Under normal circumstances, the main body of the OTDR test light curve (single tray or several trays of fiber optic cable) slope is basically the same, if the slope of a section is larger, it indicates that this section is more attenuation; if the main body of the curve is irregularly shaped, the slope is more undulating, curved or arc-shaped, it indicates that the fiber quality is seriously deteriorated and does not meet the communication requirements.
Wavelength selection and single- and two-way testing.
1550 wavelength test distance is farther, 1550nm than 1310nm fiber is more sensitive to bending, 1550nm than 1310nm unit length attenuation is smaller, 1310nm than 1550nm measured fusion or connector loss is higher. In the actual fiber optic cable maintenance work is generally tested and compared for both wavelengths. For positive gain phenomenon and over the distance line are required to carry out two-way test analysis and calculation, in order to obtain a good test conclusion.
Splice cleaning.
Fiber live connector access OTDR before, must be carefully cleaned, including the OTDR output connector and the live connector under test, otherwise the insertion loss is too large, unreliable measurement, curve more noise or even make the measurement can not be carried out, it may also damage the OTDR. avoid using other cleaning agents other than alcohol or refractive index matching solution, because they can make the fiber connector within the adhesive dissolved.
Refractive index and scattering coefficient correction.
For fiber length measurements, each 0.01 deviation in the refractive index can cause as much as 7m/km of error. For longer segments of light, the refractive index value provided by the fiber optic cable manufacturer should be used.
Identification and treatment of ghost images.
Spikes on the OTDR curve are sometimes due to echoes caused by close and strong reflections from the incident end, and such spikes are called ghost images. Identify the ghost shadow: The ghost shadow on the curve does not cause significant loss; the distance between the ghost shadow and the beginning of the curve is a multiple of the distance between the strong reflection event and the beginning, in a symmetrical shape. Eliminate ghosting: Select a short pulse width and add attenuation in the strongly reflective front end (e.g. OTDR output). If the event that causes ghosting is located at the end of the fiber, you can "make a small bend" to attenuate the light reflected back to the beginning.
Positive Gain Phenomenon Handling.
Positive gain may occur on the OTDR curve. Positive gain is caused by the fact that the fiber after the splice point produces more backward scatter than the fiber before the splice point. In fact, the fiber is fusion-lossy at this fusion point. It often occurs during the fusion of fibers with different mode field diameters or different backscatter coefficients, so it is necessary to measure in both directions and average the results as this fusion loss. In the actual maintenance of fiber optic cables, the simple principle of ≤ 0.08dB is also used to pass.
The use of additional fiber.
Additional fiber is a section of fiber used to connect the OTDR and the fiber to be measured, 300 ~ 2000m long, its main role: front-end blind processing and terminal connector insertion measurement. Generally speaking, the blind zone caused by the connector between OTDR and the fiber to be measured is the largest. In the actual measurement of optical fiber, a section of transition fiber is added between OTDR and the fiber to be measured, so that the front-end blind area falls within the transition fiber, and the beginning of the fiber to be measured falls in the linear stability zone of the OTDR curve. The insertion loss of the connector at the beginning of the fiber system can be measured by adding a transition fiber to the OTDR. If you want to measure the insertion loss of the connector at the first and last ends, you can add a transition fiber at each end.
