Wavelength-Dependent Evolution of Full-Field Transfer Matrices in Photonic Lanterns

TL;DR

This study measures and models how the transfer matrix of a photonic lantern varies with wavelength, revealing that modal phase differences dominate spectral evolution and that geometry influences this behavior.

Contribution

It provides the first quantitative physical explanation for wavelength-dependent transfer matrix evolution in photonic lanterns through direct measurement and modeling.

Findings

Measured wavelength-dependent transfer matrices across 1525-1575 nm.

Developed a propagation model that reproduces observed spectral evolution.

Identified modal phase accumulation as the key mechanism.

Abstract

A fiber-based photonic lantern can couple an array of single-mode optical fibers to the guided modes of a multimode fiber, with the mapping between the single-mode fibers and guided modes fully described by a complex-valued transfer matrix. Recent experimental studies have reported strong wavelength-dependent evolution of this matrix in non-mode-selective photonic lanterns, yet a quantitative physical explanation for this behavior has not previously been demonstrated. Here, we present direct measurements of the wavelength-dependent encoding transfer matrix of a photonic lantern across the range 1525 nm to 1575 nm using off-axis holographic imaging, enabling high-fidelity recovery of both amplitude and phase. Beyond measurement, we introduce a physically grounded propagation model and numerical simulation that quantitatively reproduces the observed wavelength evolution and provides a unified physical explanation for behavior reported in prior experimental work. The model identifies differential modal phase accumulation in the multimode section as the dominant mechanism governing spectral evolution and shows that increasing the length of the multimode end systematically accelerates the phase evolution of the transfer matrix with wavelength. These results establish a direct and predictive link between photonic lantern geometry and spectral response, providing a design framework for tailoring lanterns either to enhance sensitivity to closely spaced wavelengths or to enforce uniform response over broad bandwidths for spectroscopic and imaging applications.