Enhanced refractive index and temperature sensor based on balloon-shaped Mach-Zehnder interferometer

Highlights

• The spherical structures were added on the basis of balloon-shaped Mach-Zehnder interferometer (MZI), which can effectively excite higher order cladding mode. Thus, the performance of the sensor is enhanced.

• The sensor we proposed has high reliability and sensitivity, which are suitable for most practical sensing applications.

• The sensor we proposed has more obvious dips than the traditional balloon-shaped fiber sensors. Moreover, it has a more stable structure compared with the traditional balloon-shaped MZI.

Abstract

We demonstrated a compact fiber sensor for the Refractive Index (RI) and temperature measurement based on a balloon-shaped Mach-Zehnder interferometer (MZI) with no need of special fibers or coating sensitizer material. To enhance the sensitivity and structural stability of the sensor, a spherical structure is added on both sides of the balloon-shaped single mode fiber (SMF). The fiber length between the two spherical structures is L, three kinds of spherical MZI with L of 3 cm, 3.5 cm and 4 cm were prepared, and their sensing performance was studied. The experimental results show that the RI and temperature sensitivity of the sensor increase with the increase of L. We get maximum RI sensitivity of 965.1 nm/RIU (refractive index unit) in the RI range of 1.3350–1.3965 and a temperature sensitivity of 178.6 pm/°C in the temperature range of 30–80 °C. Compared with other balloon-shaped sensors, the proposed sensor has higher sensing sensitivity and stronger structural stability, which is conducive to promoting the development of balloon-shaped MZI to satisfy practical demands.

Introduction

Compared with the traditional electrical sensors, optical fiber sensors have the benefit features such as corrosion resistance, electromagnetic interference resistance, small size, high sensitivity and so on [1], [2], [3], [4]. They have been extensively used in temperature [5], RI [6], transverse load [7], humidity [8], biochemical [9]and other physical parameters measurement fields. In recent years, many different types of MZI sensors have been proposed. Such as the S-shaped single-mode fiber cone encapsulated in a capillary containing a liquid with high thermal and optical coefficient can be used for high sensitivity temperature sensing [10]. The SMF is roasted into a U-shaped interferometer which can be used for sensing temperature and RI simultaneously [11], [12]. A conical hollow fiber with a taper region is fused between two SMFs to form the MZI fiber strain sensor. The sensor has excellent stability and repeatability [13]. MZI all-fiber RI sensor can be fabricated by welding a dual-waist photonic crystal fiber with two single-mode fibers. The high-order cladding modes are greatly excited in the first waist region and then propagate along the cladding to the second waist region and return to the fiber core and interfere with the core mode [14]. MZI can also be formed by bending a tapered multimode fiber between two SMFs into a balloon shape. Sensitivity is significantly improved by taking advantage of tapering and bending [15]. And the optical fiber spherical structure is also suitable for sensing [16], the MZI RI sensor can be constructed by a side-polished optical fiber sandwiched between two spherical structures [17]. In addition, combining fiber bragg grating (FBG) with bent fiber structure can be used for simultaneous measurement of temperature and RI [18], [19]. The sensor, which bends standard SMF into the shape of a balloon, is simple in structure and sensitive to both temperature and RI [20], [21]. However, the balloon-shaped structure usually has the problem of light leak out of the cladding, resulting in insignificant interference effect and low sensitivity. Moreover, the transmission spectrum of the traditional balloon-shaped MZI usually appears when the bending is relatively small, which leads to the easy fracture of the fiber.

In this article, we demonstrated a compact fiber sensor for the RI and temperature measurement based on a balloon-shaped MZI, and we added two spherical structures based on the balloon-shaped MZI sensor. The spherical structure of the fiber is conducive to excite the high-order cladding mode. In the balloon-shaped MZI with spherical structure, the fiber does not need to be bent too small to appear obvious interference spectrum, which is conducive to increase the sensitivity and structural stability of the balloon-shaped sensor. The experimental results show a maximum RI sensitivity of 965.1 nm/RIU in the RI range of 1.3350–1.3965 and a temperature sensitivity of 178.6 pm/°C in the temperature range of 30–80 °C. Because this sensor enhances the inter-mode interference, the RI and temperature sensitivity are increased about four and seven times respectively by compared with the previously reported fiber RI and temperature sensors [20]. The sensor we demonstrated in this article has the advantages of simple fabrication, low cost, high reliability, good linearity, which are beneficial to the actual RI and temperature measurement.