Introduction to Fiber Optics

To anyone who reads a newspaper, it is apparent that fiber optic communications have become a huge business since the first fiber system was deployed in the late 1970s. What is not so obvious is the number of other applications of optical fiber in remote sensing, medical diagnostics and treatment, lighting, night vision appliances, and many other areas of technology previously dominated by electronics. In this section, we will address the following topics:

Some of the information will be in these notes and some will be found on the web. Be sure to visit

http://www.corning.com/opticalfiber/discovery_center/tutorials/fiber_101/

You have a piece of fiber in your OSA kit (it looks like a thin, flexible wire). If you hold one end up to a bright light source, you will see the light exiting the other end. CAUTION: if you shine your laser in one end, do not look directly into the other end! By moving your finger between the light source and the fiber end, you can make the output end of the fiber "blink"- a crude communication system.

What is optical fiber?

Optical fiber provides a method of guiding light from one place to another. It does this by total internal reflection. (See Geometric optics, Snell's law) The fiber consists of the core, or central region, the cladding, which surrounds the core, and usually buffer coating of a plastic material that protects the fiber.

The core and cladding may both be glass or they may both be plastic or (in some specialized fiber) a glass core is surrounded by a plastic cladding. Whatever the composition, the core always has a higher index of refraction than the cladding.

 

Figure 1: Structure of an optical fiber

Note that the core and cladding cannot be separated- they are really the same "piece of glass" but with different chemical composition. (Think of a glass paperweight with colored glass swirls inside- it is impossible to separate the colored glass from the clear outer globe.) The buffer layer is must be removed when the fiber is being spliced or put into a connector.

Typical cladding size (outer diameter of glass) is 125 mm- the size of a "thick" human hair. Larger (and smaller) cladding sizes are also made. The core of the fiber- where the light is trapped by total internal reflection- may be 8-10 m m for the single mode fiber used for long distance telecommunication to 62.5 m m for fiber used for local area networks, to much larger sizes for specialized purposes. (Plastic fiber used for communication over very short distances has a core size of 980 m m and a cladding size of 1000 m m, that is, 1 mm.)

What keeps light in the fiber? Recall that if light goes from a high index of refraction to a lower index of refraction, it bends away from the normal. At some angle of incidence (the critical angle) the light is totally internally reflected.

 

Figure 2: Total internal reflection in a fiber

Light which enters the end of a fiber so that it strikes the core-cladding interface at greater than the critical angle will be totally internally reflected and guided down the length of the fiber. So, total internal reflection (TIR) is what keeps the light guided in the fiber. An acceptance angle can be calculated from the core and cladding indices of refraction and any ray that strikes the end of the fiber within the acceptance angle will be guided by the fiber.

Figure 3: Acceptance angle

The acceptance angle is an important fiber parameter. A small acceptance angle fiber will need a very directional light source, since if the light from the source spreads over a wide angle much of the light will escape from the cladding and not be guided by the fiber. Fibers with small acceptance angles need laser sources.

It should be noted that manufacturers specify numerical aperture (NA) rather than acceptance angle. The NA is calculated from the equation NA = sin(acceptance angle), so it is just another way of indicating how large a cone of rays will be guided by the fiber.

Large core optical fiber is called multimode. Many paths are present for the light to follow which means a pulse of light entering at one end of the fiber is spread out at the exit. The light may take many paths, some shorter and some longer. It is similar to a race where all the runners start together (initial short light pulse), but cross the finish line over a longer span of time (longer, distorted, exit pulse). For this reason, multimode fiber is not used for long distance communications (the longer the light travels, the more the "longer" paths fall behind) or for very high speed data (the lengthened pulses run into each other, destroying the data). It can be used for short links (such as a local area network or LAN).

Graded index (GRIN) fiber is commonly a used type of multimode fiber. In the fiber shown in the sketch above, the core has one index of refraction and then there is an abrupt step to the cladding index of refraction. (This is called step index fiber.) Graded index fiber has an interesting variation in the core- the index of refraction is high in the center of the core and gradually lessens until it meets the cladding. This slows the center modes (remember high n means low velocity) relative to the modes that travel near the outside of the core. The result is that the modes that travel farther have a chance to "catch up" to the slower central modes.

GRIN fiber has less spreading of individual pulses of light so it is generally preferred over step index multimode fiber for local area networks.

Long distance, high speed data traffic is sent over single mode fiber- the core is so narrow only one light path remains. Although this eliminates multimode distortion, chromatic distortion (different wavelengths travel at different speeds) and polarization mode distortion (different polarization directions travel at different speeds) may still limit data speed or link distance.

 

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Why optical fiber?

You may hear people say that "optical fiber is faster" than copper wire. This isn't exactly correct- the speed advantage of fiber doesn't refer to how fast a signal travels from one end of the fiber to the other. The speed referred to in this sense is the data rate the fiber can transmit. Think of sending a signal by Morse code- turning a switch on and off. When the on-off speed is slow (low data rate), either copper wire or glass optical fiber will serve. (Both have advantages and disadvantages.) However, as the on-off speed increases, the nature of electricity makes the transmission over copper more difficult and, eventually at very high speeds, impossible. Modern communications are mostly digital- on/off- in nature. As the amount of data to be transferred increases, the high data rate capacity of optical fiber is a necessity.

There are other advantages to optical fiber as well. Since it is not an electrical conductor, it is immune to electromagnetic noise. A nearby lightning strike can cause havoc with electrical conductors, but unless the glass is physically harmed, the light inside is unaffected. Optical fiber does not radiate electromagnetic radiation either, so it is difficult to "listen" into. Compared to metallic conductors, it is lighter and uses lower power. It has less attenuation as well, which means a light signal on optical fiber can travel a much longer distance than an electrical signal without needing amplification.

Optical fiber does have disadvantages. It requires skill in handling that necessitates training. There are safety issues (laser sources, small bits of glass easily embedded in skin) that are not a problem with electrical conductors. In some situations the cost of an optical fiber may be more than a copper system. However, for long distance links with very high data rates, fiber has no competition. In recent years, it has become competitive in metropolitan area networks (MAN) and local area networks (LAN) as well.

 

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How is optical fiber manufactured?

For a description of the manufacture of optical fiber see http://www.corningcablesystems.com/web/college/fibertutorial.nsf/fibman?OpenForm

The important point to note is that the structure of the fiber is set when the glass preform is made. Although there are several methods of producing the preform, the finished product is a (huge) scale model of the fiber. The index of refraction of core and cladding are the same as they will be in the optical fiber. The preform is placed vertically in a drawing tower and a small cylindrical furnace heats the lower end of the preform until it forms a molten glob. Similar to the thread that forms when boiled sugar syrup is dripped from a spoon, the molten glob falls by gravity, pulling a glass "thread" behind it.

A technician breaks off the glob and threads the fiber through various diameter measuring gauges, the buffer compound and curing (UV) oven for the buffer, and around a take up spool. From then on the process is computer controlled: the take up spool continues pulling the molten fiber thread and the speed of the take up spool is controlled by the diameter measuring gauges in order to keep the cladding diameter constant. Many kilometers can be pulled from one preform approximately a meter long and 2-3 meters in diameter.

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Introduction to Fiber Optic Telecommunication Systems

A general diagram of a telecommunication system is shown below. Opticalfiber is a type of information channel, like copper wire or the atmosphere (for radio). Since optical systems have a greater information carrying capacity than copper based systems, much current research is taking place to replace as many electronic systems as possible with optical systems. For example, although fiber optic systems employing single mode fiber can carry a huge amount of data (terabit per second data rates are available commercially), the electronic switching to route traffic to its destination is much slower. The next generation of optical networks will employ all-optical switching.

Optical amplifiers are already being deployed. In the past, when an optical signal had weakened through attenuation (absorption and scattering) by the glass fiber, the light signal was converted to electricity, amplified in an electronic amplifier, converted back to light and sent back into the fiber link. The electronic processes were expensive and they introduced noise. Erbium doped fiber amplifiers (EDFAs) use principles similar to a laser to amplify light without changing it first to electricity. A section of fiber is doped with a small amount of the rare earth element Er (erbium). The signal is combined in the fiber with pump light which serves the same purpose as the pump in a laser- it raises the erbium atoms to an excited state. When the signal photons pass by, stimulated emission causes amplification of the signal light.

Where do the huge (terabit per second) data rates come from? Fiber data capcacity is increased by wavelength division multiplexing (WDM), which means that each data channel is carried on a slightly different wavelength of light. (DWDM, or "dense" WDM packs even more wavelengths on a single fiber.) These wavelengths don't interact with each other in the fiber, but travel side by side until they are split (filtered) at their destination. Note that the light used in fiber optic telecommunications is in the near infrared range, where glass is most transparent. Single mode fiber is usually used with laser sources at around 1300 nm and 1500 nm. Recent research has expanded the range of wavelengths from 1300 through around 1600 nm, including the previously unusable 1400 nm range where water impurities increased the absorption of light.

Each of the "data channels" carried on a single wavelength may be a separate data source, or it might be several sources combined (multiplexed) together by other means. In any case, most of the telecommunications traffic carried by today's communications networks is data (computer, video, fax, etc) rather than voice (telephone).

To get an idea of the order of magnitude of "terabit per second" data transmission, consider that a voice telephone call, converted to digital format, requires a data rate of 64 000 digital bits/second (64 kbps). This number is arrived at from a calculation involving the range of audible frequencies needed to understand human speech (4000 Hz, actually quite less than the range of human hearing which is around 15 000 Hz) and the digital coding scheme. A terabit/second is 1012 bits/second, so a fiber carrying a data rate of one Tbps can carry 1x1012/64 x 103 =156 million simultaneous telephone calls- on one hair thin optical fiber.

Fiber optic systems can transmit data rates so large that a new "unit" has been invented: the LOC/s or, Library of Congress/second. It is possible with current commercially available technology to send information equivalent to several "LOCs" per second.

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Other Uses for Optical Fiber

Although telecommunications optical fiber has received a great deal of attention, fiber has many other uses as well. Some examples are listed below.

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