**2.1 Realization of SOA-based synchronizers**

The synchronizers are used in the synchronous optical packet switches where the packets have a fixed size and their arrivals on each wavelength are synchronized on a time-slot basis by synchronization devices located at the ingress of the switch before the switching operation is performed. Most of the synchronizers (Chao et al., 2000; Zucchelli et al., 1998) 2 Optical Amplifier

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

**Wavelength**

**Synchronizer**

 **(WC)**

O*M*

**WC Stage**

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

The synchronizers are used in the synchronous optical packet switches where the packets have a fixed size and their arrivals on each wavelength are synchronized on a time-slot basis by synchronization devices located at the ingress of the switch before the switching operation is performed. Most of the synchronizers (Chao et al., 2000; Zucchelli et al., 1998)

r

1

**Switching**

**Control**

**Fabric**

O*1*,... O*M*

O*1*,... O*M*

*N*

*1*

**Optical Fiber**

**Optical Fiber**

**Converter**

D E M U X

> D E M U X

O*1*

SYN

SYN

SYN

SYN

O*M*

O*1*

O*1*,... O*M*

O*1*,... O*M*

SYN

and *r* shared Wavelength Converters.

**2.1 Realization of SOA-based synchronizers**

switching fabrics.

**Optical Fiber**

**Optical Fiber**

1

*N*

are composed of a series of optical switches designed to select the proper optical path and pairs of fiber delay lines with different optical lengths of *Ts* <sup>2</sup>*<sup>k</sup>* (*Ts*:time slot; *k*:integer). 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 first-stage delay lines, each of which has a different delay time of *Ts <sup>n</sup>* . The packet signal then passes through one of *m* AWG ports and the second-stage delay lines, each with delay time difference of *Ts <sup>n</sup>*×*<sup>m</sup>* . Consequently, there are optical paths with different lengths and synchronization is attained.

Fig. 2. Schematic structure of the synchronizer. WC: Wavelength converter. SOAG: SOA gate. OFC: Optical fiber circuit.

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

**3. SOA-based switching fabric**

**3.1 Single-stage approach**

Multi-Stage switching in Sections 3.1 and 3.2 respectively.

Converters. Two of them are reported in Figs 5 and 6.

1 *N+rw N N+1*

**1st SSM** 

**Stage**

O*1*

O*M*

1 *N+rw N N+1*

1 *N+rw N N+1*

1 *N+rw N N+1*

O*1*

> 1 *N*

1 *N* 1 *N* 1 *N*

**SSM 1**u**N**

O*M*

*# 1*

*# rw* **SSM 1**u**N**

O*<sup>1</sup>* **SSM 1**u**(N+rw)**

O*<sup>M</sup>* **SSM 1**u**(N+rw)**

O*<sup>1</sup>* **SSM 1**u**(N+rw)**

O*<sup>M</sup>* **SSM 1**u**(N+rw)**

**From**

**From**

**Wavelength**

**Converters**

O*1*

**Wavelength**

**Converters**

O*M*

the switching fabrics of the SPW and SPN switches.

**4th SSM** 

**SSM 1**u**N**

*# Mrw* **SSM 1**u**N**

**Stage**

Fig. 5. Shared-Per-Wavelength Single-Stage Optical Switching Fabric.

They are related to two proposed main WC sharing strategies. In the first one, referred to as Shared-Per-Wavelength (SPW) (Eramo et al., 2008; 2009a;b), the WCs are partially shared. All of the packets arriving on a given wavelength share the same pool of converters. In the second one referred to as Shared-Per-Node (SPN) (Eramo et al., 2009c; Eramo, 2010),the WCs are fully shared and all of the arriving packets share the same pool of WCs. Next we illustrate

The SPW switching fabric is illustrated in 5 and its operation mode is the following (Eramo et al., 2008; 2009a;b). A packet not requiring wavelength conversion is directly routed towards the Output Fibers (OF). On the contrary a packet needing the use of a WC will be directed to the pool of *rw* WCs dedicated for the wavelength on which the packet is arriving.

Switching fabric in Future Optical Packet Switches require high-speed optical switches (or gates). That can either be optically or electrically controlled. Such optical switches can be constructed using SOAs due to their high switching rate. The simplest method to control an SOA gate is by turning the device current on or off. The great advantage of SOA gates is that they can be integrated to form gate array. Next we illustrate SOA-based Single-Stage and

SOA-Based Optical Packet Switching Architectures 71

The structure of a switching fabric depends on the adopted sharing strategy of the Wavelength

O*<sup>1</sup>* **SSM** ,...

O*<sup>1</sup>* **SSM** ,...

**(NM+Mrw)**u**1**

**2nd SSM** 

**Stage**

1

**(NM+Mrw)**u**1**

> 1 *N*

1 *N* 1 *N* 1 *N* **SSM N**u**1** *# 1*

*# rw* **SSM N**u**1** O*1*

O*1*

**3rd SSM** 

**SSM N**u**1** *# 1*

*# rw* **SSM N**u**1** O*M*

O*M*

**Stage**

1

O*M*

O*M*

**To**

**Wavelength**

**Converters**

O*1*

**To**

**Wavelength**

**Converters**

O*M*

(Mack et al., 2008). The synchronizer is composed by *NSYN* stages. There are one 1 × 2 splitter, one 2 × 1 coupler, two SOAs, two optical bandpass filter(OBF) and one FDL in each stage. SOAs were used as the gates to select the required delay and compensate for losses. In order to suppress accumulated amplified spontaneous emission, Optical Bandpass Filters was placed inside each synchronizations stage.

Fig. 3. Fiber based synchronizer with SOA gates and Optical Bandpass Filter (OBF). Δ is the delay introduced by the fiber delay line of the 1*st* stage.
