Tunable supercontinuum in multimode fiber via bending-induced dispersion modification

Abstract

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.

Figures (14)

• Figure 1: Bending-controlled spectrally tunable HOM supercontinuum.(a) Schematic of nonlinear pulse control of a HOM in MMF via local disperison modification. (b) Schematic of experimental setup for bending-controlled HOM supercontinuum. First, a liquid-crystal SLM transforms the pump pulse from a Yb laser into a high-order fiber mode in an SI-MMF. Then, multiple motorized fiber shapers apply local bending to modulate the spectrum without scrambling the spatial profile, yielding broadband tunable Bessel-like foci at the output. (c) Dispersion engineering via launching high-order fiber modes for broadband supercontinuum evolution. The traces show experimental spectra in a 70-cm SI-MMF pumped with 400 nJ pulses. (d) Tunable supercontinuum via mechanical perturbation. A 9-meter SI-MMF and a lower pump energy of 65 nJ were used to emphasize spectral contrast resulting from mechanical perturbation.
• Figure 2: Broadband tunable Bessel-like foci via disorder-resilient LP$_{0,7}$ mode pump.(a) Broadband Bessel-like foci with an LP$_{0,7}$ mode pump in an SI-MMF. Beam profiles were measured under different bandpass filters. (b) The two-photon PSFs at 1100 nm showing axial FWHM of 4.2 µ m for the Gaussian focus and 24 µ m for the Bessel-like focus. (c) Moderate bending alters the spectrum while preserving the high-order spatial mode. Bending is quantified by the total displacement $\sum |\Delta d_i|$, where $d_i$ is the displacement of the i-th shaper with respect to the previous shaper motor. The spectra under 730 fiber shaper configurations are plotted in gray traces, with five highlighted spectra associated with the above spatial profiles. Inset (top-left): photograph of the precision laser cut motorized fiber shapers.
• Figure 3: Bending-induced dispersion modification(a-b) The 1$^{st}$ and 2$^{nd}$ order dispersion curves of the LP$_{0,7}$ mode in a 50-µ m-core SI-MMF (a) and a 5-µ m-core SMF with 0.1 NA (b). (c) Spectral decorrelation via perturbation at different distances. Left: the simulated spectral evolution (pumped by the LP$_{0,7}$ mode) showing different timings of soliton fission/dispersive wave generation and SSFS. Right: the spectral correlation maps obtained by applying mechanical perturbation at soliton fission length ($L_{fiss}$) and dispersive characteristic length ($L_{D}$). Pearson correlation coefficients were computed with a 50 nm binning width, with a total of 730 spectra for each distance. The spectral evolution simulation was performed by a MATLAB GMMNLSE solver Wright2018. (d--e) Spectral correlation maps of the supercontinuum generated by SLM modulation (d) and shaper modulation (e). Pearson correlation coefficients were computed with a 10 nm binning width, with a total of 800 and 730 spectra were for the SLM- and shaper-modulated cases, respectively.
• Figure 4: Versatile biological contrasts via broadband tunable femtosecond pulses.(a) System schematic of nonlinear microscopic imaging using broadband Bessel-like foci. Proximal end: L1--L2 are lenses with a focal length of 19 and 200 mm, respectively. Relay optics: L3--L5 are lenses with a focal length of 10, 500, and 400 mm, respectively. Beam characterization unit: L6 is a lens with a focal length of 300 mm. Multiphoton microscope: L7--L8 are lenses with a focal length of 30 and 100 mm, respectively. BS: beam splitter. QWP: quarter-wave plate. CAM: camera. BPF: bandpass filter. DM: dichroic mirror. PMT: photomultiplier tube. OBJ: objective lens. (b) Nonlinear microscopy modalities enabled by the broadband Bessel-like foci. Scale bars: 50 µ m. (c) SLAM imaging of fixed mouse whisker pad tissue using 1300 nm excitation. The emission filters are 430 $\pm$ 5 nm (THG), 530 $\pm$ 27.5 nm (FAD), and 650 $\pm$ 20 nm (SHG). The pixel size is 500 nm, and the dwell time is 400 µ s.
• Figure 5: Volumetric nonlinear imaging via broadband Bessel-like foci.(a--c) Two-photon volumetric imaging of an enteric nervous system (tdTomato-expressed ganglion network) using 1025 nm excitation. The image volume is 500 $\times$ 500 $\times$ 25 µ m$^3$ with a 500 nm lateral pixel size. The $z$ step size is 1 µ m for the Gaussian stack. (b) Fourier analysis of the images in (a). (c) Line profiles of the white-dashed cross sections in (a). (d--e) Metabolic imaging of a blood-brain-barrier model using 1100 nm excitation. The image volume is 300 $\times$ 300 $\times$ 25 µ m$^3$ with a 500 nm lateral pixel size. The $z$ step size is 1 µ m for the Gaussian stack. The inset in (d) shows a single $z$-frame. The zoomed-in views highlight astrocytes and endothelial cells in the field of view. (f--j) Multicolor imaging in a jellyfish nervous system using 1100 nm excitation. (f) shows the full jellyfish sample with a single $z$-frame. (g--h) highlight region 1, and (i--j) highlight region 2. The image volume is 300 $\times$ 300 $\times$ 30 µ m$^3$ with a 500 nm lateral pixel size and a $z$ step size of 1 µ m. The insets in (g) and (i) show single $z$-frames of the corresponding regions.