High-power Lasers

By Todd Ditmire

The innovation of the laser 50 years back has caused the recent generation that devices, through power bursts hundreds of times the of the nation’s entire electric grid

Physics technology

Lasers are exceptional devices. Their ability to develop a sharp, bright beam of light of a well-defined shade is something that amazes students to this day. It can be said that, ~ the transistor, the laser has actually been the most significant technological invention because World battle II.

Figure 1. Over their 50-year history, lasers have gone from developing powers that a few hundred watt to greater than a petawatt, or a quadrillion watts. Several an essential technological jumps have permitted researchers come compress laser beams into infinitesimally quick pulses, which amplifies their peak power. The Texas Petawatt Laser, presented here, produces pulses of around 100 femtoseconds (10-13 seconds) in duration. We may soon have the ability to make lasers v a top power of an exawatt (1,000 petawatts, or one quintillion watts) or more. (Photograph courtesy that the author.)

Photograph courtesy that Todd Ditmire.

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Lasers now uncover widespread applications in an extremely diverse variety of duties, including reading compact discs, printing paper, scanning bar codes, machining and welding, precision surgery, finding distances, permitting high-speed communications, guiding precision munitions and also driving regulated nuclear fusion. This year point out the 50th anniversary the the demonstration of the first laser, a feat perform by Theodore Maiman in his laboratory at Hughes Aircraft firm in 1960. The 50-year history of laser development since this an initial demonstration has actually had many twists, including the 30-year patent battle that surrounding the laser’s invention. However one the the most fascinating subplots in the story the laser advancement since 1960 has actually been the astonishing rise in the power that lasers can deliver.

One often thinks the lasers as transferring a constant beam of light in one color that is fine collimated (meaning the photons in the beam remain parallel and don’t diverge from each other). This is certainly the case for the miniature lasers that read compact discs or the barcode scanners in supermarkets, however there is a huge class of lasers that provide light in short pulses. Due to the fact that power is defined as energy ceded per unit that time, once the energy emerging from the laser in a pulse of light increases and the time expression of the pulse decreases, the instantaneous power throughout the pulse—described an ext accurately together “peak power”— increases. Pulsed lasers uncover application in lofty tasks, such as the production of controlled nuclear combination and laser eye surgery, under to the mundane, such as cosmetic hair removal. In fact, the high powers feasible with pulsed-laser light were noticeable from the very earliest work of laser development. Also Maiman’s very first laser was pulsed and also likely had actually a top power that a couple of hundred watts (about the same power offered by an average toaster stove today).

The huge increase in lasers’ height power v the previous 50 years has actually been mind boggling. The top power attainable in a laser pulse has actually increased by roughly a variable of 1,000 every 10 years. This power boost is much like the increase in the thickness of solid-state electronics defined by the renowned Moore’s Law—the variety of transistors in an integrated circuit doubles about every two years. Yet unlike Moore’s Law, which has actually resulted from continuous incremental enhancements in chip-manufacturing technology, the increase in lasers’ top power has actually been the result of a couple of major breakthroughs in laser technology.

The manufacturing of high power is a natural an effect of the physics behind the operation of a laser. Maiman’s first laser already delivered quite high strength by the standards of a typical consistent light source, particularly one carrying light of just one well-defined shade (in that case, at 694 nanometers, a deep red color). The ability to create high powers with lasers stems native the quantum mechanics that enable their operation.

A laser is largely an amplifier, as the “a” in laser signifies: irradiate Amplification by created Emission the Radiation. Together the latter component of the acronym implies, it functions by pumping energy into electrons of atoms in some substance, called the gain material. this atoms can be assembled in a number of forms, and also many different media suitable for lasers have been developed.

The active atoms or molecules in laser media deserve to be in gaseous form, such as the neon atom in the common helium-neon laser recognized well to students in activities classes, or the carbon-dioxide molecule in plenty of welding lasers. Lock can likewise be semiconductor materials, such together the gallium arsenide used in the diode (or solid-state) lasers of CD players. Or they deserve to be installed in crystals, such together the chromium ion in ruby, together Maiman offered in his very first laser.

In all forms, electron in the atom of the laser tool must it is in excited v energy. This feat is completed in numerous ways, such as by producing a high-voltage discharge in gases the the helium-neon laser, causing existing to circulation through the semiconductor that a diode laser, or shining light on crystals such as ruby through bright electric-arc lamps. It was this last approach that Maiman used in his first laser, where he surrounded a rod of ruby through a cut flashlamp.

When the electron in a laser material’s atoms room excited, they can only acquire discrete energies, landing in certain quantum-energy levels. When the electron are propelled to a greater quantum level, they have the right to retain that power for a period of time, dubbed the “lifetime” of the level. When light at the appropriate wavelength passes near these atoms within their lifetime window, the energy stored through the excited electron is extract by stimulating the electron come drop back to a lower-energy quantum state. Therefore the electron offers its energy to the passing light photons and effectively amplifies the passing light beam (see figure 4). One deserve to think about the laser as an energy-storage device, which releases formerly stored energy into a beam or pulse of photons. If conditions are collection up correctly, energy can it is in pumped into the electrons of the laser’s atoms over a long duration of time (roughly the life time of the laser material’s excited-energy state), and then it can be released quickly, causing the manufacturing of a pulse of light with high top power.

To accomplish this storage, the excited laser-gain product is placed in between two mirrors—a construct recognized as an optical laser cavity—so that light circulates back and forth v the laser-gain medium, extracting the power stored by electrons pumped into the top quantum state. Through coating the front encounters of the laser-gain stick so that they reflected irradiate (with a small amount allowed to leak the end on one side to deliver the laser output), Maiman produced a irradiate pulse indigenous his very first ruby laser v a duration comparable to the of his flashlamp light, about three milliseconds.

Shortly ~ the an initial laser demonstration, researcher realized that much shorter bursts could be created from such a pulsed laser. If the energy stored in the top quantum state the the laser could be extracted not over the expression of the pumping, yet instead the extraction can be stopped for a while, stored energy will naturally construct up in the laser medium. Climate if a suitably rapid switch were placed inside the cavity, all of this save on computer energy can be swiftly released in one irradiate pulse. This idea was gone after within month of Maiman’s first demonstration through his colleagues, Fred J. McClung and Robert W. Hellwarth. They inserted the ruby acquire medium between two mirrors, and also by temporarily interrupting the optical path between the two mirrors, energy could be pumped right into the ruby and stored v increasing thickness by the electron in top quantum states. Then, as soon as the interruption was promptly removed, photons could circulate and quickly extract energy from the gain medium in a laser pulse.

Instead that light power coming out of the laser over the time duration of the pumping, the energy was extract on the time scale it took photons to circulate earlier and forth a couple of times in the now-unobstructed path between the two mirrors. Since the mirror spacing in together a laser is, say, 10 come 30 centimeters—corresponding to a transit time that one or two nanoseconds in ~ the rate of light—the light pulse extracts the stored energy on this time scale, 4 to five orders the magnitude much faster than the time over i m sorry the electrons are pumped.

In the parlance of electric engineering, one can describe this procedure by saying that the quality factor of the optical cavity formed by the 2 mirrors, the “Q-factor,” is initially low. Once the obstruction is removed and a clear optical course is formed, the Q-factor is easily increased; hence the laser is “Q-switched,” permitting electromagnetic energy of the laser pulse come circulate with low losses. (The online version of this short article contains a supplementary figure on Q-switching. See the box at the finish of web page 401 for a Web link to this issue.) The very first Q-switched laser at Hughes to be demonstrated in 1961. This laser delivered pulses of only 100 nanoseconds in duration, yielding a optimal power close to 1 megawatt, a run in strength by much more than a factor of 1,000 over Maiman’s original device. Q-switching has due to the fact that been enforced in many kinds the lasers, with particularly wide applications in ones using crystals made of neodymium doped (or inserted) right into yttrium- aluminum garnet (abbreviated Nd:YAG), and Nd doped into optical glass. A modern, advertising Q-switched Nd:YAG laser can deliver pulses of energy of approximately 1 joule v a term of 1 to 10 nanoseconds, yielding top power draw close a gigawatt (109 watts).

To push this modern technology to greater pulse energy and, hence, higher power, one can naturally think about amplifying a nanosecond Q-switched laser pulse further by passing it through added laser-gain media. This build is recognized as the “master oscillator, strength amplifier,” or MOPA, architecture. It has been provided for many years in advertisement Q-switched laser solution to yield laser pulses with countless joules the energy. Quickly such laser systems began to take it up rooms and also whole buildings.

In the at an early stage 1970s, the Lawrence Livermore national Laboratory (LLNL) in California started pursuing advance of a collection of progressively larger lasers using the MOPA architecture, each an ext powerful than its predecessor. This development was motivated by the possibility of utilizing high-energy laser pulses (of numerous tens or numerous kilojoules) to implode capsules and drive nuclear blend in the laboratory (a technique known together inertial confinement fusion, or ICF). Due to the fact that of the high energies produced, these lasers compelled increasing the aperture of the laser amplifier to larger sizes and also the building of equipment with multiple MOPA beams. Every one of these solution utilized Nd-doped glass together the amplifier material because it was possible to fabricate glass into large apertures, increase to many tens the centimeters.

LLNL’s ICF laser breakthrough spanned an ext than 40 years: The 100-joule Cyclops laser was built in 1974, adhered to by the two-beam Argus laser in 1976. The 20-beam, 10-kilojoule Shiva laser was created in 1977, representing the an initial large-facility-scale, high-power laser. Shiva was adhered to by the 100-kilojoule Nova laser in 1985. This line of advance has culminated in current years in ~ LLNL with the show in 2008 of the nationwide Ignition basic (NIF), a laser that delivers nanosecond pulses through energy linked from 192 separate beams that philosophies 4 million joules—roughly a exchange rate times an ext energetic and powerful than that very first Q-switched laser.

Gas lasers have also been emerged in a MOPA design for high-energy applications, including, because that example, the building and construction of carbon-dioxide lasers in ~ Los Alamos national Laboratory in the 1970s and 1980s, and also the advancement at the Naval study Laboratory of kilojoule- energy nanosecond lasers such together the Nike laser, which operates in the ultraviolet variety using a gas the krypton fluoride. For high-energy nanosecond-laser amplification, however, Nd-doped glass has actually reigned as the king.

The story of increasing laser strength did not end with the advance of the Q-switched laser and also the MOPA architecture. The second major breakthrough in the breakthrough of high-peak-power lasers emerged essentially in parallel v the development of Q-switching. This vital development, known as setting locking, was first demonstrated in the years instantly following the very first laser. Setting locking reinvented pulsed-laser technology, permitting the manufacturing of laser pulses that space orders the magnitude much shorter than pulses produced by Q-switching. Mode-locked lasers productivity pulses v time durations shorter than one picosecond under to only a few femtoseconds (10-15 seconds) in part cases.

This an approach has fostered an entire subfield in optics dedicated to the usage of this ultrafast pulses to research physics and also chemistry top top timescales that were previous inaccessible. Ultrafast science is another of the interesting subplots in the story the the laser; it has led to remarkable insights into the way atoms move and molecules react, and work in this area got the Nobel prize in Chemistry in 1999. Ultrafast laser-pulse generation by setting locking has additionally played a main role in the march to greater laser powers.

Figure 4. In ~ the atomic level, a laser works by pumping power into electron bound come an atom, i m sorry pushes the electrons to a higher quantum power level (a). The electrons stay in the upper level because that a period of time, properly storing the pumped energy. The is extract by the i of a irradiate photon, i beg your pardon stimulates the transition of the electron back down come the lower power level (b). During this transition, the electron emits one more photon, including to the power of the passing light pulse.

Illustration through Barbara Aulicino.

The Heisenberg skepticism principle (which states that particular pairs of physical properties cannot simultaneously be known) uses to lasers and their electromagnetic waves. The principle mandates that, in bespeak to generate light pulses with quick durations, a wide bandwidth that frequencies should be current in the electromagnetic wave. One generally thinks of usual lasers as having actually only one well-defined frequency, yielding the clear shade we observe in visible lasers, such as the diode lasers provided in laser needle or argon lasers offered in laser-light shows.

However, atom in part laser media have the right to amplify a fairly broad selection of frequencies in a laser cavity. This range varies with the atom or molecule and the environment in which that is placed. Take into consideration a laser cavity through two the opposite mirrors,. Countless different irradiate wavelengths have the right to fit right into the an are between the mirrors, the only condition being the the wavelengths every must have an integral number of oscillations in the space between this mirrors. Each such electromagnetic wave is called a longitudinal “mode.”

When a pumped laser product is inserted in this cavity, any kind of modes with wavelength that fall into the range of frequencies that the material have the right to amplify deserve to thrive, and the device is said to lase “multimode.” For part laser-gain materials, this range of magnified modes deserve to be really large, involving tens of thousands of modes. This an outcome alone go not cause a short pulse the light; it merely results in a consistent beam of laser light v a broad spectrum (and random spikes of energy in time, well-known as “mode-beating”).

The key to harnessing all of this laser-light bandwidth is to force all of these modes to oscillate through one well-defined phase; in various other words, to cause every one of the settings to in march in lock step, so the peaks and also troughs in the electromagnetic tide all have actually a fixed connection to every other. Once this is done, the modes are said to be locked. When each setting is summed v all the other modes, the sinusoidal electromagnetic waves add up to productivity a short pulse in time, with duration around equal to the station of the frequency selection of the modes that are amplified in the cavity. The observable repercussion of this mode locking is the formation of a quick light pulse the travels back and forth in the laser cavity, with a small amount that the light leaking the end of one winter on every round trip. A pulse much much shorter in time than the round-trip time that the laser cavity can be formed.

Research over the years has actually led come a plethora of approaches to accomplish this setting locking, many of which autumn into among two categories, energetic and passive setting locking. Energetic mode locking, which to be demonstrated first, involves placing one optical product in the cavity that has its energy losses modulated in time by the applications of an electric signal, as is done in countless Q-switched lasers. The optical losses induced by this modulator space varied at a repetition rate that matches the price at which a mode-locked laser pulse circulates in the cavity. This pressures the laser settings to lock so that only a quick pulse is current to pass though the short temporal window of the modulator an equipment when its ns is low.

This active approach to mode locking depends on the residential property of a mode-locked laser to produce a pulse short in time; the other classification of mode-locking relies on the fact that the top intensity of the irradiate in the cavity is much higher when mode-locked than as soon as the cavity lases continuously. This passive mode-locking technique involves inserting a transmissive element in the cavity that has a loss the varies not in time but with light intensity. If this aspect absorbs light once the soot is low, but its absorb drops when the soot is high (or the absorb “saturates” with high intensity), the laser will certainly be forced to fire in a way that minimizes losses—which occurs, again, once the modes are locked. Back the energetic approach was the an initial utilized, that is the 2nd approach that has led come the generation that the shortest pulses and also has offered as the seed for the highest-power modern lasers.

Remarkably, the an initial mode-locked laser was constructed in 1964, only four years after ~ the first laser. This demonstration developed at the Bell phone call laboratories and also involved setting locking a helium-neon laser, developing pulses of around one nanosecond in duration. Mode-locking modern technology then progressed rapidly. Mode-locked lasers using lamp-pumped, solid-state gain materials created pulses that under 100 picoseconds through the so late 1960s, adhered to by a additional shortening in pulse duration in 1972 as soon as the first continuous-wave, mode-locked dye laser to be demonstrated through Erich P. Ippen, Charles V. Shank and also Andrew Dienes. Laser dyes, i beg your pardon are typically dissolved in liquid and also flowed through a container put in a laser cavity, were emerged initially in the late 1960s as obtain media that allowed a laser to it is in tuned end a range of frequencies. It to be this very broad-gain bandwidth that enabled dye lasers to generate pulses that broke through the 1-picosecond barrier, propelling ultrafast laser science right into the femtosecond routine in the early 1980s. Indeed, in 1987, Shank and his coworkers prove the manufacturing of laser pulses as brief as 6 femtoseconds indigenous a mode-locked dye laser, a document which stood because that close to 15 years.

Another breakthrough in mode-locked modern technology has made possible the very highest-power lasers. This breakthrough occurred in 1982 through Peter Moulton’s invention of titanium-doped sapphire crystals (Ti:sapphire) as a solid-state laser material. Ti:sapphire’s amazing properties make it the laser product of selection for ultrafast lasers to this day. It has an extremely broad gain bandwidth, adequate for pulses as brief as 4 or 5 femtoseconds. And also it has properties far superior to dyes: it is straightforward to work with together a solid-state crystal and also can it is in fabricated with excellent optical quality. Mode-locked pulses shorter than 20 femtoseconds can be acquired easily in a Ti:sapphire laser making use of a passive mode-locking technique, known as Kerr lens mode-locking, which to be discovered, practically by accident, in 1990 in the lab of Wilson Sibbett at the college of St. Andrews in Scotland. Due to the fact that 1982 there have been a number of other crystals uncovered (both natural and human-made) the have similar suitable properties. Mode-locked Ti:sapphire lasers, however, stay the standard device for the ultrafast laser scientist.

To get more power out of a beam, one technique is to construct up an ext energy in a laser’s acquire medium within a cavity, then extract it every at once. Initially, the high quality factor, or “Q-factor,” the the optical cavity is low, but when the cavity is opened and also a clear path is formed, the Q-factor rises quickly, and also the laser is stated to it is in Q-switched. In Q-switching, one optical shutter temporarily halts lasing if flashlamps pump energy into the laser obtain medium (top). when the shutter is removed, irradiate is suddenly allowed to circulate in the cavity. The floodgates the laser energy are opened, resulting in a light pulse that has most that the energy stored in the gain medium and which lasts just a few nanoseconds (middle). A photo shows the inside of a modern, gigawatt-power Q-switched laser (bottom).

Illustration by Barbara Aulicino. Picture courtesy of Continuum, Inc., photography by Evan Winslow Smith

The influence that these ultrafast laser sources can have ~ above the breakthrough of compact, high-peak-power lasers to be realized quickly after the show of mode-locking. Since peak power in a pulse the given power is raised by decreasing the pulse duration, amplifying additional the short pulses produced by a mode-locked laser was a course to do compact, higher-power lasers. The obvious approach here is come send a short pulse native a mode-locked laser right into a MOPA amplifier chain. It was this strategy that led to the very first trillion-watt (1 terawatt) laser, which occurred on the Janus laser in 1975 at LLNL. Mode-locked pulses of approximately 100 picoseconds in term were intensified to an energy of around 100 joules in a Nd:glass amplifier chain. Even much shorter pulse-amplified lasers were constructed employing laser dyes to further amplify the femtosecond pulses from a mode-locked dye laser. This technique led to the demonstration of tabletop-scale lasers in the early 1980s, with peak power of greater than a gigawatt and also pulse duration of under 1 picosecond.

Pushing this method to greater power levels ultimately has a limitation. For a offered beam aperture, the height intensity the the laser pulse grows as it passes v the amplifiers. When this intensity gets high enough, a variety of nonlinear effects transform the pulse in means that give high-power laser developers nightmares. The biggest problem comes from the reality that the refractive table of contents of any type of material (the price at which the bends light), including those supplied to amplify a laser pulse, increases very slightly as the soot of light is amplified. Because that light strongness encountered in most day-to-day situations, this sports of refractive index is fully ignorable. But as one amplifies quick laser pulses to high energy, this phenomenon can lead come catastrophic consequences.

Figure 7. Chirped-pulse amplification starts with a quick laser pulse native a mode-locked laser (1) and climate stretches it in time, in order to lower its intensity and also make it suitable for amplification come high energy. The color of the brief pulse are first spread out utilizing diffraction gratings (2), leading to a pulse with a move of frequencies end time. Once the pulse is safely magnified (3), this frequency sweep is reversed (4), compressing the pulse earlier to a duration near its original width (5).

Illustration by Barbara Aulicino.

Any real beam that light will certainly inevitably have little modulations of strongness on that is spatial profile, the natural an outcome of imperfections in actual optics. Due to the fact that the refractive index boosts with intensity, the center and also most intense parts of these little beam modulations will see a slightly higher refractive index, which properly slows under the step of the electromagnetic wave there. This leads to a curved, concave wavefront roughly this intensity ripple, which reasons focusing. As this now-focusing ripple propagates, its intensity further increases, and also the facility of the modulation is retarded even an ext by the an altering refractive index. This process can proceed until this ripples focus to small spots and also reach an soot high enough that the optics in the laser chain room damaged.

This so-called “small-scale self- focusing” have the right to lead to spectacular fail of laser optics, as an extreme beam will certainly break apart right into a shotgun blast of ripples, drilling numerous tiny holes in the expensive optics the a high-power laser. Till the mid-1980s it to be assumed the the only means to loss this problem and push to higher laser power was to make the beam aperture larger and also larger, minimal ultimately by the expense and an innovation of very large optics.

An elegant equipment to circumvent this trouble was designed in 1985. Functioning at the time at the college of Rochester, Gerard Mourou, aided by his graduate college student Donna Strickland, prove a systems to the nonlinear damage problem the has because revolutionized high-peak-power, ultrafast lasers. Their systems took benefit of the fact that one ultrafast laser pulse is composed of a range of wavelengths. They introduced a sequence of optics, such as prisms or diffraction gratings that spread the colors of the pulse spatially, simply as raindrops execute to the irradiate from the Sun once making rainbows. These optical devices make it feasible to manipulate this colors in time, basically by making the optical device so the each color travels a different path length. This returns a pulse the is spread out with time with a duration much longer 보다 the early ultrashort laser pulse, and also with the frequencies that the pulse distributed in time, just as the increase in ton of one audible “chirp” indigenous a bird come at our ear as a sweep in audio frequency.

Mourou’s beautiful idea to be to perform such optical chirping on one ultrafast laser pulse before it was amplified, thereby making it lot longer, numerous hundreds the picoseconds, and low enough in intensity that it is safe to amplify without the require for exorbitantly big optics. ~ amplification, the spread out of colors over time is simply reversed, utilizing the exact same devices, currently arranged so the the colors the were initially delayed arrive back at the same time as the early on colors. This procedure recompresses the pulse to practically the same brief duration as the early pulse indigenous the mode-locked laser, however with the high power that a MOPA chain deserve to supply.

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Using this “Chirped-Pulse Amplification,” or CPA, technique, Strickland and also Mourou demonstrated a compact, table laser mechanism with pulses the a picosecond in term with enhanced power of almost 100 gigawatts, a power completed at that time just by large-aperture, national-laboratory-scale lasers. This demonstration caused an explosive expansion in the height power easily accessible from lasers in the laboratory and also an remarkable proliferation the high-power lasers into research labs approximately the world. Within three years after ~ that very first demonstration that CPA, Mourou and also his partner demonstrated a compact laser delivering better than 1 terawatt of peak power, a mechanism Mourou coined the “Table height Terawatt” or “T-cubed.”

CPA an innovation was quickly implemented right into a collection of higher-power lasers roughly the world, including variety of lasers with 10s of terawatts of strength at huge labs such together LLNL, the Commissariat à l’Énergie Atomique activities at Limeil in France and also the Institute because that Laser design at Osaka in Japan. The scientific drive for building these high-power CPA lasers has been a desire to research the physics of light interactions with matter at greater and higher intensity. The intensity that can be developed with modern-day multiterawatt CPA lasers is remarkable: When concentrated to a spot of a few micrometers, the strongness of the laser pulse exceeds the intensity of sunlight on the earth by 18 or 19 order of magnitude. What wake up to issue in light of this soot is a major scientific question. For example, just how this extreme light ionizes atom has become a object of active research, and also much understanding about “strong field” physics has actually been gained with the advent of these lasers.

Also exciting are the plasmas (clouds the ionized gas) at an extremely high temperature that can be developed when together an extreme laser pulse is concentrated into a gas or onto a solid. Plasmas with temperatures of millions of degrees Celsius have the right to be created. This temperatures deserve to potentially be used to spark regulated nuclear blend in the lab, or can recreate problems that are normally discovered only in the depths of enigmatic astrophysical objects such together brown dwarfs or supernovae.

Multiterawatt, ultrashort-pulse lasers were at first all built of amplifiers utilizing Nd:glass. But because Ti:sapphire deserve to amplify light in a really broad tape of frequencies, the was shortly exploited to develop CPA lasers, which could amplify pulses that room then recompressed to durations as brief as 30 femtoseconds. Due to the fact that the first implementation of Ti:sapphire in CPA lasers in ~ laboratories in ~ the college of Michigan and Stanford University approximately 1990, Ti:sapphire has become by much the most prevalent material for high-peak-power CPA lasers. Lock are currently used global for research not only in strong-field atomic physics and also high-intensity plasma physics, but also in ultrafast chemistry and also a hold of applications as diverse as organic imaging and precision machining.