Seismic effects in ring lasers and transverse mode selection in helium-neon lasers.

Abstract

The effect of seismic activity on large ring laser gyroscopes is determined. In particular the novel use of ring laser gyros as detectors of the rotational components of earthquake waves is investigated. It is found that lasers such as C-II which are well isolated from mechanical strain of the ground are effectively sensitive only to the rotational components of seismic waves. Tilt and bulk strain in the laser body are found to be negligible. Fourier analysis of earthquake data from C-I gives very good agreement with the modelled effects. The use of the auto-regressive AR(2) model as a superior alternative to Fourier analysis is investigated. It is found that such an approach is much more sensitive and gives access to a hitherto unavailable parameter, the Sagnac linewidth. The effects on the Sagnac frequency and its linewidth are found to be in excellent agreement with earthquake data recorded from the laser C- II. Microseismic noise induced in the ground by oceanic wave action on coastlines and the possible effects on ring laser noise levels are determined. It is found that typical amplitudes of microseismic waves are many orders of magnitude too weak to be manifested in frequency spectra of ring laser gyroscopes and lie far below the ultimate quantum noise limit for such lasers. A model is developed of helium-neon lasers operating at 632.8 nm to determine the radial distribution of the population inversion density. The simplest model, requiring the least number of atomic levels to be included, is found which will account for the induction of a central minimum in the distribution due to increasing pumping rate. An extension of the gain expression for laser modes is used to determine the gains for different transverse modes. The expected variations of the first and last oscillating transverse modes in a purpose built linear laser, L1, are found to be in good agreement with the model. With the inclusion of astigmatism the model is extended to ring lasers and the expected mode behaviour is predicted and recommendations for the operation of such lasers is made. The behaviour of very large ring lasers, with perimeters of tens of metres, are investigated. A description of the design and commissioning of the large high-quality linear laser, designated Ll is given.