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Optical fiber quality detection method

2022-07-26
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In the construction of the integrated wiring project, the construction quality of the optical fiber is very important, so it is necessary to carry out the necessary inspection on the construction quality. Based on years of design and construction experience, the author summarizes a set of relatively practical detection methods and shares them with colleagues.

 

Inspection and installation

After connecting the cable and installing the connector, a continuity check of the fiber must be carried out and all the cable cores must be tested. Therefore, the person undertaking the test should have some test equipment available to test the quality of the installation. There are two kinds of measurements that must be made: a comprehensive test of the attenuation produced by the fiber and the connector; and a reflection measurement that can produce a schematic diagram of the line loss curve of the fiber connection. This measurement also tells people how long the connection is, and where things go wrong.

 

Attenuation measurement

Attenuation measurements, also called first-level measurements, are performed with a high-power calibration generator that transmits the signal and a fiber optic radiometer that measures the receive. It is possible to measure the attenuation on the line and know if the line or a section of the line is within specified tolerances. This is the first level measurement. All connectors should be cleaned before each measurement. To avoid measurement errors, the same measurement cable (eg 3 meters long) should be used for both instruments (generator and receiver). These cables should have the same characteristics as the fiber core under test.

fiber optic cable

Testing can be done in two ways: one tester with two instruments in the same place but able to loop back with the other; two testers with one tester in each place. All test procedures begin with calibrating the receiver. To this end, the generator and receiver must be directly connected together with a 3-meter long cable, and then transmit high power to the receiver, and then calibrate the receiver until 0dB appears on the liquid crystal display; test with a wavelength of 850nm, test The maximum attenuation obtained in , is equal to the fiber attenuation, which is 3.5dB/Km of this wavelength, and the attenuation generated by various connections (connectors and joints) is added above this. We believe that 90% of connected devices have an attenuation of 0.3dB up to 850nm.

 

This article specifically describes the method of testing by a tester. A tester's test requires that the different optical cables of the final distribution frame are looped through some 10-meter-long optical fiber jumpers, and then the optical cores of each pair of optical fibers are inspected one by one. The transmitter and receiver are co-located; use a 3-meter fiber optic cable to connect the transmitter to the first fiber on the cable head or fiber drawer; use a second 3-meter fiber optic cable to connect the receiver Connect to the second fiber.

 

After measuring the loopback, the tester connected the receiver to the third fiber and performed the same test, and tested the fourth, fifth and sixth fibers in the same way.

 

The test procedure is: calibrate the receiver, install the jumper cable, measure the circuit and store the test results in the computer. The transmitter and receiver are looped back with a 3-meter cable for calibration. A high power transmission should be made to the receiver and the receiver should be calibrated to display 0dB on the LCD. If the measured value exceeds the maximum expected value, the location of the fault should be determined by means of a fiber optic reflectometer.

 

The purpose of measuring with a reflection measuring instrument, also known as the second-level measurement, is to measure the physical state of the fiber core. The distribution of losses can be shown on the display.

 

The measurement principle is: the reflection measuring instrument emits a high-power calibration beam, and then observes whether there is a visible reflected power signal on the display screen. This reflection is due to imperfections in the silicon that makes up the fiber, consistent with linear attenuation. The remainder of the reflected signal is equal to the reflection of the possibly faulty part of the patch. At the same time, these losses can be located and the exact length of the cable detected.

 

These tests can detect whether the fiber is in an abnormal condition (bend radius, excessive pulling or squeezing), and can also detect whether there is a broken wire, whether the broken wire is due to improper operation. At the same time, you can also know whether the ST connector is connected correctly.

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