Octave-spanning Infrared Supercontinuum Generation in a Graded-Index Multimode Lead-Bismuth-Gallate Fiber

The generation of a broadband supercontinuum (SC) in the mid-infrared has attracted significant attention in the past few years due the wide range of possible applications in e.g. microscopy, medical diagnostics, gas sensing and imaging [1] as many molecules exhibit strong vibrational absorption bands in this wavelength range. Fibers based on fluoride, tellurite, or chalcogenide soft glasses are typically used for SC generation in the mid-infrared due to their broad transmission window and high nonlinear refractive index. These glasses, however, inherently suffer from a low damage threshold and their synthesis and processing can be challenging. In order to overcome the power limitation and scale up the SC output power, multimode fibers have attracted growing interest with SC generation demonstrations in step index multimode fluoride and chalcogenide fibers [2] . While attractive, these SC sources are nevertheless highly multimoded making them undesirable for several applications. Silicate glasses with high concentration of the heavy metal oxides on the other hand offer a good trade off between easy synthesis, broad transmission window and relatively high nonlinearity. They are also thermally stable and their composition may be tuned by changing the dopants concentration allowing to engineer their characteristics. Among those, leadbismuth-gallium silicate glasses (PBG) have shown to be promissing material for fiber fabrication with comparable nonlinearity to that of tellurite glasses and transmission up to 5.5 micron [3] . Here, we demonstrate for the first time the generation of octave-spanning supercontinuum in PBG graded-index multimode fiber using femtosecond pulses ( Fig 1 ).