Highly Sensitive Refractive Index Sensor based on Silicon Nitride Strip Waveguide Directional Coupler

We propose a refractive index sensor, based on grating-assisted directional coupler by, comprising of two coupled asymmetric strip waveguides with a top-loaded grating structure. The proposed sensor is an on-chip device with a high refractive index sensitivity of ~ 104 nm/RIU with 140nm fabrication tolerance, which is more than 2 times higher than the reported work and the sensor can also achieve the ultra-high sensitivity ~ 105nm/RIU with the fabrication tolerance 13nm. The sensor has large dynamic range from refractive index 1.0 to 1.55. We have also studied the propagation loss due to side wall roughness of the strip waveguide, and found that it does not significantly affect the sensor performance.