Fiber Optic Connectors – Standards to Ensure Physical Contact

As we know, physical contact is most important to ensure low IL and high RL for fiber connection. The evolution of endface polishing types experiences physical contact (PC), ultra physical contact (UPC) and angled physical contact (APC), as shown in Fig.1. All the endfaces are spherically polished. The UPC connector has a smaller radius of curvature than the PC connector. For the APC connector, the sphere surface is usually tilted by an angle of 8°. The RL guaranteed by PC, UPC and APC is 40dB, 55dB and 65dB, respectively.

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Fig.1 Polish types for the endfaces of fiber optic connectors

In real applications, the fiber optic connectors must be robust, which means that the precise alignment and physical contact should be kept in rigorous conditions. The robustness is insured by a series of standards. The precise alignment depends on the mechanically fabricated ceramic ferrule and the cleverly designed ceramic sleeve. The following work focuses on the standards for insurance of physical contact, including endface geometry of the ferrule and pressure loaded by the spring.

Standards for Endface Geometry The endface of the fiber optic connector is required to be spherically polished, while the fabricated products by real process are not perfect. Thus standards for the endface geometry are defined, which includes radius of curvature (ROC), apex offset, and fiber height, as shown in Fig.2. The standards by IEC are summarized in Table 1. The ROC should be a moderate value (10~25mm for PC connectors and 5~15mm for APC connectors). Ferrules with a larger ROC can’t deform enough to ensure the physical contact between fibers and a smaller ROC tends to crush the fibers after repeated plugging. The apex offset means the offset between the apex of the curvature and the axis of the optical fiber. Fig.3 shows how the apex offset affect the physical contact between optical fibers. Too much apex offset may stop the physical contact. Thus the apex offset is required to be ≤50μm. The fiber height means the height of the fiber endface relative to the ferrule endface, which may be an undercut or a protrusion. A small undercut won’t affect the physical contact because the ferrules will deform under pressure. A small protrusion won’t crush the fibers because the fibers has a certain elasticity. Thus the fiber height is required to be ranging -250~+250nm.

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Fig.2 Endface geometry of fiber optic connectors

Endface Geometry Radius of Curvature(mm) Apex Offset(μm) Fiber Height(nm)
PC 10~25 ≤50 -250~+250
APC 5~15 ≤50 -250~+250

Table 1. Standards for ferrule endface geometry by IEC

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Fig.3 Apex offset affecting the physical contact between optical fibers

As we know, the physical contact between optical fibers depends on the deform of the spherical endfaces under pressure. The pressure is loaded by some mechanical structure, whatever is the connector type. Most connectors are spring-loaded. IEC also quantitatively defined the pressure loaded by the mechanical structures.

Angled Fiber Optic Connectors In table 1, we note that the required ROC value for APC connectors is smaller than for PC connectors. The APC connectors are polished with an angled polishing plate. The alignment of the ferrule in the plate is shown in Fig.4(a). However, when the ferrule is inserted into the sleeve of an adaptor, it is aligned as Fig.4(b). the curvature apex of the endface offsets from the center, i.e. the fiber core. Fig.5 shows the mating between two APC fiber connectors. The apexes of the two endfaces are misaligned and more deformation on the endfaces is required to ensure the physical contact between optical fibers. Thus the ROC requirement for APC connectors is smaller.

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Fig.4 Alignment of the ferrule in (a) polishing plate and (b) sleeve of adaptor

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Fig.5 Mating between two APC fiber connectors

As shown in Fig.5, there is always a latch in the angled fiber optic connector indicating the direction of angle, whatever is the connector type. The indication precision of the latch will affect the apex offset of the APC connector. Meanwhile, the error in the polishing angle will also affect the apex offset. The apex offset due to different factors is shown in Fig.6, where R is the ROC of the ferrule and point “O” is the center of the endface curvature. Fig.6(a) shows a connector with perfect endface geometry, which is under measurement by an interferometer. Fig.6(b) shows a connector with a normal apex offset, which is introduced in the polishing process. Fig.6(c) shows a connector with an error Δ in the polishing angle. Apex offset is measured as d1=R·Δ when it is inserted into the 8° holder of the interferometer. Note that the measure conditions of the interferometer coincide with the real application of the connectors. Fig.6(d) shows a connector with an error δ in the direction of the latch, which may be introduced by the mechanical parts or in the assembly process. When the connector with direction error is plugged into an adaptor, the ferrule rotates. The center of the curvature rotates from “O” to “O’” and the apex of the endface rotates from “A” to “A’”, as shown in Fig.6(e). The length of OE is obtained as R·sin8° from Fig.6(d). Thus the apex offset introduced by direction error of the latch is obtained as d2=R·sin8°·sinδ.

Let’s take an example. Given R=10mm, Δ=0.1° and δ=1°, the apex offsets resulting from different factors are obtained as d1=17.5μm and d2=24.3μm. Note that the upper limitation by IEC standard is 50μm.

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Fig.6 Different factors affecting the apex offset of a fiber optic connector

Summary on Physical Contact The active connection of optical fibers rejects AR coating on the endface, due to technical and cost consideration. Thus physical contact between optical fibers is the key concept to ensure low IL and high RL. A sphere endface helps to keep physical contact. Standards for the endface geometry are defined to ensure the physical contact under rigorous conditions. The sphere endface is polished with a “soft” pad under a certain pressure. Both the hardness of the pad and the pressure loaded affect the endface geometry. The roughness of the polishing film should also be taken into account. The optimum parameters for polishing process can be obtained based on orthogonal design of experiments.

For APC connectors, more considerations are taken into account. The errors in polishing angle and latch direction affect the apex offset significantly. Thus the polishing plate should be well fabricated with precise angle. Besides the ferrule, the mechanical precision of the other parts of a connector should be guaranteed. What’s more, the latch of the connector and the corresponding slot in the adaptor should be in relatively tight matching.

Written by Zhujun Wan

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