The process of stripping the fiber plays a vital role in the long-term reliability of the fiber, and more importantly, errors created during the stripping process are often not detectable by subsequent production line tests/measurements! Even if an effective termination procedure has been developed, operator error on the production line is likely to produce a product that will pass all normal product testing and inspections, but the chance of field failures in the future will greatly increase. Therefore, while important at any manufacturing stage, strong process control is essential for all termination processes to ensure maximum product reliability and performance.
Strip a typical fiber through the following steps:
The fiber is stripped and cleaned. Prepare the epoxy and inject it into the ferrule assembly (press the ferrule into the ferrule holder) Put the ferrule (with epoxy and fiber) into the curing oven for a while to cure the epoxy .
Fiber Stripping: Fibers must be stripped of all protective coatings before inserting into a connector. Mechanical stripping, by far the most common method, involves using a tool with a sharp, precision-machined metal cutting blade to cut into the protective layer and strip it away from the fiber. It is important that the blade cuts clean and neat edges in the protective coating and that the blade does not touch or scratch the bare fiber during stripping.
If the blade becomes dull, the edges of the coating at the peel boundary may be rough and jagged. When the fiber is inserted into the ferrule holder/ferrule assembly, this rough coating edge creates lateral forces on the fiber (resulting in reduced product performance). It is best to cut the stripped edge at a clean angle and at a 90 degree angle to the fiber so that the edge is evenly butted against the back of the ferrule during potting.
optical fiber
Any nicking of the fiber during the stripping process creates a very weak weak point in the fiber. The transition point where the protective coating terminates and strips the fiber begins is particularly vulnerable and is always the weakest part of the product's entire fiber length. In most cases, this happens on the production line or in the field.
Damage to the fiber during stripping may not always result in immediate fiber breakage. Damaged fibers are likely to remain unbroken during production line processing. Unless the weakened fiber is broken during production, it is impossible to measure whether the fiber is damaged. Products with damaged fibers may pass all standard production tests (visual inspection, IL/RL, etc.) but are highly susceptible to future field failures. Controlling the quality of the stripper blade and stripping process is critical to ensuring maximum product life.
Fiber Cleaning: In order for the epoxy to properly bond to the fiber, the fiber must be clean and free of oil or other contaminants. This is usually done by wiping the bare fiber with a soft, lint-free cloth and good quality alcohol. Correct cleaning materials and techniques must be identified and strictly adhered to in the production line, as the effectiveness of cleaning cannot be confidently verified in subsequent process steps/tests.
fiber optic line
Epoxy Preparation and Curing: The epoxy resins used in the production of fiber optic assemblies are carefully selected for their adhesive properties after curing - they maintain a strong fiber/ferrule bond over the life of the fiber optic assembly. During the life of a connector, they are exposed to various environmental and mechanical stresses (changes in temperature and humidity, forces mating with another connector, etc.). Properly cured solid epoxy bonds help ensure that the fiber does not move within the ferrule, maintaining a proper fiber-to-fiber connection regardless of such stresses.
For maximum cured adhesion properties of a two-part epoxy, the two parts must be thoroughly mixed and cured at the appropriate temperature and time. Improperly mixed or undercured epoxy will generally not continue to cure at room temperature - the epoxy may appear to be fully cured and will provide enough adhesion to the product to complete the assembly process - the product will likely pass all subsequent Visual inspection and IL/RL testing. But it will never have the long-term properties of choosing epoxy in the first place.
The only way to verify that the epoxy process is suitable for the product is through destructive product testing during process development. And, since destructive testing is not possible in the production line, very robust process controls must be used in the production line to ensure that acceptable processes are followed accurately. Likewise, the proper mixing process or proper application of curing heat or curing time cannot be verified after the fact.
Epoxy Outgassing: After mixing the two parts of the epoxy, it is likely that many small air bubbles will be filled. These air bubbles must be removed prior to injection into the ferrule (production lines typically use centrifuges or vacuum chambers to remove gas/air bubbles trapped in the epoxy after mixing). Any air bubbles or voids trapped in the cured epoxy can cause excessive stress to the fiber during the life of the product (most commonly due to temperature changes, which will cause the gas trapped in the air bubbles to expand more than the surrounding epoxies are much larger). Such stress can lead to significant performance degradation and even product failure. Again, it is impossible to detect the presence of such air bubbles within the gasket after the product has cured.
Injecting epoxy into the gasket: Epoxy is usually injected into the back of the gasket until a small bead of epoxy is observed to flow out of the gasket tip. This is an effective method to ensure that the entire ferrule hole is filled with epoxy before inserting the fiber.
Often overlooked, however, is the importance of controlling the amount of epoxy remaining behind the ferrule (in the ferrule seat). This is the weakest part of the entire fiber length: the transition area where the protective coating is stripped from the bare fiber. Crucially, the entire transition area must be fully encapsulated in epoxy. The epoxy provides strength to this area, which is very fragile and prone to breakage.
Too much epoxy in the ferrule holder can cause other problems, for example, an inserted fiber will displace the injected epoxy, allowing it to flow out of the back of the ferrule holder. This can cause epoxy to flow over the springs within the connector body, locking the springs and causing damage to the connector. However, these conditions are observable/testable after curing.
Too little epoxy inside the gasket to see/test after curing. Therefore, processes and controls must be implemented to ensure a consistent and adequate amount of epoxy inside the ferrule assembly. Pneumatic epoxy dispensing systems are very common and work well. However, the amount of epoxy dispensed with this system will vary depending on the epoxy viscosity, which will vary with the pot life of the epoxy. These systems are effective, but only if the production process takes into account this change in epoxy and has proper processes and controls in place to ensure that the proper amount of epoxy has been injected.
