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The effect of external reflectors on longitudinal modes of distributed feedback (DFB) lasers is analyzed. The general case of dissimilar reflectors arbitrarily located relative to the phase of the DFB structure is considered. An eigenvalue equation for the propagation constants is derived and solved numerically for a variety of practical cases. Longitudinal mode thresholds, wavelengths, separations, and field distributions are obtained for GaAs lasers and for DFB lasers with a single reflector. It is shown that these quantities are very sensitive not only to the relative strength of the discrete and DFB, but also to the relative phases. Quite asymmetric transmitted powers are shown to occur under a variety of circumstances.