Intensity Modulation And Direct Detection Research Articles

A Chaotic Encryption Scheme in DMT for IM/DD Intra-Datacenter Interconnects

In this letter, a novel physical layer encryption method based on digital chaos is proposed for the security in high-speed intensity modulation and direct detection (IM/DD) discrete multi-tone (DMT) transmission system. In the proposed encryption scheme, a random interleaving method is used, which further enhances the ability to encrypt the quadrature amplitude modulation (QAM) constellation and improves the transmission performance. In addition, probabilistic shaping (PS) technique is also cascaded into the encrypted transmission system for the first time. Adopting the proposed algorithm, we experimentally demonstrated the transmission of 100-Gbit/s encrypted 64QAM and PS-64QAM DMT signals over 2-km standard single-mode fiber (SSMF).

Transmission Beyond 200 Gbit/s With IM/DD System for Campus and Intra-Datacenter Network Applications

Intensity modulation and direct detection (IM/DD) with pulse amplitude modulation (PAM) is a cost-effective option for intra- and inter-data center (DC) applications. Due to current demand on DC data traffic, transmission rates beyond 200 Gbit/s/ $\lambda $ are desirable. However, employing commercially available lower bandwidth components introduces severe inter-symbol interference (ISI). In addition, to achieving high-speed data rates, digital signal processing (DSP) schemes like Tomlinson-Harashima precoding (THP) and decision feedback equalizer (DFE) can increase tolerance against ISI. In this letter, we show an experimental comparison of PAM-4, PAM-6 and PAM-8 modulation formats with and without THP on the transmitter-side and Volterra nonlinear equalizer (VNLE) on the receiver-side. We provide also results with and without duobinary (DB) encoding on the transmitter side and DFE on the receiver side. The experiment is conducted using an O-band electro-absorption modulated laser over transmission distances up to 20 km.

Finite-SNR Diversity Gain Analysis of FSO Systems over Gamma-Gamma Fading Channels With Pointing Errors

Free space optical (FSO) communication systems are subject to turbulence-induced fading as a result of fluctuations in the refractive index. Gamma-Gamma (GG) distribution is commonly used in the literature to characterize moderate-to-strong turbulence conditions. The performance of FSO systems is further degraded by pointing error-induced fading as a result of building sways. While the asymptotic analysis presented in earlier works is important to understand the maximum diversity gains of FSO systems attainable over fading channels, such gains might not be available in practical range of signal-to-noise ratio (SNR). Furthermore, different systems with the same asymptotic slope may have different slopes in the range of finite SNR values. In this paper, we consider a multiple-input multiple-output (MIMO) FSO system with intensity modulation and direct detection (IM/DD). We derive approximate expression for the finite-SNR diversity order over GG turbulence channels in the presence of pointing errors. We also investigate convergence speed to full diversity. We provide numerical results to demonstrate the accuracy of our derivations and corroborate on our analytical findings.

The dual–parallel Mach–Zehnder modulator-based spectral conversion robustness research in the hybrid GPON and 5G system

Digital signal processing (DSP)-enabled dual–parallel Mach–Zehnder (DP-MZM)-based spectral converter can overcome traditional converter insufficient to realize high transparency, dynamic re-configurable, and low-cost spectral conversion. However, in practice, the converter’s driving RF signals have variables caused by physical device deviations. To explore the converter robustness, this paper, through numinous numerically simulations in the intensity-modulation and direct-detection (IMDD)-based network nodes, analyses the impact of variables on the converter performance in terms of conversion efficiency, the bit error rate (BER) performance and conversion-associated power penalty in the hybrid gigabit passive optical network (GPON) and the fifth-generation (5G) system. Simulation results demonstrate the converter robustness on driving radio frequency (RF) signals’ amplitudes, phases, and frequency acceptable variables range, meanwhile, the frequency variables restrict the user signal bandwidth and can limit the converter practical application. In addition, based on variable analysis, this paper suggests a driving approach to eliminate the frequency variables for practical application and demonstrate the simultaneous impact of amplitudes and phases variables on the converter performance.

On the Intercept Probability and Secure Outage Analysis of Mixed (α – κ – μ)-Shadowed and Málaga Turbulent Models

This work deals with the secrecy performance analysis of a dual-hop RF-FSO DF relaying network composed of a source, a relay, a destination, and an eavesdropper. We assume the eavesdropper is located close to the destination and overhears the relay’s transmitted optical signal. The RF and FSO links undergo (α-κ-μ)-shadowed fading and unified Málaga turbulence with pointing error. The secrecy performance of the mixed system is studied by deriving closed-form analytical expressions of secure outage probability (SOP), strictly positive secrecy capacity (SPSC), and intercept probability (IP). Besides, we also derive the asymptotic SOP, SPSC, and IP upon utilizing the unfolding of Meijer’s G function where the electrical SNR of the FSO link tends to infinity. Finally, the Monte-Carlo simulation is performed to corroborate the analytical expressions. Our results illustrate that fading, shadowing, detection techniques (i.e. heterodyne detection (HD) and intensity modulation and direct detection (IM/DD)), atmospheric turbulence, and pointing error significantly affect the secrecy performance. In addition, better performance is obtained exploiting the HD technique at the destination relative to IM/DD technique.

A Robust Sparse RLS-Volterra Nonlinear Equalizer Using ℓ₀-Regularization for 4 × 150 Gbit/s IMDD-Based Optical Interconnect

Volterra equalizer (VE) is a well-known and effective algorithm to deal with the linear and nonlinear distortions in optical interconnect, but the high computational complexity hinders its practical application. Generally, sparse VEs based on ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> - or ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -regularization are good ways to reduce the complexity by discarding some inessential taps. However, a tap threshold needs to be chosen in these sparse VEs like the threshold-based pruned retraining VE (TR-VE) to decide the discarded taps. And this tap threshold should be adjusted fine to balance the reduced complexity and equalization performance, especially when the testing environments alter. Thus, the reduced complexity in these sparse VEs may fluctuate. To address this issue, a robust and stable complexity reduced sparse VE using ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -regularization (ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE) is proposed in this paper. The recursive least square (RLS) algorithm is used to replace the least mean square algorithm for the faster convergence speed and better equalization performance. The complexity of this equalizer depends on its parameters but not the tap threshold. Once the equalizer parameters are determined, the complexity would not change with the system characteristics, contributing to higher practicability. In our experiment, a 150 Gbit/s PAM8 signal transmission system based on intensity modulation and direct detection (IMDD) is achieved, and a dual-drive Mach-Zehnder modulator for optical single-sideband signal generation is used to mitigate the power fading effect. The experimental results show that with the help of ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE, the reduced complexity percentage is stable at 66.18% compared with the RLS-based VE, even after 75 km standard single-mode fiber (SSMF) transmission. By using this equalizer, 4×150 Gbit/s PAM8 signals have also been successfully transmitted over 30 km SSMF at C-band. The reduced complexity variation of ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -TR-VE is >20%, but the reduced complexity of the proposed ℓ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -SR-VE is stable at 60% even after 30 km SSMF for all four lanes.

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.

Unified statistical performance of FSO link due to the combined effect of weak turbulence and generalized pointing error with HD and IM/DD

In this paper, we set up the statistical channel model of ship-to-ship (or ship-to-shore) free space optical links considering generalized pointing error and weak turbulence. We combine various pointing error models with weak turbulence and derive the composite probability density functions (PDF) for each case of pointing error model. Also, using the similarity of the composite PDFs, we obtain a unified expression for the composite PDF. Furthermore, we conduct error rate analysis based on both intensity modulation and direct detection (IM/DD) and heterodyne detection (HD). At last, the numerical results confirm that the derived average error rate gives precise prediction on error rate.

Physical Layer Encryption in DMT Based on Digital Multi-Scroll Chaotic System

Physical layer encryption based on digital chaos emerged as a promising methodology for securing data transmission. Discrete multi-tone (DMT) is a widely investigated advanced modulation format in intensity modulation and direct detection (IM/DD) systems, because of its high spectral efficiency, low cost and high dispersion tolerance. In this letter, we successfully performed a physical layer chaotic encryption method on an IM/DD system and experimentally demonstrated the transmission of 100 Gb/s encrypted 32QAM-DMT and 125 Gb/s pre-equalized encrypted 32QAM-DMT signals over 2-km standard single mode fiber (SSMF) with bit error rate (BER) under the hard decision forward error correction (HD-FEC) threshold of 3.8×10 -3 . The proposed encryption algorithm utilizes three independent pseudo-random sequences generated by the multi-scroll chaotic system for secure data transmission, and has no substantial impact on the transmission performance.

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.

Hybrid OFDM-Digital Filter Multiple Access PONs Utilizing Spectrally Overlapped Digital Orthogonal Filtering

Known hybrid orthogonal frequency division multiplexing-digital filter multiple access (OFDM-DFMA) PONs show promise of seamless and cost-effective convergence of optical and mobile networks for 5G and beyond. This paper reports, for the first time, a new hybrid OFDM-DFMA PON based on intensity modulation and direct detection (IMDD), obtained by modifying digital signal processing (DSP) algorithms embedded in both the OLT and ONUs. The proposed PON allows two spectrally overlapped sub-bands to occupy each individual sub-wavelength spectral region to independently transmit upstream ONU information. A model of the proposed PON is developed and its upstream transmission performances are numerically explored for different application scenarios. Compared with the previously published PON, the proposed PON doubles the number of supported ONUs and provides >1.7-fold aggregate upstream signal transmission capacity increases with 2.2-fold aggregate upstream signal transmission capacity increases and >0.7 dB upstream power budget improvements are achievable. The performance improvements vary by <; 18% for a transmission distance range as large as 50 km. In addition, the proposed PON is tolerant to finite digital filter tap length-induced channel interferences.

Baseband linearity and interference effects in intensity modulated and direct detected optical MIMO systems

Abstract In short reach intensity modulated and direct detected (IM/DD) optical multiple-input multiple-output (MIMO) systems linear equalization strategies are commonly applied. However, the square-law detector inherently is a non-linear system element. This work investigates this discrepancy and shows for which laser types and under which mode coupling conditions baseband linearity applies for the first time. Based on a system model that integrates a multitude of aspects, e.g. the phase state of each fiber mode and mode coupling, it is shown that the received signal consists of a linear component and one that can be considered as interference. The simulation and measurement results highlight that only MIMO setups with weak and no mode cross-talk can be considered as linear. Systems with high mode cross-talk are impaired by significant interference, resulting in a highly degraded bit-error rate performance.

16QAM spectral efficiency frequency division multiplexing transmission with Tomlinson–Harashima precoding for intensity modulation and direct detection optical links

In the spectral efficient frequency division multiplexing (SEFDM) system, in order to minimize mutual interference between subcarriers, a detector should be introduced at the receiver to handle the interference, but increases the complexity of system. In this paper, we propose a simplified precoding algorithm at transmitter instead of receiver, which can first implement higher 16QAM SEFDM transmission over standard single mode fiber (SSMF). The technique is based on interference information from non-orthogonality between subcarriers and uses nonlinear matrix operations to improve system performance with low complexity. The numerical results show that the precoding scheme achieves SSMF transmission of intensity modulation and direct detection (IM/DD) 16QAM SEFDM system under the premise of achieving higher spectral efficiency, and obtains good bit error rate performance.

Results on the Rate-Equivocation Region of the Degraded Signal-Dependent Noise Wiretap Channel

This letter studies the rate-equivocation region of a degraded signal-dependent noise wiretap channel (SDGN-WC), where the noise is Gaussian. This signal-dependent noise occurs in optical wireless communication (OWC) settings based on intensity modulation and direct detection (IM-DD). In this setup, the transmitted signals are nonnegative and are restricted by an average optical power constraint. We consider the rate-equivocation region of the degraded SDGN-WC and through solving a convex optimization problem, we establish that the support set of the optimal distributions must be countably infinite. Our result implies that when the transmitted optical signals are restricted by an average optical power: 1) the secrecy capacity is achieved by a distribution which has a countably infinite support set; 2) the channel capacity (the case with no secrecy constraints) is also achieved by a distribution having a countably infinite support set.

DSP for High-Speed Short-Reach IM/DD Systems Using PAM

Intensity modulation and direct detection (IM/DD) together with pulse amplitude modulation (PAM) is a cost-efficient solution for intra-datacenter links. However, sophisticated digital signal processing (DSP) is inevitable to reach the demanded high data rates for these short-reach applications. In this article, we compare different DSP schemes that enable rates of 200 Gb/s/λ and beyond. Those schemes include Tomlinson-Harashima precoding, partial response signaling, Viterbi equalization and post-filtering together with Volterra nonlinear equalization. Extensive experimental measurements are shown in which the DSP approaches are compared at gross rates between 180 Gb/s and 300 Gb/s. The experiments are done in C-band for fiber links of up to 2 km length. As modulation formats, PAM4, PAM6, and PAM8 are considered. Net bit rates well over 200 Gb/s are shown for all formats at a range of 1 km single mode fiber using a transmission system consisting of currently available commercial components. The optimum DSP configuration as well as the best modulation format varies along the investigated data rates. The best solution is shown to change from partial response PAM4 for lower rates to PAM6 and PAM8 at high bit rates.

Performance evaluation and enhancement of the modified OOK based IM/DD techniques for hybrid fiber/FSO communication over WDM-PON systems

In this paper, we have studied the performance evaluation and enhancement of a wavelength-division-multiplexing (WDM) for the passive optical network (PON) in the hybrid fiber/free-space optical (HFFSO) communication system using the modified on–off keying (OOK) based intensity modulation and direct detection (IM/DD) techniques. This work describes the design and analysis of the 8-channels WDM based on the PON-HFFSO transmission system at 2.5 Gbps under different atmospheric turbulence (AT) conditions. Inter-channel crosstalk (ICC), fluctuation, amplified spontaneous emission noise, AT, and impairments are investigated in the paper. A hybrid fiber/WDM-FSO over the PON structure using IM/DD and OOK significantly improves reliability link and accessibility as well as data rate, also improves the system capacity; hence the proposed model will reduce power consumption while maintaining system efficiency. Both impairments and ICC are investigated here and the results are presented in the form of bit-error-rate (BER), transmission distances, the required optical power, and electrical signal-to-noise ratios (SNRs). The system uses the modified OOK based on IM/DD to implement adaptive and electrical SNRs detection thresholds. Numerical results show that the SNR gap between the electrical-SNR-optimized detection system and adaptive detection system is 2.2 dB and 4.2 dB at a BER of 10−5 without a state offset ξ = 0 and ξ = 0.21, respectively. The proposed model enhances performance in terms of BER and electrical SNRs. An improvement in the power penalty of 2.0–4.0 dB at a BER of 10−12 can be potentially achieved using the modified OOK technique when impaired by the weak turbulence.

224-Gbps single-photodiode PAM-4 transmission with extended transmitter bandwidth based on optical time-and-polarization interleaving.

We have proposed and demonstrated the optical time-and-polarization interleaving (OTPI) technique, which can effectively extend the transmitter bandwidth for an intensity modulation and direct detection (IM/DD) optical system. The 224-Gbit/s line-rate OTPI-PAM-4 signal is successfully transmitted over a 500-m standard single-mode fiber (SSMF) in the C band, using the transmitter with a bandwidth of 25 GHz and the receiver with a single photodiode. By using a 33%-return-to-zero (RZ) pulse train, a bit-error ratio (BER) below 7% hard-decision forward error correction (HD-FEC) threshold is achieved. BER below 20% soft-decision forward error correction (SD-FEC) threshold is also realized using a carrier suppressed return-to-zero (CSRZ) pulse train. The OTPI technique can also be used for more higher-order pulse amplitude modulation (PAM) formats, making it a promising technique for next-generation high-speed optical interconnects.

Nonlinearity-Aware Adaptive Bit and Power Loading DMT Transmission Over Low-Crosstalk Ring-Core Fiber With Mode Group Multiplexing

In this article, based on a specially designed ring-core fiber (RCF) with low inter-mode group (MG) crosstalk, we experimentally demonstrate orbital-angular-momentum (OAM) MG-multiplexing (MGM) transmission with intensity modulation and direct detection (IM/DD) utilizing discrete multi-tone (DMT) modulation. Two adjacent high-order OAM MGs |l| = 2 and |l| = 3 with low inter-MG crosstalk of -24.3 dB over 1-km RCF transmission system are employed for data transmission simultaneously. In addition, a low-complexity frequency-domain polynomial nonlinear equalizer (FD-PNLE) combining with adaptive bit and power loading are implemented to mitigate the modulation/detection related nonlinearities as well as modal dispersion induced high-frequency power fading. Experimental results show that compared with linear equalization, the capacities of MGs |l| = 2 and |l| = 3 using FD-PNLE can be significantly improved by 13% and 11.1% with adaptive bit and power loading DMT modulation over 1-km low-crosstalk RCF at a received optical power (ROP) of -13 dBm, respectively. Based on the nonlinearity-aware adaptive bit and power loading DMT signal, successful 186.4-Gbit/s MGM transmission over 1-km low inter-crosstalk RCF is realized only with 20-GHz electrical signal bandwidth. Therefore, the low-crosstalk RCF-based MGM scheme combined with advanced modulation format shows great potential in large-capacity low-cost short-reach optical interconnects.

Experimental Demonstration of an SFO-Robustness Scheme With Fast OFDM for IMDD Passive Optical Network Systems

A sampling frequency offset (SFO)-robustness scheme with zero padding (ZP)-fast orthogonal frequency-division multiplexing (FOFDM) is proposed and experimentally demonstrated in intensity-modulated and direct-detection (IMDD) passive optical network (PON) systems. It can be used to mitigate the impact of SFO-induced inter-carrier interference (ICI). The performance comparison between the proposed FOFDM scheme and conventional OFDM scheme in the presence of SFO is presented. Experimental results indicate that, as SFO-induced subcarrier phase offset (SPO) is compensated within the estimated error of ±0.1 ppm, at the same spectral efficiency (SE) and bandwidth granularity, the error vector magnitude (EVM) of ZP-FOFDM is superior to the OFDM at different received optical power (ROP). In addition, when the ROP is −1 dBm, the average error vector magnitude (EVM) of all the data subcarriers for ZP-FOFDM is 3.3-dB superior to that of OFDM. It means that the ZP-FOFDM scheme can effectively improve the SFO-robustness compared with OFDM.

Demonstration of 4 × 100 Gbit/s PAM-4 Transmission Over 40 km in an IM/DD System Based on Narrow Band DMLs

We demonstrate a four-lane wavelength division multiplexing (WDM) intensity modulation and direct detection (IM/DD) system at O-band. The 3-dB bandwidth of directly modulated lasers (DMLs) in this experiment is only 15 GHz. To support 100-Gbit/s/lane PAM-4 transmission in each lane, digital pre-equalization and advanced receiver-side DSPs are adopted to compensate for bandwidth limitation. Moreover, semiconductor optical amplifier (SOA) is utilized at the receiver to compensate for the optical power loss during fiber transmission so that 40-km transmission distance can be achieved. To the best of our knowledge, it is the first time to successfully transmit 400-Gbit/s PAM-4 signals over 40-km SSMF with 15-GHz DMLs.