Abstract
Based on the use of a dual-wavelength controlled quasi-color-free laser diode (QCFLD) transmitter with either destructively interfered beating or single-sideband filtering after receiving at a remote node, the 100-km long-reach (LR) carrierless millimeter-wave over fiber (MMWoF) link with directly-encoded orthogonal frequency-division multiplexing (OFDM) at 10-15 Gbit/s in optical wired and MMW wireless domains is experimentally demonstrated. The optically heterodyned MMW OFDM data by the destructively interfered beating obtains more gain from the RF amplifier than that without carrierless operation, as the MMW carrier with reduced power no longer dominates the gain competition. The destructively-interfered-beating 16-QAM OFDM data can transmit over 100-km SMF and 10-m free-space. The modulation bandwidth can be enhanced from 2 GHz to 2.5 GHz with a raw data rate of 10 Gbit/s after the OFDM subcarrier sidelobe filtering process. The sidelobe filtering suppresses the impact of the QCFLD chirp on the signal-to-noise of the optically heterodyned beating MMW carrierless OFDM data when reducing the sidelobes of each OFDM subcarrier. Besides, the single-sideband-filtered OFDM data can also obtain a higher gain than the double-sideband one due to the relief of RF amplifier saturation. Even though the noise located at the conjugated data band involves in the frequency-down-converted data after MMW down-mixing, the single-sideband-filtered OFDM data still allows the transmission of 8-QAM single-sideband OFDM data with 2 GHz over 100-km in SMF and 10-m in free-space at a raw data rate of 6 Gbit/s in one single-sideband and 12 Gbit/s at both sidebands. In comparison, the destructively-interfered-beating scheme offers large bandwidth and high spectral usage efficiency of 4 bit/s/Hz, whereas the single-sideband-filtering scheme effectively broadens the available bandwidth by saving another spectral sideband for allocating other carriers and data, which facilitate advantages including doubled band usage and half modulation power consumption. Both schemes offer comparable performance for future long-reach carrierless MMWoF applications.
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