Propagation of Light Through Optical Fiber

Outline of Talk Acceptance angle Numerical aperture Dispersion Attenuation

Acceptance angle  is the maximum acceptance angle to the axis of the fiber at which light may enter into the fiber in order to propagate A B    c for total internal reflection Lost by radiation Acceptance cone

Different cones of acceptance Large diameter fiberSmall diameter fiber

Numerical aperture (NA) The NA defines a cone of acceptance for light that will be guided by the fiber 11 Air n 0 n2n2 n1n1 A 22 B C   =90-  2 >  c At the air core interface From the triangle ABC

Numerical aperture (NA) Using trigonometric relationship For total internal reflection,  1 =  a, and    c 90 0 n2n2 n1n1 11 Exmp. 2.1 and 2.2

Types of ray propagation in OF Skew ray follows helical path in optical fiber Meridional ray passes through the fiber axis Skew ray

Dispersion & Attenuation

Dispersion Sun light Disperse light Dispersion is a phenomenon that causes the separation of a wave into spectral components with different wavelengths, due to a dependence of the wave's speed on its wavelength.wavewavelengths Prism

Types of Dispersion Material Dispersion Modal Dispersion Waveguide Dispersion

Types of Dispersion f1f1 f2f2 n1n1 n2n2 Changing optical path length due to a changing refractive index n 1 Material Dispersion Velocity of electromagnetic wave in any medium = c/n For glass material n(), i.e, n changes with or frequency Output wave t I Input wave

Waveguide Dispersion Waveguide dispersion is chromatic dispersion which arises from waveguide effects. The origin of waveguide dispersion can be understood by considering that a guided wave has a frequency-dependent distribution of k vectors Wave propagating in core and cladding with different group velocities Field with longer wavelength leaked from core to cladding Field with shorter wavelength concentrated in the core

Definition of Modal Dispersion Only a certain number of modes can propagate in fiber optic waveguide. Each of these modes carries the modulation signal and, as each one is incident on the boundary at a different angle, they will each have their own individual propagation times. In a digital system, the net effect is to smear out the pulses, and so there is a form of dispersion called modal dispersion

Modal Dispersion t I Input light wave n1n1 n2n2 L θi =cθi =c cc Ray2 θ θaθa Ray1(axial) θ i =90 0

Effect of Dispersion on OFC n1n1 n2n L2L2 Long length fiber Input pulse n1n1 n2n L1L Short length fiber Output pulse Input pulse t t Output pulse No zero levelIndistinguishable pulse Intersymbol Interference t

Wavelength dependence of Ng and n

Source of Losses in Silica OF Losses in silica fiber are mainly occur due to two mechanisms: Intrinsic absorption mechanism (due to characteristic of glass fiber) Extrinsic absorption mechanism (due to impurities: such as OH bonds and transition metal ions (iron, cobalt, copper etc.))

Source of Losses in Silica OF Intrinsic absorption loss mechanisms are:  Material absorption: The atomic bonds associated with the core material absorb the long wavelength light.(Si-O; 9.2 m, Ge-O; 11.0  m; P-O; 8.1;  m)  Electron absorption: In the ultraviolet region, light is absorbed in order to excite the electron in a core atoms to a higher energy state.  Rayleigh scattering: Due to small irregularities in the structure of the fiber core, which are caused by density fluctuations into the glass material at manufacture.This loss reduces with forth power of (~  -4 ).

Attenuation wavelength Ch. Of Glass fiber (Early fiber) Material absorption Electron absorption Rayleigh scattering Impurity absorption Wavelength (m) Attenuation (dB/Km)

Attenuation wavelength Ch. Of Glass fiber (Advanced fiber) Conventional fiber Wavelength (m) Attenuation (dB/Km) Due to OH Dry fiber

Other scattering losses Mie scattering: Due to imperfections such as irregularities in core-cladding interface, core-cladding refractive index differences along the fiber length, diameter fluctuations, strains, and bubbles Stimulated Brilloiun Scattering: Shift in incident light frequency in the acoustic range due to scattering process, which causes reversal of propagation direction Stimulated Raman Scattering: Shift in incident light frequency in the optical range causes attenuation

Problems: Attenuation in decibels (dB) In OFC attenuation is usually expressed in dB/Km Example: When the mean optical power launched into an 8 Km length of fiber is 120 W, the mean optical power at the fiber out is 3 W. Determine: a)the overall signal attenuation in dB through the fiber assuming there are no connectors or splices; b)The signal attenuation/Km for the fiber c)The overall signal attenuation for 10 Km optical link using the same fiber with splices at 1 Km intervals, each giving an attenuation of 1 dB; d)The numerical input/out power ratio in (c). Example 3.3 Senior, self study