与激光有关的英文文献

与激光有关的英文文献

Company number：【00WT-88YT-W8CCB-BUUT-202108】

Laser technology

R. E. Slusher Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974

Laser technology during the 20th century is reviewed emphasizing the laser’s evolution from science to technology and subsequent contributions of laser technology to science. As the century draws to a close, 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 several 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)02802-0] 1. INTRODUCTION

Light has always played a central role in the study of physics, chemistry, and biology. 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 medicine, 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. There 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 ., Siegman, 1986; Agrawal and Dutta, 1993; and Ready, 1997).

Light amplification by stimulated 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 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 an 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 level

where the mean photon number in the mode is larger than one. At pump levels above this threshold condition,the system 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 spatial 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 detected 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, ., by constant current pumping 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 achieve

very high gain and low noise. In fact they presently have noise figures within a few dB of the 3 dB quantum noise limit for a phase-insensitive linear amplifier, ., 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 signal 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. HISTORY

Einstein (1917) provided the first essential idea for the laser, stimulated emission. Why wasn’t 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 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 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, 1953). The first experimentally demonstrated maser at Columbia University (Gordon et al., 1954, 1955) was based on an ammonia molecular beam. Bloembergen’s ideas for gain in three level systems resulted in the first practical maser amplifiers in the ruby system. These devices have noise figures very close 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 Arthur 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 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 mm and later at the red nm wavelength lasing transition. An excellent article 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 primarily

in terms of photons, some laser concepts 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 scotch 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 looking 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 oscillators, and parametric amplifiers is presently facilitating new research breakthroughs in physics, chemistry, and biology. 3. LASERS AT THE TURN OF THE CENTURY

Schawlow’s ‘‘law’’ states that everything lases if pumped hard enough. Indeed thousands of materials have been demonstrated as lasers and optical 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 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 quantum 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 mm that is envisioned as a defensive weapon (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 biological 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 the 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 in 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 can be efficiently coupled to

optical fibers (Choquette and Hou, 1997). VCSELs may find widespread application in optical data links.

4. MATERIALS PROCESSING AND LITHOGRAPHY

High power CO2 and Nd:YAG lasers are used for a wide variety of engraving, cutting, welding, soldering, and 3D prototyping applications. 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 edge discoloration, and a clean fused edge is obtained that eliminates

fraying 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 (computer-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 were 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 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 entire length (typically 50 m). The dopants in the core of the fiber that provide the gain can be erbium for the mm wavelength region or ytterbium for the mm region. High quality cavity mirrors are deposited directly on the ends of the fiber. These fiber lasers are extremely efficient, with overall 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 annealing 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 VLSI (very large scale integrated circuit) chips. As the IC (integrated circuit) design rules decrease from mm (1995) to 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 mercury arc radiation produced enough power at sufficiently short wavelengths of 436 nm and 365 nm for high production rates of IC devices patterned to mm and mm design rules respectively. As the century closes excimer laser sources with average output powers in the 200 W range are replacing the 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 mm design rules and the ArF laser transition at 193nm will probably be used beginning with mm design rules. At even smaller design rules, down to 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 present. 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 being considered for the 70 nm design rules anticipated for 2010 and beyond. 5. LASERS IN PHYSICS

Laser 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 resulted in great increases in precision measurement as described in an article in this volume (Ha¨nsch and Walther 1999).

Frequency-stabilized dye lasers and diode lasers precisely tuned to atomic transitions have resulted in ultracold atoms and Bose-Einstein condensates, also described in this volume (Wieman et al., 1999). Atomicstate control and measurements of atomic parity

nonconservation have reached a precision that allows tests of the standard model in particle physics as well as crucial searches for new physics beyond the standard model. In recent parity nonconservation experiments (Wood et al., 1997) Ce atoms are prepared in specific electronic states as they pass through two red diode laser beams. These prepared atoms then enter an optical cavity resonator where the atoms are excited to a higher energy level by high-intensity green light injected into the cavity from a frequency-stabilized dye laser. Applied electric and magnetic fields in this excitation region can be reversed to create a mirrored environment for the atoms. After the atom exits the excitation region, the atom excitation rate is measured by a third red diode laser. Very small changes in this excitation rate with a mirroring of the applied electric and magnetic fields indicate parity nonconservation. The accuracy of the parity nonconservation measurement has evolved over several decades to a level of %. This

measurement accuracy corresponds to the first definitive isolation of nuclear-spin-dependent atomic parity violation.