A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) Optical Approach <s> Summary form only given. We have proposed a novel optical orthogonal frequency division multiplexing technique that can overcome the spectral efficiency limitation of the conventional WDM system. This scheme permits substantial overlapping of the spectrum and can achieve the spectral efficiency up to 1 bit/s/Hz in principle. For demultiplexing, we used a newly developed optical discrete Fourier transformer (DFT) instead of electrical digital processing, which is impossible to apply in the optical frequency range. The optical DFT was realized by using a set of delay lines, a phase shifter and a coupler in the frequency domain and bit synchronization and an optical gate in the time domain. In experimental demonstration of this scheme, error-free operation was obtained with a 0.8 bit/s/Hz of spectral efficiency. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) Optical Approach <s> OFDM data with line rates of 5.4 Tbit/s or 10.8 Tbit/s are generated and decoded with a new real-time all-optical FFT receiver. Each of 75 carriers of a comb source is encoded with 18 GBd QPSK or 16-QAM. <s> BIB002
In the optical approach, an optical OFDM signal is directly generated in the optical domain through modulation of multiple optical subcarriers, without the electrical IFFT processing  . The main advantage of the optical approach is that the electronics of the ADC/DAC are eliminated. Different approaches can be used to generate OFDM subcarriers in the optical domain. BIB002 , the transmitter generates multiple optical subcarriers from a continuous-wave light source. Each optical subcarrier is then individually modulated, and finally coupled to create an optical OFDM signal, as shown in Fig. 8(a) . In creating the optical OFDM signal, the orthogonal condition is satisfied through proper pulse shaping and phase locking the optical subcarrier to orthogonal frequency, and the baud rate (symbol rate) of each optical subcarrier equals the optical subcarrier spacing. In its turn, the receiver demultiplexes each optical subcarrier by an all-optical DFT processor, which can be implemented by adding phase delays and careful arrangement of time delays in each subcarrier BIB001 , as depicted in Fig. 8(b) .