## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.3 Lithium Niobate Compound Microring Optical Waveguide Sensor  
At present, optical waveguide sensors fabricated from lithium niobate materials are used for electric field measurement. In addition, they are used in various high-performance and high-power photonic microdevices, including high-\(Q\) WGM resonators [45; 46], photonic crystal (PhC) cavities [47], hybrid lithium niobate on insulators (LNOI) platform [48; 49; 50], and microwave photonics [51; 52; 53; 54; 55; 56]. Currently, their use in biochemical sensors is relatively small. In addition, optical MRRs based on SOI technology have the advantages of strong optical confinement, small transmission losses, small bending radius, and a well-established manufacturing process. The high refractive index contrast of LNOI can result in a higher mode confinement and a lower bending radius, which enables the fabrication of small devices in integrated optics. Moreover, biosensing with a large measurement range requires ultra-high sensitivity and low detection limits, and research in this area is still ongoing [57]. [PERSON] et al. demonstrated a ring microresonator with a \(Q\) factor of only \(4.0\times 10^{3}\)[58], whereas [PERSON] et al. demonstrated an optical ridge waveguide with an optical loss of 6.3 dB/cm [59]. The highest \(Q\) factor of the LNOI microdisk resonator [60] reached \(4.7\times 10^{7}\), whereas the lowest optical loss in single- and multimode LNOI waveguides was 0.029 dB/cm [61] and 0.027 dB/cm [62; 63]. The \(Q\)-factor and optical loss in the LNOI photonic structure improved by approximately four orders of magnitude. [PERSON] et al. [64] designed and simulated a label-free optical MRR biosensor based on LNOI technology. To obtain a high sensitivity and large measurement range, the sensor uses a double MRR model and transmission spectrum obtained from 2D simulation based on the finite-difference time domain method. The refractive index of the sensing layer was between 1 and 1.45, and the LNOI consisted of an outer and inner ring of 10 \(\upmu\)m and 5 \(\upmu\)m, respectively. The sensitivity of the microring sensor is close to 68 nm/RIU, whereas the minimum \(LOD\) is \(10^{-2}\) RIU. The performance of the sensor was simulated with a glucose solution model. With the development of technology, both fluidic channels and waveguides can be laser-written in glass, thereby paving the way for 3D optofluidic chips. Because a self-trapped beam can be formed in a monolithic lithium niobate chip structure, an optical waveguide refractive index sensor on a lithium niobate substrate can be fabricated using this technology. [PERSON] et al. [65] proposed a refractive index sensor, in which a ring-shaped optical waveguide embedded in a lithium niobate substrate passes through a fluid channel, enabling accurate measurement of the refractive index of transparent liquids. A sensor was used to detect the refractive index of methanol, and the measured refractive index at 640 nm was 1.317. When the refractive index was between 1 and 1.8, the sensitivity of the sensor was approximately \(5\times 10^{-3}\). Owing to the advantages of silicon waveguides such as low loss and low cost, most integrated optical sensing devices use silicon nitride/silicon dioxide technology [36]. However, a disadvantage of the technology is the inability to externally control the light, which is necessary in defining the operating point of an interferometric device. Moreover, lithium niobate has the advantage of controlling light via the electro-optic effect and can be used to obtain waveguides with low losses and high resistance to light damage. To develop LiNbO\({}_{3}\) biochemical optical waveguide sensors, the binding process of the antibodies on the waveguide should first be studied. [PERSON] et al. [66] developed an LiNbO\({}_{3}\) biochemical optical waveguide sensor consisting of a planar waveguide fabricated on LiNbO\({}_{3}\) using the gas-phase Zn diffusion method and a low-loss optical waveguide.