| Within most ring laser gyroscopes (RLG's) there are two independent counter propagating optical waves. Rotation of the gyroscope produces a relative wavelength shift between the two waves, which can be used to measure the magnitude and direction of the rotation. At low physical rotation rates, the counter propagating waves may couple together and become frequency degenerate. This is often term "lock-in". The problem of lock-in growth of ring laser gyroscopes (RLG's), fitted with electron-beam deposited mirrors has been investigated. Experimental studies have shown that there is an increase in the skew-Hermitian and the Hermitian coupling experienced by the RLG's when operated in a locked state. Subsequent operation out of lock-in allows the gyroscopes to recover back to their original states. It is believed that the cavity mirrors are affected in some detrimental way by lock-in, which increases the amount of optical scatter originating from the mirrors. A physical model, based on the photorefractive effect, has been proposed to account for the increase and recovery of the skew-Hermitian and Hermitian coupling. Two important requirements of this model are that the cavity mirrors contain a number of optically generated mobile charge carriers, and that there is some second-order nonlinear optical activity inherent to the thin films. A review of current scientific literature has indicated the possible presence and nature of an optical charge transport mechanism. A series of second harmonic generation experiments has confirmed the presence of second-order optical nonlinearity within the films. This has also allowed estimates of the effective electrooptic coefficients to be made. The theory is consistent with the experimental observations. It has been found that the magnitude of the coupling increase is set by the concentration of impurities incorporated into the thin films. Also, the form and time scale of the recovery is very much dependent on the micro-structure of the dielectric thin films. The details of a scatterometer specifically designed to characterise RLG mirrors, with respect to RLG performance, are given in this thesis. The dynamic range of the scatterometer is 0.5sr-1 to 10-7sr-1 (in units of Bidirectional Reflectance Distribution Function) and has a stability of 0.5%. To prevent contamination from dust etc., the scatterometer is housed in a class 100 clean room. |
