Fig. 4: Proof-of-concept experiment for the clock- and frequency-referenced FDM upstream data aggregation for time-critical applications.
From: Communications with guaranteed bandwidth and low latency using frequency-referenced multiplexing

a, System diagram of our proof-of-concept experiments with three live end users combined with dummy signals to form 160-GHz-optical-bandwidth signals. Due to the availability of components, the optically distributed clock is detected by a single photodiode and sent to all the live end-user transceivers via short coaxial cables as the clock reference. Based on the clock reference, three sets of field-programmable gate arrays (FPGAs) and 4.9 GSa s–1 DACs generate SCM QAM signals and drive the corresponding IMs to generate the upstream signals. The user lasers generate CW signals with about 150 kHz linewidth and are frequency locked to neighbouring comb tones using an FLL containing a frequency detector and a PI controller, with about 10 kHz loop bandwidth. A thermoelectric controller (TEC) provides feedback for long-term stability and coarse frequency tuning. Two couplers and a 10 dB attenuator are used to emulate 1:64 remote node splitting, resulting in a total link loss of about 28 dB (including 22 km SSMF loss, WDM loss and remote node splitting loss). b, Optical spectrum (20 MHz resolution) of the upstream signals received: red (user1), orange (user2) and blue (user3). Green indicates the modulated dummy channels. c, Optical spectrum (20 MHz resolution) of the combined upstream signals, with all the live transceivers locked to 2.5-GHz-spacing tones. d, Measured power sensitivity (power per user signal into EDFA3) for different modulation formats at the SD-FEC threshold of 2 × 10–2 (15.3% overhead): cross markers (4-QAM), open markers (8-QAM), closed markers (16-QAM). e, Measured constellation diagrams of user1. f, Measured frequency deviation over 24 h using user1 locked at 193.407 THz.