Optical fiber measurement can be divided into three steps: using OTDR testers for parameter setting, data acquisition and analysis. Measuring parameters include artificial Settings.
(1) the wavelength selection (λ):
Because of different wavelengths corresponding to different features (including attenuation, slightly curved, etc.), generally following the test wavelength and wavelength corresponds to the principle of transmission communication system, the system open 1550 wavelengths, the test wavelength of 1550nm.
(2) Pulse Width:
The longer the pulse width, the bigger the dynamic measurement range, measurement of distance is longer, but blind area is bigger in OTDR curve waveform; Short pulse injection of low light level, but can reduce the blind area. Usually stand by ns for pulse width cycle.
(3) Range:
OTDR testers measurement range is refers to the maximum distance of OTDR data sampling, the choice of this parameter determines the size of the sampling resolution. Best measurement range for 1.5 ~ 2 times the distance between optic fiber network length.
(4) Average time:
The backward scattering light signal is very weak, generally USES the statistical average method to improve the signal-to-noise ratio, average, the longer the average time, the higher signal-to-noise ratio.
(5) Parameters of optical fiber
Optical fiber parameter setting including the refractive index n and Backscatter coefficient of η. Refractive index parameters related to distance measuring, backscatter coefficient effects the measurement results of reflection and return loss.
After Parameter Settings, OTDR can be sent and received by the optical fiber link light pulse scattering and reflected light, the photodetector outputs sample, get the OTDR curve, the curve is analyzed to understand quality of optical fiber.
Experience and skills
(1) Simple discriminant of fiber quality
Under normal circumstances, OTDR test the light curve of the subject (single or several plate cable) slope are consistent, if a certain section of the slope is bigger, indicates the period of decay; If subject to irregular shape curve, the slope is volatile, it is bent or arc, suggests that bulk fiber cables quality degradation seriously, do not conform to the requirements of the communication.
(2) The choice of wavelength and the test of Uni and Bi-direction:
1550 wavelengths to test distance is farther, a 1550nm – 1550nm fiber is more sensitive to bending than 1310. In an actual optical cable maintenance, compare both wavelengths to get good test results.
(3) Clean the joint:
Before accessing optical union, must be cleaned carefully, including OTDR output fiber assembly connectors and measured union, or the insertion loss will too big, otherwise don’t reliable, much noise can stop the measurement, it may also damage the OTDR. Avoid using alcohol cleaning or other refractive index matching liquid, because they can dissolve the adhesive within optical fiber connector.
(4) The use of additional optical fiber
Additional optical fiber is used to connect OTDR and optical fiber which under test, 300 ~ 2000 m long fiber, its main role is: the front insert measuring blind area processing and terminal connector.
In general, OTDR and optical fiber connectors between blind area under test is the largest. In actual measurement of the optical fiber in the OTDR with fiber and after a period of transition, the front end blind area falls within the transition of optical fiber, the fiber under test head fell on the OTDR curve in the linear region. Optical fiber system between connector insertion loss by OTDR testers and optical fiber to measure for a period of transition. As to measure the first and end connector on both ends of the insertion loss, can add a transitional fiber in every side.
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