Fiber-optic basics
Sep 21, 2008 8:00 AM
Fiber optics provides a transmission medium that surpasses coax cable in many ways, especially for long transmission runs. But now with HD-SDI, the need for high-frequency capacity and low loss brings fiber optics into the broadcast plant. Coax cabling for HD is limited to 300ft-1000ft, depending on the receiver, while some fiber-optic cables can carry 3Gb HD is excess of 30km. The following tutorial presents the basics on fiber optics as well as some practical tips on installation and troubleshooting.
Fiber-optic history
The history of fiber optics goes back to 1841, but nothing came of it until the 1950s when the fiber scope was invented. In 1956, Narinder Kapany first coined the term “fiber optic.” The invention of the fiber scope encouraged scientists to develop fiber optics as we know it today. What they came up with was a cladding made of glass that surrounds the central core glass fiber; this cladding reflects the light that escapes the core and directs it back into it.
In 1962, the first solid-state laser was created and quickly found its way into the fiber-optics field. The first fiber-optic cable had an optical loss of 1000dB/km — obviously more research was needed. In 1970, scientists at Corning succeeded in developing a fiber-optic cable with an attenuation of 20dB/km; this was a turning point when it became possible to use fiber optics for transmission runs.
The first broadcast use of fiber optics was at the 1980 Winter Olympics, in Lake Placid, NY. Video engineers requested a fiber-optic backup link; soon they realized that the backup link was more reliable and provided a better quality signal, so they made the fiber-optic path the primary link. At the 1994 Winter Olympics in Lillehammer, Norway, fiber optics transmitted the first digital video signal. Since then, fiber optics has become more reliable and easier to install.
How it works
Today’s fiber-optic cable has come a long way since its inception. Attenuation has been reduced to 0.2dB/km and splices can be as transparent as 0.1dB (using a fusion splicer). Cables are stronger and less prone to aging than earlier ones. There are basically two different types of fiber-optic cables in use today: single-mode and multimode cables. Single-mode cable has a much smaller fiber-optic core at its center, resulting in a single ray of light (mode) traveling its length. The other is multimode cable, and it is comprised of a thicker core of fiber optics at its center made of multiple strands of optical fiber. The light is dispersed as it makes its way down the cable, which results in a lower-frequency bandwidth and higher attenuation as compared to single-mode fiber. Multimode is more common because the equipment is less expensive and the larger surface area of the fiber allows more light to be transferred into and out of the fiber.
What actually makes fiber optics work is the cladding, the material that surrounds the fiber-optic strands and causes total internal reflection, where all the light striking the boundary between the fiber and the cladding is reflected back into the fiber causing it to bounce back and forth down the cable. Today, the cladding is made of glass fiber similar to the fiber-optic strands but with a lower refractive index, which is the key. The entire assembly is then surrounded by a buffer material and a protective jacket.
Typical fiber-optic receivers and transmitters accept ASI, SD-SDI or HD-SDI signals, or even analog video and audio. The SD-SDI variety can handle either embedded or separate AES digital audio.
Fiber factors
Figure 1
Click to enlarge
Factors to keep in mind when using fiber-optic cabling are attenuation and dispersion. Attenuation is caused by the light being absorbed by impurities and by scattering due to irregularities in the glass. Attenuation is very low in fiber-optic cables, with single-mode fiber being as low as 0.25dB/km and multimode fiber somewhat higher. The frequency of the light plays a major factor in the amount of attenuation for a given fiber-optic cable. (See Figure 1.)
Dispersion is caused by various light rays taking different paths at different speeds. This causes a broadening of pulses sent down the cable, which affects the frequencies, or data rates, that can be used. Different cables have been optimized for use with certain light frequencies so they will have lower attenuation and dispersion for a given frequency.
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