The possible use of lasers as weapons becomes more and more interesting for military forces. Besides the generation of
high laser power and good beam quality, also safety considerations, e. g. concerning eye hazards, are of importance. The
MELIAS (medium energy laser in the “eye-safe” spectral domain) project of ISL addresses these issues, and ISL has
developed the most powerful solid-state laser in the "eye-safe" wavelength region up to now. „Eye safety” in this context
means that light at a wavelength of > 1.4 μm does not penetrate the eye and thus will not be focused onto the retina. The
basic principle of this technology is that a laser source needs to be scalable in power to far beyond 100 kW without a
significant deterioration in beam quality. ISL has studied a very promising laser technology: the erbium heat-capacity
laser. This type of laser is characterised by a compact design, a simple and robust technology and a scaling law which, in
principle, allows the generation of laser power far beyond megawatts at small volumes. Previous investigations
demonstrated the scalability of the SSHCL and up to 4.65 kW and 440 J in less than 800 ms have been obtained. Opticalto-
optical efficiencies of over 41% and slope efficiencies of over 51% are obtained. The residual thermal gradients, due
to non perfect pumping homogeneity, negatively affect the performance in terms of laser pulse energy, duration and
beam quality. In the course of the next two years, ISL will be designing a 25 to 30 kW erbium heat-capacity laser.
The Er<sup>3+</sup>:YAG Solid State Heat-Capacity Laser (SSHCL) as a source for medium and high energy laser systems
in the “eye-safe” range is currently under investigation at ISL. The aim is to obtain a robust laser source with
low complexity, high beam quality (M<sup>2</sup> < 3) and scalable to 100 kW and beyond. In a SSHCL the laser
medium is cooled only after the laser action has ended, resulting in low temperature gradients in the laser
medium itself during operation. Previous investigations demonstrated the scalability of the SSHCL and up to
4.65 kW and 440 J in less than 800 ms have been achieved. Optical-to-optical efficiencies of more than 41% and
slope efficiencies of over 51% has been obtained. The residual thermal gradients, due to non perfect pumping
homogeneity, negatively affect the performance in terms of laser pulse energy, duration and beam quality. Using
an intra-cavity adaptive optics system, beam aberrations were limited to less than 1/10 of the wavelength for
each of the considered Zernike polynomials, and the shot duration lengthened by about 50%. In this paper
we investigate how to further increase the SSHCL pulse duration. The influence of the crystal geometry on the
pump distribution homogeneity and the pulse duration are analysed. We consider the use of a mechanical crystal
changer for extending the laser pulse duration. By using a revolver with several crystals, we demonstrated that
crystals can be correctly positioned in less than 100 ms, allowing a quasi-cw operation that can largely exceed
the time constraints imposed by the heating of the crystal. Finally, we address the problem of measuring the
laser beam quality. Since the current standard techniques are suitable only for stable cw lasers, they cannot be
used for the SSHCL. A new kind of device, capable of measuring the M2 at intervals of less than 100 ms, is
In this paper, we illustrate the latest advancement on the eye-safe Solid State Heat-Capacity Laser (SSHCL) investigated for the development of medium and high energy laser sources. Nearly all the solid-state lasers considered for defence applications in the range of 10 kW up to over 100 kW emit at a wavelength of 1.03 <i>μ</i>m– 1.06 <i>μ</i>m. Therefore, we perform research on an alternative emitting around 1.6 <i>μ</i>m, which unites many advantages in use (robustness, a simple technology, flexibility in volume and weight). The heat-capacity principle, in which the laser material is cooled only after the laser action has ended, results in low temperature gradients in the laser medium, leading to a good beam quality and a high performance. Previous investigations on Er<sup>3+</sup>:YAG SSHCL demonstrated the scalability of the heat-capacity laser principle and up to 4.65 kW and 440 J in less than 800 ms have been achieved, representing the current world record in eye-safe diode-pumped solid-state laser technology. Optical-to-optical efficiencies of over 41% and slope efficiencies of over 51% are obtained with respect to the incident pump power. In this report we further investigate the possibility of compensating any parasitic residual heating. Indeed, it has been shown that the optimal laser operation is directly coupled with the intensity distribution of the laser mode inside the laser medium. The ideal resonator configurations are those which allow an extraction of the laser energy as homogeneous as possible. Using an intra-cavity adaptive optics system beams with phase fronts as flat as possible, on the order of less that 1/10 of the wavelength for each of the considered Zernike polynomials have been generated, and the shot duration has been lengthened by 50%. The influence of the crystal geometry on the pump distribution homogeneity and the possible ways for maximizing the extraction efficiency are investigated.
In order to address the question of the possibility of a high energy laser with an emission in the “eye-safe” wavelength range, various architectures may be considered. To provide a truly scalable and efficient laser source, however, only bulk solid-state lasers using resonantly diode-pumped erbium show the necessary properties, when coupled with the solid-state heat-capacity (SSHCL) principle of operation. Although seen as nearly being impossible to realize, such a quasi-three-level laser medium can be used in heat-capacity operation. In this operation mode, the laser medium is not cooled during lasing in order to avoid the thermal lensing occurring in bulk lasers, which, in actively cooled operation, would result in a low beam quality, destabilize the laser cavity or would even cause crystal fracture. In heat-capacity mode, the laser medium will substantially heat up during operation, which will cause an increase in re-absorption for a quasi-three-level laser medium, resulting in a general drop in output power over time. However, theoretical and experimental investigations have proven that this effectis of no concern for an Er<sup>3+</sup>:YAG SSHCL. This paper presents an overview on the theoretical background of the Er<sup>3+</sup>:YAG SSHCL, experimental results including recent advances in output power and efficiency, an investigation on the laser scaling properties and recent results on intra-cavity adaptive optics for beam-quality enhancement. The effect of fluorescence re-absorption on the laser properties, especially on threshold and laser efficiency will also be discussed. Up to 4.65 kW and 440 J in less than 800 ms are achieved using optimized doping levels for upconversion reduction in this resonantly-diode-pumped Er<sup>3+</sup>:YAG SSHCL, representing the current world record in eye-safe diode-pumped solid-state laser technology. Optical-to-optical efficiencies of over 41% and slope efficiencies of over 51% are obtained with respect to the incident pump power.
Er<sup>3+</sup>:YAG eye-safe laser emitting at 1.6 μm is an interesting source for various applications such as remote
sensing, ranging, designation and free-space communications for two main reasons: its emitting wavelength
lies in a region of high atmospheric transmission and high sensor sensitivity and the resonant pumping into the
<sup>4</sup>I<sub>13/2</sub> upper laser manifold ensures highly efficient operation. The recent availability of internal grating stabilized narrow linewidth, high-power laser diodes in the 1.53 μm range, makes this laser even more appealing. The only
shortcoming to be solved for a really efficient resonantly diode pumped Er<sup>3+</sup>:YAG laser is how to have a good
overlap between the pump radiation and the laser cavity mode. Indeed, due to up-conversion processes among
the Er<sup>3+</sup> ions, to achieve efficient lasers it is necessary to use low doped samples. This requires the use of rods
with lengths of several cm that are not compatible with the low beam quality of the diode lasers. In this work, we
report on a resonantly diode pumped Q-switched Er<sup>3+</sup>:YAG laser with a crystalline fibre-like geometry emitting
at 1.64 μm. In this scheme, the pump radiation is confined into a 60 mm long crystal with a diameter of 1.2
mm thanks to the multiple total internal reflections (TIR) that occur on the barrel surface, allowing efficiently
pumping of such a long crystal. A maximum output power of more than 14 W in continuous wave mode and
pulse energies of 8 mJ in Q-switching mode have been observed, when pumped with - 40 W of absorbed power.
Even if these values are still far from the performances reported using hybrid fibre-bulk laser scheme, these
results clearly show that TIR-based Er<sup>3+</sup>:YAG fibre-shaped crystalline rod laser is a promising technology for
the development of efficient high-power and high-energy eye-safe laser. Finally, the effect of thermal lensing on
such crystalline fibre geometry is discussed.
We demonstrate cooling of a 2 micron thick GaAs/InGaP double-heterostructure to 165 K from ambient using
an all-solid-state optical refrigerator. Cooler is comprised of Yb<sup>3+</sup>-doped YLF crystal, pumped by 9 Watt near
E4-E5 Stark manifold transition.
We demonstrate first cryogenic operation in a Ytterbium doped YLF crystal by means of an optical refrigeration.
We have achieved cooling to 155 Kelvin absolute temperature with heat lift of 90 mW, exceeding performance of
multi-stack thermo-electric coolers. This progress was possible by pumping the system near the Stark-manifold
resonance of highly pure Yb:YLF crystal and careful thermal management in the cooling experiment. Detailed
spectroscopic analysis demonstrated that cooling to 110 Kelvin is currently possible if pumped exactly on that
Spectroscopic characterization of YLF crystal doped with Yb reveals the performance potential of this material in laser
cooling applications. Temperature-dependent spectra allow us to estimate the minimum achievable temperature and the
parasitic background absorption.
We discuss recent progress in the laser cooling experiments via resonant cavity. Following analysis of the cooling
efficiency, we highlight importance of wavelength dependence of the minimum achievable temperature for a given
cryocooler. Following the analysis, we utilize pump detuning along with reduction of thermal load on the sample
to achieve absolute temperature of nearly 200K, a 98.5 degree drop, starting from room temperature. Wavelength
dependent analysis suggests that further improvement is possible.
Using a cavity resonant absorption scheme we demonstrate record laser cooling for the rare-earth doped crystalline
solid Yb:YLF. A temperature drop of nearly 70 degrees is obtained with respect to the ambient. Our preliminary
results indicate that minimum achievable temperature in this material/sample is 170 K, as measured using a
modified differential luminescence thermometry technique. This indicates outstanding potential for Yb:YLF as
a cryogenic laser cooler material.
We report the successful growth and the laser cooling results of Yb<sup>3+</sup>-doped single fluoride crystals. By investigating
the mechanical and thermal properties of Yb-doped BaY<sub>2</sub>F<sub>8</sub> and LiYF<sub>4</sub> crystals and using the
spectroscopic data we collected from our samples, the theoretical and experimental cooling efficiency of fluoride
crystals are evaluated and compared with respect to those of ZBLAN. Two different methods, a thermal camera
and a fluorescence intensity ratio technique, have been used to monitor the temperature change of the samples.
The temperature change is clearly exponential, as expected from theory, and the temperature drops are 6.3
K and 4 K for Yb:LiYF<sub>4</sub> and Yb:BaY<sub>2</sub>F<sub>8</sub> respectively in single-pass configuration, corresponding to a cooling
efficiency of about 2% and 3%. This last value is slightly larger than that observed in Yb-ZBLAN in similar
We present an overview of laser cooling of solids. In this
all-solid-state approach to refrigeration, heat is removed radiatively when an engineered material is exposed to high power laser light. We report a record amount of net cooling (88 K below ambient) that has been achieved with a sample made from doped fluoride glass. Issues involved in the design of a practical laser cooler are presented. The possibility of laser cooling of semiconductor sensors is discussed.