## 3 Classification of Optical Waveguide Sensors
### MZI-Structured Optical Waveguide Sensor
#### 3.2.4 Polymer-Based MZI Type Optical Waveguide Sensor  
The refractive index of the polymer material can be adjusted, and the loss of the optical waveguide device is smaller than that of the fiber coupling. Furthermore, its preparation temperature and cost are low [107]. Therefore, polymer materials are widely used in optical waveguide devices [108; 109]. [PERSON] et al. [110] proposed polymer-based integrated optical interferometric sensors fabricated by hot-embossing and printing to realize low-cost sensors that are suitable for mass production. Polymer materials are increasingly important in the field of optical sensors owing to their diversity and compositional flexibility, as well as their ability to construct polymers using large-area printing techniques. However, special consideration must be given to polymer-based waveguide losses. In this study, an all-polymer MZI-type waveguide structure based on an inverted-ridge waveguide was proposed. The structure used a printing technique to form an inverted-ridge-groove waveguide structure in a polymethyl methacrylate (PMMA) foil. The refractive index of the core layer was in the range of 1.550-1.569. The sensor waveguide operates in a single mode, which enhances the sensitivity of the MZI sensing area to changes in the environmental refractive index and reduces the lower bending loss in the S-shaped curved waveguide. Lab-on-a-chip technology is widely used in integrated and miniaturized chemical and biological analyses. Integrated microfluidic platforms are highly portable, and the most common microfluidic devices are based on polymer substrates such as polydimethylsiloxane (PDMS), cycloolefin copolymer (COC), and PMMA. This is because they are affordable and guarantee a high degree of flexibility in design and manufacturing. At present, air pollution is increasingly becoming a concern, it is of great significance to develop gas sensors for volatile organic compounds (VOC) to detect polluted gases in the air. [PERSON] et al. [111] proposed a non-equal-width MZI optical waveguide gas sensor. By adjusting the evanescent field strength and group refractive index of the two arms of the interferometer, a refractive index sensing sensitivity of 11,624 nm/RIU, and wavelength change of 6.8 nm in the 2.3 ppm nitrobenzene gas environment were obtained through the testing of p-nitrobenzene explosive gas. [PERSON] [107] designed and fabricated a biochemical sensor based on an MZI using Si/SiO\({}_{2}\) as the substrate, and the waveguide core and upper cladding were spin-coated with 4-\(\upmu\)m wide ZPU46 and 9-\(\upmu\)m wide ZPU44 polymers, respectively. The prepared polymer MZI sensor waveguide core layer size was 4 \(\upmu\)m \(\times\) 4 \(\upmu\)m, and the bending radius and sensing window length of the Y-shaped branch structure were designed to be 3000 \(\upmu\)m and 111 \(\upmu\)m, respectively; thus, both sides of the sensing arm core layer were exposed to the solution under test, improving the sensitivity of the sensor. The refractive index ranges of 1.33-1.44 had good linearity. The average refractive index sensitivity was approximately 88 dBm/RIU, measured with glycerol aqueous solution at a wavelength of 1550 nm. [PERSON] et al. [112] added 5% dye material Disperse Red 1 (DR1) into a dilutive solution of SU8 photoresist and prepared DR1/SU8 polymer material for the waveguide. The structure was designed as an inverted ridge, reducing the waveguide loss of the sensor. The refractive index measured by the NaCl solution at 1550 nm wavelength is in the range of 1.585-1.593, which is significantly improved compared with that of SU8. [PERSON] et al. [5] proposed that the distance between the two arms of the MZI sensor and the length of the arms should be 400 \(\upmu\)m and 12,000 \(\upmu\)m, respectively.