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fiber optic connector

FO connectors provide a detachable FO plug-in connection between two optical fibres. With most FO connectors, no distinction is made between FO plug and socket, but rather the plug-in connection consists of two FO connectors that are connected to each other via a guide coupling. Since FO connectors are components of the highest precision, the slightest mechanical manufacturing tolerances or changes due to frequent plugging and unplugging can impair the essential transmission parameters.

Transmission parameters of FOC connectors

The quality of the entire optical transmission system depends on the FOC connectors. Therefore, all transmission parameters must be strictly observed. Important transmission parameters are the insertion loss and the return loss. Other properties are to be mentioned: The reproducibility of the transmission values under frequent stress, the number of mating cycles, a long service life, a stable mechanical design with easy handling, and compact dimensions.

Attenuation of fiber optic connections as a function of the air gap

Attenuation of fiber optic connections as a function of the air gap

During light transmission in a fiber optic connector, the two fibers are brought very close together in order to transfer as much light energy as possible from one fiber to the other. Light reflections and attenuation occur at the end faces of the two fibers. The attenuation that occurs depends on the size of the air gap between the two fibers, i.e. on the longitudinal offset of the two fibers. This offset is also called Radially Overfilled Launch (ROFL), the light loss is called Gap Loss.

Attenuation of LwL connections as a function of lateral offset

Attenuation of LwL connections as a function of lateral offset

In order to achieve constant attenuation values during repeated mating cycles, a gap width of about 10 µm, which is as small as possible but must be precisely maintained, is crucial. This gap is achieved by precise ferrule lengths or by hollow polishing.

Mating characteristics of LwL connectors

Furthermore, the attenuation is very much dependent on the lateral offset, the eccentricity between the two fibers. The lateral offset, i.e. the axial offset, increases the damping disproportionately. It is caused by inaccurate guidance of the ferrule in the coupling, eccentric or too large a bore in the ferrule and when the core glass is offset from the central axis. The lateral offset is also referred to as Offset Launch (OL).

Attenuation depending on the tilt angle

Attenuation depending on the tilt angle

Furthermore, the tilt angle between the two fibers should be mentioned, which has a significant influence on the attenuation of the light signal. The tilt angle occurs with imprecise connection technology, especially with poor couplings with too large a bore or spring guide. The same applies to conical connectors which are too long and wobble in the cone.

Non-parallel fiber optic cable end faces and curved or rough fiber end faces also create additional attenuation. The latter can be dirt and or scratches or stains on the FO end faces

FOC connectors for single mode, gradient and polymer fibers

The aperture of the fibers also plays a role in the connector attenuation: it is directly included in the attenuation values for the end face spacing. In the case of two fibers with different numerical apertures, the two transmission directions have different attenuation values. If the radiation comes from an FOC fiber with a high aperture and is transmitted into one with a low aperture, it is not fully absorbed by this fiber. The situation is different in the opposite direction: the light energy from the low aperture fiber is completely absorbed by the high aperture fiber.

For the reasons mentioned above, the highest demands are placed on optical fibers and on the positioning accuracy and thus on the manufacturing tolerance of fiber optic connectors. Minimal tolerance deviations have a serious effect, especially when connecting monomode fibers, since with monomode fibers an axial offset of only 4 µm already causes an additional attenuation of approx. 0.4 dB.

Overview of standard fibre

Overview of standard fibre

optic
, SFF and multi-fibre connectors

, SFF and multi-fibre connectors

For the connection technology of optical fibres there are a large number of connectors which are incompatible with each other. Standard fiber optic connectors include the ST connector, FC connector, SC connector, LC connector, MIC connector, mini-BNC connector, FSMA connector, E-2000 connector, F3000 connector, and ESCON connector. In addition, among the multitude of others, the following are also worth mentioning: SMA connector, DIN connector, PKI connector, Biconic connector, Galaxy connector and Mini-MT connector, and among multi-fiber connectors, MTRJ connector and MPO connector.

ST, FC, SC and F-SMA fiber optic connectors, photo: Siemens

ST, FC, SC and F-SMA fiber optic connectors, photo: Siemens

The most frequently used connector systems for gradient fibers are the first six mentioned. They differ in design, closure, connection type and application. For universal cabling, the cabling standard prescribes the use of the duplex SC connector if it is not an existing installation with ST connectors. In such a case, ST connectors can still be used for new connections. In addition to the connector type, the cabling standard prescribes further requirements for the FO connector. For example, only fibers with 125 µm may be used (9/125 µm, 50/125 µm, 62.5/125 µm). The possible number of mating cycles must exceed 500 and the maximum insertion loss must be below 0.5 dB.

SC connector and coupling. Photo: Huber + Suhner

SC connector and coupling. Photo: Huber + Suhner

Some of the connectors mentioned can also be used for polymer fibers with 1,000 µm cladding diameter: Thus the SMA and ST connectors. In addition, there are some plastic connectors for these fibers, such as the HFBR connector and the OVK connector.

In addition, there are the SFF connectors, which have significantly smaller form factors compared to the conventional FOC connectors and thus enable a much higher packing density. Several different SFF versions exist, one of the better known of which is the LC connector.

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Englisch: fiber optic connector
Updated at: 27.11.2017
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