We propose a novel dispersion-free interleaver using optical delay lines by accurately locating the zeros in the transfer function. It has been implemented with the design of interleaver pairs with the same amplitude responses but opposite phase responses for bidirectional dense wavelength-division multiplexed (DWDM) transmission systems. The measured results are consistent with device simulation. We have further modified the original three-port design using unidirectional amplification, and a fourport interleaver has been built and demonstrated to achieve bidirectional DWDM transmission. In this paper, we fully studied and verified the applications of our four-port interleavers in bidirectional transmission. We demonstrated a bidirectional strain-line system over 210 km and a recirculating loop transmission over a 500-km standard single-mode fiber using 10-Gb/s ON-OFF keying signals. Furthermore, we also demonstrated return-to-zero differential phase-shift keying (DPSK) and nonreturn-to-zero DPSK modulation formats for more than 230 km of transmission. For comparison, the different amplification functions, such as the erbium-doped fiber amplifier and the semiconductor optical amplifier, have also been probed in this paper. The experimental results have clearly illustrated the desirable functions of this novel bidirectional amplifier in this dispersion-free interleaver.