Size distribution of mineral aerosol: using light-scattering models in laser particle sizing

The size distribution of semitransparent irregularly shaped mineral dust aerosol samples is determined using a commonly used laser particle-sizing technique. The size distribution is derived from intensity measurements of singly scattered light at various scattering angles close to the forward-scattering direction at a wavelength of 632.8 nm. We analyze the results based on various light-scattering models including diffraction theory, Mie calculations for spheres with various refractive indices, and T-matrix calculations for spheroidal particles. We identify systematic errors of the retrieved size distribution when the semitransparent and nonspherical properties of the particles are neglected. Synthetic light-scattering data for a variety of parameterized size distributions of spheres and spheroids are used to investigate the effect of simplifying assumptions made when the diffraction model or Mie theory is applied in the retrieval.     

Research on ultrafine particle size measurement using small amounts of data

The paper puts forward a new method of ultrafine particle size measurement using small amounts of data of a dynamic light-scattering signal, and establishes an arithmetic model of the measurement by wavelet package transform. First, through the wavelet package transform, the ultrafine particle dynamic light-scattering signals were decomposed into multifrequency bands. Then, the noise of signals of different frequency bands were removed and the power spectrum of the wavelet packet coefficients of each frequency band was calculated. Finally, the ultrafine particle size distribution information could be deduced from inversing the power spectrum. The standard polystyrene particles of 100, 300, and 400 nm were measured using this method, and the inversion results indicated that this method can effectively remove noise and improve the accuracy of particle size measurement using small amounts of data.     

Measurements of laser phase fluctuations induced by atmospheric turbulence over 2 km and 17.5 km distances

A laser heterodyne system was used to measure the phase fluctuations imposed on a 1.5?μm wavelength laser beam when double-passed over long atmospheric paths. Two distances were used: 2 and 17.5?km. Results are given for intensity scintillation, phase fluctuation time series and spectra, and phase structure function. The results are found to agree well with theory: the spectrum of phase fluctuations follows the 8/3 power law predicted for Kolmogorov turbulence over 3 orders of magnitude in frequency. The methods reported here could be used to investigate large-scale temperature variations in the atmosphere.     

Broadband radio-frequency spectrum analysis in spectral-hole-burning media

We demonstrate a 20 GHz spectrum analyzer with 1 MHz resolution and >40 dB dynamic range using spectral-hole-burning (SHB) crystals, which are cryogenically cooled crystal hosts lightly doped with rare-earth ions. We modulate a rf signal onto an optical carrier using an electro-optic intensity modulator to produce a signal beam modulated with upper and lower rf sidebands. Illuminating SHB crystals with modulated beams excites only those ions resonant with corresponding modulation frequencies, leaving holes in the crystal's absorption profile that mimic the modulation power spectrum and persist for up to 10 ms. We determine the spectral hole locations by probing the crystal with a chirped laser and detecting the transmitted intensity. The transmitted intensity is a blurred-out copy of the power spectrum of the original illumination as mapped into a time-varying signal. Scaling the time series associated with the transmitted intensity by the instantaneous chirp rate yields the modulated beam's rf power spectrum. The homogeneous linewidth of the rare-earth ions, which can be <100 kHz at cryogenic temperatures, limits the fundamental spectral resolution, while the medium's inhomogeneous linewidth, which can be >20 GHz, determines the spectral bandwidth.     

Angular spectrum of light transmitted through turbid media: theory and experiment

Measurements of the angular spectrum of light transmitted through turbid slabs with monodispersions of polystyrene spheres have been performed. The results obtained are compared with theoretical calculations, based on the small-angle approximation of the radiative transfer theory. The experimental data and the theoretical results coincide with a high accuracy, which allows us to develop the laser diffraction spectroscopy of optically thick light-scattering layers.     

Proposal and testing of dual-beam dynamic light scattering for two-particle microrheology

A dual-beam dynamic light-scattering arrangement is devised to measure the time-dependent mean squared relative displacement of a pair of tracer particles with a small separation of micrometers. The technique is tested by the measurement of the relative diffusion of polymer latex spheres suspended in a simple viscous fluid. The experiment verifies the theory and demonstrates its applications. The dual-beam dynamic light-scattering technique, when combined with an optical microscope, provides a powerful tool for the study of two-particle microrheology of soft materials. The advantages of the new technique are its high statistical accuracy, faster temporal response, and ease of use.     

Mode-selective dynamic light scattering: theory versus experimental realization

We present a quantitative experimental comparison of fiber-based, single- and few-mode dynamic light scattering with the classical pinhole-detection optics. The recently presented theory of mode-selective dynamic light scattering [Appl. Opt. 32, 2860 (1993)] predicts a collection efficiency and a signal-tobaseline ratio superior to that of a classical pinhole setup. These predictions are confirmed by our experiments. Using single-mode optical fibers with different cutoff wavelengths and commercially available mechanical components, we have constructed a mode-selective detection optics in a simple and compact dynamic light-scattering spectrometer that permits an optimal compromise between signal intensity and dynamical resolution.     

Particle sizing with a simple differential light-scattering photometer: homogeneous spherical particles

A simple light-scattering photometer has been designed to measure the angular distribution of the intensity of polarized laser light scattered by micrometer and submicrometer samples. The photometer uses an ellipsoidal reflector and simple optical components to collect the He-Ne laser-scattered light and to focus it onto a 512-element photodiode array. Experimental data have been obtained for several monodisperse aqueous solutions of latex spheres of different sizes. The results have been satisfactorily interpreted on the basis of the electromagnetic scattering theory. In particular the Mie formulation has been properly reformulated to take into account the corrections to the scattered intensity determined by the apparatus design making it possible to perform particle sizing.     

Some Statistical Properties of Random Speckle Patterns in Coherent and Partially Coherent Illumination

The power spectrum of the intensity fluctuations in a random speckle pattern is derived in a simplified manner for coherent illumination. The effect of the power spectrum on the measured signal-to-noise ratio is discussed, and some preliminary measurements of the power spectrum are presented. The power spectrum of the intensity fluctuations for partially coherent illumination is derived for the special case of an aberration-free system, using quadratic filter theory.     

Influence of laser power on photoluminescence of hexagonal ZnO nanodisks

Raman scattering and photoluminescence spectra of hexagonal ZnO nanodisks were investigated. Normal Raman spectrum shows the intense E2 modes, which confirms a typical wurtzite structure. The broadened E2 vibration peak is due to surface/interface defect in ZnO crystal. The influence of laser irradiation heating effect on emission intensity and position has been investigated by power dependent PL and resonant Raman scattering. The threshold value of the power density for laser heating effect is 2 kW/cm2. The shift of LO phonon vibrational peaks from RRS with excitation laser power also confirms laser irradiation heating effect. Beyond the threshold value of power density, the PL intensity and position will be affected by laser heating effect.     

Compact laser light-scattering instrument for microgravity research

NASA has developed a compact laser light-scattering instrument that employs both static and dynamic light-scattering techniques for microgravity research. The first use of this instrument was to study the behavior of colloidal hard spheres in a reduced gravity environment during the Second United States Microgravity Laboratory space shuttle mission. We discuss the instrument design and possible improvements based on our observations of significant differences between hard-sphere behavior in Earth's gravity and microgravity.     

Optico-acoustic characterization of the fractal structure of radiation from a laser with unstable resonator

The thermooptical excitation of sound in a liquid by fluctuating laser radiation with sine-modulated intensity is studied theoretically. The intensity is randomly distributed over the beam cross section. It is assumed that the random processes are homogeneous and that the spatial spectrum of intensity fluctuations obeys a power fractal law. Possibilities for the optico-acoustic characterization of the fractal radiation structure of a laser with unstable resonator are discussed.     

Range-dependent Optical Feedback Effects on the Multimode Spectrum of Laser Diodes

Abstract

Weak optical feedback from diffusely-reflecting objects can dramatically affect the optical spectrum of homogeneously-broadened laser diodes operated near the lasing threshold. Switching between two or more oscillation modes can be controlled by as little as 1 nW of optical feedback power. The modulation effects on the optical spectrum and total power output depend on the phase of the returned light, and the strength of the effect has a range dependency that is useful for metrology applications. In the present work, experimental and theoretical analyses of these phenomena are presented, and a simple metrology tool for position sensing and remote surface profiling is proposed.     

System model identification of a Fabry-Perot fiber optic sensingsystem

A Fabry-Perot fiber optic temperature sensing system is presented in this paper. It uses a Fabry-Perot interferometer (FPI) to transform the environmental temperature into modulated reflected light. This light is directed to a patented light cross-correlator that locates the position of the maximum interference fringe intensity, which is detected by a linear CCD array. Therefore, the actual observed data is the position of the CCD pixel detecting the maximum interference fringe intensity rather than the light intensity itself. Consequently, this sensing mechanism is tolerant to the loss of light power that may result from external effects. Based on an analysis of the entire sensing system, a theoretical dynamic model was developed, which shows that the system dynamic response depends on the heat transfer process in the sensor head and the signal processing in the signal conditioner. An experimental method was developed to validate the theoretical model. Two empirical dynamic models are also obtained from the experimental data. Comparing the theoretical model with the empirical models, the poles result from the heat transfer process in the sensor head     

Raman sensitivity enhancement for aqueous protein samples using a liquid-core optical-fiber cell

We have demonstrated a sensitivity enhancement factor of 500 in aqueous solutions using a liquid core optical fiber (LCOF) Raman cell made from Teflon-AF. We were able to collect a spectrum of 54 microM lysozyme with a signal-to-noise ratio of 31 in the LCOF Raman cell using 24 mW of laser power and 3 min of integration time. The lysozyme Raman intensity was only 1% of the background Raman intensity from water, but the water-subtracted lysozyme spectrum was still shot-noise-limited and essentially free of nonrandom noise. The lack of nonrandom noise indicates that it should be possible to collect good quality Raman spectra of proteins such as lysozyme at even lower concentrations. The 2.4-microL sample volume of the LCOF Raman cell is an added benefit when limited quantities of sample are available. This volume of a 54 microM lysozyme solution corresponds to only 13 nanomoles or 1.9 microg of lysozyme.     

FBG光纤的飞秒激光刻蚀特性及传感应用

在布拉格光栅（FBG）光纤的包层中制作微槽,结合敏感材料提高了FBG光纤传感器的灵敏度,拓宽了其传感应用领域。利用飞秒（fs）激光对I型布拉格光栅光纤进行刻蚀,通过调节激光功率和辐照时间,分析光纤折射率变化以及光纤表面和内部结构的刻蚀特性,并探讨fs激光对光纤光栅微沟槽形貌和反射光谱的影响。实验结果表明,微槽表面由于碎屑的凝固而产生许多柱状结构,且随着激光能量的增加,柱状结构不断长大,槽深和形状服从高斯能量分布;随着激光功率和辐照时间的增加,反射率谱永久红移,带宽增加。利用微结构光纤增敏性能,有效提高微结构光纤光栅磁场探头和氢气探头的传感性能。     

Stability and nonlinear optical properties of ZnO nanoparticles

Photoluminescence (PL) of ZnO nanoparticles of different surface states and sizes grown by several methods has been measured. The origin of luminescence and dependence of the luminescence spectrum shape and intensity on 325 nm excitation laser power are studied. Strong ultraviolet emission at 3.26 eV, weak violet emission around 3.12 eV and weak green emission at 2.40 eV have been observed in 16 nm nanoparticles capped by octylamine grown by non-hydrolytic method. The nanoparticles are stable under high power laser radiation and their PL intensity increases nonlinearly with an increasing laser power. As the nanoparticle size decreases to 12 nm, high power laser produces nonradiative centers which may quench the luminescence in a degree. Nanoparticles of 8 nm capped by PVP and uncapped nanoparticles of 14 nm are unstable and their luminescence depends on the excitation laser power. High power laser can quench O vacancy emission and enhance ultraviolet emission in PVP capped nanoparticles while vacancy emission can not be quenched in uncapped nanoparticles.     

Stimulated photoluminescence and optical limiting in CdI2

High quality single crystal of cadmium iodide (CdI₂) grown from the melt by the refining method was studied under ruby laser excitation. The low temperature (LNT) emission spectrum of the crystal was recorded by using a spectrofluorometer. The spectrum appeared a structureless emission with its peak in the green spectral region. Analysis of photoluminescence (PL) measurements in the crystal showed a quadratic dependence of the PL peak intensity on incident laser power. This might be explained with the existence of self-trapped excitons in the crystal. In the desired energy ranges of interest, the emission intensity of CdI₂ was found to depend on the square of the laser power, indicating the biphotonic process of two-photon excitation, i.e., the nonlinear response is due to a second-order process, or two-photon absorption (2PA) process. The optical limiting properties of CdI₂ were studied using 7 ns pulses from the laser. This investigation leads to the conclusion that apart from the 2PA reverse saturable absorption (RSA) is another mechanism for optical limiting in CdI₂.     

基于F-P标准具和Music算法的FBG动态应变传感系统

提出了基于可调光纤环形激光器结构,并采用F-P标准具解调的光纤布拉格光栅(FBG)动态应变传感系统,具有高达60 dB的光学信噪比.F-P标准具用来作为一个边缘滤波器探测FBG波长的漂移,这样的解调方式具有稳定性高的优点.为了提高动态应变测量系统的分辨率,采用了Music算法进行频谱分析.实验结果显示,在700Hz时的动态应变分辨率达到了0.1με/Hz~(1/2),是传统FFT算法的10倍.     

Line narrowing effects and enhanced back scattering from ZnO colloids

The dependence of the fluorescence emission intensity from nano ZnO colloid as a function of incident laser power is investigated. Emission in the near UV region from ZnO particles in diethylene glycol medium is studied using frequency tripled radiations at 355 nm from Nd-YAG laser. The spectrum, which was broad at lower pump intensities, exhibits an increase in the intensity as well as line narrowing above a threshold. The emission occurs in all directions and varies with pumping area. Results indicate the phenomenon of random lasing action due to multiple scattering inside the highly disordered medium. Coherent back scattering experiments confirm multiple scattering and weak localization effects in these samples. These preliminary studies show that colloidal nano ZnO medium is a promising candidate for random lasers.