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Fiber Fundamentals

Nov 21, 2011 10:00 PM, By Bennett Liles

A refresher course on the latest in fiber-optic technology.


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INTO THE GLASS

To couple the emitter's output to the fiber with the maximum transfer of optical power, the transmitter's emitting area may be pigtailed to the fiber or placed in close proximity to it, or a transfer lens may be used to better focus the light into the fiber. A pigtail is usually soldered or epoxied to the transmitter case for strain relief, and it exits the case as a fiber tube that is attached to the fiber core in the cable. Care must be taken in this installation to keep any trace of dirt, dust, or other contamination from the interface. In proximity connections, the optical power transferred is determined by four factors: the light source intensity, its emitting surface area, the acceptance angle of the fiber, and attenuation from Fresnel loss and contamination.

A key parameter in fiber transmitters is referred to as “transmitter launch power.” This refers to the amount of optical power that is launched into the fiber-optic cable. Higher optical power may be necessary to transmit over longer distances, but a representative example is -8dBm or 0.158mW. Normally, the higher the transmitter power, the more light is launched into the fiber. The smaller the ratio of the area of the emitting surface compared to that of the accepting core of the fiber, the more light is transferred. The acceptance angle of the fiber forms a cone with its center at the axis of the fiber. Any light that enters at an angle inside the cone will travel down the fiber. The sine of half the acceptance angle is referred to as the “numerical aperture,” which is the term often seen in the fiber specs. The loss that occurs at any glass-to-air interface is about 4 percent, and this is called Fresnel loss. Special coupling gels can be used to reduce this loss, but they do not eliminate it. As was mentioned earlier, contamination in the transmitter or fiber can add to the loss of optical power. Loss in fiber-optic cable also varies with the wavelength of the light used, but while attenuation in copper cable goes up with the frequency of the modulated signal, in fiber, the attenuation is the same for all modulation frequencies and bit rates.

THE FIBER-OPTIC CABLE

The comparison between light mode paths in multimode step index, multimode graded index, and single-mode fiber.

The cable consists of the ultra-pure glass core that transmits the light, the glass cladding surrounding the core, and a protective sheath. Between the sheath and the cladding is usually a layer of strengthening material such as Kevlar. The cladding reflects light hitting it at an angle back into the core; it does this due to its having a different refractive index than that of the core. Fiber-optic cable is described with two numbers, which correspond to the dimensions of the core and cladding, expressed in micrometers or 1/1000 of a millimeter, commonly called a micron. A common type is 62.5µm/125µm with a core of 62.5 microns and a cladding of 125 microns. Smaller-core fiber is normally used with laser-diode transmitters for long distances in telecommunication applications.

Fiber can be classified in several ways. Either it can be stepped index fiber, in which the difference in refractive index between the core and cladding causes the light to bounce off the cladding and continue down the core material, or it can be graded index, in which the cladding’s index of refraction gradually increases with distance from center of the core until it matches the index of the cladding so that light is curved rather than bounced back into the core. The other primary type differentiation in fiber-optic cable is called the mode. To keep things simple, you can think of the mode as the path of a single ray of light through the core. A wider core allows more modes of light, but because these bounce their way down the cable, they reach the receiver at slightly different times—a factor known as “modal dispersion,” which is specified in nanoseconds per kilometer (ns/km). This limits the highest frequency that can be transmitted and thus the bandwidth of the transmission. The smaller the core, the fewer modes can be transmitted and the higher the bandwidth of the transmitted signals. Multimode fiber is typified by higher core diameters (50μm-62.5μm) and lower bandwidth, while single-mode fiber has smaller core numbers (8μm-10μm) and higher bandwidth. The bandwidth of fiber cable also decreases with length, and loss along the line is measured in decibels just as with copper cable. The loss in bandwidth due to multimode cable length is called “modal bandwidth,” and it is expressed as the frequency in megahertz times the distance in kilometers. The higher the modal bandwidth figure, the better the performance of the system, all other factor being equal.



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