Applications laser sources

Fundamental Physical Applications of Laser Spectroscopy. - Multiple Photon Dissociation. -New Sub-E)oppler Interaction Techniques. - Highly Excited States, Ionization, and High Intensity Interactions. - Optical Transients. - High Resolutionand Double Resonance. - Laser Spectroscopic Applications. - Laser Sources. - Laser Wavelength Measurements. - Postdeadline Papers. 

From 1960 onwards, fhe increasing availabilify of intense, monochromatic laser sources provided a fremendous impetus to a wide range of spectroscopic investigations. The most immediately obvious application of early, essentially non-tunable, lasers was to all types of Raman spectroscopy in the gas, liquid or solid phase. The experimental techniques. 

If may be apparenf fo fhe reader af fhis sfage fhaf, when lasers are used as specfroscopic sources, we can no longer fhink in terms of generally applicable experimenfal mefhods. A wide variefy of ingenious techniques have been devised using laser sources and if will be possible fo describe only a few of fhem here. 

Fiber laser guide star systems. It is now widely appreciated that the heat- dissipation characteristics of fibers, coupled with the high efficiencies demonstrated (> 80%) and excellent spatial mode characteristics, make fiber lasers a preferred candidate for many high power applications. Based on these features, fiber laser technologies would provide a compact, efficient, robust, turnkey laser source, ideally suited for LGS applications. 

Recent advances in instrumentation range from novel (laser) sources and highly compact spectrometers over waveguide technology to sensitive detectors and detector arrays. This, in combination with the progress in electronics, computer technology and chemometrics, makes it possible to realise compact, robust vibrational spectroscopic sensor devices that are capable of reliable real-world operation. A point that also has to be taken into account, at least when aiming at commercialisation, is the price. Vibrational spectroscopic systems are usually more expensive than most other transducers. Hence, it depends very much on the application whether it makes sense to implement IR or Raman sensors or if less powerful but cheaper alternatives could be used. 

Two types of radiation sources are used in IR sensing. Common sources are thermal broadband emitters. The second type are laser sources, mostly semiconductor lasers. The application of (monochromatic) laser sources trades the ability of multi-component detection against higher sensitivity for pre-defined target analytes. Hence, laser sources are primarily suitable for sensitive sensing in well-defined, stable systems, also because spectrally interfering substances can neither be detected as such nor compensated. 

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

This section is focused primarily on source and detector technologies. For some applications the source or the detector actually defines the entire measurement technology, for example tunable lasers (source) and array spectrographs (detector). There are other important technologies to consider, especially in the area of data acquisition, control, computer, and commuiucation technologies. These are rapidly changing areas, and if viewed genericaUy, service all forms of instrumentation. Where practical, compaiues tend to use standard platforms, but for certain applications, where performance is critical, there is still a case for proprietary solutions. 

Tunable diode-laser sensors offer considerable promise for combustion research and development and also for process sensing and control applications. These devices are rugged and relatively easy to operate and they have been demonstrated to yield simple and quantitative measurements of species, temperature, and velocity, where line-of-sight measurements are useful or preferred. These techniques will grow in use as costs of laser sources and fiber-optic components decrease and access to more wavelength regions improves. 

The metal has very little commercial use. In elemental form it is a laser source, a portable x-ray source, and as a dopant in garnets. When added to stainless steel, it improves grain refinement, strength, and other properties. Some other applications, particularly in oxides mixed with other rare earths, are as carbon rods for industrial hghting, in titanate insulated capacitors, and as additives to glass. The radioactive isotope ytterbium-169 is used in portable devices to examine defects in thin steel and aluminum. The metal and its compounds are used in fundamental research. 

To introduce the application of ultrashort laser sources in microscopy, we want to review some properties of femtosecond pulses first for a comprehensive introduction the reader may refer to one of the established textbooks on femtosecond lasers (Diels and Rudolph 2006). The most important notion is the Fourier transform relation between the temporal shape of a pulse and the spectrum necessary to create it. This leads to the well-known time-bandwidth product for the pulse temporal width (measured as full width at half maximum, FWHM) At and the pulse spectral width Av. 

It is important to select the best laser source. For practical applications the first harmonic of Nd-YAG (1,064 nm) is preferential, because such a laser is powerful and exists in ruggedized industrial versions. It was foimd that the spectra are principally the same as under Aex = 355 nm (Fig. 8.15), but the characteristic line of F is much weaker and clearly seen only after a delay time of several microseconds when the overall intensity is already low. The characteristic Une of P at 254 nm is very weak and the strongest Unes in the spectra belong to Ca ions. 

Laser sources that emit in the mid-ir region of the spectmm (2—5 pm) are useful for detection of trace gases because many molecules have strong absorption bands in that region. Other applications include remote sensing and laser radar. Semiconductor lead—salt (IV—VI) lasers that operate CW at a temperature of 200 K and emission wavelength of 4 pm are commercially available however, they have relatively low output powers (<1 mW) (120). 

In photochemical research and its applications, lasers and arc lamps are very important. Lasers will be considered in section 7.2. A few other light sources of interest will be mentioned here. 

Figure 2. Tradeoffs between polymer and crystal organic nonlinear optical materials. EO refers to applications for electro-optic waveguide devices such as modulators and switches. SHG refers to applications for frequency doubling of moderate and low power laser sources. A + indicates favored, - indicates disfavored, 0 indicates neither favored nor disfavored, and x indicates not relevant.

---