Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-01-04
2004-08-24
Gutierrez, Diego (Department: 2859)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227160, C356S032000
Reexamination Certificate
active
06781113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a strain sensor using a fiber Bragg grating to sense tension or compression.
2. Description of the Related Art
A fiber Bragg grating (FBG) is a type of Bragg diffraction element comprising an optical fiber with a core having a periodically varying refractive index, so that regions having a high refractive index N
H
alternate with regions having a low refractive index N
L
along the fiber axis. Light propagating through the fiber is reflected back if its wavelength is equal, or approximately equal, to the Bragg wavelength &lgr;
B
which is expressed as follows in terms of the effective refractive index n
e
of the core and the grating pitch d (the distance between successive regions with the same refractive index).
&lgr;
B
=2
n
e
d
(1)
An FBG may have a uniform grating pitch, or it may be chirped. In a chirped FBG, the grating pitch d varies, either continuously or in stages, along the length of the fiber. The transmission and reflection characteristics of an FBG depend on the presence or absence of chirp and on such grating parameters as the grating count (the number of alternating regions) and the degree of modulation of the refractive index (the difference &Dgr;n between N
H
and N
L
). For example, the maximum reflectivity of an FBG increases with &Dgr;n; for a given grating pitch d, the maximum reflectivity increases with the grating count; for a given grating count, the width of the reflection band increases with increasing chirp.
If an FBG is subjected to variations in temperature or strain, the effective refractive index n
e
and the grating pitch d in equation (1) change, altering the Bragg wavelength. An FBG can therefore be used as a strain sensor or a temperature sensor, by detecting the Bragg wavelength.
Strain sensors are useful in what is termed smart structure technology, in which sensors are built into buildings, bridges and other structures to sense changes in strain over time at various points. Since an FBG is sensitive to both strain and temperature, for use as a strain sensor, it must be temperature-compensated. One general method of temperature compensation that is being studied employs chirp to create a temperature-independent reflection band.
Japanese Unexamined Patent Publication No. 2000-97786 discloses several strain sensors employing this general type of temperature compensation. One of these conventional strain sensors will be described in detail here for comparison with an embodiment of the present invention to be described later. The conventional strain sensor described here, shown in a top plan view in FIG.
1
and in a side view in
FIG. 2
, has an optical fiber
10
with an FBG
12
attached by an adhesive, for example, to a tension member
44
. The tension member
44
has the general form of a rectangular plate with a tapered section
44
a
at or near the center where the FBG
12
is located. The axis &agr; of the optical fiber
10
extends longitudinally through the tapered section
44
a
. If longitudinal tension stress is applied to the tension member
44
, then the tapered section
44
a
elongates by an amount that increases with decreasing width of the taper. The FBG
12
elongates in a similar manner. As a result, the grating pitch of the FBG
12
increases toward the narrow end of the tapered section
44
a
, changing the FBG
12
from a uniform grating to a chirped grating.
FIG. 3
shows how the reflection band of the FBG
12
changes in response to strain. The horizontal axis indicates wavelength; the vertical axis indicates the relative optical power of the reflected light. Reflection spectrum
62
, which has a reflection band
64
, is observed before a certain tension force is applied; reflection spectrum
66
, which has a wider reflection band
68
, is observed after the tension force is applied. The amount of strain caused by the tension can be determined from the width of the band from &lgr;
min
to &lgr;
max
in which the reflected optical power is equal to or greater than a certain quantity. The change in this bandwidth is independent of temperature, so measurement of this bandwidth, or of the change therein, provides a way to measure strain without interference from temperature effects.
The strain &egr;
max
in the narrowest part of the tapered section
44
a
(the maximum strain), the strain &egr;
min
in the widest part of the tapered section
44
a
(the minimum strain), the tension force F, the minimum cross-sectional area A
S
of the tapered section
44
a
, the maximum cross-sectional area A
L
of the tapered section
44
a
, and Young's modulus E are related by the following equations (2) and (3).
&egr;
max
=F
/(
E·A
S
) (2)
&egr;
min
=F
/(
E·A
L
) (3)
FIG. 4
plots the changes in &egr;
max
and &egr;
min
, shown on the vertical axis, as functions of the applied tension force F, shown on the horizontal axis. As the force F increases from F
1
to F
2
, the maximum strain &egr;
max
and minimum strain &egr;
min
both increase proportionally. As implied by equations (2) and (3), however, the slope of the &egr;
max
characteristic
52
is greater than the slope of the &egr;
min
characteristic
54
.
The grating pitch d
min
in the widest part of the tapered section
44
a
(the minimum grating pitch) and the grating pitch d
max
in the narrowest part of the tapered section
44
a
(the maximum grating pitch) are related to the grating pitch d
0
when there is no strain by the following equations (4) and (5).
d
max
=(1+&egr;
max
)
d
0
(4)
d
min
=(1+&egr;
min
)
d
0
(5)
FIG. 5
plots the grating pitch d, shown on the vertical axis, as a function of longitudinal coordinates on the tension plate
44
, shown on the horizontal axis. The solid curve
56
indicates the grating pitch d when a comparatively large tension force (e.g., F
2
) is applied; the dash-dot curve
58
indicates the grating pitch d when a smaller tension force (e.g., F
1
) is applied. The coordinates x
1
and x
2
in
FIG. 5
correspond to the positions of the two ends of the FBG
12
in the optical fiber
10
. The tapered section
44
a
of the tension plate
44
is widest at position x
1
, where the minimum grating pitch d
min
occurs, and narrowest at position x
2
, where the maximum grating pitch d
max
occurs.
When a tension force F is applied, the resulting elongation of the FBG
12
varies continuously from one end x
1
and to another end x
2
of the FBG
12
, increasing from the widest end to the narrowest end of the tapered section
44
a
. The grating pitch d therefore varies continuously, as shown by curve
56
in FIG.
5
. As the tension force F increases from F
1
to F
2
in
FIG. 4
, the maximum strain &egr;
max
and minimum strain &egr;
min
in the tapered section
44
a
both increase proportionally, and the maximum grating pitch d
max
and minimum grating pitch d
min
increase according to equations (4) and (5), causing the upward shift from curve
58
to curve
56
in FIG.
5
. The difference between the maximum grating pitch d
max
and the minimum grating pitch d
min
determines the total chirp, and also determines the rate of change in the grating pitch d in the longitudinal direction.
The change &Dgr;&lgr; in the reflection bandwidth can be understood in terms of the Bragg wavelength &lgr;
max
at the end of the FBG
12
with maximum strain and the Bragg wavelength &lgr;
min
in at the end of the FBG
12
with minimum strain. From the formula for the Bragg wavelength, these Bragg wavelengths are given by the following equations (6) and (7).
&lgr;
max
=2
n
e
·(1+&egr;
max
)
d
0
(6)
&lgr;
min
=2
n
e
·(1+&egr;
min
)
d
0
(7)
Since characteristic
52
in
FIG. 4
has a greater slope than characteristic
54
, when tension force is applied, the Bragg wavelength &lgr;
max
at the end of the FBG
12
with maximum strain, corresponding to the maximum grating pitch d
max
, increases more than the Bragg wavelength &l
Courson Tania C.
Gutierrez Diego
Oki Electric Industry Co. Ltd.
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