Dominant cause of unequal scattered‑light amplitudes between symmetric and antisymmetric port readouts

Identify the dominant mechanism causing the different scattered‑light peak amplitudes observed between the symmetric‑port and antisymmetric‑port balanced homodyne detectors in the dual balanced homodyne readout of a Michelson interferometer, distinguishing between residual quadrature detuning from manual local‑oscillator phase adjustment and port‑dependent spatial mode overlap differences arising from differing propagation paths and potential aperture clipping at the Faraday isolator.

Background

In the experimental demonstration, the authors implemented a dual balanced homodyne detection scheme, reading orthogonal quadratures at the symmetric and antisymmetric ports of a Michelson interferometer to enable scattered light noise subtraction.

They expected comparable scattered‑light coupling into both balanced homodyne detector readouts based on their theoretical model, yet observed different scattered‑light peak amplitudes. They propose two plausible explanations: slight detuning from perfect quadrature due to manual phase setting of one local oscillator, and a potential difference in spatial mode overlap at the two ports due to unequal distances and an aperture in the Faraday isolator.

Resolving which effect dominates is necessary to guide improvements in implementation (e.g., phase control schemes and optical layout) to maximize scattered light suppression.