ACOUSTICAL AND OPTICAL BRANCHES IN THE SPECTRAL WAVES OF A LASER

The longitudinally multimode class-B laser with inhomogeneous broadeni ng is known to behave as a chain of coupled oscillators, each being as sociated with one mode. If emission occurs along the two eigenstates o f polarization, we show theoretically and experimentally that the wave s propagating in the spectrum are characterized by two dispersion curv es associated with oscillations in phase and in opposite phase of two close modes with different polarizations. These curves may be viewed a s the analogs of the ''acoustical'' and ''optical'' branches in one-di mensional diatomic crystals.