Bending loss insensitive fiber

September 8, 2022
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Bending loss can be an important issue when optical fibers are installed in small spaces in the access and transmission parts of the network, so the ITU developed the G.657 standard, which defines the bending resistance of two types of optical fibers. Class A covers G.652 type optical fibers used in transmission and access networks, which can have a bend radius of 10 or 7.5mm. Class B covers fibers in the access network that may not be G.652 compliant, with low loss when bent to a radius of 7.5mm or 5mm.
Bend losses occur where single-mode fiber encounters bends or tight packing, such as inside cabinets, cable ducts, risers, and bulkheads. One way to limit losses is to reduce the mode field diameter to improve light confinement. Another approach is to embed a layer of lower refractive index glass as a recessed inner cladding next to the core, or as a "trench" within the cladding. Other options include embedding subwavelength holes or nanostructures in the core.
1. Optical fiber structure for reducing bending loss and improving light guide.
thinned fiber
Reducing the thickness of the fiber allows the fiber to be squeezed into smaller volumes and bent to smaller radii without inducing the formation of tiny cracks that could cause the fiber to break. It also allows more fiber to fit into the cable. There are two options: reduce the cladding and the protective layer overlying the cladding, or reduce the protective layer only.
2. How shrinking the cladding diameter changes the size of a 10µm core single-mode fiber.
Standard fiber has an outer diameter of 125µm, which is thick compared to the 10µm core of single-mode fiber. The cladding diameter can be reduced to 80µm, which reduces the glass volume of the fiber by a factor of 2.4. The outer diameter of the shrink-clad fiber with plastic coating is about 170µm, while the outer diameter of ordinary coated fiber is 250µm.
In addition, the coating thickness applied over the standard 125µm cladding can also be reduced, so that the diameter of the coated fibers is only 200µm instead of the usual 250µm.
low water fiber
Standard fiber fabrication leaves traces of hydrogen, which combines with oxygen in fused silica fibers to form hydroxyl groups, absorbing between 1360 and 1460 nm, with a strong peak at 1383 nm. This band is negligible when the fiber optic system operates only in the 1310 and 1550nm bands, but becomes a problem for cheap coarse wavelength division multiplexing with 20nm spacing between 1270 and 1610nm.