Analysis of Structures and Performance of Various Fiber Optic Connectors

1. Introduction

When installing any fiber optic system, it is necessary to consider interconnecting optical fibers or cables with low-loss methods to achieve optical link continuity. Optical link splicing can be divided into two types: permanent and non-permanent. Permanent splicing is mostly achieved by fusion splicing, adhesive bonding, or fixed connectors; non-permanent splicing is generally achieved by using removable connectors. This article provides a brief introduction to removable connectors.

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

2. General Structure of Fiber Optic Connectors

The main purpose of fiber optic connectors is to achieve fiber splicing. Fiber optic connectors now widely used in optical fiber communication systems come in many types and various structures. However, upon closer examination, the basic structure of various types of fiber optic connectors is consistent—that is, the vast majority of fiber optic connectors generally use high-precision components (consisting of three parts: two ferrules and one coupling sleeve) to achieve fiber alignment and connection.

This method involves threading the fiber into and fixing it within the ferrule, polishing the ferrule surface, and then achieving alignment within the coupling sleeve. The outer components of the ferrule are made of metallic or non-metallic materials. The mating end of the ferrule must be polished, while the other end typically uses a bend-limiting member to support the fiber or fiber cable to relieve stress. The coupling sleeve is generally a two-half, tightly fitted cylindrical component made of ceramic or bronze, often equipped with a metal or plastic flange for easy mounting and fixing of the connector. To achieve the most precise alignment of the fibers, high machining precision is required for the ferrules and coupling sleeves.

3. Performance of Fiber Optic Connectors

The performance of fiber optic connectors primarily includes optical performance, as well as interchangeability, repeatability, tensile strength, temperature, and number of mating cycles.

(1) Optical Performance: The optical performance requirements for fiber optic connectors mainly involve two fundamental parameters: insertion loss and return loss.

Insertion Loss, also known as connection loss, refers to the loss of effective optical power in the link caused by the introduction of the connector. The lower the insertion loss, the better; generally, it should not exceed 0.5 dB.

Return Loss refers to the connector's ability to suppress reflection of optical power in the link, with a typical value of no less than 25 dB. In practical applications, the ferrule surface of connectors undergoes specialized polishing, which can achieve higher return loss, generally no less than 45 dB.

(2) Interchangeability and Repeatability

Fiber optic connectors are universal passive devices. For the same type of fiber optic connector, they can generally be used in any combination and can be repeatedly mated, with the additional loss introduced typically within a range of less than 0.2 dB.

(3) Tensile Strength

For finished fiber optic connectors, the tensile strength is generally required to be no less than 90 N.

(4) Temperature

Generally, fiber optic connectors are required to operate normally at temperatures ranging from -40°C to +70°C.

(5) Number of Mating Cycles

Currently used fiber optic connectors can generally be mated more than 1,000 times.

4. Some Common Fiber Optic Connectors

According to different classification methods, fiber optic connectors can be divided into different types. By transmission medium, they can be divided into single-mode fiber optic connectors and multi-mode fiber optic connectors; by structure, they can be divided into FC, SC, ST, D4, DIN, Biconic, MU, LC, MT, and other types; by ferrule end-face, they can be divided into PC (UPC) and APC; by fiber count, there are single-fiber and multi-fiber types.

In practical applications, we generally distinguish fiber optic connectors by their structural differences. The following is a brief introduction to some of the more common fiber optic connectors:

(1) FC Fiber Optic Connector

This connector was first developed by NTT of Japan. FC is the abbreviation for Ferrule Connector, indicating that its external reinforcement method uses a metal sleeve and the fastening method is a screw thread. Initially, FC-type connectors used a flat contact end-face on the ceramic ferrule (FC). This type of connector has a simple structure, is easy to operate, and is easy to manufacture, but the fiber end-face is sensitive to dust and prone to Fresnel reflection, making it difficult to improve return loss performance. Later, this type of connector was improved by using a ferrule with a spherical end-face (PC) while the external structure remained unchanged, resulting in significant improvements in insertion loss and return loss performance.

(2) SC-Type Fiber Optic Connector

This is a fiber optic connector developed by NTT of Japan. Its housing is rectangular, and the structural dimensions of the ferrule and coupling sleeve are identical to those of the FC type, with the ferrule end-face mostly polished in PC or APC style; the fastening method uses a push-pull latch mechanism, requiring no rotation. This type of connector is low-cost, easy to plug and unplug, has low insertion loss fluctuation, high compressive strength, and high mounting density (most commonly used on routers and switches).

(3) ST-Type Fiber Optic Connector

ST-type fiber patch cords: commonly used in optical distribution frames, with a circular housing and screw-thread fastening. (For 10Base-F connections, the connector is typically of the ST type. Commonly used in optical distribution frames.)

(4) LC-Type Fiber Optic Connector

The LC-type connector was developed by the renowned Bell Research Institute and is manufactured using a convenient modular jack (RJ) latch mechanism. The dimensions of its ferrule and sleeve are half of those used in standard SC, FC, etc., at 1.25 mm. This allows for increased density of fiber optic connectors in optical distribution frames. Currently, in single-mode SFF applications, LC-type connectors have effectively taken a dominant position, and their use in multi-mode applications is also growing rapidly.

(5) Biconic Connector

The most representative product of this type of fiber optic connector was developed by Bell Laboratories in the United States. It consists of two precision-molded cylindrical plugs with truncated conical end tips and a coupling assembly with an internal biconical plastic sleeve.

(6) DIN4-Type Fiber Optic Connector

This is a connector developed in Germany. The structural dimensions of its ferrule and coupling sleeve are the same as those of the FC type, with the end-face processed using PC polishing. Compared with FC-type connectors, its structure is somewhat more complex, with a spring in the internal metal structure that controls pressure, preventing damage to the end-face due to excessive mating pressure. In addition, this connector has higher mechanical precision, resulting in lower insertion loss values.

(7) MT-RJ-Type Connector

MT-RJ originated from the MT connector developed by NTT. It features the same latch mechanism as the RJ-45 LAN electrical connector, aligning the fibers through guide pins installed on both sides of a small sleeve. To facilitate connection with optical transceivers, the connector end-face is designed with a dual-fiber arrangement (0.75 mm spacing), making it a next-generation high-density optical connector primarily used for data transmission.

(8) MU-Type Connector

The MU connector is based on the most widely used SC-type connector and was developed by NTT as the world's smallest single-fiber optical connector. This connector uses a 1.25 mm diameter sleeve and a self-retaining mechanism, with the advantage of enabling high-density installation. Utilizing the MU's 1.25 mm diameter sleeve, NTT has developed a series of MU connectors. These include socket-type optical connectors for cable connection (MU-A series), backplane connectors with self-retaining mechanisms (MU-B series), and simplified sockets for connecting LD/PD modules with plugs (MU-SR series). With the rapid development of optical networks toward greater bandwidth and capacity and the widespread application of DWDM technology, the demand for MU-type connectors will also grow rapidly.

5. Conclusion

With the continuous development of optical fiber communication technology, especially the development of high-speed LANs and optical access networks, fiber optic connectors will be used more extensively in fiber optic systems. At the same time, higher and more demanding requirements are being placed on fiber optic connectors. The main development directions are: miniaturization of appearance, cost reduction, and increasingly higher performance requirements. In the coming period, various newly developed fiber optic connectors will coexist with traditional FC, SC, and other connectors, forming a pattern where "each excels in its own way and serves its own purpose."