Fabrication of chirped fiber Bragg gratings in a non-uniform single-core As$_{2}$Se$_{3}$-PMMA tapered fiber

Abstract

A chirped fiber Bragg grating (FBG) is fabricated using a non-uniform single-core As2Se3-PMMA tapered fiber that is pre-stretched by 0.52 mm during the inscription process. The inscription is based on a standing wave formed by two counter-propagating continuous-wave lights in the stretched non-uniform single-core As2Se3-PMMA taper. A periodical refractive index change by the standing wave in the high photosensitivity As2Se3 core is induced, which creates a FBG along the taper. A strain gradient is formed along the grating due to the non-uniform structure when the elongation is changed, which produces a varying shift in the Bragg spacing at different position along the taper leading to a tunable chirped grating. The central wavelength and bandwidth are linearly dependent on the strain change applied to the chirped grating. The wide tuning range is attributed to the low stiffness of the chalcogenide core and PMMA cladding. An exponential dependence of the amplitude of the chirping spectrum on the wave number is derived, which shows the potential for applications such as weak forces detections. Furthermore, long chirped FBGs can be easily inscribed using standing waves in As2Se3-PMMA tapered fibers, which is far beyond the practical length of phase masks and is desirable for chromatic dispersion compensation in high-bit-rate optical fiber transmission systems and distributed sensors. A 50 cm long chirped FBG with chirped wavelength range of ∼15 nm is inscribed and characterized. Fabrication of chirped FBGs in non-uniform single-core As2Se3-PMMA tapers opens the path towards the realization of novel sensors and devices.