Supercontinuum Generation in Soft-glass PCFs
The physics of SC generation, having been studied for more than four decades, is now well understood. Detailed studies of nonlinear dynamics in optical fibers have led to several breakthroughs in extending and improving the quality of SC light sources. The central role of the group velocity dispersion (GVD) in controlling these dynamics means that solid-core silica-air photonic crystal fiber, the dispersion properties of which can be extensively engineered by varying the microstructure, has become the dominant medium not only for SC generation but arguably for nonlinear fiber optics in general.
Silica-based photonic crystal fiber (PCF) has proven highly successful for supercontinuum (SC) generation, with smooth and flat spectral power densities. It suffers, however, from strong material absorption in the mid-IR (>2500 nm), and optical damage (solarisation) in the UV (<380 nm), which limits its performance and lifetime. SC generation in silica-PCF is therefore only possible between these wavelength limits. Although glasses such as chalcogenides, fluorides and heavy-metal oxides have been used to extend the spectrum out to longer wavelengths in the mid-IR, none have enhanced the UV performance. In 2015, we reported the first ZBLAN PCF with a high air-filling fraction, a small solid core, nanoscale features and a near-perfect structure. We used this PCF to generate an ultrabroadband, long-term stable, supercontinua spanning from 200 to 2500 nm [Jiang (2015)].
We have continued to explore open issues such as the physics of UV SC light generation and dispersion tailoring in novel structures, in various ZBLAN PCFs. Due to the limited access to raw materials, we have focused on simplified structures, for example a six-core ZBLAN fiber with nanobores in each core has been used for SC generation [Jiang (2016)], as shown in Fig. 24. The cores had diameters of ~1.3 μm, with nanobore diameter ~330 nm. Spectral broadening was observed when a single core was pumped in the fundamental and first higher order core-modes with 200 fs-long pulses at a wavelength of 1042 nm. The results were verified by numerical simulations.
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