Ultrasensitive and Wide-Measuring-Range Refractive Index Sensor With Optical Path Length-Tunable Vernier Effect

The Vernier effect is used to enhance the sensitivity of interferometric fiber-optic sensors. However, the optical path length (OPL) matching condition of the Vernier effect is subjected to machining errors and large variations in the OPL sensing. Due to the former, achieving high sensitivity is challenging, while the latter renders high sensitivity to be ineffective for a wide measurement range. Thus, this study proposes to realize an ultrahigh refractive index (RI) sensitivity using an OPL-tunable Vernier effect, regardless of the interference of the machining errors. Furthermore, it can maintain consistency and high sensitivity for different measuring media in a wide detection range. The proposed sensor comprises a sensing Fabry–Pérot interferometer (SFPI) and a tunable reference FPI (RFPI). The OPL matching condition of the proposed sensor was controlled by adjusting the cavity length of the RPFI. RI sensitivities of 26 160, 25 985, and 26 260 nm/RIU were achieved using RIs of 1.33, 1.55, and 1.65, respectively. The proposed method overcomes the poor linearity of the RI sensitivity of the Vernier effect sensor in a wide detection range.

I. Introduction

Recently, studies have widely investigated fiber-optic refractive index (RI) sensors in the field of environmental protection [1], [2], food safety [3], [4], and medical diagnosis [5], [6]. The development of fiber sensors with high sensitivity and linearity in a wide detection range has attracted increasing attention [7]. Fiber-optic Fabry–Pérot interferometer (FPI) sensors are widely used during RI measurements because of their simple configuration and easy fabrication [8]. The RI sensitivity of the FPI sensor is relatively constant (approximately 1100 nm/RIU) as it is subjected to the intrinsic principle of FP interference [9]. Mechanisms such as metamaterial absorption [10] and the Vernier effect [11] have been applied to the FP structure to enhance its sensitivity. Moreover, the Vernier effect can be used to enhance the sensitivity of FPI sensors. It detects the drift of the envelope, instead of high-frequency fringes of the sensor spectrum, to improve sensitivity [11]. The key to achieving a Vernier-effect FPI is to combine two FPIs that have sufficiently close optical path lengths (OPLs). The first FPI is called a sensing FPI (SFPI) for sensing; the other is a reference FPI (RFPI) for providing a reference spectrum to ensure an OPL matching. The reported combined forms of two FPIs include cascaded [11], [12], [13] and parallel FPIs [14], [15], [16]. Recently, improved Vernier effects such as enhanced Vernier effect [17] and higher-order harmonic Vernier effect [18] have been proposed to further improve sensitivity. However, the Vernier effect has a difficult OPL matching condition: the OPLs of the SFPI and RFPI need to be close but not the same. According to this matching principle, the magnification () factor of the sensitivity can be defined as follows [11]:

$$\begin{equation*} M=\frac {n_{1} L_{1}}{\vert n_{1} L_{1} -n_{2} L_{2} \vert }=\frac {n_{1} L_{1}}{\Delta } \tag{1}\end{equation*}$$ where , , and ( = 1, 2) are the cavity length, RI, and OPL, respectively, of SFPI and RFPI. is defined as the OPL mismatch factor between SFPI and RFPI. Equation (1) demonstrates that the smaller the , the higher will be the [Fig. 1(a)]. However, in practice, it is not easy to achieve a high for a Vernier-effect FPI because of several factors affecting the realization of high sensitivity. Fig. 1.

(a) Relationship between and OPL mismatch factors. During the simulation, . (b) Relationship between factor and measured RI. During the simulation, and .