Elliptical polarization effects in a chromatically addressed Faraday current sensor

Abstract

Optical polarization processes in a parallel-sided glass element used in a Faraday rotation current sensor have been considered. In such sensors the path length necessary to produce sufficient rotation of the plane of polarization is produced by a multiplicity of reflections within the glass element. It is shown that such reflections induce ellipticity of polarization and that this affects the current-sensing performance of the sensor. Two reflection cases, corresponding to total internal reflections at a glass-air interface and reflections by aluminium-coated surfaces, are considered. The latter are shown to produce higher optical attenuation but a lower degree of elliptical polarization. The implications of the induced elliptical polarization in relation to chromatically modulated polychromatic light are considered. It is shown that the resolution of the Faraday sensing is improved by minimizing the ellipticity of the polarization with the aluminium-coated reflections. However a greater dynamic range and signal strength may be achievable with the total internal reflection element.

Electric current sensors: a review

“…A sensor with three 3-D loops using total reflection at the glass surface is shown in [Ning 1995]. If total reflection is used, the optical attenuation is low, but multiple reflections cause elliptical polarization [Li 1999].…”

Improvements in difference-over-sum normalization method for Faraday effect magnetic field waveforms measurement

“…Although interferometric sensing methods provide higher sensitivity, they demand more optical components and a very complex beam path [12]. Additionally, a bulk crystal is less sensitive to vibration-and temperature-induced stress birefringence [13], thus a sensing system based on the proposed method requires less design effort and offers true miniaturization capability and robustness.…”

Linearization of chromatic change in a polychromatic current transformer

“…Dielectric reflections suffer from wavelength and temperature sensitivities with an additional sensitivity to displacements as small as 0.1 from the critical angle [14]. Consequently, metallic reflections were employed as these are far less susceptible to variations in wavelength and incident angle [13]. The effect of reflections on the sensitivity of a polarization modulated sensor may be written (10) where is the initial sensitivity, which is taken to be one, and is the resulting sensitivity due to a phase shift between and orthogonal polarization states.…”

“…The Jones matrix for a Faraday element with five reflections is (13) where angle of the analyzer relative to the initial plane of polarization; Faraday rotation angle; rotation introduced by the quartz waveplate. and are the Fresnel reflection coefficients given by (14) Fig.…”