Supercontinuum Generation in a Graded-Index Multimode Tellurite Fiber

Abstract

Supercontinuum generation (SCG) has attracted major attention due to its unique properties. These included an ultrabroad spectral bandwidth, high directionality, and under specific conditions high coherence, which makes them ideal for a number of applications in sensing and imaging. The generation of a supercontinuum results from the combination of complex linear and nonlinear effects interplay, when intense light propagates in a nonlinear medium. In guided geometries, high intensity can be maintained over extended distances, resulting in enhanced nonlinear interactions. This is why optical fibers or waveguides have become the preferred platform to generate supercontinuum sources. However, the relatively small core of conventional single-mode fibers or waveguides limits the power spectral density of SCG due to lower damage threshold as compared to fibers with large core sizes supporting a high number of propagating modes. The generation of SC sources in the mid-infrared with high power spectral density is particularly appealing as it would open up new perspectives for a wide range of applications. Indeed, many important molecules exhibit strong absorption features in this region of the electromagnetic spectrum, allowing e.g. to be detected for monitoring e.g. greenhouse gases. In order to generate a fiber-based SC in the mid-infrared, the material characteristics are key and in particular the transparency window, the dispersion properties, as well as the nonlinearity. Among soft-glass fibers which are used for SCG in the mid-infrared due to their extended transmission window as compared to silica fibers, tellurite glass fibers possess both broad transmission band as well as a high nonlinearity making them a promising candidate for mid-infrared SCG. Moreover, the recent demonstration of nonlinear spatiotemporal dynamics in large core graded-index multimode silica fibers allowing to obtain a SC with high power spectral density and near single-mode output transverse intensity distribution has triggered significant interest. However, up to now, these studies have been restricted to commercial silica graded-index fibers with limited spectral bandwidth. This thesis reports the first experimental demonstration of supercontinuum generation in a multimode graded-index tellurite fiber. Due to the high nonlinearity of the constituent glass and extended transmission window as compared to silica, the generated supercontinuum spans nearly two octave from 790 nm to 2900 nm, extending further than any supercontinuum reported in graded-index silica fibers. The supercontinuum generation process is investigated for fiber samples with different core sizes and lengths as well as in different dispersion regimes. The transverse spatial intensity distribution is also characterized showing signatures of beam self-cleaning dynamics.