Demonstration of 40 Gbit=s optical packet synchronisation using fibre Bragg gratings and fast-tunable wavelength converters

Introduction: All-optical packet switching networks [1] are becoming the solution of fast-growing telecommunication with the advantages of scalability and efficiency by reducing O=E=O conversion and higher available data rate beyond current electronic routing technologies. To ease the design of the key component, the optical packet switch, the incoming packets need to be aligned to a common time reference to eliminate the delay variations introduced by factors such as different routes in networks, fibre chromatic dispersion and thermal expansion in the links. The start of the arriving packet can be determined by a packet delineation function beforehand. Once the packet delineation has been carried out, the synchroniser can be configured by an electronic signal corresponding to the time difference between the arriving packet and the reference clock. In this Letter, a method of optical packet synchronisation based on fibre Bragg gratings (FBGs) combined with tunable wavelength converters (TWCs) [2–4] is proposed and demonstrated. The configuration is shown in Fig. 1. A series of FBGs with different centre wavelengths are deployed along the fibre to form a wavelengthdependent delay line. The header part of the incoming packet is removed before the optical synchroniser. During synchronisation the payload is converted by a rapidly-tunable WC to one of the FBG centre wavelengths according to the required delay value. The outgoing delayed payload is then forwarded to the next switching stage where it is converted to a desired wavelength by the second TWC. Here we use an InP-based Mach-Zehnder interferometric wavelength converter monolithically integrated with a fast tunable SGDBR as characterised in [4, 5]. It has an output switching window of 6 ps, extinction ratio >9 dB, less than 4 dB power penalty at 10 9 BER and low pattern dependence across a 25 nm tuning range operating at 40 Gbit=s using RZ signals, which demonstrate its feasibility for this application.