Wavelength Division Multiplexing Channels Research Articles

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SOA Based Photonic Integrated WDM Cross-Connects for Optical Metro-Access Networks

We present a novel optical metro node architecture that exploits the Wavelength Division Multiplexing (WDM) optical cross-connect nodes for interconnecting network elements, as well as computing and storage resources. The photonic WDM cross-connect node based on semiconductor optical amplifiers (SOA) allows switching data signals in wavelength, space, and time for fully exploiting statistical multiplexing. The advantages of using an SOA to realize the WDM cross-connect switch in terms of transparency, switching speed, photonic integrated amplification for loss-less operation, and gain equalization are verified experimentally. The experimental assessment of a 4 × 4 photonic integrated WDM cross-connect confirmed the capability of the cross-connect chip to switch the WDM signal in space and wavelength. Experimental results show lossless operation, low cross-talk <−30 dB, and dynamically switch within few nanoseconds. Moreover, the operation of the cross-connect switch with multiple WDM channels and diverse modulation formats is also investigated and reported. Error-free operation with less than a 2 dB power penalty for a single channel, as well as WDM input operation, has been measured for multiple 10/20/40 Gb/s NRZ-OOK, 20 Gb/s PAM4, and data-rate adaptive DMT traffic. Compensation of the losses indicates that the modular architecture could scale to a larger number of ports.

High-Radix Nonblocking Integrated Optical Switching Fabric for Data Center

To accomplish high-bandwidth and low-latency communications among tens or even hundreds of nodes with low power consumption, dual radial and angular grating optical network (DRAGON), a new integrated high-radix strictly nonblocking optical switching fabric, is proposed in this paper. The topology and routing algorithms of DRAGON are discussed, and a formal proof for the strictly nonblocking property is presented. With the loss model developed, we show DRAGON has significantly lower worst-case loss as well as average loss compared with other widely studied integrated nonblocking switching fabrics. In addition, our crosstalk analysis also suggests a lower crosstalk of DRAGON. For example, for 32 wavelength division multiplexing channels, both the worst-case loss and average loss of ${\text{180}}\times {\text{180}}$ DRAGON are around 20 dB lower than crossbar of the same size, and 28 dB lower than ${\text{128}}\times {\text{128}}$ Benes. Furthermore, due to the strictly nonblocking property, DRAGON has advantages of smaller communication latency and larger throughput compared with Benes switching fabric.

Wavelength-interleaved MDM-WDM transmission over weakly-coupled FMF

Recently mode-division-multiplexing (MDM) has been widely investigated to enhance fiber optics capacity, in which modes or mode groups in few-mode fiber (FMF) or multi-mode fiber (MMF) are exploited as different spatial channels for data transmission. For short-reach applications, significantly reducing inter-spatial-channel crosstalk to avoid coherent detection and multiple-input-multiple-output (MIMO) equalization is preferred. Currently most studies focus on the design of weakly-coupled FMFs and mode (de)multiplexers. Alternatively, in this work, a wavelength-interleaved (WI) scheme is proposed to mitigate inter-spatial-channel crosstalk by optimizing the design of direct detection (DD) MDM and wavelength-division-multiplexing (WDM) system. In weakly-coupled MDM systems, crosstalk mainly comes from the adjacent spatial channels, and the signal-to-crosstalk beat interference (SCBI) constitutes main crosstalk impairment after square-law detection. The WI scheme interleaves the WDM grids in adjacent spatial channels by half WDM channel spacing and uses an electrical low-pass filtering (ELPF) to remove out-of-band SCBI. The effectiveness of SCBI suppression is theoretically analyzed. The feasibility of WI scheme is experimentally verified by 3-mode 3-wavelength MDM-WDM transmission over 500-m OM3 MMF. Enabled by WI scheme, record 120-km 10G-per-channel MDM-WDM transmission over 2-mode FMF without MIMO equalization is successfully demonstrated. The WI scheme is promising to enhance the performance of short reach or even metro MDM optics.

Constellation Design Enhancement for Color-Shift Keying Modulation of Quadrichromatic LEDs in Visible Light Communications

Quadrichromatic light-emitting diode (QLED) cluster is a four-color solid-state apparatus suitable for simultaneous illumination and communications. Unlike traditional red/green/blue (RGB) LEDs, its extra color provides not only one new wavelength-division multiplexing data channel but also better color quality in illumination. Taking full consideration of the high quality of color rendering index (CRI) requirement with tunable color temperature (CT), this paper investigates the constellation design of color shift keying (CSK) to maximize the minimum pairwise Euclidean distance (MED) for communication performance optimization. Beyond existing works, maintaining a high-level CRI with a specified CT complicates our design optimization problem. We propose to transform the CRI requirement into a set of linear constraints on one of the LED source composition while jointly incorporating the CT constraints. Both simulation results and prototype CSK communication testbed measurements based on commercial multicolor LEDs (LUMILEDS Luxeon C) illustrate that, under the same luminous flux and CT conditions, our proposed flux independent CSK constellation for QLEDs can significantly enhance the MED, bit error rate, and illumination color qualities.

Impacts of environmental factors to bi-directional 2×40 Gb/s WDM free-space optical communication

Bi-directional short-range free-space optical (FSO) communication with bi-directional 2×4×10Gb/s wavelength division multiplexing (WDM) channel signals is demonstrated by using a transmission distance of 25m. The single-mode-fiber components are used in the optical terminals for both optical transmitting and receiving functions. The measured power penalties for the 25-m bi-directional four-channel FSO communication compared with the back-to-back link and uni-directional transmission system are less than 0.8dB and 0.2dB, respectively. The environmental factor effects, including the oblique incidence through the building window glasses, thermally induced non-uniform air index as well as rainfall on the FSO performance are investigated and analyzed. The experimental results show that rainfall is influential for free space optical transmission.

Transmission of 2.86 Tb/s data stream in silicon subwavelength grating waveguides.

In this paper, we perform an investigation of terabit-scale data transmission in silicon subwavelength grating (SWG) waveguides for wavelength-division multiplexing (WDM) optical signals. Silicon SWG waveguide is capable of decreasing the light confinement in silicon core by engineering the geometry, leading to relatively lower optical nonlinearity compared to silicon wire waveguide. We demonstrate ultrahigh-bandwidth 2.86 Tb/s data transmission through the fabricated 2-mm-long silicon SWG waveguide over a wide range of launch powers. In the experiment, 75 WDM channels are utilized with each carrying 38.12 Gb/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. With the benefit of efficient reduction on optical nonlinearity, the optimum launch power is increased by 8 dB in SWG waveguide, indicating higher tolerance to the nonlinear impairments, compared to a silicon wire waveguide with identical length. With the optimum launch power, all 75 channels exhibit bit-error rate (BER) values less than 4e-5 after SWG waveguide transmission. We also evaluate the terabit-scale data transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). The required optical signal-to-noise ratios (OSNRs) to achieve BER level of 1e-3 are around 15.27, 15.47, 16.66 and 20.38 dB, respectively.

Bit Error Rate Performance Limitations Due to Raman Amplifier Induced Crosstalk in a WDM Transmission System

AbstractAnalysis is carried out for a single span wavelength division multiplexing (WDM) transmission system with distributed Raman amplification to find the effect of amplifier induced crosstalk on the bit error rate (BER) with different system parameters. The results are evaluated in terms of crosstalk power induced in a WDM channel due to Raman amplification, optical signal to crosstalk ratio (OSCR) and BER at any distance for different pump power and number of WDM channels. The results show that the WDM system suffers power penalty due to crosstalk which is significant at higher pump power, higher channel separation and number of WDM channel. It is noticed that at a BER 10

WDM Transmission of Single-Carrier 120-GBd ETDM PDM-16QAM Signals Over 1200-km Terrestrial Fiber Links

We demonstrate the generation and wavelength-division multiplexing (WDM) transmission of single-carrier 120-GBd electronic time-division multiplexing (ETDM) polarization division multiplexed 16-ary quadrature amplitude modulation (16 QAM) optical signals. Both transmitter-side optical preemphasis and receiver-side maximum likelihood sequence estimation are utilized to mitigate the impairments caused by bandwidth limitation of the optoelectronic components. Thanks to the preemphasis and post equalization, we have successfully transmitted 12 WDM channels on 150-GHz grid, each loaded with 960-Gb/s (800 Gb/s data signals + 20% FEC emulated overhead bits) over 1200-km link based on 100 km/span TeraWave terrestrial optical fiber. The achieved data rate interface ensured spectral efficiency of 5.33 b/s/Hz. To the best of our knowledge, this is the first experiment with the highest symbol rate reported for ETDM-based 16 QAM signals.

Effect of four-wave mixing interference on spectrally separated channels in passive optical networks

The phenomenon of four-wave mixing in passive optical networks with wavelength-division multiplexing of channels has been studied. This paper presents the results of modeling the distribution of combination frequencies over the spectrum as a function of the number of channels and the channel frequency spacing. To suppress crosstalk between the channels at combination frequencies, it is proposed to divide the spectrum into two parts and select a guard band between them. The effect of combination frequencies on the signal transmission quality is considered as a function of the channel power, channel frequency spacing, and transmission length.

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.

Polarization insensitive silicon photonic ROADM with selectable communication direction for radio access networks.

We report on an experimental prototype of a low-cost silicon photonic reconfigurable optical add/drop multiplexer (ROADM). The device is able to operate with up to 12 wavelength division multiplexing channels. In order to control the polarization of the multi-wavelength signal at the input of the device, an integrated polarization controller is investigated as an alternative to the polarization diversity device architecture. The integrated ROADM is equipped with optical switches for the selection of the path direction and variable optical attenuators for optical power control. We demonstrate the polarization insensitive routing of 10 Gb/s channels between two ROADM nodes with error-free transmission.

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.

Multi-wavelength coherent transmission using an optical frequency comb as a local oscillator.

Steadily increasing data rates of optical interfaces require spectrally efficient coherent transmission using higher-order modulation formats in combination with scalable wavelength-division multiplexing (WDM) schemes. At the transmitter, optical frequency combs (OFC) lend themselves to particularly precise multi-wavelength sources for WDM transmission. In this work we demonstrate that these advantages can also be leveraged at the receiver by using an OFC as a highly scalable multi-wavelength local oscillator (LO) for coherent detection. In our experiments, we use a pair of OFC that rely on gain switching of injection-locked semiconductor lasers both for WDM transmission and intradyne reception. We synchronize the center frequency and the free spectral range of the receiver comb to the transmitter, keeping the intradyne frequencies for all data channels below 15 MHz. Using 13 WDM channels, we transmit an aggregate line rate (net data rate) of 1.104 Tbit/s (1.032 Tbit/s) over a 10 km long standard single mode fiber at a spectral efficiency of 5.16 bit/s/Hz. To the best of our knowledge, this is the first demonstration of coherent WDM transmission using synchronized frequency combs as light source at the transmitter and as multi-wavelength LO at the receiver.

High-Capacity Carrierless Amplitude and Phase Modulation for WDM Long-Reach PON Featuring High Loss Budget

This paper utilizes spectrally efficient carrierless amplitude and phase (CAP) modulation and four wavelength division multiplexing channels in formulation of a high-capacity transmission system. The dispersion and loss associated with a single-mode fiber of 60 km can result in severe power fading and insufficient loss budget, respectively. This study employed high launch power to overcome these issues and thereby achieve long-reach transmission without the need for dispersion compensation or an optical inline amplifier. Modified Volterra filtering is used to deal with the linear and nonlinear distortions within the system. This allowed for the inclusion of cost-effective 10-GHz-class electro-absorption modulators and p-i-n detectors for 4- $\lambda$ 224-Gb/s 32-CAP 60-km transmission with a loss budget of 26 dB/channel. The proposed system provides support for the 64 optical network units (ONUs) with downstream transmission of $>$ 3 Gb/s/ONU for future long-reach passive optical networks. We also examined the transmission performance of each channel under various launch powers. Increasing launch power from 9 to 18 dBm/channel increases the loss budget by 9 dB and expands the total capacity by 42%.

Constellation Shaping for WDM Systems Using 256QAM/1024QAM With Probabilistic Optimization

In this paper, probabilistic shaping is numerically and experimentally investigated for increasing the transmission reach of wavelength division multiplexed (WDM) optical communication system employing quadrature amplitude modulation (QAM). An optimized probability mass function (PMF) of the QAM symbols is first found from a modified Blahut-Arimoto algorithm for the optical channel. A turbo coded bit interleaved coded modulation system is then applied, which relies on many-to-one labeling to achieve the desired PMF, thereby achieving shaping gain. Pilot symbols at rate at most 2% are used for synchronization and equalization, making it possible to receive input constellations as large as 1024QAM. The system is evaluated experimentally on a 10 GBaud, 5 channels WDM setup. The maximum system reach is increased w.r.t. standard 1024QAM by 20% at input data rate of 4.65 bits/symbol and up to 75% at 5.46 bits/symbol. It is shown that rate adaptation does not require changing of the modulation format. The performance of the proposed 1024QAM shaped system is validated on all 5 channels of the WDM signal for selected distances and rates. Finally, it was shown via EXIT charts and BER analysis that iterative demapping, while generally beneficial to the system, is not a requirement for achieving the shaping gain.

Experimental demonstration of tunable homodyne detection of WDM and dual-polarization PSK channels by automatically locking the channels to a local pump laser using nonlinear mixing.

This Letter proposes a method for tunable automatically locked homodyne detection of wavelength-division multiplexing (WDM) dual-polarization (DP) phase-shift keyed (PSK) channels using nonlinear mixing. Two stages of periodically poled lithium niobate (PPLN) waveguides and an LCoS filter enable automatic phase locking of the channels to a local laser.

Polybinary Shaping for Highly-Spectral-Efficient Super-Nyquist WDM QAM Signals

In Super-Nyquist wavelength-division multiplexed (WDM) systems, in which WDM channel spacing is smaller than a signal baudrate, there is a tradeoff relationship between the WDM crosstalk and inter-symbol interference due to the tight spectral shaping. For optimizing the tradeoff relationship, polybinary shaping with maximum likelihood sequence estimation is introduced. In this paper, we numerically and experimentally investigate the bit-error rate characteristics of the Super-Nyquist WDM quadrature amplitude modulated signals with polybinary shaping in order to clarify the relationship between the spectral efficiency and required signal-to-noise ratio (SNR). These results indicate that the Super-Nyquist WDM technique based on polybinary shaping is effective to optimize the spectral efficiency for the target required SNR equivalent to the target transmission distance.

Effects of signal power control strategies and wavelength assignment algorithms on circuit OSNR in WDM networks

Software-defined networking is enabling wavelength-division multiplexed (WDM) networks to be programmable down to individual components. While taking into account typical gain and noise figure profiles of erbium-doped fiber amplifier (EDFA) components, the authors consider a number of signal power control strategies and compare their performance in terms of achievable lightpath optical signal-to-noise ratio (OSNR). These strategies are applied network-wide to concurrently control the gain of each individual amplifier and the signal power equalization at each reconfigurable optical add/drop multiplexer. Simulation and (in part) experimental results show that the lightpath OSNR is affected by three factors: the EDFA gain control strategy, power equalization strategy and wavelength assignment (WA) algorithm. A trade-off between lightpath average OSNR and OSNR variance across the WDM channels is also noted. Experimental work is conducted using a five-node meshed WDM network testbed proving both feasibility and effectiveness of a coordinated use of signal power control strategies and WA algorithms.

Multipass Performance of a Chip-Enhanced WSS for Nyquist-WDM Sub-Band Switching

We investigate the performance of a chip-enhanced wavelength selective switch (EWSS) in multipass add/drop experiments. The demonstrated EWSS device uses a ring-assisted Mach–Zehnder interferometer chip as a wavelength interleaver, to preprocess the incoming super channel before launch into a commercial wavelength selective switch. We show that a 4% guardband, quadrature-phase-shift-keying encoded Nyquist wavelength division multiplexing super channel can successfully pass through seven EWSS nodes. We further investigate the number of reachable nodes with varied guardbands, showing that a modest increase in guardband can translate to a significant increase in the number of reachable EWSS nodes.