With the rapid development of optical fiber production technology, the price of optical cable is gradually lower than that of coaxial cable, which makes CATV network gradually replace cable with optical cable with price guarantee. With the development of high-power optical transmitter and high-sensitivity optical receiver, the application of optical fiber technology in CATV network has been guaranteed. HFC network makes the coverage of optical fiber more and more large, and optical fiber technology is gradually used more widely.

Optical fiber connector is an indispensable passive optical component with the largest consumption. Its function is to connect optical fiber and optical fiber, optical fiber and device (including active device, passive device and optical fiber sensor), optical fiber and instrument, optical fiber and system. Wherever there is optical fiber, there is optical fiber connector. The cladding diameter of single-mode optical fiber is 125p. M (equivalent to the thickness of hair), and the diameter of its core is only 9 Dou m (the core is the part of optical fiber passing through). The task of optical fiber connector is to connect the 9 Dou m cores together accurately, so that most of the light can pass through, which is very difficult in technology. Because the optical fiber connector is also a kind of loss product, it is also required to be cheap.

In the optical fiber communication (transmission) link, in order to realize the flexible connection between different modules, equipment and systems, there must be a device that can be detachable (movable) connected between the optical fiber and the optical fiber, so that the optical signal can be transmitted according to the required channel, so as to achieve the predetermined or expected purposes and requirements, The device to realize this function is optical fiber connector. The optical fiber connector is to connect the two end faces of the optical fiber precisely, so that the optical energy output from the transmitting optical fiber can be coupled to the receiving optical fiber to the maximum extent, and the impact on the system caused by its intervention in the optical link can be minimized, which is the basic requirement of the optical fiber connector. To a certain extent, the quality of optical fiber connector also affects the reliability and performance of optical transmission system.

1 optical fiber connector structure

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 of optical fiber connectors with different structures. But in detail, the basic structure of various types of optical fiber connectors is the same, that is, the vast majority of optical fiber connectors generally use high-precision components (composed of two pins and a coupling tube, a total of three parts) to realize the alignment connection of optical fiber. In this method, the optical fiber is inserted and fixed in the pin, and the pin surface is polished to achieve alignment in the coupling tube. The outer component of the pin is made of metal or nonmetal materials. The butt end of the pin must be ground, and the other end is usually supported by a bending limiting member to release the stress. The coupling pipe is generally made of two half synthetic and fastening cylindrical components made of ceramic or bronze materials, and most of them are equipped with metal or plastic flanges to facilitate the installation and fixation of connectors.

In order to align the optical fiber as accurately as possible, the processing accuracy of the pin and coupling tube of the optical fiber connector is very high.

2 technical specifications of optical fiber connector

The technical specifications of optical fiber connector mainly include the following items:

(1) Insertion loss: it represents the optical power loss after the light passes through the optical fiber connector. Expressed in dB.

Li = – 1019 (11 / 10), where IO is the input optical power; 11 is the output optical power, which should not be greater than 0.5dB.

(2) Repeatability: the change of insertion loss after repeated plugging. Expressed in dB.

(3) Interchangeability: the change value of insertion loss after the plug and adapter are interchanged according to certain rules.

(4) Return loss: it represents the ratio of the backward reflected light to the incident light after the light passes through the optical fiber connector. Expressed in dB. The typical value should not be less than 25dB. For the practical connector, the surface of the pin has been specially polished, which can make the return loss greater, generally no less than 45dB.

LR = one log (I / IO), where 10 is the input optical power; I is the backward reflected light power.

(5) Temperature range of use: generally between – 30 and + 70. It can be used within the scope of C.

(6) Tensile strength: it indicates how many kilos of tensile force it can bear. According to the relevant standards of our country, the tensile strength of single core optical cable jumper is greater than 10kg.

(7) The number of times to plug and unplug. At present, optical fiber connectors can be plugged in and out more than 1000 times.

Three common optical fiber connectors

According to different classification methods, optical fiber connectors can be divided into different types. According to different transmission media, they can be divided into single-mode optical fiber connector and multi-mode optical fiber connector; According to the structure, it can be divided into FC, SC, St, D4, DIN, biconic, mu, LC, MT and other types; According to the pin end face of connector, it can be divided into FC, PC (UPC) and APC; According to the number of fiber cores, there are single core and multi-core.

In the practical application process, we usually distinguish according to the different structure of optical fiber connector. At present, the common optical fiber connectors are as follows:

(1) FC type optical fiber connector

FC connector adopts metal thread connection structure, and the pin body adopts precision ceramic pin with outer diameter of 2.5ram. According to the different shape of the end face of the pin, it can be divided into FC / PC with spherical contact and FC / APC with inclined spherical contact. FC connector is the most widely used type in the world, and it is also the main type in China. FC connector is widely used in optical cable trunk system, among which FC / APC connector is used in the occasions requiring high return loss, such as CATV network. The national standard of FC connector has been made in China.

(2) SC type optical fiber connector

SC type connector is designed and developed by nriti ‘company of Japan. It adopts plug-in structure. The shell adopts rectangular structure and is made of engineering plastics. It is easy to make multi-core connector. The pin body is a precision ceramic pin with outer diameter of 2.5mm. Its main features are no need for threaded connection, direct plug, small operation space, easy to install intensively. According to the shape of its pin end face, it can be divided into two structures: SC / PC with spherical contact and SC / APC with inclined spherical contact. SC connector is widely used in optical fiber subscriber network. China has formulated the national standard of SC type connector.

(3) St type optical fiber connector

St type connector is designed and developed by at & T company. It adopts bayonet locking structure with key. The pin body is a precision ceramic pin with outer diameter of 2.5mm, and the end face shape of the pin is usually PC face. Its main feature is that it is very convenient to use and widely used in optical fiber subscriber network. The national standard of St / PC connector has been made in China.

(4) Din47256 optical fiber connector

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

(5) MT-RJ connector

MT-RJ is a MT connector developed by NTT. It has the same latch mechanism as RJ-45 LAN electrical connector. It aims at 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 on the end face of the connector is designed with double core (0.75mm interval) arrangement. It is mainly used for the next generation of high-density optical connector for data transmission.

(6) LC type connector

LC type connector is researched and developed by famous bell Research Institute. The size of pin and sleeve is half of that of common SC and FC, which is 1.25 mm. This can increase the density of optical fiber connectors in the optical distribution frame. At present, in the aspect of single-mode SFF, LC type connector has actually occupied a dominant position, and its application in multi-mode is also growing rapidly.

(7) Mu connector

Mu (miniature unit coupling) connector is the world’s smallest single core optical fiber connector developed by NTT based on SC type connector which is used most at present. The connector adopts 1.25 mm diameter sleeve and self retaining mechanism, and its advantage is that it can realize high-density installation. NTT has developed a series of Mu connectors using Mu’s 1.25 mm diameter sleeve. They include socket type optical connector (mu-a Series) for optical cable connection, backplane connector (mu-b Series) with self retaining mechanism and simplified socket (mu-sr Series) for connecting LD / PD module and plug. With the rapid development of optical fiber network in the direction of larger bandwidth and larger capacity and the wide application of DWDM technology, the demand for mu connector will also grow rapidly.

4. Special faults and maintenance

Example 1: in a newly added optical node, the user reported that the signal of channel 3 was very poor and there was interference.

Analysis and maintenance. The optical power of the optical receiver measured by the optical power meter is – 1.3db, and the optical power is in the normal range. The output level of the optical receiver is detected by the spectrum analyzer, and it is found that there is no abnormality. Taking the signal from the – 20dB test port of the optical receiver and monitoring it with the TV, it is found that there are many clutter interferences in the supplement 3 channel, other channels are normal, and the fault remains after a new optical receiver is replaced, It is found that the fault point may be in the front-end computer room of the optical splitter. In the computer room, it is found that the light point is emitted by a six optical splitter, while other light points using the same optical splitter do not have this phenomenon. Exchange the light point with another normal light point splitter, and find that the fault still exists, but the signal of another light point is normal, so it is not the fault of the optical splitter. Because the optical cable is connected to the distribution box in the front end machine room of the branch, and then connected to the splitter through the movable joint, it is suspected that the movable joint is faulty, and the fault is eliminated after replacing with a new movable joint. Analysis of the above fault may be caused by the loose joint connection accuracy is not high, resulting in reflection, resulting in interference.

Example 2: a light spot user complains that the TV signal in this area is seriously snowflake.

Analysis and maintenance. The output level of the optical receiver is only 86db detected by the spectrum analyzer. After turning on the optical machine, the optical power of the optical receiver measured by the optical power meter is – 2.0db, which is in the normal range. After carefully observing the internal of the optical machine, it is found that the connection between the tail fiber connector of the photoelectric conversion module and the connector is loose. Screw off the optical fiber connector, clean it with medical cotton and absolute alcohol, and then tighten it again, The output level of Opto mechanical is 105dB, and the fault is eliminated.

With the continuous development of optical fiber communication technology, optical fiber connector will be more widely used in optical fiber system. Optical fiber connector, optical attenuator, optical isolator and other optical passive devices will play an indispensable role in optical fiber communication.

Example 3: the optical cable route needs to be changed temporarily at one time. The optical cable from ground a to ground B needs to pass through three generator and front-end computer rooms, and the optical fiber flexible connector is needed to jump in each computer room. After the link is restored, it is found that a certain data service cannot be restored.

Analysis and maintenance. Since each core has been tested with OTDR before the link is restored, and it is confirmed that it is connected, the optical power meter is used to test the optical fiber at the terminal equipment first, and it is found that there are optical signals, but it still can not be connected after it is connected back. After careful inspection again, it is found that the equipment uses 1550nm wavelength of light, while the optical wavelength is set to 1310nm when OTDR is used to test, and the 1550nm setting is immediately used to test the optical fiber again, It is found that the trace is quite different from the original. There are strong reflections in two places. After calculating the distance, they are all in the front-end machine room of two points. After cleaning the machine room again with alcohol and connecting the connector carefully, OTDR test is carried out. The reflection disappears. After connecting the equipment, troubleshooting is carried out. The cause of the failure is analyzed. It may be that the scattering echo is too strong due to the poor connection accuracy of the movable joint.

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