Direct Detection Links Research Articles

Weight-adaptive joint mixed-precision quantization and pruning for neural network-based equalization in short-reach direct detection links.

Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations. The NN connections are either directly cutoff or represented by a proper number of quantization bits by weight partitioning, leading to a hybrid compressed sparse network that computes much faster and consumes less hardware resources. The proposed approach is verified in a 50-Gb/s 25-km pulse amplitude modulation (PAM)-4 IM/DD link using a directly modulated laser (DML) in the C-band. Compared with the traditional fully connected NN-based equalizer operated with standard floating-point arithmetic, about 80% memory can be saved at a minimum network size without degrading the system performance. Quantization is also shown to be more suitable to over-parameterized NN-based equalizers compared with NNs selected at a minimum size.

Computationally efficient composite triple beat cancellation for DML-based DA-RoF fronthaul.

The digital-analog radio-over-fiber (DA-RoF) scheme offers a high-fidelity and spectrally efficient solution for future mobile fronthaul. However, to be implemented in the low-cost directly modulated laser with direct detection (DML-DD) link, both the digital and analog parts in DA-RoF modulation would suffer from the composite second-order (CSO) and composite triple beat (CTB) caused by the chirp-dispersion interaction. In this Letter, we propose and experimentally demonstrate a computationally efficient composite triple beat cancellation (CTB-C) algorithm for DA-RoF fronthaul in the dispersion-uncompensated C-band DML-DD link. The CSO and CTB are suppressed at the receiver-side DSP based on the theoretical model of these nonlinear distortions. In the proof-of-concept experiment, a 1.2-dB improvement in the recovered signal-to-noise ratio (SNR) is obtained with 5.5-GHz 1024-QAM orthogonal frequency division multiplexing (OFDM) signal after 10-km standard single-mode fiber (SSMF) transmission. The proposed CTB-C technique does not require the training process and performs close to the Volterra-based feed-forward equalizer (VFE) under the complexity constraint.

Spiking Neural Network Nonlinear Demapping on Neuromorphic Hardware for IM/DD Optical Communication

Neuromorphic computing implementing spiking neural networks (SNN) is a promising technology for reducing the footprint of optical transceivers, as required by the fast-paced growth of data center traffic. In this work, an SNN nonlinear demapper is designed and evaluated on a simulated intensity-modulation direct-detection link with chromatic dispersion. The SNN demapper is implemented in software and on the analog neuromorphic hardware system BrainScaleS-2 (BSS-2). For comparison, linear equalization (LE), Volterra nonlinear equalization (VNLE), and nonlinear demapping by an artificial neural network (ANN) implemented in software are considered. At a pre-forward error correction bit error rate of exponent-product2e-3, the software SNN outperforms LE by 1.5 dB, VNLE by 0.3 dB and the ANN by 0.5 dB. The hardware penalty of the SNN on BSS-2 is only 0.2 dB, i.e., also on hardware, the SNN performs better than all software implementations of the reference approaches. Hence, this work demonstrates that SNN demappers implemented on electrical analog hardware can realize powerful and accurate signal processing fulfilling the strict requirements of optical communications.

Effect of Higher-Order Modal Dispersion in Direct-Detection Mode-Division-Multiplexed Links

Short-reach direct-detection links using mode-division multiplexing (MDM) in multimode fiber (MMF) can employ receiver-side optical signal processing (OSP), for example, by Mach-Zehnder meshes, to compensate for crosstalk among modes within mode groups, which have nearly equal propagation constants and couple strongly during propagation. In graded-index (GI) MMF, modes in the lowest-order groups, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{01}\rbrace$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{11}\rbrace$</tex-math></inline-formula> , also have nearly equal group delays. Modes in higher-order groups, such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{02},LP_{21}\rbrace$</tex-math></inline-formula> , do not, leading to modal dispersion (MD). To enable MDM in these higher-order mode groups, we propose to also employ transmitter-side OSP to launch signals into principal modes (PMs), which are free of MD to first order in frequency. We study the performance of the proposed scheme via simulation of realistic GI-MMFs based on experimental measurements by Carpenter et al. While PM transmission eliminates MD to first order in frequency, higher-order MD limits the ability of frequency-independent OSP to mitigate inter-symbol interference and crosstalk. We quantify the impact of higher-order MD, and show that either the PM coherence bandwidth or the group-delay spread within a mode group can predict the power penalty due to higher-order MD.

Multiuser Massive MIMO-OFDM for Visible Light Communication Systems

Visible light communication (VLC) has emerged as a viable supplement to existing radio frequency (RF) communication systems. The limited modulation bandwidth of sources is one of the significant challenges in the multiuser (MU) VLC systems, which prohibits transmission at high data rates for each user. Since an array of LEDs is used to illuminate a room, a MU VLC system utilizing massive multiple-input multiple-output (mMIMO) orthogonal frequency-division multiplexing (OFDM) is proposed for achieving high data rates in this paper. Since the distances of the multiple transmitter-receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain in the OFDM technique. Complex channel vectors associated with different users are mutually spatially orthogonal as the number of transmitters increases. Therefore, inter-user interference (IUI) can be eliminated with simple linear signal processing, and more users can simultaneously communicate in the same time-frequency resource. In this paper, three linear precoding methods, including the maximum ratio transmission (MRT), the minimum mean square error (MMSE), and the zero-forcing (ZF), are investigated for VLC systems based on proposed MU-mMIMO-OFDM in intensity-modulation and direct-detection link. We evaluate and compare the performance of the aforementioned precoding methods in terms of the achievable spectral efficiency and total downlink optical power for different scenarios. Moreover, we derive a closed-form expression for the lower bound on the average achievable sum-rate, which we confirm the accuracy of the derived expression with the numerical simulation. The results demonstrate that the performance of the proposed method in this paper for all precoder techniques, i.e., ZF, MMSE, and MRT, is better than the previous works since the complex channel in the frequency domain is used in this paper. Furthermore, the ZF and MMSE methods are better than MRT when the ratio of the number of LEDs to the number of users is large. At the same time, when this ratio is relatively small, the MRT precoding technique outperforms the ZF and MMSE techniques.

Independent component analysis for impairment mitigation in direct-detected Stokes vector modulation.

In this paper we theoretically and experimentally demonstrate a novel adaptation of independent component analysis (ICA) for compensation of both cross-polarization and inter-symbol interference in a direct-detection link using Stokes vector modulation (SVM). SVM systems suffer from multiple simultaneous impairments that can be difficult to resolve with conventional optical channel DSP techniques. The proposed method is based on a six-dimensional adaptation of ICA that simultaneously de-rotates the SVM constellation, corrects distortion of constellation shape, and mitigates inter-symbol interference (ISI) at high symbol rates. Experimental results at 7.5 Gb/s and 15Gb/s show that the newly developed ICA-based equalizer achieves power penalties below ∼1 dB, compared to the ideal theoretical bit-error rate (BER) curves. At 30-Gb/s, where ISI is more severe, ICA still enables polarization de-rotation and BER < 10-5 before error correction.

A Performance Limit Estimation Framework for Multihop Repeated/Regenerated Optical Links

We propose and demonstrate a generalized framework for performance limit evaluation and comparison of multihop optical repeated and regenerated links. The model developed is implementation agnostic and applies to multihop optical links of varied forms, including fiber, free space, and underwater links. The framework estimates the best-case performance gains of deploying an all-regenerative link over an all-repeater link for any given implementation. The implementation-independent technique is then illustrated using guided and free-space optical links. An abstract model is developed first with the evolution of signal, noise power, and bit error rate down the link compared and contrasted for both cases. The model is then evaluated using physical parameters for a typical fiber optic intensity-modulated direct detection link, and the obtained all-regenerator link performance advantage is translated to extra reach and lower transmission power requirements. Further, certain approximations are provided to reduce computational complexity and improve the analytical tractability of the procedure, which could be particularly helpful when employed in specialized hardware or for dynamic reconfiguration networks. Finally, the framework&#x2019;s versatility is established by employing it in analyzing an ideal free-space link and comparing amplify and forward links against decode and forward counterparts. Similar results are also reproduced on a commercial optical link simulation suite. Detailed literature on link analysis is provided for fiber, free space, and underwater links, bringing out their similarities. We conclude by elaborating on various current and emerging application domains and certain limitations of the proposed technique.

Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System

In this work, we systematically analyze the impact of three kinds of Mach-Zehnder modulator (MZM) imbalances, including bias deviation, amplitude mismatch, and differential time skew in intensity-modulation direct-detection (IM-DD) links. It is shown that, for power fading limited transmission, the imbalances can be utilized as advantages rather than impairments. Specifically, the bias deviation with single-arm driven mode and amplitude mismatch with differential driven mode can increase the available bandwidth by shifting the frequency of fading notches. Meanwhile, time skew provides another way to avoid fading by shaping the double sideband (DSB) signal into a vestigial sideband (VSB) with an asymmetrical transfer function. In the transmission experiment, 34 Gbaud Nyquist 6/8-ary pulse amplitude modulation (PAM-6/8) signals are used for investigation in a 20 km dispersion-uncompensated standard single-mode fiber (SSMF) link. With the help of a Volterra nonlinear equalizer, all three kinds of imbalances can achieve bit-error rates (BERs) below the 7% and 20% hard-decision forward error correction (HD-FEC) thresholds for PAM-6 and PAM-8 signals, respectively. The received power sensitivity is also compared at the back-to-back (BTB) case and after fiber transmission. Both numerical simulation and experimental demonstration confirm that the dispersion-induced power fading can be effectively suppressed with bias, amplitude, or skew imbalance, providing a feasible solution for transmission distance extension of C-band DD links.

100 Gbps/λ C-Band CD Digital Pre-Compensated and Direct-Detection Links With Simple Non-Linear Compensation

In the scenario of downstream Passive Optical Networks (PON), we analyze through simulations and experiments a 100 Gbps/λ link using digital signal processing (DSP) over up to 50 km single mode fiber (SMF) in C-band. In particular, we propose chromatic dispersion digital pre-compensation (CD-DPC) and quaternary pulse amplitude modulation (PAM-4) levels optimization at the transmitter side (TX), and simple non-linear compensation (NLC) in combination with adaptive equalization (AEQ) at the receiver side (RX). Regarding NLC, we compare two approaches: a quadratic polynomial function and a square-root-like function respectively. In this paper, we analyze in detail the performances of four proposed options, namely equispaced PAM-4 levels without NLC (baseline case), equispaced PAM-4 levels in combination with NLC, optimized PAM-4 levels without NLC, and optimized PAM-4 levels in combination with NLC. We demonstrate through simulations and experiments that optimized PAM-4 levels can only offer limited enhancement when NLC is applied, and up to 3.3 dB sensitivity gain can be obtained thanks to NLC at RX when setting the optimum parameters, with respect to the baseline case.

Hybrid Coding and Filtering Technique for Optical IM-DD Link With Robustness to Multipath Interference and Bandwidth Limitation

In this work, a hybrid code division multiplexing (CDM) coding and digital filtering method for optical intensity modulation and direct detection (IM-DD) link is investigated to address multipath interference (MPI) and channel bandwidth limitation. CDM signaling already provides robustness to MPI through suppressing the unsynchronized paths by orthogonal (de)coding. As for digital filtering based on square root raised cosine (SRRC) waveforms, its advantage to IM-DD link includes robustness to channel bandwidth limitation due to pulse shaping. Additionally, matching filters achieve the best orthogonality only when they are synchronized well, thus robustness to MPI can be expected on the basis of CDM. We study the performance of such hybrid coding and filtering method over IM-DD link in the presence of MPI and bandwidth limitations through simulation. This is followed by experimental demonstrations of single point-to-point transmission with capacity at 112 Gbps. BER comparison between CDM only and hybrid filtering and coding method is performed by emulating MPI using a Mach-Zehnder interferometer. Furthermore, bidirectional transmission over a single fiber verifies its performance advantage and demonstrates potentials for reducing implementation costs of end-to-end optical communications.

Time skew enabled vestigial sideband modulation for dispersion-tolerant direct-detection transmission.

Fiber dispersion and square-law detection-induced power fading is a fundamental obstacle in intensity modulation with direct detection links. In this Letter, we propose a hardware-efficient vestigial sideband (VSB) transmitter to suppress such an impairment. By introducing an appropriate time skew between the differential arms of the Mach-Zehnder modulator, a VSB signal can be generated based on a single digital-to-analog convertor without optical filtering. At the receiver, a Volterra equalizer is utilized to mitigate the inter-symbol interference. In the proof-of-concept experiment, 32 Gbaud 4/6/8-ary pulse amplitude modulation signals can be successfully transmitted over an 80 km standard single-mode fiber with bit-error rates below the KP4, 7% hard-decision and 20% soft-decision forward error correction thresholds, respectively. The proposed scheme provides a promising and low-cost solution for high-speed metro and data center interconnect applications.

Experimental demonstration of advanced modulation formats for data center networks on 200 Gb/s lane rate IMDD links.

This work contributes experimental demonstrations and comprehensive comparisons of various modulation and coding techniques for 200 Gb/s intensity modulation and direct detection links including four-level pulse amplitude modulation (PAM-4), PAM-6, trellis-coded modulation (TCM) over PAM and discrete multi-tone (DMT) transmission. Both C-band Mach-Zehnder modulator and O-band electro-absorption modulated laser transmitters were examined for intra-data center applications based on state-of-the-art commercial components.

Probabilistically Shaped 4-PAM for Short-Reach IM/DD Links With a Peak Power Constraint

Probabilistic shaping for intensity modulation and direct detection (IM/DD) links is discussed and a peak power constraint determined by the limited modulation extinction ratio (ER) of optical modulators is introduced. The input distribution of 4-ary unipolar pulse amplitude modulation (PAM) symbols is optimized for short-reach transmission links without optical amplification nor in-line dispersion compensation. The resulting distribution is symmetric around its mean allowing to use probabilistic amplitude shaping (PAS) to generate symbols that are protected by forward error correction (FEC) and that have the optimal input distribution. The numerical analysis is confirmed experimentally for both an additive white Gaussian noise (AWGN) channel and a fiber channel, showing gains in transmission reach and transmission rate, as well as rate adaptability.

PAM-4 transmission up to 160 Gb/s with surface-normal electro-absorption modulators.

We report multi-level modulation in polarization-independent surface-normal electro-absorption modulators (SNEAMs). Four-level pulse amplitude modulation (PAM-4) at a line rate of 44 Gb/s is demonstrated on a fully packaged SNEAM with a 30 µm active area diameter and a 14 GHz electro-optic bandwidth. High-capacity PAM-4 transmission at 112 and 160 Gb/s is demonstrated on an unpackaged SNEAM chip, with a 15 µm active area diameter and ultrawide electro-optic bandwidth (≫65GHz). Fiber transmission is investigated for direct detection link lengths up to 23 km at 44 Gb/s and 2 km at 112 and 160 Gb/s, the highest multi-level modulation rates achieved on a SNEAM.

240 Gbit/s Silicon Photonic Mach-Zehnder Modulator Enabled by Two 2.3-Vpp Drivers

We recently reported for the first time 200 Gbit/s/ $\lambda$ net rate data transmission with a single silicon photonic modulator in an intensity modulation direct detection link. Our demonstration relied on an in-house designed, O-band segmented-electrode MZM. In this article, we detail the optimization of the modulator driving scheme, bandwidth/efficiency trade-off and modulation format enabling this result. We use two single-ended RF signals, each having 2.3-Vpp amplitude only, to drive two 1.5-mm modulator segments and transmit data at 80 Gbaud PAM-8 (240 Gbit/s) across 2 km under the 20% soft-decision forward error correction BER threshold of 2E-02. We further decrease the BER using maximum likelihood sequence detection (MLSD), and discuss the related implications. Additionally, we present the system margins regarding modulator drive voltage, received optical power and equalizer taps. At 240 Gbit/s line rate, the modulator energy consumption is only 73 fJ/bit at the SD-FEC threshold. We finally discuss practical implementation considerations for our system.

Electronic Dispersion Precompensation of Direct-Detected NRZ Using Analog Filtering

We demonstrate (in real-time) electrical dispersion compensation in direct detection links using analog transmit side filtering techniques. By this means, we extend the fiber reach using a low complexity solution while avoiding digital preprocessing and digital-to-analog converters (DACs) which are commonly used nowadays. Modulation is done using an IQ Mach-Zehnder modulator (MZM) which allows straightforward compensation of the complex impulse response caused by chromatic dispersion in the fiber. A SiGe BiCMOS 5-tap analog complex finite impulse response (FIR) filter chip and/or a delay between both driving signals of the MZMs is proposed for the filter implementation. Several link experiments are conducted in C-band where transmission up to 60 km of standard single-mode fiber (SSMF) of direct detected 28 Gb/s NRZ/OOK is demonstrated. The presented technique can be used in applications where low power consumption is critical.

Nonlinearity mitigation in RF-over-fiber links by chromatic dispersion modules

External modulation, such as with a Mach–Zehnder modulator (MZM), is commonly used in optical communications to reduce nonlinear signal distortions. However, in addition to the signal distortion due to chromatic dispersion, the MZM itself is another source of nonlinear distortion. Despite the fact that dispersion is a linear distortion that develops in the field domain, in low-cost direct-detection links, the detector destroys the system’s linearity. Therefore, in these channels, dispersion is another source of nonlinearity. We suggest the implementation of a dispersion module to substantially reduce the MZM-induced nonlinearity. This technique is possible only because linear dispersion mitigation filters can be implemented post detection. We show experimentally that chromatic dispersion can indeed substantially reduce the channel’s nonlinear distortion, in good agreement with theoretical predictions.

LSTM networks enabled nonlinear equalization in 50-Gb/s PAM-4 transmission links.

This paper proposes a nonlinear equalization technique enabled by long short-term memory (LSTM) recurrent neural networks. The proposed technique is implemented at the end of offline digital signal processing. And two approaches utilizing the LSTM network are experimentally tested and demonstrated in transmission of a 50-Gb/s four-level pulse amplitude modulation intensity modulation direct detection link over 100-km standard single-mode fiber. The first approach uses the LSTM network-based equalizer to directly categorize the received signal into four amplitude levels, and the second approach uses the LSTM network to estimate signal noise for compensating the received signal. The experimental results show remarkable performance improvement of the proposed method over conventional linear equalizers, and significant enhancement at high launch power compared with Volterra filtering. Also, the proposed method reveals better short-time universality.

Outage Probability Due to Intercore Crosstalk in Dual-Core Fiber Links With Direct-Detection

The outage probability (OP) in short-haul direct-detection (DD) optical links supported by weakly-coupled multicore fiber (MCF) impaired by intercore crosstalk (ICXT) and using ON–OFF keying signaling is assessed theoretically and through simulation. A closed-form expression for the OP is proposed for a single interfering core. This expression is useful to provide fast and reliable performance estimates in preliminary studies of weakly-coupled MCF-based systems where the ICXT is highly correlated along the signal bandwidth, i.e., when the skew-bit rate product is small. Particularly, the results for a 20-km-long link at 10 Gbit/s and walk-off below 1 ps/km are detailed. For high skew-bit rate products, i.e., when the ICXT is decorrelated along the signal bandwidth, the simulation results show that for a typical OP of 10−4, the tolerance to ICXT increases above 4 dB, when compared with low skew-bit rate products. It is also shown that the DD links with finite extinction ratio are more robust to outage induced by the ICXT than those with an ideal extinction ratio.

Multi-Tb/s-per-Fiber Coherent Co-Packaged Optical Interfaces for Data Center Switches

Co-packaging of optical and electronic interfaces inside data center switches has been proposed to reduce the system-level power consumption and support higher density, higher capacity optical links. However, co-locating electronics and optics increases optical losses and exposes lasers, (de)multiplexers and other components to temperature fluctuations. We propose a design for co-packaged interfaces using coherent detection and wide-passband (de)multiplexers to increase the total bit rate per fiber in the presence of demultiplexer center frequency shifts up to ±150 GHz. We compare direct and coherent detection systems within this architecture and demonstrate that unamplified coherent links can provide over 20 dB higher loss budget than direct detection links at 400 Gb/s per fiber. Moreover, coherent links can offer far higher bit rates per fiber. For example, in the presence of (de)multiplexer center frequency shifts of up to ±150 GHz, DP-QPSK links can scale to over 5 Tb/s per fiber, while four pulse amplitude modulation (PAM) links are optical bandwidth limited to ~400 Gb/s per fiber. We quantify signal path loss and optical signal-to-noise ratio budgets when optical amplifiers, limited to eye-safe output powers, are used to compensate for losses of fibers or optical switches. While the loss and SNR budgets of direct detection links may be improved by optical amplifiers, these systems cannot scale to high bit rates per fiber because large channel spacings are required to accommodate (de)multiplexer center frequency shifts. For example, amplified 4-PAM links can scale to less than 1.3 Tb/s per fiber, while amplified DP-QPSK links can scale to greater than 12.8 Tb/s per fiber.