U.S. patent application number 11/469759 was filed with the patent office on 2006-12-28 for optical fiber sensors based on pressure-induced temporal periodic variations in refractive index.
This patent application is currently assigned to VIRGINIA TECH INTELLECTUAL PROPERTIES, INC.. Invention is credited to Anbo WANG.
Application Number | 20060291768 11/469759 |
Document ID | / |
Family ID | 29406829 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060291768 |
Kind Code |
A1 |
WANG; Anbo |
December 28, 2006 |
OPTICAL FIBER SENSORS BASED ON PRESSURE-INDUCED TEMPORAL PERIODIC
VARIATIONS IN REFRACTIVE INDEX
Abstract
A optical fiber sensor for measuring temperature and/or pressure
employs temporally created long period gratings. The gratings may
be produced by a periodic change in the refractive index of the
fiber along the fiber longitudinal axis caused by periodically
spaced compressive and/or expansive forces or by spaced-apart
unbalanced forces that cause periodic fiber micro-bending. Pressure
and temperature are determined by measuring changes in both the
wavelength at which light is coupled from a mode guided by a core
to a different mode and an amount of such coupling. The gratings
are created intrinsically and extrinsically. Single and multiple
core fibers are used.
Inventors: |
WANG; Anbo; (Blacksburg,
VA) |
Correspondence
Address: |
DLA PIPER US LLP;ATTN: PATENT GROUP
1200 NINETEENTH STREET, NW
WASHINGTON
DC
20036
US
|
Assignee: |
VIRGINIA TECH INTELLECTUAL
PROPERTIES, INC.
Blacksburg
VA
|
Family ID: |
29406829 |
Appl. No.: |
11/469759 |
Filed: |
September 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10431456 |
May 8, 2003 |
|
|
|
11469759 |
Sep 1, 2006 |
|
|
|
60378351 |
May 8, 2002 |
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Current U.S.
Class: |
385/13 ;
385/37 |
Current CPC
Class: |
G01L 1/246 20130101;
G01D 5/35377 20130101; G02B 6/02071 20130101; G01D 5/35345
20130101 |
Class at
Publication: |
385/013 ;
385/037 |
International
Class: |
G02B 6/00 20060101
G02B006/00 |
Claims
1. A optical fiber sensor comprising: an optical fiber, the optical
fiber having a first core and a cladding; a light source connected
to an end of the optical fiber; a tube surrounding a portion of the
optical fiber, the tube having an interior surface and an exterior
surface, the interior surface of the tube being bonded to the
optical fiber at a plurality of spaced-apart bonding locations,
wherein a long period grating is produced in a portion of the first
core surrounded by the tube when a pressure is applied to the
exterior surface of the tube; and a processor connected to the
optical fiber, the processor being configured to determine an
amount of attenuation of the light source by the long period
grating.
2. The optical fiber sensor of claim 1, wherein an entire
circumference of the optical fiber is bonded to the tube at each
bonding location.
3. The optical fiber sensor of claim 2, wherein the circumference
is perpendicular to the core such that opposing balanced forces are
exerted on a core of the fiber at each bonding location, the
balanced forces creating changes in the refractive index of the
core at each bonding location.
4. The optical fiber sensor of claim 1, wherein the bonding
locations are periodically spaced.
5. The optical fiber sensor of claim 1, wherein the bonding
locations are chirped.
6. The optical fiber sensor of claim 1, wherein light guided by the
first core is coupled to a non-guided mode in the cladding by the
long period grating.
7. The optical fiber sensor of claim 1, wherein the optical fiber
further comprises a second core and wherein light in the first core
is coupled to a guided mode in the second core.
8. The optical fiber sensor of claim 7, wherein the first and
second cores are concentric.
9. An optical fiber sensor comprising: a first core; and a cladding
surrounding the first core, the cladding having an exterior
surface, the exterior surface having a portion including a
plurality of spaced-apart circumferential grooves formed
therein.
10. The optical fiber sensor of claim 9, wherein each of the
grooves has a direction perpendicular to the core, whereby a change
in an index of refraction of the first core is created in areas of
the core corresponding to areas of the cladding between the grooves
when the portion is exposed to a pressurized fluid.
11. The optical fiber sensor of claim 9, wherein each of the
grooves has a direction that forms an acute angle with a direction
of the first core, whereby a series of microbends is formed in the
first core when the portion is exposed to a pressurized fluid.
12. The optical fiber sensor of claim 11, wherein the groove has a
first wall, a second wall and a bottom surface and the first wall
is perpendicular to the bottom surface.
13. The optical fiber sensor of claim 9, further comprising a
second core, the second core being positioned such that light
guided by the first core is coupled to the second core when the
portion is exposed to the pressurized fluid.
14. The optical fiber sensor of claim 13, wherein the second core
is surrounded by the cladding.
15. The optical fiber sensor of claim 13, further comprising a
second cladding surrounding the second core.
16. An optical fiber sensor comprising: an optical fiber, the fiber
having at least a first core and a first cladding, the first
cladding; a first plate, the first plate having a first plurality
of ridges formed thereon and pressed against the optical fiber; and
a second plate, the second plate having a second plurality of
ridges formed thereon and pressed against the optical fiber;
wherein the first plurality of ridges and the second plurality of
ridges are offset such that a series of microbends in the core is
created when the first plate and the second plate are exerting
forces on the fiber, the microbends causing light guided by the
first core to be coupled to a second mode not guided by the first
core.
17. The sensor of claim 16, wherein the second mode is a cladding
mode.
18. The sensor of claim 16, wherein the optical fiber further
comprises a second core and the second mode is a mode guided by the
second core.
19. The sensor of claim 18, wherein the second core surrounds the
first core.
20. The sensor of claim 18, wherein the second core is spaced apart
from the first core.
21. The sensor of claim 20, wherein the second core is surrounded
by a second cladding.
22. The sensor of claim 20, wherein the second core is surrounded
by the first cladding.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional application of U.S. patent
application Ser. No. 10/431,456, filed May 8, 2003, which is based
on U.S. Provisional Application Ser. No. 60/378,351, filed May 8,
2002, the contents of both of which are hereby incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to fiber optic sensors generally, and
more specifically to fiber optic sensors employing pressure-induced
periodic gratings.
[0004] 2. Discussion of the Background
[0005] Optical fiber sensors are becoming more popular for a wide
variety of applications. Optical fiber sensors offer several
advantages over other types of sensors such as electronic and
mechanical sensors. Optical fiber sensors are generally more rugged
and have longer lifetimes than these other types of sensors, are
immune from electromagnetic interference, can often be made much
smaller than these other types of sensors, and offer multiplexing
capabilities.
[0006] One type of optical fiber sensor known in the art is the
grating-based fiber optic sensor. These types of sensors employ an
optical grating comprising a series of refractive index
perturbations spaced along an optical fiber. The spacing is
generally fixed, but "chirped" gratings with varying spacing are
also known in the art. The optical grating can be either of two
types--short period gratings (also referred to as Bragg gratings)
and long period gratings.
[0007] Short period gratings have a periodic spacing less than the
wavelength of an operating light source, typically less than one
micron. These gratings convert light traveling in the
forward-propagating guided fundamental mode to the
reverse-propagating fundamental mode; that is, light traveling in
the forward direction in the core of the fiber is reflected
backward into the core by the grating. The wavelength of the
reflected light depends upon the spacing in the grating. Therefore,
if the spacing is changed, such as by expansion of the fiber due to
a temperature increase or by compression or stretching of the fiber
due to mechanical forces, a corresponding shift in the wavelength
of the reflected light will occur. By applying broadband light to
the fiber and analyzing the spectrum of the light reflected by the
grating (or, conversely, by analyzing the spectrum of the light
that passes the grating), the change in grating spacing, and thus
the corresponding change in temperature and/or mechanical force
applied to the fiber) can be determined. Short period gratings with
periodic spacings of less than one micron have been widely used as
temperature and strain sensors.
[0008] In contrast to short period gratings, long period gratings
have a spacing greater than the wavelength of the operating light
source. Typical spacings are between 15 and 1500 microns. Unlike
short period gratings which reflect light backward into the core of
the fiber, long period gratings couple light from a forward
propagating mode in the core to another mode not guided by the
core. For example, U.S. Pat. No. 5,641,956 describes sensor
arrangements involving long period gratings in which light is
coupled from the forward propagating fundamental mode in the core
to a mode guided by the cladding of the optical fiber, where it is
attenuated due to the lossy nature of the cladding mode.
Alternatively, light traveling in the forward propagating
fundamental mode can be converted into a higher order forward
propagating mode guided by the core and subsequently stripped out
to provide a wavelength-dependent loss.
[0009] The wavelength of light for which coupling occurs in the
long period gratings is dependent upon the spacing of the grating.
Thus, by examining the spectrum of the light that continues to be
guided by a core of a fiber after passing through a long period
grating formed in the core, changes in the spacing of the grating
corresponding to changes in temperature and/or mechanical forces
can be detected and measured.
[0010] Long period gratings can be formed using photolithographic
processes involving the exposure of a doped (to increase
photosensitivity) optical fiber to ultraviolet radiation. An
example of such a process is described in U.S. Pat. No. 5,757,540.
The amount of change in the refractive index caused by such
gratings is generally permanent. The amplitude of the attenuation
resulting from such gratings generally varies little when pressure
perturbations are applied to this grating. Additionally, any
changes in the amplitude of the attenuation peaks depends on
temperature, pressure and strain, and it is therefore difficult to
use a single grating of this type to measure any of these if they
are present at the same time.
[0011] U.S. Pat. No. 6,282,341 describes optical fiber filters
employing long period gratings formed by arcing across the fiber,
such as with a commercial fiber splicer, at periodic intervals
and/or by periodically stressing the fiber such as by maintaining
pressure on a plate with milled periodically spaced ridges against
the fiber. This patent includes no description or suggestion of
employing long period gratings formed in such a manner in a
sensor.
SUMMARY
[0012] The present invention provides several novel optical fiber
sensors employing temporally created long period gratings. The
gratings may be produced by a periodic change in the refractive
index of the fiber along the fiber longitudinal axis caused by
localized, spaced-apart compressive and/or expansive forces or by
spaced-apart unbalanced forces that cause periodic fiber
micro-bending. In preferred embodiments of the invention, the
sensors simultaneously measure both pressure and temperature by
observing changes in both the wavelength at which light is coupled
from a mode guided by a core to a different mode and an amount of
such coupling.
[0013] The invention provides different ways to create the long
period gratings. In some embodiments, the gratings are created
intrinsically (that is, without the assistance of mechanical
devices external to the optical fiber) by modifying the fiber
cladding such pressure applied to the fiber will create
periodically varying stresses in the core of the fiber. In other
embodiments, the gratings are created extrinsically by mechanical
devices operating on the optical fiber.
[0014] In another aspect of the invention, some embodiments of the
invention employ a single core fiber and the long period grating
couple light from a fundamental mode in the core to a mode guided
by the cladding (also sometimes referred to as a non-guided mode).
In other embodiments of the invention, a dual core fiber is used
and the long period grating couples light from a fundamental mode
in one core to a fundamental mode in the second core. The cores may
be either concentric or spaced apart. In embodiments with
spaced-apart cores, the cores may be located inside a single
optical fiber (i.e., surrounded by a single, common cladding) or
may be located in separate optical fibers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation of the invention and many of
the attendant features and advantages thereof will be readily
obtained as the same become better understood by reference to the
following detailed description when considered in connection with
the accompanying drawings, wherein:
[0016] FIG. 1 represents a transmission spectrum diagram of a
sensor according to an embodiment of the invention.
[0017] FIGS. 2(a) and 2(b) are block diagrams of sensor systems
according to first and second embodiments of the invention.
[0018] FIG. 3 is a cross sectional view of an extrinsic optical
fiber sensor according to a third embodiment of the invention.
[0019] FIGS. 4(a) and (b) are cross sectional views of an extrinsic
optical fiber sensor according to a fourth embodiment of the
invention.
[0020] FIG. 4(c) is a side view of an extrinsic optical fiber
sensor according to a fifth embodiment of the invention.
[0021] FIGS. 5(a) and (b) are cross sectional views of an intrinsic
optical fiber sensor according to a sixth embodiment of the
invention.
[0022] FIG. 6 is a cross sectional view of an intrinsic optical
fiber sensor according to a seventh embodiment of the
invention.
[0023] FIGS. 7(a) and (b) are perspective and cross sectional
views, respectively, of an extrinsic optical fiber sensor according
to an eighth embodiment of the invention.
[0024] FIG. 8 is a side view of a concentric dual core optical
fiber for use in an optical fiber sensor according to a ninth
embodiment of the invention.
[0025] FIG. 9 is a side view of two optical fibers for use in a
dual core optical fiber sensor according to a tenth embodiment of
the invention.
[0026] FIG. 10 is a side view of a side-by-side dual core optical
fiber for use in a dual core optical sensor according to an
eleventh embodiment of the invention.
DETAILED DESCRIPTION
[0027] The present invention will be discussed with reference to
preferred embodiments of optical sensors and optical sensor
systems. Specific details are set forth in order to provide a
thorough understanding of the present invention. The preferred
embodiments discussed herein should not be understood to limit the
invention. Furthermore, for ease of understanding, certain method
steps are delineated as separate steps; however, these steps should
not be construed as necessarily distinct nor order dependent in
their performance.
[0028] As used herein, "circumferential" means around a
circumference of the optical fiber. The circumference may be
perpendicular to the core of the fiber such the circumference forms
a ring, or may be offset from the perpendicular such that the
circumference forms an oval.
[0029] Referring now to the drawings, wherein like reference
numerals designate identical or corresponding parts throughout the
several views, FIG. 1 illustrates an exemplary transmission
spectrum diagram for a sensor of the present invention. The curve
100 of FIG. 1 represents optical intensity of light transmitted
through a core of an optical fiber sensor (not shown in FIG. 1)
according to the present invention as a function of wavelength of
the light. The curve 100 includes a first order attenuation peaks
110 and higher order attenuation peaks 120, 130 as well as several
non-attenuated areas 140. The amplitude of the attenuation peaks
110-130 are indicative of an amount of light coupled from the
guided mode of the core to another mode (e.g., a non-guided, lossy
cladding mode or a guided mode in another core). The amplitude of
the peaks is a function of force exerted on the fiber. The spectral
locations of the peaks are a function of the spacing of the
gratings, which is primarily dependent on temperature. In dual core
embodiments of the invention in which light is coupled from a first
core to a second core, the second core will exhibit a spectrum with
transmission peaks corresponding to the attenuation peaks 110-130
of FIG. 1, with the amplitude of the transmission peaks being
indicative of an amount of light coupled from the first core.
[0030] An exemplary sensor system 200 is illustrated in FIG. 2(a).
The system includes a light source (e.g., a broadband light source)
210 connected to an optical fiber 220. The optical fiber 220 may
include a single core or may include two cores. The optical fiber
220 includes a sensor area 230 that is at least partially
surrounded by a force exertion device 240. The force exertion
device 240 may take a variety of forms. In some embodiments, the
force exertion device 240 comprises a tube. In other embodiments,
it comprise one or a pair of plates. In yet other embodiments, such
as those embodiments in which the grating is created intrinsically,
the force exertion device 240 takes the form of an enclosure filled
with a pressurized fluid (such as a gas or a liquid).
[0031] A spectrum analyzer 250 is connected to the optical fiber
220 to measure the spectrum of light transmitted through the sensor
region 230 of the fiber 220. As discussed above, the position of
the attenuation peaks (or the transmission peaks in the second core
of dual core embodiments of the invention) depends primarily on
temperature, and the amplitude of the peaks depends primarily on
pressure. The amplitude of the peaks may be determined by comparing
the amplitude at a wavelength at which coupling occurs with an
amplitude at a wavelength at which no coupling occurs. Thus, by
simultaneously measuring the spectral position and amplitude of the
attenuation peaks in the first core and/or the transmission peaks
in the second core with the spectrum analyzer, the temperature and
pressure can be determined simultaneously.
[0032] It should be noted that a certain level of cross-sensitivity
between the pressure and temperature measurement may occur. That
is, temperature variations may also effect the magnitudes of the
attenuation peaks and pressure changes may also affect the spectral
positions of the attenuation peaks through physical effects such as
pressure-induced dimensional changes along the fiber longitudinal
axis and temperature-induced changes to the refractive index.
However, the non-uniform refractive index variations along the
fiber longitudinal axis are primarily caused by the applied
pressure; thus, the magnitudes of the attenuation peaks (i.e., the
amount of light coupled from a mode guided by the core to another
mode) are primarily dependent upon applied pressure.
[0033] An alternative embodiment of a sensor system 201 for use
with embodiments of the invention employing dual core sensors with
each of the cores in separate fibers is illustrated in FIG. 2(b). A
light source 210 is connected to a first optical fiber 221. A
second optical fiber 222 is positioned adjacent to the first
optical fiber 221 such that some of the light guided by the core of
the first optical fiber 221 is coupled to the core of the second
optical fiber 222 when pressure is applied to the first optical
fiber 221. The spectrum analyzer 250 is connected to measure the
amount and spectral position of light coupled to the core of the
second optical fiber 222.
[0034] A cross-sectional view of an extrinsic, single core sensor
300 is illustrated in FIG. 3. A portion of an optical fiber 310
with a single core 320 surrounded by a cladding 330 is disposed
inside a tube 340. The tube 340 may be comprised of metal, glass,
or other materials. The tube is bonded to the fiber at periodically
spaced locations 350. Bonding between the tube 340 and fiber 310
may be accomplished by thermal fusion. In a highly preferred
embodiment, the tube 340 is glass and bonding between the tube 340
and the fiber 310 is accomplished by applying a laser to the tube
340 at a sufficient power to melt localized portions of the tube
340 and fiber 310 such that a bond is formed. The bonding between
the tube 340 and fiber 310 may be circumferential. In other
embodiments, the bond may be formed at localized areas on opposite
sides of the tube 340.
[0035] When the sensor 300 is exposed to pressure, the pressure
changes the density of the fiber core 320 and hence its index of
refraction over the bonded regions, while other regions where the
fiber 310 is not bonded to the tube 340 do not experience the
externally applied pressure (or experience lesser amounts of
pressure) and therefore do not experience any change (or experience
a smaller change) in refractive index. Consequently, the index of
refraction of the fiber core 320 is changed periodically, thereby
forming a long period grating. The period is determined by the
period of the bonding between the fiber 310 and the tube 340.
[0036] A cross-sectional view of an extrinsic, single core (not
shown) sensor 400 is shown in FIGS. 4 (a) and 4(b). The bonds
between the fiber 410 and tube 400 at the bonding locations 450 of
sensor 400 are offset such that the fiber 400 undergoes
micro-bending at bonding locations 450 as shown in FIG. 4(b). This
should be contrasted with the sensor 300, in which there are
opposing bonds between the tube 340 and fiber 310 at each of
bonding locations 350 such that the core 320 is compressed when the
tube 340 is exposed to pressure P, but in which micro-bending does
not generally occur.
[0037] An alternative embodiment of an extrinsic, single core
sensor 401 is illustrated in FIG. 4(c). The sensor 401 employs a
pair of plates 471, 472 with one of the plates 471 having a
plurality of ridges 471a that are offset with respect to a
plurality of ridges 472a of the second plate 472. When opposing
forces are applied to the plates 471, 472, the opposing ridges
471a, 472b form a series of micro-bends 412 in the optical fiber
410. The ridges 471a, 472a are spaced such that the micro-bends
result in a long period grating being formed that couples light
from a mode guided by the core of the fiber 410 to another
mode.
[0038] FIGS. 7(a) and 7(b) are perspective and cross-sectional
views, respectively, of yet another alternative embodiment of an
extrinsic, single core sensor 700 according to the present
invention. The sensor 700 employs a pair of plates 741, 742 in
place of the tube 340 of the sensor 300. In the sensor 700, one
plate 741 has two smooth sides 741a, 741b. The second plate 742 has
a plurality of flat-topped ridges 742a formed on a side adjacent to
the optical fiber 710. The ridges 742a are periodically spaced such
that a long period grating is created in the core 720 of the fiber
710 when opposing forces are applied to the plates 741, 742 due to
changes in refractive index in localized areas of the core 720
caused by compression of the core 720 by the ridges 742a. A groove
743 may be formed through the ridges 742a to keep the fiber 710 in
position between the plates 741, 742; however, such a groove 743 is
not strictly necessary.
[0039] In other embodiments of the sensor 700, both of plates 741,
742 may be provided with ridges 742a. In yet other embodiments, the
ridges 742a may have cross sectional shapes with angled sides
rather than sides formed at right angles to the flat tops 742(c) of
the ridges 742(b) as shown in FIG. 7. In still further embodiments,
the groove 743 may be curved (rather than straight) to produce a
chirped grating. When a straight groove 743 is used, the groove 743
may be at an angle with respect to the ridges 742a rather than
parallel as shown in FIG. 7.
[0040] FIG. 5(a) illustrates an intrinsic, single core sensor 500
according to another embodiment of the invention. Unlike the
extrinsic sensors of FIGS. 3 and 4, the intrinsic sensor of FIG. 5
does not require a mechanical device such as a tube 340, 440 to
produce a long period grating. The sensor 500 includes a fiber 510
with a single core 520 surrounded by a cladding 530. The cladding
530 has a plurality of circumferential grooves 560 formed therein
in a sensor region 531. The side walls 563 of the grooves 560 are
illustrated in phantom in FIG. 5(a).
[0041] Referring now to FIG. 5(b), which illustrates a portion of
the sensor 500, when the sensor 500 is exposed to pressure,
vertical forces P.sub.V act on each of the bottom surfaces 561 of
each of the grooves 560 and the surfaces 562 of the cladding 530
between the grooves 560. These vertical forces P.sub.V effect the
core 520 equally. Therefore, while the entire core 520 may undergo
a change in refractive index due to compression resulting from the
vertical forces P.sub.V, the forces P.sub.V do not result in a
relative change in refractive index between regions A of the core
in the grooves 560 and regions B of the core between the grooves
560. However, the horizontal forces P.sub.H acting on the side
walls 563 of the grooves 560 result in compressive forces acting on
the core 520 in regions A and expansive forces acting on the core
in regions B. The difference between forces acting on the core 520
in regions A and B results in a difference in refractive index of
the core in regions A and B, thereby producing a long period
grating when the sensor 500 is exposed to pressure.
[0042] A cross sectional view of intrinsic sensor 600 according to
yet another embodiment of the invention is illustrated in FIG. 6.
Like the sensor 500, the sensor 600 has a series of circumferential
grooves 660 formed in the cladding. In contrast to the
circumferential grooves 560 of the sensor 500, which are formed
perpendicular to the core 520, the circumferential grooves 660 of
the sensor 600 are formed at an angle offset from the angle
perpendicular to the core 620, resulting in oval-shaped
circumferential grooves. In alternative embodiments, a helical
groove is used in place of the plurality of circumferential grooves
660. In yet other embodiments, opposing pairs of groove segments,
with each groove segment being formed in only a portion of a
circumference of an optical fiber (rather than an entire
circumference as shown in FIG. 6) could also be used.
[0043] It should be noted that intrinsic sensors such as those
shown in FIGS. 5 and 6 may also be used in an extrinsic mode. This
may be accomplished by, for example, applying opposing pressure to
two smooth plates (e.g., two plates such as the plate 741 of FIG. 7
rather than one or two plates 742 with ridges 742a) adjacent to
sections of optical fiber where grooves 560, 660 are formed, or by
placing areas of fibers in which grooves 560, 660 have been formed
into tubes with smooth inner surfaces such that the inner surface
of the table compresses areas 562 between the grooves 560 when
pressure is exerted on the tube.
[0044] Each of the embodiments discussed above has involved single
core optical fibers. In such embodiments, when a long period
grating is created in the core by changing the refractive index in
localized sections of the core (whether it be through the creation
of microbends in the core or through compression/expansion of the
core), light is coupled from a mode guided by the core to a mode
guided by the cladding (which is also sometimes referred to as an
unguided mode) where it is attenuated. However, in other
embodiments of the invention, composite optical fibers with two or
more cores are used. In such embodiments, guided by one of the
cores is coupled to a mode guided by a second core when a long
period grating is created in the first core.
[0045] An example of a concentric dual core optical fiber 800 for
use in such a sensor is illustrated in FIG. 8, The optical fiber
800 includes an inner core 810 having an index of refraction
n.sub.1 surrounded by a second core 820 having an index of
refraction n.sub.2 less than the index of refraction n.sub.1 of the
inner core 810. The second core 820 is surrounded by a cladding 830
having an index of refraction n.sub.3 less than the index of
refraction n.sub.2 of the second core a
[0046] The inner core 810 supports the fundamental mode, denoted
herein as LP.sub.01(1). The second core supports more than one
mode, which shall be referred to herein as LP.sub.lm(2). When a
pressure induced periodic variation is created in the fiber,
observable optical coupling between the fundamental mode
LP.sub.01(1) in the first core 810 and some of the modes
LP.sub.lm(2) in the second core 820 can occur if two conditions are
met. One such condition is:
.DELTA.L=(2.pi.N)/.beta..sub.01(1)-.beta..sub.lm(2)| (1)
[0047] where:
[0048] .DELTA.L is the spacing of the grating;
[0049] N is an integer,
[0050] .beta..sub.01(1) is the propagation constant of the first
core fundamental mode and
[0051] .beta..sub.lm(2) is the propagation constant of the second
core modes.
[0052] The other condition is:
.intg..intg.E.sub.01)1)p(.rho.,.theta.)E.sub.lm(2)d.sigma..noteq.0
(2)
[0053] where E.sub.01(1) is the electric field profile of the first
core mode;
[0054] p(.rho.,.theta.) represents the spatial distribution of the
pressure-induced variation in the fiber index or geometry with
.rho. and .theta. representing coordinates on a fiber
cross-section;
[0055] E.sub.lm(2) is the electric field profile of a second core
mode, and
[0056] .sigma. is the fiber cross section.
[0057] Generally, the larger the integral value that the left-hand
side of equation (2) is, the stronger the cross coupling will
be.
[0058] As discussed above, coupling between the fundamental mode in
the inner core 810 and modes in the second core 820 takes place at
wavelengths that are dependent upon the periodic spacing of the
long period grating. The long period grating can be created using
several of the extrinsic and intrinsic methods discussed above.
Such methods include, but are not limited to, disposing the fiber
800 in a tube 340 such as shown in FIG. 3 or a tube 440 as shown in
FIGS. 4(a), (b); disposing the fiber 800 between a pair of plates
471, 472 as shown in FIG. 4(c) or 741, 742 as shown in FIG. 7; and
creating grooves in the cladding of 830 the fiber 800 such as the
grooves 560, 660 shown in FIGS. 5 and 6. As discussed above,
temperature and/or pressure can be determined by measuring the
spectral positions and/or amplitudes of the attenuation peaks in
the first core 810 and/or the spectral positions and/or amplitudes
of the transmission peaks in the second core 820.
[0059] With embodiments of the invention that employ concentric
dual core fibers such as the fiber 800, it is possible to design
the second core 820 such that it only supports a single or just a
few modes. This would result in fewer spectral attenuation peaks
present in the first core 810 than if the light from the first core
were coupled to a cladding mode. This can be advantageous in many
sensing applications, including applications wherein multiple
sensors are multiplexed. If multiple sensors are implemented along
a single fiber and these sensors are designed such that their
spectral attenuation peaks are located in different spectral
regions, fewer spectral attenuation peaks from each sensor imply
that more sensors could be multiplexed without interfering with
each other in a limited available spectral width.
[0060] A second dual core optical fiber sensor arrangement 900 is
illustrated in FIG. 9. The sensor 900 is comprised of two separate
fibers 980, 990. The first optical fiber 980 includes a core 982
with an index of refraction n.sub.1 and a cladding 984 with an
index of refraction n2 less than the index of refractionn, of the
core 982. Similarly, the second optical fiber 990 includes a core
992 with an index of refraction n'.sub.1 and a cladding 994 with an
index of refraction n'.sub.2 less than the index of refraction
n'.sub.1 of the core 992.
[0061] In this embodiment, both of the fibers 980, 990 are single
mode fibers supporting only the fundamental modes and coupling
occurs from one fiber core to the other. Moreover, the two fibers
980, 990 have different propagation constants (denoted as
.beta..sub.01(1) and .beta..sub.01(2)) resulting from some
difference in the fiber cores or numerical apertures or the core
structures. Because of the differences in propagation constants, no
optical coupling between the two cores occurs in the absence of
external perturbation to the fibers. However, when a pressure
induced periodic grating is generated, optical coupling between the
two cores 982, 992 takes place under the following conditions:
.DELTA.L=(2.pi.N)/|.beta..sub.01(1)-.beta..sub.01(2)| (3)
[0062] where .beta..sub.01(1) and .beta..sub.01(2) represent the
propagation constants of the fundamental modes of the cores 982,
992 and the other values have the same meaning as in Equation 1
above. Since the optical coupling occurs only between the two
modes, only one spectral attenuation peak may be obtained over a
relatively wide spectral range of interest. As discussed above,
this may be a major advantage for sensor multiplexing. The long
period gratings in these dual fiber sensor embodiments may be
created either intrinsically or extrinsically using opposing plates
rather than tubes so that the two fibers can be accommodated
between the plates. It is only necessary to create the grating in
the fiber from which coupling occurs, but it should be noted that
creating a grating in both fibers enhances coupling.
[0063] Yet another embodiment of a dual core sensor 1000 is
illustrated in FIG. 10. In this embodiment, two cores 1010, 1020
are surrounded by a single cladding 1030. The indices of refraction
n.sub.1 and n'.sub.1 of the cores 1010, 1020 are both greater than
the index of refraction n.sub.2 of the cladding 1030. As with the
sensor 900, coupling occurs between the fundamental modes
.beta..sub.01(1) and .beta..sub.01(2) of the cores 1010, 1020 when
a long period grating is created under the conditions defined in
Equation 3 above. Any of the methods described in connection with
sensor 800 of FIG. 8 may be utilized to create the long period
grating, including both intrinsic and extrinsic methods and
including both compression/expansion forces on the cores and
micro-bending of the fibers.
[0064] It should be noted that attenuation peak width and amplitude
can both be changed by varying the duty cycle of the ridges (that
is, the ratio of the top surface of the ridges as compared to the
space between ridges).
[0065] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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