Microbending loss of fiber

Both the bending loss and the microbending loss of the optical fiber are caused by the fact that the light does not satisfy the condition of total internal reflection. The following ZR Cable will introduce the microbending loss of optical fiber.

Mechanism of Microbending Loss

The so-called microbending loss is the effect of uneven stress on the fiber, such as when the fiber is subjected to side pressure or the temperature of the sheathed fiber is changed, the fiber axis is slightly irregularly bent. Microbends are random distortions with radii of curvature comparable to the cross-sectional dimensions of the fiber.

Imperfections in the geometry of the core-clad interface may cause microscopic bumps or dips in the corresponding areas. Although light travels in straight segments of the fiber, the beam encounters these imperfections and changes its direction. The light beam is initially transmitted at a critical propagation angle. After reflection at these imperfect points, the propagation angle will change. As a result, the condition of total internal reflection is no longer satisfied, and part of the light is refracted, that is, it leaks out of the fiber core. Bending loss mechanism.

The microbending loss in single-mode fiber is wavelength-dependent, that is, the sensitivity of single-mode fiber to microbending loss increases by a small amount with increasing wavelength. The physical reason for this change is that longer wavelengths will Increasing the MFD allows more power to radiate out of the core.

Microbending loss of fiber

Theoretical Calculation of Microbending Loss

The microbend attenuation is the optical power loss caused by the mode coupling between the higher-order mode and the radiation mode caused by the random distortion of the fiber. The microbend attenuation is calculated by the following formula:

Am=N<h2>a4b6Δ3EEf32(7)

In the formula: N is the number of random microbends; h is the height of the microbend protrusions; 〈〉 represents the statistical average symbol; E is the Young's modulus of the coating material; Ef is the Young's modulus of the fiber; a is the fiber Core radius, b is the outer radius of the fiber; Δ is the relative refractive index difference of the fiber.

Jeunhumme gave the following formula for the microbending loss of single-mode fiber:

asm=0.05ammk4w60(NA)4a2m(8)

where NA is the numerical aperture, am is the core radius, and amm is the microbending loss of the abrupt multimode fiber with the numerical aperture of NA and the core radius of am. This mutant multimode fiber has the same outer diameter and is in the same mechanical environment as the single-mode fiber of interest.

Several Applications of Optical Fiber Microbending Loss Effect in Detection and Automatic Control Technology

(1) Optical fiber microbend and multi-turn spiral sensor

Optical fiber microbending can have various bending deformation forms. When the measured object is affected by the outside world, the optical fiber is bent and deformed, and the measured value is determined by detecting the change of the optical power transmitted in the optical fiber. In order to improve the sensitivity, the microbend sensing fiber is made into a multi-turn helical tube shape. When the displacement is small, the displacement of the measuring plate and the change of the optical power transmitted by the fiber are basically linear.

(2) Rack sensor

In the form of this sensor structure, the inter-tooth period δ of the rack sensor can be appropriately selected to make it equivalent to the propagation coefficient between the optical fiber guided modes, and the condition δ=2πΔβ is satisfied. When the deformer is displaced by external factors, the optical fiber embedded in it causes effective coupling between adjacent modes due to bending, and the guided mode is continuously converted into the cladding mode, resulting in radiation loss, resulting in a significant decrease in the optical power transmitted in the fiber.

(3) In addition, combined with the OTDR, the microbend loss can be used to accurately find the fiber connection point, determine the fiber serial number, and determine the high loss point.

This concludes the introduction to the basic principle, theoretical calculation and utilization of microbend loss. Understanding microbend loss can be fully utilized in the design of optical fiber communication systems.