Ensure that the performance of the optical fiber cabling system complies with the standard (1)

Almost all projects require performance verification tests on the cabling system after construction. At present, copper cable testing methods, instruments and testing standards have been widely used by practitioners. Compared with copper systems, the popularity of optical fiber testing is much lower. Many projects do not perform fiber link tests, or only use a VFL light source to verify the optical connection. Using VFL light source to test the connection light transmittance is like using a breaker to test the copper link. It is impossible to know whether the performance of the test link meets the standard requirements, so that users' investment cannot be guaranteed. This article will introduce the common methods and precautions for fiber link testing, hoping to help readers.

Optical fiber test Classification

Optical fiber testing can be divided into two types: one-class testing and the other-class testing.

In a test, both ends of the optical fiber link are connected to the light source and the optical power meter respectively. The principle of the test is very short. The Light Source sends optical signals, and the power meter is used to receive optical signals. The difference between the two signal power values is the insertion loss that occurs on the fiber link ). This type of test can accurately test the loss amount and link length on the fiber link, and the measurement accuracy is high. However, the disadvantage of this method is that the user can only obtain the final test results, but the failure points cannot be analyzed and located for unqualified links.

Class 2 test, also known as OTDR test. It connects one end to the OTDR tester, and the other end opens an open circuit. It calculates the attenuation and length of the reflected signal generated by the optical signal sent by the light source in the link and generates an OTDR curve. Compared with the One-class test, this method has low accuracy for the measurement of link loss, but it has the advantage that it can locate the fault point location, so that the construction personnel can repair the unqualified tested link. This method is particularly helpful for testing the long-distance trunk cable link or the trunk cable in the park.

This article focuses on a type of testing method that is widely used to verify the performance of optical fiber cabling systems.

Purpose of Reference Value Setting

The types of optical fiber connectors for each project are different, and the models of the tested links are also different. To increase the flexibility and operability of the test and reduce the wear and tear of the light source and optical power meter ports during the measurement process, an important step in testing the fiber link is to add a test jumper to help complete the test. One end of the test jumper is connected to the light source or optical power meter, and the other end is connected to the optical fiber link to be tested. During the test, the Section connected to the light source and power meter may not be removed. As long as it is disconnected from the end of the link to be tested, continuous and repetitive testing can be completed. During user testing, as long as the end face of the test jumper is cleaned or the test jumper is replaced, the wear and dirt of the light source and optical power meter port modules are avoided. In addition, the optical fiber connection head of the Test Jumper can be configured with different types, which also increases the flexibility of the test.

After a patch cord is added to the test, the test results include not only the optical link loss to be tested, but also the loss of the test patch cord. The solution is to weigh the items that we bought together with the containers in the same way as shopping and weighing. This method is called setting the reference value, that is, the loss of the test jumper is first measured to zero, and then connected to the link under test for testing.

Test Method

Four different test methods are available for testing the optical fiber link. The following describes the test methods one by one.

1. Single jumper method 

The single jumper method uses a single jumper to set the reference value. After setting, add the orange part of the tested optical fiber link. In addition, this test method requires another test jumper CD. In this way, the measured optical fiber link loss value is L = LBX + LXY + LYC + LCD. To ensure the correctness of the test results, CD should be a known, low-loss test jumper. In general, LCD usually has little impact on the test results.

This Method has become "Method B", with the advantage that the test result is the most accurate and is the preferred Method of TIA/EIA 568-B.1 standard, is the second recommended method in the ISO/IEC 11801 standard. However, the optical fiber port module of the optical power meter provided by the test manufacturer cannot be replaced. Therefore, this method can only be used to measure Optical Fiber Links of the same type as the optical power meter port module. For example, if the optical power meter port is SC, the connectors X and Y in the Link under test must also be SC. Fortunately, we now have a tester that can be replaced by the Power Meter port module type, greatly improving the flexibility of this test method.

2. Dual jumper method 

In order to overcome the single jumper method, the test link connector type must be the same as that of the optical power meter port module. The dual jumper is proposed and adopted. In this way, you only need to test that the type of the B and C connectors of the Jumper is the same as that of the tested link X and Y. This greatly increases the flexibility of testing.

In this way, there are two patch cords, AB, CD, and a connector, to set the reference value. After the settings are complete, the connection between B and C is opened, respectively, yellow with the link under test) some connectors X and Y are connected. In this way, the tested optical fiber link loss value is L = LBX + LXY + LYC-LBC. The dual jumper Method is A common Method in North America and is also known as Method )". This method does not appear in the ISO 11801 standard. In this method, the coupling loss of connector B and C should be deducted from the test results, so the LBC value should be as small as possible to reduce the impact on the test results. If the value of LBC is similar to that of LBX or LBY, the resulting test result will lose one connection loss, which is equivalent to testing only one connection point and the loss of the optical fiber cable. Therefore, the test result obtained using this method is slightly lower than the actual loss value of The Link under test. This method is suitable for scenarios where the loss of optical fiber cables accounts for a large proportion of the total Link Loss, such as long-distance optical fiber links.

3. Three jumper method 

The Tri-jumper method is a test method proposed by FLUKE and does not appear in TIA/EIA 568-B.1 and ISO/IEC 11801 standards. This Method is also called "Modified Method B )". It is proposed to improve the single jumper method, the tested link connector and Power Meter port must be consistent with the defect, and avoid the double jumper method in the test results less than the loss of a connector.

This method sets the reference value in the same model as the dual jumper method, but adds a connector and a test jumper CD during the test. According to this model, it is not difficult to obtain that the loss value of the optical fiber link measured by the three jumper method is L = LDX + LXY + LYE + (LBC-LBE) + LCD.

The test patch cord CD must be as short as possible to reduce the impact on the test result. For example, the patch cord provided by Panda is only 0.125M in length. In this way, the LCD effect is negligible in the test results.

When using this method, you also need to note that BC and BE should use connectors of the same type and quality to achieve the best measurement results.

4. Golden patch cord Testing Method 

The golden jumper method uses three jumpers to set the reference value, and then replaces the patch cord CD with the fiber link XY to be tested. From this we can see that the CD used here should be a Jumper as short as possible, especially in the XY distance is short. The length of the CD jumper recommended by Panda is 0.125 MB. In this way, the loss of the CD jumper is negligible after the return to zero.

The "golden" patch cord method is the most flexible in all methods, and the most variability is also the strongest. during testing, the method can be independent of the port type of the optical power meter. This Method, also known as Method C, is the preferred test Method in ISO/IEC 11801, but does not appear in TIA/EIA 568-B.1.

By understanding the previous methods, the reader can conclude that the loss value of the optical fiber link measured by this method is L = LBX + LXY + LYE-(LBC + LDE + LCD ). As mentioned above, by using short jumpers, The Impact of LCD on the test results is negligible. However, in this method, LBC and LBE will have a great impact on the results, and even cause a large deviation between the test results and the actual values. Therefore, using this method, BC and BE should adopt high-quality connectors as much as possible to reduce device loss. In real time, the final test value L will be lower than the actual value, which is unavoidable. In the above four methods, testers can choose different methods based on different link types and specific applications for testing.