Adaptive Optical Technologies for Optical Transmission Systems

1. Adaptive Optical Technologies for Optical Transmission Systems Maki Nanou, George-Othon Glentis, Kristina Georgoulakis, Chris Matrakidis, Christina (Tanya) Politi, Alexandros Stavdas

2. Outline • Basic Concepts of Optical Communications • Fiber Impairments & Compensation Techniques • Optical Transmission Simulations • Results • Conclusion University of Peloponnese Dept. of Inform. & Telecommunications

3. Traffic Growth Until 2000: Voice Traffic dominates (*) Cisco Forecast 60% / year After 2004: Data Traffic dominates Traffic growth of 60% per year outstrips the growth in capacity of commercial systems. 20% / year While the entire traffic in North American core Network could be carried on a single fiber until 2008, in 2011 more than two fibers were required. Every 3 years the required number of fibers will double. Increased capacity ↔ Advanced Modulation Formats University of Peloponnese

4. Non Return To Zero – On – Off Keying Output Intensity • Most commonly used (widely deployed) quaternary point bias • OOK: switching ON and OFF the amplitude of an optical carrier signal (Intensity Modulation Only) Input Voltage t • External Modulation: biased at the quadrature point of the MZM transfer function, and driven by an electrical binary NRZ-ASK signal with peak-to-peak amplitude of Vπ. optical signal MZM LASER Vπswing • Simple Tx/Rx Configurations University of Peloponnese Dept. of Inform. & Telecommunications

5. Differential Phase Shift Keying - DPSK • Phase Modulation Only • Nearly constant envelope – higher tolerance to non linear effects • Higher receiver sensitivity due to the 3dB lower OSNR requirement to achieve a specific BER. • External MZM biased at minimum point and driven with a precoded binary data with twice the switching voltage required for NRZ – OOK (2Vπ) • More complex Tx /Rx Design Output Intensity minimum point bias t t Input Voltage optical signal MZM LASER electrical NRZ data 2Vπswing precoder University of Peloponnese Dept. of Inform. & Telecommunications

6. Fiber Impairments in Single Channel Systems Linear NonLinear SPM Dispersion Losses inserts ASE noise SMF DCF Rx Tx G G compensates dispersion compensates DCF losses compensates SMF losses University of Peloponnese Dept. of Inform. & Telecommunications

7. Chromatic Dispersion Effect Every different f travels with different velocity Some broadening Optical Fibre input pulse Dispersion Parameter: DSMF Length of Transmission: LSMF t Severe broadening ISI t • Dispersion tolerance is inversely proportional to the square of the operating bitrate and consequently limitations due to dispersion become more stringent as bit rate increases. 1 1 0 • As a linear effect, dispersion can be compensated by means of a DC fibre, providing that the exact amount of dispersion is known in advance. DDCF*LDCF=-DSMF*LSMF 1 1 0 1 1 1 University of Peloponnese Dept. of Inform. & Telecommunications

8. Electronic Equalization Electric Filter • EE attempts to reverse the distortion incurred by a signal transmitted through a channel. • It can be a simple linear filter or a complex algorithm. PIN y(n) Clock Recovery y(t) I(n) Electronic Equalizer ADC Receiver (Rx) • EE are applied after the receiver • no need in intervening in the already installed fibre links • Can cope with variable amounts of dispersion University of Peloponnese Dept. of Inform. & Telecommunications

9. Electronic Equalization • In our case we investigate the performance of the following equalizers: • Linear Transversal Equalizer – LTE • Decision Feedback Equalizer – DFE • Volterra Decision Feedback Equalizer - VDFE • All equalizers operate at supervised mode, where a training sequence, known by the receiver is transmitted, in order to train the equalizers about the channel characteristics. • Fractional spacing is employed as in this case the performance of the equalizers becomes less sensitive to the sampling phase of the receiver. University of Peloponnese Dept. of Inform. & Telecommunications

10. Linear Transversal equalizer - LTE LTE is the simplest form of electronic equalizers. The incoming signal is processed by a linear filter. In order to retrieve the transmitted sequence, FS-LTE operates according to: University of Peloponnese Dept. of Inform. & Telecommunications

11. Decision Feedback equalizer - DFE DFE consists of two parts: a Feed forward part that is driven by the received waveform and a Feedback part that is driven by the estimations of the previous symbols. FS-DFE operates according to: The performance of linear equalizers is constrained when applied to non linear systems.

12. Non Linear Photodetector • The main reason of non linearity in optical systems is induced by the detector during the conversion of optical to electrical. • Photodiode operates on a square law principle, in which the output of the detector is proportional to the intensity (i.e., the square of the input signal magnitude). • Although it is a simple circuit, it is nonlinear and as such it is difficult to correct linear distortions such as CD. University of Peloponnese Dept. of Inform. & Telecommunications

13. Volterra Decision Feedback equalizer - VDFE Simplified VDFE used: University of Peloponnese Dept. of Inform. & Telecommunications

14. Complexity Calculations University of Peloponnese Dept. of Inform. & Telecommunications

15. Simulation Setup BER Estimation w/o EDC SMF DCF Tx Rx G G equalizer Transmission Span (x N) BER Estimation with EDC • 10 spans x 100km (1000km) • 3 spans x 100km (300km) 10 Gb/s bitrate 40 Gb/s bitrate University of Peloponnese Dept. of Inform. & Telecommunications

16. Unncompensated Results 380 km 250 km 400 km 200 km 300 km 200 km 150 km University of Peloponnese Dept. of Inform. & Telecommunications

17. NRZ-OOK Results (1) 94% 70% 98% 80 % 85 % 87.5 % University of Peloponnese Dept. of Inform. & Telecommunications

18. NRZ-DPSK Results (1) OCR=70%-90% 10Gb/s & 40Gb/s DPSK University of Peloponnese Dept. of Inform. & Telecommunications

19. Upgrading Scenario Setup Operating at 40 Gb/s Operating at 10 Gb/s BER Estimation w/o EDC SMF DCF Tx Rx G G equalizer Total Length of 1000 km (10 spans x 100 km) BER Estimation with EDC 99 % OCR NRZ-OOK NRZ-OOK Dispersion Tolerance Reduces NRZ-DPSK NRZ-DPSK University of Peloponnese Dept. of Inform. & Telecommunications

20. Upgrading Scenarios Results Upgrading a system 10-40 NRZ & DPSK University of Peloponnese Dept. of Inform. & Telecommunications

21. Conclusion • Low cost, adaptivetechniques of optical transmission, consisting of optical and electronic equalization, were studied by simulating configurations with realistic link parameters. • Here, the interplay between optical and electronic techniques for physical impairment mitigation for DD optical transmission with various performance/complexity tradeoffs, is presented. • It has become evident that even in the absence of FEC, low complexity equalizers can perform sufficiently well in conjunction withoptical compensation. • Low complexity Volterra equalizers can be used to support the migration of a system from 10 to 40 Gb/s. University of Peloponnese Dept. of Inform. & Telecommunications

22. Q&A Thank you for your attention! This research was funded by the Operational Program "Education and Lifelong Learning" of the Greek National Strategic Reference Framework (NSRF) Research Funding Program: THALES PROTOMI, grant number MIS 377322. University of Peloponnese Dept. of Inform. & Telecommunications