Tuning characteristics of asymmetric inhomogeneously pumped quantum-well lasers

An analysis of the power-current and tuning characteristics of asymmetric multiple-layer inhomogeneously pumped quantum-well lasers with mode selection in the external resonator was performed. It is shown that the choice of the parameters of the barrier and active layers of the laser heterostructure and optimization of the dispersion characteristics of the selective element of the external resonator make it possible to realize continuous tuning of the lasing wavelength in a wide spectral range at a practically constant output radiation power. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 687–692, September–October, 1999.     

Picosecond Nd:BaY2F8 laser discretely tunable around 1 μm

Passive mode-locking of a diode-pumped Nd:BaY₂F₈ (Nd:BaYF) was achieved on four lines in the range 1040–1074 nm, employing a semiconductor saturable absorber mirror (SAM). Nearly Fourier-limited pulses with durations of 2.6 to 7.2 ps and output power ≈50 mW were generated in a dispersion-controlled resonator using a single prism for wavelength selection, tuning and dispersion management.     

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.     

High resolution ethylene absorption spectrum between 6035 and 6210 cm-1

A two-channel photo-acoustic spectrometer (PA spectrometer) with a near infrared diode laser was used for taking measurements of a high resolution ethylene absorption spectrum. A semiconductor TEC-100 laser with an outer resonator generates a continuous single-frequency radiation in the range 6030–6300 cm^-1. A newly designed model of photo-acoustic detector (PAD) in the form of a ring type resonator provides for measurement of weak absorption cross-section equal to 4×10^-23 cm²/mol at a laser radiation power of 3 mW. The PAD threshold sensitivity is 2×10^-9 cm^-1 Hz^-1/2 W, when the signal to noise ratio equals to 1. The ethylene absorption spectrum within the range 6035–6210 cm^-1 was measured for the first time with a spectral resolution of 10 MHz. The reported line centre positions have an uncertainty of ± 0.0005 cm^-1. The precise measurements of ethylene absorption cross-sections were carried out using the mixture of high purity ethylene and broadening gas (nitrogen) at the mixture ratio 1:50–1:200. Measurements were carried out at a mixture pressure of about 4.2 kPa. PACS 42.62.Fi; 42.55.Px     

Theoretical investigation of chirped mirrors in semiconductor lasers

This paper reports a theoretical design of chirped mirrors in 1.3-μm double-section semiconductor lasers to achieve high reflectivity and dispersion compensation over a broad bandwidth. Analytic expressions for reflectivity, group delay and group delay dispersion are derived. We use for the first time chirped air/semiconductor layer pairs as mirrors for higher-order dispersion compensation in semiconductor lasers. Our optimised calculations demonstrate that the broad-band mirrors designed consist of a total of only 12 air/semiconductor layers and achieve a reflectivity higher than 99.8%, a smooth group delay and almost stable dispersion in the laser cavity over a 100-nm bandwidth. Due to a high index contrast of both types of the layers, n _l = 1, n _h~ 3.5, a high-reflectivity bandwidth of > 700 nm is obtained in 1.3-μm semiconductor lasers. We also compare our results with that of a commercial simulation program and show a good agreement between them. As a conclusion, we assume from the theoretical results that air/semiconductor layer pairs with varying thicknesses used at one end of double-section semiconductor lasers can lead to femtosecond optical pulse generation using mode-locking techniques. An erratum to this article can be found at .     

Electron-beam-excited lasers based on A2B6 compounds (review)

The literature on electron-beam-pumped semiconductor lasers (EPSLs) based on A²B⁶ compounds is reviewed. Recent advances in the development of EPSLs in the Nineties are reflected. The literature data on the output parameters of these EPSLs are systematized. Basic parameters and characteristics of EPSLs their hardware realization, the properties of active media, and the mechanisms of amlification and degradation are discussed. The works on the use of EPSLs for pumping of other active media are reviewed. Institute of Semiconductor Engineering RWTH, 55, Templergraben, Aachen, D-52056, Germany (permanent address: B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 601–618, September–October, 1999     

Luminescence and lasing properties of neodymium- and uranium-activated inorganic laser liquids

An attempt was made to describe and show the possibilities of new inorganic neodynium- and uranium-activated laser liquids: SO₂-GaCl₃-NdCl₄; SO₂Cl₂-GaCl₃-NdCl₃-UO₂Cl₂; POCl₂-MCl_n-NdCl₃-UO₂Cl₂ for development and synthesis of direct nuclear reaction-excited lasers. Luminescence data presented in the work were used to calculate the luminescence parameters of the laser liquids such as oscillator strengths f, probability of spontaneous radiation A, intermultiplet luminescence branching coefficient β, cross-section for induced radiation σ, luminescence decay time τ, quantum yield η, and others. It is shown that the oscillator strengths of the normal absorption bands of Na³⁺, which play the main part in the pumping processes, exceed the oscillator strengths of Na³⁺ for aqueous and many other nonaqueous systems. In the luminescence excitation spectra of the Na³⁺ ion, bands are isolated in the range 400–1000 nm atλ _rec =1.06 μm. With excitation, luminescence occurs through the⁴F_3/2→⁴I_{9/2,11/2.13/2} channels. Luminescence spectral data are related to the lasing parameters. The threshold lasing energy is∼18 J/cm³. For a resonator with mirros h₁=100% and h₂=20, 40, 56, and 80%, the lasing energy is∼20–120 MJ/cm³ in the pumping energy range 18–180 J/cm³. The differential efficiency is ∼0.2% The substantial angular radiation divergence (θ∼4·10⁻² rad) and strong thermostatic distortions that occur in the active element (dn/dT≈−1.9·10⁻⁴K⁻¹) are a disadvantage of laser liquids. It is shown that operation of neodymium- and uranium-activated inorganic liquid lasers is stable under the present conditions. A. I. Gertsen Russian State Pedagogical University, Moika Embankment, 48, St. Petersburg, 191186, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 607–619, September–October, 1997.     

Linewidth studies on Cd+ 636.0 nm and 537.8 nm lines excited in a He-Cd hollow cathode discharge

Hollow cathode (HC) lasers usually operate in a single axial mode without any optical selection. This is attributed to the large homogeneous linewidth of the gain curve due to the relatively high filling pressure of these lasers. Collisional and Doppler broadening (Δν_C and Δν_D) of the Cd⁺ 636 nm and Cd⁺ 538 nm lines (laser transitions of the HeCd⁺ laser) excited in a HC discharge tube were determined using a Fabry–Perot interferometer technique. It was found that in the pressure range 7–25 mbar Δν_D was nearly constant, while, as expected, Δν_C increased linearly with pressure. The broadening constants were α(636 nm)= (47±2) MHz/mbarand α(538 nm)=(11.8±0.5) MHz/mbar. The first constant is large enough to explain single-mode operation of the red HeCd⁺ laser; but in the case of the green laser, the exact reason for the single-mode operation remained unclear. Received: 23 November 2000 / Revised version: 30 March 2001 / Published online: 7 June 2001     

Electronic structure of linear chains of fullerenes

The band structure of linear chains of fullerene molecules is calculated as a function of the intermolecular π-electron overlap integral T, which increases under increasing external pressure. Chains consisting of neutral (C₆₀) and charged (C ₆₀ ⁻ ) molecules are studied. It is shown that there is a sharp transition from a metal (or narrow-gap semiconductor) to an insulator (with band gap ∼1 eV)with increasing T. The proposed model makes it possible to describe the formation of solid-carbon struc-tures, containing chains of covalently bound fullerene molecules, with different pressure-dependent semiconductor properties. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 647–650 (25 April 1997)     

Structural, optical and electrical properties of cubic AlN films deposited by laser molecular beam epitaxy

Cubic AlN films were successfully deposited on TiN buffered Si (100) substrates by a laser molecular beam epitaxy (LMBE) technique, and their crystal structure and optical and electrical properties were studied. The results indicate that cubic AlN films show the NaCl-type structure with a (200) preferred orientation, and the lattice parameter is determined to be 0.4027 nm. The Fourier transform infrared (FTIR) pattern of the cubic AlN film displays sharp absorption peaks at 668 cm⁻¹ and 951 cm⁻¹, corresponding to the transverse and longitudinal optical vibration modes. Ellipsometric measurements evidence a refractive index of 1.66–1.71 and an extinction coefficient of about zero for the cubic AlN film in the visible range. Capacitance–voltage (C–V) traces of the metal–insulator–semiconductor (MIS) device exhibit that the cubic AlN film has a dielectric constant of 8.1, and hysteresis in the C–V traces indicates a significant number of charge traps in the film.     

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.     

Five Simultaneously Q-Switch Mode-Locked Passive Laser Modulators

Five types of passive Q-switched as well as simultaneously Q-switch mode-locked modulators: plastic dye sheets (Kodak 9850 cellulose acetate dye sheets), lithium fluoride crystals containing F₂ ⁻ color centers (LiF:F₂ ⁻), chromium-doped yttrium–aluminum–garnet crystals (Cr⁴⁺:YAG), ionic color filter glass (Schott RG1000 color filter glass), and the single crystal semiconductor wafers (GaAs, Fe-doped InP, Zn-doped InP, S-doped InP, etc.) used for modulation of the Nd:hosted(Nd:YAG, Nd:YVO₄, and Nd:LSB) lasers were investigated in detail in our research. We also investigated applications of the Q-switch mode-locked pulse train for the development of a higher resolution solid-state laser range finder.     

Electron beamexcitation of wide-aperture XeCl lasers

This paper presents the results of studies aimed at seeking efficient modes of electron beam excitation ofXeCl lasers with an active volume of some tens of liters. It has been shown that a lasing efficiency of about 4% is realized at an excitation pulse duration of 300 ns with a power of 200–400 kW/cm³. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 29–33, April, 2000.     

Dispersion of the Voigt effect in the magnetic semiconductors Cd1−x MnxTe

Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd_1−x Mn _x Te (0≤x≤0.52) in order to investigate how the exchange interaction of Mn²⁺ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x ² is independent of x and the magnetic field direction, i.e., the effect is due to the Mn²⁺ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(E _g −ℏω) ^−3.5. This can be explained by a decrease of the exchange interaction of Mn²⁺ ions with itinerant electrons with increasing distance from the center of the Brillouin zone. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 569–573 (25 April 1998)     

Spectroscopy of F 3 + -color centers in lithium fluoride crystals (review)

Results of investigations of radiative F ₃ ⁺ -color centers in lithium fluoride obtained by the methods of one- and two-photon absorption, polarization spectroscopy, and luminescence in singlet, triplet and triplet-singlet channels are reported. The scheme of energy levels of the F ₃ ⁺ -center is revised. The probabilities of singlet-triplet conversion and the rates of depletion of the lower triplet state in the range of 80–350 K are presented. Recommendations are given on the technology of radiative LiF coloration that provide an increase in the F ₃ ⁺ -center concentration and a decrease in the concentration of some other centers that prevent lasing on the LiF:F ₃ ⁺ active medium from being obtained. The phototransformations of color centers in LiF are analyzed. The parameters of lasers with LiF:F ₃ ⁺ active media that provide generation of radiation tunable in the green region of the spectrum are considered. Based on spectroscopic data the maximum attainable time characteristics of such lasers are discussed. The prospects for further investigations of LiF:F ₃ ⁺ lasers are outlined in brief. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 745–760, September–October, 1998.     

Active frequency stabilization of an ultra-high Q whispering-gallery-mode microresonator

We report on active frequency stabilization of a whispering-gallery-mode microresonator with an ultra-high quality factor exceeding 2×10⁸ to an atomic transition using the Pound–Drever–Hall technique. The critically coupled resonator is stabilized to 5.4% of its linewidth, or 206 kHz. Thermal bistability and mode-splitting are also observed and have a significant effect on the error signal.     

A diagrammatic method for analysis of GaAs and InP based optoelectronic devices

A modified version of the signal flow graphs method can be applied to reveal the topological structure of physical models describing the operation of optoelectronic devices based on heterostructures comprising A^IIIB^V semiconductor compounds. In particular, this kind of analysis is apt to reveal the presence of closed paths (feedback loops) in the causal make-up of the phenomena underlying function of the devices. The analytical apparatus associated with the diagrams affords a new formulation of criteria for the occurrence of such physical conditions as the bistability or the threshold behaviour. The approach is illustrated on the instances of injection semiconductor laser, nonlinear Fabry-Perot resonator, self-electro-optic effect device and semiconductor laser optical amplifier. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998 Support of the Grant Agency of the Czech Republic (project No. 102/99/0341) is gratefully acknowledged.     

Strong Kerr-signals from optically isotropic ZnSe nanocrystals: a study using Mach-Zehnder principles

In the present work, we show that Mach-Zehnder interferometry (MZI) can be an asset to explore the birefringence properties of semiconductor nanocrystallites (NCs). A 0.352 μm thick polymer containing ZnSe–NCs (size dispersion: 2.5×10¹⁰/cm²) was kept on either of the arms of the MZI (λ=632.8 nm) and the phase angles were measured at the output, corresponding to three selected spots of the specimen. Owing to the non-resonant excitation led transient polarization in the NCs, strong optical Kerr-signals were detected with Kerr coefficient values ~1.87 × 10^-9 m/V². The approach may be extended to other optically isotropic nanoscaled materials having a center of inversion including liquid crystals.     

Beam parameters,mode structure and diffraction losses of slab lasers with unstable resonators

The properties of unstable resonators in on-axis and off-axis geometry were investigated both theoretically and experimentally. Diffraction loss, mode structure, beam quality and misalignment sensitivity were measured in single-shot operation with a gas-cooled Nd: YAG slab laser (4×12×100 mm³) and compared with theoretical results obtained by solving the Fresnel integral equation numerically. The excellent agreement allowed detailed resonator optimization. Furthermore, the resonator performance in high-power operation was investigated with two water-cooled Nd: YAG slab lasers (6×25×163 mm³ and 7×26×191 mm³) capable of 670 W of output power. Beam quality near the diffraction limit and a maximum output power of 475 W was achieved with an on-axis unstable resonator with variable-reflectivity mirror.     

Measurement of the 45Ca β spectrum in search of deviations from the theoretical shape

The ⁴⁵Ca β spectrum has been measured with a Si(Li) detector. New limits on the mixing parameter (|U _eH |²≤0.005) for a neutrino with mass in the interval 75–100 keV are obtained. The possibility of a monochromatic peak appearing at the end of the β spectrum is analyzed. Fiz. Tverd. Tela (St. Petersburg) 66, No. 2, 81–84 (25 July 1997)