U.S. patent application number 17/552473 was filed with the patent office on 2022-04-07 for method and system for fabricating an optical fiber device for shape sensing.
The applicant listed for this patent is POLYVALOR, LIMITED PARTNERSHIP. Invention is credited to Samuel KADOURY, Raman KASHYAP, Sebastien LORANGER, Pierre LORRE, Frederic MONET.
Application Number | 20220107457 17/552473 |
Document ID | / |
Family ID | |
Filed Date | 2022-04-07 |
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United States Patent
Application |
20220107457 |
Kind Code |
A1 |
KASHYAP; Raman ; et
al. |
April 7, 2022 |
METHOD AND SYSTEM FOR FABRICATING AN OPTICAL FIBER DEVICE FOR SHAPE
SENSING
Abstract
There is described a method of fabricating an optical fiber
device, the method comprising: positioning longitudinal portions of
a plurality of optical fibers alongside each other in a given
geometrical relationship, depositing liquid coating material around
the longitudinal portions of the plurality of optical fibers; and
the liquid coating material setting up around the longitudinal
portions of the plurality of optical fibers thereby maintaining
said given geometrical relationship along the longitudinal
portions.
Inventors: |
KASHYAP; Raman; (Baie
d'Urfe, CA) ; LORANGER; Sebastien; (Montreal, CA)
; MONET; Frederic; (Montreal, CA) ; KADOURY;
Samuel; (Mount Royal, CA) ; LORRE; Pierre;
(Montreal, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POLYVALOR, LIMITED PARTNERSHIP |
Montreal |
|
CA |
|
|
Appl. No.: |
17/552473 |
Filed: |
December 16, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16666719 |
Oct 29, 2019 |
11249248 |
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17552473 |
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62751951 |
Oct 29, 2018 |
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International
Class: |
G02B 6/02 20060101
G02B006/02; G02B 6/42 20060101 G02B006/42; G02B 6/44 20060101
G02B006/44 |
Claims
1. A method of fabricating an optical fiber device, the method
comprising: positioning longitudinal portions of a plurality of
optical fibers alongside each other in a given geometrical
relationship, depositing liquid coating material around the
longitudinal portions of the plurality of optical fibers; and the
liquid coating material setting up around the longitudinal portions
of the plurality of optical fibers thereby maintaining said given
geometrical relationship along the longitudinal portions.
2. The method of claim 1 wherein said depositing includes extruding
the liquid coating material around the longitudinal portions of the
plurality of optical fibers while maintaining said given
geometrical relationship.
3. The method of claim 2 wherein said extruding includes forcing
the liquid coating material at a longitudinal position around the
longitudinal portions of the plurality of optical fibers and moving
the longitudinal portions of the plurality of optical fibers
longitudinally during said forcing.
4. The method of claim 3 wherein said moving includes
longitudinally pulling on ends of the plurality of optical
fibers.
5. The method of claim 3 wherein said moving is performed as the
longitudinal portions of the plurality of optical fibers are
longitudinally received in an opening having an inner surface
confining the plurality of optical fibers into the given
geometrical relationship.
6. The method of claim 5 wherein the inner surface of the opening
has a dimension below 1 mm, preferably below 500 .mu.m and more
preferably below 400 .mu.m.
7. The method of claim 1 wherein said depositing includes
maintaining the plurality of optical fibers parallel to one
another.
8. The method of claim 1 wherein said depositing includes melting
coating material thereby forming the liquid coating material, and
wherein said setting up includes cooling the liquid coating
material.
9. The method of claim 1 further comprising, prior to said
depositing, heating a plurality of optical fiber preforms, and
drawing the plurality of optical fiber preform into the plurality
of optical fibers.
10. The method of claim 9 further comprising, after said drawing,
inscribing one or more optical gratings along a portion of each of
the plurality of optical fibers.
11. The method of claim 10 wherein at least one of the optical
gratings has a random continuous distribution such that a return
signal propagating in said optical fiber has a full width at half
maximum bandwidth ranging between about 0.1 THz and about 40
THz.
12. The method of claim 1 wherein said positioning include
positioning at least an additional component relative to the
plurality of optical fibers, said depositing including depositing
liquid coating material also around said additional component, the
liquid coating material setting up around the additional component
as well.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority on U.S. Patent
Application No. 62/751,951 filed Oct. 29, 2018, the entire contents
of which are incorporated herein by reference.
FIELD
[0002] The improvements generally relate to fabrication of optical
fiber devices, and more particularly relate to fabrication of
optical fiber devices for two- or three-dimensional shape sensing
purposes.
BACKGROUND
[0003] Two- or three-dimensional shape sensors generally involve an
optical fiber device having two or more optical fibers extending
alongside each other in a given geometrical relationship, an
interrogator optically coupled to the optical fibers, and a
computing device communicatively coupled to the interrogator. As
the optical fiber device experiences a curvature-induced strain
along its length, a relative longitudinal tension or compression
with respect to the center of the optical fiber device will cause
the optical fibers to register positive- or negative-induced strain
changes, respectively. To calculate a local curvature, the relative
strains are measured in real-time by the interrogator and the
measured strains are processed by the computing device according to
known algorithms. Although optical fiber devices for shape sensing
applications are satisfactory to a certain degree, there remains
room for improvement, especially in providing fabrication methods
and systems which can limit variations in the positioning of the
optical fibers with respect to one another along the length of the
optical fiber device.
SUMMARY
[0004] In accordance with a first aspect of the present disclosure,
there is provided a method of fabricating an optical fiber device,
the method comprising: positioning longitudinal portions of a
plurality of optical fibers alongside each other in a given
geometrical relationship, depositing liquid coating material around
the longitudinal portions of the plurality of optical fibers; and
the liquid coating material setting up around the longitudinal
portions of the plurality of optical fibers thereby maintaining
said given geometrical relationship along the longitudinal
portions.
[0005] Further in accordance with the first aspect of the present
disclosure, said depositing can for example include extruding the
liquid coating material around the longitudinal portions of the
plurality of optical fibers while maintaining said given
geometrical relationship.
[0006] Still further in accordance with the first aspect of the
present disclosure, said extruding can for example include forcing
the liquid coating material at a longitudinal position around the
longitudinal portions of the plurality of optical fibers and moving
the longitudinal portions of the plurality of optical fibers
longitudinally during said forcing.
[0007] Still further in accordance with the first aspect of the
present disclosure, said moving can for example include
longitudinally pulling on ends of the plurality of optical
fibers.
[0008] Still further in accordance with the first aspect of the
present disclosure, said moving can for example be performed as the
longitudinal portions of the plurality of optical fibers are
longitudinally received in an opening having an inner surface
confining the plurality of optical fibers into the given
geometrical relationship.
[0009] Still further in accordance with the first aspect of the
present disclosure, said depositing can for example include
maintaining the plurality of optical fibers parallel to one
another.
[0010] Still further in accordance with the first aspect of the
present disclosure, said depositing can for example include melting
coating material thereby forming the liquid coating material, and
wherein said setting up includes cooling the liquid coating
material.
[0011] Still further in accordance with the first aspect of the
present disclosure, the method can for example comprise, prior to
said depositing, heating a plurality of optical fiber preforms, and
drawing the plurality of optical fiber preform into the plurality
of optical fibers.
[0012] Still further in accordance with the first aspect of the
present disclosure, the method can for example comprise, after said
drawing, inscribing one or more optical gratings along a portion of
each of the plurality of optical fibers.
[0013] Still further in accordance with the first aspect of the
present disclosure, at least one of the optical gratings can for
example have a random continuous distribution such that a return
signal propagating in said optical fiber has a full width at half
maximum bandwidth ranging between about 0.1 THz and about 40
THz.
[0014] In accordance with a second aspect of the present
disclosure, there is provided a system for fabricating an optical
fiber device, the system comprising: a coating material source
having liquid coating material; a die having a longitudinal conduit
receiving a plurality of optical fibers extending alongside each
other; an optical fiber positioner confining longitudinal portions
of the plurality of optical fibers in a given geometrical
relationship relative to one another; and a coating device in fluid
communication with the coating material source and with the
longitudinal conduit of the die, the coating device flowing the
liquid coating material around at least the longitudinal portions
of the plurality of optical fibers when received in the
longitudinal conduit and positioned in the given geometrical
relationship, the coating material setting up around the
longitudinal portions of the plurality of optical fibers thereby
maintaining said given geometrical relationship along the
longitudinal portions.
[0015] Further in accordance with the second aspect of the present
disclosure, the optical fiber positioner can for example be within
the longitudinal conduit of the die.
[0016] Still further in accordance with the second aspect of the
present disclosure, the optical fiber positioner can for example
include a nozzle an opening with an inner surface of a given shape,
the inner surface confining the longitudinal portions of the
plurality of optical fibers in the given geometrical relationship,
and an outer surface upon which the liquid coating material
flows.
[0017] Still further in accordance with the second aspect of the
present disclosure, the inner surface of the opening of the nozzle
can for example have a dimension below 1 mm, preferably below 500
.mu.m and more preferably below 400 .mu.m.
[0018] Still further in accordance with the second aspect of the
present disclosure, the system can for example comprise a pulling
mechanism pulling on ends of the plurality of optical fibers in a
longitudinal orientation as the liquid coating material is flowed
around the plurality of optical fibers.
[0019] Still further in accordance with the second aspect of the
present disclosure, the system can for example comprise an
inscription device upstream from said flow mechanism, the
inscription device inscribing one or more optical gratings along a
portion of each of the plurality of optical fibers.
[0020] Still further in accordance with the second aspect of the
present disclosure, the system can for example have a heater
melting solid coating material to obtain the liquid coating
material. The heater may be part of the coating material
source.
[0021] Still further in accordance with the second aspect of the
present disclosure, the system can for example have a cooler for
cooling the melted liquid coating material after deposition. The
cooler may be part of the coating material source or be a
standalone component in thermal communication with at least a
portion of the die.
[0022] Still further in accordance with the second aspect of the
present disclosure, the cooler can be at least one of a fluid flow
cooler such as a forced air flow cooler, a water-based cooler and
the like, and/or a thermoelectric unit such as a Peltier
module.
[0023] Still further in accordance with the second aspect of the
present disclosure, said positioning can for example include
positioning at least an additional component relative to the
plurality of optical fibers, said depositing including depositing
liquid coating material also around said additional component; the
liquid coating material setting up around the additional component
as well. The additional component can be one or more of any one of
the following group of components: an electrical wire, a conductive
glass fiber, a capillary fiber, a photonic crystal fiber, a laser
delivery fiber and any other suitable component.
[0024] In accordance with a third aspect of the present disclosure,
there is provided an optical fiber device having a plurality of
optical fibers each having a respective longitudinal portion
extending alongside each other in a given geometrical relationship,
a coating layer around the longitudinal portions of the plurality
of optical fibers, the coating layer maintaining the plurality of
optical fibers in the given geometrical relationship along the
longitudinal portions of the plurality of optical fibers.
[0025] Further in accordance with the third aspect of the present
disclosure, the given geometrical relationship can for example be a
triangle.
[0026] Still further in accordance with the third aspect of the
present disclosure, the triangle can for example be an isosceles
triangle.
[0027] Still further in accordance with the third aspect of the
present disclosure, the triangle can for example be an equilateral
triangle, with the longitudinal portions of the plurality of
optical fibers being adjoining to one another.
[0028] Still further in accordance with the third aspect of the
present disclosure, the optical fiber device can for example
comprise one or more optical gratings along said longitudinal
portions of each of the plurality of optical fibers.
[0029] Still further in accordance with the third aspect of the
present disclosure, at least one of the optical gratings can for
example have a random continuous distribution such that a return
signal propagating in said optical fiber has a full width at half
maximum bandwidth ranging between about 0.1 THz and about 40
THz.
[0030] Still further in accordance with the third aspect of the
present disclosure, the optical fiber device can for example
include at least an additional component relative to the plurality
of optical fibers inside said coating layer. The additional
component can be one or more of any one of the following group of
components: an electrical wire, a conductive glass fiber, a
capillary fiber, a photonic crystal fiber, a laser delivery fiber
and any other suitable component.
[0031] In accordance with a fourth embodiment of the present
disclosure, there is provided a distributed temperature and strain
sensing (DTSS) system comprising: an optical interrogator; an
optical coupler assembly having an input being optically coupled to
the optical interrogator and a plurality of outputs; and a
plurality of optical fiber devices having at least: a first optical
fiber device having a first sensing optical fiber being serially
connected to a first one of the plurality of outputs of the optical
coupler assembly; and a second optical fiber device having an
optical path extender being serially connected to a second one of
the plurality of outputs of the optical coupler assembly, the
optical path extender having an optical path length being equal to
or greater than an optical path length of the first optical fiber
device, and a second sensing optical fiber being serially connected
to the optical path extender; wherein, during use, the optical
interrogator is configured for emitting an optical signal at the
input of the optical coupler assembly, and for receiving, in
response to said emitting, a first return optical signal returning
from the first sensing optical fiber and a second return optical
signal returning from the second sensing optical fiber, the first
and second return optical signal being temporally delayed from one
another due to the difference in their corresponding optical path
lengths.
[0032] Further in accordance with the fourth embodiment of the
present disclosure, the DTSS system can comprise for example a
third optical fiber device having an optical path extender being
serially connected to a third one of the plurality of outputs of
the optical coupler assembly, the optical path extender of the
third optical fiber device having an optical path length being
equal to or greater than an optical path length of the second
optical fiber device, and a third sensing optical fiber being
serially connected to the optical path extender of the third
optical fiber device; wherein, during use, the optical interrogator
is configured for receiving a third return signal returning from
the third sensing fiber, the first, second and third return signal
being temporally delayed from one another.
[0033] Still further in accordance with the fourth embodiment of
the present disclosure, the DTSS system can for example comprise a
fourth optical fiber device having an optical path extender being
serially connected to a fourth one of the plurality of outputs of
the optical coupler assembly, the optical path extender of the
fourth optical fiber device having an optical path length being
equal to or greater than an optical path length of the third
optical fiber device, and a fourth sensing optical fiber being
serially connected to the optical path extender of the fourth
optical fiber device; wherein, during use, the optical interrogator
is configured for receiving a fourth return signal returning from
the fourth sensing fiber, the first, second, third and fourth
return signal being temporally delayed from one another.
[0034] Still further in accordance with the fourth embodiment of
the present disclosure, at least one of the first and second
sensing optical fibers can for example have a scatter increasing
device being configured for increasing scattering of the
corresponding optical signal as it propagates along the at least
one of the first and second sensing optical fibers.
[0035] Still further in accordance with the fourth embodiment of
the present disclosure, the scatter increasing device can for
example be an optical grating inscribed along a portion of a
corresponding one of the first and second sensing optical fibers,
the optical grating having a random continuous distribution such
that a return signal, caused by propagation of an optical signal
therealong, has a full width at half maximum bandwidth ranging
between about 0.1 THz and about 40 THz.
[0036] Still further in accordance with the fourth embodiment of
the present disclosure, said full width at half maximum bandwidth
can for example range between about 0.35 THz and about 7 THz.
[0037] Still further in accordance with the fourth embodiment of
the present disclosure, the grating can for example have a
coherence length ranging between about 2.lamda. and about
500.lamda. when the return signal has a scattering spectrum with a
Gaussian shape, wherein .lamda. denotes a wavelength of the optical
signal.
[0038] Still further in accordance with the fourth embodiment of
the present disclosure, the coherence length can for example range
between about 10.lamda. and about 100.lamda..
[0039] Still further in accordance with the fourth embodiment of
the present disclosure, the optical coupler assembly can for
example be a one-by-two fiber coupler.
[0040] Still further in accordance with the fourth embodiment of
the present disclosure, the optical coupler assembly can for
example have one or more a X-by-Y fiber couplers, X and Y being
positive integers.
[0041] Still further in accordance with the fourth embodiment of
the present disclosure, the optical coupler assembly can for
example have a plurality of two-by-two fiber couplers being
connected to one another.
[0042] In accordance with a fifth aspect of the present disclosure,
there is provided an optical device comprising: a length of optical
fiber configured for propagating an optical signal; and an optical
grating inscribed along a portion of the length of optical fiber,
the optical grating having a random continuous distribution such
that a return signal caused by said propagating has a full width at
half maximum bandwidth ranging between about 0.1 THz and about 40
THz.
[0043] Further in accordance with the fifth aspect of the present
disclosure, said full width at half maximum bandwidth can for
example range between about 0.35 THz and about 7 THz.
[0044] Still further in accordance with the fifth aspect of the
present disclosure, the grating can for example have a coherence
length ranging between about 2.lamda. and about 500.lamda. when the
return signal has a scattering spectrum with a Gaussian shape,
wherein .lamda. denotes a wavelength of the optical signal.
[0045] Still further in accordance with the fifth aspect of the
present disclosure, the coherence length can for example range
between about 10.lamda. and about 100.lamda..
[0046] Still further in accordance with the fifth aspect of the
present disclosure, the random continuous distribution can for
example be a random phase distribution.
[0047] Still further in accordance with the fifth aspect of the
present disclosure, the random continuous distribution can for
example be a random period distribution.
[0048] Still further in accordance with the fifth aspect of the
present disclosure, the random continuous distribution can for
example be a random amplitude distribution.
[0049] Still further in accordance with the fifth aspect of the
present disclosure, the random continuous distribution can for
example be one or more from the group consisting of: a random phase
distribution, a random period distribution and a random amplitude
distribution.
[0050] Still further in accordance with the fifth aspect of the
present disclosure, the optical grating can for example have a
length being at least in the centimeter range.
[0051] Many further features and combinations thereof concerning
the present improvements will appear to those skilled in the art
following a reading of the instant disclosure.
DESCRIPTION OF THE FIGURES
[0052] In the figures,
[0053] FIG. 1 is a schematic view of an example of a shape sensing
device having an optical fiber device, in accordance with one or
more embodiments;
[0054] FIG. 1A is a graph representing a three-dimensional model of
the optical fiber device of FIG. 1, in accordance with one or more
embodiments;
[0055] FIG. 2 is an enlarged view of a portion of the optical fiber
device of FIG. 1, in accordance with one or more embodiments;
[0056] FIG. 3 is a flow chart of an example method of fabricating
an optical fiber device, in accordance with one or more
embodiments;
[0057] FIG. 4A is an oblique view of three optical fibers extending
alongside each other in a given geometrical relationship, in
accordance with one or more embodiments;
[0058] FIG. 4B is an oblique view of the three optical fibers of
FIG. 4A having liquid coating material around longitudinal portions
of the three optical fibers, in accordance with one or more
embodiments;
[0059] FIG. 4C is an oblique view of an optical fiber device having
the three optical fibers of FIG. 4A, and a coating layer being
formed as the liquid coating material of FIG. 4B sets up, in
accordance with one or more embodiments;
[0060] FIG. 5 is a schematic view of an example of a system for
fabricating an optical fiber device, in accordance with one or more
embodiments;
[0061] FIG. 6 is a schematic view of the system of FIG. 5, showing
a draw tower and an inscription device, in accordance with one or
more embodiments;
[0062] FIG. 7A is a graph showing return loss as function of
wavelength, as measured using an example optical fiber device, in
accordance with one or more embodiments;
[0063] FIG. 7B is a graph showing amplitude as function of length
of the example optical fiber device of FIG. 7A, in accordance with
one or more embodiments;
[0064] FIG. 8A is a cross sectional view of the example optical
fiber device of FIG. 7A, in accordance with one or more
embodiments;
[0065] FIG. 8B is a top plan view of the example optical fiber
device of FIG. 7A, in accordance with one or more embodiments;
[0066] FIG. 9 is an oblique view of another example of an optical
fiber device, in accordance with one or more embodiments;
[0067] FIG. 9A includes cross-sectional and top plan views of the
optical fiber device of FIG. 9, in accordance with one or more
embodiments;
[0068] FIG. 9B is a cross-sectional view of the optical fiber
device of FIG. 9, with an optical fiber triplet maintained in an
exemplary geometrical relationship, in accordance with one or more
embodiments;
[0069] FIG. 10A is a graph showing radius of the optical fiber
triplet of FIG. 9B as function of a length of the optical fiber
device of FIG. 9, in accordance with one or more embodiments;
[0070] FIG. 10B is a graph showing angles of the optical fiber
triplet of FIG. 9B as function of a length of the optical fiber
device of FIG. 9, in accordance with one or more embodiments;
[0071] FIG. 11 is a schematic view of an example of a DTSS system,
in accordance with the prior art;
[0072] FIG. 12 is a schematic view of an example of a DTSS system,
in accordance with one or more embodiments;
[0073] FIG. 12A is a graph showing exemplary data produced by the
DTSS system of FIG. 12, in accordance with one or more
embodiments;
[0074] FIG. 13 is a schematic view of a computing device of the
controller of the DTSS system of FIG. 12, in accordance with one or
more embodiments;
[0075] FIG. 14 is a schematic view of another example of a DTSS
system, with three sensing optical fibers, in accordance with one
or more embodiments;
[0076] FIG. 15 is a schematic view of another example of a DTSS
system, with four sensing optical fibers, in accordance with one or
more embodiments;
[0077] FIG. 16 is an enlarged view of a scatter increasing device
of the DTSS system of FIG. 12, in accordance with one or more
embodiments;
[0078] FIG. 17A is a schematic view of another example of a DTSS
system, with a LUNA system configured for taking measurements
(e.g., a 42-nm wide scan) in a sensing optical fiber, in accordance
with one or more embodiments;
[0079] FIG. 17B is a graph showing return optical signal as
function of position as taken with the LUNA system of FIG. 17A for
a sensing optical fiber including i) a standard SMF-28 region, ii)
a uniform fiber Bragg grating (FBG) region, iii) a random FBG and
iv) continuous UV exposure region with no FBG;
[0080] FIG. 17C is a graph showing return optical signal as
function of position as taken with the LUNA system of FIG. 17A for
a sensing optical fiber including i) a standard SMF-28 region, ii)
a UV-exposed portion of a high numerical aperture/high
Germanium-doped core (hereinafter "HNA/high Ge core") optical
fiber, and iii) a plain portion of the HNA/high Ge core optical
fiber;
[0081] FIG. 18 is a graph showing collected scattering intensity as
function of power at 213 nm for a sensing optical fiber, showing an
increase in collected scatter intensity with UV exposure power at
constant speed of exposure;
[0082] FIG. 19 is a graph showing root mean square (RMS) noise
level as function of a length for a UV-exposed SMF-28, an unexposed
SMF-28, a UV-exposed HNA/High Ge core, and an unexposed HNA/High Ge
core;
[0083] FIG. 20A is a graph showing temperature change as function
of position for a sensing optical fiber maintained at constant
temperature;
[0084] FIG. 20B is a graph showing temperature change as function
of position for a sensing optical fiber being locally heated by a
thin wire;
[0085] FIG. 21 is a schematic view of an example of a system for
inscribing a FBG along an optical fiber using a phase mask and a
Talbot interferometer, in accordance with one or more
embodiments;
[0086] FIG. 22 is a schematic view of an example of a system for
inscribing a FBG along an optical fiber using a phase mask, in
accordance with one or more embodiments;
[0087] FIG. 23A is a graph showing backscattered intensity as
function of position for a FBG inscribed in SMF-28 fiber using the
system of FIG. 21, with 37 mW of laser power, at a writing speed of
0.2 mm/s, the measurements were taken with an 88.24 nm bandwidth on
an OBR 4600;
[0088] FIG. 23B is a graph showing return loss as function of
wavelength for the FBG of FIG. 23A;
[0089] FIG. 24 is a graph showing gain as function of writing speed
for a FBG inscribed using the system of FIG. 21 in a SMF-28 fiber
from Corning and a SM1500 fiber from FiberCore using an amplitude
of 5 V and a frequency of 20 Hz, with 22 mW and 37 mW of laser
power;
[0090] FIG. 25 is a graph showing RMS as function of bandwidth for
80 cm of unexposed SMF-28 fiber and for over 30 cm of a Random
Optical Grating by Ultraviolet Exposure fiber Bragg grating
(hereinafter referred to as "ROGUE FBG") inscribed using the system
of FIG. 21 when maintained at constant temperature and averaged
over 15 measurements;
[0091] FIGS. 26A-F include graphs showing accuracy of both
unexposed SMF-28 fiber and FBG inscribed using the system of FIG.
21, as the 115 cm fiber is being stretched from 0 to 20 .mu.m, in 1
.mu.m increments, for all scanning bandwidths provided by the OBR,
namely (a) 1.31 nm, (b) 2.61 nm, (c) 5.24 nm, (d) 10.51 nm, (e)
21.16 nm, and (f) 42.90 nm;
[0092] FIG. 27 is a graph showing spectral shift RMS error as
function of bandwidth for both the unexposed fiber and a 10 cm FBG
inscribed using the system of FIG. 21, calculated over the 20 .mu.m
stretching and across an 8 cm sensing region;
[0093] FIG. 28 is a graph showing spectral shift RMS error as
function of gain for a FBG inscribed using the system of FIG. 21,
calculated over a 20 .mu.m stretching and across an 8 cm sensing
region;
[0094] FIG. 29 is a graph showing spectral shift RMS error as
function of loss for three FBGs of different strengths inscribed
using the system of FIG. 21, calculated over a 20 .mu.m stretching
and across 8 cm of fiber;
[0095] FIG. 30A is a graph showing full width at half maximum
bandwidth of a FBG as function of length, as modeled for FBGs
having lengths ranging between 1 mm and 10 m, shown on a
logarithmic scale;
[0096] FIG. 30B is a graph showing full width at half maximum
bandwidth of a FBG as function of length, including modeled and
experimental data for FBGs having lengths ranging between 2 mm to
40 cm, shown on a linear scale; and
[0097] FIG. 31 is a graph showing RMS noise level as a function of
gauge length ranging between 0.5 mm and 20 cm, for both unexposed
fiber and ROGUE FBG, when placed in an insulated box, averaged over
15 measurements, for FWHM bandwidths of 5.24 and 42.90 nm.
DETAILED DESCRIPTION
[0098] FIG. 1 shows an example of a shape sensing system 10, in
accordance with an embodiment. As depicted, the shape sensing
system 10 has an optical fiber device 12, and an electro-optic
module 14. The electro-optic module in this example has an optical
interrogator 16 optically coupled to the optical fiber device 12
and a computing device 18 communicatively coupled to the optical
interrogator 16. The computing device 18 is shown as a part within
the electro-optic module 14 in this example, but could also be
remote therefrom.
[0099] As shown, the optical fiber device 12 has a body 20 and two,
three or more optical fibers 22 extending alongside each other
within the body 20 in a given geometrical relationship relative to
one another. As shown, when so-positioned, the optical fibers 22
are radially spaced-apart from a center of the body 20 of the
optical fiber device 12. More specifically, in this example, the
optical fibers 22 are circumferentially distributed around the
center of the body 20 of the optical fiber device 12.
[0100] In this embodiment, the optical interrogator 16 is
configured for transmitting optical signals along the optical fiber
device 12 and for receiving return optical signals from the optical
fiber device 12. Further, the optical interrogator 16 is configured
to transmit electric signals to the computing device 18, the
electric signals being representative of the received return
optical signals. Based on the received electric signals, the
computing device 18 is adapted and configured to generate a two- or
three-dimensional model representing the shape and orientation of
the optical fiber device 12 at a specific moment in time. For
instance, FIG. 1A shows a plot of a model 24 in a given coordinate
system (x, y, z) generated by the computing device 18, which
represents the shape and orientation of the optical fiber device 12
shown in FIG. 1.
[0101] Accordingly, by monitoring the model of the optical fiber
device 12 over time, the shape sensing system 10 allows the
monitoring of the shape and the orientation of the optical fiber
device 12 in real time or quasi real time. In some embodiments, the
shape sensing system 10 generally has a small footprint and is
lightweight, which can provide the ability to track instruments,
bones and/or limbs, with a millimeter-level accuracy in some
embodiments. In this embodiment, the shape sensing system 10 has
one optical fiber device, having for example a diameter of 600
microns and a longitudinal length up to a few meters, at least 50 m
or more. Alternately or additionally, the shape sensing system 10
can have more than one optical fiber device, with different
diameters and/or different longitudinal lengths.
[0102] FIG. 2 shows a portion of the optical fiber device 12 of
FIG. 1. As depicted, the optical fiber device 12 has three optical
fibers 22a, 22b and 22c which extend along a longitudinal length l
of the optical fiber device 12. In some other embodiments, the
optical fiber device 12 can have two optical fibers for
two-dimensional shape sensing, or a triplet of optical fibers (i.e.
three optical fibers), four or more optical fibers for
three-dimensional shape sensing.
[0103] In this example, the optical fibers 22a, 22b and 22c are
embedded in the body 20. As will be described below, the body 20 is
made of a coating material. Example of such coating material can
include, but not limited to, M6823MZ from Total and any other
suitable coating material. Preferably, the coating material can be
melted to a liquid form during the fabrication process, after which
it can sets up to a given shape around the optical fibers 22a, 22b
and 22c and maintain that shape over time. As shown in this
example, each of the optical fibers 22a, 22b and 22c has a
respective core 26 surrounded by one or more inner claddings 28.
The inner claddings 28 generally have a refractive index which is
lower than a refractive index of the cores 26 to allow optical
propagation therein. The refractive index of the cores 26 and/or
inner claddings 28 need not to be identical from one optical fiber
to another. As shown in this example, the optical fiber device 12
can have a sheath 30 covering the body 20 of the optical fiber
device 12, for providing at least some mechanical resistance and/or
thermal insulation.
[0104] As depicted, the optical fibers 22a, 22b and 22c are
off-axis and circumferentially spaced-apart from one another within
the body 20. In this example, the optical fibers 22a, 22b and 22c
are circumferentially spaced-apart by 60.degree. and therefore are
positioned in an equilateral triangle shape. In this specific
example, the optical fibers 22a, 22b and 22c are sized and shaped
to be single-mode for light having a wavelength of 1550 nm. In
alternate embodiments, however, each optical fiber may be sized and
shaped to be multimode.
[0105] The shape sensing system 10 involves distributed strain
measurements in each of the optical fibers 22a, 22b and 22c of the
optical fiber device 12, at different longitudinal positions li
along its longitudinal length l, to construct the model 24
discussed with reference to FIG. 1A. In this example, i is an
integer ranging from 1 and a number N of longitudinal positions.
The longitudinal increment .DELTA.l between two successive
longitudinal positions li can be in the order of the millimeter for
example. The longitudinal increments .DELTA.l between successive
longitudinal positions li need not be identical for each pair of
successive longitudinal positions li where strain measurements are
taken.
[0106] In the context of the optical fiber device 12, bending of
the optical fiber device 12 induces strain on each one of the
optical fibers 22a, 22b and 22c, which can be measured by
propagating light into each of the cores 26 during the bending and
by monitoring reflected wavelengths resulting from said
propagation. The induced strains are a function of the local degree
of bending of the optical fiber device 12. For instance, more
strain is induced in the optical fiber device 12 around its elbow
portion than in any of its straight portions. To measure strain in
a single optical fiber 22, light is sent down the core 26 which
causes light to be reflected at different longitudinal positions
along the core 26. Wavelengths of the reflected light are a
function of the strain on the core 26 and of its temperature. The
amount of reflected light can be enhanced by inscribing one or more
fiber Bragg gratings (FBGs) 23 along the cores 26 of the optical
fibers, as illustrated. As shown in this example, the cores 26 of
the optical fibers 22a and 22b have discrete FBGs 23 whereas the
core 26 of the optical fiber 22c has a continuous FBG 23' inscribed
along its length. It is intended that any one of the optical fibers
22 of the optical fiber device 12 can have at least a discrete FBG
and/or at least a continuous FBG, depending on the embodiment. It
is envisaged that using continuous FBGs, such as the ROGUE FBG
described in detail below, can provide a continuous enhancement.
Instead of using FBGs at discrete locations, the entire optical
fiber's length can provide backscattered signal. During signal
processing, using OFDR and/or OTDR sensing techniques, sensing can
thus be performed at a multitude of points along the fiber's
length. The spacing between each of those sensing points defines
the spatial resolution. Contrarily to conventional FBG sensing
schemes, those sensing points can be anywhere on the fiber, and the
sensor spacing can be tuned, e.g., depending on signal acquisition
and processing times, as well as scanned bandwidth. The shape
sensing algorithm afterwards may be similar as it not only depend
on the positioning of the discrete FBGs, but on the sensor spacing
set by the interrogator data processing. Otherwise, the reflected
light can include Rayleigh scattering, for instance. To reduce
undesirable effects of temperature during the strain measurements,
the sheath 30 of the optical fiber device 12 can provide at least
some thermal insulation.
[0107] The optical fibers 22a, 22b and 22c allow at least two
non-coplanar pairs of optical fibers to be formed. For instance, in
this embodiment, the optical fibers 22a and 22b form a first pair
of optical fibers lying in a first plane 34a, and the optical
fibers 22a and 22c form a second pair of optical fibers lying in a
second plane 34b that is not coplanar with the first plane 34a. As
having only the first pair of optical fibers would allow
reconstruction of the bending of the corresponding waveguide only
in the first plane 34a, having the two non-coplanar pairs of
optical fibers can allow reconstruction of the bending of the
corresponding waveguide in both the first and second planes 34a and
34b, thus allowing a three dimensional model of the optical fiber
device 12 to be determined based on the known geometrical
relationship between the optical fibers 22a, 22b and 22c.
[0108] It was found that limiting the variation in the positioning
of the optical fibers 22a, 22b and 22c along a length of the
optical fiber device 12 could result in more accurate shape
sensing. Indeed, should the geometrical relationship between the
optical fibers 22a, 22b and 22c vary uncontrollably along the
length of the optical fiber device 12, undesirable biases could be
introduced in the shape sensing. There is therefore described
herein methods and systems for fabricating an optical fiber device
destined for shape sensing applications and any other suitable
sensing applications.
[0109] FIG. 3 shows an example method 300 of fabricating an optical
fiber device. Description of the method 300 will be made with
respect to the optical fiber device 12 of FIG. 2 for ease of
reading, with references to FIGS. 4A, 4B and 4C.
[0110] At step 308, longitudinal portions 36a, 36b and 36c of the
optical fibers 22a, 22b and 22c are positioned alongside each other
in a given geometrical relationship 38, an example of which is
shown at FIG. 4A.
[0111] At step 310, liquid coating material 40 is deposited around
the longitudinal portions 36a, 36b and 36c of the optical fibers
22a, 22b and 22c, such as shown at FIG. 4B.
[0112] At step 312, and as illustrated at FIG. 4C, the liquid
coating material 40 sets up around the longitudinal portions 36a,
36b and 36c of the optical fibers 22a, 22b and 22c, to form the
body 20, thereby maintaining the given geometrical relationship 38
along the longitudinal portions 36a, 36b and 36c.
[0113] As such, as long as the positioning of the optical fibers
22a, 22b and 22c relative to one another is maintained until the
liquid coating material 40 set up, the given geometrical
relationship 38 can be expected to be maintained all along the
longitudinal portions 36a, 36b and 36c of the optical fibers 22a,
22b and 22c of the optical fiber device 12.
[0114] In some embodiments, the optical fibers 22a, 22b and 22c
have one or more outer claddings or jackets which are removed prior
to performing steps 308, 310 and 312. However, in some other
embodiments, the optical fibers 22a, 22b and 22c can also be
freshly drawn optical fibers. For instance, in such embodiments,
the method 300 can have a step 302 of, prior to steps 308, 310 and
312, heating optical fiber preforms and drawing the optical fiber
preforms into the optical fibers 22a, 22b and 22c, using for
instance a draw tower. In embodiments where FBGs are to be
inscribed along the cores 26 of the optical fibers 22a, 22b and
22c, a step 304 of inscribing the FBGs 23 along the cores 26 of the
optical fibers 22a, 22b and 22c can be performed before the steps
308, 310 and 312, too. In such embodiments, removing the outer
claddings and/or jackets is not necessary as the freshly drawn
optical fibers may be exempt of such outer claddings and/or
jackets. Moreover, it can be anticipated that so-fabricated optical
fiber devices may be more sensitive to induced strains, as the
absence of outer claddings and/or jackets can allow the optical
fibers within the optical fiber devices to be more flexible.
[0115] It will be appreciated that any suitable liquid coating
material deposition technique can be used. Examples of such liquid
coating material deposition techniques can include, but not
limited, extrusion, injection moulding and the like.
[0116] FIG. 5 shows an example of a system 500 for fabricating an
optical fiber device, in accordance with an embodiment. Description
of FIG. 5 is made with respect to the optical fiber device 12 for
ease of reading. As depicted, the system 500 has a coating material
source 504, a die 506, an optical fiber positioner 508, and a
coating device 510.
[0117] As can be expected, the coating material source 504 has
liquid coating material 40. In some embodiments, the coating
material source 504 has a heater melting solid coating material to
obtain the liquid coating material 40.
[0118] The die 506 has a longitudinal conduit 512 receiving the
optical fibers 22a, 22b and 22c extending alongside each other. The
optical fibers 22a, 22b and 22c are maintained in the geometrical
relationship 38 relative to one another thanks to the optical fiber
positioner 508. The length of the longitudinal conduit 512 can vary
from one embodiment to another, as it could be relatively long or
short, depending on the embodiment.
[0119] In some embodiments, the optical fiber positioner 508 can be
partially or wholly outside the longitudinal conduit 512 of the die
506. For instance, opposite ends 50 and 52 of the optical fibers
22a, 22b and 22c can be pulled away from one another, while being
maintained in the given geometrical relationship 38, by the optical
fiber positioner 508, thereby forcing the optical fibers 22a, 22b
and 22c to maintain the given geometrical relationship 38 all along
their lengths during deposition of the liquid coating material
40.
[0120] However, in this specific embodiment, the optical fiber
positioner 508 is partially within the longitudinal conduit 512 of
the die 506. More specifically, the optical fiber positioner 508
includes a nozzle 516 having an opening 518 with an inner surface
520 of a given shape, and an outer surface 522 upon which the
liquid coating material 40 can flow. It is intended that the inner
surface 520 of the nozzle 516 receives the longitudinal portions
36a, 36b and 36c of the optical fibers 22a, 22b and 22c and
confines them into the given geometrical relationship 38 during
deposition of the liquid coating material 40. As such, the
dimension of the opening 518 of the nozzle 516 can be designed to
snugly receive the longitudinal portions 36a, 36b and 36c of the
optical fibers 22a, 22b and 22c. The snugger the engagement between
the inner surface 520 and the longitudinal portions 36a, 36b and
36c of the optical fibers 22a, 22b and 22c is, the tighter the
tolerance can be on the desired geometrical relationship. For
instance, in some embodiments, the opening 518 of the nozzle 516
has a dimension below 1 mm, preferably below 500 .mu.m and more
preferably below 300 .mu.m.
[0121] The deposition of the liquid coating material 40 is
performed by the coating device 510 which is in fluid communication
with the coating material source 504 and with the longitudinal
conduit 512 of the die 506. As shown, the coating device 510 flows
the liquid coating material 40 towards the longitudinal portions
36a, 36b and 36c of the optical fibers 22a, 22b and 22c, as they
are received in the longitudinal conduit 512 and positioned in the
given geometrical relationship 38 by the optical fiber positioner
508.
[0122] As shown, the deposition step can include extruding the
liquid coating material 40 around the longitudinal portions 36a,
36b and 36c of the optical fibers 22a, 22b and 22c via the nozzle
516 while maintaining the given geometrical relationship 38. In
such an embodiment, the extruding can include a step of forcing the
liquid coating material 40 at a longitudinal position around the
longitudinal portions 36a, 36b and 36c and moving the longitudinal
portions 36a, 36b and 36c of the optical fibers 22a, 22b and 22c
longitudinally during the forcing of the liquid coating material
40. The movement can be in either longitudinal direction. In the
embodiment illustrated in FIG. 5, the movement is towards the
left-hand side of the page. As shown in this example, a pulling
mechanism 530 is used to longitudinally pull on at least the ends
50 of the optical fibers 22a, 22b and 22c. In this example, the
pulling mechanism 530 also maintains the optical fibers 22a, 22b
and 22c parallel to one another during the deposition of the liquid
coating material 40.
[0123] As can be expected, when the longitudinal portions 36a, 36b
and 36c are moved away from the die 506, the liquid coating
material 40 can cool and set up into position, thereby maintaining
the given geometrical relationship 38 between the optical fibers
22a, 22b and 22c. In some embodiments, a cooler is provided to help
the liquid coating material 40 to set up around the longitudinal
portions 36a, 36b and 36c. The cooler can be a forced airflow
cooler, a liquid cooler and/or a Peltier module.
[0124] FIG. 6 shows the system 500 as part of a larger system 600
performing the method 300 described above with reference to FIG. 3.
As shown in this example, the system 600 has a draw tower 602 which
can heat optical fiber preforms 60 and draw the optical fiber
preforms 60 to form the optical fibers 22. The system 600 also has
an inscription device 604 downstream from the draw tower 602 and
upstream from the system 500. As shown in this example, the
inscription device 604 is used for inscribing one or more FBGs 23
along a longitudinal portion of each of the optical fibers 22. In
some embodiments, described below, at least one of the FBGs 23 has
a random continuous distribution such that a return signal
propagating therealong has a full width at half maximum bandwidth
ranging between about 0.1 THz and about 40 THz. It is intended that
by inscribing the FBGs 23 and/or ROGUE FBGs 23' along the optical
fibers 22 directly after the drawing process may circumvent an
optional step of covering the optical fibers 22 with one or more
outer claddings and/or jackets, which can inconveniently impede the
flexibility of the optical fibers 22 and/or the resulting accuracy
of the optical fiber device 12 in shape sensing applications. A
step of removing the outer claddings and/or jackets can also be
omitted. In such embodiments, the body 20 of coating material can
act as the outer cladding and protect the optical fibers 22 during
use. It is intended that the FBGs can be omitted, as Rayleigh
scattering may provide sufficient return signal to perform
induced-strain measurements using the optical fiber device 12.
Example 1--Extruded Optical Fiber Triplets for 3D Shape Sensing for
Minimally Invasive Surgery
[0125] Minimally invasive surgery offers a patient the advantage of
faster recovery and reduces the risks of complications. In order to
perform such surgery, the physician needs to monitor the position
and the shape of the catheter and/or needle being used. Optical
fibers have been used as sensors that allow real time guidance to
the surgeon. Optical fibers are widely used for biomedical sensing.
With the advantage of being very flexible and electromagnetically
inactive, they can be interesting for use as sensors in flexible
needles and/or catheters and may be used with MRI at the same time.
Shape sensors usually rely on fiber strain measurement of each
fiber of a triplet, as described above. When the triplet forms a
triangle in cross-section, the relative strain difference between
the optical fibers at each point provides the curvature values and
directions along the entire length of the optical fiber device,
which can be processed to determine the shape of the optical fiber
device. Strain measurement using the optical fiber device may be
performed using various methods. The most common method is to
incorporate one or more FBGs in the optical fibers and then relate
the Bragg wavelength variation to the fiber strain. Such a method
can achieve high accuracy for simple shapes thanks to the high SNR
it may provide. In this example, a set of nine FBGs were positioned
at three longitudinal positions along the optical fibers of the
fiber triplet, with three FBGs at each longitudinal position. The
curvature information can therefore be available only at these
specific positions (e.g., typically at 3 strategic points of a 20
cm long optical fiber device), and hence the curvature has to be
inferred from these points. The limited number of monitored points
can induce errors during the shape determination, especially when
the shape cannot be approximated as a simple function. Another way
to obtain the strain of the optical fiber device is by using
Rayleigh backscatter. The Rayleigh backscatter signal in a fiber
can be related to local strain which makes the same process
possible, using OFDR, with the advantage of acquiring distributed
information along the entire length of the optical fiber sensor.
Since the Rayleigh signal is usually very low, it can lead to a
very poor SNR, and may not allow the addition of in-line optical
components for signal processing due to poor insertion loss
tolerance. It has been shown that the backscatter signal can be
significantly enhanced by UV exposure, which makes this method
promising. In some embodiments, the FBGs are provided in the form
of Random Optical Fiber Gratings written by UV or ultrafast laser
Exposure, referred to as ROGUE FBGs herein, thereby enhancing
backscatter of each of the individual optical fibers. Such ROGUE
FBGs have shown a 50 dB enhancement in backscatter in the sensing
wavelength range. Knowing precisely the position of each fiber in
the sensor can greatly improve bend sensing accuracy.
[0126] It is possible to characterize every single optical fiber in
the triplet, and correct the shape measurements accordingly in
post-processing. However, doing so could be inconveniently time-
and resource-consuming. There is thus described a method of
fabricating the triplet, which can limit variation in the
positioning of the optical fibers of the triplet along a given
length, thereby rendering moot any previous characterization steps.
In some embodiments, the method can involve an extrusion process
which can offer precision, as well as possibility to add various
sensors in the body of coating material simultaneously, producing a
complex composite protected optical fiber device in a single
extrusion step. Such a fabrication method is particularly
convenient for optical fibers having ROGUE FBGs as these are
generally written into uncoated optical fibers. Indeed, depositing
coating material therearound through an extrusion process can
greatly improve the durability of the resulting optical fiber
device.
[0127] This example proposes a process to manufacture optical fiber
devices for shape sensors which can be used in a number of
biomedical applications. In this example, a polymer extrusion
process is performed on three optical fibers, thereby forming an
optical fiber device with a diameter below 600 .mu.m. Accordingly,
the optical fiber device can be inserted into surgical needles,
catheters and the like in at least some biomedical applications. As
described above, the three optical fibers are fixed into a given
geometrical relationship to form a fiber triplet in this example.
The position of the fiber triplet within the body of coating and
the angle of the fiber triplet are parameters that can be
advantageously controlled to enhance shape measurements. The radial
and angular positions of the optical fibers in the triplet are
measured with an accuracy of 3 .mu.m and 4 degrees, respectively,
in the present example. At least within the context of OFDR
measurements, it was demonstrated that an optical fiber device
incorporating optical fibers with ROGUE FBGs could enhance shape
sensing.
[0128] Similarly to as shown in FIG. 5, the depositing of the
coating material is performed using a twin-screw extruder from
Leistritz. As depicted, the die is used in a way that the three
optical fibers are coated with the coating material simultaneously
as their relative positioning is maintained. The lower screw speed
that could be reached was 4.2 rotations per minute. The fibers were
pulled at a speed of 17 cm-s-1. The temperature profile along the
die was a humped profile starting at 130 degrees Celsius in a first
zone, gradually increasing to 195 degrees Celsius in a melt zone
and slowly decreasing to 160 degrees Celsius in the head of the
die. The optical fibers used were either uncoated SMF-28 (125 .mu.m
diameter) incorporating random gratings or polyimide coated SMF-28
(155 .mu.m diameter) optical fiber. The extrusion tip diameter was
660 .mu.m. The head exit had an opening of 940 .mu.m. Once coated,
the optical fibers were cooled using a cooler providing a forced
airflow. This cooling method can induce a turbulence at the exit of
the die which can compromise the uniform and constant positioning
of the optical fibers of the triplet. Accordingly, a cooler
providing water-cooling was used instead to solve the turbulence
challenge. The coating material used in this example is a polymer,
and more specifically the M6823MZ from Total, which is a random
copolymer made of polypropylene and ethylene. It has been chosen
for at least two reasons in this specific example. First, its
melting point is at 136 degrees Celsius, which proved to be a
convenient not to damage the optical fibers and/or erase the FBGs
inscribed therein. Secondly, its Melt Flow Index (MFI) is 30 g/10
min, which appeared to be an acceptable compromise between a
fluidity that allow the extrusion of a thin enough coating and a
melt strength high enough, so the optical fibers do not slip inside
the optical fiber positioner once it has set when the optical
fibers undergo strain.
[0129] Sensing was performed using three 35 cm long ROGUE FBGs with
a respective 45 dB, 50 dB and 25 dB backscatter enhancement on a 5
nm bandwidth centered at a wavelength of 1549.7 nm, as shown in
FIG. 7A. The backscatter measurement has been performed using an
optical backscatter reflectometer (Model OBR 4600 from Luna
Innovations Inc). The enhancement backscatter was found to be more
than sufficient to enable the three ROGUE FBGs to be connected
together to the source of the optical interrogated simultaneously
using a 1.times.3 coupler with a different delay before each ROGUE
FBG. These measurements can be seen in FIG. 7B. The scanning was
performed on a 5.24 nm wavelength range centered at 1550.5 nm.
[0130] Measurement over 20 cm showed that the angle formed by the
three optical fibers was stable within .+-.2 degrees. Furthermore,
the center of the fiber triangle is stable within .+-.1.5 .mu.m
from the center of the body of coating material. However, the
absolute values may change from one extrusion to another. FIG. 8A
shows a transversal view of an example of an optical fiber device
800, with an optical fiber triplet 802 surrounded by a body 804 of
material. Although it can be seen that the triangle is not
necessarily equilateral (which may be an optical form for shape
sensing with three fibers), it is however always isosceles. The
triangle shape can be better controlled by customizing the
extrusion head tip. The 600 .mu.m diameter triplet is small enough
to go through the 950 .mu.m diameter hole catheter. FIG. 8B shows a
top view of the optical fiber device 800. The coating diameter
(.about.600 .mu.m) has a variability of around .+-.5%. However,
even if the triangle formed by the fibers keep the same shape
within the coating and its radial position remains stable, a huge
variability was noticed (more than 30 degrees) in the angular
position of the triangle relatively to the coating center. The
turbulence generated from air cooling at the extrusion head exit
could explain this observation. Improved results can be expected
once a smoother cooling process is set up. An advantage that was
noticed was the space left in the body of coating material. Such
space could be filled with one or more optical fibers, wires and/or
capillaries in order to make a multimodal sensor/surgical tool.
[0131] FIGS. 9, 9A and 9B shows another example of an optical fiber
device 900 fabricated using the method described herein. In this
specific example, the optical fiber device 900 has an optical fiber
triplet 902 surrounded by a body 904 of coating material. In this
example, the optical fibers have a diameter of 155 .mu.m thereby
filling more of the opening of the inner surface of the nozzle
during the coating material deposition process. As the engagement
between the inner surface of the nozzle and the optical fibers is
snugger, the optical fibers were more squeezed within the inner
surface of the nozzle which forced them to be closely adjoining to
one another thereby forming the illustrated equilateral triangle
shape. It is intended that the optical fiber device 900 can include
at least an additional component 910 relative to the optical fiber
triplet 902 inside said body 904 of coating material. The
additional component can be one or more of any one of the following
group of components: an electrical wire, a conductive glass fiber,
a capillary fiber, a photonic crystal fiber, a laser delivery fiber
and any other suitable component. FIGS. 10A and 10B show radius and
angle variations along a length of the optical fiber device
900.
[0132] Strain measurement using the LUNA OBR4600 shows a stability
around of 1% (typically few micro-strain variability for a few
hundred micro-strain measurement) when repeating a measurement of a
same shape. However, the previously discussed unknown and random
twist of the triplet along the body of coating material can make
the shape reconstruction more challenging. Regardless, the
potential for excellent strain measurement accuracy will allow
highly accurate shape reconstruction results using the
so-fabricated optical fiber device.
[0133] In this example, optical fiber triplets for sensor
applications have been fabricated using direct polymer extrusion.
The extrusion process seems to make the manufacturing of fibers
triplet possible at reduced cost since one extrusion can generate
an arbitrarily long triplet with a good stability in a single
extrusion iteration with respect to geometric parameters. In some
embodiments, it is envisaged that a more effective and less
aggressive cooling method can be used to reduce any angular
position variability of the triplet within the body of coating
material. Another advantage of the extrusion process is that it is
very flexible. It would be easy to insert more optical fibers,
electrical wires sensors and/or other sensing/surgical tools within
the body of coating material for additional functionality. A single
fiber could be added in order to measure the temperature along with
a strain measurement in parallel to measure ambient changes during
shape sensing, which could thereby allow the strain measurements to
be compensated with local temperature variations. Another advantage
to such an extrusion process is that the availability of a huge
variety of polymers provides a large choice of coating properties
for specific applications.
[0134] For strain measurements, the strong backscattered signal of
the ROGUE FBG was found to be convenient. The signal may be strong
enough to accommodate an insertion loss factor of 10 dB (forward
and return loss in a 1.times.3 coupler) while still maintaining a
good SNR for accurate strain measurement, even when scanning a
relatively narrow band (5.24 nm). Furthermore, the simultaneous
measurement coupled with the narrow band scan make the response
time such that perform shape sensing in real time may be
envisaged.
[0135] Other aspects of the present disclosure may encompass
improvements generally relating to distributed temperature and
strain optical sensing (hereinafter "DTSS") systems and more
specifically relating to fiber-based DTSS systems.
[0136] Typical DTSS systems generally include an optical
interrogator which is optically coupled to a sensing optical fiber.
The optical interrogator is configured for emitting an optical
signal along the sensing optical fiber, and for receiving a return
optical signal returning from the sensing optical fiber as the
optical signal propagates along the sensing optical fiber.
Measuring temperature change(s) and/or strain change(s) to which
the sensing optical fiber are subject can be done by performing
several measurements over time. In at least some situations, it may
be convenient to monitor temperature change(s) and/or strain
change(s) using a plurality of sensing optical fibers to monitor
different environments or parts at once. To do so, it is known to
use the plurality of optical fibers concurrently with a
corresponding plurality of optical interrogators or to couple the
sensing optical fibers to a single a multi-channel optical
interrogator. In some other situations, a single optical
interrogator 1102 is sequentially optically coupled to a plurality
of optical sensing optical fibers 1104 using an optical switch
1106, as shown in the existing DTSS system 1100 of FIG. 11.
Although existing DTSS systems are satisfactory to a certain
extent, there remains room for improvement. For instance, using the
two former options can be costly whereas the latter option is only
limited to sequential measurements.
[0137] FIG. 12 shows an example of a DTSS system 1200, in
accordance with an embodiment. As depicted, the DTSS system 1200
has an optical interrogator 1202, an optical coupler assembly 1204
which is optically coupled to the optical interrogator 1202 and a
plurality of optical fiber devices 1206 which are optically
connected to the optical coupler assembly 1204.
[0138] More specifically, in this example, the optical coupler
assembly 1204 has an input 1208 being optically coupled to the
optical interrogator 1202 and a plurality of outputs 1210 to which
are optically connected the optical fiber devices 1206. As shown,
the optical coupler assembly 1204 is provided in the form of a
two-by-two (e.g., a 50:50) fiber coupler 1212 in this example. This
two-by-two fiber coupler 1212 can be referred to as a one-by-two
fiber coupler by at least some manufacturers. Nevertheless,
two-by-two fiber couplers and/or one-by-two couplers could be used
in the DTSS system 1200. Accordingly, the input 1208 is referred
herein to as an optical fiber input 1208 and the outputs 1210 are
referred to as optical fiber outputs 1210. However, other types of
optical coupler assemblies could alternately be used. For instance,
non-fibered coupler assemblies such as free-space coupler
assemblies could be used in some other embodiments.
[0139] As such, during use, the optical interrogator 1202 is
configured for emitting an optical signal at the optical fiber
input 1208 which will be propagated, at least to a certain extent,
along the optical fiber outputs 1210 and then to the optical fiber
devices 1206. The optical interrogator 1202 is configured for
receiving, in response to the emission of the optical signal, one
or more return signals returning from corresponding ones of the
optical fiber devices 1206. As shown, the optical interrogator 1202
has an optical emitter 1214 for emitting the optical signal and an
optical receiver 1216 for receiving the return signal. Examples of
such optical emitter 1214 and receiver 1216 are presented below. As
can be understood, the optical interrogator 1202 typically has an
internal optical coupler (not shown) so as to allow emission and
reception of optical signals at a single optical port 1218, to
which the optical fiber input 1208 of the optical coupler 1212 is
connected in this example. In some other embodiments, an optical
circulator could have been used instead of or in addition to the
internal optical coupler.
[0140] In the illustrated example, the optical fiber devices 1206
have first and second optical fiber devices 1206a and 1206b. The
first optical fiber device 1206a has a first sensing optical fiber
1220 which is serially connected to a first one of the optical
fiber outputs 1210 of the optical coupler assembly 1204. The second
optical fiber device 1206b has an optical path extender 1222 which
is serially connected to a second one of the optical fiber outputs
1210, and a second sensing optical fiber 1224 which is serially
connected to the optical path extender 1222.
[0141] As shown in this example, the optical path extender 1222 is
provided in the form of a length of optical fiber which extends the
optical path length of an optical signal propagating therein. The
optical path extender 1222 shown in this example can thus be
referred to as a delaying optical fiber 1226. However, in some
other embodiments, the optical path extender 1222 can be provided
in the form of a multipass cell in which the optical patch is
increased by a series of reflections on a plurality of reflective
surfaces or in the form of other types of optical path
extenders.
[0142] More specifically, the delaying optical fiber 1226 has an
optical path length which is equal to or greater than an optical
path length of the first optical fiber device 1206a. In this way,
during use, the optical interrogator 1202 is configured for
receiving, in response to the emission of the optical signal at the
optical coupler assembly 1204, a first return optical signal
returning from the first sensing optical fiber 1220 and a second
return optical signal returning from the second sensing optical
fiber 1224.
[0143] As can be understood, due to the extended optical path
caused by the presence of the delaying optical fiber 1226 in the
second optical fiber device 1206b, the first and second return
optical signals are temporally delayed from one another when they
arrive at the optical interrogator 1202. Such a configuration can
thus allow independent measurements to be taken on the first and
second sensing optical fibers 1220 and 1224 even when using an
optical signal having a single optical pulse, for instance.
Examples of such first and second return optical signals are shown
in FIG. 12A.
[0144] Still referring to FIG. 12, the DTSS system 1200 has a
controller 1230 which is communicatively coupled, via wired
communication and/or wireless communication, to the optical
interrogator 1202 for processing and/or storing data indicative of
the received return optical signal(s). As can be understood, the
controller 1230 can be provided as a combination of hardware and
software components. The hardware components can be implemented in
the form of a computing device 1300, an example of which is
described with reference to FIG. 13 whereas the software components
of the controller can be implemented in the form of a software
application.
[0145] Referring to FIG. 13, the computing device 1300 can have a
processor 1302, a memory 1304, and I/O interface 1306. Instructions
1308 for performing the method to perform distributed strain and/or
temperature measurements can be stored on the memory 1304 and
accessible by the processor 1302.
[0146] The processor 1302 can be, for example, a general-purpose
microprocessor or microcontroller, a digital signal processing
(DSP) processor, an integrated circuit, a field programmable gate
array (FPGA), a reconfigurable processor, a programmable read-only
memory (PROM), or any combination thereof.
[0147] The memory 1304 can include a suitable combination of any
type of computer-readable memory that is located either internally
or externally such as, for example, random-access memory (RAM),
read-only memory (ROM), compact disc read-only memory (CDROM),
electro-optical memory, magneto-optical memory, erasable
programmable read-only memory (EPROM), and electrically-erasable
programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM) or
the like.
[0148] Each I/O interface 1306 enables the computing device 1300 to
interconnect with one or more input devices such as the optical
interrogator 1202, a keyboard, a mouse and the like, or with one or
more output devices such as a display, a memory and the like.
[0149] Each I/O interface 1306 enables the controller 1230 to
communicate with other components, to exchange data with other
components, to access and connect to network resources, to serve
applications, and perform other computing applications by
connecting to a network (or multiple networks) capable of carrying
data including the Internet, Ethernet, plain old telephone service
(POTS) line, public switch telephone network (PSTN), integrated
services digital network (ISDN), digital subscriber line (DSL),
coaxial cable, fiber optics, satellite, mobile, wireless (e.g.
Wi-Fi, WiMAX), SS7 signaling network, fixed line, local area
network, wide area network, and others, including any combination
of these. Of course, the controller 1230 is optional as it can be
omitted in certain embodiments. The computing device 18 of the
shape sensing system 10 described with reference to FIG. 1 can be
similar to the controller 1230 described with reference to FIGS. 12
and 13.
[0150] Referring back to FIG. 12, it is noted that although the
first optical fiber device 1206a is shown without a delaying
optical fiber 1226, the first optical fiber device 1206a can have a
delaying optical fiber connected between the first one of the
outputs 1210 of the optical coupler assembly 1204 and the first
sensing optical fiber 1220 in some other embodiments. In such
embodiments, the optical path length of the delaying optical fiber
of the second optical fiber device 1206b is chosen so as to be
equal or greater than the length of the first optical fiber device
1206a, i.e., equal or greater than the length of the delaying
optical fiber of the first optical fiber device 1206a and the first
sensing optical fiber 1220 combined to one another.
[0151] Although the DTSS system 1200 described with reference to
FIG. 12 has first and second optical fiber devices 1206a and 1206b,
other embodiments can have more than two optical fiber devices.
Such examples are presented with reference to FIGS. 14 and 15.
[0152] FIG. 14 shows an example of a DTSS system 1400 having an
optical interrogator 1402, an optical coupler 1404 having an
optical fiber input 1406 connected to the optical interrogator
1402, and first, second and third optical fiber outputs 1408 to
which are connected respective ones of first, second and third
optical fiber devices 1410a, 1410b and 1410c. The first and second
optical fiber devices 1410a and 1410b of this example are similar
to the ones described with reference to FIG. 12. For clarity
purposes, the delaying optical fiber of the second optical fiber
device 1410b will be referred to as the first delaying optical
fiber 1412a.
[0153] As shown, the third optical fiber device 1406c has a second
delaying optical fiber 1412b which is serially connected to a third
one of the optical fiber outputs 1410, wherein the second delaying
optical fiber 1412b has an optical path length being equal to or
greater than an optical path length of the second optical fiber
device 1410b, including both the first delaying optical fiber 1412a
and the second sensing optical fiber 1414b. As shown, a third
sensing optical fiber 1414c is serially connected to the second
delaying optical fiber 1412b. As such, during use, the optical
interrogator 1402 is configured for receiving a third return signal
returning from the third sensing fiber 1414c in a manner that the
first return optical signal returning from the first optical fiber
device 1410a, the second return optical signal returning from the
second optical fiber device 1410b and the third return signal
returning from the third optical fiber device 1410c are all
temporally delayed from one another.
[0154] Also shown in this example, the optical coupler 1404 is
provided in the form of a three-by-three fiber coupler 1416 having
one optical fiber input 1418 and three optical fiber outputs 1420.
In other embodiments, four-by-four fiber couplers, five-by-five
fiber couplers and/or X-by-Y fiber couplers (where X and Y are
positive integers) could alternately be used.
[0155] FIG. 15 shows another example of a DTSS system 1500, in
accordance with an embodiment. In this embodiment, the optical
fiber coupler 1504 has one optical fiber input 1502 and four
optical fiber outputs 1504. In this way, four different optical
fiber devices 1506a, 1506b, 1506c and 1506d can be connected to
respective ones of the optical fiber outputs 1504.
[0156] In this example, the DTSS system 1500 has an optical
interrogator 1508, the optical coupler assembly 1504 having the
optical fiber input 1502 connected to the optical interrogator
1508, and first, second, third and fourth optical fiber outputs
1504 to which are connected respective ones of first, second, third
and fourth optical fiber devices 1506a, 1506b, 1506c and 1506d. The
first, second and third optical fiber devices 1506a, 1506b and
1506c of this example are similar to the ones described with
reference to FIG. 14.
[0157] Moreover, in this example, the fourth optical fiber device
1506d has a third delaying optical fiber 1510c which is serially
connected to the fourth optical fiber outputs 1504 of the optical
coupler assembly 1504. The third delaying optical fiber 1510c has
an optical path length which is equal to or greater than an optical
path length of the third optical fiber device 1506c, including both
the second delaying optical fiber 1510b and the third optical
sensing fiber 1512c. A fourth sensing optical fiber 1512d is
provided in a serial connection to the third delaying optical fiber
1510c. As such, during use, the optical interrogator 1508 is
configured for receiving a fourth return signal returning from the
fourth sensing fiber 1512d. Accordingly, the first return optical
signal returning from the first optical fiber device 1506a, the
second return optical signal returning from the second optical
fiber device 1506b, the third return signal returning from the
third optical fiber device 1508c and the fourth return signal
returning from the fourth optical fiber device 1506d are all
temporally delayed from one another.
[0158] In this example, the optical coupler assembly 1504 has a
cascade of two-by-two fiber couplers 1514 which are connected to
one another to provide the four distinct optical fiber outputs 1504
to which the first, second, third and fourth optical fiber devices
1506a, 1506b, 1506c and 1506d are connected.
[0159] Of course, other exemplary DTSS systems based on the present
disclosure can have more than four optical fiber devices connected
to respective outputs of the optical coupler assembly.
[0160] Referring back to FIG. 12, the inventors found convenient to
provide one or more scatter increasing devices 1232 along
corresponding one(s) of the first and second optical fiber devices
1206a and 1206b. Indeed, by providing such devices, the strength of
the first and second return signals can be increased, which in turn
allow increased sensitivity. For instance, in the illustrated
embodiment, the scatter increasing device 1232 of the second
sensing optical fiber can help distinguish a portion (low signal)
of the second return signal which returns from the delaying optical
fiber 1226 from a portion (high signal) of the second return signal
which returns from the second sensing optical fiber 1224. As such,
the DTSS system 1200 can be configured to prevent interference
between the low signal returning from the delaying optical fiber
1226 and the high signal returning from the second sensing optical
fiber 1224.
[0161] To achieve such a difference in scattering, the material of
the scatter increasing device can be different from the material of
the delaying optical fiber 1226, as these material can have
naturally different scattering properties. In such a case, a
sensing optical fiber having a material which is different from a
material of a delaying optical fiber 1226 can act as the scatter
increasing device 1232.
[0162] An example of how such an increase in scatter can be
provided includes different optical fibers with naturally different
scattering. This can be changed, for example, by a choice of
material and/or dopants (e.g., plastic versus silica, different
dopants in silica), a choice of dopant concentration, a choice of
quality as low quality optical fibers will have greater scattering
properties and provide a higher signal compared to high quality
optical fibers which will have lower scattering properties and
provide lower signal, a choice of numerical aperture (NA) as an
optical fiber having a higher NA will collect more back-scattering
than an optical fiber having a lower NA, and/or any other suitable
way to induce a change in scattering properties along between the
delaying optical fiber and the sensing optical fiber. For instance,
increased scatter optical fibers as made by Corning.RTM. can have
including Titania particles in their core, which enhance light
scatter significantly.
[0163] An example of how such an increase in scatter can be
provided includes, but not limited to, by changing a concentration
of defects along the device by UV exposure and/or or fs laser
pulses, FBG inscription, random grating inscription, ROGUE FBG
inscription, and the like. Enhanced optical fibers like the one
described in Yan, Aidong, et al. ("Distributed Optical Fiber
Sensors with Ultrafast Laser Enhanced Rayleigh Backscattering
Profiles for Real-Time Monitoring of Solid Oxide Fuel Cell
Operations." Scientific reports 7.1 (2017): 9360) could also be
used as scatter increasing devices.
[0164] FIG. 16 shows an example of a scatter increasing device 1600
similar to one of the scatter increasing devices 1232 of FIG. 12.
More specifically, the illustrated scatter increasing device 1600
is an optical grating 1602 inscribed along a portion of a
corresponding core 1604 of a sensing optical fiber 1606. Reference
numeral 1608 shows a cladding of the sensing optical fiber 1606.
The inscribed optical grating 1602 has a random continuous
distribution such that a return signal, caused by propagation of an
optical signal therealong, has a full width at half maximum (FWHM)
bandwidth ranging between about 0.1 THz and about 40 THz. In some
embodiments, the FWHM bandwidth can preferably range between about
0.35 THz and about 7 THz.
[0165] In some embodiments, the FWHM bandwidth can be related to a
coherence length of the grating, and can be defined as the length
where the visibility of fringes would drop to 1/e of its initial
intensity if the coherent wave in the grating was to interfere with
itself at another location. The coherence length is proportional to
the inverse of the FWHM bandwidth of the return signal, depending
on the spectral shape of the scatter. More specifically, the
coherence length can be equivalent to the following equation:
L c = c .lamda. 2 n .DELTA..lamda. , ( 1 ) ##EQU00001##
[0166] wherein L.sub.c denotes the coherence length, .lamda.
denotes the wavelength of the optical signal propagating in the
grating, C is dependent on the shape of the spectrum of the return
signal, n denotes the refractive index of the optical fiber and
.DELTA..lamda. denotes the FWHM bandwidth.
[0167] Accordingly, in some embodiments, the grating can have a
coherence length which ranges between about 2.lamda. and about
500.lamda. when the return signal has a scattering spectrum with a
Gaussian shape. In some specific embodiments, the coherence length
ranges between about 10.lamda. and about 100.lamda.. This can
correspond to a back-scattering FWHM bandwidth of about 1 nm to 500
nm (at a wavelength of 1550 nm) depending on the scattering
spectral shape. In other words, the back-scattering structure is
less coherent then a uniform FBG, but more coherent than a Rayleigh
scatter structure. It was found that such gratings can cause return
signals to have a FWHM bandwidth which is constant, provided that
the grating has a length which exceeds a given length. In some
embodiments, the given length is in the centimeter range. As shown,
the scatter increasing device can be inscribed in the core of the
sensing optical fiber but also in the cladding(s) of the sensing
optical fiber.
[0168] In some embodiments, the random continuous distribution of
the grating is a random phase distribution. In some embodiments,
the random continuous distribution of the grating is a random
period or wavelength distribution. In alternate embodiments, the
random continuous distribution is a random amplitude distribution.
In still further embodiments, the random continuous distribution
can involve a random phase distribution, a random period
distribution and/or a random amplitude distribution.
[0169] As will be described below, the optical grating, which is
also referred to as a ROGUE FBG in this disclosure, has a length
which is in the centimeter range. For instance, the length of the
grating can be greater than 1 cm, and preferably greater than 2 cm.
In some embodiments, the grating can be inscribed using an
inscribing wavelength which is different from a wavelength of the
optical signal which is meant to be propagated therein. However, in
some other embodiments, the grating can be inscribed using an
inscribing wavelength which corresponds to the wavelength of the
optical signal which is meant to be propagated therein. For
instance, using a point-by-point non-interferometric inscription
technique with a fs pulsed laser, the inscription of the grating is
independent of the inscribing wavelength, but rather only depends
on a repetition rate of the pulsed fs laser and on its speed.
Example 2--Scatter Based Order of Magnitude Increase in Distributed
Temperature and Strain Sensing by Simple UV Exposure of Optical
Fiber
[0170] An example is presented to improve signal strength, and
therefore increase sensitivity in DTSS by Fourier domain scatter. A
simple UV exposure of a hydrogen loaded standard SMF-28 fiber core
is shown to enhance the back-scattered light dramatically by
ten-fold, independent of the presence of a Bragg grating, and is
therefore created by the UV exposure alone. This increase in
back-scatter allows an order-of-magnitude increase in sensitivity
for DTSS compared to un-exposed SMF-28 fiber used as a sensing
element. This enhancement in sensitivity is effective for cm range
or more sensor gauge length, below which is the theoretical
cross-correlation limit. The detection of a 20 mK temperature rise
with a spatial resolution of 2 cm is demonstrated. This gain in
sensitivity for SMF-28 is compared with a high Ge doped
photosensitive fiber with a characteristically high NA. For this
latter, although of less amplitude, the UV enhancement is also
present, and enables a yet even lower noise level of sensing, due
to the fiber's intrinsically higher scatter signal.
[0171] DTSS systems are extremely useful for industrial monitoring,
since they provide real-time information along a region of interest
with low-cost optical fiber. Optical time domain reflectometry
(OTDR) using scatter has been used for decades to investigate
distributed information along a fiber. It has been demonstrated for
DTSS in long lengths of fiber (.about.km), but with poor spatial
resolution (.about.m) and very poor temperature sensitivity
(.about.10.degree. C.). On the other hand, it's Fourier Domain
(OFDR) counterpart gives the highest spatial resolution in DTSS
(.about.mm) while allowing a reasonable temperature sensitivity
(0.1 to 1.degree. C.) and remaining a rather simple and cheap
scheme, compared to other DTSS schemes. However, Rayleigh scatter
OFDR has remained quite limited in terms of sensing length (30-100
m). For this reason, other methods have been developed using Raman
scattering (ROTDR), with allows much longer reach of 1-30 km, and
Brillouin scattering (BOTDR or BOTDA), with even longer lengths of
10-100 km. Both of these techniques however show less sensitivity
(.about.1.degree. C.) and much poorer resolution (1-10 m).
Combinations of technique have also been proposed: Rayleigh and
Brillouin scattering, also known as Landau-Placzek ratio analysis,
and Rayleigh with Raman scattering.
[0172] The main limitation in the sensitivity and accuracy of
Rayleigh scattering DTSS comes from the low scattering signal at
the detector. Higher scattering medium, such as liquids in hollow
core fibers, polymer fibers with larger scattering cross-sections,
or specially designed high scattering silica fibers doped with
various impurities can be used to increase this signal, thus
increase the sensitivity. However, such schemes are non-standard
and therefore expensive to produce and to render compatible with
available optical equipment. There is suggested a simple and
affordable method to radically improve temperature and strain
sensitivity by ten-fold through a dramatic increase in scatter in
standard fiber. This increase comes from simply exposing the fiber
core to UV light, which creates a high density of scattering
defects, such as observed by Johlen et al. in their study of UV
exposure induced losses. Such enhancement in fiber can be easily
induced with any UV laser (solid state, argon) without any critical
alignment or vibration control unlike when writing FBGs. The
UV-exposure is also compared to UV writing of FBGs.
[0173] ODFR allows the measurement of a reflectivity pattern, such
as Rayleigh scattering along a fiber length. The back-scattering
effects of Rayleigh are caused by defects causing a local variation
in the permittivity. As described by Froggatt et al., such a
permittivity can be measured with the knowledge of the spectral
intensity of an interference between the fiber under test and a
reference arm:
.DELTA. .times. _ .function. ( x ) = i .times. n E 0 2 .times. rc
.times. .times. .beta. 0 .times. .pi. .times. .intg. -
.DELTA..beta. .DELTA. .times. .beta. .times. I d .function. (
.beta. - .beta. 0 ) .times. e - i .times. .times. .beta.2
.function. ( x - x 0 ) .times. d .times. .times. .beta. ( 2 )
##EQU00002##
[0174] Where I.sub.d is the measured spectral intensity of the
interference, n is the refractive index, E.sub.0 the input laser
field, r the reflection coefficient of the reference beam and
x.sub.0 the position of the reference reflection. Considering that
the system can have discrete sampling, the corresponding integral
in Eq. (2), the reflected intensity vs position (therefore in the
time domain), can be re-written as:
I ~ = 1 N .times. k = 0 N - 1 .times. I k .times. e - ikm .times. 2
.times. .times. .pi. N ( 3 ) ##EQU00003##
[0175] Where N is the total number of points within the measurement
and I.sub.k is the spectral intensity at different point k along
the scan. The measurement of temperature/strain is relative to a
reference measurement. Both are compared by doing a
cross-correlation over an integration length .DELTA.x, which is
called the sensor gauge length and corresponds to the spatial
resolution of the DTSS:
I k ( ref ) I N ' - k ( test ) * = 1 2 .times. .pi. .times. .times.
N .times. m = m 1 m 2 .times. I ~ m ( ref ) .times. I ~ m ( test )
.times. e ikm .times. 2 .times. .pi. N ' ( 4 ) ##EQU00004##
[0176] Where N' corresponds to the number of points in the
integration length .DELTA.x (as N'=m.sub.2-m.sub.1+1), which is
considered as the sensor length, or gauge length, and corresponds
to the spatial resolution of the DTSS measurement. This
cross-correlation is in the Fourier domain. When there is no strain
or temperature change, a peak is expected at zero frequency. When a
temperature or strain change is applied in the sensor gauge length,
then this peak shifts proportional to the change. Therefore, the
resulting frequency shift is a direct measurement of the observed
change in temperature or strain and such a value can be calculated
for every sensor gauge lengths .DELTA.x. It is desirable to
minimise this length, since it defines the spatial resolution.
However, the longer the sensor length, the higher the peak
intensity in the cross-correlation, giving a higher signal to noise
ratio (SNR). The noise itself is intrinsic to the calculation and
the nature of Rayleigh scattering, i.e. the random fluctuation in
.DELTA..epsilon.(x), therefore is always present. This being said,
the longer the gauge length, the more precisely can the frequency
shift be determined, thus improving the precision of the
temperature or strain measurement. The link between frequency shift
and strain or temperature is determined by a calibration constant
within the OFDR system. It can be shown that such a physical
uncertainty limit can be express as follows:
.DELTA. .times. .times. x .times. .times. res = .lamda. 4 .times.
.times. n .times. .times. .DELTA. .times. .times. xT res = .lamda.
4 .times. .times. n .times. ( d .times. .times. dT ) - 1 ( 5 )
##EQU00005##
[0177] Where .epsilon..sub.res is the strain resolution
(dimension-less) and T.sub.res is the temperature resolution. This
theoretical limit can be easily observed in the results of FIGS.
17A-C for small gauge lengths. However, at longer lengths of
.DELTA.x (1 cm or more), another limit appears: the detector
intensity noise (not taken into account in Eq. (5)) and the
temperature fluctuation along the gauge length.
[0178] To improve these limits, there is presented that a simple UV
exposure on the fiber, such that a very weak out of band reflection
grating is written has the effect of increasing the scattering
emission and collection by ten-fold, thus generating an increase in
sensitivity. UV exposure of hydrogen (or deuterium) loaded SMF-28,
as well as on high germanium content core fiber has the effect of
creating colour centers. However, why such exposure would increase
scatter, which normally comes from defects in a fiber, is still
under investigation. However, the presence of the weak grating away
from the wavelength of measurement improves the scatter signal
dramatically. An increase in back-scattering collection can also be
accounted for by an increase in the NA of the fiber. However, this
increase in collected back-scattered power is expected to be
limited to a factor of <2, for the expected refractive index
change, below 10.sup.-3 in our demonstration.
[0179] Two types of fibers were tested: a standard SMF-28
single-mode telecom fiber by Corning which was hydrogen loaded for
increased photosensitivity and a high NA (0.27) photosensitive
fiber with high Ge content from CoreActive (uvs-eps), referred to
as HNA in this example. in the latter fiber, exposure was made by
continuously exposing the fiber with UV light, while with the
SMF-28, two types of FBGs were tested and compared with the
continuously UV exposed fiber: a uniform grating and a random
grating with randomly positioned phase shifts. FBGs were measured
out-of-band for DTSS, to not be limited by the grating's bandwidth.
Although the FBGs is expected to offer more back-scattering than
simple UV exposure since there is more refractive index variation
within the fiber, surprisingly, results show that this is not the
case, as can be seen in 7B. Indeed, a continuous UV exposure
generates a 20 dB intensity increase in back-scattering return
signal, which corresponds to a ten-fold increase in local losses,
when taking into account the round-trip nature of the measurement
which squares those local losses. Back scattering in the presence
of the grating, uniform or random, also increases by the same order
of magnitude. Although a more complex structure (oscillations) is
observed when a random grating is involved, it was noted that this
did not lead to any gain in strain or temperature sensitivity.
Indeed, whether a grating is present or not, the average exposure
is the same, and an identical gain was observed in terms of
distributed temperature or strain noise. However, it is suspected
that there may be a small contribution from the presence of the
FBG, although this would require further investigation.
[0180] The UV-exposed HNA fiber shows the same improvement in
scatter signal, compared to SMF-28, as can be seen in FIG. 17A.
However, when comparing the effect of the exposure itself, i.e. the
difference in signal between non-exposed HNA and exposed HNA fiber,
the gain is not as great since this fiber already has a rather high
Rayleigh signature (.about.three times that of SMF-28) before
exposition. This signature can be due to its fabrication inducing
more defects as well as from its higher NA, which allows more
collection of the back-scatter. Nevertheless, as seen in FIG. 18B,
this UV-exposed fiber exhibits a DTSS noise level even more
advantageous than UV-exposed SMF-28.
[0181] The gain in back-scattered signal gives rise to a
considerable increase in temperature and strain sensitivity.
Indeed, with this improvement in signal to noise ratio, the
cross-correlation of Eq. (4) yields a more precise frequency shift,
thus a higher sensitivity in temperature or strain measurement.
Note that these measurements are in temperature, but the same
picture can be applied to strain with a factor of 8.32
.mu..epsilon./.degree. C. (calibration factor for silica fiber from
the LUNA system). The collected back-scatter was measured with
varying UV exposure power, as shown in FIG. 18, to understand the
optimal exposure to minimise requirements and maximise gain in
sensitivity. As can be seen, after a rather linear increase, the
gain saturates at around a ten-fold increase. Our choice of power
and exposure time for DTSS tests were based on the best exposure
conditions. The mechanics of such increase and saturation are still
under investigation.
[0182] A quantitative analysis is shown in FIG. 19 where the RMS
noise level was calculated based on a 30 cm section of 1 mm spaced
points. The sensor cross-correlation integration length, which
defines the spatial resolution of the DTSS, was varied from a long
length of 10 cm to a very short length of 1 mm. The limit defined
by Eq. (5) can be observed in these results for short sensor
lengths. The higher SNR of the UV exposed fiber seems to slightly
improve the resolution within this range, where noise is limited by
the cross-correlation on a random structure, typical of scatter.
The most important gain is in the cm range, where the detection
noise, unrelated to backscattering from the material structure and
defects, becomes the dominating limitation. In this same range, the
noise level can be expected to rise slightly as the sensor length
increases, since it becomes more sensitive to thermal fluctuation
along its length. However, the sensing fiber tested here was in a
thermally stable isolated container, which explains why the thermal
noise is more stable and actually decreases slightly with sensor
length. This shows the performance limit of the DTSS system itself,
independent of the environment. From these results in 9, there is
showed a RMS noise level of 10-15 mK in temperature for gauge
lengths of 2 cm or more for UV-exposed SMF-28. If converted to
strain, this gives a RMS noise level of 80-120 n.epsilon.. The
UV-exposed HNA fiber's performance is twice as good compared to the
SMF-28 with an RMS noise level of .about.5 mK (40 n.epsilon.), the
best performance to date with this resolution level.
[0183] The difference in noise and ease of measurement can be
appreciated in FIG. 10A, where we can see the improvement in the
measurement along the fiber, which is placed in an isothermal,
stable and isolated container. Another measurement example is shown
in FIG. 9B where the resolution and sensitivity can be appreciated.
In this case, a thin 0.2 mm diameter wire heated by a low current
of 20 to 100 mA is placed in contact with a fiber (UV-exposed
SMF-28) in a perpendicular fashion. With a 2 cm sensor integration
length, the point-spread-function of such a DTSS measurement may be
observed. The heating by 20 mK may be seen in the bottom curve in
FIGS. 20A and 20B. To further increase the resolution and
measurement quality, a spatial averaging was performed on
surrounding points. The equivalent length of this spatial averaging
was chosen as half the sensor integration gauge length so as to not
further limit the spatial resolution.
[0184] While it has been nominally noted in the past that UV
exposure increases back scatter in optical fiber, there has been no
systematic study to either understand it or to apply it. In this
example, we have undertaken to find the conditions to maximise
scatter through UV radiation and also with the writing of weak
off-band Bragg gratings, and then to use the increase in what we
believe is the first application in sensing. We have shown here
that a simple continuous UV exposure of a hydrogen loaded SMF-28,
increases the back-scattered intensity by ten-fold, thus allowing a
ten-fold increase in DTSS sensitivity. Increase in the collected
back-scatter from a gain in the NA of the fiber can account for a
factor of only 2 for the exposure used here, therefore the
remaining back-scatter signal gain comes from an increase in
scatter itself. The reason for this increase is still under
investigation. However, with the presence of a weak off-band Bragg
grating in the optical fiber, the scatter increases dramatically
due to the nature of side modes of the grating. The increase in the
signal, greatly improves the SNR at the detector, therefore pushing
down the noise floor in DTSS measurements to the theoretical
sensitivity/spatial resolution limit. Indeed, for a sensor
integration length of 2 to 10 cm, a RMS noise level of 10 mK or 80
n.epsilon. was obtained in a thermally stable environment in
standard UV exposed H2 loaded fiber, after the removal of the
hydrogen. An even lower noise floor was shown with a high NA
photosensitive fiber to 5 mK or 40 n.epsilon., the best reported
performance, to our knowledge, for a 1-2 cm range gauge length. In
comparison, Gifford et al. demonstrated recently a 1 mK resolution,
but with a 12 cm gauge length using weak semi-continuous FBG to
increase return signal. However, when using Bragg gratings in-band,
one is limited by the band-width of the grating, thus limiting the
spatial resolution. Since UV-exposure affects the entire spectrum
of scatter, our method of improvement does not involve any
theoretical bandwidth limit, except equipment limitation from the
scan range and practical consideration of measurement time.
[0185] With a saturating exposure in standard fiber, we can expect
to further push back this limit to 5 mK noise level for a 2 cm
sensor and perhaps 1 mK for a 10 cm spatial resolution, although at
this stage, applications are limited to a very stable environment.
Improving back-scatter is very simple, since it only requires a UV
laser and a focusing element. No critical alignment or vibration
stabilization is required. Although we used hydrogen loaded SMF-28
in our demonstration, the same effect was shown in photosensitive
fiber exhibiting a similar UV interaction mechanism, i.e. color
center generation, such as highly doped germanium fiber. Therefore,
UV exposure can be performed easily during the drawing process in
such a photosensitive fiber before the coating phase. It is also a
much easier technique to improve sensitivity than writing multiple
gratings along the sensing fiber, which does increase sensitivity,
while sacrificing spatial resolution and increased fabrication
costs.
[0186] Continuous UV exposure was performed using our high
precision FBG writing system without writing and with writing a
weak off-band FBG. The fiber core was illuminated with 213 nm
wavelength from the 5.sup.th harmonic of a 1064 nm solid-state
laser (from Xiton Photonics GmbH). For DTSS tests, the fiber was
exposed with 50 mW of power at 50 .mu.m per second with a spot size
of .about.200 .mu.m giving a uniform exposure time of .about.4
seconds. Continuous UV exposure was compared to an out of band OFDR
signature of FBGs written with the same exposure time and power, by
a direct holographic writing technique using the same experimental
setup. For scattering characterisation tests with exposure, power
or speed was varied along a length of 100 mm. DTSS was performed
using a commercial OFDR system from LUNA. Cross-correlation to
resolve the frequency shift was also performed by the same
commercial system. The sensing fiber was placed in a thermally
stable environment (insulated box) to eliminate thermal
fluctuations to provide a real sense of the measurement's noise
limits. The long gratings were interrogated out-of-band to allow
maximum penetration of the input light and to ensure a maximum
sweeping range of the OFDR system. In such a case, it is not the
grating resonance that is used, but the microscopic index variation
due to the periodic nature of the refractive index modulation and
which generate enhanced back-scatter. In order to ensure there is
no contribution to the scatter measurements from the molecular
hydrogen in the fiber, all measurements were in the following weeks
after the UV exposure to allow the hydrogen to diffuse out at room
temperature.
Example 3--Method of Inscribing an Optical Grating
[0187] The ROGUE FBG was written in the fiber using a Talbot
interferometer based FBG writing station. The fiber core is exposed
to a 213 nm wavelength laser, using the 5.sup.th harmonic of a 1064
nm laser. FIG. 21 presents this setup. The first two orders (+1 and
-1) are reflected by mirrors onto the fiber, where an interference
pattern is created. This interference pattern consists of very
small regions on the fiber where the UV power is high, followed by
regions where the UV power is low, in a periodic fashion. The UV
exposure increases the refractive index locally, creating an FBG.
By changing the angle of the mirrors, the interference pattern step
(and thus the FBG wavelength) can be modified.
[0188] When writing a long FBG, the fiber is moved continuously
under the phase mask. In order to preserve the interference pattern
on the fiber, a sawtooth electric wave is applied to a
piezoelectric element moving the phase mask at the same rate as the
fiber, and then bringing it back after a certain movement
amplitude. If the sawtooth frequency and amplitude match the FBG
period and fiber moving speed, the interference fringes will
overlap and a continuous, very high quality FBG will be written in
the fiber. If the sawtooth wave does not have the right frequency
or amplitude, the interference pattern will erase itself, since the
entire length of the fiber will be exposed to the UV light, instead
of specific interference fringes. However, the UV exposure is not
completely uniform because of noise in the system, and a random
interference pattern will appear in the fiber, leading to
reflectivity over many different wavelengths. This reflectivity
pattern is similar to having a very weak, very broadband Bragg
grating all along the entire length of the fiber. However, it can
be easily modeled as a multitude of very small, randomly placed
gratings, so that the ROGUE FBGs bandwidth will stay the same as
its length increases, contrary to a uniform Bragg grating, whose
bandwidth decreases as the length is increased.
[0189] In order to increase the reflectivity of the ROGUE FBG, all
that is needed is to increase the noise and slow down the rate of
movement of the fiber. In order to do so, we simply replace the
sawtooth wave by a random electric signal. This way, we are not
dependent on the noise in the system but can generate the noise
ourselves, increasing the backscattered signal by orders of
magnitude.
[0190] Another way to make such a ROGUE FBG would be to place the
phase mask close to the surface of the fiber, instead of having a
Talbot interferometer configuration, as shown in FIG. 22. The
diffraction orders +1 and -1 directly interfere and form a
near-field fringe pattern on the optical fiber, generating an FBG.
The disadvantage of this technique is that the FBG central
wavelength cannot be changed, unless the phase mask is moved as in
the case of the Talbot scheme with a piezoelectric element. The
wavelength can be changed by the speed of the fiber relative to the
movement of the piezo. However, a ROGUE FBG can be written in a
similar way as with the Talbot interferometer, by moving the fiber
and applying a random electric signal on the piezoelectric.
[0191] Two different optical fibers were studied: standard SMF-28
optical fiber from Corning, the most widely used fiber in
telecommunications, and a SM1500 highly Germanium doped fiber from
FiberCore, an intrinsically photosensitive fiber with more than 5
times more Germanium than standard optical fibers. Both fibers were
loaded with molecular hydrogen in order to increase
photosensitivity.
[0192] Using the setup explained in the previous section, we wrote
ROGUE FBGs with a backscatter intensity ranging from under 5 to
over 50 dB above standard SMF-28. FIG. 23 show the results of such
a ROGUE FBG in both, the temporal and spectral domains, written in
SMF-28 fiber. A 45 dB increase in backscattered amplitude can be
observed over the signal level of the unexposed fiber in the ROGUE
FBG in the temporal domain in FIG. 23A. The reflection spectrum of
the ROGUE FBG is shown in FIG. 23B. Its impressive spectral width
(48 nm full width, 7 nm full width at half maximum) generates an
important increase in signal over a very wide range of
wavelengths.
[0193] By modifying the random signal amplitude and frequency
applied to the piezoelectric element, the ROGUE FBG writing speed
and the laser power, the strength of the ROGUE FBG can be modified
and optimized. From our experiments, an amplitude of 5 V (the
maximum that could be applied to our piezoelectric unit,
corresponding to about 10 periods) and a frequency of 20 Hz yielded
the best results. For higher frequencies, the ROGUE FBG amplitude
decreases, most likely because the piezoelectric element cannot
follow the random signal that is applied to it, thus reducing the
effective amplitude of its movement. The writing speed and power
was found to influence the strength of the ROGUE FBG significantly,
as shown in FIG. 24. In this figure, 0 dB corresponds to the signal
level of unexposed SMF-28 fiber. Measurements were made on the OBR
4600 using a 21.16 nm scanning bandwidth. The scanning bandwidth we
established was best for sensing (as will be shown in the next
sections).
[0194] The noise levels of both, standard, unexposed SMF-28 fiber
and the ROGUE FBGs were measured by placing the fiber inside an
insulated box, in order to avoid environmentally induced
perturbations, such as air currents. In order to avoid problems
related to the OBR's dynamic range, two different fibers were used,
one in which a ROGUE FBG was inscribed, and another one that was
left untreated. Indeed, during our experimentations, we realized
that the inscription of a ROGUE FBG over one part of the fiber
influenced the measurements on the rest of the fiber. For both
fibers, a 30 cm sensing range was selected, and the spectral shift
was calculated at every 1 mm along this sensing range with a 1 cm
gauge length, leading to 300 sensor points per fiber. FIG. 25
presents the results of the root mean square (RMS) error of both
fibers, calculated over all sensor points in the selected part of
the fiber, and averaged over 15 measurements. In FIG. 25, results
directly taken from the OBR are compared to the results after
adding a correction to the OBR data treatment (the nature of the
correction is beyond the scope of this report). All measurements
relate to the sensing of strain or temperature variations and are
measured in GHz. For SMF-28, 1 GHz corresponds to a variation of
temperature of 0.801.degree. C. or to a strain variation of
6.668.mu..epsilon. (a stretching or compression of
6.668.times.10.sup.-4%).
[0195] From FIG. 25, it can be observed that, for the unexposed
fiber, the RMS error decreases when the bandwidth is increased.
That is to be expected, since an increase in bandwidth leads to a
higher signal to noise ratio (SNR) However, surprisingly, the
opposite happens in the case of the ROGUE FBG. That is most likely
due to the fact that, contrary to the unexposed fiber, most of the
signal comes from a very narrow spectrum. As such, the SNR actually
increases when the scanning bandwidth is smaller, because only the
portion of the spectrum of the ROGUE FBG is probed during the laser
scan.
[0196] In order to characterize the accuracy of our sensors, we
slowly stretched a 1.15 m fiber in increments of 1 .mu.m using our
nanometer precision stage (air bearing stage from Aerotech), and
compared the strain measured by the sensor to the calculated value.
All the scanning bandwidths offered by the OBR were tested, and
both, the ROGUE FBG and the unexposed SMF-28 fiber were compared
with the calculated values. Again, two different fibers (one in
which a grating was inscribed, the other without) were tested
separately, so that the grating did not influence the measurement
on the unexposed fiber. The results are shown in FIGS. 26A-F. As
can be seen, the ROGUE FBG yields far more accurate results than
the unexposed SMF-28. Furthermore, the ROGUE FBG remains very
similar to the calculated values for very short scanning
bandwidths, and it is not until the smallest scanning bandwidth of
1 nm is used that significant errors can be observed. Measurements
were taken while the fiber was static, in between stage movements,
so that the strain remains constant during the frequency scan.
[0197] From these measurements, the root mean square errors between
calculated and experimental values of the spectral shift were
calculated over the 20 .mu.m stretching. In the algorithm, an 8 cm
sensing region was established on both, the ROGUE FBG and the
unexposed SMF-28 fiber, with sensors at every 1 mm (80 sensors
total). The RMS error was calculated across all those sensors, over
the 20 .mu.m stretching. As FIG. 27 shows, the behavior for the
ROGUE FBG is very similar to the unexposed fiber, but the error is
significantly lower, almost by an order of magnitude for every
scanning bandwidth. In the case of the SMF-28, the error is minimal
for the largest bandwidth, which is to be expected, but in the case
of the ROGUE FBG, we see that the minimal error is actually for a
21.16 nm bandwidth. The explanation for this is the same as for the
noise level, which is that since most of the signal is from a
narrower bandwidth, a larger bandwidth does not necessarily yield a
more accurate measurement. At this bandwidth, the RMS error is 4.5
smaller than with standard SMF-28. It can be observed that, using a
ROGUE FBG, a scanning bandwidth of only 5.24 nm is sufficient to
beat the best accuracy obtained with the SMF-28.
[0198] Finally, we evaluated how the strength of the ROGUE FBG
influenced the accuracy of the measurement. In order to do this,
ROGUE FBGs of various strengths were written, and the same
measurements were performed over 20 .mu.m stretching. Only the
21.16 nm bandwidth (the one that showed the smallest error) was
used for the scan. FIG. 28 shows how the error is influenced by the
ROGUE FBG gain. Unsurprisingly, for stronger gratings the mean
error decreases. The small increase in the error as the gain of the
ROGUE FBG increases is currently under investigation. However, it
is noted that by decreasing the writing speed of the ROGUE FBG, it
becomes stronger, to the cost of a slightly smaller bandwidth.
[0199] In order to characterize the ROGUE FBGs ability to
compensate for loss in the system, we repeated the same experiment
with the three strongest ROGUE FBGs while inducing optical loss
before the grating, to see how that affected the spectral shift
accuracy. FIG. 29 presents these results as a function of loss in
measurement signal, in which the optical loss was induced by a
variable optical attenuator placed before the ROGUE FBG. As
increasing loss is induced before the grating, the error increases
slowly until a threshold is reached and a catastrophic increase in
error occurs. This occurs when the loss gets close to the ROGUE FBG
enhancement value. As such, the SNR is decreased below the
requirement for proper cross-correlation. When the noise level has
the same amplitude as the maximum of the ROGUE FBG reflectivity,
all the information is lost in noise, and the algorithm can no
longer recover the spectral shift.
[0200] Using the ROGUE FBG fabrication technique described earlier,
we were able to increase the backscattered signal by orders of
magnitude, i.e. to over 50 dB above standard SMF-28 levels. This
increase in signal turns into an improvement of over an order of
magnitude in RMS noise level, and an RMS error on accuracy in
strain measurements 4.5 times smaller than standard SMF-28 fiber.
As such, the most accurate and precise ROGUE FBGs we were able to
fabricate, exhibited an RMS noise level of 0.016 GHz (0.1 pc or 13
mK) and 0.05 GHz spectral shift RMS error (0.34 pc or 40 mK) for a
stretching range from 0 to 20 .mu.m of the 1.15 m fiber length (0
to 17.4 pc). It is important to note that, for these last
measurements, the fibers were placed inside a closed space that did
not provide an environment as controlled as the insulated box that
was used in the other set of experiments. As such, temperature
fluctuations of this magnitude can be expected, and even more
accurate readings could thus be expected when placing the fibers in
a better controlled environment. Even though we showed that
increasing the backscatter by more than 25 dB does not seem to
improve measurement accuracy, for cases where significant loss
occurs in the system, it is still worthwhile to increase the
backscatter above 50 dB, in order to compensate for that loss. In
case of loss, there is little wiggle room available with untreated
standard SMF-28.
[0201] Since the scatter enhancement technique relies on noise
during the writing process, the optical alignment is not critical,
and the experimental conditions and the equipment do not require
extensive control. Relatively fast writing speeds of 1 or even 10
mm/s yield sufficient increase in backscatter to noticeably enhance
the accuracy of such sensors without requiring an enormous amount
of laser power. The experimental setup used in this example relies
on a Talbot interferometer configuration but could as easily be
used by direct inscription of the grating with the fiber directly
behind the phase mask. As such, this technique could thus be
implemented in an industrial assembly line, or even during the
drawing process of the fiber. Arguably environmental fluctuations
or equipment vibrations during the ROGUE FBG writing could
potentially even increase the strength of the ROGUE FBG by adding
other sources of noise.
[0202] It is being noted that this enhancement is limited to the
bandwidth of the ROGUE FBG. The laser power and writing speed are
not sufficient to generate uniform Rayleigh enhancement, and as
such scanning outside the ROGUE FBG does very little to enhance
backscatter. However, scanning bandwidths of 21 nm and under are
typically more than enough for most applications, and smaller
bandwidths are usually favored for real-time applications, because
they allow faster acquisition speeds.
Example 4--Order of Magnitude Increase in Resolution of Optical
Frequency Domain Reflectometry Based Temperature and Strain Sensing
by the Inscription of a ROGUE FBG
[0203] OFDR has been investigated for two decades as a way to
replace the FBG currently used in most industries for sensing
applications, using the intrinsic Rayleigh scatter of fibers
instead. OFDR allows completely distributed strain and temperature
measurements along a fiber. The increase of backscatter using UV
laser exposition was recently reported, and was found to increase
the sensitivity in both temperature and strain sensing. We present
a technique that increases the backscattered signal amplitude by
over 50 dB, based on the writing of a ROGUE FBG, i.e. a very weak,
random grating over the entire length of the fiber. This
improvement is, to the inventors' knowledge, over 25 dB higher than
what was previously reported for UV exposure for the same
exposition power. The ROGUE FBG is generated by inducing phase
noise during the continuous writing of a FBG using a Talbot
interferometer. This leads to a grating with a very broad bandwidth
regardless of the exposure length and greatly increases the signal
without limiting the scanning bandwidth, resulting in no loss in
resolution. Using these enhanced fibers, we obtained a noise level
over an order of magnitude lower than using regular unexposed
fibers, allowing measurements of smaller temperature variations.
Fibers where such ROGUE FBGs are inscribed also allow the use of a
much smaller scanning bandwidth with similar accuracy, resulting in
faster acquisition speed.
[0204] In this example, we present a technique increasing the
backscattered signal for Rayleigh scatter based distributed sensing
by several orders of magnitude. This is achieved by writing a ROGUE
FBG (a weak, random FBG) over the entire length of the fiber,
resulting in higher reflectivity over a wide range of wavelengths.
This in turn leads to better resolution in strain and temperature
measurements when compared with standard single-mode fiber with
uniform UV exposure, and to the possibility of faster scan speeds
by using a narrower scanning bandwidth with similar accuracy.
Example 5--Influence of the Length of a ROGUE FBG on its FWHM
Bandwidth and on its RMS Noise Level
[0205] In this example, the reflectivity and bandwidth of multiple
lengths of ROGUE FBGs are measured. The results are presented in
FIGS. 30A and 30B. As can be seen, the bandwidth of the grating is
initially very large, but quickly converges to a certain value
(here, about 7 nm) as the grating's length is increased. A
probabilistic model was developed to evaluate the ROGUE FBGs
behaviour for longer gratings. As can be seen, the ROGUE FBG
bandwidth remains constant as the grating's length increases, while
the reflectivity keeps increasing. As can be appreciated, a
satisfactory agreement between the modeled and experimental data
can be observed in FIG. 30B.
[0206] The influence of the gauge length on the noise level is also
evaluated and presented in FIG. 31. It was found that a smaller
gauge length leads to a higher spatial resolution, but at the cost
of the strain resolution. From FIG. 31, it can noted that, for very
short gauge lengths, the spectral and spatial resolution seems to
be Fourier-transform limited, and an increase in gauge length
(spatial resolution) leads to a decrease in RMS noise level
(spectral resolution). However, for very large gauge lengths, a
plateau is met, and the added value of a larger gauge length is
minimal. Furthermore, it is obvious that, regardless of the gauge
length, the noise level is always much smaller with the ROGUE FBG
than with the unexposed SMF-28 fiber. Finally, in this example, a
ROGUE FBG scanned with a 5.24 nm bandwidth required a gauge length
of only 2 mm to beat the noise level of SMF-28 with a 42.90 nm
bandwidth and a 1 cm gauge length.
[0207] As can be understood, the examples described above and
illustrated are intended to be exemplary only. Although the optical
fibers are shown with a circular cross-section, the optical fibers
of other possible embodiments of the optical fiber device can have
any other suitable shape including, but not limited to,
rectangular, hexagonal, and the like. For instance, the DTSS system
described herein can be adapted to perform optical time domain
reflectometry (OTDR) in which a pulse is sent and its return is
monitored over time, and/or optical frequency domain reflectometry
(OFDR) in which each measurement includes a scan in frequency or
wavelength. The scope is indicated by the appended claims.
* * * * *