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Currently third harmonic generation (THG) in fibers is in focus of research, due to the prospects of enabling efficient generation of entangled photon triplets via the inverse, but spontaneous process. To date, microstructured and tapered fibers are the most prominent platforms to provide the high index contrast between the core and cladding material necessary to enable phase-matching (PM) to higher order modes (HOMs). Here, we employ a liquid core fiber (LCF) combining a silica cladding and a highly nonlinear core material, which can be temperature-tuned during operation [1].

Nonlinear light generation in optical fibers is an indispensable tool for generating light to access wavelengths away of the fundamental laser lines broadening the field of applications in biophotonics, metrology, and communications. However, most multi-wavelength sources rely on glass fibers which are static in their material properties after fabrication, thus post-tuning their properties is impossible. Complementary to the pressure-adjustable gas-filled hollow-core fibers in the high power laser regime [1], liquid-core fibers (LCF) offer a versatile platform for tunable nonlinear light generation and pulse control in the low- to medium-power regime. Recently, we demonstrated accurate control of the soliton fission process via temperature due to the large thermo-optical coefficient of the core liquid [2]. However, those experiments were performed with rather cost-intensive thulium laser technology.

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.

Optica 5(6), 695 - 703 (2018). doi:10.1364/OPTICA.5.000695, Liquid-core fibers offer local external control over pulse dispersion due to their strong thermodynamic response, offering a new degree of freedom in accurate soliton steering for reconfigurable nonlinear light generation. Here, we show how to accurately control soliton dynamics and supercontinuum generation in carbon disulfide/silica fibers by temperature and pressure tuning, monitored via the spectral location and the onset energy of non-solitonic radiation. Simulations and phase-matching calculations based on an extended thermodynamic dispersion model of carbon disulfide confirm the experimental results, which allows us to demonstrate the potential of temperature detuning of liquid-core fibers for octave spanning recompressible supercontinuum generation in the near-infrared., Published by OSA, Washington, DC

We show theoretically that microstructured optical fibers filled with nonlinear liquids are capable of generating photon-pairs with highly tunable properties, suitable for applications in quantum spectroscopy and as sources of heralded single photons.

Various applications in biophotonics require widely wavelength-selectable laser sources in the infrared(IR) with high spectral density rather than broad bandwidth. This requirement is barely covered by the recent advances in supercontinuum generation in static optical glass fibers, in which the nonlinear effects can mainly be controlled by the injected power [1]. Supercontinua in gas-core fibers ranging from the UV to visible wavelengths are additionally adjustable by the gas pressure but require complex fiber structures [2]. Due to their strong thermooptical coefficient liquid-core step-index fibers that have a high nonlinearity and wide transmission windows can be used for temperature-based supercontinuum tuning in the mid-infrared [3].