Adaptive Optical Technologies for Optical Transmission Systems Maki Nanou, George-Othon Glentis, Kristina Georgoulakis, Chris Matrakidis, Christina (Tanya) Politi, Alexandros Stavdas
Outline Basic Concepts of Optical Communications Fiber Impairments & Compensation Techniques Optical Transmission Simulations Results Conclusion University of Peloponnese Dept. of Inform. & Telecommunications
Increased capacity ↔ Advanced Modulation Formats 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
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) t Input Voltage 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 Vπ swing Simple Tx/Rx Configurations University of Peloponnese Dept. of Inform. & Telecommunications
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 LASER MZM electrical NRZ data precoder 2Vπ swing Differential-phase-shift-keying (DPSK) MF is a promising candidate to improve the performance of optical communication systems. In the DPSK modulation format, the information is carried by the phase of the optical carrier signal. The most interesting benefit of DPSK signals when compared to the conventional ASK format is the high receiver sensitivity due to the ~3dB lower OSNR required to achieve a given BER. DPSK can be used to extend the transmission distance, reduce optical power requirements, and relax component specifications. A commonly used NRZ-DPSK transmitter setup consists of an external MZM biased at minimum transmission and driven with a precoded binary data with twice the switching voltage required for OOK modulation 2Vπ. The advantage of DPSK over OOK is obtained at the expense of increased complexity and cost in the transmitter and receiver structure, such as a need of a differential encoder at the transmitter, a delay- interferometer at the receiver side and two photodiodes to construct a balanced receiver. University of Peloponnese Dept. of Inform. & Telecommunications
Fiber Impairments in Single Channel Systems Linear Non Linear Losses Dispersion inserts ASE noise SPM SMF DCF Rx Tx G G compensates dispersion compensates DCF losses compensates SMF losses University of Peloponnese Dept. of Inform. & Telecommunications
Chromatic Dispersion Effect Every different f travels with different velocity t 1 Optical Fibre 1 Some broadening input pulse t Dispersion Parameter: DSMF Length of Transmission: LSMF 1 Severe broadening ISI 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. 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 University of Peloponnese Dept. of Inform. & Telecommunications
Electronic Equalization 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. Receiver (Rx) Electric Filter Clock Recovery PIN y(t) y(n) I(n) Electronic Equalizer ADC 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
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
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
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.
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
Volterra Decision Feedback equalizer - VDFE Simplified VDFE used: University of Peloponnese Dept. of Inform. & Telecommunications
Complexity Calculations University of Peloponnese Dept. of Inform. & Telecommunications
Transmission Span (x N) Simulation Setup G Transmission Span (x N) Tx Rx BER Estimation w/o EDC equalizer with EDC SMF DCF 10 spans x 100km (1000km) 3 spans x 100km (300km) 10 Gb/s bitrate 40 Gb/s bitrate Here is a block diagram of the performed simulation setup. It consists of N number of identical spans. In case of 10 Gb/s the total tranmission length is 1000 km, while in case of 40 Gb/s it is 300 km. Every span has 2 amplifiers, one for compensating the SMF losses and another to compensate DCF Losses. Every amplifier has a noise figure of 5 dB. In the case of the scenario of uncompensated dispersion no DCF is used, therefore all the amount of dispersion is compensated only by the means of EE. In the scenario with variable amounts of ODCR, variable amounts of residual dispersion are accumulated at the end of the link. In all cases after the receiver, the 3 different equalizers are used in order to assess their performance. University of Peloponnese Dept. of Inform. & Telecommunications
Unncompensated Results 380 km 250 km 400 km 200 km 300 km 200 km 150 km University of Peloponnese Dept. of Inform. & Telecommunications
NRZ-OOK Results (1) University of Peloponnese 94% 70% 98% 80 % 85 % 87.5 % University of Peloponnese Dept. of Inform. & Telecommunications
NRZ-DPSK Results (1) OCR=70%-90% 10Gb/s & 40Gb/s DPSK University of Peloponnese Dept. of Inform. & Telecommunications
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 In our project we investigate a scenario of upgrading an existed link of 10 Gb/s to 40 Gb/s. 99% of its dispersion is compensated by the means of DCF as 100% DC is in practice difficult to be achieved. However even a residual dispersion of 5% becomes of major importance, since the dispersion tolerance reduces dramatically as the bitrate increases. We then deploy EE in order to investigate if the performance of the system is improved. This scenario is performed for both modulation formats. Dispersion Tolerance Reduces NRZ-DPSK NRZ-DPSK University of Peloponnese Dept. of Inform. & Telecommunications
Upgrading Scenarios Results Upgrading a system 10-40 NRZ & DPSK University of Peloponnese Dept. of Inform. & Telecommunications
Conclusion Low cost, adaptive techniques 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 with optical 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
Thank you for your attention! 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