A dye laser is a laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 which uses an organic

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 as the lasing medium, usually as a liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

. Compared to gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es and most solid state

Solid state (electronics)

Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 lasing media, a dye can usually be used for a much wider range of wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s. The wide bandwidth makes them particularly suitable for tunable laser

Tunable laser

A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range....

s and pulsed lasers. Moreover, the dye can be replaced by another type in order to generate different wavelengths with the same laser, although this usually requires replacing other optical components in the laser as well.

Dye lasers were independently discovered by P. P. Sorokin

P. P. Sorokin

Peter P. Sorokin is an American physicist and co-inventor of the dye laser. Sorokin and his colleague J. R. Lankard, at IBM Research Laboratories, used a ruby laser to excite a near infrared laser dye. Their report was quickly followed by that of F. P. Schäfer. In 1974 Sorokin received the Albert...

 and F. P. Schäfer

F. P. Schäfer

F. P. Schäfer is a German physicist co-inventor of the dye laser. His book, Dye Lasers, is considered a classic in the field of tunable lasers.-References:...

 (and colleagues) in 1966.

In addition to the usual liquid state, dye lasers are also available as solid state dye lasers

Solid state dye lasers

Solid state dye lasers were introduced in 1967 by Soffer and McFarland. In these solid state lasers, the gain medium is a laser dye-doped organic matrix such as poly , rather than a liquid solution of the dye...

 (SSDL). SSDL use dye-doped organic matrices as gain medium.

Construction

A dye laser consists of an organic dye mixed with a solvent

Solvent

A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

, which may be circulated through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to "pump"

Laser pumping

Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 the liquid beyond its lasing threshold

Lasing threshold

The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above threshold, the slope of power vs. excitation is orders...

. A fast discharge flashlamp or an external laser is usually used for this purpose. Mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s are also needed to oscillate the light produced by the dye’s fluorescence, which is amplified with each pass through the liquid. The output mirror is normally around 80% reflective, while all other mirrors are usually more than 99% reflective. The dye solution is usually circulated at high speeds, to help avoid triplet absorption and to decrease degradation of the dye. A prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 or diffraction grating

Diffraction grating

In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

 is usually mounted in the beam path, to allow tuning of the beam.

Because the liquid medium of a dye laser can fit any shape, there are a multitude of different configurations that can be used. A Fabry–Pérot laser cavity is usually used for flashlamp pumped lasers, which consists of two mirrors, which may be flat or curved, mounted parallel to each other with the laser medium in between. The dye cell is usually side-pumped, with one or more flashlamps running parallel to the dye cell in a reflector cavity. The reflector cavity is often water cooled, to prevent thermal shock in the dye caused by the large amounts of near-infrared radiation which the flashlamp produces. Axial pumped lasers have a hollow, annular-shaped flashlamp that surrounds the dye cell, which has lower inductance

Inductance

In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 for a shorter flash, and improved transfer efficiency. Coaxial pumped lasers have an annular dye cell that surrounds the flash lamp, for even better transfer efficiency, but have a lower gain due to diffraction losses. Flash pumped lasers can only be used for pulsed output.

A ring laser design is often chosen for continuous operation, although a Fabry–Pérot design is sometimes used. In a ring laser, the mirrors of the laser are positioned to allow the beam to travel in a circular path. The dye cell, or cuvette, is usually very small. Sometimes a dye jet is used to help avoid reflection losses. The dye is usually pumped with an external laser, such as a nitrogen

Nitrogen laser

A nitrogen laser is a gas laser operating in the ultraviolet range using molecular nitrogen as its gain medium, pumped by an electrical discharge....

, excimer

Excimer laser

An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....

, or frequency doubled Nd:YAG laser

Nd:YAG laser

Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...

. The liquid is circulated at very high speeds, to prevent triplet absorption from cutting off the beam. Unlike Fabry–Pérot cavities, a ring laser does not generate standing wave

Standing wave

In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

s which cause spatial hole burning, a phenomenon where energy becomes trapped in unused portions of the medium between the crests of the wave. This leads to a better gain from the lasing medium.

Operation

The dyes

Laser dye

Laser dyes are very large organic molecules with molecular weights of a few hundred mu. When one of these organic molecules is dissolved in a suitable liquid solvent it can be used as gain medium in a dye laser. Laser dye solutions absorb at shorter wavelengths end emit at longer wavelengths...

 used in these lasers contain rather large organic molecules which fluoresce. The incoming light excites the dye molecules into the state of being ready to emit stimulated radiation

Stimulated emission

In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...

, the singlet state. In this state, the molecules emit light via fluorescence

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

, and the dye is transparent to the lasing wavelength. Within a microsecond, or less, the molecules will change to their triplet state

Triplet state

A spin triplet is a set of three quantum states of a system, each with total spin S = 1 . The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.In physics, spin is the angular momentum...

. In the triplet state, light is emitted via phosphorescence

Phosphorescence

Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...

, and the molecules absorb the lasing wavelength, making the dye opaque. Liquid dyes also have an extremely high lasing threshold. Flashlamp pumped lasers need a flash with an extremely short duration, to deliver the large amounts of energy necessary to bring the dye past threshold before triplet absorption overcomes singlet emission. Dye lasers with an external pump laser can direct enough energy of the proper wavelength into the dye with a relatively small amount of input energy, but the dye must be circulated at high speeds to keep the triplet molecules out of the beam path.

Since organic dyes tend to decompose under the influence of light, the dye solution is normally circulated from a large reservoir. The dye solution can be flowing through a cuvette

Cuvette

A cuvette is a small tube of circular or square cross section, sealed at one end, made of plastic, glass, or fused quartz and designed to hold samples for spectroscopic experiments. The best cuvettes are as clear as possible, without impurities that might affect a spectroscopic reading...

, i.e., a glass container, or be as a dye jet, i.e., as a sheet-like stream in open air from a specially-shaped nozzle

Nozzle

A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

. With a dye jet, one avoids reflection losses from the glass surfaces and contamination of the walls of the cuvette. These advantages come at the cost of a more-complicated alignment.

Liquid dyes have very high gain

Gain

In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 as laser media. The beam only needs to make a few passes through the liquid to reach full design power, and hence, the high transmittance of the output coupler

Output coupler

An output coupler is a partially reflective mirror used in lasers to extract a portion of the laser beam from the optical resonator....

. The high gain also leads to high losses, because reflection from the dye cell walls, or flashlamp reflector, will dramatically reduce the amount of energy available to the beam. Pump cavities are often coated

Coating

Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wetability, corrosion resistance, wear resistance, and scratch resistance...

, anodized, or otherwise made of a material that will not reflect at the lasing wavelength while reflecting at the pump wavelength.

Narrow linewidth dye lasers

Dye lasers emission is inherently broad. However, tunable narrow linewidth emission has been central to the success of the dye laser. In order to produce narrow bandwidth tuning these lasers use many types of cavities and resonators which include gratings, prisms, multiple-prism grating arrangements

Multiple-prism dispersion theory

The first description of multiple-prism arrays, and multiple-prism dispersion, was given by Newton in his book Opticks. Prism pair expanders were introduced by Brewster in 1813. A modern mathematical description of the single-prism dispersion was given by Born and Wolf in 1959...

, and etalons.

The first narrow linewidth

Laser linewidth

←Laser linewidth is the spectral linewidth of a laser beam.Two of the most distinctive characteristics of laser emission are spatial coherence and spectral coherence. While spatial coherence is related to the beam divergence of the laser, spectral coherence is evaluated by measuring the laser...

 dye laser, introduced by Hänsch, used a Galilean telescope

Refracting telescope

A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

 as beam expander

Beam expander

Beam expanders are used in laser physics either as intracavity or extracavity elements. They can be telescopic in nature or prismatic. Generally prismatic beam expanders use several prisms and are known as multiple-prism beam expanders....

 to illuminate the diffraction grating. Next were the grazing-incidence grating designs and the multiple-prism grating configurations. The various resonators and oscillator designs developed for dye lasers have been successfully adapted to other laser types such as the diode laser. The physics of narrow-linewidth multiple-prism grating

Multiple-prism dispersion theory

The first description of multiple-prism arrays, and multiple-prism dispersion, was given by Newton in his book Opticks. Prism pair expanders were introduced by Brewster in 1813. A modern mathematical description of the single-prism dispersion was given by Born and Wolf in 1959...

 lasers was explained by Duarte

F. J. Duarte

F. J. Duarte is a laser physicist and author/editor of several well-known books on tunable lasers. He introduced the generalized multiple-prism dispersion theory and has discovered various multiple-prism grating oscillator laser configurations...

 and Piper.

Chemicals used

Some of the laser dyes

Laser dye

Laser dyes are very large organic molecules with molecular weights of a few hundred mu. When one of these organic molecules is dissolved in a suitable liquid solvent it can be used as gain medium in a dye laser. Laser dye solutions absorb at shorter wavelengths end emit at longer wavelengths...

 are rhodamine

Rhodamine

Rhodamine is a family of related chemical compounds, fluorone dyes. Examples are Rhodamine 6G and Rhodamine B. They are used as a dye and as a dye laser gain medium. They are often used as a tracer dye within water to determine the rate and direction of flow and transport...

, fluorescein

Fluorescein

Fluorescein is a synthetic organic compound available as a dark orange/red powder soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications....

, coumarin

Coumarin

Coumarin is a fragrant chemical compound in the benzopyrone chemical class, found in many plants, notably in high concentration in the tonka bean , vanilla grass , sweet woodruff , mullein , sweet grass , cassia cinnamon and sweet clover...

, stilbene, umbelliferone

Umbelliferone

Umbelliferone, also known as 7-hydroxycoumarin, hydrangine, skimmetine, and beta-umbelliferone, is a widespread natural product of the coumarin family. It occurs in many familiar plants from the Apiaceae family such as carrot, coriander and garden angelica, as well plants from other families such...

, tetracene

Tetracene

Tetracene, also called naphthacene, is a polycyclic aromatic hydrocarbon. It has the appearance of a pale orange powder. Tetracene is the four-ringed member of the series of acenes, the previous one being anthracene and the next one being pentacene.Tetracene is a molecular organic semiconductor,...

, malachite green

Malachite green

Malachite green is an organic compound that is used as a dyestuff and has emerged as a controversial agent in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper...

, and others. While some dyes are actually used in food coloring, most dyes are very toxic, and often carcinogenic. Many dyes, such as rhodamine 6G

Rhodamine 6G

Rhodamine 6G is a chemical compound and a dye. It is often used as a tracer dye within water to determine the rate and direction of flow and transport. Rhodamine dyes fluoresce and can thus be detected easily and inexpensively with instruments called fluorometers...

, (in its chloride form), can be very corrosive to all metals except stainless steel.

A wide variety of solvents can be used, although some dyes will dissolve better in some solvents than in others. Some of the solvents used are water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, glycol, ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

, hexane

Hexane

Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...

, cyclohexane

Cyclohexane

Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

, cyclodextrin

Cyclodextrin

Cyclodextrins are a family of compounds made up of sugar molecules bound together in a ring ....

, and many others. Solvents are often highly toxic, and can sometimes be absorbed directly through the skin, or through inhaled vapors. Many solvents are also extremely flammable.

Adamantane

Adamantane

Adamantane is a colorless, crystalline chemical compound with a camphor-like odor. With a formula C10H16, it is a cycloalkane and also the simplest diamondoid. Adamantane molecules consist of three cyclohexane rings arranged in the "armchair" configuration. It is unique in that it is both rigid...

 is added to some dyes to prolong their life.

Cycloheptatriene

Cycloheptatriene

Cycloheptatriene is an organic compound with the formula C7H8. This colourless liquid has been of recurring theoretical interest in organic chemistry. It is a ligand in organometallic chemistry and as a building block in organic synthesis...

 and cyclooctatetraene

Cyclooctatetraene

1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...

 (COT) can be added as triplet quenchers for rhodamine G, increasing the laser output power. Output power of 1.4 kilowatt at 585 nm was achieved using Rhodamine 6G with COT in methanol-water solution.

Excitation lasers

As already mentioned flashlamps, and several types of lasers, can be used to optically pump dye lasers. A partial list of excitation lasers include:

• Copper vapor laser
  Copper vapor laser
  Copper vapor laser uses vapors of copper as the lasing medium in a 3-level laser. It produces green laser light at 510.6 nm and yellow laser light at 578.2 nm. The pulse width is typically from 5 to 60 ns, and peak power from 50 to 5000 kW. Its pulse repetition frequencies can be 2 to 100 kHz...
  
  s
• Diode lasers
• Excimer laser
  Excimer laser
  An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
  
  s
• Nd:YAG laser
  Nd:YAG laser
  Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
  
  s (mainly second and third harmonics)
• Nitrogen laser
  Nitrogen laser
  A nitrogen laser is a gas laser operating in the ultraviolet range using molecular nitrogen as its gain medium, pumped by an electrical discharge....
  
  s
• Ruby laser
  Ruby laser
  A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960....
  
  s
• Argon ion lasers in the CW regime
• Krypton ion lasers
  Ion laser
  An ion laser is a gas laser which uses an ionized gas as its lasing medium.Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions...
  
   in the CW regime

Ultra-short optical pulses

R. L. Fork, B. I. Greene, and C. V. Shank demonstrated, in 1981, the generation of ultra-short laser pulse using a ring-dye laser (or dye laser exploiting colliding pulse mode-locking). Such kind of laser is capable of generating laser pulses of ~ 0.1 ps

Picosecond

A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....

 duration.

The introduction of grating techniques and intra-cavity prismatic pulse compressors

Prism compressor

A prism compressor is an optical device used to shorten the duration of a positively chirped ultrashort laser pulse by giving different wavelength components a different time delay. It typically consists of two prisms and a mirror. Figure 1 shows the construction of such a compressor...

 eventually resulted in the routine emission of femtosecond dye laser pulses.

Applications

Dye lasers are very versatile. In addition to their recognized wavelength agility these lasers can offer very large pulsed energies or very high average powers. Flashlamp-pumped dye lasers have been shown to yield hundreds of Joules per pulse and copper-laser-pumped dye lasers are known to yield average powers in the kilowatt regime.

Dye lasers are used in many applications including:

• astronomy (as laser guide star
  Laser guide star
  Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light...
  
  s),
• atomic vapor laser isotope separation
• manufacturing
• medicine
  Medicine
  Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
  
  
• spectroscopy
  Spectroscopy
  Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
  
  .

In laser medicine

Laser medicine

Laser medicine is the use of various types of lasers in medical diagnosis, treatment, or therapy. Types of lasers used in medicine include in principle any laser design, especially:* CO2 lasers* diode lasers* dye lasers* excimer lasers* fiber lasers...

 these lasers are applied in several areas, including dermatology

Dermatology

Dermatology is the branch of medicine dealing with the skin and its diseases, a unique specialty with both medical and surgical aspects. A dermatologist takes care of diseases, in the widest sense, and some cosmetic problems of the skin, scalp, hair, and nails....

 where they are used to make skin tone more even. The wide range of wavelengths possible allows very close matching to the absorption lines of certain tissues, such as melanin

Melanin

Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 or hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

, while the narrow bandwidth obtainable helps reduce the possibility of damage to the surrounding tissue. They are used to treat port-wine stain

Port-wine stain

A port-wine stain or naevus flammeus is a vascular anomaly consisting of superficial and deep dilated capillaries in the skin which produce a reddish to purplish discoloration of the skin. They are so called for their colour, resembling that of port wine...

s and other blood vessel disorders, scars and kidney stone

Kidney stone

A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...

s. They can be matched to a variety of inks for tattoo removal

Tattoo removal

Tattoo removal has been performed with various tools during the history of tattooing. While tattoos were once considered permanent, it is now possible to remove them with treatments, fully or partially...

, as well as a number of other applications.

In spectroscopy, dye lasers can be used to study the absorption and emission spectra of various materials. Their tunability, (from the near-infrared to the near-ultraviolet), narrow bandwidth, and high intensity allows a much greater diversity than other light sources. The variety of pulse widths, from ultra-short, femto-second pulses to continuous-wave operation, makes them suitable for a wide range of applications, from the study of fluorescent lifetimes and semiconductor properties to lunar laser ranging experiment

Lunar laser ranging experiment

The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...

s.