With the development of dye laser systems, which operated continuously and were tunable over a broad range of visible wavelengths, many new applications for lasers were introduced.
The first laser was produced by the American physicist Theodore Harold Maiman
The prescription for producing a laser had been implied in some relatively simple equations, first given by the great German American physicist and Nobel Prize winner Albert Einstein
These first lasers, and most of the lasers that soon followed, had fixed wavelengths that could not be easily changed. Visible light is often measured by millionths of a meter, or micrometers; ruby light, for example, has a wavelength of 0.694 micrometer. This wavelength may be changed only with difficulty, by changing the temperature of the ruby crystal or by applying pressure to the ruby crystal, and the wavelengths of gas lasers are even less amenable to change. For many applications, the discrete nature of the wavelength of the laser is of little consequence. In spectroscopic applications, however, there is a great need to change the wavelength of the light in a simple and rapid manner.
When compared with conventional, spontaneous light sources used in spectroscopy, lasers have intrinsic advantages of high power (energy per unit time), high intensity (power per unit area of illumination), high brightness (power per unit area and per unit angle), and high spectral brightness (brightness per unit wavelength). These advantages arise because the light waves in a laser are coherent and in unison, whereas those in a conventional light source are incoherent and scattered. If one focuses a laser beam with a good lens, the light can be squeezed down to the size of a wavelength, increasing the intensity of the laser beam enormously. Clever methods can be used to squeeze the laser into a short period or, alternatively, into a narrow band of wavelengths. If such methods could be developed to tune the laser frequency over a wide range, the intrinsic advantages of lasers could be brought to bear in the important scientific and technical areas where spectroscopy plays a role.
Spectroscopy
One of the first tunable laser sources was demonstrated in 1965 by Joseph A. Giordmaine and Robert C. Miller at Bell Laboratories.
A major advance toward broadly tunable lasers in the visible spectrum occurred with the development of the dye laser by Peter P. Sorokin and J. R. Lankard at the IBM Watson Research Center in 1966. They used the dye chloroaluminum phthalocyanine and illuminated it with pulses from a ruby laser. The dye absorbed the shorter-wavelength, higher-energy, red light from the ruby and emitted longer-wavelength, lower-energy light. Using mirrors to return the emitted light to the dye, Sorokin and Lankard produced the first dye laser. The ruby laser acted as a “pump” to produce lasing in the dye, just as the flashlamp had pumped the ruby in Maiman’s first laser. The energy lost in the emission appeared as heat in the dye, requiring the dye to be cooled. Other dyes were introduced quickly by various researchers. Dyes have broad bands of emission wavelengths, and this is an obvious advantage in a laser that is to be tuned. Although a single dye still covers only a fraction of the visible spectrum, dyes were soon developed with other emission wavelengths that spanned much of the visible spectrum.
The disadvantage of dyes is that they emit so rapidly that they require extremely intense pumping, hence the ruby pump employed by Sorokin and Lankard. The rapid, spontaneous emission of the dyes, however, produces large stimulated emission amplification, so that only a very small portion of dye is needed to produce lasing. Thus, the intense pumping is needed in only a small volume of dye that was initially circulated in a transparent cell. Scientists were fortunate that intense lasers were available for focusing to pump the dye in the dye laser. Initially, these other laser sources were pulsed lasers, including both ruby and the neodymium laser used by Giordmaine and Miller. The need to introduce another laser beam between the mirrors of the dye laser cavity and to have the means to circulate the dye to remove the excess heat complicated the optical design of the dye laser cavity.
Soon scientists replaced one of the mirrors of the dye cavity by a diffraction grating. Optical gratings select only a narrow range of wavelengths for reflection along the cavity axis, depending on the orientation of the grating. By rotating the grating within the laser cavity, the output of the dye laser could be tuned across the broadband emission of the dye, and, with a single dye, pulsed emission could be tuned over about one-tenth the visible spectrum.
The intense and concentrated pumping required of the dye laser made continuous operation seem doubtful. In 1970, however, Ottis G. Peterson and his associates, S. A. Tuccio and B. B. Snavely at the Research Laboratories of Eastman Kodak, were able to design a dye laser that allowed continuous wave (CW) operation. They used a continuous argon ion laser to pump rhodamine 6G in a water solution, after adding some soap detergent. The water has a high heat capacity, affording excellent cooling for the dye, which was circulated at very high velocity through the pump focal region. The detergent acts to prevent agglomeration of the rhodamine molecules and to maintain the dye molecule in its lasing state.
The optical cavity used for the dye laser had grown complicated, and the optical quality of the laser beam suffered as a result. In 1971, Herwig W. Kogelnik, along with Erich Peter Ippen, A. Dienes, and Charles Vernon Shank at the Bell Laboratories, published a three-mirror design for a dye laser cavity that compensated for the astigmatism of the dye cavity. Astigmatism in the dye cavity distorts and enlarges both the focused pump spot and the entrapped laser spot. For optimum dye laser operation, these two spots should overlap completely and have a minimum size. In 1974, Ippen and Shank used this design in a dye laser to produce the first laser pulse shorter than one-trillionth of a second.
The improved performance of CW dye laser still suffered because of the cell containing the dye stream, typically 1 millimeter thin and moving at several meters per second. The cell walls became contaminated by particles burned at their surface by the intensely focused beam and became damaged by the high localized heating. In 1972, P. K. Runge and R. Rosenberg, also with Bell Laboratories, designed a nozzle that allowed unconfined flow of the dye stream as a thin sheet through the air and thus eliminated the need for a dye cell within the laser. Free-flowing dye streams became the standard practice for dye laser operation. The development of a fully tunable, visible dye laser was almost complete.
The apparatus was now in place for operation of a laser system that may be tuned precisely within the visible spectrum. When, in June of 1974, J. M. Yarborough produced continuous-wave emission from a set of overlapping dyes in a single laser system over the range from 0.415 to 0.790 micrometer, the spectrum was spanned. In all, Yarborough investigated eighteen dyes, some of which had not been previously lased. This dye set served as a reference set for researchers in the years to follow.
Progress continued rapidly in improving the performance of CW tunable lasers. In 1977, H. W. Schroeder and his associates at the Institute for Applied Physics at the Technical University in East Germany used a ring-shaped laser to obtain tunable dye laser outputs, in a single frequency, at continuous powers near 1 watt. By 1982, T. F. Johnson and his coworkers at the research laboratory of Coherent, Inc., in Palo Alto, California, had produced a dye laser system that used a single argon ion gas laser to pump a set of eleven dyes, which gave a continuously tunable output that covered the range of 0.407 to 0.887 micrometer. They obtained more than 5 watts output from rhodamine 6G in the red region of the spectrum and more than 1 watt for the middle two-thirds of the visible spectrum. In addition, the rhodamine dye was capable of generating harmonic radiation in the near ultraviolet.
Continuous tunable laser systems were also developed with other types of lasers, often using techniques pioneered with dye lasers. The main requirement of any broadly tunable system is the presence of a broad, spontaneous emission curve in the active laser material. A number of solid-state and semiconductor lasers have been uncovered that meet the requirements of broadband, continuous, tunable lasers.
Among the tunable solid-state lasers that have shown promise are those whose radiating atoms are the chromium and titanium ions. The secret to the broad-tuned laser operation is finding a good-quality crystal host in which these ions give broad emission. Chromium is the radiating atom in the ruby laser discovered by Maiman. Ruby is a crystal host of aluminum oxide that contains a small fraction of chromium oxide. Without the chromium oxide, the crystal is transparent sapphire, but the small percentage of chromium produces the red color of ruby and the intense red beam from a ruby laser. With sapphire as a host for the chromium in the ruby laser, the chromium in ruby does not give a very broad emission line, but in other hosts the output produced from chromium is quite broad. The first tunable laser using chromium was discovered in 1980 in the alexandrite host, formed from beryllium aluminum oxide, and there the chromium laser output spans the deep red region from 0.70 to 0.82 micrometer.
In contrast to chromium, the titanium ion in a host of sapphire does display very broad laser emission. First studied in 1982, titanium-sapphire produces a continuously tunable output over the range from 0.70 to 0.95 micrometer, from visible red to invisible near-infrared. The crystal may be pumped with an ion laser, or with the efficient semiconductor lasers, for CW operation. It should be noted that some semiconductor lasers form useful tunable sources whose output may be tuned by variation in current, temperature, and pressure.
In addition to the visible spectrum, the dye laser opened up a broad range of tunable laser applications in the ultraviolet and infrared by using unusual optical effects possible with lasers, often referred to as nonlinear effects. The electrical field within a laser beam depends on the intensity of the beam. By focusing even a rather weak laser beam in a material, the electrical field within the laser spot may be as large as the electrical field that holds the atoms and molecules of the material together. The material now responds by distorting the laser field so that, for example, multiples of the laser frequency may be produced. This effect allows the laser frequency to be doubled and the wavelength to be cut in half, converting dye red laser to near ultraviolet. When two different laser colors or frequencies are present in the spot simultaneously, both sum and difference frequencies may be generated. If the two laser frequencies are near each other in the visible band of one dye, the difference frequency generated is infrared light. Thus, a single visible dye laser under clever control can generate tunable ultraviolet and infrared beams in addition to visible beams. With a range of dyes, the complete region from ultraviolet through infrared may be scanned.
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Bova, Ben. The Beauty of Light. New York: John Wiley & Sons, 1988. An excellent summary of light and how it is used in art, science, industry, and technology. Written simply and in nontechnical terms. Chapter 15 discusses lasers accurately, without equations, and in understandable language. The book is a thoroughly enjoyable account of the science and art of light. -
Hitz, C. Breck, James J. Ewing, and Jeff Hecht. Introduction to Laser Technology. 3d ed. New York: IEEE Press, 2001. Introductory chapter provides an overview of the applications of commercial lasers worldwide. Intended for those who are familiar with the principles of electro-optical technology. -
Johnson, T. J. “Tunable Dye Lasers.” In Encyclopedia of Physical Science and Technology, edited by Robert A. Meyers. 3d ed. San Diego, Calif.: Academic Press, 2001. A comprehensive article on tunable dye lasers. The article is technical and contains equations. The excellent illustrations and fine writing allow judicious browsing for those with a nontechnical background. -
O’Shea, Donald C., W. Russell Callen, and William T. Rhodes. Introduction to Lasers and Their Applications. Reading, Massachusetts: Addison-Welsey, 1977. This undergraduate text covers the field of lasers in a detailed manner. Intended for science and engineering students, the book is not for browsing. The level is not too advanced, however, so the book can be used for selective reference. -
Schawlow, Arthur L., ed. Lasers and Light: Readings from “Scientific American.” San Francisco: W. H. Freeman, 1969. A fine introduction to the field of lasers that gives the sense of discovery pervading the early days following development of the laser. -
Sorokin, Peter. “Organic Lasers.” Scientific American 220 (February, 1969): 30-40. Organic lasers include dye lasers. The article by the discoverer of the dye laser is understandable and authoritative. Summarizes the properties of dye lasers as they were known in the period immediately after their discovery.
Optical Pulses Shorter than One Trillionth of a Second Are Produced