depicted in
Figure 10
2o in a closed position.
Figure 12 is a cross-sectional detail elevation view of the hydrogen loading
chamber of the apparatus depicted in Figure 10.
Figure 13 is a cross-sectional elevation view of an end section of the
hydrogen
loading chamber depicted in Figure 12.
25 Figure 14 is a cross-sectional elevation view of a first embodiment of a
clamping
mechanism for the loading apparatus illustrated in Figure 10.
Figure 15 is a cross-sectional elevation view of a second embodiment of a
clamping mechanism for the vessel illustrated in Figure 10.
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Figure 16 is a top plan view of the lower block of the hydrogen loading
apparatus
illustrated in Figure 10.
Figure 17 is a cross-sectional elevation view of a third embodiment of a
clamping/sealing mechanism for the vessel illustrated in Figure 10.
Figure 18 is a side cross-sectional view of a sixth embodiment of a hydrogen
loading apparatus in accordance with the present invention.
Figure 19 is a side cross-sectional view of the apparatus depicted in Figure
18 in
the closed position.
Figure 20 is an end view and cross-sectional end view of the collet depicted
in
to Figure 18.
Figure 21 is a sequential step illustration of methods for increasing the
photosensitivity of an optical fiber in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 schematically illustrates an optical waveguide in accordance with
the present invention. Referring to Fig. lA, the waveguide is a silica optical
fiber
10 having a core 12 and one or more cladding layers 14. The core 12 in the
present
embodiment is doped with greater than or equal to three (3) mole % of
germanium
and/or other known photosensitive dopants to increase its photosensitivity.
The
cladding layers also may be doped with similar dopants and at similar levels.
2o As illustrated in Fig. 1B, in one exemplary embodiment, the optical fiber
10
includes one or more discrete and localized longitudinal regions 20 of high-
photosensitivity. The locations may be marked by a color dye, such as during
the
recoating process. The length of fiber 10 may be stored in a spool or a rotary
stage
30.
As illustrated in Fig. 1 C, in another exemplary embodiment, a plurality of
hydrogen-loaded or high-photosensitivity regions, 120, 122, 124, and 126, may
be
adj acent to each other.
The fiber 10 in Figures lA-C may include an outer coating to protect the
bare glass. In one particular embodiment, the fiber 10 is coated with a cured
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composition transparent to ultraviolet radiation, that is, a write-through
coating
such as the one discussed in co-pending, commonly-assigned U.S. Patent
Application No. 10/116,778, entitled "Cured Compositions Transparent to Ultra-
violet Radiation", which is hereby incorporated by reference. The location of
the
photosensitive sections may be marked using visual markings such as a U.V.
transparent colored dye or other methods known in the art. The use of a write-
through coating allows the writing of gratings, such as Bragg gratings, on the
sections without removal of the coating.
In one exemplary embodiment, the curable coating composition comprises
1o an organohydrogenpolysiloxane, an alkenyl functional polysiloxane, and an
ultraviolet radiation absorbing hydrosilation photocatalyst in an amount for
crosslink formation between the organohydrogenpolysiloxane and the allcenyl
functional polysiloxane. The curable coating composition crosslinks under the
influence of ultraviolet radiation to provide a cured coating having a high
level of
transparency to ultraviolet radiation. application of heat to the curable
coating
composition accelerates the rate of cured coating formation. the high level of
transparency of the cured coating allows from about 70% to about 99% of
radiation
of wavelengths from about 240nm to about 275nm to pass through the coating for
writing a refractive index grating to produce an optical fiber Bragg grating .
2o Hydrogen loading an optical fiber provides great benefit in increasing the
photosensitivity of the fiber. However, traditional hydrogen loading has the
detrimental effect of increasing the optical loss in the fiber. While much of
the
excess loss may be removed by baking out the hydrogen, there are residual
losses,
typically few tens to few thousands of dB/km, in the wavelength region between
1500 nm to 1600 mn, which will affect the device performance in optical
networks.
The present invention provides a sectionally-loaded optical fiber that reduces
the
detrimental losses related to hydrogen loading by loading only a short section
of
the fiber used to write a grating.
Below, referring to Figures 4-21, the use of high-pressure vessels to load
hydrogen into a fiber at elevated temperatures is discussed. A common theme to
many of the described vessel designs is that only short lengths of the fiber,
sufficient to write a grating, are exposed to the hydrogen atmosphere at
elevated
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temperature while the remaining portions of the fiber is maintained at a lower
temperature and pressure, thus generally not inducing the loading defects
previously described in other portions of the fiber. While these sectional
vessels
load at elevated temperatures, they are operable at lower temperatures. An
important advantage of the localized loading approach is to avoid the damage
along
the entire length of the coating as a result of exposure to high temperatures.
In
addition, there are significant additional optical performance and
manufacturing
advantages to loading only a short length of the fiber.
A traditional process for manufacturing Bragg gratings involves the
to following steps:
~ Bulk hydrogen loading of a coil of fiber (e.g., 21°C for 13 days)
~ Cold storage of the fiber coil to retain H2 content
~ Sectionalized coating removal
~ Connecting the fiber to monitoring equipment
~ Grating writing by UV exposure
~ Annealing the section of the fiber having a grating at an elevated
temperature, typically 300°C for 10 minutes
~ Recoating the stripped portion
~ Hydrogen removal of the entire spool (or alternatively of only the
loaded portions) at slightly elevated temperatures in a final bake-out
step (80°C for 3 days)
~ Packaging of the grating product
Using the present invention where only a short length of the fiber is
hydrogen loaded, allows for several steps to become more flexible or to be
eliminated altogether. For example, the bake-out step, which is used to
stabilize
the fiber by removing extra hydrogen, may not be required as any significant
hydrogen in the short length of fiber that was hydrogen-loaded is likely to be
removed during the high-temperature annealing step.
Additionally, when a fiber contains hydrogen, it cannot be fusion-spliced
because the hydrogen in the fiber causes deformation of the glass during
exposure
to the electrical arc making it impossible to achieve useable splices. If only
a shout
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length of the fiber is hydrogen loaded, fusion splicing of non-loaded portions
may
be employed at any point in the manufacturing process without an additional
step
of removing hydrogen from the ends of the fiber. Fusion splicing would most
likely be employed when monitoring the grating or in a final packaging step.
The
ability to fusion splice at any point in the manufacturing process increases
the
flexibility of the design of the package and the packaging process.
The present invention allows "just-in-time" supply of hydrogenated fiber.
Commonly-owned United States Patent No. 6,311,524, which is hereby
incorporated by reference, discusses hydrogen loading at elevated temperature
to (greater than 250°C) where the hydrogen diffusion time is quite
short, with
hydrogen diffused into the fiber in minutes as compared to hours and days. A
short
length of fiber, only the amount required, may be sectionally hydrogen-loaded
at
elevated temperatures using, for example, the clamshell vessel described below
to
achieve a controlled photosensitivity in each loaded section of the fiber.
15 Exemplary embodiments, such as that illustrated in Figure 1 B, may include
one-or more separate high-temperature HZ-loaded single short-sections. Other
alternative exemplary embodiments include waveguides having several discrete
longitudinal sections, wherein at least one section has a different
photosensitivity
than other sections. In yet another exemplary embodiment, such as the one
2o illustrated in Figure 1 C, waveguides according to the present invention
have a
plurality of discrete longitudinal portions, 120-126, adjacent to each other,
wherein
the hydroxyl band in a first portion 120 has a different attenuation than the
hydroxyl band in a second portion 122. The discrete longitudinal portions with
increased photosensitivity may be sequentially adjacent to each other and the
25 attenuation values follow a predetermined function. Being able to provide
waveguides having adjacent, periodic, or aperiodic sections of different known
photosensitivities along a continuous length allows for writing gratings of
different
specifications and/or center wavelength without altering the writing setup.
This
ability may be specially advantageous in new continuous in-line writing
processes,
3o such as reel-to-reel fiber grating writing. Additionally, new devices, such
as
sectionally-stepped chirped gratings may be possible.
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Additionally, the high temperature sectional loading approach for loading
individual short lengths of fiber affords the benefit of precise control over
the
hydrogen content in the fiber. Using the clamshell vessel, the fiber may be
loaded
for a precise amount of time at a specified temperature and pressure. When the
vessel is opened, the single fiber cools extremely quickly (seconds),
retaining
hydrogen. If the fiber is used in a reasonably short amount of time (which may
be
calculated using decay curves), the degree of photosensitization may be
estimated
very accurately. The maximum hydrogen content is determined by the solubility
of
hydrogen in the glass, which is a function of the temperature and pressure
used in
to the hydrogen loading process. With the sectional loading vessels, it is
possible to
stop the process precisely, controlling the photosensitivity of the fiber.
Fiber
compositions vary widely, and certain compositions are far more photosensitive
than others. In fact, hydrogen loading of a fiber with good photosensitivity
may
create a complication of gratings growing too fast to accurately monitor the
gratings' properties during the writing process, making it difficult to comply
exactly with specifications in a production setting. Photosensitivity control
may be
used to tailor the write time of the grating. It also may be used to equalize
the
photosensitivity of disparate fibers so the same writing conditions may be
conveniently used for a variety of fibers.
2o In comparison, traditional bulk loaded fibers are stored and used over a
long period of time. The fiber is frozen at low temperatures to help retain
the
hydrogen content by slowing the diffusion of the hydrogen out of the fiber.
Even
with the precaution of low temperature storage, the fiber is exposed
intermittently
to room temperature, which results in an unquantified loss of hydrogen which
in
turn creates a higher level of uncertainty in the degree of photosensitization
of the
fiber at the time the grating is written.
A fiber loaded under high temperature conditions produces a more stable
change in the index of refraction than a fiber loaded under lower temperature
conditions due to the increased stability of sites created at a higher loading
3o temperature. Figure 2 shows the grating growth curves in standard and high-
temperature loaded fibers with the same initial HZ concentration. While the
grating
growth in each fiber is similar, the annealing behavior is quite different as
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illustrated in Figure 3. A grating written in high-temperature loaded fiber
shows
better thermal stability when compared with the standard loaded fiber as shown
by
the smaller change in the NICC (Normalized integrated Coupling Coefficient) in
Figure 3. The NICC is a measure of the strength of the final grating measured
in
transmission normalized by the strength of the grating at room temperature
prior to
amlealing the fiber. The calculation of this parameter is well known in the
art.
When a fiber Bragg grating is annealed, the unreacted hydrogen diffuses out
and
less stable sites are erased out of the fiber and hence the change in the
index of
refraction is stabilized. The annealing process of gratings written in
standard H2-
to loaded fibers typically results in more than a 15% decrease in the UV-
induced
index change created during the grating writing process, predominately due to
the
excess unreacted hydrogen leaving the fiber. In high-temperature H2-loaded
fibers,
however, just enough hydrogen is diffused into the fiber to write the gratings
to the
desired specifications. As a result, most of the H2 in the fiber has reacted
to create
a sufficient index change per amount of hydrogen, the induced index change is
more stable than the index change induced in a fiber with a comparable
hydrogen
content loaded at lower temperatures. In the case of the high-temperature
loaded
fiber, the loss in the index change is less than 15%. A reduced index change
with
annealing has the advantage of reducing the time and/or laser power required
to
write the grating. Another benefit of the high temperature loading process is
that
less hydrogen needs to be put into the fiber to achieve the same final grating
strengths as a traditionally loaded fiber, because less hydrogen will be lost
from the
fiber prior to the writing of the grating.
Even after a grating is written, exemplary fibers in accordance with the
present invention may be identified by their residual loss measurements. High-
temperature H2-loaded germano-silicate (GS) and boro-GS fibers have measurable
residual losses in the less than X00 nm and approximately 1400 nm wavelength
regions due to the formation of germanium-related defects and hydroxyl
species.
The strength of the absorption band depends strongly on the fiber type and the
3o dopants. This characteristic feature may be utilized to accurately
determine the
loading conditions and hence to identify sectionally hydrogen-loaded fibers.
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Examples:
The absorption losses in standard (60 °C, 2000 psi (1.38x107 Pa),
3 days)
and high-temperature (260°C, 2000 psi (1.38x107 Pa), 10 min) hydrogen-
loaded
SMF-28 (GS) and TF-45 (boro-GS) fibers were measured. The results are
summarized below:
1. There is no hydroxyl band (at approximately1400 nm) in standard H2-
loaded SMF-28 fiber.
2. There is a very weak (approximately 10 dB/km) absorption band due to
hydroxyls formed in high-temperature H2-loaded SMF-28 along with an increased
to short wavelength absorption edge (SWE) at less than 800 nm. The strength of
the
SWE band decreases to about few dB/km when the excess hydrogen is removed
from the fiber in the final bake-out step (80°C for 68 hours).
3. There is a large absorption peak of approximately 2500 dB/km due to
hydroxyls formed in TF-45 fiber, which reduces to approximately 1000 dB/km
15 after the final bake-out step.
Commonly-assigned United States Patent No. 6,311,524, entitled
"Accelerated Method For Increasing The Photosensitivity Of A Glassy
Material", which is hereby incorporated by reference, describes an accelerated
method for hydrogen loading an optical medium in a high-temperature
?o enviromnent. The application discusses how the temperature that the fiber
is
exposed to in the hydrogen environment will affect the time involved in
diffusing
the hydrogen molecules into the fiber. Generally, the higher the temperature,
the
faster is the diffusion rate of hydrogen into the glassy material (e.g., an
optical
fiber).
25 Comparing similar fibers, under optimal conditions, a typical grating-
quality fiber loaded at 60°C for 3 days results in an index change of 1
x 10-3.
Under similar optimal conditions, the same fiber loaded at high temperature,
260°C, for 10 minutes exhibits an index change of 4 x 10~.
However, heating the entire fiber at high temperatures has the potential for
3o affecting both the physical integrity of the optical fibers (in particular,
of fibers
having coatings that are susceptible to damage at elevated temperatures) and
the
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optical properties of the fiber. Moreover, heating the entire fiber presents
challenges as to fiber handling and temperature ramp-up control.
The present invention loads hydrogen and/or deuterium only into the
particular portion of the fiber where the grating is to be written and where
higher
photosensitivity is desired. In a particular embodiment, the loading is done
at high-
temperatures (greater than 250°C) and/or high pressures, which
accelerates the
loading process and allows for the apparatus to be used as a stage in an in-
line
processing line.
The remainder of the fiber is not heated. Adjacent portions of the fiber may
to even be attached to a heat dissipater or sink or cooled to maintain a
cooler
temperature. This is especially useful for fibers having coatings that degrade
at
higher temperatures.
Figures 4 and 5 illustrate a first embodiment 100 and a second embodiment
200 of selective loading vessels. The selective loading vessels include the
following elements: 1) a controlled pressure and temperature chamber, which
may
withstand high temperature (greater than 250°C) and high-pressures,
where a
selected specific length of fiber can be loaded with hydrogen or deuterium; 2)
structural integrity to contain the high-pressure gases (e.g., several hundred
atmospheres of hydrogen or deuterium gas); 3) input and output ports for
2o introducing and venting pressurized gasses, 4) and mechanisms for safely
installing
and removing fibers from the vessels. As it will become apparent, similar
elements
in these embodiments generally are designated by the same last two reference
numerals.
The vessels 100 and 200 are symmetrical and each includes a center heating
tube or loading chamber 102 and 202 having a first end and a second end. In
the
depicted embodiments, both of the tube vessels are constructed from standard
high-
pressure gas supply tubing. Such tubing is commercially available and is made
from 316 stainless steel.
The heating tubes 102 and 202 are surrounded by heating blocks 110 and
3o 210, respectively. The particular heating blocks 110 and 210 are made from
aluminum or another thermally conductive material. The heating blocks 110 and
210 are designed to clamp onto the outside diameter of a center portion of the
large
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or small diameter heating tube vessels 102 and 202. The heating bloclcs 110
and
210 house a plurality of electric cartridge heaters 112 and 212, which are
controlled
via a programmable logic control system 114 and 214, such as the auto-tuning
power control system designed and fabricated by Watlow, of St. Louis,
Missouri.
Alternative embodiments may include other types of electric heaters, foil
heaters,
hot oil heaters, induction heaters, or other types of heaters.
The heating blocks 110 and 210 may be made in two halves and clamped
onto the tube, or made as a single slot collet design that clamps onto the
outside
diameter of the tube. In another embodiment, the heating blocks 110 and 210
1 o include a concentric collar that threads around the heating tubes. The
length of the
heating block 110 and 210 is about 5 cm (~2 inches), the approximate size of
the
largest "short" grating that is currently written, but could be any length
desired.
It must be noted that in the present loading vessels 100 and 200, the fiber to
be loaded, 140 and 240 respectively, is one continuous length, with the
midspan
1 s section that is to be hydrogen loaded located inside the loading chambers
102 and
202, in-between ends of this piece of fiber.
In certain embodiments, the adjacent lengths of fiber located on both sides
of the higher temperature loading zone are kept cool enough to prevent thermal
energy being conducted or radiated from the loading chamber to degrade
adjacent
2o coatings. The embodiments illustrated in Figures 4 and 5 include optional
cooling
tubes or cooling chambers, 104 and 204 respectively. The cooling tubes 104 and
204 are coupled to each one of the ends of the respective center heating tube
102
and 202.
In the embodiment illustrated in Figure 4, the length of fiber that is not
2s being loaded is not placed in a high-temperature hydrogen atmosphere, but
is
surrounded by a lower-temperature atmosphere. In alternative embodiments, gas
seals may separate the loading chamber and the cooling chambers. An inert gas,
such as nitrogen, which may be cooled, may be inserted into the cooling tube
to
inhibit combustion of organic polymeric coatings.
3o The vessels 100 and 200 are basically similar, with the differences being
the diameters of the heated and cooled tubes. The vessel 100 has a small
diameter
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heating tube 102 and a large diameter cooling tube 104. The vessel 200 has a
large
diameter heating tube 202 and a small diameter cooling tube 204.
The heating tubes 102 and 202 are connected to the cooling tubes 104 and
204 respectively by connector fittings 106 and 206. The connector fittings 106
and
206 are commercially available and also are made from 316 stainless steel. The
length of the entire vessel 100 is approximately 107 cm (approximately 42
inches).
This length was selected because traditionally gratings are written on a one
(1)
meter length of fiber. Alternative embodiments may be made longer or shorter
depending on the desired area of exposure, the type of desired grating, and
the
to optical fiber to be used.
Closure fittings 108 and 208 are placed at outer ends of the cooling tubes
104 and 204. Alternatively, the closure fittings also may be place at the end
of the
heating tubes 102 and 202. ~ne of the closure fittings includes a gas inlet,
120 and
220, for introducing the loading gases into the vessel. The other closure
fitting
includes a gas vent or outlet, 122 and 222, for exhausting the loading gases.
The
closure fittings 108 and 208 are coupled to controlled needle valves to allow
the
introduction of hydrogen and inert gasses into the vessel, and out of the
vessel at
the vent end of the vessel. The piping to any such system also may include
high-
pressure blowout disks (as a safety device), which are rated at pressures 10%
to
20% higher than the highest pressure expected during processing.
The loading process consists of purging the vessel with nitrogen 3 to 5
times before the introduction of hydrogen at high pressure, (approximately
2000
psi or 1.38 x 10' Pa). The vessel may be fitted with electrically actuated
solenoid
valves that are controlled with a PLC system for automatic gas delivery and
venting.
Although not necessary in all embodiments, the present exemplary
embodiments 100 and 200 may include cooling blocks 130 and 230 respectively.
The cooling blocks 130 and 230 are located between the heated portion of the
tube
vessel, and the end of the vessel, on both sides of the heated portion of the
vessel.
Their exact length and precise location may vary to suit the process. The
cooling
blocks 130 and 230 are made from aluminum or other thermally conductive
material and are designed to clamp onto the outside diameter of the outer
portions
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of the cooling tubes 104 and 204. The design of the clamp mechanism would be
similar to the designs used for the heating blocks 112 and 212. The cooling
blocks
130 and 230 may be helpful in in-line production applications, where heating
cycles are repeated frequently and residual heat increases the temperature of
the
entire vessel.
The cooling blocks 130 and 230 contain a series of holes or channels 132
and 232 that allow cold fluid to be pumped through them. The fluid pressure
and
temperature may be controlled via a programmable logic control system 114 and
214. The cooling blocks 130 and 230 are concentric collars or blocks that
clamp
to on or that slide over the hydrogen vessel cooling chambers 104 and 204. In
alternative embodiments, the cooling blocks may be made in two halves and
clamped onto the tubes 104 and 204, or made as a single slot collet design
that
clamps onto the outside diameter of cooling tubes. In the present embodiment,
the
length of the cooling blocks 130 and 230 is 7.6 cm (~3 inches) each, but could
be
of different length, as long as the fiber coating is prevented from combusting
or
degrading.
Figure 6 shows a convenient assembly creating cooling regions near the
heating region, in order to minimize damage to the optical fiber coating
outside the
selectively hydrogen loaded portion of the optical fiber. The exemplary
diagram
2o will be shown in reference to the first embodiment of the invention, shown
in
Figure 4, but the same principles can be readily applied to any of the
embodiments
disclosed here. The optical fiber segment 140 (not shown) is enclosed in a
tube
comprising a central heating tube 102 between cooling tubes 104. Hydrogen gas
in
introduced into the tubes with the fiber, and the outer ends of the cooling
tubes 104
are sealed with closure fittings 108. A heater block 110 is clamped around
heating
tube 102 to form the heating region. Along the tube at each side of the
heating
block 110 is attached a cooling block 130 which encloses within its body one
or
more cooling fluid chamiels 132. The cooling fluid channels 132 can be
connected
by external plumbing to a commercial water chiller or other liquid cooling
device
145. One exemplary cold fluid recirculation system is a Polyscience Model 5005
Mini-Chiller, which is a commercially available fully contained system that
can
regulate temperatures to +/-0.5°C and ranges in programmable
temperature settings
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between-5°C to +50°C. Preferably, the cooled fluid outlet 146 of
the chiller is
connected to the end of cooling fluid channel 132 that is closest to heater
block
110. The other end of cooling fluid channel 132 is comlected to the warm fluid
inlet 147 of the chiller 145. This arrangement causes that the coldest cooling
fluid
be directed nearest the heater block. This produces a steep temperature
gradient
between the heating block and the cooling blocks along the tube 104/102 which
encloses the fiber that is being hydrogen loaded. The steep temperature
gradient
helps protect the coating on the fiber outside the hydrogen loading region.
Programmable logic controller 114 can coordinate the entire loading process by
to controlling the temperature of the heating block 110, the temperature and
pressure
of the cooling fluid in the cooling blocks via the chiller 145, and the input
and
venting of hydrogen and purge gasses through valued end caps 108.
The vessels 100 and 200 allow a length of fiber 140 and 240 to be inserted
into the cooling and heating tubes, while allowing additional room to move the
fiber 140 and 240 once inside the tube vessel. In this embodiment, fiber
segments
no longer than the length of the vessels are inserted and removed from the
tube by
removal of one of the fittings located on the end of the tube vessel, which
allow
insertion or extraction of the fiber, or fibers, into or out of the vessel.
The extra length of the tube vessel (e.g., 105 cm.) as compared to the target
2o fiber length (e.g., 90 cm.), allows the fiber to move inside of the tube a
distance
that is greater than the heated length of tubing, (which is 5 cm. in this
case), to
provide a rapid transition of temperature within the fiber from hot to cool,
in the
heat affected zone. A rapid transition from the heated area to a cooled area
slows
the diffusion of hydrogen out of the fiber when loading gas pressure is
released.
A variety of mechanisms may be implemented to effect this movement. In
the embodiment illustrated in Figure 4, a magnetic body 116, such as a
magnetic or
ferrous ring, is attached to a portion of the fiber 140. By translating a
magnet 118,
having a sufficient magnetic force, along the outside of the tube in the axial
direction of the tube thus moving the fiber inside of the tube. Another method
3o would involve attaching a weight 216 onto the end of the fiber and tilting
the tube,
which will cause the weight, and attached fiber to move due to gravitational
forces,
towards the lower end of the tube.
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Following are descriptions of exemplary processes that may be utilized to
load hydrogen into an optical fiber using the vessel 100. The term hydrogen
atmosphere in the present description is intended to include atmospheres
including
H2, DZ, tritium, or molecules such as HD that combine these isotopes of
hydrogen.
The first process comprises the step of inserting (threading for non-clamping
tubes)
the optical fiber 140 into the vessel 100, and sealing the vessel 100. Several
cycles
of nitrogen, introduced through the gas inlet 120 and exhausted through the
gas
vent 122, are purged through the vessel 100 to ensure that ambient air has
been
evacuated from the vessel 100. Hydrogen is introduced, exemplarily at high
pressures, such as between 1000 psi (6.89 x 106 Pa) and 2000+ psi (1.38 x 107+
Pa).
Preferably after full pressure is reached, the heating block 110 would be
activated. The programmable logic control system 114 controls the temperature
in
the chamber by controlling the heating blocks. In applications where
considerable
heat may migrate into other portions of the fiber, the cooling blocks 130 also
may
be activated.
For high-temperature loading processes, in one exemplary process, the
portion of the fiber 140 to be loaded is stripped of its coating prior to
insertion into
the vessel 100. In yet another embodiment, the fiber includes a high-
temperature
2o resistant, hydrogen-permeable coating suitable to resist the loading
temperature.
In yet another alternative method, the coating may be selected such that it
depolymerizes into gaseous products at or below high loading temperatures. The
hydrogen atmosphere preferably is selected to not include oxygen, in order to
avoid
an oxidation/combustion process. The resulting gases are vented out of the
chamber with the heated hydrogen. This allows for both loading and stripping
of
the coating in one step. Additional detail regarding depolymerizable coatings
may
be found in commonly assigned United States Patent No. 5,939,136, "Process For
Preparation Of Optical Fiber Devices Using Optical Fibers With Thermally
Removable Coatings", and commonly assigned United States Patent No.
5,596,669, "Radiation Curable Coating Composition And Coated Optical Fiber",
which are hereby incorporated by reference.
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When the hydrogen atmosphere reaches the desired temperature, a tinier
would be started to track the time the fiber 140 is exposed to the heated
hydrogen
atmosphere. Co-assigned United States Patent No. 6,311,524, which is hereby
incorporated by reference, describes exemplary exposure and temperature
settings
for high-speed, high-temperature hydrogen loading. United States Patent Nos.
5,235,659 and 5,287,427 offer examples of other hydrogen loading parameters.
After a desired exposure time is reached, the heating blocks 110 are
deactivated. Depending on factors such as loading requirements or the heat
sensitivity of the coating of the fiber, the fiber may be immediately moved to
the
io cooling tube 104. Hydrogen pressure may be vented and nitrogen or other
inert
gases may be forced into the vessel 100. The vessel 100 is opened and the
fiber
140 removed.
A grating may be then written by exposing the selected portion to a pattern
of actinic radiation. The selected portion may then be annealed. If a coated
fiber
was used, with sectional loading, only the loaded portion, which is the same
portion that the grating is written on, will require recoating. No hydrogen
bake out
is required with sectional loading, as with bulls-loaded fiber, as the
annealing
process step removes hydrogen from the loaded area.
In a method in accordance with the present invention, the above steps may
2o be performed in a step in-line process. The fiber may be suspended in a
reel to reel
assembly, threaded through an optional coating removal station, a hydrogen
loading station, a grating writing station, an annealing station, and an
optional
recoating station.
The second exemplary process is similar, but differs at one point. The
method again comprises the step of inserting the fiber 140 into the vessel
100, and
sealing the vessel 100. Several cycles of nitrogen are forced through the
vessel to
ensure that ambient air has been purged from the vessel 100. The heating block
110 (and cooling blocks 130 if required) are activated to achieve the desired
temperature. After the nitrogen atmosphere reaches the desired temperature,
3o nitrogen is replaced by hydrogen, which may be introduced at high
pressures, such
as between 1000 psi (6.89 x 106 Pa) and 2000+ psi (1.38 x 107+ Pa). Since the
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mass of the inserted hydrogen is very small with respect to the mass of the
vessel,
the hydrogen would very quickly reach the desired temperature.
In yet another, third, exemplary process, the hydrogen is preheated in a
second pressure vessel prior to introduction into the "tube" type vessel. A
pre
y heating chamber may even be used to heat the hydrogen atmosphere prior to
introducing the hydrogen atmosphere into a loading chamber having no heating
element. The hydrogen may be preheated to the same desired temperature to
manage any "heating lag". Alternatively, the hydrogen may be preheated to a
lower temperature (to reduce the heating time, yet to allow ease of handling
when
to the desired temperature is high) or even at a higher temperature to
compensate for
expected heat loss upon insertion.
When the desired pressure and/or temperature is reached, a timer tracks the
time the fiber 140 is exposed to the hydrogen atmosphere. After this
predetermined time is reached, the heating blocks 110 are deactivated. Again,
if
15 desired, the fiber 140 may be immediately moved to the cooling tube 104.
Even
while the fiber 140 is being moved to its new position, hydrogen pressure may
be
vented, and nitrogen or another inert atmosphere may be forced into the vessel
100
to displace and purge any remaining hydrogen. After the purge, the vessel 100
may
be opened and the fiber 140 removed. In alternative process flows, if safety
and
20 equipment permits, the fiber may even be removed immediately after the end
of the
loading process (e.g., for applications using low temperature and small
volumes of
hydrogen).
Figure 7 is a schematic illustration of a reel-to-reel production assembly
300. The production loading assembly 300 includes a middle-loading vessel 301
25 including similar features to vessels 100 and 200. The assembly 300 further
includes a fiber unwind reel 350 and fiber wind up reel 352. Each reel
includes a
spool, an unwind spool 354 and a wind up spool 356 respectively. The rotation
of
the wind up spool or both of the spools is actuated by a spooling motor, such
as
electric servo motor 358. A progrannnable logic controller (PLC) 360 may be
3o electronically coupled to the motor 358 to control the entire process.
The process of loading an optical fiber using the assembly 300 comprises
loading a length of fiber 340 into the unwind reel 350. The fiber 340 is
threaded
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through the tube vessel 301 and attached to the wind up reel 352. The loading
process is similar to the ones described above; with the addition that timing
and
precise fiber advancement occurs automatically via programmed predetermined
recipes or inputs monitored by the PLC 360. With this apparatus, multiple
sections
of a longer continuous length of fiber may be hydrogen loaded, reducing the
amount of labor, and increasing the consistency of the hydrogen loading
process. If
desired, the assembly 300 may further include marking stations that identify --
such
as by visible markings, different coatings, and/or machine-readable codes--
the
areas that were hydrogen loaded. If desired, a grating may then be written in
the
to hydrogen-loaded area.
Figure 8 illustrates a fourth embodiment 400 of a high temperature
hydrogen-loading vessel. Figure 8 illustrates a cross section of the vessel
400 cut
through its axial centerline (the vessel is in the shape of a cylinder). The
vessel
400 includes a cylindrical bell-shaped body 402 capable of withstanding high-
temperatures and internal pressures. A vessel cap 404, secured to the body 402
by
vessel clamps 406 closes the open end of the body 402. The vessel cap 404
includes four pass-through ports 408 for a gas inlet/vent 410, and for
electrical
control ports 414. The electrical control ports are coupled to a programmable
heater control system, which may be used to control several electric cartridge
heaters inside of the loading vessel. The bottom of vessel 400 includes a
thermocouple port 412,
A fiber spool assembly 416, made of a material able to resist the high
temperatures, and conduct thermal energy quickly, such as aluminum, is placed
inside of the body 402. The aluminum fiber spool assembly 416 of the present
embodiment includes a number of optical fiber receiving stations 418. The
present
embodiment holds about ten (10) stations, which would allow for ten fiber
segments to be sensitized. Each station includes two individual fiber reels
420 that
retain one optical fiber segment 422. The fibers are wrapped around the
outside
diameter of each reel 420 and are held in position with flanges that extend
beyond
3o the diameter of the reel. A precise curved slot is milled into the flanges,
(on both
the top and bottom reels), that are located on the flange that is positioned
closer to
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the center of the aluminum fiber spool, and allow the fiber 422 to be routed
off of
the storage reel diameter.
Each station also includes a heater block 424 located at about the midspan
portion of each fiber segment 422. The fiber is routed out of the upper
storage reel
and back into the lower storage reel. Between the two storage reels, it is
positioned
parallel to the axis of the aluminum fiber spool, and passes through a heater
block
that is located in the center of the spool.
The aluminum fiber spool assembly 416 is attached to the vessel cap 404
where the electrical wire pass-through ports 414 are located. This attachment
to allows for easier inseution and removal of the spool assembly 416, and
provides
wire bend protection to the electrical wires. A thermocouple 428 passes
through
the thermocouple port 412 and monitors the temperature of the spool assembly.
Figure 9a illustrates a cross sectional view, (looking from top to bottom) of
the fiber 422 passing through a U-shaped channel that is cut into the heater
block
424. In the present embodiment, the heater block 424 holds an electric
resistance
cartridge heater 426, where approximately 60 degrees of the heater body is
exposed
in the bottom of the U-shaped charnel. This exposed portion of the cartridge
heater 426 provides extremely quick temperature ramps of the atmosphere that
intimately surrounds the fiber 422 in this U-shaped channel.
2o Figure 9b illustrates a second design of the heater block 424 having the
cartridge heater 426 fully embedded in the heater block 424. The outer portion
of
the heater block 424 has several ribs 427 milled into it to dissipate the heat
in a
more efficient manner. In addition to thermocouple 428, individual
thermocouples
monitors the temperature of each heater block 424, touching the heater 426, to
provide temperature signals for a PLC that would provide precise temperature
regulation of the heater 426. Additional thermocouples may be added to the
aluminum spool, positioned in an orientation to monitor hydrogen temperature.
Following are exemplary processes that may be utilized to load hydrogen
into an optical fiber using the vessel 400. The first comprises the steps of
installing
a length of fiber 422 onto the aluminum fiber spool assembly 416, inserting
the
spool assembly 416 (which is attached to cap 404) into the vessel body 402,
and
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sealing the vessel 400. Again, in high-temperature processes, the fiber 422
may be
pre-stripped or include high-temperature or gaseously depolymerizable
coatings.
Several cycles of nitrogen are purged through the gas inlet/vent 410 to
ensure that air was evacuated from the vessel 400. For high-pressure recipes,
hydrogen is introduced at high pressures, between 1000 psi (6.89 x 106 Pa) and
2000+ psi (1.38 x 107+ Pa). Depending on the type of fiber used, and the type
of
grating that is being written, pressures between 500 psi (3.44 x 106 Pa) and
2100
psi (1.45 x 106 Pa) may be used. Higher pressures (approximately 3000 psi or
2.07
x 106 Pa) would allow more hydrogen to diffuse into the fiber, and might be
1 o desirable for some applications. The heaters 426 are activated, preferably
after full
pressure is reached. When the hydrogen atmosphere reaches the desired
temperature around the fiber (as measured by the thermocouple), a timer tracks
the
time the selected portion of the fiber 422 is exposed to the high-temperature
hydrogen atmosphere.
After the selected time is reached, the heaters 426 are deactivated, and the
exposed portion of the fiber 422 is allowed to cool. Hydrogen pressure is
vented
and nitrogen or other suitable gases are forced into the vessel. In one
exemplary
embodiment, chilled nitrogen is forced into the vessel to cool the fiber and
coatings
and to reduce the diffusion rate of the hydrogen out of the optical fiber due
to the
2o venting of the hydrogen pressure. The vessel 400 is then opened, the
aluminum
fiber spool assembly 416 removed, and the fiber segments 422 removed from the
fiber spool assembly 416.
In certain embodiments, the optical fiber receiving stations 418 are
cartridges, such as those described in co-pending and commonly assigned
application US Serial No. 09/804781, "Filament Organizer", US serial No.
09/841015, "Carrier For Coiled Filaments", or US serial No. 09/907406 "An
Apparatus For Holding And Protecting Several Precision Aligned Optical
Fibers",
which are hereby incorporated by reference. In these embodiments, the entire
cartridge is removed from the fiber spool assembly 416.
3o The second exemplary process is similar to the first, but different at one
point. It again includes installing the fiber segments 422 (or fiber holding
cartridges) onto the fiber spool assembly 416, inserting the spool assembly
416 into
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the vessel body 402, and sealing the vessel 400. The vessel 400 is purged by
several cycles of nitrogen to ensure ambient air has been evacuated. At this
point,
the heater cartridges 426 are activated. When the nitrogen atmosphere around
the
fiber segments 422 has reached the desired temperature, the nitrogen is purged
and
replaced by hydrogen. It must be understood that in this and other example,
the
term hydrogen means H2, D2, or other isotopic molecules of hydrogen and/or one
or more gases, preferably inert gases, with H2 and/or other isotopic hydrogen
species. It is preferable to avoid the use of oxygen to avoid an
oxidization/combustion reaction.
1o The hydrogen is introduced, exemplarily, at high pressures between 1000
psi (6.89 x 106 Pa) and 2000+ psi (1.38 x 107+ Pa). Again, in an alternative
embodiment the hydrogen may be pre-heated. A timer tracks the time the fiber
segments 422 are exposed to the high temperature, high-pressure hydrogen
atmosphere. Due to its low mass in relation to the heater block, the hydrogen
almost immediately reaches the desired loading temperature. When a
predetermined exposure time is reached, the heaters are deactivated. Exposure
time may be calculated using the equations found in United States Patent No.
6,311,524.
The hydrogen gases may be vented, and nitrogen or another inert gas may
2o be forced into the vessel. Again, the purge gas may be cooled or chilled.
As soon
as the hydrogen pressure is released, hydrogen will begin to diffuse out of
the fiber.
The rate of diffusion is a function of temperature. The vessel may then be
opened,
the fiber spool assembly 416 removed, and the fiber segments 422 (or fiber
cartridges) removed from the spool assembly 416.
Figures 10-17 illustrate a fifth high temperature hydrogen-loading vessel
embodiment 500. The vessel 500 uses a unique split vessel design, where the
chamber that retains the high-pressure hydrogen at high temperatures is made
in
two halves.
The open position of vessel 500 is illustrated in Figure 10 and the closed
3o position in Figure 11. The primary vessel halves are the upper vessel block
502
and the lower vessel block 504. In the present embodiment, the blocks 502 and
504 are made from soft 400 series stainless steel and are annealed after
machining.
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Both the blocks 502 and 504 have pockets, upper pocket 506 and lower pocket
508
respectively, in their center areas. When closed, as shown in Figure 1 l, the
pockets
form a loading chamber 510.
In the present embodiment, the bloclcs 502 and 504 are fastened in a
precision lamination grade preloaded ball bearing precision die set 505 to
ensure
precise block aligmnent and parallelism during operation. The die sets 505 are
fastened into a hydraulic press 507, that generates enough compressive force
to
keep the two blocks 502 and 504 sealed when the vessel 500 is pressurized with
hydrogen, and can open the vessel blocks 502 and 504 wide enough to allow easy
1 o insertion and removal of optical fibers 526 between cycles.
Figures 12 and 13 illustrate enlarged cross sectional views of the hydrogen-
loading chamber 510. The enlarged views illustrate the small volume of loading
chamber 510 in greater detail. Heater bloclcs 512 and 514, surrounded by
ceramic
insulation 516 are each placed inside one of the pockets 506 and 508 in vessel
blocks 502 and 504, respectively. The insulation 516 helps to separate the hot
loading zone from the rest of the optical fiber to reduce the possibility of
damage to
the polymer coating of the fiber. Each heater block 512 and 514 includes one
or
more heaters 518, such as electric cartridge heaters. The optical fiber 526
spans the
center portion of the loading chamber 510 and is axially positioned between
the
2o two cartridge heaters 518 in the loading chamber. As illustrated in Figure
13, the
cartridge heaters 518 are in close proximity to the fiber 526 and provide fast
heating of the surrounding gas and the fiber 526. In the present embodiment,
the
cartridge heaters 518 are positioned in each heater block 512 and 514 such
that
approximately sixty degrees of the circumference of the heater cartridge 518
is
exposed to the atmosphere of the loading chamber 510.
The vessel blocks 502 and 504 include a gas inlet/vent port 520 to supply
and purge gases into the loading chamber. The gas inlet/vent port 520 may also
be
used as a wire channel to route control and data connections. Alternatively, a
second set of ports 522 may be used to allow electrical and thermocouple wires
to
3o route to the heaters 518 and one or more thermocouples 528 in the loading
chamber 510. As better seen in Figure 14, the opposing faces that come into
contact of the upper and lower blocks 502 and 504 have a radial groove cut 524
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down the x-axis centerline that is used to position and seal around a fiber
526 to be
sensitized. Two guide pins 523 that match openings in the opposite vessel
block
provide precise final aligmnent of the two blocks 502 and 504 as they come
together to make a seal.
In certain circumstances, such as in very high heat applications, or where
repeated use of the vessel caused heat buildup (e.g., in an in-line
application), the
vessel blocks 502 and 504 may include liquid cooling lines 530. The cooling
lines
530 are positioned along the y-axis near the top edge of the center pockets
506 and
508 and are used to keep the fiber polymer coating cool during loading. The
to cooling lines help to minimize the risk that the polymer coating in non-
loaded
portions of the optical fiber 526 remains below the temperature that would
cause
degradation or oxidation.
Several fiber guide plates 532 are fastened on the left and right sides of the
lower vessel bloclc to provide guidance of the fiber into the radial sealing
grooves.
15 An optional elastomer face seal 534 may be used on the vessel block contact
faces
to reduce the possibility of gas leakage during loading.
Figures 14 and 15 illustrate two alternative ways to seal around the fiber
526 as it enters and exits the loading chamber 510. The fiber 526 has a
coating 527
surrounding a glass center portion 529. In the embodiment illustrated in
Figure 14,
2o referred to as the "steel on steel clamping method", the pair of precision
machined
radial grooves 524 traverse down the x-axis centerline of the blocks 502 and
504.
The radius of the groove 524 is slightly smaller (e.g., several ten thousands
of a
centimeter), than the radius of the coating 527 of the fiber 526. An
interference-fit
between the groove 524 and the fiber 526 causes the coating 527 to compress
25 slightly when the two vessel halves are brought together, creating a tight
seal
between the fiber 526 and the grooved vessel surfaces. The glass portion 529
of
the optical fiber 526 remains undamaged. The outside surface of the coating
527
may be compressed due to the compressive forces applied, but this should not
cause optical performance problems.
3o Figure 15 illustrates a "seal-on-seal clamping method". The method may
be used with coated optical fibers as well as with bare glass optical fibers
having
no polymeric outer coatings. The embodiment includes elastomer seals 534
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installed into the faces of both vessel bloclcs 502 and 504 (the elastomer
seals cross
section are not necessarily drawn to scale). As the vessel faces are brought
together under hydraulic force, the elastomer material compresses around the
optical fiber coating creating a tight seal. The top surface of the elastomer
seals
may be pre-molded to have a groove adapted to fit the optical fibers, similar
to that
shown at 524 in Figure 14, in order to obtain a better seal along the lines
where the
sides of the fiber 526 and the upper and lower elastomer seals 534 meet upon
application of hydraulic force.
Figure 16 is a top plan view of the lower vessel block 504. This view more
to clearly shows the optical fiber path in the center of the vessel blocks x-
axis upper
surface. The fiber guides 532 outboard of the block edges provide coarse
aligmnent of the fiber 526 to the radial grooves 524 that are machined into
the
vessel block 504 upper face. The heater block 514 with surrounding ceramic
insulation 516 is centered in both axes in the center of the vessel block 504.
The
cooling lines 530 are machined in the y-axis very close to the point where the
optical fiber 526 intersects the loading chamber 510. The two guide pins 523
are
located on opposite corners of the block 504 to provide precise final
alignment of
the vessel blocks 502 and 504 before clamping occurs around the optical fiber
526.
The elastomer seal 534 is positioned to minimize or eliminate gas leakage
during
2o the loading process.
Figure 17 shows a cross-sectional elevation view of a third embodiment of
a clamping/sealing mechanism for the vessel illustrated in Figure 10. The
section
of optical fiber 526 that is to be hydrogen loaded is placed in an injection
mold 536
that forms typically a pair of mold cavities 538. These mold cavities 538
match the
cavities in upper vessel block 502 and lower vessel block 504 that are adapted
to
receive the elastomer seals 534, as shown in Figure 16. A curable seal
material
(typically an elastomer) is then injected into the mold cavities and cured
around the
fiber, forming molded seals 540 on fiber 526. The fiber is then removed from
the
injection mold and positioned on lower vessel block 504, with the molded seals
on
3o the fibers fitted into the cavities adapted to receive elastomer seals 534,
as in
Figure 16. Upper vessel block 502 is then brought into contact with lower
vessel
block 504 and pressed to form a seal around fiber 526 and molded seals 540, in
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preparation for hydrogen loading of the fiber. Alternatively, molded seals 540
may
be produced in place, using upper and lower vessel blocks 502, 504 as the
injection
mold and curing the seal material ire situ. Curing may be accomplished during
pre-
heating of the hydrogen loading cavity, before high pressure is applied within
hydrogen loading cavity 510. As shown in Figure 16, the cavity for holding
elastomer seals 534 or 540 may be separated from heater block 514 by hydrogen
loading cavity 510 and ceramic insulation 516, which is cooled by liquid
cooling
lines 530, so the elastomer will not be damaged by the very high temperature
of
hydrogen loading cavity 510 during the loading cycle.
i o Following are different exemplary processes that may be utilized to load
hydrogen into an optical fiber 526 using this vessel 500. The first process
includes
the step of locating the fiber 526 onto the fiber channel/groove 524 on the
face of
the lower vessel block 504. The blocks 502 and 504 are then clamped, such as
by
the use of hydraulic pressure. Several cycles of nitrogen may purge the
loading
chamber 510 to evacuate ambient air.
Hydrogen is then introduced. Again, the vessel 500 is designed to handle
high pressures. Depending on the type of fiber, the concentration of hydrogen
or
deuterium in the inserted loading atmosphere, the desired index change,
pressures
between approximately 500 psi (3.44 x 106 Pa) and approximately 2,200 psi
(1.52
2o x 107 Pa) have been experimentally used. The chamber is designed to
withstand
pressures up to 3,000 psi (2.07 x 107 Pa). Higher pressures are possible
depending
on the design and manufacture of the vessel. The present exemplary method uses
pressures between 1000 psi (6.89 x 106 Pa) and 2000+ psi (1.38 x 107+ Pa).
After
full pressure is reached, the heaters 518 are activated. When the hydrogen
atmosphere reaches the desired temperature around the fiber 526, a timer
tracks the
time the fiber 526 is exposed to the high-temperature hydrogen atmosphere.
After
the desired time exposure, the heaters 518 are deactivated. Hydrogen pressure
could be vented and nitrogen or another suitable inert gas (cooled or
otherwise)
may be forced into the loading chamber 510, the vessel 500 opened, and the
fiber
526 removed. Another option for the fiber removal cycle would be to open the
vessel 500 right after the time had been reached while the vessel was still
pressurized, enacting instant venting of the hydrogen, and cooling of the
fiber,
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which would ensure maximum hydrogen content in the fiber. The volume of the
vessel is so small that this procedure should be safe with reasonable
precautions,
such as minor shielding around the vessel.
The volume of an experimental loading chamber, such as the one
illustrated, was 0.3125 square inches (2.02 cm2) not including the gas feed
holes
drilled in the block, or the piping outside the block. The total gas volume
for an
optimized single fiber loading station could be as low as 0.15 square inches
(0.97
cm2). The final configuration of the vessel will dictate the total gas volume.
The average time it took for the electric cartridge heaters to reach the
1o temperature set point, (275°C), was 45 seconds, +/- 3 seconds. The
time it took for
the hydrogen gas to reach the desired set point, (260°C), depended on
the pressure
used. At pressures between 1400 psi (9.56 x 106 Pa) and 2000 psi (1.38 x 107
Pa),
the time was between 1.5 minutes to 2 minutes. At pressures between 1100 psi
(7.58 x 106 Pa) and 1400 psi (9.56 x 106 Pa), the time was between 3 minutes
to 4
minutes.
Coating delamination did not occur during any of the experimental runs.
Bulk delamination does not occur due to the sectional pressurized zone in the
two-
piece vessel.
In the second exemplary process, generally the same steps are followed,
2o with the exception that the hydrogen is introduced into the preheated
loading
chamber 510. While the small mass of the gas volume of the chamber 510
compared to the mass of the heating blocks will lead to rapid heating, in
alternative
embodiments, the hydrogen may even be preheated to or nearly to the desired
temperature.
If the fiber is written on soon after the fiber is loaded, it will not require
cold storage. Any hydrogen loaded fiber, no matter the method of loading, will
slowly diffuse hydrogen out of the fiber over time at room temperature. The
advantage with sectional loading of fiber, as compared to bulk loading,
depends on
the amounts of fiber that has been loaded. With the speed of high temperature
sectional loading, one may load only the correct amount of fiber that is to be
written in a specific time period. With the long cycle time of bulk loading,
this
becomes more difficult.
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Figures 18-20 illustrate a sixth high temperature hydrogen-loading vessel
embodiment. The vessel uses a unique tubular vessel design, having conformable
collets located at both ends of the tube that seal the end of the tube and
seal around
the fiber that passes through the tube. When the collets have sealed the tube
ends,
and have sealed around the fiber, the tube will retain the high-pressure
hydrogen at
high temperatures to enable hydrogen to diffuse into the fiber that is passing
through the tube.
The open position of the vessel is illustrated in Figure 18 and the closed
position in Figure 19. The primary vessel is a cylindrical stainless steel
tube or
1 o pipe 602 containing a precision angular chamfer 603 at each end, and gas
entry
(620) and gas vent (622) ports near the end of the tube or pipe. The tube or
pipe
has a heating jacket 610 surrounding it, and can be heated electrically, or
with hot
fluids or gasses. The tube or pipe with heater jacket assembly is contained
within a
main base bloclc 636 that is mounted to a base plate 638.
In the present embodiment, an elastomeric collet 660 is mounted to a collet
actuator plate 665 that allows linear motion of the collet plate assembly. The
shape
of the collet is a truncated cone, where the angle of the cone matches the
chamfer
603 in the tube or pipe. The linear motion of the collet actuator plate
assembly
allows the collet 660 to enter the end of the tube or pipe 602, allowing the
angular
2o surface of the collet 660 to seal against the angular surface of the
chamfer 603 in
the tube or pipe 602. It also allows the collet 660 to be withdrawn from the
tube or
pipe 602. Movement of the collet actuator plate 665 is accomplished with
hydraulic cylinders attached to the plate. These are not shown, but can be
attached
by several means, including bolts, pins, etc., in several configurations,
pushing or
pulling. Air cylinders could be substituted. Electric or mechanical actuators
could
also be used.
The elastomeric collet 660 illustrated in end view, and cross section end
view in Figure 20, has a small hole 661 in the center, which is 10% to 20%
larger
than the outside diameter of the coated fiber that is to be loaded. There are
eight
3o rectangular shaped stainless steel ribs 662 that are spaced in a 45-degree
radial
orientation about the center hole. As the collet 660 is forced into the tube
or pipe
602, the angular surface of the collet rib 662 will contact the angular
surface of the
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chamfer 603 on the end of the tube or pipe 602, transmitting this force to the
elastomer material surrounding the fiber 626, creating a seal between the
elastomer
and fiber. When the collet 660 is extracted from the tube or pipe 602, the
elastomer material returns to its stress free state, allowing the hole 661 to
open to
its original size, freeing the fiber 626. The elastomeric collet thus provides
a re-
closable seal around the optical fiber that helps contain the hydrogen
atmosphere
during the loading process. This re-closable seal can be re-opened to remove
the
fiber and, optionally, to advance the fiber, re-seal, then hydrogen load a
second
selected portion of the same fiber. (The sealing mechanisms in Figures 10, 14,
and
l0 15 would also be considered re-closable seals.)
A wind-up (656) reel, (driven by a programmable electric motor/encoder or
servo system), in which rotation of the motor is precisely controlled,
provides
accurate linear lengths of fiber to be transported through the chamber 602 at
the
desired time. There is also an unwind (654) reel, that may be used in
conjunction
with a brake of clutch, (which could be actuated with air, magnets,
electricity,
fluids, etc.), to provide precise tension on the fiber as it is transported
into and out
of the chamber 602. The process cycle would include the following steps:
~ Transport unloaded fiber into the tube or pipe, to the desired spacing.
~ The collet actuator plates position the collets into the ends of the tube or
2o pipe, sealing the vessel, and creating a seal around the fibers.
~ The vessel is purged with nitrogen or other suitable gases.
~ The vessel is pressurized with hydrogen (the heaters are already hot).
~ The fiber is kept at pressure the desired amount time.
~ The hydrogen is vented, and the vessel is purged with nitrogen.
~ The collet actuator plates are retracted.
~ The fiber is advanced.
~ Optionally, the newly-loaded section of optical fiber may have a Bragg
grating written into it, and the grating may be optionally annealed.
~ If the coating has been stripped off the loaded section of fiber by the
high-temperature loading process, the stripped section may optionally
be recoated before it reaches the wind-up reel.
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Figure 21 shows a sequential step illustration of methods for increasing the
photosensitivity of an optical fiber in accordance with the present invention,
and
for writing one or more gratings in an optical fiber. This figure corresponds
to
Figure 1 in co-assigned United States Patent No. 6,272,886 B1, "Incremental
Method Of Producing Multiple UV-induced Gratings On A Single Optical Fiber",
which is hereby incorporated by reference. An embodiment of the present
invention that has a re-closable seal, such as those shown in Figures 10 or
18, can
be substituted for coating removal station 20 in the fiber grating
manufacturing
apparatus and process shown in United States Patent No. 6,272,886 B 1. This
l0 hydrogen loading and (optionally) coating removal station 720 receives
fiber 712
from tension-controlled payoff spool 714 and alignment pulleys 716. The fiber
is
stopped when a selected portion of optical fiber 712 is positioned in hydrogen
loading (and optional coating removal) station 720. The hydrogen loading
process
is then carried out as described above. The hydrogen loaded selected portion
722
of fiber 712 is then advanced by drive capstan 718 to the grating writing
station
724, where it is clamped between clamps 726 and 728 during the writing
process.
After a grating is written in selected portion 722, this portion now
containing the
grating is advanced to an optional annealing unit 730, where the grating is
heated
to stabilize its reflectivity. If coating has been removed from the selected
portion,
2o the portion may be advanced to an optional recoat material application or
packaging station 740, then to an optional recoat curing station 750. The
selected
portion of fiber 720 containing the grating may then be advanced by means of
optional drive capstan 718 and alignment pulleys 760 to a take-up spool 762,
for
easier handling and shipping. As one selected portion of fiber 722 is
advancing
through the system, a following second selected portion of the fiber may also
be
advanced stepwise through the system in an assembly line fashion. Optional
slaclc
accumulation stations may be placed between the various processing stations if
the
spacing between gratings along the single fiber must be varied.
The present invention offers significant advantages. Selective loading
allows for only the portion of the fiber that requires gaseous loading to be
exposed.
Hydrogen loading conventional polymer coated fibers at high temperatures
greater
than 250°C may cause combustion or partial destructive depolymerization
of
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traditional coatings. If the fiber is hydrogen loaded with the traditional
bulk
loading method, the entire fiber will need to be recoated. With sectional
loading,
only the loaded portion, which is the same portion that the grating will be
written
on, will need to be recoated.
There is no need for a hydrogen balce out process step with sectional
loading, as with bulk-loaded fiber. Being that the length of the fiber where
the
grating was written, and the sectional loaded area are approximately the same
length, the aruiealing process step removes hydrogen from the loaded area.
A fiber that contains hydrogen cannot be fusion spliced. The hydrogen
to causes deformation in the glass when exposed to the electric arc, making it
impossible to achieve a useable splice. With sectional loading, a fusion
splice may
be made anywhere outside of the grating area prior to removing hydrogen from
the
fiber. This may be useful when monitoring the grating during writing, or
during a
final paclcaging step. The ability to fusion splice at any point during the
manufacturing process increases the flexibility of the processes, and
products)
being made.
Sectional loading of fiber further allows the manufacturer to tailor the
photosensitivity of each fiber loaded by precisely controlling the temperature
and
pressure of the hydrogen, and the time the fiber is exposed to that
atmosphere.
2o When the vessel is opened, the fiber cools rapidly (in less than 5
seconds), allowing
the fiber to retain the full content of hydrogen. If the fiber is used
immediately, the
exact degree of photosensitization may be known. This precise control allows
the
manufacturer to vary the hydrogen content in each fiber by varying either the
temperature, pressure, and/or time of exposure. Fiber photosensitivity can
then be
used to tailor the laser write times of gratings. It may also be used to
equalize the
photosensitivity of disparate fibers so the same writing conditions can be
conveniently used to a variety of fibers.
A fiber loaded under high temperature conditions yields less of a change in
the index of refraction of the fiber as compared to a fiber loaded at lower
3o temperature conditions. When a fiber Bragg grating is annealed, the
hydrogen in
the fiber is diffused out, changing and stabilizing the index of refraction of
the
fiber. This process often results in a 50% decrease in the UV-induced index
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change with fibers loaded at lower temperatures. The high temperature loading
approach allows has resulted in index changes as low as 15%. A reduced index
change at the anneal process has the advantage of reducing the time, and/or
laser
power required to write a grating.
Those skilled in the art will appreciate that optical waveguides in
accordance with the present invention may be used in a variety of optical
applications in addition to writing of gratings. 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 scope of
the
to 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 scope of the present invention.
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