The

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3 methods of measurement of the Kerr rotation angle:

1) Crossed- polarizers

precession: poor
sensitivity:poor
implementation: simple
distribution by optical anisotropy: very sensitive

2) Balance detector

precession: moderate
sensitivity:moderate
implementation:moderate
distribution by optical anisotropy: very sensitive

2) 2x2 switch

precession: high
sensitivity:high
implementation:moderate/difficult
distribution by optical anisotropy: not sensitive (it is very good!!!)

 

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Disturbing effect: Polarization rotation due to optical anisotropy

The polarization of light can be rotated in an anisotropic material. It is the disturbing and undesirable effect for the measurement

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Conventional setup: Crossed -polarizers

Figures 1 shows the conventional setup for measurements of Kerr rotation angle using crossed polarizers. Polarized light is reflected from the sample and it is detected by the detector. In the front of the detector there is a polarizer P2, which axis is 90 degrees with respect to the axis of the polarizer P1. In the case when there is no polarization rotation, light is blocked by polarizer P2 and it can not reach the detector. When after the reflection the polarization is rotated, some light can reach the detector. Since the magnitude of the detected light is proportional to the rotation angle (the Kerr rotation angle), the rotation angle can be evaluated. Similar to the cross-polarizer setup for measurement of Faraday rotation angle, the demerit of this setup is a low sensitivity. The reason of the low sensitivity is the same. It is zero derivation of transmission when polarizers are crossed.

 

 

 

 

 

 

 

 

 

 

 

 

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Setup with balanced detectors

Figures 2

 

 

 

 

 

 

 

 

 

 

 

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Setup with 2x2 optical switch

Zayets et al, Patent appl., 2016-126905, 2016; in Japanese; in English

Figures 3 shows the setup for measurements of the Kerr rotation angle with 2x2 optical switch. It is similar to the conventional setup shown in Fig.1, but a 2x2 fiber switch used between the sample and the detector/source. Because of the usage of the lock-in technique for the Kerr rotation measurements, the sensitivity and measurement precision of this setup can be significantly enhanced comparing to the conventional setup of Fig.1. The polarizers and quarter-wave plates are inserted after the fiber collimators. They makes input light circularly-polarized. For one input the incident light is left-circularly polarized. For another input the incident light is right-circularly polarized. Therefore, the switcher switches incident light between left and right circular polarizations. In the case of non-zero Kerr rotation, after the reflection there is a conversion between the left and right circular polarization, which can be detected using the lock-in technique and the Kerr rotation angle can be evaluated with a high precision.