An asymmetric non-uniform 3-dB directional coupler with 300-nm bandwidth

Here, we demonstrate a new type of 3-dB coupler that has an ultra-broadband operational range starting from 1300 nm to 1600 nm with low fabrication sensitivity. Overall device size is 800 micron including in/out S-bend waveguides. The coupler is an asymmetric non-uniform directional coupler that consists of two tapered waveguides. One of the coupler arms is shifted by 130 micron in the propagation direction, which reduces the phase mismatch between the odd and even mode of the coupled system resulting in a better wavelength-flattened response compared to a not shifted coupler. Moreover, compared to a long adiabatic coupler, we achieved a similar wavelength response at 16 times smaller device length. The couplers were fabricated using the silicon nitride platform of Lionix International. We also experimentally demonstrated an optical switch that is made by using two of these couplers in a Mach Zehnder interferometer configuration. According to experimental results, this optical switch exhibits -10 dB of extinction ratio over a 100 nm wavelength range (1500-1600 nm). Our results indicate that the asymmetric non-uniform directional coupler holds great promise for various applications including optical imaging, telecommunications, and reconfigurable photonic processors where compact, fabrication-tolerant and wavelength-insensitive couplers are essential.