本研究では，マルチコアファイバ（MCF）と複数のシングルコアファイバ（SCF）との結合素子となるfan-in/out（FIFO）素子をポリマー光導波路にて実現する．我々が考案したMosquito法を用い，3次元コア配線による新規構造の光導波路型FIFOを作製した．はじめにコア配置と曲げ構造の検討を行い，FIFO導波路の低損失化を目指した．
曲げ半径を小さくしつつ曲げ損失を低減するには導波路の高NA化が望まれる.はじめにコア径35μmのGI型円形マルチモードコア導波路に対して，90°曲げ構造を形成した際に，曲げ半径2mm以下で1dB以下の曲げ損失となるためのNAを，光線追跡法を用いて計算した．その結果，NAは0.35以上が望ましいことが示された．そこで，実際にNAが0.35以上となるコア・クラッド材料の組み合わせにて，鉛直方向90°曲げ(3D)マルチモードGI円形コア導波路を作製した．曲げ半径は設計通りの2mmで作製でき，曲げ損失，結合損失を含む導波路の挿入損失＝ 2dBを達成した．
続いてFIFO導波路向けにシングルモードコアによるS字曲げ構造とその際の導波路挿入損失の関係をBPM法にて計算した．その結果，S字構造を余弦関数（コサインカーブ）にて近似することで，コア曲げの曲率半径の変化を低減でき，低曲げ損失化されることが明らかとなった．
以上の計算検討にてコアの曲げ半径と損失との関係を明らかにしたところで，低曲げ損失となるS字曲げ構造の3次元FIFO導波路を設計し，Mosquito法にて10mm長FIFO導波路を実際に作製した．コアのS字曲げ部分には，計算結果に基づきコサインカーブを適用した．
作製した導波路の挿入損失値を，波長1550nmにて測定したところ，4コアすべてで2dBを下回る低挿入損失化が達成された．ニードルが通過すべき目標点を補間する様にニードルの走査プログラムを改良することで，コアがより設計値に近い理想的なコサイン曲げ構造にて吐出され，曲げ損失を低減したことが低損失化を可能にしたと考えられる．
In this research, a fan-in / out (FIFO) device, which is a coupling element between a multi-core fiber (MCF) and multiple single-core fibers (SCF), is realized with a polymer optical waveguide. Using the Mosquito method we developed, we created an optical waveguide type FIFO with a unique structure composed of three-dimensional core wiring. First, we investigated the core arrangement and bending structure, and aimed to reduce the loss of the FIFO waveguide.
In order to reduce the bending loss even under a small bending radius, it is required to increase the NA of the waveguide. First, when a 90 ° bent core structure was formed by GI type circular multimode cores with a core diameter of 35 μm, the required waveguide NA was calculated using the ray tracing method for a bending loss of 1 dB or less under a bending radius smaller than 2 mm. As a result, it was confirmed that an NA higher than 0.35 was required. Following to de design, we created multimode GI circular core waveguides with a vertically 90 ° bent cores by selecting the core and cladding materials appropriate for providing an NA higher than 0.35. The bending radius was 2 mm as designed, and the insertion loss of the waveguide including bending and coupling losses was 2 dB. Next, the relationship between the S-shaped bending structure formed by a single-mode core and the waveguide insertion loss was calculated by the BPM method for FIFO waveguides. Then, it was revealed that by approximating the S-shaped structure by a cosine function (cosine curve), the variation of the radius of the core bending can be reduced resulting in obtaining very low bending loss.
When the insertion loss of the fabricated waveguide was measured at a wavelength of 1550 nm, an insertion loss as low as 2 dB was achieved in all four cores. By improving the needle scanning program so that the target points to be passed by the needle scan should be interpolated, the core monomer is dispensed on an ideal cosine bending trajectory as close to the design structure as possible. Hence, the bending loss is reduced, to obtain a FIFO waveguide with lower insertion loss.