Ph. D. Dissertation: Studies on Laser Design and Laser Micro-Machining


Since the first laser was operated in 1960, extensive research and development have been undertaken leading to a rapid growth in both laser technology and laser applications. With its increasing use in material processing, the need for further study and improvement of the laser beam performance becomes necessary to achieve optimal processing quality. The first objective of this project, therefore, was to investigate the thermal lensing phenomenon of a laser medium which has the most significant effects on the laser beam of a solid state laser. Laser beam quality characterization, calculation and measurement of thermal lensing characteristics, study of the stability of a thermal lensing resonator, and the study of the efficiency of pumping cavities constitute the essential part of this project. The second objective of this project was to investigate the use of solid-state lasers for micro-machining of materials. As examples, two typical applications, laser micro-engraving of photomasks and laser micro-drilling of various engineering materials, are studied in the project and presented in this thesis.

The lasers used in the experimental studies are: a NEC 7W Q-switched CW Nd:YAG laser, a BMI 1.2J/12W pulsed Nd:YAG laser, a HUST 10J/pulse Nd:YAG laser, and an Exitech 80W KrF excimer laser system. The NEC laser was employed to study laser beam performance, thermal effects on the lasing medium, and laser engraving. The HUST laser was employed for the study of laser pumping efficiency. The BMI laser and the Exitech laser were employed for laser micro-drilling.

This PhD programme has led to a number of new findings in solid-state lasers and laser micro-machining. The original contributions made by the author are as follows:

(1) It is found that the beam propagation factor increases with an increase in pumping power, as well as with an increase in intra-cavity aperture size. However, the beam quality becomes worse when the aperture diameter is comparable with TEM00 beam size.
(2) An exact formula to calculate the thermal focal length in solid-state lasers is introduced. The formula relates the thermal focal length to beam propagation factor (M2) of the laser beam. Experiments show that the thermal focal lengths obtained using the new formula are longer than those calculated with the conventional formula by 14.3% to 29%. Experiments performed on a Nd:YAG laser rod confirm that the new formula is more accurate.
(3) It is found that the resonator in the stability diagram, given in the (G1, G2) plane, moves along a straight line. The line passes through two stable and three unstable zones. The two stable zones have the same width with respect to the dioptric power. Analysis shows that the mode volume (or the beam spot size) in the rod always presents a stationary point. At this point, the output power is insensitive to the fluctuations in the focal length of the thermal lens.
(4) The energy contributions from different reflective portions of the pumping cavity are significantly different. The reflection wall for 00 - 900 contributes nearly 75% of total geometrical coupled energy. It is found that lamp blockage, corresponding to the non-effective use of the reflection wall between 90o to 180o,  is a major factor in reducing the coupling efficiency. Experiments show that the pumping efficiency increases by about 50% for the single-lamp twin-rods configuration as compared to the single-lamp single-rod configuration.
(5) A theoretical expression for calculating focal spot size is derived for the optical delivery system consisting of laser resonator, beam expander, and focusing lens. Comparative experiments performed on laser engraving of iron oxide and chrome photomasks show that the excimer laser has distinct advantages over the CW Q-switched Nd:YAG laser. Engraved lines with a minimum width of 8 microns are obtained.
(6) Using the Taguchi method, the line width for Nd:YAG laser micro-engraving of  iron oxide photomasks has been optimized. The experiments reveal that the beam expansion ratio, the average laser power, the engraving speed, and the interaction between beam expansion ratio and focal length could significantly affect the engraving line width. The 95% confidence interval for the minimum line width that can be obtained is  .
(7) Four engineering materials (PEI, PI, PC, and brass) were drilled with fundamental (1064 nm), second harmonic (532 nm), and fourth harmonic (266 nm) Nd:YAG laser pulses. Experiments show that micro-drilling by the fourth harmonic generates the smallest heat-affected-zones compared to the other two wavelengths. Thus, lasers with short wavelengths are generally preferred in laser micro-drilling.
(8) Four polymers (PI, PEI, PC, and PETP) were drilled with a KrF excimer laser operating at 248 nm. It is found that the etch rate increases with an increase in fluence. The material removal rate produced by a laser pulse is also highly controllable. However, the wall angle bears no linear relationship with the fluence. Furthermore, a hole can be produced with a bigger diameter on the top surface or on the bottom surface, depending on the fluence. There exists an optimal energy density range that can be used to obtain clean and smooth high-quality holes for a given material.

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