What is Return Loss and Insertion Loss

In optical fiber communications, insertion loss and return loss are two important indicators for evaluating the quality of the termination between some optical fiber devices, including fiber optic connector, fiber jumpers, and pigtails.
What is insertion loss?
Insertion loss usually referred to as IL, mainly refers to the displacement of light lost between two fixed points in the fiber. It can be understood as the loss of optical power caused by the intervention of optical devices in the optical fiber interconnection of the optical communication system, the unit is dB. Calculation formula: IL = -10 lg (Pout / Pin), Pout is the output optical power, and Pin is the input optical power.
The smaller the value of the IL, the better the performance. For example, an insertion loss of 0.3dB is better than 0.5dB. In general, the loss of fusion splicing and manual connection is smaller than the connection between fiber optic connectors. Recommended maximum dB loss for fiber optic cabling in the data center: LC SM/MM fiber optic connector maximum 15dB, MPO/MTP multimode fiber optic connector maximum 20dB, MPO/MTP singlemode fiber optic connection The minimum is 30dB.
What is return loss?
When an optical fiber signal enters or leaves an optical device component (such as an fiber optic connector), the discontinuity and impedance mismatch will cause reflection or echo. The power loss of the reflected or returned signal is the return loss which refers RL. Insertion loss is mainly to measure the resulting signal value when the optical link encounters loss, and return loss is to measure the loss of the reflected signal 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, usually a negative value, so the larger the return loss value, the better. The typical specification range is -15 to -60 dB. According to industry standards, the return loss of Ultra PC polished fiber optic connectors should be greater than 50dB, and the return loss of bevel polishing is usually greater than 60dB. The PC type should be greater than 40dB. For multimode fiber, the typical RL value is between 20 and 40 dB.
What are the influencing factors? 1. Fiber connector Endface quality and cleanliness End-face defects such as scratches, pits, cracks and particle contamination will directly affect the connector performance, resulting in poor IL/RL. Even tiny dust particles on a 5 micron single-mode fiber core may eventually block the optical signal, resulting in signal loss.
2. Fiber breakage and poor connection
Sometimes, light still can be guided through fiber even the fiber has been broken, In this case, it will also cause bad IL or RL. Connecting the APC to the PC fiber opitic connector can cause high IL and low RL, fiber end faces are unable to be precisely butted, which prevents the light from passing normally.
3. Exceed Bending radius
Optical fibers can be bent, but too much bending will also cause a significant increase in optical loss and damage. Therefore, it is recommended to keep the radius as large as possible when coil the fiber. The general recommendation is not to exceed 10 times the diameter of the jacket. For a jumper with a jacket of 2 mm, the maximum bending radius is 20 mm.