Engineering Adiabaticity for Efficient Design of Photonic Lanterns

We present a comprehensive systematic framework for optimizing adiabaticity in photonic lanterns. The framework considers the effects of both the cross-sectional and longitudinal device parameters on adiabaticity. Photonic lanterns are adiabatic photonic devices and are therefore known to have large device lengths. A Shortcuts-to-adiabaticity (STA) protocol is employed to optimize the adiabatic taper profile in all-fibre photonic lanterns. The method tailors adiabatic propagation of light in the system by appropriately correcting the local slope of the taper profile. A quantifiable measure of adiabaticity is established, based on the adiabaticity criterion. This measure relates inversely to the device length and is a useful parameter in taper optimization. We apply the protocol to reported photonic lantern devices to obtain optimal adiabatic taper profiles having shorter device lengths. The optimized taper profile refers to that taper profile which corresponds to the minimum local inter-modal coupling at every point along the quasi-adiabatic transition for a given device length. While optimizing the adiabatic transition by minimizing inter-modal coupling is the basic aim of this work, length optimization is an extremely useful by-product. This procedure can be used to either reduce the device length or to reduce the mode-coupling losses or both. This study analyses reported three and six-core mode selective photonic lanterns as examples. We also discuss ways to make the optimum profiles practically realizable.