Component Complexity OIN Complexity

OPTICAL PACKET SWITCHES 324 single-priority arbiters and indicates among them the highest priority with its three-line output. The outputs, in addition to being forwarded to the upper layer, will also be used to inhibit the arbiters with lower priority from producing any active grants. A decoder with its outputs masked by the upper-layer grant signal is used to decompose the output of the priority encoder into seven single-line grant signals, each for a single-priority arbiter. Only the arbiter at the corresponding level of priority will receive the upper layer’s grant signal, while all the others will receive nothing but a LOW grant signal.

11.4.6 OIN Complexity

11.4.6.1 Component Complexity

The complexity of the proposed 256 = 256 OIN is determined by the numbers of optical components and of interconnection lines between optical components and between the optical and electronic components. Figure 11.39 lists the optical components used in the OIN with different available wavelengths. The number of SOA gates dominates the component and interconnection complexity. The total number of SOA gates is equal to 256 = 32 when using type-II tunable filters with 16 Ž . wavelengths i.e., W s 16 , or 256 = 24 when using type-III tunable filters. The component complexity of the 256 = 256 OIN with respect to 8, 32, and 64 wavelengths is also shown in Figure 11.39. In addition, Figure 11.39 indicates that the total number of SOA gates of the switching fabrics at the OOMs decreases as the wavelength number W increases. However, the number of SOA gates in the type-II tunable filter is proportional to the number of wavelengths. The total number of SOA gates of the OIN based on the type-II tunable filter is equal to 256 = 256rW q 256W, where the first term represents the number of SOA gates used in the switching fabrics and the second term represents the number of the SOA gates used in type-II tunable filters. It is also shown that the OIN with 16 wavelengths based on type-II tunable filters has the fewest SOA gates. If type-III tunable filters are applied, the OIN with W s 32 or 64 has the fewest SOA gates. Because the on᎐off switching of multiple wavelengths is performed at each SOA gate in the switching fabrics, the signal output of one wavelength varies according to the gain fluctuation induced by modulation of other wavelengths, even when the input power of the wavelength is constant. This is crosstalk induced by the gain saturation in the SOA gate and is often referred as cross-saturation. The impact of the cross-saturation on the performance of the OIN will become severe as the wavelength number passing through the SOA gate increases. To reduce this crosstalk in SOA w x gates, gain-clamped SOA gates 57 are necessary to provide a constant optical gain over a wider range of wavelength power as more wavelengths are used in the OIN To avoid the wavelength-dependent and polarization-dependent loss expe- Ž . w x riencing in the SOA gates, electroabsorption modulators EAMs 58 can be OPTICAL INTERCONNECTION NETWORK FOR TERABIT IP ROUTERS 325 Fig. 11.39 Component complexity of 256 = 256 optical interconnection network with different numbers of wavelengths, W. OPTICAL PACKET SWITCHES 326 Ž . used to perform faster switching ; 100 ps in multiple wavelength levels instead of the SOA gates in the proposed OIN. To compensate for the high coupling loss caused by the EAM, higher-power EDFAs or EAMs integrated w x with semiconductor optical amplifiers will be necessary 59 . Here, the SOA-based OIN is considered because the SOA can provide high gain and Ž . fast switching ; 1 ns simultaneously.

11.4.6.2 Interconnection Complexity