## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.1 Silicon-Based Microring Optical Waveguide Sensor
In this study, a three-slot structure consisting of a silicon substrate, silicon oxide bottom cladding, a 300-nm thick silicon nitride layer, and silicon oxide top cladding was used. Compared with single-slit waveguides, the effective refractive index change of the Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) multi-slit structure-guided mode can be increased by 20% as a function of refractive index change in gas or liquid. The average \(FSR\) of the multi-groove ring resonator with a radius of 70 \(\upmu\)m was approximately 1275 nm. For a cavity  
Figure 9: (**a**) Sensor structure; (**b**) cross-sectional schematic diagram of dual-mode waveguide. with a radius of 90 \(\upmu\)m and a gap of 400 nm, the \(Q\) factors of the transverse electric (TE) and transverse magnetic (TM) modes can reach 6100 and 16,000, respectively. The ring resonator losses were 28 dB/cm (\(R\) = 90 \(\upmu\)m) and 16 dB/cm (\(R\) = 110 \(\upmu\)m), respectively. To improve sensitivity, a metal-assisted silicon slit waveguide structure was added to the traditional slit waveguide structure. By inserting a layer of Ag film, the optical confinement in the groove area can be enhanced and the evanescence rate of the optical mode can be improved, thereby improving the sensitivity of the sensor. In 2020, [PERSON] et al. proposed a novel on-chip gas sensor based on refractive index sensing to detect methane and carbon dioxide gases [31]. In their proposal, metal silver (Ag) was used in the sensor structure to design the slot waveguide, and Si\({}_{3}\)N\({}_{4}\) was used to replace the SiO\({}_{2}\) material in the SOI. Because Si\({}_{3}\)N\({}_{4}\) has a lower transmission loss in the mid-infrared region than that of SOI materials, this wavelength range was selected for optical signals in the mid-infrared region. This design significantly improved the sensitivity of the refractive index sensing, and the range of evanescent waves could be extended to 0.48-0.85. The sensor structure enables real-time detection and exhibits good selectivity, making it a viable option for the design of future integrated photonic circuits. In addition, the sensing mechanism is suitable for a variety of active materials in the infrared range. Using a multimode Fabry-Perot (FP) laser instead of a passive reference ring integrated in the system to generate the reference comb eliminates the requirement for an external broadband source and yields a higher spectral power density and energy efficiency. Compared with the transmission peak of the reference ring, the linewidth of the FP laser is narrower, thus increasing the sensitivity. In 2012, [PERSON] et al. proposed a high-sensitivity integrated optical biosensor based on the cascade of FP cavity lasers and MRRs [32]. Owing to the free spectral range of ring resonators and FP laser cavities, which is slightly different, the sensitivity can be significantly improved by using the Vernier effect, and a high sensitivity of 1000 dB/RIU can be achieved. The sensor structure comprises an FP cavity laser prepared on the InP base, an MRR designed on the SOI substrate, a 2 \(\upmu\)m SiO\({}_{2}\) insulating layer with a Si layer of 220 nm, and a 1-\(\upmu\)m-wide and 40-nm-high ridge waveguide structure. In addition, the diameter of the microring was 280 \(\upmu\)m, whereas the circumference was 885 nm; the waveguide was coated with an SU8 layer. When the ring resonator is exposed to the liquid to be measured, the change in the refractive index of the liquid sample causes a large shift in the envelope function of the total transmission spectrum of the sensor, resulting in a large change in the transmission power. The \(FSR\) of the sensor limits the \(LOD\), whereas increasing the circumference of the ring results in a smaller \(FSR\), thereby improving the dispersion of the transmission characteristics. In 2017, [PERSON] et al. proposed SOI platform dispersion-enhanced critically coupled ring resonator refractive index sensing [33].