Low-loss transmission of multimode polymer optical waveguides

To deal with the exponential growth in the demands for data storage, processing, and accessibility, it is necessary for current data centers and high-performance computer systems to evolve. For example, new architectures are required to relieve bottlenecks in system bandwidth density. Optical interconnects are one promising solution to this problem because of their high transmission rate, high bandwidth density, and low power consumption compared with conventional electrical wiring. In addition, to achieve the necessary bandwidth breakthrough, electrical and optical signals need to be converted as accurately as possible in large-scale integrated circuits (ICs).

Package-board interconnects in current hierarchical packaging configurations tend to have low wiring capacities and they therefore limit the bandwidth density of multichannel optical interconnections. It has previously been demonstrated, however, that chip-scale optical transceivers—connected to a large-scale integration (LSI) device in the same package—can be used to enhance the bandwidth density. This approach can also be used to simultaneously reduce the optical channel pitch.

In our work,¹ we present the fabrication of a new silicon-based polymer optical circuit that includes a waveguide, mirror, and connector on an electrical LSI package (see Figure 1). With our hybrid optical/electrical LSI package we can achieve a high bandwidth of 2.4Tb/s and parallel multimode optical links at 1.3μm. The package also includes our novel optical input/output (I/O) module.² This I/O module consists of a silicon photonics transmitter and receiver system (on a silicon substrate), into which we have integrated a driver IC in the optical transmitter and a transimpedance amplifier in the optical receiver. Our optical I/O module for the multimode fiber (MMF) links thus provides both high-performance levels and easy coupling. Indeed, we have previously shown that our module can be used for high-speed (25Gb/s) MMF transmission (up to a distance of 300m) at a wavelength of 1.3μm for almost all conventional applications.³

Figure 1. Photograph of the experimental setup used to test the optical characteristics of the silicon (Si)-based polymer optical circuit. GI50 MMF: 50μm graded-index multimode fiber.

The substrate of our electrical package has an area of 50 × 50mm and is built from six stacks of electrical circuits and electrodes for the optical I/O module operation. The optical layer comprises 96-channel multimode polymer optical waveguides, a selectable-angle mirror, and an optical card-edge connection to the MMF. Our waveguides (with a 125μm pitch array) have three layers—i.e., under-cladding, a 35 × 35μm multimode core, and over-cladding—and their fabrication involves a number of steps. First, we deposit the transparent silicon-based polymer material directly onto the substrate and we use conventional UV photolithography to form the core and under-cladding layers (the refractive index of the core is 1.58 and the index ratio is 2.8%). We also use a dicing saw (with an angled blade) and a metal deposition technique to form the angle mirror in our waveguides. In the next step, we use spin coating and UV photolithography to form the top cladding of the waveguides. Finally, we use conventional UV photolithography (with photosensitive materials) to form the alignment pins directly on the surface of the hybrid package of the optical waveguides.

To estimate the optical characteristics of our silicon-based waveguide, mirror, and connector systems, we used a 50μm graded-index MMF. Our results indicate that the propagation loss of the multimode silicon-based waveguide is 0.3dB/cm at 1.3μm. We also evaluated the stability of the assembly to the solder-reflow process at a temperature of about 250°C, and we find that the propagation loss is almost the same before and after heating. Furthermore, we measured the bending loss of our silicon-based waveguide and obtained a minimum bending radius of 5mm (which generated 0.2dB of additional loss). In addition, we measured loss from butt-coupling and mirror-coupling of the polymer optical waveguide to the MMF as 0.4 and 1.1dB, respectively. With a passive alignment of the connector, the excess coupling loss was below 1dB. We also measured the optical characteristics of our LSI package substrate when our silicon photonics transmitter was integrated with a vertical polymer waveguide.⁴ On the basis of the output beam properties, we found that the characteristics of the insertion loss, tolerance, and crosstalk were almost the same as for the MMF-input case.

For comparison, we have also investigated the use of an epoxy-based polymer optical waveguide on an electrical LSI package substrate (integrated with an optical mirror and optical card-edge connector). With this setup we realized 25Gb/s error-free transmission at 1.3μm. At this wavelength, however, the propagation loss of the epoxy-based polymer optical waveguide was large (0.6dB/cm) compared with the silicon-based polymer waveguide (i.e., 0.3dB/cm). The topology of an epoxy-based optical circuit is therefore limited because of this large propagation loss. That is, the I/O port size of the LSI and package substrate has to be large to achieve high-bandwidth operation and low power consumption because of the required short optical waveguide.

In summary, we have developed and presented the fabrication of a new high-bandwidth hybrid optical/electrical LSI package. Our silicon-based polymer optical circuit features three-layer multimode waveguides, a selectable-angle mirror, an optical card-edge connector (for connection to an MMF), and our in-house high-performance I/O module. With this package we achieve high-bandwidth transmission (at 1.3μm) of 2.4Tb/s. The propagation loss is low (0.3dB/cm) compared with an epoxy-based polymer optical-waveguide system. In the next stages of our work we plan to use our approach to reduce the footprint of I/O ports and to help reduce bottlenecks that currently slow the improvement of high-performance computer systems.

This research was partly supported by the New Energy and Industrial Technology Development Organization.