TABLE OF CONTENTS The lasing of forced radiation in the optical region of the spectrum was obtained for the first time in a ruby laser (Cr 3+:Al 2O 3). Ruby crystals are characterised by high strength, homogeneity and heat conductivity. The optimum concentration of the impurity ions of Cr is approximately 0.05%, which corresponds to the density of the active centres of 1.6 10 19 cm -3, and at this concentration the ruby is rose-coloured. Ruby is an anisotropic single crystal with a trigonal symmetry in which the trigonal axis coincides with the optical axis of the crystal. The refractive index of ruby at the lasing wavelength is: n o = 1.763, n e = 1.755. The melting point of the ruby crystal is 2040 C, the hardness according to Moose is 9, density 3.98 g/cm 3, the thermal expansion coefficient 5.31 (4.78) 10 -6 deg -1, the thermal optical constant d n/d T = 1.0 10 -6 deg -1. The ruby crystal is characterised by the highest heat conductivity (42 W/m grad) of all other active media.
The diagram of the levels of the Cr 3+ ion in the corundum crystal is shown in Fig. 1.3 in the most widely used symbols. The gain line of ruby is uniformly broadened, and the half width of the R 1-line at room temperature is 0.53 nm and at the liquid nitrogen temperature it decreases...
