Femtosecond Laser Opening of Hollow-Filament Arrays: the Fiber Bragg Grating Opto-fluidic Sensor

In‐Fiber Switchable Polarization Filter Based on Liquid Crystal Filled Hollow‐Filament Bragg Gratings

“…When probing with a pure polarization state (parallel or perpendicular), the evolution of the spectral profile, reflection peak, and bandwidth (Figures 2b and 3a,b) have followed closely with our previous study [ 31,32 ] of isotropic liquids, being highly sensitive to both the magnitude and the sign of the refractive index contrast. The drawing of the optical field into capillaries with positive values of refractive index (i.e., NLC in Figure 3) generated much stronger and broader stopbands than for the negative index changes that pushed light out of the capillary (i.e., air‐filled FBG, Figure 2b).…”

“…An array of long and uniform filaments was placed in the core of the fiber using a point‐by‐point laser writing technique as described previously. [ 31,32 ] In this fabrication technique, long and uniform laser filaments exceeding the 125 µm fiber diameter were generated by optical aberration induced on the surfaces of a thick glass plate with a thickness up to 3 mm. The plate was positioned above the fiber with refractive index matching oil applied at the fiber‐plate contact to remove astigmatic aberration of the cylindrical fiber shape.…”

“…The drawing of the optical field into capillaries with positive values of refractive index (i.e., NLC in Figure 3) generated much stronger and broader stopbands than for the negative index changes that pushed light out of the capillary (i.e., air‐filled FBG, Figure 2b). [ 31,32 ] However, the highly birefringent grating now offers a new means for dropping a preferred polarization state in two narrow spectral reflection windows (≈5 nm amber color band for 0° and ≈1.5 nm green band for 90° in Figure 3a,b), tailored by the combination of Bragg reflection and polarization dependent NLC.…”

“…With a 10‐fold reduction in the electrode spacing, D (from 250 to 25 µm), one can anticipate a 100× increase in the dielectric torque, G ∝ E 2 ∝ 1/D 2 . Laser structuring offers one direction micro‐ or nano‐structuring of open ports within the fiber core [ 31,41,42 ] wherein fields perpendicular to the axis of holes may be applied to the limits of dielectric break down, ≈5 MV cm –1 , in fused silica, [ 43 ] if we consider the same dielectric properties for the fiber and for the NLC, see ref. [ 38 ] .…”

“…Such nano‐capillary arrays present strong, second‐order Bragg grating stopbands (i.e., λ = 1550 nm for Λ = 1.072 µm periodicity) in the C‐telecommunication band, inducing large ≈40 nm Bragg shifts for in‐fiber refractive index sensing when tested with isotropic fluids ( n = 1.0–1.67). [ 31,32 ]…”