Bragg grating fabrication in microfiber by femtosecond pulse filamentation induced periodic refractive index modification

Abstract

Fabrication of fiber Bragg grating (FBG) in single mode microfiber (core diameter: 3.75 μm, cladding diameter: 40 μm) by femtosecond laser pulse radiation is presented. Femtosecond pulse filamentation technique is employed in the pointby-point writing method to inscribe single shot periodic index modification in the core of the microfiber. Prior to writing gratings, a short length (~1.5 mm) of microfiber is fusion spliced between two standard single mode fibers (SMF) in order to improve handling and ease grating fabrication. The kilohertz femtosecond laser pulses operating at center wavelength of 800 nm were tightly focused with an objective lens (40X/ NA=0.75) to confine the pulses into a very tiny focal volume and spatially control index modification. The focused femtosecond pulses create filamentary voids at focal point. For the scanning speed of 534 nm/ Sec, the partial overlapping of void structures produces a periodic index modification in the core with a period of 534 nm and constructs the Bragg reflection spectrum centered at 1550.216 nm. Fabrication of a 1 mm long FBG takes less than 2 seconds for the scanning speed of 0.534 mm/sec. The spectral position of Bragg reflection spectrum can easily be tailored simply by changing pulse scanning speed. The performance analysis of the FBG is examined for temperature and axial strain sensitivity. The grating sensor exhibits the temperature and strain sensitivity of 10 pm/ °C and ~1 pm/ micro-strain, respectively.