Optical Loss on Fiber

One issue that isn’t much understood except by engineers and fiber technicians is optical loss on fiber. While fiber is an incredibly efficient media for transmitting signals there are still factors that cause the signal to degrade. In new fiber routes these factors are usually minor, but over time problems with fiber accumulate. We’re now seeing some of the long-haul fibers from the 1980s go bad due to accumulated optical signal losses.

Optical signal loss is described as attenuation. Attenuation is a reduction in the power and clarity of a light signal that diminishes the ability of a receiving laser to demodulate the data being received. Any factor that degrades the optical signal is said to increase the attenuation.

Engineers describe several kinds of phenomenon that can degrade a fiber signal:

  • Chromatic Dispersion. This is the phenomenon where a signal gets distorted over distance as the different frequencies of light travel at different speeds. Lasers don’t generally create only one light frequency, but a range of slightly different colors, and different colors of light travel through the fiber at slightly different speeds. This is one of the primary factors that limits the distance that a fiber signal can be sent without needing to pass through a repeater to restart and synchronize all of the separate light paths. More expensive lasers can generate purer light signals and can transmit further. These better lasers are used on long haul fiber routes that might go 60 miles between repeaters while FTTH networks aren’t recommended to travel more than 10 miles.
  • Modal Dispersion. Some fibers are designed to have slightly different paths for the light signal and are called multimode fibers. A fiber system can transmit different date paths through the separate modes. A good analogy for the modes is to think of them as separate tubes inside of a conduit. But these are not physically separated paths and the modes are created by having different parts of the fiber strand to be made of a slightly different glass material. Modal dispersion comes from the light traveling at slightly different speeds through the different modes.
  • Insertion Loss. This is loss of signal that happens when the light signal moves from one media to another. Insertion losses occurs at splice points, where fiber passes through a connector, or when the signal is regenerated through a repeater or other device sitting in the fiber path.
  • Return Loss. This is the lost of signal due to interference caused when some parts of the light are reflected backwards in the fiber. While the glass used in fiber is clear, it’s never perfect and some photons are reflected backwards and interfere with oncoming light signals.

Fiber signal loss can be measured with test equipment that measure the delay in a fiber signal compared to an ideal signal. The losses are expressed in decibels (dB).  New fiber networks are designed with a low total dB loss so that there is headroom over time to accommodate natural damage and degradation. Engineers are able to calculate the amount of loss that can be expected for a signal traveling through a fiber network – called a loss budget. For example, they know that a fiber signal will degrade some specific amount, say 1 dB just from passing through a certain type of fiber. They might expect a loss of 0.3 dB for each splice along a fiber and 0.75 dB when a fiber passes through a connector.

The biggest signal losses on fiber generally come at the end of a fiber path at the customer premise. Flaws like bends or crimps in the fiber might increase return loss. Going through multiple splices increases the insertion loss. Good installation practices are by far the most important factor in minimizing attenuation and providing for a longer life for a given fiber path.

Network engineers also understand that over time that fibers degrade, Fibers might get cut and have to be re-spliced. Connectors get loose and don’t make perfect light connections. Fiber can expand and shrink from temperature extremes and create more reflection. Tiny manufacturing flaws like microscopic cracks will grow over time and create opacity and disperse the light signal.

This is not all bad news and modern fiber electronics allow for a fairly high level of dB loss before the fiber loses functionality. A fiber installed properly, using quality connections and with good splices can last a long time.

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