Yttrium orthoaluminate (YAlO3) is an attractive laser host crystal for doping with thulium (Tm3+) ions. We report on the absorption and stimulated-emission (SE) cross-sections of this orthorhombic (sp. gr. Pnma) Tm:YAlO3 crystal for the principal light polarizations, E || a, b and c. Polarized absorption data lead to the Judd-Ofelt parameters Ω2 = 1.46, Ω4 = 2.82 and Ω6 = 1.09 [10-20 cm2]. In particular, for the 3H4 → 3H5 transition, it is found a stimulated emission cross section of 0.86×10-20 cm2 at 2278 nm corresponding to an emission bandwidth of ~12 nm (for E || b). Continuous-wave laser operation on this 3H4 → 3H5 transition is achieved with an 1.8 at.% Tm:YAlO3 crystal under laser-pumping at 776 nm. The mid-infrared Tm:YAlO3 laser generated 0.96 W at ~2274 nm with a slope efficiency of 61.8% and a linear laser polarization (E || b). Tm:YAlO3 is promising for mode-locked lasers at ~2.3 μm.
Yb:CALGO is now recognized to exhibit outstanding properties for the production of high-power and ultra-short laser pulses in the near infrared spectral range. However, various UV-visible absorption bands can be also observed due to different types of charge transfer mechanisms. Some of them are assigned to the formation of color centers due to small polarons and others to O2-→Yb3+ ligand-to-metal charge transfer (LMCT) transitions. The former can be removed by using adequate thermal treatments. The latter are intrinsic and they are very intense with cross sections of about two orders of magnitude larger that the near infrared ones. In fact, such LMCT absorption bands are responsible for relatively large changes of ionic polarizabilities and to non-negligible pseudo-nonlinear changes of refractive indices which should certainly affect the laser properties of Yb:CALGO at high pump power levels.
High optical quality rare-earth-doped LiYF4 (YLF) epitaxial layers were grown on pure YLF substrates by liquid phase epitaxy (LPE). Thulium, praseodymium and ytterbium YLF crystalline waveguides co-doped with gadolinium and/or lutetium were obtained. Spectroscopic and optical characterization of these rare-earth doped waveguides are reported. Internal propagation losses as low as 0.11 dB/cm were measured on the Tm:YLF waveguide and the overall spectroscopic characteristics of the epitaxial layers were found to be comparable to bulk crystals. Laser operation was achieved at 1.87 μm in the Tm3+ doped YLF planar waveguide with a very good efficiency of 76% with respect to the pump power. Lasing was also demonstrated in a Pr3+ doped YLF waveguide in the red and orange regions and in a Yb3+:YLF planar waveguide at 1020 nm and 994 nm.
Downconversion is investigated as a promising way to enhance silicon solar cells efficiency. The efficiency of the
downconversion process is investigated for the (Pr3+, Yb3+) codoping in two fluoride hosts: KY3F10 and CaF2. Strong
near-infrared emission from ytterbium ions after excitation of praseodymium ions at 440 nm is observed in both KY3F10
and CaF2 as a result of the efficient energy transfer from praseodymium to ytterbium. In particular, very high Pr3+ to Yb3+ energy transfer efficiencies (ETE) are achieved for low Yb3+ and Pr3+ concentrations (ETE=97% in CaF2:0.5%Pr3+-
1%Yb3+) in CaF2 in comparison with KY3F10. A low Yb3+ concentration offers the advantage to limit the Yb3+
concentration quenching which is observed in other hosts, where the Yb3+ concentration has to be larger to achieve a
high ETE for solar cell applications.
Many industrial and scientific applications need ultra-short and energetic pulses. Diode-pumped systems based on
ytterbium-doped crystals have a huge interest thanks to their good thermal and spectroscopic properties. Among them,
Yb:CaF2, shows very promising results for short pulse generation, and its long fluorescence lifetime, 2.4 ms, indicates a
high energy storage capacity.
We present a diode-pumped regenerative amplifier based on an Yb:CaF2 crystal optimized to produce short pulses for
various repetition rates ranging from 100 Hz to 10 kHz. The experiment is performed with a 2.6-% Yb doped 5-mm-long
CaF2 crystal grown by using the Bridgman technique and used at Brewster angle. To optimize the injection pulse
spectrum in terms of bandwidth and maximum gain, the seed pulses are generated by a broadband Yb:CALGO oscillator
centered at 1043 nm with a FWHM bandwidth of 15 nm at a repetition rate of 27 MHz. The pulses are then stretched to
260 ps with a transmission grating. The shortest pulse duration generated is 178-fs, and the corresponding energy is
1.4 mJ before compression (620 μJ after), at a repetition rate of 500 Hz for 16 W of pump power. The bandwidth is 10
nm centered at 1040 nm. At 10 kHz repetition rate, 1.4 W of average power before compression is obtained,
corresponding to an optical-optical efficiency of 10%. We also noticed that the pulse duration tends to increase above 1
kHz, reaching 400 fs at 10 kHz.
Cryogenic cooling is a very interesting and promising apparatus for high power lasers, especially with Yb-doped
materials. In fact, it is now well known that operating this type of laser materials at cryogenic temperatures such as 77K
(liquid nitrogen temperature) positively affects their performance, especially at high power levels, because of increased
thermal conductivities and absorption and emission cross sections. We present a high-power diode-pumped Yb:CaF2 laser operating at cryogenic temperature (77 K). A laser output power of 97 W at 1034 nm was extracted for a pump
power of 245 W. The corresponding global extraction efficiency (versus absorbed pump power) is 65%. The laser small
signal gain was found equal to 3.1. The laser wavelength could be tuned between 990 and 1052 nm with peaks which
well correspond to the structure of the gain cross section spectra registered at low temperature.
Many industrial and scientific applications need ultra-short pulses with high average power. Diode-pumped systems
based on ytterbium-doped crystals have a huge interest thanks to their good thermal and spectroscopic properties. Among
them, Yb:CALGO and Yb:CaF2, hold exceptional positions exhibiting a very atypical combination of ultrabroad
bandwidth and high thermal conductivity, therefore very promising for short pulse and high power applications.
In this paper we present an overview of the results obtained with these two crystals. First, we detail the origin of this
exceptional gathering of their broad emission bands and good thermal properties. Second, we present the results obtained
in femtosecond regime with these two crystals including a discussion on the actual limitations of Yb-doped ultrafast solid-state lasers.
We report on a diode-pumped regenerative amplifier based on Yb:CaF2 material, delivering pulses up to 1.8mJ pulse
energy at a repetition rate of 100Hz. The crystal is pumped at the zero line at 978 nm with a 10W continuous wave (CW)
fiber coupled laser diode. The pulses have a spectral bandwidth of 16nm centered at 1040 nm, which indicates a good
potential for millijoule range sub 100fs pulse duration after compression. It is also a good candidate for seeding higher
energy diode-pumped ytterbium lasers.
The presentation will give the state of the art and the results of the last advances obtained in the growth, the
characterization and the implementation of three kinds of rare-earth doped halide laser crystals: Pr3+ doped fluorides,
Yb3+ doped CaF2 and its isotypes, and Er3+ and Pr3+ doped chloride and bromide KPb2Cl5 and Tl3PbBr5.
Mid-infrared (IR) lasers are of interest for a variety of applications including environmental sensing, LIDAR and
military counter measures. However, this wavelength range lacks powerful, coherent, robust and compact sources. A
solution can lie in chalcogenide glasses as host materials for rare earth ions. With an extended infrared transparency, low
phonon energy limiting the non radiative multiphonon relaxation rates and suitable rare earth solubility, sulfide glasses
based on Ge-Ga-Sb-S system make available radiative transitions in the mid-IR range. The glasses with nominal
composition of Ge20Ga5Sb10S65 doped with Er3+ (500 to 10000 ppm) were prepared by means of conventional melting
and quenching method. The Er3+, widely studied in glass fibers for near-IR amplification, was initially selected for the
transition 4I9/2 to 4I11/2 emitting at around 4.5 &mgr;m in order to demonstrate the ability of this sulfide composition for midinfrared
fiber lasers application. In these objectives, absorption and emission spectra have been recorded and the
radiative decay lifetime of excited levels (4I9/2, 4I11/2 and 4I13/2) has been determined. These last experimental results were
compared with those obtained by Judd-Ofelt model from absorption cross-sections of all observable transitions.
Therefore, the 4I9/2 radiative quantum efficiency was estimated at 67 %. The emission cross-section was 2.6x10-21 cm2 at
4.6 &mgr;m obtained by Fütchbauer-Ladenburg theory. The product of measured lifetime and emission cross-section for 4I9/2
-> 4I11/2 transition is about 1.87x10-24 cm2.s is comparable with that for GaLaS glasses. The fiber drawing of the Er3+
doped Ge20Ga5Sb10S65 glasses and measurements of optical losses in mid-IR are currently in progress and first results
Results of diode-pumped cw and fs laser operation of an Yb3+:CaF2 single crystal are reported. With a 5-at.-% Yb3+-doped sample we obtained 5.8 W output power at 1053 nm, for 15 W of incident pump power at 980 nm. In passively mode-locked diode-pumped regime, using a Brewster-cut, 5-at.-% Yb3+-doped sample and prisms for dispersion compensation, the oscillator provided pulses as short as 150 fs, with 880 mW of average power and up to 1.4 W average output power, with pulse duration of 220 fs, centred at 1049 nm. The laser wavelength could be tuned from 1018 nm to 1072 nm in cw regime and from 1040 nm to 1053 nm in mode-locked regime. Using chirped mirrors for dispersion compensation, we obtained up to 1.74 W of average power, with pulse duration of 230 fs. For all these reasons, Yb:CaF2 crystal is showing the great potential as active medium for high average power femtosecond oscillators and as amplifier medium for femtosecond pulses.
We examine here the lasing conditions of a Ce :LiCAF laser crystal placed intracavity with a BBO nonlinear crystal and pumped longitudinally throughout an input dichroic mirror by the 532 nm radiation of a frequency-doubled diode-pumped Nd :YAG laser. The comparison with the results obtained with an off-axis configuration shows lower laser slope efficiencies but similar laser performance in terms of threshold absorbed pump fluences (around 200 mJ/cm2). A model based on revisited spectroscopic parameters is developed to account for these laser performance.
We present here the first CW high power laser operation obtained under diode-pumping with an Yb3+:CaF2 crystal. This crystal exhibits good thermo-optical properties and can easily be grown in bulk crystals or in thin films. A maximum power of 5.8 W in a diffraction limited beam has been obtained with a 5% ytterbium-doped crystal of 4 mm-long. Moreover, the laser wavelength has been tuned over 54 nm, between 1018 and 1072 nm, and the double-pass small-signal gain has been measured to be more than 1.8, showing the great potential of Yb3+:CaF2 as a gain media for ultra-short pulses operation or as amplifier.
By using laser selective excitation and low temperature time-resolved spectroscopy techniques, we have been able to experimentally identified the ion centers of tetragonal, trigonal and cubic symmetries in a low concentrated crystal as 0.03%Yb3+:CaF2. This low temperature study was then completed by an analysis of the room temperature spectroscopic properties and of the laser potential of more concentrated Yb3+ doped CaF2 single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 to 1060 nm.
An experiment has been performed to analyse a nonlinear lensing effect in a flashlamp pumped Cr3+:LiSAF laser which has been found to be adiverging lensing effect. The latter is due to the refrctive index change which is assumed to be proportional to the excited ion population. The corresponding constant of proportionality has been measured from the time variation of the laser pulses far-field divergence.
We have studied the Yb3+ (reversible reaction) Tm3+ nonradiative energy transfer processes responsible for population of the 1G4 thulium level under Yb3+ or Tm3+ selective laser excitation of YLF:Tm3+,Yb3+ crystal. The microparameters and the rates of the energy transfer via cross-relaxation schemes are determined. It is concluded that the process of populating the 1G4 thulium level is greatly affected by up-conversion processes not only from the 3H4 but also from the 3F4 level, proceeding within the static decay model.
Work being done on solid-state lasers in a single laboratory is reviewed. Recent results on broadly tunable laser systems based on transition metal ions are addressed, discussing excited-state absorption measurements and the search for new emitting centers. The main emission spectra obtained with the rare earth doped laser materials Nd(3+), Er(3+), Tm(3+), and Ho(3+) are shown and discussed.