**1. 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 [1–16], displacement [2], strain [3,4,8], tilt angle [13–16], refractive index (RI) [5–7,9,11] 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 [13–15]. 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

**Citation:** Lee, C.-L.; Chen, C.-S.; Yang, C.-R.; Zeng, R.-C. NOA61-Polymer Fiber Fizeau Interferometer with a Flexible NOA65-Polymer Taper for Simultaneous Measurement of Tilt Angle and Temperature. *Polymers* **2021**, *13*, 2798. https://doi.org/ 10.3390/polym13162798

Academic Editor: Jung-Chang Wang

Received: 1 August 2021 Accepted: 18 August 2021 Published: 20 August 2021

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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 NOA65 polymer 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 NOA61 polymer 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−<sup>4</sup> ( ◦C<sup>−</sup>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 (Δλ) 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 Δλ 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.
