Coherent WDM Research Articles

Coherent WDM Transmission Over NANF for High-Capacity Intra-Data-Center Interconnection

Coherent WDM Transmission Over NANF for High-Capacity Intra-Data-Center Interconnection

Toward Seven-Band Coherent WDM System Covering T to U Bands: Predictions of Transmission and BER Performance

Toward Seven-Band Coherent WDM System Covering T to U Bands: Predictions of Transmission and BER Performance

Coherent O-band WDM transmission of DP-16QAM over a 50-km BDFA-amplified link

We present wavelength-division multiplexed coherent transmission in an O-band amplified link enabled by bismuth-doped fiber amplifiers (BDFAs). Transmission of 4 × 25 GBd DP-16QAM (4 × 200 Gb/s) is demonstrated over a single span of 50-km length, occupying a bandwidth of 4.7 THz across the wavelengths 1323 nm to 1351 nm.

Performance investigation of geometric constellation shaping-based coherent WDM optical fiber communication system supported by deep-learning autoencoder

Geometric constellation shaping (GCS) has been proposed to enhance the performance of wavelength-division multiplexing (WDM) coherent optical fiber communication (OFC) systems. This paper presents a comprehensive simulation platform to design and simulate these systems' performance using an advanced autoencoder (AE) technique. The design of the AE is based on end-to-end deep learning and takes into account the system parameters, especially those related to the OFC channel. The system design uses identical AEs, one in each WDM channel, and only the AE of the central channel is trained to achieve the required performance target. The developed AE simulation platform is capable of assessing the bit error rate characteristics, signal-to-noise ratio, and mutual information for long-haul coherent OFC-WDM systems operating with various modulation formats. The simulation results indicate that using a multilayer perceptron neural network to design AE requires four hidden layers, sixteen nodes per hidden layer, and a “16x modulation order” batch size to achieve optimum system performance. The AE's behaviour is investigated to identify the allowed ranges of the optimal launch power using two different procedures that achieve better system performance for 2D and 4D constellations.

Multichannel Nonlinear Equalization in Coherent WDM Systems Based on Bi-Directional Recurrent Neural Networks

Multichannel Nonlinear Equalization in Coherent WDM Systems Based on Bi-Directional Recurrent Neural Networks

Achieving multi-terabit FSO capacity with coherent WDM transmission over a 1.8 km field trial

Achieving multi-terabit FSO capacity with coherent WDM transmission over a 1.8 km field trial

Analysis of SNR penalty in coherent WDM receiver system for detection of QPSK signal with component crosstalk

Analysis of SNR penalty in coherent WDM receiver system for detection of QPSK signal with component crosstalk

E-, S-, C- and L-band coherent transmission with a multistage discrete Raman amplifier.

We report for the first time an ultra-wideband coherent (UWB) WDM transmission over a 70 km standard single mode fibre (SSMF) solely using a multistage discrete Raman amplifier (DRA) over the E-, S-, C- and L-bands of the optical window. The amplifier is based on a split-combine approach of spectral bands enabling signal amplification from 1410-1605 nm over an optical bandwidth of 195 nm (25.8 THz). The proposed amplifier was characterized with 143 channelized amplified spontaneous emission (ASE) dummy channels in the S-, C- and L-bands and 4 laser sources in the E-band (1410-1605 nm). The amplification results show an average gain of 14 dB and a maximum noise figure (NF) of 7.5 dB over the entire bandwidth. Coherent transmission with the proposed amplifier was performed using a 30 Gbaud PM-16-QAM channel coupled with the ASE channels over a 70 km SMF. The ultra-wideband transmission using the tailored multistage DRA shows transmission bandwidth of 195 nm with a maximum Q2 penalty of ∼4 dB in E- and S-band, and ∼2 dB in C- and L-band.

Retraction Note to: Power Budget Analysis of an Ultra-Dense Coherent MB-OFDM WDM Metro-Access Networks

The power budget (PB) of an ultra-dense wavelength division multiplexing virtual-carrier helped coherent-detection multi-band orthogonal frequency division which multiplexes metro-access network with 10.7 Gb/s per client and per band, with groups divided by 3.125 GHz, is surveyed numerically, for channel spacings of 12.5 GHz and 25 GHz. In the light of the findings, the PB ranges between 14.8 and 20.5 dB. It was found that the minimum PB, on one hand, enables an optical distribution network (ODN) splitting ratio of 16 with standard single mode fiber (SSMF) reach of 13 km. The maximum PB, on the other hand, enables an ODN splitting ratio of 64 with SSMF reach of 12 km.

909.5 Tbit/s Dense SDM and WDM Transmission Based on a Single Source Optical Frequency Comb and Kramers-Kronig Detection

Space-division multiplexing (SDM) has become a promising technology for optical communications to seek a sustainable increment in data capacity to keep up with the ever-increasing bandwidth demand. SDM could provide massive parallelization in addition to existing WDM technology for a high capacity without compromising the signal-to-noise ratio requirement of the transmitter. However, the WDM-SDM combination requires arrays of laser sources with associated active wavelength controls. This is not favorable in terms of energy consumption and cost. Single source optical frequency comb (SS-OFC) is an appealing candidate to provide coherent WDM sources, replacing the laser arrays and surpassing the performance by providing the possibility of joint digital signal processing across multiple channels. In this paper, we have generated an SS-OFC based on highly nonlinear fibers (HNLF) for short-reach applications such as optical interconnects, taking advantage of the high pump-to-comb efficiency. The 50-GHz spaced SS-OFC across the whole C band provides 99 comb lines as WDM channel sources. Combining adaptive-rate modulation, a 7.9-km 37-core fiber, and a polarization diversity Kramers-Kronig receiver, we have successfully transmitted 3663 channels (37 SDM × 99 WDM) with reliably error-free performance, achieving 909.5 Tbit/s net rate with an aggregated spectral efficiency of 184.42 bit/s/Hz.

Proposed Power Efficient PS- QPSK Technique to Minimize Non-linearities in Coherent WDM System

Proposed Power Efficient PS- QPSK Technique to Minimize Non-linearities in Coherent WDM System

Crosstalk limitations due to fiber nonlinearities and ASE on the BER performance of a coherent WDM transmission system with a DRA.

An analytical development is presented to assess the collective effect of fiber nonlinearities such as cross-phase modulation (XPM), stimulated Raman scattering (SRS), and amplified spontaneous emission (ASE) due to a distributed Raman amplifier (DRA) on the bit error rate (BER) performance of a wavelength division multiplexing transmission system with forward pumping and a coherent optical receiver. Expressions are developed for a signal to crosstalk plus the noise ratio at the output of the optical fiber and optical receiver. The analytical results are found to match simulation results reported earlier, and coherent detection is found to be less affected by nonlinear effects compared to direct detection. XPM causes about 20-30 dB more penalty than ASE or SRS.

Frequency-Stabilized Links for Coherent WDM Fiber Interconnects in the Datacenter

The continued growth in Hyperscale data center (HDC) deployment is expected to drive world-wide Internet traffic to an astounding 21 zettabytes by 2021. This growth will place increased demands on data center interconnects (DCIs) and drive the capacity of the underlying electronic application specific integrated circuit (ASIC) switch chips that route DCI ethernet traffic, from 12.8 Tbps per chip today to 100 Tbps and beyond in the future. This astounding growth will push the limits of today's incoherent fiber link technologies that connect switches, including power dissipation, density, and practical engineering solutions. To overcome these limits, high capacity coherent WDM, traditionally relegated to the metro and long-haul networks, will need to move into the DCI. However, migrating coherent WDM into the DCI, particularly for link distances less than 2 km, will require elimination of power consuming and costly technologies like the digital signal processor (DSP). Additionally, new photonic integration technologies will be needed to co-locate the coherent optical interfaces directly with switch ASICs to alleviate the bandwidth, power, and density limits. In this article, we introduce a new approach to DSP-free coherent WDM for the DCI called FRESCO: FREquency Stabilized Coherent Optical Links for Low Energy DCIs. FRESCO utilizes spectrally pure, ultra-stable light source technology, normally associated with high-end scientific applications like atomic clocks, to enable high capacity high-order WDM QAM with low bandwidth, low power electronics normally associated with RF links. Terabits per second FRESCO links based on shared, stabilized sources and high-density coherent WDM silicon photonic coherent transceivers that are co-located with the switch ASIC will pave the way to a DSP-free coherent WDM scalable DCI solution.

Impact of fiber parameters on EDFA and/or Raman amplified high-spectral-efficiency coherent WDM transmissions.

The impact of fiber properties is investigated for coherent systems employing polarization-division multiplexed high-level quadrature amplitude modulation, wavelength-division multiplexing, and erbium-doped fiber amplifier and/or distributed Raman amplification. This is done by comparing the performances of fiber links of various attenuation coefficients and effective areas via experimentally verified analytical methods. Results show that the excess noise, which originates at amplifiers compensating for the losses of filters and switches located between fiber spans, can weaken or even diminish the performance enhancement brought about by lowering the fiber attenuation coefficient, especially if distributed Raman amplification is employed. This leads to the difference in the link performance assessment between our analytical results and some previously published figures of merit (FOM). On the other hand, increasing the fiber effective area results in the same amount of performance improvement regardless of the amplification scheme or the excess noise, which agrees with the FOMs. Since the larger effective area causes poorer pumping efficiency for systems employing distributed Raman amplification, a tradeoff between high performance and low power consumption needs to be determined for such systems.

Transmitter skew tolerance and spectral efficiency tradeoff in high baud-rate QAM optical communication systems

Residual IQ skew in a coherent transmitter severely degrades the performance of long-haul coherent optical communication systems. The impairment is particularly detrimental for a high baud-rate system using quadrature amplitude modulation (QAM). Furthermore, Nyquist pulse shaping increases the spectral efficiency for WDM systems. However, sharp roll-off of Nyquist pulse shaping further reduces the tolerance to residual IQ skew. Thus, certain trade-offs between spectral efficiency and roll-off factor should be made to improve the tolerance of residual IQ skew. We experimentally studied this trade-off and determined the optimal roll-off factor, channel spacing, receiver bandwidth, and equalizer length. The results serve as a guideline for high baud-rate coherent WDM systems.

Achievable rate degradation of ultra-wideband coherent fiber communication systems due to stimulated Raman scattering.

As the bandwidths of optical communication systems are increased to maximize channel capacity, the impact of stimulated Raman scattering (SRS) on the achievable information rates (AIR) in ultra-wideband coherent WDM systems becomes significant, and is investigated in this work, for the first time. By modifying the GN-model to account for SRS, it is possible to derive a closed-form expression that predicts the optical signal-to-noise ratio of all channels at the receiver for bandwidths of up to 15 THz, which is in excellent agreement with numerical calculations. It is shown that, with fixed modulation and coding rate, SRS leads to a drop of approximately 40% in achievable information rates for bandwidths higher than 15 THz. However, if adaptive modulation and coding rates are applied across the entire spectrum, this AIR reduction can be limited to only 10%.

Ultrahigh Count Coherent WDM Channels Transmission Using Optical Parametric Comb-Based Frequency Synthesizer

A novel optical frequency synthesizer capable of generating a multitude of narrow linewidth oscillations, fully compatible with high-speed fiber-optic transmission requirements, is presented. The new device is based on an ultra-dense and wideband parametric comb capable of generating more than 1800 spectral lines with 6.25 GHz spacing within 100 nm (C + L band) of optical bandwidth and 3 dB spectral flatness. The parametric comb generation relies on the highly efficient nonlinear interaction of the ultra-short and high peak power pulse train in the precisely controlled dispersive medium. The generated comb source is used for coherent transmission of 1520 Nyquist ultra-dense wavelength division multiplexed channels in a flex-grid compatible optical access network architecture, offering up to 30 Gb/s per user over a 50 km of standard single mode fiber with estimated aggregate capacity of 32 Tb/s. The line spacing tunablity and one-to-multiple coherent cloning capability make the proposed multicarrier generator a strong contender fully capable of replacing thousands of discrete lasers in optical data communication systems.

Symbol-Rate Dependence of Dominant Nonlinearity and Reach in Coherent WDM Links

We provide simulations of the nonlinear threshold (NLT) versus per-channel symbol rate at a constant bandwidth efficiency for a 15-channel wavelength-division multiplexing long-haul coherent optical transmission over a 20 $\times$ 100 km single-mode fiber link, both dispersion-managed (DM) and dispersion-uncompensated (DU), with several popular polarization multiplexed modulation formats. We exploit the nonlinearity-decoupling method to estimate the NLT due to each individual nonlinear effect, and, thus, show how the dominant nonlinearity changes with symbol rate. Plots of the NLT normalized to the symbol rate also reveal the symbol rate achieving the longest distance. Assuming a signal-independent Gaussian nonlinear interference, we derive a new formula yielding the system reach based on knowledge of NLT measurements on a much shorter link. We finally provide reach simulations to verify the accuracy of the new reach formula for both DM and DU links.

Simple carrier-phase estimator for high-speed RSOA-based coherent WDM PON.

It has been recently reported that reflective semiconductor optical amplifiers (RSOAs) can be used as phase modulators for the cost-effective implementation of the high-speed wavelength-division-multiplexed passive optical networks (WDM PONs). For the detection of the phase-shift-keying (PSK) signal generated by using an RSOA, we should be able to estimate its carrier phase accurately at the digital coherent receiver. However, when the baud rate of this PSK signal is set to be much higher than the RSOA's modulation bandwidth, the conventional M-th power algorithm cannot estimate its carrier phase accurately. To solve this problem, we develop a simple carrier-phase estimation technique for the high-speed (>10 Gb/s) PSK signals generated by using bandwidth-limited RSOAs. This technique estimates the carrier phase of the PSK signal by measuring the angular direction of the opening occurred in the trajectory of the phasor diagram. By using the proposed technique, we demonstrate the upstream transmission of the 25.78-Gb/s quadrature phase-shift-keying (QPSK) signal generated by using an RSOA (3-dB bandwidth: 3.2 GHz) in a 60-km reach coherent WDM PON.

Coherent Ultra Dense WDM Technology for Next Generation Optical Metro and Access Networks

Coherent optical communication has been well established as the technology of choice for long haul and high bit rate communication systems since a decade ago. Recent technology advances and ongoing price erosion further open the window of opportunity for the application of coherent optical transmission technology in other domains. This paper describes in detail the capabilities, design and implementation of a coherent ultra dense WDM technology for optical metro and access networks. Its capabilities enable a number of attractive options, such as variable downstream bit rates from 150 Mbit/s up to 10 Gbit/s per user, embedded OTDR and the coexistence with legacy systems such as GPON, EPON, XGPON or RF-Video in optical distribution networks. Due to its flexibility and capacity, it is also suitable for deployments in metropolitan networks, as well as for mobile front-haul and back-haul applications.