The properties and oscillation characteristics of very long cw xenon lasers are discussed and pertinent long laser experiments described. High-gain 3.508-microm xenon laser amplifiers were used in these experiments. Laser cavity lengths of up to 30 km were studied. Spectrum analyses revealed complex oscillation spectra exhibiting the characteristics of both homogeneous and inhomogeneous laser mode structures. Long laser Doppler experiments, Q-switching studies, and modulation experiments were performed. Applications to atmospheric pollution detection are discussed.

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This paper describes a solid state laser concept that scales to MW levels of burst power and MJ of burst energy and burst durations measured in seconds. During lasing action, waste heat is purposely stored in the heat capacity of the active medium. The paper outlines the principal scaling laws of key operational features and arrives at a conceptual design example of the laser head as well as a mobile laser system. © 1998 Cambridge University Press 0263-0346/98 $ 12.50 G.F. Albrecht et al.and average power lasers are designed not to exceed a critical tensile stress value, or the medium will fracture. Depending on how the beam propagates through this medium, the temperature and stress distributions influence the beam propagation, resulting in such phenomena as, for example, thermal focusing and birefringent stress depolarization in rods. Many techniques have been developed to mitigate the imprint of these effects on the beam. In a zigzag slab architecture, special care is taken that such thermo-optical effects are averaged out as the beam propagates through the active medium. The myriad of highly sophisticated and successful commercial systems proves that all these effects have been not only studied extensively, especially in rods and zigzag slabs, but also have been mastered very well indeed. Nevertheless, these average power heat removal effects constitute an intrinsic limit to the steady state average power that the solid state laser can put out. Single shot lasers obviously suffer none of these thermomechanical restrictions, since the medium is in thermal equilibrium with its environment (no heat flow) before the shot, and one simply waits long enough to reestablish this condition before the next shot. The beam thus travels through an active medium essentially free of gradients.The heat capacity laser is conceptually closely related to the single shot laser, in that one rapidly adds single shots at a time scale short compared to thermal diffusion times, 1 that is, short compared to times that begin to establish the thermal gradients of a steady state heat flow condition, and a near adiabatic mounting of the active medium serves to thermally isolate the active medium as much as possible. That means that the waste heat generated during lasing remains, by design, in the active medium, whose temperature now rises each shot by a small amount given by the amount of waste heat generated per unit volume, and the heat capacity of the active medium. Therefore we call it a heat capacity laser. Since the temperature of the active medium cannot rise indefinitely, lasing at some point will have to cease, and the cooling phase begins. During this cooling phase, the temperature of the active medium is again reduced to the starting temperature, and the medium will now be subject to a tensile surface stress, which needs to be managed just like in any other cooled solid state laser.Therefore, an important characteristic of the heat capacity laser is its single shot energy output; it is a rapidly pulsed single shot energy device. ...

We discuss the possibility of extending solid state laser technology to high average power and of improving the efficiency of such lasers sufficiently to make them reasonable candidates for a number of demanding applications. A variety of new design concepts, materials, and techniques have emerged over the past decade that, collectively, suggest that the traditional technical limitations on power (a few hundred watts or less) and efficiency (less than V%) can be removed. The core idea is configuring the laser medium in relatively thin, large-area plates, rather than using the traditional low-aspect-ratio rods or blocks. This presents a large surface area for cooling, and assures that deposited heat is relatively close to a cooled surface. It also minimizes the laser volume distorted by edge effects. The feasibility of such configurations is supported by recent developments in materials, fabrication processes, and optica! pumps. Two tvpes of lasers can, in principle, utilize this sheet-like gain configuration in such a way that phase and gain profiles are oniformh sampled and ID first order, vield high-quaiitv (undistorted) beams. The zigzag laser does this wi'h a single plate, and should be capable of power levels up to several kilowatts The disk laser is designed around a large number of plates, and should be capable m scaling to arbitrarily high powr levels.

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