All-optical signal processes enhanced by multimode nonlinearities

Abstract

Four-wave mixing (FWM) processes based on the third-order (Kerr) nonlinearity play a significant role in a range of classical and quantum optical phenomena, and in particular have been implemented in single mode waveguides for an array of applications. In these devices, FWM occurs between frequencies of light all of which propagate in the fundamental spatial mode. This thesis explores FWM in optical fibres when the interacting light fields are in different linearly polarized (LP) modes, and explores how this spatial degree of freedom can be used to advantage in the generation of particular FWM processes.

We initially restrict ourselves to linear propagation of the first higher-order mode (i.e. the LP11 mode) in a birefringent fibre, and use the state of polarization of input light to achieve control over the spatial intensity pattern at the fibre output.

Aspects relating to nonlinear inter-modal interactions come next. Polarization effects in inter-modal FWM are highly relevant in many applications. We will show that with two co-polarized pump waves in one spatial mode and the signal wave and the FWM generated idler field in another mode, FWM efficiency is independent of signal polarization. This polarization insensitivity is attributed to the fast evolution of random birefringence fluctuations of the fibre in combination with the input configuration employed. In FWM based parametric amplifiers, the bandwidth over which uniform gain is available is an important measure of performance. For broadband operation, our inter-modal FWM configuration relies on the frequency independence of differential group delay between the modes involved. We will report on the design, fabrication and experimental characterization of graded-index multimode fibres tailored to meet this requirement, and obtain a two-fold improvement in conversion efficiency bandwidth over previous results. Additionally, error-free and stable demodulation of an intensity modulated inter-modal FWM idler will be demonstrated. Finally, as an example of the flexibility of multimode operation, our scheme will be adapted to selectively enhance wavelength conversion of any one of multiple closely spaced signals. In the course of the above investigations, we will uncover important aspects governing the practicality of inter-modal FWM for a broad class of potential applications.

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Identifiers

ORCID for Omar Farooq Anjum: orcid.org/0000-0001-6229-4305

ORCID for Periklis Petropoulos: orcid.org/0000-0002-1576-8034

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