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2 - Optical frequency amplifiers

Published online by Cambridge University Press:  05 June 2014

Christopher C. Davis
Christopher C. Davis
Affiliation:
University of Maryland, College Park
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Summary

Introduction

When an electromagnetic wave propagates through amedium stimulated emissions increase the intensity of the wave, while absorptions diminish it. The overall intensity will increase if the number of stimulated emissions can be made larger than the number of absorptions. If we can create such a situation then we have built an amplifier that operates through the mechanism of stimulated emission. This laser amplifier, in common with electronic amplifiers, has useful gain only over a particular frequency bandwidth. Its operating frequency range will be determined by the lineshape of the transition, and we expect the frequency width of its useful operating range to be of the same order as the width of the lineshape. It is very important to consider how this frequencywidth is related to the various mechanisms by which transitions between different energy states of a particle are smeared out over a range of frequencies. This line broadening affects in a fundamental way not only the frequency bandwidth of the amplifier, but also its gain. A laser amplifier can be turned into an oscillator by supplying an appropriate amount of positive feedback. The level of oscillation will stabilize because the amplifier saturates. Laser amplifiers fall into two categories, which saturate in different ways. A homogeneously broadened amplifier consists of a number of amplifying particles that are essentially equivalent, whereas an inhomogeneously broadened amplifier contains amplifying particles with a distribution of amplification characteristics.

Homogeneous line broadening

All energy states, except the lowest energy state of a particle (the ground state) cover a range of possible energies.

Type
Chapter
Information
Lasers and Electro-optics
Fundamentals and Engineering
 , pp. 33 - 71
Publisher: Cambridge University Press
Print publication year: 2014

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References

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