An Optical Time Domain Reflectometer (OTDR) is an optical measurement instrument designed to detect faults, splices and bends in optical fiber cables. It functions by launching pulses of light into the optical fiber and measuring the back reflections created by the faults, splices, and bends. It can identify the exact location of the fault by measuring the round trip time from the launch to the detection of the reflected returning pulse. The time is determined by the speed of light in the glass core of the optical fiber.
With so many factors affecting the launch and detection process, problems such as unreliable traces of measurements are likely to be seen especially when only a little amount of light comes back to the OTDR for analysis. This occurs when trying to look at a very long length optical fiber. If you are trying to look at a very long optical fiber, it is necessary to launch a lot of power to see the end. When a lot of optical power is launched, the pulse width of the launched optical signal is increased. The large pulse width decreases the resolution of the measurement and can be as much as several hundred meters. Faults near to the launch end are then masked because of the hundreds of meters between the launch pulse and the receiver being able to see the reflected pulse. If there is a fault near the launch point, it can also create large reflections that saturate and overload the receiver. This length of fiber is sometimes called the Dead Zone because the faults are masked in the length close to the OTDR. The receiver requires an amount of time to recover from saturation. Depending on the OTDR design, wavelength, and magnitude, the OTDR may take up to 500 meters or more to fully recover.
Most OTDR manuals suggest the use of launch fibers to resolve these issues. Launch fibers place the necessary length of fiber between the OTDR and the actual fiber being measured providing time for the receiver to settle and also for the pulse width dependent resolution to be overcome. When launch fibers are used, faults close to the end of the fiber being measured can be seen by the OTDR. They do not interfere with the actual fiber being measured and thus are very secure and proven as a technique for identifying faults in the total length of fiber from the first interface to the last.
An OTDR launch fiber, often available on a small spool or within a “launch box”, is used to create the proper conditions for testing another similar optical fiber for faults. This method avoids undesirable variations in loss and distance measurements. A launch fiber will help to overcome the blind spot or Dead Zone of an OTDR brought about by high launch power or faults near the launch end of the fiber. In summary, an OTDR launch fiber provides both the time and distance required for the OTDR to effectively look at and measure the characteristics of the entire length of fiber being tested, especially the length closest to the OTDR.