IMPROVED SPECTRAL CHARACTERISTICS OF MQW-DFB LASERS BY INCORPORATION OF MULTIPLE PHASE-SHIFTS

The single-mode stability of strained-layer MQW-DFB lasers with electr on-beam written gratings containing zero, one and three phase-shifts, and with a relatively large coupling strength, has been investigated t heoretically and experimentally. The fabricated lasers with the multip le phase-shifted (MPS) structure exhibited a higher degree of stabilit y than the lambda/4-shifted lasers, and a considerably improved stabil ity for both categories with phase-shifts compared to the nonshifted l asers was obtained. These results were in good agreement with our simu lations. An investigation of the linewidths of the phase-shifted laser s is also presented. The MPS lasers had a significantly lower linewidt h floor (down to 600 kHz) than those with one phase-shift. There was n o degradation of the side-mode suppression-ratio correlated to the lin ewidth floor or rebroadening. A possible explanation for the lower lin ewidth floors obtained for the MPS lasers is that this structure is le ss sensitive to inhomogeneous carrier fluctuations. Longitudinal fluct uations in the shape of the photon density distribution will thereby b e suppressed.