At the dawn of the laser age, any color could be attained from a laser, as long as it was red. Theodore Maiman’s first laser was a ruby rod that emitted pulses at 694 nm, the long end of the visible spectrum. The first laser to emit a visible, continuous beam was a glass tube containing a mixture of helium and neon emitting at 632.8 nm, also in the red. Through most of the 20th century, the first laser beams that were most likely to be seen were red. Engineers and physicists added a handful of new visible laser colors from gases throughout this first decade. Later, new semiconductor compounds brought more shades of red and eventually blue laser light to the toolboxes of engineers and scientists, leading to new products. In the 21st century, lasers continue to reveal more colors to the human eye. In this photo from the 1970s, celebrated physicists John Emmett (left) and John Nuckolls, visionaries in early laser fusion technology, discuss flashlamp-pumped, solid-state laser technology. Courtesy of Lawrence Livermore National Laboratory. Spectra and lasers Maiman did not set out to make a beam he could see. He wanted to demonstrate that the laser concept could work, so he started with a material he knew well, ruby, which only happened to emit in the visible. He calculated that it would require a significant amount of light to excite the laser, so he chose a photographic lamp that generated extremely bright, yet brief, flashes. On May 16, 1960, at Hughes Research Labs, Maiman fired a series of pulses, turning up the energy each time, until the pump light reached the threshold and the ruby rod flashed the world’s first bright red pulse of laser light. For Maiman, the technology worked on his first demonstration. Within weeks of reporting his success, others had already replicated his results and experimented with ruby lasers. They are still in use today. The 1962 PerkinElmer/Spectra-Physics Model 110 helium-neon (HeNe) laser was among the first of the commercially available lasers with a continuous visual beam. Courtesy of Lynn Savage. At the same time, others bought large, bright flashlamps and flashed them at different materials. Peter Sorokin at the IBM Watson Research Lab picked uranium doped in calcium fluoride, which emitted at 2.5 µm, a value that he could measure with the instruments in his lab. It worked but never proved to be practical. L.F. Johnson and K. Nassau at Bell Labs used a flashlamp to excite 1.06-µm laser pulses from neodymium in calcium tungstate in 1961. The beam was not visible, but neodymium in glass and crystals would be the best-selling solid-state laser for decades. Soon afterward, Elias Snitzer at American Optical flashlamp-pumped neodymium in a clad rod of optical glass, demonstrating the first fiber laser. Flashlamp-pumped ruby was on the market by early 1961 and would remain the only visible solid-state laser on sale for decades. Yet, others wanted lasers that could emit a continuous beam. Gas lasers and gaseous electronics Semiconductors and solid-state physics were burgeoning areas of innovation in the 1960s, but gaseous electronics were a tried-and-true technology for generating light continuously by passing an electric current through gas, such as in televisions and fluorescent lights. At Bell Labs, Ali Javan thought starting with a mature technology would be the best way to make a continuous laser. “Helium-neon (He-Ne) was one of the cleanest systems I could find. It also was a medium where I could show there was gain without first having to make the laser,” Javan said in an interview1. He collaborated with three colleagues to determine the physics and optics required for a low-gain gas medium. Seven months after Maiman’s demonstration, members of Javan’s group spotted gain at 1.15 µm on their continuous-wave HeNe laser. They did not see the beam because they had designed their laser to operate at 1.15 µm, where they had expected the highest gain. The now-standard 632.8-nm neon line did not lase until 1962, after Alan White and Dane Rigden at Bell added large amounts of helium to their tube, increasing its red neon emission 50-fold. The red HeNe was both a breakthrough and an eye-opener; its pencil beam and the effects of laser coherence could be seen. The red laser light brought 3D laser holography to life when Emmett Leith and Juris Upatnieks used a factory-made HeNe laser to illuminate a toy train, and, with it, a red 3D image hung in the air in front of them. When they displayed a hologram at an Optical Society meeting, a long line of witnesses — amazed by the image — formed in the hallway. The fascination with holography would make HeNes the best-selling lasers of the 1960s and they would remain so until the 1980s. The color of argon If electricity could produce a bright red laser beam from neon, what could it produce from other gases? This question stimulated many experiments in the mid-1960s. One experimental failure led to the development of the family of rare-gas ion lasers. Disappointed with a mixture of argon and mercury, in February 1964, Bill Bridges at Hughes blew the old mercury out of the tube with helium and was surprised to see a discharge through the tube produce a blue emission at 488 nm from traces of argon ions. The unexpected laser light was not just blue — it was also very bright. Bridges soon found a somewhat stronger green argon line, at 514.5 nm. The brightness of the argon-ion laser established it as an altogether new tool for laser experimenters: The red HeNe laser peaked at only tens of milliwatts at best, but cranking up the discharge current through the argon tube could generate much more light. Laser designers eventually reached tens of watts of output, by far the brightest visible continuous beams on the market for many years. Although not as bright, krypton lasers offered other wavelengths. Output(s) could reach watts at 647 nm in the red. It also offered lines where no other lasers were yet available, including in the yellow. By combining argon and krypton in a tube, one could make a multi-colored laser light show. Laser entertainment surged in the 1970s and attracted tens of millions of people over the years. As the brightest lasers in the visible spectrum, argon-ion lasers also found a variety of other applications that needed bright beams at short wavelengths, including in medicine and avionics. Metal vapor gas lasers A systematic study for ionized metal vapors by Bill Silfvast at the University of Utah yielded the helium-cadmium (HeCd) laser, which long held the brightest visible line at 441.6 nm, in the blue. It was less powerful but more economical than argon-ion lasers, making it a more attractive option for some applications. Xerox, for example, selected a 15-mW HeCd because other drum laser printers designed for mainframe printers were insensitive to red HeNe emission. The combination of its simplicity and favorable wavelengths, Silfvast said, ultimately established the HeCd as the most successful metal vapor laser. The emergence of krypton lasers in the 1970s offered yellow laser lines. Technological developments in metal vapor lasing and, particularly, neutral copper vapor lasers, yielded strong emission at 578 nm — also in the yellow. In 2020, researchers at the Physical Research Laboratory at Gujarat University developed a compact, tunable, and ultrafast high-power yellow laser. Supported applications include those in biomedical and full-color video projection. See Reference 2. Courtesy of Varun Sharma. The highest visible power from metal vapor lasers came from a neutral copper vapor that only operates in a repetitively pulsed mode and has strong emission at 510 nm in the green and at 578 nm in the yellow. It was used for pumping dye lasers, and it was also seriously studied as a pump source for enriching uranium isotopes. Optically pumped dye lasers While experimenting with effects of firing a ruby laser into fluorescent dyes in 1966, Sorokin observed laser emission in the fluorescence. Around the same time, Mary Spaeth, then at Hughes, realized that putting dye in a suitable solvent greatly increased its absorption bandwidth — which would allow wavelength tuning in a laser cavity with dispersive optics. Spaeth performed this demonstration by pumping the dye with a ruby laser. With the idea of pumping dyes trialed, others followed with demonstrations of continuous-wave pumping, with argon-ion lasers, and pulsed pumping, with flashlamps. The tunability of dye lasers offered wavelength selection across a wide range, including through the visible, making them a boon for spectroscopists. In practice, tunable dye lasers had significant limits, and their main uses were restricted to laboratory applications; dyes were short-lived, individual dyes had limited ranges, and continuous-wave pump lasers carried a high price tag. The diode laser revolution Laser choices remained limited at visible wavelengths well into the 1980s. The only solid-state laser on the market emitting in the visible was ruby. Dye lasers required either pump lasers or flashlamps for pumping. And the most widely used visible lasers were gas. During this time, HeNe lasers were the bestsellers by far, with consumer markets in supermarket scanners as well as industry. Argon-ion laser sales were several thousand per year, hindered by cooling requirements and a wall plug efficiency of only ~0.01%. Beam powers ranged from a fraction of 1 W to as much as 40 W, with the largest lasers requiring a fire hose to cool. Annual HeCd laser sales, meanwhile, reached around 1000 in 1983. By then, researchers had cataloged more than 6000 wavelengths emitted by gas lasers. Only around 600 were in the visible, and only a handful of those were produced commercially in significant quantity. The first diode lasers, demonstrated in 1962, were made of gallium, aluminum, and arsenic and emitted at 710 to 900 nm, in the near-infrared. Adding more aluminum decreased the wavelength, but at the cost of reducing the lifetime, which was a serious problem. Not until 1970 could diode lasers operate continuously at room temperature — a feat which earned Zhores Alferov and Herbert Kroemer the 2000 Nobel Prize in physics. Tunable diode lasers — shown here using tapered semiconductor amplifiers — provide high power and more wavelengths. Their emergence sparked possibilities in industrial and medical laser applications. Courtesy of TOPTICA Photonics AG. A detailed look at a frequency-converted system diode laser. This system is tunable and can be used in a range of applications, including spectroscopy, laser cooling, holography, and interferometry. Courtesy of TOPTICA Photonics AG. Gallium-aluminum-arsenide diode laser lifetimes remained problematic at visible wavelengths through the mid-1980s. Eventually, improvements in diode structure extended to the point where red laser pointers emitting at 625 to 700 nm became essential parts of photonics presentations and diode lasers were finally established in the visible spectrum. Red laser pointers were only a small (though ubiquitous) part of the diode laser revolution. Development of high-power diode lasers emitting at 780 to 1000 nm based on gallium-arsenide technology opened the door to diode pumping of solid-state lasers. Flashlamps had long been used because of their high intensity, but they emitted across a wide range of wavelengths, meaning that minimal pump light excited light emission by the laser. Diode lasers emit at a narrow range of wavelengths, and their composition can be adjusted to match the peak absorption of the laser material. For example, diodes are designed to emit at 808 nm to be absorbed at the 808-nm absorption peak of Nd:YAG. The match between narrowband pump-diode emission and the peak absorption of the laser material makes diode pumping inherently more efficient than broadband flashlamp pumping. The higher efficiency of diode pumping has created a new class of products: continuous-wave solid-state lasers. Still, it would require further innovation to reach visible wavelengths. Nonlinear optics and new wavelengths The concept of nonlinear optics has a long history with lasers. Shortly after Peter Franken at the University of Michigan obtained one of the first commercial ruby lasers in 1961, he fired a high-energy red pulse at crystal quartz and produced a much smaller pulse of the second harmonic at 347 nm, in the UV. This experiment demonstrated that a laser beam’s combination of coherence and light intensity could produce interesting effects, including harmonic generation and producing sum frequencies from two beams. It led to much research in nonlinear optics, although it required high beam intensity and strong linearities to accomplish much in application. Diode pumping could produce the high intensities needed to make changes. Initially, efforts were small, such as the development of green laser pointers. Students in the Stanford University lab of Bob Byer pioneered another early achievement. They used pump diodes to produce a 1064-nm neodymium beam, then put it through a frequency doubler to get 532-nm green light. The impetus for the achievement was that Byer is color-blind, so he cannot see red pointers. Welding, cutting, and foil joining of high-reflectance materials are among the target applications for blue diode lasers. These sources have also proved to be vital in the manufacture of electric vehicles and battery cells. Courtesy of Laserline. Frequency doubling can accomplish much more, including stretching the spectrum of Ti:sapphire lasers into the visible range. Ti:sapphire is tunable from 660 to 1100 nm, with a pump band at 500 nm. Frequency doubling light from neodymium or other near-infrared solid-state lasers, such as ytterbium, can easily supply a pump source. Then, frequency doubling of the Ti:sapphire will cover much of the visible spectrum. These capabilities have helped Ti:sapphire replace many tunable dye lasers and meet application needs that their predecessors could not. Frequency doubling (and tripling) also can be used with fiber lasers to make both high-power green and UV lasers. Blue diode lasers The advent of blue diode lasers in the 1990s was an unexpected revolution in semiconductor lasers. Years of research had convinced many researchers that gallium indium nitride, with bandgaps from the visible green to the UV, was a material too flawed to make effective diode lasers. Imasu Akasaki at Nagoya University was unconvinced. In 1991, he pulled a bright blue diode out of his pocket and showed it at a materials research conference. A few years later, Shuji Nakamura showed off an impressively bright violet laser diode and detailed his progress before a packed conference room. What excited the photonics world initially about this innovation was the prospect of high-capacity video-discs for high-definition television using short-wavelength laser diodes. The technology worked, but streaming video won the high-definition market over Blu-ray discs. In parallel, other markets appeared for blue laser diodes, which now span 375 to 525 nm. However, the biggest success of indium gallium arsenide has been in high-efficiency solid-state lighting, for which Akasaki; Nakamura at University of California, Santa Barbara; and Hiroshi Amano at Nagoya shared the Nobel Prize in physics in 2014. A rich spectrum of visible lasers Looking at today’s laser market in the broader context of photonics, one cannot help but be amazed by the range of technology. Many years ago, it would have been unfathomable to have lasers that could be tuned across the spectrum. We have come a long way to that somewhat fanciful ideal. There is also a great variety of tools to be deployed for shaping today’s spectrum of lasers. Tailoring our selection of structures and chemistry in semiconductors, optimizing how we shape optics and resonances, and the use of nonlinear optics are just some of the available techniques. Some ideas proposed in the boom years of fiber optics — some of which did not get off the ground then — currently appear to be both possible and useful. One strength of the photonics community, beyond laser science, is the tremendous diversity of technology. As this rich selection of tools and techniques to manipulate light continues to expand, it is imperative to maintain this theme. Meet the author Jeff Hecht is a veteran science and technology writer with a specialty in optics and an Optica Fellow. He is the author of Understanding Lasers, Understanding Fiber Optics, City of Light: The Story of Fiber Optics, and Beam: The Race to Make the Laser; email: jeff@jeffhecht.com. References 1. J. Hecht (1991). Laser Pioneers. First ed. Academic Press. 2. D. Yadav et al. (2020). Watt-level, ultrafast, tunable yellow source based on single-pass, fourth-harmonic generation of Cr2+:ZnS laser at 2360 nm. Optics Letters, Vol. 45, No. 18, pp. 5109-5112.
