Linewidth, autocorrelation, and cross-correlation measurements of counterpropagating modes in GaAs-AlGaAs semiconductor ring lasers

We perform an experimental investigation of the linear autocorrelation and cross correlation properties of the two counterpropagating modes of a monolithic semiconductor ring laser (SRL). The ring laser can operate in a bidirectional regime where the two modes are active at the same time and have equal power, and also in a unidirectional regime where one of the modes is suppressed. Autocorrelation measurements are carried out using an unbalanced optical fiber Mach-Zehnder interferometer that allows us to determine the linewidth of each mode of the ring laser. Cross correlation measurements are performed using a modified optical fiber interferometric setup where the two counterpropagating modes are directly superposed on a photodetector. This measurement reveals that in the bidirectional regime the two modes are phase-locked and allows us to evaluate the degree of their correlation.