Zero-dispersion wavelength decreasing photonic crystal fibers for ultraviolet-extended supercontinuum generation

Abstract

We report the fabrication of photonic crystal fibers with a continuously-decreasing zero-dispersion wavelength along their length. These tapered fibers are designed to extend the generation of supercontinuum spectra from the visible into the ultraviolet. We report on their performance when pumped with both nanosecond and picosecond sources at 1.064 microm. The supercontinuum spectra have a spectral width (measured at the 10 dB points) extending from 0.372 microm to beyond 1.75 microm. In an optimal configuration a flat (3 dB) spectrum from 395 to 850 nm, with a minimum spectral power density of 2 mW/nm was achieved, with a total continuum output power of 3.5 W. We believe that the shortest wavelengths were generated by cascaded four-wave mixing: the continuous decrease of the zero dispersion wavelength along the fiber length enables the phase-matching condition to be satisfied for a wide range of wavelengths into the ultraviolet, while simultaneously increasing the nonlinear coefficient of the fiber.

Cascaded PCF tapers for flat broadband supercontinuum generation

“…SC generation confined to the visible 15-cmlong tapered SMF28 and tapered PCF pumped by femtosecond pulses centered at 540 nm were achieved [8] . SC generation, extending from 375 to 1 750 nm, was observed in the tapered PCFs, with 11-m-long transition for both nanosecond and picosecond 1.06-µm pump sources [9] . An ultra-broad SC (down to 280 nm in deep UV) was formed by pumping sharply tapered (5-30 mm taper lengths) solid-core PCFs at 130-fs and 2-nJ pulses at 800 nm were obtained [10] .…”

Four-wave mixing UV generation in optical microfibers

“…Ultraviolet (UV) generation has traditionally relied on crystals and gases to allow frequency conversion from longer wavelengths [1][2][3][4][5][6][7][8][9][10]. These methods involve the use of nonlinear crystals such as KTP, KDP, sapphire, K2Al2B2O7, potassium pentaborate (KB5), the exploitation of nonlinear processes in waveguides such as photonic crystal fibers or the use of hybrid schemes employing both waveguides and nonlinear crystals [1][2][3][4][5][6][7].…”

“…These methods involve the use of nonlinear crystals such as KTP, KDP, sapphire, K2Al2B2O7, potassium pentaborate (KB5), the exploitation of nonlinear processes in waveguides such as photonic crystal fibers or the use of hybrid schemes employing both waveguides and nonlinear crystals [1][2][3][4][5][6][7]. Fiberized UV generation in optical fibers typically involve the use of gas-filled hollow core microstructured fibers, or otherwise exploit nonlinear processes such as supercontinuum generation to generate a broad spectrum which extends down to the UV wavelength region [8][9][10][11][12][13][14][15].…”

Continuum generation in photonic crystal fibres by multi-wavelength amplified sub-nanosecond light pulses

“…The main nonlinear process resulting in spectral broadening of the visible pump was found to be cascaded cross-phase modulation induced by the infrared part of the continuum [2]. Other methods for covering near UV range have been proposed [3][4][5] which rely on engineering of dispersion of the PCF fibre itself and using single wavelength radiation for excitation. One of the methods proposed is shifting the ZDW of the fibre to much shorter wavelengths by tapering PCF and using shorter pump wavelength [3,4] for excitation.…”