Today, lasers are key tools in manipulating and communicating information in CD and DVD players, supermarket barcode readers and broadband telecommunications , in measurement surveying and environmental studies , chemical analysis of foods, medical specimens and materials and, increasingly, in transforming materials welding, cutting and etching, printing, and surgery. Research into lasers continues apace — new types of laser are being developed with a variety of characteristics and potential applications. In some cases, the result is a cheaper, more compact portable device designed for a specific use, or a more powerful laser used to generate power, for instance.
UK university physics departments are at the forefront of many of these areas.
A laser beam has the special property that the light waves emitted are all in step with one another — coherent — and usually of one wavelength, or colour. Items related to Principles of Lasers and Optics. Publisher: Cambridge University Press , About the Author : William S. Case study: Lasers Lasers provide the archetypal example of how a discovery in basic physics led to an invention, several decades later, that was unpredictably world-changing. In preparing the second edition the hope has been that both these aims will be better served as a result of the various improvements made. Types of Lasers.
In particular, physicists in the Central Laser Facility CLF at the Rutherford Appleton Laboratory develop novel high-powered laser systems and make them available for both pure and applied research. The laser would never have been developed without a profound understanding of an area of fundamental physics — quantum theory. Einstein had previously shown that light was composed of tiny packets of wave energy called photons the wavelength depending on the energy.
He theorised that if the atoms that make up a material are given excess energy and so emit photons, these photons could stimulate nearby atoms to emit further photons, creating a cascade effect. All the photons would have the same energy and wavelength and move off in the same direction. However, it was not until 40 years later that physicists were able to convert this idea into a practical laser.
The resulting stimulated light emission is then amplified by bouncing the light back and forth through the lasing material in a mirrored cavity, so stimulating more emission, before it escapes through a transparent mirror section as a laser beam. A device that amplified microwaves was constructed in by Charles Townes and colleagues at Columbia University. Townes shared a Nobel Prize in Physics in with Nikolay Basov and Aleksandr Prochorov of the Lebedev Institute in Moscow who independently also demonstrated what came to be called a maser.
The next few years saw a race to build the first visible light laser. When an electron absorbs energy either from light photons or heat phonons , it receives that incident quantum of energy. But transitions are only allowed in between discrete energy levels such as the two shown above. This leads to emission lines and absorption lines. When an electron is excited from a lower to a higher energy level, it will not stay that way forever.
An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, that is called " spontaneous emission ". The phase associated with the photon that is emitted is random.
A material with many atoms in such an excited state may thus result in radiation which is very spectrally limited centered around one wavelength of light , but the individual photons would have no common phase relationship and would emanate in random directions. This is the mechanism of fluorescence and thermal emission. An external electromagnetic field at a frequency associated with a transition can affect the quantum mechanical state of the atom.
As the electron in the atom makes a transition between two stationary states neither of which shows a dipole field , it enters a transition state which does have a dipole field, and which acts like a small electric dipole , and this dipole oscillates at a characteristic frequency. In response to the external electric field at this frequency, the probability of the atom entering this transition state is greatly increased. Thus, the rate of transitions between two stationary states is enhanced beyond that due to spontaneous emission.
Such a transition to the higher state is called absorption , and it destroys an incident photon the photon's energy goes into powering the increased energy of the higher state. A transition from the higher to a lower energy state, however, produces an additional photon; this is the process of stimulated emission. The gain medium is put into an excited state by an external source of energy. In most lasers this medium consists of a population of atoms which have been excited into such a state by means of an outside light source, or an electrical field which supplies energy for atoms to absorb and be transformed into their excited states.
The gain medium of a laser is normally a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission described above. This material can be of any state : gas, liquid, solid, or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy " excited " quantum states.
Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser oscillator.
In a few situations it is possible to obtain lasing with only a single pass of EM radiation through the gain medium, and this produces a laser beam without any need for a resonant or reflective cavity see for example nitrogen laser.
The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser. The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting back on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption.
If the gain amplification in the medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power the net gain gain minus loss reduces to unity and the gain medium is said to be saturated. In a continuous wave CW laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser.
If the applied pump power is too small, the gain will never be sufficient to overcome the cavity losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification.
In most lasers, lasing begins with stimulated emission amplifying random spontaneously emitted photons present in the gain medium. Stimulated emission produces light that matches the input signal in wavelength, phase , and polarization. This, combined with the filtering effect of the optical resonator gives laser light its characteristic coherence, and may give it uniform polarization and monochromaticity, depending on the resonator's design. Some lasers use a separate injection seeder to start the process off with a beam that is already highly coherent. This can produce beams with a narrower spectrum than would otherwise be possible.
Many lasers produce a beam that can be approximated as a Gaussian beam ; such beams have the minimum divergence possible for a given beam diameter. Some lasers, particularly high-power ones, produce multimode beams, with the transverse modes often approximated using Hermite — Gaussian or Laguerre -Gaussian functions.
Some high power lasers use a flat-topped profile known as a " tophat beam ". Unstable laser resonators not used in most lasers produce fractal-shaped beams. Near the "waist" or focal region of a laser beam, it is highly collimated : the wavefronts are planar, normal to the direction of propagation, with no beam divergence at that point.
However, due to diffraction , that can only remain true well within the Rayleigh range. The beam of a single transverse mode gaussian beam laser eventually diverges at an angle which varies inversely with the beam diameter, as required by diffraction theory. Thus, the "pencil beam" directly generated by a common helium—neon laser would spread out to a size of perhaps kilometers when shone on the Moon from the distance of the earth. However even such a divergent beam can be transformed into a similarly collimated beam by means of a lens system, as is always included, for instance, in a laser pointer whose light originates from a laser diode.
That is possible due to the light being of a single spatial mode. This unique property of laser light, spatial coherence , cannot be replicated using standard light sources except by discarding most of the light as can be appreciated by comparing the beam from a flashlight torch or spotlight to that of almost any laser. A laser beam profiler is used to measure the intensity profile, width, and divergence of laser beams. Diffuse reflection of a laser beam from a matte surface produces a speckle pattern with interesting properties.
The mechanism of producing radiation in a laser relies on stimulated emission , where energy is extracted from a transition in an atom or molecule. This is a quantum phenomenon discovered by Einstein who derived the relationship between the A coefficient describing spontaneous emission and the B coefficient which applies to absorption and stimulated emission. However, in the case of the free electron laser , atomic energy levels are not involved; it appears that the operation of this rather exotic device can be explained without reference to quantum mechanics.
A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course even a laser whose output is normally continuous can be intentionally turned on and off at some rate in order to create pulses of light.
When the modulation rate is on time scales much slower than the cavity lifetime and the time period over which energy can be stored in the lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall in that category. Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as continuous wave CW. Many types of lasers can be made to operate in continuous wave mode to satisfy such an application.
Many of these lasers actually lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will, in fact, produce amplitude variations on time scales shorter than the round-trip time the reciprocal of the frequency spacing between modes , typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous wave" as their output power is steady when averaged over any longer time periods, with the very high-frequency power variations having little or no impact in the intended application.
However, the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time. For continuous wave operation, it is required for the population inversion of the gain medium to be continually replenished by a steady pump source. In some lasing media, this is impossible.
In some other lasers, it would require pumping the laser at a very high continuous power level which would be impractical or destroy the laser by producing excessive heat. Such lasers cannot be run in CW mode. Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations.
Some lasers are pulsed simply because they cannot be run in continuous mode. In other cases, the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up in between pulses. In laser ablation , for example, a small volume of material at the surface of a work piece can be evaporated if it is heated in a very short time, while supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point.
Other applications rely on the peak pulse power rather than the energy in the pulse , especially in order to obtain nonlinear optical effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching. The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. In a Q-switched laser, the population inversion is allowed to build up by introducing loss inside the resonator which exceeds the gain of the medium; this can also be described as a reduction of the quality factor or 'Q' of the cavity.
Then, after the pump energy stored in the laser medium has approached the maximum possible level, the introduced loss mechanism often an electro- or acousto-optical element is rapidly removed or that occurs by itself in a passive device , allowing lasing to begin which rapidly obtains the stored energy in the gain medium. This results in a short pulse incorporating that energy, and thus a high peak power. A mode-locked laser is capable of emitting extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds.
These pulses will repeat at the round trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the Fourier limit also known as energy-time uncertainty , a pulse of such short temporal length has a spectrum spread over a considerable bandwidth.
Thus such a gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is titanium -doped, artificially grown sapphire Ti:sapphire which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration. Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales known as femtosecond physics, femtosecond chemistry and ultrafast science , for maximizing the effect of nonlinearity in optical materials e.
Due to the large peak power and the ability to generate phase-stabilized trains of ultrafast laser pulses, mode-locking ultrafast lasers underpin precision metrology and spectroscopy applications. Another method of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flash lamps, or another laser which is already pulsed.
Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high energy, fast pump was needed.
The way to overcome this problem was to charge up large capacitors which are then switched to discharge through flashlamps, producing an intense flash. Pulsed pumping is also required for three-level lasers in which the lower energy level rapidly becomes highly populated preventing further lasing until those atoms relax to the ground state. These lasers, such as the excimer laser and the copper vapor laser, can never be operated in CW mode.
In , Albert Einstein established the theoretical foundations for the laser and the maser in the paper Zur Quantentheorie der Strahlung On the Quantum Theory of Radiation via a re-derivation of Max Planck 's law of radiation, conceptually based upon probability coefficients Einstein coefficients for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. Ladenburg confirmed the existence of the phenomena of stimulated emission and negative absorption.
Fabrikant predicted the use of stimulated emission to amplify "short" waves. Lamb and R. Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave radiation rather than infrared or visible radiation.
Townes's maser was incapable of continuous output. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion. In , Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, later a main method of laser pumping.
Townes reports that several eminent physicists—among them Niels Bohr , John von Neumann , and Llewellyn Thomas —argued the maser violated Heisenberg's uncertainty principle and hence could not work. Others such as Isidor Rabi and Polykarp Kusch expected that it would be impractical and not worth the effort. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics , "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser—laser principle".
As ideas developed, they abandoned infrared radiation to instead concentrate upon visible light. The concept originally was called an "optical maser".
In , Bell Labs filed a patent application for their proposed optical maser; and Schawlow and Townes submitted a manuscript of their theoretical calculations to the Physical Review , published that year in Volume , Issue No. Simultaneously, at Columbia University , graduate student Gordon Gould was working on a doctoral thesis about the energy levels of excited thallium. When Gould and Townes met, they spoke of radiation emission , as a general subject; afterwards, in November , Gould noted his ideas for a "laser", including using an open resonator later an essential laser-device component.
Moreover, in , Prokhorov independently proposed using an open resonator, the first published appearance in the USSR of this idea.
Elsewhere, in the U. Gould's notes included possible applications for a laser, such as spectrometry , interferometry , radar , and nuclear fusion. He continued developing the idea, and filed a patent application in April The U. Patent Office denied his application, and awarded a patent to Bell Labs , in That provoked a twenty-eight-year lawsuit , featuring scientific prestige and money as the stakes. Gould won his first minor patent in , yet it was not until that he won the first significant patent lawsuit victory, when a Federal judge ordered the U. Patent Office to issue patents to Gould for the optically pumped and the gas discharge laser devices.
The question of just how to assign credit for inventing the laser remains unresolved by historians. On May 16, , Theodore H. Maiman's functional laser used a flashlamp -pumped synthetic ruby crystal to produce red laser light at nanometers wavelength. The device was only capable of pulsed operation, due to its three-level pumping design scheme. Bennett , and Donald Herriott , constructed the first gas laser , using helium and neon that was capable of continuous operation in the infrared U.
Patent 3,, ; later, Javan received the Albert Einstein Award in Basov and Javan proposed the semiconductor laser diode concept. In , Robert N. Later that year, Nick Holonyak , Jr.
Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including:. The device has potential for applications in quantum computing. Following the invention of the HeNe gas laser, many other gas discharges have been found to amplify light coherently. Gas lasers using many different gases have been built and used for many purposes. Commercial carbon dioxide CO 2 lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot.
This emission is in the thermal infrared at A nitrogen transverse electrical discharge in gas at atmospheric pressure TEA laser is an inexpensive gas laser, often home-built by hobbyists, which produces rather incoherent UV light at Like all low-pressure gas lasers, the gain media of these lasers have quite narrow oscillation linewidths , less than 3 GHz 0. Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications.
Excimer lasers are a special sort of gas laser powered by an electric discharge in which the lasing medium is an excimer , or more precisely an exciplex in existing designs. These are molecules which can only exist with one atom in an excited electronic state. Once the molecule transfers its excitation energy to a photon, its atoms are no longer bound to each other and the molecule disintegrates. This drastically reduces the population of the lower energy state thus greatly facilitating a population inversion. Excimers currently used are all noble gas compounds ; noble gasses are chemically inert and can only form compounds while in an excited state.
Excimer lasers typically operate at ultraviolet wavelengths with major applications including semiconductor photolithography and LASIK eye surgery. Solid-state lasers use a crystalline or glass rod which is "doped" with ions that provide the required energy states. For example, the first working laser was a ruby laser , made from ruby chromium -doped corundum. The population inversion is actually maintained in the dopant. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flashtube or from another laser.
The usage of the term "solid-state" in laser physics is narrower than in typical use. Semiconductor lasers laser diodes are typically not referred to as solid-state lasers. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. However, in thermal equilibrium, stimulated emission does not account to a significant extent. The reason is there are far more electrons in the ground state than in the excited states.
And the rates of absorption and emission is proportional the number of electrons in ground state and excited states, respectively. If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity. And this is called population inversion. But this process cannot be achieved by only two states, because the electrons will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission.
Then external energy is provided to excite them to level 3, referred as pumping. The source of pumping energy varies with different laser medium, such as electrical discharge and chemical reaction, etc. In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2, transferring the energy to the phonons of the lattice of the host material.
Then electrons on level 2 will decay by spontaneous emission to level 1, labeled as L, meaning laser. If the life time of L is much longer than that of R, the population of the E 3 will be essentially zero and a population of excited state atoms will accumulate in level 2. When level 2 hosts over half of the total electrons, a population inversion be achieved. Because half of the electrons must be excited, the pump system need to be very strong.
This makes three-level lasers rather inefficient. Most of the present lasers are 4-level lasers, see Fig. Laser transition takes place between level 3 and 2, so the population is easily inverted. In semiconductor lasers, where there are no discrete energy levels, a pump beam with energy slightly above the band gap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band.
At the same time, the holes generated in the valence band move to the top of the valence band. Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously and do not contribute to the overall output . Then the resonator is applied to make a positive feedback mechanism. An optical resonator usually has two flat or concave mirrors, one on either end, that reflect lasing photons back and forth so that stimulated emission continues to build up more and more laser light.
More importantly, there may be many laser transitions contribute in the laser, because of the band in solids or molecule energy levels of organics. Optical resonator also has a function of wavelength selector. It just make a standing wave condition for the photons:.
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Only wavelengths satisfying eq 2 will get resonated and amplified. The output of a laser may be a continuous constant-amplitude output known as CW or continuous wave ; or pulsed, by using the techniques of Q-switching, model-locking, or gain-switching. In many applications of pulsed lasers, one aims to deposit as much energy as possible at a given place in as short time as possible. Some dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds 10 s.
According to the gain material, lasers can be divided into the following types. Several common used lasers are listed in each type. In Albert Einstein first raised the concepts of probability coefficients later to be termed 'Einstein coefficients' for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. It was confirmed in s.