v1.0.1 / chapter 12 of 20 / 01 sep 07 / greg goebel / public domain
* One of the more unexpected inventions to come out of quantum physics was the "laser", a device for generating a powerful beam of light, as well as its microwave cousin, the "maser". The initial reaction to the laser was that death rays were around the corner; that wasn't true, but the laser would prove an extremely important invention in many other ways.
* Wartime work on electronics would lead to other discoveries besides solid-state electronic devices. Before the war, spectroscopy had generally focused on the high infrared, optical, and low ultraviolet regions of the EM spectrum, which could be picked up photographic film. That wasn't the case for the "microwave" region of the spectrum, the shortest wavelengths in the radio band; the technology didn't really exist to do a good job of mapping microwave spectral bands.
The war led to an explosive surge of development of microwave gear, and after the war physicists and chemists had a box full of new toys to probe the microwave region. Microwave spectroscopy came of age. Microwaves have much longer wavelengths than those found in the optical range, and accordingly much lower energy. While optical spectral lines are typically produced by the change of state of electrons in atoms or ions, microwave spectral lines are typically produced by changes in the configuration of molecular bonds. Microwave spectroscopy could be used to identify the "fingerprints" of various types of molecules, and also probe their behavior.
Studies of the microwave spectra of molecules quickly convinced researchers that there was a significant gap in coverage, in the "millimeter wave" region of the electromagnetic spectrum, between microwaves and the low infrared. There hadn't been much work on millimeter wave technology during the war and so there weren't many tools available for their observation. The military was also interested in millimeter waves, since they could be used for advanced radars. Millimeter waves were easily blocked by raindrops and so on and didn't have much range, but the short wavelengths of millimeter waves meant that a millimeter-wave radar could provide a high-resolution image of a target.
In the spring of 1951, at the urging of the US Navy, a physicist named Charles Townes (born 1915) from Columbia University set up a conference of experts to consider the development of millimeter-wave technology. The technologies then available to produce microwaves weren't really up to the task, and Townes felt that something entirely new was needed. While he was considering the agenda for the meeting, an idea clicked together in his head.
Since some molecules could produce spectral lines in the millimeter-wave region of the spectrum, such molecules could be used to generate millimeter waves. It was hard to figure out an efficient way of doing this, though there was a phenomenon that helped. In 1917, while Albert Einstein was following up his thoughts about the photon theory of light, he discovered a concept known as "stimulated emission". As discussed previously, if the electrons in an atom or molecule are excited to higher energy states, they will then tend to cascade back down to lower energy states, emitting a photon with an energy equal to that of the energy transition. Such emissions are generally "spontaneous", meaning that they happen at random times that follow a half-life curve, in a way very much like radioactive decay.
Einstein realized that there was a way to provoke or "stimulate" such a emission. If an atom or molecule were hit by a photon with an energy matching that of a difference between two particular energy states and the electron was in the higher energy state, it would fall down to the lower energy state and release a photon of the same energy as the one that hit the atom or molecule in the first place. In addition, the two photons would be "coherent", moving in the same direction and having the same phase.
The first problem, as Townes saw it, was that producing microwave energy using such "stimulated emission" required a batch of molecules that were mostly in the higher energy state. Since this isn't the way things normally happen, this circumstance was called a "population inversion". The second problem was that even if he could arrange a population inversion in a batch of molecules, stimulated emission wouldn't be able in itself to generate much of a millimeter-wave beam.
In 1950, two Harvard researchers, Edward M. Purcell and Robert V. Pound, had achieved a population inversion in a crystal of lithium fluoride as a "pure physics" experiment, demonstrating that a population inversion was possible. As for the limitations of stimulated emission, Townes thought he could perform it in an electromagnetic resonant cavity, a chamber precisely machine to allow electromagnetic radiation of a specific wavelength to resonate and build up as the beam bounced from one end of the cavity to the other, stimulating emission from the molecules with each pass.
Townes didn't mention the idea at the conference for fear it was still half-baked, but once he was back in New York City and Columbia University, he put a small research team to work on it. By the fall of 1951, the team had come up with a scheme for generating radiation using ammonia (NH3) molecules, with output at a wavelength of 1.25 centimeters. This was not millimeter-wave emission, but Townes and his group had found that generating millimeter waves was very difficult, partly because of the chicken-and-egg problem of a lack of millimeter-wave components. The group was trying to perform a proof-of-concept demonstration with microwaves and then move on to shorter wavelengths.
The NH3 molecule is a tetrahedron, with the nitrogen atom linked to a base of three hydrogen atoms connected in a triangle. In its ground state, the molecule is polarized, with a negative charge on the nitrogen end and a positive charge on the hydrogen base of the tetrahedron. Such a polarized molecule could be moved by an electric field. However, when an ammonia molecule is excited, the nitrogen atom has an equal probability of being on either side of the base, with the result that the molecule is no longer polarized and won't move in an electric field. That meant that ammonia molecules could be heated and then fed into a resonant cavity through an assembly that set up an electric field to shunt the ground-state molecules off to one side. By late 1953, the researchers were able to use the scheme to amplify microwaves, and by the spring of 1954 they were using it as a microwave oscillator. The achievement was announced in Columbia's quarterly report as "the first time energy has been obtained continuously from a molecular resonance."
The device was named a "maser", for "microwave amplification by stimulated emission of radiation". The term "amplification" was used instead of "oscillation" because the maser ended up being a very sensitive low-noise amplifier, often used in radio astronomy because it allowed pickup of faint cosmic radio signals. Ironically, radio telescope observations would also show that maser action occurred in nature, with stimulated emission in cosmic molecular gas clouds creating a relatively intense emission.
It should be noted that Townes wasn't the only person to think of the idea of the maser. Another American researcher, Joseph Weber (1919:2000), then a student at Catholic University, also came up with the idea in parallel, though he did not demonstrate it. In addition, two Soviet researchers, Nikolai G. Basov (1922:2001) and Alexander M. Prokhorov (1916:2002), developed the first continuous-wave maser, paralleling Townes' pioneering efforts on the other side of the Iron Curtain.
* Although the maser was ingenious and won the admiration of the physics community, from the public point of view it attracted little or no attention. It was a useful research tool, but had few or no consumer, industrial, or military applications. However, Townes had no intention of stopping at the maser, and in fact he had basically developed it as a stepping stone to a millimeter-wave system. After spending a few years getting maser technology to work reliably, in 1957 he went back to the original problem of generating millimeter waves.
Much to his surprise, his initial theoretical analysis towards this end showed that it would be about as practical to generate infrared radiation or visible light using stimulated emission in a resonant cavity as it would be to generate millimeter waves. This was a bit of a shock, since over the previous decades every order-of-magnitude reduction in the operating wavelengths of radio gear had taken the development of a new generation of technology. Now Townes saw the possibility of jumping over five orders of magnitude in a single bound; he rechecked his figures and decided that it really was possible.
He joined forces with a Canadian physicist named Arthur W. Schawlow (1921:1999), who worked at Bell Laboratories and also happened to be Townes' brother-in-law. They came up with a notion of using a lighting system to excite potassium gas in a long cylindrical tube with a mirror at one end and a half-silvered mirror at the other in which the light could build up; such a structure became known as a "Fabry-Perot resonator". In the summer of 1958, they applied for a patent on the concept and then published their ideas in the physics press.
Other people were thinking along the same lines. Robert H. Dicke (1918:1997) of Princeton wrote up some notes on the idea in 1956 but didn't follow them up. A graduate researcher at Columbia named R. Gordon Gould (1920:2005) also came up with the notion in 1957; he had dealings with Townes and suspected Townes was likely working along the same lines. Gould wrote up some notes and had them notarized in late 1957, with the notes suggesting applications such as optical radars, optical communications, and even laser-driven fusion.
After the publication of their paper, Townes and Schawlow more or less went their separate ways; townes worked on the potassium-gas optical maser, while Schawlow focused on a crystal of synthetic pink ruby. Pink ruby is a crystal of aluminum oxide, with traces of chromium atoms embedded in the crystal matrix; the chromium atoms would provide the stimulated emission. However, Schawlow decided that pink ruby wasn't very promising and gave it up.
Another Bell Labs researcher, an Iranian-born physicist named Ali Javan (born 1926), had some acquaintance with Townes and Schawlow and had read their 1958 paper. Javan had other ideas for an optical maser, believing that he could build one using an electrical discharge tube containing helium and neon and arranged as a Fabry-Perot interferometer. Gould was following up his ideas with a New York City company named TRG INC. Following a conference on quantum electronics in late 1959, a Hughes researcher named Theodore H. Maiman (1927:2007) jumped into the race, focusing his work on an optical maser using a pink ruby crystal.
Although Schawlow had abandoned pink ruby in his experiments, Maiman got it to work by inserting a pink ruby rod into a photographer's spiral flash lamp. The rod was mirrored at one end and half-mirrored at the other. Popping off the flash "pumped" the rod to emit a pulse of coherent, monochromatic, focused light. Incidentally, Einstein's discovery of stimulated emission, the basis for the laser, was a step towards his work with Bose on Bose-Einstein statistics. The photon is of course a boson, a spin-1 particle, and a maser or laser beam amounts to a sort of Bose-Einstein condensate.
Maiman got his device to work in June 1960 and Hughes hastily announced the feat at a conference in July. There was some skepticism over the claim, but a Bell Labs group quickly replicated Maiman's work. He had won the race to build the first optical maser, or "laser (light amplification by stimulated emission of radiation)", Gould's term, which would be adopted by the public. Of course, with so much activity in the field, other discoveries followed rapidly. Javan and his colleagues got their helium-neon laser to work late in 1960, announcing the development of the first "continuous" laser, as opposed to Maiman's pulsed laser. A year later, researchers at the American Optical Company developed a laser based on a glass rod doped with the appropriate atoms, providing much greater power output. Government funding soared, meetings were jammed, and the popular press sold the public on the whizzy new laser.
Townes and Gould were not at the forefront of the race at the time. Townes had been shunted off to a military advisory position, and Gould had been hobbled by a cloak of secrecy dropped over his work, greatly complicated by the fact that he had flirted with Communism in his youth and couldn't get a security clearance. However, this did not prevent them both from becoming caught up in a long string of patent lawsuits. Gould would be very persistent, even borrowing money to continue his case in the courts, and would eventually achieve legal recognition for his work -- much to the irritation of laser manufacturers, since the older patents they had been paying royalties on were just about ready to expire when they discovered that they now had to pay royalties on "new" patents.
Townes won the Nobel prize in physics in 1964 for his work on masers and lasers, sharing the prize with Basov and Prokhorov. Schawlow won the Nobel prize in physics in 1981 for his work in laser-based spectroscopy.
* There are basically three important classes of lasers: gas lasers, crystalline or glass lasers, and semiconductor lasers. There are a few other exotic types of lasers that will be discussed later.
As a rule, gas lasers are pumped by an electrical discharge, though in some rare cases they are optically pumped. Common gas laser types include:
* Crystalline and glass lasers are usually optically pumped by a flashlamp, LEDs, or laser diodes. The original red-light ruby laser is still in use, but a variant of garnet known as "yttrium-aluminum-garnet (YAG)" may be doped with rare earths such as neodymium, erbium, thulium, ytterbium, or holmium to and operate in various subbands in the infrared. Other examples include titanium sapphire lasers, which can be tuned to generate very short pulses, and high-power neodymium glass lasers.
There are some interesting variations on crystalline and gas lasers. One involves doping a section of a fiber-optic thread with the rare earth erbium; this scheme is used to boost laser signals being sent over fiber-optic communications links. Another involves using dyes in a liquid substrate, with the dyes selected to give highly specific operating wavelengths. Such "organic dye lasers" were invented in 1966 by John R. Lankard and Peter Sorokin.
* The semiconductor laser was first suggested by Basov in the late 1950s. The first semiconductor lasers were simple PN-junction devices. They worked, but they were current hogs and were not suitable for anything but lab tests. The problem was that the junction region in which lasing activity took place was simply too big: even though it was on the order of a micrometer in depth, that still made probability of stimulated emission too small.
The advance that made semiconductor lasers possible was the "double heterojunction (DH)" laser, developed in the early 1970s. It featured a thin gallium arsenide (GaAs) layer about 0.2 microns thick sandwiched between p-type and n-type aluminum gallium arsenide (AlGaAs), with the thin layer providing a much higher probability of stimulated emission, and permitting laser action at reasonable current levels. Although prototypes tended to fry themselves easily, the bugs were worked out and the DH laser was in production by the mid-1970s.
Laser diodes seemed ideal for transmitting digital data over a fiber optic link, with their crisp pulses permitting higher data rates than LEDs, but early laser diodes didn't generate light of the right wavelength to pass through a fiber optic thread with minimal attenuation. Changing the wavelength meant tweaking with the semiconductor material, using alternative alloys such as aluminum gallium arsenide antimony (AlGaAsSb) and, particularly, aluminum gallium arsenide phosphide (AlGaAsP).
The DH laser made the laser diode practical, but it still left much to be desired. Although laser action was confined to a thin layer, it was not very well confined within that thin layer. Fabricating a laser diode as simple layers of materials wasn't good enough. The next step, the "buried heterojunction (BH)" laser, was fabricated using new semiconductor etching techniques to create the lasing region as a "wedge" crammed between nonconductive materials on each side that provided "lateral confinement". Several other device configurations were also developed to provide lateral confinement.
Semiconductor lasers continue to be refined. One modern device includes a "control gate" element, in which a current adjusts the refractive index of the device's lasing channel, which adjusts the wavelength of the output. Such devices can be rapidly switched between wavelengths. Another interesting modern device is the "vertical cavity surface emitting laser (VCSEL)", in which lasing takes place between the top and bottom of the device, not between the sides as with traditional laser diodes. VCSELs can be built as arrays containing large numbers of tiny lasers in a grid, which can be used in optical image processing systems.
Although silicon is not inherently a very good material for building semiconductor lasers, there is still interest in silicon lasers because they could be integrated on the same chip with silicon microelectronics, not only potentially reducing cost of the overall system but providing such niceties as fast optical internal links for silicon microchips. For the moment, silicon lasers remain lab toys. The earliest had to be activated by a second laser, which made them of minimal usefulness, but improved devices have appeared that can be electrically driven to generate a laser beam. These devices manage to get around the limits of silicon by employing the "Raman effect", a shift in wavelength that occurs when a photon interacts with an atom in a transparent material. Free electrons in silicon tend to damp laser action, but the experimental devices use a matrix of electrodes that pull electrons out of the lasing channel.
Semiconductor lasers are traditionally low-power devices, useful for consumer and communications applications, though now there are industrial semiconductor lasers suitable for cutting and welding.
* Another form of laser is powered by a chemical reaction. The first such "chemical laser" was developed by the American physicist Jerome V.V. Kasper in 1964, with the lasing reaction set up by dissociating CF3I with an intense light pulse. Chemical lasers require a lot of "plumbing" and so are not all that useful for small-scale applications, but they are very efficient and well-suited to high-power applications, as discussed below.
* The story of the invention of the laser is exciting, at least as adventures in technology and science go, but it also had a certain humorous side to it as well. It was as though someone said: "We've just invented a coherent monochromatic light source that can generate an intense light beam!" To which the reply was: "Great! So ... what do we do with it?" The laser was called "a solution in search of a problem."
This sort of story is not all that unfamiliar in modern technology: a breakthrough is discovered, it gets wild publicity, which then fades out in a sort of sulky disappointment when nothing really happens right away. A new technology needs a lot of work to convert it from a lab toy into a product that can be manufactured; and sometimes it takes time and ingenuity to figure out what the technology is good for. Some time after the initial rush of enthusiasm fades out, products using the technology start to be introduced, and on occasion become so widespread that it would be impossible to do without them.
The laser was invented at the height of the Cold War, and to no surprise much of the initial funding was for defense applications. At the dawn of the laser age, the popular press made much of the possibility for using lasers as science fiction beam weapons, but anybody working in the field at the time knew such gadgets were a generation away at the very least: lasers were inefficient in terms of converting input power to output power, and didn't have anywhere near the output power required for a serious weapon.
However, that didn't mean that lasers didn't have military uses, and in fact their first really significant application was in warfare. When the invention of the laser was announced, some military researchers immediately saw the usefulness of the laser as a targeting system. A laser provided a straight, tightly focused beam that could be shined onto a target to "designate" it. A bomb or missile with a "seeker" head that picked up light of the particular wavelength of the laser would then be able to home in on the target. The first "laser guided bombs (LGBs)" were demonstrated by the US in the mid-1960s, and were being used in combat in VietNam in the early 1970s. They provided unheard-of accuracy, with strike aircraft crews able to put an LGB into a specific window of a building.
Since that time, laser guided bombs, missiles, and even artillery shells have become commonplace weapons in the arsenals of many nations. Laser seekers are not particularly difficult or expensive to build, and they give such weapons extreme accuracy, reducing the number (and so expense) of munitions needed to destroy a target, as well as the number of firing platforms, and also making sure that a specific target is hit and not a school, hospital, or church.
The real "brains" (and expense) of a laser-guided weapon is in the designation system. Ground forces can carry laser target designators to allow them to call in fire from aircraft or artillery on highly specific targets. Multiple designators can be used at once by coding their outputs with pulse patterns, with different laser-guided weapons programmed before launch to home in on a specific pulse pattern.
Laser designation systems carried by aircraft are much more elaborate, being often built in the form of a "pod" carried under the aircraft that is fitted with a turret containing a camera boresighted to a laser. The "weapons system operator (WSO)" in the aircraft will have a TV-type display with crosshairs on it and a joystick to move the turret; the WSO simply keeps the target in the crosshairs in the display and the weapon homes in on it. This is a bit trickier to implement than it sounds, since the target image would normally "flip over" as the aircraft passed by it, much as it would if you tipped your head over to watch something come up and pass by below. Modern targeting pods use a "position natural" system where the image always appears right-side up. The military has become fond of publicly releasing "exciting" videos of such engagements, with a LGB slamming precisely into a target and turning it into rubble. More gruesome videos along this line end up being distributed over the Internet.
The latest pods have multiple cameras to provide both day and night vision capabilities, as well as zoom magnification capability and turret stabilization systems to allow them to engage targets from long "stand-off" ranges. The pods can also be fitted with a "laser spot tracker" system that allows the aircrew to "see" a target laser-illuminated by ground forces, essentially turning the whole aircraft into a laser-guided weapon.
Laser guided weapons work well in both night and day, but laser beams are blocked by murky weather, or by clouds of dust and smoke. Since a bomb impact tends to generate large clouds of dust and smoke, it is difficult to dump a "barrage" of laser-guided weapons on a target, but since it is a "first shot first kill" type of weapon, one is often enough to do the job. Many modern laser-guided weapons have back up "Global Positioning System (GPS)" satellite navigation receiver to allow them to be used even when the laser beam can't get through, though at the expense of reduced accuracy.
Helicopters and aircraft that operate in a low-altitude battlefield environment have to deal with the threat of laser-guided weapons, and so they are often fitted with "laser warning systems" to tell the aircrew that they are being targeted by a laser. Modern "missile warning systems", used to detect that a missile has been launched, also sometimes incorporate laser warning subsystems.
* Of course, laser targeting systems can be applied to simpler weapons, the best known being the "laser sights" used on firearms. The shooter simply shines the red laser dot on the target and pulls the trigger. Bullets of course drop as they fly and can be affected slight by wind, but for close-quarters combat, laser sights provide a significant increase in accuracy. The same concept is used, less lethally, with the "laser pen" used by lecturers to point out items on slides being displayed to an audience.
As it turns out, both laser sighting and warning gear has useful applications in military training. Infantry can fit their rifles with lasers and then wear laser detectors that set off an alarm for combat training. The same concept is used in civilian "laser tag", a game in which players carry laser "pistols" and wear detector / alarm gear.
* One of the important applications of the laser in aerospace applications is in the form of the "optical gyroscope" or "laser gyroscope", used as the core element of an "inertial navigation system (INS)". An INS is carried by an aircraft or other vehicle to give its position relative to a point of origin. In modern times, navigation is primarily performed using the GPS navigation satellite system, but since GPS can be jammed, a GPS navigation system usually has an INS backup.
Early INS units developed in the 1950s used mechanical gyroscopes to determine changes in direction. Although such technology was highly refined, ultimately featuring mechanical gyros spinning on a film of gas to reduce friction that could throw off their accuracy, there was no way to eliminate friction entirely, limiting the accuracy of the INS.
The development of the laser led to a new approach, the optical gyroscope, in which a rotating loop of light was used to determine changes in direction. The optical gyroscope was not affected by friction. The basic principle was not new, going back to a paper published by the French physicist Georges Sagnac (1869:1926 )before the First World War, but the concept wasn't practical until the introduction of the laser.
An optical gyroscope is a type of optical interferometer, in which two laser beams travel in opposite directions, or "counterpropagate", over a closed path, or "optical cavity", inside the gyroscope. The two beams optically interfere with each other, and the effects of motion on the interference can be used to determine the rate and direction of rotation of the system. The first optical gyroscope, the "ring laser gyro (RLG)", was developed in 1962 by Warren Macek of Sperry Corporation, and the device was quickly incorporated into the guidance systems of missiles and other vehicles. The classic RLG is built out of a flat triangular block of "Zerodur", a type of glass that resists changes in dimensions with temperature, with a channel fabricated into it not only to act as a resonant path for the gyro's laser beam, but also to generate the laser beam itself. This is why the classic RLG is sometimes called an "active optical resonator".
The channel is filled with a lasing gas mix, usually helium-neon. Three highly reflective mirrors are placed at the squared-off corners of the triangular block. Anodes are placed on two sides of the block and a cathode is place on the third to provide laser stimulation. One of the corner mirrors is semi-transparent, to allow reading the laser light into a detector. In effect, a classic RLG is a helium-neon laser bent into a loop.
Consider the effect of rotation on the length of the path the light beams traverse in the resonant cavity. If the tube rotates in one direction, the light beam propagating in that direction sees a longer path length, while the one propagating in the other direction sees a shorter path length. This is the "Sagnac effect".
The frequency of laser light is dependent on two factors: the spontaneous light emission frequency of the lasing material, and to a lesser extent the cavity length. Lasing only occurs if there are an integral number of wavelengths of light in a cavity, and so changing this length can shift the laser's frequency from the center frequency of the lasing medium, at least up to the point where the cavity becomes an integer multiple of that center frequency again.
Under rotation, the Sagnac effect changes the path lengths seen by the counterpropagating light beams, resulting in raising the frequency of the beam moving against the rotation and lowering the frequency of the beam moving with the rotation. This frequency difference results in optical interference effects that can be measured by the output detector.
* A practical RLG incorporates a number of refinements to compensate for potential errors. The lasing system includes polarizers to ensure that the laser emits linearly polarized light, eliminating errors that might arise if the RLG were subjected to a magnetic field, which can rotate the polarization of light.
The laser medium is excited by a DC current, which can lead to gas flows that throw off the lasing center frequency through Doppler shifts. This is why the RLG has two anodes: they set up flows in opposite directions, and their effects cancel.
The RLG also is subject to a troublesome problem known as "frequency lock-in". At low rotation rates, a small amount of "backscatter" from the reflective lasing mirrors interacts with the lasing medium and causes the counterpropagating beams to stay locked together, and the output does not change until the rotation increases beyond a certain threshold, resulting in a "deadband" in the output range. The simplest technique for reducing the deadband is "mechanical dithering", in which the RLG is vibrated over a small arc at a typical frequency of 400 hertz. Dithering tends to lead to unstable output if the rates of rotation are high, as they often are in high-speed aircraft or spacecraft. Some researchers have experimented with RLGs using pulsed lasers as a potential solution.
Frequency lock-in won't occur if the laser beam is sourced outside the ring cavity, and experiments have been performed with RLGs using an external laser source. Such devices were generally relatively bulky, complicated, and expensive, and their advantages were not sufficient to replace the classic RLG.
Refinements in fiber-optic technology for communications led to the "fiber-optic gyroscope (FOG)", which uses a coil of fiber-optic thread for the laser loop. The FOG offers a much cheaper and more compact alternative to the RLG, but still has good performance. A FOG requires an external laser source, generally a LED or laser diode.
* Another early application of lasers was for rangefinding. Radar had been used for this purpose since World War II, using the simple principle of sending out a pulse of radio waves and measuring the time until an echo to return from a target. Multiplying half the time by 300,000 gave the range in kilometers.
The accuracy of radar ranging is a function of the wavelength and pulse length, with accuracy improving as both get shorter. However, sensitivity of radar is a function of total pulse power, which increases with pulse width. Lasers could provide very short pulses with short wavelengths and high power, offering the best of both worlds. Laser rangefinders were in use in military applications by the 1970s; since a laser rangefinder required a laser detector system, some aircraft used laser rangefinders that also provided a laser spot tracker capability.
Modern infantry laser target designators may include a laser rangefinding capability. The latest laser rangefinders / designators may also have a GPS receiver and a communications interface, allowing them to determine the location of a target and then relay its coordinates to a "strike platform" for attack by a GPS-guided weapon.
Laser rangefinders have scientific applications as well. The US Mars Global Surveyor spacecraft used a precision laser altimeter to obtain a highly accurate map of Martian ground levels. Earth-based laser rangefinders have been used to target reflector units left on the Moon by the Apollo manned Moon missions to determine the rate at which the Moon is receding from the Earth and the rates of continental drift.
Experimental imaging laser radar systems have been developed. They provide very high resolution compared to microwave imaging radar systems, but are limited in range by dust or fog. Yet another application of laser ranging is in laser 3D scanners, in which a laser beam is precisely scanned over an object to digitize it into a set of three-dimensional digital coordinates, possibly for generating 3D computer graphics imagery.
* Lasers have become extremely important for high-speed digital communications. The amount of information that can be crammed into a communications channel is in principle proportional to the frequency, and the high frequencies / short wavelengths of light mean that a data link using light can carry much more information than a data link using longer wavelength radio waves.
The biggest problem with using light for data transmission is of course the fact that it is easily blocked by rain or dust in the air, but the introduction of fiber-optic communications cables in the 1970s provided an unobscured long-distance path. LEDs were used for optical datalinks at first, but semiconductor lasers could generate shorter and cleaner pulses, permitting higher data transmission rates, and they now predominate.
Since there's little in space to interfere with a laser beam, optical communications links are being considered for spacecraft. Such "free space" optical links would allow spacecraft observing Mars or other planets to send large volumes of data back to Earth, to be picked up by satellites with telescopic optical receivers. Some researchers interested in searching for extra-terrestial (ET) civilizations have considered the possibility that ETs are using powerful lasers to communicate over interstellar distances and are searching for such laser emissions.
More practically, blue-green lasers, which can penetrate seawater to a substantial depth, have been used to communicate with underwater submarines, and incidentally as elements of sensor systems used to hunt naval mines.
* Of course, the original vision of a laser as an offensive weapon, a "death ray", has not died out, and in fact seems not too far away. As a first step, lasers are now being used in "directed infrared countermeasures (DIRCM)" systems carried by aircraft to dazzle heat-seeking missiles and cause them to go astray. There was some talk of using lasers to blind enemy troops as well, but the public reaction to such concepts has been loud and negative.
The next step is to use a laser to destroy the missiles, not simply dazzle them. The US military has developed prototype diode-pumped battlefield lasers to intercept small rockets and even artillery shells. Such systems are not ready for fielding yet, but progress is rapid at this time, and attack laser systems for aircraft or armored vehicles are expected to be available in the 2010:2020 timeframe.
The US Air Force is now developing a very high power chemical laser that will be shot out of a modified Boeing 747 jumbo jet to intercept missiles just after they are launched. The "Airborne Laser (ABL)" uses a "combustive oxygen & iodine laser (COIL)" system based on light emission in the burning of oxygen and iodine; earlier exercises along this line used hydrogen fluoride lasers, based on the combustion of ethylene (C2H4) and nitrogen trifluoride (NF3), the fluorine analogue of methane. The Air Force is also interested in developing a satellite platform with a high energy laser to shoot down missiles.
During the US "Strategic Defense Initiative (SDI)" or "Star Wars" effort of the 1980s, work was done on an X-ray laser system that, it was hoped, could shoot down missiles as well. A coherent X-ray beam would be very energetic and implementing shielding against it would be very difficult. The concept involved a laser system that was pumped by the detonation of a small nuclear weapon, generating the beam in the instant before the weapon destroyed itself. However, demonstration tests of the scheme did not produce useful results and it was abandoned.
* In an inverse application, lasers have been used to experimentally produce nuclear reactions, if on a small scale. As mentioned earlier, high-power laser systems have been used to perform fusion in deuterium / tritium fuel pellets as a means of performing controlled fusion.
Initially, physicists were very optimistic about laser fusion, believing that it would only take modest laser power to do the job. Unfortunately, the plasma formed around the pellet during the process had instabilities that required laser power an order of magnitude greater than expected. It didn't turn out to be so simple after all. The technology is not remotely close to producing a practical fusion reactor, but the lasers used in the experiments have been impressive. The Nova laser, at Lawrence Livermore National Laboratories in California, is one of the world's biggest, a neodymium-glass apparatus with ten arms that covers an area about the size of an athletic field and can focus a 100 terawatt pulse onto a strip of selenium, which produces an X-ray beam that is focused on a capsule of deuterium.
Development is now underway on the National Ignition Facility (NIF) at Lawrence Livermore Labs, and the Laser Megajoule (LMJ) facility near Bordeaux, being built by the French Atomic Energy Commission, in hopes of demonstrating the feasibility of inertial confinement fusion. The NIF laser system will be 60 times more powerful than the Nova laser system.
A small group of advocates has been promoting a less brute-force scheme, known as "fast ignition", which uses two relatively low-power lasers instead of one big laser system. In fast ignition fusion, one laser keeps the fuel pellet confined, while the second laser is focused into the pellet through a gold cone to drive fusion. Even the advocates admit that fast ignition is highly theoretical at this time and needs a lot of proving.
* Industrial cutting and welding was another relatively early application of lasers. High-power CO2 lasers are used for cutting and welding metals. Lasers are also used for cutting cloth patterns and "woodcut" patterns on wood boxes and the like.
Laser etching can be used to scribe a serial number on tiny polyester "microdots" a millimeter across, with batches of microdots sprayed on car parts, laptop computers, camcorders, even coins to make them traceable; the microdots can be read with a microscope under ultraviolet light. A similar application is to burn microscopic ID markings into precious gems to help protect them from theft. A new technology along such lines, known as "DataLase", involves covering fruits, vegetables, pills, and candies with an edible coating that turns dark when "written" by a laser, allowing a label to be drawn directly on the product.
Lasers can be used as a precision scalpel in medicine. They have a wide range of applications, but the most prominent are the removal of birthmarks and tattoos by laser, and "lasik" eye surgery, where the eye's lens is carefully modified to improve vision.
If lasers seemed wildly exotic back in the 1960s, they have now become commonplace. One of the earliest applications of lasers for entertainment were the "laser light shows" that became popular in the 1960s, with laser beams playing through smoke at concerts and the like to generate a dazzle effect.
Later consumer applications were subtler. One well-known application is the store barcode scanner system, in which a product barcode is swept over a scanning laser beam. The reflected laser beam is modulated by the barcode pattern, and the scanning system decodes the modulation and generates the code value to the checkout system.
Another common application is the "laser printer". The core of the laser printer is a rotating drum whose surface can be electrically charged. A semiconductor laser scans over the drum, with the electric charge dissipating where the laser beam hits; essentially, the laser scan creates a "negative" image on the page, leaving characters or figures as charged patterns on the drum. (A row of small LEDs can be used instead of a laser.) The charged patterns pick up ink or "toner" powder as the drum rotates past a toner storage cartridge, and then transfer the patterns to the paper as the drum rotates against the paper in a heater or "fuser" mechanism.
Possibly the most widespread consumer laser technology is the CD music or DVD video disc player. Both of these technologies involve data stored on discs with pits or other patterns that modulate a laser beam; a semiconductor laser is shined onto the disk as it rotates and a sensor picks up the modulation from the reflections. Coming up with higher density disk formats generally means acquiring semiconductors that operate at shorter wavelengths, and the latest systems have blue-light semiconductor lasers.
