Experimental evaluation of pilot pattern design in direct-detection optical OFDM transmission

Pilot patterns are experimentally investigated to characterize the frequency/time dependence of the optical orthogonal frequency division multiplexing (OFDM) signal transmission. The optical signal performance is evaluated in terms of error vector magnitude (EVM). It is shown that, the quality of the OFDM signals can be improved up to 4 dB in EVM, for 10 Gb/s, 16-QAM-encoded OFDM signals after 20 km single mode fiber (SMF) transmission with intensity-modulation and direct-detection (IMDD). The best performance is obtained by applying pilot tones for all subcarriers, which implies that the optical OFDM transmission is relatively quasi-static with respect to the subcarrier frequencies. The noise of the OFDM signals originated mainly from the amplitude/phase fluctuation of optical signal with time.