An acousto-optically or self-oscillation pulsed thin disk Ho:YAG laser system at 2.1 μm with an average power in the 10 W range will be presented for laser lithotripsy. In the case of cw operation the thin disk Ho:YAG is either pumped with InP diode stacks or with a thulium fiber laser which leads to a laser output power of 20 W at an optical-to-optical efficiency of 30%. For the gain switched mode of operation a modulated Tm-fiber laser is used to produce self-oscillation pulses. A favored pulse lengths for uric acid stone ablation is known to be at a few μs pulse duration which can be delivered by the thin disk laser technology. In the state of the art laser lithotripter, stone material is typically ablated with 250 to 750 μs pulses at 5 to 10 Hz and with pulse energies up to a few Joule. The ablation mechanism is performed in this case by vaporization into stone dust and fragmentation. With the thin disk laser technology, 1 to 20 μs-laser pulses with a repetition rate of a few kHz and with pulse energies in the mJ-range are available. The ablation mechanism is in this case due to a local heating of the stone material with a decomposition of the crystalline structure into calcium carbonate powder which can be handled by the human body. As a joint process to this thermal effect, imploding water vapor bubbles between the fiber end and the stone material produce sporadic shock waves which help clear out the stone dust and biological material.
A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.
The thin disk laser is a successful concept for high output power and/or high pulse energy, high efficiency and good beam quality in the 1 μm range. Holmium-doped materials are a promising approach to transform this success to the 2 μm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms) which provides good energy storage capabilities. We have realized a Ho:YAG thin-disk laser with a cw output power of 15 W at 2.09 μm and a maximum optical-to-optical efficiency of 37%. The laser was pumped with a Tm-fiber laser. Numerical simulations of the Ho:YAG thin disk laser show the potential for further scaling. As broadly tunable alternative, also a Cr:ZnSe thin disk laser was investigated. A Tm-fiber laser and a fiber coupled diode stack were tested as pump sources. A laser power of 3.5 W was achieved with diode pumping.
The thin disk laser is a successful concept for high output power and/or high pulse energy, high efficiency and good
beam quality in the 1 μm range. Holmium-doped materials are a promising approach to transform this success to the
2 μm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms)
which provides good energy storage capabilities. We have realized a Ho:YAG thin-disk laser with a cw output power of
15 W at 2.09 μm and a maximum optical-to-optical efficiency of 37%. The laser was pumped with a Tm-fiber laser.
Numerical simulations are used for further characterization.
The improvement of the security of platforms (aircrafts) with countermeasure techniques in the mid-IR especially in the
take-off or landing phase is nowadays more stringent due to upcoming threats. We report on the development of a
Tm:YLF-fiber laser (1.908 μm) pumped Ho:YAG (2.09 µm) high energy laser system with pulse energies up to 100 mJ
at pulse lengths close to 20 ns and repetition rates of 100 Hz.
A high quality laser beam leaving a platform through a variable-index-of-refraction airflow will experience wave-front
aberrations and consequently lose its ability to be perfectly focused in the far field. Two main causes of laser beam degradations
are issued in this investigation. First, there is the degradation immediately around the fuselage, referred to
aero-optic problems and second the atmospheric propagation influence via air turbulence. The aero-optic influence on
the laser beam degradation will be investigated in a laboratory experimental approach with a mid-IR laser beam traversing
a transonic free air stream relevant to a real air flow around a fuselage. The propagation characteristics of a laser
beam passing turbulent air will be numerically simulated with a multiple phase-screen method and a Fourier propagation
technique. Different turbulence degrees relevant to propagation directions especially behind aircrafts will be considered.
There is an increasing need for the generation of mid-infrared radiation in the 3 to 5-micron region especially in the
absorption minima of the atmospheric windows. Recent progress in heat seeking detector technology, operating in these
atmospheric windows, make it necessary to develop compact and reliable mid-infrared laser systems that can be installed
in airborne platforms. Future DIRCM systems will be equipped with high repetition rate/low energy per pulse
lasers as well as low repetition rate/high energy per pulse lasers. We report on the development of a Tm:YLF-fiber laser
(1.908 &mgr;m) pumped Ho:YAG (2.09 &mgr;m) high energy laser system with pulse energies up to 90 mJ at pulse lengths close
to 20 ns and operating at 100 Hz. Using single mode fiber lasers as end-pumped sources for the master-oscillatorpower-
amplifier (MOPA) system almost diffraction limited beam quality resulted. The frequency conversion into the 3
to 5-micron region is performed with a zinc germanium phosphide (ZGP) crystal in a linear or ring resonator. Propagation
of the mid-infrared laser beam through moderate turbulent atmosphere will be simulated numerically using phase
screens and Fresnel transformation.
An electron-beam controlled, pulsed carbon-dioxide laser with an average output power of more than 15 kW is presented. Laser operation has been achieved for pulse energies up to 180 J at repetition rates of 100 Hz, pulse durations variable between 3 and 10 microseconds, and bursts of 1000 shots with an unprecedented reliability.
A numerical model is presented to describe the influence of the diffraction on the temporal and spatial properties of a Q- switched carbon-dioxide laser. The model includes the interaction of the spatial distribution of the laser field with the spatial distribution of the gain medium, the diffraction of the laser field due to edge effects of the rotating chopper disc, as well as the unstable resonator configuration. An rf-excited 5 kW carbon-dioxide laser has been used to produce a Q-switched peak power of 600 kW at a pulse duration of 200 ns and an average output power up to 1.3 kW. Comparison of the numerical results with the experimental data are shown.
In an e-beam controlled carbon-dioxide laser unit the spatial, temporal and spectral properties of the small-signal gain were investigated in a single pass geometry. A smooth spatial distribution was found with peak values of 2.9%/cm near the cathode at 112.5 J/(1 bar). No reduction in the gain value was monitored up to repetition rates of 60 Hz when circulating the laser gas with a gas flow velocity of 100 m/s. A HeNe laser probe beam, being collinear to the carbon-dioxide laser beam, was used to monitor shock waves or mechanical vibrations by measuring its deflection. This investigation showed, that the small-signal gain measurements were not interfered by any disturbances.
This paper presents the status of free electron laser (FEL) research at the DLR aimed to develop a high power mm-wave source. A commercial Marx generator (max. 1 MV, 10 stages) has been installed, together with a pulse forming line (Blumlein type, 100 nsec pulse duration) and a cold-cathode diode, both being designed at the ITP. The drift tube (2 cm diameter) represents an overmoded waveguide with a conical horn antenna for outcoupling. The helical wiggler (40 periods with 2 cm pitch length) is able to produce up to 1000 Gauss on axis. At the first step of the experiment, the guide field is set to 1400 Gauss. The installation of a stronger guide field is planned for the future.