1. Introduction
When installing any fiber optic system, it is necessary to consider connecting the fibers or cables to each other in a low loss manner to achieve the connection of the optical link. The connection of fiber optic links can be divided into two types: permanent connection and active connection. Permanent connections are mostly achieved through fusion, bonding, or fixing connectors; The continuity of activities is usually achieved through activity connectors. This article will briefly introduce active connectors.

Fiber optic flexible connectors, commonly known as flexible connectors, are reusable passive devices used to connect two fibers or cables to form a continuous optical path. They have been widely used in fiber optic transmission lines, fiber optic distribution frames, and fiber optic testing instruments, and are currently the most commonly used optical passive components.

 

 

　　

2. General structure of fiber optic connectors
The main purpose of fiber optic connectors is to achieve fiber optic connections. Fiber optic connectors have various types and structures, and have been widely used in fiber optic communication systems. However, upon careful inspection, the basic structure of various types of fiber optic connectors is consistent, meaning that the vast majority of fiber optic connectors typically use high-precision components (consisting of two pins and a coupling tube composed of three parts) to achieve fiber alignment and connection.
This method involves threading and fixing the fiber optic cable into the pins, polishing the surface of the pins, and aligning it with the coupling tube. The external components of the sales are made of metal or non-metallic materials. The docking end of the pin must be grounded, and the other end is usually supported by a bending limiting component to release stress on the fiber or cable. The connecting pipe is usually made of two semi composite, fastened cylindrical components made of materials such as ceramic or bronze, and is usually equipped with metal or plastic flanges to facilitate the installation and fixation of the connector. In order to align the optical fibers as accurately as possible, there is a high requirement for the machining accuracy of the pins and coupling tubes.

3. Performance of fiber optic connectors

The performance of fiber optic connectors is primarily based on optical performance. In addition, the interchangeability, repeatability, tensile strength, temperature, and insertion and extraction time of fiber optic connectors should also be considered.

(1) Optical performance: The requirements for the optical performance of fiber optic connectors mainly include insertion loss and return loss, which are the two most basic parameters.
Insertion loss, also known as connection loss, refers to the effective optical power loss of a link caused by the introduction of a connector. The smaller the insertion loss, the better. Generally, it should not exceed 0.5dB.
Return loss (reflection loss) refers to the ability of a connector to suppress the reflection of link optical power, and its typical value should not be less than 25dB. In practical applications, the surfaces of connectors and pins have undergone specialized polishing treatment, which can increase the return loss, usually not less than 45dB.

(2) Interchangeability and repeatability
Fiber optic connectors are universal passive devices. For the same type of fiber optic connectors, they can usually be combined and reused multiple times, resulting in additional losses typically within the range of less than 0.2dB.

(3) Tensile strength
For well made fiber optic connectors, it is usually required that their tensile strength should not be less than 90N.

(4) Temperature
Usually, fiber optic connectors must be able to operate normally within the temperature range of -40 ℃ to+70 ℃.

(5) Insertion and removal time
Currently, fiber optic connectors can usually 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, including single-mode fiber optic connectors and multi-mode fiber optic connectors, depending on the transmission medium; According to its structure, it can be divided into various types, such as FC, SC, ST, D4, DIN, Biconic, MU, LC, MT, etc; According to the pin end face of the connector, it can be divided into FC, PC (UPC), and APC; According to the number of fiber cores, there are also two types: single core and multi-core.
In practical applications, we usually differentiate based on the different structures of fiber optic connectors. Below is a brief introduction to some commonly used fiber optic connectors:

(1) FC type fiber optic connector

This type of connector was first developed by NTT Corporation in Japan. FC is the abbreviation for sleeve connector, which means its external reinforcement method is metal sleeve and the fastening method is screw buckle. The earliest FC type connectors used the flat contact method (FC) as the mating end face for ceramic pins. This type of connector has a simple structure, convenient operation, and is easy to produce, but the fiber optic end is more sensitive to dust and is prone to Fresnel reflection, making it difficult to improve the return loss performance. Later, improvements were made to this type of connector by using spherical mating pins (PC) while keeping the external structure unchanged, significantly improving insertion loss and return loss performance.

(2) SC type fiber optic connector
This is a fiber optic connector developed by NTT Corporation in Japan. Its shell is rectangular, and the structural dimensions of the pins and connecting sleeves used are the same as those of the FC type. The end face of the pin is mostly grounded