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A theoretical model describing the fluence-dependent absorption of Gaussian shaped laser beams by optically thick nonlinear media is applied to the interpretation of Z-scan measurements with thick samples of chloro-aluminium-phthalocyanine. The results obtained from the measurements and the modeling have produced an enhanced understanding of the nonlinear processes operating within this type of material, as well as the role of the relaxation mechanisms and saturation effects in shaping the nonlinear absorption, and the optical limiting capability of this type of absorber. An exact first-order analysis of a five-level molecular model has been developed and explored for the case where the incident beam possesses a flat temporal profile. The predictions of the model are in excellent agreement with experimental observations and with results produced from a numerical modeling technique, previously developed, where a simple "relaxation-free" and "population-saturation-free" three-level model is used to predict the transmission properties of a thick five-level system.

The Z-scan technique is a popular method for measuring degenerate (single frequency) optical nonlinearities using a single laser beam. In order to perform reliable measurements, it is necessary to carefully characterize and control a number of experimental parameters, such as the beam quality, the power and temporal characteristics of the laser, the collection aperture size and position, the sample reflectivity, sample thickness and imperfections in the sample. Failure to control these parameters leads to inaccurate determinations of the nonlinearities. In this paper, we review the theory of Z-scan and examine each of these issues from experimental and theoretical viewpoints. This work will be of interest to anyone who performs Z-scan experiments and to those interested in optical power limiting and nonlinear optical propagation.

A device which acts as both a simple cw laser warner and anti-dazzle protection device has been designed and tested. The design is based on a single stage, double element, tunable liquid crystal Lyot filter. Laboratory tests have shown that the detection sensitivity of the device for monochromatic cw radiation is approximately 43×10−9 W/cm 2 and that the achievable contrast ratio is greater than 100:1 Automatic detection and filtering of a cw laser source has been demonstrated. Results indicate that the algorithm to automatically filter the laser radiation works well and that the extinction obtained is good. Factors which affect the contrast ratio are discussed and improved device design suggested.

We review many of the optical devices exploiting nonlinear optical and electro-optic effects which fall within the general categories of switches, limiters and discriminators.

A study has been made of the transient response of a Fabry-Perot cavity exhibiting bistability of the optical output with applied voltage. In this system the electro-optic and optical nonlinear effects act cooperatively to produce the switch, and the transient response was shown to have characteristics associated with both mechanisms. The switch was found to be composed of three distinct regions, a purely electro-optic part, followed by a slow change in the cavity intensity brought about by the electro-optic and nonlinear effects, with a final fast switch up to the high transmission state. The relative magnitude of the time duration of regions 1 and 2 was found to be dependent upon the applied voltage, initial dc biasing and optical input power. In particular, critical slowing down was evident in region 2 of the switch for values of applied voltage close to the critical value. In this case region 2 was the dominant part of the switching time. Overdriving the system resulted in the time duration of region 2 decreasing and region 1 becoming more dominant. Qualitative agreement was achieved between the experimental results and theoretical predictions of the transient response of a cavity undergoing a slow change in some external parameter.

Optical studies were carried out at 647 nm in a Fabry-Perot etalon containing the liquid crystal mixture 3/5/7 PCH. The electro-optic and nonlinear effects were brought together in two sets of experiments. Firstly, optical bistable loops were obtained at different initial cavity tunings imposed by the applied voltage through the electro-optic effect. Secondly, bistable switching was achieved by fixing the input optical power and varying the voltage to obtain “voltage bistable” loops. The behaviour of these loops was found to follow the trend predicted by theory.