Characteristics, structure and classification of optical fiber connectors

The main function of the optical fiber connector is to quickly connect two optical fibers, so that the optical signal can be continuous to form an optical path. Optical fiber connectors are movable and reusable, and they are also the most indispensable and most used passive components in optical communication systems. Through the optical fiber connector, the two end faces of the optical fiber can be precisely butted, so that the light energy output by the transmitting fiber can be coupled to the receiving optical fiber to the maximum extent, and the influence of the system due to its intervention needs to be minimized.

fiber optic connector

1、Features of fiber optic connectors:

Physical characteristics: The physical characteristics of optical fiber connectors mainly include protrusion/inner shrinkage, eccentricity, and radius of curvature.

Protrusion/inner shrinkage: the distance between the fiber end face and the ferrule pin end face. Compared with the pin section, if the fiber protrudes too much, the surface of the light may be scratched.

If it is too large, the air gap between the end faces of the two optical fibers will increase during connection, resulting in an increase in Fresnel reflection, and the light reflected back will interfere with the light source, thereby affecting the stability of the system.


Eccentricity: that is, the offset of the spherical center of the polished connector end face relative to the axis of the ferrule pin cylinder is usually controlled within 50um. When the eccentricity value is too large, it will not only connect the optical fiber

It affects the optical characteristics of the device and increases the bit error rate during information transmission.


Radius of curvature: that is, the spherical radius of the grinding end face. If the radius is too large, the end face will be too flat, it will be difficult to locate during alignment, and the connection accuracy will be reduced; if the radius is too small, it will cause stress

Excessive concentration on the small end face may cause the fiber to deform or even break.


Optical characteristics:

In addition to the physical properties such as appearance, a good optical fiber connector needs to meet the standards, and the optical properties of the device itself also need to meet certain usage standards. Generally speaking, the optical characteristics of the following two aspects need to be considered in the manufacturing process of the connector:

Return Loss, also known as return loss, refers to the number of decibels of the ratio of the portion of the incident optical power reflected back to the signal source to the incident optical power when the optical signal passes through the connector, namely


Generally speaking, RL is a negative value, and the larger the value, the better, which can effectively reduce the impact of reflected light on the performance of the light source and the stability of the optical fiber system.

Insertion Loss (Insertion Loss) refers to the ratio of output optical power to output optical power in decibels for optical fiber connectors, namely


Insertion loss is often used to measure the optical power attenuation caused by the connector to the system. Since multiple connectors need to be added to the system for connection, it will increase the power cost of the system design, so the insertion loss of the connector should be as small as possible.

2、Fiber Optic Connector Structure

The main purpose of fiber optic connectors is to realize the splicing of optical fiber. Fiber optic connectors that have been widely used in fiber optic communication systems have many types and different structures. However, in detail, 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 two pins and a coupling tube for a total of three parts) to realize optical fiber alignment connection.

Fiber optic connector docking principle

In this method, the optical fiber is penetrated and fixed in the pin, and the surface of the pin is polished to achieve 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 ground, and the other end usually uses a bending restricting member to support the optical fiber to release the stress. The coupling tube is generally made of two semi-synthetic, fastened cylindrical members made of ceramic, bronze or other materials, and is usually equipped with metal or plastic flanges to facilitate the installation and fixation of the connector. To align the optical fiber as accurately as possible, the processing accuracy of the pin and the coupling tube is very high.

With the development of application fields, the types of fiber optic connectors are diverse. Different connector types have different characteristics, different advantages and disadvantages, and different performance param

3、Classification of fiber optic connectors

According to the end face of the connector pin: According to the grinding shape of the end face, it can be divided into 3 categories.

PC type: Physical Contact, physical contact, the end face of the ferrule is ground into a microsphere, the fiber core is located at the highest point of the spherical curvature, and the highest points of the two end faces are butted during use, so that

It can effectively reduce the air gap between the optical fiber components, so that the two connected end faces can reach physical contact. PC grinding is the most common and is mainly used on telecom operator equipment.


UPC type: Ultra Physical Contact, ultra-physical end face, optimizes polishing and surface finish on the basis of PC type, making the end face more radiant and more visually present.

Strong dome shape. UPC ground fiber optic connectors are commonly used in Ethernet networking equipment, telephone systems, and more.


APC type: Angled Physical Contact, the physical contact of the inclined plane, that is, the grinding of the optical fiber end face has a specific angle, and the commonly used ones are 6 degrees and 8 degrees. Compared to the first two grinding

As far as the resulting fiber optic connector is concerned, the APC-polished connector can make the fiber end face more tightly connected, and allow the reflected optical signal to be reflected to the cladding instead of the light source through its bevel.

It effectively suppresses the impact of reflected light on the entire system and provides better connection performance. APC ground fiber optic connectors are commonly used in high wavelength range optical RF applications.


According to the connector connection method: In order to adapt to different usage scenarios, a variety of optical fiber connectors with different characteristics have been developed. The following are some common connectors:

Thread locking type: Represented by the FC type connector, the connection is realized by rotating the thread. Although the connection is tight, it cannot be assembled in pairs like a duplex connector.

Plug-in type / snap-in type: push-pull connection represented by SC type, just push it, no need to screw it.

Bayonet rotary locking type: The bayonet rotary connection represented by the ST type, after twisting, engages with the spring-loaded bayonet socket.

Modular latch type: The third-generation optical fiber connector represented by the LC type is made of a modular jack (RJ) latch mechanism that is easy to operate, and has the characteristics of small size and easy installation.


According to connector type:

FC type: metal shell, round threaded interface, can be well fixed when connecting with optical modules, has the advantages of firmness and many times of plugging and unplugging, but the installation time is long, and it is suitable for storage.

FC fiber optic connector

Storage area network, optical transceiver and other equipment

ST type: metal material, snap-on interface (there is a bayonet that can be fixed in half a circle after insertion, but it is easy to be broken), commonly used in optical fiber distribution frames

SC type: plastic material, push-pull connection (easy to operate, but easy to fall out), is a standard square connector, the shell is rectangular, with high temperature resistance, not easy to oxidize, insertion loss

The advantages of small fluctuation and high compressive strength

SC fiber optic connector

LC type: plastic material, used to connect the SFP optical module, the interface can be stuck on the optical module

LC connector

SMA type: new type connector, metal material, threaded connection, stainless steel and ceramic ferrules, high temperature resistance, suitable for large core diameter optical fibers, commonly used in equipment testing, data networks, military instruments, medical equipment and other special communications

network, military equipment, medical equipment and other special communications