Tailored Mode Coupling With Highly Localized Intracore Fiber Bragg Gratings

Abstract

Femtosecond laser-written micro void FBG's offer control of core-cladding coupling. Careful placement of localized defects within the core cross section permits tailoring of the reflection into modes of higher azimuthal order.Many sensing applications harness core-cladding coupling of FBG. In particular, fiber based spectrographs rely on slanted FBGs for coupling into cladding modes. However, strong coupling to higher azimuthal modes is limited by the extreme (over 80 degrees) tilt angles required [1]. In contrast to UV single photon refractive index changes, which are determined by the photosensitive regions of the fiber, fs pulse induced modifications can be controlled by the focussing conditions of the writing laser. Modifications can be strong (up to ∆n = 0.44), and highly localized. This enables the design of FBGs, that are not constrained by the fiber geometry. Here, we exploit this new degree of freedom for tailored core-cladding coupling. 1535 1540 1545 1550 1555 −20 −10 0 wavelength λ [nm] Transmission [dB] 1535 1540 1545 1550 1555 −20 −10 0 wavelength λ [nm] Transmission [dB] x y w h a) 20µm b) d) a) c) Fig. 1. Micrograph (a) and transmission spectrum (b) of a micro void FBG. Below, the schematic of the model cross section (c) is shown, that was used to compute the transmission spectrum (d).The FBGs for our investigation consist of a periodic chains of fs pulse induced micro voids (Fig. 1a)) in the core of an uncoated SMF-28e fiber. They were inscribed by focussing the light of an ultra short pulse laser (Spectra Physics Hurricane, pulse duration < 110 fs at 800 nm) with a 20× oil immersion objective (NA= 0.8) into the fiber core. Second order FBGs were written with pulse energies between 200 and 275 nJ [2]. Fig. 1b) depicts a typical transmission spectrum of such a grating. At wavelength below 1554 nm, the coupling to even and odd cladding modes is clearly visible. A setup similar to that of [3] was used to image the reflected cladding modes. Based on the observed a779_1.pdf a779_1.pdf OSA / BGPP 2010 BThC6.pdf BThC6.pdf

High temperature multiparameter sensor

“…Such behavior results from non-zero coupling to non-axisymmetric higher order azimuthal modes (HE l,m with l > 1), resulting in this case from non-symmetric localization of the grating within the fiber core. 27 With this in mind, the polarization dependent wavelength difference (PDλ) of the Bragg resonance was measured to be less than 15 pm, corresponding to a birefringence of only 5×10 -6 . If one aligns the various spectra using the Bragg wavelength as a baseline, the relative shift associated with each cladding mode dip becomes more apparent.…”