Silicon waveguide filter based on cladding modulated anti-symmetric long-period grating

In this paper, we demonstrate an optical filter using cladding modulated anti-symmetric long-period grating in a two-mode silicon waveguide. The filter consists of a two-mode waveguide connected with an input and output single-mode waveguide through two linear tapers. The anti-symmetric grating is formed by placing two periodic arrays of silicon squares offset by half of a grating pitch along the two-mode waveguide. Light coupling occurs between two co-propagating modes at the coupling wavelength through the grating and results in a rejection band at the output. The grating pitch, coupling coefficient, transmission spectrum and 3-dB bandwidth of the grating are investigated with the coupled-mode theory. By using a cladding modulated grating, the grating coupling strength can be controlled over a wide range by the two-mode waveguide width or separation distance between the grating and waveguide. Band-rejection filters are experimentally demonstrated in 1-μm, 0.8-μm and 0.7-μm wide two-mode silicon waveguides and rejection bands with different bandwidths and maximal attenuation contrasts larger than 15 dB (~97% coupling efficiency) have been achieved.