Compensation of Limited Bandwidth and Nonlinearity for Coherent Transponder

Abstract

Coherent optical transponders are widely deployed in today’s long haul and metro optical networks using dense wavelength division multiplexing. To increase the data carrying capacity, the coherent transponder utilizes the high order modulation format and operates at a high baud rate. The limited bandwidth and the nonlinearity are two critical impairments for the coherent in-phase quadrature transmitter. These impairments can be mitigated by digital filters. However, to accurately determine the coefficients of these filters is difficult because the impairment from the limited bandwidth and the impairment from nonlinearity are coupled together. In this paper, we present a novel method to solve this problem. During the initial power-up, we apply a sinusoidal stimulus to the coherent IQ transmitter. We then scan the frequency and amplitude of the stimulus and monitor the output power. By curve-fitting with an accurate mathematical model, we determine the limited bandwidth, the nonlinearity, the power imbalance, and the bias point of the transponder simultaneously. Optimized coefficients of the digital filters are determined accordingly. Furthermore, we utilize a coherent IQ transponder and demonstrate that the limited bandwidth is improved by the finite impulse response filter, while nonlinearity is mitigated by the memoryless Volterra filter.

Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal

“…A 92 GS/s digital-to-analog converter (DAC) with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter [27]. The root-raise cosine filter with a roll-off factor of 0.2 was applied in the following experiment.…”

Special Issue on Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

“…• Compensation of Limited Bandwidth and Nonlinearity for Coherent Transponder [6] The authors present a novel solution for optimizing the coefficients of digital filters to mitigate impairments due to limited bandwidth and nonlinearity in coherent transponders. They show that limited bandwidth is improved by the finite impulse response filter, while nonlinearity is mitigated by the memoryless Volterra filter.…”

Coupled Transceiver-Fiber Nonlinearity Compensation Based on Machine Learning for Probabilistic Shaping System

“…The transceiver nonlinearity may be compensated using static digital filters (see [8], [9] for uniform QAM constellations), although it is difficult to estimate filter parameters due to mixing of nonlinear effects from different transceiver components such as DACs/ADCs and optical modulators. Under the impact of fiber nonlinearity, other nonlinear equalizers based on inverse Volterra series transfer functions may also be deployed to partially invert the nonlinear distortion induced by the transmission link.…”