Simple judgment of fiber quality:
Under normal circumstances, the slope of the main body of the light curve (single or several cables) tested by OTDR is basically the same. If it is bent or curved, it indicates that the quality of the fiber is seriously deteriorated and does not meet the communication requirements.
Wavelength selection and unidirectional testing:
The 1550 wavelength test distance is farther, the 1550 nm fiber is more sensitive to bending than the 1310 nm fiber, the 1550 nm has less attenuation per unit length than the 1310 nm, and the 1310 nm test has a higher loss of fusion or connector than the 1550 nm. In the actual optical cable maintenance work, both wavelengths are generally tested and compared. For positive gain phenomenon and over-distance lines, two-way test analysis and calculation must be carried out in order to obtain a good test conclusion.
Connector cleaning:
Before the optical fiber joint is connected to the OTDR, it must be carefully cleaned, including the output joint of the OTDR and the joint under test. Otherwise, the insertion loss will be too large, the measurement will be unreliable, the curve will be noisy and even the measurement will not be carried out, and it may damage the OTDR. Avoid cleaning agents or index matching fluids other than alcohol, as they can dissolve the adhesive within the fiber optic connector.
Correction of refractive index and scattering coefficient:
As far as fiber length measurement is concerned, every 0.01 deviation of the refractive index will cause an error of as much as 7 m/km. For longer ray segments, the refractive index value provided by the fiber optic cable manufacturer should be used.
Recognition and treatment of ghosts:
Spikes on the OTDR curve are sometimes echoes caused by strong reflections close to the incident end. Such spikes are called ghosting. Identifying ghosts: no obvious loss is caused at the ghosts on the curve; the distance between the ghosts and the beginning along the curve is a multiple of the distance between the strong reflection event and the beginning, which is symmetrical. Eliminate ghosting: choose short pulse widths, add attenuation in strongly reflective front ends such as OTDR outputs. If the event causing the ghosting is at the end of the fiber, a "small bend" can be made to attenuate the light reflected back to the beginning.
Positive gain phenomenon processing:
A positive gain phenomenon may occur on the OTDR curve. The positive gain is due to the fact that the fiber after the splice produces more backscatter than the fiber before the splice. In fact, the fiber is splice loss at this splice point. It often occurs in the splicing process of fibers with different mode field diameters or different backscattering coefficients. Therefore, it is necessary to measure in two directions and average the results as the splicing loss. In the actual maintenance of optical cables, the simple principle of ≤0.08dB can also be adopted.
Use of additional fibers:
The additional fiber is a length of 300-2000m fiber used to connect the OTDR and the fiber to be measured. Its main functions are: front-end blind spot processing and terminal connector insertion measurement. Generally speaking, the dead zone caused by the connector between the OTDR and the fiber under test is the largest. In the actual measurement of the fiber, a transition fiber is added between the OTDR and the fiber to be tested, so that the blind area of the front end falls within the transition fiber, and the beginning of the fiber to be tested falls in the linear stability region of the OTDR curve. The insertion loss of the connector at the beginning of the optical 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 head and tail ends, you can add a transition fiber at each end.
