3. All-optical logic gates with SOA

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

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,

> <sup>1</sup> � <sup>2</sup><sup>R</sup> δð1 � RÞ I

¼ vg αint þ

q � Nth τe 

1 <sup>L</sup> ln <sup>1</sup> R

ð1Þ

ð2Þ

separate laser cavities and can have two states [7]. It is shown in Figure 2 [7].

Psw ¼ E

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

282 Optical Fiber and Wireless Communications

Nth <sup>¼</sup> <sup>V</sup>

end facets of lasers R and do not influence the memory states.

ports depending on the header information [3].

vgRτ<sup>p</sup> Lð1 � RÞ

τpΓvga

ln <sup>1</sup> R 

þ N0;

1 τp

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

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

respectively [7]:

In this section, we briefly discuss the scheme and operation principle of all-optical gates which are core logic units for the all-optical signal processing system implementation [9]. All-optical gates may be divided into two groups: without SOA and with SOA [9]. The all-optical gates without SOA are based on the change in nonlinear refraction index in silica fiber [9]. The intensity-dependent refractive index of silica results in the following nonlinear optical effects: SPM, cross gain modulation (XGM), and FWM [2, 9]. The all-optical gates without SOA based on these nonlinear optical phenomena can be realized in the following configurations: dispersion shifted fiber/high nonlinear fiber (DSF/HNLF) configuration; waveguide configuration; circular configuration; optical channel-dropping (C/D) filter configuration; multilayer waveguide configuration; double heterostructure optical thyristor (DHOT) configuration; and acousto-optical tunable filter (AOTF) configuration [9]. The detailed description and comparison between non-SOA gates are presented in Ref. [9]. For instance, DSF/HNLF, waveguide, circular, and AOTF configurations are polarization sensitive; DSF/HNLF, waveguide, circular configurations are characterized by bad or moderate integration capacity [9].

On the other hand, the SOA-based devices are mainly polarization non-sensitive and possess compact integration capacity [9]. They are highly competitive due to the high nonlinearity, low switching power, wide gain bandwidth, and compact size [8]. Recently, novel two inputs optical logic gates (NOT, AND, OR and NOR) based on a traveling wave SOA (TW-SOA) operating at 40 Gb/s had been demonstrated [10].

The implementation of all-optical gates with SOA is based on the different interferometer techniques such as ultra-high nonlinear interferometer (UNI), Sagnac interferometer (SI), MZI, and delay interferometer (DI) [9]. In these techniques, the XPM-induced phase shift is used for optical switching [4]. Typically, a weak signal pulse is divided equally between two arms of the interferometer and is undergoing identical phase shift in each arm [4]. In such a case, it is transmitted through constructive interference [4]. Consider now the situation when a pump pulse at a different wavelength as compared to the signal pulse is injected into one arm of the interferometer. As a result, the signal phase in that arm would be changed due to XPM. If the XPM-induced phase shift is close to π, the signal pulse will not be transmitted due to the destructive interference at the interferometer output [4]. An intense pulse pump can switch the signal pulse through the XPM-induced phase shift [4].

In particular, all-optical gate based on SOA-MZI can be realized with the copropagating, counterpropagating, and copropagating push-pull configurations [9]. Copropagation MZI operates on the principle of phase change caused by the light propagating through the 3 dB coupler [9]. MZI copropagating gates consist of a symmetrical MZI with two SOA placed in the upper and lower arm of the interferometer [9]. Data and clock pulses of different wavelengths are inserted into SOA operated under the gain saturation condition, where the optical gain is distributed between wavelengths according to their relative photon densities [9]. The data are transferred in the clock pulse in the inverted form in both arms of MZI [9]. After passing through the first 3 dB coupler, the phase difference π/2 is created between the upper and lower arms of clock pulse, after passing through the second 3 dB coupler the total phase shift becomes π [9]. Then, if both data have the same value, they will cancel, and at the T-port 0 will appear, if data have different value, then it will not cancel and 1 will appear at the T-port [9]. In MZI counter-propagating gates, the clock and data pulse propagate in opposite directions through MZI [9]. If any of the data is 1, then XPM between the clock and data pulse inside SOA creates the differential phase shift between the two clock components, MZI becomes unbalanced, and the clock pulse exits at T-port [9]. If both the data are the same, the total phase shift will become π, and the clock pulse is cancelled at T-port [9].

Consider now a typical all-optical logic element based on transforming of XPM into an intensity modulation and implemented as the MZI copropagating push-pull gate with SOA in the two arms shown in Figure 3 [9]. The optical fibers are used as interconnects. The SOA-based MZI with couplers at the input and output is shown in Figure 4. At the SOA-based MZI block output, there is a coupler shown in Figure 4. The outputs at the right side of this coupler are connected to the T-port and R-port shown in Figure 3.

Note that the co-propagating data streams configuration permits to avoid the SOA length restriction, and the MZI with push-pull configuration allows increasing the memory bit-rate beyond the limitation of the SOA carrier recovery time [8]. The copropagating data streams A and B of the same wavelengths are inserted into upper and lower arm of MZI shown in Figure 4.

The data A in the upper arm is ahead of one bit period to data B traveling in the lower arm of MZI, and the lower arm data B is one bit period ahead to upper arm data A [9]. As a result, a

Figure 3. MZI with push-pull configuration.

Figure 4. SOA–based MZI.

switching window for data streams occurs [9]. The clock pulse copropagating with the data streams A and B is inserted into the 3 dB coupler. Assume that the data A is 1 and data B is 0. Then, the pulse from data A splits into two parts in such a way that one pulse is pushed to the upper SOA 1 and other is delayed by the switching window. Consequently, the upper SOA 1 is switched before the lower SOA 2 [9]. The MZI is unbalanced, and clock wave is switched to the T-port. In the opposite case, when data A is 0 and data B is 1, then lower SOA 2 is switched and wave also appears at T-port [9]. Assume now that data A and data B are the same. As a result, SOA 1 and SOA 2 are equally influenced by the injected pulse. The respective push and pull pulse temporarily coincide with each other, the phase difference between the two arms of MZI equals to zero, and no switching occurs at T-port [9].

The disadvantages of the all-optical logic gates discussed above are twofold: (i) the operating speed of SOA is limited by the carrier recovery time of the order of magnitude of 100 ps; (ii) the scheme can be used only for OOK modulation format [8]. However, the SOA operation rate can be significantly increased up to 100 Gb/s by using the quantum dot semiconductor optical amplifiers (QD-SOA) [8, 11]. A theoretical model of an ultrafast all-optical signal processor based on QD-SOA MZI has been developed with limiting bit rates of 100 and 200 Gb/s at the injection currents of I = 30 mA and I = 50 mA, respectively [12].
