Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.     

Density doubling of block copolymer templated features

Block copolymers can be used to template large arrays of nanopatterns with periodicities equal to the characteristic spacing of the polymer. Here we demonstrate a technique capitalizing on the multilayered arrangement of cylindrical domains to effectively double the pattern density templated by a given polymer. By controlling the initial thickness of the film and the solvent annealing conditions, it was possible to reproducibly create density doubled lines by swelling the film with solvent until bilayers of horizontal cylinders were obtained. This process was also demonstrated to be compatible with graphoepitaxy.     

Frequency doubling of 859.2 nm radiation in a waveguide formed from Nb2O5 and Ta2O5 films on a KTiOPO4 substrate

It is proposed that second harmonic generation in a waveguide formed from Nb₂O₅ and Ta₂O₅ films on a KTiOPO₄ crystal substrate can be used to obtain blue coherent radiation. It is shown that the second harmonic generation efficiency in a thin-film stripe waveguide may reach 1010% W⁻¹·cm⁻², and the spectral phase-matching width may reach 0.45 nm·cm. Generation of the second harmonic of 859.2 nm laser radiation was observed experimentally in a Ta₂O₅-Nb₂O₅-KTiOPO₄ waveguide. Pis’ma Zh. Tekh. Fiz. 25, 26–34 (November 26, 1999)     

Passively Q-switched quasi-three-level laser and its intracavity frequency doubling

A passively Q-switched quasi-three-level Nd:YAG laser is intracavity frequency doubled to generate a blue laser. The 473-nm blue laser has a peak power of 37 W and a pulse width of 23 ns at a pumping power of 1.6 W. To model this laser numerically, we developed rate equations by taking into consideration both the quasi-three-level nature of the gain medium and the four-level nature of the saturable absorber. Good agreement was achieved between experimental and simulated results for both the fundamental and the second-harmonic output. The reabsorption loss of the gain medium is estimated under pulsed operation.     

Generator of quasi-periodic oscillations featuring two-dimensional torus doubling bifurcations

A new autonomous differential dynamical system with dimension N = 4 is introduced, which has solutions in the form of stable two-frequency oscillations and features a sequence of period-doubling bifurcations of two-dimensional ergodic tori. At the points of period-doubling bifurcations, no resonances are observed on a torus and only ergodic tori exhibit doubling.     

Angle-tuned type II external-cavity frequency doubling without temperature stabilization

We report frequency doubling of a low-power cw diode laser at 1064 nm in an external ring cavity by use of an angle-tuned KTP type II crystal. We demonstrate a new setup that requires no temperature stabilization of the crystal. An intracavity lambda/2 plate rotates the polarization of the fundamental after each cavity round trip by 90 degrees ; this provides the two eigenpolarizations of the crystal required for a type II nonlinear process. As a result, it is possible to maintain the resonance condition by use of a standard locking circuit.     

Efficient frequency doubling by a phase-compensated crystal in a semimonolithic cavity

In multiple-pass nonlinear frequency conversion devices, interacting waves may accumulate different phases, owing to dispersive elements in the system. Phase compensation is therefore necessary for efficient frequency conversion. We experimentally demonstrate phase compensation in a compact semimonolithic frequency-doubling cavity by using a periodically poled KTP crystal. The conversion efficiency of the crystal was found to decrease at high pump powers, owing to power-dependent thermal lensing. This experimental observation was supported by a theoretical calculation of the conversion efficiency in a cavity, considering the mismatch between the mode's thermally loaded and unloaded cavities. A design procedure was also presented to compensate for the thermal lensing effect. The highest conversion efficiency of 56.5%, corresponding to a second-harmonic power of 117.5 mW at 532 nm, was achieved with a cw Nd:YAG pump power of 208 mW.     

环形倍频腔系统稳定性的分析

为了实现通过腔外倍频钛宝石可调谐激光(851．1m)而得到所需波长(425．6nm)的光。首先根据二次谐波产生理论讨论了针对利用LBO晶体倍频851．1nm光时有关晶体参数的选择，然后在此基础上利用矩阵光学的方法分析了环型外谐振倍频腔的几何参数，包括腔的尺寸，反射镜的曲率半径等对系统稳定性的影响及倍频光输出功率稳定性的影响，并作了数值计算和分析。     

High-powered green light generation by intracavity frequency doubling using grating feedback optics

Grating feedback optics is shown to contribute to narrowing the spectral bandwidth of a multilongitudinal-mode laser diode to less than 0.2 nm and tuning the lasing wavelength to the peak absorption wavelength of Nd:YVO(4). A continuous green light of 31 mW was efficiently generated by intracavity frequency doubling of the Nd:YVO(4) laser with a KTiOPO(4) crystal. A relative intensity noise of less than -140 dB/Hz was obtained in the frequency region greater than 2 MHz. The noise characteristics of generated green light are discussed as compared with the case of using a single-longitudinal-mode laser diode as the pumping source.     

Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities

Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.     

Basins of attraction for chaotic attractors in coupled systems with period doubling

This article studies basins of attraction for chaotic attractors and their evolution when the attractors are restructured through variations in the control parameters. Numerical calculations show the evolution of the boundaries of the attractors, the changes within simply-connected regions, etc. Some of the regularities could be identified in a physical experiment. Pis’ma Zh. Tekh. Fiz. 23, 40–46 (February 26, 1997)     

振弦式频率读数仪频率测量值的不确定度分析

依据JJF1401-2013《振弦式频率读数仪》校准规范,对振弦式频率读数仪频率的测得值进行不确定度评定。     

Dynamics of Frequency Estimation in the Frequency Domain

The possibility of error reduction in frequency estimations with a multipoint interpolated discrete Fourier transformation (DFT) for the Hanning window is described. An estimation of the periodic parameter by the interpolation of the DFT gives the same effect as the reduction of spectrum tails. Sidelobe suppression is obtained at the cost of widening the main lobe, and this increases noise contributions. In this paper, we try to show a tradeoff between the reduction in systematic error of the frequency estimation and the uncertainty of the estimated results due to the interpolated algorithm. The number of interpolated points depends on the noise level and on the mutual positions of the frequency components of the signal.     

High-efficiency generation of a continuous-wave single-frequency 780 nm laser by external-cavity frequency doubling

We demonstrated a 670 mW continuous-wave single-frequency laser source at 780 nm by using external-cavity-enhanced second-harmonic generation of a seeded fiber amplifier in periodically poled lithium niobate. A maximum second-harmonic conversion efficiency of 58% was achieved. The source can work stably over 1 h by locking the frequency-doubling cavity, while the power stability is less than 2%.     

High-power 457-nm light source by frequency doubling of an amplified diode laser

We demonstrate the generation of 150-mW blue coherent single-mode radiation at 457 nm in a compact and inexpensive setup. The light is generated by frequency doubling the radiation of a master oscillator power amplifier (MOPA) system in an enhancement cavity with a potassium niobate (KNbO3) crystal. The MOPA consists of a 914-nm single-mode diode laser and a broad-area diode laser (BAL) as the amplifier. The BAL is a multimode laser with a specified wavelength of 938 nm. Sufficient gain at 914 nm is obtained by antireflection coating the BAL front surface and by cooling it to -10 degrees C.     

Physicochemical properties of organic nonlinear optical crystal for frequency doubling: Glycine acetamide

Non-linear optical (NLO) crystal of glycine acetamide (GA) is synthesized and grown in aqueous solution by slow evaporation technique. The cell parameters of the grown crystal are identified from single crystal XRD and the functional groups are confirmed in vibrational analysis. Temperature dependence of the material is probed using TGA/DTA. Mechanical strength of the grown material is tested by Hardness studies. UV-VIS-NIR spectral study explains the transmission ability of the crystal in visible range also the photoluminescence spectrum explains the transition mechanism of ions. The NLO activity of the crystal is confirmed by Kurtz power technique.     

微波低通高阻频率选择复合材料的设计研究

采用数值法设计了8～12GHz(X波段)具有高反射同时2～4GHz(S波段)具有高透射的频率选通复合材料.采用有限元法(FEM)计算了含导电纤维复合材料的传输和反射系数,并用自由空间法对所制备的多层凯夫拉(Kevlar)纤维增强的复合材料样板(424mm× 424mm)进行测量.测量结果与计算结果具有良好的一致性.同时发现复合材料基材的介电特性和所嵌入金属纤维的电导率对材料的传输损耗有很大影响.