2.1 Ultrafast solid-state lasers - ETH - the Keller Group

Ref. p. 134] 2.1 Ultrafast solid-state lasers 99
cases with a high gain (e.g., in fiber lasers). For this reason, grating pairs are normally only used for
external pulse compression [84Tom]. The grating pairs alone can only be used to compensate for
second-order dispersion. Higher-order dispersion limits pulse compression in the ultrashort pulse
width regime. Therefore, a combination of a grating and prism compressor was used to generate
the long-standing world record of 6-fs pulses with dye lasers [87For].
There are mainly two types of gratings: ruled or holographic. Removing material from a master
substrate with a precise instrument called a ruling engine produces ruled gratings. Replicas of the
ruled grating are then pressed and the pressings are coated. Holographic gratings are produced
by interfering two laser beams on a substrate coated with a photoresist, which is subsequently
processed to reproduce the sinusoidal interference pattern. Generally, replicas cannot be produced
from holographic masters, i.e. they are more expensive. Higher damage threshold can be obtained
with gold-coated ruled gratings (i.e. > 500 mJ/cm 2 at 1 ps). The diffraction efficiency in this
case is around 88 % to 92 % depending on the grating. Because four paths through the gratings
are required for dispersion compensation and a spatially coherent beam, this amounts to at least
about 30 % loss.
2.1.5.2.3 Prism pairs
Prism pairs [84For] are well established for intracavity dispersion compensation. Negative dispersion
is obtained with the wavelength-dependent refraction (Fig. 2.1.16c): The different wavelength
components travel in different directions after the first prism and along parallel but separated
paths after the second prism. The wavelength components can be recombined simply on the way
back after reflection at a plane end mirror (of a standing-wave cavity) or by a second prism pair
(in a ring cavity). Spatial separation of different wavelengths occurs only in a part of the cavity.
The obtained negative GDD from the geometric effect is proportional to the prism separation,
and an additional (usually positive) GDD contribution results from the propagation in the prism
material. Thus, to obtain a spatially coherent beam two paths through the prism pair are required.
The insertion loss is very small because the angle of incidence is at Brewster angle. The prism apex
angle is chosen such that the incident beam at Brewster angle is also at the minimum deviation.
Prism pairs offer two advantages. First, the pulse width can be varied by simply moving one of
the prisms which adjusts the prism insertion and therefore the amount of positive GDD from the
propagation in the prism material (see Fig. 2.1.16c). Second, the laser can be tuned in wavelength
by simply moving a knife edge at a position where the beam is spectrally broadened. Both properties
are often desired for spectroscopic applications, for example. However, the prism pair suffers
from higher-order dispersion, which is the main limitation in ultrashort pulse generation in the
sub-10-fs regime. Different prism materials introduce different amounts of higher-order dispersion.
For compact lasers with pulse durations of few tens to hundreds of femtoseconds the more dispersive
SF10-prisms are better because they require a smaller prism separation than fused quartz
prism for example. But the smaller prism separation comes at the expense of a larger higher-order
dispersion. Fused quartz is one of the best materials for ultrashort pulse generation with minimal
higher-order dispersion. The higher-order dispersion of the prism pairs is dominated by the prism
spacing, which is not changed significantly when we adjust the dispersion by inserting the prisms
into the laser beam.
More compact geometries for dispersion compensation make use of a single prism only [94Ram,
96Kop1]. In this case, the wavelength components are spatially separated in the whole resonator,
not only in a part of it. Even without any additional prisms, refraction at a Brewster interface
of the gain medium can generate negative dispersion. In certain configurations, where the cavity
is operated near a stability limit, the refraction effect can be strongly increased [99Pas2], so that
significant negative GDD can be generated in a compact cavity. The amount of GDD may then
also strongly depend on the thermal lens in the gain medium and on certain cavity dimensions.
Landolt-Börnstein
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