Publisher Summary

Ultraviolet (UV) light has become important as the various technologies that are necessary to provide practical UV laser imaging and beam delivery systems have progressed, especially the light sources. UV light has been available only from low power lamps, thereby restricting the usefulness of the technology. The discovery and development of the excimer laser made possible the availability of intense ultraviolet light. Researchers explored and uncovered the unique properties of this new light source. As various phenomena involving UV energy and material interactions were discovered and optimized, practical applications emerged. This chapter discusses the UV spectrum, early UV lamp sources, laser physics and operating principles, and development leading to the discovery of the UV laser. UV light is available from a variety of sources, including naturally occurring UV light, UV lamps, and UV lasers. UV light from the sun is abundant, but cannot be easily or economically harnessed for the applications that have practical value. To a lesser degree, UV lamps have the same problem: they supply rich amounts of the ultraviolet wavelengths, but by the time the energy is collected and transmitted through an optical system, little energy is left to meet the demands of most commercial applications.

1.1 Introduction

Ultraviolet light has become more important in recent years as the various technologies necessary to provide practical UV laser imaging and beam delivery systems have progressed, especially the light sources. Historically, UV light has been available only from relatively low power lamps, thereby restricting the usefulness of the technology. The discovery and development of the excimer laser in the 1980s made possible, for the first time, the availability of intense ultraviolet light. Researchers explored and uncovered the unique properties of this new light source. As various phenomena involving UV energy and material interactions were discovered and optimized, practical applications emerged. In this chapter, we discuss the UV spectrum, early UV lamp sources, laser physics and operating principles, and development leading to the discovery of the UV laser.

1.2 The Ultraviolet Spectrum

The “ultraviolet” is a small portion of the electromagnetic spectrum, yet within its boundaries there are three distinct regions (near, mid, and far UV), each with particular significance in terms of applications and properties. Ultraviolet light is roughly defined as that portion (400- to 100-nm wavelengths) of the electromagnetic spectrum between longer wavelength visible light (400–700 nm) and shorter wavelength x-ray (10–100 nm) energy. Figure 1.1 shows the ultraviolet, visible, and infrared spectrums, indicating the various emission lines (1).

1.3 Historical Development of the Laser

Before UV lasers, there were long wavelength, red lasers. In this section, we will discuss the origins of the UV laser back to the initial concepts of amplification of wave energy. The origins of the laser (Light Amplification by Stimulated Emission of Radiation) are traced back to the early 1900s when Albert Einstein, Niels Bohr, Max Plank, and Ernest Rutherford were developing theories about the nature and behavior of matter.

Rutherford formed the idea of the atom composed of a nucleus with orbiting electrons. The electron orbits occurred at various energy levels. Plank developed the idea of electromagnetic waves as the form taken by radiant energy, and further specified that each frequency had a fixed quantum or amount of energy.

Niels Bohr described the phenomenon of fluorescence or spontaneous emission. This occurs when an atomic electron drops from a high energy level to an unoccupied lower energy level. When this happens, a quantum of light is emitted spontaneously.

Einstein theorized that other forms of emission were possible and formulated the idea of stimulated emission. Einstein predicted that when energy was applied to atoms, the response would be emission of energy.

These scientific theories remained to be proven in the lab, and during World War II, another step was taken to identify what became known as “lasing action” or lasers. High frequency radiowave and microwave oscillators were developed that could generate electromagnetic waves of very high frequencies and short wavelengths. The RADAR (Radio Detection And Ranging) was a useful result of this work. The Microwave Amplification by the Stimulated Emission of Radiation (MASER) was also developed before the laser and proved the theory of population inversion.

An analogy of how light behaves in a laser can be taken from how sound behaves in an amplifier. If a loudspeaker is placed too close to a microphone, the sound from the speaker is reamplified through the microphone and produces a howl. The howl is caused by sound waves, placed at closely spaced intervals, reinforcing each other by being “in phase.” At a certain level, an oscillation begins which produces the howl (3). In the following section, we will explain how this same principle works with light to produce a laser beam.

1.4 How a Laser Works

Lasers are possible because of the way light interacts with atoms and mole cules. To describe how a laser works, we will first review some basic aspects of light/matter interaction. The electrons in an atom or molecule exist in very specific energy levels, called “states.” Each atom possesses electrons that are characteristic to the specific element, or combination of elements (molecules) represented.