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4D User Manual
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long distances helps, of course, but even small effects at the path from the laser outlet to the
vacuum tube will be transformed to large effects at the end of the (long) tube. Please note,
using evacuated tubes may cause drifts depending on local air pressure variations.
In addition, evacuated tubes are expensive, bulky and inconvenient to handle.
If a laser is lead between different rooms through holes in the wall temperature and pressure
differences between these rooms may lead to strong local air density gradients and thus to
strong pointing drifts. Local pressure differences between the rooms (by air conditioning sys-
tems or by laminar flow systems, e.g.) will cause a strong air flow and air density turbulences.
4.1.5 Thermal effects of optical elements
Every optical element absorbs a distinct amount of the laser beam, which is true for both re-
flecting and transmitting elements. So-called thermal lenses lead to well-known influences of
the collimation properties of the laser beam. With high quality of the materials and coatings
and/or at low intensities the focusing/defocusing effect may be negligible.
But: the (very small) absorbed power leads to a local change of the temperature itself and thus
the temperature gradient, which leads to a pointing deviation. The related time constants can
be very slow, no equilibrium may be reached in many hours.
4.1.6 Mechanically moved optical elements (delay lines, e.g.)
Sometimes optical elements have to be moved within the application:
4.1.6.1 Delay Lines
In short pulse laser systems (ps or fs durations) so-called delay lines are often used to match
or shift two laser pulses in time with respect to each others. A set of several mirrors is moved
by a motorized sleigh which has to be aligned to be exactly parallel to the optical path. This
can only be done to a certain extent; it is difficult to align better than some ten micro radiants.
In addition, the motorized rail is not perfectly straight; there will be curvatures in the order of
typically some ten up to some 100 microns, depending on the length and the price of the rail.
4.1.6.2 Motorized or Manual Telescopes or Zoom Expanders
In some applications, setups of optical lenses have to be moved (telescopes, zoom telescopes,
expanders,…). It is impossible to position and move these elements exactly at the optical axis.
Thus, a beam pointing movement will be observed during the movement of the optical element.
4.1.6.3 Switching Mirrors
In some applications, the laser beam is switched between two paths of the experiment by a
manually operated or by a motorized switching mirror. The reproducibility of the mirror position
may be very high, but will not be perfect. Residual uncertainties of approx. ten µradiant are
typical.
4.1.7 Movement of the experimental (optical) tables or vacuum chambers
Often the laser and the experimental target are mounted at different optical tables. Many ex-
periments are located in small or large vacuum chambers. Those components will be at differ-
ent and changing temperature values. This leads to relative pointing drifts, even if each ele-
ment is very stable by itself.
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