Tomoyoshi Yamashita

Intrinsic Transmission Loss of Polycarbonate Core Optical Fiber

Intrinsic transmission loss of polycarbonate (bisphenol A-type) optical fiber (PC-POF) was experimentally determined by investigating three intrinsic loss factors: electronic transition absorption (ETA), molecular vibration absorption (MVA) and light scattering caused by thermal dielectric fluctuation. The intrinsic transmission losses for optical windows are 224 dB/km at 764 nm and approx. 166 dB/km at 650-656 nm. The intrinsic ETA loss is large in the region of short wavelength, hence it is suggested that the PC-POF yellows for white light transmission. The optical anisotropic parameter of polarizability for PC is much larger than that of polymethylmethacrylate (PMMA). The intrinsic scattering loss of PC-POF results almost entirely from the anisotropic scattering caused by the strong optical anisotropy of this polymer. Present transmission loss of PC-POF is much larger than the estimated intrinsic transmission loss because of the large excess loss arising from extrinsic loss factors. The PC-POF has the potential for low transmission loss equal to that of the PMMA-POF, if it is optically purified.

Quasi-Static and Dynamic Density Fluctuations in the Glassy State II. Temperature Dependences of the Density Fluctuations for Polymethylmethacrylate

The Vυ scattering profile of glass displays two characteristics in precise light-scattering measurements. One characteristic is of speckle patterns (Vυqst) due to frozen-in (quasi-static) density fluctuation. The other is background scattering (Vυdyn) observed beneath the speckle patterns, which is caused by the dynamic density fluctuation. Temperature dependences of Vυqst and Vυdyn were investigated at several temperatures ranging from room temperature to around Tg and the wavelength of 488 nm for polymethylmethacrylate (PMMA). As a result of this experiment, it has been verified for the first time that the increase of Vυ (≈Vυiso) with increasing temperatures is the result of an increase of Vυdyn, i.e., that the increase of Vυiso results from the increase of the dynamic density fluctuation much remaining in the glassy state. On the other hand, the Vυqst dose not significantly change from room temperature to approximately Tg, and above Tg, the Vυqst drastically decreases due to thawing of the frozen-in density fluctuation.

QUASI-STATIC AND DYNAMIC DENSITY FLUCTUATIONS IN THE GLASSY STATE IV. DYNAMIC THERMAL DENSITY FLUCTUATION AND ISOTHERMAL COMPRESSIBILITY OF GLASS

The temperature dependence of isothermal compressibility {β(T) dilato} in the glassy state was measured by dilatometry at temperatures ranging from room temperature to around the glass transition temperature (T g) for polymethyl methacrylate (PMMA) and polycarbonate (PC) glasses. The results were compared with those of β(T) light obtained from isotropic light scattering caused by dynamic thermal density fluctuation (Vv iso dyn), not Vv iso (=Vv-( 4/3)Hv;Hv is anisotropic scattering), in order to test the validity of Einsteins fluctuation theory for the glassy state. Consequently, it was experimentally demonstrated for the first time that β(T) light is in very good agreement with β(T) dilato at all temperatures below T g for PMMA and PC. These experimental findings indicate that the isothermal compressibility of glass in a nonequilibrium system is thermodynamically dominated by the dynamic thermal density fluctuation having relaxation times much shorter than the experimental observation time scale. Furthermore, the isothermal compressibility of glass, in general, increases drastically as the temperature increases in the glass transition region. However, it has been suggested that this drastic increase is the result of a dramatic increase of the dynamic thermal density fluctuation, with thawing of quasi-static thermal density fluctuation.

Light Scattering Measurement in PMMA Optical Fibers

A new light scattering measuring system was developed to investigate the heterogeneous structure in plastic optical fibers (POF). Measurements were conducted on commercial PMMA POF of different diameters. The results showed that the correlation length, which characterizes the size of a heterogeneous structure, is substantially larger in POF than in a bulk PMMA specimen, and increases with decreasing fiber diameter. These facts suggest that some structural changes such as molecular orientation and residual stress might be induced during POF formation, resulting in an additional scattering loss.

Quasi-Static and Dynamic Density Fluctuations in the Glassy State III. Isotropic Scattering Related to Isothermal Compressibility of Glass and Anisotropic Scattering

For expressions of isotropic and anisotropic scattering, Vv iso and Hv, in a glass, Vv iso=Vv dyn so+Vv qst iso and Hv=Hv dyn+Hv qst were introduced from the viewpoint of dynamic and quasi-static density fluctuations. The Vv dyn iso, which is an isotropic scattering due to the dynamic density fluctuation, can be associated with an isothermal compressibility of glass measured on a comparable time scale. The isothermal compressibility estimated from the Vv dyn iso was in good agreement with that measured by dilatometry for the PC and PMMA. Therefore, it was clarified experimentally for the first time that the isothermal compressibility of glass is dominated thermodynamically by the dynamic density fluctuation largely remaining in the glassy state. Both polycarbonate (PC) and polymethylmethacrylate (PMMA) have large Vv dyn iso, and an organic polymer has much dynamic density fluctuation, irrespective of whether it has side-chains for its molecule structure. Furthermore, it is important to note that Hv dyn/Hv of PC is very small in the glassy state. Therefore, optical anisotropy of PC is attributed primarily to the quasi-static density fluctuation rather than the dynamic density fluctuation.

Effect of Radiation and Impurities on Microscopic Deformation Process in PMMA

Small-angle X-ray scattering experiments were conducted under various tensile stress levels for poly(methyl methacrylate) with different amounts of impurities and microscopic defects. Compared with a highly purified sample of optical fiber grade, the commercially available samples containing many impurities showed that the generation of submicrocracks starts at much lower stress levels. Furthermore, it was revealed that the number and size of such submicrocracks become larger in samples irradiated by γ-rays, which produce internal microscopic defects. These results suggest that such impurities and microscopic defects existing in the virgin samples tend to become nuclei for the submicrocracks upon the application of a tensile stress, causing a significant influence on the microscopic deformation process in amorphous polymers.

Current status of commercially available plastic optical fiber

Gigabit-data transmission has been made possible by using a graded index type plastic optical fiber with dopants. However, there remains the problem of bandwidth which decreases due to dopant diffusion. Recently, two kinds of dopants with a very low diffusibility, triphenylphosphate and tricresylphosphate, have been found. On the other hand, for a step index type POF (SI-POF), quite a degree of improvement in its bandwidth can be achieved by reducing the numerical aperture (NA) of the POF so that a megabit-data transmissions made possible. This low NA-POF has enough reliability for practical use. Furthermore, we succeeded in developing a new type of broadband POF consisting of a multiphase polymer, with an excellent heat resistance and a bandwidth performance much broader than that of the low NA SI-POF.