Mid-Infrared-Wavelength Generation in 2-mum Pumped Periodically Poled Lithium Niobate

A periodically poled lithium niobate optical parametric oscillator pumped by a Tm:YAG laser at 2.0124-mum wavelength has been demonstrated. A pump pulse energy of 5.1 mJ generated 0.65 mJ of signal and idler pulse energy at a 50-Hz repetition frequency with a 27.8-mum domain-period-length grating. The lithium niobate crystal at a temperature of 180 degrees C yielded 3.61- and 4.55-mum signal and idler wavelengths, respectively. Wavelength tuning over a wide range was achieved with domain-period lengths from 25.5 to 28.2 mum and crystal temperature tuning from 50 to 180 degrees C. Signal wavelengths of 3.26-3.76 mum and idler wavelengths of 4.33-5.34 mum were generated.     

Second-harmonic Generation Efficiencies in Germanium-doped Planar Waveguides: A Normal-mode Analysis

Abstract

Second-harmonic generation (SHG) in germanium-doped silica planar waveguides seeded by a fundamental wave and its second harmonic is investigated using normal-mode analysis. The effects of self- and cross-phase modulation are included in this analysis. A set of two coupled equations that describe the evolution of the amplitude of the writing second-harmonic beam along the direction of propagation is obtained. The solution to these coupled equations is used to write the effective X ⁽²⁾ grating. This grating is then used to determine the second-harmonic power generated when the beam is read with only a fundamental wave propagating in a particular waveguiding mode. Due to diffraction, the conversion efficiency saturates as a function of waveguide length. It is also found that if the reading fundamental wave is in the same mode as the writing fundamental wave, relatively significant SHG is obtained with the generated second-harmonic wave propagating primarily in the same mode as the seed second-harmonic wave. The power generated in any of the higher order modes is of the same order as that for the lowest order mode.     

Second Harmonic Generation in a Free Electron Laser

It is pointed out that in a free electron laser higher frequencies, which are very nearly equal to the harmonics, are generated. Second harmonic generation, in stimulated emission in a spatially periodic static magnetic field, is studied using the Feynman diagram technique. The ratio of gain for the second harmonic compared to that of the fundamental harmonic is of the order of 10 ^?9 for the conditions present in the free electron laser of Deacon et al. [1].     

近化学计量比铌酸锂晶体的研究进展

介绍了近化学计量比铌酸锂晶体的主要生长方法及其特点,总结了近化学计量比铌酸锂晶体的主要性能,通过与同成分铌酸锂晶体的比较,指出了近化学计量比铌酸锂晶体的主要应用优势,明确了该类晶体今后的研究重点.     

Structural Defects and Photoluminescence in Zinc-Doped Lithium Niobate Crystals

Inorganic Materials - We demonstrate that increasing the zinc concentration in Zn-doped lithium niobate crystals to their first concentration threshold (~3.0 mol % ZnO) leads to a decrease in...     

Second Harmonic Generation in Optical Fibres: Multiphoton Absorption and Translation-inversion Symmetry

Abstract

Multiphoton absorption provides a route to x ₍₂₎ grating formation in optical fibres. It explains the efficacy of ‘seeding’ the fibre with green light to induce fibre second harmonic generation. The process depends on absorption sites with spatially inverted configurations having different multiphoton absorption rates.     

Review of Second Harmonic Generation Measurement Techniques for Material State Determination in Metals

This paper presents a comprehensive review of the current state of knowledge of second harmonic generation (SHG) measurements, a subset of nonlinear ultrasonic nondestructive evaluation techniques. These SHG techniques exploit the material nonlinearity of metals in order to measure the acoustic nonlinearity parameter, \(\beta \). In these measurements, a second harmonic wave is generated from a propagating monochromatic elastic wave, due to the anharmonicity of the crystal lattice, as well as the presence of microstructural features such as dislocations and precipitates. This article provides a summary of models that relate the different microstructural contributions to \(\beta \), and provides details of the different SHG measurement and analysis techniques available, focusing on longitudinal and Rayleigh wave methods. The main focus of this paper is a critical review of the literature that utilizes these SHG methods for the nondestructive evaluation of plasticity, fatigue, thermal aging, creep, and radiation damage in metals.     

Light Propagation in Singly and Doubly Periodic Planar Waveguides

Abstract

Light propagation in singly and doubly periodic planar waveguides is investigated with respect to future applications in integrated optics. The waveguides used in our experiments reveal, in the vicinity of Bragg reflection, a strong difference between the directions of phase and group velocities, the beam steering. A clear graphical representation of the observable propagation effects is given in wave-vector diagrams, showing the directional dispersion of the elementary waves in periodic structures, the Floquet-Bloch waves. The dispersion phenomena were measured with high accuracy, using selective wave excitation. In conjunction with straight tapered transitions to smooth planar waveguides, the periodic structures show a great variety of strong frequency and direction-dependent effects such as lateral beam shifting and focusing with a frequency-variant focal length. Ray optics of Floquet-Bloch waves is used to describe these phenomena. Complex interference patterns observable in the vicinity of Bragg reflection in doubly periodic structures are explained by the superposition of Floquet-Bloch waves. Examples of applications of planar periodic structures as highly selective frequency filters, optical multiplexers as well as frequency-tunable beam narrowing, focusing and expanding devices are given together with measured data.     

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO₃ crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO₃ single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.     

Improved Long-Term DC Drift in OH-Reduced Lithium Niobate Optical Intensity Modulators

Long-term dc drift of lithium niobate optical intensity modulators is successfully reduced by exclusion of OH(-)ions from the lithium niobate substrate. In this regard, the titanium-indiffused waveguide is prepared on a commercial OH-free substrate under a dry gas atmosphere.The dc drift measured for these hermetically sealed modulators indicates a possibility for continuous operation over 20 years at 50°C.     

Investigation of Refractive-index Profiles of Ion-exchanged Planar Waveguides in Glass

Abstract

New data concerning the refractive-index profiles in Ag/Na ion-exchanged waveguides are obtained using a parabolic-segment reconstruction method. The profiles can be described as second-order polynomials, but with a mean curvature b = 2·8 which is significantly greater than the originally suggested value 0·64. The profile so obtained agrees very well with the common diffusion equation for the silver ion concentration only if one assumes a simple linear relationship between the diffusion coefficient and the concentration. This fact necessitates changes in present ideas about the ion-exchange mechanism in sodalime glasses.     

Self-induced Temperature Profile and Inhomogeneity Effect on Second Harmonic Generation Using Focused Laser Beam

The behaviour of optical second harmonic due to crystal inhomogeneity and self-induced thermal effect has been considered for a focused laser beam. Temperature profile created due to a fundamental laser beam along the crystal length shows an increase in temperature band width for various values of focusing parameter. Normalized second harmonic power in inhomogeneous crystals increases up to an optimum value of focusing parameter. Utility of focusing is limited due to formation of filament in crystal for higher values of focusing parameter.     

Angularly-resolved High-order Harmonic Generation in Helium

Abstract

Harmonics up to 119th have been generated in helium using a 1·053 μm, 1 ps chirped pulse amplification laser at intensities up to 3 × 10¹⁴W cm^?2, and their far-field spatial distributions have been measured. In the presence of considerable ionization, complex angular distributions are found for harmonics in the plateau region, while near the cut-off, their distribution narrows to approximately that predicted by lowest-order perturbation theory. The experimental results are found to be in good agreement with the predictions of a numerical propagation model which incorporates photoelectron dispersion and strong-field atomic data for the ionization rates and dipole moments.     

Efficient Sub-wavelength Plasmonic Superconducting Photodetectors in Ultra-Thin Planar Waveguides and in Optical Fibers

By using a finite-element method we investigate the efficiency of sub-wavelength plasmonic superconducting photodetectors in ultra-thin planar waveguides (the incident wave is P-polarized) and in optical fibers (a traveling wave). The unit cell of a periodic sub-wavelength planar waveguide is made by a gold strip that is encapsulated in an YBCO layer and further enclosed above by an air layer and beyond by a thick gold plate substrate. For a planar structure with a single unit cell, the power absorption in YBCO layer is very large (between 0.929 and 0.975) for a fixed wavelength λ=1.55 μm when the distance between the strips and gold substrate is varied between 17 nm and 32 nm. When the wavelength is varied between 1.1 μm and 1.9 μm, the power absorption in YBCO layer is between 0.944 and 0.965 with a maximum of 0.976 at λ=1.40 μm. The superconducting traveling wave photodetector is made by an optical fiber, which includes a gold core surrounded by a number of smaller gold cylinders at a small distance from the central gold region and that are encapsulated in an active YBCO layer and further enclosed by an air layer. In our structure of the fiber with 18 smaller gold cylinders, the imaginary parts of the effective index of plasmon modes (two nondegenerate modes and eight twofold degenerate modes) and the power absorption (between 0.904 and 0.976) in active YBCO layer are very large. The first nine modes are highly confined in the YBCO layer in the space between the neighboring gold cylinders and the other nine modes are highly confined in the YBCO layer but in the space between the small gold cylinders and the gold core. Although in our structure, the thickness of the YBCO layer is 10 times smaller in comparison with that of a recently published model with 32 small gold cylinders, the losses in active YBCO layer are also very large.     

深紫外非线性晶体KBe2BO3F2的倍频转换效率

深紫外(Duv)相干光源对于光刻技术、激光微加工、激光光谱仪等均具有重要的意义.KBe2BO3F2(KBBF)晶体是目前唯一可直接倍频产生深紫外激光的非线性光学晶体.本文通过分析KBBF晶体的光学倍频特性,分别得到了Nd:YV04激光2倍频、4倍频和6倍频以及Ti:Sapphire激光4倍频输出的相关技术参数;在此基础上,详细分析了KBBF晶体用于高斯光束的Ⅰ类和Ⅱ类倍频的转换效率.结果对于KBBF晶体用于产生深紫外全固态激光(DPL)的实验研究提供了重要的理论依据.     

Second Harmonic Generation induced by optical poling in new TeO2-Tl2O-ZnO glasses

New tellurite glasses with a large glass forming domain were elaborated within the TeO₂-Tl₂O-ZnO ternary system. The evolution of the glass transition (T_g) and onset crystallization (T₀) temperatures for such tellurite glasses was studied, in particular, as a function of the Tl₂O addition. A decrease of both T_g and T₀ temperatures was observed; the former being more affected. Structural modifications induced by the addition of the modifiers were studied by Raman spectroscopy. For a fixed ZnO concentration, the increase in the Tl₂O content leads to a destruction of the glass framework, characterized by the transformation of TeO₄ disphenoids into isolated TeO₃²⁻ trigonal pyramid-like ortho-groups. For a fixed Tl₂O concentration, the ZnO addition induces similar effects on the glass structure. The optical transmission of the ((80 − x)TeO₂-xTl₂O-20ZnO) (x = 10, 20 and 30 mol%) glasses was measured in the 300-2000 nm range. Their good transparency was evidenced and a clear reduction of the optical band-gap was noticed with the increase in the Tl₂O content. Finally, Second Harmonic Generation was unambiguously detected for each glass composition. The second order non-linearity amplitude is found to be increasing as a function of the Tl₂O concentration, in the tested range.