Mid-infrared supercontinuum generation in tapered ZBLAN fiber with a standard Erbium mode-locked fiber laser

Mid-InfraRed (MIR) broadband SuperContinuum (SC) sources are desirable for applications such as pollution monitoring, spectroscopy, and IR countermeasures due to their high spatial coherence and high power density over a broad bandwidth [1]. Conventional silica fibers cannot facilitate this need due to their intrinsic IR transmission edge at 2.4 μm, so instead soft glasses are used for MIR SC sources. The soft glass ZBLAN is in particular interesting as it has low loss out to 4.5 μm [Fig. 1(a)]. Additionally, it has a material Zero Dispersion Wavelength (ZDW) around 1.6 μm that with proper fiber design allows to generate a broadband SC using direct pumping with commercially available Erbium (Er) mode-locked fiber lasers at 1550 nm. Formation of SC is manipulated both in the UV and IR by changing the fiber dispersion and nonlinearity using tapers. This has been much studied in various silica fiber designs and is now also becoming used in ZBLAN [2], and other soft glasses such as chalcogenide [3] and tellurite [4]. The aim of this numerical work is to show how pumping tapered commercially available ZBLAN fibers with an Er mode-locked fiber laser can generate a broadband SC approaching the ZBLAN long wavelength transmission edge. The taper employed here is a 20 cm long symmetric taper with a down- and up-taper length of 2 cm and a 16 cm long waist [see inset in Fig. 1(c)] and can be easily fabricated on a conventional Vytran taper stage.