Lasers source

Infrared ultrashort laser sources with high pulse energy are most commonly used with pulse length ranging from about 200 fs to some picoseconds and a power up to few hundreds of milliwatts. These types of laser are in safety class 3B. The emitted radiation is potentially dangerous for eyes and skin. Specular reflections 

Tunability of the frequency difference of the laser beams is necessary in order to enable matching the frequency of Raman transitions, therefore one of the laser sources, normally the one delivering the Stokes beam, is tunable in wavelength. 

Up to now, the most common laser sources used for CARS microscopy are based on Tirsapphire or Nd YVO lasers with pulse durations from tens of femtoseconds up to 10 ps. Different approaches are possible in order to generate pump and Stokes beams use of two femtosecond laser sources electronically synchronized [19], pumping of an optical parametric amplifier (OPA) to produce the Stokes beam and use of the residual pump as pump beam [18], pumping of an optical parametric oscillator (OPO) to obtain the pump beam and use the residual pump light as Stokes [20], using signal and idler beams from a synchronously pumped OPO to provide directly the two excitation beams [11, 21]. 

More recently, fiber laser sources have come to an age in CARS microscopy [22-27]. This new laser sources are small and reliable, particularly suited for a future translation of the technique to the clinical environment. 

This model has been used to calculate the optical emission intensities in a DC argon glow discharge with a copper cathode, and the results were found to agree well with experimentally measured intensities [580]. Sputtering rates and depth profiles could also be calculated and were found to agree well with experimentally determined values, also in the case of an RF discharge [581]. 

With advanced measurement techniques, the results of the calculations could be readily verified, as performed with laser-excited atomic fluorescence spectrometry in the case of atom densities (Fig. 128) [582] and with laser-scattering experiments for the determination of gas and electron temperatures as well as electron number densities [583, 584]. 

Further progress in the area of atomic spectrometry with glow discharges is presented in Ref [585]. 

High-powered lasers have proved to be useful sources for the direct ablation of solids. In atomic emission spectrometry, ruby and Nd YAG lasers have been used since the 1970s for solids ablation. When laser radiation interacts with a solid, a laser plume is formed. This is a dense plasma containing both atomized material and small solid particles that have evaporated and or have been ejected from the sample due to atom and ion bombardment. The processes occurring and the figures of merit in terms of ablation rate, crater diameter (around 10 pm), and depth 

There has been renewed interest in the method, mainly due to the availability of improved Nd YAG laser systems. In addition, different types of detectors, such as microchannel plates coupled to photodiodes and CCDs, in combination with multi-channel analyzers make it possible for an analytical line and an internal standard line to be recorded simultaneously, by which the analytical precision can be considerably improved. By optimizing the ablation conditions and the spectral observation, detection limits obtained using the laser plume as a source for atomic emission spectrometry are in the 50-100 pg/g range and RSDs are in the region of 1% as shown by the determination of Si and Mg in low-alloyed steels [255, 259]. This necessitates the use of slightly reduced pressure, so that the atom vapor cloud is no longer optically very dense and the background emission intensities become lower. In the case of laser ablation of brass samples at normal pressure and direct 

From ratios of C C1 line intensities, it is possible to determine empirical formulae, which makes the method very valuable for the identification of compounds when applying element-specific detection in gas chromatography. McLuckey et al. 

There has been renewed interest in the method, mainly due to the availability of improved Nd YAG laser systems. In addition, different types of detectors, such as microchannel plates coupled to photodiodes and CCDs, in combination with mul- 

New advanced fs-lasers were recently foimd to produce less buffer-gas plasma above the sample surface and espedaUy at reduced pressure, and thus allow more accurate determinations of Cu and Zn in brass samples to be performed than with ns pulses [492]. 

Equipment for atomic emission spectrometry is now available from various manufacturers. Several types of sources [arc, spark, ICP, MIP, glow discharge (GD), etc.] are offered as indicated for the respective firms, of which most are listed below. 

Analytical Atomic Spectrometry with Flames and Plasmas. Jose A. C. Broekaert CopyrigJit 2002 Wiley-VCH Verlag GmbH Co. KGaA ISBNs 3-527-30146-1 (Hardback) 3-527-60062-0 (Electronic) 

The flow in these devices is subsonic with velocities of about 100-300 m/s. Flow pumping requirements vary from about 15 to 150 liter/s at pressures of about 0.5 torr. The static pressure of the flow within the test section is typically a few torr. The vibration-rotation populations in such devices are only 

Pulsed HF(DF), HCl(DCl), and HBr(DBr) lasers are also readily constructed. A review of such devices has recently appeared. Reaction initiation is accomplished either by photolysis or by electrical discharge dissociation. 

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O Keefe A and Deacon DAG 1988 Cavity ring-down optical spectrometer for absorption-measurements using pulsed laser sources Rev. Sol. Instrum. 59 2544-51... 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

Continuous wave (CW) lasers such as Ar and He-Ne are employed in conmionplace Raman spectrometers. However laser sources for Raman spectroscopy now extend from the edge of the vacuum UV to the near infrared. Lasers serve as an energetic source which at the same hme can be highly monochromatic, thus effectively supplying the single excitation frequency, v. The beams have a small diameter which may be... 

Barrett J J and Berry M J 1979 Photoacoustic Raman spectroscopy (PARS) using cw laser sources Appl. Phys. Lett. 34 144-6... 

The main panel of Figure B1.5.6 portrays a typical setup for SHG. A laser source of frequency to is directed to the sample, with several optical stages typically being introduced for additional control and filtering. The combination of a... 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

Johnson A M and Shank C V 1989 Pulse compression in single-mode fibres—picoseconds to femtoseconds The Supercontinuum Laser Source ed R R Alfano (New York Springer) pp 399-449... 

One of the most important teclmiques for the study of gas-phase reactions is flash photolysis [8, ]. A reaction is initiated by absorption of an intense light pulse, originally generated from flash lamps (duration a=lp.s). Nowadays these have frequently been replaced by pulsed laser sources, with the shortest pulses of the order of a few femtoseconds [22, 64]. 

The development of tunable, narrow-bandwidtli dye laser sources in tire early 1970s gave spectroscopists a new tool for selectively exciting small subsets of molecules witliin inhomogeneously broadened ensembles in tire solid state. The teclmique of fluorescence line-narrowing [1, 2 and 3] takes advantage of tire fact tliat relatively rigid chromophoric... 

Table C2.15.1 Common laser sources (s denotes solid-state lasers and g denotes gaseous lasers).

Saturation is clearly achieved more readily if states m and n are close together, as is the case for microwave or millimetre wave transitions, but, even if they are far apart, a laser source may be sufficiently powerful to cause saturation. 

Figure 5.13 shows a typical experimental arrangement for obtaining the Raman spectmm of a gaseous sample. Radiation from the laser source is focused by the lens Lj into a cell containing the sample gas. The mirror Mj reflects this radiation back into the cell to increase... 

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

From 1960 onwards, fhe increasing availabilify of intense, monochromatic laser sources provided a fremendous impetus to a wide range of spectroscopic investigations. The most immediately obvious application of early, essentially non-tunable, lasers was to all types of Raman spectroscopy in the gas, liquid or solid phase. The experimental techniques. 

If may be apparenf fo fhe reader af fhis sfage fhaf, when lasers are used as specfroscopic sources, we can no longer fhink in terms of generally applicable experimenfal mefhods. A wide variefy of ingenious techniques have been devised using laser sources and if will be possible fo describe only a few of fhem here. 

Two-photon absorption has been observed in the microwave region with an intense klystron source but in the infrared, visible and ultraviolet regions laser sources are necessary. 

If the microstmcture becomes ever finer by improved deposition technology, the domain irregularities should diminish. The SNR is limited by the shot noise of the laser source and is equal to i . In this region a high 9 is of great value. 

The compact disk player has become a very widespread consumer product for audio reproduction. The information is stored along tracks on the disk in the form of spots of varying reflectivity. The laser beam is focused on a track on the surface of the disk, which is rotated under the beam. The information is recovered by detecting the variations in the reflected light. The compact disk offers very high fideHty because there is no physical contact with the disk. This appHcation has usually employed a semiconductor laser source operating at a wavelength of around 780 nm. Tens of millions of such compact disk players are produced worldwide every year. 

Raman spectroscopy, long used for quaHtative analysis, has been revitalized by the availabiHty of laser sources. Raman spectroscopy is based on scattering of light with an accompanying shift in frequency. The amount by which the frequency is shifted is characteristic of the molecules that cause the scattering. Hence, measurement of the frequency shift can lead to identification of the material. 

Significant power is generated only below 300 GHz. Above this frequency, the expectation has always been that usehil laser sources would be eventually developed (51). Thus the most difficult region for power generation appears to be that of suhmillimeter waves or the fat infrared, ie, 300—3000 GHz (1000-100 am). 

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