Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02353504 2006-03-27
Compression-Tuned Bragg Grating and Laser
Tecbnical Field
This invention relates to fiber gratings, and more paticulady to a
compression-tuned Bragg grating and laser.
Background Art
It is lmown in the art of fiber optics that Bragg patiop embodded in the fiber
may be used in compression to act as a tunable filter or tonable fiber laser,
as is
desca'bed in US Patent No. 5,469,520, entitled "Comlmssion Tuned Fiber
Giatiaig" to
Morey, et al and US Patent No. 5,691,999, entitled "Compression Tuned F'ber
Laser"
to Ball et aL, respectively.
To avoid fiber buciding under compression, the teolmique described in ihe
aforeeneotioned US Patent Nos. 5,469,520 and 5,691,999 uses sliding fenules
around
the fiber and grating and places the feaules in a mechanical sttuctaro to
guide, align
and cwnfine the feaules and the Sber. However, it would be desirable to obtain
a
coflfiguuation that allows a fiber grating to be compressed without buckling
and
without sliding feirules and without requiring such a mechanical struedare.
Also, it is known to attach an optical fiber grating to within a glass tube to
avoid buckling under compression for providing a wavelength-stable temperature
compensated fiber Bragg grating as is desodbod in US Patent No. 5,042,898,
entitled
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"Incorporated Bragg Filter Temperature Compensated Optical Waveguide Device",
to
Morey et al. However, such a technique exhibits creep between the fiber and
the tube
over time, or at high temperatures, or over large compression ranges.
Summary of the Invention
Objects of the present invention include provision of a fiber grating
configuration that allows the grating to be compression-tuned without creep
and
without requiring sliding ferrules or a mechanical supporting structure for
the ferrules.
According to the present invention, a compression-tuned fiber optic device,
comprises: a tunable optical element, having at least one reflective element
disposed
therein along a longitudinal axis of the element; and at least a portion of
the element
having a transverse cross-section which is contiguous and made of
substantially the
same material.
According further to the present invention, the tunable element comprises an
optical fiber, having the reflective element embedded therein; and a tube,
having the
optical fiber and the reflective element encased therein along a longitudinal
axis of the
tube, the tube being fused to at least a portion of the fiber. According
further to the
present invention the tunable element comprises a large diameter optical
waveguide
having an outer cladding and an inner core disposed therein and an outer
waveguide
diameter of at least 0.3 mm.
According still further to the present invention the material is a glass
material.
According still further to the present invention the tube is fused to the
optical fiber
where the reflective element is located. According still further to the
present invention
the a plurality of the optical fibers or cores disposed in the tunable
element.
According still further to the present invention, the tunable element has a
plurality of
reflective elements encased in the tube. According still further to the
present
invention, the tunable element has at least one pair of reflective elements
disposed
therein and at least a portion of the tunable element is doped with a
raraearth dopant
between the pair of elements to form a laser. According still further to the
present
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invention, the laser lases at a lasing wavelength which changes as force on
the tube changes.
The present invention provides a Bragg grating disposed in a tunable
optical element which includes either an optical fiber fused to at least a
portion
of a glass capillary tube ("tube encased grating") or a large diameter
waveguide grating having an optical core and a wide cladding. The tunable
element is placed in compression to tune the reflection wavelength of the
grating without buckling the element.
The element may be made of a glass material, such as silica or other
glasses. The tunable element may have alternative geometries, e.g., a dogbone
shape, that provides enhanced force to wavelength shift sensitivity and is
easily
scalable for the desired sensitivity. The present invention allows a fiber
grating or
laser to be wavelength tuned with very high repeatability, low creep and low
hysteresis. Also, one or more gratings, fiber lasers, or a plurality of fibers
or
optical cores may be disposed in the tunable element.
The grating(s) or laser(s) may be "encased" in the tube by having the
tube fused to the fiber on the grating area and/or on opposite axial ends of
the
grating area adjacent to or a predetermined distance from the grating. The
grating(s) or laser(s) may be fused within the tube or partially within or to
the
outer surface of the tube. Also, one or more wavguides and/or the tube
encased fiber/gratings may be axially fused and optically coupled to form the
tunable element.
One aspect of the present invention provides for a compression-tuned
optical device, comprising: a tunable optical element, having outer dimensions
along perpendicular axial and traverse directions, said outer dimension being
at
least 0.3 mm along said transverse direction; said tunable optical element
receiving and propagating input light and having at least one reflective
element
disposed therein along an axial direction, said reflective element reflecting
a
reflection wavelength of said input light; said tunable optical element
comprising
an optical fiber, having said reflective element embedded therein and a tube
having said optical fiber and said reflective element encased therein along a
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CA 02353504 2006-10-18
longitudinal axis of said tube to prevent buckling; at least a portion of said
tunable
element having a transverse cross-section which is contiguous and comprises a
substantially homogeneous material; and said reflective element being axially
strain compressed so as to change said reflection wavelength without buckling.
In another embodiment, the present invention provides for a compression-
tuned optical device, comprising: a tunable optical element having a large
diameter optical waveguide with outer dimensions along perpendicular axial and
traverse direction, said outer dimension being at least 0.3 mm along said
transverse direction; said tunable optical element receiving and propagating
input
light and having at least one reflective element disposed therein along said
axial
direction, said reflective element reflecting a reflection wavelength of said
input
light; at least a portion of said tunable element having a transverse cross-
section
which is contiguous and comprises a substantially homogeneous material; and
said reflective element being axially strain compressed so as to change said
reflection wavelength without buckling, and wherein said buckling is prevented
by
providing a suitably wide outer cladding on said large diameter optical
waveguide.
In another embodiment, the present invention provides for a method for
wavelength-tuning an optical device, comprising the steps of: a) obtaining a
tunable optical element, having outer dimensions along perpendicular axial and
transverse directions, said outer dimension being at least 0.3 mm along said
transverse direction, said tunable optical element receiving and propagating
input
light and having at least one reflective element disposed therein along a
longitudinal axis of said element, said tunable optical element comprises a
tube,
having an optical fiber and said reflective element encased therein along a
longitudinal axis of said tube to prevent buckling, at least a portion of said
tunable
element having a transverse cross-section which is contiguous and comprises a
substantially homogeneous material; and b) axially compressing said tunable
element so as to change the reflection wavelength of said reflective element
without buckling said tunable element in said transverse direction thereby
tuning
said optical device.
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In another embodiment, the present invention provides for a method for
wavelength-tuning an optical device, comprising the steps of: a) obtaining a
tunable optical element having a large diameter optical waveguide with outer
dimensions along perpendicular axial and transverse directions, said outer
dimension being at least 0.3 mm along said transverse direction, said tunable
optical element receiving and propagating input light and having at least one
reflective element disposed therein along a longitudinal axis of said element,
at
least a portion of said tunable element having a transverse cross-section
which is
contiguous and comprises a substantially homogeneous material; and b) axially
compressing said tunable element so as to change the reflection wavelength of
said reflective element without buckling said tunable element in said
transverse
direction thereby tuning said optical device, and wherein buckling is
prevented by
an outer cladding in said large diameter optical waveguide.
The foregoing and other objects, features and advantages of the
present invention will become more apparent in light of the following detailed
description of exemplary embodiments thereof.
Brief Description of Drawings
Fig. 1 is a side view of a device for compressing a tube-encased fiber
grating, in accordance with the present invention.
Fig. 2 is a side view of an alternative device for compressing a tube-
encased fiber grating, in accordance with the present invention.
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Fig. 3 is a side view of an alternative device for compressing a tubt~encased
fiber grating, in accordance with the present invention.
Fig. 4 is a side view of a tube-encased fiber grating, in accordance with the
present invention.
Fig. 5 is a side view of a tube-encased fiber grating having an alternative
geometry for the tube, in accordance with the present invention.
Fig. 6 is a side view of a tube-encased fiber grating having an alternative
geometry for the tube, in accordance with the present invention.
Fig. 7 is a side view of a tube-encased fiber grating where the tube is fused
on
opposite axial ends of the grating area, in accordance with the present
invention.
Fig. 8 is a side view of more than one grating on a fiber encased in a tube,
in
accordance with the present invention.
Fig. 9 is a side view of two fiber gratings on two separate optical fibers
encased in a common tube, in accordance with the present invention.
Fig. 10 is an end view of the embodiment of Fig. 9, in accordance with the
present invention.
Fig. 11 is an end view of two fiber gratings on two separate optical fibers
encased in a common tube and separated by distance, in accordance with the
present
invention.
Fig. 12 is a side view of a tube-encased fiber grating where the tube is fused
on the fiber only over the length of the grating, in accordance with the
present
invention.
Fig. 13 is a side view of a tunable distributed feedback (DFB) fiber laser
encased in a tube, in accordance with the present invention.
Fig. 14 is a side view of a device for compressing a tube-encased fiber
grating
using an actuator to tune the grating, in accordance with the present
invention.
Fig. 15 is a side view of a device for compressing a tubaencased fiber grating
using a precise pressure source to tune the grating, in accordance with the
present
invention.
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Fig. 16 is a side view of a device for compressing a tubeencased fiber grating
using a precise pressure source to tune the grating, in accordance with the
present
invention.
Fig. 17 is a side view of a large diameter optical waveguide having a grating
disposed therein, in accordance with the present invention.
Best Mode for Carrying Out the Invention
Referring to Fig. 1, a compression-tuned bragg gmting comprises a known
optical waveguide 10, e.g., a standard telecommunication single mode optical
fiber,
having a Bragg grating 12 impressed (or embedded or imprinted) in the fiber
10. The
fiber 10 has an outer diameter of about 125 niicrons and comprises silica
glass (SiQ)
having the appropriate dopants, as is known, to allow light 14 to propagate
along the
fiber 10. The Bragg grating 12, as is known, is a periodic or aperiodic
variation in the
effective refractive index and/or effective optical absorption coefficient of
an optical
waveguide, such as that descnbed in US Patent No. 4,725,110 and 4,807,950,
entitled
"Method for Impressing Gratings Within Fiber Optics", to Gle:m et al; and US
Patent
No. 5,388,173, entitled "Method and Apparatus for Fonming Aperiodic Gratings
in
Optical F'bers", to Glenn.
However, any wavelength-tunable grating or reflective element embedded,
etched,
imprinted, or otherwise formed in the fiber 28 may be used if desieed. As used
herein,
the term "grating" means any of such reflective elements. Further, the
refleative
element (or grating)12 may be used in reflection and/or transmission of light.
Other materials and dimensions for the optical fiber or waveguide 10 may be
used if desired. For example, the fibe,r 10 may be made of any glass, e.g.,
silica,
phosphate glass, or other glasses, or made of glass and plastic, or solely
plastic. For
high temperature applications, optical fiber made of a glass materiai is
de3irable.
Also, the fiber 10 may have an outer diameter of 80 microns or other
diameters.
Further, instead of an optical fiber, any optical waveguide may be used, such
as, a
multi-mode, birefiingent, polarization maintaining, polatizing, multi-core, or
multi-
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cladding optical waveguide, or a flat or planar waveguide (where the waveguide
is
rectangular shaped), or other waveguides. As used herein the term "fiber"
includes
the above described waveguides.
The light 14 is incident on the gratingl2 which reflects a portion thereof as
indicated by a line 16 having a predetermined wavelength band of light
centered at a
reflection wavelength Xb, and passes the remaining wavelengths of the incident
light
14 (within a predetermined wavelength range), as indicated by a line 18.
The fiber 10 with the grating 12 therein is encased within and fusefl to at
least
a portion of a cylindrical glass capillary tube 20, discussed more
hereinaller. The tube
20 is axially compressed by a compressing device or housing 50. One end of he
tube
is pressed against a seat 51 in an end 52 of the housing 50. The housing 50
also has
a pair of arms (or sides) 54 which guide a movable block 56. The block 56 has
a seat
57 that presses against the other end of the tube 20. The end 52 and the bbak
56 have
a hole 58 drilled through them to allow the fiber 10 to pass through. An
actuator 60,
15 such as a stepper motor or other type of motor whose rotation or position
can be
controlled, is connected by a mechanical linkage 62, e.g., a screw drive,
linear
actuator, gears, and/or a cam, to the movable block 56 (or piston) which
causes the
block 56 to move as indicated by arrows 64. Accordingly, the stepper motor 60
can
set a predetermined amount of force on the block to compress the tube 20 to
provide a
20 desired reflection wavelength of the grating 12. Instead of the recessed
seats 51,57,
the tube 20 may contact the ends 52,56 with a flush contact. The stepper motor
60
may be a high resolution stepper motor driven in a microstepping mode, such as
that
described in the aforementioned US Patent No. 5,469,520, "Compression Tuned
Fiber
Grating", to Money et al, (e.g., a Melles Griot NANOMOVER).
_ Other higher or lower resolution stepper motors may be used if desired.
The stepper motor 60 is driven by a control circuit 63 which provides drive
signals on
lines 61 needed to drive the stepper motor 60, and henoe the block 56, to the
desired
position, to provide the desired Bragg wavelength Xb of the grating 12.
Instead of a
stepper motor, other actuators may be used if desired, as discussed
hereinaftsr with
Fig. 14.
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Referring to Fig. 2, instead of using the movable block 56, a housing 70 may
be used which has two end caps 72,74 and outside walls 76. In that case, the
holes 58
are in the end caps 72,74 to allow the fiber 10 to exit. The stepper motor 62
is
connected to the end cap 74 by the mechanical linkage 62. When the stepper
motor 62
pushes on the end cap 74, the walls 76 compress or deflect, the tube 20 is
compressed
and the reflection wavelength of the grating 12 shifts.
Referring to Fig. 3, another embodiment of the present invention, comprises a
cylindrical-shaped housing 90 comprising an outer cylindrical wall 98, two end
caps
95, and two inner cylinders (or pistons) 92 each connected at one end to one
of the
end caps 95. The tube 20 (with the grating 12 encased therein) is disposed
against the
other ends of and between the two pistons 92. Other cross-sectional and/or
sid&view
sectional shapes may be used for the housing 90 dements 98,95,92 if desired.
The end
caps 95 may be separate pieces or part of and contiguous with the pistons 92
and/or
the outer cylinder 98.
The stepper motor 60 applies an extemal axial force on the end cap 95 on the
left side of the housing 90. The pistons 92 have holes 94 having a diameter
large
enough to allow the fiber 10 pass through.
Between the inside dimension of the walls 98 and the outside dimension of
tube 20 and pistons 92 is an inner I-shaped chamber 100. The pistons 92, the
outer
cylinder walls 98, the end caps 95, and the tube 20 may be made of the same or
different materials.
An example of some possible dimensions for the housing 90 are as follows.
Other dimensions may be used. The tube 20 has the outer diameter d2 of about 2
mm
(0.07 inches) and a length Ll of about 12.5 mm (0.5 in.), the pistons 92 each
have
outer diameters d5 of about 19.1 mm (0.75 inches), the length L5 of each of
the
pistons 92 is about 6.25 cm (2.5 in.), the diameter of the holes 94 in the
pistons 92 is
about 1 mm (1000 microns), the overall length L4 of the housing 90 is about
12.7 cm
(5 inches), the thickness tl of the outside walls 98 is about 1.0 mm (0.04
inches), and
the gap gl between the inner dimension of the outer walls 98 and the outer
dimensions of the pistons 92 is about 1.52 mm (0.06 inches).
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The dimensions, materials, and material properties (e.g., Poisson's ratio,
Young's Modulus, Coefficient of Thermal Expansion, and other known
properties), of
the walls 98 and the pistons 92 are selected such that the desired strain is
delivered to
the capillary tube 20 at an external force. The resolution and range for
setting the
reflection wavelength are scalable by controlling these parameters. For
example, if
the overall length L4 is increased, the sensitivity DL/L will increase.
In particular, as the axial force from the stepper motor increases, the axial
length L4 of the housing 90 decreases by an amountOL due to compression and/or
deflection of the outer walls 98. A predetermined portion of the total axial
length
change AL' is seen at the tube 20 due to compression of the tube 20.
Compression of
the tube 20 lowers the Bragg reflection wavelength X1 of the grating 12 by a
predetermined amount which provides a wavelength shift. If the pistons 92 have
a
spring constant higher than that of the glass tube 20, the tube 20 will be
compressed
more than the pistons 92 for a given force. Also, for a given external force,
a
predetenmined amount of the force is dropped across the outside walls 98, and
the
remainder is seen by the tube 20.
For example, when the walls 98, pistons 92 and end caps 95 are all made of
titanium having the dimensions discussed hereinbefore, for an external force
of 2200
lbf, about 2000 lbf is dropped across (or used to compress/deflect) the
outside walls
98, and about 200 lbf is dropped across the tube 20. The cylinder walls 98 act
similar
to a diaphragm or bellows which compress or deflect due to increased extenaal
pressure.
The housing 90 may be assembled such that a pro-strain or no pre-stain exists
on the tube 20 prior to applying any outside forces.
The material of the housings 50,70, 90 and/or one or more of the components
thereof, may be made of a metal such as titanium, high nickel content alloys
such as
Inconel , Incoloy , Nimonic (registered trademarks of Inco Alloys
International,
Inc.) containing various levels of Nickel, Carbon, Chromium, Iron, Molybdenum,
and
Titanium, stainless steel, a glass material (such as discussed hereinafter for
the tube
20), or other high strength, or corrosion resistant, or high temperature or
heat resistant
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metals or alloys may be used, or other materials having sufficient strength to
compress the tube 20 may be used. Other materials having other properties may
be
used if desired depending on the application.
Referring to Fig. 14, alternatively, instead of using a stepper motor as the
actuator, the tube 20 may be compressed by another actuator 154, such as a
peizoelectric actuator, solenoid, pneumatic force actuator, or any other
device which
is capable of directly or indirectly applying an axial compressive force on
the tube 20
may be used. The actuator 154 may be disposed on a housing 150 (analogous to
the
frame 50; Fig. 1) and creates a force on a movable block 152 (analogous to the
movable block 56; Fig. 1) which moves in the direction of the arrows 155.
One end of the tube 20 is pressed against the seat 51 in an end 153 of the
housing 150. The housing 150 also has a pair of sides 157 which guide the
movable
block 152. One of the sides 157 may be removed if desired. The block 152 has
the
seat 57 that presses against the other end of the tube 20.
Also, the actuator 154 is connected to a control circuit 158 which provides
the
necessary signals on a line 156 to the actuator 154 to set the desired force
on the tube
which sets the desired Bragg wavelength Xb of the grating 12. The force may be
set by the controller 158 by providing a signal (e.g., an electrical voltage)
on the line
156 to the actuator 154 in an open loop configuration. Alternatively, the
forae may be
20 set on the actuator 154 by providing a signal on the line 156 to the
actuator 154 and
measuring the force or position of the actuator 154 on a line 160 in a closed
loop
control configuration on the actuator 154.
For single ended operation, the fiber 10 may enter on one end of the housing
150 and pass through a hole 162 in the end 153. If a feed-through (double
ended fiber)
design is used, the block 152 may have a hole 164 part or all the way through
it, and
the other end of the fiber 10 may be fed out the side or passed through a hole
166 in
the actuator 154 and in the other end of the housing 150.
One example of a closed loop piezoelectric actuator that may be used is Model
No. CM (controller) and DPT-C-M (for a cylindrical actuator) made by
Queensgate,
Inc. of N.Y. Other actuators may be used, as discussed hereinbefore.
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,. ,.r_ . .,.~ _ ._.._ _ . , .~..._.,_ . _ . ,...,.._.
CA 02353504 2006-03-27
Referring to Fig. 15, alternatively, the tube 20 may be placed in a housing
174,
and the grating wavelength set by placing a fluid pressure on the tube 20,
similar to a
pressure sensor.;
The housing 172 creates a
chamber 176 and has a port 178 that is fed to a pressum source 180, which
provides a
precise source pressure Ps. The chamber 176 may be filled with a fluid (e.g.,
one or
more gasses and/or liquids). The tube 20 may be mounted to one wa11175 or may
be
suspended in the fluid 176. The optical fiber 10 is fed into the chamber
through a
known hermetic feedthroughs and has some slack 179 to allow for compression of
the
tabe 20 over passua+e. The grating reflection wavelenglh changes as the
pressure Ps
changes, similar to the actuator embodiments discussed heminbefore; however,
in this
case, the grating wavelength is set by setting a predetermined source fluid
pressure Ps.
Refeming to Fig. 16, for example, the pressure source 180 may comprise a
hydraulic aetaator or piston 300 disposed within a chamber 301. The piston 300
is
connected by a meclunical linkage 302 to a known hydraulic drive mechanism 304
which precisely sets the position of the piston 300 to set the pressure Ps.
The
hydraulic drive 304 may be controlled electranically by a known control
circuit 308,
similar to the controlier 158 (Fig. 14), which provides a position command
signal on a
line 306 to the hydraulic controller 304 for a particular piston position and
thus
pressure Ps, and thus wavelength Ab of the grating. Other known pressure
sources
may be used if desired to set the grating wavelength.'T'he housings descn'bed
herein
50,150,70,90, and any components thereiq including the movable blocks 56,152,
may
have a circ:ular cross-soction (i.e., eylindrieal shape) or may have other
cms&secdanal
shapes, such as square, rectangular, or other shapes.
Although the invention has been described with some apeeific eanbodiments
with Figs.1-3,14,15 for compressing the tube 20, any device or fixture which
eompresses the tube axially may be used for compressing the tube 20 to tame
the
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reflection wavelength of the grating 12 to the desired wavelengths. The exact
hardware configuration is not critical to the present invention.
For any of the embodiments described herein, the axial end faces of the tube
20 and/or the seats on mating surfaces (56,50,92,74,72,153,159) may be plated
with a
material that reduces stresses or enhances the mating of the tube 20 with the
seat on
the mating surfaces. Referring to Fig. 4, the tube 20 may have an outer
diameter dl of
about 3 mm and a length L1 of about 10-30 mm. The grating 12 has a length Lg
of
about 5-15 mm. Alternatively, the length L1 of the tube 20 may be
substantially the
same length as the length Lg of the grating 12, such as by the use of a longer
grating,
or a shorter tube. Other dimensions and lengths for the tube 20 and the
grating 12
may be used. Also, the fiber 10 and grating 12 need not be fused in the center
of the
tube 20 but may be fused anywhere in the tube 20. Also, the tube 20 need not
be fused
to the fiber 10 over the entire length of the tube 20.
The dimensions and geometries for any of the embodiements described herein
are merely for illustrative purposes and, as such, any other dimensions may be
used if
desired, depending on the application, size, performance, manufacturing
requirements, or other factors, in view of the teachings herein.
The tube 20 is made of a glass material, suchas natural or synthetic quartz,
fused silica, silica (SiOz), Pyrex by Coming (boro silicate), or Vycor by
Coming
Inc. (about 95% silica and 5% other constituents such as Boron Oxide), or
other
glasses. The tube should be made of a material such that the tube 20 (or the
inner
diameter surface of a bore hole in the tube 20) can be fused to (i.e., create
a molecular
bond with, or melt together with) the outer surface (or cladding) of the
optical fiber 10
such that the interface surface between the inner diameter of the tube 20 and
the outer
diameter of the fiber 10 become substantially eliminated (i.e., the inner
diameter of
the tube 20 cannot be distinguished from and becomes part of the cladding of
the fiber
10).
For best thermal expansion matching of the tube 20 to the fiber 10 over a
large
temperature range, the coefficient of thermal expansion (CTE) of the material
of the
tube 20 should substantially match the CTE of the material of the fiber 10,
e.g., fused
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silica tube and optical fiber. In general, the lower the melting temperature
of the glass
material, the higher the CTE. Thus, for a silica fiber (having a high melting
temperature and low CTE) and a tube made of another glass material, such as
Pyrex
or Vycor (having a lower melting temperature and higher CTE) results in a
thermal
expansion mismatch between the tube 20 and the fiber 10 over temperature.
However,
it is not required for the present invention that the CTE of the fiber 10
match the CTE
of the tube 20 (discussed more hereinafter).
Instead of the tube 20 being made of a glass material, other elastically
deformable materials may be used provided the tube 20 can be fused to the
fiber 10.
For example, for an optical fiber made of plastic, a tube made of a plastic
material
may be used.
The axial ends of the tube 20 where the fiber 10 exits the tube 20 may have an
inner region 22 which is inwardly tapered (or flared) away from the fiber 10
to
provide strain relief for the fiber 10 or for other reasons. In that case, an
area 28
between the tube 20 and the fiber 10 may be filled with a strain relief filler
material,
e.g., polyimide, silicone, or other materials. Also, the tube 20 may have
tapered (or
beveled or angled) outer comers or edges 24 to provide a seat for the tube 20
to mate
with another part (not shown) and/or to adjust the force angles on the tube
20, or for
other reasons. The angle of the beveled corners 24 are set to achieve the
desired
function. The tube 20 may have cross-sectional shapes other than circular,
such as
square, rectangular, elliptical, clam-shell, or other shapes, and may have
sidaview
sectional shapes other than rectangular, such as circular, square, elliptical,
clam-shell,
or other shapes.
Alternatively, instead of having the inner tapered axial region 22, one or
both
of the axial ends of the tube 20 where the fiber 10 exits the tube 20 may have
an outer
tapered (or fluted, conical, or nipple) axial section, shown as dashed lines
27, which
has an outer geometry that decreases down to the fiber 10 (discussed more
herdnafter
with Fig. 12). We have found that using the fluted sections 27 provides
enhanced pull
strength at and near the interface where the fiber 10 exits the tube 20, e.g.,
61bf or
more, when the fiber 10 is pulled along its longitudinal axis.
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Where the fiber 10 exits the tube 20, the fiber 10 may have an external
protective buffer layer 21 to protect the outer surface of the fiber 10 finm
damage.
The buffer 21 may be made of polyimide, silicone, Teflon
(polytetraflouroethylene),
carbon, gold, and/or nidcel, and have a thickness of about 25 microns. Other
thicknesses and buffer materials for the buffer layer 21 may be used. If the
inner
tapered region 22 is used and is large enough, the buffer layer 21 may be
inserted into
the region 22 to provide a transition from the bare fiber to a buffered fiber.
Alternatively, if the axial end of the tube 20 has the extetnal taper 27, the
buffer 21
would begin where the fiber exits the tapered 27 portion of the tabe 20. If
the buffer
21 starts after the fiber exit point, the exposed bare portion of the fiber 10
may be
recoated with an additional buffer layer (not shown) which covers any bare
fiber
outside of the tube 20 and may also overlap with the buffer 21 and/or some of
the
tapered region 27 or other geometrically shaped axial end of the tube 20.
To encase the fiber 10 within the tube 20, the tube 20 may be heabed,
collapsed, and fused to the grating 12, by a laser, filament, flame, etc.
Other techniques may be used for coilapsing and fusing the tubes 20 to the
fiber 10,
such as is discussed in US Patent No. 5,745,626, entitled "Method For And
Encapsulation Of An Optical Fiber", to Duck et al.; and/or US Patent No.
4,915,467,
entitled "Method of Making Fiber Coupler Having lntegcal Precision Connection
Wells", to Berkey, or other techniques,
Altematively, ottier
techniques may be used to fuse the fiber 10 to the tnbe 20, such as using a
high
temperature glass solder, e.g., a silica solder (powder or solid), such that
the fiber 10,
the tube 20 and the solder all become fused to each other, or using laser
welding/fusing or other fusing techniques. Also, the fiber may be fased within
the
tube or par6ally within or on the outer surface of the tabe (discussed
lsrreinatber with
Fig. 11).
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CA 02353504 2006-03-27
The Bragg grating 12 may be impressed in the fiber 10 before or after the
capillary tube 20 is encased around and fnsed to the fiber 10.
If the grating 12 is impressed in the fiber 10 after
the tube 20 is encased around the grating 12, the grating 12 may be written
through
the tube 20 into the fiber 10 by any desired technique, such as is desmlbed in
US Patent No. 6,298,184.
17m grating 12 may be encased in the tube 20 hsving an initial prastrain from
the tube (compression or tension) or no pre-stiain. For example, if Pyrex or
another
glass that has a larger coefFicieat of theimal expansion (CTE) than that of
the fiber 10
is used for the tube 20, when the tube 20 is heated and fused to the fiber and
then
cooled, the grating 12 is put in compression by tha tube 20. Alternatively,
the fiber
grating 12 may be encased in the tube 20 in tension by putting the grating in
tension
during the tube heating and fusing process. In that case, when the tube 20 is
comprossed, the tension on the grating 12 is reduced. Also, the fiber grating
12 may
be encased in the tube 20 resulting in neither tension nor eompresaion on the
gcat.ing
12 when no eaternal forces are applied to the tube 20.
Refeaing to Fig. 5, the capillary tobe 20 may have a varyimg geometry,
depending on the application. For example, the tube 20 may have a "dogbone"
shape
having a nairow central sction 30 and larger outer sections 32. The nwruw
section 30
has an outer diameter d2 of about 1 mnt, and a length 12 of about 5 mm .'I7a;
large
seaaions 32 each have a diameter 0 of about 3 mm and a length L3 of about 4
mm.
Other lengths and diameters of the sections 30,32 may be used. The dogbone
shape
may be used to provide increased sensitivity in converting foroe applied by
the
stepper motor 60 or actuator 154 to wavelength ahift of the tabeencased
gcating 12.
An inner transition region 33 of the large soctions 32 may be a sliarp
vectical
or angled edge or may be curved as indicated by dashed lines 34. A curved
geometry
34 has less stress risers than a shmp edge and thus may reduce the lOcelaood
of
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CA 02353504 2001-06-01
WO 00/37969 PCT/US99/28999
breakage. Also, the sections 32 of the tube 20 may have the inner tapered
regions 22
or the outer fluted sections 27 at the ends of the tube 20, as discussed
hereinbefore.
Further, the sections 32 may have the tapered (or beveled) outer corners 24 as
discussed hereinbefore.
Also, it is not required that the dogbone geometry be symmetric, e.g., the
lengths L3 of the two sections 32 may be different if desired. Alternatively,
the
dogbone may be a single-sided dogbone, where instead of the having the two
larger
sections 32, there may be only large section 32 on one side of the narrow
section 30
and the other side may have a straight edge 37 which may have beveled corners
24 as
discussed hereinbefore. In that case, the dogbone has the shape of a "T" on
its side.
Such a single-sided dogbone shall also be referred to herein as a "dogbone"
shape.
Instead of a dogbone geometry, other geometries that provide enhanced strain
sensitivity or adjust force angles on the tube 20 or provide other desirable
characteristics may be used.
We have found that such a dimension change between the dimension d3 of the
large section 32 and the dimension d2 of the narrow section 30 provides
increased
force to grating wavelength shift sensitivity (or gain or scale factor) by
strain
amplification. Also, the dimensions provided herein for the dogbone are easily
scalable to provide the desired amount of sensitivity.
Referring to Fig. 6, alternatively, to help reduce strain on the fiber 10 at
the
interface between the fiber 10 and the tube 20, the tube 20 may have sections
36
which extend axially along the fiber 10 and attach to the fiber 10 at a
location that is
axially outside where the force is applied on the large sections 32 by
opposing end
pieces 104,105, which are equivalent to the end pieces 56,50 (Fig. 1), 74,72
(Fig. 2),
159,153 (Fig. 14), respectively, or the pistons 92 (Fig. 3). The axial length
of the
sections 36 may be about 20 mm; however, longer or shorterlengths may be used
depending on the application or design requirements. Also, the sections 36
need not
be axially symmetrical, and need not be on both axial ends of the tube 20. The
sections 32 may have the inner tapered regions 22 or the outer fluted sections
27
where the fiber interfaces with the tube 20, as discussed hereinbefore.
Alternatively,
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CA 02353504 2006-03-27
there may be a stepped section 39 as part of the sections 36. In that case,
the region 22
may be within or near to the stepped section 39 as indicated by dashed lines
38. The
regions 106 may be air or filled with an adhesive or filler. Also, the tube 20
may have
a straight constant cross-section as discussed hereinbefore and as indicated
by the
dashed lines 107 instead of a dogbone shape. Further, the hole 108 through the
end
pieces 56,50 (Fig. 1), 74,72 (Fig. 2), 152,150 (Fig. 14), respectively, or the
pistons 92
(Fig. 3) may have a larger diameter as indicated by the dashed lines 109 for
all or a
portion of the length of the hole 108.The capillary tube 20 may have other
aaial
extending geometries, such as is discussed in US Patent No. 6,519,388.
Also, more than one concentric tube may be used to form the tube 20
of the present invention, as discussed in US Patent 6,519,388.
Also, the axially extended sections 36 may be part of an inner tube.
Refemng to Fig. 7, altematively, the tube 20 may be fused to the fiber 10 on
opposite sides of the grating 12. In particulaz; regions 200 of the tube 20
are fused to
the fiber 10 and a central section 202 of the tube around the grating 12 is
not fused to
the fiber 10. The region 202 around the grating 12 may contain ambient air or
be
evacuated (or be at another pressure) or may be partially or totally filled
with an
adhesive, e.g., epoxy, or other filling mat;erial, e.g., a polymer or
silicone, or another
material or may be not filled. As discussed hereinbefore, the inner diameter
d6 of the
tube 20 is about 0.01 to 10 microns larger than the diameter of the optical
fiber 10,
e.g., 125.01 to 135 microns. Other diameters may be used; however, to help
avoid
fiber buckling in this embodiment, the diameter d6 should be as close as
possible to
the fiber 10 outer diameter. Alternatively, the same result can be achieved by
fusing
two separate tubes on opposite sides of the gcatfng 12 and then fusing an
outer tabe
across the tubes, as discussed in the aforementioned copending US Patent
Application.
We have found that the present invention provides high repeataln'liiy,low
creep and low hysteresis (e.g., about 3 picometers or less), depending on the
oonfiguration used. Referring to Fig. 8, for any of the embodiments described
herein,
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CA 02353504 2006-03-27
instead of a single grating encased within the tube 20, two or more gratings
220,222
may be embedded in the fiber 10 that is encased in the tube 20. The gratings
220,222
may have the same reflection wavelengths and/or profiles or different
wavelengths
and/or profiles. The multiple gratings 220,222 may be used individually in a
laiown
Fabry Perot arrangement
Further, one or more fiber lasers, such as that described in US Patent No.
5,666,372, "Compression-Tuned Fiber Iaser" may be
embedded within the fiber 10 in the tube 20 and compression-tuned. In that
case, the
grat.ings 220,222 form a cavity and the fiber 10 at least between the gratings
220,222
(and may also include the gratings 220,222, and/or the fiber 10 outside the
gratings, if
desired) would be doped with a rare earth dopant, e.g., erbium and/or
ytteibium, etc.,
and the lasing wavelength would be tuned accordingly as the force on the tube
20
changes.
Referring to Fig. 13, another type of tunable fiber law that may be used is a
tunable distributed feedback (DFB) fiber laser 234, such as that desen'bed in
V.C.
I.auridsen, et al, "Design of DFB Fibre Lasers", Electronic Letters, Oct. 15,
1998,
Vol.34, No. 21, pp 2028-2030; P. Vamting, et al, "Erbium Doped Fiber DGB
I.aser
With Permanent n/2 Phase-Shift Induced by W Post-Processing", IOOC'95, Tech.
Digest, Vol. 5, PD1-3, 1995; US Patent No. 5,771,251, "Optical Fibre
Distn'buted
Feedback I.aser", to Kringlebotn et al; or US Patent No. 5,511,083, "Polarized
F'ber
Laser Source", to D'Amato et al. In that case, the grating 12 is written in a
raraearth
doped fiber and configured to have a-phase shift ofN2 (where A, is the lasing
wavelength) at a predetermined location 224 near tbe center of the grating 12
which
provides a well defined resonance condition that may be continuously tuned in
single
l.ongitudinal mode operation without mode hopping, as is known. Altenoatively,
instead of a single griting, the two gratings 220,222 may be placed close
enough to
fonn a cavity having a length of (N + yZA,, where N is an integer (including
0) and the
gratings 220,222 are in rare-earth doped fiber.
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WO 00/37969 PCT/US99128999
Alternatively, the DFB laser 234 may be located on the fiber 10 between the
pair of gratings 220,222 (Fig. 8) where the fiber 10 is doped with a raro-
earth dopant
along at least a portion of the distance between the gratings 220,222. Such
configuration is referred to as an "interactive fiber laser", as is described
by J.J. Pan et
al, "Interactive Fiber Lasers with Low Noise and Controlled Output Power",
&tek
Dynamics, Inc., San Jose, CA, internet web site www.atek.com/products/
whitepapers. Other single or multiple fiber laser configurations may be
disposed on
the fiber 10 if desired.
Referring to Figs. 9 and 10, alternatively, two or more fibers 10,250, each
having at least one grating 12,252 therein, respectively, may be encased
within the
tube 20. The gratings 12,252 may have the same reflection wavelengths and/or
profiles or different wavelengths and/or profiles. In that case, thebore hole
in the
tube 20 prior to heating and fusing the tube 20 would be large enough to
contain both
fibers 10,250 and may be other than circular, e.g., square, triangle, etc.
Also, the bore
hole for the tube 20 need not be centered along the center line of the tube
20.
Referring to Fig. 11, alternatively, instead of the fibers 10,250 touching
each
other as shown in Fig. 10, the fibers 10,250 may be spaced apart in the tube
20 by a
predetermined distance. The distance may be any desired distance between the
fibers
10,250 and have any orientation within the outer diameter of the tube 20.
Also, for
any of the embodiments shown herein, as discussed hereinbefore, part or all of
an
optical fiber and/or grating may be fused within, partially within, or on the
outer
surface of the tube 20, as illustrated by fibers 500,502,504, respectively.
Referring to Fig. 12, alternatively, the tube 20 may be fused onto the fiber
10
only where the grating 12 is located. In that case, if the tube 20 is longer
than the
grating 12, the inner tapered or flared regions 22 discussed hereinbefore may
exist and
the areas 28 between the tube 20 and the fiber 10 may be filled with a filler
material,
as discussed hereinbefore. Also, the term "tube" as used herein may also mean
a block
of material having the properties described herein.
Further, for any of the embodiments shown herein, instead of the fiber 10
passing through the housing 50,70,90 or the tube 20, the fiber 10 may be
single,
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CA 02353504 2006-03-27
ended, i.e., only one end of the fiber 10 exits thehousing or the tube 20. In
that case,
one end of the fiber 10 would be at or prior to the exit point of the fiber 10
from the
tube 20 or the housing 50,70,90.
Refen-ing to Fig. 17, alteinatively, a portion of or all of the tub&encased
fiber
grating 20 may be replaced by a large diameter silica waveguide grating 600,
such as
that described in US Patent No. entitled "Large Diameter Optical
Waveguide, Grating and Laxer".
The waveguide 600 has a core 612 (equivalent to
the core of the fiber 10) and a cladding 614 (equivalent to the fused
combination of
the tube 20 and the cladding of the fiber 10) and having the grating 12
ambedded
therein. The overall length Ll of the waveguide 600 and the waveguide diameter
d2
are set the same as that descnbed hereinbefore for the tube 20 (i.e., such
that the tube
w01 not buckle over the desired grating wavelength tuning range) and the outer
diameter of the waveguide is at least 0.3 mm. An optical fiber 622 (equivalent
to the
15 fiber 10 in Fig. 1) having a cladding 626 and a core 625 which piopagates
the light
signa114, is spliced or otherwise optically coupled to one or both axial ends
628 of
the waveguide 600 using any known or yet to be developed teclmiques for
splicing
fibers or coupling light from an optical fiber into a larger waveguide, that
provides
acceptable optical losses for the application.
20 Tbe large diameter waveguide with gcating 600 may be used in the same vays
as the tube encased grating 20 is used herein where the fiber 10 is analogous
to (and
interchangeable with) the core 612 of the waveguide 600. For example, the
waveguide
600 may be etched, ground or polished to achieve the "dogbone" shape descdbed
hereinbefore with the tube 20. Altematively, the "dogbone" shape may be
obtained by
heating and fusing two outer tubes 640,642 onto opposite ends of the waveguide
600.
All otber alternative embodiments described herein for the tube 20 and the
tube-
encased grating are also applicable to the waveguide 600 where feasible,
including
having a fiber laser or a DFB fiber laser, multiple fibers (or cores), various
geometries, etc.
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. _ _....._.,r..--.-- .. .._ _
CA 02353504 2001-06-01
WO 00/37969 PCT/US99/28999
The tube-encased fiber grating 20 and the large diameter waveguide grating
600 may each also be referred to herein as a "tunable optical element". The
tub&
encased grating 20 and the large diameter waveguide grating 600 have
substantially
the same composition and properties in the locations where the tube 20 is
fused to the
fiber 10, because the end (or transverse) cross-section of the tub&encased
grating 20
and the large diameter waveguide grating 600 are contiguous (or monolithic)
and
made of substantially the same material across the cross-section, e.g., a
glass material,
such as doped and undoped silica. Also, in these locations both have an
optical core
and a large cladding.
Also, the waveguide 600 and the tube-encased grating 20 may be used
together to form any given embodiment of the sensing element described herein.
In
particular, one or more axial portion(s) of the sensing element may be a tube-
encased
grating or fiber and/or one or more other axial portion(s) may be the
waveguide 600
which are axially spliced or fused or otherwise mechanically and optically
coupled
together such that the core of said waveguide is aligned with the core of the
fiber
fused to the tube. For example, a central region of the sensing element may be
the
large waveguide and one or both axial ends may be the tube-encased fiber which
are
fused together as indicated by dashed lines 650,652, or visa versa (Figs.
1,11,31).
It should be understood that, unless stated otherwise herein, any of the
features, characteristics, alternatives or modifications described regarding a
particular
embodiment herein may also be applied, used, or incorporated with any other
embodiment described herein. Also, the drawings herein are not drawn to scale.
Although the invention has been described and illustrated with respect to
exemplary embodiments thereof, the foregoing and various other additions and
omissions may be made therein and thereto without departing from the spirit
and
scope of the present invention.
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