Measurement of Kerr nonlinearity coefficient of fibres using nonlinear polarisation rotation

Abstract

A simple and accurate method to measure the Kerr nonlinearity coefficient of optical fibres is proposed and demonstrated. The method is based on the nonlinear polarisation rotations caused by cross-phase modulation between two lightwaves of different wavelengths co-propagating in fibres. The Kerr nonlinearity coefficients of standard singlemode fibre and TrueWave TM reduced slope fibre measured with this method are in good agreement with the published data.Introduction: Optical fibre is not a linear media. It has nonlinearity, which is quantified by the Kerr nonlinear coefficient. Fibre nonlinearity is one of the main limiting factors for today's fibre-optic communication systems. Fibre nonlinearity causes self-phase modulation (SPM) within the channel and inter-channel cross-phase modulation (XPM) and fourwave mixing (FWM) in wavelength division multiplexed (WDM) systems, which could significantly degrade the performance of longhaul dense WDM systems [1]. On the other hand, fibre nonlinearity is the foundation for many optical signal processing techniques [2]. Therefore, fast and accurate measurements of the Kerr nonlinearity coefficients of fibres are desired, not only in lab testing, but also in fibre manufacturing and system implementation.There are many methods to measure fibre nonlinearity. They are usually based on the measurement of spectrum variation, which originated from SPM, XPM, FWM,. These methods have at least one of these drawbacks: 1. high requirements on pump sources, e.g. a short pulse laser or two continuous-wave (CW) lasers with precisely balanced output power; 2. strict restrictions for the chromatic dispersion (CD) profile and high sensitivity to CD variations along the fibre length; 3. complicated relation between the measured data and the nonlinearity coefficient, which usually requires sophisticated nonlinear data fitting.In this Letter, a simple technique to measure the Kerr nonlinear coefficient of fibres is presented. The technique is based on nonlinear polarisation rotation. It only requires CW lasers and can be applied for various fibre types and nonlinear components as well.