Fig. 3: BERs (BERs, bit error rates) and PAM4 (PAM4, four-level pulse amplitude modulation) eye diagrams of bidirectional WDM (WDM, wavelength-division-multiplexing) fibre-FSO (FSO, free-space optical) communications under different conditions.
a The downstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength of 1550.78 nm under different conditions. b The upstream BERs of 30 Gb/s PAM4 signal at a filtering wavelength of 1549.78 nm under different conditions. The error bars represent the standard deviations of the measured data from three experimental trials. c Clear eye diagrams at a BER of 10−9 for downstream transmission with x-polarisation and VSB (VSB, vestigial sideband)-PAM4 modulation. d Clear eye diagrams at a BER of 10−9 for upstream transmission with y-polarisation and VSB-PAM4 modulation. e Blurred eye diagrams at an upstream BER of 1.4 × 10−5 (y-polarisation; PAM4) as the tunable optical VSB filter is not used at the upstream reception side. BTB back-to-back.
