Published papers

屈折率分布型円形コアを有するポリマー並列導波路は，従来の均一屈折率コア型導波路に比べて，屈折率分布によりコア部への光の閉じ込め効果が強く，従って，より高密度に並列配線を施しても，チャネル間のクロストークが十分に低減できることを実験的に明らかにした．また，コア部への光の入射位置ずれに対しても許容が大きく，低クロストークを保てることを確認した．

ハイエンドルータやサーバ，HPCなどの筐体内光インターコネクションを実現する低損失で高密度・高速伝送ポリマー並列光導波路を初めて提案した論文である．従来型のポリマー導波路は半導体プロセスによる作製法をベースとしているため，コア部が矩形で均一屈折率コアを有するが，本提案の導波路は，円形で屈折率分布型コアを有する．このため，従来導波路に比べ，十分な低損失性と広帯域性，さらに，強いコア部への光閉じ込め効果を有することを示した．

We theoretically analyze the origin of inter-channel crosstalk in densely aligned multimode parallel optical waveguides for on-board interconnects using the Beam Propagation Method. In this paper, we demonstrate that the inter-channel crosstalk due to mode coupling is very low in graded-index (GI) circular-core waveguides because the propagation constants of the propagating modes are discrete. Additionally, it is also found that the waveguides with GI-type circular cores is sensitive to the optical confinement in the cladding: low-power cladding modes largely decrease the mode conversion.

We successfully fabricate polymer optical waveguides with graded-index (GI) circular cores whose diameter and interchannel pitch are accurately controlled using the Mosquito method: GI-core waveguides with 250-, 125- and 62.5-μm pitches are successfully obtained. The Mosquito method utilizing a microdispenser is a very simple technique for fabricating GI-circular-core polymer optical waveguides. The accurately controlled pitch is confirmed by a high connectivity with a commercially available multimode fiber (MMF) ribbon with a 125-μm pitch. Furthermore, by inserting the waveguide between two 12-channel MMF ribbons, we experimentally demonstrate 11.3 Gbps × 12 Ch. parallel signal transmission through a GI-core waveguide with a 125-μm pitch for the first time to the best of our knowledge.

We fabricate polynorbornene-based graded-index (GI) core polymer optical waveguides using the photoaddressing method, and demonstrate that the GI-core polymer waveguides are capable of realizing high-bandwidth-density on-board optical interconnects. Because of the tight optical confinement effect of the parabolic refractive index profile, the fabricated GI-core polymer waveguides exhibit superior optical properties compared to stepindex (SI) core polymer waveguides: low propagation loss, low connection loss with GI multimode fiber (MMF) with wide misalignment tolerance, and low interchannel crosstalk even under a small pitch size are experimentally confirmed. Furthermore, interchannel crosstalk due to multicore operation is experimentally evaluated, and a method to estimate the interchannel crosstalk in multimode polymer parallel optical waveguides under multicore operation is proposed for the first time, to the best of our knowledge. The photoaddressing method capable of fabricating even low-loss GI-core polymer waveguide circuits will pave the way for realizing low-cost and high-bandwidth-density optical printed circuit boards.

We demonstrate an amplitude-based bending/displacement sensor that uses a plastic photonic bandgap Bragg fiber with one end coated with a silver layer. The reflection intensity of the Bragg fiber is characterized in response to different displacements (or bending curvatures). We note that the Bragg reflector of the fiber acts as an efficient mode stripper for the wavelengths near the edge of the fiber bandgap, which makes the sensor extremely sensitive to bending or displacements at these wavelengths. Besides, by comparison of the Bragg fiber sensor to a sensor based on a standard multimode fiber with similar outer diameter and length, we find that the Bragg fiber sensor is more sensitive to bending due to the presence of a mode stripper in the form of a multilayer reflector. Experimental results show that the minimum detection limit of the Bragg fiber sensor can be as small as 3 μm for displacement sensing.

The connection loss which occurs when light transfers from one medium to another attracts additional attention in highspeed and low-power optical links. In this paper, we not only experimentally apply graded index polymer optical waveguide (GIPOW) in the optical link, but also theoretically address some key parameters which largely influence on the connection loss. The experimental setup of this optical link is composed of a VCSEL module, a GI polymer optical waveguide for the transmitter side(WGTx), a GI multimode fiber (GI-MMF), a GI polymer optical waveguide for the receiver side (WGRx), and a detector. We assume that WGTx is launched by a single mode fiber and then measure the connection losses at the interface between any two components. The results show the connection loss between the GI-MMF and the WGRx is approximately 0.4 dB higher than that of the WGTxto theGI-MMF. For the theoreticalmodeling, we start from the fundamental scalar wave equation, and apply the finite difference beam propagation method (FD-BPM) to simulate the behavior of light inside the waveguide. Furthermore, Fresnel reflection between two media and the scattering effect within polymer optical waveguides are also taken into account. The simulated results show a similar trend to the measured values. Finally, we conclude that the scattering effect could be one of the key issues of GI-POW for maintaining low connection losses with other waveguides and fibers.

A simple fabrication method for multimode polymer optical waveguides with graded-index (GI) circular cores is introduced for use in optical printed circuit boards (O-PCBs). The new method, named “Mosquito method,” utilizes a microdispenser to dispense a viscous monomer directly onto the substrates. By optimizing the dispensing conditions, 12-channel parallel waveguides with circular GI-cores (core diameter of 40 μm) are successfully fabricated using the Mosquito method. The advantages of GI-core waveguides for O-PCB applications are discussed by comparing the optical characteristics of the fabricated waveguides with those of conventional step-index (SI) square-core polymer waveguides, and even with those of silica-based GI multimode fibers (MMFs), as an ideal case. To the best of our knowledge, this is the first comparison of SI- and GI-core multimode polymer waveguides that are composed of the same polymer materials and that have similar core and pitch sizes. We experimentally demonstrate that the GI circular-core polymer waveguides fabricated by the Mosquito method have sufficiently low propagation loss (0.033 dB/cm at 850 nm), low connection loss with GI-MMFs, and low interchannel crosstalk. We observe approximately −50 dB of interchannel crosstalk in the 250-μm pitch GI-core waveguide fabricated, which is almost 10 dB lower than in the SI counterpart. Furthermore, sufficiently low crosstalk is maintained in a half-pitch GI-core waveguide fabricated by the Mosquito method.

In this paper, we demonstrate a nonlinear optical device based on a fiber taper coated with a carbon nanotube (CNT)/polymer composite. Using this device, four wave mixing (FWM) based wavelength conversion of 10 Gb/s Non-return-to-zero signal is achieved. In addition, we investigate wavelength tuning, two photon absorption and estimate the effective nonlinear coefficient of the CNTs embedded in the tapered fiber to be 1816.8 W−1km−1.

We characterize the Brillouin gain spectra (BGS) in perfluorinated graded-index polymer optical fibers (PFGI-POFs) with 62.5-μm core diameter. First, we show that the Stokes power is extremely high compared to that of a standard PFGI-POF with 120-μm core, and discuss the Brillouin threshold power. Then, we demonstrate that the PFGI-POF length has little influence on the BGS when the length is longer than 50 m. We also predict that, at 1.55-μm wavelength, it is difficult to reduce the Brillouin threshold power of PFGI-POFs below that of long silica single-mode fibers even if their core diameter is sufficiently reduced to satisfy the single-mode condition. Finally, we confirm the Brillouin linewidth narrowing effect.

For applications in high-density and high-speed optical interconnections, we propose to utilize polymer parallel optical waveguides (PPOWs) with so-called W-shaped refractive index profile in the core area. A W-shaped index profile is composed of a parabolic index distribution surrounded by a narrow index valley, followed by a cladding with a uniform refractive index. We expect that W-shaped index profiles contribute to decreasing the inter-channel crosstalk due to mode conversion in the waveguides. In this paper, we investigate how much the index difference of the index valley improves the crosstalk value. First, we fabricate polymer waveguides with various index profiles by changing the composition of the copolymer for cladding. We show the results that a 1-m long W-shaped profile PPOW has not only low propagation loss (0.027 dB/cm), but an inter-channel crosstalk (~-40 dB) lower than those of graded index (GI) core PPOW we previously fabricated. Next, we theoretically analyze the propagation loss and inter-channel crosstalk in polymer waveguides with different index profiles by means of a ray tracing model in which the light scattering effect is included. The calculation results indicate that the index valley surrounding each core works properly for preventing the power coupling from the cladding modes to the propagation modes, and consequently, very low inter-channel crosstalk is realized with W-shaped index profiles.

Stimulated Brillouin scattering (SBS) in a perfluorinated graded-index polymer optical fiber (POF) with 120 μm core diameter was experimentally observed for the first time, to the best of our knowledge, at 1:55 μm wavelength with the pump–probe technique. Compared to spontaneous Brillouin scattering previously reported, the Brillouin gain spectrum (BGS) was detected with an extremely high signal-to-noise ratio, even with a short POF (1m) and scrambled polarization state. We also investigated the BGS dependences on probe power and temperature, which indicate that SBS in a POF measured with this technique can be utilized to develop high-accuracy temperature sensing systems.

Graded-index (GI) polymer parallel optical waveguides with high-absorption, carbon-black-doped cladding are fabricated using the preform method in order to reduce the inter-channel crosstalk. The waveguides exhibit a lower inter-channel crosstalk (<-69.3dB) than optically-transparent-clad waveguides (~-33.7 dB) and maintain low propagation loss (0.029dB/cm). We characterize the waveguides with different concentration of carbon black in order to confirm the required concentration (required absorption loss) for keeping the inter-channel crosstalk low enough. In addition, carbon-black-doped waveguides are fabricated directly on a substrate by means of a soft-lithography method. Crosstalk is sufficiently decreased despite the high scattering loss of the core material, while insertion loss is not increased. Furthermore, we fabricate a waveguide with a high-scattering-loss cladding to confirm the origin of low crosstalk in carbon-black-doped waveguides. We confirm that high scattering loss of cladding is not necessarily as effective for crosstalk reduction as high absorption loss of cladding.

これまでの放物線状の屈折率分布の外周に屈折率の溝を設けたW型の屈折率分布により、さらにPOFの伝送帯域を向上できることを理論的・実験的に明らかにした初の速報。

これまで提案してきたPMMA系GI型POFの高温高湿下での伝送損失の安定性を高めた点についての報告である。PMMAと同程度の親水性・疎水性を有する低分子どーパントを適切に設計し、さらに適切な濃度を選択することにより、70oC、80%相対湿度での伝送損失上昇を抑制することに成功した。