Small-signal Absorption Research Articles

Picosecond laser study of the quasicontinuum of C2F5Cl

The quasicontinuum of the vibrationally hot (∼10 CO2 laser photons) C2F5Cl molecule was investigated using a two-laser pulse arrangement. The first laser pulse preheated the molecules into the QC (quasicontinuum) while the second picosecond CO2 laser was employed to probe the absorption characteristics of the excited molecules. The second laser could be tuned either in frequency or in pulse duration. Several experiments were performed. In the first experiment, the small signal absorption spectrum of C2F5Cl was measured and compared with the dissociation spectrum obtained by Borsella et al.1 The same ν6+ν10 hot band was observed with a considerably narrower linewidth. In the second experiment, the saturation properties of the hot band were investigated. It was found that the absorption line could be characterized as inhomogeneously broadened with no evidence for any broadening due to intramolecular damping. In a third series of experiments, the absorption of C2F5Cl at low as well as high laser energy fluences was measured as a function of the laser pulse duration between 22 and 120 ps. It was found that (i) the absorption can be characterized as predominantly intensity dependent and (ii) the small signal absorption cross section increased considerably as the pulse duration was decreased. These observations were discussed using the multitier energy level classification scheme in the QC. It was concluded that within the laser pulse duration, individual states within the first tier which interacted with the laser were not coupled to each other. The change in the absorption cross section as a function of the pulse duration is consistent with the picture that under laser irradiation, a rapid equilibrium was achieved with the final state populations remaining relatively constant in time. The final populations were dependent almost entirely on the laser intensity. Increasing the laser pulse duration had little effect on the final state distribution and the total energy absorbed by the molecule.

Multiphoton absorption and decomposition studies of chloroform-d induced by a pulsed 13CO2 laser

The multiphoton absorption (MPA) and decomposition (MPD) of chloroform-d induced by various 13CO2 [00°1 → 0°0] lines were studied for pressures from 0.133 to 13.3 kPa. MPA results show a cross-section increasing with pressure up to a plateau value which is likely due to rotational hole-filling. A MPA "spectrum" of the average number of photons absorbed versus wavenumber for different 13CO2 laser lines is shifted to lower wavenumbers compared to the small signal absorption spectrum by at least 5 cm−1 suggesting that a small amount of anharmonicity bottle-necking is occurring in CDCl3. A comparison of the MPD probability versus fluence for CDCl3 with that measured by other workers for fluoroform indicates that the CDCl3 is decomposed at a slightly lower fluence by an appropriate line of a pulsed CO2 laser.For the pressure range (0.2 to 4.0 kPa) and laser pulse-width used, the MPD is second order in CDCl3. The primary decomposition channel is elimination of DCl followed by reaction of CCl2 to give C2Cl2, C2Cl4, and Cl2. The DCl produced exchanges with a source of H2O in the reaction cell to give significant quantities of HCl. A post-pulse reaction between (HCl + DCl) and C2Cl2. produces (C2DCl3 + C2HCl3). Significant quantities of CCl4 are also produced by undetermined reactions.

Kinetic Studies of the Electron-Beam-Pumped ArF Laser Using an Ne/Ar/F2 Mixture

By experimentally optimizing the operational parameters for the electron-beam-excited ArF laser, an intrinsic efficiency of 4.4% was obtained for a 4 atm Ne/Ar/F2 mixture pumped by a 50 ns pulse at an excitation rate of 1.4 MW/cm3. Under these conditions, the measured net gain was 5.5%cm-1, which agrees well with the value of 6.0%cm-1 estimated from a modified version of Rigrod's theory. Assuming an infinite V-T relaxation rate for ArF*(B), the small-signal gain and absorption were estimated as 6.4%cm-1 and 0.35%cm-1, respectively. With an Ar/F2 mixture, it was difficult to obtain efficient ArF lasing, while with a Ne/Ar/F2 mixture, efficient lasing was readily obtainable even at low excitation rates. This is attributed mainly to the fact that the ArF* deactivation rate to form Ar2F* trimer if faster with Ar gas than with Ne gas.

Influence of the dynamic Stark effect on the small-signal gain of optically pumped 4.3-μm CO2 lasers

Experimental results are presented which show that the dynamic Stark effect significantly reduces the small-signal gain (in discharge-excited CO2) and absorption (in unexcited CO2) at the line center of 4.3-μm laser lines directly coupled to 10.4-μm sequence-band pump transitions. In unexcited CO2, population transfer by the pump radiation is negligible and the influence of coherent effects can be observed unambiguously. The results of comparing experiment and theory in this simple case are used to modify a rate-equation model of the 4.3-μm gain dynamics, and the modified calculations are shown to be in good agreement with measured 4.3-μm gain coefficients.

Comparative analysis of wide-band excimer lasers with optical and electron pumping

A comparison was made of lasers utilizing triatomic excimers and the C→A transition in the excimer XeF, pumped optically and by an electron beam. The following parameters were compared: the small-signal gain, absorption losses, and energy characteristics.

Calculation of pulse shaping in saturable media with consideration of phase memory

Starting from the complete set of Maxwell equations and density matrix equations, we derive approximate expressions for the change of pulse envelope and phase in passing samples composed of homogeneously broadened two level systems. The influence of saturation and phase memory on chirped and chirp-free pulses are discussed with respect to characteristic pulse parameters. The phase memory is found to be responsible for the dispersion of the small signal absorption (amplification), the dispersion of the saturation and the inertia of the atomic systems.

IR absorption measurements and new FIR emission lines in CH 3OH by use of a frequency-tunable CW waveguide CO 2 laser

The IR absorption in CH 3OH in the vicinity of CO 2 laser lines has been measured quantitatively by use of a 300 MHz tunable waveguide CO 2 laser with output powers of about 3 W. Information on frequency offsets from the CO 2 line centers, small signal and saturated absorption coefficients of FIR laser pump transitions is obtained. Some stronger pump transitions with frequency offsets larger than 50 MHz gave rise to the observation of 8 new FIR emission lines with wavelengths from 43 to 125 μm.

Saturable absorbers for multiband operation of a high-gain short-pulse CO_2-laser chain

It is shown that a high-gain CO2-laser chain made of rapidly excited high-pressure amplifiers can be operated without interface isolators. Saturable absorbers are then used only to prevent amplification of front-end leakage and are required to absorb solely at the working frequencies. CH3OH, CH3OD, CH3F, CCl2F2, and HCOOH have been characterized in view of pulse contrast enhancement around 9.4 μm in a multiline multiband laser chain. The optimum pressure, the saturating fluence, the small-signal absorption coefficient, and its ratio over the saturated absorption coefficient are given for many CO2-laser lines.

IR laser induced loss of fluorescence in UO2(hfacac)2⋅TMP. I. Real time detection of multiple photon excitation of the ground electronic state

The infrared multiple photon excitation and unimolecular reaction of UO2(hfacac)2⋅TMP is monitored in real time by the loss of dye laser induced fluorescence (LIF) intensity. The loss of fluorescence results from a change in the chemical structure and not from a change in the vibrational state population of the ground or excited electronic state. Absorption into the emitting states also becomes forbidden following IR excitation. The observed rates of fluorescence depletion are greater than 106 s−1 with excitation the rate limiting step over the range of laser intensities studied. At low extents of depletion the observed rate follows the single photon absorption rate. At higher yields, the observed rate is within a factor of 2 of the rate derived from the measured dependence of the yield on laser power using the analysis given by Quack. Such an analysis can provide a reasonable estimate of the reaction rate when no rate measurements can be made. The ensemble dynamics contribute significantly to the observed kinetics. The excitation and reaction behavior is characteristic of molecules in the quasicontinuum excitation regime from the absorption of the first photon. No fluence or power threshold for reaction is observed. At a fixed fluence, the yield remains constant over a change in the peak power of more than an order of magnitude and thus appears to be independent of the rate at which the photons are absorbed. In addition, the absorption profile is homogeneous with respect to reaction; the yield remains proportional to the product of fluence times the small signal absorption cross section for any laser transition within the v=0 to v=1 absorption profile of any of the accessible vibrational modes. Collision cross sections for fluorescence quenching by UO2(hfacac)2⋅TMP and by the products of the multiple photon excitation UO2(hfacac) and TMP are also obtained.

Study of the XeCl laser pumped by a high-intensity electron beam

We present the results of a detailed experimental study of the XeCl laser pumped by a high-intensity electron beam. The laser system was optimized as an oscillator for mixtures of Xe and HCl with Ne, Ar, and Kr diluents. The peak intrinsic efficiency (laser energy out/electron-beam energy deposited) was near 4.5 percent for each of these diluents. Small-signal gain and background absorption were measured as a function of electron-beam deposition rate from 0.4 to 6 MW/ cm3. The ratio of small-signal gain to absorption was found to be constant over this range with a value of ∼5. Measurements of absorption in the presence of a large photon flux indicated that there was no appreciable saturable contribution to the absorption. Measurements of fluorescence from the B and C states indicate that collisional mixing between these states is very rapid. The formation efficiencies of the B and C states are estimated to be 0.15 and 0.05, respectively. A vibrational relaxation rate of between 1 and 1.5 \times 10^{-10} cm3. s-1was determined. The effect of this finite relaxation rate is to reduce the energy available to the stimulated process by a factor of 0.67-0.75. Estimates of the XeCl* deactivation rates by HCl and electrons were also obtained. A value of 1.7 \times 10^{-9} cm3. s-1was obtained for quenching by HCl, and a value of \sim 1 \times 10^{-7} cm3. s-1was estimated for electron deactivation.

Saturation of N2F4 by a short CO2 laser pulse

Saturation parameters and absorption characteristics of N2F4 at 10.6 μm is studied using an intense 1.5 ns pump pulse and a low cw CO2 laser probe. It is found that, for short-pulse interaction, a four-level model describe satisfactorily the experimental results at pressure up to 120 Torr. The main parameters of the model are a rotational–rotational exchange constant of 47 ns Torr, a fundamental radiative cross section of 1×10−16 cm2, a contribution of 1/60 of the ground state population to absorption and a residual absorption coefficient equal to 20% of the small signal absorption at low pressure.

Temperature dependence as a probe of intramolecular relaxation in the infrared multiphoton excitation of CF2Cl2

Systematic variations have been seen as a function of temperature and irradiation frequency in the multiphoton dissociative product yields for CF2Cl2. Thermal shifts in the small signal absorption spectrum do not account for the observed effects, which are attributed instead to an interference between initial thermal vibrational excitation and laser pumping through the anharmonic bottle neck. A simple stochastic classical model for the excitation of an active oscillator coupled to an intramolecular heat bath is developed to explain our results. Temperature dependence in this model comes from the dependence of the dephasing rate in the driven system on the initial energy of the heat bath. Comparison of the model with experiment yields good agreement with characteristic times for our derived dephasing parameter of 0.1 nsec at 300 °K and 2 psec at 500 °K. It is predicted that under certain conditions of irradiation frequency and pressure, enhanced relaxation will lead to increased primary isotope separation efficiency at elevated temperatures.

Optimum output efficiency of homogeneously broadened lasers with constant loss

The exact solution of a laser power extraction model is analyzed that relates the extraction efficiency with the laser parameters such as output coupling, small-signal gain, absorption loss, and laser cavity length. The model assumes a stable optical resonator and a homogeneously broadened gain medium. Optimum output coupling and resulting maximum extraction efficiency are determined for a range of values of the small-signal gain and absorption loss per pass. A relation is derived that allows determination of the intrinsic laser parameters from output power measurements when output mirrors are utilized with three different reflectances.

Oscillator and oscillator-amplifier experiments with an HgBr2/HgBr dissociation laser

Using steady-state approximations to describe the performance of an HgBr* (B→X), uv-preionized discharge laser oscillator and amplifier, values are derived for small-signal gain g0, absorption α0, and saturation flux Isat. The averaged best fit of the experimental data to the theoretical expressions is found for g0=6.6%/cm, α0=0.35%/cm, and Isat =200 kW/cm2 for the case when the buffer gas consists of 900 Torr Ne+100 Torr N2.

Optical extraction efficiency in lasers with high Fresnel number confocal unstable resonators

Using the formulation of Moore and McCarthy for the equation describing the dominant mode in a positive branch confocal unstable resonator in the geometrical optics limit and a simple two-level kinetics model, we analyze the dependence of the optical extraction efficiency (eta) on resonator magnification (M), length (L), small signal gain (go), and (nonsaturable) background absorption (alpha). The model has cylindrical symmetry and spatially uniform small signal gain, absorption, and index of refraction. For fixed gamma= g(0)/alpha and M, a value of g(0)L that maximizes eta is found. For different gamma, the maximum obtainable value of al is found to be independent of M and to depend upon gamma in a simple way.

Asymptotic decay of periodic spherical waves in dissipative media

In studies of plane periodic waves of finite amplitude in dissipative media, it has been found that if nα1 < αn, the nth harmonic pressure component pn, decays asymptotically as exp(−nα1x), not as exp(−αnx), where αn is the small signal absorption coefficient for the nth harmonic component. Since generally nα1 < αn, this means that the nonlinearly generated nth harmonic decays more slowly than would be expected from linear theory. Moreover, save for the fundamental, no distance is ever reached at which the linear theory decay is established. In this paper the spherical wave problem is addressed. The results are based on an asymptotic solution of Burgers' equation. It is found that for both weak and strong waves the harmonics decay as exp(−nα1r)/rn. The apparent absorption is the same as in the plane wave case. The apparent geometrical spreading, however, is r−n, not r−1. The asymptotic decay is still, however, slower than would be expected on the basis of linear theory. [Work supported by NASA, AFOSR, and ONR.]

Gain and absorption measurements in a KrF* laser

The small-signal gain and absorption of an electron-beam-excited KrF* laser mix was measured by probing the excited medium with a frequency-doubled flashlamp-pumped dye laser. The mixture contained 0.3% F2, 6% Kr, and 93.7% Ar. The doubled dye laser was used to determine the single-pass gain or loss of this e-beam-excited mixture over the KrF* (2Σ1/2 →2Σ1/2) emission band. The KrF* and other relevant excited-state number densities under the condition of the experiment were determined from a comprehensive kinetic model. The effective KrF* stimulated-emission cross section was determined by dividing the measured gain per centimeter on line center by the excited-state number density.

Model for parametric acoustic sources

A theoretical model for the difference-frequency radiation from a parametric acoustic array is developed. The (two-frequency) primary wave is assumed to be radiated by a piston source and may be effectively limited in either the nearfield or the farfield of the piston. Either small-signal absorption or saturation of the primary beam may serve as the limiting mechanism. The parametric gain is defined as a complex number whose (i) magnitude is the ratio of secondary and primary source pressures and (ii) phase is an indication of how much generation occurs within the nearfield and how much within the farfield of the primary beam. The parametric gain is evaluated as a function of the primary-to-secondary frequency (downshift) ratio, the amount of small-signal primary absorption within the nearfield, and a ’’scaled’’ primary source level. Then, the parametric gain may be used to determine the beam pattern characteristics via a simple closed-form expression. The theoretical results are found to be in good agreement with other theoretical treatments (within their realms of applicability) and in fair agreement with experimental results from various types of parametric sources. The discrepancies that do exist appear to stem from the approximation that the primary nearfield can be treated as a plane wave.

Atomic absorption of thermally dissociated iodine for laser applications

An absorption cell for the 1.315 mu m iodine laser wavelength has been constructed using thermal dissociation of molecular iodine vapour to produce ground-state iodine atoms. Measurements of the small-signal absorption coefficient for iodine pressures up to 5 Torr and temperatures up to 750 degrees C have been compared with the predictions of a simple thermodynamic model of the iodine dissociation equilibrium, and satisfactory agreement obtained. The model readily allows calculation of operating conditions required for the use of the iodine absorber as a saturable absorber, for free-induction decay pulse generation and for coherent pulse propagation experiments.

Pump absorption and saturation in the CH3F 496-μm laser

Results are presented from a series of diagnostic experiments designed to measure the fundamental absorption parameters in CH3F for the P (20) CO2 laser line at 9.55 μm. These parameters include the small-signal absorption coefficient, the saturation intensity for the absorbing transition, and the homogeneous linewidth. These data provide a quantitative characterization of the pumping process for the cw CH3F laser operating at 496 μm.