Information discussion notes
Fiber Optic Distribution
by Zhizhong Li

Definitions:

Bandwidth: The amount of data that can pass through a transmission medium. For example the bandwidth of a T-1 line would be 1.544Mbps. Presently most homes have 56.6Kbps bandwidth available to them over the copper telephone local loop (i.e., copper cage). In telephone or cable TV systems, the bandwidth available is dependent on the connection ‚ e.g., twisted-pair, coax cable, or fiber optics ‚ at the clientís end, and the type of equipment at the central office or head end. But in dumb networks, the bandwidth is dependent on the transmission medium alone.

Broadband: Transmission mediums providing bandwidth equal to or greater than 1.544Mbps. Contrast with narrow-band.

Cable Modem: A modem that takes advantage of the bandwidth available from combination of coaxial and fiber-optic cables. Cable modems provide up to 40Mbps connection.

Fiber Optics: A communications medium, in which data are transmitted, using a laser, through a thin strand of transparent material, most commonly glass.

Fiber Optics Cable: A cable, used in fiber optics, which is made up of multiple strands of a transparent medium, called fiber. The most commonly used medium is glass. The light is guided down the center of the fiber called the core by reflecting off the outside of the core. The core is surrounded by an optical material called the cladding that causes the reflections. The core and cladding are usually made of ultra-pure glass, although some fibers are all plastic and some even have glass cores and plastic cladding. The fiber is coated with a plastic covering called the ìbuffer coating that protects it from moisture and other damage. More protection is provided by the cable, which has an outer covering called a jacket.

Fiber Distributed Data Interface (FDDI): An American National Standards Institute (ANSI) standard that defines a 100Mbps fiber optic Local Area Network. FDDI is based on a token ring network topology.

Fiber Miles: The measurement, in miles, of the actual length of fiber strands deployed in a network. A typical fiber-optic cable is comprised of multiple fiber strands that are bound together. Therefore, a fiber mile would be the measurement of the cable in cable miles multiplied by the number of fiber strands encased within that cable. The significance of the measure has been drastically altered by the advent of wavelength division multiplexing (WDM), which enables transmission of hundreds of separate bitstreams of information down every fiber strand.

Advantages of Fiber Optic Systems

Fiber optic transmission systems ‚ a fiber optic transmitter and receiver, connected by fiber optic cable ‚ offer a wide range of benefits not offered by traditional copper wire or coaxial cable. These include:

1. The ability to carry much more information and deliver it with greater fidelity than either copper wire or coaxial cable.
2. Fiber optic cable can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data.
3. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. In can therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind.
4. As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil and exposed to most corrosive atmospheres in chemical plants without significant concern.
5. Since the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.
6. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.
7. A fiber optic cable, even one that contains many fibers, is usually much smaller and lighter in weight than a wire or coaxial cable with similar information carrying capacity. It is easier to handle and install, and uses less duct space. (It can frequently be installed without ducts.)
8. Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.

Fiber Optic communication product applications include:

• Distance Learning
• Video Conferencing
• Security surveillance
• Traffic Management
• Studio serial digital video/audio
• Broadcast quality video/audio
• MPEG-2 and HDTV digital video
• Route protection data network
• Mission critical data network
• Mode conversion
• Network fault tolerance
• Network extension

How does it work?

The basic point-to-point fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver.

The Optical Transmitter: The transmitter converts an electrical analog or digital signal into a corresponding optical signal. The source of the optical signal can be either a light emitting diode, or a solid state laser diode. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers.

The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 2 shows a graphic representation of these two basic schemes.

The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). In a fiber optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber. In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus "every last drop" of light onto the fiber and in other cases, a fiber is "pigtailed" directly onto the actual surface of the emitter.

The Fiber Optic Cable: The cable consists of one or more glass fibers, which act as waveguides for the optical signal. Fiber optic cable is similar to electrical cable in its construction, but provides special protection for the optical fiber within. For systems requiring transmission over distances of many kilometers, or where two or more fiber optic cables must be joined together, an optical splice is commonly used.

Launching the light

Once the transmitter has converted the electrical input signal into whatever form of modulated light is desired, the light must be "launched" into the optical fiber.

When the proximity type of coupling is employed, the amount of light that will enter the fiber is a function of one of four factors: the intensity of the LED or LD, the area of the light emitting surface, the acceptance angle of the fiber, and the losses due to reflections and scattering. Following is a short discussion on each:

Intensity: The intensity of an LED or LD is a function of its design and is usually specified in terms of total power output at a particular drive current. Sometimes, this figure is given as actual power that is delivered into a particular type of fiber. All other factors being equal, more power provided by an LED or LD translates to more power "launched" into the fiber.

Area: The amount of light "launched" into a fiber is a function of the area of the light emitting surface compared to the area of the light accepting core of the fiber. The smaller this ratio is, the more light that is "launched" into the fiber.

Acceptance Angle: The acceptance angle of a fiber is expressed in terms of numeric aperture. The numerical aperture (NA) is defined as the sine of one half of the acceptance angle of the fiber. Typical NA values are 0.1 to 0.4 which correspond to acceptance angles of 11 degrees to 46 degrees. Optical fibers will only transmit light that enters at an angle that is equal to or less than the acceptance angle for the particular fiber.

Other Losses: Other than opaque obstructions on the surface of a fiber, there is always a loss due to reflection from the entrance and exit surface of any fiber. This loss is called the Fresnell Loss and is equal to about 4% for each transition between air and glass. There are special coupling gels that can be applied between glass surfaces to reduce this loss when necessary.

Losses in Optical Fiber

Other than the losses exhibited when coupling LEDs or LDs into a fiber, there are losses that occur as the light travels through the actual fiber.

Most general purpose optical fiber exhibits losses of 4 to 6 dB per km (a 60% to 75% loss per km) at a wavelength of 850nm. When the wavelength is changed to 1300nm, the loss drops to about 3 to 4 dB (50% to 60%) per km. At 1550nm, it is even lower. Premium fibers are available with loss figures of 3 dB (50%) per km at 850nm and 1 dB (20%) per km at 1300nm. Losses of 0.5 dB (10%) per km at 1550nm are not uncommon. These losses are primarily the result of random scattering of light and absorption by actual impurities within the glass.

Optical Fiber Bandwidth

As the figure above illustrates, a ray of light that enters a fiber at a small angle (M1) has a shorter path through the fiber than light, which enters at an angle close to the maximum acceptance angle (M2). As a result, different "rays" (or modes) of light reach the end of the fiber at different times, even though the original source is the same LED or LD. This produces a "smearing" effect or uncertainty as to where the start and end of a pulse occurs at the output end of the fiber ‚ which in turn limits the maximum frequency that can be transmitted. In short, the less modes, the higher the bandwidth of the fiber. The way that the number of modes is reduced is by making the core of the fiber as small as possible. Single-mode fiber, with a core measuring only 8 to 10 microns in diameter, has a much higher bandwidth because it allows only a few modes of light to propagate along its core. Fibers with a wider core diameter, such as 50 and 62.5 microns, allow many more modes to propagate and are therefore referred to as "multimode" fibers.

Optical Connectors

Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers. These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices. In operation, the connector centers the small fiber so that its light gathering core lies directly over and in line with the light source (or other fiber) to tolerances of a few then thousandths of an inch. Since the core size of common 50-micron fiber is only 0.002 inches, the need for such extreme tolerance is obvious.

Optical Splices

While optical connectors can be used to connect fiber optic cables together, there are other methods that result in much lower loss splices. Two of the most common and popular are the mechanical splice and the fusion splice. Both are capable of splice losses in the range of 0.15 dB (3%) to 0.1 dB (2%).

In a mechanical splice, the ends of two pieces of fiber are cleaned and stripped, then carefully butted together and aligned using a mechanical assembly.

A fusion splice, by contrast, involves actually melting (fusing) together the ends of two pieces of fiber. The result is continuous fiber without a break.

Fiber Optics Cable Construction

The Optical Receiver: The receiver converts the optical signal back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.

Since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal amplifiers. Because of this, optical receivers can be easily overloaded. For this reason, it is important only to the size fiber specified for use with a given system.



Design A Fiber Optic System

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