We describe the optimal design and the fabrication of a 2$ imes$2 port optical device based on the thin film filter (TFF), and also propose a 4-channel OADM module using these devices. The optical performance of the proposed OADM module is evaluated theoretically and experimentally, and is compared to that of typical OADM modules using 1$ imes$2 port optical devices for 4, 8, 16 and 32 drop channels in optical transmission systems. Since the 2$ imes$2 port optical device accomplishes the function of wavelength multiplexing and demultiplexing simultaneously in the proposed OADM module, the insertion loss of through channels can be improved by 1.2 dB compare to that of typical OADM modules using 1$ imes$2 port optical devices. In addition, both the size and the price of the module can be reduced to 40~50%.