## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.2 Chalcogenide Microring Optical Waveguide Sensor  
Chalcogenide (ChG) rectangular waveguide sensors with small evanescent fields require a large waveguide length to enhance the light-gas interaction effect. To design a more compact sensor, [PERSON] [39] proposed a mid-infrared chalcogenide (ChG)/silicon fluoride horizontal slot waveguide runway resonator sensor that detects methane (CH\({}_{4}\)). Optimization of the waveguide parameters was achieved through the use of sulfide/silicon fluoride horizontal slot waveguide structure, as shown in Figure 11. A high-power con  
Figure 10: Scanning electron microscope (SEM) image of a 5-row multilayer MRR. -finement factor of 44.63% was obtained at a wavelength of 3291 nm, which is five times higher than that of other ChG rectangular waveguides. The waveguide length was reduced by at least 30 times by optimizing the sulfide/silicon fluoride horizontal groove waveguide. The resonator has an _LOD_ of 3.87 ppm, an intrinsic waveguide loss of 3 dB/cm, an amplitude-coupling ratio of 0.1, and a response time of less than 5 \(\upmu\)s.  
Chalcogenide materials exhibit excellent application prospects in the mid-infrared wavelength range and near-infrared wavelength range. Compared with other materials such as silicon, Si\({}_{3}\)N\({}_{4}\), or polymers, the medium refractive index (RI) of the Ge\({}_{28}\)Sb\({}_{12}\)Se\({}_{60}\) chalcogenide material enables the optical field to maintain a strong confinement at small bending radii while achieving a high sensitivity and high \(Q\). [PERSON] et al. [40] proposed a liquid refractive index sensor using a Ge-Sb-Se MRR at a wavelength of 1550 nm to detect different concentrations of NaCl solutions. The chalcogenide glass material Ge\({}_{28}\)Sb\({}_{12}\)Se\({}_{60}\) enables the obtainment of a high sensitivity of 123 nm/RIU and high quality factor of 7.74 \(\times\) 10\({}^{4}\). The transmission loss of the waveguide is 4.3 dB/cm, extinction ratio is approximately 40 dB, and _LOD_ is approximately 3.24 \(\times\) 10\({}^{-4}\) RIU. Owing to the high refractive index contrast between the ChG core and SiO\({}_{2}\) substrate, the device size can be reduced to the order of tens of micrometers without adding substantial bending losses. ChG groove (ChGS) MRRs have broad application prospects in the field of mid-infrared high-sensitivity sensing. [PERSON] et al. [41] fabricated a highly sensitive sulfide slit MRR and proposed a ChGS MRR using GeSbSe thin film as the groove. The power confinement factor (PCF) was enhanced through optimization of the core layer in the MRR. Compared to the traditional evanescent-field waveguide sensor, the proposed structure optimizes the curved groove waveguide to improve the sensitivity of the sensor. By fitting the resonance peaks with a Lorentzian curve, the cavity realizes a quality factor of 1 \(\times\) 10\({}^{4}\), which is one of the highest quality factors reported thus far for chalcogenide groove MRRs. The sensor has a sensitivity of 471 nm/RIU, which results in a detection limit of 3.3 \(\times\) 10\({}^{-4}\) RIU. In ring-resonator sensors, the shift of the resonant wavelength is typically used to study the sensing phenomenon. [PERSON] et al. [42] proposed a GeAsSe sulfide slit-ring resonator for refractive index sensing. The sensitivity can reach 42 nm/RIU with the highest FWHM value of 0.7 nm and a \(Q\) value of 6.5 \(\times\) 10\({}^{3}\). When the refractive index was varied from 1.33 to 1.43, the sensitivity of the structure was 42 nm/RIU at a wavelength of 1551.03 nm. When the effective index difference was set to 0.005, the minimum _LOD_ of the slot MRR was 10\({}^{-3}\) RIU. The refractive index sensor was fabricated using photolithography and inductively coupled plasma (ICP) etching to deposit chalcogenide glass on a silicon oxide wafer.