## 3 Classification of Optical Waveguide Sensors
### MZI-Structured Optical Waveguide Sensor
#### 3.2.2 Chalcogenide MZI Optical Waveguide Sensor  
Biochemical sensors have significant research significance in the field of photonics. The increasing number of compounds has led to an increasing demand for monitoring systems with high sensitivity, wide detection range, high stability, and low response time [79; 80]. The on-chip integration of photonic sensors operating at near-infrared wavelengths has been reported [81], and miniaturization can also be considered in mid-infrared sensing. Recent advances have enabled the combination of mid-infrared (MIR) light sources (such as quantum cascade lasers), waveguides, sensors, and detectors on the same chip [82; 83; 84]. There is a large selection of waveguide materials available. Among them, chalcogenide thin films are suitable for several MIR photonic applications in sensing [85; 86; 87; 88; 89; 90]. Chalcogenide materials are particularly suitable for chemical sensing because most molecules vibrate in the infrared medium. [PERSON] et al. [91] developed a Ge-Sb-Se microsensor device for in situ monitoring of pollutants in environmental water, as shown in Figure 17. The sensor detects pollutants in natural water in the mid-infrared spectral range of organic compounds. The experimentally obtained detection of monoaromatics (BTX) using polyisobutylene (PIB) facilitates the development of chalcogenide film-based optical microsensor devices with sensitivities ranging from tens of ppb to hundreds of ppm. For the mid-infrared spectrum, quantum cascade lasers can be used as light sources owing to their broad emission wavelength range and high-power efficiency [92]. Regarding the optical platform, which is crucial to the sensor, although other MIR platforms such as silicon, germanium, and gallium arsenide have been investigated [83], chalcogenides have attracted much attention in the field of sensing because of their wide transparent windows in the infrared range. Studies have reported losses in mid-infrared integrated chalcogenide devices [93; 94] and high-quality factor microcavities [95; 96], which can detect E. coli proteins in the reflection or transmission mode [97]. In 2017, [PERSON] et al. [98] reported that on a chalcogenide mid-infrared platform, an evanescent wave was used for mid-infrared spectroscopy analysis. The cladding layer was designed as a Ge\({}_{28.1}\)Sb\({}_{6.3}\)Se\({}_{65}\) using magnetron sputtering, an ICP dry etching process, and a mid-infrared sensor with a guide layer of Ge\({}_{12.5}\)Sb\({}_{25}\)Se\({}_{62.5}\). The measured waveguide loss is approximately 2.5 dB cm\({}^{-1}\) in the 7.7 \(\upmu\)m band. Figure 17: Mid-infrared (MIR) sensor scheme with an MIR source, detector, microfluidic system, and sensor fabricated from polyisobutylene (PIB) functionalized Ge-Sb-Se waveguides. ChG is particularly suitable for the fabrication of integrated MIR optical sensors owing to its wide transparent window and low processing temperatures. [PERSON] et al. [99] proposed a suspended trough waveguide gas sensor for chalcogenide glass compounds that can achieve a power confinement factor greater than 90%. When the target gas was methane, the power-limiting factor of the proposed waveguide sensor was 93.81% at a wavelength of 3.67 um. The sensitivity was 0.4578 and the _LOD_ was 18.17 ppm. When the working wavelength was 3.291 um, the power limiting factor was 91.98%, the sensitivity was 7.151, and the _LOD_ was 1.139 ppm. Previously, a Ge-Sb-Se chalcogenide ridge waveguide sensor with a PCF of 5% was proposed for the detection of CH\({}_{4}\) and N\({}_{2}\)O at 7.7 um [100]. In 2017, Ge-Sb-Se chalcogenide glass ridge waveguide sensors with 8% PCF were used to detect CH\({}_{4}\) at 3.31 um and 7.66 um and CO\({}_{2}\) at 4.3 um [101]. Later, [PERSON] proposed a silicon horizontal double-slot ridge waveguide sensor with 65.4% PCF for the detection of CH\({}_{4}\) at 3.67 um [102]. In ref. [103], a ChG/silica sensor on a suspended silicon groove waveguide with a PCF of 85.77% was used to detect CH\({}_{4}\) at 3.291 um.