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