Fiber-Optic Ferrule

The fiber-optic ferrule is a ceramic, metal or plastic part of a connector that holds and aligns the ends of optical fibers. It plays an important role in ensuring high-quality connections that transmit light signals efficiently.

When an air gap occurs between fiber-optic cable ends, data transmission is compromised. Precise polishing and alignment are critical factors in minimizing insertion loss.

Materials

Fiber optic connectors are devices that attach to or decouple optical fibers for communication. They are based on a central component called a ferrule, which helps with optical alignment, as well as protecting and supporting the stripped ends of an optical fiber. Ferrules are available in a variety of materials, and they play an important role in minimizing the loss of light passing through the connector. They also contribute to the overall mechanical strength and durability of a fiber connector.

Ferrules are typically made from zirconia ceramic or composite plastic polymers, with zirconia ceramic providing the highest levels of dimensional control and durability. They are molded and produced to strict tolerances that meet performance standards. The end face of the ferrule is polished to a precise and smooth finish, which minimizes insertion loss and back-reflection.

Other parts of a connector include a housing and coupling mechanism. The housing, which is typically plastic, holds the ferrule and enables you to mate it with another type of connector. The coupling mechanism, which is often a screw or bayonet, allows you to snap the connector together.

Different types of fiber optic connectors use different ferrules. LC connectors, for example, have small form factor ferrules that make them more compact than other types of connectors. They are popular in datacoms because they can fit more fibers into a smaller space, making it easier to deploy high-density networks. Other fiber-optic-connectors-ferrule types of connectors use larger form factor ferrules, including SC, MU and MT-RJ.

Bore Diameter

A ferrule is the sleeve-shaped component of a fiber optic connector that holds the ends of optical fiber in place inside the body. It is designed to support the fragile fibers and prevent excessive bending that could lead to loss of signal strength. It also helps to maintain the precise alignment necessary for optimal light transmission.

The bore diameter of the ferrule is a key factor that affects performance. The bore is usually made slightly larger than the cladding of the fiber to allow for easy insertion. Ideally, the cladding should be centered within the bore and epoxied in place to avoid misalignment that may reduce signal transmission efficiency.

Singlemode ferrules require the highest tolerances of bore diameter and concentricity due to the small core size that is more sensitive to errors in lateral matching. However, multimode ferrules have a much larger core size that allows for more error in lateral matching without affecting signal transmission efficiency.

There are several types of fiber connectors and ferrules, including biconic, Biconic/BNC, D4, ESCON, FC, FDDI, LC, loopback, and straight tip (ST). Choosing the right one depends on your network requirements. Factors that influence your choice include the bore style, bore dimensions, concentricity, and features like anti-rotation notches, flats, epoxy vent holes, and any-angle chamfers. Some also feature a countersink or other features that reduce receptacle surface friction during mating.

Endface Diameter

The endface diameter of a ferrule can make or break an optical connection. Several factors impact this measurement, including bore diameter mismatch, concentricity variations, non-circularity and end separation. These microscopic variances seem insignificant, but they can significantly affect performance and reliability.

As technology advanced, so too did the need to provide better and more reliable connections for telecommunications networks and data centers. This is why we’ve seen the emergence of connectors that use angled endfaces to reduce back reflections and increase the quality of the optical signal transmitted through the connection.

For example, the early multimode SM connectors Fiber Optic assembly companies used metal ferrules and were named after microwave hardware such as the SMA (SubMiniature A). This was replaced by ceramic-ferrules that had a more glass-like expansion coefficient, so they held the fiber’s cladding in place better over extremes in temperature. It also allowed for more aggressive polishing that reduced back reflection to acceptable levels.

Today, the dominant connectors are the UPCs and LCs (Ultra Physical Contact and Light Conversion). The angled endface of these connectors reflects light into the cladding at an 8-degree angle to further decrease return loss. This solution was developed when single-mode systems became popular in long-haul applications that require extremely high bitrates and low reflections.

Polishing

The ferrule holds the optical fiber and maintains precise alignment to minimize insertion loss and back reflection, which can degrade signal strength. It also protects the fragile optical fiber and provides physical support for the connector, which is otherwise too small to be mechanically stable.

In addition to minimizing insertion loss, precision polishing optimizes the physical contact between the fiber end and the ferrule. This reduces backreflection, which can significantly decrease signal transmission capability and may result in unacceptable damage to the fiber-optic network.

To accomplish this, the ferrule is usually designed with a slightly dome-shaped surface instead of a flat end face. This increases the radius of curvature for the optical-fiber end, which makes it easier to achieve a physical contact that maximizes ferrule material compression and minimizes the potential for an air gap.

Polishing the angled endface of the ferrule requires a delicate hand-held procedure that involves carefully holding the cleaved fiber against the 12-micron polishing film and using a swivel to make small, delicate movements. The swivel allows the user to control the direction of the movement and to ensure the proper positioning of the fiber in relation to the ferrule.

The process begins with a scrubbing step that uses a fine-grit (one-micron) diamond film. The scrubbing removes scratches and prepares the endface for a final slurry-polishing step. The slurry contains a chemical agent that changes the refractive index of the surface of the fiber, which eliminates scratches and other defects. The hand-applied slurry is polished until the endface is smooth and has an acceptable radius.