Flashlamp-excited organic dye lasers

The flashlamp-excited dye laser is presently the only type of laser capable of tunable emission throughout most of the visible spectrum. Gain and power output of the device are comparable to solid-state systems although the laser performance is hindered by thermal effects, produced by spatially nonuniform excitation of the dye, and optical losses associated with the molecular triplet state. In most of the known laser dyes, steady-state lasing is prevented by triplet state effects. The analysis of the gain of the dye laser is discussed in terms of the singlet-state absorption and fluorescence and triplet-state absorption spectra. The gain analysis is used to study the influence of the triplet state upon the critical inversion, and application of the analysis to a specific system is illustrated by the detailed discussion of the rhodamine 6G laser. A criterion for the maximum permissible triplet-state lifetime consistent with CW operation is given. The quenching of the triplet state of rhodamine 6G is shown to be rapid enough to allow CW operation, although thermal effects seem to be serious. The investigation of thermal effects is reviewed and the advantages of uniform excitation of the dye are pointed out. The minimum optical excitation power required for CW operation of a 7-cm-long rhodamine 6G laser of 2 mm diameter is estimated to be 850 watts neglecting thermal effects. A catalog of dyes, with their structures, that have been used in flashlamp-excited dye lasers is given. Various methods of tuning and mode locking the dye laser are reviewed. With a single dye a tuning range of 40 nm may be obtained by substituting a diffraction grating for one of the laser mirrors. Mode locking can be produced by placing a saturable dye absorber in the cavity.     

Frequency- and time-dependent gain characteristics of dye lasers

The approach used by McCumber for treating phonon-terminated lasers is applied to investigate the gain properties of dye lasers and is extended to include effects arising from population buildup in the triplet-level system and associated triplet-triplet absorptive losses. The gain is expressed in terms of time-dependent excited-state populations and spectral emission and/or absorption functions. For a given optical-pump pulse, a computer program is used to solve rate equations for the populations up to threshold and to calculate the gain as a function of time and frequency. The gain varies with frequency over the broad fluorescence bands characteristic of dye molecules and with time until the threshold for laser action is reached. Experiments using rhodamine 6G verify the predicted dependence of the laser frequency and time of threshold on cavityQand demonstrate laser frequency tuning by adjusting the opening time of an intracavityQswitch. No variation of laser frequency is expected for fluorescing molecules exhibiting large Stokes shifts; this is observed for 7-hydroxycoumarin. Computer calculations of the gain for anthracene and rhodamine B illustrate the dependence of gain properties on the rate of intersystem crossing and triplet-triplet absorption. An estimate of the rate of intersystem crossing for rhodamine B in methanol is obtained from a comparison of predicted and observed laser threshold conditions. Requirements for achieving flashlamp-pumped laser action are shown to depend upon both flashlamp and dye properties and are analyzed using the above approach.     

Dye lasers with ultrafast transverse flow

Extension of the molecular rate equations for a dye laser to include the effect of very rapid transverse flow shows that spatial transit times comparable to the dye intersystem crossing time prevent the buildup of a large triplet state population. This new technique enables limiting the triplet-triplet absorption loss without using triplet quenching agents. An air-turbine driven rotating disk of rhodamine 6G dye in polymethyl methacrylate is used to exchange the dye5 times 10^{6}times per second by 250-m/s transverse flow through the pump beam waist in an end-pumped near-spherical cavity (W_{0} sim 25 mu). In pulse experiments, transverse flow enabled increase of power output by a factor of 2.5 with a corresponding decrease in threshold power of about 30 percent. Thermal defocusing effects are eliminated at much slower flow rates. The ultrafast transverse-flow technique should also be applicable to liquid dye lasers and would be most important for cases where no effective triplet quenching agent can be found.     

Transmission of organic laser dyes under inclusion of stimulated emission

This paper describes a theoretical study together with a few experimental results of nonlinear dye transmission under inclusion of the influence of stimulated emission, due to a high-Qoptical resonator surrounding a transversely excited dye sample. The rate equation approach takes into account spatial attenuation of the exciting radiation. Formulas are derived for both the intensity dependent dye transmission and the resultant dye laser output intensity. The study comprises all relevant facts of dye laser operation, such as reabsorption, triplet effects, and excited state absorption (ESA).     

Theory and operation of high-power CW and long-pulse dye lasers

A complete theoretical model for CW and long-pulse dye lasers is developed. The effects of excited singlet-state absorption of pump and laser fight and triplet absorption of pump and laser light are included. Expressions are derived for gain, actual dye transmission, and output power. Experimental work is presented to verify the theory and to obtain values of excited-state absorption cross sections for rhodamine 6G by matching experimental to theoretical curves.     

Scaling of dye lasers with improved laser dyes

Dye lasers are widely used in spectroscopic research because they provide tunable, coherent radiation. They employ highly fluorescent organic compounds, possessing some specific spectroscopic properties, dissolved in organic solvents. These liquid media are cooled by circulation. The liquid media, the dye solutions, have no scaling limitation. However, large dye lasers that generate high-energy and high average-power outputs do not exist because of the lack of a scalable pump source. Presently, pulsed and CW lasers and short-pulse small flashlamp are mainly used for pumping laser dyes. Only these pump sources are able to overcome the excessive triplet-state losses (TSLs) present in presently available laser dyes. These TSLs result, in part, from fairly large triplet absorption coefficients, ε_T(λ_F), of transient triplet-state dye molecules generated during excitation. Using new laser dyes that possess smaller triplet absorption coefficients should not only improve laser-action efficiencies, but should also allow pumping with large, long-pulse flashlamps and possibly with laser diode arrays and incoherent light sources. These pump sources are scalable and therefore would further increase overall outputs. This paper details TSL, vibronic spin–orbit interactions in heterocyclics, and other spectroscopic parameters affecting laser-action properties. Pump sources for dye lasers, especially flashlamps, are also reviewed. Criteria that can be used to identify candidate laser dyes for efficient long-pulse operation are provided. These are the so-called quasi-aromatics (QAs), five- and six-membered heterocyclics, and are identified as possessing small ε_T(λ_F) values. The pyrromethene–BF₂ complexes are representatives of such QAs. There should be many candidates that exhibit efficient laser action in different spectral regions, under long-pulse, large flashlamp pumping.     

Long-pulse laser emission from rhodamine 6G

The properties of the rhodamine 6G flashlamp-pumped circulating dye laser with cyclooctatetraene and cycloheptatriene as triplet-state quenchers are described. Particular attention is given to such observables as pulse duration, frequency sweep, and early termination of the laser pulse. From the experimental results and the theory of dye lasers, it is concluded that the thermal effects may not play a dominant role in such lasers.     

Spectra of the lowest excited singlet states of rhodamine 6G and rhodamine B

Excited states of laser dyes were generated in solution by an intense 6 ns laser pump pulse tuned to the absorption maximum of the longest wavelength, ground state absorption band. They were interrogated by a shorter, simultaneous, monochromatic pulse of smaller diameter, concentric and collinear with the pump, having a crossed polarization. Plots of transmission curves against intensity for the pump beams approached residual absorptions at the highest intensities. Gain or attenuation of the probe was measured throughout the region of the long wavelength absorption bands, at the 578.2 nm copper vapor laser wavelength, and for rhodamine 6G across the lasing band. Gains were determined at different probe intensities, and were extrapolated to small-signal values. The dyes have fluorescences at the ground state absorption maxima, hence pumping at these wavelengths cannot achieve 100 percent S1population. Auxiliary saturation transmissions were determined beyond the fluorescence bands and the first experiments were corrected to give a measure ofsigma_{e} - sigma^{ast}, the difference between the stimulated emission cross section and the excited state absorption cross section. Assumptions in the experiment and improvements in experimental technique are discussed.     

闪光灯泵浦染料激光器PTM运转的能量转移特性

根据由速率方程导出的一组微分-积分方程,研究了闪光灯泵浦染料激光器PTM运转时的能量转移特性。比较了PTM运转和正常运转时能量转移过程的主要区别。给出了转移到受激辐射、非受激辐射、腔损耗、三重态吸收等方面的能量与腔内光子寿命的关系。     

Excited state absorption of pump radiation as a loss mechanism insolid-state lasers

The characteristics of optical pumping dynamics in laser-pumped, rare-earth-doped, solid-state laser materials are investigated by using a tunable alexandrite laser to pump Y₃Al₅O₁₂ :Nd³⁺ in an optical cavity. It is found that the slope efficiency of the Nd laser operation depends strongly on the wavelength of the pump laser. For pump wavelengths resulting in low slope efficiencies, intense fluorescence emission is observed from the sample in the blue-green spectral region. This is attributed to the excited-state absorption of pump photons which occurs during radiationless relaxation from the pump band to the metastable state. This type of process is an important loss mechanism for monochromatic pumping of laser systems at specific pump wavelengths     

Spectroscopic and lasing characteristics of 1,3,5,7-tetramethyl-8-ethyl-pyrromethene-BF2 complex

The basic optical properties of the newly synthesized pyrromethene-BF2 derivative (Compound 1) were investigated systematically by its ultraviolet to visible (UV-visible) absorption, fluorescence and triplettriplet absorption. Its lasing characteristics were measured on a dye laser system pumped by a continuous wave Ar ion laser. The results suggest that, compared with the typical pyrromethene-BF2 dye such as P546, Compound 1 possesses stronger fluorescence and lower triplet-triplet absorption over its fluorescence spectral region. Excellent laser properties were also observed for Compound 1. It outperforms the commercially available benchmark laser dye rhodamine 6G in laser wavelength tuning range, which proves that Compound 1 is probably a potential candidate to be developed as the all-solid-state laser material.     

Efficient laser emission in concentrated Nd laser materials underpumping into the emitting level

The possibility of using concentrated Nd laser materials for efficient laser emission and for scaling to high powers is discussed. It is shown that the increased optical absorption in these materials makes direct pumping into the emitting level feasible, with a reduction of the quantum defect between the pump and emission wavelengths, which in turn can enhance the laser emission characteristics and reduce heat generation under pumping. The investigation of the effect of Nd concentration on emission decay of Nd:YAG indicates that up to quite high concentrations, the reduction of the emission quantum efficiency by self-quenching can be compensated by an increase in the pump absorption. Efficient continuous-wave laser emission is demonstrated under direct pumping into the ⁴F_3/2 emitting level of Nd:YAG crystals with up to 3.5-at.% Nd, Nd:YAG ceramics with up to 6.8-at.% Nd, and Nd:YVO₄ crystals with up to 3-at.% Nd. Superior performance as compared to traditional pumping into the ⁴F _5/2 state were obtained. It is inferred that direct pumping into the emitting level of concentrated Nd materials can improve the efficiency of solid-state lasers in the free-generation or low-storage regimes and opens the possibility of scaling these lasers to high powers     

Features of the exciplex mechanism of charge photogeneration in amorphous molecular semiconductors doped with the dye rhodamine 6G

Results of investigation of the processes of charge carrier photogeneration and relaxation within the exciplex absorption region in films of poly-N-epoxypropylcarbazole doped with the dye rhodamine 6G are given. When the films are illuminated with light from the dye absorption region, new centres of charge photogeneration appear that absorb within the exciplex absorption region. It is shown for the first time that these centres have a long lifetime at room temperature and their temporal concentration changes correlate with EPR photosignal changes. The concentration of centres is independent of the external electric field strength during their generation. These centres are identified as triplet exciplexes. © 1997 John Wiley & Sons, Ltd.     

混合染料在N_2激光泵浦下的光辐射

考察了一些混合染料的乙醇溶液在N_2激光泵浦下所发射的光谱.这些混合染料分别是Rh6G加甲酚紫、RhB加甲酚紫、Rh6G加RhB以及Rh6G加RhB加甲酚紫.讨论了其中的能量转移过程.证实混合染料对拓宽激光可调谐范围、提高某些染料的激光效率以及获得双波长激光振荡是可行的.     

Compression mechanism of subpicosecond pulses by malachite green dye in passively mode-locked rhodamine 6G/DODCl CW dye lasers

The pulse width compression effect of a malachite green (MG) dye upon subpicosecond pulses has been experimentally investigated in a CW passively mode-locked rhodamine 6G/DODCI dye laser. The pulse width reduces as MG concentration increases, and reaches 0.34 ps at1.5 times 10^{-6}M. By adding the MG dye, good mode locking is achieved in a rather wide pumping-power range. A computer simulation of pulse growth has also been carried out by using simple rate equations, in which the fast-recovery component of loss due to the MG dye is taken into account. The simulated results can explain some experimental results qualitatively such as pulse width compression and pumping-power restriction. The pulse width compression results essentially from the fast recovery of cavity loss caused by the MG dye.     

Numerical Device Simulation of Double-Heterostructure Organic Laser Diodes Including Current-Induced Absorption Processes

We investigate the impact of induced absorption caused by injected charge carriers and excited states on the threshold current density of an organic laser diode using numerical simulations. The electrical properties of the device are described by a self consistent drift-diffusion model. The optical properties are calculated using a transfer matrix method. Nonradiative annihilation processes are included employing typical rate constants. In our approach, a three-layer double-heterostructure (DH) with typical organic material properties is studied, which exhibits a threshold current density of 564 A/cm². For this virtual device, upper limits for the charge carrier and triplet-triplet absorption cross section sigma_carrier=1.53times10^-3middotsigma_SE and sigmaT ₁ TN=4.34times10^-3middotsigma_SE have been calculated as a function of the stimulated emission cross section sigma_SE. Additionally, the role of device geometry and material properties concerning induced absorptions is studied. It is shown that the impact of absorption processes is not strongly influenced by the device geometry. By increasing the charge carrier mobilities to mu = 2 cm²/Vmiddots in the transport layers and mu = 0.2 cm²/Vmiddots in the emission layer, the impact of polaron absorption can be greatly reduced. In this case, laser operation might still be possible if sigma_carrier and sigma_SE are within the same order of magnitude. Decreasing the triplet lifetime tau T ₁ is a promising way to reduce the impact of triplet-triplet absorption. For sigmaT ₁ TN and sigma_SE being within the same order of magnitude, the triplet lifetime tau T ₁ has to be reduced to 1 ns for laser operation.     

Fluorescence analysis of chromium-doped forsterite(Mg2SiO4)

The polarized absorption spectra, emission spectra, and fluorescence dynamics of Cr:forsterite are presented and analyzed in detail. At least three different kinds of emitting centers are identified by time-resolved spectroscopy. These correspond to two sorts of octahedrally coordinated Cr³⁺ ions and one Cr⁴⁺ with various possible environments. The infrared laser active center is identified with the latter species, and its infrared absorption and emission properties are attributed to excited state-ground state optical transitions without any thermalization process into the excited state     

Laser, optical, and thermomechanical properties of Yb-dopedfluorapatite

The laser performance of Yb-doped fluorapatite (Ca₅(PO ₄)₃F or FAP), is assessed by employing a Ti:sapphire laser operating at 905 nm as the pump source. We have measured slope efficiencies to be as high as 79%; the residual decrement from the quantum defect-limited efficiency of 87% is accounted for by the presence of passive loss at the 1043-nm laser wavelength. The important spectral properties of Yb:FAP were evaluated, including the absorption and emission cross sections, excited-state lifetime, and ground-state energy-level splitting. The emission and absorption cross sections of Yb⁺³ in FAP are found to be substantially larger than those of other Yb-doped media. The thermal, physical, and optical properties of the FAP host are reported as well     

Optical loss property of silica-based single-mode fibers

The optical loss property of silica fibers has been investigated theoretically and experimentally based on their Rayleigh scattering and absorption losses. The Rayleigh scattering loss for fibers has been estimated using Rayleigh scattering coefficients and power distribution in the fiber. The Rayleigh scattering coefficients are measured for preforms prepared for fiber fabrication and are discussed for GeO₂ -doped and F-doped glasses. The relationship between the optical loss and fiber parameters is clarified. Moreover, the loss increase due to residual stress which occurs during the drawing process is simulated. The optical loss limitations for GeO₂-doped and pure silica core fibers are shown     

有机材料ZnTBP/CA/PhR体系的反饱和吸收特性研究

报道了有机材料ＺｎＴＢＰ／ＣＡ／ＰｈＲ对激光的反饱和吸收特性。理论分析从该材料特有的五能级结构出发，指出三重态的第一激发态在反饱和吸收过程中的重要作用，并用两束光同时照射样品的实验方法进行了验证。该材料的反饱和吸收效应可在功率不太高的连续光下操作