Laser

• 1. A Solution Looking For A Problem

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  On the 16th of May 1960, Theodore H. Maiman switched on the first functioning laser at Hughes Research Laboratories in Malibu, California. His device used a flashlamp-pumped synthetic ruby crystal to produce red light at 694 nanometers. It could only fire in pulses. Maiman measured its power in an unusual unit. He said it had the power of one Gillette, because it could burn through a single Gillette razor blade. When the laser was first invented, people called it a solution looking for a problem. Today it is regarded as one of the greatest inventions of the 20th century. How does a device get from burning razor blades to running barcode scanners, eye surgery, and a 10-meter-diameter fusion target chamber? What makes laser light so different from the glow of a lightbulb or a star? And why did inventing it spark a legal fight that lasted twenty-eight years? The answers begin with a single triggered photon, and with the strange word built into the machine's name.

• 2. The Triggered Photon

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  Photons are the quanta of electromagnetic radiation, released and absorbed from energy levels in atoms and molecules. In a lightbulb or a star, energy escapes from many different levels at once. That gives photons a broad range of energies, a process called thermal radiation. A laser builds light the same way, but the emission is not random. Instead, the passage of one photon triggers the release of another. This is stimulated emission. For it to work, the passing photon must be similar in energy and wavelength to the one the atom could release, and the atom must already sit in a suitable excited state. The photon produced by stimulated emission is identical to the photon that triggered it. Both can then trigger emission in other atoms, opening the possibility of a chain reaction. Most materials defeat this. Atoms drop out of excited states fairly rapidly, so a cascade cannot build. Lasers use materials with metastable states, which stay excited for a relatively long time. Such a material is called an active laser medium. Population inversion is the key threshold. It means more particles sit in an excited state than in a lower-energy state, so stimulated emissions outnumber absorptions. A two-level system cannot reach it, because populating the upper level only raises the odds of stimulated emission and balances the books. Lasers therefore use three-level or higher systems, where an extra energy level lets electrons relax quickly and hold a steady inversion.

• 3. Mirrors And Screech

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  A laser consists of three parts: a gain medium, a mechanism to energize it, and something to provide optical feedback. The gain medium amplifies light of a specific wavelength as it passes through. Energizing it is called pumping, supplied as an electric current or as light at a different wavelength. Pump light may come from a flash lamp or from another laser. The most common feedback comes from an optical cavity, a pair of mirrors at either end of the gain medium. Light bounces back and forth, passing through the medium and being amplified each time. One mirror, the output coupler, is partially transparent, and some light escapes through it. Feedback favors one frequency above all others. The process is analogous to an audio oscillator with positive feedback. Place a public-address speaker near its microphone and you get a screech. That screech is oscillation at the peak of the amplifier's gain-frequency curve. A laser does the same thing with light. Because it produces light by itself, a laser is technically an optical oscillator rather than an amplifier. One observer noted, only half seriously, that the acronym LOSER, for light oscillation by stimulated emission of radiation, would have been more accurate. Saturation sets the working point. Each stimulated emission returns an atom to its ground state and reduces the gain. As beam power rises, net gain falls toward unity and the medium is said to be saturated. Below a certain pump level, gain never overcomes the cavity losses and no laser light appears. That minimum is called the lasing threshold.

• 4. What Coherence Buys

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  Coherence is what sets a laser apart from every other light source. Spatial coherence shows up as a narrow, diffraction-limited beam. It lets a laser focus to a tiny spot at very high irradiance, and it lets a beam stay narrow over great distances, a property called collimation. The pencil beam of a common helium-neon laser, shone from Earth onto the Moon, would spread to a size of perhaps 500 kilometers. A semiconductor laser does the opposite at the source, exiting its tiny crystal with a divergence of up to 50 degrees. A lens system reshapes that into a collimated beam, which is why a laser pointer built around a laser diode still throws a tight dot. Temporal coherence describes a wave at a single frequency whose phase stays correlated over a great distance along the beam, the coherence length. It permits a very narrow frequency spectrum. It can also be turned the other way to make ultrashort pulses with a broad spectrum, lasting only attoseconds. These two kinds of coherence enable nearly everything lasers do. Spatial coherence drives optical communication, laser cutting, and lithography, and it powers laser pointers and lidar. Many laser beams approximate a Gaussian shape, which carries the minimum divergence possible for a given diameter. In 1963, Roy J. Glauber showed that coherent states form from combinations of photon number states. The arrival of photons in a laser beam follows Poisson statistics, and Glauber's work earned him the Nobel Prize in Physics.

• 5. Pulses And Power

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  Femtoseconds are the timescale where some lasers do their most extreme work, with pulses as short as a few femtoseconds, or 10 to the minus 15 seconds. A laser runs in one of two modes, continuous or pulsed. A continuous-wave laser holds steady power over time and needs its population inversion replenished by a steady pump. Some media cannot sustain this, so they can only be pulsed. Pulsing concentrates energy. Since pulse energy equals average power divided by repetition rate, lowering the rate lets more energy build up between shots. In laser ablation, a short burst evaporates a thin surface layer before heat can soak into the bulk. Q-switching chases peak power. It builds up the population inversion by adding loss inside the resonator that exceeds the gain. Once the stored energy nears its maximum, the loss is rapidly removed and lasing erupts in one short, high-peak-power pulse. Mode locking reaches even shorter durations, from tens of picoseconds down to less than 10 femtoseconds, with pulses repeating at the cavity round-trip time. Unlike a Q-switched giant pulse, consecutive mode-locked pulses are phase-coherent and perfectly periodic. Titanium-doped sapphire, an artificially grown crystal, has a gain bandwidth wide enough to make such pulses. The peak numbers climb to almost unimaginable scales. The National Ignition Facility runs a 192-beam, 1.8-megajoule system reaching 700 terawatts, adjoining a 10-meter-diameter target chamber. As of 2019, the world's most powerful laser delivered 10 petawatts, located at the ELI-NP facility in Magurele, Romania.

• 6. A Zoo Of Media

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  Helium and neon filled the first gas laser, built by Ali Javan, William R. Bennett Jr., and Donald R. Herriott, the first laser capable of continuous operation, working in the infrared. Most helium-neon lasers are engineered to lase at 633 nm, and their low cost and high coherence make them common in research and teaching labs. Carbon dioxide lasers, invented by C. Kumar N. Patel at Bell Telephone Laboratories in 1964, emit at 10.6 micrometers with efficiency above 30 percent, and cut and weld across industry. Solid-state lasers use a doped crystal or glass rod, like the original ruby, which is chromium-doped corundum. Neodymium is a common dopant in crystals such as Nd:YAG, producing high infrared power at 1064 nm. Frequency doubling turns that into 532 nm green light, the basis of bright green laser pointers. Holmium-doped YAG emits at 2097 nm, strongly absorbed by water-bearing tissue, and is passed through optical fiber to resurface joints, remove tooth rot, vaporize cancers, and pulverize kidney and gall stones. Excimer lasers run on noble gas compounds that exist only in an excited state. Once the molecule hands its energy to a photon, its atoms unbind and it disintegrates, which strips the lower energy state and eases population inversion. They work in the ultraviolet, with ArF emitting at 193 nm, and they drive semiconductor photolithography and LASIK eye surgery. Stranger media exist still. Living cells genetically engineered to make green fluorescent protein have been placed between two 20-micrometer-wide mirrors and made to emit green laser light when lit with blue. And like astrophysical masers, the gases of Mars, Venus, and the object MWC 349 can amplify light as natural lasers.

• 7. The Fight Over Credit

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  In 1917, Albert Einstein laid the theoretical groundwork in the paper Zur Quantentheorie der Strahlung, deriving the probability coefficients for absorption, spontaneous emission, and stimulated emission. The word laser itself began as an acronym for light amplification by stimulated emission of radiation, and microwaves came first. In 1953, Charles H. Townes with graduate students James P. Gordon and Herbert J. Zeiger built the first maser, amplifying microwave radiation. Townes recalls that eminent physicists, among them Niels Bohr, John von Neumann, and Llewellyn Thomas, argued the maser violated Heisenberg's uncertainty principle and could not work. In 1964, Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics for the maser-laser principle. The credit for the laser proved harder to assign. In November 1957, Columbia graduate student Gordon Gould wrote down his ideas for how a laser could be made, including the use of an open resonator. His notebook held the first recorded use of the term laser and a diagram of an optically pumped device. Gould filed a patent application in April 1959. The United States Patent and Trademark Office denied it and awarded the patent to Bell Labs in 1960. That decision provoked a twenty-eight-year legal fight over rights to laser technologies. Gould won his first patent in 1977 for optically pumped laser amplifiers. It was not until 1987 that he won his first significant patent infringement claim. Many aspects of a working laser were patented by different people, and historians regard the question of who invented the laser as unresolved.

• 8. Out In The World

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  In 1974, the supermarket barcode scanner became the first widely noticeable use of lasers. The laserdisc player followed in 1978 as the first successful consumer product to carry a laser, and the compact disc player, commercialized in 1982, was the first laser device to become truly common. Laser printers came shortly after. The scale of the market has grown enormous. Global industrial laser sales in 2023 reached 21.85 billion dollars. In medicine, lasers treat cancer by shrinking or destroying tumors, most often superficial cancers on the body's surface or the lining of internal organs, and they handle eye surgery, kidney stones, and cosmetic skin treatments. As weapons, they form a class of directed-energy device. The United States Navy has tested the very short range, 30-kilowatt Laser Weapon System, described as six welding lasers strapped together, against small drones, rocket-propelled grenades, and motorboat engines. A 60-kilowatt system called HELIOS is being developed for destroyer-class ships. The danger that Maiman noticed at the start never went away. Even low-power lasers of a few milliwatts can harm eyesight, because the eye focuses the coherent, low-divergence light into an extremely small spot on the retina, causing localized burning in seconds. That is why lasers carry safety class numbers, from Class 1, which is inherently safe, to Class 4 at 500 milliwatts or more, which can burn skin and whose scattered light alone can injure. The Protocol on Blinding Laser Weapons bans weapons designed to cause permanent blindness, a reminder that a beam precise enough to repair a retina is precise enough to destroy one.