The Fiber Optic Medium

Fiber optics is a method of caring information using optic cables. An optical fiber is a thin strand of glass or plastic that serves as the transmission medium over which information is sent. It thus fills the same basic function as a copper cable carrying a telephone conversation, computer data or web pages. Unlike the copper cable however, the fiber carries light instead of electrons. In so doing, it offers many distinct advantages that make it the transmission medium of choice for applications ranging from telephone calls, television and machine control.

The basic fiber optics system is a link connecting two electronic circuits, see Figure 1. The figure shows a simple Fiber optics link.

The different parts of Fiber optics circuit are as follows:

1. The transmitter:  The transmitter unit converts an electrical signal to an optical signal. The light source is typically a light emitting diode, LED or a laser diode.  The light source performs the actual conversion from an electrical signal to an optical signal. The driving circuit for the light source changes the electrical signal into the driving current.

2. The fiber optic cable:  The fiber optic cable is the transmission medium for carrying the light. The cable includes the optical fibers in their protective jacket.

3. The receiver:  The receiver accepts the light or photons and converts them back into an electrical signal. In most cases, the resulting electrical signal is identical to the original signal fed into the transmitter. There are two basic sections of a receiver.  First is the detector that converts the optical signal back into an electrical signal.  The second section is the output circuit, which reshapes and rebuilds the original signal before passing it to the output.

Basic Building Blocks of a Fiber Optic System, Figure 1

Depending upon the application, the transmitter and receiver circuitry can be very simple or quite complex. There are three basic parts to a fiber optics system. Other components discussed in this book, such as couplers, multiplexers, optical amplifiers, optical switches provide the means for building more complex links and communications networks. But the transmitter, fiber and receiver are the basic elements in every fiber optic system.

Beyond the simple link, the fiber optic medium is the fundamental building block for optical communications.  Traditionally, the optical signal can be converted back into an electrical signal.  Today, just about every signal can be transported optically. Many optical components have been invented to permit signals to be processed optically without electrical conversion. Indeed one goal of optical communications is to be able to operate entirely in the optical domain from end to end.

Using light as a means of communications is not a new concept. Lanterns in Boston’s old north church signaled Paul Revere’s horse ride.  Navy signal men use signal lanterns to communicate among ships with Morse Code. Lighthouses have been used for naval navigation for many centuries.

Claude Chappe was the first to build an optical telegraph in France in the 1790s.  Signalmen in a successive series of towers from Paris to the Lille, relayed signals through movable signal arms over a distance of more than 230 Kilometers.  It took about 15 minutes for the messages to travel via the hand signals.  In the United States, an optical telegraph has used to link Boston and the island of Martha’s vineyard.  The systems were eventually replaced by electrical telegraphs.

In 1870, the English philosopher John Tyndall demonstrated the principles of guiding light through internal refraction. In a presentation before the royal society, he showed that light can be bent around a corner as it traveled in a stream of pouring water. Water flowed through a horizontal spout near the bottom of a container into another container.  The water flowed through the air in a parabolic path.  When Tyndall directed a beam of light through the spout along the water, the light traveled inside the curved path of water. The water guided and trapped the light inside the stream. An optical fiber works on the same principle of guiding light.

A decade later, Alexander Graham Bell patented a photo phone as seen in see Figure 2, which used light to carry speech.  A series of lenses and mirrors directed light onto a flat mirror and mouthpiece. The voice vibrated the mirror, thereby modulating the light hitting it. The receiver used a selenium detector whose resistance varied with the intensity of the light striking it.  When the light beam with the modulated voice signal struck the selenium detector, a varying current was produced in the receiver which was used to reproduce the voice.  Bell managed to transmit successfully over a distance of 200 meters. Unlike his other invention, the telephone, the photo phone lacked a practical application.

Alexander Graham Bell’s Photo Phone, Figure 2

Tyndall and Bell demonstrated two important principles, the guiding of light through a transmission medium and the modulation of light to communicate.  The two principles would eventually apply to fiber optics.  Practical applications required a better transmission medium and a better means of modulating the light.

Throughout the early twentieth century, scientists made experimental and theoretical investigations into dielectric wave-guides.  This included glass rods.  During the 1950s, image transmitting fibers were developed by Brian O’Brien at the American Optical Company and by Narinder S. Kapany and colleagues at the Imperial College of Science and Technology in London. Modern medicine now uses fiber optics for arthroscopic surgery, which gives doctors the ability to look inside the human body.  It was Kapany who invented the coated glass rod and coined the term fiber optics in 1956. The coated glass was an important step.  The outer layers served to keep the light trapped in the inner layer.  The light was then guided through the inner layer.  This fiber optic cable was not suited for communications.

In 1957, Gordon Gould, a graduate student at Columbia University, described the laser as an intense source of light. Charles Towns and Arthur’s Schawlow of Bell Laboratories helped to popularize the idea in scientific circles and to spur research into creating a working laser. By 1960, Theodore Maiman of Hughes Laboratories operated the first ruby laser. Towns also demonstrated a helium neon laser.  By 1962, lasing was observed in a semiconductor chip.  The modern lasers of today are semiconductor chips.  Based on his work in the 1950s, Gould was finally recognizes as the father of the laser and was awarded four patents in 1988.

Snell’s Law

In the early days of fiber optics the excessive amounts of loss in the optical signal as it traveled in the fiber drastically limited transmission distances.  To correct this, scientists developed glass fibers that included a separate class coating. The innermost region of the fiber, the core, carried the light, while the glass coating or cladding, prevented the light from leaking out of the core by refracting the light back into the inner boundaries of the core. Snell’s law explained this concept. It states that the angle at which a light reflects as it passes from one material to another depends on the refractive indices of the two materials.

In the case of fiber optics, this is the refractive index between the core and the cladding. Figure 3 illustrates the equations for Snell’s Law.  In figure 3, the upper region of the frame, n1, indicates a higher refractive index than the lower region n2 . The refractive index of the upper region is designated as n1 while the lower region refractive index is n2.  The figure on the right, shows the case with the angle of the indices is less than the critical angle. Note that the angle of the light travels changes at the interface between the higher refractive index, in region 1, and the lower refractive index, in region 2.  In center figure, the angle of indices has increased to the critical angle. At this point all the refracted light rays travel parallel to the interface region. In the figure on the right, the angle of indices has increased to a value greater than the critical ankle. In this case 100% of the light refracts at the interface region.

Figure 3

The advancement in laser technology next elevated the fiber optics industry. Only the light emitting diode or its higher powered counter part the laser diode had the potential to generate large amounts of light in a focused beam small enough to be useful for fiber optic transport.

Communications engineers quickly noticed the importance of lasers and their higher modulation frequency capabilities. Light has the capacity to carry 10,000 times more information than radio frequencies. Because environmental conditions such as rain, snow, and fog disrupt laser light, scientists needed to find a transmission scheme other than free space.   In 1966, Charles Kao and Charles Hockham, working at the Standard Telecommunications Laboratory presented optical fibers as an ideal transmission medium, assuming fiber optic attenuation could be kept under 20 dB per Kilometer.  Optical fibers of the day exhibited losses of 1000 dB/KM or more. At a loss of 20dB/KM, 99% of the light would be lost over only 3,300 feet.  In other words only 1/100 of the transmitted optical powers reached the receiver.

Scientists theorized that the high levels of loss where due to impurities in the glass and not the glass itself. At that time in 1970, an optical loss of 20dB/KM was within the capabilities of electronics and opto-electronic components.  Dr Robert Maurer, Donald Keck and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/KM, the limit for making Fiber optics a usable technology. Other advances of the day, such as semiconductor chips, optical detectors and optical connectors initiated the true beginnings of the fiber optic communications industry.

Optical Windows and Spectrum

Wavelength remains a significant factor in fiber optics developments. Figure 4 illustrates the wavelength “windows”.   The top section figure 4a shows the wavelength of each optical window and the typical application for Multimode or Singlemode operation.

The earliest fiber optics systems were developed at an operating wavelength of about 850nm. This wavelength corresponded to the so called “first window” in a silica based optical fiber as shown in figure 4b. This window refers to the wavelength region that will offer a low optical loss that sits between several large absorption peaks.  The absorption peaks are caused primarily by moisture in the Fiber and Rayleigh scattering, the scattering of light due to random variations in the index of refraction, caused by irregularities in the structure of the glass.

The attraction to the 850 nm region came from its ability to use low cost infrared LEDs and low cost silicon detectors.  As technology progress, the first window lost its appeal due to it’s relatively high 3 dB/KM losses. Most companies began to exploit the “second window” at 1310nm with a lower attenuation of about 0.5 dB/KM.  In late 1977, Nippon telegraph and telephone developed the “third window” at 1550nm. The third window offers an optical loss of about 0.2 dB/KM.

The three optical windows, 850 nm, 1310nm and 1550 nm, are used in many fiber optic installations today.  The visible wavelength near 660nm is used in low end, short distance, systems. Each wavelength has its advantages. Longer wavelengths offer higher performance, but always come with higher cost.

Table shown in Figure 4c provides the typical optic attenuation for each of the common wavelengths verses the fiber optic cable diameter.  A narrower core fiber has less optical attenuation.

Optical Attenuation of a Fiber Optic Cable, Figure 4

The International Telecommunications Union or ITU, an international organization that promotes worldwide telecommunications standards, has specified six transmission bands for fiber optic transmission. The first is the O Band or original band which is from 1260nm to 1310nm. The second band is the E Band or extended band which is 1360nm to 1460nm.  The third band is the S Band or short band with a range of 1460nm to 1530nm. The fourth band in the spectrum is the C Band or conventional band which is 1530nm to 1565nm.  The next band is the L Band or longer band from 1560nm to 1625nm.  The sixth band is the U band or ultra band with a range from 1625nm to 1675nm.  There is a seventh band that has not been defined by the ITU in the 850nm region.  It is mostly used in private networks is in the 850nm.   The seventh band is widely used in high speed computer networking, video distribution and corporate applications.

Researchers have attempted to develop new fiber optics that could reduce costs or improve performance; however, like most of today’s electronics, optical fiber remains silicon based, although some alternative fiber materials  find specialized usage. Plastic fiber is ideal for short transmission distances that are ideal for a home theater installations were connections are made in a single stereo cabinet. With the plunging costs of glass fiber over the last decade, the need to develop longer distance plastic fiber has diminished.  The escalating cost of copper wire over the last year has expanded glass fiber optic cable applications.

Types of Fiber Optic Material

In the last section we show the characteristics of light propagation which is the most important aspect of fiber optics.  We saw that the refraction or reflection of light depends on the indices of refraction of the two media and on the angle at which light strikes the interface. The optical fiber works on these principles. Once light begins to reflect down the fiber, it will continue to do so under normal circumstances. The purpose of this section is to describe the propagation of light through the various types of optical fibers. The next section further explains the properties of Fiber.

Keep in mind that distinction between the optical fiber and the fiber optic cable. The optical fiber is the signal carrying member, similar in function to the conductor in a wire. The optical fiber is placed in a protective covering that keeps the fiber safe from environmental and mechanical damage. This section deals specifically with the optical fiber itself.

The basic construction of the optical fiber has two concentric lawyers called core and the cladding. The inner core is the light carrying part. The surrounding cladding provides the difference in refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1% lower than that of the core. Typical values, for example, are a core index of 1.47 and cladding index of 1.46. Fiber manufacturers must carefully control this difference to obtain the desired fiber characteristics.

Fibers have an additional coating around the cladding. This coating, which is usually one or more layers of polymer, protects the core and cladding from shocks that might affect their optical or physical properties. The coating has no optical properties affecting the propagation of light within the Fiber. This coating is just a shock absorber.

Figure 5 shows the light traveling through a Fiber. Light injected into the Fiber and striking the core to cladding interface at a critical angle reflects back into the core. Since the angles of incident and reflection are equal the light will again be reflected.  The light will continue as expected down the length of the Fiber.

Internal Reflection Inside Optical Fiber, Figure 5

Light, however, striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly rapidly.  The propagation of light is governed by the indices of the core and cladding and by Snell’s law.

Such total internal reflection forms the basis of light propagation through a simple optical fiber. This analysis considers only meridional rays.  These are the rays that pass through the fiber center axis each time they are reflected. Other rays, called skew rays, traveled down the fiber without passing through the axis. The path of the skew ray is typically helical, wrapping around and around the center axis. To simply analysis, skewer rays are ignored in most fiber optics analysis.

Remember that the fiber is circular. We can define a cone, also shown in figure 6, that defines which light will be accepted and propagated by a total internal reflection. The cone is known as the acceptance cone.  Light that enters the core from within this acceptance cone refracts down the fiber. Light outside the cone will not strike the core to cladding interface at the proper angle that allows total internal reflection. This light will not propagate.

The specific characteristics of light propagation through fiber depend on many factors.  The factors include the size and composition of the fiber as well as the light source injected into the fiber. An understanding of the interplay between these properties will clarify many aspects of fiber optics.

Fiber itself has a very small diameter. The Table 1 below shows a cross section of the core and cladding diameters of four commonly used fibers.  The diameters of the core and cladding are provided below:

Core (um):            Cladding (um):
8                125
50                125
62.5                125
100                140

Table 1
To realize how small these fibers are, note that human hair has a diameter about 100 um.  Fiber sizes are usually expressed by first giving the core size, followed by the cladding size.  Thus, 50/125 means a chore diameter of 50 micros and a cladding diameter of 125 microns.

Optical fibers are classified in two ways.  One way is by the material makeup.

Glass Fiber

Glass fibers have a glass core and glass cladding.  They are the most widely used typr og fiber  The majority of the discussion in the book will center around glass fiber.  The glass used in an optical fiber is an ultra pure and transparent silicon dioxide or fused quartz.  If the water in the ocean were as clear as fiber, you could see almost 36,000 feet deep to the bottom of the Marianas Trench in the Pacific Ocean. Impurities are purposely added to the pure class to achieve the desired index of refraction.  The elements germanium and phosphorus are added to increases the refractive index of the glass.  Boron or fluorine are used to decreas the index.  There are other impurities that are not removed when the class is purified.  Theses additional impurities also affect the fiber properties by increasing attenuation from scattering or by the absorbing light.

Plastic-clad Silica PCS

Plastic-clad Silica PCS fibers have a glass core and plastic cladding. The performances is limited compared to a fiber made of all glass.

Plastic

Plastic fibers have a plastic core and plastic cladding.  Plastics fibers are limited by high optical loss and low bandwidth.  The very low cost and easy of use make them attractive for applications were low bandwidth or high losses are acceptable.   Plastic and PCS fibers do not have the buffer coating surrounding the cladding.

The second way to classify fibers is by the refractive index of the core and the modes that the fiber propagates.  Fiber can be categorized in to three general types.  Figures 7, shows the three general fibers types and their basic characteristics.

Three Types of Fiber Cable, Figure 7

First, it shows the difference between the input pulse injected into a fiber and the output pulses exiting the fiber.  The decrease in the height of the pulse shows the loss of optical signal power.  The broadening of the pulse shows the bandwidth limiting effects of the fibers.  This shows the fiber’s transport limitations.  Second, it shows the different paths the rays of light travel down the fiber.  Third, it shows the relative index of refraction of the core and cladding for each type of Fiber.  The significant properties will become apparent as we examine each type of Fiber.

Modes

Mode is a mathematical or physical concept describing the propagation of an electro-magnetic wave through any media.  In its mathematical form, mode theory derives from Maxwell’s Equations.  James Maxwell, first developed mathematical expressions for the relationship between electric and magnetic energy.  He proved that they were both a single form of electro-magnetic energy, not two different forms as was then commonly believed.  His equations also showed that the propagation of electro-magnetic energy follows strict rules.  Maxwell’s equations form the basis of Electro-magnetic theory.

A mode is a solution to Maxwell’s equations.  For purposes of this analysis, a mode is simply a path that a ray of light travels down a Fiber.  The number of modes that a given fiber will support ranges from 1 to over 100,000 individual rays of light.  This depends upon the physical properties of the fiber and fiber diameter.

Refractive Index Profile

The refractive index profile describes the relationship between the indices of the core and cladding.  Two main relationships exist: step index and graded index.  The step index Fiber has a core with a uniform index throughout.  The profile shows a sharp step at the junction of the core and cladding.  In contrast graded index has non-uniform core.  The index is highest at the center of the core and gradually decrease until it matches that of the cladding. Therefore there is no sharp transition between the core and the cladding.

By this classification, there are three types of fibers:

1. Multimode stepped index Fiber.  Commonly called step index Fiber.

2. Singlemode step index Fiber or Singlemode Fiber

3. Multimode graded index Fiber. Commonly called grated index Fiber

The characteristics of each type have important bearing on its suitability for particular applications.  The importance of each type of fiber will become apparent.

Step index multimode Fiber

The multimode step index Fiber is the simplest type. It has a core diameter from 100 to 970 microns. This fiber type includes glass, PCS and plastic fibers. The step index Fiber is the most widely used fiber type.  This is despite relatively low bandwidth and high losses.

Since light reflects at different angles for different paths, the different rays of light take a shorter or longer time to propagate down the Fiber. The ray of light that travels straight down the center of the core arrives at the other end first.  Other rays of light arrive later, since they refract back and forth in a zigzag path. Therefore rays of light that enter the fiber at the same time, exits the fiber at different times. The effect is that the light has spread out in time.

This spreading of an optical pulse is called modal dispersion. A pulse of light that began as a tight and precisely defined shape has dispersed or spread over time. Dispersion describes the spreading of light by various mechanisms. Modal dispersion is that type of dispersion that results from the varying path lengths of each mode of light as it propagates through the fiber.

If you can imagine three race cars all traveling at the same speed.  The first car follows a straight path.  This is the lowest order mode since it does not reflect as it propagates. The second car follows the longest path.  This path is the highest order mode.  While the speed of the second car does not vary from that of the first, the second card must travel back and forth from side to side. The third car follows an intermediate path. If the three card start at the same time and travel at the same speed, they will arrive at the finish line at different times.

The typical modal dispersion for a stepped index fibers ranges from 15 to 30 ns per kilometer. This means that when rays of light enter a 1 KM long Fiber at the same time, the ray of light that takes the longest path will arrive 15 to 30 ns after the ray of light that took the shortest path.

The modal dispersion of 15 to 30 billions of a second does not seem to be very much, but dispersion is a fibers main limiting factor to bandwidth. Pulse spreading results in the overlapping of adjacent pulses as shown in figure 8.  Eventually the pulses will merge so that one pulse cannot be distinguished from another.  This results in the loss of information. Reducing the modal dispersion in a fiber will increase a Fiber’s bandwidth.

Pulse Spreading due to Modal Dispersion, Figure 8.

Graded Index Multimode Fiber

One way to reduce modal dispersion is to use graded index Fiber. Here the core has numerous concentric layers of glass, somewhat like the annular rings of a tree. Each successive layer outward from the central axis of the core has a lower index of refraction. Figure 9 shows the course structure.

Graded Index Fiber Core. Figure 9.

Light travels faster in a lower index of refraction. So the further the light is from the center axes, the greater the speed. Each layer of the core refracts the light. Instead of being sharply refracted as it is in a step index Fiber, the light is now bent or continually refracted in almost a sinusoidal pattern. Those rays that follow the longest path by a traveling in the outside of the core have a faster average velocity. The light traveling near the center of the court has the slowest average velocity. As a result, all rays tend to reach the end of the Fiber at the same time. The graded index reduces modal dispersion to 1 nanosecond per kilometer or less.

Popular grated index fibers have a core diameter of 50 or 62.5microns and a cladding diameter of 125mu. The Fiber is popular in applications requiring high bandwidth, especially telecommunications, local area networks, computers and video applications.

Singlemode Fiber

Another way to reduce modal dispersion is to reduce the cores diameter until the fiber propagates only one mode efficiently. The Singlemode fiber has a very small core diameter of only 5 to 12 microns.  The standard cladding diameter is 125 um The cladding diameter was chosen for three reasons:

1. The cladding must be about 10 times thicker than the core in a Singlemode fiber. For a fiber with an 8 or 9um core, the cladding should be at least 80um.

2. It is the same size as a graded index Fiber which promotes size standardization.

3. It promotes easy handling because it makes the Fiber less fragile and because of the diameter is reasonably large so that it can be handled by technicians.

Since the Singlemode fiber only carries one mode, modal dispersion does not exist.
Singlemode fibers have a potential bandwidth of 50 to100 GHz-kilometers. Present Fiber has a bandwidth of several GHz and allowed transmissions of tens of kilometers.

Dispersion Shifted Singlemode Fibers

There are three types of Singlemode optical fibers usually found in typical applications for telecommunications and data networks.  Beyond standard Singlemode fibers, there are also dispersion shifted (DS) fibers and nonzero-dispersion shifted (NZ–DS) fibers.  The purpose of these fibers is to reduce dispersion in the transmission window having the lowest attenuation.  Normally, attenuation is lowest in the 1550nm window and dispersion is lowest in the 1310nm window.  Dispersion shifting creates a fiber that shifts the lowest dispersion to the 1550nm region.  This shifting of dispersion results in a Fiber suited for highest data rates and longest transmission distances.  In a standard Singlemode Fiber, the points of lowest loss and highest bandwidth do not coincide.  Dispersion shifting brings them closer together.

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