On-chip WDM not limited by free-spectral range

Unlike silicon-ring-resonator-based wavelength-division multiplexers (WDMs) that support multiple resonant modes and have a limited free spectral range (FSR)—a difficulty that requires the cascading of several varying-size rings to overcome—an alternative design from the University of California–San Diego (UCSD; La Jolla, CA) and Oracle (San Diego, CA) uses on-chip silicon corrugated waveguides in close proximity to perform the WDM function with a nearly unlimited FSR.

The WDM devices are fabricated on a CMOS-compatible silicon-on-insulator (SOI) platform as 250-nm-thick silicon waveguide structures on a 3-µm-thick buried oxide layer. Two waveguides of different widths are arranged closely beside each other. No coupling would normally occur between the guides because the propagation constants of their modes are unequal. However, if the guide sidewalls are sinusoidally corrugated with period Λ, coupling at a single wavelength—the wavelength where the two propagation constants differ by exactly π/Λ—is enabled and can provide add/drop functionality at this wavelength. The measured FSR is essentially across the entire telecommunications C-band (1520 to 1570 nm). By cascading four such corrugated couplers, a 1 × 4 WDM device was assembled within a narrow geometric area. Although its 16 dB crosstalk is higher than ring-resonator-based designs, the corrugated-waveguide WDM has significantly larger FSR, low insertion loss of 1 dB, and a more compact footprint.

Contact Shaya Fainman at [email protected].