Highly reflective Bragg gratings in slightly etched step-index polymer optical fiber

Abstract

During the past few years, a strong progress has been made in the photo-writing of fiber Bragg gratings (FBGs) in polymer optical fibers (POFs), animated by the constant wish to enhance the grating reflectivity and improve the sensing performances. In this paper, we report the photo-inscription of highly reflective gratings in step-index POFs, obtained thanks to a slight etching of the cladding. We demonstrate that a cladding diameter decrease of ~12% is an ideal trade-off to produce highly reflective gratings with enhanced axial strain sensitivity, while keeping almost intact their mechanical resistance. For this, we make use of Trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. FBGs are inscribed at ~1550 nm by the scanning phase mask technique in POFs of different external diameters. Reflectivity reaching 97% is achieved for 6 mm long FBGs, compared to 25% for non-etched POFs. We also report that a cladding decrease enhances the FBG axial tension while keeping unchanged temperature and surrounding refractive index sensitivities. Finally and for the first time, a measurement is conducted in transmission with polarized light, showing that a photo-induced birefringence of 7 × 10(-6) is generated (one order of magnitude higher than the intrinsic fiber birefringence), which is similar to the one generated in silica fiber using ultra-violet laser.

Intrinsic High-Sensitivity Sensors Based on Etched Single-Mode Polymer Optical Fibers

“…Though many researchers have fabricated sensors based on etched silica and polymer fibers, the impact of etching on its sensing capabilities are not investigated yet [5], [11]. Hu et al [12] reported that a cladding diameter decrease of 12% via etching is an ideal trade-off to produce highly reflective gratings by using Trans-4-stilbenemethanol-doped photosensitive step-index poly (methyl methacrylate) (PMMA) POFs. However the change in material properties due to etching and its influence on the sensors are still unknown.…”

Fast and stable gratings inscription in POFs made of different materials with pulsed 248 nm KrF laser

“…With this laser, the writing time in POFs was ranging from few to hundreds of minutes depending on the polymer material the fibers were made from, the inscription power, the structure of the fibers, and so on [8,19,21,22]. These inscription times can be significantly reduced by doping the fiber, which can be made with trans-4-stilbenemethanol (TS) that can also increase the grating reflectivity [28,29]. Benzyl dimethyl ketal (BDK) PMMA POFs also demonstrated inscription times as short as 4 minutes with a HeCd CW laser [30] and to 40 seconds with a femtosecond laser [31] or even to 1 pulse using a KrF pulsed laser [4].…”

Surface plasmon excitation at near-infrared wavelengths in polymer optical fibers

“…There are also some theoretical studies about SPR excitation with microstructured POFs [4,5]. In the meantime, the manufacturing of fiberBragg gratings (FBGs) in POFs has matured to such a degree that it is now possible to obtain deep refractive-index modulations with good uniformity and repeatability [6][7][8][9][10].…”