Presentations

当研究室では,ボード内光配線素子として,GI型円形コアポリマー並列光導波路を試作し,その優れた光学特性を示してきた.その中でも,ディスペンサを用いるMosquito法は,フォトマスク不要の「プリンテッドフォトニクス技術」の一つと位置付けることができる.このMosquito法を汎用化するためには,複数種類の樹脂材料が適用可能であることが望ましい.そこで,本研究では,最も光学的透明性の高いアクリル系樹脂の,Mosquito法への適用可能性について検討した.

25 Gbpsを超える伝送速度にて,ボード上に光配線を実現するためには,ポリマー光導波路には,今後さらなる高速・高密度配線化が求められ,加えて,光導波路リンク全体として,さらなる低消費電力化へむけた技術の進展が求められる.GI型ポリマー光導波路は,オフボード配線に用いられているGI型MMFと類似の構造をもつことから,分散,接続損失,高密度配線性などの点で,優れた特性を発現することがわかってきた.さらに,複数チャネルによる自由な光回路構成に必要とされる交差導波路についても,GI型コアの導入により,低損失化されることを実証した.このGI型コアポリマー光導波路を用いた高速・高密度オンボード光インターコネクトの実現を期待する.

本研究ではポリマー導波路－MMFにより構成される光リンクに対して，異なる構造を有する導波路とMMFとの接続損失評価を行った．その結果，従来の段階屈折率(SI)型矩形コアを有するポリマー光導波路については，MMFのコア径(50 m)よりも小コアサイズ(~35 m)化することで，導波路－MMF，MMF－導波路いずれの接続損失も低減されるという結果を実験的・理論的に確認した．一方で，当研究室にて，これまで検討を進めてきた屈折率分布(GI)型ポリマー光導波路は，コア形状(矩形・円形)に関わらずSI型矩形コア導波路に比べMMFとの接続損失を低減でき，さらにコアサイズ依存性が小さく，ミスアラインメントトレランスも大きいという，優れた接続特性を示すことを明らかにした．

In this paper, we both theoretically and experimentally demonstrate that 90 % lower optical loss is realized with multimode crossed polymer optical waveguides by introducing graded-index (GI) cores, compared to conventional step-index (SI) core counterparts. First, the light leakage losses of crossed waveguides with SI and GI cores are simulated using a ray-trace simulation. Then, we show that a drastic reduction of the leakage loss is achieved in the GI-core crossed waveguide (only a loss of 0.095 dB for 50 perpendicular crossings), which is less than one-tenth of the leakage loss in the SI-core crossed waveguide (2.5-dB loss for the same 50 perpendicular crossings). Furthermore, even in the GI-core crossed waveguide with a 30-degree crossing angle, the leakage-loss is calculated to be only 0.009 dB/crossing, while 0.086 dB/crossing for the SI-core. This low loss of GI crossed core waveguides stems from the tight optical-field confinement in GI cores. Next, we confirm the validity of the calculated results. We fabricate the GI-core polymer crossed waveguides (multiple crossings) using the photo-address method for the first time to the best of our knowledge. In this method, we use a polynorbornene resin whose refractive index could be controlled by intensity variations of UV exposure during the curing process. The experimental results in the crossed waveguide with GI profile show a very good agreement with the calculated result.

本研究では，250 mのピッチを有する既存の一般的なMMFリボンやVCSELアレイとの接続を想定し，ディスペンサ法を用いて，250 mにピッチを制御されたGI型円形コアポリマー光導波路を作製する手法を確立することを目的とした．

We introduce a simple photo-mask-free fabrication method of multimode polymer optical waveguides (PPOWs): “the Mosquito method” that utilizes a micro-dispenser. In the Mosquito method, a viscous monomer for cores is directly dispensed from a thin needle into a cladding monomer that is coated on a substrate. Because of the monomer diffusion, graded-index (GI) circular core is formed. By adjusting several parameters to control the core shape and diameter, a 15-cm long, 40-m core diameter, with 125-m pitch GI-circular core PPOWs (12 channels) are successfully fabricated by the Mosquito method. We experimentally demonstrate superior optical properties of the circular GI-core polymer waveguides: connection loss with GI multimode fibers, inter-channel crosstalk, etc. to SI-square core polymer waveguides composed of the same silicone polymer materials. Furthermore, we fabricate GI-core PPOW circuits in which perpendicularly curved waveguides are involved by applying the Mosquito method. The curved PPOWs with a 250-m pitch are obtained successfully and the curve-radius is varied from 3 mm to 20 mm by adjusting the scanning program of the needle. The insertion loss of the curved waveguides is measured for evaluating the bending losses. The waveguide with lager curve-radius shows lower insertion loss, namely low bending loss. Additionally, it is experimentally confirmed that the curved waveguides with higher numerical aperture (NA = 0.30) fabricated utilizing a different silicone resin for the cladding exhibit a bending loss as low as 0.5 dB even under 3-mm curve radius. The Mosquito-method would be a promising method for realizing high-speed and high-density on-board optical interconnects. Keywords: Optical interconnections, multimode polymer parallel optical waveguide, optical printed circuit board, bending waveguide, Graded-index core, circular core, micro-dispenser

We theoretically analyze the origin of inter-channel crosstalk in high-density multitude parallel optical waveguides for on-board interconnects using the Beam Propagation Method. In this paper, we demonstrate that inter-channel crosstalk due to mode coupling

Optical printed circuit boards (OPCBs) on which multimode polymer optical waveguides are integrated have been strongly anticipated in order to realize high-speed on-board interconnects. On the OPCBs, the surface mount technique is applied to laser and photo-diode (PD) chips, and the output/input light from/to those devices needs to be perpendicularly bent to utilize the waveguides on the OPCB. Current trends are in perpendicular light path conversion by 45-degree reflector mirrors formed at waveguide ends. By utilizing a ray-trace simulation program developed in this research, the optical loss at the reflector mirrors formed on both SI and GI waveguides is calculated.

As an innovative fabrication method of multimode polymer parallel optical waveguides (PPOWs) with graded-index (GI) circular cores, “the Mosquito method” that utilizes a micro-dispenser is introduced. We experimentally demonstrate that a 5-cm long, 40-m core diameter, and 250-m pitch GI-circular core PPOWs (12 channels) fabricated by the Mosquito method show an inter-channel crosstalk of approximately -50 dB, which is almost 10 dB lower than that of a typical SI PPOW with square cores. Furthermore, we fabricate GI-core PPOW circuits in which curved and 3-dimensional waveguides are involved by applying the Mosquito method.

We propose to apply graded-index (GI) cores into crossed multimode waveguides. We already confirmed the strong optical confinement effect of GI cores, and thus, GI cores enable to decrease the light leakage at the crossed area, compared to the step-index (SI)-core crossed waveguides. In fact, we calculate the light propagation loss in crossed waveguides with SI and GI cores by developing a ray tracing simulation program. It is found that the GI core shows remarkably lower waveguide loss (0.19 dB for 50-perpendicular crosses) than those of SI core (3.7 dB for the same crossings).

As an innovative fabrication method of polymer optical waveguides, the “Mosquito Method” is introduced, which utilizes a dispenser to extrude a viscous monomer on substrates directly. By optimizing the extruding conditions, parallel waveguides with circular GI-cores are successfully fabricated using the Mosquito Method. The optical characteristics of the polymer parallel optical waveguides fabricated by the Mosquito Method are also discussed.

ハイエンドサーバーのさらなる情報処理能力,低消費電力化に向けて,光配線板が提案されている.光配線板では,基板上に表面実装された光源・受光素子と光導波路との間に直角光路変換が必要であり,導波路端に45°ミラーを加工する手法が多く検討されている.本研究では,光線追跡シミュレーションにより,光導波路の屈折率分布,開口数(NA)やミラーの角度トレランスなどに対するミラー損失の予測を行い,ミラー損失を低減する光導波路構造についての検討を行った.結果として,ミラー反射後の光線でもGI型光導波路特有の強い光の閉じ込め効果を発揮し,SI型よりもギャップロスの低減が可能となった.

HPCのさらなる高性能化を見据え,低消費電力,高速・高密度伝送を実現できる光インターコネクト技術の導入が期待されており,多モードポリマー光導波路を用いた光配線板が検討されている.本研究では,オンボード+ボード間インターコネクトを想定した導波路ベースのリンクに対して,異なる伝送路感の接続損失に着目した.その結果,従来の研究開発対象であったSI型矩形コアポリマー光導波路においては,高密度配線化に向けた小コアサイズ化が,ボード間を接続するGI-MMFとの接続特性を改善させる効果が確認された.また,我々が検討してきたGI型円形コア導波路は,小コア径化した場合に対してもSI型コア導波路に比べて,接続特性に優れる結果が得られた.