Tunable supercontinuum in multimode fiber via bending-induced dispersion modification

Nonlinear pulse propagation in multimode fibers (MMFs) offers a compact, low-cost route to broadband, tunable femtosecond light, but most control schemes act by changing the spatial mode composition, typically resulting in irregular or speckled beams in exchange for maximal spectral tunability. Here we introduce a complementary mechanism: bending-induced local dispersion modification of a high-order mode (HOM) to steer the spectrum while keeping the spatial mode fixed. We launch an LP0,7 mode into a step-index MMF and apply programmable macrobends near the input. With a standard Yb pump at 1030 nm, this yields spatially clean, continuous spectral tuning across 700-1350 nm, while the output profile remains Bessel-like and robust to reconfiguration of controlled bends. A perturbative model explains the observed spatial-spectral decorrelation, showing that moderate curvature produces first- and second-order shifts in group delay and group-velocity dispersion of the HOM with minimal change in its modal composition; these dispersion shifts control soliton fission, dispersive-wave emission, and the soliton self-frequency shift. We further validate application utility by driving multicolor, extended-depth-of-focus multiphoton microscopy directly from this all-fiber source. To our knowledge, this is the first demonstration of bending-induced dispersion modification, rather than mode mixing, used to tune MMF supercontinuum spectra without sacrificing beam quality, laying the foundation for an alternative pathway to tunable femtosecond illumination for imaging and spectroscopy.

💡 Research Summary

The authors present a novel method for tuning the supercontinuum spectrum generated in a multimode fiber (MMF) without compromising the spatial beam quality. By launching a high‑order LP0,7 mode into a step‑index MMF and applying programmable macrobends near the fiber input, they exploit bending‑induced modifications of the mode’s local dispersion (group delay and group‑velocity dispersion, GVD) as a control knob. The experiment uses a 1030 nm Yb laser (219 fs, 400 nJ) coupled into a 70 cm, 50 µm‑core, NA 0.22 fiber. Precision laser‑cut motorized fiber shapers create bends with radii down to 1 cm, positioned within the first ten centimeters where nonlinear evolution is most sensitive.

Key observations: the output supercontinuum can be continuously shifted across 700–1350 nm while the output profile remains a clean, Bessel‑like ring with >80 % overlap with the ideal LP0,7 mode. Spectral cosine similarity exceeds 0.99 over 100 configuration switches and remains stable over several hours. The spatial mode is robust because moderate curvature introduces a slowly varying refractive‑index perturbation that primarily alters the propagation constant and its first two derivatives, leaving the eigen‑field distribution essentially unchanged. High‑order modes are intrinsically resistant to bend‑induced coupling due to large propagation‑constant separations, rapid temporal walk‑off, and cancellation of the perturbation in the overlap integral.

A perturbative analysis shows that a 1 cm bend induces a GVD change of roughly Δβ₂ ≈ 2 ps²/km for the LP0,7 mode, whereas the fundamental mode in a single‑mode fiber experiences negligible change. This shift moves the zero‑dispersion wavelength (ZDWL) from ~1300 nm (fundamental) to ~1030 nm (LP0,7), placing the pump in the anomalous‑dispersion regime. Consequently, modulation instability, soliton fission, Raman‑driven soliton self‑frequency shift (SSFS), and phase‑matched dispersive‑wave generation (DWG) are all re‑phased, allowing the spectrum to be steered by the applied bends.

The authors further explore the spatial dependence of the effect by bending the fiber at two distinct locations: early bends near the soliton‑fission length and later bends near the dispersive length. Spectral correlation maps reveal that wavelength pairs involved in the same nonlinear process shift together (high positive correlation), confirming that local dispersion engineering can selectively target specific stages of the broadband generation.

To demonstrate practical utility, the tunable source directly drives multicolor, extended‑depth‑of‑focus (EDOF) multiphoton microscopy. The Bessel‑like focus provides an axial extent of ~24 µm (six times longer than a Gaussian focus) while preserving sub‑micron lateral resolution (~0.95 µm). Multicolor imaging across the full 700–1350 nm window is achieved without additional bulk optics, highlighting the potential for compact, low‑cost femtosecond illumination in biomedical imaging.

In summary, this work establishes bending‑induced dispersion modification as a viable, deterministic control mechanism for MMF supercontinuum generation. It decouples spectral tunability from spatial mode degradation, offering a pathway to robust, high‑quality, broadband femtosecond sources for spectroscopy, microscopy, and related applications.