Pumping Schemes

In principle, the pumping schemes must fulfill the two requirements (i) high absorption of pump radiation and (ii) optimized utilization of the absorbed radiation for laser emission. At the same time, the manifestation of de-excitation processes that have no contribution to the laser emission scheme should be prevented. Besides the absorption coefficient of the laser materials, the superposition of the pump radiation spectrum with the absorption spectrum of the laser materials could also influence the pump absorption. Therefore, when selecting the pumping sources, it is necessary to consider the following (i) absorption properties of the laser materials, including spectrum peak position and FWHM and cross sections, (ii) chemical and physical properties of the laser materials, such as concentration of dopants and dimensions, and (iii) design of the laser devices, in order to have a complete absorption and proper distribution of the absorbed power. 

Usually, pumping sources with broadband or multiple emissions, e.g., flash-lamps and solar radiations, exhibit less superposition with the absorption lines of ions, so that a large portion of the pump radiation would oscillate inside the laser materials, without being absorbed by the laser-active ions. Furthermore, this type of pumping has a variety of absorption transitions, thus leading to various quantum defects. In this case, an average quantum defect is used. 

The width of the emission line of diode lasers is usually several nanometers, which is too broad for pumping some intense but narrow absorption bands of the ions 

When the resonant pumping is close to the emitting laser level, or preferably the pumping is directly into this level, low quantum defect laser schemes could be realized, thus leading to increased laser performances and reduced heat generation. The quantum defect can be further reduced, when the pumping is on the transitions 

However, the power of the diodes was too low and the absorption of the 1 at.% Nd YAG at this level was too weak, with an absorption coefficient of about 4 cm Therefore, it was later changed to pumping into the strong absorption of %/2 Fs/2 at about 809 nm, having an absorption coefficient of about 11.4 cm by using more efficient and powerful AlGaAs diode lasers. In this case, a relative quanffim defect of 0.24 was observed for the laser emission at 1.06 pm, about 30 % of which represented the upper part, without having any effect on the laser emission. 

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Various heat pumping schemes have been proposed as methods for saving energy in distillation. Of these schemes, use of the column overhead vapor as the heat pumping fluid is usually the most economically attractive. This is the vapor recompression scheme shown in outline in Fig. 14.6. 

In some applications (usually high pressure compressors using oil film seals) alternative pump schemes should be considered. It may be that the desired seal pressure is not achievable by one set of pumps or the quantity required by the seal is small relative to the main pump capacity. There are (imes when booster pumps are needed however, if the reason is energy, it would be worth reviewing the economics very carefully, because reliability tends to suffer with the booster. The booster pumps are paired into a main and standby and are configured to take suction fro lower pressure system. Sufficient interlocks have to be supplied drivers so that if the main pumps shutdown, the boosters come ... 

The active medium also determines the pumping scheme. Commonly, two types of operational schemes are used to describe laser operation /oMr-/cvc/ and three-level laser systems ... 

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

R. W. Field Each acetylene polyad contains zero-order states that are easily accessible via plausible direct or multiple-resonance A XAU — X Franck-Condon pumping schemes. Each Vib 16,000... 

In the first stage of the solution process, the advective control model seeks a pumping scheme in which the capture zone fully encompasses all control points representing the contaminant plume. The capture zone is simulated by tracking particles from extraction wells backwards through the velocity field. To represent the plume capture constraints numerically, a distance measure is used in which the minimum distance between each plume control point and all particles (see Figure 1) is constrained. When the distance between a control point and particle pathline equals zero then the plume control point lies within the capture zone. To ensure capture of the entire plume, the constraint function must equal zero for all control points. The reverse tracking formulation is stated as... 

Underflow is usually withdrawn continuously on the bases of bed level, rake torque, or underflow solids concentration in a feed-back mode. Most installations incorporate at least two of these parameters in their underflow withdrawal control philosophy. For example, the continuous withdrawal may be based on underflow solids density with an override to increase the withdrawal rate if either the rake torque or the bed level reaches a preset value. In some cases, underflow withdrawal has been regulated in a feed-forward mode on the basis of thickener feed sohds mass flow rate. Any automated underflow pumping scheme should incorporate a lower limit on volumetric flow rate as a safeguard against line pluggage. 

Fig. 8. a Multiphoton pumping scheme for Er + excited with 6500 cm photons, b, c Measured emission intensities from the %/2 (2-photon), S3/2 (3-photon), and Hg/2 (4-photon excitation) levels in (b) 1% Er + Cs3Lu2Cl9 and (c) Cs3Er2Cl9 vs pump power at 6500 cm in double-logarithmic representation. The numbers denote the slopes at low and high pump powers. Adapted from [26,27]... 

A common method for polyester formation is via trans-esterification. The procedure described below uses a procedure that is modified from that described in Sorenson. In this case, aromatic di-acid 5 and di-ester 4 monomers are reacted together in the melt. During the polymerization, the molecular weight grows while volatile side-product is continuously removed from the reaction vessel by means of a vacuum pump. Scheme 2 describes the formation of a main-chain liquid crystalline poly(ester ether) containing... 

Chemical laser action has so far been restricted to the molecules HF, HC1, HBr and their denterated analogs and to CO. Lasing has also been stated in a brief report to occur in the OH radical produced in the O3/H2 photolysis 105). The pumping scheme is likely to be... 

The carbon-monoxide chemical laser exhibits a variety of pumping reactions. Laser action was first reported by Pollack in CS2/02 photolysis 125>. The pumping scheme in this case is believed to be the following 126> ... 

CSg + O2 / hv First report of a chem. CO laser, spectroscopy, chemical pumping scheme Pollack 200)... 

Quite similar to this pumping scheme was an attempt to pump a CO2 laser by chemical means reported by Russian workers 67>. Vibrationally excited N2 was produced by flash-photolyzing hydrazoic acid (HN3) and subsequent energy transfer. However, pumping by energy transfer from hydrogen... 

Efficient, purely chemical laser operation is possible in hydrogen halide-CO2 transfer lasers, as developed by Cool and coworkers 83>. In these lasers no external energy sources are required. The systems, which operate on the mixing of commercially available bottled gases, are HC1—CO2, HBr—COg, DF— CO2, and HF—CO2. The pumping scheme for a DF—CO2 laser, for instance, is as follows ... 

Miller, et al., (1974) used MOMRIE (Microwave Optical Magnetic Resonance Induced by Electrons) complementarily with anticrossing spectroscopy in experiments on He, H2, and other molecules. Small differences in electron bombardment excitation cross-sections combined with different radiative lifetimes lead to steady-state population differences between initial and final levels of the microwave transition. By monitoring fluorescence from one of the two involved electronic states, a change in fluorescence intensity signals a microwave resonance. Electron bombardment excitation tends to produce smaller population differences than the chemical pumping scheme exploited in the CN experiments, but the MOMRIE technique is more generally applicable. 

Spence et al. employed a method by which endothelial cells were vacuum delivered into a channel that was coated with fibronectin. Li et al. described the use of a trough method for immobilizing a dopaminergic cell model. This method appears to be useful for cell types that have a tendency to cluster. Other methods have been described as well. In most cases, hydrodynamic pumping schemes have been employed for such immobilization determinations. ... 

Lavi R, Jackel S, Tzuk Y, Winik M, Lebiush E, Katz M et al (1999) Efficient pumping scheme for neodymium-doped materials by direct excitation of the upper lasing leveL Appl Opt 38 7382-7385... 

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