Since the invention of the laser in 1960 there has been an enormous increase in the number of applications of this newly available light and its spectacular properties, and there is no end to this development in sight. In many ?elds of science, technology and medicine laser photons are the driving force of progress. In the near future we will probably experience a further rapid - velopment in this ?eld as a result of the widespread industrial production of semiconductor diode lasers and new nonlinear optical materials. Light from the new lasers may become even cheaper than that from light...