Fibre optical parametric devices for large frequency-shift wavelength conversion

Abstract

In this thesis, I investigate fibre optical parametric amplifiers (OPA) and oscillators (OPO), in terms of their potential for efficient large frequency-shift wavelength conversion. The underlying physical mechanism of fibre four wave mixing (FWM) offers simultaneous up-conversion and down-conversion in frequency to arbitrary wavelengths, determined by the pump wavelength and chromatic dispersion of the fibre. Using optical pulses from an ytterbium-doped fibre master-oscillator power-amplifier (MOPA), at a wavelength of 1080 nm, I experimentally and numerically evaluate the suitability of various fibres for frequency up-conversion towards the visible spectrum. The use of an Yb-doped fibre source allows for all-fibre integration with fibre optical parametric devices, potentially making it a viable alternative to expensive bulk sources currently employed in the sub-1 µm spectral region.

To accommodate an all-fibre configuration, the first part of the thesis numerically investigates polarization maintaining (PM) as well as higher-order mode fibres for phase-matched FWM at relatively modest pump peak powers (< 1 kW). Experiments using the PM fibre in an OPA configuration, and employing multiple seeding arrangements, are subsequently presented. Here, it is found that the influence of fibre inhomogeneity, coupled with a relatively small parametric bandwidth and competing nonlinear processes, severely impairs the conversion efficiency from the 1080 nm pump wave to the anti-Stokes wave at 840 nm.

The work in the second part of the thesis reports on the use of higher-order dispersion phase-matching in a photonic crystal fibre (PCF). In the OPA configuration, this approach proves more efficient and demonstrates parametric conversion over 142 THz to an anti-Stokes wave at 715 nm. The PCF is also reconfigured into an all-fibre uni-directional ring-cavity OPO, for which the dependence on nonlinear converter length, out-coupling ratio, pump pulse duration and intra-cavity filtering are studied. Using a PCF length of 18 m and 800 ps pump pulses with sub-kW peak powers, this all-fibre OPO demonstrates, what is believed to be, a record in-fibre pump-to-anti-Stokes conversion efficiency in excess of 10% over 142 THz.

Finally, computer simulations, aimed at realising a dispersion engineered PCF for an enhanced parametric gain bandwidth, is carried out for a pump wavelength around 1 μm. PCF designs of this type, using GeO₂-doped core regions, are identified for arbitrary frequency-shift FWM. It is demonstrated that these fibres can enhance the parametric bandwidth by up to three orders of magnitude, which can lead to a significant reduction in the sensitivity of FWM to fibre inhomogeneities. An increased parametric bandwidth should furthermore enable the use of pump sources that are not currently considered to be viable. The fibres are finally also considered in terms of fabrication tolerances.

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Identifiers

ORCID for J. Nilsson: orcid.org/0000-0003-1691-7959

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