Makoto Fujimaki

Fabrication of long-period optical fiber gratings by use of ion implantation

We report the fabrication of long-period optical fiber gratings by use of a refractive-index increase induced by ion implantation. Helium ions were implanted in an optical fiber core through a metal mask that had a 170-microm -pitch grating with spacing of 60 microm . We obtained a wavelength-dependent effective transmission loss by use of the grating.

Energy states of Ge-doped SiO2 glass estimated through absorption and photoluminescence

The energy states of oxygen-deficient type defects in the vacuum ultraviolet region are discussed based on the experimental results of vacuum ultraviolet absorption, temperature dependence of the photoluminescence (PL) intensities at 4.3 and 3.1 eV, and lifetimes of the PLs. It was found that the oxygen-deficient type glass has a large absorption tail above 6 eV in addition to an absorption band around 5 eV and that the 3.1 eV PL intensity scarcely depends on temperature when excited above 6 eV. It was also found that the lifetime of the 4.3 eV PL is 9 ns and that of the 3.1 eV PL is 113 μs irrespective of the excitation photon energy. The obtained results are explainable by assuming that electrons excited into the conduction band by photons above 6 eV contribute to the PLs.

The design of evanescent-field-coupled waveguide-mode sensors

An evanescent-field-coupled waveguide-mode sensor with a multilayer structure consisting of a dielectric waveguide, a thin reflecting layer, and a glass substrate illuminated under the Kretschmann configuration operates as a sensor that is capable of detecting modifications in the dielectric environment near the waveguide surface with superior sensitivity by measuring the change in reflectivity. The sensitivity of the sensor is strongly dependent on the optical constants of the reflecting layer. Numerical simulations show that a sensor having a reflecting layer with a small value of the real part of the complex refractive index shows a good sensitivity for both S-xa0and P-polarized light. Materials with values of the real and imaginary parts of the complex refractive index of >4 and ∼0.5, respectively, are suitable for use as reflecting layers when S-polarized light excites only the lowest order waveguide mode. The simulations were experimentally confirmed using sensors with Au, Cu, Cr, W, a-Si, or Ge reflecting layers deposited by radiofrequency magnetron sputtering by observation of specific adsorption of streptavidin on biotinyl groups using an S-polarized laser beam with a wavelength of 632.8xa0nm. From the results, guidelines are given for the fabrication of preferred sensor configurations.

Effect of Ozone Annealing on the Charge Trapping Property of Ta2O5-Si3N4-p-Si Capacitor Grown by Low-pressure Chemical Vapor Deposition.

The effect of ozone annealing on the charge trapping property of Ta2O5/Si3N4/p-Si capacitors was examined by measuring high-frequency capacitance-voltage and thermally stimulated current characteristics. The results suggest that two types of electron traps exist in the Ta2O5 layer and that the ozone annealing efficiently eliminates them.

Effect of annealing on Ge-doped SiO2 thin films

Thermal annealing effects on optical and structural properties of Ge-doped SiO2 thin films prepared by the chemical vapor deposition and flame hydrolysis deposition methods were investigated. The thin film prepared by the former method showed inhomogeneous Ge distribution, and Ge oxygen-deficient centers were observed. When it was thermally annealed at temperatures higher than 800u200a°C, the Ge distribution became uniform. The concentration of oxygen deficient centers was found to decrease with the thermal annealing in an O2 atmosphere, while it increased with the thermal annealing at 1000u200a°C in N2. This suggests that improvement of the film quality can be achieved by thermal annealing. On the other hand, neither inhomogeneity of Ge distribution nor the appearance of oxygen deficient centers was observed in the film prepared by the latter method, and its film quality was scarcely affected by the thermal annealing.

Characterization of refractive index changes of silica glass induced by ion microbeam

Abstract Distributions of structural and refractive index changes of silica irradiated by H + microbeam were studied by optical and atomic force microscopes (AFM). The AFM measurements on the microbeam irradiated area show the formation of a groove on the surface. In addition, a cross sectional observation on the surface parallel to the incident plane reveals surface deformation along the ion tracks, which is deepest at the projected range of ions. Taking into account the possible structural changes of silica induced by energy deposition calculated by TRIM, the measured topological changes at the front and side surfaces result from internal compaction of silica glass. Refractive index changes were estimated from the Lorentz–Lorenz relationship using the distribution of the internal compaction estimated by the AFM measurements.

Characterization of ion-implanted silica glass by vacuum ultraviolet absorption spectroscopy

AbstractWeinvestigatethemechanismsofdefectformationandopticalabsorptioninducedbyionimplantation,forfab-rication of optical devices by radiation effects. High-purity silica implanted by H þ or He 2þ was characterized usingvacuum-ultraviolet spectroscopy and electron-spin-resonance measurement. Defect formation is suppressed by OHgroups,possiblybythereleaseofatomichydrogen.TheE 0 centerandnonbridgingoxygenholecenterwerecreatedthroughpairgenerationfromthenormalSiAOASibond.TheperoxyradicalwasgeneratedthroughthereactionoftheE 0 center with interstitial oxygen, which is a Frenkel-defect pair with an oxygen vacancy. By the Kramers–Kroniganalysis on the MeV-ion implantation-induced defects, a refractive index increase of the order of 10 4 was esti-mated. 2002ElsevierScienceB.V.Allrightsreserved. Keywords:Silicaglass;Ionimplantation;Vacuumultraviolet;Electronspinresonance 1. IntroductionFabrication of optical devices such as opticalfibergratingshasbeenreportedbyourgroupus-ing mega-electron-volt-order ion implantation onsilica-coreopticalfibers[1].Inordertoextendthistechniqueforthefabricationofsilica-basedplanaroptical devices, it is important to understand themechanismsbywhichchangesinopticalpropertiessuch as refractive index are induced by the im-plantedions.Attentionisfocusedontheelectronicstopping energy loss, since it accounts for morethan 99% of the total loss and most defects areinducedthroughthisprocessinthecaseofMeV-orderionimplantation.2. Experimental proceduresSamplesaretwotypesofhigh-puritysilicaglas-ses containing different OH contents, EDC (OHcontent <1 ppm) and ES (OH content¼ 1200ppm).Theyareplateswithanareaof100mm

Control of coupling ratio by proton implantation for a directional coupler of planar-lightwave-circuit type

By implanting protons into a cladding in the coupling region of a directional coupler of the planar-lightwave-circuit type at an acceleration energy that enables the protons to reach the center of the core, the coupling ratio of the coupler is changed periodically and continuously by increasing proton fluence. In the case of two optical waveguides formed in a planar lightwave circuit with a mutual spacing too large for optical coupling, proton implantation into the cladding between the waveguides induces optical coupling between them. These results indicate that the coupling ratio of a directional coupler can be controlled by ion implantation.

Excited-state absorption measurement in Ge-doped SiO2 glass

Optical absorption change in the microsecond order in oxygen-deficient Ge-doped SiO2 glass was measured as a function of time just after photon irradiation from a KrF excimer laser. The absorption above 3 eV was found to decay with a similar time constant as that of the luminescence at 3.1 eV. From this, it is confirmed that the observed absorption change is due to the excitation of electrons from the lowest excited triplet state to an upper state. By taking account of the energy range of the absorption, there is a high possibility that the upper state is the conduction band.

Optical Fiber Depolarizer Using Birefringence Induced by Proton Implantation

An optical fiber depolarizer was developed utilizing birefringence that was induced by ion implantation into a silica-core optical fiber. The implanted ions were protons, and their acceleration energy was chosen such that the protons reach only the center of the optical fiber core. To evaluate the depolarization effect, the degree of polarization was measured for various transmitted polarized lights with different polarization states. Several proton-implanted fibers were spliced, holding their slow axes at a mutual angle at which the degree of polarization was lowest. As a result, an optical fiber depolarizer that can reduce the degree of polarization of incident light up to 0.6 ×10-2 was obtained by splicing three ion-implanted optical fibers.