U.S. patent application number 10/003525 was filed with the patent office on 2003-06-12 for adjustable temperature compensating package for optical fiber devices.
Invention is credited to Albert, Jacques.
Application Number | 20030108286 10/003525 |
Document ID | / |
Family ID | 21706271 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030108286 |
Kind Code |
A1 |
Albert, Jacques |
June 12, 2003 |
Adjustable temperature compensating package for optical fiber
devices
Abstract
An assembly for providing thermal compensation to a fiber
optical device is described. The optical fiber device is secured in
a support structure made of a material having a negative
coefficient of thermal expansion. The securing means located at on
end of the support structure is made of a material having a
positive coefficient of thermal expansion. The securing means is
also adjustable lengthwise so as to provide longitudinal adjustment
to the tension on the optical fiber device. By selecting the
appropriate materials and dimensions the assembly can exactly
compensate for the thermal dependency of the optical device with an
overall length much smaller than assemblies based on materials with
dissimilar positive CTEs.
Inventors: |
Albert, Jacques; (Hull,
CA) |
Correspondence
Address: |
MARKS & CLERK
P.O. BOX 957
STATION B
OTTAWA
ON
K1P 5S7
CA
|
Family ID: |
21706271 |
Appl. No.: |
10/003525 |
Filed: |
December 6, 2001 |
Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G02B 6/0218
20130101 |
Class at
Publication: |
385/37 |
International
Class: |
G02B 006/00 |
Claims
1. A temperature compensating package for an optical fiber device
comprising: a support structure of a material having a negative
coefficient ot thermal expansion (CTE); and securing means located
in the support structure for securing opposed ends of an optical
fiber device passing therethrough, at least one of said securing
means being of a material having a positive CTE and adjustable
lengthwise.
2. A temperature compensating package as defined in claim 1 wherein
said at least one securing means has a threaded screw portion and a
complementary nut portion.
3. A temperature compensating package as defined in claim 1 wherein
said support structure is an alumino-silicate glass ceramic
tube.
4. A temperature compensating package as defined in claim 1 wherein
said at least one securing means is a metal with relatively high
CTE such as brass.
5. A temperature compensating package as defined in claim 1 wherein
said at least one securing means is a metal with relatively high
CTE such as aluminum.
6. A temperature compensating package as defined in claim 1 having
securing means that are adjustable lengthwise at each end of said
support structure.
7. A temperature compensating package as defined in claim 6 wherein
said securing means have different positive CTE values.
8. A method of providing thermal compensation to an optical fiber
device comprising: providing a support structure for said optical
fiber device, said support structure being of a material having a
negative coefficient of thermal expansion and having securing means
at each end, at least one of said securing means being of a
material having a positive coefficient of thermal expansion and
adjustable lengthwise; adjusting said at least one securing means
to establish a base thermal compensation value; and securing said
optical fiber device to said securing means within said support
structure;
9. The method as defined in claim 8 wherein said at least one
securing means has a threaded portion and a complementary nut
portion whereby rotation of said nut relative to said threaded
portion adjusts tension on said optical fiber device.
10. The method as defined in claim 8 wherein said optical fiber
device is a fiber grating.
11. The method as defined in claim 10 wherein said support
structure has a negative coefficient of thermal expansion which is
in excess of that which is required to compensate for thermal
dependence of said optical device.
12. A fiber optical device assembly with associated thermal
compensation comprising: an optical fiber having a fiber optical
device therein; a support structure of a material having a negative
coefficient of thermal expansion through which said optical fiber
extends; securing means in each end of said support structure, said
securing means securing said optical fiber on opposite ends of said
optical fiber device, at least one of said securing means being of
a material having a positive coefficient of thermal expansion and
being adjustable longitudinally of said optical fiber.
13. The fiber optical device assembly according to claim 12 wherein
said at least one securing means comprises a threaded portion and a
complementary nut portion wherein rotation of said nut on said
threaded portion adjusts tension on said optical fiber device.
14. The fiber optical device assembly according to claim 13 wherein
said support structure is an alumino-silicate glass ceramic
tube.
15. The fiber optical device assembly according to claim 14 wherein
said at least one securing means is made of a material with a
relatively high CTE such as brass.
16. The fiber optical device assembly according to claim 14 wherein
said at least one securing means is made of a material with a
relatively high CTE such as aluminum.
17. The optical device assembly according to claim 12 wherein said
optical fiber device is a fiber grating
18. The optical device according to claim 15 wherein said at least
one closure means is used to fine tune thermal compensation of said
optical device.
Description
FIELD OF THE INVENTION
[0001] This invention relates to fiber optic devices such as fiber
gratings and more particularly to a package and system to
compensate for the thermal dependence of such devices.
BACKGROUND
[0002] The Bragg effect is employed in optical communications
systems for, amongst other things, wavelength selective filtering.
In this implementation the filter is used in add/drop wavelength
applications and in multiplexing and demultiplexing functions.
Bragg filters are also used in Mach-Zehnder interferometer
applications for various optical communication related
functions.
[0003] A grating is a series of perturbations in an optical wave
guide precisely positioned according to a desired wavelength
effect. It is known that such gratings are thermally dependent
wherein the spacing between perturbations and the refractive index
of the waveguide materials actually increase with increasing
temperature. This temperature dependence, if not compensated for,
will change the effective central wavelength of the grating as a
function of operating temperature.
PRIOR ART
[0004] There are known methods of incorporating thermal
compensation strategies into fiber optical devices. The first such
method to be described here involves a package consisting of a
holding tube and a pair of threaded, smaller tubes designed to fit
within the holding tube. The holding tube is made of a material
that has a different coefficient of thermal expansion (CTE) than
that of the threaded tubes. The grating is fixed to the smaller
tubes in such a way that it is strained by an amount designed to
compensate for its temperature dependence when the temperature
changes. Strain arises because of the different coefficients of
thermal expansion of the two kinds of tubes. U.S. Pat. No.
5,914,972 which issued Jun. 22, 1999 to Siala et al. describes one
such package. U.S. Pat. No. 5,042,898 which issued Aug. 27, 1991 to
Morey et al. describes a similar arrangement and includes
discussion regarding the thermal dependence of a grating.
[0005] A second solution consists of fixing the grating, whether it
be a fiber Bragg grating or a Mach-Zehnder interferometer, to a
substrate and then gluing the substrate to a bi-metal plate. The
bi-metal plate is composed of two materials, each with a different
coefficient of thermal expansion, sandwiched in such a way that
when the temperature changes the bi-metal plate bends. The bending
of the bi-metal plate induces a strain on the substrate affixed to
it which is proportional to the length of the bi-metal plate. It is
this strain which compensates for the temperature dependence of the
grating. U.S. Pat. No. 5,978,539 which issued Nov. 2, 1999 to Davis
et al. describes a variant of this concept.
[0006] A third approach consists of fixing the fiber Bragg grating
or Mach-Zehnder interferometer to a special substrate that has a
negative coefficient of thermal expansion of exactly the correct
value so that it shrinks by just the right amount to compensate for
the thermal variation of the spectral response of the device. U.S.
Pat. No. 5,926,599 which issued Jul. 20, 1999 to Bookbinder et al.
gives one example of this approach.
[0007] There is a further approach described in International
application WO 00/54082 published Sep. 14, 2000 to Maaskant et al.
that describes a shaped substrate that is designed to bend in a
controlled fashion in response to temperature variations. The fiber
device is attached to the substrate in such a way that the bending
action changes the amount of tension on the fiber device in
response to temperature changes.
[0008] Another compensation technique is described in Applicant's
co-pending U.S. application filed Nov. 1, 2001 (Attorney Docket No.
11864-US) the contents of which are incorporated herein by
reference. The compensation technique of the earlier application is
based on a modification of the aforementioned bi-metal approach
whereby the use of glue to hold the substrate to the bi-metal strip
is rendered unnecessary. According to this prior design a bi-metal
element comprising two components is used. Instead of gluing a
bi-metal plate to the fiber device substrate, the substrate itself
is used as the first component of the bi-metal element and is
shaped in such a way that the second component of the bi-metal
element forces it to curve by pushing against it when subject to a
temperature increase. The curvature of the first component of the
bi-metal element changes the strain state of the fiber attached to
it. The main component of the force acting to curve the fiber
device is therefore held mechanically instead of relying on the
sheer strength of a glue.
[0009] In the aforementioned solution the combination of materials
with different coefficients of thermal expansion effectively attain
a net negative CTE of a generally proper size. The net CTE obtained
is a function of the CTE of the individual materials and of their
sizes. Therefore it is adjustable. As indicated previously an
alternative approach consists of fixing the fiber device to a
material with a negative CTE.
[0010] The solution based on materials with different CTE works
well but the package needs to be either significantly longer than
the fiber device it is designed to compensate or involves the
bending of interconnected parts, which is often difficult to
implement. Access to the fixing points (glues or solder) is also
difficult. Solutions based solely on negative CTE materials solve
these problems but introduce an additional one in the sense that
the effective CTE is an inherent property of the materials and thus
it is impossible to incorporate a fine adjustment for different
optical fiber devices. Furthermore, the negative CTE of such
materials tends to vary from batch to batch.
SUMMARY OF THE INVENTION
[0011] The present invention consists of using a material with a
CTE that is excessively negative and to combine it with a positive
material with adjustable length in such a way as to provide an
adjustable, negative CTE. In this way, the length of the package
need not be much longer than the device to be compensated and most
of the advantages associated with using negative CTE materials are
preserved while avoiding their main disadvantage (lack of
adjustability).
[0012] Therefore, in accordance with a first aspect of the present
invention there is provided a temperature compensating package for
an optical fiber device comprising: a support structure of material
having a negative CTE; and securing means located in the support
structure for securing opposed ends of an optical fiber device, at
least one of the securing means being of a material having a
positive CTE and adjustable lengthwise.
[0013] In accordance with a second aspect of the present invention
there is provided a method of providing thermal compensation to an
optical fiber device comprising: providing a support structure for
the optical fiber device, the support structure being of a material
having a negative coefficient of thermal expansion and having
securing means at each end, at least one of the securing means
being of a material having a positive coefficient of thermal
expansion and adjustable lengthwise; adjusting the at least one
securing means to establish a base thermal compensation value; and
securing the optical fiber device to the securing means within the
support structure.
[0014] In accordance with a third aspect of the present invention
there is provided a fiber optical device assembly with associated
thermal compensation comprising: an optical fiber having a fiber
optical device therein; a support structure of a material having a
negative coefficient of thermal expansion through which the optical
fiber extends; securing means in each end of the support structure
and securing the optical fiber on opposite ends of the optical
fiber device, at least one of the securing means being of a
material having a positive coefficient of thermal expansion and
being adjustable longitudinallly of the optical fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention will now be described in greater detail with
reference to the attached drawings; wherein
[0016] FIG. 1 shows the compensating package according to one
aspect of the present invention; and
[0017] FIG. 2 shows a second embodiment of the compensating
package.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The solution, according to the present invention, is
illustrated in FIG. 1. In this package the support structure 20 is
of a material having a negative coefficient of thermal expansion. A
material such as alumino-silicate glass ceramic may be used, and in
a preferred embodiment the support structure is a alumino-silicate
glass ceramic tube. The actual shape of the support structure is
not critical as long as it has a cross section which is large
enough to accommodate at least adjustment means. In FIG. 2 the
adjustment means 28 is a screw 22 made of a material having a
positive CTE. Brass is one possible material for the adjustment
means 28.
[0019] In operation an optical fiber device 24 extends through the
supporting structure and is secured at one end by securing means 26
which is attached to one end of the alumino-silicate glass ceramic
tube. At the opposite end of the tube adjustment means 28 is also
used as a securing means. The fiber 24 is held to securing means 26
by suitable means such as an adhesive or epoxy.
[0020] Adjustment and securing means 28 comprises screw portion 22
with a complimentary nut 30 which threadingly engages screw 22.
Fiber 24 is secured to screw 22 at end 32 again using an
appropriate adhesive.
[0021] It will be apparent to one skilled in the art that by
rotating nut 30 relative to screw 22 the nut facing on the end of
tube 20 will alter the effective length of the adjustment and
securing means 28 and thereby adjust the tension on fiber 24.
[0022] The tube of FIG. 1 has a negative CTE of value -A and a
length LA which signifies the length of the package. The adjustment
and securing means 28 has a positive CTE of value B. As shown in
FIG. 1 a length of screw 22 protrudes from a tube by a length LB.
The net CTE of the package of FIG. 1 is given by:
[0023] net CTE=(B.times.LB-A.times.LA)/LB+LA.
[0024] As an example, using a known ceramic for tube 20 and a brass
screw 28, then A=-80.times.10.sup.-7/.degree. C. and
B=19.times.10.sup.-7/.degr- ee. C. In this example the known
ceramic could be one of two which are commercially available: NEX-1
ceramic from Ohara (Japan), or CERSAT N-80 from Nippon Electric
Glass Japan). CERSAT N-80 has the CTE listed as A above. Setting
LA=20 mm, then, in order to get a net CTE of
-75.times.10.sup.-7/.degree. C., LB must be equal to 1.06 mm. This
yields a total package length of about 21 mm. If the required CTE
is -72.times.10.sup.-7/.degree. C. then LB would be adjusted to
1.76 mm.
[0025] It will be apparent to one skilled in the art that this
provides for fine tuning of the CTE. Furthermore, the sensitivity
of the fine tuning based on the protruding length of the screw can
be adjusted up or down by using a screw material of larger or
smaller positive CTE value. Further, the presence of the screw
allows for the fine tuning of the central wavelength of the fiber
Bragg grating by adjusting the tension slightly but not so much as
to change the CTE significantly. Obviously, some pretensioning of
the fiber to a value close to the desired value of the given
temperature is applied since the wavelength tuning requires
changing the length LB slightly.
[0026] As is known the actual CTE of negative CTE materials
available varies from batch to batch by a few parts in 10.sup.-7
and the necessary negative CTE needed for a variety of fiber
devices also varies by the same amount. For instance, material with
a nominal CTE of -70.times.10.sup.-7/.degree. C. was found to under
compensate the thermal drift of the Bragg wavelength of FBGs by +1
pm/.degree. C. while a similar material with a CTE of
-80.times.10.sup.-7/.degree. C. over compensates the same FBG by
-2.3 pm/.degree. C. Finding the right material for the fiber device
type proves to be impossible. The system described herein provides
adjustability of the negative CTE in a range which extends from the
base value of the tube material (support structure) to positive
values limited only by the length of the whole device. In general,
a very short length of protruding screw material will be needed to
achieve most design goals. For instance, for a tube having a CTE of
-80.times.10.sup.-7/.degree. C. and a grating length of 19 mm the
extra length of brass screw needed to achieve a CTE of
-75.times.10.sup.-7/.degree. C. is 1 mm. By comparison, a package
based on the two positive materials as glass and brass needs to be
43 mm long.
[0027] It will also be apparent to one skilled in the art that for
certain applications it is possible to have securing means with
length adjustment at both ends of the support structure 20 (see
FIG. 2). Further, it is within the scope of the present invention
to use adjustment screws at opposite ends of the support having
different CTE values if necessary in order to obtain an particular
net CTE. Although brass is identified in the foregoing as a
material with a positive CTE and alumino-silicate glass ceramic is
identified as a material with a negative CTE it will be apparent to
one skilled in the art that other materials of each type can be
used. For example, aluminum could be used instead of brass.
[0028] While particular embodiments of the invention have been
described and illustrated it will be apparent to one skilled in the
art that numerous changes can be made to the basic concept. It is
to be understood that such changes will fall within the full scope
of the invention as defined by the appended claims.
* * * * *