## 4 Discussion  
### Applications in the Field of Sensing  
Because optical waveguide sensors can effectively convert biological or chemical reactions into measurable signals, they are widely used in the chemical industry, biological processes, medical diagnosis, environmental monitoring, military defense, and scientific research. Optical waveguide sensors have higher sensitivity than other sensing methods [132]. Because they can be fabricated into miniaturized and integrated devices, they have been used in optical switches, sensors, and lasers [133]. Optical fibers and planar optical waveguides are common types of optical waveguides. Optical fiber sensors [134; 135; 136; 137] have been widely used in the field of biochemical sensing; however, they are difficult to miniaturize and integrate, and biochemical modification is difficult; thus, their applications are limited. Planar optical waveguides are easy to process and modify, have high sensitivity, are easy to use, and have excellent application value in the field of biochemical sensing [138]. In terms of gas and environmental monitoring, because optical waveguide refractive index sensors have easy integration, high sensitivity, and strong anti-interference characteristics, they are widely used in the fields of toxic, flammable, and explosive gas detection, and environmental protection. The change in gas concentration can be obtained by measuring the refractive index [139]. A large number of pollutants produced by industrial production have caused serious air pollution, and the sensitive detection of volatile organic gases in the air has become a research hotspot in biochemical sensing [140]. High-precision detection technology is essential for the detection of trace biochemical compounds. Optical waveguide refractive index sensors are particularly valuable for detecting these compounds optically. They have important applications in the fields of biomedicine, human health detection, and pathogen tracing, enabling the detection of a wide range of parameters including salt concentration in water, hemoglobin concentration, glucose levels, blood pressure, E. coli, and DNA in the human body. Chemical interactions between substances can be detected by measuring their concentrations, such as the interaction between proteins and nucleic acids or the binding of viruses and cell membrane receptors. In 2015, a simple, sensitive, and selective biosensor was proposed in the literature [141], which had a detection limit of 1.1 \(\times\) 10\({}^{-8}\) and a detection range of 1 \(\times\) 10\({}^{-8}\) to 1 \(\times\) 10\({}^{-6}\). In 2018, ref. [142] described the development status of optical waveguide sensors in monitoring heavy metals, organic matter, and microbial pollution, with a detection limit of 2.2 \(\upmu\)m and a response time of 5 min. In 2013, a biosensor with high optical selectivity and sensitivity was proposed in ref. [143], with a detection limit of 50 CFU/mL and a detection time of 2 h. In 2020, ref. [144] studied a sensor's ability to iden \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Performance Characteristics** & **MRR**[14; 35] & **MZI**[72; 73] & **MMI**[5; 129] \\ \hline Sensitivity (nm/RIU) & 10\({}^{2}\)–10\({}^{3}\) & 10\({}^{2}\)–10\({}^{4}\) & 10\({}^{3}\)–10\({}^{4}\) \\ Sensitivity (rad/RIU) & / & 461.6\(\pi\) & / \\ Detection limit (RIU) & 10\({}^{-2}\)–10\({}^{-5}\) & 10\({}^{-5}\)–10\({}^{-6}\) & 10\({}^{-6}\)–10\({}^{-7}\) \\ Size & Smaller & Small & Smallest \\ Fabrication & Hard & Easy & Easy \\ Loss(dB/cm) & 1–10\({}^{2}\) & \textless{}1 & \textless{}1 \\ \hline \hline \end{tabular} \({}^{\pi}\)/\({}^{\pi}\) means that the data is not given in the reference.  
\end{table} 
Table 4: Comparison of properties of three structural types. tify mutant proteins, which had high specificity and an extremely low detection limit, with a detection limit of 11.4 fM. In 2020, ref. [145] proposed an optical biosensor that can be used for point-of-care (POC) diagnosis of 10-100 virus detection within 30 min. In 2019, ref.