Fibre Optics is the channeled transmission of light through hair-thin glass fibres. The light is prevented from escaping the fibre by total internal reflection, a process that takes place when a light ray travels through a medium with an index of refraction higher than that of the medium surrounding it. In this case the fibre core has a higher refractive index than the material around the core, and the light hitting that material is reflected back into the core, where it continues to travel down the fibre.
Fibre optic technology has been applied in many areas, although it's greatest impact has come in the field of telecommunications, where optical fibre offers the ability to transmit audio, video, and data information as coded light pulses. In fact, fibre is rapidly becoming the preferred mode of transmitting communications of all kinds. Its advantages over older methods (all of which involve the movement of electrons through metallic cables) are many, and include vastly increased carrying capacity.
History
The story of communicating with light begins long before the 20th century and the drive to
develop sophisticated technology. It's an idea almost as old as human kind. Over the millennia
man used light and its properties, but really could never control it. That was until English physicist
John Tyndall discovered that light signals could be bent. Tyndall observed, that when water ran
out of a hole on the side of a rain barrel, the water transmitted the light out as well.
In Washington, a young scientist who already had an international reputation was working on what he considered his greatest invention, the Photophone. Alexander Gra-ham Bell had used electricity to carry voices in the telephone. However, Bell was intrigued by the idea of sending signals without wires. He thought of optical communication, an idea that probably goes back to signal fires on prehistoric hilltops.
In 1880 Bell demonstrated that light could carry voices through the air without wires. Bell's Photophone reproduced voices by detecting variations in the amount of sunlight or artificial light reaching a receiver. It was the first 'wireless' voice communication. However, it never proved practical because too many things could get in the way of the beam.
The 1880s also saw another important development, the first glass fibres. Charles Vernon Boys stretched molten quartz into thin fibres by attaching it to an arrow and firing it from a bow. His fibres couldn't transmit light, but they were lightweight and strong.
Important groundwork for optical communications was clearly being laid more than a century ago. Bending light, containing it in "tubes" and making glass fibres all would prove critical to creating practical optical communications systems; but the time was not yet right for these early experiments to come together. The combination of these advances in knowledge needed a catalyst.
The Laser
That catalyst was the laser. Those who foresaw the possibility of optical communications snapped
to attention on July 7, 1960, when Theodore H. Maiman announced that he had operated the first
laser at Hughes Research Laboratories in Malibu, California. The laser ( for Light Amplification
by the Stimulated Emmission of Radiation ) emits a narrow, monochromatic beam of coherent
light. Coherence - waves being precisely in phase with one another is a property that the laser
shares with many types of radio transmitters.
Light waves have an important advantage over radio waves: a much higher frequency. The higher the frequency of a signal, the more information it can carry. A significant moment in history had arrived. Maiman, Nobel Laureates Charles H. Townes and Arthur Schawlow, science writer Isaac Asimov and many others saw the vast communications potential presented by the laser. The enormous bandwidth meant substantially increased potential to transmit video signals, which require about six megahertz, 300 times the number of cycles per second needed for the transmission of music, and 2000 times more than required for voice.
But the vast potential of the laser could not be realized unless the light it emitted could be transmitted through some medium. Developing the right medium proved no easy task.
The challenge was first taken up by the telephone industry. Bell Telephone Laboratories and others began in earnest to research the technology of optical communications. Their first efforts were directed at the concept of a light pipe, with gas lenses to refract the light and send it farther down the pipe. On the other side of the Atlantic, British Telecom also was giving serious attention to optical technology.
After the invention of the laser, researches in both the United States and Britain saw the possibilities of combining fibre optic technology with laser technology to transmit communication signals. Around 1965 British Telecom was turning its attention away from hollow pipes and toward optical fibre.
British Telecom expressed its interest in optical fibre to many people including a Corning scientist. That scientist was Dr. Robert D. Maurier. Dr. Maurier headed a group of physicists which included Dr. Donald Keck and Dr. Peter Schults. This team of experts poured more energy, imagination and time into their work which lead to breaking the 20-decibel-per-kilometer barrier. In 1970, they produced fibre with attenuation of only 17 decibels per kilometer. "Eureka!" exclaimed Keck in his lab notebook. The great challenges had been met. But new ones instantly replaced them. The birth of fibre had begun.
Intro To Fibre Optics
The fibre optic system offers four basic advantages over typical metallic cable transmission
systems:
Advantages
Optical fibre is a non-metallic conductor. Therefore, it will not pick up or emit electromagnetic
(EMI) or radio frequency (RFI) interference. Crosstalk is eliminated -- a quality advantage.
There are no electrical grounding or shorting problems encountered with optical fibre, either. No
ground loops. If a fibre optic cable is broken, there are no sparks to cause fire or explosion and
no chance of electrical shock. These features make fibre optic cable a natural for use in explosive
environments such as mining, petrochemical operations, and refineries.
A single conductor fibre optic cable weighs about 9 lbs. per 1,000 ft. A comparable coaxial cable weighs 80 lbs. per 1,000 ft -- about nine times more. Weight-conscious designers can save precious pounds using fibre optics, while increasing capacity.
Before fibre optics, telephone companies used two pairs of copper wire to carry 24 two-way conversations. Now, 1344 two-way voice transmissions can be easily handled on two strands of fibre.
Typical Applications
With all the preceding benefits, these are some initial applications for fibre optic transmission:
Data Communications and
Telecommunications:
Cladding
This is the boundary zone of the fibre. Its function is to provide a different refractive index at the
core interface in order to cause reflection within the core so that lightwaves are transmitted down
the fibre.
Core
This is the transmission area of the fibre, either glass or plastic. The larger the core, the more
light that will be transmitted.
Fibre Size
The size of an optical fibre is commonly referred to by the outer diameter of its core and cladding.
Example: 62.5/125 indicates a fibre with a core of 62.5 microns, and cladding of 125 microns.
Coating is typically not referred to since it is not involved in the actual transmission of light, and is
usually removed when joining or connecting fibres.
A micron (m) is equal to one-millionth of a meter. 25 microns are equal to 1/1000 of an inch. A sheet of paper is approximately 25 microns thick.
Tensile Strength
In general it is believed that fiber is very fragile because it is made from glass. In fact research,
theoretical analysis and practical experience prove the opposite is true. While traditional bulk
glass is brittle, the ultra-pure glass of optical fibres exhibits both high tensile strength and extreme
durability.
How strong is fiber? Figures like 600 or 800 thousand pounds per square inch (kpsi) are often cited -- far more than copper's capability of 100 pounds per square inch. That figure refers to the ultimate tensile breaking strength of fibre produced today. It is fibre's real -- not theoretical -- strength . Fibre's theoretical strength is two million pounds per square inch.
Types of Fibre
Fibre can be identified by the type of paths that the lightwave rays, or modes, travel within the
fibre core. There are two types of multiple pathway fibres, or multimode fibres, step index and
graded index, and one category of single mode fibre.
Multimode Step Index
Step index multimode fibre derives its name from the sharp steplike difference in the refractive or
reflective index of the core and the cladding. Common step index fibres have core diameters of
100, 200, and 300 microns.
For short distance applications, 3,000 ft. or less and low data-rate applications, a step index multimode fibre is a cost effective and efficient answer to data transmission.
Multimode Graded Index
In graded index multimode fibre the lightwave rays are also guided down the fibre in multiple
pathways. But unlike step index fibre, a graded index core contains many layers of glass, each
with a lower index of refraction as you go outward from the axis.
The effect of this grading is that lightwave rays are speeded up in the outer layers, to match those rays going the shorter pathway directly down the axis.
The result is that a graded index fibre equalizes the propagation times of the various modes so that data can be sent over a much longer distance and at higher rates before light pulses start to overlap and become less distinguishable at the receiver end.
Graded index fibres are commercially available with core diameters of 50, 62.5, 85 and 100 microns.
Singlemode
The singlemode fibre allows only a single lightwave ray or mode to be transmitted down the core.
This virtually eliminates any distortion due to the light pulses overlapping.
The core of a singlemode (or monomode) fibre is extremely small, approximately eight or nine microns.
The monomode fibre has higher capacity and capability than either of the two multimode types. For example, monomode fibre can handle a 16 channel video system for 15 miles without a repeater.
Attenuation
In addition to physical changes to the light pulse which result from frequency or bandwidth
limitations, there are also reductions in the level of optical power as the light pulse travels to and
through the fibre.
This optical power loss, or attenuation, is expressed in dB/km (decibels per kilometer). The prime causes of optical attenuation in optical fibre systems are:
Microwaves, radar, television and radio operate in the longest wavelength areas. In between the ultraviolet and the microwave spectrums, we have fibre optic wavelengths, which are in the infrared spectrum.
Just as the speed of light slows when traveling in transparent materials, each infrared wavelength is transmitted differently within the fibre. Therefore, attenuation, or optical power loss, must be measured in specific wavelengths for each fibre type.
Wavelengths are measured in nanometers (nm) -- billionths of meters which represent the distance between two cycles of the same wave.
Losses of optical power at the different wavelengths occur in the fibre due to absorption, reflection and scattering. These occur over distance depending on the specific fibre, its size, purity and refraction indexes.
The amount of optical power loss due to absorption and scattering of optical radiation at a specified wavelength is expressed as an attenuation rate in decibels of optical power per kilometer (dB/km).
Fibres are optimized for operation at certain wavelengths. For example, less than 1dB/km loss is attainable in 50/125 (m multimode fibre operating at 1300 nm, and less than 3dB/km (50% loss) is attainable for the same fibre operating at 850 nm.
These two wavelength regions, 850 or 1300 nm, are the areas most often specified for fibre optic transmission today. These wavelengths are commercially useable with current transmitters and receivers. Optical fibres have also been optimized in the 1500 nm region for single mode transmission systems.
Microbending Loss
Without support, an optical fibre is subject to losses of optical power caused by microbending.
Microbends are minute fibre deviations caused by lateral forces which cause optical power loss
from the core.
Different types of packaging or protection for the fibre are available to minimize microbending.
Step index fibres are relatively more resistant to microbending losses than graded index.
Connector Loss
Connector loss is a function of the physical alignment of one fibre core to another fibre core.
Scratches and dirt can also contaminate connector surfaces and severely reduce system performance, but most often the connector loss is due to misalignment or end separation.
Several styles of fibre optic connectors are available from connector suppliers.
Typically, each has its own design and is generally not compatible with any other manufacturer's connectors. However, an ST type connector does offer mechanical compatibility.
Depending on connector type, different terminating techniques are used:
Splice Loss
Two fibres may be joined in a permanent fashion by fusion, welding, chemical bonding, or
mechanical joining. A splice loss that is introduced to the system may vary from as little as
0.15dB to 0.5dB.
Coupling Loss
Loss between the fibre and the signal source or signal receiver is a function of both the device and
the type of fibre used.
For example, LEDs emit light in a broad spectral pattern when compared to laser diodes. Therefore, LEDs will couple more light when a larger core fibre is used, while lasers can be effective with smaller core diameters such as in monomode systems.
Fibre core size is, therefore, a major factor in determining how much light can be collected by the fibre. Coupled optical power increases as a function of the square of the fibre core diameter.
Finally, the optical index of refraction difference between fibre core and its cladding determines the angles of approach that light can take to the core end and be accepted by the core for transmission. Referred to as the numerical aperture (NA) of the fibre, the higher the NA the more capable the fibre is of collecting wavelengths.
Today, fibre optics has matured to the point where good quality cable and terminal equipment can be easily designed into a system with minimum engineering effort. To produce a successful system from "scratch" many factors must be considered.
First Level of Fibre Protection
The optical fibre is a very small waveguide. In an environment free from stress or external forces,
this waveguide will transmit the light launched into it with minimal loss, or attenuation. However,
an unsupported fibre is subject to a loss of optical power caused by the microbending mentioned
previously. To handle this problem, two first level protections of fibre have been developed: loose
buffer and tight buffer.
Loose Tube
In the loose buffer construction, the fibre is contained in a plastic tube that has an inner diameter
considerably larger than the fibre itself. The loose tube isolates the fibre from the exterior
mechanical forces acting on the cable. For multifibre cables, a number of these tubes, each
containing single or multiple fibres, are combined with strength members to keep the fibres free of
stress, and to minimize elongation and contraction. By varying the amount of fibre inside the tube
during the cabling process, the degree of shrinkage due to temperature variation can be
controlled, and therefore the degree of attenuation over a temperature range is minimized.
Tight Buffer
The other fibre packaging technique, tight buffer, uses a direct extrusion of plastic over the basic
fibre coating. Typical plastics used for this extrusion are Nylon, Hytrel, or other thermoplastic
elastomers. Tight buffer construction serves to protect the fibre from crushing and impact loads,
and to a certain degree, from the microbending induced during cabling operations.
The tight buffer design, however, provides minimal insulation for the fibre from the stresses of temperature variations. While relatively more flexible than loose buffer, if the tight buffer is deployed with sharp bends or twists, optical losses are likely to exceed nominal specifications due to microbending.
Both constructions have inherent advantages. The loose buffer tube offers lower cable attenuation from microbending in any given fibre, plus a high level of isolation from external forces. Under continuous mechanical stress, the loose tube permits more stable transmission characteristics.
The tight buffer construction permits smaller, lighter-weight designs for similar fibre configuration, and generally yields a more flexible, crush-resistant cable.
Mechanical Protection
Normal cable loads sustained during installation may ultimately place the fibre in a tensile stress
state. The levels of stress may cause micro-bending losses which result in an attenuation increase
and possible fatigue effects. To transfer these stress loads during short-term installation and
long-term application, various internal strength members are added to the optical cable structure.
Such strength members provide tensile load properties similar to those of electronic cables, and keep the fibres free from stress by minimizing elongation and contraction. In some cases, they also act as temperature stabilization elements.
Optical fibre stretches very little before breaking, so the strength members must have low elongation at the expected tensile loads.
Aerial cable installation, for example, must be protected from excessive tensile loading from wind and ice.
Impact resistance, flexing and bending are other mechanical factors affecting choice of strength members.
Strength members which are typically used in fibre optic cable include Kevlar, fiberglass epoxy rods, and steel wire. Pound for pound, Kevlar is five times stronger than steel. It and fiberglass epoxy rods are often the choice when all-dielectric construction is required.
Toronto Hydro Application
Toronto Hydro has made major moves in the last 4 years in the area of Fibre Optic
communications. Ken Clarke of the Telecommunications Section heads the project for the
implementation of fiber at Toronto Hydro. Clarke said "In order to move ahead with all the
technological changes to our system, it is of the utmost importance that we use fibre optics to
establish a safe environment for our workers and supply a high degree of security for our control
circuit, SCADA and Networking systems."
For years Toronto Hydro has used old PILC ( Paper Insulated Lead Covered Cable ) and PCQL (Paper Core Quad Local ) copper twisted pair cables as our control communications cables.
These control cables are used for our Remote control and Direct Trip circuits plus our SCADA which involves RTUs, Vault monitoring, System Control Units, Metering, Peak Load Control, and interstation communication.
Toronto Hydro decided to install glass fiber to meet its onward going demands for all these circuits. By installing fiber we will be improving all round system performance and security.
Because of its vast underground system, Toronto Hydro is using Corning SMF-28 singlemode fully dielectric cable throughout. Since optical fiber is immune to electrical interference, it can be installed safely and confidently near high voltage power line lines.
Patrick Boshell is Supervisor, Communications Field Services at Toronto Hydro. He has worked for both British Telecom and Telecom Eireann as a fibre optic specialist and was involved from the very early stages in the transition from copper to optical fiber in Europe. Mr. Boshell is an expert in the construction and maintenance of fibre optic and copper installations and is certified to European and EEC telecom standards. Graphics courtesy of Corning.
Patrick Boshell is Supervisor, Field Services, Toronto Hydro.