## 3 Classification of Optical Waveguide Sensors
### MMI Structured Optical Waveguide Sensor
[129] presented an interferometric transducer that was based on a trimodal optical waveguide concept and generated a readout signal from the interference between the fundamental and second-order modes that propagated on a polymer waveguide. The device was fabricated using the polymer SU-8 over a SiO\({}_{2}\) substrate and exhibited a free spectral range of 20.2 nm and signal visibility of 5.7 dB. The device had a high sensitivity to temperature variations of 0.0586 dB/\({}^{\circ}\)C, which made it a promising candidate as a low-cost photonic transducer for use in sensing applications, including point-of-care. The bimodal and the trimodal waveguides are both based on the interaction between the fundamental mode and the higher-order mode. Generally, the input and output waveguides are single-mode, and the middle is the interference part of multimode. When the concentration of the material to be measured changes, the effective refractive index difference between the modes changes, which changes the phase difference, and then changes the power of the fundamental mode in the output waveguide so as to achieve concentration detection. There have been many reports on MMI fiber sensing areas in addition to the MMI planar optical waveguide sensing area. A novel leaky mode coupler of MMI fiber structure was proposed and experimentally demonstrated for surrounding refractive indices higher than that of silica fiber by [PERSON] et al. [130]. The proposed sensor showed a high sensitivity of 1682 nm/RIU and a limit of detection of 5.94 \(\times\) 10\({}^{-6}\) RIU, which are significantly better than those of conventional MMI waveguide sensors. In addition, ref. [131] also reviewed many other types of MMI fiber sensors and their applications, which will not be repeated in this paper. Table 4 compares the characteristics of refractive index sensors based on three types of optical waveguide structures: MMR, MZI, and MMI, under the same conditions. Although MMR structures are widely used, their sensitivity varies greatly, with early sensitivity being relatively low. Currently, sensor schemes have achieved certain sensitivity improvements through the improvement of waveguide structures and process optimization, but the sensitivity of MMR structures is still inferior to that of MZI and MMI structures. The detection ranges of the three structure sensors do not differ significantly and all have wide detection ranges that can meet the application requirements of sensors. In terms of size,  
Figure 23: MMI biosensor scheme. (**a**) Three-dimensional view of the MMI and its sensing area. (**b**) Side view of the waveguide coupling region. (**c**) Top view showing the region where high-order modes are excited and interact with the bioactive region. typically, MMI sensors have smaller volumes than MMR sensors, and MMR sensors have smaller volumes than MZI sensors. In terms of processing difficulty, MMR sensors may be the most challenging to process because the diameter of the MMR is usually very small and special processing techniques are required to manufacture them. In contrast, the processing difficulty of MMI and MZI sensors may be relatively lower. In terms of losses, the losses of MZI and MMI structure sensors are relatively lower than those of MMR structures. Overall, different sensor structures may require different processing techniques and process flows, depending on factors such as materials, device size, and design requirements.