Absorption Of Pump Radiation Research Articles

Influence on the generating losses on the spectral efficiency of ethanol solutions of dyes of various classes

The generation efficiency of a number of ethanol dye solutions is studied as a function of the coherent microsecond pump wavelength. It is shown that the maximum lasing efficiency is achieved by pumping not to the maximum of the main band of the absorption spectrum of dyes. In the spectral ranges determined for each dye, a decrease in the generation efficiency is observed. The reasons for this phenomenon – triplet-triplet and singlet-singlet absorption – are analised. The main role in the case under consideration is played by the absorption of pump radiation in the system of excited singlet levels with the formation of long- and short-lived photoproducts that absorb in the amplification region but do not participate in generation. A study of the luminescence kinetics of the DCM dye showed that another reason for the appearance of induced losses is the non-linear scattering of radiation in a dye solution, which occurs because of non-stationary self-diffraction of pump radiation on phase noise gratings formed as a result of heat release when light is absorbed in the channel of excited singlet levels in a bleaching dye solution. Experiments on probing induced losses, as well as picosecond spectroscopy, confirmed the appearance of thermooptical distortions of the active medium in certain spectral intervals of microsecond coherent pumping.

Decay of a laser beam under stimulated temperature scattering in toluene as a result of two-photon pump absorption and nonsteady-state laser pulse interaction with matter

The decay of the pump-beam spatial structure under conditions of stimulated temperature scattering of a pulsed laser beam in toluene as a result of two-photon absorption of pump radiation and time-dependent interaction of a laser pulse with a medium is considered.

Upconversion/back-transfer losses and emission dynamics in Nd3+-Yb3+ co-doped phosphate glasses for multiple pump channel laser

An active phosphate glass system co-doped with Nd3+ and Yb3+ rare earth, optically active ions was studied using steady-state and time-resolved luminescence spectroscopies. Because this system exhibits great potential for the development of multiple pump channel Yb3+ lasers, this work attempts to identify losses in order to establish the limits of its radiative efficiency. To achieve this, multiple upconverted visible emissions related to Nd3+ ions were observed in experiments carried out under near-infrared excitation at around 800 nm. An analysis was carried out to clarify if these upconverted emissions were governed by excited-state absorption of pump radiation or energy transfer between the rare earth ions. Another potential source of losses is related to energy back-transfer from Yb3+ to Nd3+ ions. However, no evidence of this phenomenon was observed, which is a good result for laser applications. Finally, the luminescence dynamics of the Yb3+ ions were investigated in the framework of fast-diffusion processes.

Influence of saturation of pump radiation absorption on the parameters of a planar YAG:Yb+3 amplifier

With allowances made for the saturation of pump radiation absorption, the energy parameters of planar YAG:Yb+3 amplifiers of continuous-wave-radiation and short pulses were determined. The wave-front aberrations resulting from temperature instability in the active medium were estimated. It was shown that saturation of pump radiation absorption presents an important factor affecting the parameters of planar amplifiers, particularly in the case of short pulses.

Numerical modeling of the impact of pump wavelength on Yb-doped fiber amplifier performance

Ytterbium-doped optical amplifiers have become common tools for industrial applications due to their high efficiency, relatively low cost and potentially very high output power level. The efficiency of an ytterbium-doped fiber amplifier depends mainly on the absorption of pump radiation, and, therefore, optimum pump wavelengths have been proposed such as 915 nm. However, the semiconductor pump diodes batch supplied by manufacturers may exhibit a spread in the output wavelength. This paper theoretically investigates the performance of Yb-doped amplifiers for different pump wavelengths and defines the pump power penalty when the pump source does not emit at the optimum wavelength. The penalty has been defined as normalized excess pump power required to achieve the desired gain.

Pump-probe studies of absorption saturation and optical gain in Ce:LiCaAlF6 ultraviolet active medium

Aim of this work was to investigate nonlinear absorption of pump radiation in UV spectral range of Ce3+:LiCaAlF6 crystalline active medium. We were able to evaluate excited state absorption cross-section at pump wavelength (266 nm) and recombination probabilities by means of computer simulation.

Dysprosium-doped PbGa_2S_4 laser generating at 43 μm directly pumped by 17 μm laser diode

In this Letter, we demonstrate the pulsed and CW operation of the Dy:PbGa(2)S(4) laser directly pumped by the 1.7 μm laser diode. In the pulsed regime (pulse duration 5 ms; repetition rate 20 Hz), the maximum mean output power of 9.5 mW was obtained with the slope efficiency of 9.3% with respect to the absorbed pump power. The generated wavelength was 4.32 μm, and the laser beam cross section was approximately Gaussian on both axes. Stable CW laser generation was also successfully obtained with the maximum output power of 67 mW and the slope efficiency of 8%. Depopulation of the lower laser level by 1.7 μm pump radiation absorption followed by 1.3 μm upconversion fluorescence was demonstrated. These results show the possibility of construction of the compact diode-pumped solid-state pulsed or CW laser generating at 4.3 μm in the power level of tens mW operating at room temperature.

Coherent population trapping with a train of pulses via a continuum and its application to the suppression of excited-state absorption

We discuss the coherent population trapping (CPT) effect induced in a three-level Λ-type system by a train of short pulses. Specifically, we consider a modified Λ system with the upper level replaced by a continuum of states. We show that CPT with a train of pulses can be applied to the suppression of excited-state absorption of pump radiation in crystalline laser materials. The utility of the technique for pumping a promising yellow-orange solid-state medium Ti:YAlO3 and overcoming excited-state pump absorption is analyzed.

Suppression of stimulated Brillouin scattering in high-power single-frequency fiber amplifiers

The effect of temperature variation along a high-power fiber amplifier on the stimulated Brillouin scattering (SBS) threshold is considered theoretically. We show that for an end-pumped rare-earth-doped double-clad fiber the inhomogeneous distribution of temperature, which is caused by the absorption of pump radiation, may result in total suppression of SBS even for output powers well above 200 W.

Optical in-well pumping of a semiconductor disk laser with high optical efficiency

Optical in-well pumping is shown to lead to highly efficient operation of semiconductor disk-lasers using resonant absorption or using external optics. Pump radiation absorption of 70% at 940 nm is demonstrated for a laser emitting around 980 nm. Laser output power was 1.9 W with slope efficiencies up to 35% based on the incident power.

Spectroscopy and kinetics of the population of monoclinic KYb0.5Y0.43Tm0.07(WO4)2 crystals pumped by a pulsed Nd:YAG laser

The kinetics of pump radiation absorption and luminescence of monoclinic crystals of potassium-yttrium-ytterbium tungstate doped with thulium are studied. It is shown theoretically and experimentally that due to the 'excitation multiplication' caused by absorption of pump radiation from metastable states of thulium ions accompanied by cross-relaxation, above 50 % of a total number of thulium ions can occupy the 3F4 level. The cross sections of stimulated transitions in the spectral region from 1600 to 2100 nm are calculated from luminescence spectra, and the gain in the crystal is estimated. The prospects of practical applications of the results obtained in the paper are discussed.

Excited state absorption of pump and laser radiations in NYAB non-linear crystal operating at 1.3µm for visible laser light generation

Excited state absorption of laser (≅ 1.3 µ m, corresponding to the laser channel) and pump (790-860 nm) radiation have been investigated in Nd doped YAl 3 (BO 3 ) 4 non-linear laser crystal. Laser gain slope efficiency measurements as a function of pump wavelength have been used to estimate the excited state absorption of pump radiation. On the other hand, excited state absorption cross-section in the 1300−1400 nm wavelength range has been measured by using two beam spectroscopy. We have obtained that excited state absorption cross-section for pump radiation is negligible whereas around 1.3 µ m it is comparable to the stimulated emission cross-section from 4 F 3/2 state, so that the net gain cross-section becomes negative for certain wavelengths.

Energy transfer up-conversion and excited state absorption of laser radiation in Nd:YLF laser crystals

Experiments at room temperature were carried out to study the higher order processes that produced the yellow and blue fluorescence observed in Nd:YLF laser crystals, which were pumped by lasers with wavelengths between 785 nm and 811 nm, and with the pump power density ranging from 10 to 30 kW/cm/sup 2/, or the stored energy density, 15-45 J/cm/sup 3/. The analysis of the experimental results provided an explanation for the performance degradation of a Q-switched laser with increasing pump intensity. These higher order processes were energy transfer up-conversion and excited state absorption of both pump and laser radiation. A model was also developed based upon rate equations to describe these higher order processes. This model allowed us to discriminate between these processes and to determine which one played the most important role. The experimental results agreed well with the predictions of the model. It is concluded that the energy transfer up-conversion process is the dominant mechanism for the population of the /sup 4/G/sub 7/2/ multiplet. It is also concluded that excited state absorption of the 1.05 /spl mu/m or 1.3 /spl mu/m radiation from the /sup 4/G/sub 7/2/ multiplet play a significant role in populating the /sup 2/P/sub 3/2/, /sup 2/D(1)/sub 5/2/, and /sup 2/P/sub 1/2 / manifolds, and in the performance of Nd:YLF lasers operating in the pulsed mode. The effect of excited state absorption of the pump radiation from the /sup 4/F/sub 1/2 / multiplet is much less important.

Nonlinear optical behavior of ocular tissue during laser irradiation

A pump (cw Ho-YAG laser) and probe (He-Ne laser) system was used to study the dynamics of the optical behavior of ocular tissue during laser heating. The nonlinear optical behavior of porcine corneal and vitreous-humor tissue was characterized in vitro by means of measurements of the radial profile of a He-Ne laser beam transmitted through the tissue. Temperature gradients in the tissue created by the absorption of pump radiation caused the probe beam to diverge. For constant laser power, the rate of divergence was made dependent on the spot size of the pump beam. The profile of the transmitted probe beam returned to its original magnitude and shape after the tissue was permitted to cool. This reversible change in optical behavior was attributed to the formation of a negative lens owing to thermally induced local gradients in the refractive index of the tissue.

Induced absorption of pump radiation and lasing thresholds of a POPOP solution

The cross sections for absorption of pump radiation at the 355 and 266 nm wavelengths by POPOP molecules in the lowest excited singlet and triplet states were determined. The stimulated emission thresholds for a solution of POPOP in cyclohexane, calculated using measured absorption cross sections, showed good agreement with the experimental results.

High-power pulse-periodic neodymium glass laser with a plate-shaped active element

A laser with a high-concentration KNFS phosphate-glass active element (in the form of a plate of 9 × 15 × 0.42 cm dimensions) was constructed. An average output power of 180 W was achieved at pulse repetition frequencies 5 and 10 Hz. A model description of the absorption of pump radiation in an illumination enclosure, allowing for the actual emission spectra of the pump lamps and the absorption spectrum of the active element, was developed. This model was used in an analysis of the influence of the parameters of the illumination enclosure, of the plate thickness, and of the activator concentration in the glass on the maximum attainable output power.

Parametric amplification of ultrashort 10-μm pulses by two-photon absorption of pump radiation

A numerical investigation was made of the generation of picosecond radiation pulses in the 10-μm wavelength range by parametric amplification of injected 10-μm radiation in the field of high-power ultrashort neodymium laser pulses in a nonlinear proustite (Ag3AsS3) crystal. Calculations were carried out in the approximation of plane interacting waves and they allowed for two-photon absorption of the pump pulses and phase modulation of the interacting pulses by an electron density wave. These effects lower the efficiency of parametric amplification by two orders of magnitude (when the pump pulse intensity is 1.2 GW/cm2) and they also result in pushing of the signal pulse to the front of the pump pulse and cause its shortening without a significant widening of the spectrum.

Asymmetry of the amplification of light in optically dense active media

An asymmetry effect in the amplification and efficiency of an optically dense amplifier is predicted theoretically and confirmed experimentally for opposite and concurrent collinear signal and pump paths. The effect is due to a difference in the losses as a result of the two-stage absorption of pump radiation in a system of luminescing centers described by a model of three energy levels. Calculations are made of various characteristics of this type of amplifier using the quasisteady-state approximation.

Quantitative estimates of the enhancement of the lasing efficiency of laser-pumped binary dye solutions

A study was made of the contribution of excited donor molecules to the formation of the stimulated emission flux in the acceptor band for various ethanol binary solutions of rhodamine and oxazine dyes. Estimates were also obtained of the degree of involvement of various energy transfer processes and direct absorption of pump radiation by the acceptor in the excitation of the acceptor molecules. The calculated values were compared with the experimental data. The similarity between these values confirmed the proposed theoretical model.

Excited state absorption of pump radiation as a loss mechanism in solid-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 Y/sub 3/Al/sub 5/O/sub 12/:Nd/sup 3+/ 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. >