The design of application-specific integrated circuits (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. First, we describe the key-building blocks of modern high-speed DSP-based transponders working at up to 800G per wave. Second, we show the benefits of these transmission methods in terms of system level performance. Finally, we report, to the best of our knowledge, the first long-haul experimental transmission – e.g., over 1000 km – with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2 × 800G).

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800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing

Neuromorphic photonics aims to transfer the high-bandwidth and low-energy credentials of optics into neuromorphic computing architectures. In this effort, photonic neurons are trying to combine the optical interconnect segments with optics that can realize all critical constituent neuromorphic functions, including the linear neuron stage and the activation function. However, aligning this new platform with well-established neural network training models in order to allow for the synergy of the photonic hardware with the best-in-class training algorithms, the following requirements should apply: i) the linear photonic neuron has to be able to handle both positive and negative weight values, ii) the activation function has to closely follow the widely used mathematical activation functions that have already shown an enormous performance in demonstrated neural networks so far. Herein, we demonstrate a coherent linear neuron architecture that relies on a dual-IQ modulation cell as its basic neuron element, introducing distinct optical elements for weight amplitude and weight sign representation and exploiting binary optical carrier phase-encoding for positive/negative number representation. We present experimental results of a typical IQ modulator performing as an elementary two-input linear neuron cell and successfully implementing all-optical linear algebraic operations with 104-ps long optical pulses. We also provide the theoretical proof and formulation of how to extend a dual-IQ modulation cell into a complete $N$ -input coherent linear neuron stage that requires only a single-wavelength optical input and avoids the resource-consuming Wavelength Division Multiplexing (WDM) weighting schemes. An 8-input coherent linear neuron is then combined with an experimentally validated optical sigmoid activation function into a physical layer simulation environment, with respective training and physical layer simulation results for the MNIST dataset revealing an average accuracy of 97.24% and 94.37%, respectively.

Neuromorphic Photonics With Coherent Linear Neurons Using Dual-IQ Modulation Cells

The use of spectrally shaped amplified spontaneous emission noise (SS-ASE) as a method for emulating interfering channels in optical fibre transmission systems has been studied. It is shown that the use of SS-ASE leads to a slightly pessimistic performance relative to the use of conventionally modulated interfering channels in the nonlinear regime. The additional nonlinear interference noise (on the channel under test), due to the Gaussian nature of SS-ASE, has been calculated using a combination of the Gaussian noise (GN) and enhanced GN (EGN) models for the entire C-band (4.5 THz) and experimentally shown to provide a lower bound for transmission performance.

Investigation of bandwidth loading in optical fibre transmission using amplified spontaneous emission noise.

This tutorial paper reviews advanced modulation techniques that have been proposed in the literature for the implementation of flexible (or reconfigurable) transceivers, which are fundamental building blocks of next-generation software-defined optical networks. Using a common reference multi-span propagation system scenario, the performance of transceivers employing standard quadrature amplitude modulation with variable-rate forward error correction, probabilistic constellation-shaping, and time-domain hybrid formats is assessed, highlighting the achievable flexibility in terms of continuous tradeoff between transmission rate and distance. The combination of these techniques with sub-carrier multiplexing, which enables an increase of the fiber nonlinearity tolerance thanks to the optimization of the symbol rate per sub-carrier, is also discussed.

Advanced Modulation Techniques for Flexible Optical Transceivers: The Rate/Reach Tradeoff

We briefly introduce a concept for linear an d nonlinear digital pre-distortion (DPD) of high-speed optical transmitters. As an extension to previous results, we investigate the imp act of th e DPD schemes on a wavelength-division multiplexing (WDM) trans mission of 32 GBd polariz ation-division multiplexed (P DM) M –ary quadrature amplitude modulated (M-QAM) signals. The investigated formats range from 32- to 128-QAM.

Nonlinear digital pre-distortion of transmitter components

We present a linear and nonlinear digital pre-distortion (DPD) tailored to the components of an optical transmitter. The DPD concept uses nonlinear models of the transmitter devices, which are obtained from direct component measurements. While the digital-to-analog converter and driver amplifier are modeled jointly by a Volterra series, the modulator is modeled independently as a Wiener system. This allows for a block-wise compensation of the modulator by a Hammerstein system and a pre-distortion of the electrical components by a second Volterra series. In simulations and extensive experiments, the performance of our approach for nonlinear DPD is compared to an equivalent linear solution as well as to a configuration without any DPD. The experiments were performed using M -ary quadrature-amplitude modulation ( M -QAM) formats ranging from 16- to 128-QAM at a symbol rate of 32 GBd. It is shown that the DPD improves the required optical signal-to-noise ratio at a bit error ratio of 2·10 −2 by at least 1.2 dB. Nonlinear DPD outperforms linear DPD by an additional 0.9 and 2.7 dB for higher-order modulation formats such as 64-QAM and 128-QAM, respectively.

Talha Raman

Papers

Nonlinear Digital Pre-distortion of Transmitter Components