Lasers characterization

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

Wolleschensky R, Feurer T, Sauerbrey R and Simon U 1998 Characterization and optimization of a laser-scanning microscope in the femtosecond regime Appl. Phys. B 67 87-94... 

In this chapter we review some of the most important developments in recent years in connection with the use of optical teclmiques for the characterization of surfaces. We start with an overview of the different approaches available to tire use of IR spectroscopy. Next, we briefly introduce some new optical characterization methods that rely on the use of lasers, including nonlinear spectroscopies. The following section addresses the use of x-rays for diffraction studies aimed at structural detenninations. Lastly, passing reference is made to other optical teclmiques such as ellipsometry and NMR, and to spectroscopies that only partly depend on photons. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

There are a few other surface-sensitive characterization techniques that also rely on the use of lasers. For instance surface-plasmon resonance (SPR) measurements have been used to follow changes in surface optical properties as a fiinction of time as the sample is modified by, for instance, adsorption processes [ ]. SPR has proven usefiil to image adsorption patterns on surfaces as well [59]. 

Yoo C S, Akella J and Nicol M 1996 Chemistry at high pressures and temperatures in-situ synthesis and characterization of p-SijN by DAC x-ray/laser-heating studies Advanced Materials 96 ed M Akaishi et al (Tsukuba National Institute for Research in Inorganic Materials) p 175... 

Ippen E 1997 Characterizing optical components for ultrafast laser applications Optics 1997/98 Catalog (Irvine, CA Newport Corp.) pp 8-2-8-3... 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Kellermayer et aJ., 1997] Kellermayer, M., Smith, S., Granzier, H., and Bustamante, C. Folding-unfolding transition in single titin modules characterized with laser tweezers. Science. 276 (1997) 1112-1116... 

Temporal Characteristics. Laser operation may be characterized as either pulsed or continuous. There are a number of distinctive types of pulsed laser operation having widely different pulse durations. 

An interesting variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS) (99). If two laser beams, with angular frequencies CO and CO2 are combined in a material, and if cjj — is close to a Raman active frequency of the material, then radiation at a new frequency CJ3 = 2cJ2 — may be produced. Detection of this radiation can be used to characterize the material. Often one input frequency is fixed and the other frequency, from a tunable laser, varied until matches the Raman frequency. CARS has the capabiHty for measurements in flames, plasmas, and... 

An even wider range of wavelength, toward the infrared, can be covered with quantum well lasers. In the Al Ga As system, compressively strained wells of Ga In As are used. This ternary system is indicated in Figure 6 by the line joining GaAs and In As. In most cases the A1 fraction is quite small, X < 0.2. Such wells are under compressive strain and their thickness must be carefully controlled in order not to exceed the critical layer thickness. Lasers prepared in this way are characterized by unusually low threshold current density, as low as ca 50 A/cm (l )-... 

Particle Size. Wet sieve analyses are commonly used in the 20 )J.m (using microsieves) to 150 )J.m size range. Sizes in the 1—10 )J.m range are analyzed by light-transmission Hquid-phase sedimentation, laser beam diffraction, or potentiometric variation methods. Electron microscopy is the only rehable procedure for characterizing submicrometer particles. Scanning electron microscopy is useful for characterizing particle shape, and the relation of particle shape to slurry stabiUty. 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Because high quaHty, low cost, and optimum performance are required for spray equipment, improved analytical and experimental tools are iadispensable for increasing productivity ia many competitive iadustries. In most iastances, it is no longer adequate to characterize a spray solely on the basis of flow rate and spray pattern. Information on droplet size, velocity, volume flux, and number density is often needed and can be determined usiag advanced laser diagnostic techniques. These improvements have benefited a wide spectmm of consumer and specialized iadustrial products. 

A,/2 for topography characterization much smaller for periodic structure characterization (A, is the laser wavelength used to illuminate the sample)... 

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