**1. Introduction**

66 Selected Topics on Optical Amplifiers in Present Scenario

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The service evolution and the rapid increase in traffic levels fuel the interest toward switching paradigms enabling the fast allocation of Wavelength Division Multiplexing WDM channels in an on demand fashion with fine granularities (microsecond scales). For this reason, in the last years, different optical switching paradigms have been proposed (Sabella et al., 2000): optical-packet switching (OPS), optical-burst switching (OBS), wavelength-routed OBS, etc. Among the various all-optical switching paradigms, OPS attracts increasing attention. Owing to the high switching rate, Semiconductor Optical Amplifier (SOA) is a key technology to realize Optical Packet Switches. We propose some Optical Packet Switch (OPS) architectures and illustrate their realization in SOA technology. The effectiveness of the technology in reducing the power consumption is also analyzed. The chapter is organized in three sections. The main blocks (Switching Fabric, Wavelength Conversion stage, Synchronization stage) of an OPS are illustrated in Section 2 where we also show some examples of realizing wavelength converters and synchronizers in SOA technology. Section 3 introduces SOA-based single-stage and multi-stage switching fabrics. Finally the SOA-based OPS power consumption is investigated in Section 4.

### **2. Optical packet switching architectures**

The considered optical switch architecture (Eramo, 2000; 2006; Sabella et al., 2000) is shown in Fig. 1. It has *N* input and output fibers, each fiber supports a WDM signal with *M* wavelengths, so an input (or output) channel is characterized by the couple (*i*, *λj*) wherein *i* (*i* ∈ 1, ··· , *N*) identifies the input/output fiber and *λj*, (*j* ∈ 1, ......, *M*) identifies the wavelength. In general, optical packet switches can be divided into two categories: slotted (synchronous) and unslotted (asynchronous) networks. In a synchronous switch (Eramo, 2000), as illustrated in Fig. 1 packets with fixed length are aligned (synchronized) by synchronizers before they enter the switch fabric. This type of switch generally achieves a fairly good throughput since the behavior of the packets is regulated. However, complex and expensive synchronization hardware is needed at each node. On the other hand, in an asynchronous switch (Eramo et al., 2003), the packets are not aligned and they are switched one by one on the fly. Asynchronous networks generally have lower cost, better flexibility, and robustness, but usually they have lower overall throughput than synchronous networks. The switch architecture is equipped with a number *r* of WCs which are shared according

are composed of a series of optical switches designed to select the proper optical path

SOA-Based Optical Packet Switching Architectures 69

In these architectures, however, increasing the number of switches to improve the time resolution causes additional increases in optical loss and crosstalk. To overcome loss problems SOA-based synchronizers have been proposed. Next we illustrate and explain two of them. In the first one (Sakamoto et al., 2002) synchronization is achieved by selecting one of some optical paths, each with a different length, using wavelength and space switching based on a wavelength-tunable distributed Bragg reflector laser diode (LD) and *n* semiconductor optical amplifier (SOA) gates per channel. The synchronizer has its own internal reference clock. The clock period equals the time slot duration (*Ts*) and the synchronizer aligns input packets with the time slot packet by packet. Synchronization is achieved by counting each delay of each input packet with respect to the reference time and choosing the optical paths with the appropriate length. Fig. 2 shows the schematic structure of the synchronizer. Each channel is equipped with a wavelength-divisionmultiplexing (WDM) coupler, a wavelength converter, an optical splitter, semiconductor optical amplifier (SOA) gates, two stages of fiber delay lines, an optical coupler, arrayed waveguide gratings (AWGs) for MUX/DEMUX, a delay counter, and a wavelength-tunable laser. The out-of-band optical label switching technique is used, in which optical packet and optical labels are carried on different wavelengths (Okada et al., 2001). The delay counter estimates the delay of each optical label and selects one of *m* wavelengths of the tunable laser and one of *n* SOA gates. The wavelength of each optical packet signal is converted to the laser wavelength by the wavelength converter. The wavelength-converted optical packet signal passes through one of *n* SOA gates and the

<sup>2</sup>*<sup>k</sup>* (*Ts*:time slot; *k*:integer).

*<sup>n</sup>* . The packet signal

MUX AWG

O*1*

DEMUX AWG

> O*2*

O*m*

2nd Stage Delay lines (OFCs)

and pairs of fiber delay lines with different optical lengths of *Ts*

first-stage delay lines, each of which has a different delay time of *Ts*

**control signals**

Tunable LD

> **S P L I T T E R**

time difference of *Ts*

WDM

OFC: Optical fiber circuit.

coupler EDFA WC

synchronization is attained.

Delay O/E Counter

then passes through one of *m* AWG ports and the second-stage delay lines, each with delay

**SOA #n**

1st Stage Delay lines (OFCs)

Fig. 2. Schematic structure of the synchronizer. WC: Wavelength converter. SOAG: SOA gate.

The second synchronizer has been proposed in (Mack et al., 2008) and it is illustrated in Fig. 3. Feed-forward structure with SOA-based gates is used here because of its high operation speed, large tuning range, and the potential for integration within the large SOA-based switch

**SOA #1**

**SOA #2**

*<sup>n</sup>*×*<sup>m</sup>* . Consequently, there are optical paths with different lengths and

**C O U P L E R**

EDFA

to a particular strategy (Eramo et al., 2009b). At each input line, a small portion of the optical power is tapped to the electronic controller. The switch control unit detects and reads packet headers and drives the space switch matrix and the WCs. Incoming packets on each input line are wavelength demultiplexed (DEMUXs blocks in Fig. 1). An electronic control logic, on the basis of the routing information contained in each packet header, handles packet contentions and decides which packets have to be wavelength shifted. Packets not requiring wavelength conversion are directly routed towards the output lines; on the contrary, packets requiring wavelength conversions will be directed to the pool of *r* WCs and, after a proper wavelength conversion, they will reach the output line. An example of realization of synchronizers and wavelength converters in SOA technology is shown in Sections 2.1 and 2.2 respectively. Section 3 is devoted to illustrate both SOA-based single-stage and multi-stage switching fabrics.

Fig. 1. Optical Packet Switching Architecture with *N* Input/Output Fibers, *M* Wavelength and *r* shared Wavelength Converters.
