In optical fiber communications, insertion loss and return loss are two important indicators for evaluating the quality of optical fiber devices, such as optical fiber connectors, optical jump fibers and pigtails.
What is insertion loss?
Insertion loss is often referred to as IL, mainly refers to the measurement of the 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 link 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 insertion loss, the better the performance. For example, an insertion loss of 0.3dB is better than 0.5dB. In general, the difference in attenuation (less than 0.1 dB) between fusion splicing and manual connection is smaller than the connection between fiber optic connectors. The recommended maximum dB loss of optical fiber cabling in the data center: LC multimode fiber connector is 15dB maximum, LC single mode connector is 15dB maximum, MPO/MTP multimode fiber connector is 20dB maximum, MPO/MTP single mode fiber connection is 30dB minimum.
What is return loss?
When an optical fiber signal enters or leaves an optical device component (such as an optical fiber connector), the discontinuity and impedance mismatch will cause reflection or echo. The power loss of the reflected or returned signal is the return loss( Referred to as 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 value 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.The quality and cleanliness of end- face
Defects (scratches, pits, cracks) and particle contamination on the end face of the fiber will directly affect the performance of the connector, leading to poor IL/RL. Even tiny dust particles on the 5 micron single-mode fiber core may eventually block the optical signal, resulting in signal loss.
2.Fiber breakage and poor connection
Sometimes, although the fiber has been broken, it can still guide the light through. In this case, it will also cause bad IL or RL. As the mentioned picture in the biginning of the article,the APC connector is connected to the PC connector, one is an oblique 8° angle, and the other is a micro-arc grinding angle. When the two are connected, light may pass through in a short time. But at the same time, it will also cause a large insertion loss and a low return loss, which may also cause the two optical fiber end face can’t to be precisely butted that results in the light cannot pass normally.
3. Bending radius exceeded
Optical fibers can be bent, but too much bending will also cause a significant increase in optical loss, and may also directly cause damage. Therefore, when it is necessary to coil the fiber, it is recommended to keep the radius as large as possible. The general recommendation is 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.
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