Brief introduction to common pigtails of communication pigtails

Introduction to common pigtails
You who have touched pigtails , I must have heard strange professional terms . Among them, the name of pigtail LC-LC、SC-LC、FC-FC、FC-SC Circle round , Stupid is not clear . In fact, these names are classified according to the joints of pigtails . Let's first understand the structure of optical fiber , After introducing the connector , I don't feel dizzy when talking about pigtails .

One 、 Optical fiber structure
The bare fiber of optical fiber is generally divided into three layers ： A core made of transparent optical material 、 Cladding and coating
first floor ： Fiber core , Glass core with high refractive index （ The core diameter is generally 9-10μm( Monomode ）,50 or 62.5（ multimode ）).
The second floor ： Cladding , Silicon glass cladding with low refractive index （ The diameter is generally 125μm）, Form total reflection condition with fiber core .
The third level ： Smart cover , The outermost layer is a resin coating for reinforcement , It has high strength , Able to withstand large impact , Play the role of protecting optical fiber .
The refractive index of the first layer n1 Greater than the refractive index of the second layer n2, Optical fiber communication uses the principle of total reflection , With the help of continuous total reflection , It can be conducted from one end to the other .

Two 、 Optical fiber transmission mode
According to the transmission mode of light in optical fiber, it can be divided into ： Monomode （Single-Mode）( abbreviation ：SM）, multimode （Multi-Mode） ( abbreviation ：MM） .
Single mode fiber ： Single mode core diameter ：9/125μm,10/125μm, The center glass core is thin （ The core diameter is generally 9 or 10μm）, Cladding outer diameter （2b）＝125μm, At a given operating wavelength , Only one mode of light , The intermodal dispersion is very small , Better stability . Single mode fiber is suitable for large capacity and long distance communication systems .
Multimode fiber ： The central glass core is thick （50/125μm, European standard or 62.5/125μm, American Standard ）, It can transmit multiple modes of light . But its inter mode dispersion is large , This limits the frequency of transmitting digital signals , And it will become more serious with the increase of distance . The distance of multimode optical fiber transmission is relatively close , Usually only a few kilometers . The transmission performance is worse than that of single-mode fiber .
Choice of pigtails ： The pigtail of the corresponding connector is usually selected according to the optical module interface on the communication equipment , Then, according to the required pigtail length of the on-site construction scene , Choose the appropriate length of pigtails . Common pigtails are LC-LC 10m、SC-FC 20m、LC-FC 5m、FC-FC 20m（LC, Small square head ;FC, Round head ） wait .

The optical fiber connector is a detachable connector between optical fibers ( Activities ) Connected devices , It connects the two end faces of the optical fiber precisely , So that the optical energy output by the transmitting optical fiber can be coupled to the receiving optical fiber to the greatest extent , And minimize the impact on the system due to its involvement in the optical link , This is the basic requirement of optical fiber connector . Up to a point , Optical fiber connectors also affect the reliability and performance of optical transmission systems .
Connector model of pigtail
In practical application , We usually distinguish according to the different structures of optical fiber connectors . Here are some common optical fiber connectors : FC、SC、ST、LC、D4、DIN、MU、MT And so on . among ,ST Connectors are usually used for wiring equipment ends , Such as optical fiber distribution frame 、 Optical fiber module, etc ; and SC and MT Connectors are usually used for network devices . According to the end face shape of optical fiber, there are FC、PC( Include SPC or UPC) and APC; According to the number of fiber cores, there are single core and multi-core ( Such as MT-RJ) Points . Optical fiber connectors are widely used , There is a wide variety of .

FC type （“ Round head ”）

This kind of connector was first made in Japan NTT Development .FC yes ferrule Connector Abbreviation , It shows that the external strengthening method is metal sleeve , The fastening method is turnbuckle .FC Type is divided into FC/FC and FC/PC(APC) type , FC Type of connector , The plane butt joint of the adopted ceramic pin . This kind of connector has simple structure , It is easy to operate , Easy to make , But the fiber end is sensitive to dust , And it is easy to produce Fresnel reflection , It is difficult to improve the return loss performance . later , Improved this type of connector , Use a pin with a spherical butt end surface (PC) Physical Connection, Its butt end face is in physical contact , The end face is of convex arch structure , The performance of insertion loss and return loss is greatly improved .

SC type （“ Big square head ”）

This is a kind of Japanese NTT Optical fiber connector developed by the company .SC yes Square Connector Abbreviation , Its shell is rectangular . The end face of the pin is mostly PC or APC Type grinding method ; The fastening method is plug-in pin latch type , No rotation required . This kind of connector is cheap , Easy plugging operation , The fluctuation of intervention loss is small , High compressive strength , High installation density .

LC type （“ Small square head ”）

LC Type a connectors are well known BELL( bell ) Developed by the Institute , Modular jack with convenient operation (RJ) Made of latch mechanism .SC yes Lucent Connector Abbreviation , The sizes of the pins and sleeves used are common SC、FC And so on , by 1.25MM. This can improve the density of optical fiber connectors in the optical fiber distribution frame . at present , In single mode SFF aspect ,LC Types of connectors have actually taken the lead , Applications in multimode are also growing rapidly .

ST type （“ Round head ”）

ST （Straight Tip） Clip on round type ,ST F-type optical fiber connector is a kind of metal round head with spiral notch ,ST The head coupler has a protruding detent . Commonly used in optical fiber distribution frame , The shell is round , The fastening method is turnbuckle .( about 10Base-F In connection , Connectors are usually ST type . Commonly used in optical fiber distribution frame )

DIN47256 type

This is a connector developed in Germany . The structural dimensions of the pin and coupling sleeve used in this connector are the same as that of FC Same type , The end face treatment adopts PC Grinding method . And FC Type connectors , Its structure is more complicated , There are springs in the internal metal structure to control the pressure , It can avoid damaging the end face due to excessive plugging pressure . in addition , This kind of connector has high mechanical accuracy , Therefore, the intervention loss value is small .

MT-RJ type

MT-RJ Start from NTT Developed MT The connector , With and RJ-45 type LAN The same latch mechanism as the electrical connector , Align the optical fiber through the guide pins installed on both sides of the small sleeve , To facilitate connection to the optical transceiver , The optical fiber at the end face of the connector is double core ( interval 0.75MM) Arrangement design , It is the next generation of high-density optical fiber connector mainly used for data transmission .

3、 ... and 、 The cross section of the optical fiber connector shall be divided into PC、UPC、APC

PC、APC、UPC Refers to the end face of the pin of the optical fiber connector , The difference lies in the workmanship of their connector heads and jumper losses ,UPC、APC The jumper is of fine workmanship 、 Low loss .
Return loss is a parameter of optical fiber performance . When the optical signal is transmitted in the optical fiber, it will encounter obstacles and be transmitted back to the signal transmitter , This is the echo , This is a phenomenon that is not conducive to optical fiber transmission , In order to eliminate this phenomenon , The return loss of optical fiber can eliminate the return . therefore , The larger the return loss is , The larger the echo that can be eliminated , The better the performance of the optical fiber .

PC(Physical Contact) The joint section is flat , Return loss is poor .
UPC（Ultra Physical Connectors） The joint is curved , The return loss of the industrial standard is 50dB~ 55dB.
APC（Angle Physical Connectors） section 8 Angle of inclination , To reduce reflection , The industrial standard return loss is >60dB.

Fiber attenuation
Optical fiber attenuation is an important factor that hinders the long-distance transmission of digital signals . The optical fiber loss directly affects the transmission distance or the distance between relay stations . The main factors causing optical fiber attenuation are ： Intrinsic , bending , extrusion , impurities , Unevenness and butt joint, etc .

Intrinsic ： Is the inherent loss of optical fiber , Include ： Rayleigh scattering , Inherent absorption, etc .
bending ： When the optical fiber is bent, some light in the optical fiber will be lost due to scattering , Cause loss .
extrusion ： The loss caused by slight bending when the optical fiber is squeezed .
impurities ： The impurities in the optical fiber absorb and scatter the light propagating in the optical fiber , Loss caused .
uneven ： Loss caused by uneven refractive index of optical fiber materials .
docking ： Loss caused by optical fiber docking , Such as ： Different axes ( The coaxiality of single-mode fiber shall be less than 0.8μm), The end face is not perpendicular to the axis , The end face is uneven , Mismatch of butt center diameter and poor welding quality .
Loss of optical fiber
1310 nm : 0.35 ~ 0.5 dB/Km
1550 nm : 0.2 ~ 0.3dB/Km
850 nm : 2.3 ~ 3.4 dB/Km
Optical fiber fusion point loss ：0.08dB/ spot
Optical fiber fusion point 1 spot /2km
Definition of common units in optical fiber communication ：
dB = 10 lg ( Pout / Pin )
Pout : Output power ; Pin : Input power
dBm = 10 lg ( P / 1mw)
It is widely used in communication engineering ;
It is usually expressed in 1 Milliwatts as reference optical power ;
for example : –10dBm Indicates that the optical power is equal to 100uw.
dBu = 10 log10 ( P / 1uw)