1 Lecture 5b Fiber-Optics in Digital Communication Systems & Electronic Interfaces 1. Introduction 2.Geometric Optics 3.Classification of Optical Fibers and Their Characteristics 4.Multimode Graded Index Fiber 5.Single Mode Step Index Fiber 6.Dispersion 7.Bandwidth of a Single Mode Optical Fiber 8.Fiber Optic in Point-to-Point Communication Link 9.Lasers 10.Distributed Feedback Laser Diode 11.Avalanche Photodiode 12.Conclusion 13.Appendix

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3 Geometric Optics (Physical Background) Dual Nature A.Huygens's Principle and Ray Approximation Huygens's principle states that all points on a wavefront can be taken as point sources for the production of secondary wavelets. We can assume that the wave travels through a medium in a straight line. This is the ray approximation, and it assumes that light behaves like particles traveling in a straight line. B.Diffraction When light passes through an aperture, the ray approximation is valid if the light wavelength is much shorter than the dimensions of the aperture.

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5 C. Reflection When light encounters a surface, some light will be absorbed by the surface, some light will be transmitted through the surface, and some light will be reflected by the surface.

6 D. Format’s Principle Format’s principle states that when a light ray travels between two points via surface, its path will be the one that requires the least time. This effect is called refraction.

7 Single-mode Step Index Fiber long-haul telecommunications; 100 Gbps for 1 km; repeater spacing of up to 300 km, but these capabilities continue to be improved. Axial transmission. For a given core diameter, there is a minimum wavelength c. A single mode fiber will transmit only the single mode for all wavelengths greater than the cut-off wavelength c. Cutoff wavelength c for a fiber with a 3-micrometer core diameter, a core index of refraction of 1.545, and a cladding index of refraction of

8 Single mode fiber has a core diameter 4-9 micrometers, about four times the wavelength of light, allows only one mode (single mode) to exist in the core. No bouncing, destructive or constructive interference occurs. Typical bit rate is 100 Gb/s/km. This is 5000 times the bit rate of multi-mode fiber. Theoretical BW limit is 100,000 GHz. Single mode is the highest bandwidth optical fiber and is used for long distance communications. The single mode fiber bandwidth limitations is due to different light wavelength traveling at slightly different speeds. This phenomena is called chromatic dispersion. Using single mode 100 km optical fiber between repeaters bit rate: Bandwidth of a Single Mode Optical Fiber

9 Lasers Light Amplification by Stimulated Emission of Radiation The laser uses several heavily doped layers of p ‑ type and n-type materials. When a large forward bias is applied, a large number of free holes and electrons are created in the immediate vicinity of the junction. When a hole and electron pair collide and recombine, they produce a photon of light. - +

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11 Avalanche photodiode

12 Point-to-Point Communication Link

13 Basics of semiconductor theory

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17 Laser Beam External Modulation 2k /2, Max (2k+1) /2, Min =1500 nm; L=2cm L K=0,1,2…. 2 1

18 Transphasor. indium antimonide 1000 times faster

19 By DWDM splitting of a spectrum into hundreds of channels, Decreasing of duration of pulses, Speed of transfer of the information on a separate line (one single wavelength) now managed to be up to 10 Gb per second, and Gb on main channel.

20 Orbital angular moment of photons A B C D E F G H I A A