Interference of Intensity Noise in a Multimode Nd:YAG Laser

We investigate the behaviour of the intensity noise in a multi-longitudinal mode Nd:YAG laser. This type of laser is a nonlinear system which exhibits complicated dynamics within the intensity noise. For example, antiphase dynamics is where there is cancellation of one or more collective modes of oscillation, which are distinct from the longitudinal modes, in the total output. Commonly lasers are studied experimentally to discriminate between models used to describe them. They are convenient since many external influences can be controlled and the oscillations of interest are low frequency (in the kHz regime) making their direct measurement relatively simple. In our laser, the collective modes of oscillation are excited by broadband ambient noise. Because the phase of the excitation is unknown, we develop the cross spectral technique to measure the antiphase dynamics directly and form a picture of the intensity noise interference for two to five mode operation. For three mode operation, we measure the contributions of the longitudinal modes to the collective modes. We also calculate power spectral densities of the individual longitudinal modes and the total intensity. We test relationships between these quantities, at the collective mode frequencies, which are derived from modal rate equation theory. For two mode operation, the theoretical relations are satisfied. For three mode operation, the relations are satisfied when the picture of interferences is taken into account. The cross spectral technique is therefore shown to be a sensitive test of the model developed by Pieroux and Mandel [T. Hill et al., Phys. Rev. A 66, 063803 (2002)]. The behaviour of the multimode laser operating near the threshold of a longitudinal mode is measured. Transitions in the cross spectrum are noted in some pairs of longitudinal modes, for an arbitrary but small pump rate above threshold of a longitudinal mode. It has been shown that longitudinal modes with a high threshold pump power may become more intense than those with a lower threshold [K. Otsuka et al., Opt. Lett. 23, 201 (1998), L. Stamatescu and M.W. Hamilton, (unpublished) (1999), N.B. Abraham et al., Phys. Rev. A 62, 013810 (2000), P.A. Khandokhin, E.A. Ovchinnikov and E.Yu. Shirokov, Phys. Rev. A 61, 053807 (2000)]. The AC noise component of the first two longitudinal modes to reach threshold, is found to exhibit similar properties to their intensity. The implications of the results of this thesis, on models used to describe the behaviour of solid state lasers, are also discussed.Thesis (Ph.D.)--Physics, 2003