Novel techniques and materials for optical telecommunication systems
This thesis describes work on a variety of novel fibre-based, all-optical approaches to the implementation of several key telecommunications operations. It can be divided into two distinct parts. The first part relates to the generation and coherent manipulation of short pulses in all-fibre systems. The development of an actively and harmonically mode-locked polarisation-maintaining erbium fibre ring laser operating at a repetition rate of 10GHz is first presented. Long-term stable operation is ensured by the use of an optical phase lock loop to maintain resonant operating conditions. The ring laser incorporated a long length of dispersion-shifted fibre that enhanced soliton compression of the generated pulses, and a filter to allow for tuneable operation within the erbium gain bandwidth. The output was in the form of 2 - 4ps nearly transform-limited solitons, the width of which depended on pump power. A series of characterisation measurements on this laser is presented. These include autocorrelation and optical spectrum measurements of the pulses, frequency resolved optical gating (FROG) characterisation, an interpulse noise measurement, jitter measurements using the RF spectrum of the pulses, and bit error-rate measurements. A technique that uses superstructured fibre Bragg gratings for passively manipulating short pulses in a coherent manner is presented. The technique relies on filtering in both amplitude and phase the (broad) spectrum of the input pulses, thereby producing pulses of a desired spectral, and hence temporal response. The pulses generated from the erbium fibre ring laser were used in a series of experiments that demonstrated this technique. A basic pulse encoding and decoding scheme, pulse shaping of soliton pulses into square pulses, and repetition rate multiplication from 10 to 40GHz are presented. The second part of the thesis concerns a detailed characterisation of the recently discovered optical nonlinearities associated with the reflectivity of a gallium:glass interface. It is demonstrated that such mirrors can be formed at the tip of a cleaved fibre. The reflectivity of these mirrors is shown to change at the erbium wavelengths by 30% under suitable temperature and optical excitation. An assessment of the dependence of the magnitude, dynamics and optical phase response of the nonlinearity on temperature and pump power is carried out. The reproducibility of the nonlinear effect was also examined. Gallium mirrors were then used to passively Q-switch a variety of fibre lasers. Due to their broadband nature, Q-switching of lasers of significantly different lasing wavelengths was achieved (1064 and 1550nm). Pulse energies up to 10 µJ were obtained. This performance is directly comparable to that achieved using state-of-the-art (wavelength-selective) saturable absorbers.