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A s ilico n p h o to n ic micro rin g res o n ato r mo d u lato r tran s mitter u tilizes a s eg men ted p u ls ed -cas co d e o u tp u t s tag e fo r v o ltag e lev el co n tro l to ach iev e PA M4 mo d u latio n o n a s in g le micro rin g d ev ice. Th e 65n m CMOS tran s mitter ach iev es 40Gb /s o p eratio n at 3.04mW / Gb /s wh en d riv in g d ep letio n -mo d e micror in g mo d u lato rs with 4.4Vppd s win g . Introducti onMicroring resonator devices have great potential to meet the growing the bandwidth density demands of data center and supercomputing systems due to their compact size and high -Q response allowing for inherent wavelengthdivision multiplexing (WDM). Four-level pulse-amplitude modulation (PAM4) is currently under consideration in order to enable longer transmission distances and operation with reduced circuit bandwidth relative to non -return-tozero (NRZ) modulation [1]. PAM4 modulation has been recently demonstrated with directly -modulated VCSELs [2], monolithically-integrated Mach-Zehnder modulators [3], and microring resonator modulators [4,5]. However, the previous microring resonator demonstrations operating above 20Gb/s have utilized elaborate lab setups involving arbitrary waveform generators in order to generate the PAM4 levels with the nonlinear devices. This work presents a 40Gb/s transmitter designed in 65nm CMOS that utilizes a segmented pulsed -cascode output stage for voltage level control to achieve PAM4 modulation on a single microring device. V (a) (b) (c) Fig. 1. (a) T op and cross-section views of a carrier-depletion microring modulator. Measured normalized transmission curves with different reverse bias voltages: (b) dB scale and (c) linear scale. Carrier-Depletion Ring ModulatorAs shown in Fig. 1(a), the carrier-depletion microring modulator utilized in this work has an 8μm radius with outer p+ and inner n+ doping on the ring waveguide to form a pn junction and p++ and n++ doping utilized for ohmic contacts. The device displays an 18GHz bandwidth and 7500 Q factor, as shown in the measured transmission curves of Fig. 1(b). Due to the relative small tunability of the ring modulator (~20pm/V), a high -swing (>4V) driver is necessary. Moreover, modulator non-linearity due to the voltage-to-index and index-to-intensity responses must be considered for uniform PAM4 level spacing. In order to address this, the input laser wavelength is slightly offset from the resonant wavelength to optimize the linearity of the optical output response with different bias voltages, as illustrated in the linear-scale transmission curve of Fig. 1(c). Also, the high-speed driver must have the flexibility to generate the necessary voltage levels for symmetrical PAM4 levels. Fig. 2(a) shows a block diagram of the proposed half-rate PAM4 microring modulator transmitter. The odd and even bits from a 16-bit pattern generator are serialized separately to full-rate to form the PAM4 MSB and LSB signals. This full-rate data goes through buffers and level shifters to generate the predriver signals for th...

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