A real-time implementation of a coherent optical pluggable module using digital sub-carrier (DSC) multiplexing has recently been demonstrated. Whilst these modules can support traditional point-to-point (P2P) applications, similar to e.g., 400ZR(+) – with at least the same performance, it is the point-to-multi-point (P2MP) functionality that differentiates their capabilities from a single-carrier implementation. The superior performance of coherent transmission and its insensitivity to link effects, in combination with P2MP capability, allows for new network architectures and can dramatically simplify how metro and access networks are designed. The enabling engine is the DSP ASIC, which generates and demodulates the sub-carriers in the digital domain and furthermore, handles P2MP operation-related aspects. Within the DSP, most of the functional blocks are equivalent to its single-carrier counterpart – even the DSP functionality and algorithms are very similar. Just a few additional mechanisms inside the DSP are needed to process DSCs. This paper details the related DSP design and discusses the requirements on the DSP algorithms that accommodate the low per sub-carrier symbol rate and the operation on intermediate digital carriers.

We propose an experimental method to tune SOA model parameters that yields good prediction abilities of nonlinear distortions induced on PCS-QAM signals. We show that reducing SOA nonlinearities is achieved by a trade-off between a high $P_{\text{sat}}$ and a low $\alpha_{H}$ . © 2021 The Author(s)

We demonstrate a 63.2-Tb/s throughput in a 5-span 440-km SSMF link employing real-time 400G 16QAM transponders and fully discrete C- and L-band amplifiers with a total amplification bandwidth approaching 100nm.

We experimentally compare PCS-16QAM and QPSK for 400G transmission at 128 Gbaud. A realistic, full system implementation that accounts for penalties from the FEC, distribution matcher, transceiver impairments, fiber nonlinearity, and DSP, reveals that the theoretical 0.8 dB gain of PCS- 16QAM is reduced to only 0.1 dB.

We demonstrate a 107.6-Tb/s GMI throughput in a 3-span 220-km SSMF link employing high-baudrate 95-GBaud CS64QAM signals and fully discrete C- and L-band amplifiers. A total amplification bandwidth in excess of 12 THz is achieved.

In the recent context of Software Defined Optical Network, the fast and accurate Quality of Transmission (QoT) estimation of the transmission link is essential. Gaussian Noise models are shown to yield a fast estimation of the average QoT derived from deterministic system parameters, but do not capture the QoT variability. In order to assess numerically the stochastic joint effect of Polarization Mode Dispersion (PMD) and Kerr nonlinearities, system designers generally use the Split Step Fourier Method (SSFM) based on Manakov-PMD equation neglecting the nonlinear-PMD term which is faster than using Coupled NonLinear Schrodinger Equation (CNLSE) and enough accurate for fiber with short birefringence correlation length (around less than 10m). In this work, we present insights of the way to tune the parameters of this Manakov-PMD method and its limitation when seeking an accurate estimation of the Non-Linear Interference (NLI) noise statistical distribution for all fibers potentially installed in the current optical network. In particular we compare this Manakov-PMD method results with respect to the one obtained by CNLSE while varying the fiber birefringence correlation length, PMD coefficient and the fiber type. Our results highlight a potential discrepancy of 0.5 dB in the estimation of the Q^2 factor in one span Polarization Division Multiplexed Quadrature Phase Shift Keying (PDM-QPSK) transmission with optimal launch power per channel and yield guidelines to choose the most suitable numerical estimation method.

We experimentally assess a series of four multi-dimensional modulation formats for 100 Gbaud single -span unrepeatered multi-rate applications. We show the variation of maximum acceptable span loss for entropies between 1 and 2 b/S/pol, with a granularity of 0.25 b/S/pol.

We propose a very-low complexity and high-performance algorithm for soft-demapping of multi-dimensional modulation formats. We assess its performance over the linear channel for four 8D formats, generated using binary arithmetics. This solution outperforms current algorithms in terms of complexity without loss in performances.

An ultra-low complexity soft-demapper is proposed as an implementable solution for two nonlinearity-tolerant 8D formats without losses on the post-FEC BER performances. This makes realistic the application of 8D formats for ultra-long-haul transmission systems.

We present two new nonlinearity tolerant modulation formats at spectral efficiencies lower than 4bits/4D-symbol, obtained using a simplified bit-to-symbol mapping approach to set-partition PDM-QPSK in 8 dimensions.

We demonstrate ultra-wideband operation of a low linewidth tunable laser and a polarization independent reach extender, based on the same broadband material.

For the purpose of revisiting the semiconductor optical amplifier (SOA) ability for large-band amplification in the context of wavelength division multiplexed (WDM) coherent systems, we numerically investigate nonlinear impairments brought by SOA for polarization multiplexed quadrature phase-shift keying and polarization multiplexed 16-quadrature amplitude modulation terrestrial transmissions. In order to provide a deep understanding of these impairments, we perform a step-by-step approach by assessing the transmission performance of a sole SOA cascade and an SOA-amplified transmission system using linear and then nonlinear fiber. For the same purposes, we also study these impairments for a single-channel transmission even though the final goal is to design a WDM SOA-amplified system with a larger bandwidth than the EDFA-based systems. We show some interesting results on the span-after-span accumulation of SOA nonlinear impairments, as it has been done similarly for the fiber nonlinearities, in the context of dispersion managed or unmanaged coherent systems. Design guidelines of both SOAs for amplification purposes and SOA-amplified transmission systems are derived from this investigation. Finally, we point out promising results for SOA-based transmissions, for particular system configurations that follow these guidelines.

In this paper, we present a novel procedure for the synthesis of a filter having an arbitrarily specified amplitude transmittance. The filter configuration consists of N birefringent stages placed between a polarizer and an analyzer, with each stage containing an identical section and a variable section. An additional variable section is placed in front of the analyzer. The synthesis procedure is based on the resolution of a generalized nonlinear equation system directly deducted from the Jones matrix formalism to determine the angles of each stage, the angle of the analyzer and the phase shifts of the variable sections. A typical example of a 6-stage birefringent filter having an arbitrarily non-symmetric amplitude transmittance is shown and the opto-geometrical parameters are given to demonstrate the efficiency of the proposed synthesis procedure. The results obtained show an excellent agreement with those developed in the literature.

We demonstrate that the birefringence of the liquid-crystal cell (LCC) can be varied by applying different frequency values of a single applied ac square voltage. For the experimental evaluation of the birefringence, associated with a certain wavelength λ, as a function of the frequency FLCC of the electrical signal applied to the LCC, we use, for the first time to our knowledge, what we call here a frequency-dependent transmission technique. It consists in measuring the transmission responses between crossed and parallel polarizers as a function of the frequency FLCC. Experimental tests were carried out using a 7 μm-thick E63 nematic LCC and a laser source emitting at λ = 1.55 μm with a launch power of −3 dBm. The tuning voltage VLCC applied to the LCC is an alternative square wave electrical signal whose frequency ranges from 0.5 to 15 kHz. The peak to peak amplitude of the electrical signal is 5 V. The curve of the measured variations of the optical path difference of the LCC versus the frequency FLCC has a positive slope. Application to the tuning of the center wavelength of the transmission response of a one stage hybrid birefringent filter is shown as a proof-of-principle test.

In mode-division multiplexed (MDM) transmission systems, mode coupling is responsible for inter-modal crosstalk. We consider the transmission of modulated signals over a few-mode fiber (FMF) having low mode coupling and large differential mode group delay in the presence of a non-ideal fiber connection responsible for extra mode coupling. In this context, we first analytically derive the coupling matrix of the multimode connector and we numerically study the dependence of the matrix coefficients as a function of the butt-joint connection characteristics. The numerical results are then validated through an experiment with a five-mode setup. Finally, through numerical simulations, we assess the impact of the connector on the signal quality investigating different receiver digital signal processing (DSP) schemes.

We propose a novel, low complexity, carrier phase estimation scheme for the coherent reception of PS-QPSK-modulated signals and assess its back-to-back and transmission performance.

We present a numerical investigation for the use of semiconductor optical amplifiers (SOA) as broadband amplifiers in a context of quasi-Nyquist WDM long-haul transmission using PDM-QPSK, 8-QAM and 16-QAM modulation formats. The SOAs nonlinear behavior is assessed for each format considering different numbers of channels and cumulative dispersion.

We numerically investigate the multi-channel transmission performance of Polarization Switched Quadrature Phase Shift Keying (PS-QPSK) and we compare it to the performance of Polarization-Division-Multiplexed QPSK (PDM-QPSK), using Root Raised Cosine (RRC) spectral shaping, in the context of a flexible channel grid. We point out the impact of the roll-off factor and the potential influence of different dispersion compensation scenarios. Finally, the advantage of PS-QPSK against PDM-QPSK is presented as a function of the system parameters, while we also discuss the benefit of a RRC spectral shaping against a tight filtering at the transmitter side with a 2nd order super-Gaussian-shaped filter.

We numerically investigate the nonlinear transmission performance variability as a function of the time delay between channels and their initial states of polarization, also pointing out the values yielding the best or worst transmission performances.