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Exchange-type distributed-feedback (DFB) lasers based on heterogeneous integration of III-V lasers on silicon are analyzed by using a coupled-mode model. We show that such lasers have a similar emission spectrum to their classical counterparts, in which the forward and backward modes have the same transverse profile. We demonstrate that there exists an optimal value for the coupling coefficient, which gives the maximum output slope efficiency in exchange DFB lasers. However, the maximum output efficiency is much lower than that in classical DFB lasers, due to the strong coupling between the two counterpropagating modes.