QPM grating design for novel PPLN structures

Abstract

This thesis describes a series of theoretical and experimental studies into modifying the phasematching characteristics of nonlinear parametric interactions, specifically second harmonic generation, using quasi-phase-matched structures. The use of quasi-phase-matching by periodic poling affords a flexibility in designing tailored phase-matching characteristics not offered by alternative techniques. In this work phase matching characteristics are modified to provide enhanced acceptance bandwidths, compensation for focusing effects and high power operation. The first result of this work describes the design and manufacture of 20mm long LiNbO3 aperiodic quasi-phase matched devices for the generation of stable second harmonic power across wide temperature ranges. Theoretical simulations have demonstrated constant power output over a range of 9C. Providing over 35 times the bandwidth of equivalent length periodic structures, whilst offering almost an order of magnitude efficiency enhancement over periodic devices with the same bandwidth. Experimental verification of these devices has shown that stable power can be obtained across wide temperature ranges with only slight deviation from theory. Additionally, an investigation into the effects of focusing on second harmonic generation is undertaken. In this work the Gouy phase of a focused beam has been analytically identified as the source of dephasing in bulk nonlinear interactions, causing such effects as back conversion, reduced efficiency and errors in the phase matching condition. A method to negate these effects, using a modified QPM structure has been proposed and experimentally demonstrated. Finally, simultaneous compensation of both the Gouy phase and focused intensity variation has been applied to aperiodic, wide temperature bandwidth devices. Removal of these deleterious effects has been shown theoretically to correct the experimentally observed bandwidth errors, resulting in the focused interactions performing identically to plane-wave simulations.

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Identifiers

ORCID for Peter Smith: orcid.org/0000-0003-0319-718X

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

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