1. Introduction

Optical signal processing is based on the using of linear and nonlinear optical techniques in order to manipulate and process digital, analogue, and quantum information [1]. Optical signal processing increases the processing speed of devices and reduces the energy consumption and latency of communication systems [1]. In particular, ultrafast optical nonlinearities provide a substantial speed advantage as compared to electronic techniques for simple logic: switching, regeneration, wavelength conversion (WC), performance monitoring, and analogto-digital conversion (ADC) [1]. Silicon photonics and highly nonlinear nanophotonic devices are providing strong optical nonlinearities for ultrafast processing on millimeter length scales [1].

The recent progress in optical signal processing is based on the combination of the advanced modulation techniques, coherent detection, and digital signal processing (DSP) [2]. The

interface of all-optical techniques and advanced DSP will enhance electronic processing capabilities [1]. Optical signal processing is essentially based on the following advanced technologies: coherent detection, high-speed electronics for DSP, advances in strongly nonlinear materials and devices, photonic integrated circuits (PIC), and access to four optical domains of amplitude, phase, polarization, and wavelength [2]. A simple digital modulation scheme is the on-off keying (OOK) referred to as intensity modulation with direct detection (IM/DD) [3]. In such a case, an electrical binary bit stream modulates the intensity of an optical carrier inside the optical transmitter, and the resulting optical signal is converted to the original signal in the electrical domain in an optical receiver [4]. The phase modulation combined with the coherent detection increases the spectral efficiency (SE) of optical communication systems and improves the sensitivity of optical receivers [4]. In general case, amplitude-shift keying (ASK), phaseshift keying (PSK) or M-ary quadrature amplitude modulation (QAM) can be realized [3, 4]. Polarization-division multiplexing (PDM), advanced multilevel modulation formats such as M-ary QAM, digital spectral shaping at the transmitter, coherent detection and advanced forward error correction (FEC) can increase SE of the communication system [1]. Typically, DSP must overcome deterministic signal distortions, while FEC overcomes stochastic impairments caused by noise and interference [1]. At the transmitter, DSP together with digital-toanalog converters (DAC) and FEC converts the incoming data bits into a set of analogue signals [1]. An optical coherent receiver recovers the amplitude and phase of the signal by mixing it with the local oscillator (LO) which is typically a continuous-wave (CW) laser [3, 4]. DSP, ADC, and FEC recover the data from the set of analogue electrical signals [1]. The main functions of the receiver-based DSP are equalization and synchronization [1]. Equalization must realize the polarization rotation tracking and dispersion compensation including both the chromatic dispersion and polarization-mode dispersion (PMD) [1]. Synchronization carries out the transmitter and receiver electrical and optical signal frequency and phase matching [1].

The all-optical signal processing is implemented by using the nonlinear optical phenomena such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM) related to the third-order susceptibility and sum frequency, difference frequency, second harmonic generation (SHG) related the second-order susceptibility [1–4]. The typical nonlinear elements used in optical communication systems are highly nonlinear optical fibers (HNLF), silicon waveguides, chalcogenide waveguides, photonic crystals, nonlinear optical loop mirrors (NOLM), parametric amplifiers, and semiconductor optical amplifiers (SOA) [2, 3]. SOA are characterized by the extremely strong third-order optical nonlinearity and fast response and can be integrated monolithically with other devices on the same chip [3].

Optically assisted signal processing combines optics and electronics for what each one of them does best [2]. Optical components can perform some functions very fast, while electronic components carry out complex computations with buffers and memory [2]. For instance, optically assisted network routing technique uses optical correlation on headers of Internet data packets [2]. Optically assisted signal processing can be also used for a target pattern search in large amounts of data [2]. In such cases, the data information is encoded on an optical carrier at Tb/s speed and sent to an optical correlator for pattern recognition [2]. The output at Gb/s speed is searched and processed electronically with high accuracy before being sent to the user [2].

In optical networks, the bandwidth mismatch between optical transmission and electronic routers results in the development of different optical signal processing and the investigation of optical packet switching (OPS) [5]. Some applications require selective switching of one or more bits to a different port [3]. The packet switching takes place when a packet of tens or hundreds of bits is selected from a bit stream [3]. The flip-flop memory is an essential component of the packet switch [3]. Typically, such a memory is implemented using two coupled lasers switching the output signal between two wavelengths λ<sup>1</sup> and λ<sup>2</sup> [3]. Recently, we proposed a novel architecture of an all-optical memory loop combining the ultrafast all-optical signal processor based on the Mach-Zehnder interferometer (MZI) with quantum dot (QD) SOA and the DSP block for the mitigation of dispersion and nonlinearity impairments [6].

The chapter is organized as follows. OPS in optical communication systems is discussed in Section 2. The different types of all-optical logic gates used in OPS are briefly reviewed in Section 3. The operation principle of the novel all-optical memory is described in Section 4. The QD SOA theoretical model is briefly discussed in Section 5. The numerical simulation results and conclusions are presented in Sections 6 and 7, respectively.
