Nyquist Wavelength Division Multiplexing Research Articles

Frequency-Packed Multiband-Coherent Transceiver With Symbol Rate-Adaptive Nyquist WDM Signals

In the next-generation flexible point-to-multipoint optical networks such as optical access systems, it is desirable for one transceiver to handle various Internet-of-Things traffic streams. A symbol rate-adaptive multiband-coherent-transceiver, that can transmit and receive wavelength division multiplexing (WDM) signals arranged at the Nyquist spacing after Nyquist spectrum shaping, achieves flexibility by changing the number of subbands (SB) and the symbol rate per SB. Band-asymmetric coherent access systems with wide bandwidth optical transceivers on the optical-line-terminal side and narrowband optical transceivers on the optical-networking-unit side have been reported for uplink and downlink transmissions. We also examined the multiband-coherent-transceiver for ultra-dense WDM signals with a narrow analog-device bandwidth and narrow SB spacing to improve cost and spectral efficiency. To the best knowledge of the authors, there is no other research on a Nyquist spectrally shaped WDM-passive optical network system that applies these two functions and flexibly changes the symbol rate per SB with a SB spacing less than the Nyquist spacing.

Probabilistic Shaping in Time-Frequency-Packed Terabit Superchannel Transmission

To combine the individual benefits of probabilistic-shaping (PS) and time-frequency-packing (TFP), we consider for the first time PS-TFP wavelength-division multiplexing (WDM) superchannels. However, TFP introduces inter-symbol interference (ISI) and/or inter-carrier interference (ICI). Moreover, the presence of reconfigurable optical add-drop multiplexers in the fiber links may further degrade the system performance. In this letter, we efficiently handle such challenges to present PS-TFP superchannels enabling Terabit-per-second data rates. For this, we perform a joint ISI and ICI channel estimation in tandem with turbo equalization to mitigate the interference. We investigate optimizing the parameters in the shaping and packing dimensions to achieve a desired target spectral efficiency. We show through our numerical results that such an optimized PS-TFP transmission leads up to 1.2 dB performance improvement over an unshaped Nyquist WDM system under similar conditions.

40 Gsymbol/s channel-based Nyquist wavelength division multiplexing communication in a terahertz-band using optical-domain reception signal processing

Nyquist wavelength division multiplexing (WDM) using a 40 Gbit/s channel signal is applied to terahertz (THz)-wave communication with a view to increasing its spectral efficiency and capacity. A 2 × 40 Gbit/s Nyquist WDM signal in the THz-band is both generated and demultiplexed by the assistance of optical technology. The optical-domain demultiplexing is adopted mainly due to the difficulty in both the direct THz-wave-domain and the subsequent radio-frequency (RF)-domain demultiplexing. The received THz-wave Nyquist WDM signal at an antenna is divided into two signals, which are down-converted into RF signals with heterodyne detection utilizing different frequency local sinusoidal waves. Each signal in the RF-domain is again transformed into an optical signal through an optical modulator, and each 40 Gbit/s channel is extracted from one of the generated sidebands at each optical modulator output with an optical Nyquist filter. Bit error rates (BERs) on the order of 10−5 and 10−7 (below the first KP4 forward error correction threshold) were obtained for the demultiplexed two 40 Gbit/s channels. In addition to the explanation of the experimental setup and operating principle of the optical-domain demultiplexing method for the THz-wave Nyquist WDM signal, some experimental results are reported to show the validity of our proposed demultiplexing method and factors to limit the BERs.

A low-complexity MAP detection for pattern-dependent impairments mitigation in Nyquist-WDM coherent optical systems

In the long-haul Nyquist wavelength division multiplexing (Nyquist-WDM) optical fiber communication systems, pattern-dependent impairments, such as the Nyquist filtering and fiber nonlinearity, have adverse effects on system performance. In this paper, we propose a modified maximum-a-posteriori-probability (MAP) detection based on low-complexity Euclidean distance approximate calculation to reduce the implementation complexity of pattern-dependent impairments mitigation. Comprehensive numerical simulation results show its advantages of comparable performance for pattern-dependent impairments mitigation to conventional MAP detection with lower complexity. Furthermore, the three-carrier 20GBaud/subcarrier coherent polarization-multiplexed 16 quadrature-amplitude-modulation Nyquist-WDM (Co PM 16QAM Nyquist-WDM) optical transmission off-line experiment with 7×100km fiber loops is carried out to validate the performance of the modified MAP detection. Compared with the conventional MAP, the modified MAP attain comparable performance with much lower computational complexity.

5×222 Gb/s PM-16QAM Nyquist-WDM Superchannel

This paper reports terabit Nyquist-wavelength division multiplexing (Nyquist-WDM) superchannel transmission using five subcarriers. Each sub-carrier is 222 Gb/s polarization multiplexed- 16 quadrature amplitude modulation (PM-16QAM) advance modulation format based transmitter. Each subcarrier is Nyquist-spaced using a pre filter of bandwidth 33.33 GHz, which is 1.2 ×symbol rate of the PM-16QAM based transmitter. Performance of the proposed superchannel is realized by means of bit error rate (BER), optical signal to noise ratio (OSNR), transmitted power, optical spectrum of the superchannel and individual filtered subcarrier. The proposed work exhibits reference BER of value 4×10-3 at OSNR 21 dB, which is quite remark-able. The proposed Nyquist-WDM superchannel delivers Terabit transmission capacity of 1.11 Tb/s and excellent spectral efficiency of 6.66 bit/s/Hz. Presence of Nyquist-WDM approach makes the system inter channel interference (ICI) tolerant. Hence, the proposed work is efficient enough for the next generation optical network, ultra long haul transmission and elastic net-work.

Frequency Comb Generation Using a CMOS Compatible SiP DD-MZM for Flexible Networks

On-chip frequency comb generation is a promising solution for seeding a chip-scale optical transmitter for both Nyquist wavelength-division multiplexing (WDM) and orthogonal frequency-division multiplexing. We demonstrate flexible frequency comb generation using a silicon photonic dual-drive Mach–Zehnder modulator fabricated on a CMOS-compatible process. Our on-chip comb has five lines spaced at 20 GHz with a high tone-to-noise ratio of about 40 dB after one stage optical amplification. Our back-to-back transmission achieves bit error rates (BERs) well below 2e-2, the threshold for 20% overhead forward error correction (FEC), for 800 Gb/s using 16-GBd 32QAM on five WDM channels. We also test a seamless 800-Gb/s super-channel using $5\times 20$ GBd 16QAM, with BER below the 7% overhead FEC threshold of 3.8e-3. To the best of our knowledge, this is the first demonstration of high-spectral-efficiency data carried by an all-silicon optical frequency comb. This establishes that a silicon optical frequency comb has sufficient optical signal-to-noise ratio for high-order QAM, as well as excellent stability for super-channels without guard bands, paving the way to an integrated high-spectral-efficiency multi-carrier optical transmitter.

A rigorous analysis of digital pre-emphasis and DAC resolution for interleaved DAC Nyquist-WDM signal generation in high-speed coherent optical transmission systems

The Nyquist spectral shaping techniques facilitate a promising solution to enhance spectral efficiency (SE) and further reduce the cost-per-bit in high-speed wavelength-division multiplexing (WDM) transmission systems. Hypothetically, any Nyquist WDM signals with arbitrary shapes can be generated by the use of the digital signal processing (DSP) based electrical filters (E-filter). Nonetheless, in actual 100G/ 200G coherent systems, the performance as well as DSP complexity are increasingly restricted by cost and power consumption. Henceforward it is indispensable to optimize DSP to accomplish the preferred performance at the least complexity. In this paper, we systematically investigated the minimum requirements and challenges of Nyquist WDM signal generation, particularly for higher-order modulation formats, including 16 quadrature amplitude modulation (QAM) or 64QAM. A variety of interrelated parameters, such as channel spacing and roll-off factor, have been evaluated to optimize the requirements of the digital-to-analog converter (DAC) resolution and transmitter E-filter bandwidth. The impact of spectral pre-emphasis has been predominantly enhanced via the proposed interleaved DAC architecture by at least 4%, and hence reducing the required optical signal to noise ratio (OSNR) at a bit error rate (BER) of 10−3 by over 0.45 dB at a channel spacing of 1.05 symbol rate and an optimized roll-off factor of 0.1. Furthermore, the requirements of sampling rate for different types of super-Gaussian E-filters are discussed for 64QAM Nyquist WDM transmission systems. Finally, the impact of the non-50% duty cycle error between sub-DACs upon the quality of the generated signals for the interleaved DAC structure has been analyzed.

Kalman Filtering for Carrier Phase Recovery in Optical Offset-QAM Nyquist WDM Systems

Nyquist wavelength-division-multiplexing (NWDM) systems based on offset quadrature amplitude modulation (OQAM) have been considered as interesting candidates to achieve a better spectral efficiency. However, such systems are very sensitive to laser phase noise that causes the loss of OQAM orthogonality. In this letter, we investigate carrier phase recovery using a modified extended Kalman filter (M-EKF) in OQAM-based NWDM systems. The proposed method can achieve an optical signal-to-noise ratio (OSNR) penalty lower than that of the state-of-the-art modified blind phase search (M-BPS) algorithm. More specifically, for a $10^{-5}$ normalized laser linewidth, the M-EKF method exhibits 0.8- and 1.7-dB OSNR penalties (at a bit error rate of $3.8\times 10^{-3}$ ) lower than the M-BPS method for 16- and 64-OQAM modulations, respectively. Furthermore, compared with the M-BPS method, our proposed method is of lower complexity. The required total number of multiplications and additions of the M-EKF method is reduced by about 3.3 and 6.7 times compared with that of M-BPS for 16- and 64-OQAM modulations, respectively, confirming the effectiveness of the proposed method.

Photonic Circuit Topologies for Optical OFDM and Nyquist WDM

Photonic circuits are the key to advanced functionality in future optical systems, as they efficiently process terabit/s data streams. This paper reviews how photonic circuit topologies have evolved to support high-spectral efficiency modulation formats, including: all-optical optical frequency division multiplexing (AO-OFDM), discrete Fourier transform spread OFDM (DFT-S-OFDM), Nyquist wavelength division multiplexing, (NWDM), orthogonal time division multiplexing (OrthTDM, OTDM), chirped-OFDM, and quasi-Nyquist signals. The importance of the order of components such as modulators, delays, and filters within these circuits is stressed, as simple changes to the circuit topology have significant impacts on the spectra of the signals, and the required bandwidths of the modulators.

Unrepeatered Transmission of <inline-formula> <tex-math notation="LaTeX">$10\times 400\text{G}$ </tex-math> </inline-formula> Over 370 km via Amplification Map Optimization

We demonstrate unrepeatered transmission of $10\times 400$ -Gb/s PM-16QAM Nyquist wavelength-division multiplexing (WDM) dual-carrier superchannels (75-GHz flexigrid) at the $\text {SE} = 5.33$ b/s/Hz, where each superchannel is coherently detected by a single wideband receiver. A 350-km transmission is achieved by using optimized amplification map to efficiently design remote optically pumped amplifier and hybrid amplifiers. Additional digital domain nonlinear compensation allows to increase the transmission distance up to 370 km.

Time Lens-Based Optical Fourier Transformation for All-Optical Signal Processing of Spectrally-Efficient Data

We review recent progress in the use of time lens-based optical Fourier transformation for advanced all-optical signal processing. A novel time lens-based complete optical Fourier transformation (OFT) technique is introduced. This complete OFT is based on two quadratic phase-modulation stages using four-wave mixing, separated by a dispersive medium, which enables time-to-frequency and frequency-to-time conversions simultaneously, thus performing an exchange between the temporal and spectral profiles of the input signal. Using the proposed complete OFT, several advanced all-optical signal processing schemes for spectrally-efficient systems and networks have been achieved, including all-optical generation, detection and format conversion of spectrally-efficient signals. The spectrally-efficient signals in this paper mainly refer to efficiently multiplexed signals with a high symbol rate per Hz, such as orthogonal frequency division multiplexing, Nyquist wavelength-division multiplexing (Nyquist-WDM), and Nyquist optical time division multiplexing (Nyquist-OTDM) signals.

OSNR monitoring in presence of fiber nonlinearities for coherent Nyquist-WDM system

A new method of optical signal-to-noise ratio (OSNR) monitoring in presence of fiber nonlinearities, suitable for Nyquist Wavelength Division Multiplexing (Nyquist-WDM) coherent optical transmission system, is proposed, in which the correlation function (CF) between the symbols who come from neighboring channels is used. The effectiveness of the proposed method is numerically verified in a 28 Gbaud per channel Nyquist-QPSK-WDM coherent system. The results show that, under the configuration with fiber nonlinearities, this method can achieve OSNR monitoring with the error less than 1.5dB in the OSNR range of 10–24dB, and keep valid regardless of the transmission distances, channel input powers and number of WDM channels.

On the Filter Narrowing Issues in Elastic Optical Networks

This paper describes the problematic filter narrowing effect in the context of next-generation elastic optical networks. First, three possible scenarios are introduced: the transition from an actual fixed-grid to a flexi-grid network, the generic full flexi-grid network, and a proposal for a filterless optical network. Next, we investigate different transmission techniques and evaluate the penalty introduced by the filtering effect when considering Nyquist wavelength division multiplexing, single side-band direct-detection orthogonal frequency division multiplexing, and symbol-rate variable dual polarization quadrature amplitude modulation. Also, different approaches to compensate for the filter narrowing effect are discussed. Results show that the specific needs per each scenario can be fulfilled by the aforementioned technologies and techniques or a combination of them, when balancing performance, network reach, and cost.

Distributed Aggregation of Spectrally Efficient Single- and Dual-Polarization Super-Channels by Optical Frequency Conversion in Fiber

Recently developed upgrade options for distributed ultra-dense frequency-division multiplexing toward large aggregation capacity are discussed. The concept, originally demonstrated for distributed coherent optical orthogonal frequency-division multiplexing, is based on optical frequency conversion in fiber. It enables precise frequency allocation of the successively multiplexed subcarriers within the super-channel without the need of absolute optical frequency control, i.e., allowing for use of free-running lasers at the individual multiplexing nodes. Follow-up experiments proved the applicability of this concept also to Nyquist wavelength-division multiplexing (WDM) with extremely small guard bands by successfully demonstrating the distributed aggregation of a spectrally efficient 400-Gb/s single-polarization zero-guard-band Nyquist-WDM super-channel using 4× 28-GBd 16-ary quadrature-amplitude modulation (16QAM) subcarriers. Recently, we further extended the concept to distributed generation of dual-polarization super-channels. This extension allowed for the distributed aggregation of a 400-Gb/s Nyquist-WDM super-channel using 4× 28-GBd polarization-division multiplexed quaternary phase-shift keying (PDM-QPSK) subcarriers. In this invited contribution, we review and compare in detail both experiments enabling 400-Gb/s aggregation capacity. Additionally, we discuss a viable option for operation under random fiber birefringence regardless of the polarization-dependent nature of the underlying optical frequency conversion process.

Nonlinear Impairment-Aware Static Resource Allocation in Elastic Optical Networks

This paper studies the routing, modulation format, and spectrum allocation problem in elastic fiber-optical networks for static traffic. Elastic networks, based on Nyquist wavelength-division multiplexing or optical orthogonal frequency-division multiplexing, can efficiently utilize the optical fiber's bandwidth in an elastic manner by partitioning the bandwidth into hundreds or even thousands of subcarriers. Beside the amplified spontaneous emission noise, the nonlinear impairments of each connection is explicitly considered by utilizing an analytical model to calculate the nonlinear interference from other connections propagating in the same fibers. The objective of our work is to minimize the bandwidth, i.e., the number of used subcarriers across the network, while satisfying the demands on throughput and quality for all connections. A novel integer linear program formulation and low-complexity heuristics are proposed. Simulation results are presented to demonstrate the effectiveness of the proposed approaches. Compared with transmission reach-based benchmark methods, our methods can achieve up to 31% bandwidth reduction.

Experimental Comparison of Transmission Performance for Nyquist WDM and Time–Frequency Packing

Several spectral efficient solutions for next-generation flexible optical networks are under investigation in order to cope with scalability issues while traffic demand is continuously increasing. Concerning transmission, Nyquist wavelength-division multiplexing (NWDM)—that confines the bandwidth within the Nyquist frequency of the signal—recently gained a momentum as one of the most suitable solutions for transmission over backbone networks and commercial solutions are available by now. Besides NWDM, other transmission techniques have been proposed to approach or overcome the Nyquist limit, thus further increasing the spectral efficiency (SE). Among them, time–frequency packing (TFP) is one candidate. This method builds a superchannel, whose subcarriers significantly overlap in frequency or time or both. This leads to an increased SE at the expenses of additional complexity within the transceiver to compensate for the introduced intersymbol interference. In this paper, we experimentally compare, for the first time, NWDM and TFP when employing the same identical test-bed. The experiment considered the two different terabit superchannels: a polarization multiplexed (PM)-16 quadrature amplitude modulation for the case of NWDM case, and a PM-quadrature phase-shift keying for TFP. The comparison and assessment of the results is carried out first in back-to-back configuration and, afterward, by propagating them over a recirculating loop consisting of a standard single-mode fiber, including spectrum selective switch to emulate node filtering.

Cyclostationarity-based joint monitoring of symbol-rate, frequency offset, CD and OSNR for Nyquist WDM superchannels.

Software-defined transceivers can be reconfigured based on demand and existing channel impairments, and as such, monitoring of both signal and channel parameters is necessary. We demonstrate a novel joint estimation method suitable for spectrally efficient Nyquist wavelength-division multiplexing (WDM), based on the cyclostationary property of linearly modulated signals, exploited both in the frequency and time domains. Using a Nyquist superchannel composed of three 10 GBaud channels, we experimentally demonstrate the simultaneous monitoring of symbol-rate with 100% accuracy, roll-off, frequency offset (FO), chromatic dispersion (CD) and optical signal-to-noise ratio (OSNR) with root-mean-square errors (RMSE) of 20%, 4 MHz, 200 ps/nm and 1.5 dB respectively, when the roll-off factor is larger than 0.06 for DP-QPSK and 0.3 for DP-16QAM.

Merit of Raman Pumping in Uniform and Uncompensated Links Supporting NyWDM Transmission

We propose a method to quantitatively evaluate the improvement of the optical signal-to-nose ratio ( $\text{OSNR}$ ) enabled by Raman pumping in hybrid Raman/erbium-doped fiber amplifiers (HFA) for Nyquist-wavelength-division multiplexing (NyWDM) transmission of multilevel modulation formats on the C-band, over uniform, uncompensated and periodically amplified links. It is based on the well accepted thesis that in the considered system scenario propagation effects can be approximately and conservatively modeled as an additive Gaussian disturbance called nonlinear interference (NLI), and it exploits the GN-model for analytical calculations. As an example, we apply the developed method to three typical fiber types with fiber-span loss $A_f$ from 20 to 30 dB, and show that the pure counter-propagating pumping scheme is always close to be optimal. Using this pumping scheme, the merit of Raman pumping is very similar for all the considered fibers and its maximum is enabled by full-Raman amplification with an $\text{OSNR}$ improvement that increases with $A_f$ , and goes from 4 to 5 dB in the explored range. We show that adding some copropagating pump, we obtain a further $\text{OSNR}$ improvement of only up to 0.9 dB, so, for the considered scenarios, it does not emerge any indications for the use of some copropagating pump in HFA. Moreover, for counter-propagating pumping schemes, we can obtain about 80% of the maximum Raman merit setting the Raman gain in the HFA only to 60% of the overall gain. We define as moderate pumping regime this HFA operating-mode, that, besides being convenient from an energy-efficiency point of view, it is also irrelevantly affected by pump depletion and so it is very stable in case some input channels are added/dropped, as required by reconfigurable optical networks.

Spectral Efficiency Optimization in Flexi-Grid Long-Haul Optical Systems

Flexible grid optical networks allow a better exploitation of fiber capacity, by enabling a denser frequency allocation. A tighter channel spacing, however, requires narrower filters, which increase linear intersymbol interference (ISI), and may dramatically reduce system reach. Commercial coherent receivers are based on symbol by symbol detectors, which are quite sensitive to ISI. In this context, Nyquist spacing is considered as the ultimate limit to wavelength-division multiplexing (WDM) packing. In this paper, we show that by employing a limited-complexity trellis processing at the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a denser-than-Nyquist channel packing [i.e., a higher spectral efficiency (SE)] is possible at equal reach. By adopting well-known information-theoretic techniques, we design a limited-complexity trellis processing and quantify its SE gain in flexi-grid architectures where wavelength selective switches over a frequency grid of 12.5 GHz are employed.

Linewidth-Tolerant Joint Digital Signal Processing for 16QAM Nyquist WDM Superchannel

The performance of Nyquist wavelength division multiplexing (WDM) superchannel, using 16-ary quadrature amplitude modulation (16QAM), is severely degraded due to the existence of inter-channel interference and full-response equalization-enhanced in-band noise. Here, we propose a joint digital signal processing (DSP) technique incorporating digital spectral shaping and maximum likelihood sequence detection for 16QAM Nyquist WDM superchannel with a symbol rate of 28 GBd. Compared with traditional joint DSP, we can obtain 1.8-dB required-OSNR improvement at a bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for back-to-back measurement. In addition, 0.9-dB peak Q-factor improvement is also secured after 960-km standard single mode fiber transmission. Furthermore, using the proposed joint DSP technique, linewidth tolerance is relaxed from 80 to 230 kHz given 1-dB required-OSNR penalty. Meanwhile, reduction of block-size for carrier phase recovery is also secured, taking 100-kHz linewidth into account.