Mode-locked colour-centre lasers and their application
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The passive and coupled-cavity mode locking of a LiF:F2+ colour-centre laser was studied. Pulses of less than 180 fs were obtained in the 0.85 mum spectral region by passive mode-locking using the saturable absorber dye IR140. Coupled-cavity mode locking, where a length of optical fibre was incorporated in an external control cavity, resulted in the generation of 1.5 ps duration pulses. Active and coupled-cavity techniques were applied to a KCl:TiO(1) colour-centre laser operating in the 1.5 mum spectral region. Coupled-cavity mode locking was obtained using either Fabry-Perot or Michelson interferometer arrangements. Although either arrangement generated similar pulse durations, the Michelson scheme was found to be more stable. Pulse durations of less than 100 fs were routinely obtained from either arrangement by using small-core fibre in which the dispersion minimum coincided with the laser operating wavelength. The KCl:TiO(1) laser was used to study various absorptive and refractive nonlinearities in a 1.5 mum InGaAsP optical amplifier. Pulse distortion caused by gain saturation and loss saturation was studied in the temporal and spectral domains. In addition, cross-phase modulation related to gain saturation was investigated. Self-phase modulation caused by an ultrafast refractive nonlinearity has been observed for the first time, and its nonlinear coefficient n2 was deduced to be -2 x 10<super>-11</super>cm2W-1. Coupled-cavity mode locking of the KCl:TiO(1) laser was also obtained when the InGaAsP amplifier was used, with pulses as short as 280 fs. Self-starting operation was achieved by eliminating parasitic optical feedback from the device facets. Mode locking was observed for amplifier drive currents either above or below transparency, suggesting that saturable gain and saturable loss respectively were the dominant nonlinearities exploited. The synchronous and coupled-cavity mode locking of a NaCl:OH- laser operating near 1.57 mum was investigated. Coupled-cavity mode locking using an optical fibre was achieved, but was accompanied by a sawtooth modulation in power output (or pulse duration) related to the short gain storage time of NaC1;OH-. This was circumvented by substituting the InGaAsP amplifier for the fibre to obtain self-starting mode locking.
Thesis, PhD Doctor of Philosophy
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