Carbon-coated optical glass fibers were designed to reduce the hydrogen-induced light attenuation (hydrogen darkening) in harsh environments. The carbon layer reduces the hydrogen permeability, but makes the post-treatment for direct point-by-point inscription of fiber Bragg gratings (FBG) more challenging. Moreover, the laser pulse treatment can greatly affect the tensile strength of the fiber. In this paper, we present direct point-by- point inscription of FBG through carbon-coated fiber layers with femtosecond laser pulses with a center wavelength of 400 nm, achieved via second harmonic generation of Ti:sapphire laser pulses. An array of four FBGs are successfully inscribed in a single-mode fiber. A direct comparison between the 400 nm and 800 nm inscribed FBGs in the carbon fibers is presented. The polarization dependence was examined of FBGs written with both laser processing wavelengths, as well as the mechanical stability of the processed fibers via tensile tests.
We present a novel planar polymer ridge waveguide evanescent sensor for lab-on-a-chip applications. The integrated sensor is based on post-processing of Bragg gratings for the near-infrared (NIR) wavelength range by applying a femtosecond laser point-by-point (pbp) inscription technique. In general, this pbp inscription method offers a flexible selection of the Bragg grating wavelength from UV to NIR. The optical evanescent field sensor was tested with different substances with different refractive indices, demonstrating an increased sensitivity. Finally, the sensor was coated with palladium nanoparticles. With this functional coating, the polymer Bragg grating sensor is capable of hydrogen detection up to 4% concentration.
We present a femtosecond laser written, apodized chirped fiber Bragg grating (acFBG) used for dispersion control inside picosecond all-fiber lasers. A fiber fixation setup enables a plane-by-plane (pbp) written acFBG in a standard, polarization-maintaining fiber by applying a beam-slit configuration. The spectral specifications of the acFBG are examined in detail, and the grating is validated inside a mode-locked fiber laser oscillator. This letter provides a route to the fast prototyping of acFBGs with customized parameters for use as dispersion compensating elements inside ultrafast all-fiber lasers.
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