Optical Fiber and Connection: Joints

An important aspect of fiber-to-fiber connection is ensuring that the system has low loss and minimum distortion. The main ways to achieve the aforementioned is to guarantee proper joining and termination of the transmission medium. Apart from ensuring low loss and minimum distortion, suitable connections increase the number of terminal joints permissible within the developing of optical fiber communication networks. The two major categories of fiber-fiber connections are fiber splices and demountable fiber connectors. Both the joints are designed ideally to couple all the light propagating in one fiber into the adjoining fiber with minimal optical loss.

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Fiber alignment and joint loss

A major consideration with all types of fiber-fiber connections is the optical loss encountered at the interface. Even when the two joined fiber ends are smooth and perpendicular to the fiber axes, and the two fiber axes are perfectly aligned, a small proportion of the light may be reflected back into the transmitting fiber thus causing attenuation at the joint. This phenomenon, known as Fresnel reflection, is associated with step changes in refractive index at the jointed interface. The magnitude of this partial reflection of the light transmitted through the interface may be estimated using the classical Fresnel formula for light of normal incidence. Another potentially greater source of loss at a fiber-fiber connection is caused by misalignment of the two jointed fibers. Any deviations in the geometrical and optical parameters of the two joined optical fibers will affect the optical attenuation through the connection.

Multimode fiber joints

Theoretical and experimental studies of fiber misalignment in optical fiber connections allow approximate determination of the losses encountered with the various misalignments of different fiber types. Misalignment may occur in three dimensions; the separation between the fibers (longitudinal misalignment), the offset perpendicular to the fiber core axis (lateral misalignment), and the angle between the core axes (angular misalignment). It is important to note that no loss is incurred if the receiving fiber has a larger core diameter than the transmitting one. However, when the transmitting fiber has a higher numerical aperture than the receiving fiber, then some of the emitted light rays will fall outside the acceptance angle of the receiving fiber, and they will, therefore, not be coupled through the joint.

Single mode fiber joints

The theoretical analysis of misalignment losses is based upon the Gaussian or near-Gaussian shape of the modes propagating in the single-mode fibers regardless of the fiber type. The insertion losses incurred are also strongly dependent upon the normalized frequency of the fiber. Determination of coupling loss between single-mode fibers is more accurate when it takes into account all the extrinsic factors as well as the intrinsic factor associated with the connection of fibers with unequal mode-field diameters. In contrast to the situation with multi-mode fibers, the intrinsic loss through a single-mode fiber joint is independent of the direction of propagation. Possible distortion of the transmitted signal at a fiber joint minimized by using highly coherent sources and very low dispersion fibers.

Fiber splices

A fiber splice is a permanent joint formed between two individual optical fibers. Splices may be divided into two broad categories depending upon the splicing technique utilized. These are fusion splicing and mechanical splicing. Fusion splicing is accomplished by applying localized heating at the interface between the two butted, and pre-aligned fiber ends causing them to soften and fuse. On the other hand, mechanical splicing is achieved by holding the fibers in alignment by some mechanical means. The insertion losses of fiber splices are generally much less than the possible Fresnel reflection loss at a butted fiber-fiber joint. This is because there is no large step change in refractive index with the fusion splice as it forms a continuous fiber connection, and some method of index matching tends to be utilized with mechanical splices.

Fusion splices.

A requirement with fibers intended for splicing is that they should have smooth and square end faces. The end preparation may be achieved using a suitable tool which cleaves the fiber. The fusion splicing of single fibers involves the heating of the two prepared fiber ends to their fusing point with the application of sufficient axial pressure between the two optical fibers. It is, therefore, essential that the stripped fiber ends are adequately positioned and aligned so as to achieve a good continuity of the transmission medium at the junction point. The most widely used heating source is an electric arc. The technique offers advantages of consistency and easily controlled heat. A possible drawback with fusion splicing is that the heat may weaken the fiber in the vicinity of the splice.

Mechanical splices

A number of mechanical techniques for splicing individual optical fibers have been developed. A common method involves the use of an accurately produced rigid alignment tube into which the prepared fiber ends are permanently bonded. The snug tube splice may utilize a glass or ceramic capillary with an inner diameter just large enough to accept the optical fibers. A transparent adhesive is injected through a transverse bore in the capillary to give mechanical sealing and index matching of the splice. In general, snug tube splices exhibit problems with capillary tolerance requirements. A mechanical splicing that avoids the critical tolerance requirements of the snug tube splice is the loose tube splice which uses an oversized square section metal tube that easily accepts the prepared fiber ends. Other common mechanical splicing techniques involve the use of V-grooves to secure the fibers to be joined.

Fiber connectors

Demountable fiber connectors are more difficult to achieve than optical fiber spices because of two main reasons. First, they must maintain similar tolerance requirements to splices but in a removable fashion. Secondly, the connector design must allow for repeated connection and disconnection without problems of fiber alignment. The connection must also protect the fiber ends from damage which may occur due to handling, must be insensitive to environmental factors and must cope with a tensile load on the cable. The use of an index matching material in the connector between the two joints increases the light transmission through the connection and keeps dust and dirt from between the fibers.

Fiber connectors may be separated into two broad categories, butt jointed connectors and expanded beam connectors. Butt jointed connectors rely upon alignment of the two prepared fiber ends in proximity to each other so that the fiber core axes coincide. They include cylindrical ferrule connectors, biconical ferrule connectors, double eccentric connector and duplex and multiple fiber connectors. Expanded beam connectors, on the other hand, utilize interposed optics at the joint to expand the beam from the transmitting fiber end before reducing it again to a size compatible with the receiving fiber end. They are useful for multifiber connection and edge connection for printed circuit boards where lateral and longitudinal alignment is frequently difficult to achieve. Efficient beam expansion and collimation are achieved using the graded index (GRIN) rod lens which has demonstrated loss characteristics which are independent of the modal power distribution in the fiber.