Frequency-resolved adaptive probabilistic shaping for DMT-modulated IM-DD optical interconnects

For decades, advanced modulation techniques have been proposed to increase the capacity for intensity-modulation and direct-detection (IM-DD) optical fiber interconnection systems. Typically, the frequency-resolved discrete multi-tone (DMT) modulation was proposed by loading modulations with different bit numbers to fit the channel's frequency response. Capacity can thus be better improved through finer use of the signal-to-noise-ratio (SNR) distribution in the frequency domain. For conventional DMT, the constellations loaded on individual subcarriers are all equip probability distributed. In this work, we propose a probabilistically shaped DMT (PS-DMT) modulation with adaptively loaded entropies referring to channel frequency response for short-reach optical interconnects. Achievable information rate (AIR) improvements of PS-DMT with both Maxwell-Boltzmann and dyadic distribution are investigated based on generalized mutual information (GMI). Moreover, the proposed PS-DMT has been realized experimentally over a multimode optical link using vertical-cavity surface-emitting lasers (VCSELs) with 100-m-long multimode fiber (MMF) transmission. This method can significantly improve the signaling capacity since two significant benefits are simultaneously utilized: 1) the shaping gain of PS at limited SNR condition and 2) the frequency-resolved continuous entropy loading for better fitting to the channel frequency response. Improved capacity, in terms of AIR, can thus be expected for a practical channel when using PS-DMT. This method can potentially be extended to a wide range of application scenarios, including both multimode and single-mode IM-DD fiber-optic communications.