## 1 Introduction
In label-free optical sensors, the light is mainly concentrated on the sensor surface, and the evanescent field decays exponentially from tens to hundreds of nanometers in the target analyte, so it is possible to detect the change in the refractive index; however, for biomolecules that are not recognized and For analytes far from the sensing surface, the sensing signal is weak, so the target analyte can be distinguished from the buffer solution [9]. After conducting a literature review, it was found that [PERSON] et al. [10] conducted a review in 2014 that focused on biosensors based on integrated planar optical waveguide interferometer structures. They primarily analyzed and discussed the sensing structures of both common-path and double-path waveguide interferometers and compared the advantages and disadvantages of different methods and sensing structures. However, this review only focused on two types of waveguide interferometer sensing structures and lacked a detailed discussion on the sensing detection mechanism and performance, as well as application scenarios and the detection range. Therefore, further refinement and improvement of this study was necessary for a comprehensive understanding of the future development direction and achievements in this field. In 2015, [PERSON] et al. [11] conducted a review of sensors used to measure analytes related to food safety in fields such as chemistry and microbiology. However, the paper mainly focused on sensors that use microbial cells such as enzymes and immunoglobulin antibodies as biological components and did not comprehensively introduce and analyze sensors used in the biological and medical fields. Additionally, there was limited discussion on the principles and applications of new biomarker recognition components, and no detailed description of current research trends and future development directions based on biomarker recognition sensors. In 2020, [PERSON] et al. [12] reviewed the existing problems in optical waveguide biosensors at the field of food quality control. In view of the problems that silicon waveguides cannot transmit visible light and have poor corrosion resistance, they proposed to use diamond as waveguide core material, and listed the commonly used single structure and composite structure waveguide sensors. More recently, [PERSON] et al. [13] reviewed the latest progress of sensors in environmental monitoring, mainly focusing on the detection of toxic gases, water quality, indoor environment, and natural disasters, without discussing molecular detection in other fields such as biomedicine, resulting in a limited application scope. In addition, there was a lack of introduction and classification of performance indicators such as sensor sensitivity and detection range, and there were few theoretical analyses. In addition, in 2022, [PERSON] et al. [14] reviewed some of the marked advances in the last decade in the field of optical biosensors, including the classification, typical structure, and application of optical biosensors. As optical waveguide sensor is a subclass of optical sensor, the structure and application of optical waveguide sensor are briefly introduced in this paper, and the sensing mechanism is not discussed in depth. It can be seen that the review of optical waveguide refractive index sensors in the above literature has its own focus, namely, as material, structure, application, etc. It is difficult to form a comprehensive understanding of such sensors through a single literature. Based on this, this article comprehensively reviews various waveguide refractive index sensors, introduces their applications in fields such as gas detection and biomedical engineering, and conducts detailed comparative analyses of the performance of different types of sensors, highlighting the factors that need to be considered in designing high-performance refractive index sensors.