Simulation investigation of CO₂ absorption-induced performance degradation in hollow-core fiber transmission systems and spectrum-optimized digital sub-carrier multiplexing design for circumvention

Hollow-core fiber (HCF) is developing rapidly, with its minimum attenuation already surpassing that of traditional solid-core fiber and continually setting new records. However, recent reports have pointed out that the CO2 absorption peaks present in the attenuation spectrum of HCF can severely affect the performance of high-speed signal transmission. Using a high-precision insertion loss test platform and a swept-frequency light source, we measured the attenuation spectrum of an HCF sample and found that under high-precision wavelength interval scanning, certain wavelength positions in this sample exhibited up to 0.5 dB/km extra loss due to CO2 absorption. We imported the experimental test attenuation results into VPITransmissionMaker for simulation analysis and found that in the wavelength regions where the CO2 absorption peaks are severe, the bit-error rate (BER) performance of both conventional single-carrier (SC) signals and digital sub-carrier multiplexing (DSCM) signals after 100 km transmission deteriorates significantly. To address this issue, we proposed a spectrum-optimized DSCM modulation scheme with sub-carrier center frequency optimization to circumvent the overlap between the CO2 absorption peaks and the effective components of the DSCM signal spectrum. Simulation results show that within a 150 GHz grid at a center frequency of 186.05 THz, for the 100 km HCF fiber measured in this study with pronounced CO2 absorption peaks, the proposed novel, to the best of our knowledge, DSCM scheme achieves a >10 × BER improvement compared to conventional SC and DSCM schemes, when the optical signal-to-noise ratio of the transmission system is above 26 dB.