Laser optical saturation

ALAN GELB—Physical Sciences Incorporated, Woburn, MA 01801 DAVID R. CROSLEY—SRI International, Menlo Park, CA 94025 Optical Saturation in Laser-Induced Fluorescence... 

The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us - and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condition occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and colli sional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or T1) asoirated into combustion flames and plasmas. 

Hefter, U., Ziegler, G., Mattheus, A., Fischer, A. and Bergmann, K. (1986). Preparation and detection of alignment with high m selectivity by saturated laser optical pumping in molecular beams, J. Chem. Phys., 85, 286-302. 

Figure 13 shows two kinetic traces of the formation and decay of e in methanol, (a) in the absence and (b) in the presence of a 20 ns ruby laser pulse. Saturating the e optical transition at 694 nm leads to a significant permanent loss of absorbers during the 0.4 J cm laser pulse. Figure 14(a) shows the spectra of e in methanol and 2-propanol before and during bleaching at 694 nm, and it is clear that there is a uniform loss (A.4) of intensity across the band. This loss, however, has a nonlinear dependence... 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

Ernsting NP, Kaschke M, Weller H, Katsikas L Colloidal Znl-xCdxS — optical saturation of the exciton band and primary photochemistry studied by subpicosecond laser flash-photolysis. Journal of the Optical Society of America B-Optical Physics 1990, 7(8) 1630-1637. 

The intensities of lasers exceed by many orders of magnitude those of conventional light sources. At high intensity levels, nonlinear interaction of light with atomic and molecular systems becomes pronounced. New analytical techniques involving multistep and multiphoton excitation and ionization, optical saturation, and excitation of forbidden transitions become possible. High radiation intensity also increases the sensitivity of laser analytical techniques,... 

The interaction of intense laser radiation with matter differs from that of conventional radiation sources. The high incident irradiances available with lasers can lead to a significant depletion of the initial level population. As a result, the optical absorption coefficient decreases as a function of the excitation energy and the absorbed energy tends towards a constant value. This so called optical saturation effect leads to a nonlinear dependence of the absorption signal on the light intensity. Because the amount of energy released by radiative and nonradiative relaxation processes... 

In 3D-cooling scheme, atoms are irradiated by three pairs of counterpropagating, red-detuned laser waves. In the simplest model of a two-level atom and at weak optical saturation, the radiation force on the atom, averaged over the wavelength of the laser field in the case of three-dimensional standing-wave irradiation, can be represented as a sum of three forces (eqn 5.10) ... 

CnLiSAF laser crystal does not require any cooling. As a result, relatively inexpensive femtosecond CnLiSAF lasers with optical output powers up to 45 mW, robustly modelocked with a semiconductor saturable absorber mirror (SESAM), are achievable from entbely self-contained and portable units with dimensions of 22 cm x 28 cm. [A version of this laser design incorporating one pah of pump laser diodes is shown in figure 10] A second pah of pump laser diodes can easily be incorporated onto the baseplate to maximize operational performance (figure 12). 

This number is based on the observation that in the reaction with SO], k(observed) for formation of optical density at 340 nm (where the OH adduct of benzonitrile absorbs) is proportional to [benzonitrile] up to the saturation limit ( a 20 mM, k(observed) = 5 x 10 s" ), as shown by 248 nm laser experiments... 

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