Enhanced single-mode operation in multisection DFB lasers with both localized and distributed phase shift

Multisection distributed feedback lasers with both localized (/spl lambda//8 phase shift) and distributed phase shift (produced by a phase-adjustment region) are investigated theoretically. It is shown that such a mixed phase shift, combined with a stronger coupling coefficient in the center of the cavity, may enlarge the threshold gain margin as well as the output optical power, enhance the intra cavity field flatness and produce an effective index profile leading to a low sensitivity to spatial-hole burning (SHB) (with respect to conventional /spl lambda//4 phase-shifted DFB lasers). As a consequence, the side-mode suppression ratio (SMSR) is improved and single-mode operation up to high bias currents is maintained.