Finding the Maximal Contrast of Two Elliptical Gaussian Mode Beams with Aligned Ellipticities

TL;DR

This paper derives an analytic formula for the maximum interference contrast between two aligned elliptical Gaussian beams, validated experimentally, aiding the optimization of elliptical-beam interferometers in precision measurements.

Contribution

It introduces a new analytic expression for maximum contrast of aligned elliptical Gaussian beams, validated through experimental measurements.

Findings

Predicted maximum contrast: 0.968

Experimental maximum contrast: 0.950

Discrepancies explained by practical imperfections

Abstract

Interferometric contrast is a key factor limiting the sensitivity of precision optical measurements, including the laser interferometers used in gravitational-wave detection. While standard formulas describe the interference of circular Gaussian beams, many real systems use beams with elliptical cross sections, where differing waists and radii of curvature can reduce fringe visibility. This paper derives an analytic expression for the maximum contrast achievable between two aligned elliptical Gaussian beams, written entirely in terms of their geometric and power parameters. We then test the formula using a free-space Michelson interferometer in which all beam parameters are independently measured through beam profiling and nonlinear fitting. In our experiment, the predicted maximum contrast was 0.968 while the experimentally optimized value was 0.950. The small discrepancy is consistent with expected imperfections such as beam rotation, mode mismatch, and non-Gaussian aberrations. This work provides a practical tool for modeling and optimizing elliptical-beam interferometers.