DFB lasers with π/2 phase shifter can run stably on single mode and providevery high side mode suppression ratio (MSR). This attractive feature may be degraded ifphase shift deviates from π/2. In this paper, we analyze theoretically how the output spectraand side mode suppression ratio are influenced by the deviation value of phase shift from π/2. DFB lasers with π / 2 phase shifter can run stably on single mode and providevery high side mode suppression ratio (MSR). This attractive feature may be degraded if shift shift deviates from π / 2. In this paper, we analyze theoretically how the output spectraand side mode suppression ratio are influenced by the deviation value of phase shift from π / 2.