Input Optical Module Output Optical Module Tunable Filters

OPTICAL PACKET SWITCHES 310 Fig. 11.25 256 = 256 optical interconnection network.

11.4.4.1 Input Optical Module

The IOMs carry packets at 10 Gbitrs. At Ž . Ž . each IOM, distributed Bragg reflector DBR or distributed feedback DFB laser arrays can be used as the laser sources between 1525 and 1565 nm to match the gain bandwidth of commercially available EDFAs. Each EDFA can amplify multiple wavelengths simultaneously. Each input link of an IOM is connected to a laser with fixed wavelength. To improve the chirp performance, a DFB laser diode integrated with an Ž . w x external modulator EM operating at 10 Gbitrs has been fabricated 52 . To ensure output power levels and chirp performance, an SOA and EMs can be w x integrated with the DFB laser arrays 53 . This monolithically integrated WDM source is able to provide multiwavelength capability and significantly reduce the cost per wavelength. In addition, it can also eliminate the alignment of fibers to individual lasers, reduce component count and cou- pling loss between components, and increase the reliability. OPTICAL INTERCONNECTION NETWORK FOR TERABIT IP ROUTERS 311 Fig. 11.26 Control in the jth output optical module.

11.4.4.2 Output Optical Module

Each 16 = 16 switching fabric should be capable of connecting simultaneously two or more IOMs to all tunable filters at an OOM, so it needs to have broadcast capability and to be strictly nonblocking. As shown in Figure 11.26, a space switch can meet this require- ment simply and can be constructed by using SOA gates. Ž . In addition to their fast switching function ; 1 ns , SOA gates can provide some gain to compensate the coupling loss and splitting loss caused by the splitters and combiners and the connection between discrete optical devices. Furthermore, SOA gates can be monolithically integrated with the passive couplers to enhance the reliability and loss performance between components.

11.4.4.3 Tunable Filters

Tunable filters are used to perform wavelength selection in the OIN. Three possible ways to implement the tunable filter are considered here. OPTICAL PACKET SWITCHES 312 Fig. 11.27 Type-I tunable filter. 䢇 Type-I Tunable Filter: The type-I tunable filter, as shown in Figure 11.27, performs the wavelength selection in the electrical domain. Each output of a 16 = 16 switching fabric is connected to a 1 = 16 AWG Ž . router, which is made from high-index indium phosphide InP material and is capable of demultiplexing 16 wavelengths in the 1550-nm window. Figure 11.28 shows the connectivity of a 16 = 16 AWG router. For example, if the WDM signal enters the 7th input port of the AWG Ž . router, only the 14th wavelength ␭ will be sent out through the 8th 14 output port. Each demultiplexed wavelength is detected through a high-speed signal detector. Each detector has a laser waveguide struc- ture and can be monolithically integrated with the AWG router, thus increasing the reliability and reducing the packaging cost of the AWG router. Finally, a 16 = 1 electronic selector is used to select the desired signal from the 16 detectors. The selector is controlled by the 4-bit control signal from the PAU. An alternative electronic selector is the Ž . w x InP-based optoelectronic integrated circuit OEIC receiver array 54 , which operates at 10 Gbitrs per channel and integrates 16 p᎐i᎐n Ž . photodiodes with a heterojunction bipolar transistor HBT preampli- fier. 䢇 Type-II Tunable Filter: The type-II tunable filter, as shown in Figure 11.29, performs the wavelength selection optically. It has two AWGs. The first one, 1 = 16, performs the demultiplexing, while the second, 16 = 1, performs the multiplexing. By properly controlling the SOA gates, only one of the 16 wavelengths is selected. The selected wave- length passes through the second AWG and then is converted into an electronic signal by a detector. A PLC᎐PLC direct attachment tech- w x nique 55 can be used to construct this type of tunable filter and to OPTICAL INTERCONNECTION NETWORK FOR TERABIT IP ROUTERS 313 Fig. 11.28 16 = 16 AWG router connectivity. Fig. 11.29 Type-II tunable filter. integrate the AWG routers and the SOA gates. This hybrid integration of PLC and SOA gates can reduce the coupling loss and increase the reliability. 䢇 Type-III Tunable Filter: The type-III tunable filter, as shown in Figure 11.30, performs the wavelength selection optically. In contrast with the type-II filter, it uses only one 16 = 16 AWG router. Any one of the 16 wavelengths can be selected through its specific combination of SOA OPTICAL PACKET SWITCHES 314 Fig. 11.30 Type-III tunable filter. Fig. 11.31 Connectivity of type-III tunable filter. w x gates at the input and output sides of the AWG router 56 . Figure 11.31 shows a way to choose any one of the 16 wavelengths. The 16 = 16 Ž . AWG router will route a wavelength ␭ from input port x x s 1, . . . , 16 k Ž . Ž . to output port y y s 1, . . . , 16 , where k s x q y y 1 mod 16. For example, ␭ will be selected as the output by turning on the 3rd SOA 7 gate at the input side and the 5th SOA gate at the output side of the AWG router, respectively. The number of SOA gates in the type-III Ž tunable filter is reduced by half; only 8 SOA gates 4 at the input and 4 OPTICAL INTERCONNECTION NETWORK FOR TERABIT IP ROUTERS 315 . at the output are used instead of 16 SOA gates in the type-II tunable filter. However, compared to type-I and type-II tunable filters, the type-III tunable filter has more power loss, caused by the 1 = 4 splitter and the 4 = 1 combiner.

11.4.5 Ping-Pong Arbitration Unit