Stimulated Raman scattering of picosecond light pulses in hydrogen, deuterium, and methane

Experimental results are presented on stimulated Raman scattering of short pulses of approximately 100 ps duration in H2, D2, and CH4, both in capillary waveguides and in a tight focusing geometry. Experimentally determined thresholds are in good agreement with calculation. Low thresholds (< 20 muJ) are observed in CH4and preliminary results using a mode-locked dye laser as pump indicate a useful source of tunable short pulse radiation in the near infrared.     

Stimulated vibrational Raman scattering in HD

This letter reports an observation of stimulated vibrational Raman scattering in HD gas. The Raman threshold data are compared to those of H2under similar conditions, and the difference in their gain coefficients is discussed.     

The dependence of Raman gain on pump laser bandwidth

The dependence of Raman gain upon laser bandwidth is analyzed in terms of a set of coupled mode equations. The analysis predicts that the Raman gain is independent of pump laser bandwidth within limitations set by dispersion in the Raman medium. These results are verified for rotational and vibrational transitions in H2using a variable bandwidth Nd:YAG laser.     

New lines in the UV: SRS of excimer laser wavelengths

The UV excimer lasers ArF, KrCl, KrF, and XeCl were utilized to create new families of UV lines by multiple orders of broad-band nonresonant stimulated Raman scattering (SRS) in the media H2, D2, CH4, and LN2. Mixed-media and excited-state SRS were also studied.     

Controlled stimulated Raman amplification and oscillation in hydrogen gas

The gain of theQ(1)vibrational stokes line of H2has been measured as a function of pressure in a Raman amplifier cell of variable length. The threshold power for stimulated emission of theQ(1)vibrational andS(1)rotational stokes line of H2has been measured in a transverse resonator as a function of pressure. Both geometries give results in good agreement with theory, without invoking any other nonlinear instabilities.     

The water-vapor laser

Water-vapor lasers oscillate on more than a hundred pulsed lines and over a dozen CW lines over the wavelength range from 7 to 220 μ (1300-45 cm^-1). Nearly eighty of these lines are now identified as vibrational-rotational and pure rotational transition in the low-lying vibrational states of the water-vapor molecule. The structure of the water-vapor molecule is reviewed, and the mechanisms responsible for creating population inversions in water-vapor discharges are discussed. It is shown how a model, based on perturbations between rotational levels of different vibrational states, can be used to explain the laser action and to predict new laser lines. Detailed summaries of data on all observed laser lines in H2O¹⁶, H2O¹⁸, and D2O¹⁶and their identifications are presented. Some of the lines are newly reported, and more accurate wavelength measurements have been made for many of the lines. The observation of predicted laser lines in H2O¹⁸is shown to be a strong confirmation of the perturbation model. The list of references contains a fairly complete bibliography of previous work on the water-vapor laser, and a representative bibliography of work on the energy-level structure of the water-vapor molecule.     

Dependence of rotational and vibrational Raman scattering onfocusing geometry

Stimulated rotational and vibrational Raman scattering in H₂ was investigated with a circularly and a linearly polarized XeCl laser beam and the influence of the focusing geometry on the threshold for rotational and vibrational scattering, respectively, was studied. It is shown that with a circularly polarized pump beam a high-angle focusing geometry allows to get only rotational Raman scattering, whereas a low-angle focusing geometry provides only vibrational Raman scattering for gain suppression effects     

Reaction property of diffused H2with defect Centers in GeO2-doped fiber

This paper presents the reaction property of diffused molecular hydrogen (H2) with irradiation-induced defect centers in a GeO2-doped fiber. When H2diffuses into an irradiated fiber, an OH loss increases even at room temperature. This loss increase is thought to be caused by chemical reaction between diffused H2and irradiation-induced nonbridging oxygen hole centers (NBOHC's). The hydrogen-associated defect centers having 11.9-mT hyperfine structure are formed by chemical reaction between diffused H2and irradiation-induced Ge-defect centers. Moreover, the irradiated fiber shows an OH loss increase at high temperatures even after diffused H2is removed from the fiber. The hydrogen-associated defect centers are considered to be the hydrogen source for the OH formation.     

Stimulated Raman scattering of XeF* laser radiation in H2

Stimulated Raman scattering (SRS) experiments have been performed using an ∼2 J,sim0.4 mus pulsewidth XeF* laser as the pump and high-pressure molecular hydrogen as the Raman-active medium. The SRS conversion efficiency and spectral distribution have been characterized as functions of H2pressure and pump laser focal parameters. Energy conversion efficiencies of >60 percent have been obtained, with the converted energy in first and second Stokes radiation, at 414 nm and 500 nm, respectively.     

Time-resolved output spectrum from a hydrogen fluoride laser using mixtures of SF6and HI

The time-resolved spectrum from a transverse-discharge hydrogen fluoride (HF) laser using a mixture of SF6and HI is reported. Because this spectrum matches that from a high-pressure H2- F2laser, and because the SF2-HI mixture is chemically stable, this laser should be a suitable and convenient source for probing H2- F2amplifiers.     

Very high resolution CARS spectroscopy in a molecular beam

Signal-to-noise calculations are presented for coherent anti-Stokes Raman spectroscopy (CARS) in a molecular beam. Using high peak power pulsed lasers and a high density molecular beam,S/Nfor the H2Q(1) line is expected to besim 10^{4}, with a Doppler width reduction from 1100 to 20 MHz.     

A theoretical study of the atmospheric pulsed H2+ F2chemical laser including rotational nonequilibrium effects

An atmospheric pulsed F2/H2/He chemical laser is studied numerically by a model which incorporates rotational nonequilibrium effects. The results are compared with experimental results of Chen et al. The fit between the experimental and theoretical results was found to be reasonably good. The results were also compared with those obtained from a model with an equilibrated rotational distribution. It is found that inclusion of a rotational nonequilibrated distribution which decreases the output energy by 25 percent has a certain effect on the output energy distribution among the various vibrational states but is of little importance for the lasing duration.     

HCl chemical lasing with HI and chloro compounds

HCL chemical laser action has been obtained from chloro compounds when HI was used as the source of H atoms. Laser action was produced on severalupsilon = 3-2, 2-1, and 1-0 HCl transitions when initiated by transverse pulsed discharges in HI-He mixtures containing such chloro compounds as CCl4, CHCl3, and several Freons. Both HCl and HF lasing were observed when a Freon (C2Cl2F4) containing F as well as Cl was used. The measured wavelengths for these HCl lasers correspond to excellent atmospheric windows for transmission of infrared radiation. No HCl lasing was obtained when HBr, H2S, or H2were substituted for HI.     

Gamma-ray radiation effects on hydroxyl absorption increase in optical fibers

This paper presents gamma-ray irradiation effects on OH-loss increase due to the chemical reaction of H2for GeO2-doped and GeO2. P2O5-doped fibers. When H2diffuses into irradiated fibers (γ-H2treatment), OH-loss increase occurs even at room temperature. Moreover, OH-loss increases due to dissolved H2in irradiated fibers are larger than those in unirradiated fibers at high temperatures. These OH-loss increases are attributed to irradiation-induced defects with which H2reacts to form OH ions. When H2-dissolved fibers are irradiated, the OH-loss increases are larger than those in the γ-H2-treated fibers at room temperature. These larger loss increases are thought to be caused by the reaction of the unstable radical and dissolved H2, which are excited by the irradiation.     

Far-infrared laser gain resulting from rotational perturbations

A simplified model is used to calculate the unsaturated gains of far-infrared rotational and vibrational-rotational laser transitions that are caused by rotational perturbations. Inversions are derived from the perturbation of an initial Boltzmann distribution of rotational populations, permitting the calculation of laser gains in terms of the perturbation parameters and the vibrational populations. The gain equations are applied to the three perturbations of H2O¹⁶that result in the largest number of laser lines. Numerical gain values in good qualitative agreement with experimental results are obtained.     

Transition metal-gate MOS gaseous detectors

MOS capacitors with gates of palladium, platinum, and nickel are shown to be sensitive detectors of H2, CH4, C4H10, and CO gases. 1-MHz and 10-HzC-Vcharacteristics change by - 1240 and - 215 mV when the Pd-gate MOS capacitor is exposed to H2gas at 760 and 2 × 10^-8torr, respectively. Platinum-gate MOS capacitors exhibit approximately one-half the change of Pd-gate devices. Nickel shows a response of - 120 and - 140 mV in 1 atm of H2find CO gas, respectively. When the capacitors are operated in a constant-capacitance mode by varying the bias, absorption time characteristics are obtained. Pd-gate devices absorb H2gas in 190-200 ms at 760 torr of H2but require 200 min to reach 63 percent of the total change at 2 × 10^-8torr H2pressure. Both Pt- and Ni-gate devices are slower than Pd. The detection mechanism is attributed to the change in work function upon hydrogen absorption. This is established by demonstrating no change either in the density of interface states or in the distribution across the gap or in the number of states at midgap upon absorption. Also, no accumulation of additional charges in the SiO2or Si3N4dielectric is found upon hydrogenation of devices with either Pd or Pt gates.     

Generation of extreme ultraviolet radiation at 79 nm by sum frequency mixing

High brightness tunable coherent extreme ultraviolet (XUV) radiation at 79 nm with a peak power of ∼200 mW has been generated in H2gas by sum frequency mixing of two quanta from a high spectral brightness ArF* (193 nm) source with one quantum from a tunable dye laser (∼436 nm). Spectroscopic application of this radiation has been demonstrated by observation of a broad (∼160 cm^-1) autoionizing structure in Ar and narrow (∼2 cm^-1) autoionizing features in D2. An analysis is given which identifies the dominant molecular states involved in the nonlinear susceptibility of the medium (H2). The frequency independent tuning behavior of the 79 nm output power observed over ∼300 cm^-1is related to the molecular structure and response of the nonlinear medium in the intense optical field.     

Stimulated rotational Raman scattering in the atmosphere

Theoretical and experimental information in the literature is used to calculate the gain coefficient for stimulated rotational Raman scattering by atmospheric N2and O2. The dependence on laser wavelength and polarization, as well as on the pressure and altitude is discussed. It is pointed out that because of pressure broadening, the gain coefficient is independent of altitude up to an altitude of 40-50 km, where the Raman transition becomes Doppler broadened. The 1 percent conversion threshold for vertical propagation from the ground up is calculated for various transitions and laser beam characteristics. The highest-gain transition, theS(8) transition of N2, is shown to have a 1 percent conversion intensity threshold of ∼ 1.2 MW/cm2for linearly polarized light at a wavelength of 400 nm.     

Parameter studies of pulsed HF lasers

The average output power of pulsed HF chemical lasers is found to depend strongly on the fluorine source used. The best performance was obtained from H2-C2F6-He mixes, which gave average powers of 850 mW and energies of 20 mJ/pulse. Peak gains of at least 0.036 cm^-1are observed. Average powers of 450 mW from DF and 70 mW from HCl are reported.     

A 16-µm radiation source utilizing four-wave mixing in cooled parahydrogen gas

Improvements in a 16-μm radiation source based upon the combined effects of stimulated rotational Raman scattering and resonantly enhanced four-wave mixing in parahydrogen gas are described. For this source, the input waves of which are provided by temporally and spatially coincident pulsed beams from a ruby and CO2TEA laser, it was found that cooling the parahydrogen gas from 300 K to <100 K, at constant molecular density, increased the output at 16 μm by roughly a factor 4. The maximum output was measured to besim 40 muJ/pulse, which is near the theoretical limit for the 2.5 MW CO2laser intensities that were applied.