Second harmonic generation in sodium vapour induced by a magnetic field

Abstract

A XeCl excimer laser system whose output characteristics are suitable for stimulated Raman wavelength shifting, has been developed and used in single-pass conversion experiments in CH4 and H2. The overall system, comprising the laser, Raman focusing optics and Raman cell, has been designed with simplicity and system compactness in mind, suitable for a transportable lidar application. The XeCl laser is a fast discharge pumped device, driven by a C-C transfer circuit with a N2-filled spark gap switch. Various ultraviolet preionisation schemes and cavity component arrangements, employed during the laser development, are described and compared. A general overview of the basic spark preionisation techniques commonly used in rare-gas halide lasers is also presented. Over the course of the development, the available laser output power has been increased by a factor of 100, by methods described. Laser output characteristics relevant to stimulated Raman scattering (SRS), are the emission wavelength (308nm), the output pulse energy (>100mJ), pulse duration (~6.5ns FWHM), peak pulse power (>107 W) and the output beam divergence (U x 15 mrad). The design for a new laser, using a low inductance cavity geometry and an 'automatic-integral' preionisation scheme is described. The problems of poor laser beam quality and focusability which are inherent in rare-gas halide lasers, are overcome by the use of an unstable optical resonator. Experiments with a range of 'continuously-coupled' unstable resonators, in which the laser output coupler is a simple planoconvex lens, are reported. They show that pulse duration, energy and beam divergence are systematically reduced as the round-trip resonator magnification M is increased, with the result that the focal spot area can be up to 2000 times smaller than obtainable with conventional stable-cavity optics. Furthermore, these output diaracteristics are shown to depend not simply on the overall value of M, but also on the individual one-way magnifications M12 and M21 in the resonator, between mirrors 1 and 2. The dependence has produced significant experimental differences in resonators with very similar values of M. This new observation is attributed, in part, to the shortness of the laser pulse duration in this device. Experiments have been performed in high pressure CH4 andH2 to find the pumping geometries necessary to achieve SRS. The most unstable of the range cf resonators studied, having the least divergent output and the highest beam quality, is shown to be the best configuration for the application. In addition, it is found that pump focal power densities greater than 1010W/cm2 do not automatically guarantee coherent scattering if they have been produced as a result of tight focusing, and that a relatively long Raman focusing length (>~50cm) is a more fundamental experimental requirement. Raman scattering in CH4 gas has generated output lines from the first anti-Stokes (AS1) to the fourth Stokes (S4) order, at wavelengths of 283nm, 338nm, 375nm, 421nm and 480nm. The higher Raman gain in H2 has produced a wider range of Raman-shifted orders, extending from the ultraviolet into the infrared, at wavelengths given by 245nm (AS2), 273nm, 353nm, 4l4nm, 500nm, 631nm and (S5). The relative abundance of the competing SRS and four-wave mixing processes in the Raman cell, which determine the range and distribution of energy within the Raman spectrum, has been varied by reducing the feasibility of angular phase matching in the medium. Increasing the pump beam focal length and the Raman cell pressure has enabled the overall energy conversion efficiency into the S<p> order to be optimised at ~29% in H2, for a corresponding pump depletionof ~32%. For the most intense region of the XeCl laser pump beam only, these values were measured to be 56% and 60% respectively.