# Simultaneous Transmission of Discrete-Variable Quantum Key Distribution and Classical Optical Communication in Few-Mode Fiber

## TL;DR

This paper explores how quantum and classical signals can coexist in fiber optic cables, improving quantum communication distances.

## Contribution

A new model for quantum-classical coexistence in few-mode fiber is proposed, enhancing transmission distance by optimizing fiber design.

## Key findings

- Mode coupling, number of modes, and wavelengths significantly affect spontaneous Raman scattering.
- A ring-assisted few-mode fiber design reduces noise and increases quantum signal transmission distance by up to 41.5%.

## Abstract

Based on mode crosstalk theory, this paper develops a spontaneous Raman scattering (SpRS) model for the quantum-classical coexistence system using few-mode fiber (FMF) integrated with wavelength-division multiplexing (WDM) and spatial-division multiplexing (SDM). Through numerical calculations, the influence degrees of three factors (mode coupling, the number of modes and wavelengths) on SpRS are analyzed. The investigation identifies the dominant contributors to SpRS and reveals their relative impact magnitudes. Based on these results, a ring-assisted FMF is proposed to mitigate noise impacts on quantum signals. The numerical results show that the optimized FMF enhances quantum signal transmission distance by up to 41.5%.