Abstract
Relatively high power supercontinuum sources were available from the mid 1980s based upon picosecond pulse pumping by Nd:YAG lasers of conventional step index fibres and utilizing soliton-Raman generation, however, it wasn’t until the introduction of photonic crystal fibres (PCF) and the application of master oscillator power fibre amplifier (MOPFA) configurations that high average power supercontinuum sources became practical and subsequently, a commercial success. With the Yb-doped fibre amplifier being the most extensively developed high power fibre amplifier system the vast majority of all-fibre based supercontinuum source studies utilized pumping around 1060 nm. Consequently, PCF became indispensible as the nonlinear medium since the zero dispersion wavelength of the fibre could be accurately tailored, as well as the nonlinear coefficient, to optimize pumping in this wavelength range. Early schemes provided average powers of ~2 W and spectral power densities in the visible of ~ 2 mW/nm. Over the past six years the average operational powers have gradually increased to ~50 W. However, as a result of phase and group velocity matching criteria introduced by the necessary condition of having the zero dispersion wavelength of the nonlinear fibre in the region of the pump wavelength such that modulational instability and subsequent soliton evolution played the major role in determining supercontinuum generation, the short wavelength extent of 1060nm pumped sources only extended to about 600nm. Extension to the blue was achieved by generation in cascaded systems where PCFs with gradually decreasing zero dispersion wavelength were employed, where taken to its most extreme, single fibres with tapered profiles (manufactured at the University of Bath) were utilized. This allowed spectral extension to 320 nm and spectral power densities greater than 5 mW/nm below 400 nm.
© 2011 Optical Society of America
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