Distributed brillouin sensing using correlation
View Patent ↗Methods and systems for sensing conditions of a fiber include splitting a light signal into two branches. A first branch is converted to have a mode different from that of the second branch. Both branches are mode multiplexed into a single fiber. An output of the fiber is mode demultiplexed into the two branches. The first branch is mode converted to its original mode. Brillouin scattering patterns of the two branches are compared to determine a temperature and strain profile of the fiber.
1. A method for sensing conditions of a fiber, comprising:
splitting a light signal into two branches;
converting a first branch to have a mode different from that of the second branch;
mode multiplexing both branches into a single fiber;
mode demultiplexing an output of the fiber into the two branches;
mode converting the first branch to the original mode of the first branch;
comparing Brillouin scattering patterns of the two branches to determine a temperature and strain profile of the fiber.
2. The method of claim 1 , further comprising polarizing each of the two branches to have orthogonal polarizations.
3. The method of claim 1 , further comprising coherently detecting the output of the fiber in the two branches.
4. The method of claim 1 , wherein comparing the Brillouin scattering patterns comprises determining a Brillouin frequency shift of each branch.
5. The method of claim 4 , wherein determining the temperature and strain profiles ΔT and Δε of the fiber comprises solving:
Δ
T
=
C
v
ɛ
Mode
2
·
Δ
v
B
Model
1
-
C
v
ɛ
Mode
1
·
Δ
v
B
Mode
2
C
v
ɛ
Mode
2
·
C
vT
Model
1
-
C
v
ɛ
Mode
1
·
C
vT
Mode
2
.
Δ
ɛ
=
C
v
T
Mode
2
·
Δ
v
B
Model
1
-
C
v
T
Mode
1
·
Δ
v
B
Mode
2
C
vT
Mode
2
·
C
v
ɛ
Model
1
-
C
v
T
Mode
1
·
C
v
ɛ
Mode
2
.
where Δν B Mode1 is the Brillouin frequency shift of the signal on the first branch, Δν Mode2 B is the Brillouin frequency shift of the signal on the second branch, and C νε Mode1 , C νε Mode2 , C νT Mode1 , and C νT Mode2 are proportionality constants related to an optical and acoustic refractive index and dopant concentration of the fiber.
6. The method of claim 1 , wherein the fiber is a few-mode fiber.
7. A sensor for sensing conditions of a fiber, comprising:
a light source;
a coupler configured to split an output of the light source into two branches;
a mode converter configured to convert light in a first branch between a mode shared by the second branch to a different mode;
a mode multiplexer configured to multiplex both branches onto a single fiber;
a mode demultiplexer configured to demultiplex an output of the fiber into the two branches, with light on the first branch being passed to the mode converter for conversion back to the mode shared by the second branch;
a processor configured to compare Brillouin scattering patterns of the two branches to determine a temperature and strain profile of the fiber.
8. The sensor of claim 7 , further comprising a polarization controller on each branch to polarize each of the two branches to have orthogonal polarizations.
9. The sensor of claim 7 , further comprising coherently receivers configured to detect the output of the fiber in the two branches.
10. The sensor of claim 7 , wherein the processor is further configured to determine a Brillouin frequency shift of each branch.
11. The sensor of claim 10 , wherein the processor is further configured to determine the temperature and strain profiles ΔT and Δε of the fiber by solving:
Δ
T
=
C
v
ɛ
Mode
2
·
Δ
v
B
Model
1
-
C
v
ɛ
Mode
1
·
Δ
v
B
Mode
2
C
v
ɛ
Mode
2
·
C
vT
Model
1
-
C
v
ɛ
Mode
1
·
C
vT
Mode
2
.
Δ
ɛ
=
C
v
T
Mode
2
·
Δ
v
B
Model
1
-
C
v
T
Mode
1
·
Δ
v
B
Mode
2
C
vT
Mode
2
·
C
v
ɛ
Model
1
-
C
v
T
Mode
1
·
C
v
ɛ
Mode
2
.
where Δν B Mode1 is the Brillouin frequency shift of the signal on the first branch, Δν Mode2 B is the Brillouin frequency shift of the signal on the second branch, and C νε Mode1 , C νε Mode2 , C νT Mode1 , and C νT Mode2 are proportionality constants related to an optical and acoustic refractive index and dopant concentration of the fiber.
12. The sensor of claim 7 , wherein the fiber is a few-mode fiber.