Enabling 1200-km optical DNANF transmissions via the space-time coded digital subcarrier modulation.

We fabricated 100-km double nested antiresonant nodeless fibers (DNANFs) and demonstrated their potential for long-haul hollow-core transmissions. We measured a PMD coefficient at 0.7 ps/km of the 100-km span. This elevated PMD necessitates longer multiple-input and multiple-output (MIMO) equalizer taps to compensate, thereby increasing DSP complexity. To deal with it, we introduce a coded modulation that leverages frequency-resolved equalization across multiple subcarriers to alleviate PMD impact by the reduced baud rate per subcarrier. Via implementing space-time coding (STC) on digital subcarrier modulations (DSCM), the imbalanced performance among subcarriers can be well equalized while suffering from severe frequency-selective fading for long-distance DNANF transmissions. By experimental validations on a loop configuration of 100-km DNANF spans, we demonstrated 40-Gbaud optical transmissions while investigating the proposed STC-DSCM. A 1200-km optical DNANF transmission was achieved with realizing Q-factor gain over 0.62 dB. These results underscore the feasibility of DNANF for long-haul optical communications while offering a pathway to manage PMD and frequency-selective fading efficiently by the proposed coded modulation.