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With the increasing use of internet and mobile phones, the capacity of the backbone optical communication link has been continually growing across the world and especially in India. The total optical fiber cable deployed for the BharatNet initiative of Government of India is expected to increase from 3.4 million km to 5 million km in 2024–25 just for providing last-mile connectivity. Considering this deep proliferation, this article attempts to capture the diverse research carried out in India in the domain of optical communication.

The Mach-Zehnder modulator (MZM) is widely used in high-speed optical communication, and the stability of its bias point is the key to maintain the performance of the communication system. Numerous automatic bias control methods have been proposed to stabilize the bias point, which can be classified into power-based and dither-based methods. The power-based method features low-complexity and is considered of low-precision, thus it is usually applied to low-order modulation. In contrast, dither-based method features high-complexity and is considered of high-precision, therefore it is usually applied to high-order modulation. This paper explores the bias impact mechanism and analyzes the impact of the bias and dither signal. Traditional view is that the performance of dither-based outperforms power-based ones. However, the impact of dither is ignored. Here we compare the performance of the power-based and dither-based method under fair conditions. The results show that the power-based method has higher SNR and better performance taking into account the impact of dither, which indicates that practically it is more suitable for high-order modulation.

Wireless optical coherent communication can be used as the backbone supporting technology of air-space-ground-sea-integrated network. In wireless optical coherent communication, the information can be demodulated from the carrier correctly only when the phase synchronization is accurate. Operating in different systems and diverse environments presents challenges for phase synchronization, especially lasers with different line-widths and atmospheric turbulence channels that cause scintillation and wave-front distortion. We propose a combination of adaptive fractionally-spaced blind equalization and adaptive kalman filter to realize phase synchronization under different line-widths and different scintillation and wave-front distortion. This combination which changed the limitation that the original settings of each part is not directly applicable to atmospheric channel combines spatial diversity in a natural way, jointly removes amplitude noise and phase noise more thoroughly with the optimal bandwidth, and can realize phase synchronization under time-varying conditions. The signal quality of the proposed scheme with constant parameter output can be improved 1-2dB in both mean square error (MSE) and symbol to error ratio (SER) compared to traditional equal gain combining (EGC) followed by Viterbi-Viterbi phase estimation (VVPE) and the proposed constant parameter scheme has more laser line-width options. The proposed scheme with noise parameter corrected by Autoregressive method outperforms the constant parameter scheme 1-2dB at both MSE and SER and has higher and wider line-width selections. With this design, phase synchronization can be more flexible to adapt to dynamic changes, which will be more suitable for future applications of wireless optical networks with dynamic,noisy, diverse environment.

In this work, we present an O-band dual-polarization coherent receiver optical sub-assembly (cROSA), monolithically integrated in a 0.25 $\mu$m BiCMOS technology. The receiver features 248 nm deep ultra violet compatible 2-dimensional grating couplers (2D-GRCs), and an adaptive polarization controller, suitable for mitigation of local oscillator induced power fading in self-homodyne transmission systems. The cROSA is evaluated in system experiments at 64 GBd quadrature-phase shift-keying. Experimental results are related to grating coupler induced polarization crosstalk through Monte-Carlo simulations. Second generation 2D-GRCs are proposed.

We propose a silicon polarization-diversity coherent receiver for wavelength-multiplexing transmission without using the large-footprint arrayed waveguide grating (AWG). We have integrated our proposed coherent receiver on the silicon-on-insulator (SOI) platform for high-capacity transmission. In the proposed coherent receiver, high-frequency photocurrent signals from other wavelengths are suppressed by electrical low-pass filters. Moreover, the signal-signal beat interference (SSBI) generated from each wavelength is eliminated by the balanced detection. These two features lend to the proposed coherent receiver being free of the mm-scale AWG. We have demonstrated our proposed coherent receiver to detect a 1.12-Tb/s wavelength-division-multiplexed and polarization-division-multiplexed 16-ary quadrature amplitude modulation (PDM-16-QAM) signal. The compact footprint of the silicon chip promises small-form-factor receivers for future ultra-high-capacity coherent communication systems that require a high integration level and low fabrication cost. [ABSTRACT FROM AUTHOR]

The traditional fast Fourier transform based frequency offset estimation (FFT-FOE) estimates the frequency offset by searching the spectral peak of the signal after the fourth power operation, which is suitable for multiple modulation formats. But the accuracy of FOE is limited by the number of FFT points. In this paper, we propose a high accuracy and low complexity FOE algorithm based on all phase FFT (APFFT-FOE). The accuracy of FOE can be improved effectively with lower additional computational complexity. Simulation results show that the mean square error of APFFT-FOE is reduced by more than one order of magnitude compared with the FFT-FOE. Compared with other FOE algorithms that can achieve the same accuracy, the additional computational complexity required by APFFT-FOE is reduced by more than 50$\%$. Finally, the APFFT-FOE is experimentally verified with 20 GBaud 16/64-QAM modulation signal. The experimental results show that the required OSNR for 16-QAM signal to reach 6.7$\%$ hard-decision forward-error correction is relaxed by 0.23 dB. For 64-QAM signal, the required OSNR at 20$\%$ soft-decision forward-error correction threshold is relaxed by 0.68 dB.

This research proposes the architecture of a SAC-OCDMA–OFDM–DWDM system using a flat optical frequency comb generator and coherent communication. Its aim is to create a new OCDMA communication system that is specifically intended for multiservice networks. The suggested architectural design is numerically simulated and analyzed using OptiSystem and MATLAB. To quantify the performance of the proposed system, SNR, EVM, and BER were used as metrics. Furthermore, a comparison between the two codes, EDW and RD, has been presented. Thus, it has been revealed that the RD code system has better performance in terms of robustness and number of users than the EDW code. Besides, by analyzing the BER for several symbol rates and comparing it to the pre-FEC threshold, the proposed system demonstrates its effectiveness against linear and nonlinear effects. The results also show that optical noise affects signal quality; the mathematical analysis used allowed us to determine the impact of noise on the number of potential active users. [ABSTRACT FROM AUTHOR]

Optical communication systems have evolved over the years from simple intensity modulation and direct detection systems to those involving modulation of amplitude, phase, polarization and transverse modal profile. This article provides a brief tutorial review of the different modulation schemes used in the state-of-the-art optical communication systems and the futuristic trends in this direction to improve the data rates and spectral efficiency. The additional challenges with the advanced modulation schemes are discussed and some experimental results related to the demonstration of high-speed communication with such advanced modulation schemes are presented. [ABSTRACT FROM AUTHOR]

We investigate self-homodyne coherent transmission employing space division multiplexing (SDM), where a continuous wave laser source is divided to produce spatially multiplexed signals and a co-propagating pilot-tone (PT), which serves as a local oscillator for self-homodyne detection at the receiver side. The beat interference caused by the spatial coupling between PT and the signals during propagation is investigated through derivations and numerical modelling. Both conventional multiple-input–multiple-output (MIMO) and unreplicated parallel interference canceler (UPIC)-assisted MIMO suffer from the effects of beat interference and additional mode dependent loss introduced by PT-signal crosstalk, which limits the performance of SDM-based self-homodyne coherent systems. We propose two novel MIMO processing schemes employing digital conjugate input branches to handle the beat interference and demonstrate system performance improvement in both simulations and experiments. For a 240-Gbps self-homodyne coherent system transmitting mode-multiplexed QPSK signals over a 3-mode multimode fiber, after adapting the two enhanced MIMO schemes, simulations show a up to $\sim$ 4 dB improvement in PT-signal crosstalk level tolerance, and experiments show undistorted constellations as well as BER improvements from $\sim 10^{-2}$ to $\sim 10^{-5}$ with PT-signal delay alignment after the mode demultiplexer. [ABSTRACT FROM AUTHOR]

Multiband coherent communication is being handled as a promising candidate to address the increasing demand for higher data rates and capacity. At the same time, coherent communication is expected to enter the data center domain in the near future. With coherent data links in both, data- and telecom, spanning multiple optical bands, novel approaches to coherent transceiver design and traffic engineering will become a necessity. In this work, we present a monolithically integrated silicon photonic coherent receiver for O- and C-band. The receiver features a 2 × 2 multi-mode interference coupler network as 90 $^\circ$ hybrid optimized for 1430 nm (E-band). The total power consumption is 460 mW at a footprint of approximately 6 mm $^{2}$ , and an opto-electrical bandwidth of 33 GHz. 64 GBd operation is demonstrated in O- and C-band, which is competitive to the state-of-the-art for silicon photonic coherent receiver in the C-band, and the highest symbol rate to date for O-band coherent communication. [ABSTRACT FROM AUTHOR]

A method for reducing the training time of a deep learning based digital backpropagation (DL-DBP) is presented. The method is based on dividing a link into smaller sections. A smaller section is then compensated by the DL-DBP algorithm and the same trained model is then reapplied to the subsequent sections. We show in a 32 GBd 16QAM 2400 km 5-channel wavelength division multiplexing transmission link experiment that the proposed stacked DL-DBPs provides a 0.41 dB gain with respect to linear compensation scheme. This needs to be compared with a 0.56 dB gain achieved by a non-stacked DL-DBPs compensated scheme for the price of a 203% increase in total training time. Furthermore, it is shown that by only training the last section of the stacked DL-DBP, one can increase the compensation performance to 0.48 dB. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate an elliptical-aperture multimode diversity receiver to mitigate the impairment from anisotropic turbulence, which is emulated by employing an electrically-controlled heater. The anisotropy gradually decreases as the flow-level of the introduced airflow from fans increases. In contrast to circular-aperture reception, the elliptical-aperture multimode diversity receiver can effectively resist beam wander under anisotropic turbulence, reducing outage probability from 14.63% to 0.38% for detecting a dual-polarization 30-Gbaud QPSK signal. Without the aid of an adaptive optics system, this work demonstrates the feasibility of realizing an anisotropic-turbulence-resistant FSO link using multimode and multi-aperture diversity reception employing digital coherent combining techniques. [ABSTRACT FROM AUTHOR]

We propose a cascaded neural network (NN) to simultaneously identify the modulation formats and monitor the optical-signal-to-noise ratio (OSNR). In the second-level network, it is a single deep NN (DNN) rather than multiple sub-networks, which makes the architecture more compact and can save the resource for real implementation. However, since the data set is constituted from all modulation formats, the universality can be guaranteed but not for the accuracy and the complexity. To accelerate the estimation process and improve the accuracy, we introduce the transfer learning (TL) and reconstruct the data set with a part from all of the modulation formats for universality and another part from a specific modulation format for TL to pursue higher accuracy. In the experiment, we compare the proposed cascaded single neural network (CSNN) with or without TL, cascaded multiple neural networks (CMNN) and adaptive multi-task learning (MTL) for MFI and OSNR monitoring. In the first-level NN, all of the three schemes can achieve the accuracy of MFI as 100%. In the second-level NN, the CSNN with TL (TL-CSNN) can significantly improve the training speed and decline the RMSE of 0.19 dB compared with CSNN without TL. The TL-CSNN also has faster convergence speed and is more stable compared with CMNN and adaptive MTL.

Upcoming generations of coherent intra/inter data center interconnects currently lack a clear path toward a reduction of cost and power consumption, which are the driving factors for these data links. In this work, the tradeoffs associated with a transition from coherent C-band to O-band silicon photonics are addressed and evaluated. The discussion includes the fundamental components of coherent data links, namely the optical components, fiber link and transceivers. As a major component of these links, a monolithic silicon photonic BiCMOS O-band coherent receiver is evaluated for its potential performance and compared to an analogous C-band device. [ABSTRACT FROM AUTHOR]

Recently, dynamic optical network has attracted wide concern for its high efficiency and flexible configurations. Known for high spectral efficiency and flexible allocations of frequency resources orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) is a promising scheme for future dynamic networks. Phase noise estimation (PNE) and compensation are key technologies for maintaining the performance of coherent optical OFDM/OQAM system. In this paper, a simplified phase noise (PN) model for OFDM/OQAM under channel effect is deduced according to distribution feature of intrinsic interference. Some blind PNE methods are studied and their specific implementation process for OFDM/OQAM are presented. Based on PN model and Kalman filter theory, we propose a new adaptive extended Kalman filter (AEKF) blind scheme to meet the demand of flexibility in dynamic networks. Numerical results show that AEKF can adjust the implementation complexity of PNE according to the varying laser linewidth. When the commercial laser linewidth is 200 kHz, its time complexity is only 1/3 of that of modified blind phase search with feedback loop. Besides, AEKF can achieve a stable and reliable PNE performance under varying subcarrier allocation. AEKF can achieve a stable normalized linewidth tolerance over $1.5\times 10^{-3}$ under different subcarrier allocations.

Iterative demodulation and decoding has been introduced in 2005 by Colavolpe, Barbieri, and Caire to cope with channels affected by phase noise. However, iterations between the channel decoder and the demodulator increase system’s complexity. In this letter, we propose an iterative carrier recovery scheme where the current iteration is based on the hard decisions taken after the previous iteration. The resulting algorithm is easy-to-implement and offers excellent phase noise tracking performance, as demonstrated by the numerical results presented in the letter that are obtained with phase noise generated by linewidths of practical interest and a highly spectral efficient modulation scheme as 256-QAM. [ABSTRACT FROM AUTHOR]

In this paper, on-off Keying free-space optical (FSO) communication with coherent detection and double adaptive detection thresholds is proposed. The 3-bit encoding and double adaptive detection thresholds presented in the scheme can significantly improve the system performance, and there is no need of acquiring the instantaneous channel state information and probability density function (pdf) of the turbulence model. In order to enhance the accuracy of the threshold even further, the decision-aided method is offered to set the second detection threshold. The pdfs of the detection thresholds and the average bit error rate (BER) of the system are theoretically derived. Numerical studies show that the performance of the system is comparable to the performance of the idealized adaptive detection system and the loss of the signal-to-noise ratio (SNR) performance is only 1 dB at a BER of 10-9 over a lognormal turbulence channel with normalized standard deviation of irradiance σ = 0.25 and phase noise with normalized variance $\sigma _{\phi}^2 = 0.07$. Hence, this scheme can contribute to the performance improvement of the FSO system and its practical realization.

We describe the first on-line 256 QAM digital coherent optical transmission system with an ECLD light source and DFB LD-based injection-locking, and an FPGA-based transceiver. In this system, precise optical phase control was achieved with an optical injection-locking circuit and a phase-locked loop circuit at the receiver, which helped to reduce the complexity of digital signal processing in the FPGA-based receiver. The precise polarization-demultiplexing of a 256 QAM signal was realized by using a training symbol in the FPGA. With these techniques, we successfully demonstrated the on-line detection of a QAM signal with a multiplicity as high as 256. We then present a 10 channel WDM 80 Gbit/s/ch, 256 QAM bi-directional coherent transmission as a high capacity next-generation mobile fronthaul by using the FPGA-based transmission system. We achieved error-free operations with a 14% overhead FEC for 10 km down-link (on-line) and up-link (off-line) transmissions for all channels. [ABSTRACT FROM AUTHOR]

We propose a cascaded neural network (NN) to simultaneously identify the modulation formats and monitor the optical-signal-to-noise ratio (OSNR). In the second-level network, it is a single deep NN (DNN) rather than multiple sub-networks, which makes the architecture more compact and can save the resource for real implementation. However, since the data set is constituted from all modulation formats, the universality can be guaranteed but not for the accuracy and the complexity. To accelerate the estimation process and improve the accuracy, we introduce the transfer learning (TL) and reconstruct the data set with a part from all of the modulation formats for universality and another part from a specific modulation format for TL to pursue higher accuracy. In the experiment, we compare the proposed cascaded single neural network (CSNN) with or without TL, cascaded multiple neural networks (CMNN) and adaptive multi-task learning (MTL) for MFI and OSNR monitoring. In the first-level NN, all of the three schemes can achieve the accuracy of MFI as 100%. In the second-level NN, the CSNN with TL (TL-CSNN) can significantly improve the training speed and decline the RMSE of 0.19 dB compared with CSNN without TL. The TL-CSNN also has faster convergence speed and is more stable compared with CMNN and adaptive MTL. [ABSTRACT FROM AUTHOR]

The development of the fifth generation mobile (5G) technique stimulates a higher bandwidth requirement for next generation passive optical network (PON) systems. It is a challenge to further double the data rate from 50 to 100 Gb/s/λ depending on intensity modulation direct detection due to stringent bandwidth constraints on optical and electrical components. Coherent detection technology is a promising candidate for the future 100 Gb/s/λ or higher PON systems. The parallel implementation of hardware-efficient adaptive equalization tailored to PON systems will accelerate the evolution of coherent PON systems. In view of the current situation that logic resources are abundant while hardened multipliers are few for commercial field programmable gate array (FPGA), the purpose of this work is to reduce the number of hardened multiplexers used for FPGA realized signal processing. The waveform distortion caused by chromatic dispersion (CD) and imperfect frequency response of transceiver is rough compensated by using frequency domain equalization, which alleviate the stringent filter length requirement for the following adaptive equalization. Three Coordinate Rotation Digital Computer-based vector rotators are introduced to realize polarization demultiplexing, where the state of polarization is tracing by using a simplified gradient descent algorithm. A non-butterfly equalizer with few taps for each polarization is designed to handle the residual (CD) and the frequency response difference due to external environment variations and manufacturing technology. With the proposed method, the number of hardened multipliers is reduced by 73% compared with conventional 2×2 multiple-input multiple-output equalizer under the case with the similar receiver sensitivity. [ABSTRACT FROM AUTHOR]

In the past years, nonlinear frequency division multiplexing (NFDM) has been investigated as a potentially revolutionary technique for nonlinear optical fiber communication. However, while NFDM is able to exploit the Kerr nonlinearity, its performance lags behind that of conventional systems. In this work, we first highlight that current implementations of NFDM are strongly suboptimal, and, consequently, oversensitive to noise: the modulation does not ensure a large minimum distance between waveforms, while the detection is not tailored to the statistics of noise. Next, we discuss improved detections strategies and modulation techniques, proposing some effective approaches able to improve NFDM. Different flavors of NFDM are compared through simulations, showing that (i) the NFDM performance can be significantly improved by employing more effective detection strategies, with a 5.6 dB gain in Q-factor obtained with the best strategy compared to the standard strategy; (ii) an additional gain of 2.7 dB is obtained by means of a simple power-tilt modulation strategy, bringing the total gain with respect to standard NFDM to 8.3 dB; and (iii) under some parameters range (rate efficiency η ≤ 30 % ), the combination of improved modulation and detection allows NFDM to outperform conventional systems using electronic dispersion compensation. [ABSTRACT FROM AUTHOR]

A 12-channel single-sideband transmission of 16-QAM signals at 18 GBd is experimentally demonstrated for the first time using a single quantum-dot mode-locked laser (QD-MLL) as the light source for both the modulated carriers and the continuous wave components. The Kramers–Kronig field reconstruction algorithm was used at the direct detection receiver for signal-signal beat interference cancellation. A total net data rate of >800 Gb/s is achieved over 78 km fiber of 1680 ps/nm accumulated chromatic dispersion with BER ≤ 2 × 10−3, which is lower than the threshold of hard-decision FEC with 7% overhead. Numerical simulations were also performed to assess the impact of noises from the laser source and the receiver front-end on the system performance. [ABSTRACT FROM AUTHOR]

Probabilistic-shaped quadrature amplitude modulation (PS-QAM) based on Maxwell-Boltzmann (MB) distribution has been extensively studied in recent years in the SNR limited coherent communication system to approach the Shannon capacity. However, MB distribution is not the optimal distribution when the transmission system suffers from serious laser phase noise. To improve the laser phase noise tolerance of MB distribution, a novel Maxwell-Boltzmann/Angular-distance-directed (MB/ADD) distribution is proposed in this article. The benefits of PS-QAM exist in the case of limited SNR, as well as large laser phase noise. The proposed distribution is verified through simulations and experiments. In our simulation analysis, the ADD part enables the reduction of SNR penalty when laser phase noise increases. The results are validated by experimental data, in which both digitally generated and distributed-feed-back (DFB) laser generated phase noise are employed to evaluate laser phase noise tolerance. In the scenario of large phase noise, MB/ADD shaping can achieve lower pre-FEC BER and higher GMI compared with MB shaping, without inserting pilot symbols. Moreover, the improvement of applying MB/ADD distribution can be obtained no matter whether BPS or V&V based algorithm is adopted for carrier phase estimation (CPE). Different shaping parameters are also tested, where the capability to agilely adapt to various channel conditions of the MB/ADD distribution is shown. And the proposed MB/ADD shaping can be applied with similar complexity with MB shaping, if the Kullback–Leibler (KL) divergence between the generated and the expected 2-D QAM distribution is required to be lower than 10−3. [ABSTRACT FROM AUTHOR]

Numerical and experimental investigations are carried out on the performance of parallelized Kalman filters applied for mitigation of the excess phase noise of fast tunable lasers. Based on the characterization of the phase noise of a sampled grating distributed Bragg reflector (SG-DBR) laser, the proposed carrier phase recovery (CPR) scheme using Kalman filters is introduced. By performing simulations of data transmission with various advanced modulation formats in the presence of the excess phase noise, the Kalman filter based CPR scheme shows its ability to overcome the excess phase noise and this method is suitable for parallel processing. Then the results are further demonstrated by 12.5 Gbaud QPSK and 16-QAM transmission experiments employing the SG-DBR laser. We find that the Kalman filters have better performance than the 2nd-order phase-locked loop in parallel systems due to a better phase noise tolerance. The bit error rate performance is also examined in the whole tuning range (~30 nm) of the tunable laser, which further proves the feasibility of the proposed scheme.

We report a self-injection-locked InAs/InP quantum-dash tunable laser with ~11 nm (~1602-1613 nm) tuning window for next generation multiuser ultrahigh capacity fiber/free-space optics (FSO)/hybrid fiber-FSO-based optical networks. A tunability of >18 independently locked subcarriers with ~28 dB side mode suppression ratio (SMSR) and stable (±0.1 dBm) mode power is exhibited, and an estimated small injection ratio of ~-22 dBm is found to sustain locking and SMSR. Error free transmission of 100 and 128 Gb/s externally modulated dual-polarization quadrature phase shift keying (DP-QPSK) signals over 20 km single mode fiber (SMF) and 16 m indoor FSO links are demonstrated across 8 and 4 individual subcarriers, respectively, thus covering the entire tuning range. Moreover, up to 168 (192) Gb/s successful transmission over 10 km SMF (BTB) and 176 Gb/s over 16 m FSO link is achieved on a ~1610 nm subcarrier. Finally, a 128 Gb/s DP-QPSK transmission over 11 km SMF-8 m FSO-11 km SMF hybrid system is accomplished, thus paving the potential deployment of this single-chip, cost-effective, and energy efficient tunable light source in terabits/s next-generation passive optical networks.

Coherent optical transceivers offer significant advantages over direct-detect optical transceivers. However, both intradyne coherent transmitters (ICT) and intradyne coherent receivers (ICR) are more complicated and require careful calibration. Traditionally, an ICR is calibrated through heterodyne beating using a single-polarized signal. Active stabilization to maintain a constant state of polarization for the calibration signal is necessary, leading to a relatively complicated setup. We demonstrate a novel method through heterodyne beating using a polarization-multiplexed signal. No polarization control is needed, resulting in a much simpler configuration. The calibration results obtained through the polarization-multiplexed signal match with the results using a single polarized signal. Moreover, this polarization-multiplexed signal can be generated within the intradyne coherent transmitter without using any external components. This innovative technique enables the calibration of a coherent receiver for deployment in the field throughout its lifetime. [ABSTRACT FROM AUTHOR]

Conventional optical coherent receivers capture the full electrical field, including amplitude and phase, of a signal waveform by measuring its interference against a stable continuous-wave local oscillator (LO). In optical coherent communications, powerful digital signal processing (DSP) techniques operating on the full electrical field can effectively undo transmission impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). Simpler direct detection techniques do not have access to the full electrical field and therefore lack the ability to compensate for these impairments. We present a full-field measurement technique using only direct detection that does not require any beating with a strong carrier LO. Rather, phase retrieval algorithms based on alternating projections that makes use of dispersive elements are discussed, allowing to recover the optical phase from intensity-only measurements. In this demonstration, the phase retrieval algorithm is a modified Gerchberg–Saxton (GS) algorithm that achieves a simulated optical signal-to-noise ratio (OSNR) penalty of less than 4 dB compared to theory at a bit-error rate of 2 $\;\times 10^{-2}$. Based on the proposed phase retrieval scheme, we experimentally demonstrate signal detection and subsequent standard 2 × 2 multiple-input-multiple-output (MIMO) equalization of a polarization-multiplexed 30-Gbaud QPSK transmitted over a 520-km standard single-mode fiber (SMF) span. [ABSTRACT FROM AUTHOR]

We measure the FM noise power spectral density of quantum-dot mode-locked lasers (QD-MLLs) and compare this to their measured linewidths as predictors of performance in a digital coherent system. We explain our observations in terms of the non-Lorentzian line shape of the source wherein linewidth is determined by the low frequency part of its FM noise. Investigation of system performance with simulations based on the measured phase sequences and back-to-back coherent transmission experiments show that QD-MLLs with linewidths of several megahertz can have comparable performance to that of a laser with only a few hundreds of kilohertz of Lorentzian linewidth, due to the non-white part of their FM noise. We show that spectral linewidths of lasers with similar spectral properties can underestimate their performance in coherent systems, regardless of the linewidth measurement technique used. We propose a “Lorentzian-equivalent linewidth” measure to characterize lasers with non-white FM noise and to estimate their impact in digital coherent optical systems. This measure is obtained from phase variations at frequencies higher than typical frequencies often used to characterize lasers with white FM noise and comparable to the system baud. The proposed measure is shown to be a better predictor of system performance than the measured linewidth, for lasers with non-white FM noise. The impact of non-white FM noise on the optimization of carrier phase recovery and system performance is also discussed. [ABSTRACT FROM AUTHOR]

Through theoretical analysis and simulation, we investigate the system impact due to a sinusoidal jitter tone and the resultant local oscillator (LO) laser linewidth requirement in ultra-high baud rate and long distance coherent optical systems. We also carried out experiments in 64 Gbaud, dual-polarization (DP)-16 QAM systems to verify the theoretical analysis and simulation. We have also obtained a jitter interference tolerance mask to qualify LO lasers. A jitter tone with a frequency lower than ∼1 MHz has a higher tolerance since it generally causes constant frequency or phase shift, which can be tracked by a receiver DSP. For a jitter tone with a frequency higher than ∼1 MHz, the tolerance becomes much tighter since the tone will affect laser lineshape and induce equalizer-enhanced phase noise (EEPN). Consequently, a jitter tone in the higher frequency region could severely affect the system performance. Theoretical analysis and numerical result illustrate that EVM2 due to the effect of laser linewidth and a sinusoidal jitter tone is proportional to the weighted sum of $[ \Delta \nu \times B_{s}\times \text{L}] $ and $[ \Delta f_{pp}\times B_{s}\times \text{L}] ^{2}$ , where Δν is the laser linewidth, $B_{s}$ is the baud rate, $\Delta f_{pp}$ is the laser peak-to-peak frequency deviation due to a sinusoidal jitter tone, and L is the fiber transmission length. This result is applicable for all orders of QAM constellations. The implication to future 100 Gbaud and beyond systems is delineated. [ABSTRACT FROM AUTHOR]

We propose a novel low complexity two-stage frequency offset estimation (FOE) scheme based on modified Zoom-FFT for square 16/64-quadrature amplitude modulation (QAM) coherent optical systems. The first stage roughly estimates the frequency offset through a small size FFT. After frequency shifting, the modified Zoom-FFT can select and reconstruct the spectrum of interest by time domain average. Since the average provides a good estimate of the amplitudes of the frequency components, we can accurately find the peak after the second-stage FFT. We conduct simulations and compare the normalized frequency variance of the proposed scheme with different algorithms, e.g., the single-stage FFT and the Chirp-Z transform (CZT). The simulation results show that the proposed FOE scheme has the similar normalized frequency variance as other algorithms of $4.0\times 10^{-9}$ and $1.6\times 10^{-9}$ for 16QAM and 64QAM, respectively. The numerical simulations and experimental results demonstrate that the proposed scheme can achieve comparable performance, while the complexity is reduced by 86.6% (81.2%) and 60.9% (61.2%) for 16QAM (64QAM) compared with single-stage FFT and FFT+CZT, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we present a coherent receiver based on an optical phase-locked loop (PLL) for linear phase demodulation. The receiver concept is demonstrated at low frequency. For high-frequency operation, monolithic and hybrid integrated versions of the receiver have been developed and experimentally verified in an analog link. The receiver has a bandwidth of 1.45 GHz. At 300 MHz, a spurious free dynamic range (SFDR) of 125 dB . Hz(2/3) is measured.

• This paper presents a systematic discussion of the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. • Through a thorough analysis of ASE noise correlation, we introduce a novel approach, Combined Intra Symbol Frequency-Domain Averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. • In comparison to the Full-Loaded Frequency-Domain (FL-FD) method, CISFDA enhances system robustness against ASE noise and fiber nonlinear effects significantly. Intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD) seriously degrades the performance of optical orthogonal frequency division multiplexing offset-quadrature amplitude modulation (O-OFDM/OQAM) systems. Therefore, effective channel estimations are required for such systems to maintain good transmission performance. In the previous publications, both the full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods for polarization-division-multiplexed (PDM) coherent optical OFDM/OQAM (CO-OFDM/OQAM) systems have been proposed. However, for these frequency-domain methods, the channel frequency-domain responses caused by the correlation of the amplified spontaneous emission (ASE) noise are not considered. In this paper, we systematically discuss the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. With the analysis of the correlation of the ASE noise, we propose a combined intra symbol frequency-domain averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. Compared with the full-loaded frequency-domain channel estimation (FL-FD) method, the CISFDA method promotes the system robustness against the ASE noise and fiber nonlinear effect significantly. The proposed method is validated by numerical Monte Carlo simulations of the PDM CO-OFDM/OQAM system. Our new algorithm outperforms the FL-FD method in the 36 Gb/s PDM OFDM CO-OFDM/OQAM system, and the transmission distance is improved by 500 km with the bit-error-rate (BER) target at 3.8 × 10 - 3 . [ABSTRACT FROM AUTHOR]

We report on several methods for the chromatic dispersion (CD) compensation in optical fiber offset-QAM-based filterbank multicarrier (FBMC-OQAM) systems. We show that several equalization structures, initially proposed for wireless FBMC-OQAM systems, can also be applied to optical FBMC-OQAM systems to enhance the CD tolerance. The different CD compensation algorithms are numerically validated and compared to the conventional one tap equalizer in a 30-Gbaud optical FBMC system, in terms of performance and complexity. Considering a 1-dB optical signal-to-noise ratio penalty at a bit error rate of 3.8 × 10-3 and 256 subcarriers, the results show that the maximum CD tolerance of the frequency spreading method can be enhanced by a factor 10 and 30 for 4-OQAM and 16-OQAM modulations, respectively, compared to that of the conventional one-tap equalizer, at the cost of higher complexity. Even though the other CD compensation methods provide a reduced CD tolerance compared to the frequency spreading method, they require less complexity and hence can be good alternatives.

The performance of higher order modulation formats such as 16-quadrature amplitude modulation (QAM) coherent optical communication systems are limited due to several linear and nonlinear impairments such as phase noise, frequency offset, chromatic dispersion, and fiber nonlinearities. For a satisfactory system performance, these impairments need to be eliminated either by all-optical and/or electronic means. In this paper, we use the extended Kalman filter (EKF) algorithm to jointly mitigate laser phase noise, frequency offset, and nonlinear channel impairments, namely self phase modulation and nonlinear phase noise in polarization multiplexed 400 Gb/s 16-QAM and 200 Gb/s quadrature phase shift keying (QPSK) systems. We consider a two-state EKF with phase as one state and frequency offset as another. Simulation results for transmission over 1000 km (10 $\times$ 100 km) of standard single mode fiber show a Q-factor of 12 dB for PM-16-QAM at frequency offset of 1 GHz and laser linewidth of 100 kHz and a Q-factor of 18 dB for PM-QPSK at 1 MHz linewidth. Further, phase trajectories corresponding to tracked laser phase noise and frequency offset show that EKF can suppress laser phase noise up to 10 MHz and frequency offset up to 5 GHz, thereby eliminating the requirement of using separate algorithms for phase noise and frequency offset estimation.

A low-complexity one-step digital back-propagation (DBP) scheme is proposed to mitigate intrachannel and interchannel fiber nonlinearities in high-capacity single span transmissions. Compared with conventional coherent receivers, the only additional calculation is a low-complexity nonlinear phase noise compensation stage operated between bulk chromatic dispersion (CD) compensation and adaptive equalizer. The required number of real additions and multiplications per sample are only (6N + 2) and (2N + 6), respectively, where N is the number of compensated wavelength-division-multiplexing (WDM) channels. With the proposed one-step DBP, we demonstrate that the nonlinear noise can be suppressed by >60% in single channel experiments and >40% in seven-channel WDM simulations.

Channel equalization and phase noise compensation are required in coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission systems for reliable performance. In general, redundant data such as training symbols and pilot tones are necessary for channel and phase noise estimation in quadrature amplitude modulation (QAM) based CO-OFDM systems, which reduces the spectral efficiency of the system. Especially, in the coherent passive optical network (PON), the redundancy increases as the number of subscribers increases. In this letter, we propose a differential amplitude shift keying (DAPSK)-based CO-OFDM transmission for PON system, which does not require channel equalization and phase noise compensation processes. The proposed DAPSK-based CO-OFDM transmission was experimentally demonstrated, and the performance according to receiver launched power and the number of subcarriers was compared with QAM-based CO-OFDM.

This paper presents a characterization of interchannel interference (ICI) effects in gridless Nyquist-wdm systems due to two contributions, namely, overlapping among optical carriers and stimulation of nonlinear optical fiber impairments. ICI is assessed regarding bit error rate (BER) at 16 and 32 Gbaud, as a function of several system parameters. Nonlinear impairment compensation based on the digital back-propagation algorithm is implemented in the DSP-based coherent receiver. Results demonstrated that ICI due to channels overlapped has a higher impact concerning BER than nonlinear optical fiber impairments. Besides, the use of the back-propagation algorithm improves system performance, reducing up to 3 and 0.7 orders of magnitude of BER in QPSK and 16QAM cases, respectively. It means that this algorithm can minimize nonlinear effects under varying system parameters, but its performance is limited in sub-Nyquist cases.

The sharing of lasers in space-division multiplexed multicore-fiber transmission yields correlated phase noise across the spatial channels. As a result, system performance can be improved through the use of joint-core carrier-phase estimation (CPE). However, the presence of intercore skew can reduce the potential of such schemes. This paper studies the effects of skew on pilot-aided joint-core CPE, where via simulations, it is shown that joint-core processing can be made to perform similarly to or better than separate-core processing for any amount of skew. It is shown that for a given signal-to-noise ratio and skew, the performance of joint-core CPE relative to separate-core CPE is highly dependent on the ratio of the light-source linewidth to the local oscillator (LO) linewidth. In general, the performance of joint-core CPE in the presence of skew improves as the LO linewidth decreases compared to the light-source laser linewidth, assuming that the spatial channels are digitally realigned at the receiver. [ABSTRACT FROM AUTHOR]

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The POST-forward error correction (FEC) bit error rate (BER) performance and the cycle-slip (CS) probability of the carrier phase estimation (CPE) scheme based on Viterbi–Viterbi phase estimation (VVPE) algorithm and the VV cascaded by pilot-aided-phase-unwrap (PAPU) algorithm have been experimentally investigated in a 56 Gbit/s quadrature phase-shift keying (QPSK) coherent communication system. Experimental results show that, with 0.78% pilot overhead, the VVPE + PAPU scheme greatly improves the POST-FEC performance degraded by continuous CS, maintaining a low CS probability with less influence of filter length. Comparing with the VVPE scheme, the VVPE + PAPU scheme can respectively obtain about 3.1 dB, 1.3 dB, 0.6 dB PRE-FEC optical signal noise ratio (OSNR) gains at PRE-BER of 1.8 × 10−2. Meanwhile, the VVPE + PAPU scheme respectively achieves about 3 dB, 1 dB, and 0.5 dB POST-FEC OSNR gain and improves the FEC limit from 2.5 × 10−3 to 1.4 × 10−2, from 8.9 × 10−3 to 1.8 × 10−2, and from 1.6 × 10−2 to 1.9 × 10−2 under the CPE filter length of 8, 16, and 20. [ABSTRACT FROM AUTHOR]

Coherent optical transponders are widely deployed in today's long haul and metro optical networks using dense wavelength division multiplexing. To increase the data carrying capacity, the coherent transponder utilizes the high order modulation format and operates at a high baud rate. The limited bandwidth and the nonlinearity are two critical impairments for the coherent in-phase quadrature transmitter. These impairments can be mitigated by digital filters. However, to accurately determine the coefficients of these filters is difficult because the impairment from the limited bandwidth and the impairment from nonlinearity are coupled together. In this paper, we present a novel method to solve this problem. During the initial power-up, we apply a sinusoidal stimulus to the coherent IQ transmitter. We then scan the frequency and amplitude of the stimulus and monitor the output power. By curve-fitting with an accurate mathematical model, we determine the limited bandwidth, the nonlinearity, the power imbalance, and the bias point of the transponder simultaneously. Optimized coefficients of the digital filters are determined accordingly. Furthermore, we utilize a coherent IQ transponder and demonstrate that the limited bandwidth is improved by the finite impulse response filter, while nonlinearity is mitigated by the memoryless Volterra filter. [ABSTRACT FROM AUTHOR]

Abstract In coherent optical communication systems adopting decision-aided maximum likelihood (DA-ML) phase estimation, receiver initialization requires transmission of a training sequence followed by data symbols. However, such a start-up scheme is infeasible under the scenarios like burst-mode transmission and multi-receiver scheme. We propose a novel cold-start concept for DA-ML receiver without using a training sequence. By adopting the M th-power scheme, initial data symbols are estimated and further utilized to start up the DA-ML receiver effectively. Extensive numerical simulations are conducted under different situations for comparison. Results verify the feasibility of the proposed cold-start scheme. Based on conventional DA-ML, adopting cold-start can effectively start up the DA-ML receiver as well as achieve comparative bit error rate (BER) performance under burst-mode transmission and multi-receiver scenarios, with good robustness and nearly the same system complexity. [ABSTRACT FROM AUTHOR]

Future passive optical network (PON) systems supporting more than 50 Gb/s/λ present a challenge for the use of intensity modulation direct detection (IM-DD). Since coherent technology improves the receiver sensitivity over that of IM-DD, it is a promising candidate for 100 Gb/s or higher PON systems. Introducing hardware-efficient signal processing technologies tailored to PON systems will help render coherent technology suitable for PON systems. We here review hardware-efficient signal processing technologies suitable for PON systems. We introduce two types of simplified adaptive equalization (AEQ), one which sacrifices differential group delay compensation (DGDC), and another which sacrifices some chromatic dispersion compensation but does provide DGDC. Transmission experiments on a 100 Gb/s/λ-based coherent wavelength division multiplexing (WDM) PON system showed that simplified AEQ without DGDC and with DGDC exhibited only 0.2 and 1.4 dB penalty, respectively, compared with conventional DSP. The additional penalty due to the maximum possible cumulative DGD was evaluated by numerical simulation. Conventional AEQ and the simplified AEQ with DGDC showed negligible penalty, but the simplified AEQ without DGDC showed a 1.4 dB penalty. We also introduce simplified carrier phase recovery (CPR) with interpolarization phase offset estimation, and this showed the same performance as the conventional DSP, in both experiment and simulation. Taking these results into account, 100 Gb/s/λ-based coherent WDM PON systems with the simplified AEQs in combination with the simplified CPR were shown to be able to support the loss budget required for eight ONU splits over an 80 km span of single mode fiber. [ABSTRACT FROM AUTHOR]

We propose and demonstrate an all optical wavelength conversion of Nyquist differential phase 16 quadrature amplitude modulation based on microwave photonics signal processing for flex‐grid optical networks. By analyzing the mechanism of the microwave photonics and adjusting the parameters of the components, multi‐wavelength tunable laser with smooth and adjustable optical intensity can be obtained. Then, the multi‐channel photonic signals with the minimal guard space of 5 GHz can be obtained, which provide more than 20 flex‐grids for optical transmission with transparent rate and format. The optical power of converted wavelengths, optical signal‐to‐noise ratio, error vector magnitude, and bit error rate (BER) are studied under the condition of different flex‐grids, the frequencies of which are from 15 GHz to 36 GHz. The channel frequency spacing and amplitude of the converted wavelength can be dynamically adjusted via parameter configuration. Flex‐grid spacing strategies and channel selecting strategies are given. The BER performance of all converted channels can fall below the forward error correction threshold of 3.8 × 10−3 by optimizing the optical power of the receiver end. The results show the tradeoff between the BER and the detected optical power of the receiver end. These research findings could provide solutions for spectrum allocation and routing selection when wavelength conversion is needed in optical links. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a new low-complexity hardware architecture and digital signal processing (DSP) implementation for coherent reception of Nyquist frequency division multiplexed (Nyquist-FDM, digital subcarrier multiplexing) signals in real time. Key to achieve lowest complexity is the combination of an optimized frequency domain and time domain processing block. In the frequency domain processing, we combine subcarrier equalization and timing recovery with a noninteger oversampling ratio of 16/15. In the time domain, we take advantage of polar coordinate processing for the carrier recovery to avoid complex multiplications. The receiver is optimized for flexible operation and allows the adaption of filter coefficients and modulation format between 4QAM, hybrid 4/16QAM, and 16QAM within one clock cycle. The efficiency of the DSP is demonstrated by a real-time coherent receiver implementation on a single FPGA and is experimentally evaluated. Despite of the limited hardware resources, the receiver can detect a 30 GBd Nyquist-FDM signal with four subcarriers and a net data rate of 60 Gb/s (4QAM), 90 Gb/s (4/16QAM), or 120 Gb/s (16QAM) sampled with 32 GSa/s and demodulate one of the subcarriers at a time. Transmission of 300 km through standard single mode fiber is demonstrated with a BER below the soft-decision forward error correction limit. [ABSTRACT FROM AUTHOR]

We report a successful field trial of real-time optical superchannel transport of 400Gb/s Ethernet client traffic over a 216-km flexible grid G.652 fiber link using 64GBaud flexible rate optical transponders. We demonstrated, for the first time, error free transport of 400 GbE client traffic over an ultra-efficient optical superchannel of $2\times 200 \text{G}$ using modulation formats DP–16QAM, DP–8QAM, and DP–QPSK. Error free performance of 400GbE ultra-efficient optical superchannel was achieved at different channel separations as narrow as 37.5 GHz for DP–16QAM, 39 GHz for DP–8QAM, and 50 GHz for DP–QPSK. We also successfully demonstrated non-contiguous 400GbE optical superchannel, where the optical sub-channels are in non-adjacent spectral slots across the whole ${C}$ -band (3.85 THz apart). Stable long term (overnight) error free optical performance and end-to-end 400 GbE full throughput were measured. [ABSTRACT FROM AUTHOR]

A reflective coherent orthogonal frequency-division multiple-access (OFDMA) passive optical network (PON) based on differential amplitude and phase-shift keying (DAPSK) OFDM is proposed herein. In the conventional OFDMA–PON uplink transmission, each optical network unit (ONU) signal has different channel and phase-noise characteristics. In addition, each ONU has an independent laser source; therefore, the carrier frequency offset (CFO) differs among the ONUs. These multiple noises worsen as the number of ONUs increases and adversely affect the spectral bandwidth and system complexity. The proposed system using reflective-type ONUs avoids multiple CFOs because every ONU shares the same laser source, which is transmitted from an optical line terminator. Moreover, by applying a differential modulation scheme to OFDM, we resolve the multiple channel and phase noises without spectral efficiency loss and system complexity increase. The spectral efficiency is maximized using an adaptive modulation technique. The proposed system is experimentally demonstrated in a 20-km OFDMA–PON uplink transmission link with two reflective ONUs. The results confirm the possibility of uplink multiple access without multiple-noise compensation processes. The optimal spectral efficiency in terms of bandwidth size and number of subcarriers is analyzed. [ABSTRACT FROM AUTHOR]

In the past years, nonlinear frequency division multiplexing (NFDM) has been investigated as a potentially revolutionary technique for nonlinear optical fiber communication. However, while NFDM is able to exploit the Kerr nonlinearity, its performance lags behind that of conventional systems. In this work, we first highlight that current implementations of NFDM are strongly suboptimal, and, consequently, oversensitive to noise: the modulation does not ensure a large minimum distance between waveforms, while the detection is not tailored to the statistics of noise. Next, we discuss improved detections strategies and modulation techniques, proposing some effective approaches able to improve NFDM. Different flavors of NFDM are compared through simulations, showing that (i) the NFDM performance can be significantly improved by employing more effective detection strategies, with a 5.6 dB gain in Q-factor obtained with the best strategy compared to the standard strategy; (ii) an additional gain of 2.7 dB is obtained by means of a simple power-tilt modulation strategy, bringing the total gain with respect to standard NFDM to 8.3 dB; and (iii) under some parameters range (rate efficiency η≤30%), the combination of improved modulation and detection allows NFDM to outperform conventional systems using electronic dispersion compensation.