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Capacity evolution of optical transmission
The ever increasing demand for information and communication requires capacity evolution from the metro and core networks. During the last decade, the quest for further capacity in the wavelength division multiplexing (WDM) system has implied the deployment of 10 Gbps Non-Return-to-Zero On/Off Keying (NRZ-OOK) channels operating on a 50 GHzchannel grid. The maximum capacity transported by 10 Gbps WDM long-haul transmission systems is ~1Tbps in the C-band (1530-1565 nm) corresponding to a spectral efficiency of 0.2 bit/s/Hz. Increasing further the spectral efficiency can be done either by broadening the amplifier gain bandwidth which is much lower than the total usable fiber bandwidth [Ess10], or by increasing the data rate carried by each channel. The top of Figure 2-1 represents the capacity evolution of single channel as well as WDM systems over almost 2 decades achieved in various record research experiments [Ess10]. Also shown in the bottom of this figure is the spectral efficiency attained by such systems. Around the nineteen‟s the data rate carried by a single channel was 10 Gbps. It was attained thanks to CD compensation and the implementation of forward error correction codes (FEC). Then the WDM technology developed in the mid-1990s has rapidly boosted the total capacity. Later, around 2008 the implementation of the coherent detector has enabled a data rate of 100 Gbps in a single channel. Coherent detection is considered as a technological breakthrough that allowshighdata-rate long-haul transmission. This technology is also expected to play a crucial role for any technique considered in the future in order to further raise capacity. The bottom plot of Figure 2-1 shows the evolution of the spectral efficiency, it attained 2 bits/s/Hz around 2000 and increased up to 10 bits/s/Hz in 2010. This is brought by the use of advanced modulation formats that have quickly replaced the NRZ-OOK systems in the long-haul transport.
What has permitted the EDFA invention?
The first major milestone in the evolution of optical fiber transmission systems was the invention of the erbium-doped fiber amplifiers (EDFA) at the end of the 80‟s. It replaces the expensive optical-to-electrical-to-optical (OEO) regenerators and it is able to amplify a multiplex of optical signals having their wavelength inside the amplificator bandwidth. This permits the increase of the system capacity by aggregating several wavelengths simultaneously without increasing the data rate carried by a wavelength. Multiplexing data over several wavelengths is called WDM.
Currently EDFA is the most practical way to amplify 50 GHz or 100 GHz optical channels within the ITU-T grid in the C-band. It is characterized by its low power consumption and its gain bandwidth located in the [1530 nm-1562 nm] spectral region (C-band) (Figure 2-2) that lines up with the low loss attenuation window of SSMF (minimal at 1550 nm) [Ram02]. However, a peak appeared around 1530 nm due to the high excitation levels. Thus gain flattening is required and it can be obtained through a filter designed to have an inverse profile of the gain spectrum. EDFA can be also designed to have its gain bandwidth in the Long, or L-band, from.
The coherent detection revival
The technological breakthrough that allowed the increasing of data rate was the introduction of coherent optical technology by Nortel in 2008 [Sun08]. Two things helped the implementation of this technology [Kik08]. The first is the ability to detect the signal amplitude and phase, thereby opening the way to the use of multilevel modulation formats in optical communication systems. By doing so, one can achieve spectral efficiencies much greater than intensity modulation with direct detectionwhich is limited to ~1 bit/s/Hz (0.8 bit/s/Hz in practice). Second, coherent detection can be efficiently combined with digital signal processing (DSP) algorithms able to fully compensate for CD and PMD directly in the electrical domain, at baseband. Thus coherent technology avoids the need for inline dispersion compensation fiber (DCF) in the transmission line, which reduces in turn the impact of nonlinearities and improves the received OSNR. It has been shown in [Pin12] that transmission over DCF free fiber line has a better resistance to nonlinear effects
Table of contents :
1 Introduction
2 OFDM for optical communication systems
2.1 Capacity evolution of optical transmission
2.1.1 What has permitted the EDFA invention?
2.1.2 From 2.5 to 10 Gbps based on OOK format
2.1.3 Toward 40 Gbps technologies
2.1.3.1 10 to 40 Gbps based on OOK format
2.1.3.2 The coherent detection revival
2.1.4 100 Gbps coherent DP-QPSK systems
2.1.5 Beyond 100 Gbps
2.2 Main physical impairments in long-haul optical fiber transmission
2.2.1 Attenuation
2.2.2 Chromatic Dispersion
2.2.3 Polarization mode dispersion
2.2.4 Optical amplification
2.2.5 Nonlinear effects in optical fiber
2.2.5.1 Self-phase modulation
2.2.5.2 Cross-phase modulation
2.2.5.3 Cross-polarization modulation
2.2.5.4 Four wave mixing
2.3 OFDM for optical transmission
2.3.1 Generic principles of OFDM
2.3.1.1 OFDM principles
2.3.1.2 Transmitter and receiver structures
2.3.1.3 OFDM orthogonality concept
2.3.1.4 Avoiding ISI: Cyclic prefix concept
2.3.2 OFDM for long haul coherent optical transmission
2.3.2.1 Appropriate detection technique for long-haul transmission
2.3.2.2 Typical OFDM transmitters structures
2.3.3 Multi-band optical OFDM approach
2.3.4 Optical OFDM experiments: state of the art
2.3.5 All-optical OFDM solution
2.3.6 No guard interval coherent optical OFDM
2.4 Conclusion
3 Dimensioning & signal processing architecture of the 100 Gbps OFDM format
3.1 OFDM parameters design
3.1.1 Transmission targets, hardware constraints and design choices
3.1.2 OFDM sub-band design methodology and results
3.2 Transmitter / receiver set up
3.2.1 25 Gbps Transmitter set up
3.2.2 Receiver set up
3.3 OFDM digital receiver design
3.3.1 Transmission impairments and signal processing model
3.3.2 Timing Synchronization
3.3.2.1 Impact of a timing offset
3.3.2.2 Timing synchronization algorithms
3.3.3 Frequency synchronization
3.3.3.1 Impact of CFO
3.3.3.2 CFO estimation and correction
3.3.3.2.1 Estimation of the fractional part of the CFO
3.3.3.2.2 Estimation of the integer part of the CFO
3.3.4 Channel equalization
3.3.4.1 Channel equalization for single polarization OFDM transmission
3.3.4.2 Extension to CO DP-OFDM transmission
3.3.4.2.1 Improving channel estimation
3.3.4.2.2 Time domain averaging
3.3.4.2.3 Frequency domain averaging
3.3.4.2.4 Combination of TD and FD averaging
3.3.5 Phase noise compensation
3.3.5.1 Impact of phase noise
3.3.5.2 Phase noise compensation
3.4 Conclusion
Bibliography
4 Experimental implementation of 100 Gbps MB-OFDM transmitter with polarization diversity coherent receiver
4.1 Proposed methodology
4.2 Electrical back-to-back arrangement
4.2.1 Preliminary experiment
4.2.1.1 Clipping and Pre-emphasis
4.2.1.2 Results
4.2.2 Complete experiment
4.3 Single polarization Tx/Rx design and validation
4.3.1 Experimental set-up
4.3.2 Transmitter adjustments
4.3.3 Experimental results
4.4 Dual polarization Tx/Rx design and validation
4.4.1 Experimental validation with homodyne detection
4.4.2 Experimental validation with heterodyne detection
4.5 100 Gbps transmitter set-up and validation
4.5.1 Multi-band approach for 100 Gbps OFDM signal generation
4.5.2 Performance comparison with 100 Gbps DP-QPSK system
4.6 Impact of Linear impairments impact
4.6.1 First and second PMD impact on the OFDM system performance
4.6.2 Impact of PMD and CD on the system performance
4.7 Conclusion
5 100 Gbps Transmission Experiments
5.1 Representation of the performance curves
5.2 Mixed 100 Gbps DP-MB-OFDM and 100 Gbps DP-QPSK transmission over
DCF-free fiber line
5.2.1 100 Gbps DP-QPSK transceiver description
5.2.2 100 Gbps Transmission performance
5.2.2.1 Experimental Results: BER Vs PIN SPAN
5.2.2.2 Experimental Results: BER Vs OSNR
5.3 Mixed 100 Gbps DP-MB-OFDM and 100 Gbps DP-QPSK transmission over dispersion-managed (DM) fiber line without the presence of 10 Gbps NRZ-OOK channels
5.3.1 Transmission performance comparison of the “Single-Channel” configuration with and without dispersion management
5.3.2 Performance comparison between “dispersion-managed WDM” , “Single- Channel” and “WDM” configurations
5.4 100 Gbps DP-MB-OFDM and 100 Gbps DP-QPSK transmission over legacy infrastructure including dispersion-managed (DM) fiber line and presence of 10 Gbps NRZ-OOK channels
5.4.1 Impact of the insertion of a guard band between the 10 Gbps NRZ-OOK channels and the 100 Gbps channels
5.4.2 Impact of the insertion of a power dissymmetry between the 10 Gbps NRZOOK channels and the 100 Gbps channels
5.5 Conclusion
6 Conclusion
