A comprehensive understanding of fiber optic connectors

The main function of the fiber optic connector is to quickly connect the two fibers so that the optical signal can continue to form an optical path. Fiber optic connectors are mobile, reusable, and are the most essential and most used passive components in optical communication systems.The fiber optic connectors allow the two end faces of the fiber to be precisely butt-coupled to maximize the coupling of the optical energy output from the transmitting fiber to the receiving fiber, and the effects of the system due to its intervention need to be minimized. Because the outer diameter of the fiber is only 125um, and the light-passing part is smaller, the single-mode fiber is only about 9um, and the multimode fiber is 50um and 62.5um, so the connection between the fibers needs to be precisely aligned.

Core components: ferrule

Through the role of the fiber optic connector, it can be seen that the core component that affects the performance of the connector is the ferrule. The quality of the ferrule directly affects the precision center docking of the two fibers.The ferrule is made of ceramic, metal or plastic. Ceramic ferrule is widely used, the main material is zirconium dioxide, which has the characteristics of good thermal stability, high hardness, high melting point, wear resistance and high processing precision. The sleeve is another important component of the connector, and the sleeve acts as an alignment to facilitate the mounting of the connector. The inner diameter of the ceramic sleeve is slightly smaller than the outer diameter of the ferrule, and the slitted sleeve tightens the two ferrules for precise alignment.

In order to make the end faces of the two fibers better contact, the ferrule ends are usually ground into different structures. PC, APC, and UPC represent the front end surface structure of the ceramic ferrule. PC is Physical Contact, physical contact.The PC is micro-spherical surface polished and polished, the surface of the ferrule is ground into a slight spherical surface, and the core of the optical fiber is located at the highest point of the bending so that the two fiber end faces are in physical contact.APC (Angled Physical Contact) is called a beveled physical contact, and the fiber end face is usually ground to an 8° bevel. The 8° angled bevel makes the fiber end face tighter and reflects light through its beveled angle to the cladding instead of returning directly to the source, providing better connection performance. UPC (Ultra Physical Contact), super physical end face.UPC is based on the PC to optimize the end face polishing and surface finish, the end face looks more dome-shaped. Connector connections need to be in the same end face structure, for example APC and UPC cannot be combined, resulting in reduced connector performance.

Basic parameters: insertion loss, return loss

Due to the different ferrule end faces, the performance of the connector loss is also different. The optical performance of fiber optic connectors is primarily measured by two basic parameters: insertion loss and return loss. So, what is the insertion loss? Insertion Loss (“IL”) is the optical power loss due to the connection.It is mainly used to measure the optical loss between two fixed points in the fiber, usually due to the lateral deviation between the two fibers, the longitudinal gap in the fiber joint, the quality of the end face, etc. The unit is expressed in decibels (dB). The smaller the better, the general requirement should be no more than 0.5dB.

Return Loss (“RL”) refers to the parameter of signal reflection performance. It describes the power loss of optical signal return/reflection. Generally, the larger the better, the value is usually expressed in decibels (dB). A typical APC connector has a typical RL value of approximately -60 dB and a PC connector has a typical RL value of approximately -30 dB.

In addition to the two optical performance parameters of insertion loss and return loss, the performance of the fiber optic connector should also pay attention to the interchangeability, repeatability, tensile strength and operating temperature of the fiber optic connector. , the number of insertions and so on.

Connector type

The connectors are divided according to the connection method: LC, SC, FC, ST, MU, MT, MPO/MTP, etc.; according to the fiber end face: FC, PC, UPC, APC.

LC connector

The LC-type connector is made with an easy-to-use modular jack (RJ) latch mechanism. The size of the pins and sleeves used in the LC connector is 1.25 mm, which is the size of ordinary SC, FC, etc., so the external size is only half of that of SC/FC.

SC connector

The connector of the SC connector (‘Subscriber Connector’ or ‘Standard Connector’) is a snap-on standard square connector, which is fastened by plugging and unloading, and does not need to be rotated. This type of connector is made of engineering plastics, which is inexpensive and easy to insert and remove.

FC connector

The FC fiber connector (Ferrule Connector) and the SC connector are the same size, except that the FC is made of a metal sleeve and the fastening method is a turnbuckle. The utility model has the advantages of simple structure, convenient operation, easy manufacture and durability, and can be used in a high vibration environment.

ST connector

The ST fiber optic connector (Straight Tip) has a rounded outer casing with a 2.5mm ring-shaped plastic or metal casing. The fastening method is a turnbuckle, which is commonly used in fiber distribution frames.

MTP/MPO connector

The MTP/MPO fiber optic connector is a special type of multi-fiber connector. The structure of the MPO connector is complex, connecting 12 or 24 fibers in a rectangular fiber ferrule. Usually used for high-density connection scenarios such as data centers.

In addition to the above, the connector types are MU connectors, MT connectors, MTRJ connectors, E2000 connectors, and the like. SC is probably the most commonly used fiber optic connector, mainly due to its low cost design. LC fiber optic connectors are also a commonly used fiber optic connector, especially for connection to SFP and SFP+ fiber optic transceivers. FC is mostly used in single mode and is relatively rare in multimode fiber. Complex designs and the use of metal make it more expensive. ST fiber optic connectors are typically used in long and short range applications such as campus and architectural multimode fiber applications, enterprise network environments, and military applications.