## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.1 Silicon-Based Microring Optical Waveguide Sensor
This type of sensing can be used for sensing a wide range of refractive indices (1.0-2.0) with a sensitivity of 380 nm/RIU. The sensor can measure the unknown refractive index of the analyte with a detection range of 1.67 \(\times\) 10\({}^{-2}\) RIU and a change in cladding refractive index of 0.5. Increasing the ring circumference can further reduce the detection limit to 10\({}^{-3}\) RIU without affecting the round-trip loss of the waveguide. The sensor device has a simple design, convenient operation, and a wide and accurate detection range. Sensor sensitivity can be improved by changing the waveguide structure of the MRR. [PERSON] et al. proposed a novel silicon photonic waveguide-based high-sensitivity sensor [34] by constructing an MRR which uses a silicon-on-insulator subwavelength waveguide consisting of five rows of periodic silicon square arrays with a pitch of 60 nm. The sensor achieved a higher bulk and surface sensitivity compared with that of other resonators using conventional strip waveguides. A bulk sensitivity of 579.5 nm/RIU and surface sensitivity of 1900 pm/nm can be obtained by exposing multiple MRRs to pure water and different concentrations of isopropanol solutions. Figure 10 shows a scanning electron microscopy (SEM) image of a 5-row multilayer MRR. The waveguide geometry is as follows: microring radius \(R\) = 30 \(\upmu\)m, waveguide width w = 1200 nm, waveguide thickness t = 220 nm, grating period \(\Lambda\) = 240 nm, and duty cycle \(\eta\) = 75%. The \(Q\) of the 20 \(\upmu\)m microring was significantly reduced (\(Q\) = 1700) owing to high radiation loss. The \(FSR\) of the MRR was 6 nm, whereas the group index of the multilayer waveguide at 1550 nm wavelength was \(\mathrm{n_{g}}\) = 2.15, and the bulk and surface \(LOD\) of the multilayer microring were 1.02 \(\times\) 10\({}^{-3}\) RIU and 3.13 \(\times\) 10\({}^{-1}\) nm, respectively. Compared with the recently reported subwavelength grating (SWG) MRRs, its bulk and surface sensitivities were improved by 1.5 times and 2 times, respectively. However, the \(Q\) value in water was low at 2600, which is lower than that of other SWG and silicon-based sensors. Future medical diagnostics will require biological detection using on-chip experimental techniques [35; 36]. Silicon photonics enables the fabrication of compact planar sensors using integrated CMOS processes, making it a promising approach for large-scale on-chip biological detection. [PERSON] et al. [37] proposed a novel method for on-chip sensing using a silicon-based MRR and designed a refractive index biosensor based on a silicon slit waveguide ring resonator. By detecting the ring transmission spectrum, the shift of the envelope resulted in a quality factor peak range of 2000-6000 at the wavelength of approximately 1.5 \(\upmu\)m. Furthermore, by tracking the spectral envelope wavelength shift, the sensitivity \(S\) can reach 1300 nm/RIU when a liquid with a refractive index close to 1.33 fills the tank. The extinction ratio of the critically coupled resonance is greater than 20 dB, which implies that the \(LOD\) of the helically designed MRR in SOI photonics is lower than 10\({}^{-4}\). To achieve improved sensitivity and a lower detection limit of the biosensor, [PERSON] et al. [38] proposed a silicon-based TM mode SWG waveguide MRR at a wavelength of 1550 nm. Owing to the enhanced interaction between photons and analytes, it exhibits improved sensitivity and a lower detection limit. When the coupling length is 6.5 \(\upmu\)m, the gap is 140 nm, and the silicon duty cycle is 0.7. The quality factor can reach a high value of 9800 in a water environment, and the bulk sensitivity was 429.7 nm/RIU. The corresponding detection limit was as low as 3.71 \(\times\) 10\({}^{-4}\) RIU, a reduction of 32.5% compared to the optimal value reported for other SWG microring sensors.