Dispersion-uncompensated Links Research Articles

CD-aware non-orthogonal DFT-precoding for C-band IM/DD multicarrier signals over a 50-km dispersion-uncompensated link.

In this paper, a chromatic-dispersion-aware non-orthogonal discrete Fourier transform precoding (CDA-NODFTP) scheme is proposed for CD-constrained intensity-modulation and direct detection (IM/DD) multicarrier-signal transmission systems. The performance of the proposed CDA-NODFTP scheme is experimentally evaluated and compared with conventional DFTP and NODFTP schemes over a 50-km C-band dispersion-uncompensated link, utilizing 90-Gb/s orthogonal frequency division multiplexing (OFDM) and filter bank multicarrier (FBMC) signals. Experimental results show that the proposed CDA-NODFTP with adaptive spectral compression outperforms conventional DFTP, NODFTP, and the non-precoding schemes in terms of bit error rate (BER) performance. Moreover, based on CD response only, the CDA-NODFTP-FBMC exhibits superior performance compared to both CDA-NODFTP-OFDM and adaptive bit and power loading (ABPL) FBMC, achieving receiver sensitivity improvements of over 2 dB at a BER of 3.8 × 10-3.

Complexity curtailed MLSE equalizer based on a compact look-up-table for C-band DSB IM/DD transmission.

Chromatic dispersion appears to be a major performance limiting problem in optical intensity modulation direct detection (IM/DD) transmission systems, especially for a double-sideband (DSB) signal. We propose a complexity-reduced look-up-table based maximum likelihood sequence estimation (LUT-MLSE) for DSB C-band IM/DD transmission based on pre-decision-assisted trellis compression and a path-decision-assisted Viterbi algorithm. To further compress the size of the LUT and reduce the length of the training sequence, we proposed a finite impulse response (FIR) and LUT hybrid channel model for the LUT-MLSE. For PAM-6 and PAM-4, the proposed methods can compress the size of the LUT into 1/6 and 1/4, and reduce the number of multipliers by 98.1% and 86.6% with slight performance degradation. We successfully demonstrate a 20-km 100-Gb/s PAM-6 and a 30-km 80-Gb/s PAM-4 C-band transmission over dispersion-uncompensated links.

C-band 120-Gb/s PAM-4 transmissions over a 100-km dispersion-uncompensated SSMF using joint combined pulse shaping and low-complexity nonlinear equalization.

In C-band intensity modulation and direct detection (IM/DD) systems, the frequency-dependent power fading induced by chromatic dispersion (CD) and square-law detection limits the transmission capacity and distance, especially for beyond 100-Gb/s transmissions over a 100-km dispersion-uncompensated link. To reach this goal, we propose a scheme of nonlinear pre-distortion, novel, to the best of our knowledge, combined pulse shaping, and post nonlinear equalization for four-level pulse amplitude modulation (PAM-4)-based IM/DD systems. At the transmitter, the nonlinear pre-distortion is used to generate unequally spaced PAM-4 symbols for pre-compensating the nonlinearities. While the novel pulse shaping, simply shaped by the linear combination of two inter-symbol interference (ISI)-free pulses, alters the frequency-domain power distribution of the PAM-4 signal and results in performance improvement. At the receiver, low-complexity post nonlinear equalization using an absolute-term based nonlinear equalizer with weight sharing (AT-NLE-WS) is performed to eliminate CD-induced power fading and residual nonlinear impairments. With the cooperation of these techniques, record 120-Gb/s PAM-4 signals are successfully transmitted over a 100-km standard single-mode fiber (SSMF) with the measured bit error ratio (BER) below 3.8 × 10-3, achieving >9% improvement of system capacity in comparison with the conventional pulse shaping schemes.

Multi-Rate Nyquist-SCM for C-Band 100 Gbit/s Signal Over 50 km Dispersion-Uncompensated Link

In this paper, to the best of our knowledge, we propose the first multi-rate Nyquist-subcarriers modulation (SCM) for C-band 100Gbit/s signal transmission over 50km dispersion-uncompensated link. Chromatic dispersion (CD) introduces severe spectral nulls on optical double-sideband signal, which greatly degrades the performance of intensity-modulation and direct-detection systems. Based on the prior knowledge of the dispersive channel, Nyquist-SCM with multi-rate subcarriers is proposed to keep away from the CD-caused spectral nulls flexibly. Signal on each subcarrier can be individually recovered by a digital signal processing, including the feed-forward equalizer with no more than 31 taps, a two-tap post filter, and maximum likelihood sequence estimation with one memory length. Combining with entropy loading based on probabilistic constellation shaping to maximize the capacity-reach, the C-band 100Gbit/s multi-rate Nyquist-SCM signal over 50km dispersion-uncompensated link can achieve 7% hard-decision forward error correction limit and average normalized generalized mutual information of 0.967 at received optical power of -4dBm and optical signal-to-noise ratio of 47.67dB. In conclusion, the multi-rate Nyquist-SCM shows great potentials in solving the CD-caused spectral distortions.

Performance comparison of partial-response square shaped signal with direct detection

In this work, we investigate improving spectral efficiency with direct detection based partial-response square shaping, which generalizes partial-response pulse shaping and reduces width of power spectrum of signal under both linear and non-linear model assumption. For linear model, quadrature point bias of Mach–Zender modulator is used. For non-linear model, null point bias of Mach–Zender is used, however, non-linear pre-coding and equalization are required. By comparing results obtained with linear model and non-linear model, we identify the factors influencing performance of short reach transmission. Results of experiment over a 25 km dispersion uncompensated link show following: Under non-linear model, performance issue is revealed with non-linear optical channel. Under linear model, for PAM4 modified-duo-binary with bias signal and PAM4 duo-binary, 2 b/s/Hz is achievable with 7% overhead hard decision decoding. Also, PAM4 modified-duo-binary with bias signal has 1 order lower bit error rate than PAM4 duo-binary, while theoretical analysis confirms the finding for short reach transmission, that non-linear equalization can recover information from coherent item and can suppress interference caused by self-beat item.

Direct-sequence spread spectrum time division multiple access with direct detection for latency optimized passive optical network

To improve low-latency support of passive optical networks, direct-sequence spread spectrum time division multiple access implements bi-directional byte-interleaved transmission by encoding each bit of a bi-polar sequence with orthogonal chip pattern. Consequently, guard-interval between consecutive up-link bytes can be removed and latency caused by multi-point control protocol interaction can be reduced. However, direct-sequence spread spectrum requires bi-polar signaling, which is not possible with direct detection and several solutions exist, such as, biased and differential transmission. In this work, we investigate the performance of direct-sequence spread spectrum time division multiple access with direct detection and also analyze both up-link and down-link situations. Experimental results of transmission over a 20 km dispersion uncompensated link show: The 31-bits direct-sequence reduces required received signal power to −12 dBm and the performance is limited by dark-current. With differential transmission in up-link direction, modified duo-binary has 5 dB better performance than PAM4. Also, in up-link direction, differential transmission has 3 dB better performance than biased transmission.

Fixed-State Log-MAP Detection for Intensity-Modulation and Direct-Detection Systems Over Dispersion-Uncompensated Links

In this paper, an optimized detection based on log-maximum a posteriori estimation with the fixed number of surviving states (fixed-state Log-MAP) is proposed cooperating with equalizers to deal with spectral distortions for intensity-modulation and direct-detection (IM/DD) optical transmission systems. The equalizers compensate the spectral distortions caused by limited bandwidth and chromatic dispersion, and the optimized detection decodes the useful bits from equalized symbols with severe colored in-band noise. To more accurately extract the bits, the optimized detection with larger memory length is required. However, the classical optimized detection based on maximum likelihood-sequence estimation (MLSE) requires exponential-growing computational complexity and storage space with the increase of memory length. The fixed-state Log-MAP detection can decrease the computational complexity and storage space from the exponential order to linear order. Therefore, the fixed-state Log-MAP detection can support larger memory length to more accurately extract bits with less logic gate resources and storage resources compared to MLSE. We experimentally verified the fixed-state Log-MAP detection in a C-band 64Gbit/s IM/DD on-off keying optical system over a 100km dispersion-uncompensated link. The fixed-state Log-MAP detection not only improves the receiver sensitivity of 2dB, but also reduces the computational complexity and storage space by three orders of magnitude.

Processing for dispersive intensity-modulation and direct-detection fiber-optic communications.

In intensity-modulation and direct-detection (IM/DD) fiber-optic communications, it is hard to pre- or post-compensate for chromatic dispersion (CD) by digital signal processing due to one-dimensional modulation and detection. In this Letter, we propose joint optical and digital signal processing to effectively compensate for CD-caused distortions for IM/DD optical systems. As a reasonable optical signal processing, negative chirp based on self-phase modulation can suppress a part of CD to take pressure off digital signal processing. Digital signal processing is designed based on the model of a dispersive channel to accurately compensate for CD-caused distortions. To the best of our knowledge, we present a record C-band 72 Gbit/s optical on-off keying over 100 km dispersion-uncompensated link (i.e., ∼1700ps/nm dispersion), achieving a 7% hard-decision forward error correction limit. We conclude that joint optical and digital signal processing is effective in dealing with CD-caused distortions to achieve a higher capacity-distance product in IM/DD fiber-optic communications.

56 Gbit/s OOK Signal in C-band Over 20 km Dispersion-Uncompensated Link Transmission With Receiver-Side EDC Algorithm

In this paper, we propose and experimentally demonstrate a receiver-side electronic dispersion compensation (EDC) algorithm for direct-detection 56 Gbit/s on-off keying signal in C-band over 20 km dispersion-uncompensated standard single-mode fiber transmission. The proposed receiver-side EDC algorithm includes the feed-forward equalizer, post noise-whitening filter and modified iterative detection algorithm, which is originally used in spectrally efficient frequency division multiplexing systems for inter-carrier interference cancellation. Only with the aid of the proposed EDC algorithm for the compensation of power fading effect caused by bandwidth limit and chromatic dispersion, bit-error ratio below the 7% hard-decision forward error correction limit is achieved. This scheme can keep the system cost-effective and flexible, showing potentials for optical interconnects.

Adaptive Channel-Matched Detection for C-Band 64-Gbit/s Optical OOK System Over 100-km Dispersion-Uncompensated Link

In this paper, we propose adaptive channel-matched detection (ACMD) for C-band 64-Gbit/s intensity-modulation and direct-detection (IM/DD) optical on-off keying (OOK) system over a 100-km dispersion-uncompensated link. The proposed ACMD can adaptively compensate most of the link distortions based on channel and noise characteristics, which includes a polynomial nonlinear equalizer (PNLE), a decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE). Based on the channel characteristics, PNLE eliminates the linear and nonlinear distortions, while the followed DFE compensates the spectral nulls caused by chromatic dispersion. Finally, based on the noise characteristics, a post filter can whiten the noise for implementing optimal signal detection using MLSE. To the best of our knowledge, we present a record C-band 64-Gbit/s IM/DD optical OOK system over a 100 km dispersion-uncompensated link achieving 7\% hard-decision forward error correction limit using only the proposed ACMD at the receiver side. In conclusion, ACMD-based C-band 64-Gbit/s optical OOK system shows great potential for future optical interconnects.

C-band 56 Gbit/s on/off keying system over a 100 km dispersion-uncompensated link using only receiver-side digital signal processing: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 758 (2020)OPLEDP0146-959210.1364/OL.384168.

C-band 56 Gbit/s on/off keying system over a 100 km dispersion-uncompensated link using only receiver-side digital signal processing.

In this Letter, we present a record C-band 56 Gbit/s intensity-modulation/direct-detection optical on/off keying (OOK) system over a 100 km dispersion-uncompensated link using only the receiver-side digital signal processing (DSP). The proposed DSP mainly includes an adaptive moment estimation (Adam)-based polynomial nonlinear equalizer (PNLE), autoregression (AR)-based post filter, and maximum likelihood sequence estimation (MLSE). Due to square-law detection, chromatic dispersion induces 11 nulls on the 28 GHz spectrum of the 56 Gbit/s OOK signal after the 100 km standard-single-mode-fiber transmission. Adam-based PNLE eliminates major linear and nonlinear distortions, but it cannot compensate the nulls. After the Adam-based PNLE, the AR-based post filter and MLSE further deal with the inter-symbol interference caused by the nulls to improve the system performance. The proposed C-band 56 Gbit/s OOK system shows great potential for future metro networks and data center networks.

Piecewise Linear Equalizer for DML Based PAM-4 Signal Transmission Over a Dispersion Uncompensated Link

Directly modulated laser (DML) and direct detection (DD) based pulse-amplitude modulation (PAM) for short reach optical communications has attracted lots of research interests due to its low cost and simple configuration. However, the directly modulated PAM signal usually suffers from distortions after the transmission over a dispersion uncompensated link. Due to the adiabatic chirp of the DML working in a high output power region, different intensity levels of the PAM signal correspond to different frequency offsets, thus resulting in amplitude-dependent skew after the transmission over a dispersion uncompensated link. It will degrade signal quality and limit transmission distance. In this article, we, for the first time, propose a computational efficient piecewise linear (PWL) equalizer to correct the amplitude-dependent skew due to the interaction between the DML chirp and chromatic dispersion in the fiber. By using the PWL equalizer, the amplitude-dependent skew is corrected, and we successfully transmit 56 Gb/s PAM-4 signal over 40 km and 84 Gb/s PAM-4 signal over 20 km dispersion uncompensated link in the C band, with a bit error ratio (BER) below the HD-FEC threshold (3.8 × 10− 3). To demonstrate the low computational complexity of the PWL, we compare its complexity with 2nd order Volterra equalizer for a similar BER performance. The results show that the computational complexity can be reduced by 61.4% and 40.4% for the 56 Gb/s and 84 Gb/s PAM-4 signal transmission, respectively.

Combined Optical and Electrical Spectrum Shaping for High-Baud-Rate Nyquist-WDM Transceivers

We discuss the benefits and limitations of optical time-division multiplexing (OTDM) techniques based on the optical generation of a periodic train of sinc pulses for wavelength-division multiplexing (WDM) transmission at high baud rates. It is shown how the modulated OTDM spectrum bandwidth is related to the optical comb parameters and the pulse shaping of the modulating waveforms in the electrical domain. Such dependence may result in broadening of the modulated spectra, which can degrade the performance of Nyquist-WDM systems due to interchannel crosstalk penalties. However, it is shown and experimentally demonstrated that the same technique of optical pulse train generation can be allied with digital pulse shaping to improve the confinement of the modulated spectrum toward the Nyquist limit independently of the number of OTDM tributaries used. To investigate the benefits of the proposed approach, we demonstrate the first WDM Nyquist-OTDM signal generation based on the periodic train of sinc pulses and electrical spectrum shaping. Straight line transmission of five 112.5-Gbd Nyquist-OTDM dual-polarization quadrature phase-shift keying (QPSK) channels is demonstrated over a dispersion uncompensated link up to 640 km, with full-field coherent detection at the receiver. It is shown that such a design strategy effectively improves the spectral confinement of the modulated OTDM signal, providing a minimum intercarrier crosstalk penalty of 1.5 dB in baud-rate-spaced Nyquist-WDM systems.

Nonlinear Signal–Noise Interaction in Optical Links With Nonlinear Equalization

We investigate nonlinear signal–noise interaction (NSNI) in dispersion uncompensated links by means of the enhanced Gaussian noise (EGN) model. We include a first-order perturbative description of digital backpropagation (DBP) in the EGN model, thus accounting for NSNI not just along the optical link but even inside DBP. Thanks to the EGN model, we are able to evaluate the impact of NSNI in a setup with 133 channels, thus filling the entire bandwidth of erbium-doped fiber amplifiers. We discuss the main implications of NSNI showing that it mainly impacts the part of the nonlinear interference variance predicted by the Gaussian noise model. A simple formula to extend noiseless results to the realistic case with NSNI is proposed. Finally, the reduced efficiency of DBP in presence of NSNI is investigated and discussed.

Impairment mitigation in superchannels with digital backpropagation and MLSD

We assess numerically the performance of single-carrier digital backpropagation (SC-DBP) and maximum-likelihood sequence detection (MLSD) for DP-QPSK and DP-16QAM superchannel transmission over dispersion uncompensated links for three different cases of spectral shaping: optical pre-filtering of RZ and NRZ spectra, and digital Nyquist filtering. We investigate the limits for carrier proximity of each spectral shaping technique and the correspondent performance behavior of each algorithm, for both modulation formats. For superchannels with carrier spacing close to the Nyquist limit, it is shown that the maximum performance improvement of 1.0 dB in Q(2)-factor is provided by those algorithms. However, such gain can be highly reduced when the order of the modulation format increases.

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.

Kerr nonlinearity mitigation in 5 × 28-GBd PDM 16-QAM signal transmission over a dispersion-uncompensated link with backward-pumped distributed Raman amplification.

We present experimental and numerical investigations of Kerr nonlinearity compensation in a 400-km standard single-mode fiber link with distributed Raman amplification with backward pumping. A dual-pump polarization-independent fiber-based optical parametric amplifier is used for mid-link spectral inversion of 5 × 28-GBd polarization-multiplexed 16-QAM signals. Signal quality factor (Q-factor) improvements of 1.1 dB and 0.8 dB were obtained in the cases of a single-channel and a five-channel wavelength-division multiplexing (WDM) system, respectively. The experimental results are compared to numerical simulations with good agreement. It is also shown with simulations that a maximum transmission reach of 2400 km enabled by the optical phase conjugator is possible for the WDM signal.

1.5-$\mu{\rm m}$ 10-Gb/s VCSEL Link for Optical Access Applications

We experimentally demonstrate the power budget improvement of a 10-Gb/s vertical-cavity surface-emitting laser (VCSEL) link by utilizing continuous-phase frequency-shift keying (CPFSK)/amplitude-shift keying (ASK) modulation format. We first measure the frequency modulation (FM) response of the VCSEL from 10 kHz to 10 GHz and then exploit dc-balanced line coding to utilize the flat region of the FM response. The CPFSK/ASK signals are generated by directly modulating the VCSEL and utilizing a delay interferometer (DI) for FSK-to-ASK conversion. We successfully transmit the signals over a 50-km dispersion-uncompensated link by optimizing the free-spectral range of the DI. Using the proposed scheme, we achieve > 5-dB improvement in the power budget compared with a conventional VCSEL link.

Analytical results on back propagation nonlinear compensator with coherent detection

We derive analytic formulas for the improvement in effective optical signal-to-noise ratio brought by a digital nonlinear compensator for dispersion uncompensated links. By assuming Gaussian distributed nonlinear noise, we are able to take both nonlinear signal-to-signal and nonlinear signal-to-noise interactions into account. In the limit of weak nonlinear signal-to-noise interactions, we derive an upper boundary of the OSNR improvement. This upper boundary only depends on fiber parameters as well as on the total bandwidth of the considered wavelength-division multiplexing (WDM) signal and the bandwidth available for back propagation. We discuss the dependency of the upper boundary on different fiber types and also the OSNR improvement in practical system conditions. Furthermore, the analytical formulas are validated by numerical simulations.