What are insertion loss and return loss?

In the figure below, how much insertion loss and return loss do you think will cause? In other words, whether such a connection can guide fiber light to pass normally.

In fiber optic communications, insertion loss and return loss are two important indicators for evaluating the quality of the termination between fiber optic equipments (such as fiber optic connectors, fiber jumpers and pigtails).

What is insertion loss?
Insertion loss is mainly refers to measuring the optical loss between two fixed points in the fiber. It is the optical power loss caused by the intervention of optical devices in the optical fiber link. Its unit is dB.
Calculation formula: IL=-10 lg(Pout /Pin), Pout is the output optical power, Pin is the input optical power.

The smaller the value of insertion loss, the better the performance. For example, the insertion loss is 0.3dB better than 0.5dB. Generally, the difference in attenuation (less than 0.1 dB) between fusion splicing and manual connection is smaller than the difference in attenuation between fiber optic connectors. Here is some recommended maximum dB loss for data center fiber cabling: LC multimode fiber connector is maximum 15dB; LC single mode connector is maximum 15dB; MPO/MTP multimode fiber connector is maximum 20dB; MPO/MTP single mode fiber connection is maximum 30dB.

What is return loss?
When optical fiber signals enter or leave optical equipment components (such as optical fiber connectors), discontinuities and impedance mismatches will cause reflections or echoes. The power loss of the reflected or returned signal is the return loss (abbreviated as RL). Insertion loss is mainly the signal value measured when the optical link encounters loss, and return loss is the loss value of the reflected signal measured when the optical link encounters component access.
Calculation formula: RL=-10 lg (P0/P1), P0 represents the reflected optical power, and P1 represents the input optical power.

The return loss value is expressed in dB and is usually a negative value, so the larger the return loss value, the better. Typical specifications range from -15 to -60 dB. According to industry standards, the return loss of Ultra PC polished fiber optic connectors should be larger than 50dB, and the return loss of bevel polished is usually larger than 60dB. The PC type should be larger than 40dB. For multimode fiber, the typical RL value is between 20 and 40 dB.

What causes the IL/RL?
The quality and cleanliness of fiber endface
The defects of fibe endface (such as scratches, pits, cracks) and particle contamination will directly affect the performance of the connector, resulting in poor IL/RL. Even tiny dust particles on the core of a 5 micron single-mode fiber may eventually block the optical signal, resulting in signal loss.

The fiber is disconnected or the connection is poor
Sometimes, although the fiber is damaged, it can still guide the light through. In this case, it also leads to bad IL or RL. As shown in the picture at the beginning of this article, the APC connector is connected to the PC connector, one is at an angle of 8° and the other is the grinding angle of the micro-arc. When the two are connected, light may pass through in a short time. But at the same time, this will also result in greater insertion loss and lower return loss, which may also result in the two fiber end faces not being accurately docked, and the light cannot pass normally.

Exceeding the bending radius
Fibers can be bent, but too much bending can also cause a significant increase in optical loss and can also directly cause damage. Therefore, when it is necessary to wind the fiber, it is recommended to keep the radius as large as possible. It is generally recommended not to exceed 10 times the diameter of the jacket. Therefore, for a jumper with a jacket of 2 mm, the maximum bending radius is 20 mm.