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 two coupled asymmetric strip waveguides with a top-loaded grating structure. We have studied variation of the sensitivity with the waveguide parameters to optimize the design of the sensor. The proposed sensor is an on-chip device with a high refractive index sensitivity of ~10
⁴
nm/RIU with ±140 nm fabrication tolerance, which is more than two times higher than the reported work, and the sensor can also achieve the ultrahigh sensitivity ~10
⁵
nm/RIU with the fabrication tolerance ±13 nm. 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 sensor performance.