We propose a novel optical orthogonal frequency-division multiplexing (OFDM) transmission, where the frequency spacing of subchannels is less than the symbol rate. We show experimentally that the novel OFDM transmission achieves an optical SNR (OSNR) sensitivity level and dispersion tolerance roughly equivalent to those of the conventional OFDM. We also show the influence of chromatic dispersion from the transmission channel on the OFDM signal. The chromatic dispersion causes a desynchronization of the symbol timing between subchannels and breaks the frequency orthogonality in OFDM. We confirm experimentally that it is possible to mitigate the signal degradation due to the chromatic dispersion by optimizing the symbol timing and orthogonally polarizing the subchannels to the neighboring subchannels.
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