Designing for manufacture: PAM-4 transmitters using segmented-electrode Mach-Zehnder modulators

As PAM-4 signalling becomes the norm in datacentre optics, there is increasing focus on finding optimal design and implementation of the PAM-4 transmitter PICs. We use Synopsys’ OptSim software to simulate a design using segmented-electrode Mach-Zehnder modulators and investigate how manufacturing variations affect its performance.

By Jigesh Patel, product manager, and Pablo Mena, R&D engineer, Synopsys

In datacentre optics, 4-level Pulse Amplitude Modulation (PAM-4) signalling is gradually overtaking non-return-to-zero (NRZ) signalling [1-3]. Although both schemes use intensity modulation and direct detection, PAM-4 encodes two bits into four intensity levels, reducing bandwidth requirements for a given data rate by half. In other words, with PAM-4, transmission of a 40G signal requires components with 20 GHz bandwidth (corresponding to a symbol rate of 20 GBaud).

PAM-4 strikes a good balance between data carrying capacity and cost, since it requires less complicated digital signal processing than coherent transmission methods. Since lowering energy consumption is an economic and social imperative, designers need to be mindful of energy efficiencies when exploring technologies. This article explores the design of a PAM-4 transmitter chip that provides higher bandwidth compared to NRZ, avoids using energy-inefficient electronics, and achieves PAM-4 signalling in the optical domain by using segmented-electrode phase-shifters.

Figure 1. Schematic of a PAM-4 transmitter using SE-MZM

Commonly used PAM-4 transmitter PICs often suffer from poor energy efficiency and larger footprint. Traditionally, these transmitters incorporate a Mach-Zehnder modulator (MZM) being driven by an electrical digital-to-analogue converter (DAC) with an electrical driver. But the electronics required for this is highly energy inefficient – an ever-increasing concern in datacoms. Although there are alternative designs with nested modulators and drivers, a drawback is that they typically have larger footprints.

To overcome these challenges, we can accomplish a DAC-less design using the inherent DAC capabilities of segmented phase shifters [4]. Although conventional travelling-wave MZMs (TW-MZMs) can reduce drive voltage (VπL) due to a longer interaction length (L) thereby improving energy efficiency, the longer electrodes result in higher radio frequency (RF) losses and a mismatch between the group velocities of RF and optical signals. This mismatch, in turn, impacts modulation bandwidth [5].

Figure 2. SE-MZM PAM-4 transmitter output signal (left) and spectrum (right).

The segmented approach offers the advantage of longer interaction lengths without increasing loss, by shifting the velocity matching to electronic timing circuits, which control the timing of applied electrical signals to match the optical delay between segments [6]. Figure 1 shows a schematic of a PAM-4 transmitter using an SE-MZM made from discrete PIC elements. The design is built in Synopsys OptSim – an advanced photonic circuit simulation tool, which is part of the Synopsys electronic-photonic design automation (EPDA) toolset.

The topology comprises bidirectional PIC elements such as a 1-input 2-output optical splitter and a 2-input 1-output optical combiner with user-defined power ratio, and two pairs of travelling-wave optical phase shifters. The latter are used to implement the segmented MZM. Their lengths are binary weighted, meaning that each binary word can be applied directly. This minimises the number of segments needed, making integration less complicated.

The first segment’s length is one third of the total MZM length, while the second segment is two thirds of the total length. The 20G bit sequence has already been split into separate bit patterns corresponding to the most and least significant bits (MSB and LSB, respectively). The top driver modulates the first MZM segment using the LSB pattern, and the bottom driver modulates the second MZM segment using the MSB pattern.

Each travelling-wave phase shifter has an optical waveguide and a surrounding electrical transmission line that can change the waveguide’s refractive index and propagation loss. The interaction between the electrical and optical signals is distributed along the propagation direction. The waveguide’s thermal behaviour – and hence also the thermal behaviour of the modulator – can be modelled with the derivative of effective refractive index, the parameter VπL, and propagation loss as functions of temperature [7].