DAC-Less and DSP-Free 112 Gb/s PAM-4 Transmitter Using Two Parallel Electroabsorption Modulators

Abstract

Four-level pulse amplitude modulation (PAM-4) is widely regarded as the modulation format of choice for the next generation of 400 gigabit Ethernet short-reach optical transceiver. However, generating and receiving PAM-4 at line rates of 112 Gb/s has proven challenging, without relying on power-hungry tools as digital signal processing and digital-to-analog converters, as it requires linearity from the E/O-components in the link and/or predistortion techniques. Moving the binary to multilevel conversion to the optical domain would greatly relax these requirements. Electro-absorption based transceivers would be ideally suited for this type of data center interconnects as they are capable of combining low-power and high bandwidth operation with a very compact layout, removing the need for large travelling wave structures and dedicated 50Ω terminations. In this paper, we present a novel transmitter topology for generating PAM-4 using two binary driven electro-absorption modulators in parallel. Using this approach, we achieve superior performance with respect to a single, but identical multilevel driven EAM. Finally, we demonstrate the first silicon-based modulator capable of transmitting single-lane 112 Gb/s PAM-4 over 2 km of standard single-mode fiber without any electrical DAC, DSP or long transmission line structures and terminations.

Accurate Power-Efficient Format-Scalable Multi-Parallel Optical Digital-to-Analogue Conversion

“…with b the driving Boolean value, bipolar|unipolar for COH|DD (BPSK|OOK), g the peak gate transfer factor, and a xcss MZM < 1 modeling the excess power loss of the MZM. Prior work in MPoDACs: As for MPoDAC or MPMZM earlier works in 1D constellation generation, it is solely the case S = 2 that was addressed to the best of our knowledge: see the device in Verbist et al works, [17][18][19], which would be described in our terminology as a particular case of a MPMZM oDAC for PAM4 DD generation (presently shown to differ from our own proposed realizations). This prior work 2-way (S = 2) device combines a pair of parallel EAMs incoherently, at 90 • diff.…”

“…We also propose generation of QAM constellations by IQ nesting of COH PAM 1D MPMZMs. In Section 7 we develop several variants of DD and COH MPMZMs, optimizing them in particular for PAM4 constellations, comparing our schemes with prior MPoDAC work by Verbist et al [17][18][19] and Yamazaki and Goh [20][21][22][23][24][25]. We also explain the crucial role played by chirp-free operation in order to mitigate a major source of ISI, as enabled by our MPMZM designs.…”

Silicon photonic modulators for PAM transmissions

“…The other is composed of two AMs arranged in parallel. An AM in these topologies can be an MZM, an MRM [35] and an electro-absorption modulator (EAM) [65].…”

Silicon Photonic Mach–Zehnder Modulator Architectures for on Chip PAM-4 Signal Generation

“…In this manuscript we focus on different silicon photonic (SiP) travelling wave Mach Zehnder modulator (TWMZM) architectures which enable PAM-4 signal generation in the optical domain versus generating the PAM-4 signal in electrical domain. Over the past few years, there has been significant work on PAM-4 generation using vertical-cavity surface-emitting lasers (VCSEL)s [7][8], silicon photonic ring modulators [9][10][11][12][13], silicon-germanium electrooptic modulators [14][15], III-V-onsilicon modulators [16] silicon hybrid modulators [17], TWMZMs [18][19][20][21][22][23][24], electro-optic polymer modulators [25] and LiNbO3 modulators [26]. In the majority of the presented works, PAM-4 is generated in the electrical domain either by passive power combination of two OOK signals or using digital to analog convertors (DACs) and digital signal processing (DSP).…”