Compact Mach-Zehnder interferometer based on self-collimation of light in a silicon photonic crystal

Optical waveguides based on silicon-on-insulator (SOI) and fabricated in CMOS compatible technology will likely replace electrical interconnects for high data rate circuits. In this context, a modulator based on a silicon-waveguide Mach-Zehnder interferometer (MZI) is very promising [2]. However, the requirement of producing a π phase shift in one arm of the MZI implies a millimeter scale footprint of the device, which is a serious disadvantage for integration in micro-processors. Therefore, strong efforts are made to miniaturize MZI modulators without compromising the required phase shift, particularly using slow light engineering in photonic crystals (PhCs). Here, we demonstrate a different concept for a compact MZI built from a PhC operating in the special regime of self-collimation of light. To reach this regime, we have tailored the photonic band structure such as to produce self-collimated beams. The properties of these beams are such that we can manipulate them to form the arms of the MZI in a very small area of 20×20 μm2.