A novel design for DWDM interface device interleaver is proposed and discussed, which is composed by 3×2 Match-Zehnder interferometer with thin film Gires-Tournois interferometer type optical all pass filter (GTI-OAPF) inserted. Influences on its transmission characteristics by such factors as the reflection coefficient of the partial reflector in the GTI-OAPF, detuning of the coupling angles of the directional fiber couplers from ideal values, chromatic dispersion and various losses are numerically analyzed in detail. At last, a novel six-port bi-directional interleaver, which will be used in bi-directional multi-function photonic module and bi-directional WDM transmission system is proposed as well.