why laser light travel long distance

What Is a Laser?

The letters in the word laser stand for L ight A mplification by S timulated E mission of R adiation. A laser is an unusual light source. It is quite different from a light bulb or a flash light. Lasers produce a very narrow beam of light. This type of light is useful for lots of technologies and instruments—even some that you might use at home!

How does a laser work?

Light travels in waves, and the distance between the peaks of a wave is called the wavelength .

Each color of light has a different wavelength. For example, blue light has a shorter wavelength than red light. Sunlight—and the typical light from a lightbulb—is made up of light with many different wavelengths. Our eyes see this mixture of wavelengths as white light.

This animation shows a representation of the different wavelengths present in sunlight. When all of the different wavelengths (colors) come together, you get white light. Image credit: NASA

A laser is different. Lasers do not occur in nature. However, we have figured ways to artificially create this special type of light. Lasers produce a narrow beam of light in which all of the light waves have very similar wavelengths. The laser’s light waves travel together with their peaks all lined up, or in phase . This is why laser beams are very narrow, very bright, and can be focused into a very tiny spot.

This animation is a representation of in phase laser light waves. Image credit: NASA

Because laser light stays focused and does not spread out much (like a flashlight would), laser beams can travel very long distances. They can also concentrate a lot of energy on a very small area.

This animation shows how a laser can focus all of its light into one small point. Credit: NASA

Lasers have many uses. They are used in precision tools and can cut through diamonds or thick metal. They can also be designed to help in delicate surgeries. Lasers are used for recording and retrieving information. They are used in communications and in carrying TV and internet signals. We also find them in laser printers, bar code scanners, and DVD players. They also help to make parts for computers and other electronics.

Lasers are also used in instruments called spectrometers. Spectrometers can help scientists figure out what things are made of. For example, the Curiosity rover uses a laser spectrometer to see what kinds of chemicals are in certain rocks on Mars.

This is a picture of Martian soil before (left) and after (right) it was zapped by the Curiosity rover’s laser instrument called ChemCam. By zapping tiny holes in Martian soil and rock, ChemCam can determine what the material is made of. Image credit: NASA/JPL-Caltech/LANL/ CNES/IRAP/LPGN/CNRS

NASA missions have used lasers to study the gases in Earth’s atmosphere. Lasers have also been used in instruments that map the surfaces of planets, moons, and asteroids.

Scientists have even measured the distance between the moon and Earth using lasers! By measuring the amount of time it takes for a laser beam to travel to the moon and back, astronomers can tell exactly how far away it is!

More about technology!

Learn about the Mars Rovers!

How does GPS work?

Where do satellites go when they die?

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October 8, 2008

15 min read

Rulers of Light: Using Lasers to Measure Distance and Time

A revolutionary kind of laser light called an optical frequency comb makes possible a more precise type of atomic clock and many other applications

By Steven Cundiff , Jun Ye & John Hall

Editor's note: This story was originally posted in the April 2008 issue, and has been reposted to highlight the long intertwined history of the Nobel Prizes in Scientific American.

In the blink of an eye, a wave of visible light completes a quadrillion (10 15 ) oscillations, or cycles. That very large number presents both opportunities and a challenge. The opportunities promise numerous applications both inside and outside of laboratories. They go to the heart of our ability to measure frequencies and times with extremely high precision, a skill that scientists rely on for some of the best tests of laws of nature—and one that GPS systems, for instance, depend on. The challenge has centered on the impossibility of manipulating light with the techniques that work so well for electromagnetic waves of much lower frequencies, such as microwaves.

Now, thanks to a decade of revolutionary advances in laser physics, researchers have at hand technologies that can unlock the latent potential that visible light’s high frequencies previously kept us from realizing. In particular, scientists have developed the tools to exploit a type of laser light known as an optical frequency comb. Like a versatile ruler of light with tens or hundreds of thousands of closely spaced “tick marks,” an optical frequency comb provides exquisitely precise measurements of light. Such a comb can form a bridge spanning the huge frequency gap from microwaves to visible light: very precise microwave measurements can, with an optical comb, produce equally exact data about light.

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Myriad applications are in the pipeline. Optical combs will enable a new generation of more precise atomic clocks, ultrasensitive chemical detectors and the means to control chemical reactions using lasers. The combs could greatly boost the sensitivity and range of lidar (light detection and ranging)—and also provide a vast increase in the number of signals traveling through optical fiber.

Combs will greatly simplify the task of measuring optical frequencies with extremely high precision. In the 20th century such a measurement would have required a team of Ph.D.s running rooms full of single-frequency lasers. Today a graduate student can achieve similar results with a simple apparatus using optical frequency combs. The new optical atomic clocks also spring from this simplification. Much as a pendulum in a grandfather clock requires gears to record its swings and slowly turn the clock’s hands, an optical atomic clock uses an optical frequency comb to count the oscillations of light and convert them into a useful electronic signal. In just the past year, researchers have used optical combs to surpass the cesium- based atomic clocks that have been the best system available for decades.

In some respects, the scene-changing advent of optical combs is similar to the leap forward that resulted from the invention of the oscilloscope about 100 years ago. That device heralded the modern age of electronics by allowing signals to be displayed directly, which facilitated development of everything from television to the iPhone. Light, however, oscillates 10,000 times faster than the speed of the fastest available oscilloscopes. With optical combs, the same capability to display the waveform is becoming available for light.

Optical frequency comb applications require exquisite control of light across a broad spectrum of frequencies. This level of control has been available for radio waves for a long time but is only now becoming possible for light. An analogy to music helps in understanding the required level of control. Before the development of combs, lasers could produce a single color, like a single optical tone. They were akin to a violin with only one string and no fingerboard, capable of playing only one note (ignore for the moment that musical notes are much richer than pure tones). To play even a simple piece would require many different instruments, each painstakingly tuned. Each violin would require its own musician, just as every single-frequency laser requires its own operator.

In contrast, one operator can use an optical comb to cover the entire optical spectrum, not merely like a pianist at a piano but like a keyboardist playing an electronic synthesizer that can be programmed to mimic any musical instrument or even an entire orchestra. Comb technology, in effect, enables symphonies of hundreds of thousands of pure optical tones.

Anatomy of a Comb Optical frequency combs are generated by devices called mode-locked lasers, which create ultrashort pulses of light. To understand the important features of such pulses, begin by imagining the light wave of the other chief kind of laser, a continuous-wave (CW) laser. Ideally, such a wave would be an endless stream of perfectly regular oscillations (representing the light wave’s electric field), every wave crest and trough having the same amplitude and arriving at an unchanging rate. A pulse from a mode locked laser, in contrast, is a short series of wave crests and troughs whose amplitude rises from zero to a maximum and then falls back to zero. The shortest pulses, with durations of less than 10 femtoseconds, contain just a few full oscillations of the light wave. The general outline of the pulse—its overall rise and fall—is called its envelope. One can think of the pulse as being like the earlier continuous wave (the “carrier wave”), with that wave’s amplitude multiplied by the changing height of the envelope.

The carrier wave consists of light of one pure frequency. A plot of its spectrum would have a single spike at that frequency, representing the presence of that frequency alone. You might expect that the pulse you are imagining would also consist of light only at that frequency—after all, it is just the single-frequency carrier wave with its amplitudes changed—but that is not how waves and spectra work. Instead the pulse is made up of light of many frequencies all traveling together. The frequencies form a small, continuous band centered on the carrier frequency. The shorter the pulse, the broader the spread of frequencies.

Two additional features of the pulses emitted by mode-locked lasers are keys to the development of optical frequency combs. First, shifting the envelope a little relative to the carrier wave results in slightly different pulses. The peak of the pulse envelope may occur at the same time as a crest of the carrier, but it may also be shifted to any other stage of the oscillation. The amount of displacement is called the phase of the pulse.

Second, mode-locked lasers emit trains of pulses at a very regular rate, called the repetition rate. The frequency spectrum of such a train of pulses does not form a continuum spread on each side of the carrier frequency but rather breaks into many discrete frequencies. Plotted, the spectrum looks like the teeth of a hair comb, spaced at precisely the laser’s repetition rate.

A typical repetition rate is around one gigahertz (a billion cycles per second), somewhat slower than modern computer processors. An optical comb that spanned the visible spectrum would have 400,000 teeth if they were spaced at one gigahertz. Scientists can measure repetition rates in the gigahertz (microwave) range very accurately using high-speed photodiodes, which detect each pulse in turn—and an optical comb would appear to leverage that accuracy up to visible wavelengths. Why not, then, use the teeth of the frequency comb as reference points to measure against?

There is, however, a catch. It relates to the phase. Everything is fine if the phase of every pulse in the train is exactly the same, because in that case the comb teeth will be precisely at integer multiples of the repetition rate. Thus, you would know the teeth positions once you had measured the laser’s repetition rate.

But it usually happens that the phase changes from one pulse to the next by some unpredictable but fixed amount. In that case, the comb teeth are shifted in frequency away from the exact integer multiples of the repetition rate by an amount called the offset frequency. To know the frequencies of the comb teeth, one must measure that frequency as well as the repetition rate. Measuring the offset frequency was a barrier to progress with optical combs. This barrier fell resoundingly in 2000. It took the combined efforts of scientists from two separate branches of laser research and the discovery of a new material.

Converging Disciplines For most of the past 40 years, ultrafast-laser researchers—those who focus on making and using the shortest pulses—largely ignored the pulse phase and the theoretical comblike spectrum of an ideal series of pulses. Their experiments typically only depended on the intensity of individual pulses, in which case the phase has no effect. Although the members of the ultrafast community often measured the spectrum of their mode-locked lasers, they rarely did so with sufficient resolution to observe the underlying comb spectrum; instead the lines would blend together and look like a continuous band of frequencies.

High-resolution measurements were the domain of specialists in precision spectroscopy and optical frequency metrology, wherein highly stable CW lasers reigned as the preferred tools. As mentioned earlier, a CW laser sends out a steady stream of light at a precise frequency, and its spectrum looks like one sharp spike. Not many researchers in the metrology community were cognizant of the workings of mode-locked lasers, and those who did know about them were skeptical that such lasers could produce a well-defined comb spectrum in practice. They expected that modest fluctuations in the timing or the phase of the pulses would wash it out.

But a few researchers, most notably Theodor W. Hänsch of the Max Planck Institute for Quantum Optics in Garching, Germany, had faith that mode-locked lasers could one day be a useful tool for high-precision spectroscopy and metrology. In the 1970s, while a faculty member at Stanford University, Hänsch used mode-locked dye lasers (which have a colorful liquid dye as the medium where the laser light is generated) to do a series of measurements that established the basic concept of the comb spectrum and its offset frequency. These seeds then lay dormant for almost 20 years until laser technologies had advanced enough for further progress with combs to be practical.

In the late 1980s Peter Moulton, then at Schwartz Electro-Optics in Concord, Mass., developed titanium-doped sapphire as a laser gain medium with a large bandwidth. Wilson Sibbett of the University of St. Andrews in Scotland pioneered its use in mode-locked lasers in the early 1990s. Within only a few years, titanium- sapphire lasers were routinely generating pulses shorter than 10 femtoseconds, corresponding to only three cycles of light [see “Ultrashort- Pulse Lasers: Big Payoffs in a Flash,” by John-Mark Hopkins and Wilson Sibbett; Scientific American , September 2000].

With these titanium-sapphire lasers available, Hänsch dusted off his 20-year-old idea of optical frequency combs. He performed a series of experiments in the late 1990s that demonstrated the latent potential of mode-locked lasers. In one measurement, he showed that comb lines at opposite ends of the output spectrum are well defined with respect to one another. The comb teeth were revealed to be like the marks engraved on a steel ruler and not like lines drawn along a rubber band. In another experiment, he measured the frequency of an optical transition in cesium atoms (a change in their state that absorbs or emits light at a precise frequency) using a mode-locked laser to span the difference in frequency between two CW lasers. His results inspired a group of us to undertake serious research in this arena.

At JILA, a joint institute between the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, we were in a unique position to take the technological advances in two branches of laser physics and run with them. JILA has a strong tradition in optical frequency metrology and precision spectroscopy, largely built on the ultrastable CW laser technology developed over 40 years by one of us (Hall). In 1997 another one of us (Cundiff) joined JILA, bringing expertise in mode-locked lasers and short-pulse techniques. It took many hallway and lunch table conversations before we surmounted our conceptual divide and decided to join forces, along with a pair of postdoctoral fellows: Scott Diddams, now at NIST, and David Jones, now at the University of British Columbia. The third of us (Ye) joined the fun at JILA in the summer of 1999, just as the revolution began in earnest; he soon led the way to finding applications for the new frequency combs.

Magic Fiber As impressive as Hänsch’s results were, we knew that his motivation was to dispose of most of his complex apparatus. The techniques to accomplish this simplification, however, required that a mode-locked laser produce an enormous bandwidth, preferably an octave. (An octave is a factor of two in frequency, whether it be in music, electronics or optics.) Although titanium-sapphire lasers produced impressive bandwidth at the time, they could not yet yield an octave of light.

The final puzzle piece fell into place at the 1999 Conference on Lasers and Electro-Optics where Jinendra Ranka of Bell Laboratories presented a paper on a new kind of optical fiber known as microstructure fiber. In this medium, micrometer-size airholes in the fiber guide light along its core. The fiber’s properties allow pulses at the frequencies produced by a titanium-sapphire laser to travel along it without being stretched (as occurs in ordinary fiber and most other optical media). The lack of stretching keeps the pulse intensity high, which in turn leads to much greater spectral broadening than occurs in ordinary optical fiber [see “ The Ultimate White Light ,” by Robert R. Alfano; Scientific American , December 2006]. The results are visually stunning. The output of a titanium- doped sapphire laser is in the near-infrared, just beyond the limits of human vision. It appears as a faint red color to the eye. Spectral broadening in microstructure fiber converts that faint red to visible wavelengths, causing the fiber to glow with successive colors of the rainbow.

In the fall of 1999 we managed to acquire some of this magic fiber. The timing could not have been more perfect. We had just completed a series of experiments demonstrating the use of a titanium-sapphire laser to span a gap nearly three times wider than Hänsch’s initial demonstration. We already had an operating setup into which we could almost drop the new microstructure fiber. Within two weeks of receiving the express package from Bell Laboratories, we had done a proof-of-principle experiment showing that the spectral broadening in the microstructure fiber preserved the frequency comb structure in the original laser pulse.

The importance of an octave-spanning spectrum is that it allows the offset frequency to be measured directly as a radio frequency, thus surmounting the aforementioned barrier to using combs to measure other frequencies. There are several specific methods of determining the offset frequency given an octave-spanning spectrum, many of which can be traced to techniques employed in radio engineering for measuring frequencies before high-speed counters were readily available. (Counters do the job by simply counting how many oscillations occur in a radio wave per unit of time but cannot keep up with the much higher frequencies that light has.) We will now describe the simplest and most versatile of the methods for measuring the offset frequency— self-referencing.

The key idea is that an octave-spanning spectrum enables scientists to compare the frequencies of two comb lines at opposite ends of the spectrum with each other. If the offset frequency is zero, then each line at the low-frequency end of the spectrum has a corresponding line with exactly twice its frequency at the high-frequency end. Any deviation from this exact ratio turns out to be precisely the offset frequency. The scheme is called self-referencing because one is comparing the comb’s light against itself.

Self-referencing is carried out in practice by passing some of the laser light through a so-called second-harmonic generation crystal, which doubles the light’s frequency. Thus, one can split off the light that forms the lower-frequency end of the comb using a mirror that only reflects longer-wavelength light but passes shorter wavelengths, then send it through the doubling crystal, and finally direct both it and the light of the higher-frequency end of the comb onto the same photodetector. The combined light oscillates in intensity—it “beats”—in just the same fashion as the combined sound of a tuned and a mistuned note beats. In both cases, the frequency of the beats equals the amount of mistuning. For the light pulses, the beats have the same frequency as the comb’s offset frequency because every doubled low-end line will be mistuned by that amount from a high-end line. In electronics and optics, this procedure of combining signals to get the beat frequency is called heterodyne detection.

Redefining Time The simplicity of optical frequency metrology based on optical frequency combs can only be appreciated in comparison to techniques used prior to their development. Briefly, these techniques consisted of frequency multiplication chains, where each link in the chain consisted of an oscillator that had a multiple of the frequency of the previous link. The first link in the chain was a cesium clock, a kind of atomic clock used as the international time standard that defines the second. The cesium clock is based on nine-gigahertz microwaves absorbed by cesium atoms. To reach all the way from nine gigahertz to the frequency of visible light (a factor of at least 40,000) required about a dozen stages. Each stage used a different technology, including lasers for visible light. Running these chains was resource- and personnel-intensive; just a few in the world were built, and measurements were made only intermittently. In addition, in practice the many links in the chain impaired the accuracy of the ultimate optical frequency measurement.

Once stabilized optical frequency combs were invented, it was much easier to precisely measure the frequency of a CW laser. As with a frequency chain, comb-based frequency measurements still must be referenced to a cesium clock. As we will now see, a cesium clock’s ability to measure frequencies up to about nine gigahertz is all that you need to use an optical comb to determine the frequency of a laser line. Several pieces of information involving the comb are needed. First, as we discussed earlier, the comb’s offset frequency and the spacing of its lines must be measured. From those two numbers the frequencies of all the comb’s lines can be calculated. Next, the unknown laser light is combined with the comb’s light to get the beat frequency (that is, the difference in frequency) between it and the nearest comb line.

These three frequencies are all within the microwave range that can be measured extremely accurately using a cesium clock. Recall that the comb’s line spacing is the same as the repetition rate of the pulses producing the comb. Most mode-locked lasers operate at a repetition rate of 10 gigahertz or less, making that quantity easy to measure against the cesium clock. Both the offset frequency and the beat frequency are also within range to be measured by the cesium clock because they must be smaller than the comb spacing.

Two further pieces of data must be determined: to which comb line was the unknown laser light closest and on which side of the line? Commercial wave meters can measure an optical line’s frequency to within less than one gigahertz, which is good enough to answer those two questions. In the absence of such a wave meter, you can systematically vary the repetition rate and the offset frequency to monitor how the beat frequency changes in response. With enough of those data points, you can work out where the line must be.

The simplicity of optical combs has not only increased how often scientists around the world make these extremely precise frequency measurements but also greatly decreased the uncertainty in those measurements. Such benefits may one day lead to an optical time standard replacing the present microwave cesium-based one. With this in mind, groups at NIST led by James C. Bergquist and at JILA led by Ye have been measuring frequencies relative to clocks that use light and a comb to produce the output signal. Already the uncertainties in measurements using the best of these clocks are smaller than those in measurements using the very best cesium standards. It is an exciting time, with many laboratories around the world poised to build optical frequency standards that can surpass what has been the primary frequency standard for many decades. Measurements by Leo Hollberg’s group at NIST, as well as by other groups elsewhere, suggest that the intrinsic limit of the optical comb is still a couple of orders of magnitude better than the uncertainty in current optical frequency measurements.

Higher and Higher Adopting an optical time standard remains years in the future, however. Metrologists must first carefully evaluate numerous atomic and ionic optical transitions before selecting the one that seems to be the best for a standard. In addition to the many practical applications of combs, fundamental comb research continues apace on many fronts. For example, Ye’s group can use a single comb to detect very sensitively many different transitions of atoms and molecules all at once. Thus, the whole range of energy states of an atom can be analyzed in one measurement. Alternatively, this technique can be applied to detect many trace species in a sample.

Comb technology has already had a large impact on studies of how atoms and molecules respond to the strong electric fields obtainable in intense, ultrashort light pulses. Much of this work has been led by a collaborator of Hänsch’s, Ferenc Krausz, who is now at the Max Planck Institute for Quantum Optics. Among other achievements, his group has used the response of electrons to measure the electric field of a laser’s ultrashort pulses and display the waveform, much like displaying a radio-frequency wave on an oscilloscope. Krausz used optical combs to stabilize the pulses’ phase to have an unchanging waveform from pulse to pulse.

Another very active area of research is the quest to push comb techniques to higher frequencies of the electromagnetic spectrum. (Producing lower-frequency combs, including combs that run from microwaves all the way to visible light, is straightforward.) In 2005 Ye’s group at JILA and Hänsch’s group in Garching generated a precise frequency comb in the extreme ultraviolet (not far below x-rays in frequency). Scientists are using this extended comb to study the fine structure of atoms and molecules with extreme ultraviolet laser light.

In the space of a few short years, optical frequency combs have gone from being a research problem studied by a small number of scientists to being a tool to be used across a broad gamut of applications and fundamental research. We have only begun to explore the full potential of these rulers of light.

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8.7: Lasers

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Learning Objectives

By the end of this section, you will be able to:

• Describe the physical processes necessary to produce laser light
• Explain the difference between coherent and incoherent light
• Describe the application of lasers to a CD and Blu-Ray player

A laser is device that emits coherent and monochromatic light. The light is coherent if photons that compose the light are in-phase, and monochromatic if the photons have a single frequency (color). When a gas in the laser absorbs radiation, electrons are elevated to different energy levels. Most electrons return immediately to the ground state, but others linger in what is called a metastable state . It is possible to place a majority of these atoms in a metastable state, a condition called a population inversion .

When a photon of energy disturbs an electron in a metastable state (Figure \(\PageIndex{1}\)), the electron drops to the lower-energy level and emits an addition photon, and the two photons proceed off together. This process is called stimulated emission . It occurs with relatively high probability when the energy of the incoming photon is equal to the energy difference between the excited and “de-excited” energy levels of the electron (\(\Delta E = hf\)). Hence, the incoming photon and the photon produced by de-excitation have the same energy, hf . These photons encounter more electrons in the metastable state, and the process repeats. The result is a cascade or chain reaction of similar de-excitations. Laser light is coherent because all light waves in laser light share the same frequency (color) and the same phase (any two points of along a line perpendicular to the direction of motion are on the “same part” of the wave”). A schematic diagram of coherent and incoherent light wave pattern is given in Figure \(\PageIndex{2}\).

Lasers are used in a wide range of applications, such as in communication (optical fiber phone lines), entertainment (laser light shows), medicine (removing tumors and cauterizing vessels in the retina), and in retail sales (bar code readers). Lasers can also be produced by a large range of materials, including solids (for example, the ruby crystal), gases (helium-gas mixture), and liquids (organic dyes). Recently, a laser was even created using gelatin—an edible laser! Below we discuss two practical applications in detail: CD players and Blu-Ray Players.

A CD player reads digital information stored on a compact disc (CD). A CD is 6-inch diameter disc made of plastic that contains small “bumps” and “pits” nears its surface to encode digital or binary data (Figure \(\PageIndex{3}\)). The bumps and pits appear along a very thin track that spirals outwards from the center of the disc. The width of the track is smaller than 1/20th the width of a human hair, and the heights of the bumps are even smaller yet.

A CD player uses a laser to read this digital information. Laser light is suited to this purpose, because coherent light can be focused onto an incredibly small spot and therefore distinguish between bumps and pits in the CD. After processing by player components (including a diffraction grating, polarizer, and collimator), laser light is focused by a lens onto the CD surface. Light that strikes a bump (“land”) is merely reflected, but light that strikes a “pit” destructively interferes, so no light returns (the details of this process are not important to this discussion). Reflected light is interpreted as a “1” and unreflected light is interpreted as a “0.” The resulting digital signal is converted into an analog signal, and the analog signal is fed into an amplifier that powers a device such as a pair of headphones. The laser system of a CD player is shown in Figure \(\PageIndex{4}\).

Blu-Ray Player

Like a CD player, a Blu-Ray player reads digital information (video or audio) stored on a disc, and a laser is used to record this information. The pits on a Blu-Ray disc are much smaller and more closely packed together than for a CD, so much more information can be stored. As a result, the resolving power of the laser must be greater. This is achieved using short wavelength (\(λ=405\,nm\)) blue laser light—hence, the name “Blu-” Ray. (CDs and DVDs use red laser light.) The different pit sizes and player-hardware configurations of a CD, DVD, and Blu-Ray player are shown in Figure \(\PageIndex{5}\). The pit sizes of a Blu-Ray disk are more than twice as small as the pits on a DVD or CD. Unlike a CD, a Blu-Ray disc store data on a polycarbonate layer, which places the data closer to the lens and avoids readability problems. A hard coating is used to protect the data since it is so close to the surface.

Communicating via Long-Distance Lasers Subheadline A NASA partnership made lasers viable for satellite communications

Visible light has been used to communicate for centuries: lanterns on ships and Morse code flashes allowed information to be conveyed at a distance. But now there’s a better way to use  light to communicate  over even further distances and with far more accuracy – lasers.

An infrared laser can carry information in a similar way to radio waves. By modifying the invisible beam a certain way, the varying modulation can transmit a digital signal. Space is a perfect use for the technology, because there’s no atmosphere or buildings to impede the beam’s path, and compared to other communications standards, lasers offer a wide range of benefits. Light waves can support a high data rate and take less power to run. On Earth, fiber optic cables provide similar benefits to laser communications, but using lasers over a longer distance without any physical cables has proved more challenging.

In 2013, a demonstration on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft relayed video between ground stations on Earth and the orbiter. The orbiter was able to transmit 622 megabits of data, enough to carry 30 HDTV channels. While future lunar astronauts might not need to watch reruns, the ability to transmit that much data could be a game changer for space exploration. To further explore how this technology could be developed and utilized for future applications both public and private, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, looked into partnering with the private sector.

“Transferring the optical communication technology to the private sector contributes to creating high paying jobs, heightens the competition among companies to bring the cost down, and enhances the system’s efficiency by increasing the data rate, reducing power, and mass,” said Hossin Abeldayem, senior technology manager at Goddard’s Strategic Partnerships Office.

Bridging the Communications Gap 

Denver-based BridgeComm, formerly known as BridgeSat, was founded in 2015 to dive into the opportunities presented by using lasers to communicate in space. The company had reached out to NASA centers, and pitched potential uses for lasers at a workshop on advanced communications technologies .  Bridgecomm gained some interest from Goddard’s Strategic Partnerships Office, and signed a Space Act Agreement soon afterward.

Over the course of this and the other following agreements, BridgeComm engineers met with their counterparts at NASA to discuss development of the company’s laser systems. A large portion of the collaboration was through consulting, ensuring that the researchers at BridgeComm had access to a wide knowledge base. While working with Goddard, the company also made agreements with NASA Headquarters, NASA’s Ames Research Center in Silicon Valley, California, and NASA’s Glenn Research Center in Cleveland. With all these centers on board, the company could apply NASA expertise to every part of its system.

There are a few key parts to make these systems work. Compared to the wide area a radio signal can cover, lasers from space can only be received across an area the size of a football field. To allow ground stations to reliably pick up the signals from space, BridgeComm and NASA teams brainstormed a way to ensure the beam remained trained on a spot on the planet below. By mounting the entire laser system on a gimbal and using fine steering mirrors, the engineers could ensure the beam didn’t wander as the satellite moved through its orbit. Gimballed optical modules are also used on the LCRD payload. Barry Matsumori, CEO of BridgeComm says that the expertise gained from NASA helped to make the company’s ability to precisely aim the beam and receive the reflected signal work.

Also key to the system are amplifiers. While the lasers can travel a great distance, they need to be powerful enough to travel the potentially interplanetary distances needed for NASA missions. BridgeComm’s amplifiers are able to keep the beams strong, while being small enough to fit on a satellite.

Shining Lights on the Horizon

BridgeComm’s primary customers are those that need high-speed communications but don’t want to compete for bandwidth on the already crowded radio channels. “Radio-frequency spectrum is in short supply,” said Matsumori. “By going optical, we can transmit more data without using any of it.” 

Since the completion of the Goddard agreement, BridgeComm has received additional funding from Boeing Horizon Ventures to further build out BridgeComm’s communications technology, making additional ground stations for future satellites that might use its system. There are already interested customers for the BridgeComm system as well. The company is working on laser communications equipment for satellite manufacturer ICEYE and the geospatial data company HySpecIQ, which both hope to use the light amplifiers and specialized gimbals on their new satellite constellations. With the award of the HySpecIQ contract, due for launch in 2022, BridgeComm is one of the first companies to commercialize optical communications technology in space, and Matsumori thanks NASA for the kickstart it gave to the company.

Andrew Wagner Science Writer

The demo laser system on the Lunar Atmosphere and Dust Environment Explorer orbiter could relay enough data for 30 simultaneous high-definition video streams between Earth and the Moon. The success of the test opened the door for further testing and development of “freespace” laser communications. Credit: NASA Goddard

Thanks to Space Act Agreements with various NASA centers, BridgeComm benefitted from NASA’s wide knowledge base to develop their laser communication technology. The company’s ground stations are able to transmit and receive data from satellites in orbit using visible light. Credit: BridgeComm

NASA is exploring the use of lasers to communicate over potentially interplanetary distances. Through collaboration with the private sector, future NASA missions could use a powerful beam to send data back to Earth. Credit: NASA/JPL

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Timing a space laser with a nasa-style stopwatch.

Karl B. Hille

To time how long it takes a pulse of laser light to travel from space to Earth and back, you need a really good stopwatch — one that can measure within a fraction of a billionth of a second.

That kind of timer is exactly what engineers have built at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, for the Ice, Cloud and land Elevation Satellite-2. ICESat-2, scheduled to launch in 2018, will use six green laser beams to measure height. With its incredibly precise time measurements, scientists can calculate the distance between the satellite and Earth below, and from there record precise height measurements of sea ice, glaciers, ice sheets, forests and the rest of the planet’s surfaces.

“Light moves really, really fast, and if you’re going to use it to measure something to a couple of centimeters, you’d better have a really, really good clock,” said Tom Neumann, ICESat-2’s deputy project scientist.

If its stopwatch kept time even to a highly accurate millionth of a second, ICESat-2 could only measure elevation to within about 500 feet. Scientists wouldn’t be able to tell the top of a five-story building from the bottom. That doesn’t cut it when the goal is to record even subtle changes as ice sheets melt or sea ice thins.

To reach the needed precision of a fraction of a billionth of a second, Goddard engineers had to to develop and build their own series of clocks on the satellite’s instrument — the Advanced Topographic Laser Altimeter System, or ATLAS. This timing accuracy will allow researchers to measure heights to within about two inches.

“Calculating the elevation of the ice is all about time of flight,” said Phil Luers, deputy instrument system engineer with the ATLAS instrument. ATLAS pulses beams of laser light to the ground and then records how long it takes each photon to return. This time, when combined with the speed of light, tells researchers how far the laser light traveled. This flight distance, combined with the knowledge of exactly where the satellite is in space, tells researchers the height of Earth’s surface below.

The stopwatch that measures flight time starts with each pulse of ATLAS’s laser. As billions of photons stream down to Earth, a few are directed to a start pulse detector that triggers the timer, Luers said.

Meanwhile, the satellite records where it is in space and what it’s orbiting over. With this information, ATLAS sets a rough window of when it expects photons to return to the satellite. Photons over Mount Everest will return sooner than photons over Death Valley, since there is less distance to travel.

The photons return to the instrument through the telescope receiver system and pass through filters that block everything that’s not the exact shade of the laser’s green, especially sunlight. The green ones make it through to a photon-counting electronics card, which stops the timer. Most of the photons that stop the timer will be reflected sunlight that just happens to be the same green. But by firing the laser 10,000 times a second the “true” laser photon returns will coalesce to give scientists data on surface elevation.

“If you know where the spacecraft is, and you know the time of flight so you know the distance to the ground, now you have the elevation of the ice,” Luers said.

The timing clock itself consists of several parts to better keep track of time. There’s the GPS receiver, which ticks off every second — a coarse clock that tells time for the satellite. ATLAS features another clock, called an ultrastable oscillator, which counts off every 10 nanoseconds within those GPS-derived seconds.

“Between each pulse from the GPS, you get 100 million ticks from the ultrastable oscillator,” Neumann said. “And it resets itself with the GPS every second.”

Ten nanoseconds aren’t enough, though. To get down to even more precise timing, engineers have outfitted a fine-scale clock within each photon-counting electronic card. This subdivides those 10-nanosecond ticks even further, so that return time is measured to the hundreds of picoseconds.

Some adjustments to this travel time need to be made on the ground. Computer programs combine many photon travel-times to improve the precision. Programs also compensate for how long it takes to move through the fibers and wires of the ATLAS instrument, the impacts of temperature changes on electronics and more.  

“We correct for all of those things to get to the best time of flight we possibly can calculate,” Neumann said, allowing researchers to see the third dimension of Earth in detail.

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How Far Can You See A Laser Beam?

by Biglasers | Oct 10, 2013 | High Power Laser Pointers

A common question to consider when browsing different laser pointers online is, “how far can I see the beam?” This may seem like a simple enough question to answer but may not actually be so clear to those unfamiliar with various aspects of laser technology. Here we’ll look at some basic guidelines to help answer this question and others before you buy any particular item.

1. Visible Range

How Far Can You See a Laser Beam?

The range and visible distance of any laser will depend on a few different factors. First and foremost will be the output power (mW) and secondly will be the color of the laser. Both are important factors determining how far a laser beam can be visible for. As a rule, green lasers are roughly 7X brighter than any other laser color at the same output power , making green typically the most suitable at the same power.

2. Focus Adjustable

Another factor that will determine how far a beam can be seen for is the focus adjusting design features available on some laser models. For lasers that offer the option to thin or widen the beam, you can pinpoint targets at close or long distances making it even better for long-range targeting . Condensing the beam will aid in the visibility both up close and at greater distances.

3. How Much Range?

So we return to the question: “how far can I see my laser beam for?” As mentioned there will be many different factors to consider, but here is a basic guideline. 200mW green lasers will be visible for more than 10 miles and blue lasers 1,000mW or more will also be visible for 10 miles or more on a clear line of sight. Factors like cloud coverage, fog, and if you’re at a high point of elevation should be considered, but as a basic rule, you can expect over 10 miles of visible distance on green 200mW+ lasers and 1,000mW+ blue lasers .

Laser technology has come an incredibly long way from the cheap red key chain laser pointers of two decades ago. Now, with a little bit of research online, you can find incredibly powerful and advanced lasers for purchase that will meet and exceed all of the expectations you have in mind. For the best place to Buy High Power Laser Pointers online from the #1 Trusted Laser Source Since 2005, look no further than BigLasers.com

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How Far Can a Laser Beam Travel? Exploring the Maximum Range of Laser Technology

By Happy Sharer

Introduction

Laser beams are widely used in a variety of applications, from medical treatments to military operations. But just how far can these beams of light reach? That’s what this article seeks to explore. We’ll look at the science behind light propagation, the advantages and disadvantages of long-distance laser use, and the potential for future laser technology advancements.

Exploring the Distance Laser Beams Travel in Space

When it comes to understanding the distance that laser beams can travel, it’s important to understand the basics of light propagation. Light travels in straight lines, which means that the farther away an object is, the less intense the light will be when it reaches that object. So, what is the maximum range of a laser beam?

How Does Light Propagate Through Space?

Light propagates through space by bouncing off particles in the atmosphere, such as molecules and atoms. This process is known as scattering, and it allows light to be seen even when the source is far away. As light moves through the atmosphere, it can be affected by several factors, including temperature, humidity, and air pressure. All of these factors can affect how far a laser beam can travel.

Factors That Affect Laser Beam Distance

There are several factors that can affect the distance that a laser beam can travel. These include the power of the laser, the type of material the laser is being aimed at, the atmospheric conditions, and the angle at which the laser is fired. The power of the laser is especially important, as more powerful lasers can travel farther than weaker ones.

The Science Behind How Far Light Can Go

In order to understand how far light can travel, we need to look at the science behind it. One of the most important theories when it comes to light is the wave-particle duality theory. This theory states that light behaves both as a particle and as a wave, depending on the situation. This allows light to travel great distances, as waves can spread out over a wide area.

Diffraction and Refraction of Light

Another important concept related to light propagation is diffraction. Diffraction is when light bends around corners or through small openings. This allows light to travel further than it would otherwise. Refraction is also important, as it describes how light changes direction when it passes through different materials, such as glass or water.

Reflection of Light

Reflection is another key concept related to light propagation. Reflection occurs when light bounces off of a surface, such as a mirror. This allows light to be seen even when the source is far away. In addition, reflection can also be used to focus light, allowing it to travel even further.

Analyzing the Long-Distance Capabilities of Lasers

Now that we’ve looked at the science behind light propagation, let’s take a closer look at the long-distance capabilities of lasers. Lasers have several advantages over other technologies when it comes to long-distance communication. For example, lasers can carry more data than traditional radio signals and can be directed over greater distances with greater accuracy.

Advantages of Laser Technology Over Other Technologies

Lasers also have several advantages over other technologies when it comes to long-distance communication. For example, lasers can carry more data than traditional radio signals and can be directed over greater distances with greater accuracy. Lasers also require less energy than traditional radio signals, making them more cost-effective for long-distance communication.

Disadvantages of Long-Distance Laser Use

Despite the advantages of using lasers for long-distance communication, there are some drawbacks. For example, lasers cannot penetrate clouds or fog, so they may not be suitable for certain environments. Additionally, lasers can be easily blocked by obstacles, such as buildings or mountains. Finally, lasers can be difficult to detect, making them vulnerable to interception.

What Are Some Examples of Long-Distance Laser Applications?

Lasers have many applications across a variety of industries. One of the most common applications is in telecommunications, where lasers are used to transmit data over long distances. Lasers are also used in surveying and mapping, as well as in astronomy and medicine. Finally, lasers are increasingly being used in military operations, such as target acquisition and guidance systems.

How Far Can Lasers Reach Across the Globe?

To understand the maximum range of laser technology, it’s important to consider the limitations of the technology. Lasers can theoretically reach any point on Earth, but the distance is limited by the atmosphere. Atmospheric interference can cause the beam to scatter and weaken, reducing the effective range of the laser.

Understanding the Limitations of Laser Technology

The atmosphere can absorb some of the energy from a laser beam, causing it to weaken over distance. This is known as atmospheric attenuation, and it can significantly reduce the range of a laser beam. Additionally, the curvature of the Earth limits the maximum range of a laser beam, as the beam will eventually curve away from its intended target.

Examining Existing Long-Distance Laser Systems

There are already several long-distance laser systems in use around the world. The most notable of these is the European EISCAT (European Incoherent Scatter Scientific Association) radar system, which uses lasers to measure the ionosphere over distances of up to 2,000 kilometers. There are also several satellite-based systems that use lasers for communications over long distances.

Potential for Future Laser Technology Advancements

As laser technology continues to improve, the potential for greater distances and higher data rates is becoming increasingly possible. For example, researchers are developing lasers that can transmit data at rates of up to 100 gigabits per second, which is significantly faster than existing radio systems. Additionally, scientists are exploring ways to increase the distance that lasers can travel, such as using multiple lasers or using optical fibers to guide the beam.

Calculating the Maximum Range of a Laser Beam

In order to calculate the maximum range of a laser beam, you must first determine the power of the beam and the atmospheric conditions. Once these factors are known, you can use a mathematical equation to calculate the maximum range of the beam. However, it is important to note that the actual range of a laser beam will depend on the specific environment in which it is being used.

Overcoming Atmospheric Interference

Atmospheric interference can significantly reduce the range of a laser beam. To combat this, scientists have developed several techniques to reduce the effects of atmospheric interference. These include using multiple lasers, using optical fibers to guide the beam, and using specialized lenses to focus the beam.

Understanding the Applications of Laser Beams Over Great Distances

Lasers have a wide range of applications, from military operations to medical treatments. In the military, lasers are used for target acquisition and guidance systems, while in medicine, lasers are used for surgical procedures and imaging. Lasers are also used in commercial applications, such as communications, surveying, and mapping.

In conclusion, laser beams can travel great distances, but the exact distance depends on several factors, including the power of the beam, the atmospheric conditions, and the angle at which the beam is fired. While lasers have several advantages over other technologies for long-distance communication, they also have some drawbacks, such as the difficulty in detecting them and the vulnerability to atmospheric interference. As technology continues to improve, however, the potential for greater distances and higher data rates will continue to grow.

(Note: Is this article not meeting your expectations? Do you have knowledge or insights to share? Unlock new opportunities and expand your reach by joining our authors team. Click Registration to join us and share your expertise with our readers.)

Hi, I'm Happy Sharer and I love sharing interesting and useful knowledge with others. I have a passion for learning and enjoy explaining complex concepts in a simple way.

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Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

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Laser pulses travel faster than light without breaking laws of physics

By Leah Crane

27 May 2021

No laws of physics were broken, but light seems to have moved faster than its speed limit

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The speed of light may not necessarily be constant. Light travelling through a plasma can appear to move at speeds both slower and faster than what we refer to as “the speed of light” – 299,792,458 metres per second –  without breaking any laws of physics.

Clément Goyon at Lawrence Livermore National Laboratory in California and his colleagues accomplished this using a pair of lasers fired into a jet of hydrogen and…

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Helping the public safely use consumer lasers with visible beams, laser hazard distance chart.

How divergence affects hazard distances

How laser power affects hazard distances

How wavelength affects hazard distances

• Compared with 532 nm light, the common red wavelength 635 nm appears only 27% as bright. This has a square root effect on the visual interference distances. A 532 green laser appears 4 times as bright as a 635 red laser -- but the green visual interference distances are only 2 times the red distances. (The square root of 4 is 2.)
• Compared with 532 nm light, the common blue wavelength 445 nm appears only 3.5% as bright. Again, there is a square root effect on the distances. A 532 green laser appears 29 times as bright as a 445 blue laser -- but the green visual interference distances are only 5.4 times longer than the blue distances. (The square root of 29 is 5.4.)

Some example lasers

• Comparing the top two bars, we see how color affects a visual interference distance. The 1 mW red pointer has a glare distance of 255 feet, compared to the same power green laser, which can cause glare at 490 feet.
• Similarly, the bottom two bars show that the green laser has much longer visual interference distances than the blue laser -- even though they both are the same power and thus have the same NOHD distance.
• Comparing the 5 mW green pointer to the 500 mW green pointer, we see how power affects hazard distances. The 500 mW laser is 100 times more powerful, but the glare distance is only 10 times greater. Although the numbers are not shown on this chart, the same effect happens to the NOHD. The 500 mW laser’s NOHD is only 10 times the NOHD of the 5 mW laser, despite being 100 times more powerful.

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Long-distance laser brings Mercury closer to Earth

Astronomers have set up the longest laser communication link - a distance of nearly 15m miles from Earth to a probe on its way to Mercury . It is the first demonstration of a technology that will one day be used to communicate with satellites and test some of the fundamental ideas in physics.

The laser beam was sent by Nasa to the Messenger spacecraft, which left Earth in August 2004 and will reach Mercury in 2011, to test the probe's instruments. "It is the first time it has been successfully accomplished at these kinds of distances," said David Smith, an astronomer at Nasa's solar system exploration division.

One of the main limitations to testing this technology has been the small number of lasers sent into space. "The Messenger laser altimeter is only the fourth to go beyond the moon. The first two were the Mola 1 and Mola 2 laser altimeters that went to Mars in the early and mid-90s, the third was the Laser Ranger on the Near spacecraft that went to the asteroid Eros in the mid-90s," said Dr Smith. "On all these missions we attempted the same experiment but were unsuccessful."

"The observations made late last spring were part of the instrument calibration. The experiment worked fine and the instrument is working as planned. To the best of our knowledge we are not aware of any two-way laser link over distances even as large as 500,000 km [300,000 miles]." The calibration of Messenger's instruments also allowed testing of laser technology that can reach into the deeper regions of space.

Long-distance lasers will allow communication with satellites sent into the solar system using visible wavelengths of light, which have advantages over the traditional microwave communications used in modern space probes, and can carry much higher rates of information.

"Ultimately, laser transmission over planetary distances will become easier and routine and will provide us with the higher data rates necessary for realtime video that we would really like to have when humans go to Mars," said Dr Smith.

"One of the disadvantages of optical, however, is that the laser beams are usually very narrow so must be pointed with extreme accuracy to the target. Our laser beams had divergences of under a few thousandths of a degree."

The long-distance lasers will also help test some basic physics, including Einstein's theory of relativity. "Some of these theories can only be tested over very long distances ... hundreds of millions of kilometres," said Dr Smith.

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Illustration of a shoebox-sized accelerator. An electron source and buncher/injector feeds into a sub-relativistic DLA (the device described in this article), which accelerates electrons up to 1MeV in energy. These electrons are further accelerated by SiO2 waveguide-driven relativistic DLA, and finally pass through an undulator to produce coherent free-electron radiation. (Image credit: Moore Foundation / Payton Broaddus)

Stanford researchers are getting closer to building a tiny electron accelerator based on “accelerator-on-a-chip” technology with broad potential applications in studying physics as well as medical and industrial uses.

The researchers have demonstrated that a silicon dielectric laser accelerator, or DLA, can now both speed up and confine electrons, creating a focused beam of high-energy electrons. “If the electrons were microscopic cars, it’s as if, for the first time, we’re steering and we have our foot on the gas,” said Payton Broaddus, PhD ’23 in electrical engineering and the lead author on a paper published on Feb. 23 detailing the breakthrough in Physical Review Letters .

Taking accelerators from miles to microns

Accelerators produce high-energy particle beams that allow physicists to study the properties of materials, produce focused probes for medical applications, and identify the elementary building blocks that make up all matter in the universe. Some of the earliest high-energy particle accelerators, developed in the 1930s, could fit on a tabletop. But higher particle energies were required to study more advanced physics, so scientists needed to build larger systems. (Powered up in 1966, the original linear accelerator tunnel at SLAC National Accelerator Laboratory on Stanford campus is almost 2 miles long.)

While these systems have made numerous discoveries in particle physics possible, Broaddus is motivated to build a tiny linear accelerator that could eventually rival the capabilities of machines more than a thousand times its size, at a fraction of the cost. This would also allow new applications in medicine, such as being able to attach this device to a small probe and precisely shoot an electron beam at a tumor. “There’s the ability to just completely replace every other particle accelerator with something that’s cheaper and smaller,” he said.

Thanks to advances in nano-scale fabrication and lasers, this vision is increasingly possible, said Olav Solgaard , director of the Edward L. Ginzton Laboratory and the Robert L. and Audrey S. Hancock Professor in the School of Engineering and the senior author on the paper. Traditional radiofrequency accelerators are made up of copper cavities that are pumped with radio waves, which give particles an energy boost. These pulses can heat up the metal, so the cavities need to operate at lower energy and pulse rates to dissipate the heat and avoid melting.

But glass and silicon structures can handle much higher energy pulses from lasers without heating up, so they can be much more powerful while also being smaller. About 10 years ago, Stanford researchers started experimenting with nano-size structures made of these materials. In 2013, a team led by paper co-author Robert Byer , the William R. Kenan, Jr. Professor, Emeritus, demonstrated that a tiny glass accelerator with pulsing infrared light had successfully accelerated electrons . These results led to the project being adopted by the Gordon and Betty Moore Foundation under the Accelerator on a Chip (ACHIP) international collaboration to produce a shoebox-sized mega-electron-volt accelerator.

But this first “ accelerator on a chip ” still had some kinks to work out. As Broaddus puts it, the electrons inside were like cars on a narrow road without steering wheels. They could accelerate very quickly but just as easily crash into a wall.

Scanning electron micrograph of a half-millimeter long dielectric laser accelerator through which electrons travel and accelerate. Cells labeled as black are longitudinally focusing and transversely defocusing (LFTD), while white are longitudinally defocusing transversely focusing (LDTF), which keeps the electrons on track. (Image credit: Broaddus, P., Egenolf, T., Black, D. S., Murillo, M., Woodahl, C., Miao, Y., … Solgaard, O. (2024). Subrelativistic Alternating Phase Focusing Dielectric Laser Accelerators. Phys. Rev. Lett., 132, 085001. doi:10.1103/PhysRevLett.132.085001)

Steering electrons with lasers

Now, this team of Stanford researchers has successfully shown they can also steer electrons at the nanoscale. To do this, they built a silicon structure with a sub-micron channel placed in a vacuum system. They injected electrons into one end and illuminated the structure from both sides with a shaped laser pulse that delivered kicks of kinetic energy. Periodically, the laser fields flipped between focusing and defocusing properties, which bunched the electrons together, keeping them from swerving off track.

Altogether, this chain of acceleration, defocusing, and focusing acted on the electrons for a distance of almost a millimeter. It might not sound far, but these charged particles got quite the kick, gaining 23.7 kilo-electron-volts of energy, approximately 25% greater than their starting energy. The rate of acceleration the team has been able to achieve in their prototype tiny accelerator is comparable to conventional copper accelerators, and Broaddus adds that much higher acceleration rates are possible.

While it’s a significant step forward, there’s more that needs to be done before these small accelerators can be used in industry, medicine, and research. So far, the team’s ability to steer electrons has been limited to two dimensions; three-dimensional electron confinement will be required to allow the accelerator to be long enough for greater energy gains to occur.

Electron relay race

A sister research group at Friedrich Alexander University (FAU) at Erlangen, Germany, recently demonstrated a similar device with a single laser and starting at much lower starting energy. It and the Stanford device will ultimately be part of a kind of electron relay race, said Broaddus.

This future relay would have three teammates: The FAU device would take low-energy electrons and give them an initial kick, and then they could then be fed into a device similar to the one Broaddus is developing. The last step for the electrons would be an accelerator made of glass, like the one developed by Byer. Glass can withstand even greater pummeling by lasers than silicon, allowing the accelerator to further energize and push the electrons toward the speed of light.

Eventually, Solgaard believes such a tiny accelerator will be useful in high-energy physics, exploring the fundamental matter that makes up the universe just as its larger counterparts do. “We have a very, very long way to go,” he said. But he’s still optimistic, adding, “we’ve taken the first few steps.”

Additional Stanford co-authors include Dylan Black, PhD ’21; Yu Miao, PhD ’20; and PhD students Melanie Murillo and Clarisse Woodahl; and former research engineer Kenneth Leedle. Thilo Egenolf of Institut für Teilchenbeschleunigung und Elektromagnetische Felder in Darmstadt, Germany, is also a co-author.

Robert Byer is also a professor emeritus in applied physics in the School of Humanities and Sciences and in the SLAC National Accelerator Laboratory’s Photon Science Directorate and a member of Stanford Bio-X . Solgaard is also a professor of electrical engineering; a member of Bio-X, the Stanford Cancer Institute , and the Wu Tsai Neurosciences Institute ; and an affiliate of the Precourt Institute for Energy and the Stanford Woods Institute for the Environment .

Media Contacts

Taylor Kubota, Stanford University: [email protected]