As an important link of network equipment interconnection, fiber optic patch cord is a passive optical device widely used in optical communication. Especially the connector performance at both ends of the jumper directly affects the optical transmission quality. Therefore, in order to ensure the efficient transmission of optical link signals, two key optical performance indexes:insertion loss (IL) and return loss (RL), are usually used to evaluate them.
What is Insertion loss？
In 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, which is used to represent the ratio of output optical power to input optical power of the port, in dB. 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 some signals back to the signal source during transmission due to the discontinuity of the transmission link. This discontinuity may not match the terminal load or the equipment inserted in the line. Return loss is easily misunderstood as the loss caused by return. In fact, it refers to the loss of return itself, that is, the greater the loss of return, the smaller the return. It represents the ratio of reflected wave power to incident wave power at the transmission line port, in dB, which is generally positive. Therefore, the higher the absolute value of return loss, the smaller the reflection, and the greater the signal power transmission, that is, the higher the RL value, the better the performance of the optical fiber connector.
Main factors affecting the two losses
The direct connection of a single optical fiber jumper is the most ideal optical fiber path，because the loss is the smallest. That is, a direct optical fiber without interference between ends A and B. However, usually, optical fiber networks need connectors to realize modularization and path segmentation. Therefore, the ideal low insertion loss and high return loss performance will be greatly reduced due to following three reasons.
1. End face quality and cleanliness
Obviously, fiber end defects such as scratches, pits, cracks and particle pollution will directly affect its performance, resulting in higher insertion loss and lower return loss. Any abnormal condition that hinders the transmission of optical signals between optical fibers will have a negative impact on these two losses.
The end face defects such as scratches, pits, cracks and particle pollution will directly affect its performance, resulting in higher insertion loss and lower return loss. Any abnormal condition that hinders the transmission of optical signals between optical fibers will have a negative impact on these two losses.
Figure 1: End face cleanliness comparison
2. Ferrule alignment and positioning deviation of connector
The main function of the optical fiber connector is to quickly connect the two optical fibers, ensure the accurate alignment between the two fiber cores, realize the precise butt joint of the end faces of the two optical fibers, and maximize the optical power output by the transmitting optical fiber to the receiving optical fiber. Generally, the smaller the ferrule hole diameter, the more centered the core position. If the ferrule hole is not completely centered, the core contained in it will not be completely centered. Therefore, the insertion loss and return loss will be greatly affected when the fiber cores are not accurately aligned, that is, the centering and positioning of the connector core is deviated.
3. End face physical contact air gap
The connectors are fixed through adapters, which is a physical connection, but not a real physical contact. There will be a gap between the contact at end faces of the two connectors. The smaller the end air gap, the more ideal the insertion loss and return loss. The air gap between the end faces of optical fiber connectors changes with different grinding methods. Generally, the typical insertion loss of connector by physical contact (PC), ultra physical end face (UPC) and angled physical contact (APC) , typically it is less than 0.3 dB. Among them, UPC connector has the lowest insertion loss due to the minimum air gap at the end face, while APC connector can achieve the highest return loss due to the use of inclined optical fiber end face. Choosing the right type of fiber connector can help you achieve better optical transmission quality.
Figure 3：PC vs UPC vs APC polish and alignment
How to optimize fiber optic connector loss?
The use of appropriate high-quality optical fiber connectors can help to achieve long-term stable operation of high-speed transmission system. Here are some suggestions to optimize:
- ensure the connector is clean before use. In case of contamination, use suitable tools for cleaning.
- avoid exerting any improper pressure on the optical fiber during use, and do not bend the optical fiber beyond its maximum bending radius.
- minimize bending, winding, welding and coupling of optical fiber jumpers, otherwise the optical signal may be refracted when passing through the optical fiber cladding. If it is necessary to coil the optical fiber, a large coil radius shall be maintained.
- Use factory terminated optical fibers, which are carried out under strict control and are usually guaranteed by the manufacturer.
- reasonably weigh the power loss and optical fiber cost, and the use of cheap and low-quality optical fiber may cause greater cost loss in the future.
Finally, by insertion loss and return loss, we can more accurately evaluate the optical fiber transmission efficiency and performance, and judge whether there is impedance mismatch at the pins, through holes, connectors and other discontinuities of the receiver and transmitter, which also help deploy a better optical transmission network.