ABOUT THIS CHAPTER

In this chapter, you will learn about solid-state lasers, in which light is emitted by atoms embedded in a crystal, glass, or other transparent solid. After first explaining the general operation of solid-state lasers, this chapter describes the most important types, starting with the classic ruby laser, then covers neodymium and a family of related materials in which laser emission can be tuned across a range of wavelengths. The chapter also covers fiber lasers and amplifiers, in which the solid-state laser material is drawn into an optical fiber.

8.1 WHAT IS A SOLID-STATE LASER?

The first step in understanding solid-state lasers is to recognize that the laser community uses a different definition of "solidstate" than electronic engineers or physicists. Solid-state physics occurs in a solid. Solid-state circuits are semiconductor devices that conduct electricity and perform electronic operations. In electronics, semiconductors are considered solid-state devices in contrast to vacuum tubes, in which electrons move through a vacuum rather than through a solid. However, in the laser world semiconductor devices do not count as solid-state lasers because they operate in fundamentally different ways, as you will learn in Chapter 9.

The fundamental difference arises from the way the laser medium is excited. The solids in solid-state lasers transmit light that excites atoms contained within the solid to produce a population inversion on the laser transition. The light-absorbing atoms are dispersed in a solid that transmits the pump light that excites the atoms. In Maiman's first laser, the light-absorbing atoms were chromium dispersed in aluminum oxide (sapphire). Normally, the light-absorbing atoms are present only in small concentrations, added intentionally to an otherwise transparent solid as dopants. Solids transparent at optical wavelengths are insulators and conduct electrical current poorly. In contrast, semiconductor lasers are excited by an electrical current, as described in Chapter 9.

Chromium atoms both absorb and emit light in ruby, and give the crystal its red color. The simple design of Maiman's laser, shown in Figure 8-1, clearly illustrates the principles of solid-state laser operation. Light from the spring-shaped flashlamp illuminated the little ruby rod that it surrounded. Green and violet light passed through the red-colored ruby crystal and excited chromium atoms. Some chromium atoms then emitted red light, which stimulated other excited chromium atoms to release their excess energy as identical red photons. Thin metal films coated on the ends of the rod formed a reflective laser cavity, with the beam emerging through a small hole in the middle of the film on one end. The laser pulsed only during the flash of the flashlamp.

Today's diverse family of solid-state lasers have been refined in many ways, but the same principles underlie their operation. Photons from an external source excite atoms dispersed in a solid host, producing a population inversion. Spontaneous emission triggers a cascade of stimulated emission, which oscillates between the mirrors in a laser cavity, and produces a beam.

Solid-state lasers can take various forms. Typically, the solid is shaped into a rod, but sometimes it may be a slab with mirrors arranged so light oscillates through the slab. The solid also may be in the form of an optical fiber, essentially a very long and very thin rod.

Solid-state lasers can be made from many different materials, although in practice most solid-state lasers are made of a few materials selected because they perform best as lasers. The lasers may be packaged with nonlinear devices that generate shorter-wavelength harmonics of the fundamental laser frequency. Solid-state laser materials can be used in laser amplifiers as well as in oscillators, particularly for optical amplification in fiber-optic communication systems, or to produce short high-energy pulses.

The technology has come a long way since Maiman's first laser. Sales of solid-state and fiber lasers were close to $1.2 billion dollars in 2006, according to Laser Focus World. That total is more than five times the dollar value of solid-states lasers sold in 1991, and that number is expected to continue rising.

Let us look first at some general issues of materials and optical pumping, then turn to specific types of solid-state lasers.