The first experimental demonstration of a diode-pumped passively mode-locked femtosecond laser based on an Yb³⁺:CaGdAlO₄ single crystal is reported here. Yb³⁺:CaGdAlO₄ (Yb:CALGO) seems to be one of the most interesting since it both exhibits one of the broadest and smoothest emission spectrum (from 990 to 1080 nm) among ytterbium-doped materials and a relatively high thermal conductivity. The oscillator is directly diode-pumped by a high brightness 5-W fiber coupled laser diode and pulses are produced using a SESAM. It allows the production of pulses as short as 47 fs at 1050 nm, that is to our knowledge, the shortest laser pulses ever obtained from an oscillator based on Yb³⁺-doped bulk materials. The average power was 38 mW and the repetition rate 109 MHz.

The proper selection of the parameters of the laser material parameters for efficient continuous-wave 1064 nm emission under direct diode laser pumping at 885 nm into the emitting level of Nd:YAG is discussed. High slope efficiency (0.8) in absorbed power for 1.1 and 2.5at.%Nd concentrations and the advantage of using concentrated Nd:YAG crystals for improving the absorption efficiency is demonstrated.

Emission of Yb: CALGO is constituted of a broad band lying between 990 nm and 1060 nm with relatively high emission cross section values. Since Yb³⁺ ions occupy a single crystallographic site, the disorder is related to the surrounding cations presenting different size and cationic charges. Laser experiments were performed on a 2%Yb:CALGO sample with anti-reflection coating using Ti:sapphire pumping at 979 nm. A laser oscillation was obtained in sigma polarization from 1050 to 987.6 nm using non-collinear pumping. Consequently, a quantum defect value as low as 0.8% was obtained. This is one of the smallest values ever reported in the literature. In addition the thermal conductivity values are high (k=6.3 and 6.9 W.m^-1.K^-1 for the two orientations).

The study of neodymium ions emission along the channel ⁴F_3/2->⁴I_9/2 led us to select several materials for short laser wavelength application. The results obtained with three series of compounds are presented: ABGa₃O₇ (A = Ca or Sr-B = La or Gd), ABAlO₄ (A = Ca or Sr-B = Y, Ln) and AMO₄ (A= Ca, Sr or Ba-M = Mo or W). Crystal growth of these hosts and neodymium ions spectroscopic characterizations are presented. Among all these crystals, Nd-doped BaWO₄ seems to be the most promising one for laser emission at short wavelength, combining easy crystal growth, short emission wavelength and long lifetime.

Polycrystalline ceramic Nd:YAG laser material enables new possibilities in designing the laser medium with respect to dopant, size and geometry. In this paper, a 184W continuous-wave Nd³⁺-doped ceramic Y₃Al₅O₁₂ (Nd : YAG) laser with optical to optical efficiency of about 44.3% has been developed. Laser output power characteristics as well as the thermal lensing and birefringence properties of the ceramic laser rods were investigated. The sample used in this experiment was a 3.5mm diameter, 87mm long ceramic YAG rod with 0.6% Nd³⁺ concentration. And the end faces of the rod were flat and antireflection-coated at 1064 nm. The pumping geometry used in this work was a diffusive optical cavity with narrow slits for side-pumping.

In this paper an optically pumped tunable ring-laser system based on low-loss integrated optic titanium doped sapphirelike waveguides on silicon is presented including simulation results and a characterisation of the realised system. The Al₂O₃ thin film layers are doped in situ with titanium in a PECVD deposition from metal-organic precursors. The waveguides are patterned by reactive ion etching (RIE) to obtain high quality, low-loss waveguides. Afterwards the wafer is annealed by RTP (Rapid Thermal Processing). Thereby the layers change to a sapphire-like morphology and the titanium becomes optically activated without the formation of larger crystallites so that higher losses due to scattering are prevented. For tuning the laser, two solutions are presented. The first is a ZnO based electro-optically tunable etalon which is integrated in the active ring; the second is an also ZnO based coupled ring resonator which can be tuned either by the electro-optical effect or elasto-optically by placing it on a membrane. The required directional element that ensures the propagation in only one direction of the ring is realised by a new "ratch-reel structure" which acts as an optical diode. As the optical field is shifted to the outer periphery of the ring, it is possible to realise a structure that scatters the optical field in one propagation direction, whereas in the other direction it propagates with low loss. The system is pumped by a frequency doubled Nd-YAG-laser at 532 nm which is coupled to the ring via a SiON waveguide. The output power is coupled to a tangential waveguide where the coupling coefficient is determined by distance and refractive indices.

An analytic model is developed for evaluating the extractable energy from high energy pulsed Ytterbium (Yb) doped and Ytterbium:Erbium (Yb:Er) co-doped fiber amplifiers and lasers. The energy extraction capabilities under the limitation of spurious lasing, due to amplified spontaneous emission (ASE), are mapped for various numerical apertures, single and multi transverse mode evolution and operating wavelengths. The calculation results of the analytic model show good match with experimental results carried out for various Yb doped double clad fiber amplifiers. The model provides an accurate assessment for the maximum pulse energy that can be extracted from a given fiber. In addition, for a specific pump power, the model can be used to determine the minimum repetition rate and optimal length, under which the laser source can be operated before spurious lasing occurs.

Tunable solid-state laser sources emitting in the 2 μm wavelength region are important for applications in atmospheric monitoring and laser radar because many molecular absorption lines are present there. We report on continuously tunable operation of a diode pumped Tm:YAP laser, which is among the most efficient laser materials emitting in the 1.9−2 μm spectral band, while continuous tuning of this material has not yet been reported. The 3mm long a-cut (Pbnm) crystal sample with 4% at. doping was used. The 400 μm fiber was used to deliver up to 17W of pump power to the coupling optics. The differential efficiency in respect to the absorbed pump power reached 57% while the attained optical-to-optical efficiency was 48%. The maximal output of 4.8W was only limited by the available pump power and relatively low sample absorption. The free running wavelength was measured at 1.99 μm. A 1.5mm thick birefringent plate made from quartz inserted under a Brewster's angle was used as a tuning element. The maximal output power of 3.85W in this set-up was obtained. The laser could be tuned from 1869nm up to 2036nm with a maximum at 1985 nm. The tuning range of over 160nm covers many important atmospheric absorption lines. In this wavelength range, the absorption depth in water changes from 100 μm (laser wavelength 1.94 μm) to 400 μm (wavelength 1.87 μm) making it potentially attractive for medical applications as a laser scalpel with variable cutting depth.

Pulsed Er³⁺:Yb³⁺:YAG is a novel crystalline diode-pumped laser capable of delivering high pulse energies in the 1.5- 1.7μm "eye-safe" region. This work reports efficient lasing in long-pulse and in Q-switched mode, with pulse energies in the order of 10's of mJ, employing standard 960-nm quasi-cw semiconductor laser arrays in a direct transverse diodepumping configuration. In free-running mode, 83mJ output, 15% slope efficiency and 0.24J pump threshold have been attained. Q-switched operation was realized with a "frustrated total internal reflection (FTIR)" device, yielding 12.5mJ single pulses of 41-ns FWHM duration with smooth temporal and spatial profiles. Pump temperature-tuning and pulse duration effects on laser performance were also investigated, whereas pumping parameters optimizing pulse energy and efficiency were determined.

Power performance of a compact, broadly tunable, continuous-wave (cw) Cr²⁺:ZnSe laser pumped by a thulium fiber laser at 1800 nm was investigated. In the lasing experiments, a Cr²⁺:ZnSe sample with a small-signal differential absorption coefficient of 11 cm^-1 and a fluorescence lifetime of 4.6 μs was used. An astigmatically compensated x-cavity with 15 % output coupler produced as high as 640 mW of output power at 2480 nm with 2.5 W of incident pump power. Resonator losses were investigated using three different methods, and an in-depth analysis of the results was performed. The stimulated emission cross section values determined from laser threshold data and fluorescence measurements were in good agreement with each other. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster-cut MgF² prism and a single set of optics.

Two passively Q-switched Nd:YAG monolithic microchip laser devices were prepared by Nd:YAG and V:YAG crystals diffusion bonding. Stable generation of nanosecond pulses at wavelength 1338nm was obtained. The first laser, designed for higher mean pump and output power, was based on the monolith crystal which combines in one piece a 4mm long cooling undoped YAG crystal, 12mm long active laser part (YAG crystal doped with Nd³⁺ ions), and 0.7mm long V³⁺:YAG saturable absorber. The second one was designed to obtain shorter pulse length. It consists of 4mm long Nd:YAG laser crystal and 0.7mm long V³⁺:YAG saturable absorber. The diameter of both crystals was 5 mm. The initial transmission of the V:YAG part (T₀ = 85%) and the laser resonator was the same in both crystals. Laser mirrors were deposited directly onto monolith faces. The output coupler with reflection 90% for the generated wavelength was placed on the V³⁺-doped part. The pump mirror (HT@808 nm, HR@1.3 μm) was placed on opposite monolith face. Both microchip lasers were tested under longitudinal diode pumping. The pulse length was stable for all regimes for both crystals. For longer crystal it was equal to 6.2 ns, for the shorter one it was 1.7 ns (FWHM). The wavelength of linearly polarized TEM₀₀ laser mode was fixed to 1338nm for longer crystal. In case of shorter crystal some instabilities were observed for higher mean pump power. The pulse energy depends on the mean pump power. For pulsed pumping with low duty factor the output pulse energy was equal to 131 μJ for longer crystal, and 34 μJ for shorter crystal. This corresponds to peak power 21kW and 20kW, respectively. In CW pump regime the pulse energy was 37 μJ for longer crystal (peak power 6 kW), and 16 μJ for shorter one (peak power 9.4 kW).

Tapered fibers have shown high efficiency to generate white light continua, which have many important applications such as pulse compression, spectroscopy, pump-probe measurements, and optical frequency metrology. In this paper, we discuss the principle of white light continuum generation in tapered fibers with incident pulse durations in the femtosecond and picosecond range. We are going to demonstrate some new technologies to design and improve the spectral characteristics of supercontinuum generation, which make tapered fibers very convenient for the construction of white light sources.

Nd-doped strontium and lanthanum (ASL) crystals Sr_1-xLa_x-yNd_yMg_xAl_12-xO₁₉ (0.05 ≤ x ≤ 0.5; y = 0.05) were grown by Czochralski pulling technique. Up to 1.67W of 900nm IR output laser power for an absorbed power of 2.53W was obtained under Ti:sapphire pumping at 792nm. Intracavity second harmonic generation experiments led to 320mW of blue laser power at 450nm with a 10mm-long BiB₃O₆ nonlinear crystal. Other nonlinear crystals were also evaluated such as LBO.

Low noise lasers in the UV spectrum (355 nm) are important for many applications like spectroscopy, confocal microscopy, flow cytometry, cell sorting, CD mastering, semiconductor inspection and reprographics. The UV CW laser sources currently commercially available, are restricted to excimers lasers, gas ions lasers which suffer from low power efficiency (0.01% or less), high electrical consumption and bulky dimensions. These products, which are expensive, are still waiting for practical replacements. To our best knowledge, we report in this paper the first efficient low noise diode-pumped neodymium doped solid-state laser operating at 355 nm by intra-cavity third harmonic generation (THG). The fundamental infrared laser light is generated by a diode-pumped Nd:YVO₄ crystal optically contacted with others components of the laser cavity. Intra-cavity SHG and THG are achieved with a KTP and LBO respectively. Several configurations have led to low noise 355 nm single-frequency operation at a power exceeding 10 mW. We believe that this power can still be improved.

100 mW of coherent blue light with a wavelength of 405 nm was generated utilising a BiB₃O₆ (BiBO) nonlinear crystal to frequency double a Ti:Sapphire laser. Phase match curves as well as sensitivity to angular misalignment was calculated. The BiBO crystal was found to be excellent for this application. Temperature dependance was uncritical for this crystal, while power stability was good. The pump-to-blue optical conversion efficiency was approximately 2%. Unfortunately catastrophic coating damage was observed.

Intracavity second harmonic generation (ISHG) of a continuous wave, diode-pumped, broadband Yb-doped fibre laser has been investigated. Frequency doubling of the fibre output and of the residual diode pump light, and sum frequency mixing (SFM) between the fibre output and the pump light were achieved simultaneously, resulting in three colour operation in the blue-green region.

Single crystals of BaNaB₉O₁₅ (BaNaBO) and BaCaBO₃F (BCBF) have been grown by the Czochralski pulling method in air. These compands were investigated for non linear optical applications and were expected to exhibit an extended transparency window in the ultra-violet. BaNaBO crystallizes in the non-centrosymmetric space group R3C (Z=6)^[1] Cell parameters of BaNaBO are: a=11.1003(8) Å and c= 17.404(1) Å. The borate anionic network is characterised by a three-dimensional framework built up from [B₃O₇] rings^[1]. The other compound BCBF has a hexagonal structure with P-62m space group (Z=3)^[2], its cell parameters are a=9.0489(8) Å and c=4.3257(4) Å. Its basic structural unit is [BO₃]^[2] group. The transmission spectras of the two crystals are reported. The refractive indices were measured by the minimum deviation technique and fitted to the Sellmeier equations. Second harmonic generation (SHG) phase matching angle calculations are presented.

High power operation of an ytterbium-doped multi-core ribbon fiber laser and a cladding pumped ytterbium-doped helical core fiber laser is reported. 320W laser output at 1045nm was demonstrated with a 10-core ytterbium doped ribbon fibre. Efficient single-mode operation of a cladding-pumped ytterbium-doped helical-core fiber laser with a 30 μm diameter core and a numerical aperture of 0.087 have also been demonstrated. The laser yielded 60.4 W of output at 1043 nm in a beam with M² 1.4 for 92.6 W launched pump power from a diode stack at 976 nm. Further more, the recently progress of a cladding-pumped ytterbium-doped helical-core superfluorescent fiber source is also reported. The highly efficient operation of the helical core fiber ASE source yielded 107W of amplified spontaneous emission output spanning the wavelength range from 1030-1160nm with a bandwidth (FWHM) of 37nm.

Wavelengths around 1.15 μm, 1.3 μm and 1.7 μm can be used to pump Dy-doped ZBLAN fibre in order to generate ~3 μm with high efficiency. Previously the generation of 2.9 μm from the Dy-ZBLAN fibre was demonstrated by pumping with 1.1 μm Yb-silica fibre laser sources. The laser slope efficiency and lasing threshold demonstrated was about ~5% and ~1.78 W. In this investigation, the longer wavelength absorption band (⁶H_9/2 , ⁶F_11/2) centred at 1.3 μm of Dy³⁺-doped ZBLAN is utilised and the lasing transition around ~3 μm takes places from ⁶H_13/2 → ⁶H_15/2. With this pumping scheme the Stokes' efficiency is expected to be up to ~45%. A quasi-continuous wave Dy³⁺-ZBLAN fibre laser pumped by a ~1.3 μm Nd:YAG laser and operating at 2.96 μm with a bandwidth (FWHM) of ~14 nm has been demonstrated. For a 60cm fibre length, a threshold of 0.5W and a slope efficiency of ~20% with respect to the absorbed pump power was observed. The overall pump absorption in the fibre was around 84%. The cavity reflectivities at 2.9 μm were 99% and 50%. The demonstrated slope efficiency was 45% of the Stokes' limit. The slope efficiency was around four times higher and the threshold around 3.6 times lower than the previous performance demonstrated by using the 1.1 μm Yb fibre laser pumping scheme. The higher performance achieved compared to the 1.1 μm pump scheme is due to the higher Stokes' limit, lower pump ESA losses and higher cavity reflectivity. About 590 cm^-1 Raman Stokes shift has also detected by using 514.5 nm and 488 nm Ar ion laser as excitation pump sources.

The influence of bending on the gain spectrum in a depressed-cladding erbium-doped fiber amplifier (EDFA) has been studied. In particular, by changing the bending diameter, it is possible to translate the amplifier band from C- to S-band. A 22 dB gain for an input signal power of -15 dBm has been obtained in S-band at 1504 nm and in C-band at 1545 with a bending diameter of 15 cm and 100 cm, respectively. By bending the erbium doped fiber with two different diameters, it is possible to obtain amplification in a wavelength range between Sand C-band.

A simulation-based numerical optimization method for double-clad fibres (DCFs) is presented. The technique seeks to determine the cross sectional design that offers the strongest absorption. An encoding key that transforms the twodimensional cross section into a single point in a multi-dimensional space has been developed. The coordinates of the referred point are treated as the variables of an equation-free objective function and the Nelder-Mead algorithm is deployed to search for the function minimizer which is not always found but the function is always minimized substantially. The coordinates of the minimizer or the point delivering the best performance amongst the simplex vertices is decoded to unfold the optimized cross section. During the optimization process, the evaluation of the objective function is treated as a moving boundaries problem and is addressed via an efficient numerical simulation technique operating in the geometrical optics domain. Various optimized cross section designs are generated and compared. The optimum designs are those of a spiral inner cladding with an offset core and a circular one with four embedded birefringence rods with optimized offsets. The concept of improving the multidimensional landscape instead of improving the direct search method itself increases the agility of the polytope and generates more imaginative cross sections with improved absorption characteristics.

Fiber Lasers are most powerful solid state lasers available for various applications, they are capable of providing diffraction, limited power with compact, efficient and cost effictive system. In this paper, we report a master, oscillator fiber power amplifier for pulsed operation from 20kHz to 100kHz. A 4m double cladding fiber has central core of 43um and inner core of about 600/650um is used as amplifier, the seed source is a pulsed laser with output power of 1 W at 1064nm. A fraday isolator protect the seed laser from backflection from fiber amplifier. The MOPA system is analysied by transient gain model, pulsed amplified characteristics of double cladding fiber are caulated and compared with experimantal results at different pumping power and repetition rate.The fiber amplifier is pumped by laser diode at wavelength of 795nm with pumping power of 230W, the system can emits average power upto 133.8 W with repetition rate of 100kHz.

We report experimental realization of a new type of optical parametric oscillator, in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear mixing of the two polarized beams. The mixing is performed by a type II degenerate parametric down conversion in a PPKTP crystal, pumped at 1064nm by a pulsed Nd:YAG laser. A single, linearly polarized beam is generated at the degenerate wavelength. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1nm), and is highly stable at degeneracy with respect to variations in the crystal temperature. The frequency locking is explained in terms of balanced roundtrip phase-matching condition.

We present a novel numerical model describing the continuous-wave operation of Raman lasers. This so-called 'iterative resonator model' calculates how the forward- and backward-propagating Stokes and anti-Stokes electric fields inside the Raman laser cavity grow at the expense of the intra-cavity pump fields. We show that the iterative resonator model exhibits important advantages in comparison with the rate equation model used for e.g. hydrogen-based Raman lasers.

We point out theoretically and experimentally the possibility to enhance a non-linear effect by using photonic jets. The new possibility to highly concentrate visible light with micrometric dielectric spheres is illustrated simulating the situation in water. The photonic jet is a highly energetic beam with special properties that deviate from geometrical optics. Its ability to enhance non-linear effects is experimentally demonstrated by studying the two-photon excited fluorescence of rhodamine B.

The development of femtosecond (fs) lasers has continued rapidly since the demonstration of fs Ti:Sapphire systems in 1989. Recent research has yielded lasers which offer greatly enhanced performance in all areas. In this document we describe the development of femtosecond lasers with electrical to optical efficiency > 14%, pulse repetition frequencies > 4GHz and compact and stable cavities. We further outline the use of such lasers for the generation of high power visible femtosecond pulses and their application within systems environments for ultrahigh speed data communications, ultrafast optical switching and optical analogue to digital conversion. We also describe progress in the development of femtosecond lasers based on both active and passive semiconductor quantum dot components.

We report on a simple self-starting diode-pumped passively mode-locked Er-doped fibre laser based on two semiconductor saturable absorber mirrors (SESAMs), generating sub-picosecond stable optical pulses. Pulses duration between 350 and 650 fs (FWHM) was observed for pulses central wavelengths ranging between 1540 nm and 1570 nm. The cavity basic frequency was 3.7 MHz, and stable operation up to the third harmonic (11.1 MHz) was observed when the output power of the 980 nm diode pump was increased to its maximum value of ~300 mW. The maximum average output power was 19.45 mW, which corresponded to a pulse energy of ~4 nJ. Noise characterization of the mode-locked laser source was performed, in order to estimate the phase noise of the output pulses in terms of timing jitter. All the fiber components in the cavity were polarization maintaining in order to increase long-term stability of the laser operation.

We present a systematic study of the ultrafast laser micro-machining of glass using a Ti:Spp laser with moderate pulse energy (<5 μJ) at a high repetition rate (50 kHz). Optimal conditions were identified for high resolution surface laser etching, and via drilling. Several practical applications were developed: glass templates for micro fluid diffraction devices, phase gratings for excimer laser projection techniques, micro fluid vertical channel-connectors, etc. It is demonstrated that the interaction of ultrafast laser pulses with glass combines several different processes (direct ablation, explosive material ejection, and thermal material modification). A dynamic numerical model was developed for this process. It was successfully used for modelling of laser micro-machining with arbitrary 3D translations of the target.

We present what is to our knowledge the most complete 1-D numerical analysis of the evolution and the propagation dynamics of an ultrashort laser pulse in a Ti:Sapphire laser oscillator. This study confirms the dispersion managed model of mode-locking, and emphasizes the role of the Kerr nonlinearity in generating mode-locked spectra with a smooth and well-behaved spectral phase. A very good agreement with experimental measurements of pulse energy, spectrum, and temporal width of extracavity compressed pulses is found.

In this paper, we present a method for a 3D-simulation of the population inversion and photon density in a solid state laser. The method approximates the electrical field distribution of the beam by a finite number of eigenmodes. These modes can be computed by a Gauss mode analysis. The population inversion is approximated by a finite volume discretization. Using the classical rate equations, we derive suitable rate equations for the discretized population inversion and the eigenmodes of a laser. The simulation results show the influence of the pump configuration to the distribution of the eigenmodes. Therefore, the new simulation method can be used to optimize the beam quality of a solid state laser.

Single crystals of Gd_1-xR_xCa₄O(BO₃)₃ (R³⁺ = Sc³⁺ or Lu³⁺) with large size and good quality have been grown by Czochralski method. By changing the compositional parameter x of Gd_1-xSc_xCa₄O(BO₃)₃ and Gd_1-xLu_xCa₄O(BO₃)₃ crystals, their optical birefringence can be controlled in order to achieve non critical phase matching (NCPM) in the ranges of 792-824nm along Y axis and 922-963nm along Z axis, during second harmonic generations (SHG). The chemical compositions of the grown crystals were determined and X-ray diffraction measurements have been carried out to characterize the structural changes with compositional parameter x. The solubility limits of R³⁺ ions in the Gd₄Ca₄O(BO₃)₃ crystals were also determined. Nonlinear optical properties of these new biaxial borate crystals are reported. Experimental determinations of doubled frequencies in NCPM conditions are in good agreement with theoretical predictions.

The production of high-energy ultra-short laser pulses in gas-filled hollow fibres is now a well-established technique. Below a specific critical power, the smoothness of spatial and temporal profiles of input pulses is not perturbed during propagation. However, gas ionization and pulse filamentation occur above that critical power strongly disrupting the pulse profiles. Both propagation regimes in argon are numerically studied for two different propagation configurations: free gas and gas filled hollow fibre. Below critical power it is shown the pulse is spectrally broadened while maintaining smooth temporal and spatial profiles. Otherwise, these quantities show strong variations.

In some last years mid-infrared Er:YAG laser radiation (2940 nm) found the exploitation in many applications in technology and especially in various medical branches (dentistry, dermatology, cardiology, ophthalmology). Most of them use the Er:YAG laser system working in free-running regime generating the pulses with the length of some hundreds of microseconds. In the presented work we concentrated on the development and optimization of electro-optically Q-switched Er:YAG laser and suitable delivery system. Er:YAG laser operated both in free-running or Q-switched regime was developed and optimized. LiNbO₃ Pockels cell was utilized for Q-switching the Er:YAG laser. Single giant pulses with maximal energy 67 mJ and minimal duration 53 ns FWHM were generated for maximal pump energy 131 J and for the optimal Pockels cell parameters (high voltage value 1.4 kV, delay of Pockels cell switching after the flashlamp trigger 450 μs). The Er:YAG laser radiation was effectively delivered by the special COP/Ag hollow glass waveguides with inner/outer diameters 700/850 μm or 320/450 μm, and length from 10 cm up to 1 m. The transmission of the used waveguides was measured to be from 73% to 84% according to waveguide type. With the help of delivery systems, the Er:YAG laser radiation (in the form of long pulses for free-running regime or short pulses for Q-switched regime) was applied to various biological tissue samples to study the basic laser radiation-tissue interactions and treatment possibilities.

In this study Yb-doped Y₂SiO₅ single crystals and thin films crystal field excitations and Raman active phonons characteristics have been compared using infrared absorption and Raman spectroscopy. The thin films high quality has been confirmed and their ability to adjust by co-doping various properties such as the Yb³⁺ sites occupations and Yb³⁺-Yb³⁺ pair interactions have been evidenced.

We present a numerical model of the laser system for generating a special shape of the pulse: a steep peak at the beginning followed by a long pulse tail. Laser pulses of this nature are required for various applications (laser material processing, optical breakdown spectroscopy, etc.). The laser system consists of two "overlapped" cavities with different round-trip times. The laser crystal, the Q-switching element, the back mirror, and the output coupler are shared. A shorter pulse is generated in a short cavity. A small fraction of this pulse is injected into the long cavity as a seed. It triggers generation of the longer pulse. The output emission from this hybrid laser produces a required pulse shape. Parameters of the laser pulse (ratios of durations and energies of short- and long- pulse components) can be controlled through cavity length and the output coupler reflection. Modelling of the laser system is based on a set of coupled rate equations for dynamic variables of the system: the inverse population in an active laser media and photon densities in coupled cavities. Numerical experiments were provided with typical parameters of a Nd:YAG laser to study the system behaviour for different combinations of parameters.

High power Nd:YAG laser rods may easily exceed their fracture strength as a result of thermally induced tensile stress. Strengthening of such rods is achievable by wet chemical etching in concentrated orthophosphoric acid, or in a mixture of phosphoric and sulfuric acids, at elevated temperatures. In the present study, the etching rates of Nd:YAG in both etchants were determined as a function of temperature. The calibration curves thus obtained enabled the controlled removal of the outer "damaged" layer of lasing elements. Four-point flexture strength measurements were performed on deep etched and on non-etched YAG slabs. The measurements showed a 3.5 fold increase in the strength of the etched slabs. Weibull analysis treatment was done on the basis of our fracture data.

We present a new computer numerical model of the phase-conjugate laser, utilizing an intra-cavity Stimulated Brillouin Scattering (SBS) element. The modelled laser system includes the active laser crystal which is placed between the output coupler mirror and a stimulated Brillouin scattering cell. The numerical model includes a set of rate equations for the active crystal inverse population, and for the photon density inside the laser cavity. The SBS backscattering model is based on a reduced set of coupled equations for electromagnetic fields for two waves (a pump wave and an SBS wave) propagating in opposite directions. The numerical integration of the set of equations simulates in detail the temporal dynamics of the laser. A wide range of realistic system parameters was numerically investigated. Different laser regimes (from a quasi -CW mode to a Q-switched mode) were numerically tested. The method of numerical modelling of such laser system can be efficiently used for an optimal laser design.

We have prepared an organic/inorganic hybrid using sol-gel process. The material exhibits thermal, mechanical and optical properties well adapted to the patterning by laser micromachining. We have achieved the fabrication of optical elements by this mean validating the process at micrometric scale. Optical waveguides have been fabricated in 3-4 micrometer thick hybrid sol-gel films deposited by spin coating on borosilicate glass substrates. Here laser ablation is used to remove matter on both sides of the waveguide core. The laser micromachining of hybrid sol-gel is also adapted to the fabrication of multilevel diffractive optical elements. Indeed, the ablation depth can be precisely controlled by the fluence of the laser and/or the number of pulses. This study demonstrates that laser micromachining of hybrid sol-gels permits the fast fabrication of effective optical devices, making this process well adapted to rapid prototyping or fabrication of masters.

Passively mode locked ring vanadate laser pumped by 1 W laser diode was developed. Laser threshold for free running regime was 55 mW, the bidirectional mode locked operation was obtained for incident pump power of only 630 mW. The repetition rate of the laser was 200 MHz, pulse duration 53 ps. New saturable absorber and configuration with two lenses inside the resonator improved stability of mode locking regime. Application of this laser for measuring of small intracavity phase changes is reported.

In the contribution we present briefly the manufacturing technology of the cells and the filling process and an experimental attempt to determine the iodine purity of the finished and sealed cell by an independent method. The data from manufacturing of a first sample cell are compared with the purity measurement and also with measurement of the absolute optical frequency of laser stabilized to hyperfine transitions in iodine in this sample cell. The main goal of this effort is to recognize the limits of the absolute precision of the optical frequency of iodine transitions and look for further improvements in the iodine cell manufacturing technology that may lead to even smaller frequency shifts.

Tunable mid-infrared laser radiation sources are of interest for many applications in spectroscopy, ranging, remote-sensing, medical diagnosis and treatment and also for pumping nonlinear (OPO) and laser materials. The ZnSe:Cr²⁺ is a promising laser active material for lasing in the range of 2-3 μm. Up to now number of sources have been used for pumping ZnSe:Cr²⁺ active medium: Er-fibre laser, color-center laser, Co:MgF₂ laser, Tm lasers, and Raman-shifted Nd:YAG laser [1-4]. In our study we have demonstrated, characterized and compared ZnSe:Cr²⁺ laser coherently pumped either by flashlamp-pumped Er:YAP (wavelength 1.66 μm) or diode pumped Tm:YAP (wavelength 1.97 μm) laser radiations. For the case of ZnSe:Cr²⁺ laser pumping by wavelength 1.66 μm, the Er:YAP laser was constructed. From the measured output radiation characteristics followed that the maximal ZnSe:Cr ²⁺ laser output pulse energy was 5.5 mJ (slope efficiency 23%), and the length of pulse 120-160 μs. With the help of dispersive prism inside the resonator, the generated laser radiation was tunable from 2100 nm to 2450 nm with only 2 times drop in laser efficiency. The temporal profile and spatial structure of the generated laser beam were measured. Consequently, the diode-pumped Tm:YAP laser was constructed for coherently pumped ZnSe:Cr²⁺ laser. LIMO laser diode (40 W) was used for longitudinal pumping of Tm:YAP laser. The output characteristics and tuning curves were measured for various ZnSe:Cr²⁺ laser resonator arrangements and also for various pumping radiation conditions in pulsed regime (pulse duration, repetition rate, duty cycle). The maximal obtained ZnSe:Cr²⁺ laser output pulse energy was 0.35 mJ for the Tm:YAP pump pulse energy 13.5 mJ (pulse radiation 5 ms, repetition rate 20 Hz). The generated laser radiation was tunable from 2100 nm to 2450 nm. The temporal profile and spatial structure of the generated laser beam were measured.

The population inversion of the Tm³⁺ in GLKZ glass involved in the 1470 nm emission (³H₄-> ³F₄) as a function of Tb (or Eu) concentration was calculated by computational simulation for a CW laser pumping at 792 nm. These calculations were performed using the experimental Tm->Tb an Tm->Eu transfer rates and the spectroscopic parameters of the Tm (0.1 mol%) system. The result shows that 0.2 mol% (Tb³⁺) and 0.4 mol % of Eu³⁺ ions propitiate best population inversion of Tm³⁺ (0.1 mol %) maximizing the amplification coefficient of germanate (GLKZ) glass when operating as laser intensity amplification at 1470 nm. Besides the effective deactivation of the ³F₄ level, the presence of Tb³⁺ or Eu³⁺ ions introduce a depopulation of the ³H₄ emitting level by means of a cross relaxation process with Tm³⁺ ions. In spite of this, the whole effect is verified to be benefic for using Tm-doped GLKZ glass codoped with Tb³⁺ or Eu³⁺ as a suitable material for confectioning optical amplifiers that operates in the S-band for telecommunication. PACS code: 78.50, 78.55, 71.55