When a connector is mounted on the fiber end, some loss of power will be experienced. Some of the light would be reflected back to the fiber towards the source that generated that light signal. These reflected back signals into the fiber are called Optical Return Loss (ORL). The ORL can harm the laser source can also interrupt the transmission signal. Usually the fiber connectors have different polishing types, the different polishing have different ORL levels. There are four polishing types available, each having its own ORL levels and characteristic. The polishing types are: 1. Flat surface, 2. Physical Contact (PC), 3. Ultra Physical Contact (UPC) and 4. Angled Physical Contact (APC). Each connector is evolved from one into another, let’s discuss each on detail.
Flat Fiber Connector
Originally the fiber connectors are flat surface connectors. A small gap between the two fiber faces is left when two faces of flat fibers are coupled. These flat fiber connectors are not appropriate for single mode fiber with a 9µm core size, therefore it is vital to advance into Physical Contact (PC) connectors. ORL values in this type of connectors is around -35dB.
Physical Contact (PC) Fiber Connector
Physical Contact connector is polished with a minor spherical design, this allow to decrease the general size of the end face. The spherical design helps to overcome the air gap issue experienced in flat connectors. The spherical design results in overall lower ORL, as a reduced amount of light being sent back into the fiber and also to the light source. ORL values in this type of connectors is around -45dB. But still some of the light manages to reflect back towards the source.
Ultra Physical Contact (UPC) Fiber Connector
To overcome the issue faced in physical contact connectors, the convex end face is made by extending the polishing methods on PC connectors, resulting an even finer connector that is called Ultra Physical Contact (UPC) connector. The UPC have even lower ORL as compare to PC connectors. ORL values in this type of connectors is upto -55dB. UPC fiber connector can be used with single-mode fiber as well as multimode fiber. Usually the blue color UPC connector is used for single-mode fiber and beige color UPC connector is used for multimode fiber. UPC allows more consistent signals in digital TV and telephony systems.
Previously discussed PC and UPC connectors have a low insertion loss, but ORL really depends on the surface finish of the fiber. When the connectors are repeatedly coupled and decoupled, the ORL will start to degrade. So it is essential for a connector with low back reflection and it could endure repeated coupling and decoupling without ORL degradation.
Angled Physical Contact (APC) Fiber Connector
The end faces of Angled Physical Contact (APC) connectors have curved edges but are angled at 8 degree of an industry standard. This allows even closer connection with a much smaller gap between two. The combination of angled connector with smaller gap, allows any reflected light that is reflected back into the fiber is in fact reflected into the fiber cladding. That is because of the 8 degree angled face. ORL values in this type of connectors is less then -65dB. This to be noted that these types of fiber connector can only be used with single-mode fiber.
It’s vibrant from above discussion that all types of connectors play important role and are available in market. It seems difficult to conclude that which connector is best to use. The specific application requirement chooses which one to use. For application like high accuracy optical signals APC connectors may be selected, on other hand less complex systems will work fairly well using UPC or even FC connectors.