**1. Introduction**

24 Selected Topics on Optical Amplifiers in Present Scenario

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Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks, *J. Lightwave Technol.*, Vol. 25, No. Lots of research efforts have been focused to realize all-optical high-speed switches through nonlinear optical elements, for instance, high nonlinear fibers (HNLF), nonlinear waveguides as well as semiconductor optical amplifiers (SOAs). All-optical switches incorporating SOAs is one of the particularly attractive candidates due to their small size, high nonlinearities (low switching energy required) and ease of integration. All-optical switches also keep the network transparent, enhance the flexibility and capacity in network, and offer the function of signal regeneration, therefore SOAs provide various attractive alloptical functions in high-speed signal processing in fiber communication systems (Stubkjaer, 2000; Poustie, 2007), including all-optical AND/XOR logic gates, wavelength conversion (WC), optical-time division multiplexing (OTDM) de-multiplexing, optical signal regeneration and so on, which will be essential to the implementation of future wavelength division multiplexing (WDM) or optical packet switching (OPS) networks.

However, the operation speed of SOA based switches is inherently limited by its relative slow carrier lifetime (in an order of 100 ps) (Manning et al., 2007). Various schemes have been proposed to enhance the operation speed of SOA-based all-optical devices, for instance, 160 Gb/s and 320 Gb/s wavelength conversion was reported by using a detuned narrow band-pass filter to spectrally select one of the side-bands (blue-shifted or redshifted) of the output signal (Liu et al., 2006, 2007). In this case, the SOA operation speed can be increased via the chirp effect on the SOA output associated with the SOA ultrafast gain dynamics. It has been shown that, the CW modulation response time has been reduced from 100 ps to 6 ps via filter detuning (Liu et al., 2006, 2007). Although using a detuned filter after the SOA can improve the optical signal-to-noise ratio (OSNR) of the output when comparing with the case of using a non-detuned filter (Leuthold, 2002), however the OSNR of the output signal will degrade to a large extent since the optical carrier was suppressed.

Recently, all-optical high-speed switches based on the cascaded SOAs were proposed and demonstrated. In Fig. 1, an all-optical switch incorporating two cascaded SOAs was proposed as an alternative high-speed technique, which was dubbed as "turbo-switch" (Manning et al., 2006; Yang et al., 2006, 2010), while preserving the OSNR of the output signal. An error-free wavelength conversion was demonstrated at 170 Gb/s (Manning et al., 2006). In addition, the operating speed of an all-optical XOR gate was also demonstrated at

High-Speed All-Optical Switches Based on Cascaded SOAs 27

Mørk, 1997), are adopted. Travelling-wave equations in terms of the optical amplitude/power and phase, derived from Maxwell equations and Kramers-Kronig relations, are also incorporated in the SOA model to obtain the amplitude and phase of the output optical signal propagating through the SOA (Mecozzi & Mørk, 1997; Agrawal &

Following the SOA model in (Mecozzi & Mørk, 1997), rate equations for the total carrier density *N* related to the (inter-band) band-filling effect, and the local carrier density variations *nCH* and *nSHB*, which are associated with the ultrafast (intra-band) effects: carrier heating (CH) and spectrum hole burning (SHB) processes respectively, can be expressed as

*Nzt I R N z t v gS z t v g S z t S z t t eV* (1)

0

*CH CH*

 

*n zt n zt Nzt n zt gS z t t a tt* (3)

*n zt n zt gS z t t a* (2)

2 3 *R N AN BN CN* ( ) (4)

*CH* in (2) are carrier-carrier relaxation time and gain

*<sup>g</sup> <sup>N</sup>* (5a)

*SHB* in (3) are temperature relaxation time

(5b)

(,) (,) (,) 

(,) (,) (,) (,) (,)

 *SHB SHB SHB CH*

where the first term in the right hand side (RHS) of (1) represents the increase of the total carrier density due to the injected current *I* to the SOA. Here, we have assumed a uniform distribution of the injected current along the longitude. In (1), *e* is the electron charge, and *V*

The radiative and nonradiative recombination rate due to the limited carrier lifetime in the

where *A*, *B*, *C* represent the linear, bimolecular, and auger recombination coefficients

The third and fourth terms in the RHS of (1) are used to account for the depletion of total carrier density aroused from the stimulation emission by the injected light and the amplified spontaneous emission (ASE), respectively. *vg* is the group velocity. *g* is the gain coefficient and *S* is the photon density in the active region. *gase* is the equivalent gain coefficient for ASE

To take the gain dispersion into account better, and make our model applicable in a wide optical wavelength range, a polynomial model for the gain coefficient (Leuthold et al., 2000), which combines of a quadratic and a cubic function, is used, with one modification to

, ()

2 3

*z*

 

0, ( ) 

*gg N*

 *lh z*

, , ( ) ( ) *gc N d N*

*Nz Nz*

 

0

*SHB SHB*

 

(,) ( ( , )) ( , ) (,) (,) *<sup>g</sup> g ase ase ase*

 *CH CH CH*

is the volume of the active region in the SOA.

SOA, *R*(*N*) (Connelly, 2001), can be approached by,

*CH* and

include the ultrafast effect induced by CH and SHB.

   

suppression factor caused by CH, while

and gain suppression factor caused by SHB.

*SHB* and

Olsson, 1989).

follows:

respectively.

(Talli & Adams, 2003).

85Gb/s, where dual ultrafast nonlinear interferometers (UNIs) were implemented (Yang et al., 2006, 2010) and the turbo-switch configuration was incorporated.

Fig. 1. Schematic setup of the turbo-switch, where the OBF is used to remove the pump signal. OBF: optical band-pass filter.

In this chapter, we will review the recent progress of the all-optical high-speed switches using cascaded SOAs, from both theoretical and experimental aspects. A majority of the publications (Manning et al., 2006, 2007; Yang et al., 2006, 2010) related to turbo-switch were reported, showing the high-speed experimental performances of turbo-switch over a single SOA. Apparently, a systematic theoretical turbo-switch model is necessary for the purpose of understanding the fundamental behaviors of the turbo-switch and how to further enhance the switch performance. First of all, we will present a detailed time-domain SOA model, from which the turbo-switches and switches with three or more cascaded SOAs can be evaluated. For the reason of convenience, we will refer hereafter to this kind of switch, including turbo-switch, as cascaded-SOA-switch. Then, we will focus on the relation between the overall performance of the switch and the nonlinear gain/refractive-index dynamics of the individual SOAs. The amplitude/phase dynamics of the optical output signal from the switch will be analyzed in details and compared with the experimental data. The SOA model will certainly help us not only to understand the basic principles of the switch, but also to exploit the way and the critical conditions for the switch to operate at even higher bit-rates.

The chapter is organized as follows. Section 2 presents a comprehensive theoretical analysis of the cascaded-SOA-switch, where the SOA model and the corresponding simulation method are presented. Simulation results including the gain/phase dynamics, pattern effect mitigation using turbo-switch, are shown in Section 3. Experimental demonstrations of 170 Gb/s AND gate (wavelength conversion) and 85 Gb/s XOR gate using turbo-switches are presented in Section 4. The cascaded-SOA-switches are further exploited in terms of the number of cascaded SOAs in Section 5, where the overall gain recovery time, the noise figure as well as the impact of injected SOA current of the cascaded switches are illustrated in details, as simulated by the model. Finally, conclusions will be given in Section 6.
