Optical research and the solid state laser

Abstract

It is less than three years since the first optical maser or laser was brought into operation, and spectacular progress continues to be made. Solid-state devices have not so far achieved the same spectral purity as their gaseous counterparts, but their performance as source of giant light pulses has made them ideal for experiments on non-linear optical effects, such as harmonic generation, which depend on quadratic or high powers of light intensity. The coherent output of an optical maser is generated by much the same `feedback' principle as in a radio oscillator, but the dimensions of the optical resonator are such that it is a three-dimensional momentum or mode pattern whose build-up must be considered. Comparing coherent light from a laser with ordinary light from a thermal source, we find that the advantage of the former depends very much on the type of experiment, but can be twelve orders of magnitude where both brightness and spectral purity are important. One of the main aims of the present work is to find new laser materials that can operate at new wavelengths and with low pumping power, but considerable attention is also being given to securing more nearly optimum performance from an existing material. Three of the main present limitations are divergence of the beam caused by variations in refractive index, frequency drift caused by the optical pump, and `spiking' or instability of light output. Recent progress towards solving these problems is reviewed.