An Introduction to Fiber Optic Communications


Why Fiber Optics?

In the last decade, optical fiber transmission systems have come to dominate the market for high-bit-rate transmission systems.  Indeed, they have redefined "high-bit-rate", a term which used to only apply to 45 Mbps transmission systems, but now may apply to systems carrying billions-of-bits (gigabit) of information per second.

Optical fiber has been replacing older carriers such as copper wire, coaxial cable, and microwave radio due to its incredible bandwidth.  Bandwidth is the information capacity of any carrier, and is roughly proportional to the frequency of that carrier.  The higher you go in the electromagnetic spectrum, the more information you can transmit.  With present technology, lightwaves are as far as we can go in transmitting useful information.    (Refer to Figure 1)

Early Examples of Optical Communications

There have been many attempts to harness lightwaves for communication purposes.  Some were more successful that others:

The above methods all shared one common disadvantage - they were using the Earth's atmosphere as the transmission medium.  The atmosphere is terribly unpredictable and rain or fog can block the communication path.  Line-of-sight communication is usually required, which places a sharp limit on the transmitting distance.  Finally, high-intensity optical sources can actually pose a hazard to the public!

Beginnings of Fiber Optic Communications

The first indication that "light pipes" might be possible came in the late 1870s.  A British physicist, Taylor Tyndall, discovered that a jet of water could guide a beam of light through gentle curves.  This was explained by another British physicist, James Clerk Maxwell, with his invention of electromagnetic theory.  He proved that, under certain boundary conditions between different substances (such as water and air), light would exhibit total internal reflection.

Naturally, 19th century technology did not permit development of fiber optic transmission systems.  By the 1960s, the boom in solid-state technology led to a new look at optical communications.  Within a decade, three separate pieces of the puzzle came together.  Photodiodes allowed the detection of very weak light pulses in a compact and rugged package.  Semiconductor lasers provided tiny but very intense sources of monochromatic (single color) light.  And material scientists specializing in the properties of glass learned how to mass-produce very thin, very transparent glass fibers.

The first "low loss" fiber was manufactured in the early 1970s.  (12 dB of loss per mile may not be "low loss" today, but it was a breakthrough at the time.)  Improvements followed rapidly, so that fiber attenuations of well below 0.5 dB/mi are readily available today.  These exceptional transparencies allowed the design of transmission systems with regenerator (repeater) spacings far beyond anything available with copper carriers.

Advantages of Fiber Optics

Consequently, fiber optic systems were brought to market with two great driving forces.  First, the incredible bandwidth of optical systems meant that transmission networks could now plan on megabits - or gigabits - where they had used kilobits before.  Second, fiber systems could attain regenerator spacings of up to 30 miles - compared to approximately one mile for copper.

In addition; however, there were many "bonus" features of using fiber optic transmission systems.  These include:

Basic Fiber Transmission System

Any fiber optic transmission system must contain these three basic components, a transmitter, a receiver, and optical fiber.  For systems used in telephony applications, the optical transmitter is usually a semiconductor laser (although Light Emitting Diodes (LED) may be used in short-haul systems).  The optical receiver may be a PIN diode or an avalanche photodiode.  Finally, almost all telecommunications fiber installed today is singlemode (although there is a great deal of multimode fiber in the outside plant).    Refer to Figure 2

Factors Influencing Viability

Any transmission system must make economic sense before it can be justified.  Some of the factors that had to be addressed before fiber optics became feasible included:

Sources

Detectors

Medium (Fiber Cable)

Optical Sources

Optical sources convert electrical signals into optical signals for transmission over the fiber path.  There are two types of optical sources generally available:

Light Emitting Diodes (LED)

LEDs are quite inexpensive and relatively rugged.  However, they suffer from low output power (around -18 dBm), large chromatic width (40-50 nm), and resistance to high-speed modulation.  For these reasons, LEDs have found their market in military or industrial applications where high-bit-rate or long transmission links are not required.

Semiconductor Lasers

Semiconductor lasers correct the failings of LED: they have very high output power (up to a milliwatt, or 0 dBm), very narrow chromatic width (2-5 nm), and very high modulation speed (in excess of 1 GHz).  However, they are quite expensive and relatively sensitive to environmental effects (such as temperature).  For most telecommunications applications, however, the benefits of using lasers far outweigh any drawbacks.

Optical Detectors

Optical detectors perform the reverse function - they convert incoming optical signals into electrical signals that can be processed with conventional circuitry.  Again, there are two types of optical detectors commercially available:

PIN Diodes

PIN (Positive-Intrinsic-Negative) diodes have long been the mainstay of the fiber communications industry.  They are relatively inexpensive and do not require great amounts of power.  However, they are limited in sensitivity.  PIN diodes are still an appropriate choice for many systems.

Avalanche Photodiodes

Avalanche photodiodes include amplification circuitry, so that very weak light pulses may be easily detected.  They also can respond faster than traditional photodiodes, so that higher bit rates may be transmitted.  Drawbacks include higher noise levels, increased power requirements, and significantly greater cost.

Other detection systems are currently being tested.  Some, such as coherent technology, promise to greatly increase receiver sensitivities in the near future.

Types of Optical Fibers

Listed below are three basic types of optical fiber in use today:

Step-Index Multimode Fiber

Step-index multimode is an older type of fiber that is seldom used in telecommunications links today.  It is still used in data communication and light-pipe applications, however.  It may be fabricated from plastic or glass.

The different paths shown through the fiber are the different "modes".  Obviously, since the paths have different lengths, transit times will vary between modes.  This problem of "differential mode delay" severely limits the bandwidth available with this type of fiber.    Refer to Figure 3

Graded-Index Multimode Fiber

Graded-index multimode fiber was developed to avoid the problems of differential mode delay.  The refractive index (measurement of speed-of-light) of the core now varies with the distance from the center of the fiber.  (A higher index indicates a lower speed-of-light.)  Now, therefore, the path (mode) through the center is still the shortest, but is in the region of slowest travel.  Paths near the edge of the core are longer, but the light travels faster.  When correctly optimized, the transit time of all modes is equal.

A great deal of multimode fiber was installed in the late 1970s and early 1980s.  However, the information capacity (bandwidth) was limited to approximately 150 Mbps.    Refer to Figure 4

Singlemode Fiber

Singlemode fiber represents the current state-of-the-art in fiber manufacture.  Since its commercial introduction in the early 1980s, it has come to totally dominate all fiber applications in the telecommunications industry.

By dramatically shrinking the fiber core, the number of possible paths through the fiber is reduced to one.  With only singlemode being transmitted, the self-interference of differential mode delay is eliminated.

The bandwidth of singlemode fibers is exceptionally high.  Transmission systems have been demonstrated operating at may gigabits-per-second (1 gigabit = 1 billion bits).    Refer to Figure 5

Signal Impairments

There are many causes of signal impairments in optical transmission.  These can be generally divided into attenuation (loss) and dispersion.    Refer to Figure 6

Attenuation

Attenuation, or signal loss, simply indicates that the amount of light received depends on the length of the fiber being used.  Attenuation may be caused by absorption (for example, the hydroxyl ion (OH) has a strong absorption peak near 1300 nm), by scattering from impurities in the fiber cable, or from radiation (light leakage or microbending).  In most cases, attenuation problems have been overcome by increased precision and quality control during the manufacturing phases.

Dispersion

Dispersion is the tendency of light pulses to get "blurry" after travelling through a fiber.  This limits the bandwidth of the fiber, since in extreme cases pulses begin to overlap and information is lost.  In multimode fiber, very high dispersion values are caused by differential mode delay - there are hundreds of possible paths for the light to follow through the fiber, and each path takes a slightly different amount of time.  This problem is eliminated in singlemode fiber, so dispersion values are orders of magnitude lower.  However, there is still a problem with chromatic dispersion.  The speed-of-light is slightly different for different wavelengths of light.  Although the lasers are nearly monochromatic, they actually put out a narrow range of wavelengths.  The different travel times for these components will eventually limit the amount of information that can be transmitted over singlemode fiber.

Fiber Fabrication

Currently, most fiber in North America is manufactured by the Modified Chemical Vapor Deposition (MCVD) process.  This process has two major steps:

Once the fiber is spooled and tested, it may be assembled into cables for aerial, buried, or ducted installation.    Refer to Figure 7

Typical Attenuation Characteristics

This graph of loss vs. wavelength shows why there are certain wavelengths preferred for optical transmission.  At visible wavelengths (400-800 nm, off to the left of the chart), silica-based glass exhibits very high loss.  The first fiber systems operated near 840 nm, since sources and detectors for that wavelength were readily available.  As soon as possible, manufactures began to concentrate on the 1300 nm "window" of exceptionally low loss.  Future systems may operate at even higher wavelengths, near 1550 nm.    Refer to Figure 8

The sharp peak just past 1300 nm is the "hydroxyl peak" - a region of heavy absorption.  The curve in Figure 8 demonstrates low-quality fiber (vintage 1979).  Current production fiber controls the peak values more sharply, producing a much smoother curve.

Modified Chemical Vapor Deposition (MCVD)

In modified chemical vapor deposition, several processes combine to create the preform (which is later pulled into fiber).  A hollow preform, approximately three feet long and one inch in diameter, spins rapidly on a lathe.  A computer controlled mixture of gases is pumped into one end.  Underneath, a heat source (such as an oxyacetylene torch) passes back and forth about once per minute.

Each passage of the heat source fuses a small amount of the gases to the surface.  Most of the gas is vaporized silicon dioxide (glass), but there are carefully controlled amounts of impurities, or dopants.  These cause changes in the index of refraction of the glass.  As the torch moves and the preform spins, a layer of glass is laid down inside the hollow preform.  The dopants (mixture of gases) can be changed with every layer, so the index may be varied across the diameter.

Eventually, enough layers are built up to fill the tube.  It is now a scale model of the desired fiber - but much shorter and much thicker.  It is now taken to the drawing tower to be pulled into fiber.    Refer to Figure 9