Fig. 1. FSON architecture for DCI, the tree-distributed homology architecture, and performance based on FSON. (a) Proposed FSON architecture for DCI. All laser sources are from the same ultra-stable narrow-linewidth laser within a data center. Its architecture, similar to spine-leaf, provides redundant ultra-stable lasers and a more flexible linking solution. (b) Proposed tree-distributed homology FSON architecture. Ultra-stable narrow-linewidth laser as a master laser source is cascaded and distributed to each level and every server node. (c) Phase noise measurement of ultra-stable narrow-linewidth laser source. The phase noise curves of the laser source without EDFA (noise floor) and with cascaded EDFAs are measured respectively. Tx, transmitter; Rx, receiver; EDFA, erbium-doped fiber amplifier; AOM, acousto-optic modulator; BPD, balanced photodetector; DDS, direct digital synthesizer; PNA, phase noise analyzer.

Fig. 2. Experimental setup and principle of DSP-free carrier phase recovery with PACL module in FSON architecture. (a) Experimental setup for the dual-polarization 224-Gb/s 16-QAM DSP-free carrier phase recovery based on FSON with PACL via 5-km transmission. (b) Schematic of the optical signal spectra at both the transmitter and receiver. (c) PACL phase compensation module. (d) Dynamic compensation bandwidth measurement setup with 600-kHz 10-dBm phase modulation. AWG, arbitrary waveform generator; DP-IQM, dual-polarization IQ modulator; Amp., optical amplifier; APC, auto-polarization controller; AOM, acousto-optic modulator; VCO, voltage-controlled oscillator; PACL, pilot-tone assisted Costas loop; LO, local oscillator; DSO, digital storage oscilloscope; LPF, low-pass filter; PID, proportion integration differentiation.

Fig. 3. Performance of the proposed system in the dual-polarization 224-Gb/s 16-QAM 5-km transmission case. (a) BER performance for the X- and Y-polarizations of four-tributary dual-polarization 16-QAM signals. (b) BER performance for X- and Y-polarization SNRs. (c) Eye diagrams of X- and Y-polarization 16-QAM signals. (d) Constellation diagrams with the PACL module on.

Fig. 4. Performance of PACL module frequency compensation range and dynamic compensation bandwidth. (a) I- and Q-path pilot tone amplitudes at the receiver with the PACL module on and off. (b) I- and Q-path relative power with PACL module on. (c) BER performance for the X- and Y-polarizations with a 10-dBm sine wave phase modulator in different phase fluctuation speeds. (d) Frequency difference between signal and LO with the PACL module on and off.