mechanical stress, temperature variations, vibrations, shocks
and chemical instability of the adhesives that are used to glue
the optics in their mounts. A CPA system is more sensitive to
misalignment compared to other lasers because of the unique
relationship between beam pointing and dispersion. The most
sensitive CPA lasers are the free-space systems, with all-fiber
lasers being less sensitive to misalignment.
For a free-space CPA laser, the beam pointing variations
into the stretcher and compressor will affect the dispersion
balance—which changes the pulse duration. It is also possible
that some components in the stretcher and the compressor
will also become misaligned; this will also change the pulse
duration. Dispersion-wise, some of these components are more
important than others, and sometimes misalignment in one
direction does more damage than misalignment in another. It
is very important to identify such critical optics in CPA systems
that strive for extreme beam pointing and pulse duration stability under adverse environmental conditions.
Misalignment can also be a big problem for a scientific
research laser. The following example is based on a high-end
laser: a 100 TW, Ti:S CPA system generating sub- 30 fs pulses
in a 50-mm beam. We will assume the laser uses a two-grating
compressor that is perfectly aligned in air. Suppose a compressed
pulse is focused on to the target by an off-axis parabola (OAP).
In this instance, one can achieve a diffraction-limited spot size
near 5 µm.
2. 5 mm
2. 5 mm
(Top) Final focus images obtained with Zemax ray tracing of an
ultrafast laser. A 2. 5 mm2 square detector screen is scanned
across the focus produced by an F/3.6 off-axis parabola. The
compressor is perfectly aligned. The spot is diffraction-limited
and pictured in the middle as a dot. (Bottom) Similar images
appear if the compressor is misaligned by a 0.05 degree
rotation of the 2nd grating in the plane of dispersion.
However, the experiment is performed in a vacuum. The
bending and pulling of the entire vacuum chamber under the
atmospheric pressure will determine minuscule movements of
the compressor optics. If the second grating happens to rotate
in the plane of dispersion by only 0.05 degrees—less than
1 mrad—the diffraction-limited spot will transform into an
aberrated spot. The beam pointing, dispersion balance, pulse
duration, spot size and ultimately the laser pulse intensity in
the focal spot will change.
By using a ray-tracing computer program, the laser engineer
can calculate the magnitude of these changes and, based on the
results, recommend solutions—for example, a stiffer mechanical design for the vacuum chamber. Mechanical behavior of
the optic holders in a vacuum chamber can be predicted quite
accurately with commercially available finite element analysis
software packages even before the chamber is built. Other solutions may be investigated with the ray-tracing program, such as
compensating with the vertical retro-reflector and/or the OAP,
or by changing the beam pointing into the compressor.
Stray light: some want it, some don’t
Scattered light—or stray light—is present in all lasers, but it is of
particular interest in high-energy systems. It is generated by the
laser’s optics and follows beam paths other than those for which
the laser was designed. Lenses are known for producing “back
reflections” from their surfaces. Although the reflections are very
weak, the stray light could cause damage if it is focused on the
surface of another optic in the laser. Computer models built in
the nonsequential ray-tracing mode are very useful in determining the most appropriate curvatures and positions of lenses and
telescopes relative to each other. The simple solution is to rotate
a lens to avoid back reflections to other optics. For a high-end
ultrafast laser, however, one has to use a ray-tracing program to
estimate the implications of such a small angle rotation on the
final laser spot size and pulse duration.
Scattered light can be both a saint and a demon. Many
simple, industrial CPA lasers are being used for tasks that do
not require compressed pulses with high temporal contrast.
The stray light that bounces off the stretcher and compressor
gratings at almost any angle can be visualized with an infrared
viewer. This allows for a fast and easy alignment check that is
greatly appreciated by laser engineers and technicians.
For cutting-edge scientific systems, however, scattered light
poses a grand challenge and possibly a fundamental limitation.
This problem is usually related to the light that is scattered
from the compressor grating surfaces. The mechanism is simple:
the stretched pulse hits the first grating and a very tiny part of
it will scatter. A similar process is reproduced on the second
grating, and then again on the subsequent hits after the pulse
is sent back by the vertical retro reflector.
Scattered light will end up at the intended target either
earlier or later than the compressed pulse because it does not
follow a compressible path, thus contributing to the generation
of a temporal pedestal. The spatial contrast worsens because