Presentations

HPCの高速化及び低消費電力化へ向けて既存の電気配線に代わる光配線の導入が検討されており，伝送路に同一平面内交差構造を有するポリマー交差光導波路を用いたボード内光配線が注目されている． 　当研究室では，これまでにポリマー導波路を作製する方法として，インプリント法を採用し，強い光の閉じこめ効果を有する屈折率分布(GI)型コア交差光導波路の作製を試みてきた．しかし，このインプリント法では，加工の際に生じるコア-クラッド界面の荒れ等の影響により，直線状導波路であっても十分な低損失化は達成されていない．そこで本研究では，コア側面の界面荒れによる光損失に着目し，インプリント法による低損失導波路の実現を目的とした．

演算性能増大に対応するべく，スーパコンピュータ筐体内やデータセンタ内ネットワークには，光ネットワークの導入が進んでいる．今後は，演算性能に加えて，システムの消費電力の問題から，広帯域・高密度オンボード光インターコネクト技術に期待が寄せられている． 　そこで，我々は，オンボード光インターコネクトの実現に向けて，屈折率分布（GI）型マルチモードポリマー光導波路の導入を提案し，更に，従来のポリマー光導波路とは大きく異なる，新規作製法：（Mosquito法）を考案した． この，Mosquito法の大きな特徴の一つに，容易に3次元配線が実現できる点がある． 本講演では，このMosquito法を紹介し，3次元配線など，可能になった技術について概説する．

Using the Mosquito method, the fabrication condition is investigated for multimode optical waveguides with GI circular cores. When a viscous core monomer with a viscosity of 50,000 cPs and higher is dispensed into a cladding monomer layer through a straight needle, a dip is observed in the upper perimeter of the core. Such a core-shape deviation is a concern to degrade the optical properties such as the coupling loss between an optical fiber and the waveguide. In this report, curved needles are introduced in order to form a perfectly circular core, even when a high-viscosity core monomer is dispensed: the dip could be eliminated by varying the dispensing direction of the core monomer. In fact, we evaluate the dependence of the ratio of the vertical to horizontal axes observed in the elliptical cores on the curved angle of needles after experimentally fabricating the waveguides. It is confirmed that an almost circular core shape (without a dip) is formed when the curved angle is in a range of 15-20°. In addition, the optical properties of the waveguides with and without a dip are reported.

We fabricate a pitch conversion structure in multimode polymer optical waveguides for high density interconnects on printed circuit boards (PCBs). In order to connect with a multimode fiber (MMF) ribbon which has a 250-um pitch, we design the waveguide to have a 250-um pitch on one end, while a 125-um pitch on the other end. A 12-ch. multimode pitch-conversion polymer optical waveguides with 50-um diameter graded-index (GI) circular cores are experimentally fabricated using the Mosquito method. The Mosquito method has a potential for drawing a wide variety of wiring patterns. In the fabricated waveguide, excess loss is not necessarily observed due to the bending structure. Finally, we investigate how to improve the positional accuracy of cores. Then, it is confirmed when the waveguide is connected to a 12-ch. MMF ribbon with 250-um pitch, a good optical coupling is achieved for most of the cores.

To apply to silicon photonics chip packaging, we fabricate a very low-loss single-mode polymer optical waveguide at 1550-nm wavelength using an organic-inorganic hybrid materials. The propagation loss of the single-mode polymer optical waveguide fabricated using the Mosquito method is 0.49 dB/cm at 1550 nm, which is remarkably lower than the loss of polymer optical waveguides composed of conventional organic materials at the same wavelength. Furthermore, for chip-to-chip optical interconnections, the inter-channel crosstalk of a narrow pitch (50-μm pitch) single-mode optical waveguide is evaluated and the measured crosstalk is sufficiently low as -35.0 dB at 1550 nm.

有機―無機ハイブリッド樹脂をソフトリソグラフィ法に適用し，共重合による屈折率分布形成機構を用いてGI型ポリマー光導波路の作製を目指した．得られた光導波路は，従来のアクリル樹脂－低分子ドーパントにて作製されたGI型導波路の場合に見られた伝送特性の経時劣化を抑制でき，また半田リフロー耐熱を有することが実証された．さらに，同じ材料で交差光導波路を作製したところ，一交差あたりGI型が0.0063 dB，SI型が0.0254 dBの損失であると見積もられ， GI型コア特有の強い光の閉じ込め効果による低交差損失化が確認された．