Laser Amplifier
In this chapter we will discuss the gain in energy for a laser beam passing through an optically active material. The use of lasers as pulse amplifiers is of great interest in the design of high-energy, high-brightness light sources. The generation of hig
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In this chapter we will discuss the gain in energy for a laser beam passing through an optically active material. The use of lasers as pulse amplifiers is of great interest in the design of high-energy, high-brightness light sources. The generation of high-energy pulses is based on the combination of a master oscillator and multistage power amplifier. For the purpose of illustrating the amplifier concept and principles we assume a straightforward system, as shown in Fig. 4.1. In this scheme an amplifier is driven by an oscillator which generates an initial laser pulse of moderate power and energy. In the power amplifier with a large volume of active material the pulse power can grow, in extreme cases, up to 100 times. In an oscillator-amplifier system, pulse width, beam divergence, and spectral width are primarily determined by the oscillator, whereas pulse energy and power are determined by the amplifier. Operating an oscillator at relatively low energy levels reduces beam divergence and spectral width. Therefore, from an oscillatoramplifier combination one can obtain, either a higher energy than is achievable from an oscillator alone, or the same energy in a beam which has a smaller beam divergence and narrower linewidth. Generally speaking, the purpose of adding an amplifier to a laser oscillator is to increase the brightness B[W cm- 2 sc 1 ] of the output beam p (4.1) B = Aft ' where P is the power of the output beam emitted from the area A, and f? is the solid-angle divergence of the beam. Multiple-stage amplifier systems can be built Laser amplifier
Laser oscillator Mirror
Mirror
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Laser rod
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Flashtube
Laser rod
Flashtube
Fig. 4.1. Schematic diagram of a laser oscillator-amplifier configuration
W. Koechner, Solid-State Laser Engineering © Springer-Verlag Berlin Heidelberg 1999
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4. Laser Amplifier
151
if higher amplifications are required. In extreme cases Nd: glass laser systems have been constructed in which a 1-mJ output from an oscillator is amplified to a 1-kJ beam. In the design of laser amplifiers the following aspects must be considered: -
Gain and energy extraction. Wavefront and pulse-shape distortions introduced by the amplifier. Energy and power densities at the optical elements of the amplifier system. Feedback in the amplifier which may lead to superradiance or prelasing.
Of primary interest in the design of amplifiers is the gain which can be achieved and the energy which can be extracted from the amplifier. The rod length in an amplifier is determined primarily by the desired gain, while the rod diameter, set by damage threshold considerations, is dependent on the output energy. We shall see in the following sections that the gain of an amplifier pumped at a certain inversion level depends on the intensity and duration of the input pulse. To a first approximation we can assume the growth of input energy to be exponential, since the stimulated emission is proportional to the exciting photon flux. It will be seen, however, that exponential amplification will oc
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