This article presents an optical transmitter front-end based on 2:1 analog multiplexer (AMUX) for beyond-1-Tb/s/carrier coherent optical communications systems. An AMUX is monolithically integrated with an optical modulator driver to realize an AMUX-Driver IC that can directly drive an optical modulator. The AMUX-Driver IC fabricated by using our in-house 250-nm InP double heterojunction bipolar transistors (DHBTs), which have a peak cutoff frequency (f(T)) and maximum oscillation frequency (f(max)) of 460 and 480 GHz, respectively, achieves a record bandwidth of 110 GHz and a 1.5-V-ppd linear differential output range with a <3% total harmonic distortion (THD). A coherent optical transmitter front-end module compactly integrating two AMUX-Driver ICs and an InP-based optical in-phase quadrature (IQ) modulator achieves an electro-optic (EO) bandwidth higher than 80 GHz at the 6-dB point. Beyond-1-Tb/s/carrier optical modulation based on 168-GBaud polarization-division-multiplexed (PDM) 16-ary quadrature-amplitude-modulation (16QAM) was successfully demonstrated by using this integrated module.