NOA61-Polymer Fiber Fizeau Interferometer with a Flexible NOA65-Polymer Taper for Simultaneous Measurement of Tilt Angle and Temperature

This study proposes and experimentally demonstrates a NOA61-polymer fiber Fizeau interferometer (PFFI) connected to a flexible NOA65-polymer taper (PT) for simultaneous measurement of tilt angle and temperature (T). The PT/PFFI fiber sensor consists of a taper-shaped flexible NOA65 polymer and single-mode fiber with an endface that is attached to a NOA61-polymer. The NOA61-polymer of PFFI is highly sensitive to variations of T with high repeatability and enables the simultaneous measurement of tilt angle by connecting with the highly flexible NOA65-PT. the interference fringe visibility of optical spectra in the PFFI can be highly controlled by the tilt angle of the PT and is thus capable of measuring tilt angles with high sensitivity. On the other hand, wavelength shifts of the spectra in the PFFI only occur when T varies. The proposed PT/PFFI can simultaneously detect the tilt state and the variation of surrounding T by measuring the optical spectral responses and eliminating cross sensitivity. Experimental results demonstrate the PT/PFFI can simultaneously measure tilt angles and T with good sensitivities and obtain averages of 0.4 dB/° and 0.17 nm/°C, respectively.


Introduction
Distributed and multiplexed capability of fiber optic sensors are the most important advantages that can integrate various passive optics sensors for multiple parameters and region-sensing applications. Therefore, advanced fiber optic sensors with the ability of simultaneously sensing multi-parameters have recently received much attention. These sensors can simultaneously measure any of the following parameters, pressure [1], temperature [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], displacement [2], strain [3,4,8], tilt angle [13][14][15][16], refractive index (RI) [5][6][7]9,11] and humidity [10,12]. Among the sensing parameters mentioned above, the tilt angle (φ) of structure and temperature (T) of the surroundings have important utility values in green buildings and health bridges, and therefore, attract much research interest. In achieving multiplex sensing capability, the key issue for practical applications of fiber optic sensors is not only cross sensitivity but also the complexity of sensing configurations. Thus far, several fiber optical sensors for simultaneous measurement of the tilt angle and temperature are proposed and investigated, but most are incorporated with well-known fiber Bragg gratings (FBG)-based devices [13][14][15]. For example, a simple fiber optic inclinometer is developed based on linearly chirped FBG written in both fused taper transitions [13]. Similarly, the tilt fiber sensor based on a taper-shaped polymer incorporating a FBG is proposed [14]. Simultaneous measurement of 2D tilt angles and temperature by a fiber optic sensor is also proposed and experimentally demonstrated. The sensing head consists of two FBGs to achieve various sensing performances [15]. The experimental results of the 2 of 10 above tilt angle fiber sensors show that their proposed sensing configuration responds well to the tilt angles, but the sensitivity and resolution of T responses may not be sufficiently recognizable due to the low thermal expansion coefficient of the fused fibers. In 2016, Feng et al. reported a fiber inclinometer that consists of a micro-fiber taper followed by an air-gap microcavity. The fringe contrast of the interferometer is highly sensitive to fiber bending and is thus capable of measuring tilt angles. However, the proposed fiber sensor cannot simultaneously measure T, to which the fused silica fiber is T insensitive [16]. In 2020, a novel, highly sensitive, and simple structure based on a tapered polymer as a tilt fiber sensor is developed to sensitively measure tilt angles [17]. The fiber-optic tilt sensor consists of a tapered polymer fabricated by a flexible adhesive NOA65-polymer with low modulus (20,000 psi) from Norland Products Inc. (Cranbury, NJ, USA) [18]. The NOA65polymer taper easily generates a bend in the fiber sensor that is strongly correlated with tilt angles. Experimental results of the above sensors show that the sensing configuration responds well to the tilt angles, but the sensitivity and resolution of the T responses may be not sufficiently recognizable.
In this study, we connect a T-sensitive NOA61-polymer fiber Fizeau interferometer (PFFI) with a flexible adhesive NOA65-polymer taper (PT) for simultaneous sensing of tilt (φ) and temperature (T). The proposed PT/PFFI consists of a tapered polymer made with NOA65 and the endface of a single mode fiber (SMF) attached to a UV-cured NOA61polymer to form an ultracompact microcavity. The NOA61-polymer in the PFFI is highly sensitive to T variations with high repeatability, and achieve an instantaneous measurement of φ by connecting a flexible NOA65-PT [19]. The NOA65-PT polymer is also T-sensitive with a thermal expansion coefficient (TEC) of 2.2 × 10 −4 ( • C −1 ) is similar to that of the NOA61 [20]. However, the NOA65-PT is the part of the inclinometer that merely controls the light into the NOA61-PFFI for creating interference. Therefore, the thermal expansion effect on the NOA65 is ignorable for the optical interference from NOA61-PFFI. Variation of fringe visibility (FV) and wavelength shifts (∆λ) of the spectral interference of the sensor correspond to responses of the φ and T, respectively. The interference fringe always remains unshifted during the fiber tilts, but optical power and FV showed considerable changes. On the other hand, the interference fringe shifts when T varies while the optical power of reflection is almost unchanged at a fixed φ. Thus, the proposed PT/PFFI fiber sensor can detect the tilt states and discriminate the variation of surrounding T by monitoring the FV and ∆λ removing the cross-sensitivity of φ and T. The experimental results demonstrate that the developed sensor can measure φ and T simultaneously with good measurement sensitivities and averages of 0.4 dB/ • and 0.17 nm/ • C, respectively.

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.   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 (n D = 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 n D = 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   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.
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 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.
Polymers 2021, 13, x FOR PEER REVIEW 4 of 10 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.  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.
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.

Experimental Results and Discussion
In the experiment, the proposed PT/PFFI device is placed inside the THCC, a closed space in which the tilt angle is operated from ϕ = -10°~ +10° with a step of 1° under fixed T and fixed relative humidity of 50%. Figure 5 shows the results. For the sensor with d = 46 μm, Figure 5a,b show the spectral responses to positive ϕ and negative ϕ, respectively.

Experimental Results and Discussion
In the experiment, the proposed PT/PFFI device is placed inside the THCC, a closed space in which the tilt angle is operated from φ = -10 •~+ 10 • with a step of 1 • under fixed T and fixed relative humidity of 50%. Figure 5 shows the results. For the sensor with d = 46 µm, Figure 5a,b show the spectral responses to positive φ and negative φ, respectively. These experimental results are almost similar due to the central symmetry of the PT structure. Figure 5c displays variations of FV of reflection for sensors A and B as the φ changes from −10 •~+ 10 • . When the φ of the sensor increases, the FV has high attenuation. Moreover, at high angles, the FV gradually weakens and then vanishes because the optical light almost leaks out in the great bending of PT. When different sensing structures of sensors A and B are used (as shown in Figure 1), the responses and sensitivities of φ variation from −10 •~+ 10 • are obtained and shown in Figure 5c,d, respectively. Sensor A (d = 23 µm) with thinner PT seems to be more sensitive than sensor B (d = 46 µm). However, the tilt measurement range is relatively small for sensor A. The tilt sensitivity S is defined as: These experimental results are almost similar due to the central symmetry of the PT structure. Figure 5c displays variations of FV of reflection for sensors A and B as the ϕ changes from −10°~+10°. When the ϕ of the sensor increases, the FV has high attenuation. Moreover, at high angles, the FV gradually weakens and then vanishes because the optical light almost leaks out in the great bending of PT. When different sensing structures of sensors A and B are used (as shown in Figure 1), the responses and sensitivities of ϕ variation from −10°~+10° are obtained and shown in Figure 5c,d, respectively. Sensor A (d = 23 μm) with thinner PT seems to be more sensitive than sensor B (d = 46 μm). However, the tilt measurement range is relatively small for sensor A. The tilt sensitivity S is defined as:  Figure 5d shows that high responses are achieved from tilt angles of ϕ = −10°~+10° and sensors A and B obtain the highest sensitivities of 0.72 dB/° at ϕ of ±2° and 0.6 dB/° at ϕ of ±2.7°, respectively. It is worth mentioning that in the tilt sensing, the interference power and FV highly vary but the wavelength peaks of optical interferences are nearly unshifted, as shown in Figure 6a. The inset of Figure 6a displays the detailed optical interference spectra for the variations of corresponding parameters ϕ at T = 25 °C. On the other hand, when the sensor is heating and cooling with T, the range is 20~45 °C at a fixed  Figure 5d shows that high responses are achieved from tilt angles of φ = −10 •~+ 10 • and sensors A and B obtain the highest sensitivities of 0.72 dB/ • at φ of ±2 • and 0.6 dB/ • at φ of ±2.7 • , respectively. It is worth mentioning that in the tilt sensing, the interference power and FV highly vary but the wavelength peaks of optical interferences are nearly unshifted, as shown in Figure 6a. The inset of Figure 6a displays the detailed optical interference spectra for the variations of corresponding parameters φ at T = 25 • C. On the other hand, when the sensor is heating and cooling with T, the range is 20~45 • C at a fixed φ = 0 • . The interference is red-shifted as T increases and vice versa. Figure 6b plots the T-sensitivity of the proposed PT/PFFI sensor. The detailed optical interference spectra for heating and cooling are shown in the Figure 6c,d, respectively. The results indicate that the optical responses have high repeatability, and the sensitivity is linearly proportional to T with 0.17 nm/ • C. The obtained results of the T sensitivity may not be high; however, the optical power of the spectral responses almost does not decay that can easily distinguish non-tilt variations. Based on the above sensing results, the proposed PT/PFFI can demonstrate the superiority of sensing characteristics to avoid the cross-sensitivity of the measured parameters. ϕ = 0°. The interference is red-shifted as T increases and vice versa. Figure 6b plots the Tsensitivity of the proposed PT/PFFI sensor. The detailed optical interference spectra for heating and cooling are shown in the Figure 6c,d, respectively. The results indicate that the optical responses have high repeatability, and the sensitivity is linearly proportional to T with 0.17 nm/°C. The obtained results of the T sensitivity may not be high; however, the optical power of the spectral responses almost does not decay that can easily distinguish non-tilt variations. Based on the above sensing results, the proposed PT/PFFI can demonstrate the superiority of sensing characteristics to avoid the cross-sensitivity of the measured parameters. The effectiveness for the developed sensing configuration is examined by simultaneously varying ϕ and T from their reference values of ambient (set as ϕ0 = 0° and T0 = 20 °C) to various conditions, as shown in Figures 7 and 8, respectively. The optical spectra of the initial condition of ϕ0 = 0° and T0 = 20 °C as the reference for the test sensor B are recorded at the beginning of the experimental investigations (as blue-dashed lines of Figures 7 and 8). Figure 7 presents the simultaneous measurement of the arbitrarily chosen ϕ and T for many cases. Clearly, the variation of the FV is only due to changes of ϕ and the wavelength shift (Δλ) is merely affected by T variation. The parameters of ϕ and T can The effectiveness for the developed sensing configuration is examined by simultaneously varying φ and T from their reference values of ambient (set as φ 0 = 0 • and T 0 = 20 • C) to various conditions, as shown in Figures 7 and 8, respectively. The optical spectra of the initial condition of φ 0 = 0 • and T 0 = 20 • C as the reference for the test sensor B are recorded at the beginning of the experimental investigations (as blue-dashed lines of Figures 7 and 8). Figure 7 presents the simultaneous measurement of the arbitrarily chosen φ and T for many cases. Clearly, the variation of the FV is only due to changes of φ and the wavelength shift (∆λ) is merely affected by T variation. The parameters of φ and T can be respectively estimated by using the experimental relations of FV = 2.66·exp(−0.0715·φ 2 ) and ∆λ = 0.17·T − 3.565 when these two factors are measured. Figure 8 shows the individual interference spectra of every case to compare with that of the initial φ 0 and T 0 .
The  Figure 8 and also listed in the following Table 1. Here, the second decimal point for the measured ∆λ is an artificial estimation value, so that the accuracy of the measured ∆λ as well as the obtained T is one decimal point.    Figure 8 and also listed in the following Table 1. Here, the second decimal point for the measured Δλ is an artificial estimation value, so that the accuracy of the measured Δλ as well as the obtained T is one decimal point.
The results in Figures 7 and 8 and Table 1 show that the simultaneous sensing of ϕ and T is accomplished. The simultaneously measured values of ϕ and T are very close to the setting values displayed in the THCC and tilt stages. The six setting conditions of (a)  The results in Figures 7 and 8 and Table 1 show that the simultaneous sensing of φ and T is accomplished. The simultaneously measured values of φ and T are very close The flat surface would be effectively cleaved by using a femtosecond laser to improve the measured accuracy [21].

Conclusions
This study proposes a new sensing configuration combining the high T sensitivity and repeatability NOA61-PFFI to the flexible NOA65-PT. Thus, the sensor can achieve the simultaneous measurement of ϕ and T. The experimental results show that the interference FV of the optical spectra is only correlated with ϕ and the wavelength shifts (Δλ) of the interference spectra are only affected by T variations. Consistent results illustrate that by using the proposed sensing scheme with the specific characteristics of the used materials (NOA61 and NOA65 polymers), the fiber device can simultaneously and effectively measure T and ϕ with small errors. Furthermore, the proposed PT/PFFI can independently discriminate the tilt angle state and the surrounding T variations from the measured spectra. The sensing results demonstrate that the PT/PFFI can measure ϕ and T simultaneously with good sensitivities and averages of 0.4 dB/° and 0.17 nm/°C, respectively. The obtained results of the T and ϕ sensitivities may not be high but are comparable with those based on the FBG sensors, however, the advantages of this solution are that it is simple, flexible, easy to fabricate, and does not require an expansive laser system for writing the FBGs.

Conclusions
This study proposes a new sensing configuration combining the high T sensitivity and repeatability NOA61-PFFI to the flexible NOA65-PT. Thus, the sensor can achieve the simultaneous measurement of φ and T. The experimental results show that the interference FV of the optical spectra is only correlated with φ and the wavelength shifts (∆λ) of the interference spectra are only affected by T variations. Consistent results illustrate that by using the proposed sensing scheme with the specific characteristics of the used materials (NOA61 and NOA65 polymers), the fiber device can simultaneously and effectively measure T and φ with small errors. Furthermore, the proposed PT/PFFI can independently discriminate the tilt angle state and the surrounding T variations from the measured spectra. The sensing results demonstrate that the PT/PFFI can measure φ and T simultaneously with good sensitivities and averages of 0.4 dB/ • and 0.17 nm/ • C, respectively. The obtained results of the T and φ sensitivities may not be high but are comparable with those based on the FBG sensors, however, the advantages of this solution are that it is simple, flexible, easy to fabricate, and does not require an expansive laser system for writing the FBGs.