Method for manufacturing a fiber optic current sensor with inherent temperature compensation of the faraday effect
View Patent ↗A method is provided for tuning the fiber optic retarder of a fiber optic current sensor towards a desired temperature dependence, the sensing fiber is exposed to a magnetic field or corresponding electric current and the sensor signal as well as the signal's dependence on the retarder temperature are measured. From this initial sensor signal and its temperature dependence, a target sensor signal can be determined, at which the dependence on the retarder temperature equals a desired value. Then, the retarder is thermally treated until the sensor signal reaches the target value. The method obviates the need for repetitively measuring the temperature dependence during the tuning process.
1. A method for manufacturing a fiber optic current sensor, wherein the fiber optic current sensor comprises:
a sensing fiber to be exposed to a magnetic field of a current I to be measured, wherein said sensing fiber has a Verdet constant V and a birefringent linear phase retardation δ,
at least one fiber optical retarder coupled to said fiber for converting light between linear and elliptical polarization, wherein a slow axis of said retarder is at an angle 45°−β in respect to a slow axis of said sensing fiber, and wherein a differential retardation ρ induced by said retarder is dependent on temperature where
Q
=
1
ρ
·
∂
ρ
∂
T
,
and a control unit generating a sensor signal σ dependent on a current-induced phase shift Δφ, in particular proportional to said phase shift Δφ, which phase shift can be written as
Δφ=4 S·φ F
with φ F =V·N·I, and wherein N is a number of loops of said sensing fiber and S is a scale factor,
said method comprising:
exposing said sensing fiber to a magnetic field;
in an initial step, measuring a dependence A
A
=
1
σ
·
∂
σ
∂
T
of said sensor signal a on a temperature T of said retarder;
defining a target dependence;
B
=
1
σ
·
∂
σ
∂
T
of the sensor signal a on the temperature T of said retarder;
determining, from A, B, Q, δ and β, a ratio r with
r
=
σ
fin
σ
ini
between an initial sensor signal σ ini before a thermal treatment of said retarder and a target sensor signal σ fin after a thermal treatment of said retarder; and
subjecting said retarder to said thermal treatment and measuring said sensor signal σ until said sensor signal σ has substantially changed by said ratio r.
2. The method of claim 1 , wherein said ratio r is calculated from
r
(
A
,
B
,
Q
1
,
Q
2
,
δ
,
β
)
=
S
(
B
Q
2
,
δ
,
β
)
S
(
A
Q
1
,
δ
,
β
)
wherein B is a target dependence of said sensor signal a on the temperature of said retarder, Q 1 and Q 2 are values of Q before and after said thermal treatment of retarder, respectively, and S is a normalized scale factor given by
S
=
Δϕ
4
VNI
=
Δϕ
4
φ
F
where
Δϕ
=
arctan
(
2
U
1
-
U
2
)
,
U
=
2
φ
F
tan
(
Γ
)
Γ
cos
(
ɛ
)
-
δ
sin
(
ɛ
)
sin
(
2
β
)
tan
(
Γ
)
Γ
,
and
Γ
=
[
δ
2
+
(
2
φ
F
)
2
]
1
/
2
or an approximation thereof, where
ɛ
=
-
b
+
b
2
-
4
a
c
2
a
where
a
=
1
+
1
2
X
-
sin
(
2
β
)
δ
π
4
,
b
=
π
2
+
sin
(
2
β
)
δ
(
1
+
X
)
,
and
c
=
sin
(
2
β
)
δ
π
2
-
X
,
wherein X=A/Q 1 when calculating S before thermal treatment and X=B/Q 2 when calculating S after thermal treatment, with Q 1 and Q 2 denoting values of Q before and after the thermal treatment.
3. The method of claim 2 , wherein said approximation is
S
=
1
+
1
3
δ
2
cos
(
ɛ
)
-
δ
sin
(
ɛ
)
sin
(
2
β
)
.
4. The method claim 2 , wherein said values Q 1 and Q 2 are not equal and wherein at least one of them has been determined prior to manufacturing said sensor.
5. The method of claim 2 , wherein
B
+
1
V
·
∂
V
∂
T
<
t
where t is a threshold value smaller than 7*10 −5 1/K, in particular smaller than 2*10 −5 1/K.
6. The method of claim 2 , wherein
B
+
H
+
1
V
·
∂
V
∂
T
<
t
with H being a temperature dependence of the sensor scale factor associated with effects of birefringence present in the sensing fiber or with effects from packaging the sensor for application as a measurement device, and t is a threshold value smaller than 7*10 −5 1/K, in particular smaller than 2*10 −5 1/K.
7. The method of claim 6 , wherein B is determined such that the sensor is temperature compensated for φ F <0.1 rad.
8. The method of claim 6 , wherein B is determined such that the sensor is temperature-compensated for φ F >0.3 rad.
9. The method of claim 1 , wherein during manufacturing of said retarder, said sensing fiber is substantially straight and wherein, after said manufacturing, said sensing fiber is bent to form a coil consisting of N>0 loops.