Tunable Coherent Light Sources

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

The experimental realization of these tunable coherent light sources is, of course, determined by the spectral range for which they are to be used. For the particular spectroscopic problem, one has to decide which of the possibilities summarized above represents the optimum choice. The experimental expenditure depends substantially on the desired tuning range, on the achievable output power, and, last but not least, on the realized spectral bandwidth Av. Coherent light sources with bandwidths Av 1 MHz to 30 GHz (3 x 10 -1 cm ), which can be continuously tuned over a larger range, are already commercially available. In the visible... 

Different types of tunable coherent light sources have been developed for the different spectral regions. Chapter 7 gives a brief survey of the various types, discussing their advantages and limitations. This chapter ends the second part of the book devoted to the basic concepts and the instrumentation of laser spectroscopy. 

The experimental realization of these tunable coherent light sources is of course determined by the spectral range for which they are to be used. 

There are, firstly, the improvement of frequency-doubling techniques in external cavities, the realization of more reliable cw-parametric oscillators with large output power, and the development of tunable narrow-band UV sources, which have expanded the possible applications of coherent light sources in molecular spectroscopy. Furthermore, new sensitive detection techniques for the analysis of small molecular concentrations or for the measurement of weak transitions, such as overtone transitions in molecules, could be realized. Examples are Cavity Ringdown Spectroscopy, which allows the measurement of absolute absorption coefficients with great sensitivity or specific modulation techniques that push the minimum detectable absorption coefficient down to 10 " cm ... 

Now several thousands of laser lines are known which span the whole spectral range from the vacuum-ultraviolet to the far-infrared region. Of particular inters are the continuously tunable lasers which may in many cases replace wavelength-selecting elements, such as spectrometers or interferometers. In combination with optical frequency-mixing techniques such continuously tunable monochromatic coherent light sources are available at nearly any desired wavelength above 100 nm. 

IRSFG process, IR and Raman active Intensity of newly generated light at Ct>3 = + 0)2 Surface-specific Requires tunable coherent IR source, relatively low signal intensity... 

Most XTA measurements conducted so far employed continuous energy tunability in about 1-keV range from synchrotron sources and collected X-ray absorption spectra step-by-step at each individual X-ray photon energy defined by the monochromator. The current femtosecond X-ray free electron laser (XFEL) sources provide either monochromatic or narrowly distributed energy spectra that are not continuously tunable for the step-by-step approach. For example, the Linear Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAQ currently has about a 50-eV band width centered around 7.1 keV due to intrinsic bandwidth of self-amplified-spontaneous-emission (SASE). " ... 

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

Fundamentally, the properties of laser light are concomitants of its coherence, which is in turn a consequence of the nature of stimulated emission. Most of these properties, especially brightness, monochromaticity, directionality, polarization, and coherence itself, are useful (for many applications, indis-pensible) in a spectroscopic light source. The spectroscopic potential of lasers was recognized even before they were invented. Actual applications remained very specialized until tunable lasers were devised. 

Lasers are used as an excitation source for three reasons. Because the laser output is coherent it offers special advantages in directionality and focusing. Tunable lasers allow the possibility of examining several species. Finally, lasers provide significantly higher power levels than conventional light sources. 

These few numbers illustrate the very different properties of laser systems as they exist today. The laser is,in principle, a light source with fixed and stable frequency. The emission frequency is determined by the optical transition of the laser medium and the frequencies of the modes of the laser resonator. In fact the monochromaticity and the high stability of the frequency is the basis of many spectroscopic experiments with lasers. For this reason systems with high frequency stability have been developed. In addition the spectroscopy requires, however, light sources with tunable frequency. The broad application of lasers for spectroscopy is thus closely related to the development of dye lasers, since this laser provided for the first time coherent light of broadly tunable wavelength. 

On the other hand, many possible applications of this type of spectroscopy have remained elusive because of a lack of spectral intensity, monochromaticity, tunability and of spatial coherence of the thermal light sources. This situation changed drastically with the advent of the laser that not only brought along a renaissance of classical double resonance spectroscopy, but also the development of new coherent, nonlinear spectroscopic techniques. 

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