Study On Laser Processing Mechanism And Machining Key Technologies For KDP Crystals

Potassium dihydrogen phosphate (KDP) is the unique large-scale crystal that can be used as optical switch and frequency converter in Inertial Confinement Fusion (ICF). However, owing to its inherent shortcomings such as soft in structure, high brittleness, sensitivity to temperature and stress change, easy dehiscence and deliquescence, KDP crystal is considered as one of the most difficult-to-cut materials. Until now, most of the studies have been focusing on the ultra-precision polishing of KDP crystal after being cut into slices, but there are just a few studies about the cutting of raw KDP materials. So far, researchers are still using the bladed saw or electroplated diamond wire saw to cut KDP crystals with a very low cutting speed and bad cutting safety, and fatal crystal fragmentation occurs frequently in the cutting procedure. Aiming at solving the technological difficulties and challenges existed at present, this dissertation proposes a laser separation technology of KDP crystal innovatively for the first time, and provides a new approach to the manufacturing of KDP crystal devices.Based on the characteristic that KDP has a high compressive strength (113Mpa) and a low tensile strength strength (only5-8Mpa), and the bulk absorption effect of transparent material on laser, a dual laser beam separation technology and a laser nondestructive mirror-separation technology for KDP crystal are proposed in this thesis, with a combination of "cold" processing technology of ultrafast laser and "thermal" processing technology of traditional laser. The mechanism of the technology is making a pretreating of KDP crystal by using ultrafast laser to achieve the artificial adjustment of optical absorptivity and binding capacity inside crystal, and then inducing a tensile stress or microscopic tensile stress inside the crystal by using continuous laser to achieve the crystal separation. Hence, the researches and achievements in this thesis mainly include the following aspects.(1) This thesis has set up the transmission model of focused laser inside the KDP crystal according to ray tracing and wave optics theory, moreover, has simulated and analyzed the transmission, focusing, defocus caused by birefringence and spot distortion characteristic of the laser with different parameters after focusing into the KDP crystal at different orientations. It turned out that birefringence effect of crystal had a strong impact on the transmission characteristic of E-ray, and the3-D light intensity distribution function generated in the crystal has been deduced. (2) The interaction process between ultrafast laser and KDP crystal has been studied in this dissertation. The results show that the3-D adjustment of optical absorptivity and binding capacity of KDP crystal can be achieved when peak power is low, while microscopic crack can be realized when peak power is high. This dissertation has studied the interaction between traditional continuous laser (or long pulse laser) and KDP crystal, and has confirmed the interaction mechanism and initial processing optical system which can be used for the laser separation (or nondestructive mirror-separation) of KDP crystal.(3) Based on the processing optical system and light intensity distribution function, the mathematical models of laser induced dynamic temperature distribution and thermal stress distribution inside the crystal have been established, and the laser separating mechanism and process of KDP crystal have been revealed in this thesis. The results indicate that the separating mechanism is the Y direction tensile stress or microscopic stress induced by laser. Meanwhile, this thesis has carried out the research of numerical simulation and confirmed the key laser separating parameters and the optimal processing parameter range. Moreover, the disadvantages in the separating process have been ascertained in order to improve the reliability and stability of separating process.(4) Based on the numerical simulation and results of theoretical analysis, this dissertation has set up the laser separating system and nondestructive mirror-separating system for KDP crystal. The separation, of which the relative roughness of KDP crystal is only2.684um and the partial roughness is below500nm, has been realized by using the dual laser beam separation technology, and its separation speed is20times higher than that of mechanical cutting. The nondestructive mirror-separation, of which surface roughness of KDP is4.7nm(p-v) and2.1nm(RMS), the flatness is5.433μm and the angle precision is below0.06°, has been achieved by using the laser nondestructive mirror-separation technology, furthermore, its separation speed reaches up to even more than200times higher than that of mechanical cutting technology. In addition, this thesis has analyzed the influence rules of every parameter and carried out the experimental research of laser separating technique and the processing parameters optimization. Eventually, a complete set of technical theory of laser separation of KDP crystal is achieved.