OTDR (Optical Time Domain Reflectometer, Optical Time Domain Reflectometer) is the backscatter generated by the Rayleigh scattering and Fresnel reflection in the use of light pulses in optical fibers made ​​of high-tech, high-precision photoelectric integrationinstrument. Semiconductor light source (LED or LD) output light pulse in the modulation of the drive circuit, the directional optical coupler and activities connector into the measured optical fiber cable line into the incident pulse. Incident light pulse transmission line along the Rayleigh scattering of light and Fresnel reflected light, most of the Rayleigh scattered light will be refracted into the cladding attenuation, opposite to the direction of propagation of light pulses back to Rayleigh scattering light along the optical fiber transmission line into the optical port, the directional coupler splitter fired at the photodetector into an electrical signal, low noise amplification and the number average treatment will be processed signals from the light source the back of the emission trigger signal is extracted by synchronous scanning become a reflected light pulse on the oscilloscope. Useful information returned by the OTDR detector to measure them as a time or curve fragments in different locations on the measured fiber. According to transmit signals to the time of the return signal, and then determine the speed of light in the quartz material, you can calculate the distance.

Test wavelength selection

OTDR for fiber optic communications services, before making optical fiber test first select a test wavelength single-mode fiber to select only 1 310 nm or 550 nm. 1 at 550 nm wavelength on the fiber bending loss is much more sensitive than the 1 310 nm wavelength, regardless of fiber optic cable line construction or fiber optic cable line maintenance or experiment, teaching, using an OTDR to the whole of a cable or a fiber optic transmission link The fiber optic signal backscatter curve testing, selection of more than 1 550 nm wavelength. 310nm and 550 nm, the wavelength of the test shape of the curve is the same, the measured optical fiber splice loss values ​​are basically the same. 1 550 nm wavelength testing found no problems, then the 1 310 nm wavelength measurement is certainly no problem. Select the 1 550 nm wavelength testing fiber full easy to find whether there is excessive bending of the situation. If it is found somewhere on the curve have a greater loss steps, and then the 1 310 nm wavelength retest, 1 310 nm wavelength loss steps disappear, indicating that there does exist a bend excessively, you need to find and eliminate. 1 310 nm wavelength loss steps are the same, where fiber may also have other problems, also need to look to exclude. In the single-mode fiber line test, you should try to use the 1 550 nm, so that test results will be better.

The refractive index of fiber options

Now use single-mode fiber refractive index in the range of 1.460 0 1.480 0. To accurately selected according to the actual value provided by the manufacturer of fiber optic cable or fiber. For G. 652 single-mode fiber, the actual test with a wavelength of 310 nm, the refractive index is generally choose 0 1.468; if 1 550 nm wavelength, the refractive index is generally choose to 1.468 5. The refractive index choose not allowed to affect the test length. In refractive index error of 0.001 (1), 50 000 m relay segment will generate the error of about 35 m. Cable maintenance and troubleshooting of small mistakes would bring significant error, the test must be taken seriously enough.

Three common methods of implementation of the OTDR testing

OTDR fiber optic cable and fiber optic test season testing occasions, including the maintenance tests and periodic testing of the construction test for factory testing of fiber optic cable and optical fiber, optical cable and fiber optic cable, fiber optic cable and optical fiber.

The OTDR forward one-way test method

OTDR test the direction of the optical fiber splice connector point, test instruments and testers are always ahead of the transfer of construction vehicles. Use this method to monitor, test points and splices is always only one cable length, the accuracy of the test connector attenuation, and to facilitate communications and liaison. A cable length of about 2 to 3 km, the use of walkie-talkie in the general topography can ensure that communication links. If the fiber optic cable corrugated steel protective layer can also use the magnet telephone contact.

The disadvantage of this test method is obviously an OTDR to move to each test point time and labor, and not conducive to the protection of the instrument; test point to the terrain, especially the line away from the road, more cumbersome in complex terrain. Selection of portable OTDR monitoring at close range to test the instrument dynamic range of less demanding, and small 0TDR volume small, light weight and easy to move, this can greatly reduce the workload of testers, to improve test speed and efficiency.

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