Günther Renz

Two-Micron Thulium-Pumped-Holmium Laser Source for DIRCM Applications

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.

A High Energy Tm:YLF-Fiber-Laser (1.9µm) Pumped Ho:YAG MOPA (2.09µm) Laser System

A single mode Ho:YAG MOPA (2.09 µm) laser system with 80 mJ and 20 ns at 100 Hz which is end-pumped by Tm-fiber-lasers has been developed for countermeasure applications.

Moderate high power 1 to 20μs and kHz Ho:YAG thin disk laser pulses for laser lithotripsy

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.

InP-diode laser stack pumped Ho:YAG or Cr:ZnSe thin disk lasers

In this report, InP-diode laser stacks with on-chip integrated gratings of narrow band (2 nm) emission at 1.908 μm (QPC Laser) with maximum output power of 40 W will be used to pump the laser active materials of crystalline Ho:YAG or crystalline Cr:ZnSe in a 24 pump pass thin disk module (Dausinger + Giesen GmbH).

Thin disk laser in the 2μm wavelength range

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.

A 2μm-pump-laser-based DIRCM system and aero-optics in the mid-IR

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.

Frequency stabilized injection seeded Q-switched Ho:YAG laser for 2 μm Doppler wind LIDARs

For obtaining a single frequency Q-switched laser output injecting a continuous wave single frequency master laser into a Q-switched laser is a commonly used technique [1]. For a stabilized injection seeding a Fast Fourier Transform (FFT) based single heterodyne beat signal technique will be presented.

2 µm Ho:YAG Thin Disk Laser

A Thulium fiber laser pumped Ho:YAG thin disk laser with 15W (cw) or several mJ (pulsed) operation will be presented. Additionally, a narrow (<0.5nm), tunable (30nm) cw operation near 2.09 µm, will be shown.

Spectral Broadening by Frequency Mixing Coupling in a PPLN OPG-System

We report on spectral broadening of 100% for a domain period of 30.5µm of the idler at 3.2µm in a PPLN OPG-system for broadband applications. The spectral broadening reduces the risk of crystal damage.