Narrow Results

The laser damage resistance of fused silica optics depends significantly on the surface quality. In this work, anisotropic etching with inert ion beams at various ion incident angles was performed to investigate the evolution of the fused silica surface. The results show that the surface is smoothed when the incident angle is below $30^{\circ }$. However, the fused silica surface starts to become coarse owing to the formation of nanostructures on the surface when the incident angle exceeds $30^{\circ }$. Further, ion beam etching at a large incident angle of $70^{\circ }$ removes subsurface defects and less induces nanostructures, resulting in reduction of the surface roughness. The concentrations of impurities and defects are both significantly reduced after ion beam etching. The surface quality, subsurface and surface defects, and surface impurities determine the variation in the laser damage threshold of fused silica with the ion incident angle. The results demonstrate successful application of ion beam etching to improve the laser damage resistant characteristics of fused silica optics. Ion beam etching is a very versatile tool that provides physical erosion to anisotropically mitigate surface damage of fused silica.

Photonic patterns were fabricated in fused silica, BK7, and Ge-doped borophosphosilica glass (Ge-BPSG) using a focused femtosecond (fs)-laser beam. By focusing tens to hundreds of μJ fs-laser beam with a 10x microscope objective, we inscribed the semi-circular cavity patterns on the fused silica and the BK7. The inscribed hole diameters are 28 μm (fused silica) and 11 μm (BK7) at an input fluence of 71 J/cm². This circular-cavity patterning is ascribed to the ablation via the multi-photon absorption process. For the application to functional devices, the surface relief gratings (SRGs) were made in fused silica and BK7 by focusing the fs-laser beam on the glass surface with a cylindrical lens and by translating the sample in the direction perpendicular to the focus line. The first-order diffraction efficiencies of the prepared SRGs are 34% (fused silica) and 14% (BK7). A refractive-index grating was also fabricated in the Ge-BPSG by using the two-beam interference method. The maximum index modulation of 2.5 × 10^-3 was obtained for 20,000 laser shots of 73 mJ/cm² per pulse. It is thought that the index modification occurs through the defect formation by the fs-laser irradiation.

The second-order optical nonlinearity (SON) of thermally poled fused silica was shown for the first time to be significantly improved by the implantation of multi-energy boron (B) ions, and the outcomes were compared with those obtained from the implantation of multi-energy argon (Ar) ions. Second-harmonic (SH) signals in both samples grew roughly linearly as the poling temperature rose. Samples with Ar implants showed a greater increase in SH signal with rising poling temperature. A qualitative explanation for the formation of ${\chi }^{(2)}$ in poled samples can be found in a model of charge hopping and migration through defects. The diminished SH signal in the B-implanted sample is probably caused by the negative charge trapping layer, as well as by B${}_{2\alpha }$ and E${}^{\prime }$ center defects.

Advanced LIGO will feature a quadruple suspension with a lower monolithic stage of fused silica in order to meet the seismic and thermal noise requirements. Fused silica ears are silicate bonded onto the polished flats of the test/penultimate mass and fused silica fibres, which are fabricated with a computer controlled CO₂ laser pulling machine, are then laser welded onto the ears. The whole process is robust and has been successfully demonstrated with eight test suspensions to date (six in Glasgow and two at LIGO MIT). In the second quarter of 2010 a monolithic quadruple suspension will be installed into the LIGO Advanced Systems Test Interferometer (LASTI) at MIT, Boston.