How does fiber resist bending?

(i) Bending in application

Fiber is an important long-distance signal transmission medium, and flexibility is a great advantage of fiber itself, but is it really "flexible"?

Fiber is thin enough to be "bendable" as long as it does not break. However, small bend radius (macro bend loss) can cause signal loss, so from a performance perspective, fiber cannot be bent to very small radius. Installing fiber optic cable requires experienced engineers and careful handling to   avoid small bends along the fiber path.

                      Possible bending situations encountered when installing fiber optics

(ii) Macrobending decay of optical fibers

Macrobending loss of optical fiber: Macrobending loss is the loss caused by the bending along the fiber axis. The beam propagates in the direction of the fiber axis at a critical angle to the core/packet interface; when the beam hits the core/packet boundary in the bent part of the fiber the propagation angle formed is greater than the critical value, the result is that the total internal reflection condition is not met in the bent fiber and part of the beam escapes from the core of the fiber.

Therefore, optical power loss occurs when the fiber is bent, and the optical power transmitted to the other end is less than the optical power emitted from the light source into the fiber (one of the most significant causes of total attenuation as light travels through the fiber).

Bending an optical fiber not only changes the optical transmission characteristics of the fiber, but also changes the mechanical properties. To avoid this, the installer must take certain precautions when bending the fiber. The rule of thumb for minimum bend radius is: for long-term applications, the bend radius should exceed 150 times the fiber cladding diameter; for short-term applications, it should exceed 100 times the cladding diameter. Silicon dioxide fiber cladding diameter is usually 125um, so these two values are 19mm and 13mm respectively. (c) Designing several fiber structures to reduce fiber bending performance Care is required for fiber manipulation, and in addition the design flexibility of the fiber is a special fiber (Specialty Optical Fibres ) is a major feature. Let's understand which cross-sectional designs can reduce the effect of bending on the fiber.

                                 This diagram covers several common types of bend-resistant fiber designs today

1. Reducing themode field diameter: This design is more intuitive, where the core is reduced and the refractive index of the core is increased, allowing the beam to be better bound in the fiber.

2. Reducing the cladding diameter: the diameter of the fiber is reduced to increase the resistance to bending. As mentioned before, the size of the fiber needs to be taken into account during installation (for long-term applications, the bend radius should be more than 150 times the fiber cladding diameter; for short-term applications, it should be more than 100 times the cladding diameter). The diameter of bend-resistant fibers has now been reduced from 125 microns to 80 microns, and even 60 micron OD fibers are available.

3. lowering the cladding refractive index (Depressing the cladding)

4. Adding a low index trench to the cladding (Adding a low index trench). Actually, it is somewhat similar to increasing the refractive index of the fiber core.

These designs are incremental optimizations of existing processes and can achieve limited macrobending resistance. This later one does not utilize chemical doping techniques, but rather adds symmetric holes around the core.

5. Addinga ring of symmetric holes within the cladding: Hole assisted fiber (HAF) has a very different waveguide structure compared to fibers fabricated using chemical doping techniques. Although Hole assisted fiber (HAF) is insensitive to bending, long-haul fibers are very expensive to manufacture and relatively difficult to fuse, as well as being incompatible with existing conventional standard devices.

6. nano-bubble-assisted bend-resistant fiber: This new fiber design (nano- structures) shows superior bend performance, meets the difficulty of installation in fiber-to-the-home, and is also relatively easy to mass-produce and fusion splice compatible. This design consists of a normal germanium-doped core with an additional layer of nano-structured rings (bubbles ranging from a few nanometers to several hundred nanometers) within the cladding.

In addition to communication applications, sensing applications of optical fibers are more demanding in terms of bending requirements, as fibers are often wound into various structures. Also bend-resistant fibers are not only symmetrical single-mode fibers; polarization-preserving fibers also have bending performance requirements (more to be communicated later).