• Losses due to curvature and losses caused by an abrupt change in radius of curvature are referred to as ‘bending ’
  • The sharp bend of a fiber causes significant radiative losses and there is also possibility of mechanical failure. This is shown in Fig. 2.4.1.

  • 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
  • The radiation loss from a bent fiber depends on –
  1. Field strength of certain critical distance xc from fiber axis where power is lost through
  2. 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 :

                                                                … (2.4.1)

 

where,

 

α is graded index profile.

 

∆ is core – cladding index difference. n2 is refractive index of cladding.

k is wave propagation constant .

N∞ is total number of modes in a straight fiber.

                                                                     … (2.4.2)

 

 

Microbending

 

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

2.4.2 illustrates microbening.

Macrobending

 

  • The change in spectral attenuation caused by macrobending is different to microbending. Usually there are 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 cause by large scale bending of The losses are eliminated when the bends are straightened. The losses can be minimized by not exceeding the long term bend radii. Fig. 2.4.3 illustrates macrobending.