Information
-
Patent Grant
-
6175674
-
Patent Number
6,175,674
-
Date Filed
Monday, March 8, 199925 years ago
-
Date Issued
Tuesday, January 16, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Bednarek; Michael D.
- Pittman; Shaw
-
CPC
-
US Classifications
Field of Search
US
- 385 37
- 385 12
- 385 6
- 385 13
- 385 27
- 385 28
- 385 147
- 250 22717
- 356 345
- 372 6
- 372 32
-
International Classifications
-
Abstract
An adjustable compensation device for fiber Bragg gratings. The device includes a bimetal structure and a fixture. An optical fiber provided with fiber Bragg gratings is connected to the bimetal structure. The fixture firmly holds the bimetal structure so that the bimetal structure behaves like cantilever beams to compensate pitches of the fiber Bragg gratings. Furthermore, a shim can be inserted between the bimetal structure and the fixture so as to incline the bimetal structure. The compensation for the change of the length of the optical fiber can be adjusted by changing the fixture and/or shim.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an adjus compensation device for fiber Bragg gratings.
2. Description of the Related Art
Fiber Bragg gratings (FBGs) are very important elements widely used in the fabrication of various functional devices for dense WDM networks, for example FBG stabilized laser sources and various FBG-based WDM devices for multiplexers, demultiplexers and add/drop filters. In the practical application of FBGs, however, a problem arising from changes in the surrounding temperature has been noticed. Because the pitch of the FBGs determines the central frequency of the reflected optical signal transmitted in an optical fiber, the FBGs are carefully designed and accurately manufactured. The problem is that the optical fibers elongate in a raised surrounding temperature so that the pitches of the FBGs deviate from the design value. Such a situation is not desirable.
FIG. 1A
shows a temperature compensation device for fiber Bragg gratings using a bi-metal structure which includes two arms
13
,
13
′ and two metal plates
14
,
15
. The two metal plates
14
,
15
are welded together while the two arms
13
,
13
′ are welded at the sides of the metal plates
14
,
15
. The thermal expansion coefficient of the metal plate
14
is smaller than that of the metal plate
15
.
In operation, an optical fiber
11
provided with FBGs
12
are glued onto the arms
13
,
13
′. The optical fiber
11
tends to elongate in a raised surrounding temperature. However, the temperature compensation device bends, as shown in
FIG. 1B
, due to different thermal expansion coefficients between the two metal plates
14
,
15
, so as to prevent the optical fiber
11
from elongating. By this arrangement, the change in the pitches of the FBGs
12
arising from the raised temperature can be greatly reduced.
The above-mentioned temperature compensation device for fiber Bragg gratings does lessen the influence of temperature change on the FBGs. However, the compensation in such a way is not very accurate because of the tolerances arising from manufacturing and packaging (In operations, the temperature compensation device is packaged by a box with the ends of the optical fiber fixed to the box. This can prevent an external force, such as a careless pull at the optical fiber, from destroying the temperature compensation device). For example, the tolerances may arise from over-stretching the optical fiber when: (1) gluing the optical fiber onto the bi-metal structure; (2) attaching the ends of the optical fiber to the box for packaging. It is understood that tolerances are not considered in design. However, tolerances do influence the control over the changes of the length of the optical fiber so that the compensation of the change of pitches of the FBGs is not accurate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an adjustable compensation device for fiber Bragg gratings that solves the above-mentioned problem.
In accordance with the object of the present invention, an adjustable compensation device for fiber Bragg gratings is provided. The device includes a bimetal structure and a fixture. An optical fiber provided with fiber Bragg gratings is connected to the bimetal structure. The fixture firmly holds the bimetal structure so that the bimetal structure behaves like cantilever beams to compensate pitches of the fiber Bragg gratings. Furthermore, a shim can be inserted between the bimetal structure and the fixture so as to incline the bimetal structure. The compensation for the change of the length of the optical fiber can be adjusted by changing the fixture and/or shim.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1A
shows a conventional temperature compensation device for fiber Bragg gratings using bimetal structure;
FIG. 1B
shows the temperature compensation device of
FIG. 1A
bent in a raised temperature;
FIG. 2A
is an exploded perspective diagram of an adjustable compensation device for fiber Bragg gratings in accordance with an example of the present invention;
FIGS.
2
B-
2
D show the steps of assembling the adjustable compensation device for fiber Bragg gratings in
FIG. 2A
;
FIG. 3
shows the bimetal structure of the adjustable compensation device in
FIG. 2A
in a raised temperature;
FIG. 4
shows the adjustable compensation device for fiber Bragg gratings in accordance with
FIG. 3
, used for geometrically analyzing the bent bimetal structure in a raised temperature;
FIG. 5
shows the relationship between the horizontal displacement “X” of the end of the arm and the length “L” of the flexible portion of the metal plate in accordance with the present invention;
FIG. 6
is a plan view of either of the fixing members of the adjustable compensation device in accordance with the present invention;
FIG. 7
shows the adjustable compensation device for fiber Bragg gratings in accordance with another example of the present invention, used to compensate the tolerance of pitches of FBGs arising from manufacturing and packaging; and
FIG. 8
shows the relationship between the horizontal displacement “X” of the end of the arm and the thickness “h” of the shim in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to
FIG. 2A
, an adjustable compensation device for fiber Bragg gratings in accordance with the present invention including a bimetal structure
22
, a top fixing member
23
and a bottom fixing member
24
.
The bimetal structure
22
includes two arms
223
,
223
′ and two metal plates
221
,
222
, wherein the thermal expansion coefficient of the metal plate
221
is smaller than that of the metal plate
222
. The two metal plates
221
,
222
are welded together while the two arms
223
,
223
′ are welded at the sides of the metal plates
221
,
222
. An optical fiber
11
provided with FBGs
12
is glued onto the arms
223
,
223
′.
The two fixing members
23
,
24
are I-shaped and used for clamping the bimetal structure
22
. On the bottom of the top fixing member
23
is formed a recess
231
for receiving the metal plates
221
,
222
while clamping, as shown in FIG.
2
B. Then, the two fixing members
23
,
24
are firmly screwed together.
The adjustable compensation device (i.e. the assembly of the bimetal structure
22
and fixing members
23
,
24
) is then packaged by a top cover
25
, a base
26
and a side cover
27
. On the base
26
are provided a cavity
261
and two grooves
262
. The adjustable compensation device is put into the cavity
261
of the base
26
, as shown in
FIG. 2C
, with the optical fiber
11
received in the grooves
262
. Glue is applied to the grooves
262
so as to fix the optical fiber. This can prevent an external force, such as a pull at the optical fiber, from destroying the adjustable compensation device. Then, the top cover
25
and the side cover
27
are screwed onto the base
26
as shown in FIG.
2
D.
How the present invention solves the problem arising from a temperature change is now discussed. The bimetal structure
22
bends in a raised surrounding temperature, as shown in
FIG. 3
, due to different thermal expansion coefficients between the two metal plates
221
,
222
. In the present invention, however, it is intended to control the horizontal displacement of the arm
13
or
13
′ by way of fastening the metal plates
221
,
222
with the fixing members
23
,
24
:
The bimetal structure
22
fixed by the top fixing member
23
and bottom fixing member
24
behaves like a cantilever beam. The general formula for the bending of the cantilever beam due to the change of temperature is as follows:
where
R
t
is the radius of curvature of the metal plates at temperature T
t
;
R
o
is the radius of curvature of the metal plates at temperature T
o
;
m=S
1
/S
2
, where S
1
is the thickness of the metal plate
221
and S
2
is the thickness of the metal plate
222
;
n=E
1
/E
2
, where E
1
is the modulus of elasticity of the metal plate
221
and E
2
is the modulus of elasticity of the metal plate
222
;
α
1
is the coefficient of thermal expansion of the metal plate
221
;
α
2
is the coefficient of thermal expansion of the metal plate
222
; and
S=S
1
+S
2
.
If the metal plates are flat at temperature T
o
, then R
o
=∞ and 1/R=0.
Assuming the thicknesses of the two metal plates
221
,
222
are the same. That is, S
1
=S
2
=S/2. Now referring to
FIG. 4
, from the right angle EFG, we obtain:
where
A is the deflection of the cantilever beam; and
L is the length of the cantilever beam.
Equation (2) can be rewritten as:
while subtracting equation (3) from equation (1), we obtain:
Furthermore, in
FIG. 4
, reference notation “a” is the inclined angle of the cantilever beam, reference notation “L” is the length of the arm
223
or
223
′, and reference notation “x” is the horizontal displacement of the end of the arm
223
or
223
′.
Then, we obtain:
because tan a=A/L
In equation (4), α
1
, α
2
, m, n, T
t
, T
o
and S are known. Thus, the deflection “A” of the cantilever beam depends on “L”. In equation (5), “L” is known. The horizontal displacement “x” of the end of the arm
223
or
223
′, on which the optical fiber is glued, depends on “L” and “A”. Therefore, the horizontal displacement “x” can be totally determined by “L” according to equations (4) and (5).
FIG. 5
shows the relationship between “X” and “L” according to equations (4) and (5), wherein m=l, n=1.287, L′=6.5(mm), T
t
−T
o
=55(° C.), and α
2
−α
1
=8.4×10
−6
(1/° C.).
Referring to
FIG. 6
, in the practical application of the present invention, fixing members
23
,
24
of various widths “W” are prepared in advance. Then, “L” is determined by clamping the metal plates
221
,
222
with the fixing members
23
,
24
of a proper width “W”. The compensation for the change of the length of the optical fiber due to the raised temperature can be adjusted by changing the fixing members
23
,
24
so as to generate different “L”. By this arrangement, the change of the length of the optical fiber due to the raised temperature is accurately controlled based on “L”, and therefore the change of pitches of FBGs in a raised temperature can be very well compensated.
Referring to
FIG. 6
, fixingmembers
23
,
24
of various widths “W” are produced in advance so that selecting and replacing the fixing members
23
,
24
to generate proper “L” is convenient.
The compensation device of the present invention can also be used to compensate the tolerance of pitches of FBGs arising from manufacturing and packaging. Referring to
FIG. 7
, a shim
30
of thickness “h” is inserted between the bottom fixing member
24
and the metal plate
30
so as to incline the cantilever beam.
The bottom fixing member
24
is wider than the top fixing member
23
by L″. Assuming the inclined angle of the cantilever beam “a” is very small, we obtain:
In equation (6), L′ and L″ are known. Thus, the horizontal displacement “x” of the end of the arm
223
or
223
″ is determined by the controllable parameters h.
FIG. 8
shows the relationship between “x” and “h”, wherein L′=6.5(mm) and L″=1(mm). In other words, we can generate a proper horizontal displacement “x” to compensate the tolerance of pitches of FBGs arising from manufacturing by using the shim
30
of thickness “h” according to equation (6). In the present invention, the tolerance of pitches of FBGs arising from manufacturing and packaging can also be compensated very well by using the shim
30
. That is, we can generate a proper horizontal displacement “x” to compensate the tolerance of pitches of FBGs arising from manufacturing and packaging by using the shim
30
of thickness “h” according to equation (6).
While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Claims
- 1. An adjustable compensation device for fiber Bragg gratings, comprising:a bimetal structure to which an optical fiber provided with the fiber Bragg gratings is connected; and a fixture for firmly holding the bimetal structure so that the bimetal structure behaves like cantilever beams to compensate pitches of the fiber Bragg gratings.
- 2. An adjustable compensation device as claimed in claim 1, wherein the fixture includes a first fixing member and a second fixing member holding the bimetal structure in opposite directions.
- 3. An adjustable compensation device as claimed in claim 2, wherein the first fixing member has a recess receiving the bimetal structure.
- 4. An adjustable compensation device as claimed in claim 2, wherein the second fixing member is larger than the first fixing member and the adjustable compensation device further comprises a shim inserted between the bimetal structure and the second fixing member so as to incline the bimetal structure.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
5841920 |
Lemaire et al. |
Nov 1998 |
|
6044189 |
Miller |
Mar 2000 |
|