Optical sensor with spun birefringent sensing fiber
View Patent ↗There is described an optical fiber current sensor having an opto-electronic module part for detecting an optical phase shift induced by the measurand field in a sensing fiber, a sensor head including the sensing fiber, wherein the sensing fiber is a spun highly-birefringent fiber having a length L=∫ ds defined by the line integral along the space curve given by the sensing fiber coil such that the length L of the sensing fiber is sufficiently long to suppress thermal signal instabilities due to the spun character of the sensing fiber while the effective number of fiber windings is low enough to maintain a maximum sensitivity over the full measurement range of the fiber-optical sensor.
1. An optical fiber current sensor having an opto-electronic module part for detecting a magneto-optic phase shift induced in a sensing fiber by a magnetic field of the current, and a sensor head including the sensing fiber, wherein the sensing fiber is a spun highly-birefringent fiber having a length L=∫ ds defined by the line integral along the space curve described by the sensing fiber,
wherein a sensing length is defined as L′=L for a transmission optical sensor and as L′=2L for a sensing fiber being terminated by a reflector and
L
′
>
L
EB
λ
Δλ
with
L
EB
=
L
LB
1
+
x
2
-
x
≈
2
xL
LB
and x being the spinning ratio of the spun highly-birefringent fiber,
λ
Δλ
the ratio of center wavelength λ over the spectral width Δλ of the light in the sensing fiber, and L LB being the linear beat length which is a length section of equivalent unspun highly-birefringent fiber that produces a birefringent phase shift of 2π, and
wherein
D
=
a
2
+
b
2
2
<
D
max
,
with D max being 50 cm or smaller and D max being a length characterising the available volume for the sensor head, and a and b being the two largest edge lengths of a cuboid of minimal size enclosing the sensing fiber, wherein
the magneto-optic phase shift accumulated in the fiber is reduced by having at least one of the following features:
(i) one or more sections of the sensing fiber forming loops enclosing the current but being counter-wound with respect to other sections of the sensing fiber forming loops enclosing the current,
(ii) one or more sections of the sensing fiber being at least partly magnetically shielded, and
(iii) one or more sections of the sensing fiber forming loops not enclosing the current.
2. The sensor of claim 1 , wherein the sensor head is connected by a polarization maintaining fiber to the optoelectronic module.
3. The sensor according to claim 1 , wherein the sensor is not a transmission optical sensor using counter-propagating light waves.
4. The sensor according to claim 1 , wherein the sensing fiber being terminated by a reflector.
5. The sensor according to claim 1 , wherein the sensor is a transmission optical sensor using co-propagating light waves.
6. The sensor according to claim 1 , wherein the sensing fiber has one or more sections forming loops enclosing the current or being designed for enclosing the current or a current conductor carrying the current.
7. The sensor according to claim 1 , wherein the one or more sections of the sensing fiber forming loops enclosing the current are designed for enclosing the current or a current conductor carrying the current; and/or the one or more sections of the sensing fiber forming loops not enclosing the current are designed for not enclosing the current or a current conductor carrying the current.
8. The sensor according to claim 1 , wherein the number of counter-wound loops enclosing the current differs from the number of not counter-wound loops enclosing the current.
9. The sensor according to claim 1 , having an effective winding number
N
eff
=
1
I
∫
H
→
·
d
s
→
with
N
eff
<
C
L
π
D
wherein ∫{right arrow over (H)}·d{right arrow over (s)} is the line integral of magnetic field vector H along the space curve defined by the sensing fiber over the sensing fiber length L, I being the current and |I| the absolute value of the current to be measured,
and C being a numerical factor with 0<C≤1.
10. The sensor according to claim 1 , wherein both ends of the sensing fiber are closer than 10 mm of each other.
11. The sensor according to claim 1 , wherein, in the case of a magnetically shielded sensing fiber section an unshielded end of the sensing fiber is closer than 10 mm with a beginning of the magnetically shielded sensing fiber section.
12. The sensor according to claim 1 , wherein the length L of the sensing fiber is such that expected amplitudes K of sensor signal instabilities determined in accordance with
K
=
x
-
2
exp
-
π
2
4
ln
2
(
2
L
Δ
λ
L
EB
λ
)
2
for a reflective sensor configuration, or with
K
=
x
-
2
exp
-
π
2
4
ln
2
(
L
Δ
λ
L
EB
λ
)
2
or a sensor configuration operated in transmission,
with K being below 0.5% of the average signal, and with the spinning ratio x being in the range of 1 to 20.
13. The sensor according to claim 1 , having an integer number as an effective winding number
N
eff
=
1
I
∫
H
→
·
d
s
→
wherein ∫{right arrow over (H)}·d{right arrow over (s)} is the line integral of magnetic field vector H along the space curve defined by the sensing fiber, I being the current to be measured and |I| being the absolute value of the current to be measured.
14. The sensor according to claim 13 , wherein N eff being 1 or 2.
15. The sensor according to claim 1 , wherein the spin pitch (p) of the sensing fiber is constant along the sensing fiber length L.
16. The sensor according to claim 15 , wherein the sensing fiber properties are homogenous along the sensing fiber length L.
17. The sensor according to claim 1 , wherein the spin pitch (p) is between 1 mm and 20 mm.
18. The sensor according to claim 1 , wherein the spinning ratio (x) is between 1 and 20.
19. The sensor according to claim 1 , wherein D max is 30 cm or smaller.
20. The sensor according to claim 1 , wherein a magnetic shield for shielding sections of the sensing fiber is made of magnetically soft materials.
21. The sensor according to claim 20 , wherein the magnetically soft materials is selected from the group consisting of: nickel iron alloys (e.g. Ni80Fe20), cobalt iron alloys, and amorphous magnetic alloys.
22. The sensor according to claim 1 , wherein the phase detection includes an active phase modulator.
23. The sensor according to claim 1 , wherein the phase detection includes passive optical elements for introducing a phase bias between the interfering light waves.
24. The sensor according to claim 1 ,
wherein L LB is the linear beat length which is a length section of equivalent unspun highly-birefringent fiber that produces a birefringent phase shift of 2π; and/or
wherein the spinning ratio x of the spun highly-birefringent fiber is given by
x
=
2
L
LB
p
,
wherein L LB is the linear beat length which is the length of equivalent unspun highly-birefringent fiber that produces a birefringent phase shift of 2π, and p is the spin pitch which is the length of a spun highly-birefringent fiber section that provides a full revolution of principal axes of the local linear birefringence in the spun highly-birefringent fiber.
25. The sensor according to claim 1 , wherein the magneto-optic phase shift accumulated in the fiber is reduced by having one or more sections of the sensing fiber forming loops not enclosing the current, and the one or more sections of the sensing fiber forming loops not enclosing the current are designed for not enclosing the current or a current conductor carrying the current.
26. The sensor according to claim 1 , wherein the magneto-optic phase shift accumulated in the fiber is reduced by having one or more sections of the sensing fiber being at least partly magnetically shielded, and an unshielded end of the sensing fiber is closer than 10 mm with a beginning of the magnetically shielded sensing fiber section.