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Fiber Connectors

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Definition: connectors used as terminations of optical fiber cables

German: Faserstecker

Category: fiber optics and waveguides

How to cite the article; suggest additional literature

Fiber connectors are often used as the terminations of optical fiber cables in order to provide non-permanent connections between fiber-coupled devices (a kind of removable fiber joints). They are used in a similar manner as electrical connectors.

Typically, a connector assembly comprises an adapter and two connector plugs, where one fiber is inserted into every connector plug. The process of fitting a connector plug to a fiber is more delicate than for most electrical connections:

fiber connector

Figure 1: A fiber connector at the end of a fiber cable. The photograph has been kindly provided by NKT Photonics.

These termination assembly operations are often performed in factories, but with suitable equipment they can also be done in the field, preferably in a reasonably clean environment.

Although fiber connectors give some protection to the inserted fibers, that protection is quite incomplete, e.g. concerning moisture. Therefore, connectors may be put into protective enclosures. Hermetically sealed enclosures protect most reliably against moisture and dirt, but can hinder the dissipating of heat (e.g. from connector losses in high-power operation). Dust caps are often used during times where a connector is not plugged in.

Characteristics of Optical Fiber Connectors

The following characteristics can be relevant for the selection of fiber connectors:

Types of Optical Fiber Connectors

The most common fiber connectors are those of ST, FC, SC and LC type, and there are various versions of those. A short overview is given in the following:

ST Connectors

ST connectors are most popular for fiber-optic networks based on multimode fibers, as are usually used within buildings. The fiber is placed in a relatively long cylindrical keyed ferrule (often made of ceramic), providing a tight contact (physical contact) of the fiber ends. ST connectors contain a spring-loaded bayonet mount.

FC Connectors

FC connectors (“fixed connection”) are a popular type for single-mode fibers. They contain a floating ferrule with 2.5 mm diameter, into which the fiber is inserted (with the risk of scratching the fiber end). They provide a good mechanical isolation of the contact, allowing the use in high-vibration environments, but need to be handled quite carefully, with alignment of the ferrule's key when plugging it in. That key prevents rotation of the fiber when the plug is screwed into the receptacle. This is important for angled fiber ends (see below) and for polarization-maintaining fibers. Unfortunately, there are two mutually incompatible versions with key widths of 2 mm (reduced version, “type R”) and 2.14 mm (“type N”), respectively.

The standard version FC/PC involves a standard “physical contact” with a slightly rounded fiber surface; the fiber tip is polished after insertion into the ferrule. The ferrule is spring-loaded, providing a well-defined contact force when the the connector is plugged in. Higher-quality fiber tip polishing is used in FC/SPC and FC/UPC connectors (with “super” or “ultra” polish). This provides higher return loss and lower insertion loss.

FC/APC connectors use angled fiber ends in order to obtain a high return loss (even in the disconnected state), but normally at the cost of increased insertion loss. Of course, fiber ends with a carefully controlled tilt angle are more difficult to make, and the two angled fibers need to be inserted properly such the surfaces fit together.

FC connectors are often used for single-mode fibers including polarization-maintaining fibers, but are increasingly replaced with SC and LC connectors.

SC Connectors

SC connectors (“subscriber connectors”) are general purpose connectors of push/pull snap-in type for single-mode fibers. The fiber lies in a ceramic ferrule, and its orientation is controlled with a key. The rectangular shape of the connector leads to a defined orientation concerning rotations (in contrast e.g. to FC connectors). The snap-in mechanism makes the handling less critical than with FC connectors.

SC connectors are increasingly used in network applications, particularly with single-mode fibers. They are also available in duplex versions.

LC Connectors

LC connectors (“Lucent connectors”) are particularly small, belonging to the “small form factor” connectors (SFF connectors). It contains a ferrule with only 1.25 mm diameter. It is often used for single-mode fibers. Duplex versions are also available.

Applications of Fiber Connectors

As fiber optics are intensively used for optical fiber communications, many fiber connectors are also used in that context – for example, in central offices of telephone companies, at Internet backbones or in fiber to the home installations.

Laboratories also often use fiber cables with connectors, e.g. for transporting light to diagnostic instruments such as fiber-optic powermeters and spectrometers. For complex setups such as mode-locked fiber lasers, for example, it is less common to use fiber connectors, partly because of the detrimental effects even of very small return losses. Mechanical splicing or fusion splicing is then often a better option.

Multimode fiber connectors are often installed in the field. Single-mode connectors, having tighter tolerances, are usually combined with their fibers in factories. In the field, one may fusion-splice such pig-tailed single-mode connectors to cables. (Single-mode connectors installed in the field tend to have higher losses.)

See also: fiber cables, fibers, fiber optics, fiber joints, fiber to the home, fiber collimators
and other articles in the category fiber optics and waveguides

In the RP Photonics Buyer's Guide, 74 suppliers for fiber connectors are listed.

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