The residual stress field of fused silica induced by continuous wave CO2 laser irradiation is investigated with specific photoelastic methods. Both hoop stress and axial stress in the irradiated zone are measured quantitatively. For the hoop stress along the radial direction, the maximum phase retardance of 30 nm appears at the boundary of the laser distorted zone (680-μm distance to center), and the phase retardance decreases rapidly and linearly inward, and decreases slowly and exponentially outward. For the axial stress, tensile stress lies in a thin surface layer (<280 μm) and compressive stress lies just below the tensile region. Both tensile and compressive stresses increase first and then decrease along the depth direction. The maximum phase retardance induced by axial tensile stress is 150 nm, and the maximum phase retardance caused by axial compression stress is about 75 nm. In addition, the relationship between the maximum axial stress and the deformation height of the laser irradiated zone is also discussed.
The laser-induced bulk damage and stress behaviors of fused silica are studied by using a neodymium-doped yttrium aluminum garnet laser operated at 1064 nm with pulse width of 11.7 ns. Three zones of bulk damage are defined: columned cavity zone, compacted zone, and crack zone. The damage morphology and stress distribution are characterized by a three-dimensional digital microscope and a polarizer stress analyzer. The results show that the stress in the columned cavity zone and compacted zone is approximately zero. From the laser beam center to fringe, both tensile and compressive stresses in the crack zone increase abruptly and linearly and then decrease exponentially. Thermal annealing is used to prove the phase retardation caused by the residual stress. The formation mechanism of bulk damage is also discussed.
Two-photon polymerization is a powerful technique in the area of functional micro/nano device fabrication. The greatest limiting factor in widespread use of this technique is the low efficiency because the structure is fabricated by point-by-point scanning. In recent years, computer generated hologram is used for parallel fabrication via multi foci. In this paper, we proposed a new rapid fabrication method which use desirable multi-focus pattern as scanning cell instead of single focus point or foci array to polymerize. We establish a femtosecond laser experimental setup involved in a liquid crystal spatial light modulator. The computer generated hologram pattern on spatial light modulator is used to produce desirable foci array. The position and intensity of each focus in the pattern can be controlled well by optimal design. We use multi foci in a line as scanning cell to fabricate some revolving structure and the Fresnel lens can be expected. This work provides a new method to greatly improve the efficiency of two-photon polymerization production in fabricating revolving structures.