OTDR is the main instrument in the field of optical fiber testing technology. It is widely used in the maintenance and construction of optical cable lines, and can measure fiber length, fiber transmission attenuation, joint attenuation and fault location. OTDR has the advantages of short test time, fast test speed and high test accuracy.
An optical time domain reflectometer (OTDR) uses light time domain measurement to emit light of a certain pulse width into the fiber under test by detecting Rayleigh scattering and Fresnel reflection optical signal power along time. Axis distribution, plot OTDR curves to measure various fiber optic cable and joint parameters to locate fiber fault points, and to understand cable loss distribution. The following is an example of the OTDR tester of Shenzhen Xiaguang Communication Measurement Technology Co., Ltd. (referred to as "Xiaguang").
Five parameter settings to ensure OTDR accuracy
Xiaguang OTDR parameter setting:
Wavelength: the test wavelength, the average loss of 1310nm in the fiber is larger than that of 1550nm.
Range: the range, the recommended range value is 1.5 times the fiber length.
Pulse: The pulse width is selected according to the actual situation.
Duration: Recommended value is 60 seconds.
IOR: The refractive index, supplied by the cable or fiber manufacturer.
1. Test wavelength selection
Since the OTDR is for fiber-optic communication, the test wavelength is selected before the fiber test. The single-mode fiber is only selected for 1310 nm or 1550 nm. Since the 1550nm wavelength has a much more sensitive effect on the bending loss of the fiber than the 1310 nm wavelength, the OTDR is used to carry out the entire fiber backing of an optical cable or a fiber transmission link, whether it is cable line construction or cable line maintenance or experimentation or teaching. Scatter signal curve test, generally use 1 550 nm wavelength.
The shape of the test curves of the two wavelengths of 1310 nm and 1550 nm is the same, and the measured value of the fiber joint loss is also substantially the same. If no problems are found in the 1550 nm wavelength test, then the 1310 nm wavelength test is certainly no problem.
By selecting the 1550 nm wavelength test, it is easy to see if there is excessive bending over the entire fiber. If there is a large loss step at a certain point on the curve, and then retest with the wavelength of 1310 nm, if the loss step disappears at the wavelength of 1310 nm, it indicates that there is excessive bending at this place, which needs further search and elimination. If the loss step is as large at 1310 nm, there may be other problems with the fiber at that point, and it is necessary to find the exclusion. In the single-mode fiber line test, the 1550 nm wavelength should be used as much as possible, so the test results will be better.
2. Test range selection
The range of the OTDR is the maximum distance that the OTDR can reach on the abscissa. During the test, the range should be selected according to the length of the fiber to be tested, and the range is preferably 1.5 times the length of the fiber to be tested. When the range selection is too small, the display of the optical time domain reflectometer is not comprehensive; when the range selection is too large, the horizontal coordinate compression on the display of the optical time domain reflectometer cannot be clearly seen.
According to the actual experience of engineers and technicians, the test range selection can make the backscatter curve account for about 70% of the OTDR display. Both the length test and the loss test can get better direct vision and accurate test results. .
In the fiber-optic communication system test, the link length is several hundred to several thousand kilometers, the length of the hop section is 40-60 km, and the length of the single-disc cable is 2 to 4 km. A reasonable selection of the range of the OTDR can obtain good test results.
3. Test pulse width selection
Setting the width of the light pulse too large produces a strong Fresnel reflection, which will increase the blind zone. Although the narrower test light pulse has a smaller dead zone, the optical power is definitely too weak when the test light pulse is too narrow, and the corresponding backscattered signal is weak, and the backscattered signal curve will be undulating and the test error is large. The set optical pulse width must ensure that there is no excessive blind spot effect, and that the backscattered signal curve has sufficient resolution to see every point along the fiber.
Generally, according to the length of the fiber to be tested, an appropriate test pulse width is selected first, and after one or two pre-tests, an optimum value is determined therefrom. When the distance of the fiber under test is short (less than 5,000 m), the dead zone can be less than 10 m; when the distance of the fiber under test is longer (less than 50 000 m), the dead zone can be less than 200 m; the distance of the fiber under test is very long. When (less than 2 500 000 m), the blind zone can be as high as 2 000 m or more.
In a single-disc test, proper selection of the light pulse width (50 nm) can make the dead zone below 10 m. By averaging through two-way testing or multiple tests, the blind zone has less impact.
4. Fiber refractive index selection
The refractive index of the single-mode fiber currently used is basically in the range of 1.460 0 to 1.480 0, which is precisely selected according to the actual value provided by the fiber optic cable or fiber manufacturer. For G.652 single-mode fiber, the refractive index is generally chosen to be 1.468 0 if the wavelength is 1310 nm in actual test; if the wavelength is 1550 nm, the refractive index is generally chosen to be 1.468 5 . The refractive index selection is not accurate and affects the test length.
Small errors in cable maintenance and troubleshooting can cause significant errors, and must be taken seriously when testing.
5. Average time selection
Since the backscattered light signal is extremely weak, multiple statistical averaging methods are generally used to improve the signal to noise ratio. The OTDR test curve samples the reflected signal after each output pulse, and averages the multiple samples to eliminate random events. The longer the averaging time, the closer the noise level is to the minimum value, and the larger the dynamic range. The dynamic range obtained by averaging for 3 min is 0.8 dB higher than the dynamic range obtained by averaging for 1 min.
Generally speaking, the longer the averaging time, the higher the test accuracy. In order to improve the test speed and shorten the overall test time, the test time can be selected within 0.5 to 3 minutes.
In the fiber-optic communication connection test, a satisfactory result can be obtained by selecting 1.5 min (90 s).
If using the Xiaguang OTDR for testing, the recommended duration is 1 min (60 s).
Only by setting the basic parameters of the test accurately can you create conditions for accurate testing.
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