Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF FORMING A GRATING IN AN OPTICAL WAVEGUIDE
FIELD OF THE INVENTION
The present invention relates generally to the production of fiber optic
components. More specifically, the present invention relates to methods for
forming a
Bragg reflection grating in an optical waveguide whereby the optical fiber is
protected
from contamination during fabrication.
BACKGROUND OF THE INVENTION
The sensitivity of optical waveguide fibers to light of certain wavelength and
intensity has been known since the late 1970's. It was found that the loss
characteristic
and refractive index of a waveguide fiber could be permanently changed by
exposing
the waveguide to light of a given wavelength and intensity, allowing periodic
variations
in the refractive index of a length of optical fiber to be formed. A periodic
variation in
refractive index of the waveguide along the long axis of the waveguide is
commonly
known as an optical waveguide grating. A fiber Bragg grating is an optical
waveguide
grating in a waveguide fiber which will selectively filter propagated light
having a
wavelength which is twice the period of the grating. Such a fiber Bragg
grating is
useful as a wavelength filter.
Fiber Bragg gratings may be formed by a multiple step process which includes
writing with actinic radiation, etching, or other mechanisms for making
periodic
perturbations. Side writing is a technique for forming a grating in an optical
waveguide
fiber wherein light, such as actinic radiation, is caused to form a periodic
series of
alternating light and dark fringes along the long axis of the waveguide. An
example of
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such a periodic series is an interference pattern formed on the side of a
waveguide fiber
and along a portion of the long axis of a waveguiding fiber. The periodic
light intensity
pattern, produced by the light interference, induces a periodic change in
refractive
index along a portion of the long axis of the waveguide fiber.
It is recognized that during the fiber grating processing steps, exposure of
the
bare fiber to contaminants can lead to failure of the fiber grating device and
reduced
reliability. Additionally, there may be some difficulty holding the fiber with
sufficient
stability to prevent grating degradation due to fiber slippage. Such slippage
may occur
because it is necessary to hold the fiber by its polymer coating utilizing a
small amount
of tension.
Accordingly, it would be highly advantageous to provide a process for making a
fiber Bragg grating which protects and secures the stripped portion of the
optical fiber
during the process of writing the grating.
SUMMARY OF THE INVENTION
The present invention provides an advantageous method for forming a grating
in an optical waveguide. By placing a photosensitized optical waveguide into a
package prior to writing the grating into the optical waveguide, the present
invention
allows the optical waveguide to be securely held and protected from
contamination
during the fabrication process steps. According to one aspect of the
invention, the
method includes placing an optical waveguide within an enclosing structure,
sealing the
structure so that the waveguide is secured within the structure, and forming a
grating
within a portion of the waveguide.
In alternative aspects, an embodiment of the invention may include the steps
of
photosensitizing the optical waveguide, testing the spectral performance of
the grating,
tuning the grating within the sealed structure, and annealing the grating and
the
structure.
The optical waveguide may have many specific forms including that of, for
example, a single mode or a multimode optical fiber, a multicore optical
fiber, a
channel waveguide, or a planar waveguide.
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A more complete understanding of the present invention, as well as further
features and advantages of the invention, will be apparent from the following
detailed
description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross sectional view of a grating package in accordance with the
present invention;
Fig. 2 is a flowchart of a method of forming a fiber grating in accordance
with
the present invention;
Fig. 3 is a graph of a reflectance curve of a fiber grating formed in
accordance
with the present invention;
Fig. 4 is a graph of a transmittance curve of a fiber grating formed in
accordance with the present invention; and
Fig. 5 is a graph of a transmission spectrum of a fiber grating at multiple
time
intervals formed in accordance with the present invention.
DETAILED DESCRIPTION
The present invention now will be described more fully with reference to the
accompanying drawings, in which several currently preferred embodiments of the
invention are shown. However, this invention may be embodied in various forms
and
should not be construed as limited to the exemplary embodiments set forth
herein.
Rather, these representative embodiments are described in detail so that this
disclosure
will be thorough and complete, and will fully convey the scope, structure,
operation,
functionality, and potential of applicability of the invention to those
skilled in the art.
Referring to the drawings, Fig. 1 shows a cross-sectional view of a grating
package 10, formed by methods described below, in accordance with the present
invention. For purposes of illustration, an embodiment in which the waveguide
is an
optical waveguiding fiber and the package is generally tube-shaped is shown
and
discussed. It will be understood, however, that the method of the invention
may also be
used with other types of waveguides, with suitable modification of package
shapes and
process steps. An optical fiber 12 is partially enclosed by a tube-shaped
structure 14
formed of material that is transparent to actinic radiation, such as
ultraviolet (UV) light.
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Boron-doped silica or other glass which is transparent to UV light are
suitable materials
for the structure 14. The tube-shaped structure 14 has an inner diameter "a"
(e.g., 255-
1000 Vim), an outer diameter "b" (e.g., 3.0 mm) and a length "c" (e.g., 70
mm). The
optical fiber 12 including its coating 16 has an outer diameter "d" (e.g., 250
Vim). The
coating 16 has been stripped from a length of the optical fiber 12 which is
contained
within the hollow tube 14. The optical fiber 12 has written into it a fiber
grating 18
along a portion of the length which has been stripped of the coating 16.
Two seals 20, 21 disposed at each end 22, 23 of the hollow tube 14 tensionally
maintain and support the region of the optical fiber 12 containing the fiber
grating 18.
The seals 20, 21 may be frits, which include copper glass or other suitable
material.
The package 10 also includes two plugs 24, 25 of epoxy or other suitable
material,
disposed at each end 22, 23 of the tube-shaped structure 14. The ends 22, 23
of the
structure 14 are funnel-shaped at an angle of, for example, 45° to
facilitate placement of
the plugs 24, 25 and insertion of the optical fiber 12.
While presently preferred materials and dimensions are disclosed herein, one
skilled in the art would appreciate that the grating package 10 of the present
invention
may include a variety of materials and sizes, and should not be construed as
limited to
the embodiments or dimensions shown and described herein, which are exemplary.
Further details of other grating packages and packaging methods suitable for
use with
the present invention are provided in U.S. Patent Application (Attorney Docket
No.
Carberry 6), filed on September 16, 1999, entitled "Method And Apparatus For
Packaging Long-Period Fiber Grating" which is incorporated by reference herein
in its
entirety.
Fig. 2 shows a method 30 of forming a waveguide grating in a package (such as
the grating package 10) in accordance with the present invention. In a
sensitizing step
32, a waveguide such as, for example, an optical fiber is photosensitized. An
example
of an optical fiber suitable for use with the present invention is a high-
delta, germanium
doped, step-index fiber with an index delta of substantially 2%. As used
herein, the
term index delta refers to the relative refractive index difference between
the core and
the cladding of the optical fiber and is expressed as a percentage. An example
of a
process suitable for photosensitizing the optical fiber includes exposing the
optical fiber
to a hydrogen atmosphere at 100 atmospheres of pressure for two weeks. A
section of
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the optical fiber is then flood exposed to ultraviolet light. A UV laser
operating at 248
nm pulsed at 15 Hz has been found suitable for this flood exposure. The
exposure may
be at a pulse fluence of 75 millijoules/cm2 for 30 minutes. The optical fiber
is then
annealed for 24 hours at 125° C. Another process suitable for
photosensitizing for use
with the present invention is described in U.S. Patent Application No.
09/252,151, filed
on February 18, 1999 entitled "Optical Waveguide Photosensitization" which is
incorporated by reference herein in its entirety.
Next, in a packaging step 34, the optical fiber is placed within a hollow tube
(such as the hollow tube 14) and sealed to form a package. The package
securely holds
and protects the optical fiber from contamination during the process steps. In
a grating
writing step 36, a grating is written onto the optical fiber. Any of a variety
of side
writing techniques may be used to write the grating into the optical fiber. In
one
exemplary technique suitable for use with the present invention, an excimer-
pumped,
frequency-doubled dye laser system operating at substantially 240 nm
(nanometers) is
used as the source of the ultraviolet (UV) light. The 240 nm beam produced by
the
laser is first passed through silica slits. Suitable silica slits are
described in greater
detail in U.S. Patent Application Serial No. 09/081,912, filed on May 19,
1998, entitled
"Spatial Filter For High Power Laser Beam" which is incorporated by reference
herein
in its entirety. After the 240 rmn beam has passed through the silica slits,
it passes
through a phase mask and then onto optical fiber within the silica tube. The
phase
mask may be a transmission diffraction grating, a component whose structure
and
characteristics are known in the art. A phase mask may also be a substrate
having a
series of periodically spaced openings. During the grating writing step 36,
the tube 14
in the illustrated embodiment is located approximately 4 millimeters from the
phase
mask. The pulsed exposure from the beam of the excimer laser is at a
repetition rate of
10 Hz for 25 minutes. The laser fluence, or intensity, at the position of the
optical
fiber is approximately 75 millijoules/cm2. Exemplary reflectance and
transmittance
curves of the resulting grating are shown in Figs. 3 and 4, respectively. The
average
refractive index change during exposure to the laser beam was approximately 2
x 10-4.
Next, in a first testing step 38, the spectral performance of the grating is
tested.
Spectral performance is adjusted, or tuned, in a first tuning step 40. In the
first tuning
step 40, the grating is flood exposed to UV light provided by, for example, an
excimer
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laser system operating at substantially 248 nm for 5 minutes in order to meet
the
spectral target required for the grating. At the position of the optical fiber
in the
illustrated embodiment, the laser fluence is approximately 75 millijoules/cm2
and the
repetition rate is 15 Hz. Fig. 5 shows an exemplary transmission spectrum of
the fiber
grating at multiple time intervals during the flood exposure. During the
exposure time,
a total wavelength shift of approximately 0.15 nm occurs. As seen in Fig. 5,
the
transmission minimum increases as exposure time increases due to a decrease in
the
amplitude of the grating modulation. The decrease in grating modulation is the
result
of an increase of the refractive index in the previously light exposed troughs
of the
grating. In the illustrated embodiment, this is followed by a first annealing
step 42. In
this step 42, the package is annealed for 24 hours at 125° C.
In one embodiment, this may be followed by further testing, tuning and
annealing steps. In a second testing step 44, the spectral performance of the
grating is
tested to see if the grating meets the spectral target. If the grating does
not meet the
spectral target, then in a second tuning step 46 the grating is flood exposed
to UV light
by an excimer laser system operating at substantially 248 nm in order to tune
the
grating. As illustrated, at the position of the optical fiber, the laser
fluence is
approximately 75 millijoules/cm2 and the repetition rate is 15 Hz. In a final
annealing
step 48, the package is annealed for 24 hours at 125°C.
It will be apparent to those skilled in the art that various modifications and
variations can be made in the present invention without departing from the
spirit and
scope of the present invention. Thus, it is intended that the present
invention cover the
modifications and variations of this invention provided they come within the
scope of
the appended claims and their equivalents.
