The strength of optical signals transmitted through a fiber can be degraded due to various factors like absorption, scattering, bending loss, etc. Fiber bending losses are the radiative losses that occur in optical fibers due to its bending as shown in figure 1. It occur when the fiber optic cable is bent too tightly or too sharply, causing some of the light to escape from the fiber core. This results in a loss of signal strength and a decrease in overall performance.

Figure 1: Bending loss in an optical fiber

Optical fibers confine and transmit optical signals in core region by total internal reflection, which occurs above certain critical angle for a given fiber. Bending a fiber can change the incident angle at core-cladding interface to be less than the critical angle. This causes the light propagating in a core region to be coupled to a cladding region or to be transmitted out of the fiber through cladding depending on the bend radius. While some bend losses are unavoidable, it is important to minimize them as much as possible to ensure the best possible performance.

The degree to which fiber optic cables are susceptible to bend losses depends on a number of factors. These include the diameter of the fiber core, the wavelength of the light being transmitted, and the material properties of the fiber itself. In general, smaller diameter fibers are more susceptible to bend losses than larger diameter fibers because they have a smaller area for the light to travel through, which increases the amount of light that comes in contact with the fiber's cladding. And also, shorter wavelengths of light are more susceptible to bend losses than longer wavelengths as they have a higher refractive index and experience more bending when traveling through the fiber.

There are several different types of fiber bend losses, including macro-bending losses, micro-bending losses, and radiation losses.

Macro-bending losses

Figure 2: Macro Bending Loss

Macro-bending losses occur when the fiber is bent over a large radius, such as around a corner or a tight bend. It happens when the fibers are bent to a degree that exceeds the critical angle. In this case, the bend radius is much larger than the diameter of the fiber. A biggest problem with macro bending loss is that it can be difficult to detect. In some cases, the fiber may look perfectly fine, but still be bent at a large radius to cause significant signal loss. This can result in a decrease in network performance or even complete signal failure. The higher order modes are not as tightly bound to the fiber core as the lower order modes, thus they will be the first to radiate out of the fiber. Macro bending loss is depicted in figure 2 and figure 3 shows the graph between bend radius and macro bending loss.

Figure 3: Graph depicting bend radius versus macro bending loss

Micro-bending losses

Figure 4: Micro Bending Loss

Micro bends are tiny bumps or curves that happen in the radius of curvature of the fiber axis. They can be caused by unevenness when making the fiber or from uneven pressure when the fiber is put together. Micro-bending losses occur when the fiber is subject to small, repetitive bends, such as those caused by vibration or tension. Here, the bend radius is comparable to the diameter of the entire optical fiber. i.e., fiber is having sharp bends. This can happen as a result of poor cabling or manufacturing defects. Micro bending causes more signal loss because the fiber curves repeatedly, which makes energy transfer between different modes in the fiber again and again. Micro bending loss is illustrated in figure 4.

Radiation losses

Figure 5: Radiation Loss

Radiation losses occur when light leaks out of the fiber due to physical defects or impurities in the material. It occurs when the light in the fiber core encounters a change in refractive index, such as a boundary between the core and cladding or the interface between the cladding and surrounding medium. When the angle of incidence of the light exceeds a critical angle, the light will be emitted from the fiber core and into the cladding or surrounding medium. This results in a loss of light energy and a decrease in signal strength. It depends on the core diameter, the numerical aperture of the fiber, and the refractive indices of the core, cladding, and surrounding medium. Larger core diameters and higher numerical apertures will result in higher levels of radiation loss. Figure 5 shows the radiation loss in a fiber.

Different ways to reduce fiber bend losses

To minimize fiber bend losses, it's important to follow certain best practices when installing and handling fiber optic cables. One of the most important things to do is to avoid bending the cable beyond its minimum bend radius. Every fiber optic cable has a minimum bend radius, which is the smallest radius that the cable can be safely bent without causing damage. Going beyond this radius can cause significant bend losses and may even damage the cable itself.

Fibers with a larger core diameter are used to avoid macro-bending losses. This allows the light to travel through the fiber with a greater margin of error, reducing the amount of signal loss caused by bending. Another approach is to use specialized fiber designed specifically to resist bending, such as bend-insensitive fiber.

Excessive tension or pulling on the cable is to be avoided, as this can cause micro-bending losses. Also, it is important to use high-quality cables and connectors that are designed to minimize bend losses. Finally, the fiber optic cables are to be handled carefully to avoid kinking or crushing the cable during installation or maintenance. To reduce micro-bending losses, cover the fiber with a compressible jacket. When pressure is put on it, the sleeve will bend, but the fiber inside will stay straight.

Fibers with a lower numerical aperture are used to reduce radiation losses. This can be achieved by using a fiber with a smaller core diameter or by using a material with a lower refractive index for the cladding. Also, special coatings are used on the fiber from a variety of materials, including polymers, metals, and ceramics to prevent the light from escaping the fiber core. They are designed to reflect the light back into the core or absorb it, thereby reducing the amount of energy lost through radiation.