An analytic approach to the theory of the optical defect modes in chiral liquid crystals (CLC) is developed. The analytic study is facilitated by a special choice of the problem parameters. The dispersion equation determining connection of the defect mode frequency with the isotropic (defect) layer thickness is given. Analytic expressions for the transmission and reflection coefficients of the defect mode structure (CLC-defect layer-CLC) are presented and analyzed for nonabsorbing, absorbing and amplifying CLC. The effect of anomalously strong light absorption at the defect mode frequency is revealed. It is shown that in DFB lasing in a defect structure adjusting of the lasing frequency to the DM frequency results in a significant lowering of the lasing threshold. Numerical solutions of the DM dispersion equation are performed for typical values of the related parameters.