Yoshifumi Suzaki

Thermal tuning of mechanically induced long-period fiber grating

We have developed a wideband tunable optical filter that uses a long-period fiber grating (LPFG) in which both resonance wavelength and its signal attenuation can be adjusted. We create the grating mechanically by pressing a spring coil to an optical fiber. We achieve continuous fine tuning of wavelength and attenuation by varying the temperature of the LPFG. The adjustable ranges of the LPFG are more than 200 nm in resonance wavelength and more than 10 dB in signal attenuation.

Crystallization behaviour of amorphous Si1-xCx films prepared by r.f. sputtering

Abstract The crystallization behaviour of amorphous Si1-xCx films with x = 0.11, 0.38, 0.48 and 0.75 prepared by r.f. sputtering was studied using IR spectroscopy and transmission electron microscopy, where films were annealed isothermally at 700–1100°C for 1 h. The broad transmission bands in IR spectra at 700–800 cm-1 for as-deposited films became sharper and their peak positions shifted to the higher frequency side with increasing annealing temperature. The wavenumber of the peak positions reached finally 818 cm-1 for x = 0.11 and 800 cm-1 for x = 0.38, 0.48 and 0.75, the corresponding microstructures being polycrystalline β-SiC and silicon, and β-SiC and amorphous graphite respectively. The crystallization temperature of stoichiometric SiC film was the highest, and the greater the deviation of the composition from stoichiometry, the lower the crystallization temperature. From the kinematic analyses of the results of IR measurements for the films with x = 0.11, the activation energy of the crystallization process was found to be about 7.1 eV.

Experimental Study of Temperatures of Atmospheric-Pressure Nonequilibrium Ar/N2 Plasma Jets and Poly(ethylene terephtalate)-Surface Processing

To understand the mechanism of surface processing using atmospheric-pressure nonequilibrium plasma jets, we measured the vibrational and rotational temperatures in the plasmas by optical emission spectroscopy. Plasma was excited using a high-frequency pulsed power supply, using a gas mixture of Ar (20 L/min) and N2 (0.1 to 0.5 L/min) as the plasma gas, and changing the flow rate of N2 gas at an input power of 100 W and plasma frequencies of 5 and 10 kHz. The measured vibrational and rotational temperatures in plasma were approximately 0.18 to 0.26 eV and 0.21 to 0.28 eV, respectively. We also carried out a plasma surface processing of polyethylene terephtalate film to measure the changes in water contact angle before and after the processing. We found a monotonic decrease in the contact angle of the processed poly(ethylene terephtalate) (PET) film as plasma rotational temperature increased. It is concluded that the hydrophilicity of the PET surface increases with plasma rotational temperature.

Concentration and thermal release of hydrogen in amorphous silicon carbide films prepared by rf sputtering

Concentration and thermal release of hydrogen in hydrogenated amorphous SiC a-SiC:H films were studied. The films were prepared . onto Si 111 wafers at room temperature by rf planer magnetron sputtering in a gas mixture of argon at partial pressures of 0.33 Pa and hydrogen from 0.065 to 1.3 Pa. The IR measurements conducted on the films annealed at various temperatures for 3600 s suggested that the hydrogen was released from Si-H and C-H bonds in the films at the temperatures above 600 and 850 K, respectively. In-situ . isochronal annealing for 300 s at various temperatures from 323 to 1123 K in the ERDA Elastic Recoil Detection Analysis system was carried out for the specimen having the hydrogen concentration of 7.1 = 10 27 atomsrm 3 . It was revealed that three types of hydrogen exist in the films; hydrogen bonded to Si or C atoms and unbonded hydrogen, with the concentrations of 2.8 = 10 27 , 1.9 = 10 27 and 2.4 = 10 27 atomsrm 3 , respectively. The concentration of unbonded hydrogen decreases with increase of the hydrogen partial pressure. q 1997 Elsevier Science S.A.

Fabrication of transparent antifouling thin films with fractal structure by atmospheric pressure cold plasma deposition.

Antifouling surface with both superhydrophobicity and oil-repellency has been fabricated on glass substrate by forming fractal microstructure(s). The fractal microstructure was constituted by transparent silica particles of 100 nm diameter and transparent zinc-oxide columns grown on silica particles by atmospheric pressure cold plasma deposition. The sample surface was coated with a chemically adsorbed monomolecular layer. We found that one sample has the superhydrophobic ability with a water droplet contact angle of more than 150°, while another sample has a high transmittance of more than 85% in a wavelength range from 400 to 800 nm.

Wavelength evolution of fiber Bragg gratings fabricated from hydrogen-loaded optical fiber during annealing

The shift in the central wavelength of fiber Bragg gratings (FBGs) during annealing of hydrogen-loaded optical fiber exposed to ultraviolet (UV) laser irradiation through a phase mask is studied and shown to be caused by thermal diffusion of hydrogen out of the fiber for both short-period and long-period FBGs. Reloading FBGs with hydrogen followed by a second annealing cycle without exposing the fiber to the UV laser reproduces the details of the wavelength evolution observed during first annealing following UV irradiation. This shows that the wavelength shift of the grating during annealing is determined by diffusion of hydrogen gas out of the optical fiber for both short-period and long-period FBGs.

Apodization Method Owing to the Finite Length of UV Laser Coherence in Fabricating Fiber Bragg Gratings

A very effective apodization, in which side lobes are suppressed to below a particular noise level, is found to operate automatically in a two-beam interference device for fabricating fiber Bragg gratings when a pair of mirrors deflecting two beams are off parallel to each other. This is considered to be a consequence of the short temporal coherence length of the KrF excimer laser used.

Evolution of Optical Fiber Temperature during Fiber Bragg Grating Fabrication Using KrF Excimer Laser

The temperature distribution in an optical fiber during the fabrication of fiber Bragg gratings (FBGs) using a KrF excimer laser with a phase mask has been analyzed experimentally for typical fabrication conditions. The fluence of UV laser light at the fiber surface has been varied up to 420 mJ/cm2. These experiments show that (1) scanning electron microscope (SEM) images of the fiber surface facing the excimer laser beam reveal partial physical damage apparently owing to partial melting, so that the surface temperature has increased beyond the softening point of silica glass, which is approximately 1200°C, (2) the optical spectrum transmitted through the FBG during laser irradiation contains spikes coincident in time with the laser pulses that correspond to a near instantaneous shift of the FBG spectrum to a higher temperature spectrum representing fiber core heating of approximately 8°C, and (3) analysis of the energy absorbed by the fiber indicates a bulk temperature rise of approximately 3°C in the fiber. The resulting large variation in the temperature rise over the fiber cross section from a few °C to 1200°C, along with partial physical damage on the surface, will certainly induce large internal stresses in the fiber material and reduce the mechanical strength of the FBG.

Dynamic evolution of the spectrum of long-period fiber Bragg gratings fabricated from hydrogen-loaded optical fiber by ultraviolet laser irradiation

Long-period fiber Bragg gratings fabricated by exposure of hydrogen-loaded fiber to UV laser light exhibit large-scale dynamic evolution for approximately two weeks at room temperature. During this time two distinct features show up in their spectrum: a large upswing in wavelength and a substantial deepening of the transmission minimum. The dynamic evolution of the transmission spectrum is explained quantitatively by use of Malos theory of UV-induced quenching [Electron. Lett. 30, 442 (1994)] followed by refilling of hydrogen in the fiber core and the theory of hydrogen diffusion in the fiber material. The amount of hydrogen quenched by the UV irradiation is 6% of the loaded hydrogen.

Surface Modification of Si Wafer by Low-Pressure High-Frequency Plasma Chemical Vapor Deposition Method

In recent years, a flexible type of solar cell that can maintain various shape changes and that is applicable to virtually all products has attracted global attention. In the present research, we describe equipment for the production of thin-film material for flexible type solar cells that uses a high-frequency plasma chemical vapor deposition (CVD) method. This equipment is now at the development stage, and in order to clarify the cardinal trait of the plasma, we performed a plasma treatment on the surface of a Si wafer. Using X-ray photoelectron spectroscopy and contact angle meter measurements, we identified one index that clarifies the simple cardinal trait of plasma CVD.