Optical Networks - _12_2 Synchronization_134

Optical Networks - _12_2 Synchronization_134 - 668 Photonic...

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668 Photonic Packet Switching Birefringent fiber l l Filter ld l + l + l + l - dl Figure 12.11 Block diagram of a soliton-trapping logical AND gate. to group velocity dispersion (Section 2.6), a pair of orthogonally polarized soliton pulses propagate in birefringent fiber without walk-off. The quantitative analysis of this phenomenon is beyond the scope of this book, but qualitatively what occurs is that the two pulses undergo wavelength shifts in opposite directions so that the group velocity difference due to the wavelength shift exactly compensates the group velocity difference due to birefringence! Since the two soliton pulses travel together (they do not walk off), this phenomenon is called soliton trapping . The logical AND operation between two pulse streams can be achieved using this phenomenon if the two pulse streams correspond to orthogonally polarized soliton pulses. Most high-speed TDM systems use soliton pulses to minimize the effects of group velocity dispersion so that the soliton pulse shape requirement is not a problem. The orthogonal polarization of the two pulse streams can be achieved by appropriately using polarizers (see Section 3.2.1). The logical AND operation is achieved by using an optical filter at the output of the birefringent fiber. Figure 12.11 shows the block diagram of such a soliton-trapping AND gate. It consists of a piece of birefringent fiber followed by an optical filter. Figure 12.12 illustrates the operation of this gate. When pulses of both polarizations are present at the wavelength λ , one of them gets shifted in wavelength to λ + δλ , and the other to λ . The filter is chosen so that it passes the signal at λ + and rejects the signal at λ . Thus the passband of the filter is such that one of the wavelength-shifted pulses lies within it. But the same pulse, if it does not undergo a wavelength shift, will not be selected by the filter. Thus the filter output has a pulse (logical one) only if both pulses are present at the input, and no pulse (logical zero) otherwise. 12.2 Synchronization Synchronization is the process of aligning two pulse streams in time. In PPS networks, it can refer either to the alignment of an incoming pulse stream and a locally available
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12.2 Synchronization 669 Figure 12.12 Illustration of the operation of a soliton-trapping logical AND gate. (a) Only one pulse is present, and very little energy passes through to the filter output. This state corresponds to a logical zero. (b) Both pulses are present, undergo wavelength shifts due to the soliton-trapping phenomenon, and most of the energy from one pulse passes through to the filter output. This state corresponds to a logical one.
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Optical Networks - _12_2 Synchronization_134 - 668 Photonic...

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