A simple heterodyne frequency offset locking system for pulsed lasers

As a new approach the heterodyne frequency offset locking system which is applicable to a broad range of pulsed lasers is described. A 100 MHz wide-band and all-digital frequency discriminating technique is presented and an offset locking loop in which the frequency discriminator works accordingly is designed. The result of the experiment on offset locking between a CW CO₂ laser and an electro-optically Q-switched CO₂ laser with an optical pulse width 300 ns and its repetition rate 20 kHz showed that the beat frequency stability was kept within ±2 MHz.     

超宽光谱掺钛蓝宝石飞秒激光器时域频域特性的实验研究

系统地研究了超宽光谱（Δλ>100nm）情况下,掺钛蓝宝石飞秒激光振荡器时域和频域特性,观测到腔内光谱从40nm带宽的sech²型演化至180nm带宽的多峰结构,以 及脉冲在时域相应的成型过程.实验表明随着光谱的加宽,脉冲的变窄主要体现在其中部,而脉冲底部并未变窄,其包含的能量反而相对增加. 关键词： 掺钛蓝宝石激光器 飞秒脉冲 超宽光谱 时频特性     

钛宝石激光器端面抽运Nd:YAG陶瓷激光器热沉积理论和实验研究

通过热沉积系数研究在激光提取条件下掺杂原子分数为1.0%的Nd:YAG陶瓷激光器中热沉积问题.热沉积系数定义为热沉积功率与激光器输出功率之比.在理论分析基础上，通过测量激光器斜率效率来间接测定热沉积系数，实验测定的热沉积系数值为0.63.建立激光提取条件下Nd:YAG陶瓷发热模型，讨论了影响热沉积系数的主要因素.结果表明：热沉积系数对Nd:YAG陶瓷的辐射量子效率、交叠效率以及激光提取效率的变化非常敏感.为有效减少介质内热沉积，在激光器优化设计中交叠效率和激光提取效率是需要着重考虑的参数.所得结果可为进一步研究陶瓷激光器中热效应提供参考. 关键词： 热沉积 Nd:YAG陶瓷 固体激光器     

Numerical simulation of gain narrowing control by hybrid amplifiers chain based on Ti:sapphire and Ti:chrysoberyl

A hybrid amplifier chain composed of Ti:chrysoberyl and Ti:sapphire is proposed to control spectral gain narrowing in this letter. The Ti:chrysoberyl crystal is employed as the host material of the frontier amplification stage with the polarization parallel to a axis. According to the numerical simulation, the seeding pulse with a proper wavelength center is required to generate a broader spectrum, and spectra broader than 120 nm are available with total net gain from 10⁴ to 10¹⁰. The simulation proves that the amplified spectrum can even support pulse shorter than the seed pulse.     

Comparative study of spectral narrowing of a pulsed Ti:Sapphire laser using pulsed and CW injection seeding

We report a comparative study of a pulsed as well as continuous-wave (cw) injection seeding of a Ti:Sapphire laser pumped by aQ-switched frequency-doubled Nd³⁺:YAG laser for achieving narrow spectral bandwidth. The results have indicated that the Ti:Sapphire laser using either a pulsed or a cw injection seeding could achieve efficient energy extraction in a narrow spectral bandwidth. In the case of pulsed injection seeding, the injection energy required for the complete injection seeding critically depended upon the timing of the Ti:Sapphire laser with respect to the delayed onset of the slave laser. On the other hand, in the case of cw injection seeding, the spectral bandwidth of the Ti:Sapphire laser was efficiently narrowed down to approximately 0.01 cm^–1 with an injection power of less than 1 mW. In both types of injection seeding, characteristics observed experimentally were compared with those obtained by a numerical simulation code based on the one-dimensional rate-equation model.     

A comprehensive equivalent circuit model for the study of thermal and spectral characteristics in laser diodes

An improved electrical equivalent model for multimode laser diode incorporating thermal effects and spectral behavior is developed from the multi mode rate equations. The temperature dependent optical power, wavelength shift, spectral widths are determined in the dc sweep simulations. The output power, frequency chirp and temperature rise are evaluated under pulse modulation conditions. Further, the laser diode bandwidth dependence on temperature is also evaluated. Analysis is carried out in the temperature range of 283–343 K, by simulating the equivalent circuit, using PSpice circuit simulator.     

频域标定飞秒脉冲干涉自相关迹及钛宝石振荡器实时啁啾监测

采用频域处理的方法首次在频域标定飞秒激光脉冲干涉自相关迹的时间扫描.计算被测脉冲的均方根宽度和啁啾参数，并以此为依据在实验中优化钛宝石振荡器的调节从而获得接近变换极限的115fs脉冲.该方法只涉及到数值积分和快速傅里叶变换，在实验中完全可以用于实时监测振荡器的运行情况. 关键词： 频域标定 钛宝石振荡器 飞秒脉冲 干涉自相关迹     

Cross-relaxation and spectral broadening of gain for Nd/Cr:YAG ceramic lasers with white-light pump source under high-temperature operation

Spectral broadening of the fluorescence of a Nd/Cr:YAG ceramic at the 1064 nm lasing wavelength was observed, and the amplification properties at a high temperature were investigated by considering cross-relaxation. These ceramics are promising for use as a solid-laser material pumped with solar or lamp light. It has been found that whenever the temperature of a laser medium is high and a spectral shift occurs, a high small-signal gain remains owing to the broad spectral band and the cross-relaxation. This optical property is remarkably different from that of a Nd:YAG laser. For a conventional Nd:YAG laser, the bandwidth at 1064 nm is 0.45 nm, and a reduction in small-signal gain occurs at a temperature of 373 K because the spectral peak shift is 0.005 nm/K. However, for the Nd/Cr:YAG ceramic, the bandwidths are 1.2 and 1.9 nm in the case of 0.1% Cr ion doping and 3% Cr ion doping, respectively, owing to the existence of excited Cr ions and the shortening of the effective Nd ion coherence time. It is prospected that the laser medium can be used at a high temperature of 600 K.     

一种适用于脉冲Nd^3＋：YAG激光器的新型横模控制谐振腔

本文报道了一种新型横模控制谐振腔——圆形空心波导谐振腔。实验表明这种新型横模控制诣振腔与腔内加小孔光阑的横模控制谐振腔相比,有基模TEM_∞输出效率明显增高和远场发散角大大减小等优点,文中对其限横模机理进行了分析和讨论。     

Laser cleaning of gilded wood: A comparative study of colour variations induced by irradiation at different wavelengths

There is a growing interest by art conservators for laser cleaning of wood artworks, since traditional cleaning with chemical solvents can be a source of decay, due to the prolonged action of chemicals after the restoration. In this experiment we used excimer and Nd:YAG lasers, emitting radiation in the ultraviolet (248 nm), visible (532 nm) and near infrared (1064 nm), to investigate the effect of laser interaction on gilded wood samples at different wavelengths. Increasing fluence levels were tested to assess threshold values both for surface damage and colour changes. Detailed colorimetric analyses of the irradiated samples show that cleaning effectiveness is related to the emission wavelength, the fluence and the number of pulses.     

Micro‐LED pumped polymer laser: A discussion of future pump sources for organic lasers

Optical pumping conditions for organic solid‐state lasers (OSLs) are discussed with particular emphasis on the use of gallium nitride based light‐emitting diodes (LEDs) as pump sources. LEDs operate in a regime where the pump should be optimized for a short rise time and high peak intensity, whereas fall time and overall pulse duration are less important. Lasers pumped with this approach need to have very low thresholds which can now be routinely created using (one‐dimensional) distributed feedback lasers. In this particular case stripe‐shaped excitation with linearly polarized light is beneficial. Arrays of micron‐sized flip‐chip LEDs have been arranged in an appropriate stripe shape and the array dimensions were chosen such that the divergence of LED emission does not cause a loss in peak intensity. These micro‐LED arrays have successfully been used to pump OSLs with thresholds near 300 W/cm² (∼9 ns rise time, 35 ns pulse duration), paving the way for compact arrays of indirectly electrically pumped OSLs.     

Bio-inspired optimization algorithms for optical parameter extraction of dielectric materials: A comparative study

Particle swarm optimization(PSO) and invasive weed optimization(IWO) algorithms are used for extracting the modeling parameters of materials useful for optics and photonics research community. These two bio-inspired algorithms are used here for the first time in this particular field to the best of our knowledge. The algorithms are used for modeling graphene oxide and the performances of the two are compared. Two objective functions are used for different boundary values. Root mean square(RMS) deviation is determined and compared.     

Influence of valence electron concentration on elastic, electronic and optical properties of the alkaline-earth tin oxides A3SnO (A=Ca, Sr and Ba): A comparative study with ASnO3 compounds

By employing first principles method of the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA), the correlation between valence electron concentration and structural, elastic, electronic as well as optical properties of A₃SnO and ASnO₃ compounds where A=Ca, Sr and Ba are investigated. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young's moduli for ideal monocrystalline and for polycrystalline A₃SnO and ASnO₃ aggregates. Band structures reveal that alkaline-earth tin oxides A₃SnO are direct energy band gap (G-G) materials.The hardness of these compounds was explained using chemical bonding properties and Milliken charges transfer. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV. We have found that the static dielectric constants of all these compounds are in good agreement with Penn model.     

A comparative study of magnetic resonance venography techniques for the evaluation of the internal jugular veins in multiple sclerosis patients

Background and Purpose

The use of magnetic resonance imaging (MRI) to assess the vascular nature of diseases such as multiple sclerosis (MS) is a growing field of research. This work reports on the application of MR angiographic (MRA) and venographic (MRV) techniques in assessing the extracranial vasculature in MS patients.

Materials and Methods

A standardized MRI protocol containing 2D TOF-MRV and dynamic 3D contrast-enhanced (CE) MRAV was run for 170 MS patients and 40 healthy controls (HC). The cross-sectional area (CSA) of the internal jugular veins (IJVs) was measured at three neck levels in all subjects for both MRV techniques to determine the presence of venous stenoses. All data were analyzed retrospectively.

Results

For the values where both methods showed signal, the 3D method showed larger CSA measurement values compared to 2D methods in both IJVs, in both MS and HC subjects which was confirmed with student paired t-tests. Of the 170 MS patients, 93 (55%) in CE-MRAV and 103 (61%) in TOF-MRV showed stenosis in at least one IJV. The corresponding numbers for the 40 HC subjects were 2 (5%) and 4 (10%), respectively. Carotid ectasias with IJV stenosis were seen in 26 cases (15%) with 3D CE-MRAV and were not observable with 2D TOF-MRV. Carotid ectasias were not seen in the HC group. In the 2D TOF-MRV data, banding of the IJVs related to slow flow was seen in 58 (34%) MS cases and in no HC cases. MS patients showed lower average CSAs than the HC subjects.

Conclusion

The 3D CE MRAV depicted the vascular anatomy more completely than the 2D TOF-MRV. However, the 3D CE MRAV does not provide any information about the flow characteristics which are indirectly available in the 2D TOF-MRV in those cases where there is slow flow.     

Coherent control of stimulated emission inside one dimensional photonic crystals: strong coupling regime

The present paper discusses the stimulated emission, in strong coupling regime, of an atom embedded inside a one dimensional (1D) Photonic Band Gap (PBG) cavity which is pumped by two counter-propagating laser beams. Quantum electrodynamics is applied to model the atom-field interaction, by considering the atom as a two level system, the e.m. field as a superposition of normal modes, the coupling in dipole approximation, and the equations of motion in Wigner-Weisskopf and rotating wave approximations. In addition, the Quasi Normal Mode (QNM) approach for an open cavity is adopted, interpreting the local density of states (LDOS) as the local density of probability to excite one QNM of the cavity; and therefore rendering this LDOS dependent on the phase difference of the two laser beams. In this paper we demonstrate that the strong coupling regime occurs at high values of the LDOS. In accordance with the results of the literature, the emission probability of the atom decays with an oscillatory behaviour, so that the atomic emission spectrum exhibits two peaks (Rabi splitting). The novelty of this work is that the phase difference of the two laser beams can produce a coherent control of both the oscillations for the atomic emission probability and, as a consequence, of the Rabi splitting in the emission spectrum. Possible criteria to design active delay lines are finally discussed.     

Ultrasonic assisted synthesis of two urea functionalized metal organic frameworks for phenol sensing: A comparative study

Two pillared metal-organic frameworks containing urea functional groups were synthesized by a sonochemical method and characterized by scanning electron microscopy, X-ray powder diffraction, IR spectroscopy and elemental analysis. The time of sonication and concentration of starting materials have been optimized to synthesize nanoparticles of TMU-31 and TMU-32. These two frameworks are interesting candidates for a comparative fluorescence study. Thus, their potential abilities for phenol sensing were investigated. This investigation revealed the prominent roles of hydrogen bond donating urea groups inside the pore cavity in the ability of these structures in phenol sensing.     

Ab initio electronic structure,bonding and optical properties of two relatively new ceramics Hf2Al3C4 and Hf3Al3C5: A comparative study

The structural, electronic and optical properties of the ternary carbides Hf₂Al₃C₄ and Hf₃Al₃C₅ are studied via first principles orthogonalized linear combination of atomic orbitals (OLCAO) method. Results on crystal structure, interatomic bonding, band structure, total and partial density of states (DOS), localization index (LI), effective charge (Q*), bond order (BO), dielectric function (ε), optical conductivity (σ) and electron energy loss function are presented and discussed in detail. The band structure plots show the conducting nature of Hf₂Al₃C₄ and Hf₃Al₃C₅ carbides. DOS results disclose that the total number of states at Fermi level N(E_F) are 1.89 and 2.38 states/(eV unit cell) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The Q* calculations show an average charge transfer of 0.723 and 0.711 electrons from Hf and 0.809 and 0.807 electrons from Al to C sites in Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The BO results provide the dominating role of Al–C bonds with BO value of 6.62 (BO%?=?59%) and 6.66 (BO%?=?49%) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively and are considered responsible for the crystals cohesion. The LI results reflect the presence of highly delocalized states in the vicinity of the Fermi level. The dielectric function plots of the real (?₁(?ω)) and imaginary (?₂(?ω)) parts show the anisotropic behavior of Hf₂Al₃C₄ and Hf₃Al₃C₅. The results on optical conductivity (σ) support the trends observed in dielectric functions. The electron energy loss functions reveal the presence of sharp peaks both in ab-plane and along c-axis around 20?eV in Hf₂Al₃C₄ and Hf₃Al₃C₅ ternary carbides.     

A study of X-ray luminescence and spectral compatibility of europium-activated yttrium-vanadate (YVO4: Eu) screens for medical imaging applications

The X-ray luminescence efficiency of laboratory-prepared YVO₄: Eu screens and their spectral compatibility to common optical detectors were studied under medical fluoroscopy conditions. YVO₄: Eu screens were prepared by sedimentation and with different coating thickness. Luminescence efficiency of the YVO₄: Eu screens was measured at various X-ray tube voltages (50–250 kVp) and for screens of different coating thicknesses (20–180 mg/cm²). Spectral response was also measured and spectral matching factors between the YVO₄: Eu screens and some common optical detectors (photocathodes, photodiodes, photographic emulsion) were calculated. Experimental results on efficiency were fitted by formulas of the theoretical model developed by Hamaker and Ludwig in order to determine phosphor intrinsic X-ray-to-light conversion efficiency and intrinsic optical characteristics, such as coefficients related to light scattering and absorption. Although the luminescence efficiency of YVO₄: Eu screens was found to be relatively low (3–11 M s/mR m²), the matching factor of YVO₄ : Eu screens with some red sensitive optical detectors was excellent, of the order of 0.96. High spectral compatibility may indicate that YVO₄: Eu scintillators could be used in medical image detectors.     

Ab-initio study of Li based chalcopyrite compounds LiGaX2 (X= S,Se, Te) in tetragonal symmetry: A class of future materials for optoelectronic applications

Structural, electronic, optical, and thermoelectric aspects of chalcopyrite LiGaX₂ (X = S, Se and Te) compounds have been investigated by density functional theory (DFT) based Wien2k simulator. The optimized ground state parameters are calculated by Wu-Cohen generalized gradient approximation (WC-GGA) and electronic structures, which have been further improved by modified Becke-Johnson (mBJ) potential. Moreover, a comparative study is given among the contribution of three anions (S, Se and Te) in the same symmetry of tetragonal phase. The calculated band gaps of the studied compounds are 3.39, 2.83, and 1.96 eV for LiGaS_2, LiGaSe₂ and LiGaTe₂, respectively. The observed band gaps consider the studied compounds are potential materials for optoelectronic devices. In addition, the optical response of the studied materials has been analyzed in terms of dielectric constants, refraction, absorption, reflectivity and energy loss function. We have also reported the thermoelectric properties like Seebeck coefficient, thermal and electrical conductivities, and figure of merit as function of temperatures by using BoltzTrap code. The high thermal efficiency and absorption spectra in the visible region make the studied materials multifunctional for energy applications.