Philipp Koopmann

2 µm Laser Sources and Their Possible Applications

The wavelength range around 2 μm which is covered by the laser systems described in this chapter is part of the so called “eye safe” wavelength region which begins at about 1.4 μm. Laser systems that operate in this region offer exceptional advantages for free space applications compared to conventional systems that operate at shorter wavelengths. This gives them a great market potential for the use in LIDAR and gas sensing systems and for direct optical communication applications. The favourable absorption in water makes such lasers also very useful for medical applications. As it can be seen in figure 1, there is a strong absorption peak near 2 μm which reduces the penetration depth of this wavelength in tissue to a few hundred μm.

Efficient diode-pumped laser operation of Tm:Lu 2 O 3 around 2 μm

We report on the first diode-pumped laser operation of thulium-doped Lu2O3. With a very compact setup an output power of 75 W and slope efficiencies of around 40% with respect to the incident pump power were achieved at room temperature. Free running laser operation was observed at wavelengths of 2065 nm and 1965 nm. With a birefringent filter the wavelength could continuously be tuned from 1922 nm to 2134 nm. The thermal conductivity of Tm:Lu2O3 was measured for different dopant concentrations and is compared to the one of thulium-doped YAG.

175 fs Tm:Lu 2 O 3 laser at 2.07 µm mode-locked using single-walled carbon nanotubes

Single-walled carbon nanotube saturable absorbers were designed and fabricated for passive mode-locking of bulk lasers operating in the 2 μm spectral range. Mode-locked lasers based on Tm:Lu2O3 single crystals containing different Tm3+-doping concentrations were studied. Nearly transform-limited pulses as short as 175 fs at 2070 nm were generated at 88 MHz repetition rate.

Directly diode-pumped high-energy Ho:YAG oscillator

We report on the high-energy laser operation of an Ho:YAG oscillator resonantly pumped by a GaSb-based laser diode stack at 1.9 μm. The output energy was extracted from a compact plano-concave acousto-optically Q-switched resonator optimized for low repetition rates. Operating at 100 Hz, pulse energies exceeding 30 mJ at a wavelength of 2.09 μm were obtained. The corresponding pulse duration at the highest pump power was 100 ns, leading to a maximum peak power above 300 kW. Different pulse repetition rates and output coupling transmissions of the Ho:YAG resonator were studied. In addition, intracavity laser-induced damage threshold measurements are discussed.

Multi-watt laser operation and laser parameters of Ho-doped Lu 2 O 3 at 2.12 μm

We present spectroscopic investigations and the first laser operation of Ho:Lu2O3. Laser operation was obtained with two different setups at room temperature: In a 1.9 μm diode pumped setup a maximum output power of 15 W was achieved. With a Tm-fiber laser pumped setup the maximum output power was 5.2 W and the slope efficiency was 54% with respect to the absorbed pump power. Absorption measurements revealed absorption cross sections of up to 11.7 · 10−21 cm2 at 1928 nm. In the 2.1 μm range a maximum emission cross section of 4.5 · 10−21 cm2 at 2124 nm was determined, which remains the highest gain peak even for high inversions. The fluorescence lifetime of the 5I7-manifold was found to be 10 ms.

Passively mode locked femtosecond Tm:Sc 2 O 3 laser at 2.1 μm

We report on the passive mode locking of a Tm3+:Sc2O3 laser at 2.1 μm using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. Transform-limited 218 fs pulses are generated with an average power of 210 mW. A maximum output power of 325 mW is produced during mode locking with the corresponding pulse duration of 246 fs at a pulse repetition frequency of 124.3 MHz. A Ti:sapphire laser is used as the pump source operating at 796 nm.

Holmium-doped Lu 2 O 3 , Y 2 O 3 , and Sc 2 O 3 for lasers above 2.1 μm

Efficient room-temperature laser operation was obtained in the wavelength range from 2117 nm to 2134 nm with Ho:Lu(2)O(3) and Ho:Y(2)O(3) as the active materials. With an FBG-stabilized Tm-doped fiber laser as the pump source, the maximum slope efficiency and output power of the Ho:Y(2)O(3) laser were 63% and 18.8 W, respectively. With Ho:Lu(2)O(3) the respective values were 76% and 25.2 W. With Ho:Sc(2)O(3) as the active material the accessible wavelength range could be expanded to 2158 nm in a diode-pumped setup.

Q-switched Ho:Lu 2 O 3 laser at 2.12 μm

We present the first diode pumped Q-switched Ho:Lu₂O₃ laser. At room temperature the maximum pulse energy exceeded 5 mJ at 100 Hz pulse repetition rate while the maximum peak power was 23 kW.

High power diode pumped 2 µm laser operation of Tm:Lu 2 O 3

We report the first diode pumped laser operation of thulium-doped lutetia in the 2 µm wavelength range. Output powers of more than 40 W and slope efficiencies of up to 42 % with respect to the incident pump power were achieved at room temperature.

Highly efficient, broadly tunable Tm:Lu 2 O 3 Laser at 2 µm

Lasers with a wavelength of 2 µm have a wide variety of applications in different fields of medicine, material processing, and LIDAR-systems. Due to the strong absorption in water, this laser radiation can be considered eye-safe. The rare-earth-ion thulium allows the emission of 2 µm radiation while it can be pumped at about 800 nm. The excitation energy is transfered to the upper laser level by a cross relaxation process which leads to a quantum efficiency of up to 2.