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

Abstract

Abstract-In this work, we experimentally validate and characterize the first phase-shifted polymer optical fiber Bragg gratings (PS-POFBGs) produced using a single pulse from a 248-nm krypton fluoride laser. A single-mode poly (methyl methacrylate) optical fiber with a core doped with benzyl dimethyl ketal for photosensitivity improvement was used. A uniform phase mask customized for 850-nm grating inscription was used to inscribe these Bragg structures. The phase shift defect was created directly during the grating inscription process by placing a narrow blocking aperture in the center of the UV beam. The produced high-quality Bragg grating structures, presenting a double dips, reject 16.3 dB (97.6% reflectivity) and 13.2 dB (95.2% reflectivity) of the transmitted power, being therefore appropriate for sensing or other photonic applications. Its transmission spectrum possesses two sharp transmission notches, allowing a significant increase in measurement resolution compared to direct interrogation of a single grating. The reflection and transmission spectra when multiple phase shifts are introduced in the fiber Bragg grating structure are also shown. The PS-POFBG's strain, temperature, pressure, and humidity characteristics have been experimentally analyzed in detail to assess their potential usage as sensors.

Inscription of Bragg gratings in undoped PMMA mPOF with Nd:YAG laser at 266 nm wavelength

“…However, until now, PMMA remains the preferred material for POFs composition. Moreover, different grating structures such as chirped [18][19], tilted [20], phase-shift [21] were investigated for sensing applications with optimal characteristics. Due to the low photosensitivity of PMMA material, different kinds of dopants were used to improve the photosensitivity of the POF, such as benzyl dimethyl ketal (BDK) [22][23], or trans-4-stilbenemethanol (TS) [24] with the increase of the transmission loss as the main drawback.…”

Latest Achievements in Polymer Optical Fiber Gratings: Fabrication and Applications

“…The obtained grating showed 0.035 nm bandwidth and 8 dB in the rejection band, with a high level of flexibility and no need for strain accuracy. At the 850 nm wavelength region, PS FBG was also obtained directly during the grating fabrication, by placing a narrow blocking aperture in the center of the UV beam (see Figure.6b) [107]. A high-quality Bragg grating structure was obtained with -16.3 dB and -13.2 dB dips in transmission.…”

“…Dispersion compensation using chirped POFBG was first proposed in 2005 [107], but the experimental demonstration of a largely tunable dispersive device based on a chirped FBG in mPOF was published by Min et al [115]. Figure 10 depicts the reflected spectral power with a bandwidth from 0.11 to 4.86 nm, as well as the group delay variation under strain, which corresponds to a tunable dispersion from 513.6 to 11.15 ps/nm, respectively, with potential applications in both optical communications and microwave photonics systems.…”

“…PS-FBG fabrication by 248 nm irradiation: (a) Moiré overlapping method (image adapted from[106]); (b) a narrow blocking in the center (image adapted from[107]). …”

Largely tunable dispersion chirped polymer FBG

“…Since the first fiber Bragg grating (FBG) in a POF was reported in 1999 [17], FBGs in a POF have become attractive devices for optical communication and sensing. Some examples include phase-shifted FBGs [18], tilted FBGs [19], and FBGs in low loss cyclic transparent amorphous fluoropolymers (CYTOP) POFs [20,21], as reported in recent literature. CFBGs in a POF were theoretically proposed for a tunable dispersion in 2005 [22] by applying both the strain and temperature, showing a potential large dispersion tuning range from 110 to 2400 ps/nm.…”