Sofia B. Amado

Fully Blind Linear and Nonlinear Equalization for 100G PM-64QAM Optical Systems

We demonstrate fully blind processing and reduced-complexity nonlinear equalization (NLE) of a 100G PM-64QAM optical channel in a 50 GHz WDM grid, achieving a maximum reach of 1524 km over pure silica core fiber at a bit error rate of 2.7 × 10-2. The equalization of linear polarization-dependent effects is performed by a radius-directed constant modulus algorithm (RD-CMA), enabled by a multiradii training stage, yielding a very small penalty (<;0.1 dB in terms of Q2 factor) relatively to data-aided CMA. Applying a simplified Volterra series nonlinear equalizer (simVSNE), operating in the frequency domain, we demonstrate a reach extension of ~27% relatively to linear equalization. Due to its lower spatial resolution requirements, the simVSNE technique is shown to provide a more efficient NLE than the well-known back-propagation split-step Fourier method, both in terms of latency and number of complex multiplications per sample. The potential benefit of NLE for fully blind processing of high-order QAM optical signals is demonstrated by an incremental reduction of the RD-CMA penalty to <;0.04 dB.

Time-Domain Volterra-Based Digital Backpropagation for Coherent Optical Systems

We propose a novel closed-form time-domain (TD) Volterra series nonlinear equalizer (VSNE) for the mitigation of Kerr-related distortions in polarization-multiplexed (PM) coherent optical transmission systems. The proposed TD-VSNE is obtained from the inverse Fourier analysis of a frequency-domain VSNE based on a frequency-flat approximation. Employing novel TD approximations, we demonstrate the equivalency between the VSNE algorithms formulated in time and frequency domains. In order to enhance the computational efficiency, we insert a power weighting time window in the TD-VSNE, yielding the weighted VSNE (W-VSNE) algorithm. We demonstrate that the convergence of the W-VSNE to its maximum performance is much faster than that of the TD-VSNE, thus requiring fewer parallel filters. Through numerical simulation of a 224-Gb/s PM-16QAM optical channel, we compare the performance/complexity tradeoff of the W-VSNE with the well-known split-step Fourier method (SSFM) and with the computationally optimized weighted SSFM (W-SSFM). Enabled by the use of fewer iterations and only two parallel W-VSNE filters, we demonstrate a reduction of up to ~45% on computational effort and ~70% on latency, in comparison with the W-SSFM.

Low Complexity Advanced DBP Algorithms for Ultra-Long-Haul 400 G Transmission Systems

We experimentally assess the performance versus complexity tradeoff of advanced split-step and Volterra-based digital backpropagation (DBP) techniques, applied to aWDM (75-GHz flexigrid) ultra-long-haul (ULH) transmission system composed of five dual-carrier PM-16 quadratic-amplitude modulation 400 G superchannels. Using the recently proposed weighted Volterra series nonlinear equalizer (W-VSNE), we demonstrate a maximum reach improvement of 600 km, obtained at the expense of only six DBP steps for an entire >5000-km optical link (1000 km/step), representing a ~90% reduction on the total number of DBP steps relatively to the widely used split-step Fourier method (SSFM) implementation. The W-VSNE technique is also shown to be more accurate than the correspondent weighted SSFM (W-SSFM) algorithm when applied in medium (10-50 DBP steps) and low complexity (<;5 DBP steps) regimes. The achieved reduction of computational effort (up to 88%), together with the associated latency savings (up to 90%), demonstrates the feasibility of low complexity and efficient DBP for high-speed ULH transmission systems.

Real-Time Bidirectional Coherent Nyquist UDWDM-PON Coexisting With Multiple Deployed Systems in Field-Trial

We experimentally characterize a bidirectional 2.5 Gb/s UDWDM-PON system based on Nyquist shaped DQPSK with digital signal processing in real time. The optical distribution network power budget of this system is evaluated and in coexistence of UDWDM channels with upstream TWDM-PON and RF video overlay signals, a set of optimum parameters and their impact on the network operation is driven. Additionally, we report the first field-trial of bidirectional coherent Nyquist UDWDM-PON using commercial real-time FPGA-based transceivers, coexisting with the deployed GPON, RF video overlay, and NG-PON2 technologies. A -44.5 dBm receiver sensitivity is achieved for 64 × 2.5 Gb/s DQPSK downstream channels and a 17 dB tolerance in dynamic power range of upstream channels is performed.

Multicarrier Digital Backpropagation for 400G Optical Superchannels

Optical superchannels and digital nonlinear mitigation are key technology options to be considered for the deployment of next-generation optical 400G transmission systems. In this paper, we experimentally assess the performance and complexity of multicarrier digital backpropagation (DBP) for dual- and triple-carrier 400G superchannels based on polarization-multiplexed (PM) 16QAM and 64QAM modulation. As an alternative to the widely used nonlinear compensation based on total-field DBP, we demonstrate that a coupled-equations DBP (CE-DBP) approach can be more computationally efficient and also more robust to a nonideal equalization of the receiver front-end in scenarios with limited sampling rate and electrical bandwidth. Employing a triple-carrier PM-16QAM superchannel placed in a 75-GHz slot of a WDM system, we demonstrate an ultralong-haul signal reach of 6600 km using CE-DBP, corresponding to 32% increase relatively to chromatic dispersion equalization (CDE). Targeting an highly spectral-efficient solution for 400G transmission in metro optical networks, these results are extended to a triple-carrier PM-64QAM superchannel placed in a 50-GHz slot, yielding a maximum signal reach of 1750 km through the use of CE-DBP in single-superchannel propagation, corresponding to approximately 60% increase over CDE.

Coherent Nyquist UDWDM-PON With Digital Signal Processing in Real Time

Nyquist shaping UDWDM-PON combined with digital coherent optical systems has been recently proposed as a promising technology for future optical access networks. In this paper, we report real-time transmitter and receiver for a future coherent PON with high spectral efficiency and low-cost DSP implementation. Using field-programmable gate array (FPGA)-based 8-bit DSP, we experimentally demonstrate a high performance system based on 16 × 2.5 Gb/s QPSK signals in terms of receiver sensitivity and fiber transmission. An optimal transmitted power of -7 dBm per channel is achieved for 50-km standard single-mode fiber in a 2.5-GHz grid. The experimental results show the feasibility of coherent PON scenarios based on DSP supported by FPGAs.

Experimental demonstration of the parallel split-step method in ultra-long-haul 400G transmission

We experimentally demonstrate a parallel split-step Fourier method in an ultra-long-haul dual-carrier 400G transmission system. For a reach enhancement of 500-800 km over linear equalization, we demonstrate a 5× reduction of the step-size when compared to the standard split-step Fourier method.

Impact of TWDM on Optional Real-Time QPSK WDM Channels

We experimentally demonstrate in real-time mode operation the coexistence of a digital coherent PON architecture based on UDWDM QPSK with a NGPON2 system based on TWDM OOK. Using simple DSP-based ONU, guard bands of 100 GHz are achieved.

Transmission of PM-64QAM over 1524 km of PSCF using fully-blind equalization and Volterra-based nonlinear mitigation

Using fully-blind equalization and Volterra-based nonlinear mitigation we demonstrate the transmission of 10×124.8 Gb/s PM-64QAM over 1524 km of PSCF. The Q2 penalty due to blind pol-demux is kept below 0.1 dB and the reach extension due to nonlinear compensation is ~27%.

Distributive FIR-Based Chromatic Dispersion Equalization for Coherent Receivers

We propose a novel and efficient multiplierless finite-impulse response (FIR)-based filter architecture for chromatic dispersion equalization (CDE) in coherent optical communication systems. After quantizing the FIR coefficients, we take advantage of the high multiplicity of their real and imaginary parts, employing the distributive property of multiplication over addition to sharply reduce the number of multiplication operations, obtaining the distributive FIR-CDE (D-FIR-CDE). Furthermore, the implementation of multiplication operations with shifts and additions allows us to obtain a multiplierless D-FIR-CDE (MD-FIR-CDE). The proposed equalizers are experimentally validated in a 100G polarization-multiplexed (PM)-QPSK long-haul optical link and compared against benchmark FIR-CDE and frequency-domain (FD)-CDE implementations. We demonstrate computational resources savings of over 99% in number of multiplication operations and 40% in number of additions, relatively to the FIR-CDE implementation. In addition, the D-FIR-CDE is also shown to compare favorably relatively to the most widely used FD-CDE, achieving significant gains both in terms of required chip area and latency: more than 99% and 30% fewer multipliers and additions, respectively, and a latency reduction of over 90%. We have also experimentally demonstrated that the performance penalty imposed by the coefficient quantization tends to decrease with increasing propagation length, rendering it as an attractive solution for efficient and high-performance chromatic dispersion compensation in long-haul optical fiber links.