6th Optical Fibre Measurement Conference, Girton College Cambridge, September 26-28, 2001 73 A simple non-linear coefficient measurement scheme based on Four-Wave Mixing Bostjan Batagelj Laboratory of Optical Communications Faculty of Electrical Engineering, University of Ljubljana Trzaska 25, 1000 Ljubljana, Slovenia Tel: +386 (0)1 4768 423, Fax: +368 (0)1 4768 424, email: bostjan.batagelj@fe.uni-lj.si Abstract We propose a novel measurement scheme of n 2 /A eff using FWM technique for an optical fibre. A simple measurement scheme uses only one laser source, avoids polarization dependence and is highly sensitive and accurate. Introduction As new fibres are being designed for non-linear optics applications, a consistent approach to measurement of optical non-linearity is required for systematic materials engineering and optimisation. Simultaneously, as overall transmitted data estimates on WDM and soliton systems increase, the effects of fibre non-linearity come to play an even more decisive role in the design and performance of optical communications links. Additionally, more complicated span designs incorporating dispersion management may consist of several lengths of different fibre types, each having different non-linear coefficients. For this reason it is important to be able to simply and accurately measure the non-linear coefficient of fibres of different compositions. The non-linear coefficient is defined as the ratio between the non-linear refractive index n 2 and the effective cross section area A eff . Several methods have previously been proposed for the measurement of n 2 /A eff [1,2,3], but the results have not provided to be consistent and the techniques generally tend to require at least two laser sources. Some of them require special pulse sources [4,5], suffer large uncertainties or are specific to certain fibre types [6]. Four- Wave mixing is a reliable technique for determining the third-order non-linear susceptibility χ (3) . The standard measuring setup for non-linear coefficient measurement based on Four- Wave Mixing is made up of two lasers. In order to achieve maximum FWM efficiency the polarization controllers are required. [7] We have devised and demonstrated a technique, which uses only one laser source, eliminates polarization dependence, is simple, accurate, highly sensitive and applicable to all currently available fibre types. Measurement of fibre non-linear coefficient by simple scheme based on FWM The experimental configuration for our method is shown in Fig. 1. The key element of the setup is a Mach-Zehnder (MZ) LiNbO 3 guided-wave intensity modulator. The output from a single frequency laser is modulated with a pulsed RF signal of a fixed frequency 10GHz. The light source could be any single frequency laser, for example DFB, fibre or external-cavity laser. The pulse modulator driven by the 1MHz pulses produces a train of 10GHz RF pulses as shown on Fig. 2. Pulse modulation is used to increase the optical peak power, while the RF