Optical fibers Dispersion induced limitations .pptx

Geometric optic view of waveguide, numeric aperture
Symmetric planar dielectric Slab waveguide
Modal and waveguide dispersion in palnar waveguide
Rectangular waveguide, effective index method

1. 16.711 Lecture 3 Optical fibers Last lecture Geometric optic view of waveguide, numeric aperture Symmetric planar dielectric Slab waveguide Modal and waveguide dispersion in palnar waveguide Rectangular waveguide, effective index method

2. 16.711 Lecture 3 Optical fibers Today Fiber modes Fiber Losses Dispersion in single-mode fibers Dispersion induced limitations Dispersion management The Graded index fibers

3. 16.711 Lecture 3 Optical fibers Fiber modes --- single mode and multi-mode fibers V-number Number of modes when V>>2.41 Normalized propagation constant for V between 1.5 – 2.5. Mode field diameter (MFD)

4. 16.711 Lecture 3 Optical fibers Examples --- single mode and multi-mode fibers 1. Calculate the number of allowed modes in a multimode step index fiber, a = 100 m, core index of 1.468 and a cladding index of 1.447 at the wavelength of 850nm. Solution: a < 2.1 m 2. What should be the core radius of a single mode fiber that has the core index of 1.468 and the cladding index of 1.447 at the wavelength of 1.3m. Solution: 3. Calculate the mode field diameter of a single mode fiber that has the core index of 1.458 and the cladding index of 1.452 at the wavelength of 1.3m. Solution:

5. 16.711 Lecture 3 Optical fibers Fiber loss Material absorption silica electron resonance <0.4  m OH vibrational resonance ~ 2.73  m Harmonic and combination tones ~1.39  m 1.24  m, 0.95  m Rayleigh scattering Local microscopic fluctuations in density C~ 0.8dB/km  m4 0.14dB loss @ 1.55 m Bending loss and Bending radius

6. 16.711 Lecture 3 Optical fibers Dispersions in single mode fiber Material dispersion Example --- material dispersion Calculate the material dispersion effect for LED with line width of 100nm and a laser with a line width of 2nm for a fiber with dispersion coefficient of Dm = 22pskm-1nm-1 at 1310nm. Solution: for the LED for the Laser

7. 16.711 Lecture 3 Optical fibers Dispersions in single mode fiber Waveguide dispersion Example --- waveguide dispersion n2 = 1.48, and delta n = 0.2 percent. Calculate Dw at 1310nm. Solution: for V between 1.5 – 2.5.

8. 16.711 Lecture 3 Optical fibers chromatic dispersion (material plus waveduide dispersion) material dispersion is determined by the material composition of a fiber. waveguide dispersion is determined by the waveguide index profile of a fiber

9. 16.711 Lecture 3 Optical fibers Polarization mode dispersion fiber is not perfectly symmetric, inhomogeneous. refractive index is not isotropic. dispersion flattened fibers: Use waveguide geometry and index profiles to compensate the material dispersion

10. 16.711 Lecture 3 Optical fibers Dispersion induced limitations For RZ bit With no intersymbol interference For NRZ bit With no intersymbol interference

11. 16.711 Lecture 3 Optical fibers Dispersion induced limitations Optical and Electrical Bandwidth Bandwidth length product

12. 16.711 Lecture 3 Optical fibers Dispersion induced limitations Example --- bit rate and bandwidth Calculate the bandwidth and length product for an optical fiber with chromatic dispersion coefficient 8pskm-1nm-1 and optical bandwidth for 10km of this kind of fiber and linewidth of 2nm. Solution: Fiber limiting factor absorption or dispersion?

13. 16.711 Lecture 3 Optical fibers Dispersion Management Pre compensation schemes Prechirp Gaussian Pulse:

14. 16.711 Lecture 3 Optical fibers Dispersion Management Pre compensation schemes Prechirp Prechirped Gaussian Pulse:

15. 16.711 Lecture 3 Optical fibers Dispersion Management Prechirp With T1/T0 = sqrt(2), the transmission distance is:

16. 16.711 Lecture 3 Optical fibers Dispersion Management Examples: 1. What’s the dispersion limited transmission distance for a 1.55 m light wave system making use of direct modulation at 10Gb/s? D = 17ps(km-nm). Assume that frequency chirping broadens the guassian-shape by a factor of 6 from its transform limited width. Solution:

17. 16.711 Lecture 3 Optical fibers Dispersion compensation fiber or dispersion shifted fiber Why dispersion compensation fiber: for long haul fiber optic communication. All–optical solution Approaches longer wavelength has a larger index. make the waveguide weakly guided so that longer wavelength has a lower index.

18. 16.711 Lecture 3 Optical fibers The Graded index fibers Approaches General case Intermode dispersion Only valid for paraxial approximation Calculate the BL product of a grade index filber of 50 m core with refractive index of n1 = 1.480 and n2 = 1.460. At 1.3 m. Solution: