High repetition rate pulsed laser of twin wavelengths from KTiOPO4 optical parametric oscillation

In this paper, some key techniques of KTiOPO_4 optical parametric oscillation (OPO) laser with repetitive frequencies and twin wavelengths output have been analysed theoretically and studied experimentally in detail. An intracavity optical parametric oscillator (IOPO) has been applied. Operating under 20Hz, laser output energy of more than 38mJ with twin wavelengths 1.57μm and 1.06μm has been achieved. Results of the experiments agree well with the theoretical discussion.     

Crystalline Phase Formation of Poly(vinylidene fluoride) from Tetrahydrofuran/N,N‐dimethylformamide Mixed Solutions

The present work focused on the effect of the interactions between poly(vinylidene fluoride) (PVDF) chains and solvent molecules on the structure and crystallization behavior of PVDF in films obtained by solution casting. In a single solvent system, the film cast from the good solvent of N,N‐dimethylformamide (DMF), showed dominantly β‐phase crystals with the highest PVDF crystallinity (50.6%) and the largest spherulite size, about 4 μm, at the top surface. The samples deposited from good swelling agents, such as tetrahydrofuran (THF) and methyl ethyl ketone (MEK), exhibited mainly the original α phase with some amount of β‐phase crystals; the crystallization behavior and the morphology of the surface were similar to the original PVDF resin, because of the only partially dissolved PVDF chains in these two solvents. In a mixed solvent system (THF/DMF), the β phase formation linearly increased as the DMF component increased, determined by Fourier transform infrared spectroscopy (FTIR) techniques, owing to increased interactions between PVDF chains and DMF molecules. The film surface consisted of β spherulites with average size of about 3 μm, which were smaller than those grown from pure DMF, because of the increased crystallization rate in the mixed solvent.     

Local phase-structure modification inside of lithium silicate glass by combined double-wavelength laser action

The process of local structure modification inside of lithium silicate glass under the combined laser action of two different wavelengths is considered. The first step is laser irradiation of ultrashort laser pulses with 532 nm wavelength, which is used to create of nucleation centers inside of the optically transparent glass. The crystallization of the structural modification areas was carried out by a photothermal action of CO₂ laser radiation with a 10.6 μm wavelength. The range of crystallization temperatures was defined and the kinetics of the phase transformations of the modified regions inside of the glass were studied. Duration of crystallization was about 10 min with a slow heating and 25 s at the fast heating to crystallization temperature.     

固体发动机羽烟的激光透过率测试

为获得羽烟对激光透过率的影响,用烟箱法对2种配方的缩比发动机羽烟在1.06 μm、10.6 μm激光波段的透过率进行测试.采用1.064 μm激光调制发射、接收、数据采集系统对1.06 μm激光波段烟雾透过率测试;用黑体、光谱辐射计、数据采集系统可测出2 μm ～13 μm 的光学透过率,从中提出10.6 μm激光波段烟雾透过率,得到不同推进剂配方、不同烟雾浓度情况下10.6 μm光波和1.06 μm光波的烟雾透过率测试数据.烟箱1.8 m烟道上的测试数据表明:配方2推进剂优于配方1推进剂,10.6 μm光波的烟雾透过率96%～97%大于1.06 μm光波的烟雾透过率92%～93%.     

Structural and Near-Infrared Properties of Nd<Superscript>3+</Superscript> Activated Lu<Subscript>3</Subscript>NbO<Subscript>7</Subscript> Phosphor

Undoped and Nd³⁺ doped lutetium niobate phases have been prepared by a conventional solid state reaction method using lutetium acetate and niobium oxide at 1250 °C for 6 h. X-ray diffraction patterns of the 6 mol% Lu₃NbO₇ sample exhibited a cubic fluorite single phase. Phase structure exhibited interesting crystallization behaviour depending on increasing Nd³⁺ concentration which led to a Lu₃NbO₇ single phase formation during the heat treatment process. SEM investigations were also in agreement with the XRD results. Morphologies of Nd³⁺ doped lutetium niobate powders exhibited oval like shapes and grain sizes varied between 0.3 and 5 μm. Near-infrared luminescence properties of Nd³⁺ doped Lu₃NbO₇ were also studied. 1.06 μm laser transition characteristics of Nd³⁺ doped lutetium niobate have been observed. Concentration quenching phenomenon was not detected depending on increasing Nd³⁺ doping concentrations at room temperature.     

Radiation effects on poly(methyl methacrylate) induced by pulsed laser irradiations

Poly(methyl methacrylate) (PMMA) was irradiated using a medical UV-ArF excimer laser operating at the fundamental wavelength of 193 nm. Characterized by a beam diameter of 1.8 mm and energy of 180 mJ with a Gaussian energy profile, it operates in a single mode or at 30 Hz repetition rate. Mechanical profilometry was carried out on ablation craters in order to study the rugosity and the ablation yield in the various operative conditions. Optical transmission and reflection measurements at six wavelengths were conducted in order to characterize the optical properties of the irradiated surfaces. Measured crater depths in PMMA were lower with respect to the forecasted ones in corneal tissue, while the lateral crater aperture was maintained. The rugosity produced at the crater bottom after irradiation was about 0.3 μm, and the ablation yield was about 10¹⁵ molecules/laser pulse, while etching depth and diameter show a roughly linear dependence on the number of laser shots. These experiments constitute a base for deeper clinical investigations.     

烟雾激光透过率测试系统关键技术研究

为准确评估0.808μm、1.06μm、10.6μm激光对于推进剂包覆层烟雾的透过率,研制了激光烟雾透过率测试系统。国内首创10.6μm CO2激光器烟雾透过率测试系统,方案新颖,测试结果准确。经现场测试,该系统的精度、稳定度等指标均达到了设计要求。     

Measurements of light scattering by a characterized random rough surface

Abstract

Measurements are presented of the angular distribution of four wavelengths of light scattered by a one-dimensional random rough surface, whose probability density function is Gaussian with a standard deviation σ=1.22±0.02 μm and whose lateral correlation function is also Gaussian with 1/e width τ=3.17±0.07 μm. The wavelengths used are 0.63, 1.15, 3.39 and 10.6 μm. The surface is used in two forms: coated with gold and as an almost lossless dielectric. The results are compared to those predicted by a double scattering form of the Kirchhoff formulation. Agreement is good at small angles of incidence but less good at larger angles of incidence.     

In-band pumping of Nd-vanadate thin-disk lasers

We investigate the output performance of the 1.06 μm ⁴F_3/2→⁴I_11/2 transition in Nd:GdVO₄ and Nd:YVO₄ thin-disk lasers under multi-pass pumping with diode lasers at 0.81 μm and at 0.88 μm, which corresponds to direct in-band pumping of the ⁴F_3/2 emitting level. The use of a pump module with 24 passes through the crystal allowed the realization of an in-band pumped Nd:GdVO₄ thin-disk laser with 14.9 W of continuous wave (cw) output power at 1.06 μm; the overall optical-to-optical efficiency was 0.50 and the slope efficiency with respect to the incident pump power was 0.52. Intracavity frequency-doubling of the Nd:GdVO₄ thin-disk laser with a LiB₃O₅ (LBO) nonlinear crystal yielded 9.1 W of cw output power in the green at 0.53 μm with an overall optical-to-optical efficiency of 0.31.     

Limits of applicability of the laser-location equation in the optical probing of clouds

By simultaneously solving the transient transport equation and the equation of laser location, the effect of the multiple-back-scattering background in a monostatic cloud-probing system on the accuracy of the optical measurement of back-scattering coefficients is analyzed. The transport equation is solved by the Monte Carlo method. Calculations are carried out for the most promising optical-probing wavelengths λ = 0.6943; 2.36 and 10.6μ, and a comparative analysis of their efficiencies is presented. The optical characteristics of a polydispersed liquid-drop cloud (one with a wide spread of particle sizes) are presented for the wavelengths in question.     

Infrared absorption in a high density electron-hole plasma in germanium at low temperatures

Measurements of the absorption in the temperature range 1.8–9 K have been made for wavelengths between 3.1 and 4.2μm, in germanium highly excited by a 1.06μm laser. The shape of the absorption curve is noted to be similar to that observed by Pokrovskii and Svistunova at lower excitation levels, and somewhat broader than that calculated by Smith, Chen and McGill for exciton absorption. A cross section of 4.5 (±1.1) × 10^-17 cm² at 3.39μm is deduced from normalization with the theoretical curves of Smith et al.     

Efficient 1.06 μm laser operation in an unprocessed Nd3+:BaGd2(MoO4)4 cleavage microchip

A 1.2 mm thick microchip cleaved from a 1.2 at.% Nd³⁺: BaGd₂(MoO₄)₄ crystal was used as the laser gain medium directly. The laser cavity mirrors were coated on the microchip without cutting and polishing. In the 1.06 μm quasi-CW laser operation, the microchip was pumped by a Ti:sapphire laser with a duty cycle of 10%, the threshold is 10 mW, and the slope efficiency is 50%. In the CW laser operation pumped by a diode laser, the threshold and slope efficiency are 140 mW and 33%, respectively. The results demonstrate that the unprocessed surfaces of the cleaved microchip have good enough flatness and parallelism for laser operation.     

Ultra-high resolution spectral domain optical coherence tomography using supercontinuum light source

An ultra-high resolution spectral domain optical coherence tomography (SD-OCT) was developed using a cost-effective supercontinuum laser. A spectral filter consists of a dispersive prism, a cylindrical lens and a right-angle prism was built to transmit the wavelengths in range 680–940 nm to the OCT system. The SD-OCT has achieved 1.9 μm axial resolution and the sensitivity was estimated to be 91.5 dB. A zero-crossing fringes matching method which maps the wavelengths to the pixel indices of the spectrometer was proposed for the OCT spectral calibration. A double sided foam tape as a static sample and the tip of a middle finger as a biological sample were measured by the OCT. The adhesive and the internal structure of the foam of the tape were successfully visualized in three dimensions. Sweat ducts was clearly observed in the OCT images at very high resolution. To the best of our knowledge, this is the first demonstration of ultra-high resolution visualization of sweat duct by OCT.     

Effect of PMMA on crystallization behavior and hydrophilicity of poly(vinylidene fluoride)/poly(methyl methacrylate) blend prepared in semi-dilute solutions

Films of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend were derived from a special procedure of casting semi-dilute solutions. Hydrophilic character and crystallization of PVDF were optimized by variation of PMMA concentration in PVDF/PMMA blends. It was found that a PVDF/PMMA blend containing 70 wt% PMMA has a good performance for the potential application of hydrophilic membranes via thermally induced phase separation. The films presented β crystalline phase regardless of PMMA content existed in the blends. Thermal analysis of the blends showed a promotion of crystallization of PVDF with small addition of PMMA which induced larger lamellar thickness of PVDF, leading to the largest spherulitic crystal of PVDF (10 wt% PMMA) is about 8 μm. SEM micrographs illustrated no phase separation occurred in blends, due to the high compatibility between PVDF and PMMA.     

Correction of small imperfections on white glazed china surfaces by laser radiation

A laser-assisted technique has been developed for correction of small diameter (1 mm) and shallow (0.5 mm) imperfections on the surface of gloss fired porcelain. To study the physics and establish the important parameters, artificially made holes in a porcelain sample have been first filled with correction material, then covered with raw glaze and treated by a pulsed, 7 kHz repetition rate CO₂ laser at 10.6 μm. The modification of the surface and the surrounding area have been quantified and studied with a large range of parameters of incident laser power (1-10 W), width of the laser pulses (10-125 μs) and duration of laser heating (60-480 s). Although the shine of the treated area, defined as the distribution of micro-droplets on the surface, is very similar to the untreated surfaces, the surroundings of the treated area usually show cracks. The measurement of both the spatial temperature distribution and the temporal cooling rate of the treated surface has revealed that a simple melting process always results in high gradient temperature distribution within the irradiated zone. Its inhomogeneous and fast cooling always generate at least micro-cracks on the surface within a few seconds after the laser was turned off. The duration and intensity of the laser irradiation have been then optimized in order to achieve the fastest possible melting of the surface, but without producing such high temperature gradients. To eliminate the cracks, more elaborated pre-heating and slowed-cooling-rate processes have been tried with prosperous results. These achievements complete our previous study, making possible to repair the most common surface imperfections and holes of gloss fired china samples.     

Interaction of fine-dispersed aluminum particles in an argon medium with powerful laser radiation at different wavelengths

We have experimentally studied the dynamics of the extinction coefficients of laser radiation at wavelengths 10.6, 1.06, and 0.53 μm in a gas-dispersed medium based on aluminum aerosol in an argon atmosphere. Based on measurement results, we have derived calculation relationships for the concentration of aerosol particles and their size and analyzed the interaction efficiency of the radiation at these wavelengths with particles of the fine-dispersed medium.     

Watt-level, all-fiber supercontinuum source based on telecom-grade fiber components

Supercontinuum (SC) generation in a standard telecom fiber using 1 ns pulses of a 1,550-nm DFB laser amplified in a cascade of erbium and erbium/ytterbium fiber amplifiers is reported. The SC source operated at 200 kHz repetition rate and delivered up to 2 W of average output power in the band of 1,300–2,500 nm with a diffraction limited beam. For the wavelengths over 1,650 nm, the output power of 1.1 W was recorded. The spectrum was very flat with the flatness of <5 dB in the wavelength interval of 1.6–2.18 μm. To the best of our knowledge, it is the first report on W-level SC generation obtained only in a standard single-mode fiber (SMF-28) with almost the entire spectrum in the eye-safe spectral region (λ > 1.4 μm) permitted by silicate glass transparency.     

Morphology and Pore Size Distribution of Biocompatible Interconnected Porous Poly(L-lactic acid) Foams with Nanofibrous Structure Prepared by Thermally Induced Liquid–Liquid Phase Separation

Biocompatible, highly interconnected microporous poly(L-lactic acid) (PLLA) foams with nanofibrous structure, containing pores with average diameter below 1 μm and fibers with diameters of 10² nm scale, were prepared through the thermally induced liquid–liquid phase separation (TIPS) method consisting of quenching of the PLLA solution, freeze extraction with ethanol, and vacuum drying. Diverse foam morphologies were obtained by systematically changing parameters involved in the TIPS process, such as polymer concentration, solvent composition, and quenching temperatures. The morphology of different foams was examined by scanning electron microscopy to characterize the pore size and the pore size distribution. The results showed that most porous foams had a nanofibrous structure with interconnected open pores. In the case of using tetrahydrofuran (THF) as solvent, the higher the PLLA concentration, the smaller the average pore diameter and the narrower the pore size distribution. In the case of using the mixed solvents of THF/DOX (1,4-dioxane) with higher than 6/4 volume ratio, there appeared a maximum value of average pore diameter and a widest pore size distribution at 0.09 g/mL PLLA concentration. The average pore diameter of the foams increased with increasing DOX content in the mixed solvent and ranged from 0.2 to 0.9 μm depending on the process parameters. When the DOX content reached 60% by volume, the morphology of the foams contained some large closed pores with diameter ranging from 1 to 10 μm. By decreasing the quenching temperature, the average pore diameter of foams decreased and the pore size distribution became narrower. All the pore size distribution fit F-distribution equations.