In-depth The Birth of Lasers and Fiber Optics

Introduction: Since the 1950s, an optical revolution has been taking place in science and technology, with the birth of lasers and optical fibers, bringing revolutionary breakthroughs. In fact, the birth of lasers and optical fibers is also in the realm of the first quantum revolution, as the development of these technologies is based on the observation and application of quantum laws. This article is from the first issue of OPTICA in the New Year, originally titled "Two Breakthroughs in the Optical Revolution," in which the central point is that the development of information technology is not driven by any single breakthrough. This is still an important guideline for today, when the second quantum revolution was initiated. Unlike the first quantum revolution, the second quantum revolution is the active modulation and manipulation of quantum states by humans, thus enabling quantum information technology.

 

The shared history of lasers and optical fibers illustrates that the development of truly revolutionary breakthroughs are similar.

 

In 1950, optics was a lagging field in physics. World War II had spurred major advances in spectroscopy and infrared technology, but optics seemed an unattractive world of lenses and optical instruments. When Jay Last graduated from the Institute of Optics at the University of Rochester in 1951, his professor, Parker Givens, said it was in solid-state physics that this happened.

 

Last said, "Optics is necessary, but it doesn't rock; and solid-state electronics is creating something new every day." After his doctorate, he moved to California in 1956 to co-found Fairchild Semiconductor, which put him at the center of Silicon Valley's integrated electronics initiative. There, he would become part of the semiconductor frontier, one of the few major breakthroughs that formed the basis of modern information technology, which in turn provided new capabilities and enhanced performance for machines and products in the power industry era.

 

The great breakthroughs themselves were the product of a series of advances. First was the programming of machines, which began with the invention of the Jacquard loom in 1804 and developed through geared calculators and vacuum tubes; second was semiconductor electronics, which began with transistors and progressed to integrated electronics at a breakneck pace. What made these two technologies great breakthroughs was their ability to multiply their own progress, thus opening up vast new possibilities. Jacquard looms repeatedly weave threads into elaborate patterns; today's software controls the complex operations of rocket launches or climate simulations. Semiconductor electronics began as miniature versions of vacuum tubes, but have grown exponentially into powerful computing machines.

 

However, programming and integrating electronics is only one part of information technology. If computing machines cannot communicate with each other, they remain functionally limited; transporting the enormous amount of information they process and generate requires a global, high-capacity network.

 

Building such a network is the work of light, made possible by two other great breakthroughs: lasers and optical fiber. Lasers provide tremendous control over light, making it monochromatic and tightly focused to the femtosecond in units of space and time. Optical fibers channel light that would otherwise dissipate into waveguides and transmit it around the world through hair-thin fibers made of almost completely transparent glass.

 

Remarkably, the power of computers and the transmission capacity of optical communications have grown rapidly. Since the 1980s, the growth in the number of transistors on a chip has followed Moore's Law, paralleling the growth in fiber-optic transmission capacity. The evolution of information technology has not been driven by any single breakthrough, but by the evolution of a complementary combination of four major breakthroughs: software, semiconductor electronics, lasers, and optical fiber.

 

Left: Optical fiber, Bell Labs, 1976. Right: General Electric's laser-assisted processing, 1975.

 

Mid-20th Century: The Optical Renaissance

 

The first tremors in optical breakthroughs occurred in the mid-1950s. The first glass-clad optical fiber made its debut in December 1956, and in February 1957, a doctor tested a fiber-optic gastroscope in a patient's throat. Later that year, efforts to build a laser began, and the first laser was demonstrated with the first laser in 1960. Fiber optics could direct light around corners. The laser created a new form of light: coherent, and focused in a narrow beam. The old light was white and spread in all directions; the new light was tightly controlled in wavelength and direction.

 

In 1841, the Swiss physicist Jean-Diel Colladon demonstrated for the first time a total internal reflection light guide that directed light into a stream of water; in 1884, his illustration of a "light fountain" was published in the journal Nature.

 

Both of these breakthroughs emerged from the new physics of the late 19th and early 20th centuries. Fiber optics had its roots in the traditional concept of total internal reflection, but the concept of waveguides was derived from Maxwell's theory of electromagnetic waves. Lasers arose from the concept of stimulated emission, which Einstein introduced in his 1916 analysis of Max Planck's radiation law. Both optical fibers and lasers slowly matured as theoretical and experimental knowledge expanded, and as technology developed.

 

Of course, the historical story of optical fibers and lasers has been told many times. But they deserve to be studied together again. Looking back, we can certainly point to milestones: the first glass-clad fiber in 1956, the first laser in 1960, and the first low-loss fiber in 1970. However, what transformed optics into a major technology was a series of important advances, made over decades by many people standing on the shoulders of giants.

 

Total internal reflection and optical fibers

 

Total internal reflection was known in the early 17th century, but fluctuation theory was not able to explain it until the early 19th century. The first practical light guide was a bent glass rod, used in the early 1900s to illuminate a dental patient's mouth with light from a heat lamp, which may have led to the idea of bundling glass fibers together to transmit images. in 1930, Heinrich Lamm, a German medical student, transmitted a bright image of an incandescent bulb filament through a bundle of loose glass fibers. His goal was to create a flexible endoscope to view the stomach, but he could not go further.

 

After World War II, others tried to transmit images through a loose, exposed bundle of fibers, but were hampered by high losses. The first to propose a solution was Brian O'Brien in 1951, when he was both president of OSA (now Optica) and director of the Institute of Optics at the University of Rochester. With degrees in electrical engineering and physics, O'Brien recognized that optical fibers were the optical counterparts of the plastic dielectric rods used to transmit radio signals because both are nonconductive waveguides that conduct electromagnetic waves along their length. He realized that cladding an optical fiber with a lower refractive index material would reduce light leakage into the air.

 

The problem was to find a low-refractive-index transparent material suitable for the cladding. in 1956, Larry Curtiss, an undergraduate student at the University of Michigan, succeeded by inserting a high-refractive-index glass rod into a low-refractive-index tube and melting them together. After he pulled out 40 feet of fiber, he could see the glow from the melting furnace through the fiber. His experiment opened the door to practical medical endoscopy, an early major application of fiber optics.

 

Larry Curtiss had the idea for a practical medical endoscope when he was a physics student at the University of Michigan.

 

An American eyewear manufacturer, American Optical Company, which also innovated in other areas of optics, took a different path: stacking many thin fiber rods together and drawing them into rigid fused rods. When the individual fiber cores reached a few microns in diameter, Will Hicks, who worked at the company, noticed strange patterns that his colleague Elias Snitzer identified as modal patterns, all the way down to single mode. The discovery of single-mode fiber transmission later proved to be important for fiber optic communications.

 

1961, the first fiber laser

 

Experiments in the late 1920s proved the existence of excited radiation. Later, in search of higher frequency sources of microwave spectroscopy, Charles Townes discovered how to amplify the excited emission. Microwave spectroscopy was his main interest at Columbia University, USA, and in 1951 he had the bright idea that molecular transitions might provide the required high frequencies. in 1954, his student James Gordon created the first maser source.

 

In 1957, Townes turned to making an optical version of the maser, which presented different challenges: what luminescent materials to use, how to excite atoms or molecules to high energy levels, and how to design a resonant cavity. Others followed, but the problem was a thorny one. theodore Maiman, like O'Brien, had degrees in engineering and physics, and found success by exciting his familiar ruby with white light from a commercial photographic flash.

 

Theodore Maiman (The actual flash used by Maiman in the first laser was another, smaller flash.)

 

Maiman announced the invention of the laser at a press conference on July 7, 1960, making optics the front page of newspapers around the world. Although the tabloids called it "the ray gun of science fiction," it was immediately recognized as a breakthrough in generating new light sources. IBM soon used the flashlight to make lasers out of other materials. In December of the same year, Bell Labs demonstrated the first gas and continuous wave laser. 1961 saw the first fiber laser when Snitzer of the American Optical Company produced a laser pulse from a glass rod with a neodymium-doped core.

 

As new lasers proliferated, engineers and scientists sought ways to use them. Early tests showed that lasers could drill holes in diamonds, measure distances and produce nonlinear effects. Among other potential applications, the most important is laser beam communication.

 

Laser and Optical Communications

 

Broadcast and long-distance telephone calls grew steadily after World War II, and the communications industry wanted to transmit signals at higher carrier frequencies, providing wider bandwidth.

 

In the 1950s, Bell Systems, a regulated monopoly providing telephone service in the United States, began developing a system that transmitted at 50 GHz, which had to be buried in hollow metal waveguides because the atmosphere would absorb signals at those frequencies.

 

While consulting for Bell Labs, Townes got them interested in the greater potential transmission capability of optical frequencies for next-generation telephone systems. Bell began working on optical waveguides after early tests showed unstable transmission of laser beams through the atmosphere. Rudolf Kompfner, head of transmission research at Bell, first thought of using hollow waveguides, like those used in 50 GHz systems. He also asked one of his employees to find the loss of the clearest fiber on the market; the answer was about 1,000 dB/km, clearly not enough for communications, so Bell pursued hollow optical waveguides.

 

Chinese-born physicist Charles Kao measures the transparency of fused quartz at the Standard Telecommunications Laboratory in the United Kingdom.

 

Corning Glass Works in the United States has developed ultra-pure quartz glass that can withstand high baking temperatures. When Corning's Robert Maurer heard about Kao's research on ultra-clear glass, he and his colleague Frank Zimar began a small project: modifying heat-resistant glass to achieve low attenuation. When that project went well, he hired young scientists Donald Keck and Peter Schultz to spend more time on it, and in 1970 they reported a fiber with a loss of 17 dB/km on a red He-Ne laser line.

 

This was a game-changing breakthrough, but the first low-loss fibers were too fragile for practical use. in 1972, Corning reported a reduction in loss to 4 dB/km at 850 nm, and the addition of germanium to the core made the fiber more durable. in 1976, Masaharu Horiguchi of NTT's Ibaraki Laboratory in Japan and Fujikura Cable's Hiroshi Osanai of Fujikura Cable, Japan, opened the transmission window at the zero-dispersion wavelength of 1.3 µm and the fiber loss minimum of 1.55 µm. Their losses were below 0.5 dB/km at both wavelengths, which shifted work on long-haul fiber to these bands. By that time, Bell had completed field testing and quietly abandoned the 50 GHz system.

 

Another breakthrough: diode lasers

 

A breakthrough in fiber loss would not have made sense without a breakthrough in laser performance. Early gas and solid-state lasers were bulky and inefficient, so the invention of the diode laser in 1962 gave a major boost to laser communications. It came so soon after researchers at the Massachusetts Institute of Technology (MIT) Lincoln Laboratory reported a jump in light emission from gallium arsenide diodes that one lecture attendee thought they violated the second law of thermodynamics.

 

A close-up of an early diode laser at the Massachusetts Institute of Technology's (MIT) Lincoln Laboratory.

 

The MIT team didn't do this, but it showed that GaAs is a very good candidate for diode lasers. Within a few weeks, the team and three other teams added resonators to make the first diode lasers. Diode lasers, made from semiconductors, are the hottest technology in electronics and are starting to look very attractive for laser communications.

 

However, the first diode lasers could only emit short bursts of pulses at low temperatures. It took a series of advances over a decade to achieve reliable room temperature operation. The first step was the Nobel Prize-winning invention of semiconductor heterostructures by Zhores I. Alferov and Herbert Kroemer, but it was not until 1970 that Alferov's group and an independent group at Bell Labs demonstrated room-temperature continuous-wave operation of diode lasers. It took another seven years before Bell Labs produced a gallium arsenide diode laser capable of operating at room temperature for one hundred years.

 

Ironically, around the same time, the opening of the long-wavelength fiber window shifted the ideal wavelength from GaAs's 850 nm band to the 1310 nm band, where quartz fibers have zero dispersion and low attenuation. Fortunately, for compound semiconductors, changing the mixture of elements in the diode can alter its emission line. In this case, adding indium and phosphorus to gallium arsenide increased its wavelength, so it didn't take long to produce an InGaAsP laser that emitted at 1310 nm, and later at 1550 nm. Although there were some minor trade-offs, the process developed for GaAs was effective for most of the long wavelengths of InGaAsP.

 

The technological breakthrough that followed: single-mode fiber

 

Kao's initial proposal called for single-mode fiber because, based on his experience with 50 GHz buried millimeter waveguides, multimode transmission could lead to serious noise problems. However, the small cores required for single-mode transmission make it difficult to connect two sections of fiber without missing most of the light. An 850 nm step-index fiber must have a core diameter of less than about 5 µm to be used for single-mode operation, which makes optical coupling lossy.

 

Shifting transmission to 1310 nm changes the rules. At that wavelength, the single-mode core diameter is about 9 microns, and the tolerance of the mechanical connection is improved. But the biggest benefit was the elimination of modal dispersion and reduced attenuation, which limited data rates and transmission distances. In the early 1980s, single-mode systems jumped from 45 Mbit/s and 10 km to 400 Mbit/s and 30 km; by the 1990s, with a maximum data rate of 2.5 Gbit/s, a wave of technological breakthroughs began to take shape.

 

The great foundation of transmission systems: fiber-optic amplifiers and WDM

 

Long-distance fiber optic transmission in the 1980s relied on electro-optical repeaters spaced 30 to 50 kilometers apart. Each repeater converted the incoming optical signal into electronic form, electronically amplified it, and then used the electronic output to drive a laser that would send the signal to the next span. Quartz optical fibers are close to their minimum possible attenuation, and no better fiber material could be found. Semiconductor optical amplifiers (diode lasers without resonators) seemed like the logical next step, but their signal quality proved inadequate.

 

 

David Payne in his lab.

 

The solution to this problem comes from another branch of the laser family: fiber lasers.

 

David Payne, of the University of Southampton in the United Kingdom, found that erbium added to a fiber core can be emitted in the 1550-nm band, where silica is most transparent. At 980 nm pumping, it is a good fiber amplifier, a wavelength efficiently produced by InGaAs diode lasers.

 

Better yet, erbium has a broad gain band, so it can amplify a range of wavelengths around 1550 nm, enabling wavelength division multiplexing (WDM). Shifting fiber transmission to 1550 nm involves some complex engineering to get the details right. The gain is not uniform across the erbium band, so precision optics are required to adjust the gain to achieve uniform power across all wavelengths. The shift to 1550 nm also requires compensation for dispersion, which is zero at 1310 nm. But dealing with these complications increased the bandwidth of a single fiber by a factor of nearly 100, at a time when other improvements had increased the fiber bandwidth by a factor of four to 10 Gbit/s.

 

The timing seemed perfect, because in the 1990s, Internet traffic was growing by leaps and bounds, and both users and operators needed more bandwidth. This growth led to a boom in the telecommunications market in the 1990s. Stocks of previously obscure optical companies soared, and market gurus predicted a glorious future. Ultimately, however, the tremendous advances in optical technology did what no one thought possible: provide too much bandwidth.

 

For many years thereafter, operators could buy dark step refractive index single-mode fiber installed during the bubble for a few cents to provide more bandwidth. They got a bargain because most fibers could carry many wavelength division multiplexing channels. Initially, they used 10 Gbit/s per wavelength, but the introduction of coherent transmission and digital signal processing first increased capacity to 100 Gbit/s per channel, and more recently to 400 or 800 Gbit/s to handle cloud computing and streaming video. New large-mode area fibers are needed to carry the highest capacity signals, but they are essentially improved versions of the ancient step-refractive-index single-mode fibers that have proven to be a great foundation for advanced transmission systems.

 

Looking back at history, lasers, fiber optics become cornerstones of technological society

 

The past half century has seen a series of advances in the field of lasers and fiber optics.

 

A closer look at the design of fiber optic systems over the generations reveals the continued evolution of the fiber, the transmitter, the receiver, the transmission format and the system itself. New technologies have been added: coherent transmission and digital signal processing have replaced dispersion management to improve transmission capabilities; in addition, they have extended the usable life of fibers already buried underground, a significant advantage since installation costs are typically higher than the cables themselves.

 

Laser technology is also evolving. In the early days, most lasers turned no more than a few percent of the input energy into output. Now, diode and fiber lasers can convert more than half of the input energy into an output beam. Optical pumping and nonlinear optics have increased the variety of wavelengths available. The more we explore, the more we learn, the more lasers and fiber can do for us; these are breakthroughs that keep advancing.

 

Modern lasers and fiber optics are not the product of a single invention, but the cumulative creation of generations of scientists and engineers. Contributors stood on the shoulders of giants; other contributors are now in turn standing on the shoulders of giants. This article can only list a few of the founding fathers.

 

Some advances predate the era addressed in this article. snitzer invented the fiber laser in 1961, but the technology did not come into play until diode lasers were mature enough to provide a pump source; Bell Labs demonstrated coherent optical communication in hollow waveguides in the 1960s, but it only became practical around 2010 with the advent of digital signal processing; Corning The company's materials science for purifying fused silica for cookware was the basis for low-loss optical fibers; research in solid-state physics opened the door to semiconductor electronics, which in turn opened the door to compound semiconductor and diode lasers ......

 

By revolutionizing optics, lasers and optical fibers laid the foundation for much of what is done in optics today. In medicine, lasers and fiber optics can make sensitive measurements or perform life-saving surgery. In industry, they can position objects on production lines and weld thick sheets of metal. Lasers have discovered gravitational waves from the distant universe and created fascinating and powerful tools, such as frequency combs.

 

And the breakthroughs that brought us lasers and optical fibers have not only revolutionized optics; they have made optics an essential element of our technological society.

 

Reference links：

https://www.optica-opn.org/home/articles/volume_34/january_2023/features/two_breakthroughs_that_revolutionized_optics/