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.
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.
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