Gee Whiz! Lasers Are Everywhere
In the fifty years since its discovery, the laser has become an indispensible tool in our daily lives. The laser's many uses stem from its unique properties; for example, the ability to achieve high power while being focused to a pinpoint makes the laser ideal as a precision scalpel in medicine or as a means to slice through thick plates of steel.
What is really so amazing is the diverse and unique applications scientists continue to find for the laser.
From catching a speeding vehicle to detecting ripples in the fabric of space-time, some of the ways we use lasers are just plain cool!
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Think of it as laser ultrasound. Optical Coherence Tomography is a method of mapping below the surface in translucent or opaque materials, such as human tissue. When the laser is applied, it penetrates the material, and then bounces back when it hits a sub-surface feature. The depth and intensity of the returning lasers are recorded, and an image is constructed from the data. Multiple scans over a region yields a 3-D image.
In addition to being safer than radiation techniques such as X-ray, the images turn out much less cloudy. When the laser is scattered back in a different direction from its origin, the phase change is noted by the receiver, and filtered out of the final image, yielding a much clearer, more accurate picture.
Both lasers and fiber optics have independently become vital components of many industries. When the two are combined, their potential skyrockets. Inside fiber optic cables, bundles of long, thin, transparent fibers are surrounded by a highly reflective material. Light enters one end, bounces through the flexible cable via total internal reflection, and out to an optical receiver on the other. Analogous to a complicated version of Morse code, the light can carry with it vast amounts of data over long distances, much more so than traditional electrical wires. Lasers are frequently used as the incoming light signal, or to amplify existing non-laser signals. Although most notably present in telecommunications and computer networks, fiber optic cables carrying laser light have been instrumental in medical procedures in which lasers are needed within the human body in order to kill tumors or other unwanted growths.
We have all looked at a long line of our image in a pair of nearly parallel mirrors. The images trail off the edge of the mirrors and get increasing dim, as light is lost on each reflection. A good metal mirror only reflects ~90% of the visible light that strikes it, so on average the light only makes 10 passes before being lost. However, one can now purchase mirrors, made by stacks of alternating layers laid down on a highly polished window, that reflect ~99.999% of the light, so that the light would, on average, make 100,000 reflections before being lost. Such mirrors have been key to the development of an extremely sensitivity way of observing very faint absorptions, known as Cavity Ring-Down Spectroscopy. Here one puts light through one mirror, which is possible due to the very high brightness and coherence of lasers, filling the space between two or more mirrors with light. The mirrors form an optical cavity much like a guitar is an acoustic cavity. When one abruptly shuts off or blocks the laser light from entering the cavity, that the intensity of light leaving the cavity slowly decays with time and the rate of this decay can be measured with high precision. If a gas between the mirrors absorbs or scatters some of the light on each pass, the rate that the light decays increases. With care, one can detect an absorption of one part in a billion (109) in the light on each pass of the cell. By changing the wavelength of the light used to excite the optical cavity, one obtains an absorption spectrum of a sample with extreme sensitivity.
Cavity Ring-Down Spectroscopy has proven useful in many applications. It is used in laboratories to study new chemical compounds and chemical reactions. Commercial instruments are used to detect tiny levels of some molecules, such has H2O or CH4 (even as low as parts per trillion). Such impurities can cause defects in semiconductor wafers that can destroy entire chips. It is used to monitor trace components, such as the highly reactive OH free radical, that play important roles in the atmosphere but are present in tiny levels. It is used to monitor the complex chemistry that occurs during combustion and will hopefully lead to cleaner and more efficient engines or can handle a greater range of fuels. Recently, medical applications have been proven, such as rapidly detecting components in human breath that act as markers for a range of diseases or allow doctors to monitor the effectiveness of treatments.
James Bond has demonstrated laser cutting more than once, starting as early as 1964 in Ian Flemmings "Goldfinger". Though it takes more than just a beam from some canon like device or a special watch from the 007 agent's outfit, it is true that laser light can cut through both paper and 1 1/4 inches of stainless steel.
Laser systems for cutting paper, panels of wood, textiles, plastics or metal foils use laser optical output power of up to a few hundred watts. This power level is sufficient to vaporize these materials. In sheet metal processing, laser cutting machines are standard equipment and have laser power of up to 7 Kilowatts. They can cut anything from sheet metal — from single pieces to small and medium batch sizes. The machines are easily programmed using the CAD-files of the desired part and do not require any other tool besides the laser cutting head which operates without contact and wear. With more than a million watts per square centimeter in the focal area of the laser beam, it melts and partially vaporizes the metal resulting in a needle-like hole reaching tens of millimeters deep into the material.
What the movie producers missed: To cut metal efficiently you need a nozzle to blow away the melted metal. Without the nozzle and cutting gas you would witness another standard laser process: welding.
You have seen and touched laser marked goods a thousand times, but probably without noticing. Lasers are used for marking keyboards and electronic devices, cables, switches and automotive parts, medical instruments and animal ear tags. They encode manufacturing data to trace faulty parts, paint logos or write labels. Unlike ink, it doesn't wear off and unlike scribing with a needle it can mark without creating grooves on the material surface. You might say the only thing it doesn't do is draw pictures in full color.
Laser marking uses a laser and a scanner unit that quickly deflects the beam over the surface of the workpiece. The combination of switching the laser light on and off and moving it above the surface lets you write data or draw graphics, such as a data matrix code with a high resolution. The data can be generated on the fly so that each piece can be marked with a time stamp, serial number or even an individual scale depending on the acquired calibration data.
The variety of available laser light wavelengths from IR to UV allows almost any material to be marked, even those that are transparent: glass, ceramics, plastics, metal, wood, etc. Laser applied marks are typically very durable because they result from a modification of the material or from removing material from the surface.
The pictures taken by Eadweard Muybridge in 1872 of a trotting horse enabled him to capture it flying through the air. The camera's shutter speed for such a visible object was appropriately set at a thousandth of a second. For atoms and molecules the time and length scales are vastly different—the atom dimension is about 0.1 nm and atoms move in matter's transformations at speeds as large as a few kilometers in one second.
With the invention of lasers, and their mode-locking, bursts of light could be produced with femtosecond duration, a millionth of a billionth of a second. With these stroboscopic pulses shined on a beam of isolated molecules it was possible to witness the motion of atoms as one substance changes to another. The shutter speed was nearly ten orders of magnitude faster than that of Muybridge's camera.
In 1999, Ahmed Zewail of Caltech was awarded the Nobel Prize for developing the field of femtochemistry. On such a time scale one is able to observe in real time the transition states of atoms in motion and uncover the fundamental elementary processes of matter's transformations. It is also feasible to control the outcome of reactions. Currently, the generation of attosecond pulses and the plethora of theoretical and experimental advances in femtoscience represent frontiers of research in atomic, molecular, and biological sciences.
In 1974, the first public laser was introduced in the BarCode scanner found at supermarkets. BarCode scanners use a laser beam that is scanned back and forth so rapidly that it appears as a line to the human eye. A photodiode measures the intensity of the light reflected back from the black and white BarCode pattern, generating a signal that is used to measure the widths of the bars and spaces in the BarCode.
These simple, pocket-sized lasers are used to highlight important areas during presentations, and first became available in the 1980s. A red pointer is simply a battery-powered laser diode, which produces light when electricity passes through it. The now popular green laser pointer includes a special crystal that doubles the frequency of an infrared laser into the visible part of the spectrum. They appear so bright because the human eye is most sensitive to green light.
Photolithography is the process of using light to print patterns onto a surface or silicon wafer, and is the main technique for micro structuring of electronic devices (such as semiconductors) found in computer chips. Current lithography relies on laser light in the deep ultraviolet produced from an excimer laser. The smallest feature that can be produced with photolithography is related to the wavelength of the light, which led to the adoption of excimer lasers for the light source, as they can produce large amounts of light at very short wavelengths.
Computer chip manufacturers are in continual pursuit of smaller, faster, and more efficient semiconductors. The benefit of being tiny is pretty simple: finer lines mean more transistors can be packed onto the same chip. The more transistors on a chip, the faster it can make your Facebook and World of Warcraft!
The laserdisc player, introduced in 1978, was the first mass-market consumer product to include a laser. Although this format never really caught on, the compact disc (CD), introduced in 1982, became the audio format of choice. The CD player became the first laser-equipped device readily found in the home. The laser acts as a precise disc-reading mechanism. Its beam of light is reflected off information stored on the disc in a series of tiny pits. The reflected light strikes a photodetector, which converts the information into digital 1s and 0s that is further processed into an audio signal. CD-ROMS use the same technology to store digital data other than audio.
Have you ever been dazzled at a laser light show or in awe of holographic art? The vivid bright colors of lasers make them popular in the entertainment industry in the form of laser light shows and holographic art. Laser shows produce visual displays by using beam effects; either by switching a stationary beam on and off or by creating dynamic beam effects, including fans of colored beams, beam sequences, sheets of light, cones, tunnels of beams, and some moving diffraction grating effects.
Holograms are typically created by reflecting laser light from an object and combining it with light from a reference beam. The resulting interference pattern is recorded onto a film, resulting in an apparent three-dimensional image that changes slightly when viewed from different angles.
Shaping your eyeballs with a sculptor's finesse—doesn't sound like LASIK (an acronym for Laser-Assisted in Situ Keratomileusis), but that's exactly what happens. In laser eye surgery, physicians use a pulsed laser beam to gently reshape the surface of the cornea (corneas that are too flat or pointy cause far and near-sightedness). By returning the cornea to its ideal shape, light entering your eyes is bent correctly, enabling clearer and sharper vision.
The first use of a laser in medicine occurred in the early 1960s, when physicians at Columbia-Presbyterian Hospital used a laser on a human for the first time, by destroying a retinal eye tumor with a ruby laser. The use of lasers in medicine has grown steadily since, as laser technologies become less expensive.
The microwave 'laser' or maser showed that microwave background noise in the universe is a remnant of the Big Bang. In 1965, Arno Penzias and Robert Wilson discovered cosmic microwave radiation using a massive satellite antenna containing a maser built by Bell Labs.
When they began to use the antenna for astronomical research, they found there was a background "noise", like static in a radio. This annoyance was a uniform signal in the microwave range, seeming to come from all directions; it was later dubbed "cosmic background radiation". Penzias and Wilson were awarded the 1978 Nobel Prize in physics for their work.
Lasers can be used to measure distance to exceedingly high precision, using the principle of interference in devices called interferometers. Scientists are now using laser interferometers to probe the fundamental nature of gravity. LIGO, the Laser Interferometer Gravitational Wave Observatory in Washington and Louisiana, hopes to sense gravitational waves or ripples in the fabric of space-time by the measuring the tiny, barely discernable changes they cause in the distance between mirrors separated by 4 km.
The laser interferometer makes a measurement by bouncing high-power laser light beams back and forth between test masses in each arm of the L-shaped detector, and then interfering the two arms' beams with each other. The slight change in distance between the test masses throw the two arms' laser beams out of phase with each other and disturb their interference, thereby revealing the passing gravitational wave. This spectacular research device can sense changes in distances a billion times smaller than a commercial interferometer, a distance much smaller than the size of a proton.
Lasers are at the forefront of energy-related technologies, especially potential clean energy sources like fusion, which attempts to recreate conditions in the sun in order to generate a clean source of nuclear energy. At the National Ignition Facility (NIF), scientists built the world's largest laser in an attempt to create the same fusion energy process that powers the sun.
In 2010, the NIF will focus the intense energy of 192 giant lasers (nearly two million joules of ultraviolet laser energy!) on a small pellet filled with hydrogen fuel. During the short time the laser pulse is on (30 ns), the NIF will use an enormous amount of power. The resulting fusion reaction will release energy many times greater than the energy needed for the reaction.
Mapping ice flows, monitoring storm erosion damage to beaches, changes along shorelines, measuring chemicals in the atmosphere—all can be done with LIDAR, a laser-based remote sensing technology. In airborne LIDAR technology for mapping, lasers capable of recording elevation measurements at a rate of 2,000-5,000 pulses per second are mounted onto aircraft. As the aircraft flies over a particular area, say for example, a shoreline, the LIDAR sensor records the time difference between the emission of the laser beam and the return of the reflected laser signal (light reflected off the earth's surface) to the aircraft.
LIDAR' data is then used in mapping or reconstruction to produce extremely accurate topographical information. LIDAR isn't just relegated to Earth—NASA's Phoenix Lander used LIDAR to detect snow in the atmosphere of Mars.
If you've ever gotten a seemingly impossible speeding ticket, blame LIDAR. Quickly replacing the radar as a choice tool by police officers, LIDAR measures a vehicle's speed by calculating the changing time it takes to catch sight of reflecting pulses of highly focused laser light over a certain time period. LIDAR has the distinct advantage of being able to pick out one vehicle in a cluttered stream of traffic; so keep that urge to speed in check!
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. It typically consists of a rotating wheel mounted so that its axis can turn freely in all directions. Ring laser gyroscopes replace a spinning wheel with laser light traveling around a loop.
In a typical laser gyroscope, two lasers travel in opposite directions on a triangular route. When the gyroscope is rotated on its axis, one of the laser beams' rotational path is shortened while the other beams' path is lengthened. The difference in time between the two laser beams is its angular change. The navigation of airplanes and ships, alignment of telescopes, survey of landscapes, and high precision measurements all require the use of gyroscopes.
"Twinkle twinkle little star, how I wonder what you are..." almost everyone can recall this cute little rhyme from their childhood days—but those twinkling stars drive astronomers crazy! Whenever light from stars passes through the atmosphere, it becomes distorted by layers of air with different temperatures and densities. What appear like sharp, shimmering stars to the eye look more like smeared blobs by the time they are imaged by ground-based telescopes.
To solve this problem, astronomers use a technique called laser guide star adaptive optics. Adaptive optics irons out the wrinkles in light that cause atmospheric distortion, so that stars, galaxies, and other celestial objects can be clearly viewed through telescopes. In order to work, adaptive optics needs a bright reference star, near the astronomical object of interest. Astronomers use a powerful laser to create an artificial "laser guide" star exactly where and when they need it.
They utilize a layer of sodium atoms that floats about 100 km above the Earth's surface, illuminating it with a laser tuned to excite these atoms (the same yellow color as sodium lamps so common in parking lots). The atoms fluoresce, and look like a small yellow star to the telescope. This set up allows the object of interest to be imaged while cancelling out the atmospheric distortion. (setup)
It's a familiar movie scene—bright red laser beams crisscross some area, protecting precious jewels or expensive works of art. Of course masked bandits attempt to enter, with grandiose schemes to avoid touching the beams.
Yet laser based security systems are being used in real life! The presence and movement of intruders can be detected using a laser beam-interruption system. In some systems, even the number of intruders crossing a given beam can be detected. The one difference from the movies is that laser beams are invisible when traveling through the air. Most photographs of laser beams are time exposures where the photographer has passed a card along the beam while the camera shutter is open.
Optical Tweezers use laser light to hold and rotate microscopic objects, much like the way tweezers are used for picking up objects too small or delicate to be handled by human hands. Almost any small transparent object can be held with optical tweezers, including living things like bacteria. Even individual molecules can be manipulated by attaching them to a micron-sized glass or polystyrene bead. When a laser beam hits the bead its light bends and exerts a small force on the bead, pulling it directly into the center of the beam.
This creates an "optical trap" which is able to hold the small particle at its center. Optical tweezers have been used to trap everything from viruses, bacteria, and living cells to small metal particles, and even strands of DNA. What's next? You guessed it: optical scissors.
When you place an order over the internet and go to a "secure" site to enter your credit card information, you are relying on the secrecy of a cryptographic system that encodes your information so that it would look like a random stream of numbers to an eavesdropper.
Using the quantum properties of light, it is now possible to send information that is quantum encrypted and is guaranteed secure against eavesdropping by the laws of physics. Using a scheme envisioned in the 1980s, single packets of light (photons) are sent through an optical fiber, with the information encoded in their polarization. The receiver makes measurements of the polarization of the photons they receive, and because of the properties of quantum mechanics, they can be sure that no one has been listening in on the conversation. Quantum cryptographic systems are now commercially available, but will most likely be used for very special communications (like bank-to-bank) for the near term.
In 1994 a mathematician, Peter Shor, proved that the mathematical basis for all our standard encryption (like those credit card purchases) could be totally obviated if we could build a "quantum computer" — a computer that exploits some of the weird properties of quantum mechanics. Although we are still many years away from such a device, lasers have a prominent role in many of the technologies under development, such as individual ions trapped by electric fields. The quantum states (qubits) are manipulated by carefully tailored laser pulses. A working quantum computer may eventually be composed of a network of smaller processors, all coupled with laser light through optical fibers.
Light travels really, really fast. In fact, we know that nothing in the universe can move faster than the speed of light in a vacuum, thanks to Einstein's theory of relativity. If you prefer sluggishness and leisure over speed, you'll be glad to know there is no limit on how slow light can travel. A decade ago, Dr. Lene Vestergaard Hau and a team from Harvard University were the first to slow light to about 17 meters per second, using an ultra cold gas of sodium atoms bathed in laser light. In 2001 Hau and others went a step further and momentarily stopped a laser beam..
When light travels through material it always slows down, but usually by a small amount, less than a factor of two. In these experiments, researchers used a technique called "quantum interference", engineering the response of the atoms to slow down light by huge factors. In the future, slow light could play a big role in optical technology, including the potential to send and store data, sound, and pictures using much less space and power.
How do you photograph something happening at a millionth of a billionth of a second? It's easy if you have the right equipment. Ultrafast photography is an imaging technique that uses laser pulses to capture processes that happen so quickly they can't be snapped with regular camera shutters, like a electron escaping from an atom. Lasers have allowed scientists to break the femtosecond barrier, opening the door to real-time observation of physical, chemical, and biological processes at the atomic level.
Time resolution is determined by the duration of "pump" and "probe" laser pulses. A pump pulse triggers a reaction, and a probe pulse follows, acting like a strobe. Strobes are used for synchronization in high shutter speed cameras, in order to photograph a rapidly moving object, such as a bullet, for such a short duration that it will appear to be standing still. Using femtosecond long pulses, the motion of atoms and molecules can be frozen.
Laser designation is the practice of shining a laser beam on a target (illuminating it) in order to mark it for destruction. The newest laser designators use infrared lasers (invisible to the human eye) so as to avoid the spot being noticed by the targets, and can encode messages in their laser light pulses for verification purposes. A laser designator emits a beam of laser light that is used to mark or "tag" a specific place or object, usually for precision-guided weapons in the military, such as "smart" bombs.
Laser-guided bombs have an internal system that detects laser energy and guides the weapon to the target highlighted by the laser designator. When a target is marked, the beam is invisible and does not shine continuously. Instead, a series of coded pulses of laser-light are fired. These signals bounce off the target into the sky, where they are detected by laser guided weapon, which steers itself towards the center of the reflected signal.
If I ask you how well your watch is running and give you one minute to compare it to a good clock, you will not be able to make a very good judgment, but can do much better if I give you a month for the comparison. The longer scientists observe, the more precise measurements can be. Laser cooling uses lasers of a specific color to greatly slow atoms, ultimately allowing for longer observations and higher precision. The world's best atomic clocks, which are used to keep the planet's time, use laser cooled atoms.
It has been known since the 19th century that light carries momentum and can exert a force on an object, whether it is a mirror or an individual atom. Laser cooling an atom is analogous to slowing a bowling ball by bouncing thousands of ping pong balls off of it. Each ping pong ball (or photon) slightly reduces the speed of the bowling ball (atom). Bounce off enough of them and the ball will slow appreciably.
In 1997, Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips won the Nobel Prize for developing methods to cool atoms to temperatures just barely above absolute zero. Chu's 1985 experiment at Bell Labs consisted of six laser beams arranged in such a manner that they cooled sodium atoms in a vacuum. The light in all six laser beams was slightly red-shifted compared to the typical color absorbed by a stationary sodium atom. It was as if the atoms moved in a thick liquid (although it was only light), hence the name "optical molasses".
Laser cooling is a critical first step in the creation of Bose-Einstein Condensate, a form of matter where gaseous atoms all drop to the lowest possible energy state. The Bose-Einstein condensate occurs when atoms at a particular temperature and density, on the removal of some energy, fall into lockstep with one another, each in exactly the same quantum state.
Conventional particle accelerators are bulky structures, often kilometers long, which achieve acceleration by generating enormous electric fields inside microwave cavities. Seeking smaller, more efficient particle accelerators, scientists turned to lasers. Dubbed a "paser" for particle acceleration by stimulated emission of radiation, these unique particle accelerators speed up bundles of electrons using the same principles as a laser, except the output is accelerated electrons that are traveling in the same direction.
Packets of electrons are fired into a cloud of excited gas. As in a laser, the gas releases a large number of identical photons. But those photons are instantly absorbed by the passing electrons, which get an energetic kick, enabling them to move much faster. Some even say that free electron lasers, which require a particle accelerator to start, may one day take us to great heights, serving as a power source for future space elevators. We are still a number of years away from everyday applications of laser accelerators, but it is a definitely a field to watch!