1、Laser technologyR. E. Slusher Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974Laser technology during the 20th century is reviewed emphasizing the lasers evolution from science to technology and subsequent contributions of laser technology to science. As the century draws to a c
2、lose, lasers are making strong contributions to communications, materials processing, data storage, image recording, medicine, and defense. Examples from these areas demonstrate the stunning impact of laser light on our society. Laser advances are helping to generate new science as illustrated by se
3、veral examples in physics and biology. Free-electron lasers used for materials processing and laser accelerators are described as developing laser technologies for the next century.S0034-6861(99)02802-01. INTRODUCTIONLight has always played a central role in the study of physics, chemistry, and biol
4、ogy. Light is key to both the evolution of the universe and to the evolution of life on earth. This century a new form of light, laser light, has been discovered on our small planet and is already facilitating a global information transformation as well as providing important contributions to medici
5、ne, industrial material processing, data storage, printing, and defense. This review will trace the developments in science and technology that led to the invention of the laser and give a few examples of how lasers are contributing to both technological applications and progress in basic science. T
6、here are many other excellent sources that cover various aspects of the lasers and laser technology including articles from the 25th anniversary of the laser (Ausubell and Langford, 1987) and textbooks (e.g., Siegman, 1986; Agrawal and Dutta, 1993; and Ready, 1997). Light amplification by stimulated
7、 emission of radiation (LASER) is achieved by exciting the electronic, vibrational, rotational, or cooperative modes of a material into a nonequilibrium state so that photons propagating through the system are amplified coherently by stimulated emission. Excitation of this optical gain medium can be
8、 accomplished by using optical radiation, electrical current and discharges, or chemical reactions. The amplifying medium is placed in an optical resonator structure, for example between two high reflectivity mirrors in a Fabry-Perot interferometer configuration. When the gain in photon number for a
9、n optical mode of the cavity resonator exceeds the cavity loss, as well as loss from nonradiative and absorption processes, the coherent state amplitude of the mode increases to a levelwhere the mean photon number in the mode is larger than one. At pump levels above this threshold condition,the syst
10、em is lasing and stimulated emission dominates spontaneous emission. A laser beam is typically coupled out of the resonator by a partially transmitting mirror. The wonderfully useful properties of laser radiation include spatial coherence, narrow spectral emission, high power, and well-defined spati
11、al modes so that the beam can be focused to a diffraction-limited spot size in order to achieve very high intensity. The high efficiency of laser light generation is important in many applications that require low power input and a minimum of heat generation.When a coherent state laser beam is detec
12、ted using photon-counting techniques, the photon count distribution in time is Poissonian. For example, an audio output from a high efficiency photomultiplier detecting a laser field sounds like rain in a steady downpour. This laser noise can be modified in special cases, e.g., by constant current p
13、umping of a diode laser to obtain a squeezed number state where the detected photons sound more like a machine gun than rain. An optical amplifier is achieved if the gain medium is not in a resonant cavity. Optical amplifiers can achievevery high gain and low noise. In fact they presently have noise
14、 figures within a few dB of the 3 dB quantum noise limit for a phase-insensitive linear amplifier, i.e., they add little more than a factor of two to the noise power of an input signal. Optical parametric amplifiers (OPAs), where signal gain is achieved by nonlinear coupling of a pump field with sig
15、nal modes, can be configured to add less than 3 dB of noise to an input signal. In an OPA the noise added to the input signal can be dominated by pump noise and the noise contributed by a laser pump beam can be negligibly small compared to the large amplitude of the pump field.2. HISTORYEinstein (19
16、17) provided the first essential idea for the laser, stimulated emission. Why wasnt the laser invented earlier in the century? Much of the early work on stimulated emission concentrates on systems near equilibrium, and the laser is a highly nonequilibrium system. In retrospect the laser could easily
17、 have been conceived and demonstrated using a gas discharge during the period of intense spectroscopic studies from 1925 to 1940. However, it took the microwave technology developed during World War II to create the atmosphere for thelaser concept. Charles Townes and his group at Columbia conceived
18、the maser (microwave amplification by stimulated emission of radiation) idea, based on their background in microwave technology and their interest in high-resolution microwave spectroscopy. Similar maser ideas evolved in Moscow (Basov and Prokhorov, 1954) and at the University of Maryland (Weber, 19
19、53). The first experimentally demonstrated maser at Columbia University (Gordon et al., 1954, 1955) was based on an ammonia molecular beam. Bloembergens ideas for gain in three level systems resulted in the first practical maser amplifiers in the ruby system. These devices have noise figures very cl
20、ose to the quantum limit and were used by Penzias and Wilson in the discovery of the cosmic background radiation.Townes was confident that the maser concept could be extended to the optical region (Townes, 1995). The laser idea was born (Schawlow and Townes, 1958) when he discussed the idea with Art
21、hur Schawlow, who understood that the resonator modes of a Fabry-Perot interferometer could reduce the number of modes interacting with the gain material in order to achieve high gain for an individual mode. The first laser was demonstrated in a flash lamp pumped ruby crystal by Ted Maiman at Hughes
22、 Research Laboratories (Maiman, 1960). Shortly after the demonstration of pulsed crystal lasers, a continuouswave (CW) He:Ne gas discharge laser was demonstrated at Bell Laboratories (Javan et al., 1961), first at 1.13 mm and later at the red 632.8 nm wavelength lasing transition. An excellent artic
23、le on the birth of the laser is published in a special issue of Physics Today (Bromberg, 1988).The maser and laser initiated the field of quantum electronics that spans the disciplines of physics and electrical engineering. For physicists who thought primarilyin terms of photons, some laser concepts
24、 were difficult to understand without the coherent wave concepts familiar in the electrical engineering community. For example, the laser linewidth can be much narrower than the limit that one might think to be imposed by the laser transition spontaneous lifetime. Charles Townes won a bottle of scot
25、ch over this point from a colleague at Columbia. The laser and maser also beautifully demonstrate the interchange of ideas and impetus between industry, government, and university research. Initially, during the period from 1961 to 1975 there were few applications for the laser. It was a solution lo
26、oking for a problem. Since the mid-1970s there has been an explosive growth of laser technology for industrial applications. As a result of this technology growth, a new generation of lasers including semiconductor diode lasers, dye lasers, ultrafast mode-locked Ti:sapphire lasers, optical parameter
27、 oscillators, and parametric amplifiers is presently facilitating new research breakthroughs in physics, chemistry, and biology.3. LASERS AT THE TURN OF THE CENTURYSchawlows law states that everything lases if pumped hard enough. Indeed thousands of materials have been demonstrated as lasers and opt
28、ical amplifiers resulting in a large range of laser sizes, wavelengths, pulse lengths, and powers. Laser wavelengths range from the far infrared to the x-ray region. Laser light pulses as short as a few femtoseconds are available for research on materials dynamics. Peak powers in the petawatt range
29、are now being achieved by amplification of femtosecond pulses. When these power levels are focused into a diffraction-limited spot, the intensities approach 1023 W/cm2. Electrons in these intense fields are accelerated into the relativistic range during a single optical cycle, and interesting quantu
30、m electrodynamic effects can be studied. The physics of ultrashort laser pulses is reviewed is this centennial series (Bloembergen, 1999). A recent example of a large, powerful laser is the chemical laser based on an iodine transition at a wavelength of 1.3 mm that is envisioned as a defensive weapo
31、n (Forden, 1997). It could be mounted in a Boeing 747 aircraft and would produce average powers of 3 megawatts, equivalent to 30 acetylene torches. New advances in high quality dielectric mirrors and deformable mirrors allow this intense beam to be focused reliably on a small missile carrying biolog
32、ical or chemical agents and destroy it from distances of up to 100 km. This star wars attack can be accomplished during the launch phase of the target missile so that portions of the destroyed missile would fall back on its launcher, quite a good deterrent for these evil weapons. Captain Kirk and th
33、e starship Enterprise may be using this one on the Klingons!At the opposite end of the laser size range are microlasers so small that only a few optical modes are contained in a resonator with a volume in the femtoliter range. These resonators can take the form of rings or disks only a few microns i
34、n diameter that use total internal reflection instead of conventional dielectric stack mirrors in order to obtain high reflectivity. Fabry-Perot cavities only a fraction of a micron in length are used for VCSELs (vertical cavity surface emitting lasers) that generate high quality optical beams that
35、can be efficiently coupled to optical fibers (Choquette and Hou, 1997). VCSELs may find widespread application in optical data links.4. MATERIALS PROCESSING AND LITHOGRAPHYHigh power CO2 and Nd:YAG lasers are used for a wide variety of engraving, cutting, welding, soldering, and 3D prototyping appli
36、cations. rf-excited, sealed off CO2 lasers are commercially available that have output powers in the 10 to 600 W range and have lifetimes of over 10 000 hours. Laser cutting applications include sailclothes, parachutes, textiles, airbags, and lace. The cutting is very quick, accurate, there is no ed
37、ge discoloration, and a clean fused edge is obtained that eliminatesfraying of the material. Complex designs are engraved in wood, glass, acrylic, rubber stamps, printing plates, plexiglass, signs, gaskets, and paper. Threedimensional models are quickly made from plastic or wood using a CAD (compute
38、r-aided design) computer file.Fiber lasers (Rossi, 1997) are a recent addition to the materials processing field. The first fiber lasers were demonstrated at Bell Laboratories using crystal fibers in an effort to develop lasers for undersea lightwave communications. Doped fused silica fiber lasers w
39、ere soon developed. During the late 1980s researchers at Polaroid Corp. and at the University of Southampton invented cladding-pumped fiber lasers. The glass surrounding the guiding core in these lasers serves both to guide the light in the single mode core and as a multimode conduit for pump light
40、whose propagation is confined to the inner cladding by a low-refractive index outer polymer cladding. Typical operation schemes at present use a multimode 20 W diode laser bar that couples efficiently into the large diameter inner cladding region and is absorbed by the doped core region over its ent
41、ire length (typically 50 m). The dopants in the core of the fiber that provide the gain can be erbium for the 1.5 mm wavelength region or ytterbium for the 1.1 mm region. High quality cavity mirrors are deposited directly on the ends of the fiber. These fiber lasers are extremely efficient, with ove
42、rall efficiencies as high as 60%. The beam quality and delivery efficiency is excellent since the output is formed as the single mode output of the fiber. These lasers now have output powers in the 10 to 40 W range and lifetimes of nearly 5000 hours. Current applications of these lasers include anne
43、aling micromechanical components, cutting of 25 to 50 mm thick stainless steel parts, selective soldering and welding of intricate mechanical parts, marking plastic and metal components, and printing applications.Excimer lasers are beginning to play a key role in photolithography used to fabricate V
44、LSI (very large scale integrated circuit) chips. As the IC (integrated circuit) design rules decrease from 0.35 mm (1995) to 0.13 mm (2002), the wavelength of the light source used for photolithographic patterning must correspondingly decrease from 400 nm to below 200 nm. During the early 1990s merc
45、ury arc radiation produced enough power at sufficiently short wavelengths of 436 nm and 365 nm for high production rates of IC devices patterned to 0.5 mm and 0.35 mm design rules respectively. As the century closes excimer laser sources with average output powers in the 200 W range are replacing th
46、e mercury arcs. The excimer laser linewidths are broad enough to prevent speckle pattern formation, yet narrow enough, less than 2 nm wavelength width, to avoid major problems with dispersion in optical imaging. The krypton fluoride (KF) excimer laser radiation at 248 nm wavelength supports 0.25 mm
47、design rules and the ArF laser transition at 193nm will probably be used beginning with 0.18 mm design rules. At even smaller design rules, down to 0.1 mm by 2008, the F2 excimer laser wavelength at 157 nm is a possible candidate, although there are no photoresists developed for this wavelength at p
48、resent. Higher harmonics of solid-state lasers are also possibilities as high power UV sources. At even shorter wavelengths it is very difficult for optical elements and photoresists to meet the requirements in the lithographic systems. Electron beams, x-rays and synchrotron radiation are still bein
49、g considered for the 70 nm design rules anticipated for 2010 and beyond.5. LASERS IN PHYSICSLaser technology has stimulated a renaissance in spectroscopies throughout the electromagnetic spectrum. The narrow laser linewidth, large powers, short pulses, and broad range of wavelengths has allowed new dynamic and spectral studies of gases, plasmas, glasses, crystals, and liquids. For example, Raman scattering studies of phonons, magnons, plasmons, rotons, and excitations in 2D electron gases have flourished since the invention of the laser. Nonlinear laser spectroscopies have result
