Laser Power Control Units. White Paper0 pages

Laser Power Control
Units
s-pol

Laser capability to accomplish extremely precise
tasks by applying high power impact within tiny
spatial domain, e.g. diffraction limited spot, has lead
laser power control to become an important variable
that helps to optimise yield and secure successful
completion of the task. There are various methods to
control the power of solid state lasers which will be
reviewed in this article.
The simplest way to control laser output power is by
controlling its gain medium pump power and in DPSS
lasers it means adjustment of the current feeding laser
diode. However such way of power control cannot
ensure smooth and continuous laser power control
because of several reasons. First of all, changing laser
diode current introduces temperature shift in the diode
which changes the band gap between conduction and
valence bands, thus central wavelength of the pump
is shifted, altering the conditions in solid state gain
medium. In case of CW operation this string of related
events introduce fluctuations in laser output power
and it takes time to stabilize the power again. But if the
laser is pulsed (e.g. passively Q-switched), not only the
output power fluctuations are introduced, but also PRR
and pulse duration would be altered with changing the
pump power. What is more, when laser diode current is
changed during the operation, additional degradation
mechanisms emerge [1]. Thus this type of power
control is only acceptable for the least power-sensitive
applications.
Extra-cavity laser power control is yet the most popular
method of laser power management, for instance
absorbing neutral density glass filters are useful to
discretely attenuate relatively low power beams and
are quite effective up to 1 J/cm² energy densities within
the nanosecond regime. ND filters provide rather even
attenuation over wide spectral range from UV to NIR.
Quite similar are reflective neutral glass filters which
are coated with metallic layer that transmits a portion
of the radiation while reflecting and absorbing the rest
of it. Advantage provided by metallic semi-reflecting

p-pol

Polarizing beam splitter cube

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56°
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Brewster TFP
Half-wave plate
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72°
p-pol
Ultrafast TFP

EO modulator

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p-pol

Glan polarizer
Fig. 1. Options of polarization manipulation based attenuator.

coating is the possibility to deposit thickness gradient
of metallic layer allowing continuously variable density
along filter cross-section.
Laser sources with polarized output provide subtile
attenuation option – output power control through
manipulation of the polarization. Such method
incorporates polarization rotator and polarizer.
Polarization rotator is usually a waveplate or electrooptic (EO) modulator that rotates input polarization.
Then the beam passes onto the polarizer, where
orthogonal polarizations S and P are separated into
two separate S and P polarized beams. Intensity ratio
between the two beams is controlled by polarization

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