Raman-Accelerated Power Depletion of Fundamental Mode in a Few-Mode Fiber in the Visible Spectral Range

TL;DR

This work addresses how Raman scattering governs nonlinear mode dynamics in a few-mode fiber excited by visible ultrafast pulses. It combines precise on-axis coupling, holographic mode decomposition, and spectral analysis to quantify intermodal energy transfer from the fundamental $LP_{01}$ mode to higher-order modes under increasing input power. Experimental observations show rapid depletion of $LP_{01}$ with transfer to $LP_{11a}$/$LP_{11b}$, $LP_{21a}$/$LP_{21b}$ and $LP_{02}$, accompanied by asymmetric spectral broadening dominated by Raman redshift and the emergence of OAM-like near-field profiles; cascaded Raman further enriches the spectrum. Numerical simulations solve the generalized multimode nonlinear Schrödinger equation (GMMNLSE) with a Raman response, confirming the essential roles of group-velocity matching and modal overlap in determining energy pathways and reproducing the measured modal distributions and spectra. The results provide mechanistic insight for tailoring ultrafast light in FMFs, with practical relevance for high-power laser delivery and mode-division multiplexed optical communications.

Abstract

We experimentally and numerically investigate Raman-driven power depletion in the fundamental mode of few-mode fibers (FMFs) excited by visible ultrashort pulses. Using a tunable femtosecond laser and SMF-28 fibers operated below the single-mode cutoff wavelength, we explore nonlinear mode dynamics through precise coupling, holographic mode decomposition, and spectral analysis. Experiments on 12-meter and 50-meter fibers reveal significant energy transfer to higher-order modes via intermodal Raman scattering. These findings advance our understanding of nonlinear propagation in FMFs, with important implications for high-power laser delivery and next-generation optical communication systems.

Figures (10)

• Figure 1: A schematic of the experimental setup. VNDF: Variable neutral density filter; L1-L3: Lens; BPF: Bandpass filter; HWP: half-wave plate; LP: Linear polarizer; FM: Flip mirror; NF $\&$ FF camera: Near and Far field camera; SLM: Spatial light modulator.
• Figure 2: Experimentally measured mode power distributions for fiber length of $50\text{ m}$ at center wavelength of $\lambda \:= \:700\text{ nm}$ for input average powers of a) $P_{in}\:= \:0.1\text{ mW}$, b) $P_{in}\:= \:1.6\text{ mW}$, c) $P_{in}\:= \:4.5\text{ mW}$, and d) $P_{in}\:= \:5.0\text{ mW}$.
• Figure 3: Experimental mode power distributions at the fiber output for two center wavelengths and varying input powers: (a–c) At $\lambda \:= \:800\text{ nm}$: a) $P_{in}\:= \:0.1\text{ mW}$, b) $P_{in}\:= \:1.1\text{ mW}$, and c) $P_{in}\:= \:6.6\text{ mW}$. (d–f) At $\lambda \:= \:900\text{ nm}$: d) $P_{in}\:= \:0.1\text{ mW}$, e) $P_{in}\:= \:1.8\text{ mW}$, and f) $P_{in}\:= \:5.0\text{ mW}$, for 50-m-long fiber.
• Figure 4: Experimental spectra obtained from a 50-m-long Alcatel SMF-28 as a function of the input guided power measured at the input for a laser pulse centered at $900\text{ nm}$.
• Figure 5: A fundamental mode reshaping near-field profile in a $50\text{ m}$-long fiber with input wavelengths of (a) $700\text{ nm}$, (b) $800\text{ nm}$, and (c) $900\text{ nm}$. (d) With an offset input, the same as (c), for different input average power $\text{P}_{\text{In}}$.