The two major conditions for lasers are sufficient pumping to obtain population inversion,and minimized quenching.Energy transfer between simultaneously present lanthanides,chromium(III) or manganese(II) are useful for feeding energy to the emitting level, and storing energy beyond its intrinsic life-time .Quenching of lanthanide J-level emission is well understood since the multi-phonon de-excitation was rationalized by Weber(1967) but can also involve cross-relaxation and other energy transfer (including resonant migration to dark traps).Both emission spectra and quenching (especially with increasing temperature,sometimes already above 100 K) is far more complicated in Cr(III) than in Mn(II) and can be related to the 16-dimensional Born-Oppenheimer potential surface of local MX6 clusters. Glasses preserve a dispersion of nuclear positions from their hot melt, enhancing quenching (it impedes energy transfer much less) with an optimized exception of low Cr(III) content in lithium lanthanum phosphate glass.Tunable Cr(III) lasers seem now accessible with limpid glass-ceramics.

Our interest for lanthanide hexaaluminates began about twenty years ago. At this time, we were investigating the alumina-lanthanide oxide phase diagrams.New compouns with approximate composition "LnAl11O18", Ln = La-Sm, and X-ray powder patterns very similar to the β-alumina one were found [1]. β-alumina - in fact a sodium aluminate with theoretical formula NaAl11017 - has also been subjected to detailed studies in this laboratory because of its high ionic conductivity [2]. At the end of the seventies, a serie of papers by Verstegen and Stevels from Philips laboratory have appeared [3-5]. They reported the highly efficient luminescent properties of rare-earth and transition-metal activated n-alumina and magnetoplumbite. The magnetoplumbite family, to whom belongs for instance CaA112O19 is structurally related to β-alumina. The Philips'papers decided us to come back to the "LnAl1 1O18" phases. Their thermal stability were very poor, but we remarked that β-alumina structure has an oxygen sublattice in O17 while magnetoplumbite has an O19 one. Therefore, we tried to stabilize the "LnAl11O18" phases by incorporating one MO unit in it, to reach the O19 lattice of MP compounds. With M = Mg, the phase turned to have the formula LaMgAl11O19, latter referred to as LMA.

Laser holmium belongs to a family of rare earth doped ions emitting in the near or mid-IR spectral range. Its 2.1 μm laser emission has potential applications in many fields as will be discussed below. In this review we will concentrate on the following topics: A. General characteristics of Ho3+ laser and hosts. B. Significant milestones in holmium laser development. C. Mechanism of basic processes. D. Engineering considerations E. Applications F. Trends and future. A. General Characteristics The main characteristics of holmium laser are as follows: 1-A. Its emission wavelength originates from the 517→518 transition (≈2.1 μm) 2-A. The main laser hosts used are: oxide crystals such as YAG (Y3Al5O12), YAlO3 or fluorides such as YLF (YLiF4) or HoBaYb28. 3-A. Energy sensitizers such as Cr3+, Tm3+, Er3+ are used in order to increase the laser efficiency and to better utilize the lamp emission spectrum. 4-A. Holmium laser needs liquid nitrogen cooling for efficient operation. At ambient temperature it behaves as a quasi three-level system with high lasing threshold and low slope efficiency. 5-A. The laser can be operated both in CW or pulsed modes. 6-A. It has high gain cross section and a long lifetime of 5I7 level which results in an efficient Q-switched operation. 7-A. Applications: Medical Free space communication Eye-safe range finders or Target illuminators Remote sensing Tunable operational amplifier The most popular hosts for holmium laser are the aPHo:YAG (erbium-thulium-sensitized Ho:YAG) and aPHo:YLF. Tables 1 and 2 summarize the mechanical and optical properties of YLF, YAG and GSGG (gadolinium scandium galium garnet), respectively. The mechanical and thermal properties of YAG are better than those of GSGG and superior relative to YLF - see Table 1. From Table 2 it is inferred that YLF has a negative derivative of its refraction index with temperature, implying that YLF may show a lower thermal lensing effect than YAG in spite of its lower thermal conductivity. YLF does not exhibit uv induced damage (solarization) as is the case in YAG, and has lower multiphonon rates.

This presentation is divided in two parts. In the first part it is done a rapid but fairly exhaustive description of all the active media which have been studied in the recent years for their potential use as tunable solid-state laser systems. It is shown their superiority on the other existing ones, in relation with the applications they allow consider. I mention too some of their drawbacks and, linked to these drawbacks, the optical studies and the search for new materials which are still necessary. As a matter of fact, in the second part, a more precise description is made of these optical studies in relation with the theoretical models, and more particularly with the predictions of the Single Configuration Coordinate model -for it is the easiest one to work up from classical experimental data-, which have been proposed to account for processes such as the non-radiative and the excited-state absorption processes.

Recent developments in diode pumped 2 and 3 μm rare earth lasers are reviewed.

Due to wavelength matching between emission lines and transmission windows of glass fibers for optical communications, all kind of Er3+ doped matrices are being considered more and more frequently in litterature, namely : semi-conductors, glass fibers, crystals. The review shall present a number of recent breakthroughs of significance to me : electro-luminescence and laser effect of Er3+ in III-V correspounds ; CW laser effect of 3-levels Er3+ transitions in connection with recent availability of powerful GaAlAs laser diodes as optical pumping sources ; CW excited laser effect of Er3+ at 2.7 [μm] in crystals and in fluoride glasses. The purpose of the lecture shall be to discuss in a critical way the respective advantages and limitations of such Er3+ doped materials for future optical communications.

In ruby, some connection is found between the main lines in the phonon replicas spectrum and Raman lines of pure sapphire (corundum). In Alexandrite, no such connection exists.

The existence of Br22- pseudomolecules has been pointed out in the quasi one dimensional material CsCdBr3 and such centers have been observed for the first time at the ground state and at room temperature. The electronic and molecular orbitals involved in the optical transitions have been respectively identified by the Nishimura model and by two-photon excitation experiments. The origin of these centers has been related to stacking faults located at the interfaces of the cubic microdomains of CsMBr3 (M=Pb or Sn) by the studies of the doping effect by Pb2+ or Sn2+ and thermal treatments on the concentration of the centers Br22-.

Military solid state lasers always need a simultaneous optimization of the brightness, the efficiency and the compacity, even though these targets are opposed. In this paper, we will show how to optimize these three caracteristics by using classical technologies. This lecture only deals with flash pumped garnet materials and with moderate average power (below 100 watts). We first describe thermal effects in cylindrical geometry. Then we will show the consequences of thermal effects on laser beam caracteristics, for the cylindrical and the slab geometries. The last part compares YAG and GGG performances from the efficiency and thermal effects points of view.

A new resonator configuration which combines in a single laser crystal the operations of a narrow beam oscillator and a large volume amplifier is presented. The polarization feature of the laser beam is used to perform coupling of the beam into and out of the resonator in order to utilize efficiently the whole volume of the active medium. As a result, a high brightness laser beam is generated with a high output energy contained in a small divergence angle.

Fluoride glasses incorporating rare earths are proposed as integrated systems for fiber optic communications in the visible and infrared parts of the spectrum. Such devices may be obtained from rare earths in fluoride glasses in direct contact with undoped glass of the same composition without the light-emitting lanthanide. Radiative and nonradiative transitions of rare earth ions in fluoride glasses are presented and from these data the laser cross-section and laser threshold power is calculated.

Fundamental optical properties of heavy metal fluoride glasses are presented as well as the different families of these new optical materials. The main characteristics of rare-earth doped glasses such as emission, energy transfer, non radiative loss, are discussed. A survey of the recent laser operations obtained on bulk and especially on fiber is reported.

Rare earths incorporated in solids are the most popular solid laser devices to date, covering the infrared range from about one micron to about three microns. These consist of single lasing species, such as holmium, neodymium and erbium, or of a combination of different species to enable the system to lase at higher efficiency at room temperature. An example of such systems is the recent demonstration of a room temperature system lasing at about 2 microns, which utilizes a sophisticated scheme of three-stage energy transfer from chromium to holmium and thulium [1]. Recently an Er( III) glass laser operating on two- and three-fold upconversion has been demonstrated [2]. Of the above-mentioned systems by far the most important commercially is the Nd(III) laser, lasing at 1.04-1.07 μm, which is known to operate in a multitude of glasses and crystals [3-4]. Importance of this ion for laser industry prompted an intense worldwide research aimed toward increase of slope efficiency of the Nd(III) lasers and toward decrease of the lasing threshold. In the present paper we shall examine briefly the main advantages and disadvantages of the Nd(III) laser and propose the ways to overcome the disadvantages illustrated by practical examples.

Much work is presently ongoing investigating the codoping of crystals with acceptors and donors to achieve new, more efficient laser materials. In this paper, we investigate the possibility that in codoped materials, acceptors and donors are correlated on a microscopic scale even when the macroscopic doping appears to be uniform. A theoretical model for this transfer is presented; data from the most common solid-state laser materials are shown to exhibit this effect.

Among the prospects of new solid state laser-materials the garnet-like structure hosts have a privileged place due to good chemical, thermal and optical properties respectively. The oxyde compounds with usual general formula C3A2D3O12 show three sites inside the cubic structure: C is the dodecahedral site A the octahedral site and D is the tetrahedral site. So that easy substitutions with either rare-earth ions (Ce3+, Ne+, Er3+, Tm3+, He+) in the dodecahedral site or metal transition ions (Cr3+) in the octahedral site may be done with high concentrations and rods may be grown by the standard Czochralski method technique. The most known singlecrystal is Y3A15O12:Nd3+(YAG) which is, to day, the reference of Nef-doped solid state laser materials emitting an emission line at 1064nm. But a new one Gd3Sc2Ga3O12:Cr3+-Nd3+(GSGG) which has recently shown better laser performances, could be commercialized in the future due to an efficient Cr3+-Nd3+ energy transfer process under visible broad band Xe-flash lamps pumping [1-2-3]. Unfortunately, this crystal contains expensive and rare Sc3+cations and then another approach has been tested to replace SO+ cations by more common ions as for example Ga3+ions (Gd3Ga5O12)(GGG) or Al3+ions (Gd3Al2Ga3012) or also Ca2+, mg2+ and Zr44- cations in substituted GGG (Ca2+, mg2+, Zr4+ ) with formula Gd3-xCaxGas-x-2yMgyZrx+yO12 [4-5-6-7]. This paper summarizes our own approach in the search on such oxyde garnet-like structure laser materials, restricting the results with Nd3+ activator ions and codoped by Cr3+ or Ce3+ sensitizer ions and we also give the trends of this class of solid state laser materials

The main goal of such a study is to understand the fluorescence processes occurring in these systems. Many different mechanisms are generally involved and it is important to really identify the main ones and to try to determine the exact transfer route. Models valuable for a given system are too often imprudently generalized to other systems without considering that the efficiencies of the energy transfers strongly depend not only upon the nature of the ions, but also on the active ion concentrations and on the intensity and range of excitation. A good knowledge of the excited state dynamics should be helpful for the determination of the best doping densities for a given laser emission and for given excitation domain and mode. Optimized materials present lower laser threshold energy and higher power conversion slope efficiency. The solid-state laser materials containing Er3+, Tm3+ and Ho3+ ions are particularly interesting for infrared emissions and many infrared laser actions have been demonstrated in the past/1/. Recently these systems have been proposed as mid infrared diode-pumped solid state lasers/2,3/ because recent progress make an attractive alternative to lamp-pumped operation/4/ and also because of very good spectral match exists between the GaAlAs diode emission and the Er3+ and Tm3+ absorption near 795 nm. Ho3+ ions do not absorb this radiation but its 2μm laser emission in which we will be greatly interested in the following can be sensitized by Er3+ and Tm3+ ions. The systems investigated in the present study are Er3+, Tm3+ and He+ doped LiYF4 single crystals and fluoride glasses. The so-called ZBLA, BATY and BIZYT glasses which are investigated are described in detail in the LUCAS's paper of the present book.

Inertial confinement fusion (ICF) research puts severe demands on the laser driver. In recent years large, multibeam Nd:glass lasers have provided a flexible experimental tool for exploring fusion target physics because of their high powers, variable pulse length and shape, wavelength flexibility using harmonic generation, and adjustable focus. Advances in solid-state laser technology indicate that (1) Nd:glass lasers can be scaled up to provide a single-pulse, multi-megajoule, high-gain laboratory microfusion facility, and (2) gas-cooled slab amplifiers with laser diode pump sources are viable candidates for an efficient, high repetition rate, megawatt driver for an ICF reactor. In both applications, requirements for energy storage and energy extraction drastically limit the choice of lasing media. Nonlinear optical effects and optical damage are additional design constraints. New laser architectures applicable to ICF drivers and possible laser materials, both crystals and glasses, are surveyed.

(Ho,Tm,Er):YAG single crystals of high optical quality suitable for 2.09μm laser operation have been grown by the Czochralski method. Accurate crystal composition was obtained by new analytical technique. This technique enables the determination of the effective distribution coefficients for both yttrium as well as the other rare earth ions incorporated into the garnet lattice. Lattice parameters and absorption spectra of the grown crystals are reported, together along with results of CW laser performances.

Single crystals of LaMgAll1O19 (magnetoplumbite-like structure) doped with Pr3+, Sm3+, Dy3+, Ho3+, Er3+ have been obtained by the flame fusion process. For each ion, the limit solubility has been determined. Optical absorption, emission and fluorescence excitation have been performed on cleavage platelets and the position of the fluorescent levels determined. The possible sensitization of the two Pr3+ fluorescent levels (3P0 and 1D2) by Dy3+ and Sm3+ respectively has been investigated. Energy transfer is important for instance if the lamp pumping of these solid state lasers is considered.

A perylene derivative dye, BASF 241, was impregnated into a composite sol-gel glass, and tested as a laser material. Laser tunability was obtained in the range 568 - 583 nm using a 532 nm pump beam (a frequency doubled Nd:YAG laser). Maximum efficiency of 7.4% was obtained at 575 nm. The laser threshold, was about 60 μJ/pulse.