Bragg grating-based Fabry–Perot interferometer fabricated in a polymer fiber for sensing with improved resolution

Abstract

We demonstrate for the first time a Bragg grating-based Fabry–Perot interferometer (FPI) fabricated in the polymer fiber with a core made of PMMA/PS copolymer and pure PMMA cladding. The FPI was formed by two gratings with the same Bragg wavelength, λB = 1312 nm, separated by a small gap. The FP cavity was created directly during the grating inscription process by placing a narrow blocking aperture, in the center of the UV beam. Good long-term stability was achieved by fabricating the gratings of type II with long irradiation time (8 min). By choosing an appropriate width of the blocking aperture, we could control the number and the width of interference fringes visible in the grating’s reflection spectrum. Sharp fringes were obtained, of 3 dB width within the range of 50–100 pm, allowing for a significant increase in measurement resolution compared to direct interrogation of a single grating. The proposed interferometer was tested in measurements of strain and temperature in the range of 0–20 mstrain and 20 °C–70 °C, showing the sensitivity of 1.074 nm mstrain−1 and −25.1 pm °C−1, respectively.

Phase-Shifted Bragg Grating Inscription in PMMA Microstructured POF Using 248-nm UV Radiation

“…In recent years, the polymer optical fibe Bragg gratings (POFBGs) fabrication technology has been intensively established using uniform FBGs [16]- [19], tilted FBGs [20], chirped FBGs [21], and FBGbased Fabry-Perot interferometers [22]. Specifi material properties, such as low Young's modulus (about 3 GPa compared to 72 GPa for silica), a very large breaking strain, as well as biological compatibility, opens a variety of new applications unattainable for silica fibers In the published literature, one can fin studies reporting on applications of single FBGs in POFs as sensors of strain [23], acoustic waves [24], temperature [23], [25], humidity [25], accelerometer [26] or liquid level [27], [28].…”

Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

“…[42], when two separate and identical Bragg gratings are written in the fiber core, with a small gap between, they form an FP structure. This structure manifests unique properties of having two spectral peaks within its main reflection band [22] and has better measurement resolution than a single direct grating [43]. The structure of a grating-based FP in a standard fiber (125 µm cladding diameter), is shown in Figure 1, which consists of two identical uniform Bragg gratings of length g, separated by a distance e. Following are the coupled mode equations derived using the coupled mode theory [44], to study the behavior of the grating and related FP structure.…”

Study of the Vernier Effect Based on the Fabry–Perot Interferometer: Methodology and Application

“…In recent decades, fiber optic FPI sensors have been studied for their excellent performance benefits, such as low cost, fast response, anti-electromagnetic interference ability, and durability in harsh environments [5][6][7][8]. The structure of the fiber optic FPI sensor can be grouped into two categories: (i) an intrinsic Fabry-Perot interferometer (IFPI), where the light is reflected by the reflector inside the fiber, a typical example is how the IFPI can be formed by a pair of Bragg gratings separated by a small gap [9], and (ii) an extrinsic Fabry-Perot interferometer (EFPI), where the light exits the fiber and propagates inside the external cavity. Such an external cavity can be made of polymer or an air cavity encapsulated by a diaphragm [10,11].…”