ABOUT THIS CHAPTER

Many different types of lasers have been developed. They often are classified by the type of active medium, but that is not the whole story. In this chapter, we will look at lasers in a variety of ways, including not just the nature of the medium but the way in which they are operated. We will start by comparing the fundamental nature of oscillators and amplifiers, then move on to media type, gain levels within the laser, and laser bandwidth. The chapter closes with a section on laser-like light sources. Chapters 7 through 10 will examine major types of lasers in more detail.

6.1 LASER OSCILLATORS AND OPTICAL AMPLIFIERS

The distinction between amplification and oscillation is a fundamental one in the laser world. Although the acronym "laser" specifies only light amplification by the stimulated emission of radiation, the real definition of "laser" includes the requirement for internal feedback that sustains oscillation. A laser acts as an oscillator in that it generates its own beam, at a wavelength that depends on the laser's physical characteristics. In contrast, an optical amplifier merely amplifies light from an external source, and lacks mirrors that form a resonant cavity

This difference leads to a fundamental difference in behavior. A laser oscillator sets its own wavelength, but the wavelength of an optical amplifier depends on the input signal that stimulates emission. Laser oscillation is relatively easy to measure because the output power is low below threshold, but increases rapidly in power and decreases in linewidth once the device exceeds threshold. Amplification is more difficult to measure, particularly if the gain is low.

Early developers focused their attention on laser oscillators because they wanted devices that generated beams internally. Optical amplifiers (sometimes called laser amplifiers) have found some applications in boosting the power available from pulsed laser oscillators, but today the main application of optical amplifiers is for boosting the signal strength in fiber-optic communication systems.