1.  Introduction

when installing any optical fiber system, the optical fiber or optical cable must be connected with each other in a low loss way to realize the connection of optical link. The connection of optical fiber link can be divided into two types: permanent and active. Permanent connection is mostly realized by fusion, bonding or fixed connector, while movable connection is generally realized by movable connector. This article will make a brief introduction to the movable connector. Optical fiber movable connector, commonly known as flexible joint, commonly known as optical fiber connector, is a reusable passive device used to connect two optical fibers or optical cables to form a continuous optical path. It has been widely used in optical fiber transmission lines, optical fiber distribution frames, optical fiber testing instruments and meters, and is currently the most used optical passive device.

 2. The general structure of optical fiber connector the main purpose of optical fiber connector is to realize the connection of optical fiber. Now it has been widely used in optical fiber communication system. There are many kinds and structures of optical fiber connectors. However, the basic structure of various types of optical fiber connectors is the same, that is, most of the optical fiber connectors generally use high-precision components (consisting of two pins and a coupling tube in three parts) to achieve optical fiber alignment connection. This method is to insert and fix the optical fiber in the pin, polish the pin surface, and then realize the alignment in the coupling tube. The outer components of the pin are made of metal or non-metal materials. The butt end of the pin must be grinded, and the other end is usually used to support the optical fiber or the optical fiber flexible cable to release the stress. The coupling pipe is generally made of two half synthetic and tight cylindrical components made of ceramic, bronze and other materials, most of which are equipped with metal or plastic flange to facilitate the installation and fixation of the connector. In order to align the optical fiber as accurately as possible, the machining accuracy of the pin and coupling tube is very high.

3. The performance of optical fiber connector the performance of optical fiber connector, first of all, the optical performance, in addition to the interchangeability, repeatability, tensile strength, temperature and plugging times of optical fiber connector.

(1) Optical performance: for the optical performance requirements of optical fiber connector, the two most basic parameters are insertion loss and return loss.           

Insertion loss refers to the loss of effective optical power of the link caused by the import of the connector. The smaller the insertion loss is, the better. Generally, it should not be more than 0.5dB.           

Return loss (reflection loss) refers to the connector's ability to suppress the reflection of link optical power, and its typical value shall not be less than 25dB. The practical connector and the pin surface have been specially polished, which can make the return loss larger, generally no less than 45dB.

(2) Interchangeability and repeatability           

Optical fiber connector is a general passive device. For the same type of optical fiber connector, it can be used in any combination, and can be used repeatedly. Therefore, the imported additional loss is generally less than 0.2db.

(3) Tensile strength           

For a good optical fiber connector, its tensile strength is generally required to be no less than 90N. (4) Temperature           

Generally, the optical fiber connector must be able to operate normally at the temperature of - 40oC to + 70oC. (5) Number of plugging           

At present, the optical fiber connector can be plugged and unplugged more than 1000 times.

4. Some common optical fiber connectors

According to different classification methods, optical fiber connector can be divided into different types, single-mode optical fiber connector and multi-mode optical fiber connector according to different transmission media; FC, SC, St, D4, DIN, biconic, mu, LC, MT and other types according to different structures; FC, PC (UPC) and APC can be divided according to the end face of connector pin; single core and multi-core can be divided according to the number of optical fiber cores The difference. In the process of practical application, we usually distinguish according to the different structure of optical fiber connector. The following is a brief introduction of some common optical fiber connectors:

 (1) FC type optical fiber connector           

The connector was first developed by NTT in Japan. FC is the abbreviation of ferrale connector, indicating that the external reinforcement method is metal sleeve, and the fastening method is turnbuckle. In the earliest FC type connectors, the butt end face of ceramic pins was plane contact (FC). This kind of connector is simple in structure, convenient in operation and easy to manufacture, but the fiber end is more sensitive to dust, and it is easy to produce Fresnel reflection, so it is difficult to improve the return loss performance. Later, this type of connector was improved by using a pin (PC) with a spherical butt end, while the external structure was unchanged, which greatly improved the insertion loss and return loss performance.

 (2) SC type optical fiber connector           

This is an optical fiber connector developed by NTT company of Japan. Its shell is rectangular, and the structure size of the pin and coupling sleeve is the same as FC type, in which the end face of the pin is mostly grinded by PC or APC type; the fastening method is plug pin latch type, without rotation. This kind of connector has the advantages of low price, convenient insertion and removal, small fluctuation of insertion loss, high compressive strength and high installation density.

(3) Biconic connector           

The most representative product of this kind of optical fiber connector is developed by Bell Laboratories in the United States. It consists of two cylinder plugs with truncated conical ends and a coupling assembly with double tapered plastic sleeves inside.

(4) Din47256 optical fiber connector           

This is a connector developed by Germany. The structure size of the pin and coupling sleeve used in the connector is the same as that of FC type, and the end face is treated by PC grinding. Compared with FC connector, its structure is more complex. There is a spring controlling the pressure in the internal metal structure, which can avoid the damage of the end face due to the excessive plug-in pressure. In addition, the mechanical accuracy of the connector is high, so the insertion loss value is small.

(5) MT-RJ connector           

MT-RJ started from the MT connector developed by NTT. It has the same latch mechanism as RJ-45 LAN electrical connector. It aligns the optical fiber through the guide pins installed on both sides of the small sleeve. In order to connect with the optical transceiver conveniently, the optical fiber at the end face of the connector is designed as a double core (0.75mm interval) arrangement, which is the next generation of high-density optical connector mainly used for data transmission.

(6) LC type connector           

LC connector is developed by Bell Research Institute. It is made of modular jack (RJ) latch mechanism. The size of the pin and sleeve used is half of the size used for common SC, FC, etc., which is 1.25mm. This can increase the density of the optical fiber connector in the optical distribution frame. At present, in the single-mode SFF, LC type connector has occupied the leading position in practice, and its application in multi-mode is also growing rapidly.

(7) Mu connector           

Mu (minimum unit coupling) connector is the world's smallest single core optical fiber connector developed by NTT on the basis of the most commonly used SC type connector. The connector uses a 1.25mm diameter sleeve and self retaining mechanism, and its advantage is that it can achieve high-density installation. NTT has developed a series of Mu connectors using Mu's l.25mm diameter sleeve. They are socket type optical connector (mu-a Series) for optical cable connection, base plate connector (mu-b Series) with self retaining mechanism, and simplified socket (mu-sr Series) for connecting LD / PD module and plug, etc. With the rapid development of optical fiber network to the direction of larger bandwidth and larger capacity and the wide application of DWDM technology, the demand for mu connector will also grow rapidly.

5. With the continuous development of optical fiber communication technology, especially the development of high-speed LAN and optical access network, the application of optical fiber connector in optical fiber system will be more extensive. At the same time, it also puts forward more and higher requirements for optical fiber connector. Its main development direction is: miniaturization of appearance, low cost, but higher and higher requirements for performance. In the future, a variety of newly developed optical fiber connectors will form a pattern of "each shows its own advantages, each has its own use" together with traditional FC, SC and other connectors.