## 3 Experiments and Results
### MZI Probe  
In a real-life application, it is more convenient to use a probe-type sensor structure. The sensor fabrication method is similar as that illustrated in Section 3.1. The only difference is that only one micro taper is fabricated, and the sandwiched SMF is cut smoothly using a cleaver. A schematic diagram of an MZI probe based on the micro-fiber taper is shown in Figure 7a. The experimental setup of temperature measurement with the probe-type MZI is shown in Figure 7b. The incident light travels through the circulator to the sensor probe. At the end of the fiber sensor probe, due to the Fresnel reflection, the reflected light passes through the circulator again and is detected by an OSA. The advantage of the probe structure is that it can be inserted directly into the object or solution to be tested without being affected by strain. Four MZI probes were made, and the effects of the interference arms on the performance of the sensor probe were investigated using different arm lengths \(B\) = 1, 1.5, 2, and 3 cm. The diameter and micro taper length of the sensor were \(A\) = 80 \(\upmu\)m and \(b\) = 500 \(\upmu\)m, respectively. The corresponding transmission spectra are shown in Figure 8a-d, respectively. As the length of \(B\) increases, the fringe-free spectral range (FSR) decreases, and the extinction ratio of all probes also exceeds 10 dB.  
A probe-type MZI with a length \(B\) of 2 cm was selected to test temperature response. As can be seen from Figure 9a, we measured the stability of the sensor at a constant temperature (200, 500, and 950 \({}^{\circ}\)C), each for 30 min. As shown in Figure 9a, the sensor had a wavelength shift of less than 0.3 nm at 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, indicating the good stability of the sensor. However, when the temperature increased to 950 \({}^{\circ}\)C, the wavelength had a significant blue shift of 13.13 nm within 30 min, indicating that the sensor is not suitable for high temperature measurements. This problem can be addressed by an annealing process [29]. In our experiments, an annealing process at 950 \({}^{\circ}\)C for 2 h was conducted. It can be clearly seen that the wavelength had very small shifts (\(<\)0.3 nm) at 950 \({}^{\circ}\)C after annealing. The temperature response of the sensor after annealing is shown in Figure 9b, with the temperature varying from 89 to 950 \({}^{\circ}\)C. There are three measured curves in Figure 9b, namely the heating circle, the cooling circle, and the heating circle after 2 days. Linear fitting to the three measurement results were conducted, and the results show that temperature sensitivity of the three linear fittings are the same as 0.11 nm/\({}^{\circ}\)C, which is ten times as that of traditional Fiber Bragg gratings (FBG) and a single mode-multimode-single mode fiber [35]. The correlation coefficients \(R^{2}\) of the linear fitting curves are 0.99433, 0.99388, and 0.99211, respectively, showing a good linearity of the results.  
Figure 6: (**a**) Relationship between micro-stain and wavelength. (**b**) Relationship between RI and wavelength.  
Figure 8: The transmission spectra of the proposed probe-type MZI with different \(B\) (interference length); (**a**–**d**) are 1, 1.5, 2, and 3 cm, respectively.  
Figure 7: **(a) Schematic diagram of the probe-type MZI. (b) Experimental setup. The inset figure is the optical microscope image of the probe-type MZI. BBS: Broadband source; OSA: Optical spectrum analyzer; SMF: Single mode fiber.**