Lect. 15: Optical Fiber

Transcription

1 3-dimentioanl dielectric waveguide? planar waveguide circular waveguide optical fiber

2 Optical Fiber: Circular dielectric waveguide made of silica (SiO ) y y n n 1 n Cladding Core r z Fiber axis SiO :Ge Loss in rectangular metal waveguide The step index optical fiber. The central region, the core, has greater refractive index What than is special the outer about region, fiber? the cladding. The fiber has cylindrical symmetry. We use the coordinates r,, z to represent any point in the fiber. Cladding is normally - Extremely much low thicker loss: than 0.dB/km shown. - Can be very long: 100 s of km?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

3 Lect. 15: Optical Fiber Optical Communication Networks Total undersea fiber length: 0.4 billion km ( 68 round trips between earth and moon)

4 r Buffer tube: d = 1mm n n 1 n Protective polymerinc coating Cladding: d = m Core: d = 8-10 m for single-mode fiber (For multi-mode fiber, d: several tens of m) The cross section of a typical single-mode fiber with a tight buffer tube. (d = diameter)?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

5 Vapors: SiCl 4 + GeCl 4 + O Fuel: H Burner Deposited soot Drying gases Porous soot preform with hole Furnace Target rod Deposited Ge doped SiO Rotate mandrel (a) (b) Clear solid glass preform SiCl 4 + O ->SiO + Cl GeCl 4 + O ->GeO + Cl Sintering at deg C

6 Preform feed Thickness monitoring gauge Furnace 000 C Polymer coater Ultraviolet light or furnace for curing Take-up drum Capstan

7 n n 1 a b b k n n 0 1 n LP 01 LP 11 Solving for guided modes for circular dielectric waveguide problem in (r,, z) coordinate is very complicated LP 1 LP 0 0. It can be shown that with a little approximation, LP (linearly polarized) mode solutions are obtained. 0 jlm E E (, r )e LP lm z V k a( n n ) ( a : fiber core radius) V

8 (a) The electric field of the fundamental mode (b) The intensity in the fundamental mode LP 01 (c) The intensity in LP 11 (d) The intensity in LP 1 Core Cladding E E 01 r The electric field distribution of the fundamental mod in the transverse plane For LP to lm the mode, fiber axis z. The light intensity m represents is greatest the at number the center of of peaks the fiber. along Intensity r, patterns l represents in LP 01, LP the 11 number and LP 1 of modes. peaks along?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

9 Loss in fiber Loss (db/km) Rayleigh scattering OH - absorption peaks 1310 nm 1550 nm Lattice absorption Wavelength ()

10 Lattice Absorption: EM waves cause vibration of ions inside fiber. Peak absorption occurs at around = 9 m in Silica fiber. Si and O ions E x Light direction k z

11 Rayleigh scattering A small portion of EM waves get directed away from small dielectric particles that are due local fluctuation of fiber refractive index. More scattering with smaller wavelength (inversely proportional to 3 ). A dielectric particle smaller than wavelength Incident wave Through wave Scattered waves

12 Dispersion in Fiber - Multi-Mode Fiber: Modal Dispersion n O 1 3 n 1 n (a) Multimode step index fiber. Ray paths are different so that rays arrive at different times. O Each mode has its own group velocity If a light pulse is decomposed into many modes, n the pulse gets dispersed (broadened) after propagation O' O'' Prevention: Use single mode fiber! n n n 1 (b) Graded index fiber. Ray paths are different but so are the velocities along the paths so that all the rays arrive at the same time.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

13 But single mode fiber also has small but non-zero dispersion - Waveguide dispersion Waveguide solution depends on wavelength - Material (or chromatic) dispersion: Refractive index of any material depends on wavelength (frequency) v g for SMF depends on wavelength(frequency) Intensity Intensity Spread t Dispersive SMF t

14 Dispersion exists because is not not linear with Mathematically, 1 ( ) ( ) ( 0) 1( 0) ( 0) = ( 0) vg( 0) v g 0 In Silica fiber, ~ 0 ps /km at =1.5m With 0, increases as increases v g

15 Often, dispersion parameter D is used. Intensity 1 D 1 c ( kc c) D c D~16 ps at 1.5m km nm Intensity t Spectrum ( Dispersive medium (L,D) Intensity Spread ( ~ D L t

16 Homework