Fluorescence versus laser effect: Towards a unified theory

A single evolution equation is established to treat the mechanisms of fluorescence and the laser effect, using a mixed representation, classical for the electromagnetic field and quantum for the dipoles. The model approach takes advantage of the principle of conservation of energy for the system of electromagnetic field and dynamic dipole. A resulting nonlinear differential equation is derived and is shown to sustain two fixed points associated with fluorescence and laser emission. The existence of a pumping threshold is confirmed for the laser effect. In particular the pumping rate determines whether light will be emitted by fluorescence or by the laser effect, and there is no pumping that engenders fluorescence and laser emission simultaneously. The initial physical conditions, chosen to integrate the evolution equation in time, assume a nonvanishing electric polarization of the emitting dipole rather than a nonvanishing electromagnetic field. The distribution function accounting for the thermal fluctuations of the random initial polarization is also calculated.     

Excimer laser spectroscopy: influence of tissue ablation on vessel wall fluorescence

Limited steerability and injury to the normal vessel wall are major drawbacks of laser coronary angioplasty. To overcome these limitations a new generation of laser systems has been developed which allows not only to eliminate the atherosclerotic plaque but to guide the laser beam by analyzing the laser induced tissue fluorescence (= spectroscopy) for the treatment of the atherosclerotic vessel. An excimer laser (MAX 10 LP, 308 nm, Technolas, Munich, Germany) was used with an emitting (phi 1070 microns) and a detecting (phi 130 microns) optical fiber to induce tissue fluorescence which was analyzed quantitatively by a computerized system. Specimens from the descending (thoracic) aorta were obtained from 24 patients (mean age 68.1 years, range 44-92). Tissue fluorescence was induced with ablating (26-30 mJ/mm2) and nonablating (3 mJ/cm2) laser activations. The emitted fluorescence (range 380-575 nm) was normalized to a wavelength of 380 nm; as a measure of tissue fluorescence the intensity ratio at 500 nm divided by 400 nm was calculated in normal (n = 78), mildly atherosclerotic (n = 40), and severely atherosclerotic (n = 48) tissue samples. Repeated laser activations were carried out and tissue fluorescence was checked until the fluorescence spectrum was normalized. All tissue samples were analyzed histologically by a semiquantitative score. Normal tissue samples showed the highest intensity ratios (5.9 +/- 3.4), whereas mildly (2.9 +/- 1.3) and severely atherosclerotic (2.1 +/- 1.0) samples elicited a significantly reduced fluorescence. Repeated tissue ablations were associated with a normalization of fluorescence intensity ratios in the mildly (7.0) as well as in the severely diseased (4.9) vessels. A curvilinear relationship between intensity ratio and the semiquantitative score was observed (r = 0.66) as well as between intensity ratio and intimal wall thickness (r = 0.62). No gender related differences were found but there was an inverse relationship between fluorescence intensity ratio and age (r = 0.56) as well as between intimal thickness and age (r = 0.41). Excimer laser spectroscopy allows reliable detection of atherosclerotic vessel alterations. Fluorescence intensity ratio is inversely proportional to the intimal wall thickness and the severity of the histologic alterations. There is an age dependency of fluorescence intensity ratio which can be explained by an increase in intimal wall thickness. Successful tissue ablation can be obtained by laser angioplasty and allows determination of the optimal point where complete tissue ablation is achieved by laser activation. Thus, excimer laser spectroscopy is an effective method for selective tissue ablation by laser angioplasty.     

Laser spectroscopy : 3. Linear laser spectroscopy

The three basic methods of linear laser spectroscopy are reviewed: absorption, opto-acoustic, and fluorescence spectroscopy. The characteristics of each method and their areas of application are presented and compared.     

Performance characteristics of a Nd: glass laser amplifier from fluorescence emission studies

The dependence of cavity transfer efficiency of a Nd: glass laser amplifier on flashlamp current density is obtained from the analysis of amplified fluorescence. The cavity transfer efficiency decreases as the current density through the flashlamps is increased. The use of the fluorescence method in optimizing the flashlamp pulse duration for achieving maximum gain in the amplifier is illustrated.     

Frequency measurements of saturated-fluorescence-stabilized CO2 laser lines: comparison with an OsO4-stabilized CO2 laser standard

4 frequency grid, to test the performance of the saturated fluorescence method for CO₂ laser frequency stabilization. The Allan variance for the saturated-fluorescence-stabilized CO₂ laser reached 60 Hz for an integration time of 300 s. We also performed three absolute measurements of CO₂ laser frequencies stabilized by the saturated fluorescence method, with a sub-kHz accuracy. We obtained a 1.7 kHz redshift of the CO₂ saturated fluorescence lines, which we attribute essentially to pressure. This is in contradiction with the previous observation of a blueshift [K.L. SooHoo, C. Freed, J.E. Thomas, H.A. Haus: Phys. Rev. Lett. 53, 1437 (1984)]. Furthermore, significant deviations of the measured frequencies from the data of Maki et al. [A.G. Maki, C.-C. Chou, K. Evenson, L.R. Zink, J.T. Shy: J. Mol. Spec. 167, 211 (1994)] demonstrate the need for a new calibration of the CO₂ frequency grid. Received: 1 December 1997     

Annealing effect on Cr,Yb:YAG laser crystal

The absorption spectra, fluorescence spectra, and lifetimes of as-grown and annealed Cr,Yb:YAG crystal grown by Czochralski technique have been measured. The broad absorption bands in the visible region increase in intensity and shift to long wavelength after annealing, and the additional absorption around 482 nm may be possibly due to new octahedral Cr4+ center in the crystal, and the increase in the infrared (IR) region is due to the increase of Cra+. The increase of Cr4+ also results in the groud state absorption and the concentration quenching of Ybs+ in Cr,Yb:YAG crystal after annealing, the fluorescence intensity is reduced to 75% and the emission lifetime is shortened from 1.40 to 0.44 ms.     

Optimizing the Yb:YAG thin disc laser design parameters

Based on quasi-three-level system, a numerical model of continuous wave thin disc laser is proposed. The fluorescence concentration quenching (FCQ), refractive index depending concentration effects and temperature distribution in the gain medium have been taken into account in the model. The first and second phenomena are not included in previously models. The model is used to determine optimum design parameters and to calculate the influence of various parameters like temperature, number of pump beam passes, active ions concentration and the crystal thickness on the operational efficiency of the laser. This model shows that for higher doping concentrations (>15%) the optical efficiency is decreased due to fluorescence concentration quenching. Our results are excellently in agreement with experimental results.     

Experimental spectra study of Tm:GdVO4 microchip laser at room temperature

A high efficient diode-pumped Tm:GdVO4 microchip laser is demonstrated. Output power of 310 mW at 1917 nm with slope efficiency of 22% is produced when pumped by 2 W of power at 802 nm. Unpolarized fluorescence spectra near 1800 nm pumped by laser diode are measured. In addition, the most important upconversion fluorescence from 1D2 to 3F4 and 3H6 levels is discussed.     

Photophysical properties and laser performance of photostable UV laser dyes

The photophysical properties such as singlet absorption and fluorescence spectra, the fluorescence quantum yield and fluorescence decay time as well as the laser performance data such as the tuning range, the conversion efficiency, and the photochemical stability of 12 sterically hinderedp-quaterphenyls have been measured in ethanol and/or dioxane at room temperature. The sterically hinderedp-quaterphenyls exhibit shorter laser dye emissions in the 330–380 nm range than the parent compoundp-quaterphenyl. The conversion efficiencies of the sterically hinderedp-quaterphenyls range between 1 and 21%. The photochemical stability of the sterically hinderedp-quaterphenyls is by a factor of 10 to 20 better in dioxane than in ethanol. 2-Methyl-5-tert. butyl-p-quaterphenyl and 2,5,2,5-tetramethyl-p-quaterphenyl are among the most stable UV laser dyes known today.     

激光感生荧光中的共振光声效应

本文描述了CDCI_3介质中的激光感生荧光中的共振光声效应(简称荧光声共振效应)。指出这种荧光声共振效应与通常的光声效应的区别。给出了利用荧光声共振效应确定介质中的声速、第二维里系数、声吸收系数、分子内模弛豫时间、粘滞系数和热导率的理论公式,以CDCI_3为例,给出了上述各参数的具体数值。     

A proposed ablation laser

A new type of eximer laser is proposed; it involves the simultaneous sublimation and excitation of a frozen film by means of a relativistic electron beam. Calculated beam requirements for noble gases are ～ 10 kA/cm² of 400 kV electrons for periods of 2 to 12 ns. Predicted advantages of this laser are high gain and the elimination of wavelength limiting optical windows. Preliminary experimental fluorescence spectra have been obtained from xenon films. The emission from the solid phase consists primarily of two 100 Å wide bands centered at 1690 Å and 1730 Å the fluorescence lifetimes are 4 ± 2 ns for both bands.     

Increasing efficiency of laser fluorescence diagnostics of microbial diseases

The laser-induced fluorescence spectra of five types of mycobacteria are studied in various associations. The results obtained are used to demonstrate the possibility of a diagnostic system for the differential diagnostics of microbial diseases. The method employed for the specific analysis of microbial species in the substrate that contain various associations is based on a regression of the spectral characteristics of the fluorescence intensity to the principal components. The diagnostic efficiency is increased when the fluorescence spectra are reconstructed with regard to the optical characteristics of the medium. The detergent effect on the parameters of microbial fluorescence diagnostics is studied.     

激光诱导荧光雷达探测系统仿真研究

激光诱导荧光技术是一种应用广泛的新型遥测技术,为了指导实际激光诱导荧光探测系统的搭建,介绍了荧光检测原理,设计了激光诱导荧光雷达探测系统。以测量水面石油类污染物为例,通过分析激光诱导荧光雷达探测系统中发射系统及接收系统主要参数对系统信噪比的影响,可根据测量条件选择合适的系统参数。在实验室条件下,选用单脉冲能量为50 μJ的Nd:YAG激光器,接收视场角0.1 mrad,滤光片带宽60 nm的接收系统,可以检验浓度为10个/L的粒子。     

Tunable room-temperature cw laser action in Cr3+: GdScGa-Garnet

We report for the first time on tunable room temperature cw laser action of a transition metal laser material. In Cr³⁺:GdScGa-garnet crystals almost the total fluorescence is channelled into the broad band⁴ T ₂→⁴ A ₂ transition. The fluorescence lifetime is τ=120 μs. Laser pumping in the blue-green and yellow-red spectral range yields pump thresholds around 250 mW and slope efficiencies up to 11%. The wavelength of the free running Cr³⁺ laser is centered at 777 nm.     

High efficient diode-pumped Tm:YAP laser at room temperature

A high efficient diode-pumped Tm:YAP laser is reported. The maximum output power at 1981 nm is 5.2 W and the slope efficiency is 30%. Unpolarized absorption near 800 nm and unpolarized fluorescence spectra near 1800 nm pumped by laser diode (LD) are measured. In addition, the relationship between operation temperature and output power is discussed.     

Stimulated radiative laser cooling

Building a refrigerator based on the conversion of heat into optical energy is an ongoing engineering challenge. Under well-defined conditions, spontaneous anti-Stokes fluorescence of a dopant material in a host matrix is capable of lowering the host temperature. The fluorescence is conveying away a part of the thermal energy stored in the vibrational oscillations of the host lattice. In particular, applying this principle to the cooling of (solid-state) lasers opens up many potential device applications, especially in the domain of high-power lasers. In this paper, an alternative optical cooling scheme is outlined, leading to the radiative cooling of solid-state lasers. It is based on converting the thermal energy stored in the host into optical energy by means of a stimulated nonlinear process, rather than a spontaneous process. This should lead to better cooling efficiencies and a higher potential of applying the principle for device applications.     

Absorption and laser excited fluorescence of Ho3+:LaF3

Pramana - The absorption and fluorescence spectra of Ho3+:LaF3, in the wavelength region from 4000 Å to 7500 Å have been reinvestigated incorporating polarization features, for the first...     

Application of a femtosecond self-sustaining mode-locked Ti:sapphire laser to the field of laser scanning confocal microscopy

Developments in ultrafast Ti:sapphire laser technology can be applied in the investigation of nonlinear optical processes. We describe the application of a self-sustaining femtosecond Ti:sapphire laser as an illumination source in the field of confocal laser scanning fluorescence microscopy (LSM). We present spectra for various fluorescent stains under two-photon excitation and present LSM images of stained samples under mode-locked illumination. The potential for such a system as a non-destructive technique for studying live cells in biomedical research is discussed.     

Time resolved spectroscopy in laser grade organic dyes

Time resolved spectroscopy is discussed along with the theoretical and experimental methods used to measure the fluorescence lifetimes in various laser grade organic dyes. The role of solvents and their viscosities in the fluorescence life time is also discussed. Different photophysical parameters have been obtained in rigid and non-rigid dye molecules.     

Fluorescence feedback control of a diode pumped solid state laser

We implement active feedback control in a Q-switched diode-pumped Nd:YVO₄ laser by monitoring the fluorescence intensity from the laser crystal. When the initial inversion level indicated by the detected fluorescence has reached a predetermined value, Q-switching is initiated. The Q-switched pulse energy is stabler with our feedback scheme based on the fluorescence intensity than that with the conventional Q-switching when pumping source is not stable.