What is a Laser?

This is a continuation from the previous tutorial - linear lightwave propagation in an optical fiber.

Lasers are devices that generate or amplify coherent radiation at frequencies in the infrared, visible, or ultraviolet regions of the electromagnetic spectrum. Lasers operate by using a general principle that was originally invented at microwave frequencies, where it was called microwave amplification by stimulated emission of radiation, or maser action. When extended to optical frequencies this naturally becomes light amplification by stimulated emission of radiation, or laser action.

This basic laser or maser principle is now used in an enormous variety of devices operating in many parts of the electromagnetic spectrum, from audio to ultraviolet. Practical laser devices in particular employ an extraordinary variety of materials, pumping methods, and design approaches, and find a great variety of applications. The study of laser and maser devices and their scientific applications is often referred to as the field of quantum electronics.

From an electronics-engineering viewpoint, the developments that followed the operation of the first ruby laser in 1960 suddenly pushed the upper limit of coherent electronics from the millimeter-wave range, using microwave tubes and transistors, out to include the submillimeter, infrared, visible, and ultraviolet spectral regions (and soft X-ray lasers are now on the horizon). All the familiar functions of coherent signal generation, amplification, modulation, information transmission, and detection are now possible at frequencies up to a million times higher, or wavelengths down to a million times shorter, than previously. But it has also become possible for engineers and scientists, in fields of technology ranging from microbiology to auto manufacture, to perform an almost unlimited variety of new and unexpected functions made possible by the short wavelengths, high powers, ultrashort pulsewidths, and other unique characteristics of these laser devices.

In the twenty-odd years since the first appearance of coherent light, lasers have become widespread and almost commonplace devices. The importance and the excitement of the laser and its applications, however, still can hardly be overestimated. The objective of this book is to explain in detail how lasers work, what the performance characteristics of typical lasers are, and how lasers are employed in a wide variety of applications.

Lasers, broadly speaking, are devices that generate or amplify light, just as transistors and vacuum tubes generate and amplify electronic signals at audio, radio, or microwave frequencies. Here "light" must be understood broadly, since different kinds of lasers can amplify radiation at wavelengths ranging from the very long infrared region, merging with millimeter waves or microwaves, up through the visible region and extending now to the vacuum ultraviolet and even X-ray regions. Lasers come in a great variety of forms, using many different laser materials, many different atomic systems, and many different kinds of pumping or excitation techniques. The beams of radiation that lasers emit or amplify have remarkable properties of directionality, spectral purity, and intensity. These properties have already led to an enormous variety of applications, and others undoubtedly have yet to be discovered and developed. 

Essential Elements of a Laser

The essential elements of a laser device, as shown in Figure 1.1, are thus: (i) a laser medium consisting of an appropriate collection of atoms, molecules, ions, or in some instances a semiconducting crystal; (ii) a pumping process to excite these atoms (molecules, etc.) into higher quantum-mechanical energy levels; and (iii) suitable optical feedback elements that allow a beam of radiation to either pass once through the laser medium (as in a laser amplifier) or bounce back and forth repeatedly through the laser medium (as in a laser oscillator).

These elements come in a great variety of forms and fashions, as we will see when we begin to examine each of them in more detail.

Laser Atoms and Laser Pumping

For simplicity we will from now on use "atoms" as a general term for whatever kind of atoms or molecules or ions or semiconductor electrons may be used as the laser medium. A pumping process is then required to excite these atoms into their higher quantum-mechanical energy levels. Practical laser materials can be pumped in many ways, as we will describe later.

For laser action to occur, the pumping process must produce not merely excited atoms, but a condition of population inversion (Figure 1.2), in which more atoms are excited into some higher quantum energy level than are in some lower energy level in the laser medium. It turns out that we can obtain this essential condition of population inversion in many ways and with a wide variety of laser materials—though sometimes only with substantial care and effort.

Laser Amplification

Once population inversion is obtained, electromagnetic radiation within a certain narrow band of frequencies can be coherently amplified if it passes through the laser medium (Figure 1.3). This amplification bandwidth will extend over the range of frequencies within about one atomic linewidth or so on either side of the quantum transition frequency from the more heavily populated upper energy level to the less heavily populated lower energy level.

Coherent amplification means in this context that the output signal after being amplified will more or less exactly reproduce the input signal, except for a substantial increase in amplitude. The amplification process may also add some small phase shift, a certain amount of distortion, and a small amount of amplifier noise. Basically, however, the amplified output signal will be a coherent reproduction of the input optical signal, just as in any other coherent electronic amplification process.

Laser Oscillation

Coherent amplification combined with feedback is, of course, a formula for producing oscillation, as is well known to anyone who has turned up the gain on a public-address system and heard the loud squeal of oscillation produced by the feedback from the loudspeaker output to the microphone input. The feedback in a laser oscillator is usually supplied by mirrors at each end of the amplifying laser medium, carefully aligned so that waves can bounce back and forth between these mirrors with very small loss per bounce (Figure 1.4). If the net laser amplification between mirrors, taking into account any scattering or other losses, exceeds the net reflection loss at the mirrors themselves, then coherent optical oscillations will build up in this system, just as in any other electronic feedback oscillator.

When such coherent oscillation does occur, an output beam that is both highly directional and highly monochromatic can be coupled out of the laser oscillator, either through a partially transmitting mirror on either end, or by some other technique. This output in essentially all lasers will be both extremely bright and highly coherent. The output beam may also in some cases be extremely powerful. Just what we mean by "bright" and by "coherent" we will explain later.

The next tutorial introduces lenses for image formation and manipulation.