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A general method for evaluating the feedback sensitivity of semiconductor lasers is proposed based on Green's functions approach. The rate equations derived in this paper generalize works already published to any type of laser cavities such as those with axially varying parameters. The variation of the lasing frequency occurring under external optical feedback is then used to predict the coherence collapse threshold. The approach is validated for conventional DFB lasers by comparing the calculated feedback sensitivity with those obtained from analytical expressions. Both feedback sensitivity and coherence collapse thresholds are then calculated and analyzed for DFB lasers with a chirped grating. A remarkable agreement on the critical feedback level between simulations and measurements is obtained for all the lasers under study.