Explosive growth in hyperscale computing and Internet traffic has forced designers to rethink intra-datacenter optics. Coherent fiber optics inside data centers are still expensive and the binary non-return to zero (NRZ) intensity modulation has reached its limit in terms of bandwidth.
As a result, 4-level pulse amplitude modulation (PAM-4) is being widely considered for short-reach data center interconnects (DCIs). 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 blog post explores the design of a PAM-4 transmitter chip that provides higher bandwidth compared to NRZ, avoids use of energy-inefficient electronics, and achieves PAM-4 signaling in the optical domain by using segmented-electrode phase-shifters.
A photonic PAM-4 transmitter often includes a Mach-Zehnder modulator (MZM) driven by a digital-to-analog converter (DAC) that requires energy-inefficient electronics. Implementations with nested modulators and drivers also exist, but they typically have larger footprints. An alternative approach is to create a DAC-less design by using inherent DAC capabilities of segmented phase-shifters.
The longer interaction length in conventional travelling-wave MZMs (TW-MZMs) helps reduce drive voltage due to technology-dependent VπL. However, longer electrodes mean higher RF losses and mismatch in group velocities between RF and optical signals, which in turn impact modulation bandwidth.
The segmented approach offers longer interaction lengths without increased loss by shifting the velocity matching to electronic timing circuits to control timing of applied electrical signals to match the optical delay between segments.
The schematic in Figure 1, modeled in Synopsys OptSim software, shows a PAM-4 transmitter using a segmented-electrode Mach-Zehnder modulator (SE-MZM) made from discrete photonic IC (PIC) elements.
Figure 1: Schematic of a PAM-4 transmitter using SE-MZM
The topology includes bidirectional PIC elements including an optical splitter 1×2 and combiner 2×1 with user-defined power ratio and two pairs of traveling wave optical phase shifters. The phase shifters are used to implement the segmented MZM. The lengths of the phase-shifter are binary-weighted so that each binary word can be applied directly. The number of segments is minimized, which helps with integration.
The first segment is one-third of the total MZM length, and the second segment takes up the remaining two-thirds. The 20 Gbps bit sequence has already been split into separate bit patterns corresponding to the most- and least-significant bits (MSB and LSB, respectively), with the top driver modulating the first MZM segment using the LSB pattern, and the bottom driver modulating the second MZM segment using the MSB pattern.
Each traveling wave phase shifter has an optical waveguide and a surrounding electrical transmission line that introduces change in the waveguide’s refractive index and propagation loss. The interaction between the electrical and optical signals is distributed along the propagation direction. Thermal behavior of the waveguide (and hence modulator) is modeled with the derivative of effective index, parameter VπL, and propagation loss as functions of temperature.
Each arm of the MZM also has a phase tuner near the combiner which sets the modulator at quadrature. A simple 90-degree phase-shifter model from OptSim in one of the arms could have accomplished the same result; however, in a packaged product, external controls are preferred to account for additional tuning that may be required for factors such as ambient noise and manufacturing tolerances.
The CW light is modulated by an electrical signal derived from PRBS data followed by an electrical driver. The inverter model provides push-pull electrical bias to one of the electrodes compared to the other. At the output of the SE-MZM PIC, a scope is connected to observe transmitter output.
Figure 2 shows the PAM-4 modulated optical signal (left) and its spectrum (right).
Figure 2: SE-MZM PAM-4 transmitter output signal (left) and spectrum (right)
Figure 3 shows output at the transmitter.
Figure 3: Optical eye diagram at the transmitter output
The optical eye diagram shows four intensity levels with nice eye-openings.
Now that we understand the design motives and operation of a DAC-less optical PAM-4 transmitter using SE-MZM, we can use this segmented design concept to analyze performance with respect to the deviations in segment-to-segment distance, driver time-delay and response time – all of which have direct impact on overall yield of the SE-MZM-based high-speed transmitter PICs. For more information on this topic, please refer to https://www.synopsys.com/content/dam/synopsys/photonic-solutions/documents/whitepapers/ps-semzm-white-paper.pdf