Recent year, they require the high performances of laser as a light source in variety application area. For instance, those
are a shorter wavelength, a shorter pulse width. In order to serve those needs, an improvement of the laser damage
threshold value of optical element used in the laser applications is required. And they reported that a surface-roughness
of glass substrate as used coated optical element exert also influence that. Currently, the chemical mechanical polishing
method (CMP method) is general used as the polishing method of optical element. This method is a friction method.
Therefore, the reduction of the surface-roughness is prevented by generation of scratches and digs that keep happening
by contamination in slurry. In order to solve this problem, we propose the optical near-field etching method (ONE). The
ONE is operated by irradiation of a SHG light (λ=532nm) of Nd:YAG laser on glass substrate in chlorine gas
atmosphere that have a optical absorption band edge of 400nm. The radical formation of the chlorine molecular is
created by non-adiabatic photochemical reaction due to optical near-field occurred in glass surface. And the etching is
progressed in the projection of glass surface. With this processing, we can achieve the reduction of Ra value of surfaceroughness
from 0.2nm to 0.13nm. In addition, we gave the mirror coating to the glass substrate to which the surfaceroughness
was improved by ONE and measured the laser damage threshold value. Accordingly, we obtained 14.0J/cm2as the laser damage threshold value. The laser damage threshold value of the glass substrate without ONE is 8.2J/cm2. It
is shown that the laser damage threshold increase by 1.7 times by ONE.