Tunable four-channel wavelength division (De) multiplexer based on cascaded long-period waveguide gratings

To match the growing demand for data transmission capacities, silicon photonic tunable multichannel filters with architectures like ring resonators or Bragg gratings are the leading technology. However, their performance, especially the channel count, is often limited by the free spectral range or fabrication difficulties due to small size grating teeth (~ 150 nm). The long-period waveguide gratings (LPWGs) with periodicity ~ μm can be a promising alternative to overcome these limitations. In this context, we propose a four-channel wavelength division (de) multiplexer (WDM) based on cascaded LPWG geometry, operating in the C-band of telecommunication. The proposed structure consists of four cascaded LPWGs in which each LPWG comprises two parallel waveguides made of silicon and titanium dioxide (TiO₂) with apodized gratings placed in between having a length of 720 μm. The 3-dB bandwidth of each channel is found to be 1.59 nm with a small insertion loss of -0.1 to -0.6 dB owing to the unique spectral property of LPWG. The channel spacing is decided through the efficient tuning with the metallic heaters placed on top of each grating due to the high and opposite thermo-optic coefficient of silicon and TiO₂. A minimum spacing of 268 GHz between two consecutive channels is achieved with a power consumption of 30 mW. The cross-talk between the channels is found to be < -20 dB even with ± 10 nm random errors in the waveguide’s width along the propagation direction. The proposed structure possesses great potential towards dense WDM application.