## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.1 Silicon-Based Microring Optical Waveguide Sensor
The width of the ridge waveguide used in this study is 0.45 \(\upmu\)m, while the wavelength is 1550 nm. The detection limit of the sensor can be improved by maximizing the quality factor of the resonant cavity and increasing the interaction of light and biomolecules attached to the waveguide surface to maximize the average resonance wavelength shift per detection. In 2009, [PERSON] et al. proposed a silicon-on-insulator ring resonator [27] based on slot waveguides. Compared with ordinary waveguides, slot waveguide resonators are used as biosensors in the fabrication of SOI rings. The sensitivity of the resonator is increased by a factor of 3.5 in detecting proteins. The area of the structure is 13 \(\upmu\)m \(\times\) 10 \(\upmu\)m, whereas the sensitivity measured using a saline solution with a refractive index at a wavelength of 1550 nm is 298 nm/RIU, and the detection limit (the change in the refractive index of the top cladding) is 4.2 \(\times\) 10\({}^{-5}\) RIU. The ring resonator consists of a groove of width 104 nm and an SOI groove waveguide with a width of 268 nm. The device is fabricated using 193 nm optical lithography. The bending radius is set to 5 \(\upmu\)m.  
In addition, in silicon-based microring optical waveguide refractive index sensing, the sensitivity is generally not high for the refractive index sensing of a single waveguide structure, such as a slot waveguide, slit waveguide, or one-dimensional photonic crystal microring. Therefore, researchers are now designing optical waveguide refractive index  
Figure 8: (**a**) SEM image of the microring cavity; (**b**) measurement device. sensors using composite structures, such as subwavelength grating and one-dimensional photonic crystal groove waveguide microrings. In 2018, to solve the problems of low figure of merit (FOM) and low sensing performance of traditional microring sensors, [PERSON] proposed a one-dimensional photonic crystal slot waveguide microring refractive index using a high-quality factor sensor [28]. The sensing structure incorporates a ring-shaped photonic crystal slot waveguide into the traditional single-waveguide MRR. In addition, it confines the light field in the slot waveguide to the middle of the air hole and thereby improves the interaction between the light field and the substance under test. Optimizing the radius of the air hole on the microring slot waveguide resonator, the width of the slot waveguide, and the coupling distance between the resonator and waveguide can affect the sensing performance and improve the quality factor. The refractive index sensor consists of an annular photonic slot waveguide, a quality factor that can exceed \(10^{5}\), sensitivity of 97.47 nm/RIU, and an FOM value of 6.63 \(\times\)\(10^{3}\). However, the structural model proposed in this study has only been simulated, and no actual sensor device was fabricated and tested. In 2021, [PERSON] et al. proposed a method to obtain the dispersion turning point (DTP) in the dual-mode region by optimizing the core size of the metal-clad ridge waveguide (MUCRW). This approach enables ultra-high sensitivity in refractive index sensing of the nearby metal-clad ridge waveguide mode [29], as shown in Figure 9. The sensitivity of the refractive index sensing can reach 90 \(\upmu\)m/RIU, and the FOM value can reach 4500. The sensors considered in this study have great potential for biosensing applications owing to their ultrahigh volume and adhesion layer sensitivity. Similar to the ridge waveguide, light is guided into the high-refractive-index material. However, the sensor also has shortcomings in biosensing applications. The interaction between light and the surrounding medium is significantly small, resulting in a small change in the index of the surrounding medium, which makes sensing and detection difficult. Therefore, [PERSON] et al. proposed a ring resonator based on vertical multi-slot silicon nitride waveguides [30].