mean value of DGD is √ 8 / (3 π ) $_{D$_{p}$}$√$_{n}$$_{E}$$_{L}$$_{. A typical PDL is 0}$$_{.}$$_{1dB, where}$ PDL[dB] = − 20 log( k$_{l}$ ). We consider both QPSK and 16-QAM modulations. The data rate is 25G symbols/s, i.e., 100Gb/s for QPSK and 200Gb/s for 16QAM. Figure 2 illustrates the PAPR performance of the two systems for QPSK and 16-QAM. It is seen that the OFDM system exhibits a much higher PAPR than the DFT-spread OFDM system. In general 16-QAM has a higher PAPR than QPSK. However, their PAPRs are similar in the OFDM system. Amplitude clipping is a typical method to lower the PAPR in OFDM systems. We used a clipping ratio (CR) of 3dB, defined as CR = 20 log$_{10}$( A/P ), where P is the power of transmitted signal, and A is the maximum transmitted signal magnitude. It is seen that clipping can indeed significantly reduce the PAPR in the OFDM system; however, the PAPR in the clipped OFDM system is still much larger than that of the DFT-spread OFDM system. On the other hand, clipping substantially degrades the bit error rate (BER) performance. As shown in Fig. 3, the DFT-spread OFDM and the OFDM systems have the same BER performance. But after the 3dB clipping on the OFDM signal, a 0.8dB loss is incurred at the BER of 10 − 3 for QPSK, and for 16QAM the loss due to clipping is about 8dB. In summary, compared with the traditional optical OFDM system that employs amplitude clipping, the DFT-spread OFDM system offers both a lower PAPR and a better BER performance. Discussions: Note that the effect of nonlinearity depends on the instanta- 9