Optical Local Area Networking

Transcription

1 Optical Local Area Networking Richard Penty and Ian White Cambridge University Engineering Department Trumpington Street, Cambridge, CB2 1PZ, UK Tel: , Fax: , Acknowledgements: Many others in the Photonic Communications group Intel Research (Derek McAuley and Madeleine Glick) through SOAPS project Bookham Technology for providing tunable lasers Photonic Communications Research

2 Outline Introduction The WDM Revolution WDM for Datacommunications Optical LANs for Computing - Recent advances in low cost photonics Background Work to Date Athermal WDM Lasers SOA Based Optical Add/drop Switch Initial Results of a LAN Network Node Demonstrator Conclusions and Future work Photonic Communications Research

3 WDM Transmission Technology Evolution Post 2001

4 DWDM for Transmission ITU-T G.692 DWDM specification defines grid of 80 wavelengths with 50GHz spacing around 1550nm. Commercial systems with 2.5Gb/s and 10Gb/s per channel (40Gb/s in laboratory) Precise wavelength grid requires control of channel spectral content - wavelength locking to few GHz (mk and ma range control) - high SMSR (~40dB) - external modulation to meet required spectral efficiency - results in high cost per wavelength Current demand doesn t require many wavelength per fibre Strongest trend is towards lower cost Solution - remove cost by relaxing tolerances on wavelength Photonic Communications Research

5 Advantages of Coarse Wavelength Spacing No need for precise temperature or wavelength control - TEC one of the major costs of a DWDM transceiver Higher DFB laser yield possible - large intra and inter wafer wavelength variations allowed - significantly relaxed SMSR requirement Low cost MUX/DEMUX components possible - wider wavelength spacing allows smaller mux/demux components - grating or interference filter components preferred - mux/demux technologies allow MMF systems CWDM systems typically are unamplified - entire spectrum of fibre can be used if required Photonic Communications Research

6 CWDM Specifications Emerging standards have specified uncooled CWDM channel spacing of around 20nm Wide channel spacing necessary due to wavelength drift of DFB laser diodes - typically around 0.1nm/ C i.e. 10nm over 100 C operating temperature range Thus typical channel spacing are specified at ~20nm ITU-T G694.2 specifies 18 channels from nm

7 Athermal WDM - Motivation Would be very attractive to have a CWDM system that isn t quite so coarse Few (4x8) wavelengths inside amplifier spectrum (either EDFA or SOA) would transform applications Wavelength spacing of 1-4nm probably acceptable if cost advantages of CWDM can be retained. Constant wavelength operation can be achieved by; The use of high current peltier effect cooler in conjunction with a tunable photodetector External fibre Bragg grating wavelength locker Etc. Allows the number of channels in WDM systems to be increased but; Expensive Power consumption is high Often complicated, involving many components Can we develop a simple, low cost, low power consumption technique to achieve athermal operation?

8 Uses a tunable laser, for an active approach to constant wavelength operation Current supplied to tuning section depends upon the temperature of the device submount Athermal Laser Operation Tuning Section Current (ma) No need for high current peltier effect cooler or expensive optics Operating wavelength can be adjusted by the user to correct wavelength registration issues Wavelength (nm) Constant wavelength operation? T1 T2

9 Experimental Details Computer Control Optical Power Meter Current Source V I V I Voltage Measurement Optical Spectrum Analyser Temperature Sensor Tunable laser is three section device Consists of grating, phase and gain sections Computer control allows currents to be adjusted depending upon: Temperature Front and back facet power measurement Forward voltage across device at each contact Optical Power Meter

10 Wavelength (nm) Wavelength Control Uncontrolled Controlled Temperature ( C) Uncontrolled wavelength drifts from 1556 to 1563 nm between 20 and 70 C Under control the wavelength remains at ± 0.30nm Allows at least an order of magnitude reduction in CWDM channel spacing Photonic Communications Research

11 SMSR (db) Single-Mode Performance Temperature (C) SMSR typically > 40 db but reduces slightly at higher temperatures always > 35dB Ripple can be eradicated with more sophisticated control techniques also further reduction of wavelength variation (subject to patent application)

12 Suppression of Mode-Hopping DBR Filter Longitudinal Modes Gain Spectrum Initial results show that DBR filter has been athermalised Modes continue to drift with temperature however Need keep mode aligned with filter to eradicate modehops Use of phase shifting section Photonic Communications Research

13 Optimised Device Structure Phase Current (ma) Wavelength (nm) C 20 C 40 C 60 C 70 C Grating Current (ma) Temperature ( C) Longer phase sections are necessary to allow wavelength control with continuous changes in control currents Novel laser designs exhibit contours of controlled mode-hop free output extending up to 70 C Designs currently being fabricated Photonic Communications Research

14 Issues for Optical Networking Using Athermal WDM How do you design an efficient optical packet switched data network? - using currently available technology Network aspects will stay electronic for near future Header processing Routing Security QoS Classification What can we gain using photonics (WDM, optical switch)? Capacity Power consumption Noise immunity Latency Cost (eg leveraging off low cost datacomm transceivers) Photonic Communications Research

15 Wavelength Striped Semisynchronous LAN We need - nanosecond optical switch - WDM channel spacing ~nm HUB Payload Header Control logic TERMINAL Photonic Communications Research

16 SOAs as Optical Gates Input wavelength gain Gain / loss λ / µm saturated loss unsaturated loss Advantages Optical gain gain (possibly up up to to ~30dB) Large Large extinction ratio ratio (~50dB) Fast Fast gating gating time time (( ~ns) ~ns) Photonic Communications Research Disadvantages Limited cascadability because of of ASE ASE Gain Gain saturation can can lead lead to to ER ER degradation Limited input input power power dynamic range range -- distortion

17 Optical Crosspoint Switch 2x2 Array 2x2 and 4x4 arrays constructed - scalable to 32 x 32 and above 8 individually controllable SOAs 850µm x 850µm chip 5 quantum well InGaAsP structure On chip switch gain >8dB Crosstalk < -50dB Switching time < 2ns Operating wavelength 1550nm, optical bandwidth >40nm Broadcast operation possible

18 Add,Drop & Pass Through Output port Input port Pass-through mode Output port Add mode Add port Input port Drop mode

19 Optical Add-Drop HUB HUB TERMINAL 1300nm TERMINAL Media access via blue control wavelength at 1310nm High capacity CWDM data at 1550nm 8 x 10Gb/s FPGAs or pulse gens Driver HUB DROP ADD 15xxnm 15xxnm 1300nm 2x2 optical switch

20 Initial Test of Switch Node 8 x 10Gb/s data transmitted through add-drop node Routing data provided by 100Mb/s channel on separate wavelength Electronic control provided by Xilinx Spartan II FPGA FPGA also generates packet, including preamble, destination address, data and slot delimiters

21 Packet Switching Results Input Waveform Through Packets Drop Packets 70ns guard bands, limited by timing jitter from FPGA 30 byte preamble for clock acquisition High extinction packet routing demonstrated

22 BER Performance Error free operation demonstrated with low penalty Penalty dominated by clock acquisition/recovery rather than switch

23 Conclusions Use of CWDM reduces cost by stripping out control etc - At the expense of large (20nm) channel spacing - Not possible to have amplified (and hence switched) systems High capacity packet switched LANs require - Few nm channel spacing but retaining cost advantages of CWDM - Nanosecond scale optical switching Athermal laser demonstrated - Wavelength varies by < ± 0.30nm up to 70 C grating or interference filter components preferred 2x2 add/drop SOA based switch demonstrated - 2ns switch time with <-50dB crosstalk and 8dB gain 80 Gb/s CWDM LAN router controlled by separate 100 Mb/s channel Photonic Communications Research

24 Research Directions Within CII - Low cost technologies for achieving > 100 Gb/s transfer rates within computer LANs - Study potential of multichannel techniques and optimum level of granularity - Will WDM have as great and impact in Datacomms as it had in Telecomms? - Should packets always have headers? - Study relative importance of bandwidth and latency - Study potential of overlay architectures for the long reach problem - Study protocol independence - Study security issues?