Incoherent laser sources

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

Replacement of the conventional incoherent light source with a laser has allowed CD measurements to be made on extremely small samples, such as those encountered in capillary electrophoresis. In addition, the unique spatial properties of the laser are utilized to advantage in thermal lens spectroscopy. TL detection of CD, either via a single beam, or a differentially configured arrangement, has demonstrated significant improvements in the measurement SNR. Application to HPLC detection, or time-resolved studies, are currently under investigation. 

Equations (3.19) and (5.27) represent two extremes, where the result is eitfc fully coherent or fully incoherent. It is enlightening to consider [188] the effeqf partially coherent laser sources through the example of the pump-dump scenario]) Section 3.5 in the case where the laser is not fully coherent. 

In addition, a novel generation of lamps with promising features for photochemical applications has been developed to industrial maturity over the last decade, the so-called incoherent excimer radiation sources (Eliasson et al., 1988). Note that these lamps are not laser sources. In contrast to well-known excimer lasers, excimer lamps are operated under different physical conditions and they emit incoherent electromagnetic radiation. Whereas pulsed laser radiation can reach very high irradiances, E up to 100 MW m , the irradiance E of excimer lamps is only in the range of 1000 W m . ... 

In contrast to coherent radiation emitted by laser sources, incoherent or non-co-herent radiation is characterized by elementary electromagnetic waves that do not have any phase relation in space and time. The production of incoherent excimer radiation in the UV or VUV region of the electromagnetic spectrum is made pos-... 

A 10- to 20-Hz Nd YAG laser is very convenient as an excitation source for this experiment, since the doubled output at 532 nm is near the broad 550-nm ruby absorption and the laser pulse is short (5 to 10 ns) compared to the excited Cr radiative lifetime. The student should note the cavity construction of such a laser and, following the directions of the laboratory instructor, adjust the doubling crystal for optimum green output. A 532-nm output of 0.1 to 1 mJ is adequate for the experimeut, so a small and relatively inexpensive flashlamp- or diode-pumped pulsed laser is sufficient. Also suitable as an excitation source would be a dye laser operated near 550 nm and pumped by a nitrogeu or excimer laser. (An incoherent pulsed source such as a strobe light can also be used if the pulse is about 10 /ts or less and if appropriate band-pass filters are used.) For all laser experiments, safety goggles must be worn to minimize hazard due to the high intensity of these sources. The instructor will provide instructions about any special features of the lasers and their safe operation. 

This article reviews direct and indirect (e.g., afterimage, flash blindness) light hazards from common incoherent light sources. For direct hazards specific to lasers and other specialized coherent sources, the reader is referred to organizations such as the Laser Institute of America and the International Electrotechnical Commission. 

In hollow-optical fibers for laser delivery, usually only some low order modes are excited because of small divergence angle of incident laser beam. In contrast, many high order modes are excited in hollow-optical fibers for spectroscopic applications because usually an incoherent light source like an arc lamp is used as the light source. Therefore, ray optic theory that can handle a wide divergence beam is used for evaluation of optical properties in this section. 

The varieties of exposure sources that have found applications in UV and visible light optical lithography can be broadly divided into two groups (i) high-pressure arc lamp or incoherent sources and (ii) laser sources or temporally coherent sources. In the laser-type sources, we include all techniques and devices for radiation generation that have their basis in stimulated emission of radiation. 

In order to illustrate the advantages of absorption spectroscopy with tunable lasers, we first compare it with conventional absorption spectroscopy, which uses incoherent radiation sources. Figme 1.1 presents schematic diagrams for both methods. 

Contrary to radiation sources with broad emission continua used in conventional spectroscopy, tunable lasers offer radiation sources in the spectral range from the UV to the IR with extremely narrow bandwidths and with spectral power densities that may exceed those of incoherent light sources by many orders of magnitude (Vol. 1, Sects. 5.7, 5.8). 

Because of the much higher spectral power density of most lasers compared to incoherent light sources the detector noise is generally negligible in laser... 

Since the fluorescent transitions must obey certain selection rules, it is often possible to populate a selected level m) by fluorescence from the laser-pumped upper level (Fig. 5.1b). Even with a weak pumping intensity large population densities in the level m) may be achieved. In the pre-laser era, the term optical pumping was used for this special case because this scheme was the only way to achieve an appreciable population change with incoherent pumping sources. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

Dye lasers are very flexible in their operation. They can be pumped by incoherent fight from a flash lamp (less common today) or by radiation from a laser source. They can be run both pulsed and CW, and they offer a very broad range of wavelengths, with tuning over more than 100 nm for some specific dyes. Laser output of many watts in CW mode is achievable. 

Although these new techniques have proved to be very fruitful, the really stimulating impetus to the whole field of spectroscopy was given by the introduction of lasers. In many cases these new spectroscopic light sources may increase spectral resolution and sensitivity by several orders of magnitude. Combined with new spectroscopic techniques, lasers are able to surpass basic limitations of classical spectroscopy. Many experiments that could not be performed with incoherent light sources are now feasible or have already been successfully completed recently. This book deals with such new techniques of laser spectroscopy and explains the necessary instrumentation. 

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