Fused tapered fibre optic devices
This thesis is concerned with the realisation of an all-fibre acousto-optic device which can be used as a switch, filter, frequency shifter, polarisation controller or amplitude modulator, depending on the configuration. The devices are based on the acousto-optic interaction in fused tapered couplers and as such are completely compatible with fibre networks. For the first time polarisation insensitive devices have been realised by precisely controlling the degree of fibre fusion in an asymmetric coupler. This thesis details the theoretical considerations required to fabricate and operate all-fibre acousto-optic devices. The acousto-optic theory of circular cross-section devices is developed and extended to non-circular cross-sections. In this way all practical devices with non-circular cross-sections can be accurately modelled, and the optical bandwidth, switching speed and polarisation dependence can be tailored to a specific requirement. Polarisation independent operation is a serious obstacle in the development of practical photonic devices. This issue was resolved in this device by using geometric birefringence in the interaction region, produced by fusing the fibres of the null coupler to a precise degree of fusion. Polarisation independent operation is achieved with this technique without compromising other device parameters such as drive power. In order to fabricate these devices with the required uniformity, the tolerance on the manufacturing process was theoretically and experimentally studied. A new coupler rig was designed to operate within these tolerances. The coupler rig was capable of the manufacture of uniform devices both in the degree of fibre fusion and in the area of the device cross-section. The rig was optimised to produce low excess loss devices with high isolation between the output ports. In addition, it was possible to manufacture the length of the uniform tapered region over three times longer than previously possible. The incorporation of the non-circular operational theory enables non-destructive measurements of both the degree of fibre fusion and the device cross-sectional area, anywhere along the tapered region. Unlike competing technologies, geometric birefringence produces polarisation insensitive devices without compromising device parameters. This technique allows the following characteristics to be simultaneously achieved: 1) lower drive power, < 0.25 mW, 2) improved polarisation insensitivity, < 0.01 dB, 3) improved crosstalk, < -40 dB, 4) better excess loss, < 0.001 dB, 5) switching speeds of < 40 µs for a 25 mm long device, and 6) only one fabrication step.