Fiber optics have paved the way for rapid data transmission across longer distances. As we have explained in our introduction to this 2-part series, the connection performance of fiber optic cables must be at its maximum to meet this demand.

This requires extreme attention to detail, as connectivity issues can depend on very minute variances. In Part 1, we will discuss bore diameter, core concentricity, and circularity, including what they are and how they affect fiber optic power. 

Bore Diameter

Ferrules come with varying bore diameters depending on the size of the optical fiber it will host. One reason for the variance is that the diameter is purposefully slightly larger than normal from a “standard” fiber diameter to allow for differences in cladding. 

Cladding is defined as a layer of either glass or plastic that surrounds the inner core with a lower index of refraction. Through reflection, it confines light to the core to improve light transmission.

Ferrules are manufactured using injection-molded ceramic. After the initial molding step, the ferrules are put through a precision boring process and then polished to remove any machining marks, nicks, or scratches. The ferrules then have the bore diameter measured and sorted into different “grades."

Matching the grade of the ferrule to the application and desired performance level is a key step in preparation, along with specifying a fiber with a low variation of the cladding diameter.

Singlemode ferrules require the highest tolerance on bore diameter due to the high tolerances necessary to guarantee the correct alignment of the fibers (dictated by the small size of the optical core, typically 9 microns in diameter).

Even a tiny mismatch between two single-mode optical fibers can produce large losses in light transmission. Multimode ferrule terminations can be more forgiving due to larger core sizes (50 microns, 62.5 microns, or up to 100 microns) and are not as sensitive to mismatch.

Bore diameter is the first step in a quality termination. Unwanted variation in bore diameters will prevent the optical fibers from being fixed along the center axis due to variation in placement. This variation will contribute to a mismatch in core alignment, possibly increasing the connection loss.

The bore diameter depends on extremely precise and minute factors. Fiber optics is a technology that requires extreme accuracy and attention to detail. If this factor falls short to a tiny degree, then loss of light and data occurs. The greater the discrepancy, the lower the connectivity performance.

Concentricity

Concentricity is the positioning of one cylindrical item along the axis of another cylindrical item. When lining up two terminated ferrules, the optical fibers must line up for light to be transmitted from one to another.

Any amount of mismatch (where one core is not concentric to another) will contribute to optical loss.

Figure: A model of a typical single-mode optical fiber with cladding (in red) over a single-mode core (in blue), inserted into an LC ferrule. As shown, the optical fiber has poor placement within the ferrule bore (note the white gap along the edge).

In fiber optics, there are three important items to consider: 

1. The concentricity of the ferrule bore: Is the hole centered concerning the outside of the ferrule?
2. The concentricity of the fiber core: Is it centered within the fiber cladding?
3. The concentricity of the fiber within the ferrule: Is the main axis of the cladding along the main axis of the bore opening? 

First, UCL Swift uses high-quality equipment to make the bore opening in the ferrules and to minimize the amount that the bore opening deviates from the true center of the ferrule. ULC Swift inspects the ferrules to ensure quality and performance using a V-groove-based microscope system.

As seen below, each ferrule is placed in the groove of the V-block with the end of the fiber facing the light source. The ferrule is rotated in steps as images are taken. At each step, the center coordinates of the ferrule core are recorded. This creates an average of how concentric the ferrule core is, providing you with an accurate measurement.

Second, we use high-quality fiber from reputable sources with tight tolerances on core concentricity. We rely on our partners to provide us fiber that meets or exceeds the tolerances we set, and we review the documentation and inspect the fiber to make sure it meets our standards.

Third, Ilsintech inspects each completed assembly after fiber insertion for placement of the fiber within the bore. Ilsintech holds the tolerances to be tight on the fiber and the bore diameter, which minimizes the amount that the fiber can move within the bore. Minimizing the movement will reduce the contribution towards insertion loss.

Figure: Inspection microscope view of a fiber in a ferrule. Note the black crescent across the top – this is epoxy filling the area between the clad fiber and the ferrule.

Circularity/Cylindricity

The term circularity refers to how close any given object is to a perfect circle, which would satisfy the requirement that the circumference equals the diameter multiplied by Pi (3.14159). The tolerance, or tolerance zone, refers to an acceptable range of deviation away from perfection. 

When referring to a ferrule, you will be measuring cylindricity, as this is the 3-dimensional version of circularity (cylindricity is the circularity extended along the main axis of the ferrule). Cylindricity is an important factor in many manufacturing processes because it typically affects the quality of a product and ultimate the quality of the transmission between two ferrules.

In this case, you are specifically measuring the cylindricity of the ferrule itself as well as the bore that goes through the ferrule. Rather than simply measuring the circularity on one plane, you’ll have to record measurements of many locations along the length of the cylinder to obtain an average. The average deviation away from perfection must be within tolerance.

If the cylindricity of the ferrule and the ferrule bore is not within tolerance, then the core of the transmitting ferrules and the optical fibers could be misaligned. A loss of light will occur as the transmit fiber optic cable transfers data to the receive cable.