The main job of the coating is to protect the glass fiber, but this goal has many complicated problems. Glass fiber’s performance and protection have been optimized by a carefully formulated coating material.
1. Coating Function
For standard-sized fibers with a cladding diameter of 125 µm and a cladding diameter of 250 µm, the polymer cladding accounts for 75% of the three-dimensional fiber volume. The core and cladding glass account for the remaining 25% of the total volume of the coated optical fiber. Coatings play a key role in helping optical fibers meet environmental and mechanical specifications, as well as some optical performance requirements.
If the fiber is stretched without being coated, the outer surface of the glass cladding will be exposed to air, moisture, other chemical contaminants, nicks, collisions, abrasion, small bends, and other hazards. These phenomena can cause defects on the glass surface. Initially, these defects may be small or even microscopic, but over time, the applied stress, and exposure to water, will become larger cracks and eventually lead to failure.
In other words, even using the most advanced manufacturing processes and the highest quality materials, it is impossible to produce fibers that are completely defect-free. Fiber manufacturers do their best to process preforms and control drawing conditions to minimize the size and distribution of defects. In other words, there will always be some tiny defects, such as nano-scale cracks. The function of the coating is to protect the glass surface and protect the glass surface from damage by external factors, such as handling and abrasion.
Therefore, all fibers have a protective coating when stretched. Between the fiber leaving the bottom of the performing furnace and entering the first coated cup on the drawing tower, the uncoated fiber only has a short span on the drawing tower. This uncoated interval is just enough to allow the fiber to cool down for the coating.
2. Coating Dimensions
As mentioned above, most standard communication fibers have a cladding diameter of 125µm and a UV-curable acrylate polymer coating to increase their outer diameter to 250µm. A typical acrylic coating consists of a two-layer system, with the inner layer being softer than the outer layer, and the outer layer is harder. Recently, some companies have developed 200-μm or even 180-μm cladding diameter communication fibers for high-density and high-count cables. This development means that the coating is thinner, but it also means that the coating must have different bending and mechanical properties.
On the other hand, depending on the types of specialty fibers and their applications, specialty fibers have more varieties in terms of fiber size, coating diameter, and coating materials. The glass-clad diameter of the special fiber can be less than 50µm to more than 1000µm (1mm). This list is also based on a wide range of fiber applications and coating materials. Some coatings may be as thin as 10 microns, while others are hundreds of microns thick.
Some special fibers use the same acrylic coating as communication fibers. Other companies use different coating materials to meet the requirements of sensing, harsh environments, or as secondary coatings. Examples of non-acrylate specialty fiber coating materials include carbon, metals, nitrides, polyimides and other polymers, sapphire, silicon, and complex compositions of polymers, dyes, fluorescent materials, sensing reagents, or nanomaterials. Some of these materials, such as carbon and metals, can be applied in thin layers, supplemented by other polymer coatings.
At present, the annual output of communication fibers is close to 500 million fiber kilometers, and UV-curable acrylic coatings account for the vast majority of optical fiber coatings (maybe more than 99%). In the acrylic coating family, the main suppliers provide different types of stretching tower curing systems, environmental requirements, and optical and mechanical properties, such as optical fiber bending specifications.
3. The key performance of optical fiber coating
Important parameters of the coating include:
1) Modulus
Modulus is also called “Young’s modulus” or “modulus of elasticity”, sometimes referred to simply as “e”. This is a measure of hardness, usually expressed in MPa. Modulus can be as low as a single digit for primary coatings. The secondary coating can reach more than 700mpa.
2) Refractive Index
The refractive index is the speed at which light passes through a material, expressed as the ratio of the speed of light in a vacuum. A telecom fiber coating (such as that used by DSM) has a refractive index of between 1.47 and 1.55. DSM and other companies also provide low refractive index coatings, usually used with specialty fibers. The refractive index changes with temperature and wavelength, so the refractive index of the coating is usually reported at a specific temperature, such as 23°C.
3) Temperature Range
The temperature range is usually from -20°C to +130°C, and many widely used uv-curable acrylates for telecommunication optical fibers. The higher range is suitable for harsh environments. It can be used with other coating materials, such as polyimide or metal, if the temperature exceeds +200°C.
4) Viscosity and curing speed
Viscosity and curing speed are related to the characteristics of the coating film. These properties are also related to temperature. The control of coating parameters is an important job for drawing engineers, including the control of coating temperature.
5) Adhesion and resistance to delamination
Adhesion and delamination resistance are important characteristics to ensure that the primary coating does not separate from the glass cladding and the secondary coating does not separate from the primary coating. TIA FOTP-178 “Coating Peeling Force Measurement” is a standardized test procedure for measuring the peeling resistance of coatings.
6) Strip-ability
Strip-ability is essentially the opposite of delamination resistance-you don’t want the coating to fall off when using fibers, but you do want to be able to remove short-length coatings during processes such as splicing, installing connectors, and making fusion splices. In this case, the technician will use a special tool to control the length.
7) Micro-bending performance
Micro-bending performance is a situation where the coating is crucial in helping the glass fiber maintain its optical properties, especially attenuation and polarization properties. Slight bends are different from large bends that are visible to the naked eye. The bending radius of a large bend is measured in millimeters. The bending radius of the micro-bend is several hundred microns or less. These bends may occur during manufacturing operations, such as cables, or when the fibers come into contact with microscopic irregular surfaces. In order to minimize the problem of micro-bending, paint manufacturers have developed a system of low modulus primary coating and high modulus secondary coating. There are also standardized tests for micro-bending, such as TIA FOTP-68 “Optical Fiber Micro-bending Test Procedure”.
8) Abrasion resistance
For some special optical fiber applications, wear resistance is critical, and most communication optical fibers get extra protection from buffer tubes and other cable components. The technical article describes different perforation and abrasion resistance tests. For applications where this is a critical parameter, the fiber or coating manufacturer can provide detailed information on the test method.
4. Tensile strength
The key strength parameter of the fiber is the tensile strength, which is the ability of the fiber to resist breaking when it is pulled. The units are Pascals (MPa or GPA), pounds per square inch (KPSI), or Newtons per square meter (N/m2). All fibers have been tested to ensure that they meet the minimum tensile strength. After being stretched and coated, the fiber is passed through a testing machine and a predetermined fixed tensile load is applied to the fiber. The size of the load is determined by the optical fiber specifications, especially in the case of most communication optical fibers, by international standards.
In the verification test, due to some defects in the glass, the fiber may break at a point in a weak area. In this case, the fiber that passed the test device before breaking passed the verification test. Has the smallest tensile strength. The broken fibers are also sieved through the machine in the same way. One problem is that such breakage affects the continuous length of fiber stretch. This may be a problem for some special fiber applications, such as gyroscopes with polarization-maintaining fibers, where the connectors are unacceptable. Fractures can also reduce the output of fiber manufacturers. An excessive number of breaks may indicate other problems in the performing and stretching process2.
How does the coating affect the tensile strength? A typical coating cannot increase the strength of the fiber. If the defect is large enough to cause a fracture in the verification test, the coating cannot prevent the fracture. But as mentioned earlier, glass has unavoidable defects, which are small enough to allow the fiber to pass the test. This is what the coating does-it helps the fiber maintain its minimum strength during its lifetime. The coating protects tiny defects from external factors and other hazards, preventing the defects from becoming large enough to cause fiber breakage.
There are some tests to describe how the coated fiber will withstand changes in tensile load. The data from these tests can be used to simulate life performance. One of the standardized tests is TIA-455 “FOTP-28 measures the dynamic strength and fatigue parameters of optical fibers through tension.” The description of the standard says: “This method tests the fatigue behavior of the fiber by changing the strain rate.
A dynamic tensile test such as FOTP 28 is destructive. This means that the fiber segment used for testing cannot be used for any other purpose. Therefore, such a test cannot be used to characterize the fibers of every preform. Instead, these tests are used to collect data on specific fiber types in specific environments. The test results are considered applicable to all specific types of fibers, as long as the same materials and processes are used in the manufacturing process.
A parameter derived from the dynamic tensile strength test data is called “stress corrosion parameter” or “n value”. It is calculated from measurements of applied stress and failure time. The n value is used for modeling to predict how long it will take for fiber to fail under certain circumstances when it is stressed. The test is performed on coated fibers, so the value of n will vary with different coatings. The coating itself does not have an n value, but the n value data of the fiber of a specific coating can be collected and reported by the coating supplier.
5. Coating characteristics and specialty fibers
When choosing a coating material, what are the most important parameters? The answer depends on what kind of fiber you are making and its use. Telecom fiber optic manufacturers use a two-layer system to optimize high-speed stretching, high strength, and excellent micro-bending performance. On the other hand, telecommunications fiber does not require a low refractive index.
For special fibers, coating specifications vary greatly with fiber type and application. In some cases, strength and mechanical properties-high modulus and high value are more important than the refractive index. For other special fibers, the refractive index may be the most important. Below are some comments on coating considerations for special fibers.
6. Rare earth doped fiber for fiber laser
In some fiber lasers, the primary coating serves as the secondary cladding. The goal is to maximize the optical pump power coupled to the fiber. For fiber lasers, the emission of pump power to the cladding helps stimulate the gain region of the fiber’s doped core. The low refractive index coating allows the fiber to have a higher numerical aperture (NA), which means that the fiber can accept more pump power.
These “double-clad” fibers usually have a hexagonal or octagonal glass cladding, followed by a circular low-index polymer secondary cladding. The glass cladding is formed by flattening the sides on the preform and then applying a low-index coating/secondary cladding on the puller. Because this is a low-index coating, a harder outer coating is also needed. A high refractive index outer coating helps the fiber meet strength and bending requirements
7. Power transmission fiber
In addition to rare-earth-doped fibers for lasers, there are other special fibers in which low-refractive-index coatings can be used as cladding to improve optical performance. For example, some medical and industrial laser systems use large core fibers to provide laser power, such as for surgery or material processing. Like doped fiber lasers, low-index coatings help increase the NA of the fiber, allowing the fiber to receive more power. Note that fiber delivery systems can be used with many types of lasers-not just doped fiber lasers.
Polarization maintains fiber. PM fiber represents a category, and there are multiple fibers designed for multiple applications. For example, some PM fibers contain rare earth dopants for fiber lasers. In these cases, a low refractive index coating can be used as the secondary cladding, as described above.
Other PM fibers are designed as tight coils for use in gyroscopes, hydrophones, and other sensors. In these cases, the coating may need to meet environmental requirements, such as the low-temperature range, and the strength and micro-bending requirements associated with the winding process.
For some interferometric sensors, such as gyroscopes, one goal is to minimize crosstalk, that is, to minimize the coupling of power from one polarization mode to another. In wound coils, soft coatings help avoid crosstalk and micro-bending problems, so a low modulus primary coating is specified. A harder secondary coating is specified to address the mechanical risks associated with entangled fibers. For some sensors, the fiber must be tightly wound under high tension, so strength requirements are critical in the secondary coating.
In the case of another PM fiber, some gyros require small diameter fibers so that more fibers can be wound into a compact “puck”, a cylindrical shell. Specifically, the fiber used in this case has an outer (cladding) diameter of 80 μm and a cladding diameter of 110 μm. In order to achieve this, only one layer of paint is needed-that is only one layer. Therefore, this coating must strike a balance between the softness required to reduce cross-talk and the hardness required for protection.
Another consideration for PM fibers is that the fiber coils are often encapsulated in a sealed package with epoxy resin or other materials. This places additional requirements on the temperature range of the coating and its stability when in contact with other chemicals.
8. Conclusion
The idea of providing a “perfect” coating for any particular type of fiber is impossible, if not impossible. As a result, various parameters are taken into consideration, including refractive index, modulus, temperature performance, and draw tower requirements, when selecting a coating composition. In order to solve the mixing needs, paint manufacturers are conducting research and development plans to improve the performance of their resins and achieve a better balance of multiple parameters.
