Attenuation of the Laser Beam

The actual signal arising from any aerosol concentration, used in the evaluation, is associated with the two aforementioned scattering parameters, i.e. the volume back-scattering coefficient fi and the extinction coefficient a. The latter appears in the exponential factor exp[ 2 f oc R, A) dR] and accounts for the total atmospheric attenuation of the laser beam... 

The crucial point is that the signal is proportional to exp[-a(o))2R], which means that it measures the attenuation of the laser beam from 0 to R. Gating the receiver alternatively at delay times t = 2R/c and t + At = 2(R+aR)/c allows measurement of the difference AS = S(t+At) -S(t) which yields the attenuation between R and R + aR. 

Equation (1.4). Assuming that the beam size is conserved, the attenuation of the laser power is given by... 

The use of suspensions can be circumvented by adding carbon black or CuO (28). The function of dark (black) additives appears to be related to a reduction of the penetration depth of the laser beam, causing an attenuation of reflected or refractive radiation, which is scrambled relative to polarization. 

The principle of photoacoustic Raman spectroscopy (107) is similar to that of CARS. When two laser beams, vp (pump beam) and vs (Stokes beam), impinge on a gaseous sample contained in a cell (Fig. 3-43), these two beams interact when the resonance condition, vp — vs = vM, is met, where vM is a frequency of a Raman-active mode. This results in the amplification of the Stokes beam and the attenuation of the pump beam. Each Stokes photon thus... 

One application of silicone isolator is the pick up attenuator for the laser beam systems of compact disk player. 

The 488.0 nm Coherent Radiation Model 520-B argon-ion laser of 300 mW power was focused on the center of the drop and the Raman light is detected perpendicular to both the laser beam and the sample tube. Qualitative polarization measurements were made by rotating the plane of polarization of the laser beam with a half-wave plate. In most of the spectra, it was not necessary to use a spike filter to attenuate the plasma lines. The spectra were recorded using a modified Cary 81 spectrophotometer employing a 9558A EMI photo-multiplier counting detection system. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

The critical propagation problems which are being addressed at the various research centres are Stimulated Raman scattering by air molecules thermal blooming, due to laser heating of the air electrical breakdown of the air, which could generate plasmas that attenuate the beam atmospheric turbulence, which could perturb the focusing of the laser beam. 

Without absorption cell the intensity of the laser beam decreases after one roundtrip by the attenuation factor exp[—2(1 - / )]. For q roundtrips with the absorption cell the transmitted intensity has decreased to 7oexp[—2 (1 — R) 2qaL. ... 

Instead of measuring the attenuation of the probe beam, the absorption can also be monitored by the laser-induced fluorescence, which is proportional to the ab-... 

The absorption coefficient a(a>) assumes nonnegligible values only within the small spectral ranges Aco of absorption lines (a few gigahertz around the center frequencies coo). On the other hand, the scattering cross section, which is proportional to ct)" for Rayleigh scattering, does not vary appreciably over these small ranges Aco. Therefore, measurements of the laser beam attenuation at the two different frequencies coi and co2 inside and just outside an absorption profile yields the ratio... 

Laser Safety Class OSHA requires employees potentially exposed to laser beams to have laser safety goggles with adequate filtering capacity for the wavelength and intensity of the laser beams. Table 28-3 gives minimum attenuation requirements for various laser intensities. 

Fig. 1 shows the Kretschmann configuration of the attenuated total reflection (ATR) method. The ATR method was reported in previous papers [l]-[6]. An Ar laser beam at 488.0 nm was used to excite SPs on metal thin films in this measurement and the p-polarized beam was directed onto the samples on the substrates through the half-cylindrical prism. The prism was mounted on a rotating stage. The incident angles of the laser beam were scanned using a computer-controlled pulse motor. The reflected intensities of the laser light, that is, the ATR... 

Because IR absorption measurements discussed in this book are all performed in optically extremely dilute samples, such as those occurring in molecular beams or tandem mass spectrometers, action spectroscopy is employed instead of direct absorption IR spectroscopy. In conventional direct absorption IR spectroscopy, the attenuation of the light beam transmitted through the sample is measured as a function of the frequency of the IR radiation. By measuring the IR laser beam intensity before and after the sample, the absorbance as a function of the frequency of the IR photons is obtained according to the Lambert-Beer law ... 

A half-wave plate (HWP) and a polarizer (GLP) are positioned after the oscillator and are used to variably attenuate the laser output power to the desired input power required by specific experiments. Using a beam sampler (M ), a small portion of the laser beam is directed into a beam diagnostic unit (AC). In it, the laser pulse is characterized both in the time and frequency domains by employing an autocorrelator and a spectrometer. The laser beam is then expanded to match or overfill the back aperture of the objective lens. This is accomphshed using two positive lenses with the appropriate focal lengths. At the focal point of the first lens, a pinhole (SF) is carefully positioned to spatially filter the laser beam. An electro-mechanical shutter (S), used to control laser exposure times in the sample, is placed before this assembly. If exposure times shorter that a few milliseconds are required, faster response shutters such as acousto- or electro-optic modulators can be used. 

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