Wavelength Division Multiplexing Channels Research Articles

Broadband 1×8 channel silicon-nanowire-waveguide WDM filter based on point-symmetric Mach-Zehnder interferometric optical couplers in the O-band spectral regime

Novel broadband operating 1×8 channel wavelength-division-multiplexed (WDM) optical filter is proposed and experimentally demonstrated. The proposed 1×8 channel WDM filter is composed of third-stage delayed interferometers with point-symmetric Mach-Zehnder type wavelength insensitive couplers (PS-MZ-WINCs). We verify that the PS-MZ-WINC operates as a key component for the broadband operating WDM optical filter in terms of good interchannel balance and low crosstalk without accompanying with an additional excess loss. The fabricated silicon-nanowire 1×8 channel WDM filter with the channel spacing of ∼9-nm exhibited a clear demultiplexing filter response over the wavelength range of >100-nm in the O-band regime, together with interchannel balance of <0.5 dB, and low excess loss of <0.9 dB. The proposed scheme could be very effective for increasing channel count and/or channel spacing for several kinds of WDM applications.

XPM Mitigation in WDM Systems Using Split Nonlinearity Compensation

We propose a simple method to mitigate the cross phase modulation (XPM) effect in wavelength division multiplexing (WDM) systems, which combines the split nonlinearity compensation (NLC) and the receiver-side phase recovery algorithm. In the proposed method, the split NLC can suppress the conversion from the nonlinear phase noise (NLPN) to the Gaussian-like nonlinear interference noise (NLIN), and together with the phase recovery algorithm, which can compensate for the NLPN, the impact of the XPM on the signal is finally mitigated. This method is demonstrated by a transmission simulation with 11 WDM channels and 32 Gbaud 16 QAM modulations. The simulation results show that, due to the XPM mitigation, the proposed method can provide 0.4 dB gain for 500-km transmission, 0.3 dB gain for 2000-km transmission, and about 0.2 dB gain for 3000-km to 10000-km transmissions. The influence of the transmission distance on the gain is also discussed. Moreover, the selection of the phase recovery algorithm for different transmission distances is investigated, and the results show that for a short distance the blind phase search (BPS) is better, but for a long distance the V&V algorithm is more appropriate.

Robust Multidimensional Optical Modulation Based on Hybrid Subcarrier/Amplitude/Phase/Dual Polarization for Wavelength-Division Multiplexing Systems

Here, we propose a novel scheme based on advanced techniques of digital modulation in optical communications to achieve a single-channel transmission rate above 100 Gb/s. We utilize a hybrid scheme amplitude/phase/frequency/dual polarization, combined with multidimensional dual lattice and a low-density parity-check-coded modulation. The Stokes parameters are applied to the proposed scheme to map the four-dimensional classical polarization I X , Q X , I Y , Q Y in a three-dimensional space. In addition, in the proposed system, the packing theory is applied to the bit interleaver process. Three wavelengths are packaged before being transmitted over a wavelength-division multiplexing optical channel. This modulation process is carried out using symmetrical geometric shapes, such as a hypercube or a polyhedron, based on the molecular links theory using a grouping of 12 and 13/15 bits for the cubic and spherical lattices, respectively. The proposed technique is evaluated in the context of long-distance communications over distances up to 100km. The bit error rate (BER) results showed that the optical signal-to-noise ratio was approximately 4dB over a distance of 50km. In addition, the power spectral efficiency was found to be 3 lambdas, which is considered good performance considering the effects of distance and the non-linear effects influencing the number of lambdas. Also, we use an optical time-division multiplexing scheme (OTDM) in order to achieve a transmission rate beyond 1Tbit/s, where the speed effect is evaluated, taking into consideration that the power spectral efficiency is degraded.

Suitability of FBG for Gain Flatness of 64 × 10 Gbps DWDM System Using Hybrid (EDFA+YDFA) Optical Amplifier in C + L Band up to 50 GHz (0.4 nm) Channel Spacing

Abstract Gain flatness of erbium-doped fiber amplifier (EDFA) is an important aspect to support high speed multimedia applications for smart devices, cloud computing, big data analysis and high definition television (HDTV) as it achieves equal power for all dense wavelength division multiplexing (DWDM) channels. In this paper, we compare 32 and 64 channels 10 Gbps DWDM system using hybrid EDFA+ Ytterbium-doped silica fiber amplifier (YDFA) in C + L band for 0.4, 0.8 and 1.6 nm wavelength spacing along with FBG for gain flattening. The system performance has been analyzed in terms of optical signal to noise ratio and spectral gain fluctuation observed from the optical power spectrums. It is observed that YDFA although extends the amplification window of EDFA from 1530 to 1610 nm (C to L band), but gain fluctuates over the operating wavelength range. Gain flatness is enormously improved (5–10 dB) by adding FBG as a gain flattening filter to the hybrid (EDFA+YDFA) amplifier. It is found that as the channel spacing decreases (from 1.6 to 0.4 nm) and the number of channels increases (from 32 to 64), the gain reduces due to inter-channel crosstalk and four wave-mixing effect.

Compact photonic crystal multichannel drop filter for CWDM systems

A novel photonic crystal channel drop filter (PCCDF) is proposed to drop four coarse wavelength division multiplexing (CWDM) channels at 1490 nm, 1510 nm, 1530 nm and 1550 nm. A special coupled dual-loop design was used for multichannel forward/backward dropping for the first time. Due to the reduced number of the dropping resonators with this design, the proposed multichannel PCCDF has a very compact size (338 μ m2). High transmission efficiencies at the four ITU wavelengths (87.87%, 93.99%, 85.40% and 84.24%) can be obtained simultaneously. Furthermore, the same dimension parameters (lattice constant and rod radius) of the dual-loop resonators are helpful for easy fabrication. The proposed device could be used in future photonic integrated circuit-based CWDM communication systems.

Nonlinear distortion estimation in dual-carrier PM-QPSK signals for uncompensated fiber links

Abstract Kerr nonlinearity is particularly detrimental in coherent optical communications over long-haul, sub-sea and transatlantic links. The correct and accurate determination of the penalty due to fiber nonlinearity is critical for estimating the link capacity and optical signal to noise ratio margins. Two distinct approaches for calculating the power in the nonlinear field are proposed. The first approach utilizes the nonlinear variance at the decision gate after coherent detection, while the second attempts to extract the electrical field of the nonlinear interference noise. Subsequently, the power spectral density of the nonlinear interference noise is integrated to estimate the nonlinear power within a pre-defined signal-to-nose ratio criterion. Based on the assumptions in this study, a temporal resolution of 16 samples per symbol or (6.25 ps) is required to achieve a signal-to-noise ratio sensitivity of 40 dB on the nonlinear power estimate for a 10 Gbaud signal. Furthermore, a minimum pattern length of 128 symbols is required to obtain an adequate estimate of the nonlinear noise power within 30 dB signal-to-noise ratio for two adjacent wavelength-division-multiplexed (WDM) channels. The influence of the pulse shaping roll-off factor and number of WDM channels on the required pattern length and the nonlinear power estimation is also investigated.

340-Gb/s PolSK-DP-DQPSK optical orthogonal modulation format with coherent direct detection for high capacity WDM optical network

Abstract In this paper, a 340-Gb/s polarization shift keying-dual polarized-differential quadrature phase shift keying (PolSK-DP-DQPSK) orthogonal modulation for next generation high-capacity optical transmission systems has been proposed. The NRZ-PolSK and DP-DQPSK are considered to investigate the performance for higher order optical modulation in 50 GHz spaced wavelength division multiplexing (WDM) optical network. In the system architecture, eight 340-Gb/s PolSK-DP-DQPSK orthogonal modulated WDM channels have been transmitted over 1000 km of SMF + DCF. The influence of different network parameters such as input power, channel spacing, transmission distance etc. are considered to analyze the performance of orthogonal modulated signals in WDM optical network. This new modulation scheme has high spectral efficiency and very low phase noise in WDM optical network. The WDM system achieved the spectral efficiency of 6.8b/s/Hz for the 400 GHz bandwidth. Further, increase in the spectral efficiency represents the most attractive approach in the transport capacity of future long-haul optical transmission systems.

$4\times100$ -Gb/s PAM-4 FSO Transmission Based on Polarization Modulation and Direct Detection

For the first time, we experimentally demonstrate a 400-Gb/s [ $4\times100$ -Gb/s wavelength-division multiplexing (WDM)] pulse-amplitude-modulation-4 (PAM-4) transmission based on free space optics (FSO) utilizing the polarization modulation (PolM) and direct detection (DD) scheme. Each WDM channel comprises a polarization modulator, a polarizing beam splitter (PBS), and two photodetectors (PDs). Four wavelengths each carrying a 100-Gb/s PAM-4 signal (aggregate 400 Gb/s) are transmitted over the free space. At the receiver side, the PBS separates the two orthogonal polarizations, and then, the two polarized signals are detected by two PDs. Then, the two detected signals are combined by a subtractor, followed by digital signal processing (DSP). The PolM-DD system can achieve higher data rate while maintaining the required forward-error-correction (FEC) threshold compared with intensity modulation and direct detection (IM-DD) scheme. Moreover, owing to the stable polarization state in the free space and the mitigation of the common mode noise by the subtraction, the PolM-DD system provides enhanced tolerance to the amplitude fluctuations attributable to weather turbulence, which makes it more reliable than the conventional IM-DD FSO system. To the best of our knowledge, this is the first experimental demonstration of a 400-Gb/s FSO system using the PolM-DD scheme with the transmission distance of 1.2 m.

An Experimental Demonstration of Rate-Adaptation Using Shaped Polar Codes for Flexible Optical Networks

In this paper, we propose a polar-coded transmission system with adjustable data rate for coherent optical transmission employing quadrature amplitude modulation (QAM). The proposed system is based on a many-to-one constellation shaping method for achieving a range of data rates with arbitrarily small rate steps. An implicitly punctured polar-coded modulation system is then designed by combining polar coded bit-interleaved coded modulation system with many-to-one labelings. The scheme is experimentally evaluated in a wavelength division multiplexed (WDM) system with five carriers modulated at 16 GBaud with polarization multiplexed (PM) 256QAM. Data rates ranging from 121 to 182 Gb/s per carrier are experimentally demonstrated and the system can be directly extended to achieve higher data rates. Synchronization and equalization of PM-256QAM received symbols are performed with a pilot rate of 5%. The experimental results show 1.2 dB of shaping gain over the conventionally punctured polar codes in the optical back-to-back scenario. The maximum transmission system reach is increased by 200 to 400 km w.r.t the conventionally punctured polar codes. It is shown that the rate adaptation does not require a change of modulation format and/or underlying forward error-correction code. Finally, the performance of all five WDM channels is validated for the optimal input power.

Reconfigurable optical generation of nine Nyquist WDM channels with sinc-shaped temporal pulse trains using a single microresonator-based Kerr frequency comb.

Sinc-shaped temporal pulse trains have a spectrally efficient, rectangular Nyquist spectrum. We demonstrate the simultaneous and reconfigurable optical generation of multiple Nyquist-shaped wavelength-division-multiplexed (WDM) channels having temporal sinc-shaped pulse trains as data carriers. The channels are generated through the insertion of coherent lines using cascaded continuous-wave amplitude modulation around the spectral lines of a microresonator-based Kerr optical frequency comb. For each of nine Kerr frequency comb lines, we insert sub-groups of uniform and coherent lines to generate nine WDM channels. The deviations from ideal Nyquist pulses for the nine channels at repetition rates of 6 and 2GHz are between 4.2%-6.1% and 2%-4.5%, respectively. Each WDM channel is modulated with on-off keying (OOK) at 6 Gbit/s. In addition, we show the reconfigurability of this method by varying the number of WDM channels, the generated sinc-shaped pulse train repetition rates, the duration, and the number of zero-crossings.

Design and Provision of Traffic Grooming for Optical Wireless Data Center Networks

Traditional wired data center networks (DCNs) suffer from cabling complexity, lack flexibility, and are limited by the speed of digital switches. In this paper, we alternatively develop a top-down traffic grooming (TG) approach to the design and provisioning of mission-critical optical wireless DCNs. While switches are modeled as hybrid optoelectronic cross-connects, links are modeled as wavelength division multiplexing capable free-space optic channels. Using the standard TG terminology, we formulate the optimal mixed-integer TG problem considering the virtual topology, flow conversation, connection topology, non-bifurcation, and capacity constraints. Thereafter, we develop a fast yet efficient sub-optimal solution, which grooms mice flows (MFs), mission-critical flows (CFs), and forward on predetermined rack-to-rack (R2R) lightpaths. On the other hand, elephant flows (EFs) are forwarded over dedicated server-to-server express lightpaths whose routes and capacity are dynamically determined based on the availability of wavelength and capacity. To prioritize the CFs, we consider low and high-priority queues and analyze the delay characteristics such as waiting times, maximum hop counts, and blocking probability. As a result of grooming, the sub-wavelength traffic and adjusting the wavelength capacities, numerical results show that the proposed solutions can achieve significant performance enhancement by utilizing the bandwidth more efficiently, completing the flows faster than delay sensitivity requirements, and avoiding the traffic congestion by treating EFs and MFs separately.

Semi‐passive optical front‐haul supporting channel monitoring and link protection for the cloud radio access network

To accommodate the extremely large traffic load on the front-haul of a cloud radio access network, optical fibres along with wavelength division multiplexing (WDM) technology have become the de facto solution. In this study, the authors propose the model of a semi-passive front-haul device that can overcome the vulnerability of the optical fibre by using duplicated optical fibres and optical power monitoring for each WDM channel. Experiments conducted on the implemented prototype demonstrate that the link protection mechanism satisfies the regulation imposed by the international standard and that the optical power monitoring results are accurate. Thus, the proposed semi-passive front-haul device provides a high link reliability, fault reporting capability, and low operating cost.

Wavelength division multiplexing of continuous variable quantum key distribution and 18.3 Tbit/s data channels

Quantum key distribution (QKD) can offer communication with unconditional security and is a promising technology to protect next generation communication systems. For QKD to see commercial success, several key challenges have to be solved, such as integrating QKD signals into existing fiber optical networks. In this paper, we present experimental verification of QKD co-propagating with a large number of wavelength division multiplexing (WDM) coherent data channels. We show successful secret key generation over 24 h for a continuous-variable QKD channel jointly transmitted with 100 WDM channels of erbium doped fiber amplified polarization multiplexed 16-ary quadrature amplitude modulation signals amounting to a datarate of 18.3 Tbit/s. Compared to previous co-propagation results in the C-band, we demonstrate more than a factor of 10 increase in the number of WDM channels and more than 90 times higher classical bitrate, showing the co-propagation with Tbit/s data-carrying channels.

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.

Optical Mitigation of Interchannel Crosstalk for Multiple Spectrally Overlapped 20-GBd QPSK/16-QAM WDM Channels Using Nonlinear Wave Mixing

Optical mitigation of interchannel interference (ICI) for multiple spectrally overlapped data channels is experimentally demonstrated without multichannel detection and channel spacing estimation. The ICI mitigation takes place in three stages of periodically poled lithium niobate (PPLN) waveguides. In the first PPLN stage, the conjugate copies of channels are generated. In the second stage, the overlapped signals are coherently multiplexed with different complex taps. In the third stage, the concurrent nonlinear processes and the coherent multiplexing of the signals with their delayed copies compensate the ICI. The bit error rate and the constellation diagrams of wavelength division multiplexed (WDM) channels carrying quadrature phase-shift keying (QPSK) or 16 quadrature amplitude modulation (16-QAM) formats demonstrate the potential capability of the proposed method to reduce the ICI and its possible modulation format transparency. The effect of channel spacing on the performance of the method is also demonstrated. After optical ICI mitigation, a reduction of almost 4 dB is achieved for the value of optical signal-to-noise ratio at BER of 10–3 for 20-GBd QPSK signals with a channel spacing of 17.5 GHz. The overlapped WDM system of 20-GBd 16-QAM signals with channel spacing of 17.5 GHz is also ICI mitigated and error vector magnitudes are reduced by almost 28%.

Use of Same WDM Channels in Fiber Network for Bidirectional Free Space Optical Communication With Rayleigh Backscattering Interference Alleviation

In the paper, we present and demonstrate experimentally a 10 Gbit/s on-off keying (OOK) bidirectional wavelength-division-multiplexing (WDM) free space optical (FSO) communication architecture integrated in fiber network together with 5 m free space transmission length and 25 km fiber link, respectively. To achieve bidirectional FSO link, the all WDM wavelengths can be used simultaneously to serve as the downstream and upstream traffics in the proposed FSO-WDM system. Here, four pairs of line terminations (LTs) and optical wireless units (OWUs) are integrated arbitrarily in fiber access network for confidential and bidirectional FSO connection based on the requirement of practical environment. In the measurement, the relationship of FSO signal performance and detected tolerance are also discussed and analyzed. Moreover, the WDM-FSO transmission can also avoid the Rayleigh backscattering (RB) beat noise in fiber connection.

All-Optical Encryption Using Multi-Channel Spectral Shuffling

We propose a novel all-optical encryption technique that shuffles spectral slices of multiple wavelength-division multiplexing (WDM) channels. In particular, we investigate the application of this technique to two WDM 56-Gb/s quadrature phase-shift keying signals. Simulation results reveal that the encrypted signals could be propagated through transparent optical networks with diameters larger than 500 km and properly decrypted at the receiver. The technique enhances security because it forces eavesdroppers to discover the shuffled slices and to monitor multiple channels to recover data from just one of them.

Crosstalk Impact on Continuous Variable Quantum Key Distribution in Multicore Fiber Transmission

We investigate the excess noise induced by crosstalk in a trench-assisted homogeneous 19-core fiber for a continuous variable quantum key distribution (CV-QKD) receiver. Crosstalk is studied for both 30 wavelength division multiplexing channels of 24.5-Gbaud PM-16QAM with 100-GHz channel spacing and for an amplified spontaneous emission signal with 37.5-GHz bandwidth. We show that in-band crosstalk from neighboring cores prohibits secret key generation. However, it is possible to place CV-QKD channels at wavelengths not used by the classical channels even though wideband amplified spontaneous emission noise is present. The excess noise induced by crosstalk from a neighboring core is shown to be time-dependent.

Polarization-Tracking-Free PDM Supporting Hybrid Digital-Analog Transport for Fixed-Mobile Systems

For fixed services such as hybrid fiber-coaxial and passive optical network with deployed fiber networks, it is desirable to exploit the potentials to support wireless services in fifth-generation (5G) mobile data communications on the top of the existing fixed networks. We propose a polarization-division multiplexing (PDM) scheme to achieve the co-delivery of independent signals for fixed-mobile convergence. This is achieved by removing unwanted optical carrier in each polarization, and thus the proposed system can resolve the polarization-tracking issue in conventional PDM. It has been demonstrated in this letter that the polarization-tracking-free PDM system can support the co-transmission of respective 70-Gbps digital and analog signals over 20-km single mode fiber. Experimental results have shown low crosstalk between two polarizations. The proposed PDM system is compatible with wavelength-division-multiplexing (WDM) network and is capable of enhancing the spectral efficiency of WDM channels. The inherent single-sideband feature of the proposed PDM also enhances long-range services.

Investigation on Pointing Error in Multi-Beam Free Space Optical Communication System

Abstract Free space optical (FSO) communication has fascinated a lot of attention for a variety of applications in telecommunication area. It is dream of every researcher and telecommunication society to make it a real alternative solution for the last mile problem, to replace fiber optics. FSO is much favored because of its low maintenance cost and deployment time. Pointing error is one of the main challenges in FSO communication system which affects its performance especially at high data links that leads to significant performance degradation. In this paper, the impact of pointing error for multi-beam Hybrid Wavelength Division Multiplexing (HWDM) FSO system is investigated. Then the effect of link distance and Bit Error Rate (BER) are estimated by accounting the pointing error and atmospheric attenuation. The effect of change of link distance is examined while changing the number of beams between the transmitter and receiver as well. In this attempt, the HWDM system is considered by combining eight Dense Wavelength Division Multiplexing (DWDM) channels and four Coarse Wavelength Division Multiplexing (CWDM) channels. In addition, the influences of BER and receiver sensitivity are analyzed for the proposed system by incorporating Erbium Doped Fiber Amplifier (EDFA) at the receiver end. The data rate 2.5 Gbps is considered for both CWDM and DWDM channels for investigation.