Nonlinear optical components and systems for quantum technologies and communications

Abstract

This thesis describes a series of experimental research primarily revolving around the applications of periodically poled lithium niobate (PPLN). Initially, a set of work is described regarding the fabrication and characterisation of PPLN ridge waveguides designed for frequency doubling into the blue/UV range. These waveguides are fabricated via diffusion of zinc into the crystal structure. A precision dicing process creates the sidewall definition for modal confinement and also optical facet preparation. Preliminary results with the most efficient waveguide fabricated produced a maximum conversion efficiency of 6.9%/W in a 14 mm-long device, generating 391 nm light. This was performed with a novel near-IR VECSEL; designed and built as part of a collaboration with the research undertaken in this thesis to investigate new routes for miniaturised UV sources. The development of a non-destructive route to characterise the physical dimensions of a ridge waveguide for nonlinear applications is also presented. This presented methodology uses white light interferometry and standard edge recognition algorithms to determine the waveguide width profile. In combination with numerical propagation constant data for these waveguides, a numerical phasematching spectra can be calculated. A comparison to the experimental phasematching spectra of the second-harmonic generation process enables the discrimination between issues in the fabrication processes. Using bulk PPLN, a final application is presented for communications using mid-IR radiation and all commercial components. This configuration involves the use of differencefrequency generation followed by sum-frequency generation to create the transmitter and receiver systems, respectively. This enables final detection of 809 nm light (firmly within the silicon absorption band). Bit error ratios as low as 10−7 from single-pass nonlinear conversion for both systems without the need for an additional optical amplifier in the receiver. Finally, the propagation of relative intensity noise within the system as the source lasers propagate through a two step nonlinear conversion is presented and discussed within.

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