Polarization optical detection with enhanced accuracy in the high-signal regime

Patent metadata

Field	Value
Publication number	US-11143678-B2
Application number	US-201716954451-A
Country	US
Kind code	B2
Filing date	Dec 22, 2017
Priority date	Dec 22, 2017
Publication date	Oct 12, 2021
Grant date	Oct 12, 2021

Abstract

In order to carry out the polarimetric detection of a measurand, light of two polarization states is passed through a sensing element, where the two states suffer a differential phase shift depending on the value of the measurand. In order to compensate for only imperfections of the device, a method is proposed that is based on calibration values obtained in a low-value regime of the measurand only. Yet the method can still be used for accurately determining higher values of the measurand.

First claim

The invention claimed is:

1. A method for measuring a value Z of an alternating measurand by means of a sensing device, said method comprising:
sending two different polarization states of a light beam through a sensing element, where the two different polarization states experience a measurand-dependent differential phase shift Δφ,
splitting light having passed at least once through said sensing element into at least two channels and carrying out different polarimetric measurements in the two channels, thereby generating two raw signals S 1,raw ·S 2,raw depending on the phase shift Δφ as follows
S 1,raw =( S o1 /2)·[1+ K 1 sin(Δφ+θ 1 )],

S 2,raw =( S o2 /2)·[1+ K 2 sin(Δφ+θ 2 )],

wherein
S o1 , S o2 are parameters depending at least on a light source power and optical losses in the sensing device,
K 1 , K 2 are parameters of the sensing device,
θ 1 , θ 2 are parameters depending at least on the sensing device and having absolute values much smaller than π/2,
normalizing the two raw signals S 1,raw , S 2,raw , to two sensor signals S 1 , S 2 of equal amplitudes Y o /2
S 1 =( Y o /2)·[1+ K 1 sin(Δφ+θ 1 )],

S 2 =( Y o /2)·[1+ K 2 sin(Δφ+θ 2 )],

calculating a normalized combined signal S using a normalization value C as
S =( S 1 −S 2 )/ C

wherein the normalization value C is calculated from previous values S 1 ′, S 2 ′ of the sensor signals S 1 , S 2 , near zero crossings of the alternating measurand, by C=S 1 ′+S 2 ′, or wherein C is calculated in an iterative approach,
further calculating said value Z using an arcsine function, using
K 1 ,K 2 ,θ 1 , and/or θ 2 .

2. The method of claim 1 , wherein θ 1 ≠0, θ 2 ≠0, θ 1 −θ 2 ≠0, K 1 ≠1, and/or K 2 ≠1 at at least one temperature in an operating temperature range of the sensing device and/or wherein the absolute phase shift |Δφ| reaches values larger than 0.1 rad, larger than 0.5 rad, and larger than 1 rad.

3. The method of claim 1 , wherein the correction value C is given by a sum of S 1 ′ and S 2 ′ with S 1 ′, S 2 ′ being the previous values of S 1 , S 2 corresponding to phase shifts Δφ having absolute values smaller than a threshold, the threshold being below 0.1 rad.

4. The method of claim 3 wherein S 1 ′ and S 2 ′ are determined as time-averaged values of S 1 ′ and S 2 ′ during time periods where |Δφ| is below said threshold and over at least one half-cycle of the alternating measurand.

5. The method of claim 3 wherein, for determining said normalization value C, an extremum of a sum of S 1 +S 2 within at least one half-cycle of the alternating measurand is determined by averaging the obtained extrema over more than one half-cycle.

6. The method of claim 1 , wherein the normalization value C is calculated in an iterative approach, wherein in an iteration n with n=0, . . . , N−1:
C is derived from C(n), with C(n) being a function of Δφ(n), wherein Δφ(n) is an iterative value for the phase shift Δφ, and Δφ(n+1) is calculated from C(n) by calculating iterative values S(n) for S and Z(n) for Z from C(n).

7. The method of claim 6 wherein C(n)=(S 1 +S 2 )·k 0 /k(n) with k 0 =1/[1+k·cos(θ)·sin(Δθ/2)] and k(n)=1/[1+K·cos (θ+Δφ(n)·sin(Δθ/2)], with K being an effective fringe contrast and θ=(θ 1 +θ 2 )/2 and Δθ=θ 1 −θ 2 .

8. The method of claim 7 , wherein
N=1 or larger, and in a first iteration Δφ(0) is set to a starting value comprising zero.

9. The method of claim 8 , wherein the value Z is calculated as

Z
=

1
A

⁢

f
⁡

(

θ
1

⁢

θ
2

,

K
1

,

K
2

)

⁢

arcsin
⁡

[

1

f
⁡

(

θ
1

⁢

θ
2

,

K
1

,

K
2

)

]

+
H

with
A being a scale factor;
f being a function of at least one of the parameters θ 1 θ 2 , K 1 , K 2 ; and
H is an offset value comprising a value of Z near zero-crossings of said measurand.

10. The method of claim 9 , wherein the value Z is calculated with f(θ 1 θ 2 , K 1 , K 2 )=koKcos[Δθ/2], with
ko= 1/[1+ K ·cos(θ+Δφ( n )·sin(Δθ/2)]

K being an effective fringe contrast of S, θ=(θ 1 +θ 2 )/2, Δθ=θ 1 −θ 2 ;
and wherein the offset value His given as
h=−So/A

with So being a signal offset at a zero value Z, and said scale factor A is given as
A=A ′/cos θ

with A′ being a scale factor determined by calibration in a regime of shift |Δφ|<<π/2, including |Δφ|<<0.1, and A′ being obtained from calibration data as a function of a temperature (T) of at least part of said sensing device.

11. The method of claim 9 , wherein said parameters θ 1 , θ 2 , θ=(θ 1 +θ 2 )/2, Δθ=θ 1 −θ 2 , K 1 and/or K 2 , or parameters dependent on at least one of them, are calculated from calibration data as a function of a temperature of at least part of said sensing device or of a value indicative of the temperature of at least part of said sensing device.

12. The method of claim 11 wherein said parameters K 1 and K 2 are assumed to be 1 or are calculated from further parameters of the sensing device.

13. The method of claim 1 wherein, for normalizing the raw signals S 1,raw , S 2,raw , time-averaged amplitudes S o1,ac , and S o2,ac at an alternating frequency of said measurand are calculated and said signals S 1 , S 2 are calculated as
S 1 =S 1,raw ·S o2,ac
S 2 =S 2,raw ·S o1,ac .

14

Assignees

• Abb Power Grids Switzerland Ag

Inventors

• Bohnert Klaus
• Frank Andreas
• Muller Georg
• Yang Lin

Classifications

• G01R15/247

  Details of the circuitry or construction of devices covered by G01R15/241 - G01R15/246 · CPC title

• G01R33/0322

  using the Faraday or Voigt effect · CPC title

• G01R15/246 Primary

  based on the Faraday, i.e. linear magneto-optic, effect · CPC title