OPERATION OF A COHERENT ENSEMBLE OF 5 DIODE-LASERS IN AN EXTERNAL CAVITY

A spatial filter in an external cavity has been used to coherently couple five separate diode lasers with operating wavelengths that differed by as much as 6 nm when they were operated individually in the external cavity. The coherent ensemble of these diode lasers, which have one facet antireflection coated, operated CW with a spectral linewidth less than the 7.5 MHz resolution of the available spectrum analyzer. When the ensemble is brought from below to above threshold, by applying a current pulse to only alternate elements of the ensemble, with the other elements operated at constant current, the spectral chirp of the output has been reduced to below 10 MHz. The loss in the cavity due to the spatial filter has been calculated for a variety of distributions of output power from the individual diodes of the ensemble. The single-pass transmission of the spatial filter decreased by nearly a factor of two when there was no output from alternate elements. The theory of operation of the five-element coherent ensemble in the external cavity has been used to determine the optimum design of the spatial filter. A lens focuses the ensemble outputs on the spatial filter plane. The spacing between each of the 40 slits in the filter is given a priori by the Fourier transform of the geometry of the ensemble plane. Theoretically, a slit opening that is 30-40% of the slit spacing provides the best frequency selectivity and cavity output stability. This slit opening is also optimum in the tradeoff of the power output from the cavity and the difference between threshold for the ensemble and for individual external cavity lasers. The feedback from the passive portion of the cavity and the amplification in the diode elements have been analyzed in detail. A self-consistent solution of the cavity operation was obtained using a computer simulation, and the optimum filter design was determined.