Select only 1 310 nm or 550 nm single-mode fiber OTDR for fiber-optic communication services, first select the test wavelength before performing optical fiber test. 1 550 nm wavelength fiber bending loss is much more sensitive than 1 310 nm wavelength, and so whether the cable line construction or maintenance of fiber optic cable lines or OTDR experiments, teaching, and use of a cable or a fiber optic transmission link for the entire The fiber-optic signal backscatter curve testing, selection of more than 1 550 nm wavelength. 1 310nm and 1 550 nm wavelength test the shape of the curve is the same optical fiber splice loss values measured. If no problems are found in the 1 550 nm wavelength test, then the 1 310 nm wavelength test and certainly no problem. Select 1 550 nm wavelength testing, you can easily find the optical fiber throughout the existence of bending over. If it is found somewhere in the curve larger loss of stairs, and then 1 310 nm wavelength retest in 1 310 nm wavelength loss steps disappear, indicating that the premises do exist the bending excessive need to find and exclude. 1 310 nm wavelength loss steps the same, where the optical fiber may also have other problems, need Find exclude. 1 550 nm wavelength in single-mode optical fiber line testing, you should try to use, so that the test results will be better.
Now use single-mode optical fiber refractive index of the basic 1.460 0 to within 1.480 0,, according to the actual value provided by cable or fiber manufacturers to choose exactly. For G. 652 single-mode fiber, the actual test 1 310 nm wavelength, refractive index generally choose 1.468 0; 1 550 nm wavelength, the refractive index generally choose to 1.468 5. Refractive index choose not allowed to affect the test length. Medium refractive index, in the formula . If the error is 0.001, then at 50 000 m of hops will produce the error of approximately 35 m. Small mistakes in the cable maintenance and troubleshooting will bring about a significant error, the test must be taken seriously enough.
Assembly set the width of the light pulse to produce a strong Fresnel reflection, make blind intensify. Smaller blind spot in the a narrower test light pulse, but the test light pulse is too narrow time power is too weak, the corresponding back-scattered signal is also weak backscatter signal curve undulating, test error. Light pulse width set to be able to ensure that not too blind spot effect, but also to ensure the backscatter signal curve has sufficient resolution to be able to see the point of the fiber along. Generally according to the length of the fiber under test, first select an appropriate test pulse width, the pre-test after one or two, from which to determine an optimal value. The shorter distance of the fiber under test (less than 5 000m), the blind spot in less than 10 m; distance measured fiber long (less than 50 000 m), the blind spot in less than 200 m; distance of the fiber under test long (less than 2 500 000 m), blind spots can be up to 2 000 m or more. In the single-disk test, the proper choice of the light pulse width (50 nm) can make the dead zone below 10 m. Through the the bidirectional test or several tests averaged blind spot impact would be smaller.
OTDR range of OTDR the abscissa can reach the maximum distance. Test shall be based on the length of the fiber under test selection range, the range is 1.5 times the length of the fiber under test. Range selection is too small, optical time domain reflectometer display point of view is not comprehensive; range selection is too large, the OTDR display to see clearly on the abscissa compression. According to the actual experience of the engineering and technical personnel, test range selection backscatter curve can account for around 70% of the OTDR screen, regardless of the length of the test or loss testing can get a good look directly at the effects and accurate test results. Test of optical fiber communication systems, the length of the link in the hundreds to thousands of kilometers, and the length of the hops 40 ~ 60 km, 4 km the single disk cable length to collectively select the range of the OTDR can get good test results.
Backscattered light signal is extremely weak, several statistical average method commonly used to improve the SNR. The OTDR test curve is after each output pulse reflected signal sampling, and sampled many times do the averaging process in order to eliminate the random event, the averaging time is longer, more noise level close to the minimum value, the greater the dynamic range.The average time for 3 min dynamic range than the averaging time of 1 min, the dynamic range is increased by 0.8 dB. Generally averaging time is longer, the higher the accuracy of testing. In order to improve the test speed, shorten the overall test time, test time can be selected within 0.5 ~ 3 min.
Select 1.5 min (90 s) in the continue testing of fiber optic communication, satisfactory results can be obtained.
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