Saturday, March 3, 2018

Signal Distortion in Optical Fibers Part 4: Bending Loss

. Losses due to curvature and losses caused by an abrupt change in radius of curvature are referred to as bending losses'
. The sharp bend of a fiber causes significant radiative losses and there is also possibility of mechanical failure. This sis shown in Fig. 1.

. As the core bends the normal will follow it and the ray will now find itself on the wrong side of critical angle and will escape. The sharp bends are therefore avoided.
. The radiation loss from a bent fiber depends on –
i ) Field strength of critical distance xc from fiber axis where power is lost through radiation.
ii) The radius of curvature R.
. The higher order modes are less tightly bound to the fiber core, the higher order modes radiate out of fiber firstly.
. For multimode fiber, the effective number of modes that can be guided by curved fiber is given expression :

α is graded index profile
Δ is core – cladding index difference
nc  is refractive index of cladding.
K is wave propagation constant (2π/λ).
N is total number of modes in a straight fiber.

1. Microbending
. Microbending is a loss due to small bending or distortions. This small microbending is not visible. The losses due to this are temperature related, tensile related or crush related.
. The effect of microbending on multimode fiber can result in increasing attenuation (depending on wavelength) to a series of periodic peaks and troughs on the special attenuation curve. These effects can be minimized during installation and testing Fig. 2. illustrates microbending.

2. Macrobending
. The change in spectral attenuation caused by macrobending is different to microbending.usually there no peaks and troughs because in a macrobending no light is coupled back into the core from the cladding as can happen in the case of microbends.
. The macrobending losses are caused by large scale bending of fiber. The losses are eliminated when the bends are straightened. The losses can be minimized by not exceeding the long term bend radii. Fig. 3 illustrates macrobending.

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