Fiber Optics Sep 14, 2009

The first commercial fiber optic installation was for telephone signals in Chicago, installed in 1976.

The copper cable has about 1000 pairs of conductors. Each pair can only carry about 24 telephone conversations a distance of less than 3 miles. The fiber cable carries more than 32,000 conversations hundreds or even thousands of miles before it needs regeneration. Then each fiber can simultaneously carry over 150 times more by transmitting at different colors (called wavelengths) of light.

Applications of Fiber Optics

Why use Fiber Optics? The biggest advantage of optical fiber is the fact it can transport more information longer distances in less time than any other communications medium. Fiber is unaffected by the interference of electromagnetic radiation which makes it possible to transmit information and data with less noise and less error. Fiber is lighter than copper wires which makes it popular for aircraft and automotive applications.

Inexpensive The cost of transmitting a single phone conversation over fiber optics is only about 1% the cost of transmitting it over copper wire! Large bandwidth Single mode fiber used in telecommunications and CATV has a bandwidth of greater than a terahertz. Standard systems today carry up to 64 channels of 10 gigabit signals - each at a unique wavelength.

Where is the bulk of telephone cabling? Only 10% is in long distance networks, which were the first links converted to fiber - years ago. Another 10% is local loop (metropolitan) connecting central offices and switches - now mostly converted to fiber too. Fully 80% of all fiber cabling is subscriber loop - the last mile that connects the end user to the system. After 20 years of fiber optic installations, virtually all long distance and local loop connections are already fiber. Only the last mile is still copper, and much of it is very old and incapable of carrying modern high bandwidth digital signals.

Optical fiber is comprised of a light carrying core surrounded by a cladding which traps the light in the core by the principle of total internal reflection. Most optical fibers are made of glass, although some are made of plastic. The core and cladding are usually fused silica glass which is covered by a plastic coating called the buffer which protects the glass fiber from physical damage and moisture. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Single-fiber construction.

FIGURE 24-2 (a) Basic principle of the law of refraction; (b) total internal reflection prevents any light from leaving medium 1. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Refraction of light.

Critical angle.

Modes of propagation.

FIGURE 24-3 Light paths and dispersion in (a) step index, (b) graded index, and (c) single-mode fibers. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Types of optical fiber.

Graded-index fiber.

Light Used in Fiber Optics

The ultra-pure glass used in making optical fiber has less attenuation (signal loss) at wavelengths (colors) in the infrared, beyond the limits of the sensitivity of the human eye. The fiber is designed to have the highest performance at these wavelengths. The particular wavelengths used, 850, 1300 and 1550 nm, correspond to wavelengths where optical light sources (lasers or LEDs) are easily manufactured. Some advanced fiber optic systems transmit light at several wavelengths at once through a single optical fiber to increase data throughput. We call this method wavelength division multiplexing.

Upper portion of electromagnetic spectrum

Figure 18-4 The electromagnetic wavelength spectrum.

The input end of a WDM system is really quite simple. It is a simple coupler that combines or multiplexes all the signal inputs into one output fiber. The demultiplexer separates the light at the end of the fiber. It shines the light on a grating (a mirror like device that works like a prism and looks similar to the data side of a CD) which separates the light into the different wavelengths by sending them off at different angles. Optics capture each wavelength and focuses it into another fiber, creating separate outputs for each wavelength of light. Output Fibers DWDM Demultiplexer

Cables will include strength members, typically a strong synthetic fiber like Kevlar, which takes the stress of pulling the cable. The thin yellow fibers in the photo are the strength members. The outside of the cable is called the jacket. It is the final protection for the fibers and must withstand extremes of temperatures, moisture and the stress of installation. Some cables even have a layer of thin metal under the jacket to prevent rodents from chewing through the cable.

FIGURE 24-4 Power loss versus wavelength of a typical fiber; other fiber formulations have differing wavelengths of minimum and maximum loss. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. 830 to 860 nm 1290 to 1350 nm 1530 to 1600 nm 850 nm1310 nm1550 nm

FIGURE 24-5 Three types of fiber misalignment, causing power loss at a splice. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Sources of connection loss.

Figure (continued) Sources of connection loss.

FIGURE 24-6 Geometrical meaning of numerical aperture and acceptance cone (courtesy of Belden Wire and Cable Corp.). Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Total Internal Reflection By making the core of the fiber of a material with a higher refractive index, we can cause the light in the core to be totally reflected at the boundary of the cladding for all light that strikes at greater than a critical angle determined by the difference in the composition of the materials used in the core and cladding.

(a) Development of numerical aperture; (b) acceptance cone.

FIGURE 24-8 Relative output power versus wavelength (and bandwidth) of LED and laser diode. William Schweber Electronic Communication Systems, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Spectral response of a p-i-n diode.

LED modulating circuit is relatively simple to interface to data. Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

FIGURE PIN diode receiver circuit includes a trans-impedance amplifier, secondary-stage amplifier, threshold comparator, and digital circuit at receiver. William Schweber Electronic Communication Systems, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Fiber optic transmission systems all consist of a transmitter which takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light is ultimately coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment.

FIGURE Link function schematic and budget analysis of a basic point-to-point fiber optic system shows all factors and resultant maximum and minimum loss and signal power from source to receiver (courtesy of Belden Wire and Cable Corp.). Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Figure Fiber connectors.

Fiber-optic communication system.

Attenuation versus wavelength for typical high-quality fiber.

Semiconductor laser.

Light output versus bias current for a laser diode.

LED modulator.

P-i-n diode.

Avalanche photodiode.

Splice technique using alignment pins.

Fiber-optic emitter/detector/connector assembly. (Courtesy of General Electric Company.)

Bus topology optical LAN.

A fiber link showing emitter, detector, connectors, and fiber cable.

Figure System design.

Figure A graphical view of the system design problem shown in Figure

Figure An alternative view of the system design problem.

Figure An OTDR trace of an 850-nm fiber.

Figure (continued) An OTDR trace of an 850-nm fiber.