Dual Mach–Zehnder Interferometer Based on Side-Hole Fiber for High-Sensitivity Refractive Index Sensing

Abstract

A unique type of refractive index (RI) sensor is proposed using a dual Mach-Zehnder interferometer (MZI) structure based on side-hole fibers (SHFs). The MZI structure contains two single-mode fibers (SMFs), two coreless fiber (CLF) sections and an SHF section, which are spliced together in the order of SMF-CLF-SHF-CLF-SMF. The SMFs and the CLFs enable light lead in/out and beam splitting/combining, respectively. As a special feature of the structure, one hole of SHF is exposed for liquid filling and to form two MZIs as well. Three types of sensors are fabricated, namely S1, S2 and S3. Numerical simulation and experimental studies have been conducted to characterize the sensing performance. The RI sensitivity of the S1 using the ∼550 μm long SHF section reaches 14,000 nm/RIU. When a shorter SHF section is used in S2, the detectable RI range is broadened due to larger FSR. When the closed hole of SHF in S3 is filled with liquid to introduce the Vernier effect, the sensitivity can be further enhanced to over 44,000 nm/RIU (i.e., Refractive Index Unit), which corresponds to the detection limit at the level of 1.0 × 10 −5 RIU. This sensor design is original and easy to package, which gives it potential for label-free biochemical analyses.

Advanced Fiber Sensors Based on the Vernier Effect

“…To produce the Vernier effect, some scholars assemble different typed MZIs together. Similar to the case of FPIs employed to introduce the Vernier effect, the MZIs can be assembled in cascade [ 9 , 43 , 44 , 45 ] or in parallel [ 46 , 47 , 48 , 49 , 50 ]. As depicted in Figure 5 a, for the cascaded inline structures, the two MZIs can be physically connected [ 45 , 46 , 47 , 48 , 49 , 50 , 51 ] or separated by a section of SMF [ 9 , 43 , 44 ].…”

“…Similar to the case of FPIs employed to introduce the Vernier effect, the MZIs can be assembled in cascade [ 9 , 43 , 44 , 45 ] or in parallel [ 46 , 47 , 48 , 49 , 50 ]. As depicted in Figure 5 a, for the cascaded inline structures, the two MZIs can be physically connected [ 45 , 46 , 47 , 48 , 49 , 50 , 51 ] or separated by a section of SMF [ 9 , 43 , 44 ]. For the paralleled structures, MZIs can be assembled by two 3 dB couplers [ 50 ] or integrated into a single fiber by micromachining [ 46 , 47 , 48 , 49 ], shown in Figure 5 b.…”

“…The simplest way to produce the Vernier effect by connecting two MZIs in parallel is to connect MZIs with two 3 dB couplers [ 50 ]. To simplify the structure, the researchers integrated the parallel structure into one fiber [ 46 , 47 , 48 , 49 ]. As a representative, in 2018, Lin et al proposed a gas pressure sensor based on dual side-hole fiber (DSHF) interferometers with the Vernier effect [ 47 ].…”

Temperature Sensor Based on Fiber Ring Laser With Cascaded Fiber Optic Sagnac Interferometers

“…Lots of traditional optical fiber sensor schemes have been proposed over the last twenty years. Temperature sensors are designed and fabricated based upon the principles of fiber Bragg grating (FBG) [1][2][3], Mach-Zehnder interferometer (MZI) [4][5][6], and Fabry-Perot interferometers (FPI) [7][8][9]. Recently, there is a growing interest in all-fiber laser sensors, since it has the advantages of high sensitivity, good stability, low insertion loss and high signal-to-noise ratio.…”

Anomalous Sensitivity Enhancement of D-Shaped Fiber-Based Sandwiched Structure Optofluidic Sensor

“…Side polished D-shaped fiber [5], fiber Bragg grating [6], long-period fiber grating [7] based SPR sensors were mostly used optical fiber-based SPR sensors over the past years. Interferometric based optical fiber sensors were also employed for successful measurement of the chemical and biochemical analyte [8,9]. The fabrication and designing of interferometric sensors are complex and the optimization of the fabrication process is crucial to achieving superior performance.…”