Compact On-Chip Dual Grating Filter Using Backward Mode Conversion

A compact on-chip dual grating filter using subwavelength and multimode waveguide grating-assisted contra-directional mode conversion has been designed and demonstrated experimentally. The device features a multimode waveguide grating (MWG)-assisted contra-directional mode converter with apodized gratings for a high side lobe suppression ratio. The apodization is followed by an adiabatic subwavelength grating-assisted mode converter for output in the fundamental mode. The filter is designed to operate at a center wavelength of 1545 nm and has a 3 dB bandwidth as large as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 6~~\rm {nm}$</tex-math></inline-formula> . The two stages of the filter have a side-lobe suppression ratio as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;30~~\rm {dB}$</tex-math></inline-formula> and a cross-talk as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt; -30~~\rm {dB}$</tex-math></inline-formula> . With the filter having a device footprint of only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$450 \times 40~\mu ~\rm {m}^{2}$</tex-math></inline-formula>, it is suitable for photonic on-chip integrations and can be scaled up for designing wavelength division multiplexing systems for local area network applications.