Experimental reconstruction of spatial Schmidt modes for a wide-field SU(1,1) interferometer

TL;DR

This paper demonstrates a method to reconstruct the spatial mode structure of a wide-field SU(1,1) interferometer using intensity covariance measurements, revealing a large number of modes suitable for advanced phase sensing.

Contribution

It introduces a simple intensity-based technique for reconstructing spatial modes in a multimode nonlinear interferometer, enabling practical spatial mode analysis.

Findings

Reconstructed radial and azimuthal mode content using intensity covariance.

Identified a large number of spatial modes suitable for phase measurement.

Demonstrated the feasibility of mode analysis with minimal measurement complexity.

Abstract

We study the spatial mode content at the output of a wide-field SU(1,1) interferometer, i.e. a nonlinear interferometer comprising two coherently-pumped spatially-multimode optical parametric amplifiers placed in sequence with a focusing element in between. This device is expected to provide a phase sensitivity below the shot-noise limit for multiple modes over a broad angular range. To reconstruct the spatial modes and their weights, we implement a simple method based on the acquisition of only intensity distributions. The eigenmode decomposition of the field is obtained through the measurement of the covariance of intensities at different spatial points. We investigate both the radial and azimuthal (orbital angular momentum) modes and show that their total number is large enough to enable applications of the interferometer in spatially-resolved phase measurements.