## 1 Introduction  
With the rapid development of modern information technologies, such as the Internet of Things and big data, sensor technology, as the foundation of an intelligent information society and one of the three pillars of modern information technology, is currently receiving significant attention worldwide. High-sensitivity optical sensing technologies are applicable in industrial process control, national security, and other fields. With the development of optical integration technology, optical sensor devices have expanded from the study of optical fiber sensors to optical waveguide sensors that can be easily integrated [1]; in particular, we see an uptake in the use of optical waveguide technology for target detection in the fields of biology and chemistry. Optical waveguides have advantages such as small size, simple structure, and easy integration, and they are sensitive to changes in the refractive index, absorption, and chemical processes. These changes modulate the light transmitted in the waveguide, and the properties of the optical waveguide can be used to fabricate various sensors [2]. Optical waveguide refractive index sensing uses optical phenomena, such as resonance and interference, to detect the shift of the resonance peak or the attenuation of optical power according to the properties of different concentrations of target analytes. Compared with traditional sensors, optical waveguide refractive index sensors have advantages such as small size and low power consumption [3], and can rapidly, sensitively, label-free, and detect substances to be tested in a non-destructive manner. Hence, they have become a research field in the field of integrated optoelectronics in recent years. Optical waveguide biochemical sensing is a combination of optical waveguide technology and biological and chemical technologies. Biosensors are a class of chemical sensors. Optical biochemical sensing can be divided into labeled optical biochemical sensing and unlabeled optical biochemical sensing (e.g., refractive index sensing) [4]. In the optical waveguide biochemical sensor, the evanescent field of the waveguide is altered by the changes in the refractive index, absorption coefficient, and other parameters, which is typically caused by the change in the phase of the transmitted light, resulting in a different effective refractive index of the optical transmission mode. The change in effective refractive index can be calculated by mode coupling theory, which is applied to sensors such as surface plasmon resonance (SPR) sensors and grating sensors. It can also be calculated based on phase changes, which is applied to devices such as microring resonators (MRRs), Mach-Zehnder interferometers (MZIs), multimode interferences (MMIs), fiber Bragg gratings, photonic crystals, and other differential interferometers [5; 6]. Based on the refractive index change or absorption spectrum, optical waveguide refractive index biochemical sensors can be used for qualitative or quantitative analysis of gas, liquid, and solid substances to be tested [7]. Furthermore, they can be used for the detection of harmful gas concentrations in the environment, food and water quality monitoring, and bacteria and virus detection for public health issues and chemical threat perception [8]. The mechanism of biochemical sensors that use label-free detection is mainly based on the change of refractive index caused by molecular interaction to measure chemical substances, viruses, or bacteria, and it is related to the target sample concentration or surface density, which is easy to measure and low in cost. Sensing measurements are possible due to a change in the refractive index near the sensor surface due to the difference in refractive index between the target analyte and the buffer solution.