The laser is a device that uses the principle of stimulated emission to produce light. The qualities of the light generated by a laser are significantly different from that generated by a conventional source such as an incandescent light bulb or fluorescent light tube. These major differences include:
- Divergence: the laser generally emits a pencil thin beam of light whose divergent angle is closely related to the wavelength and limiting aperture size.
- Bandwidth: the light emitted by the laser generally consists of a very narrow range of wavelengths, or color.
- Intensity: the output from a laser is typically orders of magnitude higher in intensity (measured in watts per square meter) than a conventional light source.
- Coherence; the output from a laser is generally coherent; that is, the peaks and troughs of the light waves all correspond, allowing the light to form clear interference patterns.
Background and history
In the 1950s, there was a push by scientists to develop sources of coherent electromagnetic radiation at wavelengths shorter than vacuum tubes could provide. Charles Townes and co-workers at Columbia University, New York, developed the ammonia maser (microwave amplification by stimulated emission of radiation) in 1954, a device which produced coherent microwaves. In 1958, Townes and Art Schawlow published the principles of a maser operating in the visible region of the electromagnetic spectrum. The first successful demonstration of a laser followed in 1960 by Theodore Maiman of Hughes Laboratories, who operated a pulsed ruby laser that generated several kilowatts of optical power.
In the following few years, several different laser systems were demonstrated in gases (helium neon mixture, carbon dioxide, argon, krypton) and solids (uranium, samarium, neodymium, nickel, cobalt, and vanadium ions implanted in electrically insulating crystalline hosts). Since that time, laser action has been demonstrated in many different materials, involving all four states of matter (solid, liquid, gas, and plasma), covering the range of wavelengths from x rays to submillimeter waves. Only a few types of laser find widespread use because of issues such as efficiency, ease of use, reliability, and cost.
How it works
The laser consists of the following components: the pump, which is the source of energy to drive the laser, the active medium, which is where the stored energy is converted into the laser light through the process of stimulated emission, the optical cavity, which is usually made up of a pair of mirrors, one whose reflectivity at the wavelength of the laser is as close to 100 percent as possible and the other with a reflectivity less than 100 percent, through which the output beam propagates.
Stimulated emission is a process similar to absorption, but operates in the opposite direction. In absorption, an incoming photon is absorbed by an atom, leaving the atom in an excited state and annihilating the photon in the process. In stimulated emission, an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon. If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.
For laser action to occur, a majority of the atoms in the active medium must be excited into an energetic state, creating a population inversion of energized atoms ready to emit light. This is generally accomplished by pumping the atoms optically or electrically. As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light. The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, for example) and delivers a large signal (many photons, all identical to the first) at the output. This amplification, or gain, is provided by stimulated emission; hence the term laser, which is actually an acronym for light amplification by stimulated emission of radiation.
To illustrate laser operation, consider the helium neon (HeNe) laser, once a staple of supermarket scanners. The active medium is a mixture of helium and neon gases, enclosed in a glass tube a few inches long, with an electrode and a mirror at each end. The atoms in the gas mixture are excited, or pumped, by an electrical discharge that runs through the gas, in much the same way that a neon sign is lit. The conditions in the HeNe laser have been optimized so that the maximum number of neon atoms are in the correct state to emit light at the familiar red wavelength, 633 nm (nanometers).
Stimulated emission alone is not sufficient to produce laser output. The emission from the atoms occurs in all directions; to produce the highly directional output that makes the laser such a useful tool, the light must be trapped in an optical cavity that provides feedback, i.e., forces the light to travel in a desired direction. An optical cavity is free conduction band electrons recombine with holes in the valence band, emitting photons equal in energy to the band gap of the semiconductor. The only suitable semiconductors are the so-called direct gap materials, such as gallium arsenide and indium phosphide. By using alloys of different compositions, the laser band-gap can be engineered to produce light over a wide range of wavelengths (426 to 1,550 nm). Silicon, having an indirect gap, is not suitable for diode lasers.
Diode lasers are typically very small (the laser chip, with wires running into it, is mounted on a copper block only 6 mm wide). The cavity is formed by cleaved crystal facets that act as mirrors. The cavity length is typically between 100 microns and 1 mm, and the emitting volume has a cross-section of about 1 micron high by 1 to 200 microns wide. A diode laser can produce power in the range 1 mW to 1 W, depending on the size of the active volume. Standard semiconductor processing techniques can be used to form arrays of individual lasers in order to generate higher powers: an array with an emitting area of 1 cm × 1 cm can produce several kW of optical power.
Solid state lasers
Although the first commercial laser demonstrated was a solid state ruby laser, for many years the most common commercial systems were gas lasers such as helium neon lasers and argon ion lasers, or lasers based on organic dyes. Helium neon lasers were frequently limited in output power, argon ion lasers required expensive, sophisticated power supplies and cooling sources, and the dyes used in dye lasers were messy and often toxic. In the past decade, solid state lasers and diode lasers have become the dominant players in the commercial marketplace.
In a solid state laser, the active species is distributed throughout a solid, usually crystalline, material, although glass can also be used as a host. The lasers are robust and frequently tunable, though heat dissipation can sometimes be an issue. Certain types of solid state crystals, for example neodymium-doped yttriumaluminum garnet (Nd:YAG), can be pumped by diode lasers instead of by other lasers or by flashlamps, which is often the case for other materials. Such diode-pumped, solid state systems are reliable, economical, compact, and easy to operate–in fact, many commercial systems are turnkey, needing only to be plugged in and turned on to operate.
Solid state lasers are available from mid-infrared to ultraviolet wavelengths, and at a variety of output powers. Many solid-state lasers are tunable, providing output across a range of wavelengths. Some of the most sophisticated laser systems developed are solid-state-based, including ultrafast systems that produce light pulses just 6 femtoseconds (10-15 seconds) long, and are used to study molecular dynamics. Other uses for solid state lasers include medical applications, materials processing, and remote sensing.
Another common laser class is that of gas lasers, which includes helium neon (HeNe) lasers, carbon dioxide lasers, nitrogen lasers, and so on. The helium neon laser, widely used until the advent of the diode laser, was one of the first types developed and commercialized. As described above, it is a discharge-pumped gas laser, which generally produces an output measuring a few mW in power.
Like the HeNe laser, the carbon dioxide laser is a gas laser powered by an electrical discharge. However, the photon generated by stimulated emission arises from a vibrational transition within the molecule, rather than an electronic transition within an atom. As a result, the photon energy is much less than the laser systems described so far, and the wavelength is correspondingly longer at 10.6 æm. The carbon dioxide laser is one of the most efficient laser sources, having an efficiency in excess of 10 percent. Since the gas can flow through the discharge tube, the gas can easily take away excess heat and the laser can be cooled very effectively; this allows the carbon dioxide laser to operate at high average powers up to around 10 kW.
When it was first invented, the laser was called “a solution looking for a problem” because few good applications could be found for it. This is no longer the case, and the laser has found its way into many uses in every day life. In 2004, according to Laser Focus World, about 131,000 lasers (excluding diode lasers) were sold in the world, with about 733 million diode lasers sold. The major application areas for the laser are in communications, materials processing, optical data storage, surgery, defense, and scientific research.
Communications (diode lasers)
A pulsed infrared laser beam traveling down a single strand of optical fiber can carry thousands of times the information that can be carried by an electrical signal over copper wire. Not only can a single optical signal travel more rapidly than an electrical signal, but also optical signals of different wavelengths, or colors, can travel down a fiber simultaneously, and without interfering. This technique is known as wavelength division multiplexing (WDM). Using WDM, the capacity of an optical network can be increased by a factor number of wavelengths, or channels used—if two channels are used, the network capacity doubles, without the need for an additional fiber; if four channels are used, the network capacity quadruples. To offer an analogy, a WDM network is like a high-speed superhighway compared to the one-lane country road that is copper wire.
Low-loss optical fiber is lighter, more compact, and less expensive than the more traditional copper wire. Transcontinental and transoceanic optical fiber systems installed for telephone systems use directly modulated diode lasers operating at 1550 nm to generate the signals. Because even optical fiber generates some loss, the signals must be amplified periodically by repeaters, which also use laser technology. In the repeaters, a piece of optical fiber doped with erbium is pumped by a diode laser to optically amplify the original signal, which is launched back into the fiber link to continue its journey.
Materials processing (CO2 and Nd:YAG)
Since a laser beam can be focused down to a very small spot of light that can be absorbed very well at the surface of a material (be it metal, plastic, textile, etc.), the material can reach very high temperatures up to 9,032°F (5,000°C) and melt or even vaporize. In factories, laser systems are used to measure parts, inspect them for quality, and label, cut, weld, or resurface materials ranging from plastic film to sheet steel a quarter of an inch thick.
Lasers form the basis of precision-measuring tools called interferometers that can measure distances less than 1/100th the thickness of a human hair, and are as useful on construction sites as in laboratories. Such instruments can be scanned over objects to create images, and are used on highways to identify vehicles automatically, or on NASA spacecraft to map the surface of the moon and asteroids, for instance. In semiconductor manufacturing, ultraviolet lasers provide the exposure source for optical lithography, a technique used to produce computer chips with features as small as one hundred thousandth of an inch (0.25 microns).
Optical data storage (laser diodes)
The fact that a laser beam can be focused down to a very small spot has been used by the information industry and implemented in the form of the compact disc. The area needed to store a bit of information on a disc surface is smaller when optical reading is used than when magnetic reading is used. Much more information can be stored on a compact disc (several gigabytes) than on an equivalent magnetic disc (several hundred megabytes). This represents a savings in space and cost, which has opened up new markets for optical data storage-music CDS, CD-ROM, and optical storage libraries for large computer systems.
Surgery (Nd:YAG, CO2, holmium, erbium)
Lasers have a variety of applications in the field of medicine. Using laser beams whose wavelength is absorbed strongly; surgeons can cut and remove tissue with great precision by vaporizing it, with little damage to the surrounding tissue. The systems are used in minimally-invasive surgical techniques such as angioplasty (removing plaque from artery walls) and lithotripsy (the destruction of kidney stones in the bladder), in which a hair-thin optical fiber feeds light into the body through a tiny incision; the small opening required for the fiber results in reduced scarring and shorter healing times. In a technique called photorefractive keratectomy, lasers are routinely used to correct myopia and astigmatism. Other medical applications include removal of birthmarks, tattoos, and small varicose veins from the skin; welding tissue and sealing blood vessels during surgery; or resurfacing skin to minimize wrinkles or sun damage.
Cutting-edge researchers are using visible and infrared lasers to increase blood flow to oxygen-starved regions of the heart, to visually detect cancer cells on the skin's surface, or to image through the skin to monitor blood flow or detect cancerous regions. In a technique called photodynamic therapy, laser light activates a drug that accumulates in certain types of tumors, causing the drug to destroy the cancerous tissue. A sophisticated diagnostic technique called spectroscopy takes advantage of the fact that cancerous tissue reflects light differently than non-cancerous tissue, allowing doctors to identify skin cancer and cervical cancer. A laser-based optical mammography system is even under development.