Development of rare-earth doped microstructured optical fibres

Abstract

This thesis describes the development of novel optical fibres, microstructured optical fibres (MOFs), and demonstrates device applications based on these structures. A particular emphasis is made on incorporating rare-earth ions within these fibres in order to realise novel active devices. Together with the development of the fabrication technique, characterisation and applications of these radically different fibre types are presented.

First, the fabrication techniques of MOFs, which heavily rely upon fibre drawing, are studied. A mathematical model developed for the capillary drawing process is experimentally examined. Good agreement is obtained whilst it is also found that the model provides useful physical insights for determining the fibre draw parameters even for MOFs with complex geometries. Details of the fabrication techniques developed to optimise fibre structures are also presented.

Transmission properties of highly nonlinear MOFs are then studied experimentally. It is found that the transmission losses are strongly influenced by the core dimensions due to the high Rayleigh scattering coefficient that originates from the holey cladding. A simple model is used to explain the observations. In addition, a continuous effort towards reducing OH-induced losses of this fibres type is outlined.

Rare-earth doped highly nonlinear MOFs are fabricated and characterised. Then, three device demonstrations are carried out for the first time. These include a mode-locked ytterbium doped MOF laser, a nonlinear amplifier based on an ytterbium doped MOF, and a continuous wave erbium doped MOF laser with a very low threshold and high efficiency. Using the ytterbium doped MOF, wide tunability of ultrashort pulses from 1µm to 1.58µm is demonstrated using the soliton self frequency shift effect. For the erbium doped MOF, a pump power threshold of 0.5mW and a slope efficiency of 57% are demonstrated.

Novel cladding pumped fibres, air clad MOFs, which use a conventional inner cladding and a holey outer cladding, are developed aiming at improved performance of cladding pumped fibre lasers. Wide tunability over 110nm and pure three level operation at 980nm of ytterbium doped cladding pumped fibre lasers are demonstrated.

Finally, the fabrication and characterisation of large mode area microstructured fibres (LMA-MOFs) are described, and a comparison with conventional counterparts is made in terms of bend losses and corresponding effective mode areas. The results show that a slight refractive index difference introduced in the core region of this fibre type strongly modifies its waveguide characteristics. By applying this knowledge, a novel ytterbium doped cladding pumped fibre, which uses different sizes of air holes to define the inner and outer cladding, is developed. A continuous wave output power in excess of 1W is obtained. Results concerning various forms of pulsed laser operation using this fibre are presented and future possibilities are discussed.

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