A Linear Modulator Driver With Over 70-GHz Bandwidth 21.8-dB Gain and 3.4-Vppd Output Swing for Beyond 120-GBd Optical Links

This article investigates a linear differential modulator driver for optical communication links beyond 120 GBd. The driver was implemented in a 55-nm SiGe BiCMOS technology with an
$f_{T}/f_{\text {MAX}}$
of 330/370 GHz, featuring a traveling wave output stage driven by a lumped variable gain amplifier (VGA) and a predriver. The fabricated IC achieved a maximum low-frequency gain of 21.8 dB with more than 10-dB gain control capability and a 3-dB bandwidth of over 70 GHz. Power measurement results show the driver is able to deliver a constant output swing of 3.56 Vppd while keeping the THD below 2%, with the differential input swing varying from 0.3 to 1.03 Vppd at 1 GHz. In the large-signal measurement, the chip delivered 72, 106, 112, and 128 GBd pulse amplitude modulation-4 (PAM-4) signals with differential output swings of 4, 3.59, 3.434, and 3.425 Vppd, respectively. The chip’s total power consumption is 725 mW, resulting in a power efficiency of 2.83 pJ/bit. Overall, this is the first time an optical modulator driver reported the ability to accommodate such a wide input swing and demonstrated the operation at a data rate of 256 Gb/s. These features make it a desired candidate for beyond 120-GBd optical links.