Accurate polarization measurements with a dual photoelastic modulator

Abstract

Measurements of the polarization effects in multimirror experiments by using a dual photoelastic modulator are described. The effect of single and multiple mirrors in polarization measurements in two and three dimensions is discussed, and experimental results show how symmetrical placement of mirrors in three-dimensional geometry can eliminate changes in the polarization. Calibration procedures for a dual photoelastic modulator and potential error sources such as misalignment of analyzer, signal dc offset, and neglect of aperture size are presented. Mirror-surface evolution and how it can disturb the polarization measurement are also addressed.

Preparations for the motional Stark effect diagnostic on EAST

“…This encodes the polarization direction into intensity modulation in the transmitted light. 9 The PEMs are operated at 20 and 23 kHz, respectively, and γ (the angle between the polarization direction and PEM axis) can be obtained from the ratio A 70-m-long fiber bundle is used to guide the encoded light into the diagnostic room. The fiber bundle has 19 fibers with a tight hexagonal packing at the filterscope end, and on the other end, the fibers are arranged vertically in 2 columns for a better spatial resolution.…”

Hybrid complete Mueller polarimeter based on phase modulators

“…This polarized beam arrives to the PSA in order to be measured. The PSA is basically a dual-PEM Stokes polarimeter [20][21][22] where the optical elements are placed conveniently in order to have signals related to the fundamental and second harmonic PEM's resonant frequencies. The Mueller matrix representing the PSA module results from…”

The MAST motional Stark effect diagnostic

“…The PEMs are mounted with their axes 45°apart and are followed by a linear polarizer set at 22.5°from each PEM. This arrangement encodes polarization information as intensity modulation in the transmitted light, 6 which can then be coupled into optical fibers ͑which do not preserve polarization͒ and brought outside the tokamak radiation shielding to the sensitive optical detectors.…”