Fabrication and applications of lead-silicate glass holey fibre for 1-1.5 microns: nonlinearity and dispersion trade offs

Abstract

This thesis describes the development of novel microstructured optical fibres (MOFs), namely holey optical fibres (HFs), based on a commercial highly nonlinear non-silica glass: lead silicate glass, for their specific incorporation in optical devices based on fibre nonlinearity and dispersive properties. For example, supercontinuum (SC) generation experiments using these structures are demonstrated. Together with the development of the novel fabrication techniques, the characterisation and applications of these fibres are presented, proving the advantages of HF technology in the implementation of highly nonlinear optical devices, as well as discussing their limitations.

At the time of this study, soft glass HFs (SG-HFs) had just recently been identified as an alternative route towards extremely high nonlinearity HFs. Thus, very little was understood about the soft glass HF fabrication, and the factors that influence the optical properties of the HFs: propagation loss, fibre nonlinearity and dispersion. There had been little development towards SG-HF fabrication techniques that could be used to produce good quality fibres, nor the characterisation techniques that could be used to accurately measure the optical properties. Both the fabrication and characterisation techniques are essential for future SG-HF research and development, and for assessing what benefits SG-HFs may offer over their conventional counterparts and pure silica HFs in the small core, single mode regime.

This research is targeted on the fabrication of HFs made by glasses other than silica, to further enhance fibre nonlinear effects and to tailor novel dispersion properties, such as anomalous dispersion in the visible and near-IR regions. This behaviour arises from the large contribution, in HFs, of the waveguide dispersion, to the total dispersion value. The use of soft glass with low softening temperatures (~420°C), has allowed the development of a novel fabrication technique for HF structured preforms: Extrusion, where a glass disc is forced through a die at elevated temperature near the softening point, whereby the die orifice determines the preform geometry. Wagon wheel (WW) structure was chosen as our design, where its microstructured region consists of three large air holes surrounding a very small core. We have established the maximum nonlinearity that can be achieved in this fibre at telecommunications wavelengths, and developed high nonlinearity fibre with dispersion-shifted characteristics suited for SC generation applications at 1.06 µm. Although the material loss in the soft glass is normally higher than its counterpart, silica, the massive nonlinear effects possible in HF made from the non-silica glass are counterbalanced by its high loss, of the order of few dB/m.

The complex structure, high nonlinearity SG-HFs are developed, aiming at shifting the zero dispersion wavelength (ZDW) to the 1.55 µm regime and offering an improved dispersion profile. A novel fibre fabrication approach: Structured Element Stacking Technique (SEST), which combined the best features of both extrusion and stacking and allows the production of the more complex preforms required to achieve good dispersion control in SG-HFs, is demonstrated. We demonstrated the fabrication of an SF57 SEST fibre with a ZDW within the C-band, a 3.2 dB/m loss, and a gamma value of 170 W^-1km^-1.

By taking the advantages of the zero dispersion of the HFs at 1 µm and 1.55 µm, the SC studies are carried out using these HFs (WW HFs and SEST HFs). Both of the fibre SC studies combined experimental and numerical results, and the simulations and experiments were in qualitative agreement. We observed a spectrum spanning over 1000nm by using just ~98 pJ energy pulses in a 60cm piece of the WW HF. This demonstrated the advantage of SG-HFs in terms of compact devices and low power requirements.

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