## 3 Classification of Optical Waveguide Sensors
### MZI-Structured Optical Waveguide Sensor
#### 3.2.1 Silicon-Based MZI Optical Waveguide Sensor
Therefore, using the basic EH polarization mode (hybrid mode in which the electric and magnetic field components are mixed together, defined as EH mode) of the SOI ridge waveguide, [PERSON] et al. [75] proposed an MZI biochemical sensor based on a large-cross-section SOI ridge waveguide, as shown in Figure 15. At a working wavelength of 1550 nm, a uniform sensitivity of 7296.6 %/RIU was obtained. Its _LOD_ can reach 2.74 \(\times\) 10\({}^{-6}\) RIU, which is lower than that of the refractive index of the Si\({}_{3}\)N\({}_{4}\) ridge waveguide (less than 7 \(\times\) 10\({}^{-6}\) RIU). Figure 14: **(a)** Schematic diagram of the proposed sensor; **(b)** cross-section of a single-mode waveguide; **(c)** cross-section of a dual-mode waveguide and the electric field intensity distribution of the two transverse modes. [PERSON] et al. [76] proposed a sensor design rule based on MZI, as shown in Figure 16. By properly selecting the MZI arm length, sensitivity can be adjusted to a value that is higher than that of any other traditional MZI sensors. Using SOI materials, a low-cost optical sensing system can be realized via proper selection of MZI arms with a device sensitivity as high as 10\({}^{6}\) nm/RIU and a detection limit as high as 10\({}^{-6}\) RIU. Conventional MZI sensors that use intensity interrogation require highly sensitive power sensors and highly stable systems to achieve low noise levels and high sensitivity. Although sensors with ring-resonator structures can overcome this problem by determining the wavelength shift with high precision, a high-resolution spectrometer is required. For the reported sensors based on two cascaded ring resonators, the Vernier effect was used to significantly improve the sensitivity. However, the extremely high sensitivity resulting from a significantly large Vernier factor has some disadvantages. [PERSON] et al. [77] realized ultra-high-sensitivity silicon photonic biosensors using cascaded MZIs and ring resonators with the Vernier effect. The sensitivities of the MZI and MZI ring sensors obtained were as high as 2870 and 21,500 nm/RIU, respectively. Biosensing applications were demonstrated by monitoring the interaction between goat and anti-goat immunoglobulin G (IgG). The measurement results showed that 1 ng/mL IgG resulted in a wavelength shift of 0.035 and 0.5 nm for the MZI and MZI ring sensors, respectively. For the MZI and MZI ring-based sensors, the measured temperature drifts were 76 and 271 pm/\({}^{\circ}\)C, respectively, and highly sensitive sensors are anticipated to be used in medical diagnosis. Exposure to pyridine vapor can inhibit the nervous system, leading to poisoning. Therefore, there is an urgent need to develop a safe and reliable sensing technology to monitor its concentration in real-time. [PERSON] et al. [78] proposed an MZI microsensor based on a CMOS-compatible silicon nitride (Si\({}_{3}\)N\({}_{4}\)) waveguide with bipolar polycarbonate as  
Figure 16: MZI sensor. Figure 15: Schematic diagram of the MZI sensing platform based on a large-section silicon-on-insulator (SOI) rib waveguide. the sensitive cladding material. The waveguide width and cladding thickness were varied to tune the group index and evanescent field. In the final experiment, the sensitivity of pyridine vapor can reach 63 pm/ppm, \(LOD\) was as low as 476 ppb, and the response time was significantly reduced. The designed MZI sensor exhibited ultra-high sensitivity, and the refractive index sensing sensitivity was greater than 30,000 nm/RIU in both the TE and TM modes.