Output performances of Nd-based lasers that were pumped with diode lasers directly into the 4F3/2 emitting level has
been investigated at various wavelengths of emission. The end-pumping scheme was used to demonstrate efficient laser
emission in the watt-range at 0.94 µm, 1.06 µm and 1.34 µm in Nd:YAG pumped at 885 nm. Nd:YVO4 and Nd:GdVO4
thin-disk lasers with high power emission at 1.06 µm were realized under multi-pass pumping at 880 nm, and for the first
time laser emission at 0.91 µm was achieved in this configuration. Intracavity frequency-doubled lasers with emission in
the green and blue spectral ranges were obtained. Thus, an end-pumped Nd:GdVO4 laser with 5 W output power at
0.53 µm, and Nd:YVO4 and Nd:GdVO4 thin-disk lasers with output power in excess of 4 W at 0.53 µm were realized.
Nd-vanadates thin-disk lasers with ~1 W power into 0.45 µm 'deep-blue' visible spectra were demonstrated.
Comparative results with the traditional pumping at 0.81 µm into the highly absorbing 4F5/2 level are presented, proving
the advantage of the direct 4F3/2 pumping.
The spectroscopic investigation of the highly transparent rare earth (Pr3+, Nd3+, Ho3+, Er3+, Tm3+, Yb3+) doped Sc2O3
ceramics produced by the solid-state synthesis technique indicate that these materials could substitute the single crystals
in construction of solid-state lasers. These studies indicate also that the rare earth doped transparent ceramics could
extend considerably the variety and performances (new active systems, wavelength ranges or emission schemes) of these
lasers.
The information on the variety, nature and structure of the centers formed by the rare earths ions doped in the transparent laser ceramics of garnets and cubic sesquioxides, acquired from high-resolution spectroscopy and emission decay is analyzed. The quantum states (energy levels, transition probabilities) of several doping rare earth ions, their distribution at the available lattice sites, the interactions between ions, and energy transfer processes are also presented. It is inferred that from spectroscopic point of view these materials could substitute the melt-grown single crystals in construction of solid-state lasers and extend considerably their capabilities.
The high resolution and polarized spectroscopic investigation of Nd3+ in Mg-compensated strontium lanthanum aluminate Srl-xLax-yNdyMgxAl12-x019 (Nd: ASL) function on composition makes possible the elucidation of the nature of non- equivalent centers and enables the selection of composition and pumping conditions that grant the efficient quasi-three-level laser emission around 900 nm. Based on a proper selection of these conditions, 900 nm laser emission with slope efficiencies of 0.74 and 0.84 at 792 nm 4F5/2 and respectively 865 nm 4F3/2 pumping of Nd3+ is demonstrated.
Highly transparent Nd- or (Nd,Yb)-doped Sc203 ceramics are produced by the method of solid-state mixing of oxides. High-resolution spectroscopy indicates the spectroscopic and structural properties of the doping ions in these ceramics are similar with those of single crystals, although the maximum doping could be much higher. Very efficient Nd-to-Yb energy transfer is observed, which can be used for sensitisation of Yb laser emission.
Nd-doped strontium and lanthanum (ASL) crystals Sr1-xLax-yNdyMgxAl12-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.
Single crystals of Gd1-xRxCa4O(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 Gd1-xScxCa4O(BO3)3 and Gd1-xLuxCa4O(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 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.
A diode end-pumped Nd:YAG laser passively Q-switched by Cr4+:YAG saturable absorber and intracavity frequency-doubled by a LBO nonlinear crystal is described. Using a linear cavity the maximum average power at 1.06-μm fundamental wavelength was 3.3 W, 22.5 kHz repetition rate and 25.2-ns pulses duration. The maximum peak power was 12 kW for an average power of 1.8 W, laser pulses of 17.5-ns duration and 209.3-μJ energy. Intracavity frequency-doubling employing a V-type laser resonator yielded 532-nm green pulses of 226-μJ energy and 86-ns width at 4.2-kHz rate of repetition. The theoretical calculations based on a model of rate of equations show good agreement with the experiments at both fundamental and second-harmonic wavelengths.
The paper discusses the basic possibilities for enhancement of the output power of the self-frequency-doubling devices based on Nd-activated nonlinear crystals, by acting on the factors that determine the intracavity intensity at the fundamental frequency. It is inferred that a very important factor of improvement is the increase of the quantum defect ratio at the fundamental frequency by pumping directly into the emitting level. The relation between the conditions that enhance the intracavity intensity and the frequency-doubling performances is discussed and verified by the marked enhancement of emission at 530 nm in Nd:GdCa4O(BO3)3 with direct pumping at 887 nm.
Highly efficient one-micron laser emission in Nd-doped vanadates under direct pumping into the 4F3/2 emitting level is reported. A 1.0-mm-thick, 1.0-at.% Nd:YVO4 crystal operated with 80% slope efficiency (79% overall optical-to-optical efficiency) under Ti:Sapphire pumping and 75% slope efficiency versus absorbed power under diode laser pumping at 880 nm. Slope efficiency of 80% (overall optical-to-optical efficiency of 77%) under Ti:Sapphire pumping and 66% with respect to the absorbed power under diode laser pumping at 879 nm is obtained from a 3.0-mm-thick, 1.0-at.% Nd:GdVO4 crystal. These values, which were superior to those obtained by pumping into the 4F5/2 level, were explained consistently by the effect of the quantum defect between the pump and laser radiation, the superposition of pump and laser mode volumes, the pump level efficiency and the residual optical losses.
The possibility of basic enhancement of the global efficiency of frequency-doubling devices for the one-micron continuous-wave Nd lasers by direct pumping into the emitting level and by the use of concentrated laser materials is discussed. This possibility is demonstrated for 1.0 and 2.4-at.% Nd:YAG crystals pumped by a Ti:Sapphire laser at 885 nm. A slope efficiency of 0.79 in absorbed power was obtained for the 1064 nm emission of the 1-at.% Nd:YAG. The effect of enhancement of intracavity emission on the second harmonic emission is manifested in a drastically reduction of emission threshold and in an enhanced dependence on the absorbed power. The use of concentrated Nd:YAG crystals enables a better use of the pump power and increases the overall efficiency of the frequency-doubling devices.
The effect of energy transfer on the VUV-to-VIS quantum cutting in Praseodymium-activated phosphors for Xe dimer luminescence lamps is discussed. From the modeling of emission process in presence of energy transfer it is inferred that the failure to observe efficient energy transfer-assisted quantum cutting in these phosphors is caused by the low efficiency of transfer with respect to the radiative de-excitation of the metastable VUV state 1So of Pr at the low Pr concentrations specific to these systems.
The paper discusses some directions for improvement of efficiency and power scaling of Nd lasers: (1) the direct pmping into the emitting level 4F3/2; (2) the use of concentrated Nd laser materials; (3) new laser materials, particularly ceramic materials; (4) advanced laser resonator design.
Efficient quasi-three-level emission at 946 nm in Nd:YAG requires a proper balance between the laser emission and the reabsorption induced by the residual thermal population of the terminal laser level. The paper shows that the emission properties can be increased by direct 885 nm pumping into the emitting level (slope efficiency in absorbed power equal to 0.68, compared with 0.48 for the traditional pumping at 809 nm) and infers that the reduced pump absorption can be overcome by using laser resonators with multi-pass of pump radiation inside the laser material.
The paper presents a modeling of heat generation by non-radiative de-excitation processes in pumped laser materials. This model explains consistently the differences in heat generation in Nd laser materials in presence and in absence of laser emission and its dependence on the emission wavelength, on laser extraction efficiency and on Nd concentration.
Spectroscopic and emission decay investigation on single crystal Nd:YAG (with up to 3at.%Nd) and transparent ceramic (up to 9at.%Nd) samples indicate that the state of Nd ions (the sites occupied by the Nd ions, the crystal field and electron-phonon interactions, energy transfer between the Nd ions) are identical. Coupled with the similarity of the thermo-mechanical properties and with specific features of ceramics, such as a high production yield, possibility to produce very large components with uniform Nd doping up to very high concentrations, reduced production cost, these studies show that the transparent ceramic Nd:YAG can be used for construction of lasers. These conclusions are supported by efficient laser emission data.
KEYWORDS: Quantum efficiency, Ions, Neodymium, Nd:YAG lasers, Energy transfer, Crystals, Data modeling, Luminescence, Temperature metrology, Energy efficiency
The paper discusses the problem of Nd concentration dependence of the emission quantum efficiency in Nd:YAG at low pump intensities, based on a modeling which uses the emission decay data. The measurements on crystalline or ceramic Nd:YAG samples with Nd concentrations up to 9at.%Nd show a marked dependence on concentration, induced by the energy transfer self-quenching. The parameters of ion-ion interactions that determine the transfer are used for an analytical modeling of the concentration dependence of the emission quantum efficiency, which gives a very good description of the experimental data.
Strontium lanthanum aluminates crystals (ASL) doped with Nd3+ with formula Sr1-xNdyLax-yMgxAl12-xO19 are interesting crystals for lasers operating in i.r. (~900 nm range) or blue spectral ranges. This paper uses high-resolution spectroscopy, at low temperatures, for investigation of the influence of composition and Nd3+ doping of ASL crystals on emission properties. Preliminary spectral characteristics of two non-equivalent centers were determined. From these data and their dependence on composition, structural models for the two centers are proposed.
Experimental data presents a pump intensity dependence of the 4F3/2Nd3+ level emission in various hosts; these effects were assigned to the up-conversion processes. A new formalism that takes into account both up- and down-conversion processes and the statistical distribution of acceptors around donor in the case of direct processes is presented. This involves a time dependent of transfer function and two interaction types: dipolar and superexchange. The profile of excitation beam is considered in order to compare theoretical calculations with experimental data. Calculations concerning quantum efficiency as a function on the ratio between up- and down- conversion rates are presented.
Most of the RE3+ optical lines in YAG, at low temperature and concentrations, present two interesting features: a multisite structure and Lorenz lineshape of the inhomogeneous broadened lines. The satellite structure of RE3+ ions optical spectra in high temperature YAG present a group of P satellite lines of almost equal intensity that was assigned to nonstoichiometric occupation of Al3+ octahedral sites (a) by Y3+. Up to now, no reliable explanation of the anisotropic effects of Y3+(a) on RE3+ spectra was given. In this paper an attempt is made to explain the multisite structure and lineshapes in terms of the strain field (induced by point defects) theory. The model is illustrated for the absorption line 4I15/2 implies 4S3/2 of Er3+ (1.at.%) doped in YAG.
The garnets with disordered structure such as calcium niobium gallium garnet (CNGG) doped with Nd3+ are interesting materials for diode pumping. Spectroscopic investigations on Nd3+:CNGG have revealed a multisite structure and this is reflected in laser emission characteristics. The purpose of this paper is to present several new spectral aspects, a modeling for the main nonequivalent centers, energy transfer characteristics and an attempt to connect the laser characteristics to centers structure. Samples were grown by Czochralski method with Nd3+ from 0.1 to 5 at.% and the high resolution spectral and decay measurements were performed at 10 K. Similar considerations were made in the case of Nd3+ doped in calcium lithium niobium gallium garnet (CLNGG).
An analysis of low temperature lineshapes for various absorption lines of several RE3+ ions from the beginning Nd3+ and end of lanthanide series Er3+, Tm3+ embedded in the same crystal - YAG known as important laser systems for 1-3 micrometers emission is presented. New aspects of the dependence of electron-phonon coupling on RE3+ ions and matrix are observed from the analysis of many transitions of these ions at low temperature. These data show that eh electron-phonon coupling presents a symmetric behavior in the lanthanide series, larger toward beginning and end. With only very few exceptions, less than 2 percent, the broadenings or splittings of zero-phonon liens could be connected to optical Raman phonons of undoped lattice. That means that the parity of 4fn states could still be considered a good parent quantum number, even if the local symmetry has no inversion, and the near resonant coupling involves the even part of the electron-phonon coupling operator. Besides, the one phono relaxation involve the undoped lattice phonons. For ions with small spin-orbit coupling, the vibronic contribution to the line intensity given by ir phonons through a Van Vleck mechanism could be important for spin forbidden transitions.
In this paper we discuss the upconversion processes responsible for violet luminescence from the Erbium levels 2P3/2 and 2H29/2 in diluted Er:YAG crystals, excited with pulsed as well with cw lasers. Besides the general need for new spectroscopic data concerning laser materials, the interest for this study is generated by the fact that energy levels of very different quantum efficiencies can produce, function of the pumping conditions, fluorescence spectra of comparable intensities or even with reversed intensity ratio.
The paper discuses the influence of cross-relaxation parameter on the characteristics of the sensitized photon avalanche. A system Pr3+ as activator ion and Yb3+ as sensitizer ion were considered for the avalanche process. By numerical modelling of a system of rate equations, time dependencies and pump rate dependencies of populations of energy levels were calculated for three different values of cross-relaxation rate. Correlation between cross-relaxation rate and threshold value of pump intensity was established.
Laser crystals with disordered structure present a series of advantages for diode pumping or short pulse generation as compared with ordered crystal. Such a system proved to be calcium lithium niobium gallium garnet (CLNGG) doped with Nd3+. The papers present the growth and x-ray analysis of specially doped CLNGG. Very few spectroscopic data on this system have been previously published. The paper present low temperature absorption spectra, site selective excitation and lifetime measurements of Nd3+ in CLNGG. At least four distinct nonequivalent centers are observed, the spectral lines for every center presenting a disordered Gaussian shape. based on structural, spectral static and emission dynamics data an attempt of modeling the structure of these centers is proposed.
The paper discusses the effect of pump intensity variations caused by absorption in a fiber system with upconversion avalanche fluorescence or laser emission. The numerical modeling shows that in given pump conditions the initial part of the fiber (up to a distance zth) is pumped above the avalanche threshold while the remaining part lies below it. This induces a strong variation of populations of the excited levels along the fiber. The characteristics of the avalanche emission are connected to the spectroscopic parameters of the system and to the conditions of pump. It is also shown that the features of the global luminescence emission at the end of fiber depend strongly on the fiber length for the short fibers (L <EQ zth) but for the long ones this dependence diminishes; thus the avalanche emission of a long fiber could be different from that predicted by a constant rate modeling. The effect of distribution of local pump intensities on laser emission in avalanche-driven upconversion fiber lasers is also discussed.
In this paper we discuss the heat produced by the non-radiative (multiphonon) transitions in Er:YAG laser rods, during the optical pumping, using the mathematical modeling. Three energy transfer processes (up-conversion from 4I15/2, up- conversion from 4I13/2, and cross-relaxation from 4S3/2) are included in the rate equations. The main result of our simulation is that for usual optical pump conditions (Xenon flash-lamp, pumping pulse duration 200 - 300 microsecond(s) ) more than 50% of the absorbed energy is lost as heat in the active medium and only a small fraction (approximately 1%) is lost as fluorescent emission.
A mathematical model, based exclusively on spectroscopic data concerning radiative, non-radiative, and energy transfer processes, is proposed and used to simulate the Q-switch regime of three-micron Er:YAG laser. The connection between the main energy transfer mechanisms that make possible generation on the self-saturated transition 4I11/2 yields 4I13/2 (up- conversion from 4I13/2 and 4I11/2, cross- relaxation from 4S3/2) and the giant pulse characteristics is discussed. The radiative as well as non- radiative losses during optical pumping and giant pulse generation are defined and evaluated. A particular attention is given to FTIR Q-switch which demonstrated real qualities for 3- micrometers Erbium lasers. The reasons responsible for experimental performances of Q-switched Er:YAG lasers inferior to those predicted by the mathematical modeling are analyzed.
The sensitization with Yb3+ of other rare earth ions emission is intensively used in the last years for increasing excitation efficiency of infrared (diode laser) pumped solid state lasers. It proved as a good sensitizer for Pr3+ emission in various matrices. The spectral investigations for the elucidation of the nature of multisite structure of Pr3+ in YAG and of the Yb3+ co-doping on Pr3+:YAG are presented in this paper. In Pr3+:YAG two types of multisites have been observed: perturbed Pr3+ sites by nonstoichiometric defects and strongly coupled Pr3+ - Pr3+ near neighbor pairs. Pr3+ spectra in Yb3+, Pr3+:YAG show new lines assigned to near neighbor Pr3+ - Yb3+ pairs and Pr3+ - Yb3+ - Yb3+ triads.
A generalized model of the cw three-micron emission in concentrated Er systems, which takes into account the whole variety of de-excitation processes is presented. The analytical equation for the flux of light enables a direct correlation of the parameters of emission (threshold, slope efficiency) with the spectroscopic parameters and pump conditions. The effect of excited state absorption is, also, put in evidence.
The features of a new upconversion mechanism of excitation, the sensitized photon avalanche, are discussed on the basis of a rate equation modeling. In this process the excited state absorption (ESA) from a metastable level (reservoir) of the active ion is enhanced by a cross-relaxation between the emitting level of this ion and the sensitizer ion which is able to transfer subsequently the accepted excitation back into the reservoir level of the activator; the donor act of the cross-relaxation ends on this reservoir level too. When the efficiencies of these two energy transfer processes are larger than specific threshold values related to the intrinsic de-excitation rates of the emitting level of activator and of the excited level of sensitizer, a process of population of the reservoir level with a quantum efficiency larger than the unit for each act of de-excitation of the emitting level takes place. Further on, if the ESA rate exceeds a threshold value determined by the intrinsic de- excitation of the reservoir and by the efficiencies of the energy transfers an avalanche population of these levels could take place. This process could be used for infrared pumping of visible solid state lasers. The theory is illustrated for Yb3+- sensitized visible emission of Pr3+ under infrared pump (835 nm).
This paper presents a theoretical treatment of electron-phonon interaction problem in the case of quasi resonant interaction for rare earths ions in laser crystals. Expressions for the absorption or emission lineshapes (asymmetries or splittings of zero-phonon lines) are obtained. The theory is applied to explain some spectral features of Yb3+ transitions in YAG; it allows also the estimation of the vibronic coupling strengths, phonon energies and the `unsplit' electronic levels. A new electronic energy level scheme of Yb3+ in YAG is proposed.
The effects of co-doping with Cr3+ on the absorption and emission characteristics of double doped Cr3+, Nd3+:YAG (yttrium aluminum garnet) are analyzed. The multisite structure due to co-doping (samples with a large range of Cr3+ content were used) is separated from that in single doped crystals. At least seven new lines have been separated in high resolution spectra and a tentative assignment in terms of structural data is proposed. These data suggest that the energy transfer data should be analyzed in terms of inhomogeneous system. However, the measurements indicate strong migration effects. The global donor and acceptor data at 300 K show a linear dependence on donor concentration, but the decays cannot be described in terms of published theoretical models.
The medical laser equipments made at NILPRP have been exploited intensively for more than 10 years at CMH. The availability and reliability of the first like-on equipment have increased, following improvements in optical delivery system and cooling circuit. This paper shows the impact of technical advances on the development of ophthalmologic laser therapy. Intraocular tumors pose special problems of diagnosis and treatment. Diagnostic methods include addition to systemic and ophthalmologic examinations, ancillary examinations, such as transillumination, fluorescence angiography, ultrasonography, radioactive phosphorus uptake tests, radiology, computerized tomography and fine-needle aspiration biopsy with cytological analyses. The enucleation of the involved eye used to be a generally accepted management of malignant tumors. Improved therapeutic methods such as photocoagulation and better surgical techniques now provide a variety of therapeutic alternatives. This study covers 31 cases of intraocular tumors that were managed either by Argon Laser photocoagulation and/or by Nd:YAG laser surgical treatment. Four cases were intraocular metastasse and 17 cases were primitive intraocular tumors. Argon laser therapy proved to be totally ineffective for intraocular metastasse but very adequate therapy for primitive tumors. Tumor extirpations (choroidal, cillary body or iris tumors) using Nd:YAG laser lancet proved to be more suitable than classic surgery.
KEYWORDS: Resonators, Laser resonators, Mirrors, Nd:YAG lasers, High power lasers, Solid state lasers, Diffraction, Thermal effects, Near field, Mathematical modeling
In order to describe the transversal mode construction in a real laser resonator, a mathematical model was investigated. The analysis use Kirchhoff-Fresnel diffraction equation and amplification processes in the active medium, that was supposed as sum of thin discs with gain and losses constants on every disc's length. Influence of additional phenomenon on laser beam characteristics, as phase-shift caused by active medium thermal induced refractive power and curvature of the resonator mirrors were considered.
Admittance matching is generally used in the design of optical components which require in the same time high reflectance and high transmission bands. The matching is done at the both sides of the basic stack by synthesizing an equivalent layer with symmetrical three layer periods. In order to obtain a broad transmission band, a quasi-matching is necessary around the matching wavelength. The paper studies the solutions provided by analytical synthesis of the matching stack for a number of one to three periods.
The paper briefly presents an original method for design of inhomogeneous thin films which takes into consideration from the beginning the finite character of the optical thickness. At the beginning, the best solution is obtained in terms of the value of a complex function on a certain spectral grid. At the second step, the design is converted in terms of refractive index profile. Special care is taken to minimize the variations of this profile. Then, a program using this method, written as Windows application, is described together with a design example providing a spectral dependence of the reflectance having the shape of Romanian Athenaeum building.
This paper presents the design and realization of the cavity mirrors for a 2.94 micrometers Er laser, longitudinally pumped by a diode laser emitting in 0.97 micrometers range, used in stomatology applications. The total reflection mirror is a dichroic one with the third harmonic in reflectance spectral characteristic suppressed. The study on the harmonics suppression that we present was based on the theory of the finite thickness inhomogeneous layers and the results were used in the mirrors design. The theoretical and experimental reflectance spectral dependencies are presented.
The paper presents the design of a dichroic mirror used in a Nd:YAG high power laser to reflect the 1.44 micrometers radiation and to transmit the 1.064 micrometers one. In order to obtain a wide transmission band, all the solutions for matching basic stack with the substrate, consisting in a number of periods less or equal than three, were investigated and the best was selected. The solutions were obtained by analytical inversion of the equations for the three layer equivalent system.
The paper describes the design and realization of a dichroic mirror for a diode pumped YAG:Nd laser. The mirror is deposed on an optical glass substrate and works in optical contact with the laser crystal. The design was performed by admittance matching of the basic stack with the adjacent media. Proper matching stacks were selected between all possible solutions computed by analytical inversion of the system equation for the equivalent layer. Deposition conditions and experimental results are given.
The various processes leading to loss of efficiency and to heating effects in YAG:Nd laser crystals are discussed. It is thus shown that there are several unavoidable intrinsic sources of loss such as the spectral inhomogeneities in the melt grown crystals and the concentration quenching of emission and these impose upper limit quantum efficiency function on the activator concentration. The estimations made within these models are in a good agreement with the data on quantum efficiency of emission or on fractional thermal loading.
The mutual static crystal field perturbations produced by near sensitizer and activator ions shift the energy levels and modify the radiative and energy transfer characteristics. The selective and discrete nature of these effects transforms the ensembles of sensitizer and activator ions into inhomogeneous systems and modify the spectral and temporal characteristics of emission from activator ions in sensitized laser crystals. These effects are exemplified for Cr3+ or Fe3+ sensitized emission of Tm3+:YAG.
The performances of a two-rods super-Gaussian positive branch unstable resonator and one rod unstable resonator with amplifier stage for Nd:YAG solid-state lasers were investigated. A glass deformable pressure mirror was used for thermal lens compensation in the resonators.
The paper reports the design of two dichroic mirrors with a broadband high reflectance in the region 700 - 800 nm, and a very good transmission at 532 nm. These mirrors are designed by the admittance matching method, using dielectric materials ZrO2 - SiO2 and TiO2 - SiO2.
This paper presents a general design method for graded reflectivity mirrors, which uses a rotating mask and works even if the distance mask-substrate is no more negligible. The method was tested with a model for the deposition process which considers the mask aperture as an extended plane source and for several superGaussian orders.
In this paper the design of two different graded reflectivity mirrors is described. Using the thickness variation of the middle layer of a symmetrical (H L)2 H stack, a superGaussian reflectance profile was obtained. The dependence of the superGaussian parameters on the deposition geometry was investigated and the results are discussed by comparison with an other graded reflectivity mirror, for which the thickness of the overall high reflecting coating presents a radial variation. The design of a graded reflectivity mirror with a parabolic reflectance profile is also given. The dependence of the parabola parameter on the deposition geometry was calculated for the same deposition conditions.
The paper discusses the factors affecting the pump quantum efficiency for three-micron stationary emission in concentrated Er systems, driven by the up-conversion processes from the terminal laser level. It is shown that in the stationary regime analytical formulae for the quantum efficiency could be obtained as a function of pump conditions and global efficiency of the cross-relaxation processes. These formulae enable a selection of the active medium and pump conditions in order to obtain high pump efficiency.
The spectroscopic properties of F3+ ions in tetrahedral sites in garnets, together with the good packing of these sites around the dodecahedral position occupied by rare-earth ions, favor a very efficient energy transfer from Fe3+ to TM3+. Since Fe3+ does not introduce additional losses, this ion is a potential sensitizer for Tm3+ laser emission.
A study on Nd:YAG media active laser pumped with millisecond variable pulses in free- running generation and passive Q-switched regime is presented. Different resonator configurations were used: stable plane-plane and spherical-plane resonators usually with reflectivity mirrors or with super-Gaussian variable reflectivity mirrors (VRM) as output couplers and super-Gaussian unstable resonators. In Q-switching operation, laser emission consists in trains of 20 - 50 nanosecond pulses with repetition frequency of 20 - 200 KHz. Peak power pulses of 0.5 - 1 MW were obtained and the laser energy contained in a train of pulses varies from 1 to 8 J depending on the length and energy of the pumping pulse. The beam quality of the Q-switched laser was studied comparatively with free-running laser operation.
The paper describes the design of the antireflection coating and the high reflectance stack for variable reflectance mirrors. A deposition model which takes into account the collisions of the evaporated molecules was studied in order to obtain the radial thickness profile. For a certain multilayer system, deposed with a fixed mask, a map relating the parameters of the reflectance profile to the deposition geometry was calculated. The effect produced on this map by the deposition using a rotating mask with shaped holes was also investigated.
The possibility of sensitization of rare-earth laser active ions with infrared (especially two- micron) emission by Fe3+ is discussed. The spectroscopic and luminescence decay measurements on Fe, Tm or Fe:Tm doped GGG or YAG show that a very efficiency energy transfer from Fe3+ in tetrahedral sites to Tm3+ takes place showing prospects for new sensitization schemes.
The characteristics of energy processes in concentrated Er3+ system and their effects on the stationary three micron laser emission are discussed. It is shown that in certain conditions, valid for a broad range of concentrations, the laser equations could be solved analytically, the solution allowing to study the influence of various parameters on the laser emission. This revealed that a crucial role in determining a positive pump effect is played by a figure of merit that depends on the ratios of up-conversion rates from 4I11/2 and 4I13/2 levels and of the fractional thermal populations for the crystal field components involved in emission. In given pump conditions, the restrictions imposed on the figure of merit limit the emission wavelength range and the efficiency. The emission in Er3+ systems co-doped with Cr3+ is also discussed. Cr3+ co- doping not only improves the pump efficiency, but modifies the whole chain of energy transfer processes, leading to the extension of the laser wavelength range.
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