On-site testing of optical cable construction is very important, as it prepares for the overall commissioning of the optical terminal. The purpose of optical cable testing is the quality of factory products; the test after construction and deployment is to check whether there is any damage during the deployment process, and is used as a check before connection; the test during connection is to check whether the joint reaches low loss; the unit is formed after connection The purpose of testing the optical cable segment is to check whether the design requirements for total transmission attenuation and total baseband response are met, as preparation for the overall commissioning of the connected optical terminal.
Single-mode optical fiber is characterized by dispersion coefficient. The dispersion coefficient of single-mode optical fiber is originally very low. The limit for a 140Mbit/s system is 300ps/nm. Therefore, when the trunk length is less than 50km, there is a large margin for this limit and there is no need to measure it during the construction process; 565Mbit/s five times The limit of the group is 120ps/nm, so it is necessary to consider it in the design and conduct verification measurements after construction.
When an optical signal is transmitted along an optical fiber, the loss of optical power is the attenuation of the optical fiber. The attenuation A is measured in decibels (dB).
A=10lgP1/P2（dB）
In order to obtain accurate measurement results, it is necessary to ensure that the power distribution is a steady mode, so an injection system is added between the light source and the optical cable under test. The injection system is a simulation device composed of a mode scrambler, a mode filter and a cladding mode stripper; for multi-mode optical fiber, an optical fiber of more than 1km with a certain curvature radius can be used.
CCITT recommendation G.651 recommends 3 test methods. That is, shearing method and backscattering method. The shearing method has high accuracy but is destructive; the insertion loss method is non-destructive and is not as accurate as the shearing method; and the backscattering method, which is measured with an optical time domain reflectometer (OTDR), has full functions, high accuracy and non-destructiveness. Measurement data Can be printed directly.
Testing with an optical time domain reflectometer (OTDR) only needs to be carried out at one end of the optical fiber. This instrument can not only measure the attenuation coefficient of the optical fiber, but also provide details of the attenuation characteristics along the length of the optical fiber and detect physical defects or break points of the optical fiber. position, determine the attenuation and position of the connector, and the length of the optical fiber under test. This instrument is equipped with a printer that can print out the measured curve.
On-site optical fiber splicing is monitored by OTDR. After splicing a core, the splicer will give an estimated attenuation value of the splicing point. The estimation is generally based on visual monitoring of the local fiber core, that is, by observing the quality of the fiber core docking. Attenuation value. Whether the connection work is intact or not will be measured by the supervisor and then notified to the connection worker. The advantages of this method: First, the OTDR is fixed. The vehicles, manpower and material resources required for instrument transfer are omitted; secondly, the test point is selected in a place where there is mains power without the need for a generator; thirdly, the test point is fixed, reducing optical cable stripping.
(1) Select the appropriate range: OTDR has different ranges. The operator should choose a more appropriate range based on the length of the optical cable being tested so that the test curve is displayed in the middle of the screen as much as possible, so that the reading can be accurate and the error will be small.
(2) Select the appropriate pulse width: OTDR can select the optical pulse width parameter injected into the optical fiber under test. When the amplitude is the same, the energy of the wide pulse is greater than the energy of the narrow pulse, and it can test longer distances, but the error is larger. Therefore, the operator should select an appropriate pulse width based on the length of the optical fiber to be tested so that it can test the longest possible distance while ensuring accuracy.
(3) Select the appropriate refractive index: Due to the different materials used for optical fibers from different manufacturers, the transmission speed of light in the optical fiber is different, that is, different optical fibers have different refractive indexes. Therefore, the appropriate refractive index should be selected during testing, so that It is only accurate when measuring fiber length.
(4) The selection of test points should be reasonable: At present, most OTDR test joint losses use the 5-point method. During the test, the cursor should be positioned on the joint point as one point, and the remaining 4 points should correspond to the two sides of the joint point respectively. Fiber optic properties. Only in this way can the joint test be accurate.
This method is to place an OTDR at the beginning of the connection direction and perform unidirectional testing on all joint points.
When the length of the trunk section is short and there are not many optical cable connectors, such as local telephone trunk optical cables, and the attenuation requirements of the connectors are not very precise, you can use an optical time domain reflectometer to monitor from one end and instruct the connector to adjust the connector to achieve a relatively optimal value. It can be formally continued. The waveform at point c appears with a small "step", and the size of the attenuation can be estimated by the size of the "step".
This method is not as accurate as the comparison method, but it is simple and requires only one point of monitoring and two points of coordination. It is suitable for the construction of optical cable sections with large attenuation margins in the trunk section, and can speed up the progress.
This method is to short-circuit two optical fibers at the beginning of the connection direction to form a loop, and the OTDR performs bidirectional testing on all joint points at a point before the connection start point. Since the loopback point is added, the bidirectional value of the connection loss can be measured on the OTDR. The advantage of this method is that it can accurately evaluate the quality of the joint.
Due to the testing principle and optical fiber structure, the phenomenon of false gain will appear when using OTDR for one-way monitoring, and accordingly the phenomenon of false large attenuation will appear. For a connector, the mathematical average of the attenuation values in the two directions is used. In order to accurately reflect its true attenuation value. The standards for optical fiber attenuation constants are: at the 1310mm wavelength, the average attenuation value should be less than or equal to 0.36dB/km, and the maximum attenuation value should be less than or equal to 0.4dB/km; at the 1550mm wavelength, the average attenuation value should be less than or equal to 0.22dB/km. The maximum attenuation value should be less than or equal to 0.25dB/km; when optical fiber is connected, its bidirectional average joint loss shall not be greater than 0.08dB.
After completion, use a light source and optical power meter to conduct a two-way test on the entire process. The attenuation value must meet the design requirements. And use OTDR to check whether the backscattering curve meets the requirements in both directions.
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