On the possibility of mid-IR supercontinuum generation in As-Se-Te/As-S core/clad fibers with all-fiber femtosecond pump source

We propose and optimize theoretically a supercontinuum (SC) laser source in the mid-IR based on using As-SeTe/As-S core/clad step-index fibers and a femtosecond all-fiber laser system at 2 μm. Numerically simulated spectra extending from ~1 μm to more than 8 μm are demonstrated for pump energy of order 100 pJ in a fiber with a core diameter of 2 μm. To the best of our knowledge, the possibility of such long-wavelength spectral conversion of pump pulses at the wavelength of 2 μm in optical fibers is demonstrated for the first time. The theoretical calculations are performed on the base of real low loss step-index As-Se-Te/As-S glass fibers with various coreclad diameter ratios. Introduction The mid-infrared (IR) ultra-broadband coherent light sources have important applications in remote sensing, biophotonics, homeland security, and so on. The photonic technologies based on mid-IR fibers providing these properties are very promising [1]. Chalcogenide glasses (ChG) have the broadest transmittance windows and the highest third-order nonlinear refractive indices (n2) among all optical glasses, ~200-1000 times higher than that for silica [1, 2]. These characteristics make them ideal candidates for mid-IR nonlinear fiber optics where short sample lengths or ultralow pulse energies are sufficient to elicit nonlinear optical behavior. SC generation in ChG fibers with various compositions, geometrical design, and pumping wavelengths are constantly reported (see [1] and references therein). To date, spectral SCs in the 1.4-13.3 μm and 2-15.1 μm ranges have been demonstrated in AsSe core step-index fibers pumped by optical parametric amplifiers (OPA) at 6.3 μm [3] and at 9.8 μm [4], respectively. An As-Se commercial fiber has allowed generating SC in the 3-8 μm range using an erbium-doped ZrF4-based in-amplifier SC source spanning from 3 to 4.2 μm [5]. SC spectrum spanning 1.5-14 μm has been achieved by pumping a Ge-As-Se-Te fiber at 4.5 μm [6]. An As-S-Se/As-S fiber pumped at 4.8 μm has been used to generate the spectrum in the 1-5 μm range [7]. SCs spanning 1.7-7.5 μm and 0.9-9 μm has been obtained with microstructured As-Se fibers pumped by OPA at 4.4 μm [8] and by SC from ZBLAN fiber in the 0.9-4.1 μm range respectively [9]. SC with red boundary beyond 5 μm has been reported for all-solid microstructured As-Se/As-S fiber pumped at 3.4 μm by OPA [10]. However, despite the substantial efforts which have led to startling results over the past several decades, the full potential of ChG fibers has not been achieved yet [2]. For instance, in the majority of the works devoted to mid-IR SC generation, the pump source initially operates in the mid-IR. But it would be great to start from the standard robust fiber laser systems in the near-IR in order to simplify a scheme and make a photonic device more suitable and convenient for applications. To the best of our knowledge, the reached red boundary of the SC spectra in ChG-based fibers pumped by fiber laser systems in the range of 1.5-2 μm is shorter than 4 μm [11-13]. But our numerical study demonstrates a possibility of SC generation with red boundary beyond 8 μm in As-Se-Te/As-S core/cladding step-index fibers pumped by a femtosecond all-fiber laser system at 2 μm. Such a kind of laser system at 2 μm built on standard telecom components and Er-doped and Tm/Yb co-doped silica fibers has been previously used by us for SC generation in the 2-3 μm range in a germano-silicate fiber [14] and for Raman soliton shifting up to 2.65 μm in tellurite fibers [15]. Here we have synthesized experimentally high-purity As39.4Se55.3Te5.3 glass for core and As39.4S60.6 glass for cladding and manufactured low-loss fibers with various diameters that can be pumped by this all-fiber system for SC generation. We study numerically nonlinear dynamics of the ultrashort pulses launched into ChG fibers with different core diameters d and tellurium content in the core glass. It is well known that not only fiber nonlinearity, depending on n2 and effective mode field area, impacts on SC generation, but dispersion is also of great importance. The ChGs are chosen with a large refractive index difference (Δn) between them. This allows tailoring dispersion by core diameter variation due to the strong waveguide contribution. The content of Te in As-Se-Te glasses affects refractive index, and can be also varied for dispersion management. Besides, As-Se-Te glasses have a number of advantages over binary arsenic chalcogenides. They offer lower phonon energies, higher values of refractive index (2.9-3), higher nonlinear refractive indices (n2), better transmission in the long wavelength mid-IR range, and high stability against crystallization. So, we believe, that the proposed fiber design is suitable for ultra-broadband spectral conversion due to the above listed advantages of physical and chemical properties.