## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.1 Silicon-Based Microring Optical Waveguide Sensor  
In 2008, to realize the sensing and detection of weakly interacting gases, [PERSON] et al. [25] realized a slotted Si/SiO\({}_{2}\) micron resonator. The enhanced effect of the slot waveguide on the optical field is combined with the refractive index sensitivity, and its scanning electron micrograph (SEM) images are shown in Figure 7. It combines the enhancement effect and refractive index sensitivity of the slit waveguide to the MRR. Furthermore, the relative refractive index of acetylene gas is detected under different pressures by measuring the change in the microring resonance wavelength. The slit width is 40 nm, resulting in a significant high electric field enhancement in the slit region. As a result, the refractive index (RI) sensitivity can reach 490 nm/RIU (refractive index unit), demonstrating that the light-liquid interaction is greater than that obtained using Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) groove waveguides. A resonator with a \(Q\) factor of 5000 was used to detect the refractive index change on the order of \(10^{-4}\). In 2007, [PERSON] et al. [23] proposed a label-free biosensor based on an SOI racetrack-shaped microring cavity. This biosensor can quantitatively measure trace substances and has the advantage of rapid preparation. The raceway shape has greater control over the coupling coefficient. Furthermore, the use of transverse waveguide coupling simplifies the fabrication process; SOI provides a high refractive index contrast, and the enhanced light field interacts with the substance under test in the cavity to increase the sensitivity and reduce the size of the sensor. It has an area of less than 100 \(\upmu\)m\({}^{2}\), a high \(Q\) factor, and a significantly small radius of 5 \(\upmu\)m. With a refractive index sensitivity of \(10^{-5}\), owing to its small size, the device exhibits excellent sensitivity for absolute molecular mass sensing,  
Figure 7: Scanning electron micrography (SEM) image of silicon slit MRR. (**a**) Ring slot waveguide; (**b**) cross-sectional SEM image of the slot waveguide; (**c**) waveguide quasi-TE mode profile. detecting protein concentrations as low as 10 ng/mL. In addition, it can be mass-produced and fully integrated on a chip, as shown in Figure 8.  
For the SOI platform, silicon is used as the layer material for the device; however, considering that silicon nitride has low refractive index and low temperature sensitivity and has a weak binding ability to light, sensitivity of silicon nitride devices can be improved. Therefore, in 2020, [PERSON] [21] proposed a refractive index sensor based on a few-mode silicon nitride microring. This sensor detects liquid components by measuring the resonance wavelength shift by changing the refractive index of the object under test. The design parameters of the few-mode microring in the waveguide structure include a ring waveguide width of 2 \(\upmu\)m and a microring waveguide thickness of 0.4 \(\upmu\)m. The layer thickness of the silicon nitride wafer is 400 nm, whereas the wavelength and microring radius are 1550 nm and 30 \(\upmu\)m, respectively. The refractive index sensor can obtain a detection limit of \(10^{-5}\) RIU, sensitivity of 91 nm/RIU, and a \(Q\) value of \(2\times 10^{4}\) or higher. When a substance is attached to the ring waveguide, the effective refractive index (\(n_{\mathit{eff}}\)) changes, thereby affecting the optical output power, which enables the measurements of the object under test. In 2014, [PERSON] et al. proposed a microbiological sensor ring resonator based on an SOI planar optical waveguide [26]. The SOI optical waveguide adopts a ridge structure, the single-mode ridge waveguide is coupled with the ring waveguide to form a ring resonator, and its \(Q\) value can reach magnitudes of \(10^{3}\). According to the optical waveguide theory, when the waveguide width is larger than 1 \(\upmu\)m, light is confined to the waveguide. This is not conducive to the coupling of the ring resonator.