Tea CO2 laser beam absorption in polyethylene

The absorption coefficient of polyethylene sheets has been measured for two wavelengths (0.6328 μm, He-Ne laser; 10.6 μm, CO₂ laser). For TEA CO₂ laser power densities up to 10⁴ W cm⁻², the absorption coefficient was found to be 35.36 cm⁻¹. The measured absorption coefficient shows that polyethylene sheets are good and simple attenuators for CO₂ laser radiation.     

Trace-gas sensor based on mid-IR difference-frequency generation in PPLN with saturated output power

The development and characterization of a compact pulsed mid-IR laser source for sensitive on-line trace-gas analyses in the 3–4 μm wavelength range is reported. The source is based on an advantageous difference-frequency mixing configuration in periodically poled LiNbO₃ (PPLN) with a cw external-cavity diode laser (ECDL; 810–830 nm) for broad and accurate tunability and a diode-pumped passively Q-switched Nd:YAG laser (1064 nm) for high mid-IR peak power. With 5 mW cw pump and 4.7 kW signal peak power incident on a 19 mm long PPLN crystal, a maximum of 360 μW mid-IR peak power was generated. The narrowband (∼150 MHz) radiation was saturated by a factor of 15 compared with the nonsaturated case due to depletion of the pump laser radiation. This results in a very high amplitude stability of the generated mid-IR power and thus in a high detection sensitivity. A minimum detectable absorption coefficient of 2.8×10⁻⁸ cm⁻¹ was achieved in combination with a 36.2 m multipass cell in an averaging time of 20 s, as demonstrated by on-line analyses of formaldehyde traces near 3.53 μm.     

Interaction of the radiation of the high-power ytterbium-fiber laser with inhomogeneous dielectric targets

The action of the radiation of the ytterbium-fiber laser (λ = 1.07 μm) on the Nd³⁺Y₂O₃ target with nonuniform transparency in the course of the nanopowder production is studied. It is demonstrated that the laser irradiation leads to an extremely rough surface with the stalagmite roughness due to a relatively large melting depth. The resulting powder consists of two fractions. The first fraction (99% of the total mass of the powder) consists of nanoparticles with a mean size of 29 nm (BET data). The second fraction consists of micro- and submicroparticles that represent circular drops condensed from the melt and shapeless debris of the target. The peaks on the diameter distribution of the drops at 2, 8, and 80 μm are determined by different effects. The laser heating of the inhomogeneous target with the nonlinear refractive index is numerically analyzed. It is demonstrated that the melting of the target is initiated at a mean laser power of 700 W, a power density of 5.6 × 10⁵ W/cm², and an irradiation time of 150 μs.     

Optical properties of sapphire in its opacity range

We have studied the reflection spectrum of leucosapphire in the range of 2.5–25.0 μm. Based on the analysis of the known frequency and temperature dependences of the absorption coefficient in the range of 2.5–7.0 μm, its extrapolation to the range of 10.6 μm has been performed. We have shown that the absorption coefficient in the range of 10.6 μm and in the temperature interval 300–2300 K can be evaluated as β = (2–3) × 10⁴ cm^–1. It has also been shown that the value of this characteristic correlates with the results of investigations of the action of a powerful radiation pulse of a СО₂ laser on the surface of anodized aluminum. These data can be used in the development of technologies of laser processing of articles made of sapphire and ceramics based on aluminum oxide, as well as anodized articles made of aluminum alloys.     

Particular features of the transmission of laser radiation with wavelength 0.438 µm and intensity (3–7)·1014 W/cm2 through an undercritical plasma from polymer aerogels

The velocities of energy transport in an undercritical plasma of polymer aerogel with and without copper nanoparticles were measured. Transmission of the laser light through targets of different thicknesses such as submicron three-dimensional polymer networks with densities below the critical value (0.13–0.52 N _cr) for a wavelength of 0.438 μm and intensity of (3–7)·10¹⁴ W/cm² at a half-height pulse duration of 0.32 ns was studied. The transfer of a heating laser radiation was registered on the rear side of the target. It ranged from a level of ∼0.5% for the thickness of a low-density layer of 400 μm and density of 9 mg/cm³ (mass per unit square of 0.36 mg/cm²) up to 50–60% for a thickness of 100 μm and density of 2.25 mg/cm³ (mass per unit square of 0.02 mg/cm²). The time dependences of the optical emission from the rear side of the targets were measured. They appear to be indicative of the plasma dynamics in two-layer targets (polymer foam on Al foil) and enable the estimation of the absorption depth for the laser light in an undercritical plasma. __________ Translated from Preprint No. 8 of the P. N. Lebedev Physical Institute, Moscow (2007).     

Decay and recondensation of electron-hole liquid in germanium under CO2 laser pulse irradiation: Investigation by microwave conductivity measurements

In microwave conductivity investigations of photoexcited germanium at low temperatures under CO₂ laser pulse irradiation the evaporation of EHL and e-h plasma formation have been observed. This plasma irreversibly vanishes at high CO₂ laser intensities I_CO2 >4 × 10⁵ W cm^?2 but recondenses at low intensities. It was found that complete and irreversible disappearance of EHL is due to the e-h plasma throw out to the crystal boundaries by phonon wind, generated in 10.6 μm radiation absorption whereas at I_CO2 > 10⁶ W cm^?2 it is connected with the crystal lattice heating over the condensation critical temperature. A theoretical analysis of the CO₂ laser produced phonon wind interaction with e-h plasma is briefly presented. By comparing with experimental data on recondensation process the phonon wind efficiency is estimated.     

Application of a characterized difference-frequency laser source to carbon monoxide trace detection

A tunable continuous wave(cw) mid-infrared(MIR) laser based on difference-frequency generation(DFG) in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported.Two visible and near-infrared diode lasers were used as pump and signal sources.The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm.The phase-matching(PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths.The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U,et al.(Willer U,Blanke T and Schade W 2001 Appl.Opt.40 5439).Then the calculated PM temperatures are compared with the experimental values.The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide(12C16O) absorption line in a laboratory-fabricated absorption cell.The enhanced sensitivity of about 0.6×10 4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.     

Growth and spectral-luminescent study of SrMoO<Subscript>4</Subscript> crystals doped with Tm<Superscript>3+</Superscript> ions

SrMoO₄ crystals doped with Tm³⁺ ions have been produced from a melt using the Czochralski method; their spectral-luminescent characteristics have been studied, and laser radiation has been generated at the wavelength of 1.94 μm using laser-diode excitation. The high absorption section at the wavelength of 795 nm, the fairly high luminescence section, the long lifetime at the upper laser level ³F4 of 1.5 ms, and a wide luminescence band allow one to hope for developing efficient tunable Tm³⁺: SrMoO₄ crystal lasers with diode pumping in the range of 1.7–2.0 μm, which are capable of implementing SRS self-transformation of radiation into the middle IR band.     

Nonlinear transmittance of ZnSe:Fe<Superscript>2+</Superscript> crystal at a wavelength of 2.92 μm

ZnSe crystals doped with Fe²⁺ ions are produced with the diffusion method under the conditions for thermodynamic equilibrium of solid ZnSe, solid Fe, and vapors (S_ZnSe-S_Fe-V). The transmittance of the samples is varied from 7 to 50% (in the absence of the antireflection coating) for a wavelength of about 3 μm. It is demonstrated that the transmittance of the ZnSe:Fe²⁺ crystals increases with an increase in the energy density of the high-power laser radiation with a wavelength of 2.92 μm. An equation that describes the propagation of the resonant radiation in a medium with saturable absorption at an arbitrary ratio of the radiation pulse duration to the relaxation time of the medium is introduced for the analysis of the experimental dependence of the transmittance on the energy density.     

The temperature-dependent absorption spectrum of the v3 band of SF6 at 10·6 μm

Mixtures of SF₆ diluted by argon were heated behind an incident shock front in the range 400 to 1500K. Absorption measurements at 11 wavelengths near 10·6 μm were made on the post-shock, equilibrated gas using a CO₂ laser. A band contour model, which makes use of known spectroscopic constantd and the experimental results, satisfactorily accounts for the observed temperature dependence of the absorption coefficient. We are able to estimate the identity of the transitions responsible for absorption of the laser radiation.     

Subnanosecond optical switching of far infrared radiation

Switching of 119 μm CH₃OH laser radiation in less than 1 ns was achieved by irradiating high resistivity Si (>10⁴ Ω cm) and GaAs (>10⁷ Ω cm) samples with 1.4 ns, 150 μJ pulses from a 337 nm TEA nitrogen laser. Modulation degrees of up to 100% were observed both in reflection and transmission.     

Nonlinear absorption of non-chain HF laser radiation in germanium

The transmission of high-power HF(DF) laser radiation through Ge single crystals of different quality has been experimentally and numerically investigated. Based on the experimental data for the lasing spectrum, the two-photon absorption coefficient in Ge is estimated to be K₂ =55±10 cmGW?1 at λ=2.8μm. The transmission dynamics of high-power radiation with λ=2.6?3 μm through germanium crystals is numerically simulated.     

Growth of high quality non-linear optical crystal zinc germanium phosphide for Mid-infrared optical parametric oscillator

ZnGeP₂ single crystals were grown from Vertical Bridgman method. High-quality near-stoichiometric ZnGeP₂ single crystals were obtained in the diameter of 30 mm and length of 120 mm. The results showed that after thermal annealing of the crystals the optical absorption coefficient was below 0.03 cm^?1 at 2.05 μm, and ～0.02 cm^?1 at 3–8 μm. The low absorption loss ZnGeP₂ samples with dimension of 6 × 6 × 18 mm³ were cut from the annealed ingots for 3–5 μm optical parametric oscillation (OPO) experiments. For OPO experiment, we obtained up to 8.7 W output in the 3–5 μm wavelength range (with signal of 3.80 μm and idler of 4.45 μm, respectively) pumped by a 16.3 W 2.05 μm Tm,Ho:GdVO₄ laser at pulse repetition rate of 10 kHz, which corresponded to a conversion efficiency of 53.4% and slope efficiency of 64.8%, respectively.     

Tunable infrared generation in KTA and applications

Noncollinear difference frequency mixing of dye laser and Nd:YAG second harmonic (fundamental) radiation from a commercial laser system is employed for the generation of 2.7–5.3 μm (1.6–1.7 μm) radiations in a flux-grown KTiOAsO₁ crystal. The generated radiation is used to scan the methane absorption in the fundamental (v ₃) and its first overtone (2v ₃) band at pressure 90 torr in a laboratory made single pass gas cell of length 33 cm.     

<Emphasis Type="Italic">K</Emphasis><Subscript>α</Subscript> radiation of foil under interaction with laser pulse of relativistic intensity

An analytical model of K _α radiation of thin laser targets has been developed. It has been shown that, for such targets, the motion of fast electrons is significant not only in the target itself but also in vacuum. The considered dependences for the free path length of a fast electron and for the absorption coefficient of laser radiation on the laser intensity with allowance for the electron motion in vacuum make it possible to match the results of the proposed model with the experimental data on generation of K _α radiation in wide ranges of laser intensities (10¹⁸–10²¹ W/cm²) and thicknesses (1–100 μm) of targets.     

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO₂ and Ar clusters with high-intensity femtosecond laser pulses with q _las≃10¹⁸ W/cm² are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters. Zh. éksp. Teor. Fiz. 115, 2051–2066 (June 1999)     

Dual-beam laser analyzer for inhomogeneities in active regions of multielement photodetectors

We describe equipment and a procedure for measuring inhomogeneities in active regions of solar cells, photoelectric transducers, and multielement photodetectors, based on a computer-controlled dual-beam laser scanner including a Pentium III personal computer, an HP-34401A digital multimeter with RS-232 serial interface;, a platform movable along the Y coordinate with the sample multielement photodetector to be tested and a microprobe device for picking up the photocurrents, an optical head with laser photodiodes in the visible (λ₁ = 0.68 μm) and IR (λ₂ = 0.82 μm) ranges scanning along the X coordinate, and a control unit for focusing the laser beams λ₁, λ₂, stabilizing the laser radiation power, and controlling the step motors for the X,Y coordinates. The equipment and procedure enable laser scanning of sample multielement photodetectors, with external dimensions 10 × 10 μm2 to 150 × 150 mm², along the X, Y coordinates at a maximum rate of 100 mm/sec; minimum radiation power of the laser diodes, 10 mW; range of laser beam diameters, 2–50 μm; range of scanning steps along the X, Y coordinates, 5–100 μm; current sensitivity 1·10⁻⁸ A, voltage sensitivity 1·10⁻⁷ V; measurement and analysis accuracy at least 0.5%; storage of color plots of the inhomogeneities in the active regions of the multielement photodetectors on the hard disk of the personal computer, with output of analysis results to a monitor and color printer, 32 color gradations. Software written in Delphi 7.0. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 277–280, March–April, 2006.     

Optical properties of a new IR converter for laser beam analysis

This work presents the temporal and spacial resolution of a new infrared (IR) converter based on thermal radiation emission. Using this converter, it is possible to measure the intensity distribution of laser beams with a wavelength between 1 and 20 μm. For this purpose, the laser radiation (for example, 10.6 μm) is converted into a wavelength coverage of 800–1100 nm. In the actual converter thin metal foils provide the basis of this method. The metal foils are heated to a temperature of 600–800 K. The emitted radiation of the foils defers into the near-infrared (NIR) area, thus enabling detection by camera systems based on silicate. Additional heat input of the laser results in a local temperature increase, and then the increase in radiation intensity can be measured. Typical thicknesses of converter metal foils are <5μm. Foil materials with a low thermal conductivity, good absorption of the measured laser beams, and a high melting temperature are particularly suitable. These parameters are well shown by using stainless steels, such as INOX (stainless steel 1.4310 CrNi steel). Using this material, it is possible to gain a maximum spatial resolution of 250 μm and a temporal resolution of 12.5 Hz, by a measurement range from 1 to 100 W/cm². The maximum measured intensity is 125 W/cm².     

Dual excimer and CO2 laser etching studies of polyethylene terephthalate

Polyethylene terephthalate (PET) films preheated with a pulsed CO₂ laser have been ablatively etched with an XeCl laser. The observed reduction in ablation threshold, from 170 to 140 mJ cm^–2, is consistent with a thermal mechanism for XeCl laser ablation of PET. Transient changes in the UV absorption coefficient of PET caused by heating with pulsed CO₂ laser radiation have also been studied and a significant increase in absorption observed at 308 nm. Permanent changes in the ultraviolet absorption of PET following exposure to low fluence XeCl laser radiation are also reported.