Using OTDR to quickly locate optical cable line faults

OTDR (Optical Time Domain Reflectometer) is the abbreviation of optical time domain reflectometer. It has the unique advantages of non-destructive, single-ended intervention, intuitive and fast testing of optical fibers. Fiber testing with OTDR can be divided into three steps: parameter setting, data acquisition and curve analysis. Manually set measurement parameters include wavelength selection, pulse width setting, measurement range, averaging time, etc.

(1) Wavelength selection: Because different wavelengths correspond to different light characteristics, the test wavelength should generally follow the principle of corresponding to the system transmission communication wavelength. That is, if the system opens a 1310 nm wavelength, then the test wavelength is 1310 nm.

(2) Pulse width setting: The longer the pulse width, the larger the dynamic range and the longer the measurement distance, but a larger test blind area is generated at this time. When short pulses are injected, the test distance is short, but the blind area can be reduced.

(3) Measurement range: The OTDR measurement range refers to the maximum distance at which the OTDR acquires data samples. The choice of this parameter determines the sampling resolution. The optimal measurement range is about 1.5 times the length of the optical fiber to be measured.

(4) Averaging time: Since the backscattered light signal is extremely weak, statistical averaging is generally used to improve the signal-to-noise ratio. The longer the averaging time, the higher the signal-to-noise ratio.

The general idea of ​​fault location in optical transmission systems is: external first, then transmission. That is to say, when locating a fault, first exclude possible external factors, such as cable breakage, power outage, etc., and then consider the transmission equipment. Therefore, how to accurately locate the fault point is very important. When a line fails, you can use OTDR to test the line. By comparing the test curve, it is easy to find out whether the line loss is normal and the distance. The line with large loss should be maintained to achieve the best state. If a cable break is found, it is easy to find the breakpoint and repair it by running along the actual route of the optical cable according to the test distance.

The method of quickly locating the fault point of the optical cable line is summarized as follows:

(1) Correctly master the use of the instrument. ① Correctly set the parameters of the OTDR. When using the OTDR for testing, the instrument parameters must be set first, the most important of which is to set the refractive index of the test fiber and the test wavelength. Only by accurately setting the basic parameters of the test instrument can the conditions for accurate testing be created.

② Select an appropriate test range. For different test ranges, the distance resolution of OTDR test is different. When measuring the fiber obstacle point, you should choose a test range that is larger than the measured distance and closest to it, so that you can make full use of the instrument's own accuracy. ③ Use the instrument's zoom function. Using the OTDR's zoom function, you can accurately set the cursor on the corresponding inflection point. Use the zoom function key to enlarge the graph to 25m/grid, so that you can get a relatively accurate test result with a resolution of less than 1m.

(2) Establish accurate and complete original data. Accurate and complete optical cable line data is the basic basis for fault measurement and location. Therefore, it is necessary to attach importance to the archiving of line data.

(3) Establish an accurate correspondence between the actual route of the optical cable line and the geographical location markers, and record the actual curve, loss, etc. of the optical cable.

(4) After each breakpoint is repaired, the correspondence between the measured cable length and the pole number and place name (landmark) must be recorded and archived in a timely manner to facilitate future maintenance and management.

The above are some summaries and experiences from my daily work. If there are any deficiencies or errors, please criticize and correct me so that we can promote and improve together.