# Partial Pre-Emphasis for Pluggable 400 G Short-Reach Coherent Systems
## 1 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 400 ZR by optical internetworking forum (OIF) [1]. Due to costly rack space in data centers, it is imperative to implement integrated coherent transceivers to fit small-sized pluggables such as Quad Small Form Pluggable-Double Density (QSFF-DD) to provide high switch-port density. This can be achieved using either silicon or InP-based technologies. The silicon photonics (SiPh) platform has an edge over its InP counterpart in terms of cost-effectiveness, since it leverages the complementary metal-oxide semiconductor (CMOS)-compatible process in silicon foundries. Silicon photonic traveling wave modulators (TWMZMs) have been actively studied and show good potential for coherent applications [2; 3; 4; 5; 6; 7; 8; 9]. To meet the low power requirements in 400 G SiPh coherent systems, short TWMZMs with low V\({}_{\pi}\) are attractive, which allow for low-power radio frequency (RF) driver amplifiers (DA). However, 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]. Furthermore, this is aggravated by the cascaded low-pass frequency response of the arbitrary waveform generator (AWG), which often possesses a comparably low bandwidth in the entire channel. It is known that linear equalizers at the receiver can compensate for the frequency response of the entire channel, including the severely band limited Tx, but also incur undesired noise enhancement. 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]. However, 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. Moreover, DB FFE has either modulo loss due to modulo operation or increased DSP complexity due to the use of MLSE. Trellis coded modulation (TCM), though promising, increases transmitter and receiver complexity significantly [13]. So, a finite-impulse response (FIR) pre-emphasis filter is usually used at the Tx side to pre-compensate the Tx frequency response. Several works have been conducted on transmitter pre-emphasis targeting high-speed coherent systems [14; 15; 16; 17]. Authors in [14; 15] proposed a pre-emphasis filter for the AWG frequency response, and in [16] the authors presented an adaptive digital pre-distortion (DPD) algorithm to mitigate the undesirable linear and non-linear distortions of the AWG, digital to analog converter (DAC), and dual-polarization Mach-Zehnder modulator (DP-MZM). However, 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, thus degrading the transmitter signal-to-noise-ratio (SNR). We demonstrate in this work that by using a partial pre-emphasis filter, a lower signal PAPR is achieved and a higher signal power is obtained from the AWG. The work is part of the quest to understand and, ultimately, develop low-power transceivers that do not depend on the modulator driver, i.e., radio frequency amplifications. In this paper, we first numerically study the change of the signal PAPR when the pre-compensation of the transmitter is reduced to varying degrees for a 50 Gbaud 16 QAM signal. It is found that the effective number of bits (ENoB) requirement can be reduced by around 0.5 by using a partial pre-emphasis filter for the same transmitter signal SNR. Next, using discrete optical equipment, we experimentally characterize the overall performance penalty of partial pre-emphasis for a 50 Gbaud 16 QAM short-reach coherent system with extremely bandlimited Tx.