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We provide a correction due to an erroneous repetition rate of one of the laser systems (90 fs pulse duration) in our previously published paper [Opt. Express28, 25037 (2020)10.1364/OE.399771].
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Supercontinuum generation enabled a series of key technologies such as frequency comb sources, ultrashort pulse sources in the ultraviolet or the mid-infrared, as well as broadband light sources for spectroscopic methods in biophotonics. Recent advances utilizing higher-order modes have shown the potential to boost both bandwidth and modal output distribution of supercontinuum sources. However, the strive towards a breakthrough technology is hampered by the limited control over the intra- and intermodal nonlinear processes in the highly multi-modal silica fibers commonly used. Here, we investigate the ultrafast nonlinear dynamics of soliton-based supercontinuum generation and the associated mode coupling within the first three lowest-order modes of accurately dispersion-engineered liquid-core fibers. By measuring the energy-spectral evolutions and the spatial distributions of the various generated spectral features polarization-resolved, soliton fission and dispersive wave formation are identified as the origins of the nonlinear broadening. Measured results are confirmed by nonlinear simulations taking advantage of the accurate modeling capabilities of the ideal step-index geometry of our liquid-core platform. While operating in the telecommunications domain, our study allows further advances in nonlinear switching in emerging higher-order mode fiber networks as well as novel insights into the sophisticated nonlinear dynamics and broadband light generation in pre-selected polarization states.
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Geometrically induced birefringence represents a pathway for precisely engineering the modes in fibers and is particularly relevant for applications that crucially depend on modal dispersion. Here liquid core fibers (LCFs) with elliptical cores are analyzed in view of modal properties and third-harmonic generation (THG) numerically and experimentally. Using finite element modeling, the impact of ellipticity on phase matching, inter-modal coupling, electric field distribution, and birefringence are investigated. Significant THG in practically relevant modes, in accordance with phase-matching calculations, was measured in inorganic solvent-based LCFs.
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Third harmonic generation in a circular liquid core step-index fiber filled with a highly transparent inorganic solvent is demonstrated experimentally using ultrafast pump pulses of different durations in the telecom domain for the first time. Specifically we achieve intermodal phase matching to the HE13 higher order mode at the harmonic wavelength and found clear indications of a non-instantaneous molecular contribution to the total nonlinearity in the spectral broadening of the pump. Spectral power evolution and efficiency of the conversion process is studied for all pulse parameters, while we found the greatest photon yield for the longest pulses as well as an unexpected blue-shift of the third harmonic wavelength with increasing pump power. Our results provide the basis for future studies aiming at using this tunable fiber platform with a sophisticated nonlinear response in the context of harmonic generation.
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Accurate dispersion management is key for efficient nonlinear light generation. Here, we demonstrate that composite-liquid-core fibers-fibers with binary liquid mixtures as the core medium-allow for accurate and tunable control of dispersion, loss, and nonlinearity. Specifically, we show numerically that mixtures of organic and inorganic solvents in silica capillaries yield anomalous dispersion and reasonable nonlinearity at telecommunication wavelengths. This favorable operation domain is experimentally verified in various liquid systems through dispersion-sensitive supercontinuum generation, with all results being consistent with theoretical designs and simulations. Our results confirm that mixtures introduce a cost-effective means for liquid-core fiber design that allows for loss control, nonlinear response variation, and dispersion engineering.
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Supercontinuum generation is a key process for nonlinear tailored light generation and strongly depends on the dispersion of the underlying waveguide. Here we reveal the nonlinear dynamics of soliton-based supercontinuum generation in case the waveguide includes a strongly dispersive resonance. Assuming a gas-filled hollow core fiber that includes a Lorentzian-type dispersion term, effects such as multi-color dispersive wave emission and cascaded four-wave mixing have been identified to be the origin of the observed spectral broadening, greatly exceeding the bandwidths of corresponding non-resonant fibers. Moreover, we obtain large spectral bandwidth at low soliton numbers, yielding broadband spectra within the coherence limit. Due to the mentioned advantages, we believe the concept of resonance-enhanced supercontinuum generation to be highly relevant for future nonlinear light sources.
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We show that the ultrafast nonlinear dynamics in supercontinuum generation can be tailored via mixture-based liquid core fibers. Samples containing mixtures of inorganic solvents allow changing dispersion from anomalous to normal, i.e., shifting zero dispersion across pump laser wavelength. A significant control over modulation instability and four-wave mixing has been demonstrated experimentally in record-long (up to 60 cm) samples in agreement with simulations when using sub-psec pulses at 1.555 µm. The smallest concentration ratio yields indications of soliton-fission based supercontinuum generation at soliton numbers that are beyond the coherence limit. The presented dispersion tuning scheme allows creating unprecedented dispersion landscapes for accessing unexplored nonlinear phenomena and selected laser sources.
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Due to their unique properties such as transparency, tunability, nonlinearity, and dispersion flexibility, liquid-core fibers represent an important approach for future coherent mid-infrared light sources. However, the damage thresholds of these fibers are largely unexplored. Here we report on the generation of soliton-based supercontinua in carbon disulfide (CS2) liquid-core fibers at average power levels as high as 0.5 W operating stably for a long term (>70 h) without any kind of degradation or damage. Additionally, we also show stable high-power pulse transmission through liquid-core fibers exceeding 1 W of output average power for both CS2 and tetrachloroethylene as core materials.
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We demonstrate that exposed-core microstructured optical fibers offer multiple degrees of freedom for tailoring third-harmonic generation through the core diameter, input polarization, and nanofilm deposition. Varying these parameters allows control of the phase-matching position between an infrared pump wavelength and the generated visible wavelengths. In this Letter, we show how increasing the core diameter over previous experiments (2.57 µm compared to 1.85 µm) allows the generation of multiple wavelengths, which can be further controlled by rotating the input pump polarization and the deposition of dielectric nanofilms. This can lead to highly tailorable light sources for applications such as spectroscopy or nonlinear microscopy.
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The nano-bore optical fiber geometry represents a new waveguide platform that uniquely allows studying the interaction of low-index fluids and light inside the core of an optical fiber while maintaining total internal reflection as a light guidance mechanism. Here, we have analyzed several application-relevant properties of this novel geometry experimentally and from the simulation perspective, including the analysis of the power fraction inside the bore, the determination of radius-dependent cutoffs, and the identification of single-mode operation domains. The obtained results will pave the way for new application of fiber optics in fields such as optofluidics, nonlinear light generation, and bioanalytics.
