Do you know the fiber test method based on OTDR technology?

OTDR is the main instrument in the field of optical fiber testing technology, which is widely used in the maintenance and construction of optical fiber cable, and can be used to measure the length of the fiber, transmission attenuation of the optical fiber, joint attenuation and fault location. The OTDR has the advantages of short testing time, fast testing speed and high precision of testing.

Optical fiber communication is a transmission medium with the optical wave as the carrier. Optical fiber communication, as the main means of information transmission, is the cornerstone of “information superhighway” because of its long transmission distance, large information capacity and high quality of communication. Optical fiber testing technology is the most extensive and basic technology in the field of optical fiber application. OTDR is the main instrument in the field of optical fiber testing technology, which is widely used in the maintenance and construction of optical fiber cable, and can be used to measure the length of the fiber, transmission attenuation of the optical fiber, joint attenuation and fault location. The OTDR has the advantages of short testing time, fast testing speed and high precision of testing.

OTDR tests fiber optic cables and optical fibers, test occasions include fiber optic cable and fiber factory testing, fiber optic cable and fiber optic cable construction testing, fiber optic cable and fiber maintenance testing and regular testing.

The method of testing the connection is: OTDR – fiber optic connector -1st Optical cable – 2nd Optical fiber cable, the terminal does not connect any equipment. According to the actual testing work, there are three main methods:

OTDR Back test method

This method is mainly used to monitor the connection of an optical cable, and the optical cable must be equipped with a special optical fiber fusion machine and a light time domain reflector (OTDR). The welding machine will usually give the estimated attenuation value of the contact after it has been fused to the core.

There are three advantages of this method test:

1. OTDR fixed, omitted the instrument transfer required vehicles and a large number of human and material resources;
2. The test pilot is selected in the place where the electricity is not required with the petrol generator;
3. The test point is fixed, reducing the fiber optic cable stripping.

The method also has two shortcomings:

1. Due to distance and terrain constraints, sometimes can not guarantee the smooth contact;
2. with the increasing of the continuous distance, the test range and precision of OTDR are limited.

There are currently three ways to solve these problems:

1.  the use of mobile phones in both the city and the suburbs allows testers and staff to keep in touch, facilitate organization and coordination, and help improve efficiency.
2.  the telephone for contact. Make sure that you use a fiber (such as blue Fiber Unit red fiber) to connect to a light telephone line. Of course, the last fiber used for contact is not able to be monitored because it cannot be contacted when it is fused and spun. Even so, the likelihood of problems will be greatly reduced (if the 24-core optical cable, the probability of the problem will fall below 1/24 of the original).
3.  The OTDR moves forward when the optical cable continues to reach a relay distance.

Test practice has proved that these monitoring methods are effective to ensure quality and reduce rework.

OTDR forward one-way test method

OTDR in the optical fiber connection direction before a joint point to test, with construction vehicles will be the test instrument and test personnel always ahead of the transfer. Using this method to monitor, the test point and the connecting point always have only one optical cable length, the test joint attenuation consumption accuracy is high, and facilitates the communication contact. At present, a disk cable length of about 2~3km, the general terrain under the use of walkie-talkie to ensure communication contact. If the cable has a wrinkle steel belt, you can also use a magnet phone to contact.

The disadvantage of this test method is also obvious, OTDR to move to each test point of time-consuming, and not conducive to the protection of the instrument, the test point is also subject to terrain restrictions, especially the line away from the road, complex terrain more trouble. Using portable type OTDR for monitoring, near-distance testing on the dynamic range of instrumentation is not high, and small 0TDR volume small weight easy to move, this can greatly reduce the workload of testers, improve testing speed and efficiency.

OTDR forward two-way test method

The OTDR position is still monitored with “forward-one-way”, but the two fibers are shortened respectively at the beginning of the connecting direction, forming a circuit. This method can satisfy the relay section fiber test, also can carry on the monitoring to the optical fiber. The optical fiber test of the relay section can clearly see the incident light pulse, the reflected light pulse, the joint point, the rupture point, the fault point and the attenuation distribution curve on the display of the optical time domain reflector.

When the optical fiber connection is monitored, the loopback point is increased, so the bidirectional value of the continuous attenuation can be measured on the OTDR. The advantage of this method is the ability to accurately assess the quality of the joints.

Due to the test principle and fiber structure reasons, with OTDR unidirectional monitoring will appear false gain phenomenon, accordingly, there will be false large attenuation phenomenon. For a fiber optic connector, the mathematical mean of the two directions of attenuation can accurately reflect its true attenuation value. For example, a connector from A to B measured the attenuation of 0.16 dB, from B to A measured -0.12 dB, in fact the head of the attenuation of [0.16 + (- 0.12)] / 2 = 0.02 dB.

Conclusion

OTDR, as the main instrument of optical fiber communication, plays an important role in the fields of scientific research, teaching, factory, construction, and maintenance. At present, OTDR whether imported equipment or domestic equipment, testing accuracy and blind spot two key issues will be due to the test of the technology play a certain difference. With the passage of time and the progress of science and technology, the use of a new generation of artificial intelligence OTDR fiber parameters automatic testing, the rate will be faster, the effect will be better.