W. Lüthy

150 mW unsaturated output power at 3 µm from a single-mode-fiber erbium cascade laser

We report on an erbium cascade laser in a fluorozirconate fiber. Lasing on the transition 4I11/2 -> 4I13/2 at 2.71 µm is supported by colasing on the transition 4S3/2 -> 4I9/2 at 1.72 µm. This recycles the excitation that is lost via excited-state absorption and avoids the saturation of the output power. Threshold at 2.71 µm is 33 mW launched pump power at 791 nm. The measured slope efficiency of 22.6% is relatively close to the 29.1% stokes-efficiency limit. An output power of 158 mW is obtained, limited only by the 1.43 W power available from the Ti:sapphire pump laser. Output power is 15 and slope efficiency 2.5 times higher than reported in previous publications.

Efficiency of erbium 3-/spl mu/m crystal and fiber lasers

The population dynamics of erbium 3-/spl mu/m crystal and fiber lasers are compared experimentally and theoretically. Laser slope efficiencies of 40% in Er:LiYF/sub 4/ and 23% in a fluorozirconate fiber are experimentally demonstrated under Ti:sapphire pumping. These are both to our knowledge the highest values reported so far for the different host geometries. On the basis of the excitation and loss mechanisms, the best pump wavelengths are determined to be 970 mm in LiYF4 and 791 nm in a fluorozirconate fiber. The theoretical limit of the slope efficiency with respect to absorbed pump power is redefined as depending on the major population mechanisms of the system. Calculated values are 56% in Er:LiYF4 and 27% in a fluorozirconate fiber.

Properties of laser‐assisted doping in silicon

Ohmic contacts and p‐n junctions in p‐ and n‐type silicon are generated with the aid of a laser. Doping was achieved by covering the surface of the silicon with a layer of dopant and melting locally with pulses from either a Nd : YAG or a CO2 laser. Typical residual resistances of the Ohmic contacts are of the order of 0.1–1 Ω cm2 and backward/forward resistance ratios of 104 were measured for the diodes. A model which takes account of segregation during the cooling process is discussed and shown to agree with the resulting distribution of dopant. Highly doped material was found in a surface layer of a thickness less than 0.5 μm. This thickness was independent of laser parameters.

The effective absorption coefficient in double-clad fibres

Abstract Pumping of a fibre laser with a powerful multimode diode laser array is most efficient if the receiver area of the fibre is enhanced by the use of a double-clad configuration. In this case the pump light is guided in the inner multimode cladding of the fibre and is only weakly absorbed by the rare-earth doped monomode core. In a first approximation the absorption coefficient of the core material is reduced by the core/inner cladding area ratio. The effective absorption coefficient has been determined for different lengths of a neodymium doped double-clad fibre. It is shown that in our configuration the effective absorption coefficient for a fibre length of 50 cm is up to 3 times larger than for long fibre lengths, where values close to those predicted by the first approximation are reached. The experimental results are described using a simple model.

Saturation of the 2.71 µm laser output in erbium doped ZBLAN fibers

The saturation of the 2.71 μm laser output power has been investigated in an erbium doped ZBLAN single-mode fiber with an Er3+ concentration of 5000 ppm mol. The bleaching of the ground state, the absorption coefficient at the pump wavelength and the fluorescence intensities over a wide wavelength range have been measured simultaneously during laser emission. In a computer simulation the rate equation system of the Er3+ system in ZBLAN fibers has been analyzed. The saturation of the 2.71 μm laser output is explained by simultaneous lasing at 850 nm.

Diode pumping and laser properties of Yb:Ho:YAG

Abstract The rare-earth ion Yb3+ is an efficient sensitizer for Ho3+ crystal lasers. It has a broad absorption band lying in the emission range of commercially available InGaAs diode lasers and an efficient energy transfer to Ho3+. We have investigated four Yb3+:Ho3+:YAG laser crystals with 16.1 and 30.3 at.% Yb3+, both with 2 at.% Ho3+ and two different crystal thicknesses. The pump absorption coefficient α is about 15.1 cm−1 for 16.1 at.% Yb3+ in YAG at λ=936 nm. Due to the strong absorption, relatively thin crystals with a thickness of 0.5 and 1 mm were used in active mirror configuration. Room temperature cw laser emission has been achieved in all these crystals. The laser thresholds have been measured as a function of the crystal temperature between 0 and 60°C. Thresholds and slope efficiencies of the different crystals are compared. Under diode laser pumping room temperature quasi cw emission up to 370 mW has been achieved in a 15-mm-long resonator for the first time.

UV laser polishing of thick diamond films for IR windows

Abstract Diamond has very suitable properties for infrared (IR) window applications. The rugged surface of a free-standing 320 μm thick diamond film grown by chemical vapour deposition (CVD) is polished with the UV light of an ArF excimer laser (λ=193 nm). The angle of incidence is 85° and irradiation is subsequently performed at three planar directions around the perpendicular axis. Scanning electron microscopy (SEM) is used to visualise the surface before and after the polishing procedure. With a profilometer (tip radius 5 μm) the improvement of the average roughness of the surface is measured. Polishing is analysed in dependence on the size of surface irregularities by means of fast Fourier transformation (FFT) of the signal of the profilometer. Surface roughness measurements are also performed on the polished surface with an atomic force microscope (AFM). With IR transmittance measurements the optical quality is analysed over a wavelength range of 2.5 to 25 μm before and after polishing. The oxidative removal of the laser induced graphitic layer is studied with optical spectroscopy techniques.

Investigation of diode-pumped 2.8-µm laser performance in Er:BaY2F8

Laser operation at 2.8 mm in BaY2F8 with erbium concentrations of 7.5% and 20% is investigated under laser-diode pumping at 967 nm. Output powers as high as 250 mW and slope efficiencies as high as 24% are obtained. Results are comparable with those of Er3+:LiYF4 under the same pump conditions. Slope efficiencies above 30% are predicted for optimized erbium concentrations.

Acoustooptic Q-switching of erbium lasers

Experiments were performed on Q switching of erbium lasers at 3 mu m with an acoustooptical modulator. Different laser crystals of YAG:Er and YSGG:Cr:Er have been investigated. The highest fraction of single pulse Q-switch energy to free-running mode energy was 1%. Pulse durations of 50 ns were obtained. Multiple switching with up to 14 Q-switch peaks per flashlamp pulse could be generated, containing up to 19% of the energy of the free-running mode. >

Emission properties of an optimised 2.8 μm Er3+:YLF laser

Abstract A Ti:sapphire-pumped Er 3+ :YLF laser is built. The samples are grown at our institute and are compared with a commercially available Er 3+ :YLF. The dopant concentration and the polarisation of the pump beam is optimised. Slope efficiencies up to 50% and thresholds as low as 42 mW are achieved. The emission properties of the laser are studied. The cw laser emission is linearly polarised. More than 90% of the output power is emitted on a single Stark transition.