Abstract
The free-electron density is very important to investigate the nonlinear absorption phenomenon, which strongly relates to the
joining characteristics such as mechanical strength in fusion micro-welding of glass material by an ultrashort pulsed laser. In this
study, the free-electron density distributions for different numerical apertures (N.A.) were simulated, and photoelasticity observation
and the mechanical strength in micro-welding of glass was experimentally investigated. N.A. 0.45 has a large difference of free-
electron density in beam axis, and the area of large principle stress is generated. On the other hand, N.A. 0.65 can obtain small
difference of peak electron density in the upper and lower parts of molten area, which led to continuous molten area formation. This
joining characteristic resulted in small area of large principal stress difference and high mechanical strength.
