The concept of fiber optic connector insertion loss and return loss

What is insertion loss?

In the field of telecommunications, insertion loss refers to the loss of signal power due to the insertion of a device somewhere in the transmission system, usually refers to attenuation, and is used to express the ratio of the output optical power of the port to the input optical power, in decibels (dB) for the unit. Obviously, the lower the insertion loss value, the better the insertion loss performance.

What is return loss?

Return loss refers to the power loss caused by the reflection of part of the signal back to the signal source due to the discontinuity of the transmission link. This discontinuity may be a mismatch with the termination load, or with equipment inserted in the line. The return loss is easy to be misunderstood as the loss caused by the return. In fact, it refers to the loss of the return itself, that is, the more the return is lost, the smaller the return. It represents the ratio of the reflected wave power to the incident wave power at the transmission line port, in decibels, and is generally a positive value. Therefore, the higher the absolute value of the return loss, the smaller the amount of reflection, the greater the signal power transmission, that is, the higher the RL value, the better the performance of the fiber optic connector.

Factors Affecting Insertion Loss and Return Loss

The direct connection of a single fiber jumper is the most ideal fiber path, and the loss is the smallest at this time, that is, a direct connection fiber between the two ends of A and B without interference. Typically, however, fiber optic networks require connectors for modularity and path segmentation. Therefore, ideal low insertion loss and high return loss performance is compromised for three reasons.

End face quality and cleanliness

Obviously, fiber end face defects such as scratches, pits, cracks, particle contamination directly affect its performance, resulting in higher insertion loss and lower return loss. Any abnormality that prevents the optical signal from traveling between fibers will adversely affect both of these losses.

Connector ferrule alignment deviation

The main function of the optical fiber connector is to quickly connect two optical fibers, ensure the accurate alignment between the two fiber cores, realize the precise butt joint of the two optical fiber end faces, and maximize the coupling of the optical power output by the transmitting optical fiber into the receiving optical fiber. Typically, the smaller the ferrule hole diameter, the more centered the core is. If the ferrule hole is not perfectly centered, the fiber core contained in it will naturally not be perfectly centered. Therefore, when the cores are not precisely aligned, that is, when the alignment of the connector ferrule is misaligned, both insertion loss and return loss will be greatly affected.

End face physical contact air gap

The optical fiber connectors are fixed by adapters, which belong to physical connection, but are not in real physical contact. There will be a gap between the contact end faces of the two connectors. The smaller the end-face air gap, the better the insertion loss and return loss. The air gap between the end faces varies with the grinding method used in the fiber optic connector. Typically, fiber optic connectors have a typical insertion loss of less than 0.3 dB using physical contact (PC), ultra-physical end face (UPC) and bevel physical contact (APC) grinding methods. Among them, UPC connectors have the lowest insertion loss due to the smallest end-face air gap, and APC connectors can achieve the highest return loss among the three due to the use of beveled fiber end faces. Choosing the right type of fiber optic connector can help you achieve better optical transmission quality.

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