Description
22
4.3
Principle of Operation
Laser beam source
The laser source consists of a Q-switched Nd:YVO4 laser (neodymium-doped yttrium va-
nadate) whose emission wavelength 1064 nm is converted by frequency multiplication
with optically nonlinear crystals in the visible or ultraviolet spectral range.
The pulsed laser radiation of the basic wavelength 1064 nm is generated in a resonator
by the optically pumping of a Nd:YVO4 crystal and modulated with an acousto-optic
Q-switch. Depending on the configuration frequency conversion takes place inside a
resonator with a SHG (second harmonic generation) crystal in the visible spectral range
at 532 nm (
DPL
Nobilis
Marker
) or by another THG (Third Harmonic Generation) crystal
at 355 nm (
DPL
Lexis
Marker
).
First-pulse suppression
Q-switched solid-state lasers possess the characteristic that at high pulse frequencies in
the kHz range, the pulse energy of the first pulses is often significantly greater than the
pulse energy of the following pulses. For extremely sensitive materials, in particular, this
pulse increase results in a strong reaction on the material, which is revealed in the form
of broadened laser structures at the beginning of the vector contours. In order to minimise
(or prevent) this effect, first-pulse suppression (FPS) can be activated. FPS reduces the
pulse energy of the first pulses so that all the pulses in a pulse train possess virtually the
same energy.
The FPS setting is made in the
Magic
Mark
software by specifying a time in
μ
s, which
should be indirectly proportional to the pulse repetition frequency. This value should al-
ways be set (or optimised) depending on the material involved. The reciprocal value of the
pulse repetition frequency - plus 20% of this value - is recommended as the start value for
optimising this time. For example, for a pulse repetition frequency of 40 kHz, this would
result in a start value of 30
μ
s to optimise the FPS. The FPS is not activated if a time of
0
μ
s is specified.
Содержание DPL Lexis Marker
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