Yutaka Kuroiwa

Arbitrary micropatterning method in femtosecond laser microprocessing using diffractive optical elements

We successfully developed an arbitrary micro-patterning method with femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. The large chromatic dispersion effects of DOE resulting from the spectral bandwidth of femtosecond pulses can be reduced with the appropriate DOE focal length and the proper distance between the DOE and the focusing lens. The method was verified through optical and processing experiments. A partial periodic structure was formed at the designated position. Microstructures were precisely formed on the SiO2 glass surface and inside the glass by irradiating the constructed beam. The points were evenly dispersed with a separation of 5 mum.

Fabrication of high-efficiency diffraction gratings in glass

We investigated a microfabrication process for optical gratings with periods of micrometer order that use ultrafast laser pulses in semiconductor-doped glass. ZnS- or PbS-doped SiO2-A12O3-B2O3-CaO-ZnO-Na2O-K2O glass was prepared by a melting method. Glass transmission diffraction gratings with a high refractive-index difference were fabricated with femtosecond laser pulses. The first-order diffraction efficiencies of these gratings were approximately 80%, and the first-order diffraction angles of these gratings were 8 degrees at telecommunication wavelengths.

Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses

A microfabrication process using ultrafast laser pulses in glass was investigated. We investigated the formation of semiconductors by the irradiation of glasses with femtosecond laser pulses. ZnS- or PbS-doped SiO(2)-Al2O(3)-B(2)O(3)-CaO-ZnO-Na(2)O-K(2)O glasses were prepared by a melting method and irradiated by femtosecond laser pulses. Periodic structures in the sample glasses with a high refractive index difference were produced by femtosecond laser pulses. The maximum relative refractive index difference between the irradiated area and the nonirradiated areas was 20%. Diffraction gratings were also fabricated inside the ZnS- or PbS-doped silicate glasses. The diffraction efficiency of these gratings was approximately 90% in the infrared region.

Highly ytterbium-doped bismuth-oxide-based fiber

Thermally stable highly ytterbium-doped bismuth-oxide-based glasses have been investigated. The absorbance increased linearly with Yb(2)O(3) concentration, reaching 7800 dB/m with 3 mol-% of Yb(2)O(3). An ytterbium-doped bismuth-oxide-based fiber has also been fabricated with a fiber loss of 0.24 dB/m. A fiber laser is also demonstrated, and it shows a slope efficiency of 36%.

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.

Fusion spliceable and highly efficient Bi/sub 2/O/sub 3/-based EDF for short-length and broadband application pumped at 1480 nm

A conventional fusion splicing to SiO/sub 2/ fiber was achieved in W/sub 2/O/sub 3/-based Er doped fibers (EDF). High slope efficiency of 43 % and broadband gain spectrum were achieved at 1490 nm pumping amplification for only 26 cm-length.

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.

Quenching treatment for improvement of nonlinear optical response time in high-χ(3) CuCl-doped glasses

Abstract Quenching treatments were performed on CuCl microcrystallite-doped glasses prepared by a melting method. Radiative decay time measurement and χ (3) measurement for the quenched glasses were performed using ps and ns laser. The glasses showed that the radiative decay time were shortened by one order of magnitude without a decrease of third-order nonlinear susceptibility χ (3) after the quenching treatment from above 300°C. The distorted structure of CuCl microcrystallites quenched from high temperature was considered to be most effective for the shortening of the lifetime of Z 3 exciton.

3D patterning method in femtosecond laser microprocessing using diffractive optical elements

We demonstrated a simple chromatic dispersion reduction method of 3-dimensional (3D) patterning of femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. Our method increases flexibility of femtosecond laser microprocessing. With appropriate focal length of the DOE and distance between the DOE and the focusing lens, large chromatic dispersion of the DOE resulting from spectral bandwidth of a femtosecond pulse can be reduced, and 3D focusing pattern of femtosecond pulse can be obtained not only controlled in focal plane but also in focal depth. The method was verified through optical and processing experiments with laser pulses of 400 fs duration and of 40 nm bandwidth. The focal length of the DOE and the objective lens was 1600 mm and 10 mm, respectively. Partially periodical structure of focusing points was formed at designed position and its focal depth were much smaller than that focused with only the DOE. By irradiating the constructed beam, microstructure was formed precisely inside SiO2 glass. The processed points are clearly separated each other with a separation of 5 mm and the spot sizes were almost same as those irradiated without the DOE.