Developing flexible WDM networks using wavelength tuneable components A. Dantcha 1, L.P. Barry 1, J. Murphy 1, T. Mullane 2 and D. McDonald 2 (1) Research Institute for Network and Communications Engineering, School of Electronic Engineering, DCU, Dublin 9, IRELAND. Email : liam.barry@dcu.ie (2) Intune Technologies Ltd, 9c Beckett Way Park West, Dublin 12, IRELAND Abstract: This paper outlines the usefulness of wavelength tunable laser modules for developing wavelength packet switched optical network employing wavelength division multiplexing techniques. We investigate how the excitation of spurious wavelengths during a transition period may influence the performance of a complete WDM system employing wavelength tunable transmitters. Our results show a bit-error-rate floor of 4 x 10-5 is introduced due to the spurious emission from the wavelength tunable laser. 1. Introduction The phenomenal growth in information and communications technology (ICT) over the last decade can be largely attributed to research and development of optoelectronic technologies, which enable the enormous capacity of optical fibre to be exploited. Optoelectronic technology is the dominant carrier of information in the world today. It is also central to the realisation of future networks that will have the capabilities demanded by society. These include virtually unlimited capacity to carry communication services of almost any kind. They will also require full transparency that allows terminal upgrades in capacity and flexible routing. As optical fibre communication systems become increasingly important for transporting information at ever increasing data rates between different areas, a new generation optical Internet is gradually being developed. Although most of the packets of information transmitted over the Internet are sent in the optical domain, the time-consuming process of determining where the packet must be sent is still undertaken using electronic routers This electronic routing of the data packets thus requires optical-to-electronic conversion, followed by electronic routing, and then another electrical-to-optical conversion to send the information on to its destination over the optical fibre. As the capacity of these networks increases, it is expected that the electro-optic conversions will prove too costly, in the sense that the efforts involved with electronic routing at high bit rates will enter the region of diminishing returns, from an economic point of view. Recent advances in electronic switching have shown that 10 Gb/s backplane switching is feasible, but bit rates
beyond this are technically not feasible from an electronic perspective. It will thus become attractive to have one layer of the overall network in which the routing of packets is undertaken; the optical domain. One possible solution for optical packet switching may be to transmit different data packets on different optical wavelengths, and then use wavelength selective filtering to determine where the packets are sent. This wavelength packet routing technology may prove to be a suitable technique for achieving a high utilization level of wavelength capacity in future photonic networks. The key element in a wavelength packet switched network will be a wavelength tuneable transmitter, as this will be responsible for sending the information packets on the different wavelengths (determined by destination of the information). In addition, the parameters of the wavelength tuneable transmitter (tuning speed, optical linewidth, tuning range, etc.) will determine the overall network design. There are a number of different approaches that may be employed for developing a wavelength tuneable laser, with all techniques involving a change in the optical path length of the lasing cavity. The most efficient way to achieve wavelength tunability from a laser is to change the current applied to the laser such that the emission wavelength is varied. Tuneable lasers of this type normally have multiple sections, with one to generate the light signal and another to carry out the wavelength selection. Additional sections may be added to further improve the device performance, and one of the most commonly used wavelength tuneable lasers is the Sampled Grating Distributed Bragg reflector (SGDBR) [1,2]. This device normally has four electrical sections and by correct control of the current applied to the individual sections it is possible to tune the device over 40 nm. The aim of this work is demonstrate the usefulness of a wavelength tuneable laser for developing wavelength packet switched WDM networks. The basic idea of wavelength packet switching [3, 4, 5, 6] involves the use of wavelength tuneable transmitters to route optical packets to different destinations. Fig. 1 details how this technique operates. Tuneable Transmitter λx Receiver Tuneable Transmitter Coupler Fibre Array Waveguide Grating λy Receiver Tuneable Transmitter λz Receiver Fig. 1 Basic architecture for wavelength packet-switched network
Each transmitter can tune its emission wavelength to transmit packets of information at a number of different wavelengths. The information from all the transmitters can then be multiplexed together and sent over optical fibre. As the information is basically been sent on multiple wavelengths over a single fibre we are essentially using wavelength division multiplexing in the wavelength packet switched network. The composite wavelength signal then passes through an Array Waveguide Grating Router (AWGR), which sends each of the incoming wavelengths to one specific output fibre port. In general it is possible to write the wavelength connectivity in the form of a matrix. Thus by choosing an appropriate wavelength on the transmitter side, the laser selects the output port to which the information packet is sent. The tuneable transmitters, together with the optical coupler and the AWG become a strictly non-blocking switch fabric with a switching speed limited only by the laser s wavelength tuning speed. 2. Experimental set-up The experimental test-bed (Fig. 2) employs an INT1100 fast wavelength-switching module from Intune Technologies for the present tests. The module is controlled via a PC and is set to transition between different wavelengths. In addition to this laser, a HP wavelength tunable external cavity laser that can emit light from 1480 to 1570nm is employed. Electrical information from a pattern generator is encoded onto the optical signal from the HP ECL laser using an external modulator (with a bandwidth of 8 GHz). A polarization controller (PC) is required before the modulator to ensure that the input signal has the correct polarization. The data signal from the pattern generator (with a peak-to-peak voltage of 2.5 volts) is fed directly into the RF port of the modulator while the bias port is connected to a voltage supply (this is optimized during the experiments to achieve the clearest eye opening). Tunable Computer laser HP External Cavity Tuneable Laser Pol Cont. MOD COUPLER EDFA Pattern Generator & Error Analyser Tunable optical filter VAR. ATT. PIN Diode RF AMP Scope Fig. 2 Experimental set-up for investigating usefulness of wavelength tuneable lasers in WDM based wavelength packet switched network
This data signal is then coupled together with the optical output from the INT1100 that is switching back and forth between two specific wavelengths. The wavelength carrying the information can be set to any wavelength between the two output wavelengths from the tunable laser. A characteristic of this module is that as it transitions between two specific channels it may excite other wavelength channels that are being used for data transfer in an overall WDM network, and the purpose of this work is to determine how this would affect the performance of this information transfer. 3. Experimental Results Fig. 3 displays the composite wavelength signal after the fibre coupler. This signal then passes through an optical filter, with a bandwidth of 0.28nm that selects out the central wavelength channel carrying the information. The optical data signal is then detected using a 50 GHz pin diode and can be displayed on an oscilloscope or inputted into the error analyzer to determine the BER of the received signal. Fig. 3 Optical Spectrum of composite signal after coupler. Spectrum shows the 1533 and 1538 nm signals from the INT1100 that is transitioning between these wavelengths, and the data channel at 1535.5 nm
To determine whether the signals excited by the tunable INT1100 module affect the system performance, as it transitions between two wavelengths on either side of the data channel, it is first of all necessary to plot the back to back performance of the 1535.5 nm data channel on it s own. The BER vs. received power for the back-to-back case is shown in Fig. 4. We then proceed to measure the BER vs. received power for the case when the data channel is coupled together with the tunable laser output before being filtered out (also shown in Fig. 4). one channel 3 channels operating 1.00E-01 1.00E-02 1.00E-03 BER 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08-14 -13-12 -11-10 -9-8 Received optical power [dbm] Fig. 4 BER vs. received power for back-to-back case, and for case when the data channel is multiplexed with output from the INT1100 that is transitioning between two wavelengths. The associated eye diagrams of the received data signals for single channel case, and the case when the data channel is multiplexed with the tunable laser (that is switching between two wavelengths), before being filtered out and detected, are shown in Fig. 5
Time, 100 ps/div Fig. 5 Received eye diagrams for the 1535.5 nm channel for the back-to-back case (left hand figure), and the case when the data channel is multiplexed with the tunable laser output before being filtered out and detected (right hand figure). When the data channel is multiplexed with the output from the tunable laser (that is switching between two wavelengths), before being filtered out and detected, the BER vs. received power curve presents an error floor at around 4 x 10-5. The explanation for this is as follows: As the laser transitions from 1533 to 1538 nm it does excite light at intermediary wavelengths for small periods of time (as can be seen from the eye diagram of the filtered channel at 1535.5 nm). In our experiment, for the time interval that the tunable device generates an optical signal at the same wavelength as the 1535.5 nm data signal, all the logical zeros transmitted will be received in error. So by looking at the error floor it is possible to actually estimate the length of time that the tunable laser module emits light at one of the intermediary channels. There is a transition every 9.5 microseconds and since the information is being sent at 2.5 Gbit/s, in 9.5 microseconds there are approx. 23750 bits sent. With a BER of 4 x 10-5, this means that 1 of the 23750 bits sent in 9.5 microseconds is received in error due to the excitation of the intermediary wavelength channel. However given that the signal generation from the tunable laser only gives an error for a sent 0, and given that unbiased data is being used (equal number of 1 s and 0 s ), we can assume that the intermediary wavelength is on for approx. 2 bit periods of the 2.5 Gbit/s data signal (800 ps). Similar error floors were obtained when the data channel was set to wavelengths corresponding to the other spurious wavelengths that are generated during the transition period of the INT1100. 4. Conclusions Wavelength tunable lasers are becoming a mainstream component in photonic networks. In addition to providing cost saving for WDM networks with respect to inventory reduction, these tunable devices may also be used for implementing extremely efficient bandwidth utilization in WDM networks by employing wavelength packet switching architectures. With this technique,
WDM optical packets are generated by a fast tunable light source in conjunction with an external modulator. The wavelength packets can then be routed to specific nodes in the optical network by using optical filtering techniques. One problem that may be encountered with the tunable devices is that during a transition between two wavelengths, light may be generated at spurious wavelengths. We have examined how the excitation of spurious wavelengths during the transition period may influence the performance of a complete WDM system employing wavelength tunable transmitters. Our work shows that cross channel interference caused by the spurious wavelengths during a transition introduce an error floor into the performance characteristic of other data channels. Recent advances in integration technology [2] have the ability of allowing the output of the laser to be blocked during the wavelength-switching event itself. This greatly improves the overall error rate performance of such switch architectures, and may reduce cross channel interference effects during the routing process. References 1. Larry Coldren, Monolithic Tunable Diode Lasers, IEEE Journal of Selected Topics in Quantum Electronics, vol.6 No.6, Nov 2000. 2. Beck Mason et al.,, Charnacterisitics of samples grating DBR lasers with Integrated Optical Amplifiers, OFC2001, paper Tul6. 3. K. Shrikhande et al., Performance demonstration of fast-tuneable transmitter and burst-mode packet receiver for HORNET Optical Fibre Communications Conference, Technical Proceedings, ThG2-1, Annahein, USA, 2001. 4. I. White et al., Wavelength switching components for future photonic networks, IEEE Communications Magazine, Vol. 40, Issue 9, Page(s): 74 81, Sept. 2002. 5. Chun-Kit Chan, K. Sherman, M. Zirngibl, A fast 100 channel wavelength tunable transmitter for optical packet switching, IEEE PTL, pp 729 731, July 2001. 6. M. Duser, I. De Miguel, P. Bayvel, D. Wischik, Timescale analysis for wavelength-routed optical burst-switched (WR-OBS) networks, OFC 2002, paper WG7.