Optimization of PAM-4 transmitters based on lumped silicon photonic MZMs for high-speed short-reach optical links

Abstract

We demonstrate how to optimize the performance of PAM-4 transmitters based on lumped Silicon Photonic Mach-Zehnder Modulators (MZMs) for short-reach optical links. Firstly, we analyze the trade-off that occurs between extinction ratio and modulation loss when driving an MZM with a voltage swing less than the MZM's V_π. This is important when driver circuits are realized in deep submicron CMOS process nodes. Next, a driving scheme based upon a switched capacitor approach is proposed to maximize the achievable bandwidth of the combined lumped MZM and CMOS driver chip. This scheme allows the use of lumped MZM for high speed optical links with reduced RF driver power consumption compared to the conventional approach of driving MZMs (with transmission line based electrodes) with a power amplifier. This is critical for upcoming short-reach link standards such as 400Gb/s 802.3 Ethernet. The driver chip was fabricated using a 65nm CMOS technology and flip-chipped on top of the Silicon Photonic chip (fabricated using IMEC's ISIPP25G technology) that contains the MZM. Open eyes with 4dB extinction ratio for a 36Gb/s (18Gbaud) PAM-4 signal are experimentally demonstrated. The electronic driver chip has a core area of only 0.11mm2 and consumes 236mW from 1.2V and 2.4V supply voltages. This corresponds to an energy efficiency of 6.55pJ/bit including Gray encoder and retiming, or 5.37pJ/bit for the driver circuit only.

Silicon Photonic IQ Modulators for 400 Gb/s and Beyond

“…The phase shifter can be modeled as a series of lumped capacitors and resistors [5,11], whose values depend on the applied voltage. We conduct 2D simulation in Lumerical Mode and Device software at wavelength  =1550 nm to predict the electro-optic response of the phase shifter and to calculate the circuit components.…”

“…Despite recent demonstrations in higher-order modulation formats, with direct detection [2][3][4][5][6] or coherent detection [7][8][9][10], there exists a gap between optimization efforts in the design of SiP modulators and in the system-level performance. A joint optimization, of both modulator design and system parameters, could further push performance.…”

Modeling and trade-off analysis of a capacitive silicon Mach-Zehnder modulator for telecom applications

“…The inductance (L 1 ) as a function of the operation frequency in the transmission line were calculated according to the coplanar waveguide (CPW) model proposed by Heinrich [11]. Finally, in order to provide higher bandwidth, the input characteristic impedance was set to 5 Ω, as proposed in [12]. In TABLE II, it is possible to note that the electro-optical bandwidth increases with the gate thickness.…”

Electronic ICs for Silicon Photonic Transceivers

“…This structure has the potential to achieve very low power consumption as energy is only required when switching capacitors, unlike conventional driving approaches which continuously draw current through load and termination resistors. One implementation using 65nm CMOS is given in [2]; up to 18Gbaud PAM-4 was demonstrated at 5.4pJ/bit. The EIC was flip-chipped onto the Si PIC using 50m solder jetted microbumps [3].…”