When installing any fiber optic system, consideration must be given to connecting optical fibers or cables to each other in a low-loss method to achieve optical link splicing. The connection of optical fiber links can be divided into two types: permanent and active. Permanent connection is mostly realized by welding method, bonding method or fixed connector; active connection is generally realized by movable connector. This article will give a brief introduction to active connectors.
Optical fiber active connectors, commonly known as live joints, generally called optical fiber connectors, are reusable passive devices used to connect two optical fibers or optical cables to form a continuous optical path. They have been widely used in optical fiber transmission lines and optical fiber distribution frames. It is the most widely used optical passive device among optical fiber test instruments and meters.
2. General structure of fiber optic connectors
The main purpose of fiber optic connectors is to connect optical fibers. Optical fiber connectors, which have been widely used in optical fiber communication systems, have many types and different structures. But on closer inspection, the basic structure of various types of fiber optic connectors is the same, that is, most fiber optic connectors generally use high-precision components (composed of three parts, two pins and a coupling tube) to realize optical fiber. alignment connection.
In this method, the optical fiber is inserted and fixed in the ferrule, and after the surface of the ferrule is polished, alignment is achieved in the coupling tube. The outer components of the pins are made of metal or non-metal materials. The butt end of the ferrule must be ground, and the other end usually uses a bend limiting member to support the fiber or fiber optic cable to relieve stress. The coupling tube is generally made of two semi-synthetic and fastened cylindrical members made of ceramic or bronze and other materials, and is often equipped with a metal or plastic flange to facilitate the installation and fixation of the connector. In order to align the fibers as precisely as possible, the processing precision of the pins and coupling tubes is very high.
3. Performance of fiber optic connectors
The performance of the optical fiber connector, first of all, the optical performance, in addition to the interchangeability, repeatability, tensile strength, temperature and insertion and removal times of the optical fiber connector.
(1) Optical performance: The requirements for optical performance of optical fiber connectors are mainly the two most basic parameters of insertion loss and return loss.
Insertion Loss (Insertion Loss) is the connection loss, which refers to the loss of the effective optical power of the link caused by the introduction of the connector. The smaller the insertion loss, the better, and the general requirement should be no more than 0.5dB.
Return Loss (Return Loss) refers to the ability of the connector to suppress the reflection of the optical power of the link, and its typical value should not be less than 25dB. In the practical application of the connector, the surface of the pin has been specially polished, which can make the return loss larger, generally not less than 45dB.
(2) Interchangeability and repeatability
Optical fiber connectors are general passive devices. For the same type of optical fiber connectors, they can generally be used in any combination and can be used repeatedly. The additional loss introduced by this is generally within the range of less than 0.2dB.
(3) Tensile strength
For a well-made fiber optic connector, it is generally required that its tensile strength should not be less than 90N.
(4) Temperature
Generally, the optical fiber connector must be able to be used normally at the temperature of -40℃~+70℃.
(5) times of plugging and unplugging
The fiber optic connectors currently used can generally be plugged and unplugged more than 1000 times.
4. Some common fiber optic connectors
According to different classification methods, fiber optic connectors can be divided into different types. According to different transmission media, they can be divided into single-mode fiber optic connectors and multi-mode fiber optic connectors; according to different structures, they can be divided into FC, SC, ST, D4 , DIN, Biconic, MU, LC, MT and other types; according to the pin end face of the connector, it can be divided into PC (UPC) and APC; according to the number of fiber cores, there are single-core and multi-core points.
In the actual application process, we generally distinguish according to the structure of the optical fiber connector. The following is a brief introduction to some of the more common fiber optic connectors:
(1)FC fiber optic connector
This connector was first developed by Japanese NTT. FC is the abbreviation of Ferrule Connector, indicating that its external reinforcement method is a metal sleeve, and the fastening method is a turnbuckle. The earliest, FC type connector, the mating end face of the ceramic pin used is the plane contact method (FC). Such connectors are simple in structure, easy to operate, and easy to manufacture, but the fiber ends are more sensitive to dust, and are prone to Fresnel reflections, making it difficult to improve return loss performance. Later, this type of connector was improved, and the contact pin (PC) with spherical end face was adopted, and the external structure was not changed, so that the performance of insertion loss and return loss was greatly improved.
(2) SC type fiber optic connector
This is a fiber optic connector developed by Japan's NTT company. Its shell is rectangular, and the structure and dimensions of the pins and coupling sleeves used are exactly the same as those of the FC type. The end faces of the pins are mostly ground by PC or APC type; rotate. This type of connector is cheap, easy to plug and unplug, has small fluctuation in insertion loss, high compressive strength, and high installation density (the most used on router switches).
(3)ST type fiber optic connector
ST-type optical fiber jumper: commonly used in optical fiber distribution frame, the outer shell is round, and the fastening method is turnbuckle. (For 10Base-F connections, the connectors are usually ST type. Often used in fiber optic distribution frames)
(4) LC type fiber optic connector
The LC-type connector was developed by the famous Bell Research Institute and is made of a modular jack (RJ) latch mechanism that is easy to operate. The size of the pins and sleeves used is half the size of ordinary SC, FC, etc., which is 1.25mm. This can increase the density of fiber optic connectors in the optical distribution frame. At present, in terms of single-mode SFF, LC-type connectors have actually occupied a dominant position, and the application in multi-mode is also growing rapidly.
(5)Biconic Connector
The most representative product of this type of fiber optic connector was developed by Bell Labs. It consists of two precision-molded cylindrical plugs with frusto-conical ends and a double-conical plastic sleeve inside. composed of coupled components.
(6) DIN4 fiber optic connector
This is a connector developed in Germany. The pins and coupling sleeves used in this connector have the same structural dimensions as the FC type, and the end face treatment adopts PC grinding. Compared with the FC type connector, its structure is more complicated, and there is a spring that controls the pressure in the internal metal structure, which can avoid damage to the end face due to excessive insertion pressure. In addition, the mechanical precision of this connector is high, so the insertion loss value is small.
(7) MT-RJ type connector
MT-RJ started from the MT connector developed by NTT. It has the same latching mechanism as the RJ-45 type LAN electrical connector. It is aligned with the optical fiber through the guide pins installed on both sides of the small sleeve. The optical fiber on the end face of the connector is designed in a double-core (0.75mm interval) arrangement. It is the next-generation high-density optical connector mainly used for data transmission.
(8)MU type connector
The MU connector is the world's smallest single-core optical fiber connector developed by NTT based on the SC-type connector that is currently most used. The connector adopts a 1.25mm diameter sleeve and a self-retaining mechanism. Its advantages lie in High-density installation is possible. Using MU's 1.25mm diameter sleeve, NTT has developed a series of MU connectors. They are available as receptacle-type optical connectors (MU-A series) for optical cable connection, backplane connectors with self-retaining mechanism (MU-B series), and simplified receptacles for connecting LD/PD modules to plugs (MU-SR series) )Wait. With the rapid development of fiber optic networks in the direction of larger bandwidth and larger capacity and the wide application of DWDM technology, the demand for MU-type connectors will also increase rapidly.
5. Conclusion
With the continuous development of optical fiber communication technology, especially the development of high-speed local area network and optical access network, the application of optical fiber connectors in optical fiber systems will be more extensive. At the same time, more and higher requirements are also put forward for optical fiber connectors. The main development direction is: miniaturization of appearance and low cost, but the requirements for performance are getting higher and higher. For a period of time in the future, various newly developed optical fiber connectors will form a pattern of "each showing their strengths and each having their own uses" together with traditional FC, SC and other connectors.
