100Gbit/s transmission data rate is very attractive for the next generation Ethernet transport systems, and this kind of high-speed channel based WDM systems is very attractive for constructing cost-effective optical transport networks. However, it is quite difficult to achieve such high bit-rate by using the conventional amplitude shift keying (ASK) technology. For the subcarrier modulation of the orthogonal frequency division multiplexing (OFDM) scheme, it is possible to use various modulation formats. It is relatively easy to use such an advanced format in the microwave domain compared to the optical domain. Therefore, it is possible to increase the spectral efficiency relatively easily by using the OFDM technology. In this master thesis, 100 Gbit/s WDM system with OFDM is discussed. The performance of 8 channels 100Gbit/s OFDM system was evaluated. The simulation result showed that the BER of all channels were below 10-3 after 500 km transmission, and the performance could be improved by using the FEC. This result shows the possibility of 100Gbit/s transmission system using the OFDM technology. The major concern of WDM-PON system is the cost. For WDM-PON system, it needs several light sources for downstream signals and upstream signals. For the practical implementation of WDM-PON, it is essential to develop a low-cost light source in the optical network unit (ONU). In order to develop a simple efficient ONU, a promising solution is re-modulated the downstream signal from central office (CO) as the upstream signal at ONU using reflective semicounductor optical amplifier (RSOA) so that there is no need to setup an additional light source at the ONU side. For the conventional fiber-to-the-home (FTTH) system, the modulation format for both downstream signal and upstream signal is intensity modulation with direct detection (IM/DD). The demerit of the IM/DD scheme for the FTTH system is that the performance of the re-modulated upstream signal is limited by the interference of the downstream signal. In order to overcome this issue, the IM/DD format of downstream should be replaced by some different formats such as differential phase shift keying (DPSK). As there is no amplitude modulation for the DPSK downstream signal, the re-modulated signal will not be affected by the downstream signal. Therefore, this master thesis is focusing on comparing the re-modulated signal quality for both IM/DD downstream and DPSK downstream. The experimental results shows that the upstream signal of the DPSK downstream case shows better performance than the IM/DD downstream case, and the performance difference of the re-modulated signal is 17dB.