2.3.1. Structural Design

Since the refractive index sensitivity response of the SMBMS type structure sensor was still very low, in order to further improve the refractive index sensitivity, the SMBMS type structure sensor was improved. As shown in Figures 6 and 7, the structure of the ESMBMS is based on the structure of the SMBMS, and the middle multimode segment is etched in hydrofluoric acid. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 6 of 19

**Figure 6.** Schematic diagram of the Etched Single mode–Multimode–Bitaper–Multimode–Single mode Mach–Zehnder Interferometer (ESMBMS MZI). **Figure 6.** Schematic diagram of the Etched Single mode–Multimode–Bitaper–Multimode–Single mode Mach–Zehnder Interferometer (ESMBMS MZI).

**Figure 7.** Schematic diagram of the ESMBMS MZI with the position of the bitaper set to one third of the sensing arm. **Figure 7.** Schematic diagram of the ESMBMS MZI with the position of the bitaper set to one third of the sensing arm.

When the sensing arm was etched, the diameter of the interference arm decreased,

The etching solution often used in the optical fiber chemical etching method is a hydrofluoric acid solution. The cladding of optical fibers is generally made of silicon dioxide (SiO2) material. Hydrofluoric acid dissolves silica mainly for the following two reasons: First, hydrofluoric acid solution contains fluoride ions, which combine with silicon ions in SiO2 to form complex ions. Second, the hydrogen ions in the hydrofluoric acid solution form water with the oxygen ions in the SiO2. The reaction equation can be expressed as

SiO2 **+** 6HF = 2H3O <sup>+</sup> **<sup>+</sup>**Si <sup>2</sup> F6

dish, and connected the broadband light source and the spectrometer at both ends. According to past experience, the corrosion rate of 40% hydrofluoric acid at 23 °C is about 2 µm/min [18,19]. After etching the MZI for 20 min, the corrosion surface was relatively

In the experiment, we first fixed the sensing unit of the interferometer in a plastic

Figure 5 shows the MZI waist-enlarged bitaper before etching, and Figure 8 shows the bitaper after etching. The 40% hydrofluoric acid solution was directly dropped on the optical fiber sensing arm and etched for 20 min. The diameter of the sensing arm of the 40/125 multimode fiber is reduced to 85 µm. The diameter of the fiber bitaper is reduced from 152 µm to 112 µm, and the length of the waist-enlarged bitaper shows little change.

<sup>−</sup> (6)

tive index. The change of the refractive index caused the change of the phase difference of the transmitted light in the fiber, and then the change of the refractive index was measured by measuring the change of the interference spectrum. In this experiment, the sensing arm was used to sense the change of refractive index, which was reflected on the spectrum, that is, the wave valley moved. By measuring the drift of the valley, the change in the

refractive index was measured.

follows:

smooth.

2.3.3. Manufacture

2.3.2. The Principle of Chemical Corrosion

When the sensing arm was etched, the diameter of the interference arm decreased, which increased the coupling degree between the cladding mode and the ambient refractive index. The change of the refractive index caused the change of the phase difference of the transmitted light in the fiber, and then the change of the refractive index was measured by measuring the change of the interference spectrum. In this experiment, the sensing arm was used to sense the change of refractive index, which was reflected on the spectrum, that is, the wave valley moved. By measuring the drift of the valley, the change in the refractive index was measured.

### 2.3.2. The Principle of Chemical Corrosion

The etching solution often used in the optical fiber chemical etching method is a hydrofluoric acid solution. The cladding of optical fibers is generally made of silicon dioxide (SiO2) material. Hydrofluoric acid dissolves silica mainly for the following two reasons: First, hydrofluoric acid solution contains fluoride ions, which combine with silicon ions in SiO<sup>2</sup> to form complex ions. Second, the hydrogen ions in the hydrofluoric acid solution form water with the oxygen ions in the SiO2. The reaction equation can be expressed as follows:

$$\text{SiO}\_2\text{+6HF} = 2\text{H}\_3\text{O}^+\text{+SiF}\_6^{2-} \tag{12}$$

In the experiment, we first fixed the sensing unit of the interferometer in a plastic dish, and connected the broadband light source and the spectrometer at both ends. According to past experience, the corrosion rate of 40% hydrofluoric acid at 23 ◦C is about 2 µm/min [18,19]. After etching the MZI for 20 min, the corrosion surface was relatively smooth.
