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For realizing wavelength-division multiplexing (WDM) transmission systems, one of the most important devices is the erbium-doped fiber amplifier (EDFA), which exhibits a flat gain, a high pumping efficiency, and a low noise figure (NF). To meet these requirements, we have optimized the hybrid EDF configurations composed of cascaded Al codoped EDF and P/Al codoped EDF. Their characteristics have been theoretically and experimentally examined, and it has been shown that the hybrid EDF is superior in terms of both the gain flatness and the pumping efficiency, but the NF tends to be worse than conventional EDF configurations. Thus a midway isolator should be inserted. As a result of the design optimization, three-section hybrid EDF configurations have been proposed, and two types of EDFAs have been developed for use as in-line amplifiers and preamplifiers. Both types exhibit a passband spanning about 15 nm. In experiments employing a five-cascaded in-line amplifier chain, a total gain deviation of less than 2 dB has been demonstrated, while the total loss budget is about 125 dB.

Erbium-doped fiber amplifier (EDFA) featuring a spectrally flattened gain is considered to be one of the most important key devices for the wavelength-division multiplexing (WDM) transmission systems. For this purpose, we have developed hybrid EDFs composed of Al doped EDF and P/Al co-doped EDF and examined their characteristics comparing with the conventional EDFs and those with passive gain equalizers. As a result, hybrid EDFs have been shown to exhibit both a flat gain and high pumping efficiency, but their noise characteristics tends to be worse than conventional EDF configurations. Thus a midway isolator should be inserted. As a result of the design optimization, three-section hybrid EDF configuration has been proposed, and three types of EDFAs have been developed employing this configuration for use as in-line amplifiers or preamplifiers. All types exhibit the passband spanning about 15nm.

An optimum fiber structure for a practical dispersion-compensating fiber (DCF) has been explored theoretically and experimentally, taking dispersion, attenuation, bending loss characteristics, polarization mode dispersion, and nonlinear effects into account. A high figure of merit (FOM) of 280 ps/nm/dB has been successfully achieved with a simple matched cladding design. Furthermore, it was confirmed that the optimized structure with enhanced FOM also benefits self-phase modulation (SPM) suppression.

We have proposed a novel configuration of erbium-doped fiber amplifier (EDFA) which realizes both a flat spectral gain and high output power for analog transmission. This EDFA uses hybrid EDF which consists of serially cascaded EDFs with different glass compositions, based on the fact that EDFs with certain glass compositions exhibit gain tilt of opposite signs in the wavelength range of interest. It is theoretically shown that, in the hybrid EDFA, the gain flattening under any required optical input/output conditions is possible without sacrificing output power by optimizing the constituent EDF lengths. By using thus designed hybrid EDFAs, 50-channel amplitude-modulated vestigial-sideband signals were successfully amplified in four stages with no significant degradation of signal distortions.

Accelerated tests of Er-doped fibers in hydrogen environments are carried out and as the evidence of their long-term utilization, the degradations in loss and gain are estimated. The hydrogen-induced loss-increase in Er-doped fibers with 0 to 3 percent aluminum (Al) content is formulated. As the results of quantitative investigations, it is understood that during 25 years of use, there is almost no transmission loss increase in 1.55 μm wavelength region and at hydrogen partial pressure of 0.001 atm and 40°C if there is no Al content. In case of 1 wt% Al content, the loss increase is 0.04 dB/20 m while it is 0.08 dB/20 m if the Al content is 3 wt%. Further, it is possible to describe the gain degradations, under hydrogen environment, by the increase of background losses. Under the forementioned conditions, the gain degradation is estimated to be 0.07 dB if the Al content is 1 wt% and 0.15 dB if the content is 3 wt%. Therefore, it becomes clear that their characteristics are immune to hydrogen environment in real applications.

Feasibility of repeaterless transmission systems over non-dispersion-shifted fiber (NDSF) using directly modulated DFB lasers and dispersion compensating fiber (DCF) was examined. The transmission characteristics were investigated by simulations, considering the effects of the laser chirping, fiber chromatic dispersion, and the self-phase-modulation (SPM) occurring in both NDSF and DCF. As a result, it was revealed that the post-compensation scheme is the most suitable and that the SPM in DCF dominantly deteriorates the transmission characteristics in this scheme. Following the predictions by the simulations, 2.5 Gbit/s repeaterless transmission over 306-km pure-silica-core fiber has been demonstrated with no power penalty, using a directly modulated DFB laser with a bulk active layer and dispersion compensating fibers.

We have developed a fiber-based frequency comb system consisting of a simple mode-locked fiber laser and a backward pumping amplifier combined with a highly nonlinear fiber with a short zerodispersion wavelength. As a result, the signal to noise ratio of the obtained carrier-envelope-offset frequency beat is larger than 45 dB at a bandwidth of 100 kHz. Furthermore, we have succeeded in measuring the optical frequencies of a 1542-nm acetylene-stabilized laser and a 532-nm iodinestabilized Nd:YAG laser continuously for more than one week using the fiber-based comb system. The long-term measurement revealed that the frequency stability of the iodine-stabilized laser was 5.7 x 10(-15) with 100 000 s averaging.

Bismuth (Bi) -doped silicate glass for future photonics applications was proposed by Y. Fujimoto et al . in 2001 [1]. And its broadband fluorescence characteristic at 1.3um band was demonstrated for the first time. From this report, several studies such as clarifying the fluorescence mechanism, surveying the optical properties of Bi-doped glass with various host glass materials and so on have been done. However, there was no report on fabricating Bi-doped silicate glass fiber (BiDF) for the demonstration of optical amplification and laser oscillation. In 2005, Haruna et al . succeeded in fabricating the BiDF by Modified Chemical Vapor Deposition (MCVD) method for the first time, and evaluated its absorption and fluorescent characteristics [2]. In this report, the absorption bands around 0.5 um, 0.7 um, 0.8 um and 1.0um were shown, and the very wide fluorescence band with a FWHM of 192 nm centered at 1.06um was also indicated. The background loss of the fiber was as low as

Local structure of Er/sup 3+/ in the Er-doped optical fiber (EDF) was explored by the X-ray absorption fine structure (XAFS) measurement. We have successfully observed variations of Er-O coordination number and Er-O distance by changing the Al concentration.

We have successfully developed silica-based Bismuth-doped fiber (BiDF) by using the conventional fiber fabrication technique for the first time. Fluorescence over a 150nm bandwidth is observed in the 1100nm wavelength band by using fabricated fibers.

The full-vector finite element method with perfectly matched layer has realized accurate modeling of chromatic dispersion and bending loss of the hole-assisted lightguide fiber for the first time. Optimal design for large anomalous dispersion fiber is also presented.

We propose a novel holed fiber based on lightguiding by glass index difference and dispersion enhancement by holes. Due to the reduced power fraction in the holes, a record-breaking low loss of 0.82 dB/km at 1550 nm is achieved with extraordinarily large anomalous dispersion of +34 ps/nm/km.

Usage of air holes in optical fibers has become a hot subject in fiber optics because of the possibilities for novel transmission properties. Although photonic crystal fibers based on photonic bandgap guidance are the most drastic innovation in this subject, optical fibers containing air holes but not having photonic crystal structures are also being intensively studied. Such air-silica microstructured fibers are more practical than the photonic bandgap fibers because the lack of photonic crystal structure makes the fabrication far easier. Even without the photonic bandgap, the microstructured fibers can exhibit valuable properties in terms of group velocity dispersion and nonlinearity, because the index contrast between air and silica is 10 or more times as large as that of the conventional optical fibers based on doped silica glasses. However, one of the major challenges for practical applications of the air-silica microstructured fibers has been their high transmission losses, which have been several tens to hundreds times higher than those of the conventional fibers. As a solution to this problem, we have proposed a more practical structure called hole-assisted lightguide fiber (HALF). In addition to the air holes for realizing novel optical properties, this structure has a material index profile for waveguiding, and hence is closer to the conventional fibers than the other microstructured fibers are. As a result, novel optical properties can be realized without severe degradation in transmission loss. In experiments, an anomalous group velocity dispersion as large as +35 ps/nm/km at 1550 nm wavelength, which would be unattainable in the conventional fibers, has been realized with a loss of 0.41 dB/km, which is comparable to those of the conventional fibers. Analyses of the losses of the fabricated HALFs suggest that the loss should be lowered by mitigating the effect of the drawing tension and minimizing the power fraction in the holes. It is also shown that the full-vector finite element method realizes accurate modeling of the properties such as dispersion and macrobend loss.

Dispersion shifted fiber with a high nonlinearity coefficient (20.4 W/sup -1/ km/sup -1/) and low attenuation (0.51 dB/km) was successfully fabricated. 500 m of the fabricated fiber efficiently generated four-wave mixing, and nearly constant wavelength conversion efficiency over 20 nm was demonstrated.

Without gain equalizers, newly-designed hybid silica-based EDFAs exhibit relative gainnon-uniformity as small as 5.8% over 2Snm-bandwidth , which is comparable to those of fluoride-based EDFAs.

Since erbium-doped fiber amplifiers (EDFAs) are being installed in undersea optical communication systems, long-term reliability of EDFA becomes important. Especially, durability of EDFs against γ-ray irradiation is of concern because EDFs which generally contain large amount of germanium(Ge) and aluminum(Al) in the core may be more sensitive to γ-ray-induced damage than standard fibers.

Dispersion-compensating fibers (DCFs) with large negative chromatic dispersion in the 1.55-µm region are attractive for upgrading 1.3-µm-optimized transmission lines using erbium-doped fiber amplifiers.1 For putting DCFs into practical use, a figure of merit (FOM), which is defined as a ratio of the absolute value of the chromatic dispersion to the transmission loss at 1.55 µm, should be enhanced. In this paper an optimum range of the refractive index of the core for maximization of the FOM is investigated for the matched-cladding-type profile,2 and a DCF with a FOM of 250 ps nm−1dB−1 is presented. An analog transmission experiment using this DCF is also demonstrated.

System performance of Pr-doped fluoride fiber amplifier was examined for multichannel AM-VSB video signal transmission for the first time. For CNR of 51 dB, the loss budget of 15.1 dB was obtained with the amplifier.

The local structure around Er(3+) in Er-doped optical fiber (EDFs) was explored by X-ray absorption fine structure (XAFS) measurements. Using several new approaches such as a novel sample preparation, we successfully measured the XAFS. The intensities near the 8.36 keV peaks were observed for the first time using X-ray Absorption Near Edge Structure (XANES) analysis. The intensities increased in the order of Er, Er(2)O(3), and EDF samples, indicating that Er(3+) in the EDFs existed as an oxide state. Extend X-ray Absorption Fine Structure (EXAFS) analysis also showed that the oxygen coordination number of Er(3+) increased as the Al concentration increased and that the Er-O distances of EDF with Al co-doping is longer than that of EDF without Al co-doping.

Hole-assisted lightguide fiber (HALF) is a microstructured fiber comprising a material index profile for waveguiding and air holes for modifying optical properties. Anomalous dispersion larger than those of the conventional fibers can be realized without severe degradation in optical loss, because of low power fraction in the holes and structural simplicity. We investigate into the causes of the loss of the fabricated HALFs, and show that a GeO2-doped core, in addition to the low power fraction, is desirable for low loss. The fabricated HALF exhibits a loss as low as 0.41 dB/km and a large anomalous dispersion of +35 ps/nm/km at 1550 nm wavelength.

Flexible Bi-Pb-Sr-Ca-Cu-O glass fibers were prepared by the glass-drawing method. They showed superconducting properties after crystallization by heat treatment. The T c and J c values of the fibers were very sensitive to starting compositions and heat treatment conditions. The fiber with the starting composition of Bi2.1Pb0.9Sr2Ca2Cu3O y showed superconductivity with a J c=1480 A/cm2 (77 K, zero magnetic field).

Bi-Pb-Sr-Ca-Cu-O glass fibers were drawn from melt-quenched glass preforms by the method that is ordinarily applied to optical fiber fabrication. The drawn fibers were very flexible, and their surfaces were smooth. The glass fibers were crystallized by annealing and had superconducting properties. The annealed (1133 K, 240 hour) Bi1.6Pb0.4Sr2Ca2Cu3O x fiber exhibited superconductivity with a T c(zero) of 68 K and a critical current density (30 K, zero magnetic field) of 22 A/cm2.

Since ductile cast iron was invented, cast iron has been used for various purposes. However, because of its poor corrosion resistance, the use of cast iron in corrosive environments was restricted. As pre-viously reported, the authors succeeded in obtaining a prominent protective film on steel by silicon impregnation (so-called siliconizing).In the present work, 3 kinds of specimens, i.e. nodular cast irons having ferrite and pearlite matrix, respectively, and gray cast iron, were siliconized by means of a mixed gas of silicon tetrachloride and oxygen free argon in accordance with the silicon impregnation of steel. The results obtained were summarized as follows.(1) Microscopic examination showed that porous films were formed on nodular cast iron with ferrite matrix, and particularly, on gray cast iron. On the contrary, when nodular cast iron with pearlite matrix was siliconized at a relatively high temperature for a short time, the film obtained had comparatively less pores in number.(2) Immersion tests of the siliconized cast irons in dilute sulfuric acid showed that the siliconized film protected the underlying cast iron core from the attack of acid though it was porous. Above all, the film which had been formed on nodular cast iron with pearlite matrix by siliconizing at a high temperature for a short time showed the best corrosion resistance.

Bi1.5SrCaCu2O x glass preforms were prepared by the melt-quenching method, and their thermal characteristics and viscosity were investigated to determine the possibility of drawing them into glass fibers. Furthermore, Bi1.5SrCaCu2O x glass fibers were drawn from glass preforms by the method that is ordinarily applied to optical fiber fabrication. The drawn fibers were very flexible, and the maximum length was 1200 mm.