Lateral field instability and six-wave mixing in a diode laser with broad active area

The electromagnetic field inside a nonlinear active medium of a laser is considered as a system of counterpropagating waves. Such an approach changes radically an earlier studied behavior of the lateral field instability due to self-deformaion (or self-focusing). In our calculations we used an expression for a laser field in the form of two “strong” counterpropagating waves whose complex amplitudes have weak perturbations. Amplitude perturbations of each of the “strong” waves can be presented by two spatial harmonics corresponding to two weak perturbation waves with wave vectors making some tilted angle ±φ with the cavity axis. Thus six waves would participate in the interaction: two counterpropagating strong waves and two pairs of weak waves. Using this approach, we have developed a theory for the propagation of four “weak” perturbation waves in a nonlinear amplifying medium in the presence of two counterpropagating “strong” waves. It is shown that perturbation waves with tilted angle φ⋍0.5–1.2° inside the active region, and respecively, with the side lobes of the far-field pattern at ∼1.7–4°, have the greatest growth increment. These perturbation waves produce lateral intensity modulation with period 10–30 μm for the 0.85 μm lasing wavelength. The appearance of such waves corresponds to the instability threshold of a homogeneous lateral distribution of optical power in a diode laser. The present theory makes it possible to investigate the stability of the homogeneous lateral optical intensity distribution in a diode laser of any design. This allows one to choose a suitable design of a laser with a homogeneous lateral distribution at high radiation power. Translated from Preprint No. 43 (1992) of the Lebedev Physics Institute, Russian Academy of Sciences.     

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).     

Kinetics of photon generation and propagation in a scintillation layer

Experimental results of an investigation of the kinetics of photon generation and propagation in a scintillation layer (n-terphenyl in polystyrene) are presented in this paper for the case of relativistic μ-meson passage through the layer. The duration of the photon emission Δt_em has been measured as a function of the scintillation layer thickness (l=0.05–0.5 m). The total duration of the photon emission and exit from the scintillation layer Δt_tot measured with a side coating having diffuse light reflection coefficient ρ=0.9 and 0.95 is given. A kinetic model of the photon generation and propagation in the scintillation layer is described. Satisfactory agreement of the expected values of Δt_em and Δt_tot with the exeeriment is indicated. Ivanovo State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 73–77, June, 1999.     

Modeling the cw lasing regime for diode-pumped solid state lasers

In terms of a system of balance equations, we propose a model for describing the operation of lasers based on solid-state quasi-three-level active media in the cw lasing regime with longitudinal pumping by emission from diode lasers, taking into account the spatial distribution of the pump and lasing light beams. We model the operation of lasers based on the active crystals Yb³⁺:NaLa(MoO₄)₂ and Yb³⁺:YVO₄, taking into account the proposed spatial distribution function of the intensity and the emission spectrum of the pump. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 55–60, January–February, 2007.     

Spectroscopic Properties of Er3+/Yb3+-codoped PbO–Bi2O3–Ga2O3–GeO2 Glasses

We investigate the spectroscopic properties of the 1.5-μm emission from the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions in PbO–Bi₂O₃–Ga₂O₃–GeO₂ glasses for applications in broadband fiber amplifiers. The measured emission peak locates at 1,532 nm with a full width at half-maximum of ∼45 nm. The glasses exhibit a large stimulated emission cross-section of 0.89 × 10⁻²⁰ cm² and a large product of 40.0. Infrared-to-green upconversion occurs simultaneously upon excitation of the 1.5-μm emission with a commercially available 980 nm laser diode. The green-upconversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. Energy transfer processes and nonradiative phonon-assisted decays could account for the population of the ²H_11/2 of Er³⁺. The results indicate the possibility towards the development of lead–bismuth–gallate–germanate based glasses as photonics devices.     

Synthesis and study of IR luminescence of Ln<Subscript>2</Subscript>O<Subscript>2</Subscript>S:Yb (Ln = Y,La, Gd) under laser excitation

The ranges of the existence of solid solutions in binary systems Ln₂O₂S-Yb₂O₂S(Ln = Y, La, Gd) were found and main features of variations of Stokes IR luminescence intensity of Ln₂O₂S·Yb in the range of 0.96–1.1 μm were determined depending on the composition during IR excitation. Based on the established features, a new Y₂O₂S:Yb monospectral IR phosphor was developed, which, upon excitation by a 0.940-μm laser, is characterized by an increased Stokes IR luminescence intensity in the range of 0.96–1.1 μm in comparison with that of known commercial phosphor.     

Phase perturbation investigation of laser diode for the laser diode to single-mode fiber coupling system

A novel phase-perturbation method as well as an experimental system were established to study the deviation of the laser diode (LD) far-field intensity distribution from the ideal Gaussian. An analytical expression of the LD far-field intensity distribution and a numerical method based on the Monte Carlo calculation were developed to fit the measured results. The loss of the LD to the single-mode fiber (SMF) coupling system caused by this kind of deviation of the LD far-field intensity distribution was quantitatively analyzed.     

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.     

Boltzmann spectral distribution or “infrared catastrophe” in the resonance radiation of a gas

The purely thermal infrared emission spectra of a resonance medium (sodium vapor) are investigated experimentally. It is shown that the emission intensity in the 2–3 μm range at temperatures of 600–1200 K is several orders of magnitude higher than the intensity obtained from the standard theory of resonance radiation transfer. This phenomenon can be conventionally termed an “infrared catastrophe.” The form of the recorded spectra and the absolute intensity of the emission in both the infrared and visible regions of the spectrum are in agreement with the theory developed by Yu. L. Zemtsov and A. M. Starostin, Zh. éksp. Teor. Fiz. 103, 345 (1993) [JETP 76, 186 (1993)], in which the Boltzmann spectral distribution of the population of the resonance level is proportional to exp(−ħω/T). Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 807–811 (10 June 1997)     

Determination of microphysical parameters of erythrocytes in human blood from laser radiation scattering

We have studied the sensitivity of the correlation between the unidirectional scattering coefficients β(θ) and the microphysical parameters of erythrocytes in human blood the number concentration (N) and the volume concentration (C_V), the modal radius (r), and the half-width (Δr) of the size distribution function of the erythrocytes, the real part of their refractive indices n) to variations of the latter in the ranges of their possible values at wavelengths λ = 0.3–1.2 μm. We have obtained the multiple regression equations for the microphysical parameters and β(θ) in the transparency window of blood (λ = 0.65 μm). We have shown that the procedure for determining the microphysical parameters, based on these equations, allows us to rapidly determine the microphysical parameters rapidly and highly accurately within their possible ranges. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 652–658, September–October, 2007.     

Structure of the free exciton luminescence band of heteroepitaxial ZnSe/GaAs layers

The exciton reflectance and photoluminescence spectra of epitaxial ZnSe/GaAs layers with a thickness of 2–4 μm are investigated in the temperature range 10–120 K. It is shown that one of the causes of the formation of the doublet structure of the A _n=1 photoluminescence band is interference of the exciton radiation at the boundaries of the near-surface dead layer. Fiz. Tverd. Tela (St. Petersburg) 40, 881–883 (May 1998)     

Twisted lines and small-scale structures in generation spectra of Yb-doped media excited by focused radiation of LiF:F 2 + color center laser

Spectral lines of Yb lasing in 1.03–1.05 μm region structured by 50–200 μm spots were found at focusing a pulsed LiF:F₂⁺ color center laser of 0.5–5.0 GW/cm² intensity on highly doped Yb:YAG or Yb:glass plates in a resonator. Small spots at the spectrograph located ≈ 1 m apart from the resonator indicated a “sub-diffraction” directivity of Yb generation, 1–2 orders better than the diffraction limit 10⁻³–10⁻² rad determined by the pumped volume dimensions. Observed features of Yb emission are explained assuming off-axis oscillations in Yb laser on phase-synchronized photons due to a strong spatial-angular selection of radiation in the resonator. Propagation of near diffraction free beams at angles to the axis built at the spectrograph slit for every 10–15 ns pulse of Yb generation a magnified “image” of a structure of generating channels in the active medium. This image projection brought a corresponding structure of spots in Yb spectra. It was found that channels may be formed due to a high-frequency spatial modulation (micrometers scale) of the refractive index profile in samples caused by the oscillating amplitude of thermoelastic stresses in the pumped medium. Obtained data demonstrate a possibility to study (with high spatial and temporal resolution) non-equilibrium stales of materials in small volumes using laser radiation emerging from these objects. This study results evidence for the novel concept of the spatial distribution of electromagnetic field of a photon: not in the form of a “travelling” wave but in the form of a wave with maxima and nodes located at fixed positions along the photon propagation direction.     

Low-Noise single-frequency He-Ne laser with stable output power at a wavelength of 1.52 µm

The results of investigation of the energy and spectral characteristics of a He-Ne laser (λ_rad=1.52 μm) with transverse microwave discharge are presented. A single-frequency generation mode at a pressure above 6.0 mm Hg was obtained with radiation power 7.0 mW and low level of amplitude noise (10⁻⁵ Hz^−1/2). Active stabilization of the laser power was carried out, which allows reduction in radiation intensity fluctuations from 1–5 to 0.1 %. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 127–128, January–February, 2000.     

Spectroscopy of GaAs/AlGaAs microstructures with submicron spatial resolution using a near-field scanning optical microscope

A near-field scanning optical microscope (NSOM), which we built, is used to investigate 1–5-μm wide stripes with a 10-nm thick layer—a quantum well — on a GaAs surface. A map of the photoluminescence intensity is obtained synchronously with the topographic profile of the structures. The measured spatial distribution of the photoluminescence intensity is described satisfactorily in a model that takes into account carrier diffusion in the layer and the existence of a region with a short carrier lifetime near the side boundaries of the layer. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 523–527 (10 April 1996)     

Stimulated Raman conversion of radiation from a YAG:Nd laser with wavelengths of 1.319, 1.338, and 1.357 µm in a barium nitrate crystal

We have studied stimulated Raman (SRS) conversion to the first Stokes component of multimode (M₂ = 8) radiation from a YAG:Nd laser with lasing wavelengths of 1.319 μm, 1.338 μm, and 1.357 μm in a barium nitrate crystal. We have obtained pulses of converted radiation with energies up to 120 mJ. We have achieved conversion efficiencies greater than 40% for each of the three Raman laser wavelengths of 1.530 μm, 1.556 μm, and 1.582 μm with divergence of the beams of converted radiation close to the diffraction-limited value (M₂ < 1.5). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 330–334, May–June, 2006.     

Variations of the multiple scattering contribution to the transmitted acoustic radiation intensity

A problem of acoustic radiation propagation through a 500-m layer of the plane-stratified turbulent atmosphere is solved by the Monte Carlo method. Quantitative estimates of variations of the multiple scattering contribution to the transmitted radiation intensity are presented. Calculations were performed for mid-latitude regional models of the atmosphere in winter and summer in clear and cloudy days for a point-sized omnidirectional source placed at altitudes H_s = 5–35 m above the Earth’s surface. Results of calculations for sound frequencies in range F = 500–4000 Hz with a step of 500 Hz and for outer scale of turbulence L₀ = 1–5 m with a step of 1 m and 10–100 m with a step of 10 m demonstrate significant regional and seasonal variability of the multiple scattering contribution. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 86–90, October, 2007.     

Comparison of selected optical properties of oxyfluoride glass fibers doped with Er3+ and co-doped with Er3+ Yb3+

The method of manufacturing and spectroscopic evaluation of the Er³⁺ ions doped and Er³⁺–Yb³⁺co-doped SiO₂–Al₂O₃–Na₂CO₃–CaO–PbO–PbF₂ oxyfluoride glass fibers is presented in the paper. Both optically active elements erbium and ytterbium were introduced into the batch in the form of fluorides. The X-ray diffraction (XRD) technique was applied at each stage of fibers manufacturing in order to control an amorphous structure of the preforms and fibers. Optical studies of glass preforms and fibers (reflection/transmission, absorption, emission, and excited state absorption (ESA)) were directed to examine their suitability as fiber amplifiers at 1.55 μm band.     

Stress-Induced Alteration of Chlorophyll Fluorescence Polarization and Spectrum in Leaves of <Emphasis Type="Italic">Alocasia macrorrhiza</Emphasis> L. Schott

The value of intrinsic chlorophyll fluorescence polarization, and the intensity in emission spectrum were investigated in leaf segments of Alocasia macrorrhiza under several stress conditions including different temperatures (25–50°C), various concentrations of NaCl (0–250 mM), methyl viologen (MV, 0–25 μM), SDS (0–1.0%) and NaHSO₃ (0–80 μM). Fluorescence emission spectrum of leaves at wavelength regions of 500–800 nm was monitored by excitation at 436 nm. The value of fluorescence polarization (P value), as result of energy transfer and mutual orientation between chlorophyll molecules, was determined by excitation at 436 nm and emission at 685 nm. The results showed that elevated temperature and concentrations of salt (NaCl), photooxidant (MV), surfactant (SDS) and simulated SO₂ (NaHSO₃) treatments all induced a reduction of fluorescence polarization to various degrees. However, alteration of the fluorescence spectrum and emission intensity of F₆₈₅ and F₇₃₁ depended on the individual treatment. Increase in temperature and concentration of NaHSO₃ enhanced fluorescence intensity mainly at F₆₈₅, while an increase in MV concentration led to a decrease at both F₆₈₅ and F₇₃₁. On the contrary, NaCl and SDS did not cause remarkable change in fluorescence spectrum. Among different treatments, the negative correlation between polarization and fluorescence intensity was found with NaHSO₃ treatments only. We concluded that P value being measured with intrinsic chlorophyll fluorescence as probe in leaves is a susceptible indicator responding to changes in environmental conditions. The alteration of P value and fluorescence intensity might not always be shown a functional relation pattern. The possible reasons of differed response to various treatments were discussed.     

Phase composition and defect substructure of nickel alloyed with boron and copper by electric explosion of conductors

Using methods of electron microscopy, we have discovered that combined electroexplosive alloying of nickel with boron and copper gives rise to a multilayer structure. An outer amorphous crystalline layer incorporating 2–3 nm nickel-and copper-boride, oxide, and boride-oxide crystallites is formed on the alloyed surface. The intermediate (subsurface) layer 1–2 μm thick is made up of NiB ₁₂ and Ni ₄ B ₃ crystallites 120–130 nm in size, with boron-and copper-oxide particles observed along the grain boundaries. The underlying thick layer exhibits a cellular Ni-Cu-B melt crystallization structure grading initially into a high-rate dendritic crystallization structure and then into a granular structure. The electroexplosive alloying process is found to bring into existence a high scalar dislocation-density substructure in nickel crystallites both in the alloyed zone and in the adjacent heat-affected zone. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–7, March 2007.     

Radiation scattering by finite dielectric cyclinders

The volumetric integral equation formalism (VIEF) is used to determine characteristics of scattering of radiation falling along the axis of dielectric cylinders that scatter the radiation. The spectral dependence of the extinction efficiency factor Q is studied with changes in the length l of the cylinder (300–2200 nm), its width d (100–300 nm), and the refraction index m (1.33–1.65). In the range of angles θ=0–180°, for a cylinder with l=700 nm, d=100 nm, and m=1.33, angular intensity distribution functions i_i and i₂ are calculated for the components of the scattered radiation that are polarized perpendicular and parallel to the plane of observation, respectively. No effect of scattered-radiation depolarization is found. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 256–260, March–April, 1998.