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Achieving high data rate with simple system configuration and low power consumption is of great interest in short-reach intensity-modulation direct-detection (IM-DD) fiber links for future datacenter networks, business local area networks and so forth. For external modulation-based IM-DD, recently remarkable modulator performance has been achieved in terms of 3dB bandwidth, exploiting different materials/platforms. However, due to the relatively-high RF-to-RF (or end-to-end) signal loss, optical amplifiers (OA) and/or RF amplifiers (RFA) were typically needed for IM-DD links, which increases the complexity and power dissipation. In this work, we present a more comprehensive figure of merit (FoM) for modulator (and link) design, namely "slope efficiency (SLE)", which takes into account not only the bandwidth but also the half-wave voltage and the RF/optical waveguide losses, and can provide a guiding principle of the modulator design toward lower RF-to-RF signal loss. With a high-SLE modulator, both OA and RFA can be omitted even when supporting broadband signals with CMOS-level driving voltages. We theoretically investigate the impact of modulators with different technology and SLE on the IM-DD pulse amplitude modulation (PAM) link. Next, we report a series of PAM transmission experiments with a high-SLE Lithium Niobate intensity modulator prototype, which enables sub-1 Vpp driving, up-to-432Gb/s PAM transmission over up-to-600m single-mode fiber in C-band with OA&RFA-free reception and low digital signal processing (DSP) complexity. For instance, for 384Gb/s PAM8 over 300m, the transmitter was DSP-free while a symbol-spaced decision-feedback equalizer (DFE) with 61 feedforward taps and 1 feedback tap was sufficient for the receiver considering 16.7% hard-decision (HD)-FEC. Furthermore, we experimentally show that the high-SLE modulator supports ultra-wide fiber transmission bands (O-, S-, C- and L-band).
Zhu et al. (2024) studied this question.