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As the bandwidth requirements of board-to-board and rack-to-rack computer interconnects continue to increase, the need for optical solutions that maximize bandwidth per unit area, power consumption, and cost is becoming more urgent.Parallel optics [1,2] and integrated coarse wavelength-division multiplexing (CWDM) [3,4] are two techniques to increase optical link density. The MAUl project, a DARPA supported collaboration between AgilentLabs and the University of Southern California, aims to develop a parallel multiwavelength optical subassembly (PMOSA) that combines parallel optics and CWDM in a compact chip-scale package that has the component density advantages of two-dimensional parallel optics and the connector and cabling density advantages of CWDM.In the present work, we present the fIrst fully-functional 48-channel parallel-wavelength-division-multiplexed (PWDM) transmitter, receiver and link results at a per-channel data rate of 5.21-Gb/s, giving us an aggregate link bandwidth of a quarter terabit per second. Exploded views of the MAUl PMOSA transmitter (TX) and receiver (RX) are shown in fIgure l(a). The base of the PMOSA is a 48-channel driver or receiver IC with wirebond pads around the perimeter for differential digital I/O and power supply. A 4 x 12 array of optoelectronics is flip-chip-bonded in the center, surrounded by a silicon spacer. The TX has four bottom-emitting 1x12 VCSEL arrays, with nominal wavelengths of 990, 1020, 1050, and 1080 nm. The RX has a substrate-illuminated 4x12 PIN photodiode array, with integrated lenses. The PWDM multiplexer and demultiplexer (4A x 12 fIber) are attached to the spacer for optical coupling between the 12 fIbers and 48 optoelectronic devices, and a mechanical lid with MT-ferrule alignment pins sits atop the subassembly. Each PMOSA has a 5 mm x 8 mm footprint and is pre-aligned to mate to a standard 50-llm-core 12-fIber ribbon cable with MT-ferrule termination. Photographs of a TX and RX PMOSA are shown in fIgure l(b) and l(c). (b) (c) arrays Fig 1. MAUl PMOSA: (a) Exploded view of transmitter (left) and receiver (right). (b) Photograph ofTX PMOSA wirebonded to an evaluation board and optically connected to a fiber-ribbon. (c) Photograph ofRX PMOSA on a dime for size reference.The PWDM multiplexer and demultiplexer are photolithographically-defIned micro-optical devices that are based on the same basic optical zig-zag architecture that has been used in single-fIber CWDM demultiplexers [4]. In the PMOSA, however, we have extended this architecture to apply to a 12-fIber ribbon with a 250-llm pitch. Figure 2 shows ray-tracing models of a single-fIber cross-section for the PWDM multiplexer and demultiplexer.