OPTICAL COMMUNICATIONS

Transcription

1 L21-1 OPTICAL COMMUNICATIONS Free-Space Propagation: Similar to radiowaves (but more absorption by clouds, haze) Same expressions: antenna gain, effective area, power received Examples: TV controllers, inter-building and interplanetary links Earth Evanescence outside θ i > θ c glass fiber Optical fibers guide waves Rays inside fiber impact wall beyond critical angle total reflection and wave trapping Little attenuation 0.5 < λ < 2 microns (can go >100 km) Guided Wave Propagation: Devices: Detectors: phototubes, photodiodes, avalanche photodiodes Sources: LED s, laser diodes, fiber amplifiers, gas lasers Modulators: amplitude and frequency, mixers, switches Other: filters, spectral multiplexers and combiners Mars

2 L21-2 UNDERSEA OPTICAL FIBER CABLES Fiber Communications Around the Globe 2.5 Gbp/s WDM 560 Mbp/s Other regenerative Non-repeated 5.0 Gbp/s 2.5 Gbp/s 280 Mbp/s Terrestrial systems too numerous to depict Tyco Submarine systems, : undersea about 1.5, some are dark Figure by MIT OpenCourseWare. Fiber optics dominates long-distance telecommunications In-line Erbium-Doped Fiber Amplifiers (EDFA s) make extremely wideband transoceanic transmission possible without repeaters Without fiber communications there would be no World Wide Web

3 L21-3 WDM MULTIPLEXED LINK WAVELENGTH DIVISION MULTIPLEXING (WDM): Multiple wavelengths combined onto one fiber All wavelengths amplified simultaneously and independently in each Optical Amplifier (OAMP) λ 1 ~80 km of fiber λ 1 λ 2 λ 3 MUX OAMP OAMP OAMP DEMUX λ 2 λ 3 λ n passive multiplexing e.g. prism λ n

4 WAVES IN FIBERS Optical Fiber Simple Picture: 6 μm glass cladding ε 1 glass core ε 2 = ε 1 + Δε Δε / ε 0.02 evanescence E(r) 125 μm Total internal reflection in the higher ε glass core traps light Small Δε very shallow reflection angles. Only certain angles are allowed: waves must interfere constructively modes (characterized by Bessel functions) Mode velocity = f(ε s, core size, mode) L21-4

5 L21-5 OPTICAL WAVEGUIDES Dielectric slab waveguide example: Waves reflect if θ i > θ c Glass/air θ c = sin -1 (n g -1 ) n g 1.5 θ c 41.8 Cladding/core θ c = ~sin -1 (0.98) θ c 78.5 Slab waveguide fields: sinkxx jk = zz E ye ˆ o e x d cosk x x αx jk z E = ye ˆ e for x > d, 1 +αx jk z E =± ye ˆ e for x < d 1 Boundary conditions for TE n : z z TE odd x Slab ε > ε o z θ > θ c TE or TM E // and Ey x continuous E = zˆ Ey x xˆ Ey z = H t E y 2d x TE 1 TE 2 TE 3 +d -d 0 slab

6 L21-6 ELECTROMAGNETIC FIELD DISTRIBUTION Magnetic Field: H ( E) j o = ωμ Inside the slab, x < d: 2d sinkxx coskxx jkzz H= ( Eo ωμ o ) xk ˆ z zjk ˆ x e coskxx sinkxx x jk Outside, x > d: ( )( ) zz H= E1 ωμo xk ˆ z zj ˆ α e α Propagation Matching Boundary Conditions at x = d: Dispersion relations: k x2 + k z2 = ω 2 μ o ε inside the slab, x < d -α 2 + k z2 = ω 2 μ o ε o outside, x > d [let μ = μ o ] jkzz -αd-jkzz Continuity of E : Eo coskxd e = E1e for TE1,3,5... jk ( ) zz αd jk ( ) zz Continuity of H: jkxeo ωμ o sinkxd e = jαe1 ωμo e Therefore: k x tan k x d = α (ratio of continuity equations) k x2 + α 2 = ω 2 μ o (ε - ε o ) (from dispersion equations) E H x z TE odd

7 L21-7 DIELECTRIC SLAB WAVEGUIDES TE odd n Field continuity equations: k x tan k x d= α k x2 + α 2 = ω 2 μ o (ε - ε o ) (ratio of continuity equations) (from dispersion equations) Transcendental equation, graphical solution: 2 ωμo( ε εo) tankxd = 1 2 kx x half-te 2 mode Increasing ω σ = E(x) 0 π/2 3π/2 5π/2 TE 1 mode TE 3 mode TE 5 mode k x d

8 FIBER WAVEGUIDE DESIGN Loss mechanisms: Rayleigh scattering from random density fluctuations Loss f 4 (scattering makes sky blue) Infrared absorption dominates for λ > ~1.6 microns Minimum total attenuation 0.2 db km -1 Fiber structure: Typical: 10-μm core in 125-μm diameter glass, with 100-μm-thick plastic protective cladding (bundled in cables) Manufacturing: solid or hollow preform grown by vapor deposition of SiO 2 and GeO 2 (using e.g. Si(Ge)Cl 4 + O 2 = Si(Ge)O 2 + 2Cl 2 ) Architecture: various single or multimode, polarization-selective Attenuation (db km -1 ) H 2 O Rayleigh ~1.5 THz Infrared λ Clad Multimode core Single-mode core Single polarization cladding L21-8

9 MIT OpenCourseWare Electromagnetics and Applications Spring 2009 For information about citing these materials or our Terms of Use, visit: