Bi-doped glass optical fibers: Is it a new breakthrough in laser materials?

It has been shown recently, that Bi-doped glass optical fibers are a very promising active laser medium. Luminescence of Bi-doped glasses takes place in a spectral region of 1150–1500 nm, where no efficient fiber lasers (or any other efficient lasers) exist. The glasses have very broad luminescence bands (200–400 nm) and long lifetime (200–700 μs). The Bi absorption bands are situated in a spectral region of 500–1100 nm where long-lived high-brightness laser diodes developed for the pumping of lasers and amplifiers are available. Besides, strong luminescence has been observed in a variety of glass compositions. In this paper the recent results on the new laser material – Bi-doped glasses and optical fibers are reviewed. First, luminescence properties of various Bi-doped glasses are discussed. Then we describe the Bi-doped silica-based optical fiber fabrication and the absorption and luminescence properties of the fibers. At last some results on Bi-doped fiber lasers and their applications are presented.     

Chemical processing of rare earth chalcogenides

Chalcogenides are compounds containing sulfur, selenium and tellurium and are well known for their semiconducting and optical properties. Their characteristic absorption at low frequencies render these materials excellent for long wavelength transmission. In the past few years, the need for far infrared transmitting materials to be used in target acquisition equipment formed the basis for identifying several new crystalline sulfide materials which transmit into the extended far infrared (IR) spectral region. Rare earth sulfides exhibiting excellent transmission into the far infrared (8-14 μm) emerged as a new group of infrared window materials. This paper reviews the status of binary and ternary rare earth sulfide compounds for optical window application. Conventional and non-conventional techniques used for processing these materials will be discussed in complete detail. The use of metallorganic precursors and modification of the precursor chemistry to tailor the composition of the final ceramic will be described. The potential of these emergent low temperature chemical processing techniques such as sol-gel, precipitation and co-precipitation for synthesis of non-oxide ceramics will also be addressed, particularly in light of their successful application in processing of novel electronic oxide ceramics, glasses and composites.     

Preparation and properties of ultratransparent NaCl single crystals

The preparation of up to 100 mm diameter single crystalline ingots of ultratransparent NaCl by injecting the reactive atmosphere is reported. The resulting material exhibits high optical quality and bulk absorption coefficient at 10.6 μm of 0.91 × 10⁻³ cm⁻¹, close to the intrinsic value, which makes it suitable for transmissive components in high-power CO₂ lasers.     

InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition

Long-wavelength vertical cavity surface emitting lasers (VCSELs) are considered the best candidate for the future low-cost reliable light sources in fiber communications. However, the absence of high refractive index contrast in InP-lattice-matched materials impeded the development of 1.3–1.5 μm VCSELs. Although wafer fusions provided the alternative approaches to integrate the InP-based gain materials with the GaAs/AlAs materials for their inherent high refractive index contrast, the monolithic InP-based lattice-matched distributed Bragg reflectors (DBRs) are still highly attractive and desirable. In this report, we demonstrate InP/InGaAlAs DBRs with larger refractive index contrast than InP/InGaAsP and InAlAs/InGaAlAs DBRs. The switching between InP and InGaAlAs layers and growth rate control have been done by careful growth interruption technique and accurate in situ optical monitoring in low-pressure metal organic chemical vapor deposition. A 35 pairs 1.55 μm centered InP/InGaAlAs DBRs has the stopband of more than 100 nm and the highest reflectivity of more than 99%. A VCSEL structure incorporating 35 pairs InP/InGaAlAs DBR as the bottom mirror combined with a 2λ thick periodic gain cavity and 10 pairs SiO₂/TiO₂ top dielectric mirrors was fabricated. The VCSELs lased at 1.56 μm by optical pumping at room temperature with the threshold pumping power of 30 mW.     

激光加热基座技术生长超细单晶光纤研究

单晶光纤(SCF)是具有纤维结构的“准一维”功能晶体材料,在高能激光、高温传感、辐射探测、信息通信等国防民生领域展现出了巨大的应用前景。本文采用激光加热基座技术成功制备出直径60~100 μm、长径比>6 000∶1的超细Al₂O₃、YAG单晶光纤,单晶光纤平均直径起伏<4%,展现出良好的柔韧性与波导特性,为单晶光纤器件的小型化与集成化创造了条件。     

Growing of bulk crystals and structuring waveguides of fluoride materials for laser applications

We report on recent progress in the synthesis, the crystal growth and the epitaxial growth of fluoride and other laser materials. Results on the fabrication of single crystalline waveguides for dielectric down - and upconversion lasers pumped by semiconductor diode lasers are summarized. Epitaxial growth (molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), pulsed laser deposition (PLD)) and surface modifying techniques (high energy ion implantation, ion diffusion) have been applied in several laboratories. Progress in techniques fabricating optical waveguides from glassy media is addressed as well. Particular emphasis is given on the structuring (wet etching, chemical polishing, ion beam etching) of fluoride crystals for the purpose of obtaining 2-D and 1-D optical waveguides. Results on the structuring of LiYF₄ by wet and ion beam etching are reported. With respect to laser action, the generation of short wavelength light by upconversion (UC) processes, stimulated Raman scattering (SRS) and second harmonic generation (SHG) is discussed. Reports on the first crystalline waveguide lasers of fluoride crystals LiYF₄ and LaF₃, both doped with neodymium, are presented.     

Upgraded X-ray topography and microtomography beamline at the Kurchatov synchrotron radiation source

An upgraded X-ray Topography and Microtomography (XRT-MT) station is described, the parameters of the optical schemes and detectors are given, and the experimental possibilities of the station are analyzed. Examples of tomographic reconstructions are reported which demonstrate spatial resolutions of 2.5 and 10 μm at fields of view of 2.5 and 10 mm, respectively.     

Janus型二维磁电材料CrXX’(X/X’=S,Se,Te)第一性原理研究

自石墨烯被发现以来,各种具有新奇特性的二维材料受到了越来越多的关注。Janus型二维材料具有不对称的表面特性,这种特殊的结构往往具有独特的电学、磁学与光学性质,使其成为近年来材料科学领域研究的热点。本文搭建了Janus型结构CrXX’(X/X’=S,Se,Te)(CrSSe, CrSTe, CrSeTe),研究了体系的电学、磁学、光学性质,并探究了双轴应变对其电学、磁学、光学性质的影响。结果表明,CrSSe、CrSTe与CrSeTe均呈现金属性,都是电子的优良导体,三种体系的电子结构对外加应变具有很好的鲁棒性。CrXX’(X/X’=S,Se,Te)具有本征铁磁性,并且通过施加双轴应变可对其磁矩进行调控。此外,三种体系均具有较高的居里温度,特别是CrSTe的居里温度可达310 K。CrXX’(X/X’=S,Se,Te)还具有优异的可见光与紫外光吸收性能,应变可对其光吸收系数进行调控,并且压应变与拉应变可分别使其吸收谱线向短波与长波方向移动。本文的工作为进一步研究二维Janus单层CrXX’(X/X’=S,Se,Te)在新型室温自旋电子器件领域的应用提供了理论支持。     

Epitaxial growth on optical gratings for distributed feedback GaAs injection lasers

Good quality epitaxial overgrowth of Al_0.3Ga_0.7As on corrugated GaAs and Al_0.12Ga_0.88As surfaces has been achieved by molecular-beam epitaxy. The electrical properties of the interface appear to be equivalent to those prepared by LPE without growth interruption. The corrugations were third order Bragg gratings of ～0.37 μm period and ～0.20 μm depth and were formed by ion milling. Separate confinement heterostructure injection lasers with these periodic corrugations in the optical cavity have demonstrated lasing behavior characteristic of distributed feedback. They have operated with room-temperature threshold current densities as low as 2.2 kA/cm². These results suggest that the MBE overgrowth is a useful technique for the fabrication of integrated optoelectronic structures which include active devices.     

Infrared fibers based on Te-As-Se glass system with low optical losses

Unclad optical fibers based on high-purity Te-As-Se glasses prepared by chemical and physical methods of purification have been drawn. The optical, thermal and mechanical properties of glasses and fibers were investigated. The minimum optical losses were 0.07 dB/m at 7.3 μm for Te₂₅As₄₀Se₃₅ glass fiber and 0.04 dB/m at 6.7 μm for Te₂₀As₃₀Se₅₀ glass fiber. Sixty five percent of input power of a tunable CO₂ laser emitting at 9.3 μm was transmitted through a 1 m long fiber with diameter of 900 μm.     

Epitaxial AlGaAsSb?GaSb (AlGaSb) heterostructures for injection lasers

A possibility is shown of obtaining nearly perfect heterojunctions in the GaAlAsSb—GaAlSb system which do not contain any remarkable density of lattice mismatch defects at the heteroboundaries. Application to low threshold DH injection lasers for 1.4–1.8 μm spectral region for optical communications, moist-o-graphs etc.     

LPE growth and characterization of InP/InGaAsP ridge-waveguide lasers at 1.3 μm-wavelength

This paper presents the fabrication and the lasing characteristics of 1.3 μm-wavelength ridge-waveguide laser. The epitaxial material used in this study was grown applying the step-cooling technique of liquid phase epitaxy (LPE). The growth conditions for InGaAsP layers lattice-matched to the (001) InP-substrate are reported for lattice compositions corresponding to photoluminescence peak wavelengths of 1.07, 1.14, and 1.31 μm. We have used a conventional multiple-bin sliding boat to grow the LPE layers and a second apparatus for achieving batches of melts of uniform compositions. In the LPE apparatus the various batch melts (In–Sn In–Zn; In–Ga–As of different composition) were saturated with phosphorus using the seed dissolution technique. The epitaxial layers were grown by a single phase technique at a constant temperature. This LPE growth technique is useful for the fabrication of double-heterostructure wafers with an uniform alloy composition and a well-defined layer thickness. Using these epitaxial materials, metal-clad ridge-waveguide (MCRW) lasers have been prepared with stripe widths of 3.5 μm. CW threshold currents of 18 mA at room temperature are achieved for 200 μm long cavities. These lasers have T₀ values ranging from 50 to 70 K and well linear L-I-characteristics.     

GeS/MoS2异质结电子结构及光学性能的第一性原理研究

以MoS₂、GeS为代表的二维层状材料在光学、电学等方面表现出优异的物理性能。如何将两者的优良性能结合,同时获得具有新的协同功能的复合材料对电子器件的发展和应用具有重要意义。本文采用密度泛函理论的第一性原理计算方法,对GeS/MoS₂异质结的电子结构及光学性质进行了系统研究,并探索了界面间距、应变和电场对异质结电子结构和光学性能的影响。研究结果表明,GeS/MoS₂异质结是Ⅱ型能带排列,该能带排列有利于光生电子-空穴对的分离。进一步研究发现,通过应变和电场等手段可以实现对GeS/MoS₂异质结能带排列及光吸收系数的有效调控。该研究结果表明GeS/MoS₂异质结在光催化、光电器件等领域具有潜在的应用,为设计与制备GeS/MoS₂相关的光电器件提供了理论指导。     

Zinc oxide – analogue of GaN with new perspective possibilities

Zinc oxide due to specific electrical, optical and acoustic properties is the important semiconductor material, which has many various applications. There is growing interest in ZnO due to its potential applicability for optoelectronic devices such as light‐emitting diodes, laser diodes and detectors for UV wavelength range. ZnO properties are very close to those of widely recognized semiconductor GaN. The band gap of ZnO (3.37 eV) is close to that of GaN (3.39 eV) but ZnO exciton binding energy (60 meV) is twice larger than that of GaN (28 meV). Optically pumped UV lasing have been demonstrated at room temperature using high textured ZnO films. The excitonic gain close to 300 cm^–1 was achieved. ZnO thin films are expected to have higher quantum efficiency in UV semiconductor laser than GaN. The physical properties of ZnO are considered. PEMOCVD technology was used to deposit piezoelectric and highly transparent electroconductive ZnO films. Their properties are discussed. The experiments on polycrystalline ZnO films deposited by RF magnetron sputtering at different partial pressure of oxygen are presented. AFM images were studied in tapping mode for deposited films. The investigated films were dielectric ones and had optical transparency within 65‐85% at thickness in the interval 0.2‐0.6 μm. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High quality Ba2NaNb5O15 characterization and optical absorption coefficient measurements

Significant improvements have been made in the quality of barium sodium niobate crystals and material with very high optical quality and low optical loss has been produced. Comparisons with previously available material can be made by means of schlieren photography and parallel light interference methods. A technique for determining absorption coefficients by differential temperature measurements has been developed and optical absorption coefficients as low as 0.0003 cm^-1 at 1.06 μm have been measured.     

Infrared transmitting glasses and glass-ceramics

The demand for infrared materials which exhibit higher performance is still growing; both on the thermal and mechanical side as well as on the width of the optical window, with an extension of the transparency towards the long wavelength region to satisfy the requirements of space applications. Glasses as well as glass-based ceramics offer the advantage of unique rheological properties allowing molding and fiber drawing. In controlling the nucleation/growth process in a chalcogenide glass modified by an alkali halide, it has been possible to develop a new generation of glass ceramics containing nanosize grains and transparent in the mid infrared. For the optical exploration of the universe, new low phonon glasses transparent in the 20 μm region and beyond are needed and to reach this goal a new family of telluride glasses was developed in combining Te with Ge with the addition of Gallium or Iodine for stabilization. These new optical glasses are transparent from 2 to 20 μm and can be drawn into fiber.     

Methods for measuring light scattering in germanium and paratellurite crystals

Methods for studying the scattered light in germanium and paratellurite (α-TeO₂) crystals are considered. Investigations of the light scattering in Ge crystals were performed in the infrared wavelength range by the photometric-sphere method (in the range 2–3 μm) and by measuring the line-scattering functions (at 10.6 μm). In the visible range, the paratellurite single crystals were investigated by recording and analyzing images of laser beams transmitted through the samples. It is shown that small-angle Mie scattering is characteristic of both materials. Some conclusions about the sizes and the physical nature of scattering inhomogeneities are drawn. The effect of high-temperature annealing on the scattering intensity is studied.     

基于单体石墨纤维的场发射特性研究

利用化学气相沉积(CVD)法,以甲烷为碳源在管式炉中合成了单体石墨纤维(MGF)。选取长度为3.426 mm,顶端球面半径为11.26 μm的单体石墨纤维直立于圆铜片上作为阴极,以导电ITO玻璃作为阳极,采用二极管结构在真空室中进行直流场发射测试,证实MGF的开启场强为0.477 5 V/μm。基于有限元仿真软件ANSYS进行电磁场分析,计算了MGF在不同电压下的有效发射面积。结果表明,当电压为5.36 kV时,MGF达到最大发射面积为796.226 μm²,在实验测量电压范围内,平均发射电流密度可以达到46.069 A/cm²,单体石墨纤维具有良好的场发射特性。     

Nondestructive concentration profiling of fiber optic preforms by analysis of Raman spectra

We have demonstrated that Raman scattering may be used to determine P and Ge concentration profiles of optical fiber preforms. For Ge, the technique has an absolute accuracy of about 0.5 mol.% Ge, and spatial resolution to 40 μm was achieved. When used in a nondestructive mode, spatial resolution of 100 μm is possible. The results clearly show that phosphorus as well as germanium burn out as the tube is collapsed, with the spatial extent of the phosphorus burnout region being about twice as great as that for germanium. Our measurements on a graded-P preform provide a determination of the Ge incorporation ratio across the preform with the surprising result that it increases toward the edge, where the temperature is higher and P content is lower.The spectra also provide clear evidence for two types of Ge sites in silica-germanium glasses, where one type of site, possibly associated with network defects, is predominantly occupied at low concentration. The 710 cm^?1 line associated with these defect sites is not detected when either P or B is present, and we speculate that this may be related to the dependence of the Ge incorporation ratio on the phosphorous concentration.     

Prospective evaluation of the use of various materials as matrices for active elements of lasers

Methods of prospective evaluation of the use of various materials as active elements for quantum generators for certain applications are considered. The way of evaluation for all kinds of dielectric crystal lasers with respect to optical quantum generators, applicable in technical designs is given. The influence of material properties on the prospective technical use is considered. The results are summarily analyzed.