## 3 Classification of Optical Waveguide Sensors
### Cascaded Structured Optical Waveguide Refractive Index Sensors  
#### 3.3.1 MZI Coupled Microring Structure  
[PERSON] et al. [117] proposed a novel high-sensitivity biosensor based on a cascaded MZI and ring resonator with a wavelength-interrogated Venier effect. Using wavelength interrogation, sensors based on MZI only and cascaded MZI rings with the Vernier effect can achieve sensitivities of 1960 nm/RIU and 19,100 nm/RIU, respectively. With the free spectral range of 600 nm, the sensitivity can be as high as 60,000 nm/RIU. A SiN sensor  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Unit**} & \multirow{2}{*}{**Construction**} & \multirow{2}{*}{\(S\) **(nm/RIU)**} & \multicolumn{2}{c}{_LOD_} \\  & & & & **(RIU)** & \(Q\) \\ \hline \multirow{4}{*}{2014} & Saint Petersburg & Single-mode silicon & & & \\  & Polytechnic University & waveguide double & & & \\  & [73] & transverse mode helical structure & & & \\  & & & & & \\
2013 & Bari Polytechnic University [74] & Vernier effect andVernier effect enhanced by SOI & 1000/2500 & \multirow{2}{*}{\(10^{-7}/10^{-8}\)} & \multirow{2}{*}{/} \\  & University [74] & material MZI & & & \\
2014 & Tsinghua University [75] & Large cross-section SOI & 7396.6 & & \\  & [75] & ridge waveguide MZI & \%/RIU & 2.74 \(\times\) 10\({}^{-6}\) & / \\
2020 & Zhejiang University [76] & SOI material MZI & 10\({}^{6}\) & 10\({}^{-6}\) & / \\
2014 & Zhejiang University [77] & Venier effect MZI ring & & & \\  & University of & & & & \\  & Electronic & & & & \\
2022 & Science and & & & & \\  & Technology of China & & & & \\  & [78] & & & & \\
2021 & Ningbo University [99] & & & & \\  & [90] & tank waveguide & & & \\
2013 & University of & Single-mode porous & 13,000 & \multirow{2}{*}{\(10^{-5}\)} & \multirow{2}{*}{/} \\  & Maryland [106] & silicon integrated MZI & rad/RIU-cm & & \\
2015 & Southeast University [107] & Polymer MZI sensor & 88 & / & / \\  & University of & & & & \\
2021 & Electronic & Polymer-based & & & \\  & Science and & non-monospaced MZI & 11,624 & / & / \\  & Technology [111] & & PMMA film & 791 & / & \\
2017 & Jilin University [114] & asymmetric MZI sensor & dB/RIU & & \\  & & Asymmetric MZI double & & & \\
2022 & Henan Normal & suspended GaAs & 854.5 & / & 208.2 \\  & University [115] & waveguide Sensor & & & \\ \hline \hline \end{tabular} \({}^{\pi}\)/\({}^{\pi}\) means that the data is not given in the reference.  
\end{table}
Table 3: Performance comparison of MZI optical waveguide sensors.  
chip composed of an MZI and an array waveguide grating spectrometer was realized with a detection limit of 0.6 \(\times\) 10\({}^{-5}\) RIU.  
[PERSON] et al. [118] proposed a new low-cost optical waveguide polymer material, polysiloxane photonic polymer material (PSQ-Ls), and applied it to biochemical sensing of the refractive index. By introducing an MZI-type tunable coupler into the passive MRR, optical characteristic tuning under lower power consumption and three different coupling states are realized. Through experiments, it was found that UV soft imprinting technology has good compatibility with PSQ-Ls materials, and the process is simple and can be repeated. The \(Q\) values of the device in air and water can reach 5 \(\times\) 10\({}^{4}\) and 2.7 \(\times\) 10\({}^{4}\), respectively, and the obtained absorption loss is 50 dB/cm.