Fig. 3: Clock and carrier frequency distribution.
From: Communications with guaranteed bandwidth and low latency using frequency-referenced multiplexing
a, A CW laser seeds two stages of comb generator, yielding 1.25-THz-bandwidth 2.5-GHz-spacing comb signals with 10 dB flatness. The comb signals are sent to the end users for clock and carrier frequency synchronization. VOA, variable optical attenuator. b, Spectrum of the 25-GHz-spacing comb signals output from the first stage. c, Spectrum of the generated comb signal output from the second stage. d, Demultiplexed comb signals using a 200.0 GHz WDM each containing 64 tones of 2.5 GHz with about 10 dB spectral flatness. e, RF spectrum of the detected 2.5 GHz clock signal using channel 4 as an example (ITU ch35, 193.4–193.6 THz). f, Jitter of the 50 MHz reference clock for end-user transceivers at different received optical powers. The spur at around 1.8 GHz (about 60 dB below to the 2.5 GHz clock tone) is attributed to the slightly saturated RF amplifier, and it does not affect the phase noise as the divider that locks to the 2.5 GHz tone has <100 kHz loop bandwidth. The increased jitter value from –5 to 0 dBm is due to the saturation of the electronic amplifier and the decreased jitter value from –5 to –16 dBm is due to the reduced power. g, Measured phase noise of the distributed reference clock signals to different WDM channels, showing a maximum r.m.s. jitter of <2 ps, integrated over 1 kHz to 10 MHz.
