title: Determination of femtosecond-laser-induced refractive-index changes in an optical fiber from far-field measurements.
abstract: A new method for direct writing of localized, circularly symmetric refractive-index changes in optical fibers with a femtosecond laser is demonstrated. The refractive-index changes are characterized using a novel approach employing comparison of numerical simulations to the measured far-field profiles of unmodified and modified fibers. From the analysis, a negative refractive-index change of -0.015±0.005 within a radius of (0.6±0.1)  μm is determined.
authors: Juha-Matti
authors: JM
authors: Savolainen
authors: Lars
authors: L
authors: Grüner-Nielsen
authors: Poul
authors: P
authors: Kristensen
authors: Peter
authors: P
authors: Balling