Strongly coupled multicore fiber with FBGs for multipoint and multiparameter sensing

Highlights

• An optical fiber sensor is proposed for multipoint and multiparameter sensing.

• The SMF-SCMCF structure was used as a vibration gauge with high precision.

• Two FBGs are used for multipoint transverse load and temperature sensing.

• Two FBGs and the supermode coupler can be interrogated simultaneously.

Abstract

A compact optical fiber sensor by embedding fiber Bragg gratings (FBGs) in strongly coupled multicore fiber (SCMCF) is proposed for multipoint and multiparameter sensing. To build the device, two FBGs with different peak wavelengths were inscribed in a segment of SCMCF. Then one end of the SCMCF was fusion spliced to a single-mode fiber (SMF) and the other end of the SCMCF was cleaved. In the SMF-SCMCF structure, two supermodes are excited, as a result, the reflection spectrum exhibits a sinusoidal pattern with two sharp peaks. The wavelength position of the FBGs and the supermode coupler can be extracted simultaneously. Two distinct FBGs inscribed in different positions of the SCMCF were used to demonstrate quasi-distributed multipoint sensing in the proposed structure. To the best of our knowledge, this is the first demonstration of an optical fiber sensor that combines FBGs with SCMCF. The sensor here proposed has the advantage of compact size, low-cost, good mechanical strength and ease of interrogation.

Introduction

Over the past decades, optical fiber sensors have been widely used in the fields of civil engineering, mechanical manufacturing, robotics and aeronautical engineering due to its unique characteristics such as immunity to electromagnetic interference, resistant to corrosion, high sensitivity and compact size [1], [2]. So far, various optical fiber sensors have been developed based on different sensing principles, such as long period gratings, fiber Bragg gratings (FBGs), Fabry-Perot interferometers, Mach–Zehnder interferometers, and so on [3], [4], [5], [6]. Among them, FBG-based sensors are one of the most representative and promising optical fiber sensing technique due to their mature manufacturing process and multiplexing capability.

A variety of FBG-based sensors have been validated in different types of optical fibers, such as single-mode fiber (SMF), multimode fiber, photonic crystal fiber, microstructure fiber, polymer fiber and multicore fiber (MCF) [7], [8], [9], [10], [11], [12], [13], just to mention a few. Compared with other types of optical fiber, MCF comprises more than one core within the same cladding. Consequently, several identical or different sensors can be written at the same position along the length of the MCF, which is beneficial for simultaneous measurement of multiple parameters [14], [15], [16]. Therefore, multicore fiber Bragg grating (MCFBG) sensors have been proposed and successfully validated in a variety of mechanical parameters sensing, such as bending, twist, inclination, acceleration, or 3D shape sensing [17], [18], [19], [20], [21], [22]. For instance, Gander et al. reported the first demonstration of bend sensing using a pair of FBGs written in a two-core MCF [23]. Soon afterwards, a two-axis bend measurement was realized by writing FBGs into three separate cores of a multicore fiber [24]. Apart from single-point measurement, Barrera et al. proposed a multipoint two-dimensional curvature sensor by inscribing arrays of apodized highly reflective FBGs in non-twisted homogeneous four-core MCF [25]. Fender et al. reported a two-axis temperature-insensitive accelerometer by inscribing pairs of FBGs into MCF to measure differential strain [21]. Distributed temperature and 3D shape sensing with high accuracy have been demonstrated by using continuous multicore fiber grating arrays [26]. It is noteworthy that the MCFBG-based sensors exhibit excellent ability to compensate the variation of environmental temperature. This is because all cores are embedded in the same cladding and, consequently, have same temperature response. So far, the majority of MCFBG-based sensors were fabricated by using weakly coupled MCF, where the coupling between neighboring cores can be negligible. In this case, the use of expensive fan-in/out devices to extract sensing information from individual cores is inevitable. Thus, weakly coupled MCFBG sensors tend to complex and more costly.

Recently, a new type of MCF known as strongly coupled multi-core fiber (SCMCF) has attracted research attention. In an SCMCF, the separation between the cores is small enough; hence, the evanescent fields of the guiding cores coupled to each other. Therefore, all cores of the SCMCF participate in the sensing task. An important advantage of SCMCFs is their simple interrogation as it can be carried out with a conventional SMF. This drastically simplifies the interrogation of SCMCF-based sensors. So far, several compact sensors based on SCMCFs have been reported for high-temperature, vibration or strain measurement [27], [28], [29]. However, such sensors are not suitable for multiparameter sensing. To solve this issue, a hybrid structure by combining FBGs with various interferometer has been proven to be an effective and practical method [30], [31], [32].

In this paper, we propose and demonstrate a novel optical fiber sensor for temperature, transverse load and vibration sensing. The proposed sensor is fabricated by inscribing two cascaded FBGs in an SCMCF, which is then spliced to SMF to form a SMF-SCMCF structure. In the latter configuration, two supermodes supported by the SCMCF gives rise to a well-defined sinusoidal pattern. The SMF-SCMCF structure was used as a vibration gauge, while two cascaded FBGs were dedicated to measure transverse load and temperature at different locations. The experimental results demonstrate that two consecutive FBGs can accurately measure the temperature and transverse load at two adjacent positions, while the periodic shift of reflection spectrum can be used to monitor vibration with high precision.