Four tests to ensure the quality of fiber optic patch cords, you have to know!

In order to provide customers with high-quality fiber optic patch cords , manufacturers conduct a series of tests during the design and manufacturing process. These fiber optic patch cord tests are critical to any type of fiber optic network. Not only suppliers, end users also need to understand these fiber patch cord tests in order to better judge the quality of fiber patch cords and ensure the feasibility of their applications. This article will introduce four kinds of tests: 3D test, insertion loss (IL) test and return loss (RL) test, and end face test. Generally, the fiber patch cords that have passed these four tests are of good quality, and end users can use them with confidence.

3D testing: guarantee of high-quality connector end faces

3D testing is a key test to ensure the performance of optical fiber connectors. When producing fiber optic patch cord components, the supplier will use a 3D interferometer (an optical interferometer) to inspect the end face of the fiber optic connector and strictly control the size of the connector end face. The 3D test mainly measures the radius of curvature, vertex offset and fiber height. Details are as follows:

Radius of curvature

The radius of curvature refers to the radius of the ferrule axis to the end face, as shown in the figure below, which is the radius of the curve of the end face of the ferrule. The curvature radius of the end face of the high-quality fiber jumper connector should be controlled within a certain range. If the radius of curvature is too small, greater pressure will be applied to the optical fiber, while too large a radius of curvature will not be able to apply pressure to the optical fiber, resulting in an air gap (ie, air gap) between the connector and the fiber end face. Regardless of whether the radius of curvature is too large or too small, it will cause light scattering or insufficient physical contact, which cannot guarantee the best transmission performance. Only a proper radius of curvature can ensure the correct pressure and the best transmission performance.

Figure 1: The radius of curvature of the 3D test

Vertex offset

Apex offset refers to the distance from the highest point of the ferrule end surface curve after grinding and polishing to the axis of the fiber core. This is a key item in the polishing process, and inaccurate polishing will cause the vertex to shift.

Figure 2: Vertex offset of 3D test

In technical standards, it is generally required that the vertex offset of the fiber jumper is ≤50μm. If the tip offset is large, an air gap will be formed, resulting in high insertion loss (IL) and return loss (RL) of the fiber jumper. Under ideal conditions, the apex offset of PC and UPC optical fiber connectors is almost zero, because they make the end face of the ferrule perpendicular to the polishing surface during the polishing process, and the apex coincides with the core axis. But for the APC type optical fiber connector, the end face and the optical fiber axis form an angle of 8 degrees, which is not completely perpendicular. For more information about PC/UPC/APC, please visit "Which Connector to Choose: PC vs UPC vs APC? ".

Fiber height

The fiber height is the distance from the fiber end face to the ferrule section, that is, the extension height from the fiber core to the ferrule end face. Likewise, the fiber height cannot be too low or too high. If the height of the fiber is too high, the pressure in the fiber will be increased when the two fiber connectors are connected, thereby damaging the fiber; if the height of the fiber is too low, there will be a gap when the two fiber connectors are connected, resulting in increased insertion loss. This must be avoided for transmissions with strict requirements for insertion loss.

Figure 3: 3D test fiber height

Different polishing methods and types of fiber optic patch cords use 3D interferometer to test the values, but the tested fiber optic patch cords should meet or exceed the industry-recognized end-face geometric size standard. The following table is based on the geometric size requirements of the end face of the MTP single-mode optical fiber connector of IEC/PAS 61755-3-31 and IEC/PAS 61755-3-32.

IL and RL testing: key testing for optical deployment

Insertion loss (IL) refers to the loss of signal power due to the insertion of a device somewhere in the transmission system. Return loss (RL) is the power loss caused by part of the signal reflected back to the signal source during transmission due to the discontinuity of the transmission link. To learn more about the definition of insertion loss and return loss, please visit "Analysis of Optical Connector Insertion Loss and Return Loss".

Whether it is the manufacturing process or the installation process, the insertion loss and return loss tests are very important. For fiber optic cable suppliers, the insertion loss and return loss of the provided fiber jumpers should meet the corresponding standards. For example, the TIA standard clearly specifies that the maximum insertion loss of an optical fiber jumper is 0.75dB (that is, the maximum acceptable value). For most fiber jumpers on the market, the normal range of insertion loss is between 0.3dB and 0.5dB, and some low insertion loss ranges between 0.15dB and 0.2dB. For optical fiber manufacturers, insertion loss testers and return loss testers are usually used to check whether they are within the normal range so that end users can receive qualified products.

For end users, in addition to using the insertion loss and return loss values ​​shown in the product specification table as a reference to design fiber links and select other equipment or components, if there are available test tools, they can also test themselves. This can effectively help installers to quickly troubleshoot and identify faulty system components. Optical time domain reflectometer (OTDR) and optical frequency domain reflectometer (OFDR) are common instruments used to measure the echo insertion loss.

End face test: to ensure the cleanliness and smoothness of the end face

The so-called optical fiber cleaning actually refers to the cleaning of the end face of the optical fiber connector. Whether it was forty years ago or today, the cleaning of the end face of an optical fiber connector is an essential step in optical fiber maintenance. Manufacturers usually use optical fiber end-face testers to perform end-face inspections to confirm whether there are contaminants, scratches, or cracks on the end face of the optical fiber connector. For fiber optic engineers, fiber cleaning tools (such as fiber cleaning pens, cassette cleaning boxes, etc.) are usually used to clean the fiber end face during wiring to ensure no pollution.

Figure 4: End face test

Why do end-face testing? Because having a good fiber connector end face is the basic condition to ensure high-quality fiber connections. If the end face of the optical fiber connector has contaminants (such as dust, etc.), scratches or even deformed, it will increase the return loss, and may even permanently damage the optical fiber connector, rendering it unusable. In addition, the dust between the end faces will scratch the surface, causing the air gap or the core to be misaligned, thereby reducing the transmission quality of the optical signal. Since these contaminants cannot be identified with the naked eye, if the end face is not tested and cleaned, it will contaminate the socket connected to it. Therefore, even if the supplier has tested and cleaned the end face of the optical fiber connector before shipment, the end face of the optical fiber connector needs to be cleaned every time before and after plugging or unplugging the optical fiber connector. At the same time, if you will not use it again temporarily, you need to cover the end surface with a dust cap.

to sum up

All in all, the optical fiber industry improves the quality of optical fiber connectors by exploring the key parameters that need to be measured, and industry associations and committees have always been committed to determining the manufacturing standards for optical fiber quality assurance. If the optical fiber patch cord passes the above four tests, and the test results meet the standard, then they will help to achieve high-quality optical signal transmission. For end users, it is necessary to check whether the supplier has performed these tests and whether they can provide relevant test reports to confirm that their parameter values ​​are within the correct range.