Partial Pre-Emphasis for Pluggable 400 G Short-Reach Coherent Systems

Ahmad Abdo,Mahdi Parvizi,Naim Ben-Hamida,Claude D’Amours,Md Samiul Alam,David Plant,Xueyang Li

doi:10.3390/fi11120256

Abstract

Pre-emphasis filters are used to pre-compensate for the transmitter frequency response of coherent systems to mitigate receiver noise enhancement. This is particularly essential for low-cost, low-power coherent transceivers due to having an extremely bandlimited transmitter. However, the pre-emphasis filter also increases the signal peak-to-average power ratio (PAPR), thus posing a higher effective number of bits (ENoB) requirement for the arbitrary waveform generator (AWG). In this paper, we first numerically study the PAPR impact of partial pre-emphasis filters. We show that with partial pre-emphasis, an ENoB reduction from 5 to 4.5 bits is attainable at the same signal-to-noise ratio (SNR) out of the AWG. Next, we experimentally investigate the overall performance penalty of partial pre-emphasis in a 50 Gbaud 16QAM coherent system. A manageable Q factor penalty of around 0.5 dB is found for both single-polarization and dual-polarization systems with a 0.8 dB PAPR reduction.

Highlights

The high-capacity, high-spectral efficiency and low operating expenses (OPEX) of coherent systems have made them attractive candidates for 400 G data center interconnects (DCI) as standardized in 400ZR by optical internetworking forum (OIF) [1]
We demonstrate in this work that by using a partial pre-emphasis filter, a lower signal peak-to-average power ratio (PAPR) is achieved and a higher signal power is obtained from the arbitrary waveform generator (AWG)
We summarize that partial pre-emphasis provides the following advantages for coherent system design at the expense of manageable performance penalty: (i) this leads to less signal distortion from the nonlinearity of the radio frequency (RF) driver amplifiers (DA) and modulator due to reduced PAPR

Summary

Introduction

The high-capacity, high-spectral efficiency and low operating expenses (OPEX) of coherent systems have made them attractive candidates for 400 G data center interconnects (DCI) as standardized in 400ZR by optical internetworking forum (OIF) [1]. To meet the low power requirements in 400 G SiPh coherent systems, short TWMZMs with low Vπ are attractive, which allow for low-power radio frequency (RF) driver amplifiers (DA) This is obtained by trading off the modulator E-O bandwidth, which leads to the transmitter (Tx) low-pass filtering that is significantly stronger than the receiver (Rx) [8]. One way to deal with the low bandwidth of the system is to use partial response signaling like duobinary (DB) modulation format or Tomlinson–Harashima precoding (THP) [10,11,12,13] Both these solutions require an increased number of signal levels such that higher signal-to-noise ratio (SNR) is required for the same forward error correction (FEC) threshold. Pre-compensating for the entire Tx low-pass filtering leads to increased signal peak-to-average ratio (PAPR) and reduced average signal power out of the AWG, degrading the transmitter signal-to-noise-ratio (SNR).

Silicon Photonics Modulators

Design Trade-Offs

Conclusions