This paper briefly reviews some of the spectroscopical results obtained for the Eu²⁺ and Ce³⁺ ions in the cubic perovskite KMgF₃. Both linear and non-linear spectroscopic techniques have been employed to assess the role played by the crystal-field and the lattice dynamics in modifying the optical properties of these rare-earth ions.

A survey of previous and newest results of the excited state absorption (ESA) measurements is presented and discussed. Author describes the measurements and interprets the ESA characteristics in case of transition metal ion-based systems. A special stress is put upon distinguishing the direct type ESA from more complex ESA schemes. Interpretations of the ESA spectra involve such phenomenon as charge redistribution via the d-dp transition, Jahn- Teller effect, existence of other centers contributing to the ESA, and thermal effects.

We have evaluated a prototype glass doped with titanium from the point of view of its basic spectroscopic characteristics. Measurements of the luminescence lifetime at various wavelengths, luminescence spectra and time resolved luminescence spectra have been performed. Using the obtained characteristics we have attempted to interpret the de-excitation mechanism in the Ti:glass. For sufficiently high excitation energy the luminescence involves the following centers: Ti³⁺, emitting at 780 nm, Ti⁴⁺, emitting at 460 nm and an unidentified luminescence center emitting at approximately 560 nm.

Electron paramagnetic resonance of tow new paramagnetic centers, formed by non-Kramers ions of iron group in synthetic forsterite is studied at frequencies range 65-350 GHz. The measurements were performed at 4.2 K in magnetic fields up to 0.8 T. It was established that the first center is due to Ni²⁺ in the M1 position with inverse symmetry. The angles between projections of the principal magnetic axes of four magnetically non-equivalent centers and axis b of crystal lattice are +/- 49 degrees and +/- 7 degrees in the bs and ab planes respectively. The fine structure parameters of this center were measured to be D equals 230.5 Hz and E equals 4 GHz. For the second center the only resonance transition was observed with zero field splitting 96.5 GHz. We suppose that it is transition between +2 > and -2 > states of Fe(superscript 2+ ions in the M1 position. For this center magnetic axes are misaligned from the a axis on +/- 3.5 degrees and +/- 12.5 degrees in the ab and ac planes respectively. Direct measurement of the ZFS for non-Kramers ions in forsterite revealed additional signals at zero field.

The method of determination of the upper laser level lifetime (tau) _sp, based on the dependence of the lasing threshold of the active medium of solid-state tunable lasers on the repetition period of pump pulses when they approach one another by an interval comparable with (tau) _sp, is proposed. An experimental setup for realizing the method proposed is described.

This paper deals with the synthesis and optical properties of a new compound Cr⁴⁺:Li₂MgSiO₄ which looks promising for tunable lasers applications. The emission is intensive and centered around 1190 nm, covering the range 1100 to 1400 nm. At low temperature, the fluorescence spectrum represents two zero phonon lines at 1109 nm and 1205 nm, attributed to the ³T₂ and ¹E levels, ¹E lies 712 cm^-1 below ³T₂. The fluorescence lifetime exceed 100 s at room temperature and 400 microsecond(s) at 50 K. This is roughly one order of magnitude higher than the values obtained for all the others Cr⁴⁺ activated materials. A thermalization between the ³T₂ and ¹E levels can explain these outstanding properties.

The spectroscopic characteristics of chromium-doped gahnite glass ceramics have been evaluated. The material exhibits multi-site spectra, including a complicated emission spectrum and room temperature anti-Stokes emission. Careful analysis of the room and low temperature spectra, supported by luminescence decay measurements, proves the existence of a t least two high-field sites associated with single Cr³⁺ ions and pairs of Cr³⁺ ions. The R line of the highest field sites is accompanied by phonon side bands. Estimates of the associated phonon energies have also been made.

A new class of room-temperature, diode-pumped solid state lasers, that are broadly tunable in the mid-IR spectral region, has been conceptualized and demonstrated. These lasers are based on intra-ion transitions of divalent transition metals placed in substitutional cation sites of tetrahedral symmetry in large bandgap chalcogenide semiconductor crystals. These combinations of laser-ions and host crystals are seen to provide favorable radiative and non-radiative transition processes for the realization of the desired laser performance characteristics. Spectroscopic data for candidate schemes are reviewed and divalent chromium doped zinc chalcogenides are identified as potentially superior laser candidates. Preparation of laser quality Cr²⁺:ZnSe crystals is described and experimental results to date for a diode-pumped laser are given. Remaining laser development issues are discussed briefly.

Excited state absorption spectra of Cr⁴⁺-doped garnets and wurtzite-like crystals were measured between 400 nm and 2000 nm. In Cr⁴⁺:Y₃Al₅O₁₂, a strong polarization dependence of the excited state absorption is observed, while it diminishes for Cr⁴⁺:Y₃Sc_0.5Al_4.5O₁₂ and is almost not observed in Cr⁴⁺:Y₃Sc₂Ga₃O₁₂. FOr Cr⁴⁺:LiAlO₂ and Cr⁴⁺:LiGaO₂ the region of the emission is overlapped by excited state absorption and nonlaser oscillation at room temperature is expected.

Dysprosium doped inorganic glasses and crystals are reviewed with special emphasis on applications in lasers and optical fiber amplifiers. The results of spectroscopic studies of Dy³⁺ doped LiNbO₃ and SrLaGa₃O₇ crystals are presented. Absorption, emission, excitation and lifetime measurements have been performed and discussed in the framework of Judd-Ofelt approach. The stimulated emission cross sections of the strongest transitions of Dy³⁺ ion have been estimated.

We discuss how the interactions of the 5d orbital with the conduction band of the host medium play a crucial role in determining whether rare earth containing materials can serve as useful laser materials, based on their 5d-4f transition. To explore this issue, we examine the pump-probe spectra of Sm²⁺, Eu²⁺, and Ce³⁺ dopants in various fluoride and chloride crystals. In addition we suggest that the luminescence properties are also profoundly impacted by this interaction. The outstanding UV laser performance achieved by the Ce:LiSrAlF₆ crystal is rationalized in terms of the reduced overlap of conduction band states with the cerium ions.

THe spectroscopic properties of trivalent praseodymium doped Be₂La₂O₅ crystals have been investigated at various temperatures. The absorption, emission, excitation and fluorescence lifetime measurements for a sample containing 1 atomic percent of activator have been performed and discussed. The Judd-Ofelt theory has been applied to obtain (Omega) _i intensity parameters.

Overview of basic spectroscopic properties of several crystals belonging to two wide families of compounds is presented. Both the families form tetragonal crystals with layered structure and show certain structural disorder resulting from random distribution of divalent A atoms and trivalent B atoms. After presenting some details concerning the crystal growth and structural investigation, the nature of activator sites in the matrices is discussed. Then, the emission spectra as well as relaxation dynamics of Nd³⁺, Yb³⁺, and Tm³⁺ are analyzed. After that, the basic spectroscopic features of chromium doped crystals are given. In conclusion, suitability of the crystals for the design of tunable lasers is discussed.

Ultrabroadband nanosecond red-IR oscillation of up to 370 nm from longitudinally pumped Ti³⁺-sapphire laser has been achieved. The 'spatially-dispersive' laser resonator employed a prism pair, an intracavity cylindrical lens, and a diaphragm allowed to maintain the lasing conditions simultaneously for different spectral components in different areas of acting element. Second harmonic radiation from a Q-switched YAG:Nd³⁺ laser with 532 nm oscillation wavelength and 200 mJ pulse energy was used as a pumping source. Stable giant pulse operation of ultrabroadband Ti³⁺ sapphire laser in 680-1050 nm range with pulse energy of up to 30 mJ, pulse repetition rate of up to 10 Hz and 15 percent pumping to lasing efficiency have been obtained.

The origin of excited-state absorption (ESA) that plagues the performance of Ti:YAlO₃ as a laser material is discussed. It is proposed that two populations of Ti³⁺ exist in YAlO₃. Those that have a lower photoionization threshold have to be eliminated to prevent the ESA from affecting lasing.

Polarized differential absorption spectra of Cr⁴⁺- doped forsterite have been studied using a picosecond pump- probe technique. For pump beam and probe beam polarization parallel to the axis, an excited state absorption in the 450-800 nm region is observed and assigned to the ³T₂yields³T₁(³P) transition of Cr⁴⁺ ion. The lifetime of the ³T₁(³F) state is estimated to be less than 15 ps.

Luminescence decay curves have been measured from the ⁴S_3/2 excited state of erbium(III) in the elpasolite lattice for a range of concentrations and temperatures. Cross relaxation occurs from this state involving a relaxation at the donor to the ⁴I_9/2 state and a transition at the acceptor from the ground state to the ⁴I_13/2 state. The decay curves are strongly temperature dependent, the cross relaxation becoming important at 80 K and above. A comparison with the decay curves calculated using a shell model of discrete acceptor distribution within the lattice is unsuccessful due to migration of the excitation energy within the lattice.

Low temperature, high resolution emission spectra have been measured for the pure and diluted erbium elpasolites, Cs₂NaEr_xY_1-xCl₆ from the ⁴G_11/2, ²G_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 excited states of Er³⁺ under 364 nm argon laser excitation. A number of these spectra show some unusual features which are consistent with migration of the excitation energy within the lattice leading to excitation of luminescence from defect sites. The nature of the defect sites is discussed and possible mechanisms leading to the migration of the excitation energy within the lattice are explored.

The luminescence decay curves of the ⁵D₀ excited state of the cubic hexachloroelpasolite Cs₂NaEuCl₆ have been measured at 80 and 295 K for samples prepared by several methods. The decay curves are non-exponential and the rate of decay depends on the method of preparation. This is attributed to energy migration between the majority sites followed by energy transfer to defect sites associated with the presence of oxide ions in the lattice. A new method of preparation is devised which minimizes the concentration of oxide and other impurities. Crystals prepared by this method have lifetimes that are several times longer than that prepared by the standard procedure.

Excitation into the ⁵I₈yields⁵F₅ transition of Cs₂NaHoCl₆ near 15 633 cm^-1 at 300 K results in strong, long lived emission from the ⁵F₅ state wit an almost exponential decay curve. The decay constant of this emission decreases slightly in Cs₂NaY_0.99Ho_0.01Cl₆ the decay becoming exactly exponential, the difference between the decay constants is greater at lower temperatures. This suggests that there is migration in the host lattice followed by energy transfer to defect sites with short ⁵F₅ lifetimes in the pure Ho compound and this hypothesis is confirmed by measurements of the relaxation of the ⁵F₅ state in Cs₂NaNd_0.01Ho_0.99Cl₆ where the decay constant is greatly increased but the decay curves are exponential. Under the same conditions and at temperatures from 1.2K to 300K there is emission in the ultraviolet, green and blue regions from Cs₂NaY_1-xHo_xCl₆. The decay curves are complex with more than one rise and decay processes reflecting a variety of up-conversion processes. The mechanisms of these processes are discussed.

Growth and optical properties of Er³⁺ doped single crystals 5NaF 9YF₃ are investigated. The low temperature absorption and emission spectra were sufficiently resolved to allow building up the Stark sublevels diagram. The measured absorption line strengths at room temperature were used for a Judd-Ofelt analysis. The resulting radiation rates are tabulated and compared with the experimental lifetimes. The possibility to use such material for both IR and visible laser emissions is discussed.

Luminescence decay curves from the $_5)D₁ state of Eu³⁺ in the cubic hexachloroelpasolite crystals Cs₂NaSm_xEu_0.1Gd_0.9-xCl₆ have been measured at 80 K and 300 K. At this concentration of Eu³⁺, cross-relaxation to nearby Eu³⁺ acceptor ions and energy-transfer to the ⁴G_5/2 state of Sm³⁺ occur at both temperatures. Excellent agreement between the experimental decay curves and the a priori calculated curves is achieved by using a recently developed discrete shell model for energy transfer.

The cubic system Cs₂NaEr(NO₂)₆ provides an interesting comparison with the corresponding hexachloroelpasolite, having a similar structure but, in addition, there are high frequency phonons present since the one phonon spectrum extends up to 1 600 cm^-1. The absorption spectrum at low temperatures shows a beautiful progression in the NO₂ bending mode in the near UV in addition to the typical f-f erbium centered transitions. Under red excitation at 80 K into the ⁴I_15/2 yields ⁴F_9/2 transition strong, but short lived emission is observed from both the ⁴F_9/2 and ⁴I_9/2 states. At high irradiation densities intense emission is also observed in the blue at 21 460 and 21 280 cm^-1 and at other wavenumbers in the UV and green regions. These emissions have different complex decay curves and appear to arise from energy migration to defect sites followed by up- conversion at those sites. In the up-conversion process, the luminescent defect sites are destroyed but appear to regenerate on long term storage of the crystals at room temperature.

Ho₃₊ ions emitting in the near IR range, can be sensitized either by Tm³⁺ or by Cr⁴⁺ ions, as energy resonances between these ions are possible. In the Cs₂Al₂SiO₇ (CAS) host, the shortest distance between two RI³⁺ cations is 3.56 angstrom while this distance could be reduced to 3.14 angstrom when Cr⁴⁺ and RE³⁺ ions are considered. In this study, the materials are prepared in the form of small single crystal by the melting zone process in order to perform spectroscopic investigation. In the CAS host, the energy transfer processes are in competition with the very efficient non radiative multiphonon relaxation. Strong Tm yields Ho energy transfer enhanced the Ho³⁺ near IR emission around 2 micrometers while the Ho yields Tm back transfer probability is quite low. For Cr⁴⁺:CAS, due to a strong electron-phonon coupling, Cr⁴⁺ emission suffers from strong thermal quenching and almost vanished at room temperature. In the Cr⁴⁺:RE³⁺:Ca₂Al₂SiO₇ crystals, interactions between Cr⁴⁺ and Er³⁺, Ho³⁺ or Tm³⁺ occur and lifetime measurements are used to quantify the energy transfers probabilities.

Color centers in alkali halide crystals possess a peculiar optical cycle, which delivers an emission very often with quantum efficiency near 1, a few of them having the qualities required by active media. Among these the F_A center was the first one to lase, and it is the most stable, efficient and easy to be produced. However up to now, as far as its basic properties are concerned, it remains a rather elusive point defect.Indeed recent results brought up again for discussion the old classification between F_A(I) and F_A(II) centers, the last ones only being optical active centers. It has ben found that both species can coexist in the same host crystal, their relative number being a function of temperature. This phenomenon opens up new perspectives for studying in detail relaxation, thermal excitation, and energy transfer processes at a microscopic level after optical excitation.

The peculiarities of energy levels structure of laser-active F₃⁺ color centers in LiF crystals were studied. The dependencies of polarization and fluorescence intensity of LiF crystals with F₃⁺ centers against polarization of powerful laser irradiation which originates two-photon excitation of F₃⁺ centers, were measured and calculated. It was shown that the two-photon transition leads to the excitation of unknown previously spin-singlet electronic state, possessing ¹A₁ wavefunction symmetry.

New spin-triplet electronic states of laser-active F₃⁺ color centers in LiF crystals are discovered. The triplet-triplet absorption spectra from low laying ³E state to the high laying states, triplet-triplet ³T_(alpha ) yields ³E fluorescence spectrum and ³E yields ¹A₁ triplet-singlet phosphorescence spectrum are measured. Temperature dependencies of nonradiative singlet-triplet ¹E yields ³E and triplet- singlet ³E yields ¹A₁ transitions probability are found. The influence of spin-triplet states on the LiF:F₃⁺ color center laser characteristics is discussed. The new way of obtaining of tunable oscillation in wide visible range using triplet-triplet transition is offered.

The theoretical and experimental investigations of temporal dynamic of LiF:F₂^- color center laser oscillation are presented. It was found that due to the high amplification cross section, the lasing temporally overlaps with the pump pulse. The peculiarities of LiF:F₂^- lasing under various pump sources are under consideration.

We review our recent results in the development of broadly tunable picosecond and femtosecond color center lasers operating in the near IR wavelength region. As active materials we use color centers with high optical gain in NaCl, KCl, and KBr hosts. All systems can be efficiently pumped with Nd:YAG laser lines. Using synchronous mode- locking, pulses with typically 5-10 picosecond duration and nanoJoule energy levels are obtained. The combined tuning range of these actively mode-locked lasers extends from approximately 1.4-2.4 micrometers without gap. In all cases the pulses are nearly bandwidth limited, with the notable exception of the NaCl laser. We show that coherent photon seeding can be used in this case to drastically improve the pulse quality by reducing high-frequency noise and spectral bandwidth. Pulses with femtosecond time duration, operating in the 1.5-1.6 micrometers range, are realized using passive mode- locking with semiconductor saturable absorbers. Using the absorbers in single- and coupled-cavity configurations, these lasers are particularly simple in that they require only a continuous-wave pump. Finally we demonstrate that the laser active color center systems can be used also as high- gain amplifiers. Employing a single two-pass amplification scheme, femtosecond pulses from a 1.5 micrometers oscillator are amplified to microJoule energy levels and used to generate femtosecond IR spectral continua.

Different conditions of the F₃⁺ color centers production are investigated. Thermo-annealing and photo- irradiation are proposed to increase the F₃⁺ centers concentration. Data of polarization spectroscopy, triplet states and energy level diagram are presented. The characteristics of laser on the base of F₃⁺ centers in lithium fluoride are given.

Luminescence emission and excitation spectra of a number of natural spodumene crystals have been measured at temperatures down to 12K. Both Cr³⁺ and Mn²⁺ centers were identified: Mn²⁺ is shown to be mainly in Li-sites rather than Al-sites and gives rise to abroad emission centered at 600 nm. In most spodumenes Cr³⁺ R-line emission with phonon sideband is observed at temperatures below 100K. In green hiddenite crystals Cr³⁺ emission is dominant at room temperature where the R-lines are superimposed on a broad-band emission. Calculation of the crystal-field splitting of the Cr³⁺ energy levels has been carried out assuming a C_2v pseudo-symmetry for the Al-sites. In the more Mn- rich crystals multiple R₁ lines are seen at temperatures below about 40K which are possibly associated with adjacent point defects due to charge compensation effects.

The analysis of simultaneous second and fourth harmonic generation of superbroadband LiF:F₂^- color center laser radiation in different nonlinear crystals is presented. The experiments have shown high laser and harmonic conversion efficiencies. Simultaneous generation of continuous spectra in near IR, visible and Uv spectral regions, and multiline oscillation with high spectral resolution (Delta) v < 5 cm^-1 have been obtained.

The results of investigation of phosphorescence of benzaldehyde in porous matrix are presented. As matrix was used porous glass obtained by sol-gel technology. Spectra of phosphorescence and its decay kinetics have been obtained for solvent-containing samples. The influence of the matrix on the above-mentioned parameters of phosphorescence has been revealed. It is shown that the character of this influence is determined by the interaction of the activator molecules with matrix walls and depends on the nature of used solvent.

The migration of electronic excitation energy over the triplet levels of benzaldehyde in an ethanol solution in porous soda-borosilicate glass and matrix porous glass- polymethylmetacrylate is studied by capture in traps under steady-state excitation conditions. The probability of excitation capture by a trap as a function of donor concentration is obtained for different trap concentrations and temperatures. The change of the character of these dependencies without the change of the critical concentration value for benzaldehyde in matrix ethanol- porous glass is observed. The strong increase of the critical concentration of benzaldehyde in the matrix polymethylmetacrylate-porous glass takes place. The experimental result are analyzed in the framework of percolation theory.

Lasing characteristics of laser-active material-doped microdroplets containing highly scattering fat emulsion Intralipid-10 percent are studied experimentally. Noteworthy findings are than well-defined lasing threshold can be observed and appreciable magnitude-enhancement of emission intensity with suitable conditions of the Intralipid mixing ratio, in comparison with original neat-active-material- doped microdroplets. We present and discuss the measured results of input-output intensities for different Intralipid mixing ratios in these active material-soft scatterer system. It was found that almost no lasing neat- microdroplets can achieve lasing by substituting suitably certain amounts of the Intralipid causing multiple light scattering. Spectral measurements of light emission from the active material-Intralipid microdroplets show the tendency of spectral narrowing with the increase of the pumping intensity and with the optimum Intralipid concentration. It is our belief that the present results make this novel method of active material-Intralipid microsystem very attractive for a variety of future applications including diagnostic tools for highly sensitive detecting and identification of small quantity objects and species embedded or hidden in highly scattering media such as biological tissues and cellular structures.

Supramolecular photochemistry is an emerging and rapidly growing research area, most attractive from fundamental point of view and very promising for a variety of applications. An appropriate assembly of molecular components capable of performing light-induced functions can be called a photochemical molecular device (PMD). The most functions that can be performed by PMDs are (i) generation and migration of electronic energy, (ii) photoinduced vectorial transport of electric charge, (iii) photoinduced conformational changes, and (iv) control and tuning of photochemical and photophysical properties. On the other hand, by the low-temperature sol-gel process are produced rigid and porous xerogel materials. The important point is that the sol-gel materials are transparent, therefore can be used as hosts for supramolecular photo-chemical devices.

The high concentrated silica glasses doped with Ce and Sm ions were prepared by the sol-gel technique. The possibility of preparation of the Ce-doped glasses with a low absorption coefficient in the optical range above 350 nm and the luminescence quantum yield >= 90 percent is reported. Such glasses seem to be promising as filters for laser heads. The relatively high light scattering for Sm-doped glasses has been also observed.

A new type of solid state lasers tunable in the visible rang has been developed recently. Incorporation of perylimide and pyrromethene dyes into glasses prepared by the sol gel method allows to design new types of visible, stable solid lasers. These can be prepared either in the form of slabs or rods or as waveguiding active media deposited on glass or polymer supports. The bulk lasers are prepared by two methods: (1) By one phase procedure by preparation of dye- ORMOSIL samples including the photostable laser dyes. (2) By incorporation of the laser photostable dyes into pre- prepared porous glasses by the sol gel method followed by polymerization in situ in the glass. We summarize her the laser properties obtained by different methods and when possible compare the performance of the solid laser with the solutions of corresponding laser dyes.

The emission of Cr-doped silica glass obtained by the sol- gel method is characterized by an orange broad band with a maximum at 610 nm. Its nature is examined by the absorption, excited state absorption, emission, excitation and lifetime measurements over a wide range of temperature and for different concentration of Cr ions. Our measurement show that in spite of fact that the absorption properties of Cr- doped silica sol-gel glass are predominantly associated with Cr⁴⁺ centers, the observed in visible range emission can be assigned neither to Cr³⁺ nor to Cr⁴⁺ ions. The discussion of the nature of observed emission was carried out for all possible valencies of the Cr ions. In conclusion is suggested that it may be ascribed to the transitions on the monovalent Cr¹⁺ ion. The reducing agents occurring during the sol-gel process and leading to lowering the Cr valency are discussed.

Preparation of silica glasses containing Eu₂(CO₃)₃ 4H₂O by means of sol-gel technology was described. The absorption, emission, and excitation of Eu₂(CO₃)₃ 4H₂O compounds in powder and entrapped in silica sol-gel glass were measured in a broad temperature range. It was found that the optical properties of europium ion direct environment were not preserved in silica glass. A broad band, in UV range, characteristic to Eu²⁺ ions or C-T transition, was also observed. The intensity of this broad band is controlled by temperature an concentration of the active ions. Following the energy diagram of f-f transitions derived from Eu₂(CO₃)₃ H₂O system, the symmetry of Eu³⁺ ion was discussed in precursor and gel spectra. The effect of anisotropy of europium ion was considered, too.

An optical probe of structure of metal ion center in sol-gel systems based on the Eu³⁺ and Pr³⁺ emission and absorption measurements have been developed. It was tested how far the substrates affect the structure of metal centers in the glasses obtained by two different methods. Splitting of the levels, as well as their electronic transition probabilities at room and liquid nitrogen temperatures were used for considerations of the structure and symmetry of metal center. It was shown that the structure of metal centers was partially preserved up to 450 K. Correlation of the thermogravimetric measurements of substrates with the gels dried and heated up to 1100 K enables to control spectroscopically possible clustering processes occurring in sol-gel silicas. The effect of organic modification of water retention and dopant aggregation was investigated too.

Lanthanum beryllate, new efficient solid-state laser material for laser diode pumping has been investigated. Slope efficiency comparison for BEL:Nd and YAG:Nd under the same cavity conditions was carried out. Two wavelength lasing regime for fundamental mode and tunable four wavelength lasing regime for intracavity doubled mode were observed.

Disordered crystal with Ca-gallogermanate structure are one of the most appropriate laser materials for compact systems with different special lasing regimes. Output power dependence on laser diode pump power and pumping wavelength temperature tuning of Nd:La₃Ga_5.5Nb_0.5O₁₄ laser, one of a large group of Ca-gallogermanates, under laser diode pump are presented in this research. Maximum output power of 68 mW under 250 mW pump power was reached. Slope efficiency was 41 percent.

Influence of light feedback in laser diode end-pumped Nd³⁺:KGd(WO₄) laser with intracavity doubling as well as wavelength temperature variations of the pump source on stability of solid-state laser output power and efficiency have been investigated. Threshold and slope efficiency were measured under different output coupler transparencies and pumping beam waist sizes. Maximum output power of 130 mW for fundamental mode and 25 mW for SHG was obtained.

We built three monolithic lasers pumped longitudinally by laser diodes. All lasers had the same dimensions and worked in CW mode. The lasers were based on neodymium doped crystals. They appeared to have quite good power stability. Comparison of the parameters of built lasers is presented in this work.

Application of the metallic thin-film selector to tunable, single frequency oscillation of diode pumped lasers has been investigated theoretically and experimentally. Tunable, single frequency output was achieved in Nd:YAG, Nd:YVO₄ and Nd:YLF crystals. The best results were obtained in case of Nd:YVO₄ crystal namely up to 0.6 W output power for 2 W pumping laser diode with optical slope efficiency > 40 percent. Single frequency operation, slow smooth tuning or chirping within range 0.5-10 GHz was realized by means of output coupler movement with the piezoceramic transducer. In case of Nd:YLF crystal single frequency generation on both 1047 nm and 1053 nm lines was accomplished. Physical and technical limitations caused by the wide gain bandwidth, thermal effects and mechanical vibrations of cavity elements were discussed.

The renewal of second order non-linear crystals is very strong in laser materials optics. We are involved in the search of new systems based upon highly non-linear niobate crystal family. Among these crystals, Ba₂NaNb₅O₁₅ (BNN) and K₃Li_2-xNb_5+xO_15+2x (KLN, 0.15 < x < 0.5) with a tungsten bronze-type structure are very efficient materials for the obtention of respectively green and blue laser light by frequency doubling or by self-frequency doubling after doping by Nd³⁺ ions. Unfortunately, BNN and KLN are difficult to grow by conventional techniques such as Czochralski without cracks caused by a strong lattice change occurring during cooling of the crystals. In addition, microtwins in BNN crystals and compositional inhomogeneities in KLN ones are often encountered. The obtention of good quality and crackless BNN and KLN single crystals doped with different amounts of Nd₂O₃ is reported. The crystals were grown by the laser heated pedestal growth technique. This method allowed to obtain good quality and homogeneous crystals of BNN and KLN overcoming the problems encountered with other growth techniques. More particularly, for BNN crystals, a structural transition between orthohombic and tetragonal symmetry was found to occur for a Nd³⁺ amount between 2 and 3 atomic percent avoiding the microtwinning which disturbs the optical properties of the crystals. Low temperature spectroscopy revealed that Nd³⁺ ions substitute both Ba²⁺ and Na⁺ ions in the pentagonal and square sites of the tungsten bronze-type structure of BNN whereas they substitute only K⁺ ions in the pentagonal sites of that of KLN. Non- critical phase matching at room temperature was also investigated for both materials.

In this paper we report the optical properties of Er-doped silica fibers. The fibers were produced from glasses obtained by sol-gel method. Luminescence spectra of Er³⁺ ion were measured in monolithic glass samples after excitation by a xenon lamp, as well as in the fiber after excitation by an IR 980 nm diode laser. It was found that under the IR excitation, a green emission at 540 nm was observed due to the up-conversion effect. This effect is associated with the aggregation of Er ions and constitutes an important factor reducing the efficiency of 1.55 micrometers emission.

This paper reviews ultrafast Kerr Lens Mode-locked solid- state lasers with particular emphasis on all-solid-state diode-pumped laser technology which has the potential to provide low-cost compact devices for ultrafast instrumentation, particularly for biomedical applications.We have demonstrated the use of ultrafast solid-state lasers for 3D imaging through turbid media using time-gated photorefractive holography, and for fluorescence lifetime imaging.

An experimental study of parametric amplification of light beams with screw dislocations is presented. It is shown that a vortex can be parametrically amplified preserving its topological charge. The reversal of the vortex in an optical parametric amplifier is demonstrated experimentally. It is shown also that OPA can be used for the vortex reversal and its frequency conversion. Experimental result are in agreement with theoretical predictions.

The generation of tunable ultrashort light pulses in the mid-IR spectral region using optical parametric oscillators is demonstrated. In particular, the operation of a continuous-wave mode-locked silver gallium selenide optical parametric oscillator is reported. The OPO was synchronously pumped by 1.55 micrometers signal radiation from a noncritically phase-matched KTP-OPO operating at a repetition rate of 75.4 MHz with a pulse length of 40 ps. The tuning range extended from 1.94 micrometers to 2.56 micrometers for the signal and idler wave, respectively. A total average output power of up to 520 mW was generated. The possibility to generate tunable fs-pulses in this spectral range with high average output power using the same nonlinear material is discussed.

Tuning range limitation in the temperature-tuned 90 degrees lithium niobate OPO synchronously pumped at 532 nm by long trains of 30 ps pulses is investigated. It is shown that the threshold intensity for oscillation at a signal wavelength shorter than 710 nm quickly rises. The main reason for this is two-photon absorption of the picosecond pump pulses, which increases at higher lithium niobate temperatures and so really limits the tuning range of this OPO.

A tunable Cr:Forsterite laser with intracavity Nd:YAG pump and its experimental results are presented. To our best knowledge, this is the first tunable intracavity pumped Cr⁴⁺ ion activated laser. A theoretical model of saturable absorber operating both as a laser and an intracavity Q-switch for its pumping laser is presented and discussed. A good agreement of the numerical calculations with the experimental results has been obtained.

The tunability of a diode pumped Yb:YAG laser is presented. With the thin disc laser concept good efficiencies with scalable output power are obtained. Up to 147 W output power, optical efficiencies of 64 percent and diffraction limited beam quality have been achieved.

A novel type of laser diodes and amplifiers based on asymmetric quantum-well heterostructures having a set of active layers of different thickness and/or compositions has been considered. In contrast to ordinary laser heterostructures, for such modified quantum-well systems, it is possible to change the gain spectrum in a wide range and to control the set of definite amplification frequencies due to the selecting of the width and composition of quantum wells and barrier regions and of their doping and arrangement as well. New heterostructure schemes for tunable spectrum light-emitting sources have been proposed.

The singly resonant operation of a BBO OPO pumped by the third harmonic of the Q-switched Nd:YAG laser was demonstrated. Output energies of the visible pulses higher than 100 mJ, and total conversion efficiencies higher than 50 percent at multiaxial pumping with average power of few watts were achieved. The OPO was tunable over the range 405- 2900 nm.

In this paper we show that 2 micrometers Tm³⁺:Ho³⁺ silica fiber lasers have promising properties for applications in laser medicine and in lidar applications. 2 micrometers lasers are interesting for measurements in the atmosphere since in this range there are several absorption lines of water vapor. 2 micrometers lasers can be used for lidar applications if their wavelength can be controlled. In contrast to crystal lasers glass fiber lasers can be tuned continuously over a range of about 80 nm. The excitation can be performed in a wide range of pump wavelengths and with different geometrical arrangements. With Ti:sapphire pumping a laser output of 71 mW with a slope efficiency of 17.5 percent has been reached. The possibility of side-pumping together with the development of fibers with lower losses will allow to reach the power levels that are necessary for the mentioned applications.

Stable continuous wave operation of a Cr-forsterite laser pumped by cw ND:YAG laser has been demonstrated at 288 K. No choppers were used to limit duty cycle of the cw system. The proper adjustment of the pump and cavity mode overlap eliminates the output power decrease at the high level pump power. The output power 1.1 W and slope efficiency 26 percent were derived.