Laser cross-section

Wavelength Laser cross-section Radiative lifetime (upper level) Decay lifetime (upper level)... 

From Tx, determined by oscillator strength measurements in absorption, it is possible to obtain the laser transition oscillator strength from table 5 and eq. (123). It has been verified that the result is nearly constant for non-centrosymmetric crystals (Auzel and Michel 1975), as shown in table 6 where the integrated cross-section has been obtained using eqs. (117) and (121). Using the average value of 3.7x 10 cm, an estimate of the laser cross-section for any Nd -doped material can be obtained as (Auzel 198T)... 

This relationship gives the laser cross section from a single integrated absorption measurement at about 0.750 im and from the emission effective linewidth at 1.06 (Am, with good accuracy (Deutschbein 1976). 

Radiative transition probabilities and laser cross sections of rare... 

All glass lasers developed to date have used a rare earth as the active ion and optical pumping for excitation (Stokowski, 1982). Of these, flash-lamp-pumped neodymium glass lasers are the most frequently used and the most widely investigated. The spectroscopic data needed for estimation of the laser characteristics are usually obtained from small samples (Reisfeld and Jprgensen, 1977). The data include absorption, emission, nonradiative relaxation, energy transfer probabilities and laser cross sections. Laser operation predictions can be made from such data without actually demonstrating laser action. 

The small cross-sectional area covered by a laser light beam coupled with the energy density in the beam leads to power levels reaching from milliwatts to many hundreds of kilowatts per square meter. 

The cavity of a laser may resonate in various ways during the process of generation of radiation. The cavity, which we can regard as a rectangular box with a square cross-section, has modes of oscillation, referred to as cavity modes, which are of two types, transverse and axial (or longitudinal). These are, respectively, normal to and along the direction of propagation of the laser radiation. 

Spatial Profiles. The cross sections of laser beams have certain weU-defined spatial profiles called transverse modes. The word mode in this sense should not be confused with the same word as used to discuss the spectral Hnewidth of lasers. Transverse modes represent configurations of the electromagnetic field determined by the boundary conditions in the laser cavity. A fiiU description of the transverse modes requires the use of orthogonal polynomials. 

Fig. 10. Cross-sectional drawing of a vertical cavity surface emitting laser (VCSEL). Proton implantation is used to channel the current through a small active region. Light is emitted in the direction perpendicular to the plane of the wafer. This makes preparation of two-dimensional arrays quite easy.

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

The strength of a photon—molecule interaction is deterrnined by the frequency-dependent cross section 0 (v), expressed in cm for absorption and related to a(y) in equation 1 or by the differential cross section (k5(y) jin units of cm /sr for scattering (14). The latter specifies the likelihood that active species scatter some portion of the incident laser fluence (photons /cm ) into a viewing soHd angle, AQ, measured in steradians (Fig. 1). The cross sections can be expressed as in equation 5 ... 

As illustrated in Fig. 3.41, several laser schemes can be used to ionize elements and molecules. Scheme (a) in this figure stands for non-resonant ionization. Because the ionization cross-section is very low, a very high laser intensity is required to saturate the ionization process. Scheme (b) shows a simple single-resonance scheme. This is the simplest but not necessarily the most desirable scheme for resonant post-ionization. Cross-... 

A very important characteristic of laser radiation is the beam shape. So far most LA experiments have been performed with Gaussian laser beams. Lasers with uniform distribution of the beam cross-section have been used only recently to achieve high lateral and depth resolution. Specially designed beam homogenizers must be used for this purpose [4.226-4.228]. The Cetac LSX-200 system has a flat-top distribution of the laser beam. 

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