Based on the self-collimation effect of light propagating inside a photonic crystal, we demonstrate a novel concept for a
compact Mach-Zehnder interferometer. The properties of these self-collimated beams are such that we can manipulate
them to form the beam splitters and mirrors of the Mach-Zehnder interferometer in a very compact area of 20x20 μm<sup>2</sup>.
We obtain the unidirectional output behaviour characterized by the high contrast in the telecommunication-wavelength
signal at the two outputs of the photonic crystal Mach-Zehnder interferometer. The experiments are done using optical
transmission spectroscopy and far-field optical microscopy. This photonic crystal Mach-Zehnder interferometer holds a
promise for a compact Mach-Zehnder modulator, inspired by recent reports of NEMS-based photonic crystal membrane.