As we know, there are a large number of fiber optic cables used between devices in optical communications, and the optical connectors of fiber cables are required to be reliable for high-performance optical fiber networks. When measuring the attenuation effects of the fiber connectors, insertion loss (IL) and return loss (RL) are two essential parameter measurements.
In telecommunications, insertion loss refers to the loss of signal power, calculated as a ratio in dB (decibel), resulting from inserting a device in a transmission line or optical fiber. It can also be referred to as attenuation, which indicates how much the signal loss is by comparing the input power to the output power. A lower insertion loss value indicates a better insertion loss performance. For example, an insertion loss of 0.3dB is better than 0.5dB.
When a signal is transmitted through a transmission line, some signal power is always reflected or returned to the source due to discontinuities in the transmission line. The discontinuity can be a mismatch with the terminating load or with a device inserted in the line. Return loss refers to the loss of reflected signal power. Therefore, the higher the return loss is, the lower the amount of reflection will be. That is to say, the fiber connector will have better performance with a higher value of RL.
Ideally speaking, if the fiber patch cable has no connections, then the minimum loss will be realized—a continuous, straight-through glass fiber from Point A to Point B with no interruptions. However, the fiber optic networks are requiring connectors for modularity. The desired low IL and high RL performance are reduced consequently on account of the following three factors:
Evidently, fiber end-face defects like scratches, pits, cracks, and particle contamination will have a direct impact on the performance, contributing to poor insertion/return loss. Any irregularity that impedes light transmission from one fiber to the other will negatively affect IL and RL.
Figure 1: End-face Cleanliness
The main task of the connector is to hold the fibers precisely, ensuring the core of one fiber will align neatly and accurately with the core of the other fiber, so as to make every connector to mate with another connector with precise core alignment and core-to-core contact. Normally speaking, the smaller the ferrule hole diameter, the more precisely the fiber will be centered in the ferrule. If the ferrule hole is not perfectly centered, the fiber it holds will obviously never be perfectly centered. Therefore, the misalignment between two cores often happens when the fibers with the light-carrying cores are not aligned perfectly with each other, resulting in poor IL/RL.
Figure 2: Two Cases of Misalignment Situations
In order to achieve the desired low IL and high RL, optimized core-to-core contact must be achieved and maintained. Different polishing styles of fiber connectors have varied core-to-core contact performance regarding the connector's insertion loss and return loss. Usually, the insertion loss of PC, UPC, and APC connectors is less than 0.3dB. However, UPC connectors have the lowest IL due to the smallest air gap while APC connectors have the highest RL due to the beveled fiber end-face. PC vs UPC vs APC Connector will help you to select the proper fiber connect type.
Figure 3: Core-to-core Contact of PC vs UPC vs APC
Adopting the well-tested and high-quality connectors may help the network installers to deliver high-speed connections that perform well over the long term. Here are some tips for you to optimize the value of the insertion/return losses:
Keep all the fiber connectors clean, especially before and after the installation and testing. Using the proper tools to clean the connector ferrules.
Minimize the number of tight bends, coils, splices, and connectors, which may cause light to refract through the fiber cladding. If coil fiber is required, then keep the radius as large as possible.
Utilize factory terminated cabling. These terminations are made under stringent guidelines and typically include a manufacturer warranty.
Make a wise budget decision: your "power loss" budget or your cable inventory budget. Purchasing cheap fiber optic cables with the poor quality may bring larger costs further down the line.
In summary, the assessment of efficiency and performance will be more accurate if we combine the measurement parameters of both insertion loss and return loss. The measurements can judge whether there is an impedance mismatch at the pins of the receiver and transmitter as well as the vias, connectors, and various other discontinuities.