Fiber optic communications
Fiber optic communications are simple: an electrical signal is converted to light, which is transmitted through an optical fiber to a distant receiver, where is converted back into the original electrical signal. Fiber optic communications have many advantages over other transmission methods. A signal can be sent over longer distances without being boosted; there are no interference problems from nearby electrical fields; its capacity is far greater than for copper of coax cable systems; and the fiber itself is much lighter and smaller than copper systems.
The major limiting characteristics in an optical communications system is the attenuation of the optical signal as it goes through the fiber. The important thing is that the information contained in the light sent down the fiber is received and converted back to its original form. Light is attenuated in a fiber as it travels along due to Rayleigh scattering(explained late). If too much light is lost(or attenuated)then the signal may be too weak at the far end for the receiver to distinguish between pulses in the signal. If the signal is too weak at the receiver then we must boost the transmitter output power, increase the receiver sensitivity, or decrease the distance between the transmitter and receiver to compensate for the excessive attenuation. It is important to know how much light is lost in a length of fiber before it is put into use in a communications system. If the overall attenuation is too high, then corrective action must be taken.
Testing Optical Fiber For Loss
The best way to measure overall attenuation in a fiber is to inject a known level of light in one end and measure the level when it comes out the other end. The difference in the two levels —measured in decibels, or dB — is the end-to-end attenuation (sometimes called “insertion loss”). The most accurate way to make this measurement is with a calibrated light source and optical power meter. But a light source and power meter measurement does not indicate if the attenuation is high along the entire fiber or is localized in one trouble-spot. It does not indicate where a problem may be in a fiber.
On the other hand, an OTDR provides a plot of distance versus signal level in a fiber, and this information is extremely useful in knowing where to find a problem in the fiber.
Other Fiber Tests
The most important test for most fibers is an accurate measurement of the attenuation characteristics. But other tests may be needed for high-speed or very long fiber systems. A dispersion test measures how the information carrying capacity of a fiber may be affected due to the differential speed of light in the fiber. That is, some parts of the light that represents the information being transmitted can travel faster than other parts. In multimode fiber this is called a bandwidth measurement. Dispersion and bandwidth tests are not done with an OTDR.
OTDRs are widely used in all phases of a fiber system’s life, from
construction to maintenance to fault locating and restoration. An OTDR
is used to:
~Measure overall (end-to-end) loss for system acceptance and commissioning; and for incoming inspection and verification of specifications on fiber reels
~ Measure splice loss — both fusion and mechanical splices — during installation, construction, and restoration operations
~ Measure reflectance or Optical Return Loss (ORL) of connectors and mechanical splices for CATV, SONET, and other analog or high-speed digital systems where reflections must be kept down
~Locate fiber breaks and defects Indicate optimum optical alignment of fibers in splicing operations
~Detect the gradual or sudden degradation of fiber by making comparisons to previously-documented fiber tests