In complex-coupled DFB lasers with both index and gain gratings, I have studied the effect of the phase difference between index and gain gratings on the single-mode yield considering the threshold gain difference (side-mode suppression ratio) and the optical field uniformity (spatial hole burning). I have obtained the optimum values of: (1) the phase difference /spl Delta//spl Omega/ between index and gain gratings, (2) the coupling strength (/spl aleph/L)/sub i/ of gain grating, and (3) the coupling strength (/spl aleph/L)/sub r/ of index grating, in order to obtain a high single-mode yield regardless of the relative positions of both facets, The used theory is based on the coupled-mode theory and includes the spatial hole burning correction and the standing wave effect. /spl Delta//spl Omega/=/spl pi//4 (and 3/spl pi//4) DFB lasers with HR-AR facets have the highest single-mode yield and should have /spl sim/0.6/spl les/(/spl aleph/L)/sub i//spl les//spl sim/1.5 and (/spl aleph/L)/sub r/</spl sim/1.25 in order to obtain above 50% yields. Even above 90% yields can be obtained with the range of /spl sim/1.1/spl les/(/spl aleph/L)/sub i//spl les//spl sim/1.4 and /spl sim/0.5/spl les/(/spl aleph/L)/sub r//spl les//spl sim/0.85. The superior yield characteristics of /spl Delta//spl Omega/=/spl pi//4 (and 3/spl pi//4) DFB lasers, which is above 2.4 times higher than that of /spl Delta//spl Omega/=0 (and /spl pi/) DFB lasers, comes from their better field intensity uniformity. The results presented in this paper provide insight into the variation of the threshold gain difference and the optical field uniformity with /spl Delta//spl Omega/, (/spl aleph/L)/sub r/, (/spl aleph/L)/sub i/, and (/spl rho//sub l/, /spl rho//sub r/).