Optical frequency-division multiplexing (OFDM) is a promising method to exploit the vast bandwidth of singlemode optical fiber. A key issue in OFDM implementation is the absolute frequency accuracy of each individual channel. The generation of high-quality and free-controllable multi-carrier lightwave source based on frequency shift property of discrete Fourier transform (DFT) is proposed. A 4x100Gb/s all-optical OFDM system by employing polarization division multiplexed (PDM) non return-to-zero (NRZ) quadrature phase shift keying(QPSK) modulation format is designed based on our proposed source. The required optical signal-to-noise ratio (OSNR) at bit error rate (BER) of 1x10-3 for the 400Gb/s all-optical OFDM signal is simulated to be ~23.25dB.