## 6 Conclusions  
In this study, the basic principles of optical waveguide refractive index sensing and various design structures of refractive index sensors are introduced. The performance and development prospects of optical waveguide refractive index biochemical sensors with different structures and materials were studied by analyzing the principles and detection mechanisms of two common optical waveguide refractive index sensors.  
With the rise of intelligence and informatization, the demand for biochemical sensors is constantly increasing in both biomedicine and environmental monitoring. Integrated, convenient, and accurate biochemical sensors will be the main development direction in the future. Biochemical sensors based on optical waveguide refractive index MRRs and MZI structures have the advantages of high sensitivity, high quality factor, strong anti-interference ability, easy mass production, and excellent research value. In this study, we summarized and analyzed the structural characteristics and performance indicators of two optical waveguide refractive index sensors. In particular, we compared performance indicators such as sensing sensitivity, detection range, and quality factor. The results show that the sensitivity and quality factors of the optical waveguide refractive index sensor are mutually restricted. Compared with the microring design, the MZI sensor has higher sensitivity, whereas the MRR sensor has a higher quality factor. Optical waveguide refractive index sensors exhibit improved performance indicators; however, their structures are relatively complex.  
Finally, the applications of refractive index sensing in the biochemical field for material detection are discussed. They have significant advantages over other types of biochemical optical sensors. Further research is necessary to realize sensors with high sensitivity, high quality factors, and low losses. At present, silicon photonics technology and photonics integration technology are still undeveloped; however, owing to their significant advantages, such as ultra-high refractive index contrast, wide transparent window, higher sensing sensitivity, and high integration density, they are expected to become mainstream solutions for sensing technology. Miniaturized and high-sensitivity chip sensors exhibit good prospects for development.  
Conceptualization, C.P., L.L. and H.Z.; methodology, C.P., L.L. and H.Z.; writing--original draft preparation, C.P. and H.Z.; writing--review and editing, L.L. and C.P.; visualization, C.P., C.Y. and H.T.; validation, L.L., C.Z. and L.Q.; supervision, L.L. and C.C.; project administration, L.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key R&D Program of China (2020 YFB2205902); the National Natural Science Foundation of China (62090050, 62121005, 62227819, 62274164, 62275245, 62090054); the Science and Technology Development Project of Jilin Province (202001069 GX, 20200501006 GX, 20200501008 CX); the 17 th batch of innovative and entrepreneurial talents Project in Jilin Province (2021Y008); and the Dawn Talent Training Program of CIOMP.  
Not applicable.  
The authors would like to thank [PERSON] for helping with this article.  
The authors declare no conflict of interest.