Light Budget is one of the key items for designing fiber optic network. in order to create a product that will meet application requirements. To adequately characterize the budget loss, the following key parameters are generally considered:
- Transmitter: Output power, temperature and aging
- Fiber connections: Active connection and splices
- Fiber Cable: fiber attenuation and temperature effect
- Receiver: Detector sensitivity
- Others: Safety margin and repairs
When one of the above-listed variables fails to meet specifications, the performance of the network can be greatly affected or worse, the degradation can lead to network failure. Unfortunately, not all the variables can be controlled with ease during the deployment of the network or the maintenance stage; however, there exists one component—the connector—that is too-often overlooked, sometimes overused (test jumpers) but that can be controlled using the proper procedure.
Connectors are key components that interconnect the entire network elements, which is why maintaining them in good condition is essential to ensure that all the equipment operates to their maximum performance—to avoid catastrophic network failure. Since connectors are susceptible to damage that is not immediately obvious to the naked eye—the inspection phase is vital.
When proceeding with the inspection of connectors, there are two main components to inspect: the connector itself and the ferrule.
One of the advantages with connectors is that when connector failure occurs, it can be rapidly dealt with since its main cause is often traced to the end-face (also called the “ferrule”) or the mechanical section of the connector. Connector failure is most frequently the result of a dirty or damaged end-face. Figure 1 illustrates the parts of a SC-type fiber optic connector
In the connector, the element that holds the fiber and provides the alignment positioning is the ferrule. The ferrule is the part of the connector that connects the cable either to another cable, a transmitter or a receiver. Made of either glass, plastic, metal or ceramic, the ferrule is composed of three principal zones (see Figure 2 a): (1) Zone A, which is defined as the core of the fiber where the light travels; (2) Zone B, called the “cladding”, is the outer optical material surrounding the core that reflects the light into the core and (3) the buffer coating protects the fiber from damage and moisture as it surrounds the cladding and is normally made of plastic.
Since the core and cladding are the two main sections of the ferrule, it is critical that they be maintained in good condition—to minimize the loss that occurs when two ferrules are mated together. In order to properly carry out connector maintenance, the connector end-face must first be visually inspected. As shown in Table 1, the core diameter of a single-mode fiber is less than 10 microns, which means that without the proper inspection tool, it is impossible to tell if the ferrule is clean, making it essential to have the right tools.
To properly inspect the connector end-face, it is recommended to use a microscope that is specially designed for the fiber-optic connector end-face. There are many types of inspection tools on the market, but they all fall into two main categories: fiber inspection probes (also called “video fiberscopes”) and optical microscopes. When considering the purchase of an inspection tool, there are a number of points that need to be considered to ensure that the tool purchased suits the applications and connector types that will be inspected. Table 2 below lists the main characteristics of fiber inspection probes and optical microscopes:
When inspecting a connector ferrule, two types of problems can be encountered: a damaged end-face or a dirty end-face.
Physical damage to the connector end-face are, in general, permanent and will, in most cases, require a connector replacement—unless the damage is not detrimental to the endface. In order to determine whether the damage is detrimental or not, a good rule of thumb is to discard or replace any connector that has scratches near or across the fiber core (see Figure 5 a), since these scratches can generate high loss and affect the connector
performance. For physical damage, including chipped cladding (see Figure 5 b), worn connectors and/or excessive epoxy residue on the cladding, the connector must be replaced .
In an ideal world, free of contaminants, connector end-faces would always be clean and would not require in-depth maintenance; however, this is not the present reality, and many fiber-optic connector contaminants exist. For example, a 1 μm dust particle on a singlemode core can block up to 1% (0.05 dB loss) of the light—imagine what a 9 μm dust particle can do. Another important reason for keeping end-faces free of contaminants is the effect of high-power components on the connector end-face—some of today’s telecommunication components can produce signals with a power level up to +30 dBm (1 W), which can have catastrophic results when used with a dirty or damaged connector end-face (e.g., fiber fuse).
Dust, isopropyl alcohol, oil from hands, mineral oils, index matching gel, epoxy resin, oil-based black ink and gypsum are among the contaminants that can affect a connector end-face. Some of these contaminants are single soil or they may come in complex soil combinations. Note that each contaminant appears differently, but
regardless of its appearance, the most critical areas to inspect are the core and cladding regions—as contamination in these regions can greatly affect the quality of the signal. Figure 7 illustrates the end-face of different connectors that has been inspected with a video inspection probe.
Good practice for avoiding connector end-face damage or contamination is to always keep a protective cap on the unused connector—thereby stressing the importance of storing unused protective caps in a sealed container to prevent contamination. When inserting the protective cap on a ferrule, do not insert it all the way since small dirt particles can accumulate at the bottom of the cap and if the bottom of a contaminated cap comes into contact with the connector end-face, it can contaminate the connector end-face. Note that outgassing from the manufacturing process of the dust cap can leave a residue of the mold release agent or materials in the cap. Therefore, the presence of a dust cap does not guarantee cleanliness; it is a protective device to prevent damage. Another interesting fact about test jumper and connectors, which you take right out of the sealed bag from the supplier, is that they are not always clean before sealing of the bag and therefore will be dirty. Fortunately, using the proper cleaning tools and cleaning procedures can effectively clean a soiled connector.