Dynamical control of one- and two-dimensional optical fibre scanning

Abstract

This thesis investigates the dynamical control of one- and two-dimensional optical fibre scanning. One dimensional scanning is performed with a mechanically biaxial polarisation-preserving fibre mounted on a piezoelectric transducer with one of its principal mechanical axes aligned parallel to the excitation direction. The addition of an apertured reflector in front of the imaging lens allows a position sensing mechanism based on intermittent optical feedback to be integrated into the scanner. Over-scanning the lens generates timing pulses interlaced with back-scattered signals from the target. The timing information can be used for closed loop control of the phase and amplitude of vibration. Suitable control algorithms are developed and their convergence and stability is studied.

This thesis also investigates the construction of fibres with enhanced mechanically asymmetry and their dynamical properties during two-dimensional imaging based on Lissajous scan patterns. Dip-coating is proposed as a method of forming two-cored waveguide cantilevers from two separate, parallel fibres that are encapsulated in a plastic coating. The frequency ratio between the two orthogonal bending mode resonances can be controlled with number of coatings. An exact image reconstruction algorithm based on Lissajous scanning is proposed. Latency, transient response and steady-state phase errors are all shown to cause dramatic deterioration of the reconstructed image. Solutions are provided by ensuring the correct starting time for data acquisition and introducing a drive phase correction to one of the axes. Two methods of resolution enhancement are demonstrated. The first is based on combining data sets obtained during separate scans carried out with deliberately applied phase offsets. The second operates by combining data sets from separate imaging operations carried out using the two different fibre cores. Finally, this thesis demonstrates potential applications in optogenetics by combining the two operations of imaging and writing, using different light sources that may also have different wavelengths.