Polarizers, optical bridges and Sagnac interferometers for nanoradian polarization rotation measurements

TL;DR

This paper compares polarizer-based, Sagnac interferometer, and optical bridge methods for detecting extremely small polarization rotations near the quantum shot noise limit, demonstrating a bridge's superior noise rejection and proposing new Sagnac geometries for various rotation types.

Contribution

It introduces novel Sagnac geometries sensitive to specific non-reciprocal and reciprocal polarization rotations, expanding capabilities for high-precision optical measurements.

Findings

Bridge achieves shot noise limited detection at high light intensities.

Measured a 3 nrad rotation induced by a 0.9 nT magnetic field.

Sagnac interferometer can discriminate between different symmetry rotations.

Abstract

The ability to measure nanoradian polarization rotations, $\theta_F$, in the photon shot noise limit is investigated for partially crossed polarizers (PCP), a static Sagnac interferometer and an optical bridge, each of which can in principal be used in this limit with near equivalent figures-of-merit (FOM). In practice a bridge to PCP/Sagnac source noise rejection ratio of $1/4\theta_F^2$ enables the bridge to operate in the photon shot noise limit even at high light intensities. The superior performance of the bridge is illustrated via the measurement of a 3 nrad rotation arising from an axial magnetic field of 0.9 nT applied to a terbium gallium garnet. While the Sagnac is functionally equivalent to the PCP in terms of the FOM, unlike the PCP it is able to discriminate between rotations with different time ($T$) and parity ($P$) symmetries. The Sagnac geometry implemented here is similar to that used elsewhere to detect non-reciprocal ($\overline{T}P$) rotations like those due to the Faraday effect. Using a Jones matrix approach, novel Sagnac geometries uniquely sensitive to non-reciprocal $\overline{TP}$ (e.g. magneto-electric or magneto-chiral) rotations, as well as to reciprocal rotations (e.g. due to linear birefringence, $TP$, or to chirality, $T\overline{P}$) are proposed.