My Research and Inventions

### non-linear constants

Measurements of 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!!!)

### 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

Conventional setup for measurements of Kerr rotation angle using crossed polarizers

## Commercial setup

Conventional setup for measurements of Kerr rotation angle. Polarized light from the source is reflected from the sample and it is detected by the detector. The angle between axes of polarizer (P1) and analyzer (P2) is 90 degrees and light is blocked. Only when the polarization is rotated after the reflection, light reaches the detector and the rotation angle can be evaluated. Click on image to enlarge it

# 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.

 Fig.2. Setup for measurements of Kerr rotation angle using balanced detectors . Click on image to enlarge it

# Setup with balanced detectors

Figures 2

 Fig.3. Setup for measurements of Kerr rotation angle using a 2x2 optical switch The setup for measurements of Kerr rotation angle. The angle between axes the polarizers P1, P2 is 45 degrees. The angle between axes quarter-wave plates QW1 and P1 is + 45 degrees. The angle between axes quarter-wave plates QW2 and P2 is -45 degrees. Click on image to enlarge it

# 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.

 Fig.4. Setup for measurements of Kerr rotation angle using a 2x2 optical switch The setup for measurements of Kerr rotation angle. The 2x2 switch is switching between two opposite directions of light propagation. The quarter wave plates QW1, QW2 and the polarizers P1,P2 makes light right circular polarized. However, the polarization rotation is different for two switching states.(1) when switch is off, light is right circular polarized (shown in red) and (2) when switch is on, light is right circular polarized (shown in green). The lock-in amplifier measures a small difference between absorption of the right-circular polarized light between two opposite directions of light propagations. Click on image to enlarge it