Fig. 1. Structure of 3rd-order cascaded resonator feedback Delta-sigma modulator

Fig. 2. Frequency response of the NTF of Delta-sigma modulator with different OSRs

Fig. 3. Experimental setup of unrepeated coherent transmission based on Delta-sigma modulation

Fig. 4. The flowcharts of transmitter and receiver DSP

Fig. 5. Measured optical spectra with different OSRs

Fig. 6. SNR of the QPSK optical waveform as a function of launch power

Fig. 7. SNR of the QPSK optical waveform as a function of unrepeated transmission distance

Fig. 8. Recovered SNR of the original high-order format signal as a function of OSR

Fig. 9. EVM of the high-order format signal as a function of OSR

Fig. 10. Recovered constellations of original signals

Fig. 11. Measured SNR of the QPSK optical waveform as a function of optical SNR at back-to-back scenario

Fig. 12. Measured BER of the QPSK optical waveform as a function of optical SNR at back-to-back scenario

Fig. 13. Measured recovered SNR of the original high-order format signal as a function of optical SNR at back-to-back scenario