• YLiF4
• Q-switching
• pulse
• laser
• infrared
• BaY2F8

The great interest for diode pumped solids state laser sources (DPSSL), in the mid- and
far-infrared is principally related to the absorption spectrum of water. The latter shows
strong absorption for wavelength longer then 1.6μm, making these sources suitable for a
wide range of applications. Infrared lasers above 1.6μm in general, fall in the so called
eye safe region, for this reason pulsed 2μm sources are widely used in communication
technologies as well as in remote sensing, e.g. LIDAR systems and atmospheric physics.
The short penetration depth in biological tissues, due to the high absorption in water,
makes these sources suitable also as laser scalpel in medical applications. Moreover, they
are used as laser sources for Optical Parametric Oscillators for down conversion to the
mid-infrared by nonlinear crystals.
Rare Earths (REs), such as thulium and holmium, are commonly used in solid state gain
media thanks to their peculiar spectroscopic proprieties. They show sharp peaks in spectra
and metastable excited states, typical of free ions behaviour, also when inserted in bulk
hosts. These proprieties, clearly, are strongly related with their electronic configuration
and this thesis I give an idea of why REs have this configuration and of how to calculate
the free ions energy levels scheme. We describe also the characteristic ion-ion coupling
of the REs and the resulting non radiative energy transfer mechanism. Such phenomena,
when resonant, allow an efficient conversion of the radiation: halving the wavelength via
up-conversion, or doubling it by cross-relaxation.
Fluorides are widely used as crystal hosts in gain media REs based systems thanks to
their low maximum phonon energy. In this work I focus on BaY2F8(BYF) and YLiF4
(YLF) crystals, I introduce the concepts necessary to characterize laser crystals: ground
state absorption cross section, stimulated emission cross section and gain cross section.
The main goal is to investigate the characteristics of amplification of thulium and
thulium-holmium doped fluorides in the 2μm region for application in passive Q-switching,
PQS. This technique, simply obtained by intracavity insertion of a saturable absorber,
allow me to obtain short pulses (tens of ns long) and high peak power (up to tens of kW),
without the need to use active devices and thus considerably reducing costs and complexity.
The parts of a general DPSSL system, and in particular the details of the DPSSL used
in this work, are described. The role of the saturable absorber (SA) is to introduce time
modulated losses. It is engineered in order to prevent stimulated emission at the begin
the pumping phase and to be transparent when the fluence reaches the level of the SA
saturation fluence value allowing the pulse to be emitted. The saturable absorbers, used in this work, are large bandgap semiconductors (ZnS and ZnSe) doped with doubly ionized chromium ions. The bandgap ensures a wide transparency window and the chromium doping a
smooth absorption band from 1.5μm to 2.1μm.
The laser crystals have been tested in different configurations and in combination with
different absorbers in order to maximize the peak pulse and minimize the pulse duration.
Comparing the Tm:BYF performance obtained with the results in literature achieved with
other Tm-doped fluorides, we found out that the performance of BYF are lower. The
problem is the lower damaging threshold of BYF that makes it very difficult to work
without damage. Changing the setup in order to decrease the energy density on the
Tm:BYF crystals the damaging problem was still present. Instead working with- Tm-
Ho:YLF crystals was easier, mainly because in the co-doped system the energy densities
on the optical elements are several times lower. Comparing the data achieved with the
literature we demonstrated with this material for the first time sub μs pulse operation at a
room temperature (40ns of pulse duration), consequently improving also the peak power.
The improvement could be mainly imputed to the use of the Cr2+:ZnSe in place of the
Cr 2+ ZnS that shows a higher absorption cross section in the 2050μm region.
As regards Tm:BYF the next step could be the use BaYLuF8 instead of the BYF follow-
ing the same philosophy between YLF and LLF. Substituting the yttrium with lutetium
the thermo-mechanical proprieties could be improved and consequently the damage prob-
lem reduced. For Tm-Ho:YLF the next natural step is to perform the same experiments
with LLF that for single doped thulium system shows the best results to date.