Fused silica optics are widely used in high-power laser systems and ultraviolet optical systems because of their excellent properties. Laser-induced damage threshold (LIDT) of fused silica is a key problem that limits the improvement of optical systems. Atmospheric pressure plasma processing (APPP) has great promise for improving LIDT because it involves lowcost, non-contact, and high-efficiency material removal based on its pure chemical etching mechanism. However, the deteriorated surface morphology after atmospheric plasma etching limits the further improvement of LIDT of fused silica. This study analyzes the exposure and passivation processes of subsurface scratches during atmospheric pressure plasma processing. The evolution process of interfacial contours related to the etching and deposition process is described. At the end, a series of etching experiments under different dwell times are carried out and a flexible medium polishing processing is used to improve the deteriorated surface quality. The results indicate that the LIDT of fused silica is significantly increased from 8.1 J/cm2 to 30.8 J/cm2 by APPP and flexible medium polishing processing. APPP is expected to be effective in improving the LIDT of fused silica.
Spatial period is an important characteristic parameter in the design and fabrication of continuous phase plate (CPP). The smaller the minimum spatial period, the more freedom of CPP design and the more difficult of CPP manufacture. The minimum spatial period of continuous phase plate determines removal function size of tool in high-precision machining of continuous phase plate. Based on chemical reaction, atmospheric pressure plasma processing (APPP) is a non-contact and high efficiency material removal method. The removal function of APPP is nonlinear with dwell time because of chemical reaction rate affected by temperature. The dwell-time algorithm of variable removal function was proposed in order to solve the nonlinear removal function. APPP for fabrication of continuous phase plate with small spatial period is introduced in this paper. Finally, APPP with variable removal function dwell time algorithm was used to fabrication a continuous phase plate with spatial period of 8mm, surface peak-valley (PV) more than 790nm, wavefront gradient root-mean-square (RMS) of 1.07um/cm. The results show that residual surface error between designed surface and measured surface root-mean-square (RMS) is down to 50 nm. The variable removal function in APPP for fabrication of continuous phase plate with small spatial period is validated.
Laser damage of fused silica is a critical problem that limits the fluence increase of high-power laser systems. The reaction of plasma processing of fused silica was analyzed. The redeposited layer was removed and scratches of the fused silica were passivated by plasma to increase the laser-induced damage threshold (LIDT). It was found that the deposition generated during the etching process adheres to the surface of fused silica to limit the further increase in the LIDT. The LIDT of fused silica tends to increase and then decrease as fused silica is processed by atmospheric pressure plasma. Accordingly, a competitive evolution for etching and deposition affecting the LIDT of fused silica was established. The etching and deposition relationship analyzed by the model can effectively increase the LIDT of fused silica without deteriorating the original surface of fused silica by choosing an appropriate dwell time.
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