Silicon fibres for nonlinear wavelength conversion

Abstract

Nonlinear optics has been one of the most important research areas in optics since the invention of lasers. Within this field, nonlinear fibre optics has gained notable attention as the guided light can be confined tightly within small core cross sections over extended lengths, thus enhancing the nonlinear interactions. In recent years, silicon core optical fibres have emerged as a versatile platform for nonlinear applications due to the high nonlinearity of the silicon material and their ability to be integrated with other fibre-based components. The motivation of this project is to build novel nonlinear photonic devices using silicon core fibres. In this thesis, the polysilicon core fibres were fabricated via a conventional molten core drawing method and subsequently tapered down to small core diameters in a range of 0.7 μm to 2 μm to enhance the nonlinearity. The linear and nonlinear properties of tapered silicon core fibres were characterised to ensure excellent nonlinear performance after the tapering process. Then the tapered fibres were used for nonlinear applications. Firstly, Raman amplification was demonstrated using a silicon core fibre for the first time. A Raman gain of 1.1 dB was obtained by using a continuous wave pump source in the telecom band. Following the initial Raman amplification demonstration, an enhanced peak Raman gain of 30.4 dB was achieved by using a pulsed pump laser in the 2 μm regime, taking advantage of the lower nonlinear losses in this region. Secondly, wavelength conversion based on four-wave mixing was investigated. Following a discussion on the roles of the dispersion profile and fibre length in the wavelength conversion processes, measurements were conducted to confirm the expected efficiencies of four-wave mixing in the tapered silicon core fibres. A maximum conversion efficiency of -30 dB was measured and a wavelength conversion range of 280 nm was achieved. Moreover, the possibility to enhance the four-wave mixing processing using the high Raman gain was also studied in silicon core fibres. A maximum Raman enhancement of ∼15 dB was achieved experimentally by using tapered silicon core fibres with a suitable dispersion profile and length. Finally, undetected-photon imaging was demonstrated using photon beams generated via four-wave mixing processes in the silicon core fibres, which was the first such imaging example to make use of a third-order nonlinear process. Due to the strong correlation of photon beams, both high-quality phase and amplitude images were measured with a resolution of 1.4 mm. These demonstrations show the wide-ranging potential of silicon core fibres for use in nonlinear processes that extend the wavelength coverage beyond traditional glass fibre systems.

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Identifiers

ORCID for Anna Peacock: orcid.org/0000-0002-1940-7172

ORCID for Radan Slavik: orcid.org/0000-0002-9336-4262

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