ABSTRACT
The invention of the laser, which stands for light amplification by stimulated emission of radiation, can be dated to 1958 with the publication of the scientific paper, Infrared and Optical Masers, by Arthur L. Schawlow, then a Bell Labs researcher, and Charles H. Townes, a consultant to Bell Labs [158, 159] . The work by Schawlow and Townes, however, can be traced back to the 1940s and early 50s and their interest in the field of microwave spectroscopy. In 1953, Townes, Gordon, and Zeiger demonstrated a working device, which Townes called the maser, which stands for microwave amplification by stimulated emission of radiation [160]. Then, Townes realized that the shorter wavelengths beyond those of microwaves - the wavelengths of infrared and optical light - probably offered even more powerful tools for spectroscopy than those produced by the maser. Schawlow’s idea was to arrange a set of mirrors, one at each end of the cavity, to bounce the light back and forth on-axis, which would eliminate amplifying any beams bouncing in other directions. In the fall of 1957, they began working out the principles of a device that could provide these shorter wavelengths. While Schawlow was working on the device, Townes worked on the theory. After eight months of work, the collaboration was fruitful. In 1958, the two men wrote the aforementioned paper, although they had not yet made an actual laser. Two years later, Schawlow and Townes received a patent for the invention of the laser; the same year a working laser (a pulsed ruby laser at λ=694 nm) was built by Theodore Maiman at Hughes Aircraft Company [161]. Since then, lasers have been developed spanning the spectral range from the far infrared to the vacuum ultraviolet region. They have proved to be invaluable tools not only for the solution of a myriad of scientific problems but also for countless technical applications. In this chapter, we discuss the basic physical principles of lasers as well as the most important classes of both direct laser systems and laser-based coherent radiation sources, with particular regard to the applications in the field of optical frequency measurements.
