Polymer-functionalized Si3N4 waveguide Mach-Zehnder interferometer for hazardous pyridine vapor sensing

Abstract

CMOS-compatible Si3N4 waveguide offers the advantage of compact size, low cost, and integrate-ability in optical fiber sensing network. To develop a Si3N4 waveguide chip for the sensing of toxic and carcinogenic pyridine vapor, this work demonstrated the following two aspects in waveguide engineering. The first effort was waveguide material engineering: the functionalization of Si₃N₄ waveguide by simple spin-coating a sensitive and specific porous dipolar polymer for pyridine detection. The other effort was the waveguide device engineering in virtue of evanescent field and group refractive index regulation to realize the high sensitivity of waveguide. For the Mach-Zehnder interferometer (MZI) with width/height of 1.5 μm/300 nm and 5 mm length, the sensitivities at TE and TM modes were 31587 nm/RIU and 30293 nm/RIU, both of which are exceptionally high for refractive index sensing. Experimentally, the sensor chip was fabricated by standard CMOS process including photolithography and ICP etching. We conceptually demonstrated the sensing of hazardous pyridine vapor with packaged Si3N4 MZI pyridine sensor in optical fiber transmission system. The sensing performance showed that the limit of detection was 1.92 ppm (lower than the threshold limit of 5 ppm for adverse effects) and the sensitivity was as high as 15.6 pm/ppm with a large linear dynamic range of 2.5-400 ppm. And in the subsequent improvement, we obtained the sensitivity of 63 pm/ppm and the detection limit of 476 ppb by optimizing the thickness of sensitive cladding.