Nonlinear properties of optical microfibres

Abstract

Glass microfibre waveguides offer an intriguing platform for the investigation of nonlinear effects, due to their high effective nonlinearity which arises from the tight modal confinement down to dimensions comparable to the wavelength of guided light. This thesis presents theoretical and experimental work towards achieving efficient third and second harmonic generation in silica microfibres, as well as in microfibre loop resonators for enhancing the conversion. Since microfibre resonators themselves exhibit interesting nonlinear behaviour, the polarisation dependent properties of microcoil resonators were also studied. Efficient third harmonic generation is possible through intermodal phase matching, and experiments using short tapers have demonstrated significant efficiencies up to 10-3 over a uniform 4 mm waist. On the other hand, the interrogation of longer tapers, in which the harmonic generation occurs within the taper transition regions of several cm rather than the waist, allows for a broadband conversion observed to exceed 36 nm at the 5 dB bandwidth level. A straightforward technique to improve efficiency using microfibre loop resonators was also investigated. Near resonance, the recirculation of the pump power inside the resonator was experimentally shown to increase conversion by 7.7 dB higher than that of the straight microfibre, similar to simulated predictions, and by optimising the loop geometry the resonant efficiency enhancement can potentially reach 20 dB.
Simulations on second harmonic generation in microfibres indicate that the second order nonlinearity originates primarily from the structural anisotropy at the glass-air boundary (which exploits the high surface electric field strength of microfibres) as well as multipolar effects within the bulk. To overcome the inherent weakness of these effects, experiments focused on conversion enhancement via the aforementioned technique using loop resonators, with a measured 7.6 dB efficiency improvement.
In addition, an accurate polarisation dependent model for microcoils was developed for both the linear and nonlinear regimes by incorporating the effects of fibre twist and birefringence. The coupling between orthogonally polarised modes propagating along the microcoil results in a strongly polarisation sensitive transmission, especially near resonances, which in turn influences the nonlinear hysteresis characteristics.

More information

Identifiers

ORCID for Timothy Lee: orcid.org/0000-0001-9665-5578

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

Catalogue record

Export record