2. Optical packet switching (OPS)

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

280 Optical Fiber and Wireless Communications

response and can be integrated monolithically with other devices on the same chip [3].

user [2].

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

OPS process requires many components for buffering, header processing, and switching [3]. Each packet begins with a header containing the destination information [3]. When a packet arrives at a node, a router reads the header and sends it toward its destination [3]. The basic element of an optical router is a packet switch directing incoming packets to the corresponding output ports depending on the information in the header [3].

Consider the architecture and operation principle of the all-optical packet switch. The scheme of the 1 2 all-optical packet switch is shown in Figure 1 [3, 5, 7]. The all-optical packet switch consists of three functional blocks: the all-optical header-processing block, the all-optical flipflop memory block, and the WC block [7]. All-optical header processing can be realized by using the different methods such as tunable Bragg gratings, FWM in a SOA, terahertz optical asymmetric demultiplexers (TOAD), two-pulse correlation in a semiconductor laser amplifier in a loop optical mirror (SLALOM) [5, 7].

Figure 1. System concept for 1 2 all-optical packet switches.

Figure 2. The all-optical flip-flop memory based on two coupled lasers.

The high-speed memory is necessary for OPS networks in order to avoid the packet collisions during packet routing [8]. The all-optical flip-flop memory is based on two coupled lasers with separate laser cavities and can have two states [7]. It is shown in Figure 2 [7].

In state 1, light from laser 1 suppresses lasing in laser 2 emitting CW light at wavelength λ1, while in state 2, light from laser 2 suppresses lasing in laser 1 emitting CW light at wavelength λ<sup>2</sup> [7]. The output pulse of the optical header processor is used to set the optical flipflop memory into the desired wavelength [7]. The amount of light Psw which is necessary for the change of states, the threshold carrier number Nth, and photon lifetime τ<sup>p</sup> are given by, respectively [7]:

$$P\_{sw} = E \frac{v\_g R \tau\_p}{L(1 - R)} \ln\left(\frac{1}{R}\right) \left(1 - \frac{2R}{\delta(1 - R)}\right) \left(\frac{I}{q} - \frac{N\_{\text{fl}}}{\tau\_e}\right) \tag{1}$$

$$N\_{\hbar t} = \frac{V}{\tau\_p \Gamma v\_{\otimes} a} + N\_0; \frac{1}{\tau\_p} = v\_{\otimes} \left( a\_{\rm int} + \frac{1}{L} \ln \left( \frac{1}{R} \right) \right) \tag{2}$$

Here, E is the photon energy, R is the reflectivity at the end facets of lasers, δ is the coupling constant between the two laser cavities, vg is the group light velocity, L is the length of the active region in the laser, I is the injection current, q is the electron charge, τ<sup>e</sup> is the carrier lifetime, V is the volume of the laser cavity active region, Γ is the confinement factor, a is the gain factor, N<sup>0</sup> is the carrier number at transparency, and αint is the internal laser cavity losses factor. Note that the outputs of the lasers on the left side are defined by the reflectivity at the end facets of lasers R and do not influence the memory states.

WC component converts the incoming data packet wavelength to the output wavelength of the flip-flop memory [3]. The demultiplexer directs output at different wavelengths to different ports depending on the header information [3].
