Spectral purity of high-intensity laser beams

Three experiments are described in which high-intensity ruby and Nd: glass laser beams were analyzed for spectral purity. The aim of the experiments was to verify whether the nonlinear relation for photon energy $\epsilon =\frac{h\nu }{[1-{\beta }_{\nu }f(I\left)\right]}$, previously used to explain ionization phenomena in gases irradiated with laser beams, admitted an interpretation in terms of change of light frequency as a function of intensity. Standard methods of frequency measurement, namely spectroscopic, interferometric, and filtering methods, were used. The results consistently exclude the idea of monochromatic laser beams and seem to indicate that, at the intensities used in the experiments, which ranged from a few hundred kilowatts/${\mathrm{cm}}^2$ to a few gigawatts/${\mathrm{cm}}^2$, the laser line is broadened by several hundred angstroms.