Abstract
We successfully fabricate a polymer optical waveguide with multiple graded-index (GI) cores directly on a substrate utilizing the soft-lithography method. A UV-curable polymer (TPIR-202) supplied from Tokyo Ohka Kogyo Co., Ltd. is used, and the GI cores are formed during the curing process of the core region, which is similar to the preform process we previously reported. We experimentally confirm that near parabolic refractive index profiles are formed in the parallel cores (more than 50 channels) with 40 microm x 40 microm size at 250-microm pitch. Although the loss is still as high as 0.1 approximately 0.3 dB/cm at 850 nm, which is mainly due to scattering loss inherent to the polymer matrix, the scattering loss attributed to the waveguide's structural irregularity could be sufficiently reduced by a graded refractive index profile. For comparison, we fabricate step-index (SI)-core waveguides with the same materials by means of the same process. Then, we evaluate the inter-channel crosstalk in the SI- and GI-core waveguides under almost the same conditions. It is noteworthy that remarkable crosstalk reduction (5 dB and beyond) is confirmed in the GI-core waveguides, since the propagating modes in GI-cores are tightly confined near the core center and less optical power is found near the core cladding boundary. This significant improvement in the inter-channel crosstalk allows the GI-core waveguides to be utilized for extra high-density on-board optical interconnections.
Highlights
As computing performance has continued to evolve rapidly, optical interconnections including on-board communications have been drawing much attention [1,2]
Multimode polymer optical waveguides integrated on printed circuit boards (PCBs) have been one of the potential candidates for feasible, low-cost interconnection media
We demonstrated that the parabolic refractive index profiles (GI cores) led to excellent optical properties: reduction of the scattering loss due to the imperfection on the core-cladding boundary, high-speed transmission capability, and low inter-channel crosstalk under very narrow pitch (−37 dB in 120 μm pitch)
Summary
As computing performance has continued to evolve rapidly, optical interconnections including on-board communications have been drawing much attention [1,2]. Multimode polymer optical waveguides integrated on printed circuit boards (PCBs) have been one of the potential candidates for feasible, low-cost interconnection media. The polymer optical waveguides have rectangular-shaped cores with step refractive index (SI) profile, because they have been fabricated by means of the conventional photo lithography or imprinting processes. In the case of poly methyl methacrylate (PMMA)-based polymer waveguides, heat-drawing is carried out at a temperature of 200 to 250 °C, which means that the PMMA-based waveguides obtained would be unstable for the lamination and solder reflow processes of PCBs carried out at 200 to 260 °C. In order to address these problems, in this paper, we utilize a UV-curable and crosslinkable monomer applicable to the imprinting process, from which the GI-core waveguides can be directly fabricated on-board. As it is possible to fabricate a normal step-index (SI)-core waveguide with the same material system by adjusting the fabrication process, the advantages of GI-core waveguides over their SI-core counterparts are demonstrated
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