All-Optical Signal Processing for High Spectral Efficiency (SE) Optical Communication
Optical Communication 344 electronics required advanced modulation, coding and digital equalization  . Recently, coherent optical communications attracted a large interest due to the feasibility of the high SE, the large bandwidth and multilevel modulation formats  . Coherent optical systems using multilevel modulation formats can increase SE up to M b/s/Hz where M is the number of bits per symbol for a given modulation format  . Coherent optical OFDM (CO-OFDM) has been recently
... een recently proposed in order to increase receiver sensitivity, SE, and especially, provide the dispersion compensation at high data transmission rates tending to 100Gb/s , , ,  . Generally, OFDM is a kind of multicarrier modulation (MCM) in which the data information is carried over many lower rate subcarriers  . In OFDM spectra of individual subcarriers overlap, but due to the orthogonality, the subcarriers can be demodulated without interference and without analog filtering for the received subcarrier separation  . The signal processing in the OFDM system can be carried out by using the Fast Fourier Transform (FFT)/Inverse Fast Fourier Transform (IFFT) , ,  . In CO-OFDM systems the digital signal processing is used in order to mitigate the channel dispersion, nonlinearity and different types of noise. CO-OFDM system combines the following advantages of coherent detection and OFDM modulation essential for the high-speed optical fiber communication systems: (i) in CO-OFDM systems, chromatic dispersion and PMD can be mitigated; (ii) the SE of CO-OFDM systems is high due to the partial overlapping of OFDM sub-carrier spectra; (iii) linearity in radio frequency (RF)-to-optical (RTO) up-conversion and optical-to-RF (OTR) downconversion; (iv) the electrical bandwidth requirements for the CO-OFDM transceiver can be greatly reduced by using direct up/down conversion which results in the low cost of the high-speed electronic circuits , ,  . Recently, all-optical FFT scheme enabling Tbit/s real-time signal processing has been proposed  . The method based on only passive optical components realizes the highest speed signal processing without the power consumption where electronics cannot be used. This approach combines the advantages of the electronic high precision processing of the low bit rates and the optical processing of high bit rates  . However, OFDM is characterized by the inter-symbol-interference (ISI) and inter-carrierinterference (ICI) caused by a large number of subcarriers  . In the RF systems ISI is mainly due to multipath channel delay spread ,  and ICI is mainly due to the carrier frequency offset  . In the case of CO-OFDM, ISI and ICI are caused by channel chromatic dispersion and PMD ,  . A so-called cyclic prefix (CP), i.e. the cyclic extension of the OFDM waveform into the guard interval (GI) G , has been proposed in order to prevent ISI and ICI  . If the GI is long enough to contain the intersymbol transition, then the remaining part of the OFDM symbol satisfies the orthogonality condition and receiver crosstalk occurs only within GI  . The addition of CP requires an increase of a bandwidth and sampling rate of analog-to-digital converter (ADC) and digital-to-analog converter (DAC). CP appeared to be an easily recognizable feature of an OFDM system making the signal vulnerable to interception by surveillance receiver  . The elimination of CP reduces the probability of interception and improves SE . All-Optical Signal Processing for High Spectral Efficiency (SE) Optical Communication 345 The need for CP can be avoided if the wavelet packet transform (WPT) is used in CO-OFDM systems instead of discrete Fourier Transform (DFT) and inverse DFT (IDFT)  . The sinusoidal functions are infinitely long in the time domain while wavelets have finite length being localized in time and in frequency domains  . Wavelet signal analysis can be a base for an effective computational algorithm which is faster and simpler than FFT  . Wavelets have been used in optical communications for time-frequency multiplexing and ultrafast image transmission  . A signal may be expanded in an orthogonal set of wavelet packets (WPs) as the basis functions, each channel occupies a wavelet packet (WP), and IDWPT/ DWPT are used at the transmitter and receiver, respectively  . In this chapter, we consider the CO-OFDM based on WPT and its influence on the optical communication network performance. The chapter is constructed as follows. In Section 2, we review the coherent optical communication systems. In Section 3, we discuss high SE CO-OFDM system. In Section 4, we discuss the OFDM based on WPT and present the original results for the WPT-OFDM system performance. In Section 5, we present the original results concerning the simulations of the structure and operation mode of the novel passive components for all-optical signal processing based on Si-on-insulator (SOI) structure, and a novel hierarchical architecture of the 1Tb/s transmission system based on WPT-OFDM  . In Section 6, conclusions are presented.