fiber optics introduction 3

dogear · 发表于 2004-7-13 04:14:00

The Optical Fiber: 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.

As previously mentioned, there are two methods whereby light is coupled into a fiber. One is by pigtailing. The other is by placing the fiber’s tip in very close proximity to an LED or LD. 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.

The Optical Fiber-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.

The core of an optical fiber is made of ultra-pure low-loss glass. Considering that light has to pass through thousands of feet or more of fiber core, the purity of the glass must be extremely high. To appreciate the purity of this glass, consider the glass in common windowpanes. We think of windowpanes as "clear," allowing light to pass freely through, but this is because they are only 1/16 to ¼ inch thick. In contrast to this clear appearance, the edges of a broken windowpane look green and almost opaque. In this case, the light is passing edgewise into the glass, through several inches. Just imagine how little light would be able to pass through a thousand feet of window glass!

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 1550 nm are not uncommon. These losses are primarily the result of random scattering of light and absorption by actual impurities within the glass.

Another source of loss within the fiber is due to excessive bending, which causes some of the light to leave the core area of the fiber. The smaller the bend radius, the greater the loss. Because of this, bends along a fiber optic cable should have a turning radius of at least an inch.

The Optical Fiber: Optical Fiber Bandwidth

All of the above attenuation factors result in simple attenuation that is independent of bandwidth. In other words, a 3 dB loss means that 50% of the light will be lost whether it is being modulated at 10 Hz or 100 MHz.

There is an actual bandwidth limitation of optical fiber however, and this is measured in MHz per km. The easiest way to understand why this loss occurs is to refer to Figure 6

As Figure 6 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.

Typical bandwidths for common fibers range from a few MHz per km for very large core fibers, to hundreds of MHz per km for standard multimode fiber, to thousands of MHz per km for single-mode fibers. And as the length of fiber increases, its bandwidth will decrease proportionally. For example, a fiber cable that can support 500 MHz bandwidth at a distance of one kilometer will only be able to support 250 MHz at 2 kilometers and 100 MHz at 5 kilometers.

Because single-mode fiber has such a high inherent bandwidth, the "bandwidth reduction as a function of length" factor is not a real issue of concern when using this type of fiber. However, it is a consideration when using multimode fiber, as its maximum bandwidth often falls within the range of the signals most often used in point-to-point transmission systems.

The Optical Fiber: Fiber Optics Cable Construction

Fiber optic cable comes in all sizes and shapes. Like coaxial cable, its actual construction is a function of its intended application. It also has a similar "feel" and appearance. Figure 7 is a sketch of a typical fiber optic cable.

The basic optical fiber is provided with a buffer coating which is mainly used for protection during the manufacturing process. This fiber is then enclosed in a central PVC loose tube which allows the fiber to flex and bend, particularly when going around corners or when being pulled through conduits

Around the loose tube is a braided Kevlar yarn strength member which absorbs most of the strain put on the fiber during installation. Finally, a PVC outer jacket seals the cable and prevents moisture from entering.

Basic optical fiber is ideal for most inter-building applications where extreme ruggedness is not required. In addition to the "basic" variety, it is also available for just about any application, including direct buried, armored, rodent resistant cable with steel outer jacket, and UL approved plenum grade cable. Color-coded, multi-fiber cable is also available.

fifa · 发表于 2004-7-13 23:49:00

fiber optics introduction 3

The Optical Fiber: Fiber Optics Cable Construction

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