Yuki Kondo

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses

We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500 degrees C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique will be useful for fabrication of fiber gratings with a superior aging characteristic.

Three-Dimensional Microdrilling of Glass by Multiphoton Process and Chemical Etching

We demonstrate the three-dimensional microdrilling of glass by the multiphoton process with nonresonant femtosecond laser pulses and by the subsequent chemical etching. We use photomachinable glass which is sensitive to cw UV light of a wavelength shorter than 320 nm. After the focused irradiation of nonresonant femtosecond laser pulses at 400 nm and subsequent heat treatment, crystallites of Li2OSiO2, which are more soluble in dilute hydrofluoric acid than matrix glass, precipitate in the focused area of the laser within the glass sample. After etching the crystallites, three-dimensional holes are formed in the glass sample. We produce straight and Y-branched holes in the glass sample. This technique can be applied to the fields of microoptics, microelectronics and microchemicals.

Three-Dimensional Microscopic Crystallization in Photosensitive Glass by Femtosecond Laser Pulses at Nonresonant Wavelength

We investigated three-dimensional microscopic crystallization by irradiation of femtosecond and nanosecond laser pulses at a nonresonant wavelength of 630 nm in a photosensitive glass containing Ag+ and Ce3+. The pulse widths of the lasers were 150 fs and 3 ns, respectively. By focusing the femtosecond laser beam and following heat-treatment, spot crystallization within the glass of about 10 ?m successfully occurred without surface crystallization, while the use of the nanosecond laser beam produced no crystallization. By using the femtosecond laser, it is possible to create three-dimensional structured crystallization because of the use of transparent light and the presence of a threshold for crystallization.

Infrared Femtosecond Laser Induced Visible Long-Lasting Phosphorescence in Mn2+-Doped Sodium Borate Glasses

We report on the long-lasting phosphorescence phenomenon in Mn2+-doped sodium borate glass. After scanning by a focused 800 nm femtosecond laser for 10 s, the path traversed by the focal point of the laser in the glass emits reddish long-lasting phosphorescence visible to the naked eye in the dark five minutes after the removal of the activating laser. Absorption spectra of the glass showed that defect centers were induced after laser irradiation, and the absorption due to the laser-induced defect centers decayed with time. The long-lasting phosphorescence is considered to be due to the thermostimulated recombination of holes and electrons at traps induced by the laser irradiation, and energy transfer from the composite defect centers to Mn2+ ions.

Space-selectively crystallized fiber with second-order optical nonlinearity for variable optical attenuation

We have fabricated BaO-TiO2-GeO2-SiO2-based glass fibers with the oriented space-selectively crystallized structure by laser irradiation and also demonstrated variable optical attenuation induced by electro-optical birefringence change based on second-order optical nonlinearity. The transmittance of a polarized signal is controlled by an electric field applied to the fiber, and the electro-optic fiber devices are operated with extremely low nanowatt electric power dissipation.

Wavelength dependence of photoreduction of Ag+ ions in glasses through the multiphoton process

We have investigated the wavelength dependence of the photoreduction of Ag+ ions in glass irradiated by visible femtosecond pulses. These pulses, issued at wavelengths ranging from 400 to 800 nm, were nonresonant with the glass absorption. In this article, a relationship between threshold powers, wavelengths, and linear and nonlinear refractive indices is described. The nonlinear refractive index of Ag+-doped glass was measured by an optical Kerr shutter method. The wavelength dependence of threshold powers of the photoreduction is explained by considering linear and nonlinear refractive indices. The mechanism of the photoreduction is also discussed.

Third-order optical non-linearities of CuCl-doped glasses in a near resonance region

Third-order optical non-linearities of CuCl microcrystallites with sizes of 6.8-11.0 nm in glasses were investigated. Absolute values of third-order non-linear susceptibilities, |χ (3) (-ω 1 : ω 1 , -ω 1 , ω 1 )|, were measured in the resonance region by a degenerate four-wave mixing method and |χ (3) (-ω 2 : ω 1 , -ω 1 , ω 2 )| were measured in the near resonance region by a non-degenerate four-wave mixing method. |χ (3) (-ω 1 : ω 1 , - ω 1 , ω 1 )| was strongly dependent on the frequency, ω 1 , of incident beam, while |χ (3) (-ω 2 : ω 1 , - ω 1 , ω 2 )| was less dependent on the frequency, ω 2 , of incident beam under the resonant excitation condition. In the non-degenerate four-wave mixing configuration in which the pumping frequency is resonant with the confined excitons, the figure of merit, |χ (3) |/α, was enhanced to more than an order of magnitude compared with that in the degenerate four-wave mixing configuration in a near resonance region.

Ultraviolet irradiation effect on the third-order optical nonlinearity of CuCl-microcrystallite-doped glass

We investigate the effect of irradiation by a UV pulse laser on the third-order optical nonlinearity of CuCl-microcrystallite-doped glass as a function of the irradiation’s shot number, energy density per pulse, and wavelength. The response time τ is estimated from the luminescence decay time of the CuCl excitons. The absolute value of the third-order nonlinear susceptibility χ(3) is measured by degenerate four-wave mixing. Values of both τ and χ(3) decrease after UV irradiation. The figure of merit for the optical nonlinearity, χ(3)/ατ, where α is the absorption coefficient, is almost constant before and after UV irradiation and is approximately 500 esu cm s-1 in the case of 8-nm CuCl microcrystallites. This value is 1 order of magnitude larger than that of CdSxSe1-x-microcrystallite-doped glasses. The mechanism by which τ and χ(3) are decreased is discussed, considering defects levels formed at the interface between CuCl microcrystallites and glass matrix.

Bi2O3-based glass for S-band amplification

Bi2O3-based thulium (Tm3+) doped glass fiber (Bi-TDF) for S-band amplification was investigated. Tm3+ was doped in Bi2O3-SiO2 based glass and melted using a conventional method. Emission spectra of the 3H4 - 3F4 were measured with pumping at a wavelength of 792 nm using laser diode (LD). Full width of half maximum (FWHM) of the emission is 1.4 times and 1.1 times broader than that of fluoride glass and tellurite glass, respectively. Moreover, the emission peak shifted towards longer wavelength as compared with fluoride and tellurite glasses. Single mode Bi-TDFs with Tm3+ concentrations of 2000 ppm, 3900 ppm and 6000 ppm were fabricated and evaluated with fusion splicing to SiO2 fibers. Gain profiles were measured with bi-directional pumping using 1047 nm Yb fiber lasers. The gain-peaks observed around 1470 nm shifted towards longer wavelength with increasing Tm3+ concentration. Gain properties of Bi-TDF were improved by additional pumping at the wavelength of 1560 nm with Tm3+ concentrations of 2000 ppm and 3900 ppm. A maximum gain over 10 dB of Bi-TDF was obtained using a fusion spliceable Bi-TDF with a length of only 100 cm.

Band-gap dependence of optically encoded second-harmonic generation in Bi2O3–B2O3–SiO2 glasses

Abstract The dependence of photoinduced second-harmonic generation (SHG) on the band gap of glass was investigated using Bi 2 O 3 –B 2 O 3 –SiO 2 glasses. The experiments showed that the saturation intensity of the IR preparation beam for the photoinduced SHG and the decay of the SHG depended strongly on the band gap of the glasses. These glasses exhibited lower saturation intensities of the 1064-nm preparation beam for the photoinduced SHG compared to that of germanosilicate glass.