Long-period gratings written in large-mode area photonic crystal fiber

Abstract

ABSTRACT We report to the best of our knowledge for the first time on the fabrication and characterization of CO2-laser written long-period gratings in a large-mode area photonic crystal fi ber possessing a core diameter of 25 µm. The gratings have low insertion losses ( 10 dB) at the resonant wavelengths, making them particularly interesting for high power applications. 1. INTRODUCTION Long period gratings (LPG) are characterized by a longitudinal periodic variation of the core or cladding effective index of a waveguide which leads to a resonant coupling between two co-propagating modes provided that a spatial overlap is given and the phase-matching condition (n eff,1 – n eff,2 ) = O/ is fulfilled [1]. n eff,1/2 denotes the effective indices of the two modes, / is the grating period and O the resonant wavelength. Similar to FBGs the most common technique to inscribe LPGs is the illumination of a photosensitive fiber by a UV-laser through a phase mask [1]. Using a LPG a fiber-integrated spectral filter with designable characteristics can be achieved by coupling a guided core mode to a leaky or lossy cladding mode. Depending on the wavelength range LPGs have been written into fibers with core diameters of less than 10 µm, i.e. into standard single-mode fibers. Howe ver, due to performance limitation by nonlinear effects and fiber damage the generation and delivery of high power and high brightness laser radiation relies on fibers with significantly larger mode area in order to reduce the intens ity inside the fiber core. Diff raction-limited high power fiber laser sources with powers well above the kW barrier have been reported [2]. Such systems ask for fiber-integrated components compatible to their fiber dimensions and geometries. Photonic crystal fibers (PCF) [3] offer due to a micro-structured cladding region an enhanced index control capability , hence, larger intrinsically single-mode fiber cores with low numerical apertures are possible. Sing le-mode Yb-doped PCFs with core diameter of larger than 50 µm have been reported [4]. A wavelength-selective attenuation can be achieved by periodic collapsing (at least partly) the holey cladding structure by illumination with a CO