Experimental demonstration of original optical filter based on multiply coupled waveguides.

We present an experimental demonstration of an optical filter based on multiply coupled waveguides that has previously been demonstrated only numerically. The experimental results show a good match to numerical modeling using a 2D finite-difference time-domain method that utilizes a modified effective index method (MEIM) approximation. The MEIM correctly describes both the phase and the group indices of 3D silicon wire, providing the means to study complicated and large photonic structures with moderate computer resources and simulation time. An optical filter with a set of 12 directional couplers, constructed on a SOITEC silicon-on-insulator wafer with 220 nm of silicon on a 2-μm-thick buried oxide layer, provides at optical wavelength 1.59 μm a free spectral range about 16 nm and 3 dB linewidth about 1.6 nm. These parameters are limited by the radius of curvature used (5 μm) and the small structure sizes available (40  μm×200  μm). Fabrication imperfections, such as sidewall roughness, cause moderate variations in the waveguide width, approximately 2 nm, which results in parasitic responses in the transmission spectrum of the drop spate. This effect could be decreased through improvements in technology and by decreasing the length of the connecting waveguides. Our results prove that proposed filter can be manufactured by modern CMOS compatible technology and is promising for a range of applications in photonics.