Laser, fixed-wavelength

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

PLIF has become a very useful technique to measure local concentrations. In PLIF, a laser sheet is formed and a high-speed CCD camera measures the excited hght from a fluorescent dye that is mixed into the flow. Most commonly used is a simple laser at a fixed wavelength (532 nm for Nd YAG). The excited light has a longer wavelength and all Hght reflections from the laser are filtered out The camera is usually 10,12,... 

Recent advances in constructing single-mode tunable cw dye lasers with a bandwidth of 20 Mc/sec or less 98a) open the way for high resolution absorption sepectroscopy without the limitations to fixed wavelengths 98b) One may observe either the total fluorescense intensity, which is proortional to the total absorption, or the decrease of the transmitted laser intensity. 

Dyes have the important role in dye lasers of allowing a fixed wavelength laser input to be tuned to a wide range from which a selection can be made appropriate to the end use of the dye laser. 

There is no inherent sample size restriction, large or small, but is fixed by the optical components used in the instrument. The diffraction limit of light, roughly a few cubic micrometers depending on the numerical aperture of the optics used and the laser s wavelength, sets the lower bound.7 In a process application, the type of fiber optics used also affects sample volume examined. Macroscopic to microscopic samples can be measured with the appropriate selections of laser wavelength, laser power, and optics. 

The experimental conditions for the excitation and detection of all the species are listed in Table I along with the radiative lifetimes of the excited states. Under the narrow detection bandwidth conditions for these measurements the quench term is much greater than T 1 for the species studied and the fluorescence efficiency varies as x T1/2. Thus with fixed geometry, laser excitation wavelength, and detection parameters, the fluorescence intensity in Equation (1) simplifies to... 

A laser is a radiation source which produces a very high spectral radiance in a small spectral range at a fixed wavelength. A laser combines a radiation source with spectral isolation of its radiation - two important components of a spectrometer. The word laser is an acronym which stands for light amplification by stimulated emission of radiation. The essential elements of a laser are an active medium a pumping process to produce a population inversion and a suitable geometry or optical feedback elements (Moore et al., 1993). Most lasers are essentially Fabry-Perot interferometers whose cavities contain... 

If we impose an assumption that the chroniophore molecule is excited by an ultrashort laser, then the pulse covers all the frequencies. Consequently, the whole initial wavefunction is promoted to the excited state, hi the experiment, however, a cw laser with a fixed wavelength is usually used. Within the Wigner trajectories approach we have, therefore, to filter the Wigner distribution function in such a way that the energetic relation... 

DE Goeringer, GL Glish, SA McLuckey. Fixed-wavelength laser ionization/tandem mass spectrometry for mixture analysis in the quadrupole ion trap. Anal Chem 63 1186—1192, 1991. 

Previously, a multiple, fixed-wavelength sensor for measuring the gas temperature of detonation products was reported [1]. While this sensor provided important results, the rapid, broad-wavelength-scanning capabilities of advanced diode-laser technologies allow for a simpler and more robust technique to measure both temperature and pressure simultaneously [4, 5]. This technique, described below, provides gas temperature and pressure histories, spanning 2000-4000 K and 0.5-30 atm, respectively, with microsecond time response. 

Previously, a 3.39-micrometer He-Ne absorption scheme was used to measure propane concentration [7]. Sensors based on this scheme have the advantage of offering strong absorption because many fundamental bands of propane are located near the 3.39-micrometer wavelength region, but the fixed-wavelength lasers available in this region limit this sensor s applications in practical systems where interference absorption and noise are common problems. 

Recently, development of a more accurate and robust propane sensor by replacing the fixed-wavelength He-Ne laser with a tunable diode laser was initiated. The strongest absorption of propane in the near-infrared region occurs near 1.68 /xm. Figure 10.4 shows the first known high-resolution spectral data of propane near 1.68 /xm, from which one can identify the optimum detection strategy. In this work, the diode laser was scanned across the absorption peak at 1.6837 /xm at 500 Hz to mecisure propane concentration. This sensor was applied to the head-end of the PDE at Stanford University, as shown in Fig. 10.5. The initial results of propane measurements are shown in Fig. 10.6. These results contain a wealth of information... 

Baer, D. S., Nagali, V, Furlong, E. R., Hanson, R. K., and Newfield, M. E. "Scanned- and Fixed-Wavelength Absorption Diagnostics for Combustion Measurements Using a Multiplexed Diode-Laser Sensor System." AIAA Journal 34 (1996) 489-93. 

Because most common UV lasers operate at fixed wavelengths, the range of application of laser-induced fluorescence may be somewhat limited. The He-Cd laser provides additional versatility because two excitation wavelengths are available, 325 nm and 442 nm. This feature Is Illustrated in Figure 3, taken from the recent work of Guthrie, Jorgenson, and Dluzneskl (41), where a solvent-refined coal fluid has been analyzed using an open tubular... 

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