**2. Sensor Fabrication and Principle**

The configuration of the PT/PFFI sensor is based on a PFFI formed by a T-sensitivity NOA61-polymer combined with a NOA65-polymer taper (PT) with good flexibility, as presented in Figure 1.

**Figure 1.** (**a**) Configuration of the proposed sensor with (**b**) taper-shaped NOA65-polymer with L/d = 85 μm/78 μm and (**c**) sensor A with d = 23 μm or (**d**) sensor B with d = 46 μm NOA61-PFFI.

The taper-shaped NOA65 polymer is the main part of the inclinometer that controls the light into the NOA61-PFFI for generating low-finesse interference. Here, the NOA65 (nD = 1.52) [18,19] polymer materials used are based on a type of optical adhesive, ultraviolet (UV)-cured polymer, with good elongation and is more elastic than conventional fiber taper-based tilt sensors [14,15]. In addition, the NOA61-polymer with the high refractive index of nD = 1.56 can produce high FV that is more suitable for this study [10]. The fabrication is monitored by a charge-coupled device (CCD) microscope. Figure 2 shows the fabrication steps of the PFFI combined with PT. The NOA65-polymer taper is simply fabricated with the assistance of three-axis translation stages at exact alignment [17]. After accomplishing the desired taper shape of the PT, the monitored translation stages are used to attach a thick film of NOA61-polymer onto the endface of the SMF (Corning@ SMF-28e) to form the PFFI (Figure 2e). The thickness of NOA61 can be carefully controlled by the number of attachment times, as plotted in Figure 2c,d. Here, Figure 1b shows the structure of the NOA65-PT with the tapered region (L)/waist diameter (w) of L/w = 85 μm/78 μm and Figure 1c,d shows the NOA61-PFFI with a cavity length of d = 23 and 46 μm for sensor A and B, respectively.

**Figure 2.** Fabrication of the PT/PFFI by the processes of (**a**) cleaving fiber flat, (**b**) NOA65 attached to fiber endface, (**c**) two fiber-NOA65 aligned, (**d**) contact and pull into a tapered shape to form the NOA65-PT, and (**e**) sensor endface with NOA61 attached to form the NOA61-PFFI.

Figure 1a also illustrates the principle of the proposed PT/PFFI. When the NOA65- PT is bent due to the tilt, light propagating into the NOA61-PFFI intensely decreases with deviation from the center of the fiber axis to instantly weaken the FV of the optical interferences. Figure 3 displays the experimental setup with a broadband light source (BLS, BLS-GIP Technology) and a 2 × 1 optical coupler, which reflects off the endfaces of the PFFI, and returns to the coupler. Finally, the spectral response readouts are directly measured by an optical spectrum analyzer (OSA, Advantest Q8381 A). The PT/PFFI is located inside a temperature and humidity controlling chamber (THCC, LABSON, No. LA-85R) for varying the T and φ with fixed humidity.

**Figure 3.** Experimental setup for simultaneously measuring φ and T.

The used THCC with temperature (T) accuracy of ~0.2 ◦C and relative humidity (RH) accuracy of ~2% that are applied to control the T and RH. Since the used polymer device can affected by the surrounding humidity [10], all the measurements are accomplished under a fixed RH = 50%. Therefore, reactions of tilt sensing results can be readily obtained by monitoring FV values or optical power of the reflective interference only from signals of the NOA61-PFFI. If the proposed sensor tilts at a fixed T, then the interference power and FV can considerably decay. Moreover, when the sensor is under T variation at a fixed φ, the interference spectra are red-shifted as T increases and are blue-shifted as T decreases. In the reflection spectra, interference signals of two cavities of the NOA65-PT and NOA61- PFFI are superimposed and collected by the OSA. However, the interference signal of NOA65-PT is much weaker than that of the NOA61-PFFI. Analyses of the optical responses from the combined interferences are accomplished using the fast Fourier transform (FFT) method, which is used to separate multiple interferences in spatial frequency into two individual spatial frequencies for the NOA65-PT and NOA61-PFFI. Figure 4 shows the optical responses of the superimposed interference, separating into the spectra through the simple FFT method. The superimposed interference plotted in Figure 4a is processed by the FFT to obtain the spatial frequency spectra, as shown in Figure 4b, indicating that the signal of NOA61-PFFI is higher than that of the NOA65-PT. Subsequently, the spatial frequencies for the NOA65-PT and NOA61-PFFI can be individually separated by the inverse FFT with signal processing, as plotted in Figure 4c,d respectively. In this study, only the interference signals of NOA61-PFFI must be measured to achieve multiple parameters sensing.

**Figure 4.** (**a**) Optical response of superimposed interference measured by the OSA; (**b**) superimposed spectra processed by FFT; separated optical spectra of (**c**) NOA61-PFFI and (**d**) NOA65-PT.
