Analysis of common problems in the optical fiber testing phase

　　As an important transmission medium for an integrated wiring project, the performance of optical fiber is particularly important, especially for optical fiber cables laid in the air or underground pipelines. Therefore, before laying optical fiber, we must test the optical fiber and optical fiber equipment. What problems will we encounter during the test?

　　1. Why do we need to use a dedicated reference jumper to set the reference value when testing optical fiber?

　　The accuracy of the test results is closely related to the setting of the reference value. If the reference value is not set properly, the test results will be inaccurate or negative. The recommended method is to use a reference light jumper and adapter when setting the reference value.

　　In the TIA/EIA 568-B.3 and ISO 11801 standards, the loss requirement for a fiber coupling is less than 0.75dB. The performance of fiber optic patch cords and adapters produced by most manufacturers can meet or even exceed this requirement. However, many people do not know that the standard has special requirements for reference patch cords and adapters. The IEC 14763-3 standard stipulates that when reference patch cords and adapters are used, the coupling loss of multimode fiber is less than 0.1dB and that of single-mode fiber is less than 0.2dB. Conventional fiber optic patch cords cannot meet this requirement at all. Therefore, it is recommended to purchase special reference patch cords from manufacturers and test instrument manufacturers for testing.

　　When setting the reference value, you need to use a fiber adapter made of zirconium oxide ceramic sleeve material to obtain the best coupling effect. The conventional practice is to use a single-mode fiber adapter to set the reference value to obtain the best coupling effect, regardless of whether you are testing a multimode or single-mode fiber link.

　　2. When testing fiber link loss, why is it important to preheat the tester?

　　Generally, the higher the temperature of the light source module, the greater the light source power value it emits. During the test, the light source module needs to warm up for a period of time before the light source power value it emits can reach a stable value. If the reference value is set before the light source module is warmed up, as the temperature of the light source module rises, the test result will have a gain, thus affecting the accuracy of the test result.

　　For example, when the reference value is initially set, the power value received and stored by the optical power meter is -6.00dB. At this time, if the reference value setting model is maintained and the test is performed directly without adding the link under test, a test result of 0.00dB should be obtained. However, after the optical power module is preheated, the power emitted will increase, and the power value received by the power meter may rise to -6.20dB. If the test is performed again at this time, a gain of -0.2dB will be obtained.

　　The preheating time of the light source module is related to the temperature of the test environment. The method to check whether the light source module has reached stability is very simple. After completing the reference value setting, test the reference value model. The test value obtained is between -0.04dB and 0.04dB, which is acceptable; if it exceeds this value, you need to wait for a while and reset the reference value.

　　3. Why does a negative value appear when testing loss? Does the link under test not only have no loss, but also generate gain?

　　When testing a single-mode fiber link, if the length of the link under test is less than 100m and the entire link is connected by pigtail fusion splicing, the loss of the entire link may be only 0.15dB. In this case, insufficient preheating time for the light source module, large changes in the test environment temperature, the coupling effect between the reference jumper and the test instrument, and inaccurate reference value setting may all result in a negative test result, such as -0.03dB. At this time, the machine needs to be fully preheated and the reference value reset.