This paper looks at whether hollow-core fiber (HCF) can extend the use of simple, low-cost optical links that use intensity modulation and direct detection (IMDD). IMDD is widely used inside data centers and in short links because the transceivers are much cheaper and use less power than full coherent systems. But IMDD struggles with chromatic dispersion, a signal-smearing effect that currently limits reach to a few kilometres. The authors present a system-level analysis showing that HCF can change that picture and make IMDD practical at metro distances.

The authors compare anti-resonant HCF with standard single-mode fiber (SMF) and quantify three main advantages. First, HCF has much lower chromatic dispersion (about 2–4 ps/(nm·km) versus 17 ps/(nm·km) for SMF). That moves the first dispersion-induced power-fading null at 40 km from roughly 10 GHz to 20–28 GHz, which the authors say extends dispersion-limited reach by 4–8 times. Second, HCF’s nonlinearity is roughly 1,000 times lower than silica, allowing much higher launch power (+10 to +20 dBm) and contributing an extra 7–17 dB of link budget. Third, HCF’s group index is near one (ng ≈ 1.003), which reduces one-way propagation latency by about 31% compared with SMF.

The paper also studies the main impairment unique to HCF for IMDD: inter-modal interference (IMI). IMI happens when light leaks into higher-order modes in the larger hollow core and then interferes with the main signal. The authors show that if the fiber has differential modal attenuation (DMA) above about 12 dB per kilometre, IMI-induced crosstalk can be pushed below the −30 dB multipath-interference threshold that is considered acceptable for PAM4 signaling. They also find that the lower dispersion of HCF reduces the number of taps needed in a feed-forward equalizer (FFE) by roughly 3–6×. Fewer taps reduce the digital signal processing (DSP) complexity and the extra noise that equalizers can add.