Determination of the Auger upconversion rate in fiber-coupled diode end-pumped Nd:YAG and Nd:YVO4 crystals

An analytical model is developed to study the influence of the Auger upconversion process on the thermal loading under lasing and nonlasing conditions. With the developed model, Auger upconversion rates can be determined by comparing theoretical calculations with experimental results for the ratio of the thermal loading under lasing and nonlasing conditions. The upconversion rates obtained with the present method are compared with the results measured from the fluorescence decay experiment. Received: 7 June 1999 / Revised version: 3 August 1999 / Published online: 30 November 1999     

Temporal and spectral properties of picosecond two-photon pumped cavity lasing of an organic dye HEASPS

The operation of cavity lasing of a two-photon absorption (TPA) organic dye, trans-4-[p-(N-hydroxyethyl-N-ethylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HEASPS), by using a picosecond infrared laser as the pump source, is reported. The lifetimes of TPA fluorescence of this dye in different solvents were measured. Temporal profiles of cavity lasing show obvious oscillations and magnification of the feedback light. By using the difference of re-absorption coefficients at different conditions, we explained the blue shift for cavity lasing and superradiance compared with TPA-induced fluorescence. The long lifetime of fluorescence is helpful to the generation of cavity lasing. Received: 18 June 2001 / Revised version: 5 November 2001 / Published online: 17 January 2002     

Two-photon absorption-induced optical properties of a new lasing dye in two solvents

Pumped by an infrared lasing and its frequency-doubling source, the fluorescence and superradiance emission properties of a new lasing dye trans-4-[4^′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated DEASPS), have been studied in benzyl alcohol and chloroform. The two-photon absorption (TPA) and emission properties of DEASPSare influenced by the solvents used. The emission wavelength of the dye in benzyl alcohol is redshifted relative to that in chloroform. The lifetime of two-photon absorption-induced fluorescence is about 529 ps in benzyl alcohol, whereas it is 340 ps in chloroform. Correspondingly, the one-photon-excited fluorescence lifetimes in the two solvents are also given. The upconversion efficiency of DEASPS in chloroform is higher than in benzyl alcohol. Finally, the effective molecular two-photon absorption cross-sections were measured by the nonlinear transmittance method. It is found that the optical limiting effect of the dye in benzyl alcohol is stronger than that in chloroform. Received: 3 May 2001 / Revised version: 6 July 2001 / Published online: 19 September 2001     

Laser performance studies of para-(phenylene-ethynylene) polymers in organic solvents

The lasing performance of the polymers poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-thienyl) (OPT) and poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-pyridinyl) (OPP) in organic solvents was studied. A two-mirror resonator was transversally pumped using single pulses of the second harmonic of a mode-locked ruby laser. OPT in tetrahydrofuran lased in the region from 480 nm to 525nm and OPP in tetrahydrofuran showed laser action in the region from 460 nm to 490nm. The polymer solutions are characterized by absorption and fluorescence spectroscopic analysis. Effective stimulated emission cross-sections are reported. High excited-state absorption cross-sections and residual ground-state absorption reduce the lasing efficiency. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fresnel diffraction model for mode-mismatched thermal lens with top-hat beam excitation

A theoretical model, based on the Fresnel diffraction integral, is developed to describe the thermal lens (TL) signal in a mode-mismatched collinear configuration, which is optimized for a near field detection scheme and excitation by a cw modulated laser beam with a top-hat profile. The TL amplitudes obtained with both top-hat and Gaussian beam excitations are numerically computed and compared, and the dependence of the TL amplitude on the experimental parameters is discussed. Numerical results show that the top-hat beam TL instrument is more sensitive than the Gaussian beam TL instrument, with a potential doubling of the sensitivity. The use of the top-hat beam excitation with TL detection is a significant improvement because a top-hat beam can be easily obtained with a low-cost, wide-spectral emission white-light source. The use of incoherent light sources as the excitation sources would substantially expand the applicability of the TL technique to the general area of chemical analysis.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Lasing without inversion due to cooling subsystem

The new possibility of inversionless lasing is discussed. We have considered the resonant interaction of a two-level system (TLS) with photons and the adiabatic interaction with an ensemble of Bose particles. It is found out that a TLS with equally populated energy levels amplifies the coherent light with Stokes-shifted frequency. This becomes possible as photon emission is accompanied by Bose particles excitation. The energy flow from the TLS to the photon subsystem is realized due to the Bose subsystem being at finite temperature and playing the cooler role. The advantage of this new lasing principle is discussed. It is shown that lasing conditions strongly differ from conventional ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Towards new lasers in Ti:Er:LiNbO3 waveguides: a study of the excited Er3+ states

Detailed excited state absorption measurements under pumping at 980 nm and 1.5 μm together with conventional absorption and emission spectroscopy is employed to investigate optical transitions of Er³⁺ in Ti:LiNbO₃ channel-waveguides. The experimental data were evaluated using the Judd–Ofelt method giving parameters close to those in the bulk. The good agreement between theoretical prediction and measurements allows us to calculate cross section, lifetimes, and branching ratios. Based on these results we developed and tested a model which is able to predict the conditions for which laser operation at 550 nm and 2.7 μm will be possible under 980 nm pumping. Received: 9 December 1998 / Revised version: 8 January 1999 / Published online: 24 March 1999     

Theoretical investigation of photoprocesses in complex organic compounds upon high-power laser excitation

A procedure of theoretical investigations into the dynamics of energy level populations of organic molecules upon high-power laser excitation is developed that allows information on stimulated emission of molecules, gain and lasing spectra, dynamics of lasing pulses, populations of the ground and excited states, and influence of the induced absorption on lasing of the organic molecules to be obtained. Laws of changing of the above-indicated processes are established for two pyran substitutes. Results of theoretical and experimental investigations are well correlated. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 51–60, March, 2009.     

Random laser action in ZnO

We report a direct evidence of random laser in optically pumped ZnO powder. Discrete lasing modes are observed above threshold. The laser emission spectra depend on the angle of observation and are random. The lasing action is attributed to the coherent feedback due to recurrent light scattering in the powder. The lasing threshold intensity depends on the excitation volume. Received: 15 September 1999 / Revised version: 2 February 2000 / Published online: 5 July 2000     

Frequency degenerate oscillation in semilinear photorefractive oscillator with reflection gratings

The oscillation intensity of a semilinear photorefractive oscillator with reflection gratings is calculated for different cavity losses, different pump intensity ratios and different crystal orientations. Distinct from an oscillator with transmission gratings, this oscillator always has a soft excitation in the vicinity of the threshold. The theoretical results are in reasonable agreement with that of experimental studies with BaTiO₃:Co as a photorefractive crystal.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Proposal for Observation of a Hidden Nuclear Population Inversion

Observation of a hidden nuclear population inversion, as a precursive experimental proof of the concept of recoil-assisted gamma-ray lasing, can be accomplished by pure optical methods due to spectral shift of atomic transitions. This paper presents numerical estimates and a proposal for such a proof-of-principle experiment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tunable wavelength conversion between picosecond pulses using cascaded second-order nonlinearity in LiNbO3 waveguides

Tunable wavelength conversion between picosecond pulses is demonstrated by exploiting cascaded secondorder nonlinearity in periodically poled LiNbO₃ waveguides when the pump pulse with 40-GHz repetition rate and 7.5-ps pulse width is adopted. No external continuous-wave input is required in the proposed wavelength converter. The converted signal wavelength can be tuned from 1519 to 1562.6 nm as the lasing wavelength is changed from 1534.5 to 1572.1 nm.This revised version was published online in August 2005 with a corrected cover date.     

Efficient and highly photostable solid-state dye lasers based on modified dipyrromethene.BF2 complexes incorporated into solid matrices of poly(methyl methacrylate)

We report on the laser action of modified dipyrromethene.BF₂ complexes both dissolved in poly(methyl methacrylate) and copolymerized with methyl methacrylate. All recently synthesized dyes have the chromophore core of laser dye pyrromethene 567, differing in the substituent in position 8. The new dyes showed important and most significant increases in both lasing efficiency and photostability. When the laser samples were pumped transversely at 534 nm, laser emission was obtained with efficiencies of over 30% in most cases, to be compared with a lasing efficiency of 12% exhibited by reference dye pyrromethene 567 under the same experimental conditions. Some of the materials exhibited highly photostable laser action, with no sign of degradation in the laser output after 60000 pump pulses at the same position of the sample at a repetition rate of 10 Hz, whereas the laser emission of reference dye pyrromethene 567 dropped to zero after less than 40000 pump pulses. Received: 19 November 2002 / Revised version: 20 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +34-91/564-2431, E-mail: acostela@iqfr.csic.es     

Thermal effect on multiphonon-assisted anti-Stokes excited upconversion fluorescence emission in Yb3+-sensitized Er3+-doped optical fiber

The temperature effect upon infrared-to-visible frequency upconversion fluorescence emission in Yb³⁺-sensitized Er³⁺-doped germanosilicate optical fibers excited with cw radiation at 1.064 μm is investigated. The experimental results revealed an eightfold enhancement in the visible upconversion emission intensity as the fiber temperature was increased from 17 °C to 180 °C. The fluorescence emission enhancement is attributed to the temperature-dependent multiphonon-assisted anti-Stokes excitation process of the ytterbium sensitizer. A theoretical approach that takes into account a sensitizer absorption cross-section, which depends on the phonon occupation number, has proven to agree very well with the experimental data Received: 6 April 1999 / Revised version: 27 August 1999 / Published online: 27 January 2000     

Energetic and thermal performance of high-gain diode-side-pumped Nd:YAG rods

We investigated the energetic and thermal performance of a diode-side-pumped Nd:YAG rod laser with up to 50 W power deposited as excess heat into a 3-mm-diameter, 10-cm-length rod. The rod design produces an extremely flat gain profile resulting in “textbook” expressions of thermal lensing and birefringence. Thermal and energetic measurements are compared to corresponding “textbook” theoretical expressions. Discrepancies between various published thermo-mechanical YAG parameters are resolved by a self-consistent set of measured and calculated data for rod thermal lens focal lengths, birefringence depolarization and ratio of heat to stored energy (χ). Measured thermal and energetic performance under lasing and nonlasing conditions are presented, which agree with published theoretical expressions and measurements. Compensation of rod thermal lensing with simple spherical concave lenses is demonstrated. In addition various methods for compensating birefringence depolarization are theoretically and experimentally analyzed and compared. Received: 19 July 1999 / Revised version: 22 October 1999 / Published online: 23 February 2000     

Population inversion induced by collisions in a two level system under nonresonance optical excitation

When the gas particles, which can be considered as a two level quantum system, are affected by frequent collisions with buffer gas particles (thermal reservoir), the spectral densities of the Einstein coefficients for absorption and induced emission are unequal to each other far from the absorption (emission) line. The relationship between the spectral densities at the specified condition reveals new physical phenomena. The mostly convincing example is the fact that population inversion is established in a two level system under nonresonance optical excitation. This effect is experimentally observed as a lasing on sodium resonance transitions.     

Stationary and transient analysis of photoconductivity and photorefractivity in CdZnTe:Ge

The photopyroelectric (PPE) method in a non-contact configuration was proposed to study water migration in starch sheets used for biodegradable packaging. A 1-D theoretical model was developed, allowing the study of samples having a water profile characterized by an arbitrary continuous function. An experimental setup was designed or this purpose which included the choice of excitation source, detection of signals, signal and data processing, and cells for conditioning the samples. We report here the development of an inversion procedure allowing for the determination of the parameters that influence the PPE signal. This procedure led to the optimization of experimental conditions in order to identify the parameters related to the water profile in the sample, and to monitor the dynamics of the process. Received: 1 June 1999 / Final version: 13 March 2000 / Published online: 7 June 2000     

3D Modeling of amplification processes in CPA laser amplifiers

We present here chirped pulse amplification simulations allowing for the extension of the generally used 1D model. The importance of the beam spatial profiles, diffraction and thermal effects in simulations is clearly shown in order to describe with accuracy the gain saturation. Experimental measurements performed on a 100 TW laser validate this calculation. The effects of dispersion and self-focusing are also studied.This revised version was published online in August 2005 with a corrected cover date.     

Extremely selective modal oscillation of random lasing induced by strong multiple scattering

We report on a new physical aspect of random lasing, an extremely selective modal oscillation by using a two-dimensional calculation model. The developed model consists of two theoretical subsystems, two-dimensional scattering model for describing multiple scattering process and rate equation model for describing lasing process. Using this model, we show how emission spectrum behaves as the scattering state of photons inside the system changes. It is shown that specific and strong modal oscillation takes place in a closed loop path of emitted photons, efficiently supported by the background multiple scattering. With the increase of multiple scattering events around the closed loop path, the system starts to oscillate with an extremely strong mode without any ASE (amplified spontaneous emission) noise.     

Simple parameterization of dark-resonance line shapes

We have studied the dispersion and absorption line shapes of coherent population trapping resonances (dark resonances) in thermal cesium vapor. Outside the time-of-flight regime the line shapes can be described by a simple phenomenological model: a sum of a dispersive and an absorptive Lorentzian of identical width and position, whose relative weights depend on the detuning of the laser frequencies from the optical resonance. This functional form can be derived from a theoretical model for the full multilevel structure of an alkali atom. The analytical expressions for the parameters of the line shape model show the same behavior as the experimental data. Received: 4 October 2002 / Revised version: 30 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch