Spectral Properties of a Single-Mode Ruby Laser: Evidence of Homogeneous Broadening of the Zero-Phonon Lines in Solids

Abstract

According to several recent studies, the width of the sharp zero-phonon lines in the electronic spectra of impurities in solids, such as the $R$ lines of ruby, is due to Raman scattering of phonons by the impurity ions. The question whether such zero-phonon lines are homogeneously broadened is an important one, both for its own interest and for optical-maser applications. From a detailed study of the stimulated-emission spectra of a ruby laser operating in the traveling-wave modes, we obtain conclusive evidence that such zero-phonon lines are homogeneously broadened. Because the $R_1$ line of ruby is essentially homogeneously broadened at room temperature, the traveling-wave laser oscillates for all practical purposes only in a single mode. The spectral width of the laser emission is extremely sharp. It is less than 0.005 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature as compared to a width of the order of 1 ${\mathrm{cm}}^{-1\mathrm{}}$ for conventional ruby lasers of comparable intensity. In addition, the output intensity no longer shows random spikes but shows a predictable train of regularly spaced spikes which agree well with theory.