1 Department of Photonics Engineering, Technical University of Denmark2 Fiber Sensors and Supercontinuum Generation, Department of Photonics Engineering, Technical University of Denmark
The nonlinear pulse broadening phenomenon of supercontinuum generation in optical fibres is appreciated as one of the most striking in nonlinear physics. Thanks to the unique combination of high brightness and octavespanning spectra, modern “white-light” supercontinuum lasers have found numerous applications in areas such as spectroscopy and microscopy. In this work, we exploit the tremendous design freedom in air hole structured photonic crystal fibres to shape the supercontinuum spectrum. Specifically, the supercontinuum dynamics can be controlled by clever engineering of fibres with longitudinally varying air hole structures. Here we demonstrate supercontinuum generation into the commercially attractive deep-blue spectral region below 400 nm from an Yb laser in such fibres. In particular, we introduce the concept of a group acceleration mismatch that allows us to enhance the amount of light in the deep-blue by optimising the fibre structure. To this end, we fabricate the first single-mode high air-fill fraction photonic crystal fibre for blue-extended supercontinuum sources. The mechanisms of supercontinuum broadening are highly sensitive to noise, and the inherent shot-to-shot variations in long-pulsed supercontinuum sources are a limiting factor for several applications. We investigate different approaches to quantify and lower the spectral noise. Specifically, we characterise the spectral noise in the framework of statistical higherorder moments, which provides insight into the nature of the noise across the spectrum. We further investigate the possibilities of reducing the spectral noise by modulating the pump with a weak seed, which makes the broadening dynamics increasingly deterministic rather than driven by noisy modulation instability. Particular attention is paid to the commercially relevant high power regime. Finally, we examine passive noise reduction in photonic crystal fibres with longitudinally varying air hole structures.