Daniele Alderighi

High-efficiency, high-power and low threshold Yb 3+ :YAG ceramic laser

We present a high-power, high-efficiency and low threshold laser prototype based on doped ceramic Yb(3+):YAG. We achieved an output power of 9 W with a slope efficiency of 73% and a threshold of 1 W at 1030 nm in quasi-Continuous Wave (QCW). Moreover, we obtained an output power 7.7 W with a slope efficiency of 60% in Continuous Wave (CW). Finally, a characterization of a low losses tunable cavity for several laser wavelengths with an output power exceeding 5 W is reported.

Effects of the excitation density on the laser output of two differently doped Yb:YAG ceramics

We report the behavior of two Yb(3+) doped ceramics (i.e. 10% at. and 20% at.) under quasi-continuous wave laser pumping. Two different behaviors are found depending on the density of Yb(3+) in the excited level. Experimental results show that at low population inversion density, the maximum output power and the efficiency are almost independent on the doping concentration. In particular, an output power as high as 8.9 W with a corresponding slope efficiency of 52% with respect to the injected pump power was reached with the 20% at. sample. Conversely, at high population inversion densities, the 20% doped sample shows a sudden decrease of the laser output for increasing pump power, due to the onset of a nonlinear loss mechanism. Finally, we report a comparison of the experimental results with numerical simulations for the evaluation of the inversion density and of the temperature distribution.

High efficiency room temperature laser emission in heavily doped Yb:YLF.

We report the tunable, CW and quasi CW laser operation at room temperature of an highly doped (30% at.) Yb:YLF crystal longitudinally pumped by a fiber coupled laser diode array. The CW output power is 1.15 W vs. an absorbed pump power of 6 W, with a slope efficiency of 31%. In quasi-CW operation (20% duty factor @10 Hz) an output power of 4 W with an absorbed power of 9.5 W, and a slope efficiency of 62.8% were obtained. The tuning range spans from 1022 to 1075 nm. To our knowledge, these are among the best experimental results obtained at room temperature with Yb doped YLF.

Direct Comparison of Yb 3+ :CaF 2 and heavily doped Yb 3+ :YLF as laser media at room temperature

We report an extensive comparison of the laser performances of diode-pumped Yb(3+):YLF (30% at.) and Yb(3+):CaF(2) (5% at.) crystals, lasing at room-temperature and operating in two different operation mode, i.e. Continuous Wave (CW) and quasi-CW. An in-depth investigation of the crystals behavior by changing the pump power, clearly shows the crystal absorption depends on the lasing conditions. Therefore, we report an unambiguous definition of the slope efficiency calculated taken into account the real measured crystal absorption under laser action. Finally, we present a study of problems related to thermally induced losses which are expected influencing the laser performance.

A ceramic based Yb3+:YAG laser

We report the performance of a laser based on a 9.8% doped ceramic Yb3+:YAG operating in quasi-Continuous Wave (QCW) and in Continuous Wave (CW) at room temperature. The maximum output power of 9 W with a slope efficiency of 73% at 1030 nm was achieved in QCW. We investigated the problems originated from thermo-mechanical properties of the ceramic and we studied their effects on the laser per-formance finding that either the slope efficiency or the laser threshold are thermal-losses independent. Finally, we report a characterization of a low losses tunable cavity for several laser wavelengths.

Experimental evaluation of the cw lasing threshold for a Ce:LiCaAlF6 laser.

We present an experimental technique that allows the direct measurement of the continuous wave (cw) lasing threshold and the slope efficiency of a Ce:LiCaAlF6 (Ce:LiCAF) laser source by means of time-resolved measurement in the pulsed regime. We used a long-pulse-duration source to pump a tunable laser and a high-efficiency nondispersive laser in a quasi-stationary lasing regime. We compare the experimental results with earlier theoretical evaluations, and we demonstrate the feasibility of a cw Ce:LiCAF laser. Under the conditions discussed here, our technique can be applied to all the active media that achieved pulsed laser emission to investigate their potential as cw laser active media.

Measurement of upper level lifetime in presence of radiation trapping: A revisitation of the pinhole method

In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.

Diode‐Pumped Yb3+:YLF and Yb3+:CaF2 Laser Performance

We report an extensive comparison of the laser performances of diode‐pumped Yb3+:YLF (30% at.) and Yb3+:CaF2 (5% at.) crystals, lasing at room‐temperature and operating in two different operation modes, i.e. Continuous Wave (CW) and quasi‐CW. An in‐depth investigation of the crystals behavior by changing the pump power, clearly shows that the crystal absorption depends on the lasing conditions. Therefore, we report an unambiguous evaluation of the slope efficiency calculated taken into account the real measured crystal absorption under laser action. Finally, we present a study of problems related to thermally induced losses which are expected influencing the laser performance.

Development of UV solid state laser sources based on cerium activated crystals

In this work we present some laser experimental achievements obtained with the Ce:LiCaA1F6 (Ce:LiCAF) active medium. The Ce:LiCAF belongs to the family of fluorides Ce3+ activated, which emit in the near UV with tunable emission. In particular it emits in the band 280-315 nm. The laser action was obtained by pumping Ce:LiCAF with frequency quadrupled Nd:YLF and Nd:YAG lasers. We developed a multi-kHz tunable laser with high quality beam, tunable in the range 280-315 nm and with an average power exceeding hundred of mW, pumping with a pulsed Nd:YLF laser. Due to the high quality of Ce:LiCAF crystal we grew, we were able to obtain the highest ever reported slope efficiency. We achieved also the highest ever reported energy of 26 mJ for a Ce:LiCAF tunable oscillator pumping with a single low repetition rate Nd:YAG IVth harmonic laser. The wavelength coverage is further extended in the range 225-230 nm by means of nonlinear frequency mixing of the Ce:LiCAF output with the residual emission of the fundamental wavelength of the Nd:YAG laser.

An experimental study on ultrafast lamps for an ultraviolet solid state laser

In this work we present an accurate experimental study of Xenon filled flash-lamps developed to achieve a fast submicrosecond light pulse with high emission in the ultraviolet (UV) spectral range. This study has been initiated to obtain an efficient pump for solid state lasers with Ce3+ activated crystal as active medium. For Ce3+ doped fluorides the absorption bands lie in the UV spectrum, in the region between 248 and 288nm. Due to the very short lifetime (-4Ons) of the upper level ofthe laser transition (5d->4f), Cerium activated hosts must be pumped with a fast rising light pulse. To our best knowledge, no direct ultraviolet laser action from a Ce3+ activated host with incoherent pumping has been reported. To achieve a threshold population inversion, an unusually fast lamp pulse is required making necessary the development of a lamp system opposite to what it is usually required in terms of duration and stability. We present here a detailed characterization of the lamp and ofthe discharge circuit parameters. Furthermore we discuss the opportunity to use an excimer mixture or a Silicon-Xenon based gas mixture to optimize the spectral coupling of lamp emission to the active medium adsorption.