Design and simulation of 1.28 Tbps dense wavelength division multiplex system suitable for long haul backbone

Wavelength division multiplex (WDM) system with on / off keying (OOK) modulation and direct detection (DD) is generally simple to implement, less expensive and energy efficient. The determination of the possible design capacity limit, in terms of the bit rate-distance product in WDM-OOK-DD systems is therefore crucial, considering transmitter / receiver simplicity, as well as energy and cost efficiency. A 32-channel wavelength division multiplex system is designed and simulated over 1000 km fiber length using Optsim commercial simulation software. The standard channel spacing of 0.4 nm was used in the C-band range from 1.5436-1.556 nm. Each channel used the simple non return to zero - on / off keying (NRZ-OOK) modulation format to modulate a continuous wave (CW) laser source at 40 Gbps using an external modulator, while the receiver uses a DD scheme. It is proposed that the design will be suitable for long haul mobile backbone in a national network, since up to 1.28 Tbps data rates can be transmitted over 1000 km. A bit rate-length product of 1.28 Pbps.km was obtained as the optimum capacity limit in 32 channel dispersion managed WDM-OOK-DD system.Comment: Accepted for publication in Journal of Optical Communications - De Gruyte

Why Noise and Dispersion may Seriously Hamper Nonlinear Frequency-Division Multiplexing

The performance of optical fiber systems based on nonlinear frequency-division multiplexing (NFDM) or on more conventional transmission techniques is compared through numerical simulations. Some critical issues affecting NFDM systems-namely, the strict requirements needed to avoid burst interaction due to signal dispersion and the unfavorable dependence of performance on burst length-are investigated, highlighting their potentially disruptive effect in terms of spectral efficiency. Two digital processing techniques are finally proposed to halve the guard time between NFDM symbol bursts and reduce the size of the processing window at the receiver, increasing spectral efficiency and reducing computational complexity.Comment: The manuscript has been submitted to Photonics Technology Letters for publicatio

Optical phase conjugation in fiber-optic transmission systems

As the data rate of long-haul transmission links is increased, the design and realization of the transmission link becomes more difficult. As a result, more sophisticated methods are required to improve the transmission quality. The robustness of a transmission link can be increased and its structure greatly simplified by the use of mid-link optical phase conjugation (OPC). OPC is a promising technology to compensate for deterministic, phase related impairments (i.e. the Kerr effect and chromatic dispersion) in long-haul transmission systems. This thesis assesses the regenerative capabilities of OPC for the compensation of distortions that occur in modern transmission systems. The focus of the research is on transmission systems where OPC is employed to compensate for both chromatic dispersion and nonlinear impairments. The dispersion map (i.e. the dispersion as a function of the transmission distance) of such a transmission system is completely different from that of a conventional transmission system. The accumulated dispersion along the link of an OPCbased transmission system is significantly higher than that of a conventional transmission system. We investigated the influence of the dispersion map of OPC on nonlinear impairments using the non return-to-zero amplitude-shift-keying (NRZ-ASK) modulation format. As a result, it is shown that the peak powers that occur in the OPC transmission system are at a 10-Gbit/s/channel data rate significantly higher than the peak powers that occur in a conventional transmission system. The higher peak powers in the OPC based transmission system lead to an increased self-phase modulation (SPM) penalty. Through phase conjugation most of the SPM impairments are compensated for. However, when multiple wavelength division multiplexed (WDM) channels at narrow channel spacing are used for transmission, cross-phase modulation (XPM) is the dominating transmission impairment. Although XPM is principally a deterministic distortion, it must be treated as non-deterministic due to the dispersion of the transmission link. With simulations and experiments we show that because of this, the XPM compensation through OPC is marginal. At a 40 Gbit/s data rate, the peak powers that occur in the OPC-based transmission system are similar to those that occur in the conventional transmission system. In 40 Gbit/s WDM transmission systems the influence of XPM is relatively low. These transmission systems are rather limited by intra-channel nonlinear impairments such as SPM, intrachannel XPM (IXPM) and intrachannel FWM (IFWM). We show experimentally that in this case, the performance of the OPC transmission system is better than that of the conventional transmission system. When OPC is used to compensate for the chromatic dispersion, the OPC must be placed in the middle of the transmission link. This technique is often referred to as "midlink OPC". However, in some transmission links it is not possible to place the OPC exactly in the middle. Therefore, several configurations with a transmission length of 700 km to 900 km were assessed where the OPC was placed 100 km from the middle of the transmission link. In this experiment practically no bit-error ratio (BER) degradation was observed in the off-center configuration. Recently, strong interest has been shown in phase-shift keying modulation (PSK) formats such as differential phase-shift-keying (DPSK). DPSK’s main advantages over ASK are that it is more robust to narrowband optical filtering and has a 3 dB higher sensitivity in combination with balanced detection. However, unlike ASK signals PSK signals can be distorted by nonlinear phase noise (NPN). For long-haul transmission systems, the impact of NPN is so severe that the performance of DPSK is in some cases even worse than that of ASK. The impact of nonlinear phase noise is studied for 10.7-Gbit/s DPSK in an 800-km transmission link. In this experiment it is shown that impairments due to nonlinear phase noise can be significantly reduced using optical phase conjugation. The dependence of the location of the OPC within the transmission link is assessed as well. Allowing a penalty of 1 decade in BER from the optimum, the OPC-unit can be varied over a wide range, from nearly 1/3 to 2/3 of the transmission link. The combination of mid-link OPC is assessed with 21.4-Gbit/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) in an ultra long-haul transmission experiment. Error-free transmission after FEC is realized over 10,200 km for all 22 WDM channels. In this experiment, a single OPC-unit is used in the middle of the link to compensate for an accumulated chromatic dispersion of over 160,000 ps/nm. Along the transmission line, the dispersion accumulates in this experiment to more than 80,000 ps/nm. This is significantly higher than the maximum accumulated dispersion in the conventional transmission system (approximately 3,000 ps/nm). The high accumulated dispersion results in an extreme overlap of the pulses along the transmission line. With this experiment we show that despite the high dispersion, the feasible transmission distance of the OPC based transmission system is 44% greater than that obtained in the conventional transmission system. By doubling the data rate and keeping the 50-GHz channel spacing, a 0.8-bit/s/Hz spectral efficient WDM transmission system is realized. At 42.8-Gbit/s RZ-DQPSK, transmission over 5,000 km was realized with mid-link OPC. Compared to the feasible transmission distance obtained at 21.4-Gbit/s, the feasible transmission distance is reduced by about 50%. This reduction of transmission distance with 50% corresponds to the 3-dB OSNR penalty that is present between 21.4-Gbit/s and 42.8-Gbit/s RZ-DQPSK in the back-toback configuration. For the conventional transmission system, a greater reduction in the feasible transmission distance (factor of 2.4) is measured due to increased penalties that result from a combination of self phase modulation and nonlinear phase noise. Comparing the feasible transmission distance of the OPC to the conventional transmission system an improvement of 60% is observed in this experiment

Advanced DSP for coherent optical fiber communication

In this paper, we provide an overview of recent progress on advanced digital signal processing (DSP) techniques for high-capacity long-haul coherent optical fiber transmission systems. Not only the linear impairments existing in optical transmission links need to be compensated, but also, the nonlinear impairments require proper algorithms for mitigation because they become major limiting factors for long-haul large-capacity optical transmission systems. Besides the time domain equalization (TDE), the frequency domain equalization (FDE) DSP also provides a similar performance, with a much-reduced computational complexity. Advanced DSP also plays an important role for the realization of space division multiplexing (SDM). SDM techniques have been developed recently to enhance the system capacity by at least one order of magnitude. Some impressive results have been reported and have outperformed the nonlinear Shannon limit of the single-mode fiber (SMF). SDM introduces the space dimension to the optical fiber communication. The few-mode fiber (FMF) and multi-core fiber (MCF) have been manufactured for novel multiplexing techniques such as mode-division multiplexing (MDM) and multi-core multiplexing (MCM). Each mode or core can be considered as an independent degree of freedom, but unfortunately, signals will suffer serious coupling during the propagation. Multi-input−multi-output (MIMO) DSP can equalize the signal coupling and makes SDM transmission feasible. The machine learning (ML) technique has attracted worldwide attention and has been explored for advanced DSP. In this paper, we firstly introduce the principle and scheme of coherent detection to explain why the DSP techniques can compensate for transmission impairments. Then corresponding technologies related to the DSP, such as nonlinearity compensation, FDE, SDM and ML will be discussed. Relevant techniques will be analyzed, and representational results and experimental verifications will be demonstrated. In the end, a brief conclusion and perspective will be provided

Coherent 100G Nonlinear Compensation with Single-Step Digital Backpropagation

Enhanced-SSFM digital backpropagation (DBP) is experimentally demonstrated and compared to conventional DBP. A 112 Gb/s PM-QPSK signal is transmitted over a 3200 km dispersion-unmanaged link. The intradyne coherent receiver includes single-step digital backpropagation based on the enhanced-SSFM algorithm. In comparison, conventional DBP requires twenty steps to achieve the same performance. An analysis of the computational complexity and structure of the two algorithms reveals that the overall complexity and power consumption of DBP are reduced by a factor of 16 with respect to a conventional implementation, while the computation time is reduced by a factor of 20. As a result, the proposed algorithm enables a practical and effective implementation of DBP in real-time optical receivers, with only a moderate increase of the computational complexity, power consumption, and latency with respect to a simple feed-forward equalizer for dispersion compensation.Comment: This work has been presented at Optical Networks Design & Modeling (ONDM) 2015, Pisa, Italy, May 11-14, 201

Performance limits in optical communications due to fiber nonlinearity

In this paper, we review the historical evolution of predictions of the performance of optical communication systems. We will describe how such predictions were made from the outset of research in laser based optical communications and how they have evolved to their present form, accurately predicting the performance of coherently detected communication systems

Wavelength-division-multiplexed Transmission Using Semiconductor Optical Amplifiers And Electronic Impairment Compensation

Over the last decade, rapid growth of broadband services necessitated research aimed at increasing transmission capacity in fiber-optic communication systems. Wavelength division multiplexing (WDM) technology has been widely used in fiber-optic systems to fully utilize fiber transmission bandwidth. Among optical amplifiers for WDM transmission, semiconductor optical amplifier (SOA) is a promising candidate, thanks to its broad bandwidth, compact size, and low cost. In transmission systems using SOAs, due to their large noise figures, high signal launching powers are required to ensure reasonable optical signal-to-noise ratio of the received signals. Hence the SOAs are operated in the saturation region and the signals will suffer from SOA impairments including self-gain modulation, self-phase modulation, and inter channel crosstalk effects such as cross-gain modulation, cross-phase modulation, and four-wave mixing in WDM. One possibility to circumvent these nonlinear impairments is to use constant-intensity modulation format in the 1310 nm window where dispersion is also negligible. In this dissertation, differential phase-shift keying (DPSK) WDM transmission in the 1310 nm window using SOAs was first considered to increase the capacity of existing telecommunication network. A WDM transmission of 4 x 10 Gbit/s DPSK signals over 540 km standard single mode fiber (SSMF) using cascaded SOAs was demonstrated in a recirculating loop. In order to increase the transmission reach of such WDM systems, those SOA impairments must be compensated. To do so, an accurate model for quantum-dot (QD) SOA must be established. In this dissertation, the QD-SOA was modeled with the assumption of overall charge neutrality. Static gain was calculated. Optical modulation response and nonlinear phase noise were studied semi-analytically based on small-signal analysis. The quantitative studies show that an ultrafast gain recovery time of ~0.1 ps can be achieved when QD-SOAs are under high current injection, which leads to high saturation output power. However more nonlinear phase noise is induced when the QD-SOAs are used in the transmission systems operating at 10 Gbit/s or 40 Gbit/s. Electronic post-compensation for SOA impairments using coherent detection and digital signal processing (DSP) was investigated next in this dissertation. An on-off keying transmission over 100 km SSMF using three SOAs at 1.3 [micrometer] were demonstrated experimentally with direct detection and SOA impairment compensation. The data pattern effect of the signal was compensated effectively. Both optimum launching power and Q-factor were improved by 8 dB. For advanced modulation formats involving phase modulation or in transmission windows with large dispersion, coherent detection must be used and fiber impairments in WDM systems need to be compensated as well. The proposed fiber impairment compensation is based on digital backward propagation. The corresponding DSP implementation was described and the required calculations as well as system latency were derived. Finally joint SOA and fiber impairment compensations were experimentally demonstrated for an amplitude-phase-shift keying transmission