03-04-2012, 12:28 PM
Laser Micromachining Seminar
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Population Inversion
A laser requires a “population inversion” to sustain a continuous or even a pulsed output. A population inversion
exists between two lasing energy states when there are more species occupying the upper state than the lower. The
process used to excite lower energy atoms or molecules to their excited states is called “pumping”.
Essential Elements of a LASER Oscillator
A laser requires a lasing medium, pump process and a resonator cavity to sustain oscillation. The lasing medium
can be a gas, solid, liquid, or in the case of semiconductor lasers, electrons. The pump process excites the atoms or
molecules of the lasing medium to their upper energy states by electronic means or kinetic energy transfer. Laser
transmission is initiated by spontaneous emission and amplified by stimulated emission along the axis of the
resonator cavity. The cavity mirrors reflect a portion of the photons back and forth through the laser medium for
increased amplification.
Characteristics of the laser cavity:
· Rear resonator mirror is fully reflective
· Front optic is partially reflective and partially transmissive - for a helium-neon laser, the
front optic is about 98.5 % reflective; for an excimer laser, the front optic is near 10%
reflective because of the high gain in the laser medium
· Resonator optics can be concave, as in a helium-neon laser, or flat, as in an excimer laser
· Lasing medium must have high stimulated emission cross section so more photons are
produced than absorbed
· Methods of laser pumping: gas discharge, optical (flashlamp), chemical pump, laser
pump, electron collision excitation
CO2 Laser Operational Theory
A carbon dioxide laser uses a gas mixture of CO2:N2:He. The CO2 molecules constitute the active lasing medium,
the N2 gas serves as an energy transfer mechanism and the He atoms enhance the population inversion by
depopulating the lower energy states. The population inversion and lasing transition in a CO2 laser is established
between vibrational and rotational energy states. Most CO2 lasers are pumped by a gas discharge
Autofocus -
the lens is focused automatically by a stepper
motor controlled by the system computer at the touch of
a key or through a process program.
Automagnification -
the mask and lens is slewed
according to algorithms derived from the thin lens
equation.
Rotary or linear mask control -
a rotary mask or linear
mask assembly is actuated by a computer controlled