A laser, a device that meticulously orchestrates light through a process of optical amplification, is a marvel of physics. It operates on the principle of stimulated emission of electromagnetic radiation, a concept so fundamental it gave the device its very name: LASER, an acronym for Light Amplification by Stimulated Emission of Radiation. The genesis of this remarkable invention traces back to 1960, when Theodore Maiman, working at Hughes Research Laboratories, brought the first laser into existence. This achievement was built upon the theoretical groundwork laid by luminaries like Charles H. Townes and Arthur Leonard Schawlow, further refined by the optical amplifier patents of Gordon Gould.
Unlike the chaotic effusions of ordinary light sources, a laser distinguishes itself through its coherence. This spatial coherence allows for the precise focusing of light into an incredibly tight spot, a capability that underpins a vast array of applications, from the intricate pathways of optical communication to the precise cuts of laser cutting and the microscopic etchings of lithography. Furthermore, this focused beam maintains its narrowness over considerable distances, a phenomenon known as collimation. This property is what makes laser pointers, lidar systems, and free-space optical communication possible. Beyond spatial coherence, lasers can also exhibit remarkable temporal coherence, emitting light with an exceptionally narrow frequency spectrum. This temporal coherence can be harnessed to generate ultrashort pulses of light, measured in attoseconds, even when those pulses encompass a broad spectrum.
The applications of lasers are as diverse as they are transformative. They are indispensable in fiber-optic and free-space optical communications, powering optical disc drives, and illuminating the inner workings of laser printers and barcode scanners. In the realm of semiconductor manufacturing, lasers are crucial for photolithography and etching processes. Medical science benefits immensely, with lasers employed in laser surgery and skin treatments. Industry relies on them for cutting and welding materials, while military and law enforcement agencies utilize them for target marking, range finding, and speed measurement. Even entertainment benefits from the spectacle of laser lighting displays. It is no exaggeration to say that the laser stands as one of the most significant inventions of the 20th century.
Terminology
The lineage of the laser can be traced back to its predecessor, the maser, which operated at microwave frequencies. The term "maser" itself is an acronym for Microwave Amplification by Stimulated Emission of Radiation. When similar devices operating in the optical spectrum were developed, they were initially termed "optical masers." Eventually, "microwave" was replaced by "light" in the acronym, giving us "laser." Today, any device operating at frequencies above microwaves, roughly above 300 GHz, is classified as a laser, encompassing infrared, ultraviolet, X-ray, and even gamma-ray lasers. Devices operating at microwave or lower radio frequencies continue to be called masers.
The verb "to lase" has become common parlance within the field, signifying the emission of coherent light, particularly from the gain medium of a laser. When a laser is actively producing light, it is said to be "lasing." The terms "laser" and "maser" are also applied to naturally occurring phenomena, such as astrophysical masers and atom lasers.
It's worth noting that a laser, in its purest form, functions as an optical oscillator rather than a simple amplifier, as its acronym might suggest. Some have humorously pointed out that "LOSER" (Light Oscillation by Stimulated Emission of Radiation) might have been a more accurate, albeit less inspiring, acronym. The term "laser" itself is often considered an anacronym—an acronym so widely adopted as a noun that its origin as an abbreviation is no longer overtly recognized.
Fundamentals
The fundamental process by which a laser produces its distinctive beam of light, often of a single wavelength, is rooted in the quantum mechanical behavior of photons and their interaction with atoms and molecules. In a conventional light source, such as a lightbulb or a star, photons are emitted and absorbed across a broad range of energies, resulting in a spectrum of light that is essentially random and incoherent. This is known as thermal radiation.
In stark contrast, the photons emitted by a laser are not the product of random thermal processes. Instead, their emission is precisely triggered by the presence of another photon of similar energy. This is the essence of stimulated emission. For this to occur, the triggering photon must possess an energy, and thus a wavelength, that precisely matches a potential transition within the atom or molecule, and that atom or molecule must be in a specific, excited state.
When a photon stimulates the emission of an identical photon, both photons can then go on to stimulate further emissions, creating a chain reaction. This amplification process requires a significant population of atoms or molecules to be in the proper excited state. Most materials quickly shed their excitation energy, making this chain reaction difficult to sustain. Lasers, however, utilize materials with metastable states – states that remain excited for a comparatively longer duration. When combined with a continuous energy source, known as a pump, to maintain this excited state, a laser can achieve a self-sustaining chain reaction of stimulated emission.
The defining characteristic of laser light is its coherence. Spatial coherence means that the light waves are in phase across the beam's cross-section, allowing it to be focused to a very small spot with high irradiance or to remain narrow over vast distances. Temporal coherence refers to the predictable phase relationship of the light wave over time, which translates to a very narrow frequency spectrum and high monochromaticity. Incoherent light sources, on the other hand, exhibit random fluctuations in phase and amplitude, resulting in a short coherence length.
While many lasers are designed to produce a single wavelength, most emit light in several closely spaced modes. The temporal coherence, while high, does not always translate to perfect monochromaticity. Similarly, not all laser beams are perfectly collimated; some exhibit greater divergence than theoretically ideal due to multimode operation. Nevertheless, all these devices are classified as lasers because their light generation relies on stimulated emission. Lasers are employed precisely because they can produce light with a degree of spatial or temporal coherence that cannot be achieved by simpler, less sophisticated technologies.
Design
The architecture of a laser is elegantly simple, yet profoundly effective. At its core lie three essential components: a gain medium, an energy source for pumping, and a mechanism for optical feedback. The gain medium, a material possessing the unique ability to amplify light through stimulated emission, is the heart of the laser. When light of a specific wavelength traverses this medium, its power increases. The feedback mechanism, typically a pair of mirrors, ensures that this amplified light is reflected back and forth through the gain medium, further stimulating emission and preferentially amplifying light at the peak of the medium's gain curve. Over successive passes, one particular frequency will dominate, solidifying the formation of a coherent beam.
This process can be likened to an audio oscillator: when a microphone is placed too close to a speaker, a piercing feedback screech occurs at the amplifier's peak gain frequency. In a laser, the mirrors act as the resonant cavity, bouncing the light and allowing it to build in intensity.
The gain medium requires energy input, a process termed pumping. This energy can be supplied as an electric current or as light from another source, such as a flash lamp or another laser.
The most common laser design utilizes an optical cavity formed by two mirrors positioned at either end of the gain medium. Light bounces between these mirrors, traversing the gain medium repeatedly and amplifying with each pass. One of these mirrors, the output coupler, is partially transparent, allowing a fraction of the amplified light to escape as the laser beam. The nature of the mirrors – whether flat or curved – influences how the emitted light spreads out or forms a narrow beam. This resonant cavity structure earns the laser the designation of an optical oscillator.
Beyond these fundamental components, practical lasers often incorporate additional elements to precisely control the characteristics of the emitted light, such as its polarization, wavelength, and beam shape.
Laser Physics
The behavior of electrons within atoms and their interaction with electromagnetic fields forms the bedrock of our understanding of laser physics. The process of stimulated emission, a cornerstone of laser operation, is a quantum mechanical phenomenon.
Stimulated Emission
In the classical view, electrons orbit atomic nuclei at varying distances, with orbits farther from the nucleus possessing higher energy. However, quantum mechanics dictates that electrons occupy discrete energy levels within an atom, arranged in specific orbitals. An electron can only absorb energy, in the form of photons or phonons, if that energy corresponds precisely to the difference between two energy levels. This means that a specific transition within an atom will only absorb a particular wavelength of light. When a photon with the correct energy strikes an atom with an electron in a lower energy state, that electron can be excited to a higher energy level, consuming the photon in the process.
When an electron is excited to a higher energy state (ΔE), it doesn't remain there indefinitely. Eventually, it will spontaneously transition back to a lower energy state, emitting a photon with energy ΔE. This is known as spontaneous emission. This quantum mechanical process, directly linked to the Heisenberg uncertainty principle, results in photons being emitted in random directions. Fluorescence and thermal emission are manifestations of spontaneous emission.
However, a photon with the precise energy to be absorbed can also stimulate an electron in a higher energy state to drop to a lower energy state, emitting a second photon. Crucially, this emitted photon is identical to the stimulating photon in terms of wavelength, phase, and direction. This is stimulated emission.
Gain Medium and Cavity
The gain medium, the material at the heart of the laser, is excited to a higher energy state by an external energy source. This pumping process, whether through an electrical current or light absorption, raises electrons to these excited quantum states. Particles can then interact with light through absorption or emission. While spontaneous emission is random, stimulated emission is directed and coherent. When the number of particles in an excited state exceeds those in a lower energy state, a condition known as population inversion is achieved. In this state, stimulated emission outpaces absorption, leading to light amplification. A system exhibiting this property is an optical amplifier. When this amplifier is placed within a resonant optical cavity, it becomes a laser.
For exceptionally high-gain lasing media, a phenomenon called superluminescence can occur, where light is amplified significantly in a single pass without the need for a resonator. While sometimes colloquially referred to as lasers, these devices lack the spatial and temporal coherence of true lasers. They are, in essence, high-gain optical amplifiers.
The optical resonator, often referred to as an optical cavity, is typically composed of two mirrors. These mirrors reflect the light back and forth through the gain medium, allowing for repeated amplification. As the beam power increases, the gain medium becomes saturated, meaning that the rate of stimulated emission matches the rate of loss from the cavity. In a continuous-wave (CW) laser, a balance is struck between the pump power, gain saturation, and cavity losses, establishing a stable operating point and output power. If the pump power is insufficient to overcome cavity losses, lasing will not occur. The minimum pump power required to initiate lasing is known as the lasing threshold. The gain medium amplifies any photons passing through it, but only those propagating in specific spatial modes supported by the resonator will experience significant amplification.
The Light Emitted
The genesis of lasing typically begins with spontaneous emission into a specific lasing mode. This initial light is then amplified through stimulated emission within the gain medium. The process of stimulated emission ensures that the emitted light is coherent with the stimulating light, possessing the same direction, wavelength, and polarization. The optical resonator further refines this light, contributing to its characteristic coherence, uniform polarization, and monochromaticity, depending on its design. The fundamental linewidth of the light emitted from the lasing resonator can be dramatically narrower than that of the passive resonator itself. Some advanced lasers employ an injection seeder to initiate the process with a highly coherent beam, enabling even narrower spectral output.
The theoretical framework for understanding coherent light was significantly advanced by Roy J. Glauber in 1963, work for which he was awarded the Nobel Prize in Physics. His research demonstrated that coherent light states are formed from combinations of photon number states, and that the arrival rate of photons in a laser beam follows Poisson statistics.
Many lasers produce beams that can be approximated as Gaussian beams, exhibiting the minimum possible divergence for a given beam diameter. High-power lasers, however, may produce multimode beams, with transverse modes often described by Hermite–Gaussian or Laguerre–Gaussian functions. Some specialized lasers generate a flat-topped profile, known as a "tophat beam." Unstable resonators, less common than stable ones, can produce beams with fractal geometries. Advanced optical systems can even create more complex beam shapes, such as Bessel beams and optical vortices.
Near its focal region, or "waist," a laser beam is highly collimated, with planar wavefronts and no divergence. However, due to diffraction, this perfect collimation is limited to the Rayleigh range. A single transverse mode (Gaussian beam) laser beam will eventually diverge, with the angle of divergence inversely proportional to the beam diameter, as dictated by diffraction theory. This means that even a tight beam from a common helium–neon laser would spread considerably when shone over vast distances. Conversely, light from a semiconductor laser, which typically exits with significant divergence, can be transformed into a collimated beam by a lens system, as seen in a laser pointer. This capability stems from the single spatial mode nature of the light. This property of spatial coherence is a hallmark of laser light, setting it apart from conventional light sources.
A laser beam profiler is an essential instrument for measuring the intensity profile, width, and divergence of laser beams.
The diffuse reflection of a laser beam from a matte surface results in a speckle pattern, a phenomenon with intriguing characteristics.
Quantum vs. Classical Emission Processes
The mechanism by which lasers produce radiation is fundamentally quantum mechanical, relying on stimulated emission. This process, where energy is extracted from atomic or molecular transitions, was predicted by Albert Einstein. He established the relationship between the coefficients describing spontaneous emission and those governing absorption and stimulated emission. In the case of the free-electron laser, a more exotic type, atomic energy levels are not directly involved, and its operation can be explained without recourse to quantum mechanics.
Modes of Operation
Lasers can be broadly categorized by their operational modes: continuous-wave (CW) or pulsed. This distinction depends on whether the laser emits a steady stream of light or a series of discrete pulses. Even a CW laser can be intentionally modulated to produce pulses, but the classification typically refers to the inherent nature of the output.
Continuous-Wave Operation
Certain applications demand a laser output that remains constant over time. Such lasers are termed continuous-wave (CW) lasers. Many laser types can be configured for CW operation. While some CW lasers may exhibit high-frequency amplitude variations due to beats between multiple longitudinal modes, they are still considered CW if the output power is stable over longer periods. Mode-locked lasers, designed to produce very short pulses, are an exception to this naming convention.
For continuous-wave operation, the population inversion within the gain medium must be continuously sustained by a steady pump source. This is not achievable for all lasing media, and in some cases, it would require impractically high pumping power, potentially damaging the laser or generating excessive heat.
Pulsed Operation
Pulsed laser operation encompasses any laser that is not a continuous-wave laser, meaning its output manifests as pulses of light. This wide category serves a multitude of purposes. Some lasers are inherently pulsed because CW operation is not feasible.
In other instances, applications require pulses with the highest possible energy. This can be achieved by reducing the pulse repetition rate, allowing more energy to accumulate between pulses. For processes like laser ablation, where material is vaporized from a surface, a rapid energy delivery is crucial to prevent heat from dissipating into the surrounding material.
Other applications prioritize peak pulse power, particularly for inducing nonlinear optical effects. To maximize peak power for a given pulse energy, pulses of the shortest possible duration are desired. Techniques like Q-switching are employed for this purpose.
The optical bandwidth of a pulse is inversely related to its duration. Extremely short pulses necessitate a broad spectral bandwidth. Certain dye lasers and vibronic solid-state lasers possess gain bandwidths wide enough to generate pulses lasting only a few femtoseconds.
Q-switching
In a Q-switched laser, the population inversion is allowed to build up by introducing significant loss within the resonator, effectively lowering its quality factor (Q). Once the stored energy in the gain medium reaches its maximum, the loss mechanism is rapidly removed, allowing lasing to commence. This results in a single, short pulse that carries the stored energy, thus achieving very high peak power.
Mode locking
A mode-locked laser is capable of generating extremely short pulses, ranging from tens of picoseconds down to less than 10 femtoseconds. These pulses repeat at a rate corresponding to the time it takes light to travel one round trip within the laser cavity. According to the Fourier limit, shorter pulses inherently possess broader spectral bandwidths. Consequently, the gain medium must have a sufficiently wide gain bandwidth to amplify these frequencies. Materials like titanium-doped sapphire are well-suited for this, enabling the generation of femtosecond pulses.
Mode-locked lasers are invaluable tools for studying ultrafast phenomena in physics and chemistry. Their high peak powers also facilitate nonlinear optical effects. Unlike the single giant pulse from a Q-switched laser, pulses from a mode-locked laser are phase-coherent, meaning they are identical and perfectly periodic. This coherence, combined with the immense peak powers, makes them essential for cutting-edge research.
Pulsed Pumping
Another method to achieve pulsed laser operation is by using a pulsed energy source to pump the laser medium. This can involve electronic charging of flash lamps or using another pulsed laser as the pump. Historically, pulsed pumping was employed with dye lasers, which have very short excited-state lifetimes. High-energy, fast pump pulses were necessary to achieve population inversion. This was often accomplished by discharging large capacitors through flash lamps. Pulsed pumping is also essential for three-level lasers, where the lower energy level quickly becomes populated, inhibiting further lasing until atoms relax to the ground state. Lasers like the excimer and copper vapor lasers can only operate in pulsed mode.
History
Foundations
The theoretical underpinnings for the laser and maser were laid in 1917 by Albert Einstein. In his seminal paper "Zur Quantentheorie der Strahlung" (On the Quantum Theory of Radiation), he re-derived Max Planck's law of radiation by postulating probability coefficients for absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. Rudolf W. Ladenburg later confirmed the existence of stimulated emission and negative absorption in 1928. Valentin A. Fabrikant predicted the amplification of "short" waves using stimulated emission in 1939. In 1947, Willis E. Lamb and R. C. Retherford observed apparent stimulated emission in hydrogen spectra, marking the first experimental demonstration. Alfred Kastler, who would later receive the Nobel Prize in Physics in 1966, proposed the method of optical pumping in 1950, which was experimentally verified two years later by Brossel, Kastler, and Winter.
Maser
The path to the laser involved an intermediate step: the maser. In 1951, Joseph Weber proposed a microwave amplifier based on stimulated emission. Charles H. Townes, along with his graduate students James P. Gordon and Herbert J. Zeiger, successfully built the first maser in 1953, a device amplifying microwave radiation. However, Townes's initial maser was incapable of continuous output.
Independently, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov were developing their own quantum oscillators. They addressed the continuous-output challenge by employing systems with more than two energy levels. This approach allowed for stimulated emission between an excited state and a lower excited state, rather than the ground state, facilitating the maintenance of population inversion. In 1955, Prokhorov and Basov proposed optical pumping of multi-level systems as a method for achieving population inversion, a technique that would become central to laser pumping.
Notably, prominent physicists like Niels Bohr and John von Neumann initially expressed skepticism about the maser, arguing it might violate Heisenberg's uncertainty principle. Others, such as Isidor Rabi, viewed the endeavor as impractical. Nevertheless, in 1964, Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov were jointly awarded the Nobel Prize in Physics for their foundational work in quantum electronics, which led to the development of maser and laser oscillators and amplifiers.
Laser
The conceptual leap to the laser occurred in stages. In April 1957, Japanese engineer Jun-ichi Nishizawa filed a patent application for a "semiconductor optical maser." Around the same time, Charles H. Townes and Arthur Leonard Schawlow, then at Bell Labs, began a focused investigation into infrared "optical masers." Their research evolved, leading them to shift their focus from infrared to visible light.
Simultaneously, Gordon Gould, a graduate student at Columbia University, was independently exploring the energy levels of excited thallium. Gould and Townes had discussions about radiation emission in general, though not their specific research. In November 1957, Gould documented his ideas for a "laser," including the crucial concept of an open resonator and a diagram of an optically pumped laser. His notebook also contained the first recorded use of the acronym "laser," derived from "light amplification by stimulated emission of radiation," along with potential applications.
In 1958, Bell Labs filed a patent application for Schawlow and Townes's proposed optical maser, and their theoretical calculations were published in the Physical Review. Prokhorov independently published the concept of the open resonator in the same year.
At a 1959 conference, Gordon Gould formally presented the acronym "LASER" in his paper "The LASER, Light Amplification by Stimulated Emission of Radiation." Gould envisioned a system of "-ASER" acronyms for different spectral regions, but "LASER" ultimately became the universally adopted term for devices operating at wavelengths shorter than microwaves.
Gould's notes also detailed potential laser applications, including optical telecommunications, spectrometry, interferometry, radar, and nuclear fusion. He filed a patent application in April 1959, but it was initially denied. The U.S. Patent and Trademark Office awarded a patent to Bell Labs, sparking a protracted legal battle over laser technologies and applications that lasted for twenty-eight years. Gould eventually secured his first patent in 1977 for optically pumped laser amplifiers and won his first significant infringement claim in 1987. The question of who truly deserves credit for the invention of the laser remains a subject of historical debate, as various individuals contributed key elements.
On May 16, 1960, Theodore H. Maiman achieved a significant milestone by operating the first functional laser at Hughes Research Laboratories. This groundbreaking device, utilizing a flashlamp-pumped synthetic ruby crystal, produced red laser light at a wavelength of 694 nanometers. It was, however, limited to pulsed operation due to its three-level pumping scheme. Later that year, Ali Javan, along with William R. Bennett Jr. and Donald R. Herriott, developed the first gas laser, using helium and neon, capable of continuous operation in the infrared spectrum. Javan later received the Albert Einstein World Award of Science in 1993. In 1962, Robert N. Hall demonstrated the first semiconductor laser, made from gallium arsenide, emitting in the near-infrared. Nick Holonyak Jr. followed with the first visible-emitting semiconductor laser that same year, though it required cooling to liquid nitrogen temperatures and operated only in pulsed mode. The breakthrough for continuous-wave, room-temperature operation of semiconductor lasers came in 1970, independently achieved by Zhores Alferov in the USSR and by Izuo Hayashi and Morton Panish at Bell Labs, utilizing the heterojunction structure.
Recent Innovations
The relentless pursuit of improved laser performance has led to a continuous stream of innovations, optimizing lasers for various parameters such as wavelength, output power (both average and peak), pulse duration, linewidth, efficiency, and cost.
In 2015, researchers unveiled a "white laser," a device whose light is modulated by a synthetic nanosheet capable of emitting red, green, and blue light in adjustable proportions, each spanning 191 nm.
A significant development in 2017 came from Delft University of Technology, where researchers demonstrated an AC Josephson junction microwave laser. Operating in the superconducting regime, this laser offered enhanced stability and held potential for quantum computing applications. In the same year, the Technical University of Munich showcased the smallest mode-locking laser, capable of emitting pairs of phase-locked picosecond pulses at repetition frequencies up to 200 GHz.
Further pushing the boundaries of precision, researchers at the Physikalisch-Technische Bundesanstalt (PTB), in collaboration with US scientists from JILA, established a new world record in 2017 for laser linewidth, achieving an astonishing 10 millihertz with an erbium-doped fiber laser.
Types and Operating Principles
The diversity of laser applications is matched by the variety of laser types, each operating on distinct principles and optimized for specific performance characteristics.
Gas Lasers
Following the pioneering work on the HeNe gas laser, numerous other gas discharge configurations have been developed for coherent light amplification.
The helium–neon laser (HeNe) is a common and relatively inexpensive laser, often engineered to lase at 633 nm. Its high coherence makes it a staple in optical research and educational settings. Industrial applications, particularly for cutting and welding, rely on high-power carbon dioxide (CO 2 ) lasers, which can deliver hundreds of watts in a single spatial mode at a wavelength of 10.6 μm. These lasers boast an impressive efficiency of over 30%. Argon-ion lasers operate at several transitions between 351 and 528.7 nm, with the 488 nm and 514.5 nm lines being particularly common. The nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is a more accessible option, often home-built, producing ultraviolet light at 337.1 nm. Metal ion lasers, such as Helium–Silver (HeAg) at 224 nm and Neon–Copper (NeCu) at 248 nm, generate deep ultraviolet wavelengths. Due to their narrow oscillation linewidths (less than 3 GHz), they are suitable for applications like fluorescence-suppressed Raman spectroscopy.
The phenomenon of lasing without inversion was demonstrated in sodium and rubidium gases, where external masers were used to induce "optical transparency" by interfering ground electron transitions, effectively canceling the likelihood of ground electrons absorbing energy.
Chemical Lasers
Chemical lasers derive their power from the rapid release of energy during a chemical reaction. These high-power lasers are of particular interest to the military, and continuous-wave versions, fed by gas streams, have found industrial applications. Examples include the hydrogen fluoride laser (2700–2900 nm) and the deuterium fluoride laser (3800 nm), powered by reactions involving hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. The first chemical laser, a hydrogen chloride laser operating at 3.7 micrometers, was demonstrated in 1965 by Jerome V. V. Kasper and George C. Pimentel.
Excimer Lasers
Excimer lasers are a specialized type of gas laser energized by an electric discharge. Their gain medium consists of excimers or exciplexes – molecules that exist only when one atom is in an excited electronic state. Upon emitting a photon, the molecule dissociates, rapidly reducing the population of the lower energy state and facilitating population inversion. Common excimer molecules include ArF (193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm), operating in the ultraviolet spectrum. These lasers are crucial for semiconductor photolithography and LASIK eye surgery. The molecular fluorine laser, emitting at 157 nm in the vacuum ultraviolet, is sometimes misclassified as an excimer laser, though F₂ is a stable compound.
Solid-State Lasers
Solid-state lasers utilize a crystalline or glass rod doped with ions that provide the necessary energy levels for lasing. The inaugural laser was a ruby laser, made from chromium-doped corundum. The dopant ions are optically pumped, absorbing energy from a shorter wavelength light source, such as a flash tube. It's important to distinguish that semiconductor lasers, while also solid-state devices, are typically classified separately.
Neodymium is a prevalent dopant in various solid-state laser crystals, including yttrium orthovanadate (Nd:YVO₄), yttrium lithium fluoride (Nd:YLF), and yttrium aluminum garnet (Nd:YAG). These lasers, operating around 1064 nm in the infrared, are widely used for material processing like cutting, welding, and marking, as well as for spectroscopy and pumping dye lasers. Frequency doubling, tripling, or quadrupling these lasers can produce green (532 nm), UV (355 nm), and even deeper UV (266 nm) beams. Frequency-doubled diode-pumped solid-state (DPSS) lasers are the source of the bright green laser pointers and are also used in medical lasers.
Other common dopants in solid-state lasers include ytterbium, holmium, thulium, and erbium. Ytterbium-doped crystals like Yb:YAG operate around 1020–1050 nm and offer high efficiency and power potential due to a small quantum defect. Extremely high peak powers can be achieved with Yb:YAG in ultrashort pulses. Holmium-doped YAG crystals emit at 2097 nm in the infrared, a wavelength strongly absorbed by water-containing tissues, making Ho:YAG lasers useful in surgery for procedures like joint resurfacing, tooth decay removal, cancer vaporization, and gallstone pulverization, typically operated in pulsed mode.
Titanium-doped sapphire (Ti:sapphire) lasers are highly tunable in the infrared and are widely used in spectroscopy. They are also notable for their role in mode-locked lasers, generating ultrashort pulses with exceptionally high peak power.
Optical parametric oscillators (OPOs) can shift the wavelength of solid-state lasers across a broad spectrum, from UV to infrared. Non-critically phase-matched OPOs can achieve high conversion efficiencies, leading to the development of highly efficient lasers at "eyesafely" wavelengths, reducing the risk of accidental eye damage.
Thermal management is a critical challenge in solid-state lasers. Unconverted pump energy generates heat, which, coupled with a high thermo-optic coefficient, can lead to thermal lensing and reduced efficiency. Thin disk lasers, where the gain medium is significantly thinner than the pump beam diameter, overcome these limitations by allowing for more uniform temperature distribution and efficient heat dissipation, achieving output powers up to one kilowatt.
Fiber Lasers
Fiber lasers are a type of solid-state laser where the light is guided within a single-mode optical fiber through total internal reflection. This waveguiding property allows for very long gain regions, which, combined with the high surface-area-to-volume ratio of fibers, enables efficient cooling. This leads to reduced thermal distortion of the beam. Erbium and ytterbium ions are common active species in these lasers.
Many fiber lasers are designed as double-clad fibers, featuring a core, an inner cladding, and an outer cladding. The refractive indices are chosen such that the core guides the laser emission in a single mode, while the outer cladding efficiently guides the pump light into the active inner core. This design allows for high pump power delivery with convenient launching conditions.
Fiber disk lasers, or stacks thereof, offer another approach to efficient pumping. However, like other optical media, fiber lasers can be susceptible to photodarkening when exposed to certain wavelengths, leading to material degradation and loss of functionality over time. The specific mechanisms and effects of photodarkening vary with the material, often involving the formation of color centers.
Photonic Crystal Lasers
Photonic crystal lasers utilize nanostructures that confine light modes and manipulate the density of optical states (DOS) to provide the necessary feedback for lasing. These lasers are typically micrometer-sized and their emission wavelength is tunable based on the photonic crystal's band structure.
Semiconductor Lasers
Semiconductor lasers, commonly known as laser diodes, are electrically pumped devices. The recombination of electrons and holes injected by an applied current generates optical gain. Reflection from the crystal facets typically provides the optical resonance, although external cavities can also be employed.
Commercial laser diodes operate across a wide wavelength range, from 375 nm to 3500 nm. Low-to-medium power diodes are ubiquitous in applications like laser pointers, laser printers, and optical disc players. They also serve as efficient pump sources for other types of lasers. High-power industrial laser diodes, reaching up to 20 kW, are used for cutting and welding.
The development of high-power green laser diodes (515/520 nm) by companies like Nichia and OSRAM in 2012 has provided a competitive alternative to traditional diode-pumped solid-state lasers.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers that emit light perpendicular to the wafer surface. They typically produce a more circular output beam than conventional laser diodes. While 850 nm VCSELs are widely available, research and commercialization of 1300 nm and 1550 nm devices are ongoing. VECSELs are external-cavity VCSELs. Quantum cascade lasers are a distinct class of semiconductor lasers that utilize intersubband transitions within quantum wells.
The development of silicon lasers is a key area in the field of optical computing, aiming to integrate electronic and photonic components on a single chip. Silicon's inherent properties present challenges for lasing, but researchers have achieved lasing in hybrid silicon lasers by combining silicon with other semiconductor materials like indium phosphide or gallium arsenide. Monolithically integrated nanowire lasers directly on silicon also show promise for chip-level optical interconnects, capable of emitting phase-locked picosecond pulses. Raman lasers, which leverage Raman scattering, offer another pathway for silicon-based lasing.
Quantum dot lasers use quantum dots as their active medium, exhibiting performance characteristics closer to gas lasers and overcoming some limitations of traditional semiconductor lasers. Improvements in modulation bandwidth, lasing threshold, noise, and temperature insensitivity have been observed. The wavelength emission can be tuned by adjusting the size and composition of the quantum dots, enabling the fabrication of lasers at previously inaccessible wavelengths.
Dye Lasers
Dye lasers use organic dyes as their gain medium. The broad gain spectrum of available dyes allows for high tunability or the generation of ultrashort pulses, on the order of femtoseconds. While typically found in liquid form, researchers have also developed solid-state dye lasers using dye-doped polymers.
Bubble lasers employ a bubble as the optical resonator, generating a frequency comb through whispering gallery modes. The spacing of these modes is directly related to the bubble's circumference, making them sensitive pressure sensors.
Free-Electron Lasers
Free-electron lasers (FELs) produce coherent, high-power radiation across an exceptionally wide range of wavelengths, from microwaves to X-rays. Their operation differs from conventional lasers in that they use a relativistic electron beam as the lasing medium, rather than bound atomic or molecular states.
Exotic Media
The quest for high-quantum-energy lasers has led to extensive research into transitions between isomeric states of atomic nuclei, aiming for gamma-ray lasers. Despite ongoing optimism and significant international research efforts, an operational gamma-ray laser remains elusive. Early studies explored methods like neutron excitation and Mössbauer effect narrowing, while more recent conjectures involve coupling collective electronic oscillations to nuclear transitions.
Speculation about using positronium annihilation to drive a powerful gamma-ray laser emerged in 2007, with proposals suggesting such a laser could potentially ignite nuclear fusion reactions. Space-based X-ray lasers pumped by nuclear explosions have also been conceptualized as antimissile weapons, albeit as one-shot devices.
Remarkably, laser light has even been produced by living cells. Genetically engineered cells expressing green fluorescent protein, placed between mirrors, have been shown to emit directed green laser light when illuminated with blue light.
Natural Lasers
Similar to astrophysical masers, irradiated planetary or stellar gases can amplify light, creating natural lasers. This phenomenon has been observed on Mars, Venus, and in the object MWC 349.
Uses
When the laser first emerged, it was famously described as "a solution looking for a problem." However, its potential was recognized early on, with numerous applications envisioned. Today, lasers are ubiquitous, permeating every facet of modern society, from consumer electronics and information technology to science, medicine, industry, law enforcement, entertainment, and the military. Fiber-optic communication, a cornerstone of the internet, relies heavily on multiplexed lasers in dense wavelength-division multiplexing (WDM) systems to transmit vast amounts of data over long distances.
The supermarket barcode scanner, introduced in 1974, was one of the first widely recognized laser applications. The LaserDisc player, launched in 1978, was an early consumer product featuring a laser, but it was the compact disc player, commercialized in 1982, that truly brought lasers into the mainstream home. Laser printers followed shortly thereafter.
Beyond these examples, lasers find application in:
- Communications: In addition to fiber-optic communication, lasers are used for free-space optical communication, including laser communication in space.
- Medicine: Detailed below.
- Industry: Laser cutting, converting thin materials, welding, heat treatment, parts marking (engraving and bonding), additive manufacturing (3D printing) processes like selective laser sintering and melting, laser metal deposition, non-contact measurement, 3D scanning, and laser cleaning.
- Military: Target marking, munitions guidance, missile defense, electro-optical countermeasures (EOCM), lidar, and anti-personnel applications.
- Law Enforcement: LIDAR traffic enforcement, and forensic identification through latent fingerprint detection.
- Research: Spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry, lidar, laser capture microdissection, fluorescence microscopy, metrology, and laser cooling.
- Commercial Products: Laser printers, barcode scanners, thermometers, laser pointers, holograms, and bubblegrams.
- Entertainment: Optical discs, laser lighting displays, and laser turntables.
- Informational Markings: Projecting informational markings onto surfaces for sports fields, roads, runways, or warehouse floors.
In 2004, the global market for lasers (excluding diode lasers) was valued at approximately 3.20 billion. By 2023, global industrial laser sales reached an estimated $21.85 billion.
In Medicine
Lasers have revolutionized medical practice, offering precise and minimally invasive treatment options. Applications include:
- Laser Surgery: Particularly in eye surgery, where they can correct vision problems.
- Therapeutic Applications: Including photobiomodulation therapy (laser healing), kidney stone treatment, and ophthalmoscopy.
- Cosmetic Treatments: Such as acne treatment, cellulite and striae reduction, and hair removal.
Lasers are also employed in cancer treatment to shrink or destroy tumors, especially superficial cancers. They are used for basal cell skin cancer and early-stage cervical, penile, vaginal, vulvar, and non-small cell lung cancers. Often used in conjunction with other therapies, lasers offer greater precision than traditional surgical methods, leading to less damage, pain, bleeding, swelling, and scarring. However, specialized training for surgeons and potentially higher costs are considerations. Laser-induced interstitial thermotherapy (LITT) uses lasers for hyperthermia treatment, damaging or killing cancer cells with heat.
Low-level laser therapy (LLLT), using low-power lasers or LEDs, is applied to the body's surface, with claims of stimulating healing, relieving pain, and enhancing cell function. While its effectiveness is still under investigation, repeated low-level red light therapy shows promise for myopia control in children. Several LLLT devices have received FDA clearance, and research is ongoing for conditions like rheumatoid arthritis and oral mucositis.
As Weapons
A laser weapon is a directed-energy weapon that utilizes lasers to inflict damage. The practical deployment of high-performance laser weapons remains a subject of development, with challenges such as atmospheric thermal blooming requiring further solutions. The U.S. Navy has tested the short-range (1 mile), 30-kW Laser Weapon System (LaWS) against targets like small UAVs and rocket-propelled grenades, describing it as a combination of six welding lasers. A more powerful 60-kW system, HELIOS, was under development for destroyer-class ships.
The YAL-1, a Boeing 747 equipped with a laser weapon, was a notable project that was ultimately canceled.
Lasers can also serve as incapacitating non-lethal weapons. Direct exposure to the eyes can cause temporary or permanent vision loss, even with low-power lasers, raising ethical concerns and leading to the Protocol on Blinding Laser Weapons. Weapons designed for temporary visual impairment, known as dazzlers, are used by military and law enforcement.
Hobbies
In recent years, lasers have garnered interest among hobbyists. While generally Class IIIa or IIIb, some enthusiasts create their own Class IV lasers. This hobby carries inherent risks and costs. Hobbyists often salvage laser diodes from discarded electronics like DVD players and Blu-ray drives. Surplus lasers from retired military applications are also repurposed for applications such as holography.
Examples by Power
The power output of a laser is tailored to its specific application. Continuous-wave or average power is a key metric for many uses, while pulsed lasers are also characterized by their peak power, which can be orders of magnitude higher than their average power.
| Power | Use |
|---|---|
| 1–5 mW | Laser pointers |
| 5 mW | CD-ROM drive |
| 5–10 mW | DVD player or DVD-ROM drive |
| 100 mW | High-speed CD-RW burner |
| 250 mW | Consumer 16× DVD-R burner |
| 400 mW | DVD 24× dual-layer recording |
| 1 W | Green laser in Holographic Versatile Disc prototype development |
| 1–20 W | Majority of commercially available solid-state lasers for micro machining |
| 30–100 W | Typical sealed CO₂ surgical lasers |
| 100–3000 W | Typical sealed CO₂ lasers for industrial laser cutting |
Pulsed systems can achieve extremely high peak powers:
- 700 TW (700 × 10¹² W): National Ignition Facility, a 192-beam system used for inertial confinement fusion research.
- 10 PW (10 × 10¹⁵ W): The world's most powerful laser as of 2019, located at the ELI-NP facility in Romania.
Safety
The potential hazards of lasers were recognized even with the first device, which was noted to have the power to burn through a razor blade. Today, it is understood that even low-power lasers can pose a risk to eyesight if the beam directly strikes the eye or reflects off a shiny surface. The eye's lens can focus laser light onto the retina, causing localized burning and permanent damage.
Lasers are classified into safety categories based on their potential hazard:
- Class 1: Inherently safe, typically due to containment.
- Class 2: Safe during normal use due to the blink reflex (up to 1 mW).
- Class 3R: Small risk of eye damage within the blink reflex time (typically up to 5 mW).
- Class 3B: Can cause immediate eye damage upon exposure (5–499 mW).
- Class 4: Can burn skin, and scattered light can also be hazardous (≥ 500 mW).
These power limits are for visible-light, continuous-wave lasers; pulsed lasers and invisible wavelengths have different limits. Protective eyewear is essential when working with Class 3B and Class 4 lasers.
Infrared lasers above approximately 1.4 micrometers are often termed "eye-safe" because the cornea absorbs the light, protecting the retina. However, this label can be misleading, as high-power or pulsed lasers at these wavelengths can still cause severe corneal burns.
Lasers also pose a safety risk to aviation, potentially distracting or blinding pilots. Cameras, particularly those based on charge-coupled devices, can be more susceptible to laser damage than human eyes.