## 3 Classification of Optical Waveguide Sensors
### MMI Structured Optical Waveguide Sensor  
A novel platform for chemical and biosensing based on optical MMI couplers was presented in a 2005 article by [PERSON] et al. [123]. A proof-of-concept humidity sensor was implemented, using hybrid sol-gel planar technology for integration onto a silicon substrate. The experiments showed that the sensor was linearly responsive to relative humidity and  
Figure 21: Schematic diagram of the sensor. Figure 20: Schematic diagram of the refractive index sensor. the sensitivity could be adjusted using MMI design technology. [PERSON] et al. [124] also presented a waveguide sensor based on MMI in 2010. The sensor layer was used as the cover of the multimode section in the modeled systems. By selecting a material sensitive to specific external conditions as the cover of the multimode section, the sensor could be adapted to detect different physicochemical quantities. The study also investigated and compared systems based on waveguides with step index profile and gradient distribution of refractive index. It was demonstrated that the sensitivity of these systems could be adjusted by properly selecting the dimensions of the multimode section. [PERSON] et al. [125] characterized the sensing performance of a refractometric sensing platform based on MMI couplers using a low-cost process with photocurable organic-inorganic hybrid sol-gel materials. The sensing principle is based on the sensitivity of the optical field distribution in the MMI coupler to changes in the refractive index of its environment. The length of the MMI coupler section was found to be important, with longer platforms being more sensitive. A porous sol-gel humidity-sensing enrichment layer was coated on the MMI coupler, achieving a resolution of 0.097% for relative humidity detection. The platform's refractive index resolution was determined to be \(2\times 10^{-6}\) RIU, making it a generic and label-free biosensor platform with immense potential for future applications. [PERSON] et al. [126] discussed an integrated bimodal waveguide interferometric biosensor for label-free analysis, as shown in Figure 22, which was highly sensitive with a detection limit of \(2.5\times 10^{-7}\) RIU for homogeneous sensing. It was fabricated using standard silicon technology and successfully monitored biomolecular interaction in real-time. The interferometric devices were simple and could be integrated into lab-on-a-chip platforms for point-of-care diagnostics, making them a powerful tool for biochemical analysis. [PERSON] et al. [127] proposed and demonstrated a new evanescent wave biosensor that utilized the interaction between the fundamental and second-order modes, leading to a high sensitivity behavior. The device was designed to only allow propagation of these two modes, avoiding excitation of the first order mode, as shown in Figure 23. This design resulted in stronger interaction between the evanescent field and the outer surface compared to previous designs. The authors predicted that the limit of detection for the polymer device in bulk will be approximately \(7.34\times 10^{-7}\) RIU. Figure 22: Schematic diagram of sensing waveguide structure and interaction between electromagnetic field and surface sensing molecules. [PERSON] et al. [128] described an integrated optical waveguide refractive index sensor that had ultra-high sensitivity in the communication wavelength range. The proposed sensor can be integrated with other sensing elements on a single chip. It had a sensitivity ranging from \(\sim\)1370 to 33,850 nm/RIU for TM and 1160-26,050 nm/RIU for TE polarized input light, and this is the highest sensitivity reported for MMI-based sensors. Due to the different length of the tail of TM- and TE-mode evanescent field extending out of the waveguide core, the sensitivity is different. [PERSON] et al.