This disclosure is concerned with optical add/drop modules. In one example, he drop portion of the add/drop module is accomplished by using thin film filters or thin film interleavers. The add portion of the add/drop module uses fused fiber interleavers for the less critical stages of the multiplexing process. In a final stage, fused fiber interleavers can be placed in series. A thin film interleaver having a flattop frequency response may also be used for the critical stage where the multiplexed channels are more closely spaced. The frequency response of the thin film interleaver is relatively constant across a bandwidth of a channel while having a drop off at the channel edge to reduce cross talk.