Folded Mach-Zehnder interferometer based on photonic crystal self-collimation effect

A Folded Mach-Zehnder interferometer (FMZI) in a two-dimensional photonic crystal is proposed. The FMZI consists of one splitter and several mirrors. Light propagates between them employing self-collimation effect. Its two interfering branches have different path lengths. The two complementary transmission spectra at two FMZI output ports are both in the shape of sinusoidal curves and have a uniform peak spacing in the frequency range from 0.255c/a to 0.270c/a. The peak spacing becomes smaller when the length difference between the two branches is increased. As self-collimation light beams can cross each other without coupling, this FMZI is much smaller than non-folded interference-type filters in photonic crystals. This FMZI may work as a wavelength division demultiplexer in high-density photonic integrated circuits.