Long-haul Optical Networks Research Articles

Performance analysis of NGPON using hybrid GPON-XGPON based robust long-haul passive optical communication network

Abstract For high data rates and extended reach capabilities, communications networks need to be enhanced by employing passive optical networks (PONs). NG-PON plays a key role in achieving the modern demands. This paper proposed novel hybrid G-PON and XG-PON based robust long-haul Passive Optical Communication Network. In this proposed network hybrid semiconductor optical amplifier (SOA) and EDFA are used to enhance the system performance for downstream (D.S.) and upstream (U.S) signals transmission. This paper analyses a hybrid G-PON and XG-PON bi-directional optical fiber link up to length of 100-km with a 128 (1:64 and 1:64) way-splitter to increase the coverage area and to enhance the quality of transmitted data. The qualitative analysis of the proposed network shows an improved performance in terms of transmission distance, quality factor and eye-diagrams.

5G Mobile Wireless Access and Digital Channeling with RF Over Fiber for Long-Haul 64-QAM Communication

This paper presents the experimental results of a novel radio-over-fiber (RFoF) architecture that is capable of transmitting data at 20 Gb/s over a 600 km fiber. Such high bit rates required by 5G systems for long-haul communication. This is accomplished by overcoming the limitation of single mode fibers (SMF) of power attenuation and chromatic dispersion. This is achieved by interspersing sections of the SMF with a dispersion compensating filter (DCF) and embedding midway of the fiber length with chirped fiber Bragg grating (CFBG). Moreover, the proposed approach is shown to enhance the total bandwidth over a long-haul gigabit passive optical network (GPON) using integrated channel coarse wavelength division multiplexing (CWDM). In the experiment, a 64-quadrature amplitude modulation (64-QAM) signal was modulated onto CWDM signal. With the proposed RFoF architecture the signal power received over a 600 km fiber is greater than a conventional optical architecture by 19 dB for identical applied input optical power of 20 dBm. This translates to significant power savings that can be achieved with the proposed architecture over existing architectures that should reduce the carbon footprint of long-haul optical communications systems. The optical receiver is shown to have a sensitivity of −28.3 dBm for a bit-error rate (BER) of 10−9. These results demonstrate the viability of the proposed RFoF architecture in providing significant improvement in the bit rate over a transmission distance.

Reconfigurable multichannel amplitude equalizer based on cascaded silicon photonic microrings

A compact on-chip reconfigurable multichannel amplitude equalizer based on cascaded elliptical microrings is proposed and demonstrated experimentally. With the optimized structure of the elliptical microring with adiabatically varied radii/widths, the average excess loss for each channel in the initialized state is measured to be less than 0.5 dB, while the attenuation dynamic range can be over 20 dB. Flexible tunability through the overlapping of the resonance peaks of adjacent wavelength-channels enables even higher attenuation dynamic ranges up to 50 dB. Leveraging the thermo-optic effect and fine wavelength-tuning linearity, precise tuning of the resonance peak can be implemented, enabling dynamic power equalization of each wavelength-channel in wavelength-division-multiplexing (WDM) systems and optical frequency combs. The proposed architecture exhibits excellent scalability, which can facilitate the development of long-haul optical transport networks and high-capacity neuromorphic computing systems, while improving the overall performance of optical signals in WDM-related systems.

Position-Controlled Telecom Single Photon Emitters Operating at Elevated Temperatures.

A key resource in quantum-secured communication protocols are single photon emitters. For long-haul optical networks, it is imperative to use photons at wavelengths compatible with telecom single mode fibers. We demonstrate high purity single photon emission at 1.31 μm using deterministically positioned InP photonic waveguide nanowires containing single InAsP quantum dot-in-a-rod structures. At excitation rates that saturate the emission, we obtain a single photon collection efficiency at first lens of 27.6% and a probability of multiphoton emission of g(2)(0) = 0.021. We have also evaluated the performance of the source as a function of temperature. Multiphoton emission probability increases with temperature with values of 0.11, 0.34, and 0.57 at 77, 220 and 300 K, respectively, which is attributed to an overlap of temperature-broadened excitonic emission lines. These results are a promising step toward scalably fabricating telecom single photon emitters that operate under relaxed cooling requirements.

Performance Analysis of a Linear Gaussian- and tanh-Apodized FBG and Dispersion Compensating Fiber Design for Chromatic Dispersion Compensation in Long-haul Optical Communication Networks

This paper investigates a novel compensation technique of dispersion effect mitigation using a combination of three- and four-stage-apodized fiber Bragg gratings (FBG) and dispersion compensating fiber (DCF) designs. Two designs using three-stage and four-stage FBG and DCF in combination have been proposed and compared for their performance in mitigating chromatic dispersion effects at 100 km SMF. The performance of each design has been evaluated using Q-factor results using linear Gaussian- and tanh-apodized fiber Bragg gratings. Each profile manifested different Q-factor results over a range of 5 dBm, 7.5 dBm, and 10 dBm of CW laser power over FBG grating lengths from 4 mm to 8 mm. The results obtained using the three-stage and four-stage FBG and DCF designs showed that an apodization profile using a tanh function can be used successfully with FBG lengths from 4 mm to 8 mm, regardless of the CW launched power. In contrast, the results using a Gaussian apodization profile for three- and four-stage FBG and DCF designs are applicable to FBG lengths from 5 mm to 8 mm. Designs using three-stage FBG and DCF generated higher Q-factor results than designs using only four-stage FBG and DCF, regardless of the launched power. The highest Q-factor of 18.58 was obtained for three-stage tanh-apodized FBG and DCF used in combination for an FBG length of 6 mm. The highest result obtained for a three-stage Gaussian-apodized FBG and DCF design was a Q factor of 17.13 using an FBG length of 8 mm. The proposed method was also compared to current similar works and can be successfully implemented in long-haul optical communication.

The design and implementation of a software based gain control for EDFAs used in long-haul optical networks

The optical gain spectra of erbium-doped fiber amplifiers (EDFAs) have a nonlinear structure in the C-band region. Therefore, when the optical amplifier chain is used, in other words, when successive amplifications are made, the gain values obtained at some wavelengths will differ. As a consequence, some wavelengths will be reduced and will disappear before reaching the receiver side in long-haul optical networks. To overcome this, it is attempted to bring the gain values to the same level for all wavelengths used. Electronically, the dynamic gain value is achieved by controlling the power of the pump laser. Pump lasers are controlled based on two parameters consisting of current and temperature. In this study, an EDFA operating in the C-band region was designed, and an electronic board that would control the gain of this EDFA and software to control it via computer were implemented. In conclusion, an automatic control of the gain of the EDFA operating in the C-band region was achieved, and gain ripple values in the range of 0.17–0.30 dB were obtained.

Hybrid compensation of polarization-multiplexed QPSK optical format for high bit rate networks

<span lang="EN-GB">Transmitting the highest capacity throughput over the longest possible distance without any regeneration stage is an important goal of any long-haul optical network system. Accordingly, Polarization-Multiplexed Quadrature Phase-Shift-Keying (PM-QPSK) was introduced lately to achieve high bit-rate with relatively high spectral efficiency. Unfortunately, the required broad bandwidth of PM-QPSK increases the linear and nonlinear impairments in the physical layer of the optical fiber network. Increased attention has been spent to compensate for these impairments in the last years. In this paper, Single Mode Fiber (SMF), single channel, PM-QPSK transceiver was simulated, with a mix of optical and electrical (Digital Signal Processing (DSP)) compensation stages to minimize the impairments. The behaviour of the proposed system was investigated under four conditions: without compensation, with only optical compensator, with only DSP compensator and finally with both compensators. An evidence improvement was noticed in the case of hybrid compensation, where the transmission distance was multiplied from (720 km) to more than (3000 km) at 40 Gb/s.</span>

Experimental Demonstration of Dual O+C-Band WDM Transmission Over 50-km SSMF With Direct Detection

In this article, we experimentally demonstrate dual O+C-band direct-detection wavelength division multiplexed (WDM) transmission over 50 km of standard single-mode fibre, wherein 4 × 40-Gb/s Nyquist on-off keying (OOK) modulation is used in the O-band whilst 4 × 40-Gb/s single-sideband (SSB) subcarrier modulation (SCM) with Kramers–Kronig (KK) detection is used in the C-band. A bismuth-doped fibre amplifier (BDFA) is used as the pre-amplifier in the O-band to extend the reach of the direct-detection system. It is demonstrated that the O-band exhibits superior receiver sensitivity relative to the C-band and its transmission performance is limited by the chromatic dispersion (CD), especially when longer reaches and wavelengths in the longer edge of the band are explored. In contrast, due to the adoption of the SSB format, the C-band shows enhanced tolerance to CD effects. In this case, some receiver sensitivity is sacrificed to accommodate the adoption of the KK detection which is used to eliminate the signal-to-signal beating interference of the SSB-SCM format. The inferior receiver sensitivity in the C-band is caused by (1) the higher CSPR requirement of the KK detection and (2) the restriction of small-signal modulation for the dual-drive Mach–Zehnder modulator to generate optical SSB signals. The results indicate the feasibility to further extend the reach of O-band transmission systems, which enables the possibility to realise dual O+C-band WDM transmission in intermediate- to long-haul optical networks.

Ultra-low-loss and large-effective-area fiber for 100 Gbit/s and beyond 100 Gbit/s coherent long-haul terrestrial transmission systems

Ultra-low-loss and large-effective-area fiber has been successfully applied in transoceanic transmission, which is considered as a promising candidate for 100 Gbit/s and beyond 100 Gbit/s coherent long-haul terrestrial optical networks. Several theoretical and experimental investigations have been reported, including provincial terrestrial field trial. To support long-haul terrestrial application, it is urgent to prove that the ultra-low-loss and large-effective-area fiber after terrestrial deployment can significantly enhance the performance of long-haul transmission over 1000 km compared with the conventional single mode fiber. In this paper, we extended our previous work and summarized design methods for complex terrestrial environment. To verify the fiber characteristics in long-haul terrestrial transmission, we installed the longest terrestrial ultra-low-loss and large-effective-area fiber link in the world with a total length of 1539.6 km. The results show that the transmission performances of wavelength-division-multiplexed signals with per-channel data rates of 100 Gbit/s, 200 Gbit/s, and 400 Gbit/s over the ultra-low-loss and large-effective-area fiber are all obviously improved, demonstrating that this fiber is more suitable for ultrahigh-speed long-haul terrestrial transmission.

Comparative Simulation of Thermal Noise Effects for Photodetectors on Performance of Long-Haul DWDM Optical Networks

Abstract Long-haul 16-channel dense-wavelength division multiplexing networks employing two different avalanche photodiode (APD) structures (Si and InGaAs) and positive-intrinsic-negative (PIN) photodetectors are simulated and compared under thermal noise effects for different fiber lengths. The effect of thermal noise level on the transmission quality with a variation of amplifying section length, number of amplifying sections and channel speed is discussed. The impact of thermal noise on the system performance is analyzed by varying input power from −5dBm to 20dBm for both 25 km and 50 km amplifying section at 100 km fiber length. The performance is evaluated for both 5 Gb/s and 10 Gb/s data rates over transmission distances up to 500 km. A comprehensive comparison is developed based on signal-to-noise ratio (SNR), quality factor (Q-factor) and bit error rate (BER). It is found that both APD structures achieve superior performance up to distance of 350 km comparing to PIN photodetectors for 50 km amplifying section. The system provides optimum performance at input power P in = 10dBm in case of 50 km amplifying section, but then afterwards, the performance is degraded rapidly due to nonlinearities. The results revealed that the worst performance scenario is at 10–18 W/Hz thermal noise in terms of higher BER and lower Q-factor. Finally, the desirable BER of 10–12 is achieved at Q-factor of 6.78 and SNR of 23 dB.

A Memetic based Approach for Routing and Wavelength Assignment in Optical Transmission Systems

In optical networks, Routing and Wavelength Assignment (RWA) problem is one of the major optimization problems. This problem can be solved by different algorithms such as Genetic Algorithm (GA), Artificial Bee Colony (ABC), Ant Colony Optimization (ACO), etc. Shuffled Frog Leaping Algorithm (SFLA) is implemented in the proposed work, to solve the RWA problem in long-haul optical networks. The goal is to use minimum number of wavelengths and to reduce the number of connection request rejections. Cost, number of wavelengths, hop count and blocking probability are the performance metrics considered in the analysis. Various wavelength assignment methods such as first fit, random, round robin, wavelength ordering and Four Wave Mixing (FWM) priority based wavelength assignment are used in the analysis using SFLA. Number of wavelengths, hop count, cost and setup time are included in the fitness function. The SFLA algorithm proposed, has been analyzed for different network loads and compared with the performance of genetic algorithm.

Transmission Performance Analysis of WDM Radio over Fiber Technology for Next Generation Long-Haul Optical Networks

This paper presents a detailed study of N-channels Wavelength Division Multiplexing (WDM) Optical transmission system using Radio over Fiber (RoF) technology. The study was applied to optical long-haul networks to overcome the nonlinearity effects, chromatic dispersion, and signal loss. For this purpose, Fiber Bragg Grating (FBG) has been implemented in 4-channels, 8-channels, and 16 channels WDM transmission system network at 10 Gb/s to compensate the dispersion and the nonlinear distortion. The use of erbium-doped fiber amplifiers (EDFA) has been also investigated to improve the quality of the transmission system. In Digital RoF, the impact analysis of modulation types such as Differential Phase-Shift Keying (DPSK) and Quadrature Amplitude Modulation (QAM) is also introduced. Constellation diagrams, received optical power, types of modulation, fiber dispersion, channel spacing variation, and laser power were considered to validate the study with the existing studies. All results achieve good reliability performance and prove the efficiency of the presented model.

Minimization of Nonlinear Impairments and Its Impact on Transmission Performances of High-Capacity Long-Haul Optical Networks

Abstract High-capacity and long-haul transmission gained great significance in modern communication networks. Optical fiber communication system is good enough to face the high demand of current telecom terrific. This paper will propound the theoretical model showing nonlinear factors which degrade the transmission performances of high-capacity long-haul network. The postulatory model will be validated using simulation of key nonlinear factors such as effective area, launch power, refractive index and fiber length. The transmission performance of the high-capacity long-haul optical network would be analyzed on the basis of some key parameters such as bit error rate and signal-to-noise ratio. Mitigation of nonlinear impairments shows significant impact on transmission performances of high-capacity long-haul optical networks.

Filtered Carrier Phase Estimator for High-Order QAM Optical Systems

We investigate, using Monte Carlo simulations, the performance characteristics and limits of a low-complexity filtered carrier phase estimator (F-CPE) in terms of cycle-slip occurrences and signal to noise ratio (SNR) penalties. In this work, the F-CPE algorithm has been extended to include modulation formats whose outer-ring symbols have a quadrature phase shift keying (QPSK) symmetry, and which are applicable to metro and long-haul optical networks: QPSK, 8, 16, and 64 quadrature-amplitude modulation (QAM). The proposed joint-polarization approach, where the number of non-null symbols in a simplified QPSK partition is increased, shows a further improvement in robustness against cycle slips, resulting in cycle-slip-free operation at symbol rate 32 GBd and laser linewidths up to 900 kHz, for the range of investigated SNRs. In addition, it reduces SNR penalties for only a small incremental complexity. We also propose a method for constellation alignment that exploits F-CPE computational blocks to minimize the electronic footprint, in order to compensate for an arbitrary rotation, introduced by F-CPE. We further show that F-CPE can successfully recover the phase of a 32QAM signal that does not have the outer-ring QPSK symmetry, albeit with increased penalties and higher cycle-slip rates. A performance comparison between F-CPE, the blind phase search algorithm, and a data-aided phase estimator, is also presented.

A New Paradigm in High-Speed and High-Efficiency Silicon Photodiodes for Communication—Part I: Enhancing Photon–Material Interactions via Low-Dimensional Structures

Photodetectors (PDs) used in communication systems require ultrafast response, high efficiency, and low noise. PDs with such characteristics are increasingly in demand for data centers, metro data links, and long-haul optical networks. In a surface-illuminated PD, high speed and high efficiency are often a tradeoff, since a high-speed device needs a thin absorption layer to reduce the carrier transit time, whereas a high-efficiency device needs a thick absorption layer to compensate for the low absorption coefficient of some semiconductors such as Si and Germanium (Ge) or SiGe alloys at wavelengths near the bandgap. In this part of this review, we present the recent efforts in enhancing the photon–material interactions by using low-dimensional structures that can control light for more interaction with the photoabsorbing materials, slow down the propagation group velocity and reduce surface reflection. We present recent demonstrations of high-speed PDs based on nanostructures enabled by both synthetic bottom-up or transformative top-down processing methods. In particular, we detail a CMOS-compatible ultrafast surface-illuminated Si PD with 30-ps full-width at half-maximum, and >50% efficiency at 850 nm. A complementary discussion on device challenges and the integration of low-dimensional structures will be presented in the part II of this review.

Optimized Hybrid Raman/EDFA Amplifier Placement for DWDM Mesh Networks

The minimization of the number of expensive 3R regenerators required at intermediate nodes is a key task when planning long-haul optical transport networks. Combining Raman amplification with Erbium-doped fiber amplifiers (EDFAs) is one of the strategies that can be exploited to achieve this objective, especially in the presence of long fiber spans. Given that hybrid Raman/EDFA amplifiers are more expensive than EDFAs, their deployment should also be minimized. However, determining the number and location of these amplifiers in a mesh optical network is a complex problem because of the overlap of lightpaths with different source and destination nodes at each link. A method to optimize the placement of hybrid Raman/EDFA amplifiers in mesh optical networks is described in this paper. The proposed method analyzes the network as a whole and determines where to introduce Raman amplification such that the 3R regenerator count is minimized while also keeping the number of hybrid Raman/EDFA amplifiers at a minimum. It is shown that, in a reference mesh optical network, the proposed method clearly outperforms the simpler placement approach based solely on fiber span attenuation.

Resource Allocation for Elastic Optical Networks Using Geometric Optimization

Resource allocation with quality of service constraints is one of the most challenging problems in elastic optical networks that is normally formulated as a mixed-integer nonlinear optimization program. In this paper, we focus on novel properties of geometric optimization and provide a heuristic approach for resource allocation that is much faster than its mixed-integer nonlinear counterpart. Our heuristic consists of two main parts for routing/traffic ordering and power/spectrum assignment. It aims at minimization of the transmit optical power and spectrum usage constrained to the quality of service and physical requirements. We consider three routing and three traffic ordering procedures and compare them in terms of total transmit optical power, total received noise power, and total nonlinear interference including self- and cross-channel interferences. We propose a posynomial expression for the signal-to-noise ratio in which fiber nonlinearities and amplified spontaneous emission noise have been addressed. We also propose posynomial expressions that relate the modulation spectral efficiency to its corresponding minimum required signal-to-noise ratio. We then use the posynomial expressions to develop six geometric formulations for the power/spectrum assignment part of the heuristic that are different in run time, complexity, and accuracy. Simulation results demonstrate that the proposed solution has very good accuracy and much lower computational complexity in comparison with mixedinteger nonlinear formulation. As an example, for the European Cost239 optical network with 126 transmit transponders, the geometric formulation can be more than 22 times faster than its mixed-integer nonlinear counterpart. Numerical results also reveal that, in long-haul elastic optical networks, considering the product of the number of common fiber spans and the transmission bit rate is a better goal function for routing/traffic ordering that can reduce the total transmit optical fiber (total accumulated noise and nonlinear interference) up to 20% (15%).

Review of Comparative Booster Performances of Semiconductor Optical Amplifier and Erbium-doped-fiber Amplifier for use in Future Long-haul Optical Networks

This study reviews the performance of the Semiconductor Optical Amplifier (SOA) photonic device with the classic Erbium-doped-fiber-amplifier (EDFA). In certain situations, we report that the SOA out-performs the EDFA, particularly in terms of gain and Q-factor and BER of the post-amplified received signal. This substantiates its potential as a viable replacement for the EDFA in future all-optical, long haul networks.

Large-Area Low-Loss Fibers and Advanced Amplifiers for High-Capacity Long-Haul Optical Networks [Invited

This paper reviews recent progress on ultra-large-area low-loss fibers for next-generation high-capacity terrestrial long-haul optical networks. The key optical fiber properties of new class fibers are described, and their impact on the transmission performance for 400 Gb/s polarization-multiplexed multilevel modulation coherent transmissions is discussed. The practical consideration of the large-area fibers, such as splicing and cabling for terrestrial transport systems, is also briefly addressed. In addition, we describe two advanced optical fiber amplifier technologies that will improve the efficiency in utilization of optical networking and reduce total system costs. The design and performance of an arrayed optical fiber amplifier using a compact ribbonized Er-doped fiber (EDF) for next-generation reconfigurable optical add/drop multiplexer nodes are discussed, and the performance characteristics of complementary Raman/EDFA, which has +70 nm bandwidth for seamless C+L-band transmissions, are described. Finally, we present the experimental demonstration of transmission of 34.6 Tb/s in 70 nm single band over 2400 km fiber.

Advanced modulation formats for 400-Gbps short-reach optical inter-connection.

Besides the long-haul optical networks covering over thousands of kilometers for backbone transmission, short reach optical networks (SR-ONs) are widely deployed in metro-area for aggregation and accessing. The SR-ONs include the metro optical transport networks (Metro-OTN), optical access networks or other optical inter-connection systems with even shorter distance. As predicted, the growing bandwidth demanding from SR-ONs will be much more than that from the long-haul optical networks in the near future. Besides, there are tremendous amounts of optical terminals and end-users in SR-ONs compared with the long-haul transmission systems and thus will induce large cost and huge energy consumption. So, the power and cost efficiency should be the key consideration for SR-ONs besides the transmission performance. To improve the power and cost efficiency in SR-ONs, advanced modulations and detection techniques based on low power, low cost and integrated optical modulators should be utilized. In this paper, different advanced modulation formats have been discussed. 56Gbps PAM4, 112Gbps poly-binary and 100Gbps DMT that can be used to realize 400-Gbps SR-ONs for different applications have also been demonstrated respectively. In addition, low-cost and low-power opto-electronic components suitable for SR-ONs, the impairments induced by all kinds of defects and bandwidth limitation of opto-electronic components and the corresponding compensation techniques based on DSP algorithms have also been discussed in the experiments.