Forward Error Correction Limit Research Articles

Quantum Dash Passively Mode-Locked Lasers for Coherent Wavelength Conversion System

We demonstrate an all-optical wavelength conversion system based on four-wave mixing in semiconductor optical amplifiers for quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM) signals using tunable dual-correlated pumps provided by the combination of a quantum dash passively mode-locked laser (QD-PMLL) and a programmable tunable optical filter. We investigate the effects of additional phase noise transfer to the wavelength converted idler due to non-ideal correlation between the comb lines of the QD-PMLL. Our studies of the wavelength conversion of QPSK and 16-QAM signals at 12.5 Gbaud using the proposed scheme find that the bit error rate performance was below the 7% forward error correction (FEC) limit over a range of pump spacing exceeding 300 GHz for QPSK signals, and conversion of 16-QAM signals was limited by the wavelength conversion scheme though was nonetheless below the 20% FEC limit.

10-Gb/s Transmission Over 10-m SI-POF With <inline-formula> <tex-math notation="LaTeX">${M}$ </tex-math> </inline-formula>-PAM and Multilayer Perceptron Equalizer

We demonstrate the gigabit-per-second transmission over a step-index plastic optical fiber (SI-POF) of 10-m length with a pulse-amplitude modulation (PAM). A multilayer perceptron-based equalizer is used to mitigate an intersymbol interference and non-linearity in the system. Using this equalizer with 32-PAM, a data rate of 10 Gb/s is achieved over the 10-m SI-POF at a bit error rate of 10−2, which is below the 20% forward error correction limit.

Low Noise Frequency Combs for Higher Order QAM Formats Through Cross-Phase Modulation of Modelocked Laser Pulses

An active, harmonically modelocked fiber laser with phase unstable output pulses from its long cavity is shown transformable into a low noise frequency comb suited for phase noise sensitive communications. Cross-phase modulation (XPM) of its output pulses with a continuous wave (CW) probe laser in a highly nonlinear fiber (HNLF) produces a frequency comb with noise at its minimum for low timing jitter pulses. Experiments with 2 ps pulses at 40 GHz repetition show that spectral lines obtained from within a bandwidth of 30 lines achieve low bit error rate (BER) after their individual modulation with 96 Gb/s dual polarization 64-QAM signals, reaching below the limit of hard-decision forward error correction (FEC), and close to that of a narrow linewidth CW laser. Its suitability as a seed for even broader comb generation is demonstrated by addition of a second stage HNLF, enabling a larger bandwidth of nearly 50-comb lines to reach a BER below the FEC limit for the same signal. The benefit of XPM for improving the phase noise of a modelocked semiconductor laser frequency comb is also shown, with limitation from larger pulse timing jitter. The results highlight the capability for enabling new applications of otherwise unusable pulse sources.

Multilevel quadrature amplitude multiplexing using coherently coupled orbital angular momentum modes

Information can be encoded onto transverse spatial light modes, providing a platform that can improve the spectral efficiency of a communications link. Switching speed between these spatial modes and detection methods limit the information capacity of such links. This work demonstrates high speed capabilities of such a link. Transverse modes are created and switched at high rates by coherently coupling twisted light modes using passive optical elements in line with electro-optical modulators. Here we demonstrate the encoding and detection of two coherently coupled modes encoded with 0.5 Gbaud quadrature amplitude modulated (16- and 32-QAM) signals, for a 4X and 5X increase in spectral efficiency by exploiting both phase and amplitude of the coherently coupled modes. The receiver is able to successfully recover the signal with error rates below the forward error correction limit using passive optical techniques. The data rate of the system used is only limited by hardware, but similar devices are available that are capable of multi-Gigahertz operation.

Real-time 100 Gbps/λ/core NRZ and EDB IM/DD transmission over multicore fiber for intra-datacenter communication networks.

A BiCMOS chip-based real-time intensity modulation/direct detection spatial division multiplexing system is experimentally demonstrated for both optical interconnects. 100 Gbps/λ/core electrical duobinary (EDB) transmission over 1 km 7-core multicore fiber (MCF) is carried out, achieving KP4 forward error correction (FEC) limit (BER < 2E-4). Using optical dispersion compensation, 7 × 100 Gbps/λ/core transmission of both non-return-to-zero (NRZ) and EDB signals over 10 km MCF transmission is achieved with BER lower than 7% overhead hard-decision FEC limit (BER < 3.8E-3). The integrated low complexity transceiver IC and analog signal processing approach make such a system highly attractive for the high-speed intra-datacenter interconnects.

Enhanced noise tolerance for 10 Gb/s Bi-directional cross-wavelength reuse colorless WDM-PON by using spectrally shaped OFDM signals

Abstract Spectrally shaped orthogonal frequency division multiplexing (OFDM) signal for symmetric 10 Gb/s cross-wavelength reuse reflective semiconductor optical amplifier (RSOA) based colorless wavelength division multiplexed passive optical network (WDM-PON) is proposed and further analyzed to support broadband services of next generation high speed optical access networks. The generated OFDM signal has subcarriers in separate frequency ranges for downstream and upstream, such that the re-modulation noise can be effectively minimized in upstream data receiver. Moreover, the cross wavelength reuse approach improves the tolerance against Rayleigh backscattering noise due to the propagation of different wavelengths in the same feeder fiber. The proposed WDM-PON is successfully demonstrated for 25 km fiber with 16-QAM (quadrature amplitude modulation) OFDM signal having bandwidth of 2.5 GHz for 10 Gb/s operation and subcarrier frequencies in 3–5.5 GHz and DC-2.5 GHz for downstream (DS) and upstream (US) transmission respectively. The result shows that the proposed scheme maintains a good bit error rate (BER) performance below the forward error correction (FEC) limit of 3.8 × 10−3 at acceptable receiver sensitivity and provides a high resilience against re-modulation and Rayleigh backscattering noises as well as chromatic dispersion.

Faster-Than-Nyquist Precoded CAP Modulation Visible Light Communication System Based on Nonlinear Weighted Look-Up Table Predistortion

In this paper, we have experimentally demonstrated faster-than-Nyquist carrierless amplitude and phase modulation quadrature phase-shift keying visible light communication system over 1.5-m free space transmission. Weighted look-up table is used to mitigate nonlinear impairment caused by amplifiers and electro-optical components. At the receiver side, recursive least square is employed to improve the decision precision. By combining these techniques, a data rate of 1.01 Gb/s is experimentally achieved with a bit error rate less than 7% forward error correction limit of 3.8 × 10 -3 . To the best of our knowledge, this is the first time that weighted look-up table is proposed and used in visible light communication system.

Underwater wireless optical communication using an arrayed transmitter/receiver and optical superimposition-based PAM-4 signal.

In this work, we propose an underwater wireless optical communication (UWOC) system using an arrayed transmitter/receiver and optical superimposition-based pulse amplitude modulation with 4 levels (PAM-4). At the transmitter side, we design a spatial summing scheme using a light emitting diode (LED) array, which is divided into two groups in a uniformly interleaved manner. With on-off keying (OOK) modulation for each group, optical superimposition-based PAM-4 can be realized. It has enhanced tolerance to the modulation nonlinearities of LEDs. We numerically investigate the feasibility of the proposed spatial summing scheme in various underwater channels via Monte Carlo simulation. With the increase of divergence angle of LEDs and link distance, the optical power distribution tends to be more uniform at the reception plane. It can significantly relax the requirement on the link alignment. Furthermore, we conduct a proof-of-concept experiment employing two blue LEDs. A multi-pixel photon counter (MPPC), containing an array of single-photon avalanche diodes (SPADs), is used as the detector. It has a much higher sensitivity and can further relax the requirement for pointing. Over a 2-m tap water channel, data rates of 6.144 Mb/s, 8.192 Mb/s, and 12.288 Mb/s were achieved by using the PAM-4 signal generated by optical superimposition, within a 2.5-MHz system bandwidth. With 0.570-mg/L Mg(OH)2, the measured optical power is just 12.890 µW after a 2-m underwater channel. The corresponding bit error rate (BER) of the 12.288-Mbs PAM-4 signal is 2.9 × 10-3, which is still below the forward error correction (FEC) limit of 3.8 × 10-3. It implies that the UWOC system based on the high-sensitivity MPPC with array structure has superior power efficiency and robustness.

Nonlinearity-aware 200 Gbit/s DMT transmission for C-band short-reach optical interconnects with a single packaged electro-absorption modulated laser.

We experimentally demonstrate the transmission of a 200Gbit/s discrete multitone (DMT) at the soft forward error correction limit in an intensity-modulation direct-detection system with a single C-band packaged distributed feedback laser and traveling-wave electro absorption modulator (DFB-TWEAM), digital-to-analog converter and photodiode. The bit-power loaded DMT signal is transmitted over 1.6km standard single-mode fiber with a net rate of 166.7Gbit/s, achieving an effective electrical spectrum efficiency of 4.93bit/s/Hz. Meanwhile, net rates of 174.2Gbit/s and 179.5Gbit/s are also demonstrated over 0.8km SSMF and in an optical back-to-back case, respectively. The feature of the packaged DFB-TWEAM is presented. The nonlinearity-aware digital signal processing algorithm for channel equalization is mathematically described, which improves the signal-to-noise ratio up to 3.5dB.

Thresholding Scheme Based on Boundary Pixels of Stripes for Visible Light Communication With Mobile-Phone Camera

In mobile-phone camera-based visible light communication (VLC) system, column matrix selection and threshold setting are very important steps for signal demodulation. As the blooming effect of complementary metal-oxide-semiconductor camera fluctuates grayscale values of pixels, thus causing a decrease in the accuracy of distinguishing data logic 0 and 1. Facing grayscale value fluctuation, using traditional polynomial fitting as the threshold to distinguish data logic 0 and 1, the achievable system performance is not very good. This limits applications of the VLC system. Therefore, advanced thresholding schemes are needed to increase the accuracy of distinguishing data logic 0 and 1, thereby improving the system performance. In this paper, we propose and experimentally demonstrate for the first time a thresholding scheme based on boundary pixels of stripes for improving the VLC system performance. First, we describe the process of boundary pixel position acquisition for the thresholding scheme in detail. Then, we analyze and discuss the selection of the optimum column matrix for signal demodulation. Finally, we demonstrate experiments to verify the feasibility of our proposed thresholding scheme. Experimental results show that our proposed thresholding scheme can achieve a good performance even when grayscale values fluctuate greatly. Compared with the conventional third-order polynomial fitting scheme, our proposed thresholding scheme has advantages in both of transmission distance and data rate. With aid of the optimum selected column matrix and our proposed thresholding scheme, a data rate of 4.5 Kb/s (2.5 Kb/s) is successfully achieved with the transmission distance of 50 cm (175 cm) under the 7% forward error correction limit (bit-error-rate = $3.8\times 10^{-3}$ ) for the VLC system with mobile-phone camera.

Demonstration of 15-M 7.33-Gb/s 450-nm Underwater Wireless Optical Discrete Multitone Transmission Using Post Nonlinear Equalization

In this paper, we experimentally demonstrate an underwater wireless optical communication (UWOC) system using a 450-nm gallium nitride (GaN) laser and adaptive bit-power loading discrete multitone (DMT). To enhance the system capacity, a post nonlinear equalizer based on the simplified Volterra series is employed at the receiver to mitigate the nonlinear impairments of the UWOC system. By combining the adaptive bit-power loading with nonlinear equalization, 7.33-Gb/s DMT-modulated UWOC under 15-m tap water is achieved at a bit error rate below the 7% hard-decision forward error correction (FEC) limit 3.8 × 10–3. The electrical signal bandwidth is 1.25 GHz, which corresponds to an electrical spectrum efficiency of ∼6 bit/s/Hz. The capacity-distance product reaches 109.95 Gb/s-m in a single channel UWOC system with tap water. Compared with the linear equalization case, the system capacity at the FEC limit for 15-m underwater transmission is improved by ∼18% with the nonlinear equalization. Furthermore, the impact of turbidity on the performance of UWOC system is investigated by measuring the signal-to-noise ratio (SNR) under different suspension concentrations of Al(OH)3 and Mg(OH)2. The results show that significant SNR gains (>3 dB for transmission distance up to 11 m) can be obtained by the nonlinear equalization over a wide range of water turbidity levels representing “clear ocean,” “coastal ocean,” and “harbor water,” which demonstrates the robustness of the proposed scheme in various ocean environments.

60 GHz 5G Radio-Over-Fiber Using UF-OFDM With Optical Heterodyning

Millimeter-wave communications may be harnessed to help meet future 5G network bandwidth requirements, but the generation of these frequencies in the electronic domain can be difficult and costly. Photonic techniques, such as optical heterodyning, can be exploited to generate millimeter-wave frequencies in a manner which is efficient and compatible with fiber transport infrastructure. In this letter, analog radio-over-fiber transmission of five bands of UF-OFDM, with optical heterodyning around 60 GHz, is experimentally demonstrated over 25 km SSMF, and performance well below the forward error correction limit is achieved for an aggregate raw bit rate of 4.56 Gb/s. Additionally, the importance of precise optical phase correlation in these systems is highlighted with respect to future 5G system requirements.

Gigabit LED-based visible light transparent transmission from free-space to a 100-m ultra-large effective area pure silica fiber

We experimentally demonstrate the LED-based visible light transmission (VLC) over a 100-m special designed low-loss ultra-large effective area (diameter >1 mm) pure silica fiber (ULEAPS fiber). Three-channel wavelength multiplexing is achieved by using a single chip RGB LED as the light source. The total insertion loss after 100-meter transmission for the red light, green light and blue light are 1.015, 1.582, and 3.660 dB, respectively. A total data rate of 3.1 Gb s−1 is experimentally achieved over 5-m ULEAPS fiber and a data rate of 2 Gb s−1 is achieved over 100-m fiber with the bit error rate (BER) below 7% forward error correction (FEC) limit of 3.8 × 10−3. The results obtained here may also serve as a valuable information on how to realize LED based VLC transparent transmission from free-space to a pure silica fiber utilizing a commercial illumination LED as the light source.

An Adaptive Activated ANN Equalizer Applied in Millimeter-Wave RoF Transmission System

We experimentally demonstrate a 60-GHz radio-over-fiber (RoF) system using an adaptive activated artificial neural network nonlinear equalizer (ANN-NLE) to improve the bit error rate (BER) performance. We propose a variable- $\alpha $ activation function ANN-NLE with a faster convergence speed and a stronger equalization ability to solve the nonlinear problems generated from the RoF link. To the best of our knowledge, this letter is the first to apply an adaptive activation function ANN to solve a classification problem and equalize the signals in a 60-GHz RoF system. Using the proposed ANN-NLE, a higher convergence speed and better BER performances can be achieved. Comparisons between our proposed ANN-NLE and nonlinear Volterra equalizer in polynomial structure are reported. Using the proposed ANN-NLE, the system successfully transmits a 5-Gbps BPSK signal through a 10-km fiber and 1.2-m wireless link under a forward error correction limit of $10^{-3}$ with much higher power sensitivity than the Volterra equalizer.

Simplified Maximum Likelihood Detection for FTN Non-Orthogonal FDM System

In this letter, we first apply simplified maximum likelihood detection (MLD) with QR decomposition and M-algorithm (QRM-MLD) to reduce inter-carrier interference (ICI) in a faster-than-Nyquist non-orthogonal frequency division multiplexing (FTN-NOFDM) system. A QRM-MLD algorithm has lower computational complexity than an MLD algorithm owing to the decrease of searched path number. In the experiment, 10-Gb/s FTN-NOFDM with the QRM-MLD algorithm was transmitted over 25-km standard single mode fiber. For 20% bandwidth saving, FTN-NOFDM with the QRM-MLD algorithm can achieve almost the same performance as FTN-NOFDM with the MLD algorithm when the path with minimal metric is not discarded before detecting the last subcarrier, meanwhile the bit error rate of FTN-NOFDM with the QRM-MLD algorithm or the MLD algorithm can achieve 7% forward error correction (FEC) limit, but that with an iterative detection (ID) algorithm only can achieve 20% FEC limit. For reducing ICI, the QRM-MLD algorithm has lower computational complexity but can obtain almost the same performance as the MLD algorithm, and the QRM-MLD algorithm also has much better performance than the ID algorithm.

A 60-GHz Radio-Over-Fiber Fronthaul Using Integrated Microwave Photonics Filters

To lower the cost and to improve the reliability of next generation fronthaul links, we propose and demonstrate an analog 60-GHz radio-over-fiber fronthaul link employing two wavelength-tunable integrated microwave photonics filters for the first time. An integrated notch filter is cascaded with an integrated bandpass filter to offer the optical filtering function, resulting in a dual-passband filter profile with frequency separation of 60.25 GHz. We prove that the integrated optical filters are able to provide acceptable system performance through both experiment and numerical simulation. 4QAM signal with a bit rate of 5 Gb/s is transmitted in a fiber-based fronthaul link achieving a receiver sensitivity of −5.8 dBm at the forward error correction limit with 7% overhead.

Using pre-distorted PAM-4 signal and parallel resistance circuit to enhance the passive solar cell based visible light communication

Using solar cell (or photovoltaic cell) for visible light communication (VLC) is attractive. Apart from acting as a VLC receiver (Rx), the solar cell can provide energy harvesting. This can be used in self-powered smart devices, particularly in the emerging “Internet of Things (IoT)” networks. Here, we propose and demonstrate for the first time using pre-distortion pulse-amplitude-modulation (PAM)-4 signal and parallel resistance circuit to enhance the transmission performance of solar cell Rx based VLC. Pre-distortion is a simple non-adaptive equalization technique that can significantly mitigate the slow charging and discharging of the solar cell. The equivalent circuit model of the solar cell and the operation of using parallel resistance to increase the bandwidth of the solar cell are discussed. By using the proposed schemes, the experimental results show that the data rate of the solar cell Rx based VLC can increase from 20 kbit/s to 1.25 Mbit/s (about 60 times) with the bit error-rate (BER) satisfying the 7% forward error correction (FEC) limit.

Bit- and Power-Loading—A Comparative Study on Maximizing the Capacity of RSOA Based Colorless DMT Transmitters

We present a comparative study of the capacity increase brought by bit- and power-loading discrete multi-tone (DMT) modulation for low-cost colorless transmitters. Three interesting reflective semiconductor optical amplifier (RSOA) based colorless transmitter configurations are compared: First, an amplified spontaneous emission (ASE) spectrum-sliced source; second, a self-seeded RSOA fiber cavity laser (FCL) and third, an externally seeded RSOA. With bit- and power-loaded DMT, we report record high line rates of 6.25, 20.1 and 30.7 Gbit/s and line rates of 4.17, 10.1 and 24.5 Gbit/s in a back-to-back and in a 25 km nonzero dispersion shifted fiber (NZDSF) transmission experiments for the three transmitter configurations, respectively. In all the experiments, BER (bit error ratios) below an FEC (forward error correction) limit of 7.5 × 10−3 were achieved.

Gigabit free-space multi-level signal transmission with a mid-infrared quantum cascade laser operating at room temperature.

Gigabit free-space transmissions are experimentally demonstrated with a quantum cascaded laser (QCL) emitting at mid-wavelength infrared of 4.65μm, and a commercial infrared photovoltaic detector. The QCL operating at room temperature is directly modulated using on-off keying and, for the first time, to the best of our knowledge, four- and eight-level pulse amplitude modulations (PAM-4, PAM-8). By applying pre- and post-digital equalizations, we achieve up to 3 Gbit/s line data rate in all three modulation configurations with a bit error rate performance of below the 7% overhead hard decision forward error correction limit of 3.8×10-3. The proposed transmission link also shows a stable operational performance in the lab environment.

Terahertz Wireless Data Transmission With Frequency and Polarization Division Multiplexing Using Resonant-Tunneling-Diode Oscillators

High-capacity short-distance wireless communications in the terahertz (THz) range are anticipated and therefore have been intensively studied. Higher data rates are made possible by the introduction of frequency division multiplexing (FDM) and polarization division multiplexing (PDM) schemes in the THz range. In this paper, wireless data transmissions using 2-channel FDM in the 500 and 800 GHz ranges and PDM in the 500 GHz range are demonstrated using resonant-tunneling-diode oscillators with different frequencies and polarizations integrated into one chip. Transmissions at a data rate of 28 Gbit/s were achieved in each channel with an error rate below the forward error correction limit in both multiplexing systems. The ratios of the leakage of the transmitted signal in one channel into the other channel were about −40 and −30 dB in FDM and PDM modes, respectively.