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

A sensing device for measuring a value Z of an alternating measurand, the sensing device comprising:

• a beam-splitter comprising an integrated-optic beam splitter connecting at least three downstream terminals to at least one upstream terminal,
• a light source connected to a first downstream terminal of the at least three downstream terminals,
• a first polarizer and a first light detector arranged to measure a first raw signal S1,raw from light returning from a sensing element at a second downstream terminal of the at least three downstream terminals, and
• a second polarizer and a second light detector arranged to measure a second raw signal S2,raw from light returning from said sensing element at a third downstream terminal,
• wherein said first polarizer and said second polarizer are orthogonally polarized, and
• wherein said upstream terminal is connected to said sensing element;
• and
• a signal processing unit, wherein during operation, the signal processing unit causes the sensing device to perform operations comprising:
• sending two different polarization states of a light beam through the 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 S1,raw, S2,raw depending on the phase shift Δφ as follows
• S1,raw=(So1/2)·[1+K1 sin(Δφ+θ1)],
• S2,raw=(So2/2)·[1+K2 sin(Δφ+θ2)],
• wherein
• So1, So2 are parameters depending at least on a light source power and optical losses in the sensing device,
• K1, K2 are parameters of the sensing device,
• θ1, θ2 are parameters depending at least on the sensing device and having absolute values much smaller than π/2,
• normalizing the two raw signals S1,raw, S2,raw to two sensor signals S1, S2 of equal amplitudes Yo2/2
• S1=(Yo/2)·[1+K1 sin(Δφ+θ1)],
• S2=(Yo/2)·[1+K2 sin(Δφ+θ2)],
• calculating a normalized combined signal S using a normalization value C as
• S=(S1−S2)/C
• wherein the normalization value C is calculated from previous values S1′, S2′ of the sensor signals S1, S2, near zero crossings of the alternating measurand, by C=S1′+S2′, or wherein C is calculated in an iterative approach,
• further calculating said value Z using an arcsine function, using K1, K2, θ1, and/or θ2.