developed continues the most recent series on optical fiber manufacturing processes, providing an overview of coatings for a wide range of standard communication and specialty optical fibers. The main job of films would be to safeguard the glass fiber, but there are many intricacies to this objective. Coating materials are carefully formulated and tested to optimize this protective role as well as the glass fiber overall performance.
To get a regular-dimension fiber having a 125-µm cladding diameter as well as a 250-µm covering size, 75Percent in the fiber’s 3-dimensional volume is the polymer covering. The core and cladding glass take into account the other 25% from the covered fiber’s total volume. Films play a key part in helping the fiber meet environmental and mechanical specifications as well as some optical overall performance requirements.
If a fiber were to be drawn and not coated, the external surface of the glass cladding would be exposed to air, dampness, other chemical contaminants, nicks, protrusions, abrasions, microscopic bends, along with other risks. These phenomena can result in imperfections in the glass surface. Initially, such defects may be small, even microscopic, but with time, applied anxiety, and contact with water, they can become bigger breaks and ultimately lead to malfunction.
That is certainly, even with state-of-the-artwork production processes and top-high quality components, it is far from easy to create FTTH cable production line with simply no imperfections. Fiber producers go to excellent measures to procedure preforms and manage pull problems to minimize the defect sizes and their distribution. That said, there will almost always be some microscopic imperfections, like nanometer-scale breaks. The coating’s work is to preserve the “as drawn” glass surface and protect it from extrinsic factors which could damage the glass surface including handling, abrasion and so on.
Therefore, all fiber gets a defensive coating when it is driven. Uncoated fiber happens for only a quick period around the draw tower, between the time the fiber exits the base of the preform your oven and gets into the very first coating mug on the pull tower. This uncoated interval is just long enough for the fiber to cool so the coating can be applied.
As observed above, most standard communication fibers have a 125-µm cladding diameter along with a Ultra violet-cured acrylate polymer covering that boosts the outside diameter to 250 µm. Typically, the acrylic covering is a two-coating coating “system” having a softer internal layer called the main coating as well as a tougher external layer referred to as secondary coating1. Lately, some companies have developed interaction fibers with 200-µm or even 180-µm covered diameters for packed high-count cables. This development means thinner coatings, but it also indicates the coating will need to have different bend and mechanical qualities.
Specialty fibers, on the other hand, have several much more versions in terms of fiber size, coating size, and coating components, dependant upon the form of specialty fiber and its application. The glass-cladding size of specialized fibers can range from lower than 50 µm to greater than 1,000 µm (1 mm). The amount of coating on these fibers also shows a large range, based on the fiber application and the coating materials. Some films may be as thin as 10 µm, yet others are many 100 microns thick.
Some specialty fibers use the exact same acrylate films as interaction fibers. Other people use various covering components for specifications in sensing, harsh environments, or becoming a supplementary cladding. Examples of non-acrylate specialty fiber coating materials consist of carbon, metals, nitrides, polyimides and other polymers, sapphire, silicon, and complicated compositions with polymers, chemical dyes, luminescent materials, sensing reagents, or nanomaterials. A few of these components, including carbon dioxide and steel, can be used in slim levels and supplemented with other polymer coatings.
With communication fibers currently being produced at levels close to 500 million fiber-km each year, the UV-treated acrylates signify the huge majority (probably a lot more than 99%) of all the coatings applied to optical fiber. Inside the group of acrylate films, the main suppliers offer several versions for many different draw-tower curing systems, ecological requirements, and optical and mechanical performance qualities, such as fiber twisting specifications.
Key properties of optical fiber coatings
Important guidelines of films include the following:
Modulus is also called “Young’s Modulus,” or “modulus of suppleness,” or sometimes just “E.” This is a measure of hardness, usually reported in MPa. For main coatings, the modulus can be in solitary digits. For supplementary films, it can be more than 700 MPa.
Index of refraction is the velocity in which light goes by from the material, expressed as a proportion to the velocity of light within a vacuum. The refractive index of commonly used Sheathing line from significant suppliers such as DSM ranges from 1.47 to 1.55. DSM along with other companies also provide lower index coatings, which can be used in combination with specialty fibers. Refractive index can vary with heat and wavelength, so covering indexes usually are reported in a particular heat, such as 23°C.
Temperature range usually extends from -20°C to 130°C for lots of the widely used UV-treated acrylates combined with telecom fibers. Greater can vary are for sale to severe surroundings. Can vary extending above 200°C can be found with other coating materials, like polyimide or metal.
Viscosity and cure speed issue coating characteristics when being applied to the draw tower. These properties are temperature centered. It is necessary for the draw engineer to regulate the covering guidelines, which include control over the covering heat.
Adhesion and potential to deal with delamination are important characteristics to ensure that the primary coating will not apart from the glass cladding which the supplementary covering fails to outside of the main coating. A standardized test process, TIA FOTP-178 “Coating Strip Force Measurement” is used to appraise the effectiveness against delamination.
Stripability is essentially the opposite of effectiveness against delamination – you may not want the covering in the future off whilst the fiber is in use, but you will want so that you can remove brief lengths from it for methods including splicing, installation connectors, and making merged couplers. In such cases, the technician strips away a controlled length with special tools.
Microbending performance is a case where the coating is critical in aiding the glass fiber maintain its optical properties, specifically its attenuation and polarization performance. Microbends are different from macrobends, that are visible using the nude eye and possess flex radii measured in millimeters. Microbends have flex radii around the order of hundreds of micrometers or less. These bends can happen throughout manufacturing procedures, including cabling, or when the fiber contacts a surface with tiny irregularities. To reduce microbending issues, coating manufacturers have developed systems integrating a small-modulus main coating as well as a higher-modulus supplementary covering. There are also standardized tests for microbending, such as TIA FOTP-68 “Optical Fiber Microbend Test Procedure.””
Abrasion resistance is critical for many specialty fiber applications, while most communication fiber gets additional defense against buffer pipes along with other cable elements. Technical posts explain various assessments for pierce and abrasion resistance. For applications in which this is a critical parameter, the fiber or covering manufacturers can offer particulars on check techniques.
The key power parameter of fiber is tensile power – its effectiveness against breaking up when becoming pulled. The parameter is indicated in pascals (MPa or GPa), lbs per square ” (kpsi), or Newtons per square meter (N/m2). All fiber is proof tested to assure it meets a minimum tensile power. Right after being drawn and coated, the fiber is run through a evidence-testing machine that places a pre-set repaired tensile load in the fiber. The volume of load depends on the fiber specifications or, particularly in the case of the majority of communication fibers, by international specifications.
Throughout evidence testing, the fiber may break with a point using a weak area, because of some flaw within the glass. Within this case, the fiber that went through the screening equipment prior to the break has gone by the proof test. It provides the minimum tensile power. Fiber following the break is also approved through the machine and screened in the same fashion. One problem is that such breaks can change the continuous period of fiber driven. This can become a problem for a few specialized fiber programs, including gyroscopes with polarization-sustaining fiber, in which splices usually are not acceptable. Smashes also can lower the fiber manufacturer’s yield. As well as an excessive number of smashes can suggest other problems within the preform and draw processes2.
Just how do films impact tensile strength? Common coatings cannot improve a fiber’s strength. When a defect is large enough to cause a break during proof screening, the coating are not able to prevent the break. But as observed previously, the glass has unavoidable imperfections which can be small enough to permit the fiber to pass through the proof test. Here is where films use a part – improving the fiber sustain this minimal strength more than its life time. Coatings do that by safeguarding minor flaws from extrinsic aspects and other risks, preventing the flaws from becoming large enough to result in fiber breaks.
You will find assessments to characterize just how a coated fiber will withstand alterations in tensile loading. Data from this kind of tests can be employed to design lifetime performance. One standard test is TIA-455 “FOTP-28 Measuring Powerful Power and Exhaustion Parameters of Optical Fibers by Stress.” The standard’s description states, “This technique tests the fatigue behavior of fibers by varying the stress price.”
FOTP 28 along with other powerful tensile tests are destructive. This means the fiber segments utilized for the tests can not be employed for anything else. So this kind of tests are not able to be utilized to characterize fiber from each and every preform. Rather, these tests are employed to gather information for particular fiber kinds in particular environments. The exam effects are considered applicable for many fibers of a particular kind, as long as the exact same materials and procedures are utilized in their manufacturing.
One parameter produced from powerful tensile power test data is referred to as “stress corrosion parameter” or perhaps the “n-value.” It is actually calculated from dimensions in the applied stress and also the time and energy to failure. The n-value is used in modeling to calculate how long it will require a fiber to fall short when it is below anxiety in certain surroundings. The testing is done on covered fibers, and so the n-principles will be different with various coatings. The coatings themselves do not have an n-worth, but data on n-values for fibers with specific coatings can be gathered and reported by coating providers.
Covering characteristics and specialized fibers
What is the most important parameter when deciding on coating components? The solution depends on what kind of fiber you happen to be making along with its application. Telecom fiber manufacturers use a two-layer system optimized for top-velocity draw, higher strength, and superior microbending performance. In the other hand, telecom fibers usually do not need a low index of refraction.
For specialized fibers, the coating specifications differ significantly with the type of fiber as well as the application. Sometimes, strength and mechanised performance-higher modulus and n-worth – are more essential than directory of refraction. For other specialized fibers, directory of refraction may be most significant. Here are some comments on coating considerations for chosen types of specialized fibers.
Rare-earth-doped fiber for fiber lasers
In a few fiber lasers, the main covering works as a supplementary cladding. The aim would be to take full advantage of the amount of optical pump power combined into fiber. For fiber lasers, water pump energy launched into the cladding helps stimulate the acquire area in the fiber’s doped primary. The low index coating provides the fiber an increased numerical aperture (NA), which suggests the fiber can accept a lot of the water pump power. These “double-clad” fibers (DCFs) usually have a hexagonal or octagonal glass cladding, then this circular low-index polymer secondary cladding. The glass cladding is formed by milling flat sides onto the preform, and then the reduced-directory covering / supplementary cladding is used in the pull tower. Since this is a reduced-directory covering, a tougher external coating is also essential. The top-index external covering assists the fiber to satisfy power and bending specifications
Fibers for energy shipping
In addition to rare-planet-doped fibers for lasers, there are more specialized fibers when a low-directory covering can serve as being a cladding coating and improve optical overall performance. Some medical and commercial laser beam techniques, for example, use a large-primary fiber to offer the laser beam energy, say for surgical treatments or material processing. Similar to doped fiber lasers, the low-index covering serves to increase the fiber’s NA, allowing the fiber to accept more power. Note, fiber delivery techniques can be applied with many types of lasers – not just doped fiber lasers.
Polarization-sustaining fibers. PM fibers represent a category with cable air wiper for several programs. Some PM fibers, for instance, have uncommon-planet dopants for fiber lasers. These cases may utilize the reduced-index coating being a secondary cladding, as explained above. Other PM fibers are intended to be wound into tight coils for gyroscopes, hydrophones, as well as other detectors. In these cases, the coatings may must meet environmental requirements, such as reduced heat can vary, as well as power and microbending specifications linked to the winding process.
For a few interferometric detectors such as gyroscopes, one goal is to reduce crosstalk – i.e., to minimize the amount of power coupled from one polarization mode to another. Inside a wound coil, a smooth coating helps steer clear of crosstalk and microbend problems, so a low-modulus main covering is specified. A harder supplementary coating is specific to address mechanised risks ictesz with winding the fibers. For a few detectors, the fibers should be firmly covered below high stress, so power requirements can be critical in the supplementary coating.
In another PM-fiber case, some gyros require small-size fibers to ensure that more fiber can be wound into a lightweight “puck,” a cylindrical real estate. In this particular case, gyro makers have specific fiber with the 80-µm outdoors (cladding) diameter along with a covered size of 110 µm. To accomplish this, a single covering is utilized – that is certainly, just one coating. This coating therefore must balance the softness necessary to reduce go across speak from the hardness needed for safety.
Other things to consider for PM fibers are that this fiber coils frequently are potted with epoxies or any other materials inside a sealed bundle. This can location extra requirements in the coatings in terms of temperature range and balance below contact with other chemical substances.