Abstract

Development of supercontinuum (SC) fiber-based light sources spanning the mid-infrared molecular fingerprint region is currently of great scientific interest, in particular for spectroscopy applications and gas sensing. Supercontinuum as broad as 4000-nm bandwidth was generated in a sub-cm long tellurite microstructured fiber in 2007 by means of a near-infrared femtosecond regime pumping [1]. More recently, mid-infrared SC spanning from 1.5 μm to 13 μm into short pieces of ultra-high numerical-aperture step-index chalcogenide glass optical fiber was reported by means of a mid-IR femtosecond regime pumping [2]. However, it still remains very challenging to confirm such dramatic spectral broadenings in quasi-single-mode fibers and using pump lasers at moderate energy levels. In this work, we demonstrate 400-THz (0.6–3.3 µm) bandwidth SC generation in a 10 cm-long sample of tapered tellurite suspended core fiber pumped by an optical parametric oscillator (OPO). In our case, the OPO delivers 200-fs pulses at a repetition rate of 80MHz. The increased nonlinearity and dispersion engineering extended by the reduction of the core size are exploited for SC optimization on both frequency edges, while keeping efficient power coupling into the untapered fiber input (195 mW average power injected at 1730nm). This pump wavelength was used to pump our tellurite fibers in the anomalous dispersion regime and close to its ZDW (for untapered fiber regions). Without our moderate tapering ratio, a similar SC experiment performed in the same length of uniform suspended core fiber revealed a 100-THz decrease of SC bandwidth. Resulting SC is collected by a fluoride fiber and spectrally analyzed by two OSA (350nm-1200nm and 1.2µm–2.4µm) and by a FTIR (2.4µm–4µm). The suspended core fiber used in our experiments is elaborated from a synthesized tellurite 80TeO₂-10ZnO-5Na₂O-5ZnF₂ glass (mol. %). The fiber has an outer diameter of 190 µm and a central solid core diameter of 3.7 µm, held to the clad through three surrounding holes by means of three thin struts. It exhibits a zero-dispersion wavelength (ZDW) close to 1.6 µm (see Fig.1a). The tapered suspended core fiber exhibits smooth and fast transition regions from the full fiber diameter to the small waist and back again (transitions are about 55 and 5 mm, respectively). The 5-mm-long taper waist has an outer diameter of 65 µm and a core diameter of 1.3 µm. The corresponding dispersion curve then exhibits two ZDWs at 1.2 µm and 3.1 µm (see Fig.1a). Experimental SC spectra obtained in both uniform and tapered fibers are reported in Fig. 1b-c, respectively. Corresponding numerical simulations based on the generalized nonlinear Schrödinger equation taking into account longitudinal variations of both dispersion and nonlinearity, but also fiber losses as well as self-steepening and analytical model of the Raman gain spectrum, fit quite well the dynamics observed. However we cannot extend the SC bandwidth in the mid-IR region due to extra OH-absorption around 3 µm [3].

© 2015 IEEE