For both transverse magnetic and transverse electric modes, the governing equations for the guided‐guided wave interaction in a dielectric thin film waveguide which is etched with a cosinusoidal surface corrugation have been developed by using a quasioptical technique. These governing equations are used to investigate the effect of the phase shifter on the characteristics of the distributed feedback laser. Phase shifters either at the center or at the ends of the laser eliminate the gain degeneracy and make the power distribution inside the laser more uniform. When the amount of phase shift is appropriately chosen, the gain margin increases and reaches a maximum value. The roles of the other parameters in achieving a single‐mode operation for the laser have been investigated by means of numerical examples. It has been found that the characteristics of the distributed feedback laser with phase shifters are quite similar for both transverse magnetic and transverse electric modes. ©1995 American Institute of Physics.