Origin of fiber optics
Even though it may seem new, the origin of fiber optics actually that dates back several centuries ago.
In 1790, French engineer Claude Chappe invented the first "optical telegraph," It was an optical communication system which consisted of a series of human operated semaphores mounted on top of a tower. This was a signalling device for sending information over distances using mechanically operated arms or flags to represent alphabetical letters. This was a simple yet effective method of communication to relay messages from one place to the next. Over the course of the next century great strides were made in optical science.
In 1870, Irish philosopher and physicist, John Tyndall, demonstrated to the Royal Society, "that light used internal reflection to follow a specific path." He set up a tank of water with a pipe outlet on the one side. He let the water flow from the pipe while he shone a bright light from inside the tank into the water stream. As the water fell, an arc of light followed the water down. This simple experiment, illustrated in Figure 1, marked the first research into the guided transmission of light.
As technology progressed so did applications of optics. Alexander Graham Bell, who is credited for the invention of the telephone, patented an optical telephone system called the "photo phone" in 1880. This unique device used no wires to connect the transmitter and the receiver. The "photo phone" was an optical voice transmission system that used light to carry a human voice. Bell focused direct sunlight on a mirror and then talked into a mechanism that vibrated the mirror. A detector picked up the vibrating beam, decoded it back into voice similarly to the phone did with electrical signals. Unfortunately without direct sunlight the "photo phone" did not work and Bell ended his research on this invention.
"In the same year William Wheeler invented a system of light pipes lined with a highly reflective coating that lit up homes by using light from an electric arc lamp placed in the basement and directing the light around the home with the pipes.
In 1888, the medical team of the company Roth and Reuss of Vienna used bent glass rods to illuminate body cavities. The French engineer Henry Saint-Rene designed a system of bent glass rods in 1895. These rods were for guiding light images in an attempt at early television. In 1898, American David Smith applied for a patent on a dental illuminator using a curved glass rod."
During the 1920's, the Scottish engineer and inventor, John Logie Baird patented the idea of using arrays of transparent rods to transmit images for television and the American, Clarence W. Hansell patented the idea for facsimiles. Baird's 30 line images were the first demonstrations of television using the total internal reflection of light.
In 1930, a German medical student, Heinrich Lamm was the first person to assemble a bundle of optical fibers together to carry an image. During these experiments, he transmitted an image of a light bulb filament through the bundle of optical fibers. "Lamm's goal was to look inside inaccessible parts of the body". Unfortunately the image was of poor quality and his effort to register his patent was denied, because of Hansell's patent.
By 1951, Holger Moeller applied for a Danish patent on fiber optic imaging in which he uses cladding the glass or plastic fibers with transparent low-index material. This patent was also declined because of Hansell's patents.
In 1954, the Dutch scientist Abraham Van Heel and British scientist Harold H. Hopkins separately published papers on imaging bundles. Hopkins delivered his paper on imaging bundles of unclad fibers while Van Heel reported on simple bundles of cladded fibers that greatly reduced signal interference and "crosstalk between fibers." Van Heel covered a bare fiber with a transparent cladding of a low refractive index. This process protected the fiber reflection surface from outside distortion and greatly reduced interference between fibers. At the time, the greatest obstacle was to find a way in achieving the lowest signal (light) loss, in order for the fibre optics to be workable.
"In 1961, Elias Snitzer of American Optical published a theoretical description of single mode fibers, a fiber with a core so small it could carry light with only one wave-guide mode." Snitzer's idea was acceptable for a medical instrument looking inside the human, but the fiber had a light loss of one decibel per meter. Unfortunately communication devices needed to operate over much longer distances and required a light loss of no more than 10 or 20 decibels per kilometer.
"Charles Kao and George Hockham, of Standard Communications Laboratories in England, published a paper in 1964 demonstrating, theoretically, that light loss in existing glass fibers could be decreased dramatically by removing impurities."
By the late 1970s, telephone companies began to incorporate the use of optical fibers into their communications infrastructure. In 1970, one team of researchers began experimenting with fused silica, a material capable of extreme purity with a high melting point and a low refractive index. Corning Glass researchers Robert Maurer, Donald Keck and Peter Schultz invented fiber optic wire or "Optical Waveguide Fibers" (patent #3,711,262) capable of carrying 65,000 times more information than copper wire, through which information carried by a pattern of light waves could be decoded at a destination even a thousand miles away. The team had solved the problems presented by Dr. Kao.
The Corning breakthrough was among the most dramatic of many developments that opened the door to fiber-optic communications. In that same year, Morton Panish and Izuo Hayashi of Bell Laboratories worked with a group from the Ioffe Physical Institute in Leningrad (now St. Petersburg) and made the first semiconductor diode laser capable of emitting continuous waves at room temperature.
"In 1973, Bell Laboratories developed a modified chemical vapour deposition process that heats chemical vapours and oxygen to form ultra-transparent glass that can be mass-produced into low-loss optical fiber. This process still remains the standard for fiber-optic cable manufacturing."
In 1977, Corning joined forces with another technological giant, Siemens Corporation, to form Corning Cable Systems. Corning's extensive work with fiber, coupled with Siemens' cabling technology, helped launch a new era in the manufacturing of optical fiber cable and associated products. Today, Corning Cable Systems is a world leader in the manufacture of fiber optic cabling system products for voice, data, and video communications applications.
The method and materials invented by Maurer, Keck and Schultz opened the door to the commercialization of fiber optics, first for long-distance telephone service, and later for computer-related telecommunications (such as the Internet) and even medical devices (like the modern endoscope).
The first non-experimental fiber optic link was installed by the Dorset police in England in 1975. Two years later, the first live telephone call using fiber optics takes place in Long Beach, California.
In the late 1970's and early 1980's, telephone companies began to use fibers extensively to rebuild their communications infrastructure. In 1975, the United States Government decided to link the computers in the NORAD headquarters at Cheyenne Mountain using fiber optics to reduce interference.
In 1977, the first optical telephone communication system was installed about 1.5 miles under downtown Chicago, and each optical fiber carried the equivalent of 672 voice channels.
"The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by eliminating the need for optical-electrical-optical repeaters, was invented in 1986 by David Payne of the University of Southampton and Emmanuel Desurvire at Bell Labratories. Based on Desurvire's optimized laser amplification technology, the first transatlantic telephone cable went into operation in 1988."
In 1991, Desurvire and Payne used optical amplifiers and built it into the fiber optic cable. This system could carry 100 times more information than cable with electronic amplifiers.
In that same year photonic crystal fiber was developed, because of its ability to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber. This fiber guides light by means of diffraction from a periodic structure rather than total internal reflection which allows power to be carried more efficiently then with conventional fibers therefore improving performance.
The first all-optic fiber cable, TPC-5, that uses optical amplifiers was laid across the Pacific Ocean in 1996. The following year the Fiber Optic Link Around the Globe (FLAG) became the longest single-cable network in the world and provided the infrastructure for the next generation of Internet applications.
Driven by the development and growth of the internet and communication systems, fiber optic technology and applications continue to grow, including the medical, military, telecommunication, data storage, networking and broadcast industries.
What is Fiber Optics?
Fiber optics is extremely thin strands of purified glass that carry information from one point to another in the form of light. Unlike copper wire, fiber optics does not use electricity during transmission. Optical fibers can be either glass or plastic tubing capable of transmitting light, which is then converted into sound, speech or information. Fiber Optic cables transmit a digital signal via pulses of light through the very thin strand of glass.
A basic fiber optic system consists of:
- a transmitting device, which generates the light signal,
- an optical fiber cable, which carries the light,
- a receiver, which accepts the light signal that was transmitted.
A fiber optic strand is about the thickness of a human hair, about 120 micrometers in diameter and can carry as many as 20 billion light pulses per second. The fibers are bundled together to form optical bundles, which transmit the light signals over long distances up to 50 km without the need for repeaters.
Each optic fiber is made up of three main parts:
- The core is in the center of the optical fiber and is the very thin strand of glass that carries the light.
- The cladding is the optical material which reflects the light signal back into the core. This keeps the light from escaping and allowing it to travel through the fiber.
- The buffer coating or jacket is a plastic material that protects the optical fiber from any damage such as corrosion, moisture and external damage.
There only two main types of fiber optic cables:
- Glass fibers are more common because they allow longer-distance transmission and are more efficient.
- Plastic optical fibers are used in less technical applications and in extremely short-length transmissions.
How are Optical Fibers made?
The glass core is made of silica and is purified to minimize the loss of signal. It is then coated to protect the fibers and to contain the light signals. The signals carried by optical cable consist of electrical signals that have been converted into light energy.
The silica must first be purified before it can be spun into glass fibers. This is a long process in which the silica is heated to extreme temperatures and distilled to purification. All elements have a different atomic weight and will change states at different temperatures. The sand gets heated to a temperature that will change silica to a gaseous state. Then it will be combined with other materials called dopants, which will react with the silica (in a gaseous form) to form glass fibers. All the solid impurities are removed and the gas is cooled, forming the fiber material.
"Once the silica has been purified and the rod is formed, it must be drawn. The silica rod is placed in a furnace or crucible and heated to melting point. It is then dripped in a thread, through a thickness gauge that reduces the thickness to 125 microns. As the fiber is drawn, it is sintered, forming an outside layer called cladding. The fiber is drawn for a distance of about 6 meters (20 feet), first through a coating dye that covers the fiber with a polymer coating, then through a polymer curing oven for the coating. When the fiber reaches the bottom of the tower, it has been coated and cooled and is ready to be spooled. The fiber will be coated again with a buffer, strengthening fiber wrap and outside coating before it can be bundled with hundreds or thousands of other optical fibers in an optical fiber cable for use in the field. Every step of the manufacturing process must be very closely controlled to avoid contamination of the fiber."
How Are Optical Fibers Made?
Now that we know how fiber-optic systems work and why they are useful -- how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.
Making optical fibers requires the following steps:
- Making a preform glass cylinder
- Drawing the fibers from the preform
- Testing the fibers
Making the Preform Blank
The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).
In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:
- The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
- The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.
The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).
The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.
Fiber-optic lines have revolutionized long-distance phone calls, cable TV and the Internet. It enables the long-distance transmission of data in light signals. Find out all about fiber optics.
The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.
Testing the Finished Optical Fiber The finished optical fiber is tested for the following:
- Tensile strength - Must withstand 100,000 lb/in2 or more
- Refractive index profile - Determine numerical aperture as well as screen for optical defects
Fiber-optic lines have revolutionized long-distance phone calls, cable TV and the Internet. It enables the long-distance transmission of data in light signals. Find out all about fiber optics.
Photo courtesy Corning
Finished spool of optical fiber
- Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform
- Attenuation - Determine the extent that light signals of various wavelengths degrade over distance
- Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)
- Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)
- Operating temperature/humidity range
- Temperature dependence of attenuation
- Ability to conduct light underwater - Important for undersea cables
Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.
Types of Optical Fibers
There are two types of optical fibers:
Single Mode Fiber
Single-mode fibers have small cores (9 microns Ø) and transmit infrared laser light (wavelength = 1,300 to 1,550 nm).Single mode fibers transmit a single data stream. The core of the glass fiber is much finer than in multi-mode fibers. Data transmission modes are higher, and the distances that single mode fiber can cover can be over 50 times longer than multi-mode fibers.
Multi-mode fibers have larger cores (50 micron and 62.5 micron Ø) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs). Multi-mode fibers allow different data streams to be sent simultaneously over a particular fiber. The glass fiber has a slightly larger diameter to allow light to be sent through the fiber at different angles. Both 50 micron and 62.5 micron fiber optic cables use an LED or laser light source. They are also used in the same networking applications. The main difference between the two is that 50 micron fiber can support 3 times the bandwidth of 62.5 micron fiber. The 50 micron fiber also supports longer cable runs than 62.5 micron cable.
Simplex cable consists of a single fiber optic strand. Data is transmitted in only a single direction, transmit to receive. Duplex cable consists of two fiber optic strands side-by-side. One strand goes from transmit to receive and the other strand connects receive to transmit. This allows bi-directional communication between devices.
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs. Due to their inferior optical properties, plastic fiber optic (POF) strands and cables are not suitable for extended data transmission.
How does a fiber optic cable work?
Traditionally when we sent data transmissions over copper cables we transmit electrons over a copper conductor. In fiber optic cables we transmit a digital signal via pulses of light through a very thin strand of glass. The fiber strands are extremely thin, no thicker than a human hair.
The basic fiber optic transmission system consists of three basic components:
- fiber optic cable
At the one end, the fiber cable is connected to a transmitter. The transmitter converts electronic pulses into light pulses and sends the optical signal through the fiber cable. At the other end, the fiber cable is plugged into a receiver which decodes the optical signal back into digital pulses.
The core of the cable is surrounded by a cladding which reflects the light back into the core and eliminates light from escaping the cable. This is called total internal reflection.
When light is sent through the core of a fiber optic cable, the light constantly bounces off the cladding, which is highly reflective, like a mirror-lined wall. The cladding does not absorb any light allowing complete internal reflection and allowing the light to travel far distances without losing its intensity.
The discovery of lasers influenced the development of fiber optics. Lasers and LED's can generate an enormous amount of light in a very small area, which can successfully used in fiber optics.
Laser diodes are complex semiconductors that convert an electrical current into light. The process of converting the electrical signal into light is far more efficient because it generates less heat than an ordinary light bulb.
Reasons for using laser diodes in fiber optics:
- laser diodes are very small
- laser diodes are highly reliable and have a long life
- laser diodes have high radiance
- laser diodes emit light into a very small area
- laser diodes can be turned on and off at very high speeds
Advantages of fiber optics
Disadvantages of fiber optics
Applications of fiber optics
Today, a variety of industries including the medical, military, telecommunication, industrial, data storage, networking, and broadcast industries are able to apply and use fiber optic technology in a variety of applications.
Sintered To bond metal particles or to use pressure and heat below the melting point to bond and partly fuse masses of metal particles, or be bonded in this way Furnace a device or oven in which heat is produced by an industrial process such as smelting metal or glass.
Crucible a heat-resistant container in which ores, metals or glass are melted Dopant a substance such as arsenic or antimony that is added in small quantities to a semiconductor material in order to change its electrical characteristics. Dopants are added during the manufacture of semiconducting diodes and transistors.
nm Nanometer, is a unit of spatial measurement that is 10-9 meter, or one billionth of a meter. It is the most common unit to describe the wavelength of light.
micron micron or a micrometre is a metric unit of length equal to one millionth of a meter or equivalently one thousandth of a millimetre or one thousand nanometres (Symbol : µm)LED Light-emitting diodes
Semaphores signalling devices for sending information over distances using mechanically operated arms or flags to represent alphabetical letters
Lathe turning tool, a machine which rotates while the product is fed through and a cutting tool is applied to the product