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.
Pulsed erbium lasers operating in the eye-safe spectral band around 1.6 μm can find numerous defense and civil applications that often require high pulse energy, reasonable pulse repetition frequency (100 Hz), specific wavelength and last not least very good beam quality. Even though resonant pumping shifts a significant part of thermal load from gain medium to pumping diodes, fulfillment of all these requirements is still rather difficult, what can be attributed to spectroscopic limitations of erbium doped crystalline gain media as well as to low spatial brightness of available InP pumping diodes. In the paper we report recent breakthroughs in the field of pulsed erbium lasers. Main difficulties towards multi-ten-mJ output from systems based on the TIR (total- internal-reflection) pump scheme arrangement will be defined and solutions proposed. We also demonstrate for the first time to the best of our knowledge a Q-switched Er3+:YAG laser operating at the repetition rate of 100 Hz with truly diffraction limited beam quality (M2 =1) and pulse energy of up to 24mJ (damage free).
A new heat extraction geometry for resonantly-diode-pumped Er3+:YAG lasers has been proposed. With this approach
heat extraction from a laser rod is symmetrised and improved significantly, thus thermal lensing and thermo-induced
aberrations of the active crystal are reduced. For proposed approach more than 10 W average has been generated both in
CW and QCW mode of operation at comparable pumping conditions with nearly diffraction limited beam. Investigations
on diffraction effects inside the fiber-like laser rod have been performed and theoretical background of observed
phenomena have been defined. Finally results of further investigations on actively Q-switched laser will be presented.
The study describes the efficient, acousto-optic Q-switching of the Er:YAG laser at the 1645 nm 'eye-safe' wavelength. For longitudinal pumping at the wavelength of 1532 nm, a linearly-polarized 10 W erbium fiber laser radiation was used. The investigated Er:YAG crystal was 40 mm long and its erbium concentration was 0.25 %. The active crystal was mounted in a copper heat-sink maintaining a 15°C temperature of coolant water. For giant pulse generation, the fused-silica acousto-optic modulator was inserted inside the Er:YAG laser oscillator near the output mirror of the resonator.
Laser output characteristics were performed depending on the parameters of output coupler reflectance (R= 95%, 90%, 85%) and the repetition rate (from 0.1 to 10 kHz). In free running experiments almost 2.8 W of output power with 55% slope efficiency with respect to incident pump power was obtained. In Q-switching regime the high peak power generation was demonstrated. For maximum incident pump power of 7.8 W, pulse energy up to 4 mJ was generated with a pulse duration less than 40 ns at a 500-Hz repetition rate, which corresponds to nearly 110 kW of peak power. This laser source can find application as a transmitter in 'eye-safe' rangefinders, ladars etc.