Lecture9-228a

Lecture9-228a - ECE 228A Fall 2011 Daniel J. Blumenthal 9.1...

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Unformatted text preview: ECE 228A Fall 2011 Daniel J. Blumenthal 9.1 Lecture 9 - Polarization Mode Dispersion and Fiber Nonlinearties ECE 228A Fall 2011 Daniel J. Blumenthal 9.2 Polarization So called single mode fber is not really single mode. There are two degenerate modes (or example, vertical and horizontal polarization). Fiber is in general bireringent due to core ellipticity or strain so the two polarizations travel at dierent velocities. The polarization evolves in time. The distance over which it repeats is the beat length (visible by looking at the fber). p.36 ECE 228A Fall 2011 Daniel J. Blumenthal 9.3 Polarization Mode Dispersion (PMD) An input optical pulse is randomly coupled, along the fber, with the two local orthogonal states o polarization The two states has slightly dierent group velocities This eect is called PMD Related to PMD delay PMD will broaden pulses in the same way other dispersion mechanisms do PMD changes instantly along fber as a unction o time, temperature and wavelength ECE 228A Fall 2011 Daniel J. Blumenthal 9.3 Polarization Mode Dispersion (PMD) An input optical pulse is randomly coupled, along the fber, with the two local orthogonal states o polarization The two states has slightly dierent group velocities This eect is called PMD Related to PMD delay PMD will broaden pulses in the same way other dispersion mechanisms do PMD changes instantly along fber as a unction o time, temperature and wavelength Power penalties associated with PMD are time varying ECE 228A Fall 2011 Daniel J. Blumenthal 9.4 Poincare Sphere A way to represent all polarizations states. Equator: Linear polarization states Poles: Right and left circular polarization Half wave plates rotate around the pole. Quarter wave plates go through the poles, ECE 228A Fall 2011 Daniel J. Blumenthal 9.5 Poincare Sphere ECE 228A Fall 2011 Daniel J. Blumenthal 9.6 Real Polarization Controllers ECE 228A Fall 2011 Daniel J. Blumenthal 9.7 PMD limit A (quite approximated) formula that shows the PMD limit is the following (see Optical Fiber Communications IIIa, I. Kaminov, T. Koch, Academic Press) 2 2 max 2 2 02 . 02 . B PMD L PMD L B = Bit rate = 10 Gbit/s Bit rate = 40 Gbit/s PMD=0.1 ps/km 0.5 L max =20.000 Km L max =1250 Km PMD=1 ps/km 0.5 L max =200 Km L max =12.5 Km New bers have PMD values of the order of 0.1 ps/km 0.5 PMD is an issue on ultra long distance only Installed ber often have PMD values close to 1 ps/km 0.5 In these cases, PMD may be a fundamental issue even at 10 Gbit/s ECE 228A Fall 2011 Daniel J. Blumenthal 9.8 Polarization Control Polarization maintaining fber (or) Polarization preserving fber: I the index o the two directions is signifcantly dierent, then they do not couple Deorm the preorm or Introduce stress (bow tie or PANDA fber) Single polarization fber Only one polarization is guided. ECE 228A Fall 2011 Daniel J. BlumenthalECE 228A Fall 2011 Daniel J....
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Lecture9-228a - ECE 228A Fall 2011 Daniel J. Blumenthal 9.1...

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