Lasers have grown larger and smaller, more potent and less expensive over the past ten years. The technology has expanded in terms of both the types of materials used and the number of wavelengths. Lasers are now used in both mundane and fantastical applications. A December 2018 study by MarketsandMarkets estimated that the laser market would reach over $12.9 billion by the end of 2018.
Lasers have become indispensable in a wide range of applications and industries, greatly enhancing the impact of light. Today, laser-powered lithography is essential to producing semiconductors; according to research and consulting firm Gartner, revenues from this sector reached $477 billion in 2018. Laser-based ranging systems give autonomous vehicles the data they need for secure navigation. The market for these vehicles is small today, but according to Allied Market Research (AMR) projections, it could reach $550 billion by 2026.
Cardiovascular, dermatological, and eye-related treatments are among the most popular medical applications. According to networking giant Cisco, the market for data centers and long-haul fiber, in which lasers and optical connections carry data traffic, is expected to grow by 26% annually through 2022.
This post will look at a brief history of laser technology.
A. Einstein adds stimulated emission to the well-known processes of light absorption and spontaneous emission of light as a fundamental light-matter interaction process.
In his discussion of “negative absorption,” or amplification, Richard Tolman explains that the radiation emitted would be coherent with the radiation input.
Rudolph W. Landenburg has confirmed the existence of stimulated emission.
In his Ph.D. thesis, Fabrikant proposes a method for producing a population inversion. For maser/laser operation, population inversion is required.
Alfred Kastler proposes an “optical pumping” method to orient paramagnetic atoms or nuclei in the ground state. This was a significant step toward developing lasers, for which Kastler received the Nobel Prize in Physics in 1966.
In a nuclear magnetic resonance experiment, Edward Purcell and Robert Pound observe inverted populations of states. Population inversions are required for maser and laser action.
Nikolay Basov and Alexander Prokhorov explain the Maser principle (Microwave Amplification by Stimulated Emission of Radiation).
C. H. Townes, J. P. Gordon, and H. J. Zeiger developed the first maser that uses an excited ammonia molecule beam to produce microwave amplification by stimulated emission at 24 gigahertz (GHz).
Charles H. Townes and Arthur L. Schawlow introduce the laser concept.
In his paper “The LASER: Light Amplification by Stimulated Emission of Radiation,” Gordon Gould coined the term laser.
T. H. Maiman observed laser action in Ruby. It is now recognized as one of the most challenging laser systems. At IBM, Sorokin and Stevenson create the first four-level solid-state laser. Ali Javan, William Bennett, and Donald Herriott created the first helium-neon (He: Ne) gas laser at Bell Labs.
Elias Snitzer describes the operation of a neodymium glass laser, which is currently the leading candidate for use as a fusion laser source. Charles Campbell and Charles Koester used the ruby laser to destroy a retinal tumor in the first medical application of the laser. P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich demonstrate the generation of second harmonic light by passing pulses from a ruby laser through a quartz crystal, transforming red light into green in the first example of efficient nonlinear optics.
Bell Labs researchers report the development of the first yttrium aluminum garnet (YAG) laser, which continues to dominate material processing applications. General Electric, IBM, and MIT Lincoln Laboratory scientists created a gallium arsenide laser that converts electrical energy directly into infrared light. F. J. McClung and R. W. Hellwarth invent a laser Q-switching technique that generates laser pulses with short duration and high peak powers. Four groups in the United States (M. I. Nathan et al., R. N. Hall, T. M. Quist, N. Holonyak, and S. F. Bevacqua) nearly simultaneously developed the first pulsed semiconductor diode lasers. Semiconductor diode lasers are a critical first step in the development of optical communication, optical storage, optical pumping of solid-state lasers, and various other applications.
L. E. Hargrove, R. L. Fork, and M. A. Pollack reported the first modelocked operation of a helium-neon laser with an acoustic-optic modulator. The femtosecond pulsed laser is based on mode-locking. Herbert Kroemer and his colleagues Rudolf Kazarinov and Zhores Alferov independently propose ideas for building semiconductor lasers from heterostructure devices, earning them the Nobel Prize in Physics in 2000. At Bell Labs, C. K. N. Patel creates the first carbon dioxide laser.
The Nobel Prize in Physics was awarded to C. H. Townes, N. G. Basov, and A. M. Prokhorov for their fundamental work in Quantum Electronics; Townes for demonstrating the ammonia (NH3) maser and subsequent work in masers and lasers, and Basov and Prokhorov for their contributions to the development of masers and lasers. William B. Bridges invent the first noble gas ion laser. J. E. Geusic, H. M. Marcos, and L. G. Van Uitert create a laser based on neodymium-doped yttrium aluminum garnet (Nd: YAG). This is the most commonly used solid-state laser, with applications ranging from cutting and welding to medical applications and nonlinear optics. E. Snitzer and C. J. Koester create neodymium-doped fiber amplification. Fiber amplifiers are used in communications as well as in high-power lasers. Arno Penzias and Robert Wilson use a maser amplifier to detect 3K cosmic background radiation, proving the Big Bang’s existence. In 1978, they were awarded the Nobel Prize in Physics.
George C. Pimentel and Jerome V. V. Kasper demonstrate the first chemical laser. Chemical lasers, which can now produce megawatts of power, derive their energy from chemical reactions and are among the most powerful lasers in the world. The laser compact disk is invented by James Russell (CD player). The authors, Anthony J. DeMaria, D. A. Stetser, and H. A. Heynau, present the first generation of picosecond laser pulses produced using a neodymium glass laser and a saturable absorber.
The first widely tunable organic dye laser, built by Peter Sorokin and John R. Lankard, is now used in ultrafast science and spectroscopy. Standard Telecommunications Laboratories in England’s Charles K. Kao and George Hockham publish seminal papers demonstrating that optical fiber can transmit laser signals while reducing the loss if the glass strands are pure enough. The Nobel Prize in Physics is awarded to Alfred Kastler for “discovering and developing optical methods for studying Hertzian resonances in atoms.”
NASA launched the first laser-equipped satellite.
NASA’s Lunar Laser Ranging experiments begin, led on Earth by American physicist Carroll Alley and using retroreflectors placed on the moon by Neil Armstrong and Buzz Aldrin. Scientists on Earth use these mirrors to bounce lasers off the moon, measuring its orbital motions and determining fundamental gravitational and relativistic constants with extreme precision. The first continuous-wave chemical laser is invented by D. J. Spencer, T. A. Jacobs, H. Mirels, and R. W. F. Gross. High-power chemical lasers generate megawatts of power, prompting proposals for laser weapons. It was invented the pulsed dye laser.
Excimer lasers, which are important in photolithography and laser eye surgery, are developed by Nikolai Basov, V. A. Danilychev, and Yu. M. Popov of the Lebedev Physical Institute in Moscow. The Ioffe Physical Institute’s Zhores Alferov group and Bell Labs’ Mort Panish and Izuo Hayashi create the first continuous-wave room-temperature semiconductor lasers, paving the way for commercializing fiber optics communications. In Australia, the world’s first laser-driven lighthouse is dedicated (Point Danger). Corning Glass Work’s Robert Maurer, Peter Schultz, and Donald Keck prepare the first batch of optical fiber hundreds of yards long to carry an optical signal. J. Beaulieu develops a machining laser with transversely excited atmospheric (TEA) pressure CO2. O. G. Peterson, S. A. Tuccio, and B. B. Snavely create a continuous-wave dye laser, ushering in a new spectroscopy and ultrafast science era. Arthur Ashkin demonstrates the use of laser beams to manipulate microparticles, establishing the field of optical tweezing and trapping, resulting in significant advances in physics and biology.
Dennis Gabor received the Nobel Prize in Physics for developing the holographic method.
A barcode scanner in a grocery store scanned the first item. The sub-picosecond mode-locked CW dye laser is created by E. P. Ippen and C. V. Shank, paving the way for ultrafast optical science.
Laser Diode Labs creates the first commercial room-temperature continuous-wave semiconductor laser. Telephone conversations can be transmitted using continuous-wave operation.
Stanford University’s John Madey and colleagues demonstrate the first free electron laser (FEL). FELs use a beam of electrons accelerated to near-light speed while passing through alternating magnetic fields instead of a gain medium. Due to the tunable magnetic field, the forced undulating motion produces a coherent photon beam with the widest tunable frequency range of any laser type.
In Long Beach, California, General Telephone and Electronics transmitted the first live telephone traffic via fiber optics at a rate of 6 Mbit/s.
The Nobel Prize in Physics was awarded to N. Bloembergen and Arthur Schawlow for their contributions to masers, nonlinear optics, and spectroscopy.
Kanti Jain published the first paper on excimer laser lithography, which is still widely used in today’s computer and electronics industries. P. F. Moulton creates a titanium-sapphire laser that almost completely replaces the dye laser in tunable and ultrafast laser applications.
Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips create laser-based methods for cooling and trapping atoms. Their research contributes to the study of fundamental phenomena and the precise measurement of important physical quantities. In 1997, they were awarded the Nobel Prize in Physics. Grard Mourou and Donna Strickland show how chirped pulse amplification, or CPA, works. They used gratings to lengthen laser pulses before amplification and then to shorten them after amplification to their original length. This allows for much higher powers without causing damage to the amplifying material itself. Later, CPA was used to produce ultrashort, high-intensity (petawatt) laser pulses.
Steven Trokel, an ophthalmologist, performs the first laser eye surgery with an excimer laser. Emmanuel Desurvire, David Payne, and P.J. Mears demonstrate fiber-optic cable optical amplifiers.
James Wynne, Samuel Blum, and Rangaswamy Srinivasan investigated how biological materials were affected by the ultraviolet excimer laser. Further research showed that the laser produced clear, accurate cuts perfect for delicate surgeries. Amplification devices are only required roughly every 40 miles on the first transatlantic fiber cable because the glass used is so transparent. E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, and B. C. McCollum created a double-clad fiber laser. These solid-state lasers with high power are employed for machining.
Norman F. Ramsey received the Nobel Prize in Physics for inventing the separated oscillatory fields method and its application in the hydrogen maser and other atomic clocks. In contrast, Hans G. Dehmelt and Wolfgang Paul received the prize for developing the ion trap technique.
Eric Betzig, Ray Wolfe, Mike Gyorgy, Jay Trautman, and Pat Flynn create a magneto-optic data storage technique capable of cramming 45 billion bits of data into a square inch of disk space.
The first quantum cascade laser, proposed by Rudy Kazarinov and Robert Suris in 1971, was demonstrated by Bell Labs’ Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho.
S. Nakamura and colleagues create semiconductor lasers from GaN (Gallium nitride) and InGaN. (Indium gallium nitride).
The Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips for developing methods to cool and trap atoms using laser light. MIT researchers invented the first atom laser.
The Nobel Prize in Physics was awarded to Zhores I. Alferov and Herbert Kroemer for “basic work on information and communication technology” and “development of semiconductor heterostructures used in high-speed and optoelectronics.” The optical frequency comb techniques John Hall and Theodor Hansch developed are used in research, precision metrology, and time measurement. This work earned them the 2005 Nobel Prize in Physics.
The Nobel Prize was awarded to Eric A. Cornell, Wolfgang Ketterle, and Carl E. Wieman for achieving Bose-Einstein condensation in dilute gases of alkali atoms and for early fundamental studies of the properties of the condensates.
The Nobel Prize was awarded to J. L. Hall and T. W. Hansch for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique, and to Roy Glauber for his contribution to the quantum theory of optical coherence. INTEL creates a chip with eight continuous Raman lasers using relatively standard silicon processes rather than the somewhat expensive materials and processes required for laser manufacturing today.
Charles Kao shared the Nobel Prize in Physics with Willard S. Boyle and George E. Smith of Bell Labs for their work in fiber optics, which led to the development of the CCD (charge-coupled device), which enabled digital photography. The SLAC Linac Coherent Light Source created the first coherent, hard X-ray beam. The ultrafast laser pulses are powerful enough to produce images of moving molecules or atoms.
Scientists at the University of Konstanz created a 4.3-fs single-cycle light pulse at 1.5-m wavelength using an erbium-doped fiber laser, as reported in the January issue of Nature Photonics. Short laser pulses, they claim, could help with frequency metrology, ultrafast optical imaging, and other applications.
The National Nuclear Security Administration announced in January 2010 that the NIF had successfully delivered a historic level of laser energy — more than 1 MJ — to a target in a few billionths of a second and demonstrated the target drive conditions required to achieve fusion ignition, a project planned for the summer of 2010. The peak power of the laser light is approximately 500, which the United States uses at any given time.
In January, researchers at Northwestern University led by Professor Manijeh Razeghi reported a breakthrough in quantum cascade laser efficiency, hitting 53% compared to the previous best of less than 40%. According to Razeghi, this efficiency figure meant that the device produced more light than heat. The lasers emitted at 4.85 m in the mid-IR (3 to 5 m) region are useful for remote sensing.
Rainer Blatt and Piet O. Schmidt of the University of Innsbruck in Austria demonstrated a single-atom laser with and without threshold behavior by varying the strength of atom-light field coupling on March 31, 2010.
According to a paper published in the Journal of Applied Physics on July 15, physicists at Lawrence Livermore National Laboratory used ultrafast laser pulses to probe basic material properties. The laser pulses created shock waves in a diamond anvil cell, pushing the pressure in argon and other gases up to 280,000 atmospheres.
Researchers at ETH Zürich, a Swiss Federal Institute of Technology division, worked with Hans Zogg to create the first vertical external-cavity surface-emitting laser (VECSEL), which operated in the mid-IR at a wavelength of about 5 m. Spectroscopic applications benefit from this wavelength range. Members of the research team were inspired by the potential of VECSELs to locate Phocone as a company to commercialize the technology.
Researchers Malte Gather and Seok Hyun Yun from Harvard University presented a living laser and discussed the development in the June issue of Nature Photonics. They produced a novel green fluorescent protein (GFP) material by genetically engineering cells, making jellyfish bioluminescent. They next put a cell with a diameter of 15 to 20 m inside an optical resonator and pulsed blue light through it. Medical and biophotonics applications are now possible because the cell lased without suffering any damage.
Zinc oxide nanowire waveguide lasers were created by researchers at the University of California, Riverside, under the direction of Professor Jianlin Liu. Their research was published in Nature Nanotechnology’s July issue. The team created a p-n junction diode by figuring out how to make p-type material. This diode caused the nanowires to glow from their ends when powered by a battery. Compared to other ultraviolet semiconductor diode lasers, nanowire lasers may be more affordable, have a higher power output, and have a shorter wavelength.
A Yale University team developed a random laser. These sources are made of disordered materials and emit low spatially coherent emissions despite being as bright as a conventional laser. According to researchers Brandon Redding, Michael Choma, and Hui Cao in an April Nature Photonics paper, a random laser could help full-field microscopy and digital light projection because this characteristic eliminates noise or speckles.
A new record was set in July: peak power of over 500 trillion watts. The 1.85 MJ of energy was delivered by 192 UV laser beams at Lawrence Livermore National Laboratory’s National Ignition Facility and struck a target only 2 mm in diameter. The energy level enabled the investigation of matter states, such as those found in the centers of planets and stars, and hydrogen fusion as a potential power source. The brief burst of energy also replicated conditions inside a modern nuclear device, allowing simulations to be validated without actual testing. The facility’s energy level was 85% higher than in March 2009.
A laser zapped a rock on Mars in August. NASA’s Curiosity rover was getting to work. The rover launched its two-year mission in September. Curiosity’s instrumentation produced light at 1.067 m using an Nd: KGW crystal. Using a telescope, the light was then focused on a spot 1 to 7 meters away. The rock produced a plume due to repeated light pulses, allowing laser-induced breakdown spectroscopy to determine its composition.
Random lasers have advantages, but they also have disadvantages. For example, they have an irregular and chaotic spatial emission pattern. A team led by Professor Stefan Rotter of the Vienna University of Technology developed a control scheme. Because light bounces back and forth among the particles during amplification, the researchers discovered that the layout of the granular material in a given laser determines the emission direction. Pumping the material in a nonuniform manner that matches this layout can thus be used to set the emission direction, making the random laser more useful, according to the researchers, who published their findings in Physical Review Letters in July.
The entire world’s information, including everything from financial transactions to cat videos, is carried by laser pulses moving down fiber optic cables. Researchers Camille Brès and Luc Thévenaz from the Ecole Polytechnique Fédérale de Lausanne (EPFL) demonstrated how to squeeze as many as ten more pulses into a fiber in a paper published in Nature Communications in December. The researchers modulated the lasers to create rectangular pulses with little to no dead space between them that had frequencies of equal intensity. A Technical University of Munich group, led by Benedikt Mayer, demonstrated near-IR-emitting room-temperature laser nanowires. The nanowires were built in a core-shell arrangement, and the researchers reported their findings in Nature Communications in December. They pointed out that while the nanowires could be grown directly on silicon chips, this was a plus; they also needed optical pumping, a drawback because many applications call for electrically injected devices.
In a paper published in the October issue of Applied Optics, physicists Yuri Rezunkov and Alexander Schmidt suggested that lasers might help rockets fly higher. Rocket propulsion using laser ablation has long been proposed. This technique involves striking a surface with a laser to produce a plasma plume that, as it leaves, produces thrust. The speed at which the plume exits is increased by integrating laser ablation with a gas steering system to flow close to the interior walls of a spacecraft’s nozzles, increasing thrust and making the technique more useable. The data leaped forward significantly in November. The European Space Agency and collaborators used lasers to transmit gigabits of data over about 45,000 kilometers between two satellites in geosynchronous and low Earth orbit. According to them, future scaling of the design could reach 7.2 Gb/s. Data could move between satellites and eventually reach the ground more quickly because the link was faster than what was previously available. The previous system could only transmit to specific ground stations when the satellite was in range. Geosynchronous satellite connections filled in these gaps. Physical Review Letters published a report from the Lawrence Berkeley National Laboratory on a new world record for a small or “tabletop” particle accelerator: 4.25 GeV. The energy gradient that accelerated the electrons was 1000 times greater than that of conventional particle accelerators because this was accomplished in a 9-cm-long tube. Sub-petawatt laser pulses were launched into the plasma by the researchers. The light energy pulses, close to a quadrillion (1015 or a million billion) watts in power, propelled the electrons like a surfer riding a wave, bringing them to well within 0.01% of the speed of light.
A group led by Texas A&M University physicist Brett Hokr added another random element to the amazing light toolkit in May. The researchers described a random Raman laser that can generate a wide-field, speckle-free image with a strobe time of roughly a nanosecond in a presentation at CLEO 2015. Tests revealed that the random Raman laser pulse had a spectral width of about 0.1 nm and lasted only a few nanoseconds. The melanosomes, which are organelles found in animal cells and are the location of the synthesis, storage, and transport of the light-absorbing pigment melanin, are seen forming a cavitation bubble in the full-frame, a speckle-free microscopic image created by the researchers using these pulses. Laser printing that is too small to be seen by the unaided eye could be used to encode data, according to research from the Technical University of Denmark’s Anders Kristensen and colleagues published in the December issue of Nature Nanotechnology. They bent 100-nm-diameter columns with laser beams, producing color when illuminated. The researchers used this to produce a “Mona Lisa” replica that is 50 m wide and 10,000 m smaller than the original. According to researchers, potential applications include producing tiny serial numbers, barcodes, and other data.
Research on cells ingesting microresonators was simultaneously published by two groups in Nature Photonics (Harvard Medical School) and Nano Letters (University of St Andrews). These tiny plastic beads trap light by directing it in a circular path along their circumference. The resonators can lase optically while optically pumped by nanojoule light sources without harming the cell. Each cell has a unique microlaser that has a different spectral makeup. The researchers pointed out that this might open new avenues for cell tracking, intracellular sensing, and adaptive imaging for millions or even billions of cells.
ASML, a semiconductor lithography toolmaker, announced the readiness of EUV (extreme ultraviolet) lithography technology at the February SPIE Advanced Lithography Symposium in San Jose, Calif. ASML backed the laser-produced plasma approach after years of development that stalled because the light source was insufficiently bright. In this method, an infrared CO2 laser fires a concentrated pulse at a microscopic molten tin droplet. After filtering the resulting emission burst, a 13.5-nm, or EUV, light pulse was produced. This technology and its resulting wavelength, much shorter than the 193-nm-deep UV lasers used in semiconductor production, are critical to continuing semiconductor manufacturing advancements.
Researchers from Cardiff University, University College London, and the University of Sheffield reported growing quantum dot lasers on silicon in the March issue of Nature Photonics. The lasers were electrically pumped, emitted at 1300 nm, and were shown to operate for up to 100,000 hours at temperatures as high as 120 °C. The team’s goal was to integrate photonics with silicon electronics.
In September, the Laser Guide Star Alliance took third place in the 2016 Berthold Leibinger Innovation Prize. Modern telescopes use optical wavefront correction to eliminate the atmospheric oscillations that cause stars to twinkle. As a result, you can see anything possible in space. However, the achievement requires guide stars that are bright enough to allow for the correction. When there are no visible guide stars, astronomers create artificial ones by exciting a sodium layer at about 90 kilometers. The Laser Guide Star Alliance used Raman amplification to generate the required wavelength at a record power level significantly higher than 20 W for the Very Large Telescope in Chile’s Atacama Desert. The team used diode and fiber lasers in an eight-year development effort.
In a February release, NASA’s Jet Propulsion Laboratory in Pasadena, California, stated that lasers could give space communications a “broadband” moment. Radio has been the primary mode of communication since the dawn of the Space Age. It translates best to connections that operate at a few megabits per second. A spacecraft orbiting Mars, for example, has a maximum radio transmission rate of 6 Mb/s. A laser could boost the rate to around 250 Mb/s. Conversely, Lasers are susceptible to cloud interference, require more precise pointing, and require a ground-based infrastructure to support them. Missions planned for 2019 and 2023 will put the technology to the test, determining whether lasers have a future in space communications.
Rugged fiber lasers have grown in power and application since their commercial debut in the 1990s, with one example being a weapon developed by Lockheed Martin for the US military. During March testing, the system produced a single beam of 58 kW, a world record for this type of laser. In 2015, a laser half this power disabled a truck from a mile away in tests. According to reports, the laser achieves the 60-kW threshold by combining several beams and operating close to the diffraction limit. The laser system is also efficient, converting over 43% of the energy consumed into the light.
Researchers from the University of St Andrews, the University of Wurzburg, and the Technical University of Dresden created a fluorescent protein polariton laser by borrowing a molecule from nature. Previous polariton lasers required cryogenic cooling, but these new lasers are based on green fluorescent protein, which causes jellyfish to emit bright green light. According to the scientists, the molecule was just the right size to strike the optimal balance between not losing energy and quenching and being able to squeeze as many molecules as possible into the jellyfish’s light-emitting cells. According to the researchers, the new laser could be a biocompatible, bio-implantable light source. The findings were published in the August 16 issue of Science Advances.
In July, the National Ignition Facility laser system at Lawrence Livermore National Laboratory set a new record: 2.15 MJ. This was more than 10% higher than the previous high of March 2012. In an August Optica paper, NIST (National Institute of Standards and Technology) researchers demonstrated that commercial laser ranging could provide 3D images of objects melting in a fire. The NIST team measured 3D surfaces on chocolate and a plastic toy with 30-m precision from a distance of two meters. The ability to precisely and safely measure burning structures as they collapse could help us understand the destruction process and later reconstruct what happened.
A nanoscale manipulation technique described in a September paper in Nature Communications may make random lasers less random. A team from the Tampere University of Technology in Finland, Case Western Reserve University in Ohio, and others demonstrated that the output of a random laser based on a liquid-crystal medium could be steered with an electrical signal. According to the researchers, this control capability has brought random lasers closer to practical applications.
The Shanghai Superintense Ultrafast Laser Facility scientists set their sights on a 10-petawatt shot, nearly doubling their previous record of 5.3 petawatts (PW) (5.3 million billion watts). Wenqi Li and colleagues reported significant progress toward that threshold in a paper published in the November issue of Optics Letters, with a nearly 340-J output centered at 800 nm. Peak power was estimated to be 10.3 PW when compressed to a 21-fs pulse. The goal is to reach 100-PW status by 2023. This level of power would be sufficient to create matter from nothing.
MIT researchers proposed using lasers to deliver whispers to listeners. A 1.9-m wavelength thulium laser was used to excite water molecules near a microphone, transmitting an audible signal. The signal was roughly the volume of normal conversation. The technique could enable the transmission of secret messages, with applications in the military and advertising. A paper was published in the January issue of Optics Letters.