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. It can measure optical fiber length, optical fiber transmission attenuation, joint attenuation, and fault location. OTDR has the advantages of short test time, fast test speed, and high test accuracy.
Optical Time Domain Reflectometer (OTDR), using the method of optical time domain measurement, emits a certain pulse width of light into the tested fiber, and detects the Rayleigh scattering (Rayleigh scattering) and Fresnel reflection (Fresnel reflection) optical signal power along the time Axis distribution, draw OTDR curve, to measure various optical cable and joint parameters to locate optical fiber fault points, and understand the distribution of optical cable loss. The following takes fiber-mart.com OTDR tester as an example to analyze and explain.
1. Test wavelength selection
Since OTDR is for optical fiber communication, select the test wavelength before performing fiber test. Only 1310 nm or 1550 nm is selected for single-mode fiber. Since the 1550nm wavelength has a much more sensitive influence on the bending loss of the fiber than the 1310nm wavelength, it does not matter. Whether it is fiber optic cable line construction or fiber optic cable line maintenance or experimentation and teaching, use OTDR to test the entire fiber backscatter signal curve of a certain optical cable or a certain optical fiber transmission link. Generally, the wavelength of 1 550 nm is used.
The shape of the test curves at 1310nm and 1550nm wavelengths is the same, and the measured optical fiber connector loss values are basically the same. If no problems are found in the 1550 nm wavelength test, then the 1310 nm wavelength test must be no problem.
Choosing the 1550 nm wavelength test, you can easily find whether there is excessive bending of the fiber throughout the entire process. If a large loss step is found somewhere on the curve, repeat the measurement with a wavelength of 1310 nm. If the loss step disappears at a wavelength of 1310 nm, it means that there is indeed an excessive bending situation, which needs to be further searched and eliminated. If the loss step is the same at the 1310 nm wavelength, there may be other problems in the fiber, which need to be found and eliminated. In the single-mode optical fiber line test, the 1550 nm wavelength should be selected as much as possible, so that the test effect will be better.
2. Test range selection
The range of the OTDR refers to the maximum distance that the abscissa of the OTDR can reach. The measurement range should be selected according to the length of the optical fiber under test. It is better for the measurement range to be 1.5 times the length of the optical fiber under test. When the range is selected too small, the display of the optical time domain reflectometer is not comprehensive; when the range is selected too large, the abscissa on the display of the optical time domain reflectometer is compressed and can not be seen clearly.
According to the actual experience of engineers and technicians, when the test range is selected so that the backscatter curve accounts for about 70% of the OTDR display screen, both the length test and the loss test can get a better direct-view effect and accurate test results.
In the optical fiber communication system test, the link length is several hundred to several thousand kilometers, the relay section length is 40-60 km, and the single-reel optical cable length is 2 to 4 km. A reasonable selection of the OTDR range can get good test results.
3. Test pulse width selection
Setting the light pulse width too large will produce strong Fresnel reflection, which will increase the blind area. Although the narrow test light pulse has a small blind area, the light power of the test light pulse is too narrow, and the corresponding backscatter signal is also weak. The backscatter signal curve will be undulating and the test error is large. The set optical pulse width must not only ensure that there is no excessive blind zone effect, but also ensure that the backscattered signal curve has sufficient resolution and can see every point along the optical fiber.
Generally, according to the length of the fiber under test, an appropriate test pulse width is selected first, and an optimal value is determined from the pre-test once or twice. When the distance of the tested fiber is short (less than 5 000 m), the blind area can be less than 10 m; when the distance of the tested fiber is longer (less than 50,000 m), the blind area can be less than 200 m; the distance of the tested fiber is very long (Less than 2 500 000 m), the blind area can be as high as more than 2 000 m.
In single-disk testing, proper selection of the optical pulse width (50 nm) can make the blind zone less than 10 m. By taking the average of two-way test or multiple tests, the impact of the blind zone will be smaller.
Generally speaking, the longer the averaging time, the higher the test accuracy. In order to increase the test speed and shorten the overall test time, the test time can be selected within 0.5 to 3 min.
In the optical fiber communication connection test, a satisfactory result can be obtained by choosing 1.5 min (90 s).
If Fiber-mart OTDR is used for testing, the recommended duration is 1min (60s).
Only by accurately setting the basic parameters of the test can conditions be created for accurate testing.