Thesis Type:PhD thesis
Ultrashort optical pulses are widely and increasingly used in many diverse fields of science and technology. By providing high temporal resolution they enable investigation and measurement of fundamental physical, chemical and biological phenomena that occur on picosecond time scales or shorter. In addition ultrashort pulses are an essential enabling tool for high speed optical communications and data processing technologies, as well as in advanced manufacturing and photomedicine applications. In all of these areas precise measurement and control of ultrashort optical pulses is vital - advances in ever shorter pulse generation must be accompanied by new methods to characterise and manipulate them.
This thesis presents work on the ongoing development of an ultrashort pulse measurement and manipulation technique known as time-to-space conversion. Time-to-space conversion uses sum-frequency generation between spectrally resolved ultrashort pulses to transfer information from the time domain to the space domain; in other words to create the real-time spatial image of an ultrashort pulse. Mapping the pulse temporal intensity envelope and phase onto a quasi-static spatial image allows high resolution measurement of these quantities, overcoming the difficulty of optoelectronic detection of ultrashort pulses directly in the time domain. Furthermore, the spectrally resolved nature of time-to-space conversion results in a large time window of operation. This enables a series of ultrashort pulses to be simultaneously transferred to spatially separated locations via interaction with a single reference pulse, thereby performing an all-optical demultiplexing operation.
The two main developments introduced in this thesis are: a) greater feasibility of time-to-space conversion for all-optical demultiplexing of a high speed optical communications channel by demonstrating the technique in a planar nonlinear waveguide and b) the demonstration of full-field characterisation of ultrashort pulses by using interferometric detection after the time-to-space conversion. The practicality of time-to-space conversion for all-optical demultiplexing depends on minimising its optical power consumption. This can be achieved by implementation of the conversion process in the guided-wave regime, as opposed to the free-space regime in which it has previously been demonstrated. The first three papers presented in this thesis describe the preliminary steps towards this goal, namely the demonstration of non wavelength-degenerate and background-free collinearly phase-matched time-tospace conversion and the demonstration of time-to-space conversion in a planar nonlinear waveguide. Full-field characterisation of ultrashort pulses by time-to-space conversion is enabled by the quasi-monochromaticity of the output sum-frequency signal, a feature which follows from the unique geometry of the oppositely dispersed waves of the pulse to be measured and the reference pulse. The quasi-monochromatic converted signal can be mixed with a narrow linewidth local oscillator for interferometric measurement of the ultrashort pulse field amplitude and phase. The final two papers included here describe the first time demonstration of full-field measurement of bandwidth-limited and chirped pulses by time-to-space conversion and of single-shot coherent detection of a phase modulated ultrashort pulse train.
Taken together, the work presented in this thesis has achieved an increase in the utility of time-to-space conversion as an ultrashort optical pulse measurement and manipulation technique, with potential applications in optical communications and data processing and in the field of ultrashort pulse measurement.