## 3 Classification of Optical Waveguide Sensors
### MZI-Structured Optical Waveguide Sensor
#### 3.2.1 Silicon-Based MZI Optical Waveguide Sensor  
The MZI optical waveguide sensor has the advantages of high sensitivity, small size, and strong anti-interference ability, and has attracted increasing attention in the field of biochemical sensing [70; 71]. In the literature, the refractive index of the MZI optical waveguide sensor is approximately \(2\times 10^{-8}\), and the lowest concentration detection value can reach \(10^{-6}\)[35]. Moreover, CMOS circuits based on SOI materials have the advantages of low power consumption, high integration density, strong anti-interference ability, and improved performance compared to those of traditional silicon-integrated circuits. With advantages of miniaturization, integration, and high sensitivity, new applications of biochemical sensors can be realized. [PERSON] et al., from Tsinghua University, proposed a biochemical sensor based on an SOI planar optical waveguide [72]. The micron-sized sensor chip used CMOS technology. The sensitivity of the sensor can reach magnitudes of \(10^{-7}\) and the corresponding changes of the output optical power in the order of \(10^{-3}\) magnitude. A conventional MZI sensor consists of two channels: a beam splitter and a coupler connected by a sensing arm and reference arm. To prevent evanescent coupling between the waveguides, the arms must be relatively wide apart, and to recombine the signals through the channels, additional bending or coupling structures are required, all of which increase the size of the device. To reduce the size of the sensing area, a single-mode helical waveguide is used in a conventional dual-channel MZI; however, balanced helical arms and additional structures are still required for splitting and combining light. [PERSON] et al. [73] proposed a compact single-channel MZI biochemical sensor that consisted of two single-mode silicon waveguides and used dual transverse mode helical waveguides for sensing. The discontinuous node output is shown in Figure 14. Different concentrations of NaCl solutions were injected into the sensor to measure the bulk refractive index sensitivity of the sensor, which was 461.6 \(\pi\)/RIU. The surface sensitivity of the sensor was found to be 1.135 \(\pi\)/ng mm\({}^{-2}\). By sampling the optical response of the stable region, the sensor exhibited a resolution of 0.01 \(\pi\) and a detection limit of 2.2 \(\times\) 10\({}^{-5}\) RIU. Finally, the biosensing ability of the sensor was verified by measuring biotin-streptavidin interactions at different concentrations. To improve the sensitivity and detection limit of the sensor, this study utilized the high sensitivity of the MZI and the architectural improvement introduced by the Vernier effect. In 2013, [PERSON] et al. [74] proposed an ultra-high-sensitivity chemical optical waveguide sensor using the MZI-enhanced Venier effect, and the sensor chip was fabricated from compact SOI material. A CO\({}_{2}\) sensor was designed, capable of detecting gas concentrations as low as 5000 ppm, with significantly low detection limits, less than 5.5 \(\times\) 10\({}^{-7}\) RIU, and a sensitivity greater than 1000 \(\upmu\)m/RIU. In addition, an ultra-high-sensitivity MZI-enhanced Vernier effect NH\({}_{3}\) sensor for aqueous solutions was designed. The sensor can detect ammonia concentrations as low as 2 ppm, with a sensitivity as high as 2500 \(\upmu\)m/RIU, and an ultra-low _LOD_ of 8.79 \(\times\) 10\({}^{-8}\) RIU. For the MZI sensing platform to be easily integrated with optical fiber communication systems and optoelectronic systems, the coupling efficiency of the large-section SOI ridge waveguide and the standard single-mode glass fiber must be high. Furthermore, the matching must be improved, and the size of the MZI branch must be small. The sensing platform is compact and can achieve a micron level. According to the analysis, the trench-based MZI structure exhibits an improved performance compared to conventional configurations.