Published papers

Isothermal aging has been carried out on low-loss PMMA glass at 18℃ below the glass transition temperature (Tg). The effect of enthalpy relaxation on light scattering loss of PMMA glass which had no excess scattering was investigated by the use of differential scattering calorimeter (DSC) and light scattering measurement. The light scattering intensity of PMMA glass did not increase with increasing aging time. It is seen that the light scattering loss of glassy PMMA is not affected inherently by physical aging as a result of its approach to thermodynamic equilibrium at a temperature 18℃ below its Tg.

Organic dyes and rare earth chelates have been chosen and incorporated into the core regions of graded index (GI) polymer optical fiber (POF) as gain media. Since the synthetic polymer shows much better compatibility with organic fluorescent materials and is amenable to high active dopant concentrations, the doped POFs produce high gains in short length of fiber. We demonstrated that organic dye-doped polymer optical amplifier (POFA) cover a wide spectral range in the visible with a best observed gain of 37 dB in a Rhodamine B-doped POFA pumped at 550 nm. Also, with a GI POF doped with europium chelate of hexafluoroacetylacetone in tris form, we observed organic dyes and rare earth chelates offers optical amplifiers and superfluorescence source for a variety of communication and sensor applications.

We have proposed a highly scattering transmission (HSOT) polymer for use as a high luminance light source medium. This polymer contains specified microscopic heterogeneous structure for controlling light scattering properties. A LCD backlight using the HSOT polymer has twice the brightness of the conventional transparent PMMA-based backlight because of multiple scattering phenomenons inside the HSOT polymer. In this paper, such a light scattering phenomenon was quantitatively analyzed by a ray tracing based on Mie scattering and Monte Carlo simulations. Subsequently, we propose the new HSOT backlight system having higher luminance and uniformity of white color illumination.

Recent progress of the graded-index polymer optical fiber (GI POF) is reviewed. Because the low attenuation of light transmission in the perfluorinated (PF) polymer based GI POF enables 500 to 1000m transmission, more accurate analysis is required in the refractive index profile to realize the order of giga bit transmission. Furthermore, recent improvement of the thermal stability of the GI POF is introduced. We clarify that the stability of the refractive index profile and the attenuation under high temperature and high humidity atmosphere can be improved by selecting the suitable dopant materials.

This paper describes the high speed transmission characteristics of the Perdeuterated PMMA (PMMA-d8) based GI POF. The PMMA-d8 base GI POF was obtained by the interfacial-gel polymerization technique. We succeeded in 1Gb/s 160m transmission at 650-nm wavelength in the 160m GI POF link. In general, the possible transmission distance of the PMMA POF link is limited to approximately 100m because of high attenuation. However, substitution of hydrogen atoms in the polymer for deuterium atoms decreases the intrinsic absorption loss. The total attenuation of the PMMA-d8 base GI POF at 650nm wavelength was about 60dB/km which is approximately half of the attenuation of PMMA base GI POF. Therefore, the low-loss PMMA-d8 base GI POF enables longer distance communication. We also analyzed the dispersion property of the PMMA-d8 base GI POF by utilizing WKB method in which both modal and material dispersion were taken into consideration. It was clarified that approximately 2Gb/s transmission can be achieved by PMMA-d8 base GI POF with 200m length when 650nm wavelength is adopted as the signal wavelength. The PMMA-d8 base fiber has two optical windows of light transmission which are located around 650-680nm and 780nm wavelength. As the material dispersion decrease with increasing the signal wavelength, the use of the light source operating at 780nm is effective for higher bit rate transmission. It was also confirmed that the PMMA-d8 base GI POF can transmit about 3Gb/s for 200m when the signal wavelength is 780nm. These experimental and theoretical results indicate that the PMMA-d8 base GI POF could be one of the promising candidates of the physical layers in the high speed multimedia network.

Bandwidth characteristics of the large core GI POFs are theoretically and experimentally clarified in this paper. The refractive index profile of the GI POF was controlled by the interfacial-gel polymerization to investigate the relation between the index profile and the bandwidth characteristics. It was experimentally confirmed that the maximum bandwidth of the PMMA base GI POF is at most 3GHz for 100m transmission with using the typical laser diode emitting at 650-nm wavelength (3nm source spectral width) when its refractive index profile is optimized. The maximum bandwidth theoretically estimated by considering both modal and material dispersion is approximately 3GHz which is exactly the same as the measured value, while higher than 10GHz for 100m was expected if only modal dispersion was taken into account. The optimum refractive index profile of the PMMA base GI POF is theoretically and experimentally clarified by further considering the profile dispersion.

Rear-earth chelate-doped GI POFs are proposed and fabricated. The attenuation loss was measured to be 0.4dB/m at 650nm for a GI POF doped with 1wt% of Eu chelate. Lifetime shortening and spectral narrowing verified the occurrence of Superfluorescence at 614nm in the Eu chelate-doped GI POF pumped with xenon flashlamps. The demonstration of Superfluorescence shows that rare-earth chelate-doped GI POFs are appealing as optical amplifiers and superfluorescent sources in a variety of communication and sensor applications.

Birefringence is caused by both orientation of polymer chain and photoelasticity. These birefringence were compensated by random copolymerization of negative birefringent MMA and positive birefringence BzMA, but orientational and photoelastic zero birefringence were achieved by quite different compositions of the copolymer. Note that the birefringence of the copolymer that occurred in the process of injection molding was almost completely eliminated with a composition for orientational zero birefringence. The orientational and photoelastic zero-birefringence copolymer processed enough transparency as optical materials that the total scattering losses were 30.4 and 19.5 dB/km, respectively, competing with the transparency of homopolymers.

低分子ドーパント系GI型POFにおいて懸念された耐熱性について、ドーパントによるマトリックスポリマーのガラス転移温度の低下について検討し、屈折率分布、伝送損失が85oC以下でも安定に維持されるGI型POFの作製に成功したことを報告した。

最近のポリマー光ファイバーの応用とその原理

低分子ドーパント系GI型POFにおいて懸念された耐熱性について、ドーパントによるマトリックスポリマーのガラス転移温度の低下について検討し、屈折率分布、伝送損失が85oC下でも安定に維持されるGI型POFの作製に成功したことことこ報告した。

Amplification of high signal gain and high output power was achieved in a Rhodamine B (RB)-doped GI POFA based on a prediction in which absorption and emission cross section data were used. The absorption and emission cross section spectra of RB, R6G, and DCM in PMMA bulk were determined. The cross section data enable us to predict gain characteristics of RB, R6G, and DCM-doped GIPOFA. The wide spectral coverage in the visible is achievable with GI POFAs doped with several selected dyes.

We have proposed a new brighter backlight using highly scattering optical-transmission (HSOT) polymer. This polymer material consists of specified microscopic heterogeneous structures inside the polymer. The high order scattering phenomenon in the HSOT polymer was exactly investigated by Monte Carlo simulation based on Mie scattering theory. We obtained a good agreement between the calculated and the experimental results. The scattering property inside the HSOT polymer can be optimized for the LCD backlight by using this simulation program. As the result, our back-light system consisting of the HSOT polymer plate has simpler system, brighter luminance, and better luminance profile. The property of our backlight system which was actually applied to the LCD is shown.

パーフルオロ非晶質樹脂CYTOPを用いたGI-POFを開発中であり、試作したGI-POFの伝送特性を測定した結果、650-1.300nmの広い波長域で使用でき、長波長では100m以上の伝送が可能であること、および300MHZ・KM以上の高帯域を有していることを実証した。

In quest of high-power optical amplifier in the visible, we incorporated organic dyes into POFs. Organic dyes were chosen as gain media based on the following features: a large emission cross section, which allows the achievement of high gains in a short length and a broadband fluorescence spectrum that provides wide tunability. A Rhodamine B-doped POFA, 1m in length and 0.3mm in core diameter, has exhibited more than 20dB gain at signal wavelength from 560nm to 600nm when optically pumped at a wavelength of 532 nm. Furthermore, we have obtained the optical amplification covering most of the spectral range from 560 to 650nm. The spectral range in the visible can be continuously covered with the POFAs doped with several selected dyes. The broad spectral coverage demonstrates the versatility of the organic dye-doped POFA.

The fluctuation theory indicates that the isotropic scattering loss of amorphous polymer glass, which is used as a material for POF, decreased with the decrease in isothermal compressibility β and refractive index. In order to prepare low light-scattering-loss polymer glass, it is necessary to know the isothermal compressibility of polymers. However, the β for most polymers has not yet been measured precisely using a mechanical procedure. In this work, we estimate the isothermal compressibility of amorphous polymer glass from its molecular structure and calculate its isotropic light scattering loss.

ポリマー固体内部に不均一構造を有し、光の導波と拡散出射の二つの機能を有する光散乱ポリマー導光体を提案した。またMonte Carlo 法によってその内部の多重散乱現象を理論的に解析し、液晶用バックライトに応用するために最適化を行った。その結果、輝度の均斉度が低下することなく市販品よりも高輝度なバックライトを作製した。（解説）

The status of the POF for high-speed data communication is described. Very recently, the low-loss and high-bandwidth perfluorinated GI POF which has no serious absorption loss from visible to 1.3µm wavelength was successfully prepared at Keio University. Since the core diameter of GI POF is Much larger than that of the multimode silica fiber was virtually eliminates, resulting in stable giga bit order data transmission with inexpensive couplers and connectors.

Rhodamine B (RB), Rhodamine 6G (R6G), Oxazine 4 perchlorate (O4PC), and 4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl-4H-pyran)-doped GI POFAs were successfully prepared by novel technique to prevent degradation of organic dyes and to enhance solubility of the dyes in MMA and in PMMA. High-power and high-gain amplification was achieved for RB-doped GI POFA’s based on the spectral investigation of dyes in PMMA bulk. It should be noted that amplification in the low-loss region of POF was achieved in the R6G- and O4PC-doped GI POFA.

The status of the POF for high-speed data communication is reviewed. Very recently, the low-loss and high-bandwidth perfluorinated GI POF which has no serious absorption loss from visible to 1.3µm wavelength was successfully prepared at Keio University. Since the core diameter of GI POF is much larger than that of the multimode silica fiber, the serious modal noise in the conventional multimode silica fiber was virtually eliminated in stable giga bit order data transmission with inexpensive couplers and connectors.