Understanding Lasers

Chapter 4: Laser Characteristics

ABOUT THIS CHAPTER

Now that we have seen what goes on inside lasers, it is time to look at the important characteristics of lasers and laser light. In this chapter, we will learn about coherence, wavelength, directionality, beam divergence, power, modulation, polarization, and related properties. We also will learn about the factors that determine laser efficiency.

4.1 COHERENCE

Coherence probably is the best-known property of laser light. Light waves are coherent if they are in phase with each other, that is, if their peaks and valleys line up, as shown in Figure 4-1. Coherence requires that the waves start in phase and that their wavelengths match to keep the waves from drifting out of phase with distance. Laser light starts out coherent because stimulated emission has the same phase and wavelength as the light wave or photon that stimulates it. The stimulated photon, in turn, can stimulate the emission of other photons, which are coherent with both it and the original wave.

In reality, laser light is not perfectly coherent. Not all stimulated emission is produced from a cascade triggered by a single spontaneously emitted photon, and even the stimulated emission generated by a single original photon is not perfectly in phase or exactly identical in wavelength. Differences in gain and spontaneous emission rates between types of lasers lead to differences in their coherence, which affects how the lasers are used. Let us look closer.

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