Ultra-dense WDM Research Articles

Performance of different hybrid dispersion compensation modules (DCMs) in long reach ultra dense WDM passive optical networks

Abstract Long reach Passive optical network (LR-PON) is an attractive solution to fulfill the ever-increasing bandwidth requirements due to propelling internet applications and competent to serve distant optical network units (ONUs). Wavelength division multiplexed (WDM) PON systems experience distance and performance limiting constraint termed as Dispersion. In order to compensate dispersion effects, Fiber bragg gratings (FBGs) and Dispersion compensation fibers (DCFs) are incorporated extensively in PONs. Performance of DCF is better than FBG in terms of dispersion compensation, but it comes at the cost of 3 $/m (very expensive). Therefore, long reach ultra dense WDM-PON systems are needed with incorporation of economical and high performance DCMs. Three newly constructed hybrid DCMs are investigated such as FBG-DCF (module 1), OPC-DCF (module 2), and FBG-DCF-OPC (module 3) in WDM-PON to get optimal DCM in terms of dispersion compensation efficiency (DCE) and economical operation. As per author’s best knowledge, DCE calculations and performance enhancement with cost reduction using hybrid DCMs in ultra dense WDM-PON, is not reported so far. WDM-PON consists of 32 channels at 25 GHz channel spacing is analyzed for 300 km link distance at 10 Gbps/channel using different hybrid DCMs. It is perceived that highest DCE of 70% is given by module 3 with maximum cost reduction of 19.84%. DCE performance of three modules is as follows: Module 3 (DCE 70%), Module 1 (DCE 55%), Module 2 (DCE 45%) and cost reduction/increase from conventional module by 19.84% reduction (Module 3), 19.05% reduction (Module 1), and increase 10.5% (Module 2). Hence, Module 3 is preferred for long reach WDM-PON to get high performance with lesser cost.

Performance Evaluation of Diverse Hybrid Pulse Width Reduction Modules in WDM systems

Long reach wavelength division multiplexing (WDM) systems are required to cater the remote locations and to cover the maximum distances. Dispersion is prominent factor for distance limitation and should be addressed with high efficiency as well as using low cost pulse width reduction (PWR) modules. In this research work, hybridization of different PWR is proposed such as FBG-DCF, OPC–DCF (optical phase conjugation), FBG-DCF-OPC to analyse the PWRE and total cost of the modules. Further, in order to get optimal hybrid PWR module and amplification unit, different optical amplifiers are also investigated at 10 Gbps over 300 km. An ultra dense WDM system with 25 GHz channel spacing using 32 channels is demonstrated and PWRE is analysed using different hybrid PWR modules. Results revealed that FBG-DCF having least cost with PWRP of 55%, OPC-DCF show PWRE of 45% with moderate cost, and FBG-DCF-OPC provide maximum PWRE of 70% with little bit higher cost. Performance sequence of investigated PWR modules is give as: FBG-DCF-OPC (70% PWRE)> FBG-DCF (55% PWRE)>OPC-DCF (45% PWRE)> and performance of amplification unit with FBG-DCF-OPC is depicted as EDFA>RFA>SOA). This module can be used in passive optical networks for long reach systems.

Performance enhancement of ultra-dense WDM over FSO hybrid optical link by incorporating MIMO technique

Abstract Free-space optical (FSO) communication is a wireless optical data transmission technology with a high data transmission rate. It has received much attention in recent years as it is cost-effective and has license free operation. It is line of sight free-space communication technique where optical signal severely degraded from atmospheric losses especially due to weather conditions; hence it restricts the link range and data carrying capacity. Therefore, a 16-channel ultra-dense wavelength division multiplexing–free space optics (UWDM–FSO) system each having each 10 Gb/s data rate is proposed to enhance the capacity and performance of FSO system. To authenticate the performance of the proposed system, investigation for different modulation formats such as nonreturn to zero (NRZ), return to zero (RZ), carrier suppressed return to zero (CSRZ) and duo binary (DB) are reported. Further, to reduce the atmospheric interference, multiple input multiple output (MIMO) technique is integrated into the proposed system. The outcomes of MIMO–UWDM–FSO link revealed a significant improvement in the bit error rate (BER), eye diagram and Q-factor, under different weather conditions. It is also observed that NRZ modulation formats perform better than RZ, CSRZ and DB formats.

Performance enhancement of Raman + EYDFA HOA for UD-WDM system applications

Abstract In this work, we study and analyze the performance of Raman + Erbium-Ytterbium codoped fiber hybrid optical amplifier (HOA) for an ultradense wavelength division multiplexing (UD-WDM) system having 100 channels. The system has been investigated considering initial values of channel spacing and data rate of 0.1 nm (12.5 GHz) and 100 GB/s, respectively. Initially, the two important WDM system parameters—wavelength and channel spacing—have been selected and then optimization of the proposed HOA has been performed in terms of EYDFA length, pump power and Er+ concentration to achieve higher values of average gain, Q-factor and lower gain variation ratio. The optimized configuration of the HOA results in the achievement of higher value of average gain, Q-factor and gain variation ratio of 47 dB, 14 and 0.14, respectively, which confirms its viability for UD-WDM system applications.

Flexible coherent UDWDM-PON with dynamic user allocation based on limited-tunability lasers

Coherent wavelength division multiplexing (WDM) technologies have leveraged the optical communication systems in core networks, increasing the fiber capacity by transmission with advanced modulation formats and mitigation of impairments with digital signal processing. However, these solutions are too expensive for access networks, where cost, power budget, and footprint are limited. Hence, the key technology will be developing low-cost coherent transceivers providing an excellent selectivity and giving high sensitivity, which allows high splitting ratios. This paper reports an experimental design of a low-cost coherent ultra-dense WDM passive optical network (UDWDM-PON) with 6.25 GHz channel spacing. The users’ optical network unit (ONU) is built employing coherent transceivers with two paired low-cost distributed feedback (DFB) lasers, one as the local oscillator and another as the transmitter, offering simplicity and low-cost hardware; likewise, the optical line terminal (OLT) at the central office can profit from the same design. The ONU DFB lasers have wavelengths with limited thermal tunability, controlled by a thermo-electric cooler, which is used to allocate the wavelengths. A medium access control (MAC) at the OLT manages the spectrum channel allocation for ONUs, demanding connection when activation is requested: the OLT furnishes an optical carrier wavelength for the ONU to obtain connection by a control algorithm, assigning a down-channel and another paired up-channel assigned to the ONU DFB transmitter. The MAC can reassign the channels because of interference or collision in a dynamic wavelength allocation. Measures in an activation process and in channel reassignment have been performed in environmental conditions, including control signals and the physical parameters of DFB lasers, demonstrating the practical viability of the PON scaling from 32 up to 256 wavelength channels.

To Decrease Maintenance Issues using FWM in Ultradense WDM Systems and Enhancing Optimum Placement of Optical Phase Conjugation

AbstractIn this work, a maintenance issues resolved suppression method of four-wave mixing (FWM) is proposed using optical phase conjugation (OPC) modules and different placements of OPCs are investigated in ultradense wavelength division multiplexing (WDM) system with coherent orthogonal frequency division multiplexing (OFDM) incorporation. Moreover, comparison of the placements of OPC in six different cases has been done and compared with conventional WDM (no-OPC) system in terms of Q-factor, BER and induced FWM. Analysis has been done at different launched power levels. It is observed that placement of OPC after each laser source performs exceptionally well and suppresses FWM with ease of maintenance.

DSP Enabled Optical Detection Techniques for PON

Some of the different aspects of the DSP enabled receivers in access networks will be addressed in this article. A revision of the main topics which may impact DSP design will be discussed through application examples. Modulation formats, KK receivers, and coherent receivers are analyzed at light of the DSP enabled receivers’ impact. Also, relevant are FEC and burst mode which are debated at the light of PON requirements. Simplified analog coherent receivers are presented as a use case to serve as simplification path for non-DSP high performance receivers. On the other end, aiming at discussing current PON limits, an ultra dense wavelength division multiplexing (UDWDM) use case is also presented where several advanced topics like pulse shaping, high density, and simplified DSP operations are considered.

Investigation the Effect of Channel Spacing for Long Distance Communication

In this paper, the impact of different channel spacing on proposed system setup is investigated for long distance communication. This wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM) and ultradense wavelength division multiplexing (UDWDM) is evaluated by considering the signal quality factor, bit error rate, optical gain, and received power for different signal input power and for distance. It is observed that at -5 dBm of signal input power the system covers 130 km with acceptable BER (10-8) and Q-factor (14dB).

A Novel Scheme for UDWDM-PON Broadband Access Network Using Injection-Locked Phase-to-Intensity Modulation Converter

Abstract In this paper, we have proposed and analyzed a novel cost-effective ultradense wavelength division multiplexed (UDWDM) passive optical network (PON) which uses phase modulation (PM) of the optical carriers in the optical line terminal (OLT) in order to achieve inherent noise immunity in the network. In the demodulation of the optical PM signal at the optical network unit (ONU), we have proposed to use a simple injection–locked Fabry–Perot (FP) laser diode (LD) for achieving PM to intensity modulation (IM) converter which is followed by a photodiode to recover the specific channel information. A detailed analysis of PM–IM conversion in an injection–locked FPLD has been carried out and the viability of the proposed scheme has been established. The proposed UDWDM–PON which is a broadband access network is cost-effective and long reach.

5G RAN architecture based on analog radio-over-fiber fronthaul over UDWDM-PON and phased array fed reflector antennas

This manuscript introduces a 5G radio access network architecture concept based on ultra-dense wavelength division multiplexing (UDWDM) and incorporating an optical fronthaul network that uses a novel wireless antenna system for radio frequency transmission and reception. A ring topology is proposed where optical signals travel within the 5G UDWDM passive optical networks and millimeter waves are generated in the optical line terminals by optical heterodyning. The wireless transmission of the millimeter waves is conducted by an innovative phased array fed reflector antenna approach for mobile communications that grants high antenna gain due to highly focused radiation characteristics, as well as multiplexing gain by multiple beam generation. Furthermore, beam steering is provided by a radio frequency analog beamformer network. Finally, implementation options synthesizing the total system are discussed.

Design of low-loss and near-zero ultraflattened dispersion PCF for broadband optical communication

A 2D finite difference time domain method is used to design a single-mode photonic crystal fiber (PCF) with engineered parameters that is studied with the aim of simultaneous achievement of low loss (below 10−14dB/km) and low chromatic dispersion over a broad band for future optical communication. The superiority of the proposed structure over recently reported structures is shown by the near-zero ultraflattened chromatic dispersion of 1.69±0.08 ps/(km nm) obtained over a broad wavelength range from 1 to 2 μm, covering the entire optical communication bands (O, E, S, C, L, and U). Also, the large effective area of more than 130 μm2 and as a result the low nonlinearity in addition to the near-zero ultraflattened chromatic dispersion and ultralow loss achieved for the proposed PCF make the structure a good candidate for high-capacity optical communication with a high bitrate over long distances for future ultradense wavelength-division multiplexing.

320 Gbps Free Space Optic Communication System Deploying Ultra Dense Wavelength Division Multiplexing and Polarization Mode Division Multiplexing

Abstract This work aims to investigate the integrated ultra-dense wavelength division multiplexing (UDWDM) and polarization division multiplexing (PDM) schemes incorporated in the free space optic (FSO) communication system. Erbium-doped fiber amplifiers (EDFAs) are used as post and pre-amplifiers in the proposed UDWDM–PDM–FSO system to boost the transmission power for increasing the distance. Thirty-two channels are transported over the FSO link to realize the total data transmission of 160 and 320 Gbps with 0.08 and 0.1 nm channel spacing, respectively. Results are also reported with non-return to zero modulation schemes. The performance of the proposed UDWDM–PDM–FSO transmission system is measured in terms of bit error rate (BER) and eye diagrams. The transmission distances are 100 and 87 km under clear weather and have acceptable BERs with 0.08 and 0.1 nm channel spacing, respectively. The weather conditions may worsen due to the increase in atmospheric turbulence. Hence, the transmission distances decrease to 3.93 and 2.6 km for heavy rain weather conditions. The UDWDM–PDM–FSO communication system exhibits an excellent performance due to the use of EDFAs with high achievable maximum distance and good and acceptable BER performance. The UDWDM–PDM–FSO communication system considerably enhances the transmission distance compared with the UDWDM–FSO communication system.

Real-time bidirectional coherent ultra-dense TWDM-PON for 1000 ONUs

We propose a bidirectional coherent ultra-dense time/wavelength division multiplexing passive optical network (UD-TWDM-PON) scheme for 1000 optical network units (ONUs). Then we experimentally demonstrate a bidirectional coherent UD-TWDM-PON using real-time FPGA-based transceivers. The power budget of this system is evaluated based on 1000 downlink ultra-dense wavelength division multiplexing (UD-WDM) channels with 5 GHz channel spacing in C band. The power budgets are 28 dB for DP-QPSK format with 10 Gb/s per user and 30 dB for PS-QPSK format with 7.5 Gb/s per user after 40 km SSMF transmission. The uplink transmission over 40 km SSMF for the 1000 users is also achieved based on 200 channels with 10 Gb/s per channel under channel spacing of 12.5 GHz in L band. Time division multiplexing is performed for the uplink optical signals, where 5 ONUs share one channel in the uplink. The 24-hour real-time performance testing for the bidirectional transmission is also conducted to evaluate the feasibility of the real-time FPGA-based transceivers in the proposed UD-TWDM-PON scheme.

Field-Trial of a High-Budget, Filterless, $\lambda$ -to-the-User, UDWDM-PON Enabled by an Innovative Class of Low-Cost Coherent Transceivers

We experimentally demonstrate an innovative ultradense wavelength division multiplexing (UDWDM) passive optical networks (PON) that implements the full $\lambda \text {-to-the-user}$ concept in a filterless distribution network. Key element of the proposed system is a novel class of coherent transceivers, purposely developed with a nonconventional technical approach. Indeed, they are designed and realized to avoid D/A–A/D converter stages and digital signal processing in favor of simple analog processing so that they match system, cost, and power consumption requirements of the access networks without sacrificing the overall performance. These coherent transceivers target different use case scenarios (residential, business, fixed, wireless) still keeping perfect compatibility and co-existence with legacy infrastructures installed to support gray, time division multiplexed PON systems. Moreover, the availability of coherent transceivers of different cost/performance ratios allows for deployments of different quality service grades. In this paper, we report the successful field trial of the proposed systems in a testbed where 14 UDWDM channels (and one legacy E-PON system) are transmitted simultaneously in a dark-fiber network deployed in the city of Pisa (Italy), delivering real-time and/or test traffic. The trial demonstrated filterless operations (each remote node selects individually its own UDWDM channel on a fine $\text{6.25}\text{-}\text {GHz}$ grid), real-time GbE transmissions (by using either fully analog or light digital signal processing), multirate transmission ( $\text{1.25}\; \text{and}\; \text{10}\,\text {Gb/s}$ ), high optical distribution network loss (18–40 dB) as well as a bidirectional channel monitoring system.

Effect of DWDM Channel Spacing on the Signal Quality for DWDM/CWDM FTTx Network

In this paper, two models of fiber-to-the-x (FTTx) networks are provided, one of hybrid dense wavelength division multiplexing / coarse wavelength division multiplexing (DWDM/CWDM) withhigh subscriber’s allocated bandwidth, and the other of hybrid ultra-dense wavelength division multiplexing / coarse wavelength division multiplexing (UDWDM/CWDM) with high network capacity. The effect of the fiber chromatic dispersion on the signal quality was demonstrated. The behavior of the network with respect to the changing of the DWDM channel spacing and the relation between the channel spacing and the system bit rate was discussed

Coherent UDWDM-PON With Dual-Polarization Transceivers in Real-Time

We report a real-time experimental demonstration of a long-reach coherent ultra-dense wavelength-division multiplexing (WDM)-passive optical network (PON) system with quadratic-amplitude modulation (QAM)-based dual-polarization optical transceivers. Using field-programmable gate array-based digital signal processing, the transmission of 20-WDM channels in 2.5-GHz grid, employing both 2.5-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 3.75-Gb/s DP-8PSK signals, is successfully demonstrated with bit-error rate in real-time. The performance is evaluated over 100-km standard single mode fiber in terms of receiver sensitivity and ODN power budget for both modulation formats, showing the feasibility of coherent PON scenarios using flexible dual-polarization strategies supported by software-defined transceivers.

Experimental Demonstration of Ultra-Dense WDM-PON With Seven-Core MCF-Enabled Self-Homodyne Coherent Detection

We demonstrate an ultra-dense wavelength-division-multiplexing (UDWDM) passive optical network (PON) in a spatial-division-multiplexing system utilizing our home-made seven-core multicore fiber (MCF) with a self-homodyne coherent detection (SHCD) scheme. In the SHCD system, we transmit 12 channels of 40-Gb/s UDWDM-PON signals using polarization-division-multiplexing quadrature phase-shift keying (PDM-QPSK) with a 12-GHz grid through the six outer cores of the seven-core MCF with the central core used to transmit the pilot tone. An optical frequency comb generator is employed to supply the multi-carriers for cost saving. After 37-km low-crosstalk seven-core MCF long-reach transmission with compact fan-in/fan-out multiplexers, a large transmission capacity of 2.88 Tb/s and a power budget of 26 dB are obtained. At the optical network unit side, a narrow linewidth local oscillator laser is no longer necessary since SHCD is adopted in our system. The complexity of the digital signal processing procedure at the coherent receiver is also greatly reduced as the carrier phase recovery and carrier frequency offset estimation are not needed.

Demonstration of 16QAM-OFDM UDWDM Transmission Using a Tunable Optical Flat Comb Source

A new approach for designing broad and flattened spectrum multicarriers optical sources is presented leading to a 32 spectral lines source by using a dual-arm Mach-Zehnder modulator (MZM) and a 41 spectral lines source from two-stage MZM. A modified simulated annealing-based optimization method is applied to derive the necessary settings allowing the optical flat comb source (OFCS) to be ultraflat. The OFCS is mooted as a technology to enhance the overall capacity of an access optical network by increasing the number of wavelength division multiplexing (WDM) channels. Here, we demonstrate an ultradense WDM (UDWDM) transmission for application to passive optical networks with (11 × 12.5 Gbps) quadrature amplitude modulation (QAM) based on a 4 b/s/Hz spectral efficiency orthogonal frequency division multiplex (16QAM-OFDM) transmitter and direct detection. We use an OFCS to generate the 11 subcarriers spaced by 6.25 GHz, made of a two-stage MZM. We study the performance of three filtered channels in terms of error vector magnitude in back-to-back (B-to-B) conditions and after propagation through 25 and 100 km standard single mode fiber.

Link Optimization and Performance Analysis of a 40 Gbps DQPSK Modulated Ultra DWDM System with 32 Orthogonally Polarized channels

Abstract The demand for higher speed at a lesser transmission cost per bit resulted in growth of optical networks with improved spectral efficiency even at narrower channel spacing. Incorporation of multilevel modulation formats in DWDM system led to the exploration of the 25 GHz channel grid, popularly known as Ultra Dense Wavelength Division Multiplexed (UDWDM) systems, opening new research frontiers. At such channel spacing's, non-linear optical effects impose severe system impairments and hence robust modulation schemes are currently being investigated. This paper presents a simulative model to implement and analyze alternative polarized DQPSK modulated UDWDM system to evaluate its resilience to XPM and fiber nonlinearity. An UDWDM system supporting 32 channels, each operating at 40 Gbps and spaced at 25 GHz is studied numerically for long-haul optical communication system using OptiSystem simulator to estimate OSNR penalties to mitigate XPM effects. The major detrimental factors encountered in link design have been estimated to evaluate the system performance in terms of Q value for different number of channels and with varied input power. The analysis reports acceptable performance for DQPSK format up to a link distance of 1500 Km and also claims a better tolerance to dispersion and nonlinearities at higher input power levels.

Theoretical Analysis of the Four-Wave Mixing Effect in Virtual Carrier-Assisted DD MB-OFDM Ultradense WDM Metropolitan Networks

A theoretical method (TM) to evaluate the four-wave mixing (FWM) power in multiband orthogonal frequency division multiplexing (MB-OFDM) ultradense wavelength division multiplexing (WDM) metropolitan networks using virtual carriers (VCs) and direct detection is proposed and validated numerically. It considers a band spacing of 3.125 GHz and 10-Gb/s data rate per band. A simplified TM (STM) to obtain the FWM power for MB-OFDM systems employing high-dispersion fiber is also proposed to relax the computation complexity. Both methods consider that FWM products generated from the mixing between two VCs and an OFDM band are dominant. Analytical expressions for the maximum allowed average power of the worst band (the band most impaired by FWM) are also derived. The TM and STM enable estimating the average power level per WDM channel corresponding to the threshold error vector magnitude (EVM) (threshold average power—TAP), where the threshold EVM is the maximum allowed EVM due to FWM, which induces a 2-dB EVM penalty to the EVM at the forward error correction threshold. Results show that, when a total data bit rate of 450 Gb/s is transmitted (nine WDM channels with five bands each) along the standard single-mode fiber (SSMF), the discrepancy between the numerical simulation-estimated and TM-estimated TAPs does not exceed 0.5 dB. By defining the upper bound of the power limit (UBPL) as the TAP obtained with 17 WDM channels, the discrepancy between the TM-estimated and STM-estimated UBPLs does not exceed 1 dB for transmission along up to 10 SSMF spans and for channel gaps up to 22.81 GHz.