U.S. patent application number 15/789214 was filed with the patent office on 2018-02-15 for fiber bragg gratings in carbon-coated optical fibers and techniques for making same.
This patent application is currently assigned to OFS Fitel, LLC. The applicant listed for this patent is Adam Hokansson, David J. Kudelko, Yaowen Li, Joanna Y. Ng, Debra A. Simoff. Invention is credited to Adam Hokansson, David J. Kudelko, Yaowen Li, Joanna Y. Ng, Debra A. Simoff.
Application Number | 20180044234 15/789214 |
Document ID | / |
Family ID | 54209614 |
Filed Date | 2018-02-15 |
United States Patent
Application |
20180044234 |
Kind Code |
A1 |
Hokansson; Adam ; et
al. |
February 15, 2018 |
Fiber Bragg Gratings in Carbon-Coated Optical Fibers and Techniques
for Making Same
Abstract
A technique is described for fabricating one or more optical
devices in a carbon-coated optical fiber. A photosensitive optical
fiber is provided having a hermetic carbon coating. Further
provided is a laser having a beam output that is configured to
inscribe one or more refractive index modulations into the optical
fiber through the hermetic carbon layer while leaving the hermetic
carbon layer intact. The laser is used to inscribe one or more
optical devices into the optical fiber through the hermetic carbon
layer.
Inventors: |
Hokansson; Adam; (Granby,
CT) ; Kudelko; David J.; (Rockaway, NJ) ; Li;
Yaowen; (Princeton, NJ) ; Ng; Joanna Y.;
(Warren, NJ) ; Simoff; Debra A.; (Simsbury,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hokansson; Adam
Kudelko; David J.
Li; Yaowen
Ng; Joanna Y.
Simoff; Debra A. |
Granby
Rockaway
Princeton
Warren
Simsbury |
CT
NJ
NJ
NJ
CT |
US
US
US
US
US |
|
|
Assignee: |
OFS Fitel, LLC
Norcross
GA
|
Family ID: |
54209614 |
Appl. No.: |
15/789214 |
Filed: |
October 20, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15144563 |
May 2, 2016 |
9822035 |
|
|
15789214 |
|
|
|
|
14169541 |
Jan 31, 2014 |
9353001 |
|
|
15144563 |
|
|
|
|
61784347 |
Mar 14, 2013 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 26/50 20151001;
C03B 2205/20 20130101; B23K 26/359 20151001; G02B 6/02395 20130101;
G02B 2006/02161 20130101; C03C 25/1062 20180101; B23K 26/364
20151001; C03C 25/106 20130101; C03C 2218/152 20130101; C03B
2203/22 20130101; C03B 37/027 20130101; C03C 25/6208 20180101; G02B
6/02114 20130101; C03B 2201/31 20130101; C03B 2201/22 20130101;
G02B 6/02138 20130101; C03B 2201/21 20130101 |
International
Class: |
C03C 25/10 20060101
C03C025/10; C03B 37/027 20060101 C03B037/027 |
Claims
1. A method for fabricating one or more optical devices in a
carbon-coated optical fiber, comprising: providing a photosensitive
optical fiber having a hermetic carbon coating and a secondary
outer coating over the hermetic carbon coating, wherein each of the
hermetic carbon coating and the secondary outer coating has a
respective absorption of light at a given wavelength that allows a
laser operating at the given wavelength to inscribe one or more
optical devices into the fiber through the secondary outer coating
and the hermetic carbon coating, while leaving them intact;
providing a laser having a beam output that is configured to
inscribe one or more refractive index modulations into the optical
fiber through the hermetic carbon layer and the secondary outer
coating while leaving the hermetic carbon layer and the secondary
outer coating intact; and using the laser to inscribe one or more
optical devices into the optical fiber through the hermetic carbon
layer and the secondary outer coating.
2. The method of claim 1, wherein the one or more optical devices
comprise one or more fiber gratings.
3. The method of claim 2, wherein a phase mask technique is used to
inscribe the one or more gratings into the fiber.
4. The method of claim 2, wherein a holographic technique is used
to inscribe the one or more gratings into the fiber.
5. The method of claim 1, wherein the one or more optical devices
are written into the fiber during draw, subsequent to the
application of the carbon coating and secondary outer coating onto
the fiber.
6. The method of claim 1, wherein the carbon coating and secondary
outer coating are applied to the fiber during draw, and wherein the
one or more optical devices are inscribed into the fiber during a
post-draw process.
7. The method of claim 1, wherein the secondary outer coating
comprises a polymer.
8. The method of claim 1, wherein the laser is an ultraviolet
laser, and wherein each of the hermetic carbon coating and the
secondary outer coating has a respective ultraviolet absorption
that is sufficiently low so as to allow the ultraviolet laser to
inscribe one or more optical devices into the fiber through the
secondary outer coating and the hermetic carbon coating.
9. The method of claim 1, wherein the laser is a femtosecond laser,
and wherein the secondary outer coating has an infrared absorption
that is sufficiently low so as to allow the femtosecond laser to
inscribe one or more optical devices into the fiber through the
secondary outer coating and the carbon coating.
10. The method of claim 1, wherein the carbon layer has a thickness
ranging from 0.02 to 0.08 .mu.m.
11. A method for fabricating a photosensitive, carbon-coated
optical fiber, comprising: doping a fiber to have a high degree of
photosensitivity; applying a hermetic carbon coating to the fiber
cladding; and applying a secondary outer coating over the hermetic
carbon coating, wherein the secondary outer coating has an
absorption at a given wavelength of light that is sufficiently low
so as to allow the one or more optical devices to be inscribed into
the fiber, in a post-secondary-coating process, through the
secondary outer coating and the carbon coating, using a laser
operating at the given wavelength.
12. The method of claim 11, wherein the fiber core or fiber core
and part of fiber cladding are doped with one or more rare-earth
dopants.
13. The method of claim 12, wherein the one or more photosensitive
dopants comprises germanium.
14. The method of claim 13, wherein hydrogen or deuterium loading
is used to increase the photosensitivity of the fiber.
15. The method of claim 14, wherein hydrogen or deuterium is loaded
into the fiber subsequent to the application of the carbon coating
by heating the fiber to a temperature at which the hydrogen or
deuterium to be loaded passes through the carbon coating.
16. A method for fabricating a photosensitive, carbon-coated
optical fiber, comprising: doping a fiber to have a high degree of
photosensitivity; applying a hermetic carbon coating to the fiber
cladding, wherein the carbon layer is hermetic to both water and
hydrogen, and wherein the carbon layer has a thickness that is
sufficiently transparent to light at a given wavelength so as to
allow an optical device to be written into the fiber through the
carbon layer; and using a laser operating at the given wavelength
to write an optical device into the fiber.
17. A method for fabricating one or more optical devices in a
carbon-coated optical fiber, comprising: providing a photosensitive
optical fiber having a hermetic carbon coating and a secondary
outer coating over the hermetic carbon coating; providing a laser
having a beam output that is configured to inscribe one or more
refractive index modulations into the optical fiber through the
hermetic carbon layer while leaving the hermetic carbon layer
intact; in a selected region of the fiber, stripping off the
secondary outer coating, while leaving the carbon coating intact;
and using the laser to inscribe one or more optical devices into
the optical fiber through the hermetic carbon layer.
18. The method of claim 17, further including: after using the
laser to inscribe one or more optical devices into the optical
fiber, applying a protective coating over the selected region of
the fiber.
19. A system for mass producing carbon-coated optical fiber
gratings, comprising: a draw tower; a furnace assembly at the top
of the draw tower for receiving an optical fiber preform, and for
heating the preform so as to allow an optical fiber to be drawn
therefrom; a carbon reactor assembly located in the draw tower
below the furnace assembly for receiving the drawn fiber and for
applying thereto a hermetic carbon layer; a grating inscription
assembly located in the draw tower below the carbon reactor
assembly for receiving the carbon-coated optical fiber and for
using a laser to inscribe gratings into the carbon-coated optical
fiber through the hermetic carbon coating, while leaving the
fiber's carbon coating intact; and an outer coating applicator
assembly for applying an outer coating over the carbon-coated
optical fiber and the gratings inscribed therein.
20. A system for mass producing carbon-coated optical fiber
gratings, comprising: a draw tower; a furnace assembly at the top
of the draw tower for receiving an optical fiber preform, and for
heating the preform so as to allow an optical fiber to be drawn
therefrom; a carbon reactor assembly located in the draw tower
below the furnace assembly for receiving the drawn fiber and for
applying thereto a hermetic carbon layer; an outer coating
applicator assembly for applying a secondary outer coating over the
carbon-coated optical fiber, wherein the secondary outer coating
has an absorption at a given wavelength that is sufficiently low so
as to allow one or more optical devices to be written into the
optical fiber through the carbon layer and secondary outer coating;
and a grating inscription assembly located in the draw tower below
the outer coating application assembly for receiving the optical
fiber and for using a laser to inscribe gratings into the optical
fiber through the carbon coating and second outer coating, while
leaving the fiber's carbon coating intact.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of co-pending U.S.
patent application Ser. No. 15/144,563, filed on May 2, 2016.
[0002] U.S. patent application Ser. No. 15/144,563 is a divisional
of U.S. patent application Ser. No. 14/169,541, filed on Jan. 31,
2014, which issued as U.S. Pat. No. 9,353,001 on May 31, 2016.
[0003] U.S. patent application Ser. No. 14/169,541claims the
priority benefit of U.S. Provisional Patent Application Serial No.
61/784,347, filed on Mar. 14, 2013, now expired.
[0004] All of the above applications are owned by the assignee of
the present application, and are incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0005] The present invention relates generally to the field of
fiber optics, and in particular to fiber gratings and like optical
devices for use in harsh environments.
Background Art
[0006] The demand for optical fibers and fiber-based devices has
dramatically increased over the past decade. However, in humid or
hydrogen-rich environments, fiber-based devices, such as sensors or
other devices employing fiber Bragg gratings (FBGs), typically
require shielding in order to address mechanical robustness and
reliability issues, as well as issues relating to optical
functionality. The reliability and optical performance of these
devices are of primary importance to designers of optical
components and systems.
[0007] There is thus an ongoing need in the art for FBGs and other
fiber-based devices that are capable of reliable performance in
harsh environments.
SUMMARY OF INVENTION
[0008] An aspect of the invention is directed to a method for
fabricating gratings or like optical devices in a carbon-coated
optical fiber. A photosensitive optical fiber is provided having a
hermetic carbon coating. Further provided is a laser having a beam
output that is configured to inscribe one or more refractive index
modulations into the optical fiber through the hermetic carbon
layer while leaving the hermetic carbon layer intact. The laser is
used to inscribe one or more optical devices into the optical fiber
through the hermetic carbon layer.
[0009] Further aspects of the invention are directed to structures
and techniques for mass production of carbon-coated optical
devices. In one practice of the invention, optical devices are
written into a carbon-coated optical fiber in a
post-secondary-coating process. In another practice of the
invention, optical devices are written into a fiber on the draw
tower during the optical fiber drawing process, subsequent to the
application of a carbon coating onto the fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1 and 2 show diagrams, respectively, of a test optical
fiber and a grating inscription setup used in an experimental
demonstration of the invention.
[0011] FIGS. 3-6 shows a series of reflection spectra illustrating
the results of tests conducted using the fiber and the grating
inscription setup shown in FIGS. 1 and 2.
[0012] FIG. 7A shows an isometric view of a carbon-coated optical
fiber configured for use in a post-secondary-coating inscription
technique, in accordance with an aspect of the invention.
[0013] FIG. 7B shows a cross section of the carbon-coated optical
fiber shown in FIG. 7A through the plane 7B-7B.
[0014] FIG. 8 shows a schematic diagram of an automated grating
writing system for use with the fiber shown in FIGS. 7A and 7B.
[0015] FIG. 9 is a schematic diagram of an automated system
according to an aspect of the invention for inscribing gratings
into a carbon-coated optical fiber as part of the draw process.
[0016] FIG. 10 shows a flowchart of a general technique according
to the invention.
DETAILED DESCRIPTION
[0017] Aspects of the invention are directed to optical devices,
such as fiber Bragg gratings (FBGs) and the like, that are
fabricated in an optical fiber having a hermetic carbon coating.
The structures and techniques described herein can be used, for
example, to fabricate FBGs for use as sensors in a harsh
environment. In addition, the described structures and techniques
can be used to fabricate FBGs displaying improved fatigue
resistance.
[0018] As used herein, the term "harsh environment" generally
refers to an environment in which a hermetic carbon coating is
useful in protecting an optical device. Such an environment
includes, for example: a high-humidity environment, a hydrogen-rich
environment; an environment that is both high-humidity and
hydrogen-rich; a high-water-content or aqueous environment, such as
liquid water; or the like. Generally speaking, hermeticity to water
improves mechanical reliability in a harsh environment, and
hermeticity to hydrogen improves optical reliability.
[0019] As used herein, the terms "hermetic carbon coating" and
"carbon coating" refer to a coating applied to a fiber surface so
as to hermetically seal the portion of the fiber contained within
the carbon coating. The carbon coating serves a number of purposes,
including, for example: providing the glass fiber surface with a
seal against water molecules, including condensed water, water
vapor at a wide range of temperatures, and the like; protecting
against ingress of hydrogen or deuterium to .about.130.degree. C.
or higher, depending on the coating structure; and extending the
expected life of a given fiber in a given environment by retarding
fatigue and by reducing stress corrosion failures.
[0020] In one carbon-coating technique, a carbon layer is applied
onto the outer surface of an optical fiber in a draw tower as part
of the drawing process. The draw tower is equipped with a carbon
reactor, through which optical fiber travels after it has been
drawn from a preform. Within the carbon reactor, a chemical vapor
deposition (CVD) technique is used to deposit a hermetic layer of
amorphous carbon at a temperature of 1200.degree. C. onto the outer
surface of the optical fiber. A typical thickness for the carbon
coating is .about.0.02 .mu.m to .about.0.08 .mu.m.
[0021] The CVD technique is implemented by causing reactant gases
to flow through the carbon reactor in the absence of oxygen. A
pyrolytic reaction occurs at the glass surface of the drawn fiber,
resulting in thermal deposition of the carbon onto the fiber
surface. A minimum thickness of deposited carbon is required in
order for the carbon coating to be hermetic. Typically, a polymer
coating is then applied over the carbon layer.
[0022] A carbon-coating technique is described in greater detail,
for example, in Lindholm et al., "Low Speed Carbon Deposition
Process for Hermetic Optical Fibers," International Wire and Cable
Symposium (1999).
[0023] For a number of reasons, carbon coatings have not been used
in connection with FBGs and like optical devices. First, gratings
may not be inscribed into a fiber prior to the application of a
carbon coating because the high temperatures required to apply the
carbon coating may destroy the gratings. Second, previously, it has
generally been believed that gratings cannot be inscribed into a
fiber subsequent to the application of a carbon coating, because of
the carbon coating's lack of transparency with respect to a UV
laser beam.
[0024] The present invention is based on the insight that, if a
fiber has a sufficiently high degree of photosensitivity, and if
the inscribing laser is configured to operate within a suitable
power range, it is possible to apply a hermetic carbon coating onto
the fiber's cladding and then subsequently inscribe gratings into
the fiber through the carbon coating without breaching the hermetic
seal.
[0025] Generally speaking, the inscription of gratings into
carbon-coated optical fibers can employ any of the configurations
and techniques that are used for inscribing gratings into optical
fibers without carbon coatings. Thus, gratings can be inscribed
into a carbon-coated optical fiber using a holographic technique, a
phase-mask technique, or the like. Depending upon a particular
situation, some adjustments or modifications may be necessary.
[0026] As discussed below, the inscription of gratings into a
carbon-coated optical fiber can be performed as part of the initial
draw process or in a separate, post-secondary-coating process.
Photosensitivity
[0027] As noted above, in order for gratings to be written into a
fiber through a carbon coating, the fiber must have a suitable
degree of photosensitivity. The degree of photosensitivity required
in a given situation depends upon a number of factors, including
(1) the strength of the gratings to be inscribed and (2) the
interaction between the inscribing laser beam and the carbon
coating through which the gratings are to be written.
[0028] According to an aspect of the invention, a suitable degree
of photosensitivity in a given fiber is achieved through the use of
one or more dopants that are known to create photosensitivity.
These dopants include, for example, germanium, as well as fluorine,
boron, or the like. Depending upon a particular application, it may
be desirable to employ one or more co-dopants in one or more fiber
regions in order to arrive at a fiber design having a desired
degree of photosensitivity as well as a particular refractive index
profile or other desired property.
[0029] A suitable fiber for a given application of the present
invention may be provided in any of a number of different ways. For
example, in certain situations it may be possible to employ an
already existing fiber design, if the fiber has a suitable degree
of photosensitivity. Alternatively, an already existing fiber
design may be modified by making suitable adjustments to its
photosensitivity. In some situations, it may be necessary or
desirable to custom design a suitable optical fiber. The design of
such an optical fiber will be understood adequately by a
practitioner in the art to include, e.g.: core, trench, clad
dimensions and refractive index, as well as the possibility of
incorporating other structures, such as multiple cores,
polarization-maintaining stress rods, a star-shaped or octagonal
cladding, a rectangular core, one or more coatings of various
types, and the like.
[0030] According to a further aspect of the invention, hydrogen
loading or deuterium loading may be used to enhance the
photosensitivity of the carbon-coated fiber, and thus the
reflectivity (i.e., strength) of gratings written into the fiber.
As mentioned above, a carbon coating protects against hydrogen
ingression to .about.130.degree. C. or higher, depending on the
coating structure. Thus, it is possible to load a carbon-coated
optical fiber with hydrogen or deuterium, prior to inscription, by
placing the fiber in a pressurized chamber containing hydrogen or
deuterium at a suitable concentration and maintaining a fiber
temperature above .about.130.degree. C. for a suitable amount of
time. The parameters for such a conditioning schedule (e.g., time,
temperature, and pressure) are dependent upon the required
photosensitivity of the fiber, the hermeticity of the carbon
coating, and the capability of the secondary coating to survive
this exposure.
[0031] Gratings are subsequently inscribed into the fiber in
accordance with the techniques described herein. According to a
further aspect of the invention, once the gratings have been
inscribed into the fiber, they are then annealed at a temperature
of greater than .about.130.degree. C. to allow the excess loaded
hydrogen or deuterium to escape from the fiber.
Experimental Confirmation
[0032] Aspects of the invention have been confirmed in a series of
experiments, in which gratings were successfully inscribed into a
carbon-coated test fiber. The successful outcome of these
experiments has led to the development of a number of manufacturing
techniques, discussed below, that can be used for mass production
of carbon-coated gratings.
[0033] FIGS. 1 and 2 show diagrams, not drawn to scale, of the test
fiber 10 and an inscription station 20. The test fiber 10 was a
single-core fiber comprising a core region 12, a cladding region of
80 .mu.m in diameter surrounding the core 14, a hermetic carbon
layer 16, and a protective polyimide coating 18 surrounding the
carbon layer 16. The fiber core region 12 was doped with germanium
at a concentration resulting in a relatively high degree of
photosensitivity and a numerical aperture of 0.21. The carbon
coating 14 was applied during the draw process and had a thickness
of approximately 0.029 .mu.m. The polyimide coating 18 was also
applied during the draw process, subsequent to the application of
the carbon layer, and had a thickness of approximately 15
.mu.m.
[0034] It is noted that, depending upon a particular application, a
carbon coating having a thickness as high as 0.08 .mu.m, or
thicker, may be considered.
[0035] The fiber 10 was loaded into the inscription station 20,
which included means (not shown) for holding and positioning the
fiber relative to the output of laser 22 and beam delivery optics
24. Prior to inscription, the polyimide coating 18 was stripped
from the fiber, leaving the carbon coating 16 intact.
[0036] In the FIG. 2 inscription station 20, the laser 22 is
implemented using an excimer UV laser manufactured by TuiLaser,
which provided a laser beam having a wavelength of 248 nm. A phase
mask 26 was used to generate a periodic interference pattern from
the UV laser beam, which was used to inscribe a grating 28 into
fiber core 12, or fiber core 12 and part of cladding 14, depending
on the fiber design. It will be appreciated that the present
invention can be practiced with other suitable types of lasers,
depending upon a given application.
[0037] It is noted that there is a relatively wide range of
wavelengths at which a carbon coating is sufficiently transparent
to allow gratings to be written therethrough. These wavelengths
include 193 nm, 244 nm, and 248 nm. Longer wavelengths may be
employed, such as when using a "cold-writing" technique, a
femtosecond laser, or other approaches. In the tests, a number of
gratings were inscribed into the carbon-coated test fiber using
different inscription parameters. The performance of the inscribed
gratings was measured using a setup having a broadband source, a
3-port circulator and an optical spectrum analyzer (OSA). FIGS. 3-5
show a series of reflection spectra illustrating the test results.
Among other things, the reflection spectra confirm that it is
possible to inscribe gratings into a carbon-coated optical fiber.
In addition, the reflection spectra confirm that, generally
speaking, stronger FBGs can be written by longer exposure to the
inscribing laser light and/or increasing the concentration of
germanium (or other suitable dopant or combination of dopants) in
the fiber into which the grating is written.
[0038] FIG. 3 shows a reflection spectrum 30 generated by an 8 mm
grating inscribed into a length of the test fiber, as described
above. The TuiLaser excimer laser was used to provide a 40 Hz
pulsed laser beam having a wavelength of 248 nm. The laser energy
density in the inscription region was .about.25 mJ/cm.sup.2. The
exposure time was 60 seconds, after which time the grating did not
change significantly. The laser energy density was chosen because
at .about.25 mJ/cm.sup.2 the carbon coating showed no visual damage
when inspected under a microscope. Experiments have shown that a
power density level of 80 mJ/cm.sup.2 will burn or damage the
carbon coating. The exact upper limits for the inscription
parameters have not yet been determined, but are expected to be
between 25 mJ/cm.sup.2 and 80 mJ/cm.sup.2 for the 248 nm TuiLaser
excimer laser. For other wavelengths or types of lasers, the laser
energy density ranges may be different.
[0039] As shown in FIG. 3, the 8 mm grating reflection spectrum
reaches a peak of .about.-11 dB, which corresponds to a peak
reflectivity of .about.8%.
[0040] FIG. 4 shows the reflection spectrum 40 of an FBG written
into a carbon-coated optical fiber by a single laser pulse (i.e., a
"one-pulse" inscription process). The setup used was exactly the
same as the one used to inscribe the FBG in FIG. 3. The one-pulse
FBG achieved a peak reflectivity of .about.0.29% (.about.-25.5 dB).
This result is significant because it implies that the
carbon-coated fiber gratings could be fabricated by using a draw
tower grating writing technique.
[0041] The grating illustrated in FIG. 3 is the stronger of the
two, and is suitable for use, for example, in a wavelength division
multiplexing (WDM) based interrogator. The weaker one-pulse grating
illustrated in FIG. 4 is suitable for use, for example, in a weak
grating interrogator based on time division multiplexing/wavelength
division multiplexing (TDM/WDM) techniques. In a TDM/WDM system,
the reflectivity of individual FBGs can be as low as -35 dB (i.e.,
a reflectivity of .about.0.03%).
[0042] FIG. 5 shows the reflection spectra 50 of a series of four 8
mm gratings written into the carbon-coated test fiber using the
same testing setup, but with a different number of laser pulses
(one pulse 51, two pulses 52, three pulses 53, and five pulses
54).
[0043] A further experiment was conducted to determine whether the
inscription of gratings in the test fiber resulted in damage to the
carbon coating not visually observed under a microscope.
[0044] Six FBGs similar to the one shown in FIG. 3 were inscribed
into the test fiber. Their wavelengths were measured at room
temperature after annealing for 64 hours at 85.degree. C. The six
FBGs were then placed into a deuterium exposure chamber having a
pressure of 4,400 psi pressure and a temperature of 50.degree. C.
After 20 days, the FBGs were removed from the chamber and their
wavelengths were measured again at room temperature. Deuterium gas
was chosen because the setup for creating the above deuterium
environment was readily available. It is expected that the same
results should be obtained by replacing deuterium gas with hydrogen
gas.
[0045] Compared to the wavelengths before the loading, the
wavelength shifts from these FBGs ranged from -12 pm to 0 pm with
an average wavelength shift of -7.2 pm. Given a variation in room
temperature of .about.0.5.degree. C. and the relatively small
wavelength shifts, it can be concluded that deuterium did not enter
into the fiber core with those loading conditions. It can also be
concluded that the grating writing did not damage the carbon
coating.
[0046] FIG. 6 shows a reflection spectrum 60 for a grating with a
length of 10 mm that was written into another carbon-coated fiber,
in which the carbon coating thickness was .about.0.056 .mu.m, the
cladding diameter was 125 .mu.m, and the numerical aperture was
0.16. The laser conditions were the same as those in the tests of
the gratings inscribed into carbon-coated optical fibers, in which
the carbon coating had a thickness of .about.0.029 .mu.m. In
reflection spectrum 60, the 0.6% peak reflection 61 is referenced
to the cleaved fiber end reflection of 4%. Compared to the results
obtained from the 0.029 .mu.m carbon coated fiber, this lower
reflectivity is believed to be mainly due to both the less
photosensitive fiber and the thicker carbon coating. To verify
this, gratings were also written in a fiber with the same glass
structure as that of the 0.056 .mu.m thick carbon coated fiber, but
a 0.024 .mu.m thick carbon coating. The results showed that 1.7%
peak reflectivity was obtained for a 10 mm grating. It is believed
that with higher germanium doping in the fiber core and extra
photosensitization of the fiber, much higher grating peak
reflectivity can be obtained even in the thick carbon coated
fiber.
Mass Production of Carbon-Coated Optical Fiber Gratings
[0047] The above encouraging results have given rise to a number of
possible scenarios for mass-producing FBGs in carbon-coated optical
fibers. In one approach, FBGs are inscribed into a carbon-coated
optical fiber in a post-secondary-coating process. In a second
approach, FBGs are inscribed into a fiber as part of the draw
process, after the application of a carbon coating. Each of these
approaches is described in turn.
Post-Secondary-Coating Inscription
[0048] As used herein, the term "post-secondary-coating
inscription" refers to an inscription process that is performed
subsequent to the application of one or more secondary coatings
over a carbon coating layer. A "post-secondary-coating inscription"
technique can be performed in a draw tower as part of an optical
fiber drawing process, or can be performed in an inscription
station after a fiber has been removed from a draw tower.
[0049] FIG. 7A shows an isometric view of a carbon-coated optical
fiber 70 that is configured for use in a post-secondary-coating
inscription technique, in accordance with an aspect of the
invention. FIG. 7B shows a cross section of the carbon-coated
optical fiber shown in FIG. 7A through the plane 7B-7B.
[0050] Fiber 70 comprises a core region 72, a cladding region 74,
and a hermetic carbon layer 76. Surrounding the hermetic carbon
layer is a protective "write-through" coating 78 that is
substantially transparent to ultraviolet light. Such a coating is
described in U.S. Pat. No. 5,620,495, which is owned by the
assignee of the present application, which is incorporated herein
by reference in its entirety. As disclosed therein, the outer
coating comprises a polymer with low UV absorption. Such a coating
may comprise, for example, a polymer selected from the group
consisting of acrylates, aliphatic polyacrylates, silesesquioxanes,
alkyl-substituted silicones and vinyl ethers. As used herein, the
terms "acrylates" and "aliphatic polyacrylates" include
methacrylate, poly(meth)acrylate, urethane acrylate, and the
like.
[0051] FIG. 8 shows a schematic diagram of an automated FBG writing
system 80 for use with fiber 70. System 80 comprises a programmable
controller unit 81, an automated fiber winding system 82, an
automated fiber clamping system 83, and a UV laser 84. The beam
delivery optics 85 can be based on phase mask or holographic
techniques. Fiber 70 is loaded into the automated fiber winding
system 82, and gratings are written into the fiber 70 in a series
of inscription cycles. In each cycle, the clamping system 83 clamps
a portion of fiber 70 in position relative to the output of UV
laser 84, as shaped by the beam delivery optics 85.
[0052] An FBG array fabricated using fiber 70 and grating
inscription system 80 maintains pristine fiber strength. Further,
as discussed above, since the fiber is carbon-coated, it can be
configured for use in humid, hydrogen-rich, or other harsh
environments, such as those described above. The
"post-secondary-coating" approach allows for greater degrees of
manipulation to achieve strict optical spectrum requirements not
normally achievable with draw-tower grating writing.
Pre-Secondary-Coating Inscription
[0053] As used herein, the term "pre-secondary-coating inscription"
refers to an inscription process that is performed subsequent to
the application of a carbon coating layer, but prior to the
application of a secondary coating over the carbon coating
layer.
[0054] In an exemplary practice of a pre-secondary-coating
inscription technique according to an aspect of the invention,
gratings are automatically inscribed into a carbon-coated optical
fiber in a draw tower as part of the draw process. FIG. 9 is a
schematic diagram of an automated system 90 according to this
aspect of the invention.
[0055] System 90 comprises a draw tower 91 having a furnace 92 at
its top that is configured to receive a fiber preform from which a
fiber 93 is drawn. The drawn fiber descends through the following
stages: a carbon coating applicator 94, in which a hermetic carbon
coating is applied to the fiber 93; a grating inscription stage 95,
in which gratings are inscribed into the fiber; a secondary coating
die 96, in which an outer coating is applied over the carbon
coating; and a fiber winding system 97 for collecting the finished
fiber and winding it onto a bulk reel.
[0056] Grating inscription stage 95 comprises a UV laser 951 and
laser beam delivery optics 952 that are configured to deliver an
inscription laser beam 953 to a mask 954 and its mounting fixtures.
Of course, other grating inscription techniques, such as a
holographic technique, can also be implemented here. The amount of
time required to inscribe each grating is sufficiently short, such
that it is possible to inscribe gratings into fiber 93 without
having to stop the fiber as it descends through the draw tower
91.
[0057] Because the outer coating is applied subsequent to the
inscription of gratings into fiber 93, it is not necessary for the
outer coating to be a write-through coating. Thus, the described
approach provides flexibility with respect to the outer coatings
that can be applied by the secondary coating die 96. Any of a
number of different outer coatings may be used, including, for
example, acrylate, polyimide, vinyl ether, silicone,
silsesquioxane, epoxy, and even metal.
General Technique
[0058] FIG. 10 is a flowchart illustrating a general technique 100
according to the above-described aspects of the invention.
[0059] Technique 100 comprises the following steps:
[0060] 101: Provide a photosensitive optical fiber having a
cladding onto which has been applied a hermetic carbon coating.
[0061] 102: Provide a laser having a beam output that is configured
to inscribe one or more refractive index modulations into the
optical fiber through the hermetic carbon layer while leaving the
hermetic carbon layer intact.
[0062] 103: Use the laser to inscribe one or more optical devices
into the optical fiber through the hermetic carbon layer.
Conclusion
[0063] Fiber Bragg gratings fabricated in carbon-coated fibers in
accordance with the present invention have a number of advantages,
including: (1) resisting hydrogen ingression below a certain
temperature; (2) increasing long-term reliability when the fiber is
under stress and in a humid environment.
[0064] In both post-secondary-coating and pre-secondary-coating
inscription of FBGs in carbon-coated optical fibers, another
advantage is that the FBG-inscribed carbon-coated optical fiber
will have almost the same breaking strength as that of the pristine
carbon-coated optical fiber (.about.550 kpsi), i.e., prior to
inscription of the FBG therein.
[0065] While the foregoing description includes details that will
enable those skilled in the art to practice the invention, it
should be recognized that the description is illustrative in nature
and that many modifications and variations thereof will be apparent
to those skilled in the art having the benefit of these teachings.
It is accordingly intended that the invention herein be defined
solely by the claims appended hereto and that the claims be
interpreted as broadly as permitted by the prior art.
* * * * *