WDM-filters fabricated with hydrogenated amorphous silicon ring and racetrack resonators

Wavelength-division multiplexing filters are photonic key components for the distribution of multiple optical channels. Microring and racetrack resonators in add/drop configuration can be employed to perform the signal routing. In this work, low-loss hydrogenated amorphous silicon (a-Si:H) was employed as waveguide core material as it is transparent in the near infrared and can be fabricated at low costs and with low temperatures (<;300°C). The advantages of using a high refractive index material like a-Si:H (n=3.48) as photonic layer are the low device footprint (<;250 μm² for eight channel ring resonators) and the large free spectral range (FSR) which supports multiple parallel channels and hence high optical bandwidth. Compared to SOI, the plasma deposition process is CMOS-backend compatible and the a-Si:H photonic layers can be directly deposited e.g. on top of integrated microchips at the last process steps. In this paper, four and eight channel WDM-filters which are based on cascaded racetrack and ring resonators are presented. The design, the fabrication, and the optical characterization are discussed. In order to match the desired filter characteristics the circumference length of each resonator was slightly tuned, whereas the gap separation between bus and ring waveguide were fixed. The photonic wire dimensions were chosen to be 480×200 nm to ensure single-mode operation for the qTE-mode. The racetracks were designed with a radius of 20 μm, a coupling gap of 300 nm, and a straight coupling section of 6 μm. The 5 μm ring resonators were separated with a 150 nm gap from the bus waveguides. The channel spacings were targeted to meet a 100-Ghz grid (0.8 nm) which resulted in a successive length difference of 5.2 nm for each resonator that was obtained by determining the group refractive index with FEM-simulations and numerical calculations. The measured FSRs of the racetracks are 4.6 nm and 18 nm for the ring resonators, respectively. Thus, four channel filters were fabricated with the racetracks and up to eight channel filters were realized with the microrings.