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Adaptive moment estimation (Adam) is a popular optimization method to estimate large-scale parameters in neural networks. This paper proposes the first use of Adam algorithm to fast and stably converge large-scale tap coefficients of polynomial nonlinear equalizer (PNLE) for 129-Gbit/s PAM8-based optical interconnects. PNLE is one of simplified Volterra nonlinear equalizer for making a trade-off between complexity and performance. Different from serial least-mean square (LMS) adaptive algorithm, Adam algorithm is a parallel processing algorithm, which can obtain globally optimal tap coefficients without being trapped in locally optimal tap coefficients. Timing error is one of the main obstacles to the PAM systems with high baud rate and high modulation order. Owing to parallel processing and global optimization, Adam algorithm has much better performance on resisting the timing error, which can achieve faster, more-stable and lower-MSE convergence compared to LMS adaptive algorithm. In conclusion, Adam algorithm shows great potential for converging the tap coefficients of PNLE in PAM8-based optical interconnects.

Coherent optical communication systems are still too expensive for metro and short reach networks in the near future. Coherent detection with an intensity modulator replacing a costly dual-polarization in-phase and quadrature modulator could offer a good trade-off between cost and performance for metro networks. The major performance limit of such systems is the nonlinearity. To the best of our knowledge, this Letter offers the first known investigation on nonlinearity of a high speed intensity modulation and coherent detection system using an integrated laser and electro-absorption modulator. Advanced nonlinearity mitigation digital signal processing (DSP) are proposed including a 2×1 multiple-input single-output complex transversal Volterra feed-forward equalizer (FFE) filter, a first proposed post-FFE noise cancellation filter taking into account both pre- and post-cursor noise correlations based on a simple autocorrelation coefficient calculation, and a maximum likelihood sequence estimator. The receiver DSP needs no carrier and phase recovery that is required by a conventional coherent receiver DSP. Results show that, compared to a simple linear FFE equalization, the proposed nonlinear DSP enables over 8 and 4 dB improvement in optical signal to noise ratio (OSNR) sensitivity at 7% overhead hard-decision feed-forward error correction (FEC) and 20% overhead soft-decision FEC threshold bit error rates (BERs), respectively, for a 56 Gb/s intensity modulation and coherent detection system. To the best of our knowledge, a record 18 dB OSNR sensitivity at a hard-decision FEC threshold BER of 3.8×10-3 is also achieved.

We present, to the best of our knowledge, the first known detailed analysis and fair comparison of complexity of a 56 Gb/s multi-band carrierless amplitude and phase (CAP) and discrete multi-tone (DMT) over 80 km dispersion compensation fiber-free single-mode fiber links based on intensity modulation and direct detection for data center interconnects. We show that the matched finite impulse response filters and inverse fast Fourier transform (IFFT)/FFT take the majority of the complexity of the multi-band CAP and DMT, respectively. The choice of the multi-band CAP sub-band count and the DMT IFFT/FFT size makes significant impact on the system complexity or performance, and trade-off must be considered.

We experimentally demonstrate the generation and successful transmission of a single-lane 180 Gb/s (90 GBd) duo-binary four-level pulse-amplitude modulation (DB-PAM-4) signal over an 80 km standard single-mode fiber with a net data-rate of 150 Gb/s, enabled by a dual-drive Mach-Zehnder modulator-aided dispersion pre-compensation. A net data rate of 168 Gb/s is also achievable, for ranges up to 40 km. To mitigate bandwidth limitations and error spreading, pre-coded DB-PAM-4 is used; the pre-coding provides a gain of approximately 1 dB. A Volterra filter and a maximum likelihood sequence estimator are used at the receiver side to reduce the linear and nonlinear distortions.

Extensive numerical investigations are undertaken to analyze and compare, for the first time, the performance, techno-economy, and power consumption of three-level electrical Duobinary, optical Duobinary, and PAM-4 modulation formats as candidates for high-speed next-generation PONs supporting downstream 40 Gb/s per wavelength signal transmission over standard SMFs in C-band. Optimization of transceiver bandwidths are undertaken to show the feasibility of utilizing low-cost and band-limited components to support next-generation PON transmissions. The effect of electro-absorption modulator chirp is examined for electrical Duobinary and PAM-4. Electrical Duobinary and optical Duobinary are power-efficient schemes for smaller transmission distances of 10 km SMFs and optical Duobinary offers the best receiver sensitivity albeit with a relatively high transceiver cost. PAM-4 shows the best power budget and cost-efficiency for larger distances of around 20 km, although it consumes more power. Electrical Duobinary shows the best trade-off between performance, cost and power dissipation.

The first known experimental demonstrations of a 10 Gb/s hybrid CAP-2/QAM-2 and a 20 Gb/s hybrid CAP-4/QAM-4 transmitter/receiver-based optical data link are performed. Successful transmission over 4.3 km of standard single-mode fiber (SMF) is achieved, with a link power penalty ∼0.4 dBo for CAP-2/QAM-2 and ∼1.5 dBo for CAP-4/QAM-4 at BER=10(-9).

Simulations have been performed to compare the system capacity and power dissipation of NRZ, CAP-64 and 64-QAM-OFDM systems over FEC enhanced POF links using LEDs, for both unidirectional and bidirectional transmission. It is shown that CAP-64 outperforms NRZ and 64-QAM-OFDM in terms of system capacity and supports a record high 3.5 Gb/s bidirectional and 2.1 Gb/s unidirectional transmissions over 50 m POF. The CAP-64 transceiver consumes similar power compared with NRZ whilst the 64-QAM-OFDM transceiver consumes about twice as much.

A performance comparison between a recently proposed novel technique known as fast orthogonal frequency-division multiplexing (FOFDM) and conventional orthogonal frequency-division multiplexing (OFDM) is undertaken over unamplified, intensity-modulated, and direct-detected directly modulated laser-based optical signals. Key transceiver parameters, such as the maximum achievable transmission capacity and the digital-to-analog/analog-to-digital converter (DAC/ADC) effects are explored thoroughly. It is shown that, similarly to conventional OFDM, the least complex and bandwidth efficient FOFDM can support up to ~20 Gb/s over 500 m worst-case multimode fiber (MMF) links having 3 dB effective bandwidths of ~200 MHz × km. For compensation of the DAC/ADC roll-off, a power-loading (PL) algorithm is adopted, leading to an FOFDM system improvement of ~4 dB. FOFDM and conventional OFDM give similar optimum DAC/ADC parameters over 500 m worst-case MMF, while over 50 km single-mode fiber a maximum deviation of only ~1 dB in clipping ratio is observed due to the imperfect chromatic dispersion compensation caused by one-tap equalizers.

Based on a comprehensive theoretical optical orthogonal frequency division multiplexing (OOFDM) system model rigorously verified by comparing numerical results with end-to-end real-time experimental measurements at 11.25Gb/s, detailed explorations are undertaken, for the first time, of the impacts of various physical factors on the OOFDM system performance over directly modulated DFB laser (DML)-based, intensity modulation and direct detection (IMDD), single-mode fibre (SMF) systems without in-line optical amplification and chromatic dispersion compensation. It is shown that the low extinction ratio (ER) of the DML modulated OOFDM signal is the predominant factor limiting the maximum achievable optical power budget, and the subcarrier intermixing effect associated with square-law photon detection in the receiver reduces the optical power budget by at least 1dB. Results also indicate that, immediately after the DML in the transmitter, the insertion of a 0.02nm bandwidth optical Gaussian bandpass filter with a 0.01nm wavelength offset with respect to the optical carrier wavelength can enhance the OOFDM signal ER by approximately 1.24dB, thus resulting in a 7dB optical power budget improvement at a total channel BER of 1 × 10(-3).

Detailed investigations of the transmission performance of adaptively modulated optical orthogonal frequency division multiplexed (AMOOFDM) signals converted using reflective semiconductor optical amplifiers (RSOAs) are undertaken over intensity-modulation and direct-detection (IMDD) single-mode fiber (SMF) transmission systems for WDM-PONs. The theoretical RSOA model adopted for modulating the AMOOFDM signals is experimentally verified rigorously in the aforementioned transmission systems incorporating recently developed real-time end-to-end OOFDM transceivers. Extensive performance comparisons are also made between RSOA and SOA intensity modulators. Optimum RSOA operating conditions are identified, which are independent of RSOA rear-facet reflectivity and very similar to those corresponding to SOAs. Under the identified optimum operating conditions, the RSOA and SOA intensity modulators support the identical AMOOFDM transmission performance of 30Gb/s over 60km SMFs. Under low-cost optical component-enabled practical operating conditions, RSOA intensity modulators with rear-facet reflectivity values of >0.3 outperform considerably SOA intensity modulators in transmission performance, which decreases significantly with reducing RSOA rear-facet reflectivity and optical input power. In addition, results also show that use can be made of the RSOA/SOA intensity modulation-induced negative frequency chirp to improve the AMOOFDM transmission performance in IMDD SMF systems.

The fastest ever 11.25Gb/s real-time FPGA-based optical orthogonal frequency division multiplexing (OOFDM) transceivers utilizing 64-QAM encoding/decoding and significantly improved variable power loading are experimentally demonstrated, for the first time, incorporating advanced functionalities of on-line performance monitoring, live system parameter optimization and channel estimation. Real-time end-to-end transmission of an 11.25Gb/s 64-QAM-encoded OOFDM signal with a high electrical spectral efficiency of 5.625bit/s/Hz over 25km of standard and MetroCor single-mode fibres is successfully achieved with respective power penalties of 0.3dB and -0.2dB at a BER of 1.0 x 10(-3) in a directly modulated DFB laser-based intensity modulation and direct detection system without in-line optical amplification and chromatic dispersion compensation. The impacts of variable power loading as well as electrical and optical components on the transmission performance of the demonstrated transceivers are experimentally explored in detail. In addition, numerical simulations also show that variable power loading is an extremely effective means of escalating system performance to its maximum potential.

The wavelength dependent transmission performance of adaptively modulated optical OFDM (AMOOFDM) signals is investigated, for the first time, over optical amplification- and chromatic dispersion compensation-free IMDD SMF systems using semiconductor optical amplifiers (SOAs) as intensity modulators. A theoretical SOA model describing both optical gain saturation and gain spectral dynamics is developed, based on which optimum SOA operating conditions are identified for various wavelengths varying in a broad range of 1510 nm- 1590 nm. Results show that, SOA intensity modulators operating at the identified optimum conditions enable the realization of colourless AMOOFDM transmitters within the aforementioned wavelength window. Such transmitters are capable of supporting >30 Gb/s signal transmission over 60 km SMFs.

Detailed investigations are undertaken, for the first time, of the transmission performance of recently proposed novel Adaptively Modulated Optical OFDM (AMOOFDM) modems using Subcarrier Modulation (AMOOFDM-SCM) in single-channel, SMF-based IMDD links without optical amplification and chromatic dispersion compensation. The cross-talk effect induced by beatings among subcarriers of various types is a crucial factor limiting the maximum achievable AMOOFDM-SCM performance. By applying single sideband modulation and/or spectral gapping to AMOOFDM-SCM, three AMOOFDM-SCM designs of varying complexity are proposed, which achieve >60Gb/s signal transmission over 20 km, 40 km and 60 km. Such performances are >1.5 times higher than those supported by conventional AMOOFDM modems.

The impact of Adaptive Cyclic Prefix (ACP) on the transmission performance of Adaptively Modulated Optical OFDM (AMOOFDM) is explored thoroughly in directly modulated DFB laser-based, IMDD links involving Multimode Fibres (MMFs)/Single-Mode Fibres (SMFs). Three ACP mechanisms are identified, each of which can, depending upon the link properties, affect significantly the AMOOFDM transmission performance. In comparison with AMOOFDM having a fixed cyclic prefix duration of 25%, AMOOFDM with ACP can not only improve the transmission capacity by a factor of >2 (>1.3) for >1000 m MMFs (