High-power solid-state laser systems

Usual lasers respect Stokes law, the pump excitation being at a higher energy than emission. Because present-day high-power semiconductor laser pumping sources are limited to the red or infia-red region, one anticipates that it is impossible to obtain directly an all-solid-state laser system in the visible. To circumvent this limitation one can make use of second-harmonic generation, APTE (see sect. 3.4.2), or ESA when optical confinement increases its probability. These processes are basically of an anti-Stokes nature. 

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

Gas lasers use CO as a medium. They use high-voltage, low-current power sources to supply the energy needed to excite the CO gas mixture. A rigid lens and mirror system is used to deliver the beam. Power outputs for gas lasers can be much higher than solid-state lasers, reaching 25 kW. 

The Nd-YAG laser is one of the most widely used solid-state lasers. The lasing medium consists of neodymium ion in a host crystal of yttrium aluminum garnet. This system offers the advantage of being a four-level laser, which makes it much easier to achieve population inversion than with the ruby laser. The Nd-YAG laser has a very high radiant power output at 1064 nm, which is usually frequency doubled (see page 175)... 

Rare earth laser action has been obtained for two groups of liquids metallo-organic and inorganic aprotic liquids. The first group are chelate lasers and are reviewed by Lempicki and Samelson (1966) research on aprotic materials and systems for high-power, pulsed liquid lasers are reviewed by Samelson and Kocher (1974). Stimulated emission in both liquids occurs between 4f states of trivalent rare earths. Optical pumping is via xenon-filled flashlamps in optical cavities and resonators similar to those used in solid-state lasers. Rare earth liquid lasers have only been operated pulsed. 

The future prospects for selective chemistry research as an aspect of laser development are not as bright as several years ago. With the exception of the quest for a visible chemical laser, most new laser research is oriented towards systems that are patently nonmolecular. The leading candidate for very high power applications is the free electron laser (PEL). In this device, the interaction of a relativistic electron beam with a periodic magnetic structure produces coherent radiation. Such devices on paper can have substantially higher power and efficiency than electric molecular lasers. For moderate power applications, advances in solid state lasers, nonlinear optical conversion processes, and tunable solid state media offer the prospect of broadly tunable compact sources. At low powers, diode lasers and diode laser arrays are gaining increasing application and hold out the promise, when used with solid state media, of versatile tunable sources. 

Four-level lasers offer a distinct advantage over tlieir tliree-level counterjiarts, (figure C2.15.5). The Nd YAG system is an excellent example of a four-level laser. Here tlie tenninal level for tlie laser transition, 2), is unoccupied tlius resulting in an inverted state as soon as any atom is pumped to state 3. Solid-state systems based on tliis pumping geometry dominate tlie marketplace for high-power laser devices. 

Recent developments in ultrashort, high-peak-power laser systems, based on the chirped pulse amplification (CPA) technique, have opened up a new regime of laser-matter interactions [1,2]. The application of such laser pulses can currently yield laser peak intensities well above 1020 W cm 2 at high repetition rates [3]. One of the important features of such interactions is that the duration of the laser pulse is much shorter than the typical time scale of hydrodynamic plasma expansion, which allows isochoric heating of matter, i.e., the generation of hot plasmas at near-solid density [4], The heated region remains in this dense state for 1-2 ps before significant expansion occurs. 

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