Standard Single-mode Fiber Transmission Research Articles

A novel in-band OSNR monitoring technique based on fractional Fourier transform of LFM signal

We propose a novel in-band optical signal to noise ratio (OSNR) monitoring technique in digital coherent receiver based on fractional Fourier transform (FrFT), which employs the power aggregation property of linear frequency modulation (LFM) signal in fractional domain. In our method, the LFM signal with extremely low power is added on the data signal. Thus, the received signal exists a peak in fractional domain, which can be used to indirectly differentiate the data signal power from noise thereby calculating the OSNR. Meanwhile, the proposed technique does not occupy additional spectrum resource and avoids any additional hardware on top of the standard digital coherent receiver. Experimental results of a 10 Gbaud quadrature phase-shift keying (QPSK) system show that the proposed technique is highly accurate, with the monitoring error less than 0.55 dB in the OSNR range of 9–29 dB for back-to-back transmission and 0.9 dB in the OSNR range of 11–25 dB for 960 km standard single mode fiber (SSMF) transmission.

Modulation format identification assisted by sparse-fast-Fourier-transform for hitless flexible coherent transceivers.

For hitless flexible coherent transceivers based next-generation agile optical network, efficient modulation format identification (MFI) is an essential element in digital signal processing (DSP) flow at the receiver-side (Rx). In this paper, we propose a blind and fast MFI scheme with high identification accuracy at low optical signal-to-noise ratio (OSNR) regime. This is achieved by first raising the signal to the 4th power and calculate the peak-to-average power ratio (PAPR) of the corresponding spectra to distinguish 32 quadrature amplitude modulation (QAM) from quadrature phase shift keying (QPSK), 16 and 64QAM signals. Then, followed by iterative partition schemes to remove signals with phase ±π4,±3π4 (or QPSK-like phases) based on the signal amplitudes, the PAPR of the remaining signals is calculated to distinguish the other three formats. Additionally, by frequency offset (FO) pre-compensation, the spectrum can be obtained using sparse-fast-Fourier-transform (S-FFT), which greatly reduces the total complexity. The MFI performance is numerically and experimentally investigated by 28 Gbaud dual-polarization (DP) coherent optical back-to-back (B2B) and up to 1500 km standard single mode fiber (SSMF) transmission system using QPSK, 16QAM, 32QAM, and 64QAM. Results show that high identification accuracy can be achieved, even when OSNR is lower than that required for the 20% forward error correction (FEC) threshold of BER=2×10-2 for each format. Furthermore, fast format switching between 64QAM-32QAM and 32QAM-16QAM are demonstrated experimentally for B2B scenario and 900 km SSMF with the proposed MFI technique, respectively.

Comparison of Bit-Loading DMT and Pre-Equalized DFT-Spread DMT for 2-km Optical Interconnect System

In this paper, we experimentally compared the performance of bit-loading discrete multi-tone (DMT) and pre-equalized discrete-Fourier-transform spread (DFT-Spread) DMT in intensity modulation with direct detection (IM/DD) system for 2-km optical interconnect for the first time. Pre-equalization is applied to compensate the high-frequency power fading in wideband communication system. However, it remains to be verified whether zero-forcing (ZF) channel estimation based complete pre-equalization can achieve the best performance for system with serious high frequency fading. In an IM/DD system with 120 Gbit/s data rate, we demonstrated that the bit error rate of DFT-Spread DMT signal with optimized quadratic curve fitting based partial pre-equalization outperforms ZF based complete pre-equalization after 2-km standard single mode fiber (SSMF) transmission. What's more, we experimentally compared the performance of pre-equalized DFT-Spread DMT and Chow–Cioffi–Bingham (CCB) algorithm-based bit loading DMT. The results show that the receiver sensitivity of pre-equalized DFT-Spread DMT is improved by 2.5 dB compared with CCB algorithm-based bit-loading DMT at hard decision forward error correction threshold of 3.8 × 10−3 after 2-km SSMF transmission.

Experimental demonstration of an adaptive SFO pre-compensation scheme for OFDM-PON

An adaptive sampling frequency offset (SFO) pre-compensation scheme is experimentally demonstrated for orthogonal frequency division multiplexing-based passive optical network (OFDM-PON). Different QAM modulation formats and SFO effects for four optical network units (ONU) are investigated in OFDM-PON. In addition, the length of cyclic prefix (CP)/cyclic suffix (CS) and the number of OFDM symbols per OFDM frame are discussed for the OFDM-PON with the adaptive SFO pre-compensation scheme. And the signal-to-noise ratio (SNR) penalty with respect to SFO estimation offset is also studied. The experimental results show that, the SFO-induced ISI in each ONU can be avoided using the adaptive SFO pre-compensation scheme. After 20 km standard single-mode fiber transmission, the SFO estimation accuracy can be within ±1 ppm in the presence of SFO ranging from -200 to 200 ppm. The SNR penalty for each ONU is below 1 dB under different SFO combinations and different QAM modulation formats.

Modulation format identification and OSNR monitoring using density distributions in Stokes axes for digital coherent receivers.

We experimentally demonstrate a modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring method for digital coherent receivers by using the specific features of received signals' density distributions in Stokes axes combined with deep neural networks (DNNs). The features of received signals' density distribution fitting curves in S1 and S2 axes depend on the signal's modulation format and OSNR. The proposed technique extracts the features of these fitting curves' first-order derivation for MFI and OSNR monitoring, in order to improve the probability of format correct identification and OSNR estimation accuracy. Experimental results for 28Gbaud/s polarization-division multiplexing (PDM) quadrature phase-shift keying (QPSK), PDM 8 quadrature amplitude modulation (PDM-8QAM), PDM-16QAM, and 21.5Gbaud/s PDM-32QAM signals demonstrate OSNR monitoring over back-to-back transmission with mean estimation standard errors (SEs) of 0.21dB, 0.48dB, 0.35dB and 0.44dB, respectively. The MFI results over back-to-back transmission show that 100% identification accuracy of all these four modulation formats are achieved at the OSNR values lower or equal to their respective 7% forward error correction (FEC) thresholds. Similarly, 100% identification accuracy also is obtained for PDM-QPSK, PDM-8QAM, PDM-16QAM, and PDM-32QAM after 2000km, 2000km, 1040km, and 400km standard single-mode fiber (SMF) transmission within practical optical power ranges launched to the fiber, respectively.

High-Precision and Low-Complexity Symbol Synchronization Algorithm Based on Dual-Threshold Amplitude Decision for Real-Time IMDD OFDM-PON

We propose a low-complexity and high-precision symbol synchronization algorithm based on dual-threshold amplitude decision for field programmable gate array-based real-time intensity modulation direct detection orthogonal frequency division multiplexing-passive optical network (OFDM-PON) system. In addition, a new criterion to judge symbol synchronization accuracy is proposed first based on bipolar non-return-to-zero codes in time domain. Experimental results show that 95% synchronization precision rate can be achieved after 25 km standard single-mode fiber transmission with the received optical power as low as –21 dBm. Compared with the previously proposed one threshold symbol synchronization algorithm, the synchronization performance has been greatly improved, which brings a maximum signal bit rate gain of 35% when the received optical power is lower than –16 dBm. Furthermore, it is verified that the proposed algorithm is robust. It has wide transmission range which is applicable for OFDM-PON optical access networks within 100 km and has good sampling frequency offset (SFO) tolerance, which can cover the SFO up to 500 ppm without any discard on synchronization precision rate. Finally, complexity analysis shows that our algorithm has the lowest computational complexity and hardware resource consumption, which account for only 0.9% of system resources without any multipliers.

50-km C-Band Transmission of 50-Gb/s PAM4 Using 10-G EML and Complexity-Reduced Adaptive Equalization

In this paper, we experimentally demonstrate the transmission of 50-Gb/s four-level pulse-amplitude modulation (PAM4) signal over 50-km standard single mode fiber (SSMF) in C-band using electroabsorption modulated laser and PIN/APD detector with 8.5/6 GHz bandwidth. A simplified Volterra-DFE filter is proposed, which reduces the required tap number by ∼70% compared with traditional DFE and Volterra filtering. Experimental results show that by using 86-tap equalization, a sensitivity of –15 and –14 dBm can be achieved at the FEC limit of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> in back to back (BtB) and 50-km SSMF transmission cases. In addition, by replacing the PIN-TIA with a 10-Gb/s APD-ROSA for detection, the sensitivity can be further increased by 5.5 and 3 dB for BtB and 50-km SSMF transmission cases, respectively.

Performance Investigation of a Cost- and Power-Effective High Nonlinearity Tolerance OFDMA-PON Scheme Based on Sub-Nyquist Sampling Rate and DFT-Spread

We investigate the performance of a cost- and power-effective high nonlinearity tolerance orthogonal frequency-division multiple access passive optical network scheme based on sub-Nyquist sampling and discrete Fourier transform spread. The analog-to-digital converter with a sampling rate below the Nyquist sampling rate is used for signal detection for each optical network unit. The complicated pre-process procedure is located in the optical line terminal. Only Low-point FFT and IDFT procedures are required in the ONU, reducing the computational complexity and power consumption in the ONU, and the sampling rate of ADC, and improve the cost- and power-efficiency effectively. Moreover, the peak-to-average power ratio is reduced effectively with the scheme. The simulation results show that the measured bit error ratio of each ONU signal well less than the forward-error correction limit of 3.8 × 10 -3 is obtained after 40-km standard single mode fiber transmission.

Inter-Symbol Differential Detection-Enabled Sampling Frequency Offset Compensation for DDO-OFDM

Sampling frequency offset (SFO) between digital-to-analog converters and analog-to-digital converters will seriously degrade the receiver performance of the asynchronous direct-detection optical OFDM (DDO-OFDM), due to the SFO-induced inter-symbol interference, inter-channel interference, and phase rotation. In this letter, a simple and overhead-free SFO compensation scheme enabled by inter-symbol differential detection (ISDD) is proposed and experimentally demonstrated in a short-reach and low-cost directly modulated laser-based DDO-OFDM system with an optimal fast Fourier transform size. The experimental results exhibit that the SFO up to ±200 ppm can be compensated with acceptable error vector magnitude (EVM) penalties after 20-km standard single-mode fiber transmission. Moreover, the EVM performance can be further enhanced by using a decision-aided residual phase compensation method after ISDD when SFO is large.

Real-Time 10 Gbps Polarization Independent Quasicoherent Receiver for NG-PON2 Access Networks

In this paper, we propose and test experimentally a real-time 10 Gbps polarization independent quasicoherent receiver for NG-PON2 access networks. The proposed 10 Gbps quasicoherent receiver exhibits a sensitivity of −35.2 dBm after 40 km standard single mode fiber (SSMF) transmission with a commercial generic EML as transmitter. This sensitivity means a 14.9 dB improvement over a direct detection scheme with a photodiode after 40 km SSMF transmission. Therefore, the use of the proposed 10 Gbps quasicoherent receiver with the tested EML will provide a power budget of 35.64 dB (class E2) and a splitting ratio of 128 after the 40 km SSMF transmission. Finally, the proposed 10 Gbps quasicoherent receiver allows a colorless and optical filterless operation because wavelength selection is done by tuning the local oscillator wavelength and using electrical intermediate frequency filtering.

150-Gb/s SEFDM IM/DD transmission using log-MAP Viterbi decoding for short reach optical links.

Spectral efficient frequency division multiplexing (SEFDM) can improve the spectral efficiency for next-generation optical and wireless communications. In this work, we apply SEFDM in beyond 100-Gb/s optical intensity modulation and direct detection transmissions and propose a low-complexity logarithmic-maximum-a-posteriori (log-MAP) Viterbi decoding algorithm to achieve the maximum likelihood (ML) detection. We evaluate the likelihood of detections using a posteriori probability instead of Euclidean distance by taking both noise and inter-carrier interference into consideration. In order to balance the performance and complexity, we then employ Viterbi decoding principle to retain only certain paths with ML detections (a.k.a., the surviving paths) while discarding the others during the decoding procedure. Results show that the proposed log-MAP Viterbi decoding scheme achieves optimal performance due to the precise likelihood evaluation, which guarantees the retention of the global ML detection. By using the proposed decoding scheme, the data rate of SEFDM signals can reach 150-Gb/s in a 2-km standard single mode fiber transmission, using only 28-GHz bandwidth and 16-QAM modulation. Experimental results show that the 16-QAM modulated SEFDM signal with a bandwidth compression factor of 0.8 outperforms 32-QAM modulated OFDM, while both signals have the same bandwidth (28-GHz) and data rate (140-Gb/s), which demonstrate the superiority of SEFDM in optical short reach applications.

A structure-simplified and cost-effective coherent receiver for high splitting downstream transmission in next-generation PONs

Abstract In this paper we demonstrate a structure-simplified and cost-effective coherent receiver scheme for high splitting downstream transmission in next generation passive optical networks (NG-PONs). In such scheme, the simple 3dB coupler is used to replace 9 0 0 optical hybrid and the electrical local oscillator (LO) is utilized to down-convert signal, and there by only one balanced photodiode (BPD) is required for each polarization. Hence the cost is reduced effectively for the high splitting PON downstream transmission scenario. The comprehensive numerical simulation results show that the receiver sensitivity for both 14G Baud PM-QPSK and PM-16QAM system is comparable to that of the conventional coherent receiver using 9 0 0 hybrid. Furthermore, the effect of fiber nonlinearity on maximum power budget performance is investigated. In addition, the high receiver sensitivity of the scheme is also experimentally validated for PM-QPSK and PM-16QAM system under the scenario of 14G Baud BtB, 20km and 40km standard single-mode fiber (SSMF) transmission. The coexistent advantages of low-cost and high receiver sensitivity make the proposed scheme an attractive candidate for NG-PONs.

Polar coded optical OFDM system with chaotic encryption for physical-layer security

Abstract The security of the transmission system based on optical orthogonal frequency division multiplexing (OFDM) has been an important research direction recently. Polar code is also a key technique for reducing bit error rate (BER) as a promising forward error correction (FEC) scheme. In the paper, a polar coded optical OFDM system using chaotic encryption is proposed. In the proposed system, a two-dimensional Henon map is utilized to generate chaotic sequences to encrypt subcarriers data for enhancing physical layer security. Meanwhile, polar code can improve the reliability and performance of the system. The simulation results show that the polar code can achieve ∼ 7.4 dB gain at the BER of 1 0 − 3 compared to the conventional case after 60 km standard single mode fiber (SSMF) transmission, and chaotic encryption can effectively protect the signal from illegal attack. Hence, the proposed method is capable of enhancing the physical layer security and simultaneously achieves lower BER.

Theoretical and Experimental Investigation of Direct Detection Optical OFDM Systems with Clipping and Normalization

A data clipping and normalization technique is employed to improve the performance of the overall direct detection optical orthogonal frequency division multiplexing (DCO-OFDM) system. A detailed analysis of clipping distortion introduced by digital clipping and normalization is provided. The normalization operation amplifies the clipped data signal to the maximum input amplitude of a digital-to-analog converter (DAC). Based on the analysis, a BER formula of the proposed scheme is derived over the AWGN channel and single fiber channel. Performance of an optical clipped OFDM with normalization is assessed through numerical simulations and Monte Claro simulation over the AWGN channel. Theoretical analysis and simulation results both show that the clipping and normalization scheme can greatly improve the BER of an optical OFDM. In particular, BER performance of the proposed transmission scheme was measured in a practical OFDM transmission platform. The measured experimental results show that the clipped and amplified OFDM signal exhibits superior performance in comparison with the conventional OFDM signal. The received sensitivity at a BER of 10−3 for a 4 Gsamples/s (2.6667 Gbits/s) clipped and normalized OFDM signal with clipping ratio of 4 after 100 km standard single-mode fiber (SMF) transmission was improved by 4.3 dB when compared with the conventional OFDM system. The measured results also showed that the clipped OFDM signal exhibits superior performance in comparison with the conventional OFDM signal. Therefore, a clipping and normalization at the transmitter is most effective, and a substantial performance improvement can be obtained by a simple normalization after clipping.

Integrated germanium-on-silicon Franz-Keldysh vector modulator used with a Kramers-Kronig receiver.

We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz-Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800 μm×200 μm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully re-constructed with a single-ended photodiode in a recently proposed Kramers-Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.

Machine Learning Adaptive Receiver for PAM-4 Modulated Optical Interconnection Based on Silicon Microring Modulator

Modulation nonlinearity can severely distort multilevel modulation, and signal processing to mitigate the distortion is highly desirable. In this paper, we demonstrated a machine learning method for adaptive detection of 4-level pulse amplitude modulation (PAM-4) signals modulated by silicon micro-ring modulator (Si-MRM). The very limited linear modulation range of Si-MRM leads to serious modulation nonlinearity distortion for high-level modulations like PAM-4 with the consideration of wavelength drift. Our approach is based on the support vector machine (SVM) method, which can learn the nonlinear distortion of Si-MRM during PAM-4 modulation. Thus, the detection can be made adaptive for PAM-4 signals with nonlinear levels and level dependent noise. The modulation nonlinearity distortion of PAM-4 has been characterized in terms of level deviation (LD) with respect to wavelength drift. Up to 2.7-dB receiver sensitivity gain is obtained at about 26% LD by using the proposed SVM machine learning method. The receiver sensitivity-float range can be squeezed to be within 0.3 dB even with up to 30% LD, which indicates a stable detection of PAM-4 signals along with wavelength drift. Up to 3.63-dB receiver sensitivity improvement has been experimentally achieved at 50 Gbps for PAM-4 signals modulated by a Si-MRM and after 2-km standard single mode fiber transmission. The stable operation of Si-MRM is very difficult and very important. The proof-of-concept results indicate the very promising capability of machine learning method for stable detection of PAM-4 signals modulated by Si-MRM, which is of great significance for practical application of Si-MRM in optical interconnection.

60 Gbps 4-PAM VCSEL-based Raman assisted hyper-scale data centre: In context of spectral efficiency and reach extension

Abstract Cost effective large-scale installation of hyper-scale data centres has attracted extensive research recently, due to the exponential bandwidth demand over extended reach. Low cost, power and spectral efficient techniques for increasing network capacity, as well as extending the transmission reach while maintaining duplex connectivity are still key challenges facing current big data networks such as hyper-scale data centre networks. In this work, a low cost energy efficient technique for maximizing carrier spectral efficiency and extending transmission reach is experimentally demonstrated. This is achieved through combined adoption of vertical cavity surface emitting lasers (VCSELs), four level pulse amplitude modulation (4-PAM), dense wavelength division multiplexing (DWDM) and Raman amplification. Three class 10 G VCSEL channels are directly modulated with a 20 Gbps 4-PAM data signal therefore maximizing carrier spectral efficiency by doubling the data rate per channel. The channels are then converged to a single fibre link through DWDM technique at 0.4 nm spacing, achieving a total aggregated data rate of 60 (3 × 20) Gbps. By exploiting the 8.6 dB flat gain of a forward Raman pump, a 77.12 km of standard single mode fibre (SMF) transmission is experimentally achieved. To the best of our knowledge, this is the longest error free fibre transmission distance reported using VCSEL-based 60 Gbps 4-PAM, without any forward error correction (FEC), equalization and pre-emphasis mechanisms. Results show that a real time uncorrected BER

Carrier Phase Recovery for Set-Partitioning QAM Formats

The set-partitioning quadrature amplitude modulation (SP-QAM) formats are highly preferred for the next generation fiber optical transmission system, due to the Euclidean distance increment of constellation and its power efficiency. However, the existing carrier phase recovery (CPR) scheme of SP-QAM is independently implemented between two polarization signals, and the laser linewidth tolerance of SP-QAM is restricted the same as that of conventional QAM signals. Here, we propose a novel four-dimensional (4-D) CPR scheme for the SP-QAM formats. After the phase offset estimation and precompensation between two polarizations, we implement blind phase search (BPS) algorithm in the 4-D space. Meanwhile, traditional decision circuit is replaced by 4-D detection of SP-QAM and 4-D distance is calculated as the cost function for identifying the desired test phase angle. We carry out numerical simulations under conditions of 28 Gbaud SP-128-QAM, SP-512-QAM, and SP-2048-QAM coherent transmissions. The CPR performance of our proposed 4-D BPS scheme is much better than that of conventional BPS. Given the 1 dB required optical signal-to-noise ratio penalty at ${\rm{BER}} = 1 \times {10^{-3}}$ , the linewidth times symbol duration products of $5.2 \times {10^{-4}}$ , $2 \times {10^{-4}}$ , and $7 \times {10^{-5}}$ are tolerable for SP-128-QAM, SP-512-QAM, and SP-2048-QAM, respectively. We further experimentally verify the performance of the proposed 4-D BPS under the scenarios of 28 Gbaud SP-128-QAM back-to-back and standard single-mode fiber transmission from 750 to 1500 km, indicating of the robustness to the amplified spontaneous emission noise and transmission impairments.

Direct detection of polarization multiplexed single sideband signals with orthogonal offset carriers.

We propose a novel direct detection (DD) scheme for polarization division multiplexed (PDM) single sideband (SSB) signals with two orthogonal carriers located at the opposite sides. Polarization diversity is realized with a pair of optical filters that are used to suppress the unwanted orthogonal carrier component. A PDM-SSB DD receiver is thus constructed without polarization de-rotation. The intra-polarization signal-signal beat interference (SSBI) can be mitigated by Kramers-Kronig detection or iterative SSBI cancellation. For inter-polarization SSBI mitigation, we propose a joint iterative SSBI cancellation method. The proposed PDM-SSB DD scheme is validated with a principle experiment of 40Gbaud PDM-SSB 16-ary quadrature amplitude modulation (16-QAM) signals. After 80km standard single-mode fiber (SSMF) transmission, the bit-error rates (BERs) achieve 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10-2. The performance of iterative SSBI cancellation, Kramers-Kronig detection, and joint iterative SSBI cancellation are evaluated for PDM-SSB signals with different carrier-to-signal ratios (CSPRs) through numerical simulations. Moreover, a multi-input-multi-output (MIMO) equalization scheme is proposed and validated with numerical simulation, which can suppress the linear inter-polarization crosstalk and relax the sharpness requirement of optical filter edges.

Limitations of intra datacentre communications exploiting optical PCIe interconnections

Abstract To support the huge intra-datacentre traffic growth, network architectures based on Peripheral Component Interconnect Express (PCIe) communication protocol have been proposed in alternative to Ethernet ones. To cover the entire datacentre area and to promote the recently released PCIe Gen4, operating at 16 Gb/s, the employment of optical fibre communication is mandatory. We demonstrate, by physical layer measurements, the compliance to the PCIe standard in terms of eye diagram mask and jitter for both Gen3 and Gen4 optical PCIe links, considering up to 10-km standard single mode fibre (SMF) propagation and transmission in the second window of optical communications. On the other hand, we perform an analysis to evidence that the maximum fibre reach is limited by the propagation latency due to the PCIe protocol ACK/NACK operation. For 1-lane PCIe Gen3 link we experimentally verify, through a protocol analysis, the reach limits expected by our evaluation; moreover, for longer distances, we detect retransmissions which reduce the actual transmitted capacity, without error introduction.