Cascaded-mode interferometers: spectral shape and linewidth engineering

TL;DR

This paper introduces a new type of interferometer that uses multiple transverse modes in a single waveguide to engineer spectral shapes and linewidths, enabling advanced optical filtering and sensing applications.

Contribution

The authors develop a novel framework for spectral engineering using multimode waveguides with corrugated gratings, allowing arbitrary spectral shape control in a compact device.

Findings

Demonstrated narrow-linewidth spectra with tunable free spectral range.

Achieved independent spectral shaping for different transverse modes.

Validated the approach through theoretical analysis and experimental results.

Abstract

Interferometers are essential tools to measure and shape optical fields, and are widely used in optical metrology, sensing, laser physics, and quantum mechanics. They superimpose waves with a mutual phase delay, resulting in a change in light intensity. A frequency-dependent phase delay then allows to shape the spectrum of light, which is essential for filtering, routing, wave shaping, or multiplexing. Simple Mach-Zehnder interferometers superimpose spatial waves and typically generate an output intensity that depends sinusoidally on frequency, limiting the capabilities for spectral engineering. Here, we present a novel framework that uses the interference of multiple transverse modes in a single multimode waveguide to achieve arbitrary spectral shapes in a compact geometry. Through the design of corrugated gratings, these modes couple to each other, allowing the exchange of energy similar to a beam splitter, facilitating easy handling of multiple modes. We theoretically and experimentally demonstrate narrow-linewidth spectra with independently tunable free spectral range and linewidth, as well as independent spectral shapes for various transverse modes. Our methodology can be applied to orthogonal optical modes of different orders, polarization, and angular momentum, and holds promise for sensing, optical metrology, calibration, and computing.