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"SNR, Signal-to-noise ratio.

signal (Q factor) of a 5900-km-long 5-Gb/s NRZ transmission test bed for various scrambling frequencies between 40 kHz and 10 GHz (Heismann 1995). The largest SNR improvements of more than 4 dB are obtained with high-speed scrambling at 5 and 10 GHz. where the detection of the high-speed digital data is not affected by undesired intensity modulation arising from interactions of the polarization scrambling with polarization-dependent loss in the system components (Taylor and Penticost 1994). However, the highest SNR improvement of about 5.7 dB is obtained when the SOP is modulated at 5 GHz phase synchronously with the high-speed digital data. The increased performance with bit-synchronous scrambling is attributed to nonlinear pulse compression in the transmission fiber that arises from a sinusoidal frequency chirp introduced by the scrambler (BerThe scrambling frequency for optimal transmission performance usually depends on the length of the system and its particular design. Electrooptic polarization scramblers offer the great advantage of an extremely broad range of modulation frequencies, from a few kilohertz to several gigahertz. Alternative schemes for fast polarization scrambling allow only a narrow range of modulation frequencies: scramblers based on piezoelectric fiber stretchers and acoustooptic polarization scramblers usually operate at fixed resonance frequencies of a few hundred megahertz (Kersey and Dandridge 1987: Taylor 1993: Noe et al. 1994), whereas depolarized light sources based on two polarization-multiplexed transmitters of slightly different optical wavelengths typically modulate the polarization state at rates of more than tens of gigahertz (Burns et al. 1991; Bergano. Davidson, and Li 1993).

Automatic polarization controllers are mostly used to transform the fluctuating output polarization at the end of a transmission fiber continuously into the preferred SOP of a polarization-sensitive optical component, such as a polarization-dependent optical switch or demultiplexer (Heismann, Ambrose, et al. 1993; Heismann, Hansen, et al. 1993). A particularly important application of such polarization controllers is found in polarization-multiplexed transmission systems, where they allow automatic demultiplexing of the two orthogonally polarized signals at the optical receiver, as shown schematically in Fig. 9.20. Polarization multiplexing is an attractive scheme for increasing the transmission capacity of lightwave systems by transmitting simultaneously two independent optical signals at the same wavelength in orthogonal polarization states (Hill, Olshansky, and Burns 1992; Evangelides et al. 1992). Although it is fairly easy to multiplex the two orthogonally polarized signals at the transmitter, it is far more difficult

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20 Gb/s

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Automatic Polarization Demultiplexer

Fig. 9.20 Schematic diagram of a polarization-multiplexed 20-Gb/sNRZ transmission system with an automatic polarization demultiplexer, comprising a fast polarization controller and a conventional polarization splitter-combiner (PolSplit).

Automatic Polarization Demultiplexer

Fig. 9.20 Schematic diagram of a polarization-multiplexed 20-Gb/sNRZ transmission system with an automatic polarization demultiplexer, comprising a fast polarization controller and a conventional polarization splitter-combiner (PolSplit).

to demultiplex them at the receiver because their absolute polarization states are not preserved in the transmission fiber and may even change with time. This problem can be solved by transforming the unknown polarization states of the two signals simultaneously into two orthogonal linear states, which can then easily be separated by a conventional polarization splitter. Such automatic demultiplexing has indeed been successfully demonstrated in 20-Gb/s NRZ transmission systems as well as in ultralong 10-Gb/s soliton systems (Heismann, Hansen, et al. 1993; Mollenauer, Gordon, and Heismann 1995).

In these applications it is essential that the polarization controller does not interrupt, at any time, the continuous flow of the optical signals through the system. The controller therefore has to be fast enough to allow instantaneous compensation of the random and potentially large polarization fluctuations in the fiber. Moreover, to guarantee continuous adjustment of the output SOP, the controller has to have an effectively infinite transformation range. Once again, electrooptic polarization controllers on LiNb03 have shown superior performance over alternative schemes: polarization controllers using mechanically rotated bulk-optic or fiber optic wave plates are inherently slow and hence unsuitable for practical applications (Imai, Nosu, and Yamaguchi 1985; Okoshi 1985), whereas control systems based on electromagnetic fiber squeezers and liquid crystal polarization rotators are usually limited in speed by complicated drive algorithms that are required to reset the various control elements periodically (Noe, Heidrich, and Hoffmann 1988; Walker and Walker 1990; Rumbaugh, Jones, and Casperson 1990). Electrooptic polarization controllers based on the principle of cascaded, rotating wave plates exhibit an inherently unlimited transformation range and have demonstrated control speeds that clearly exceed the speeds of natural polarization fluctuations in long transmission fibers (Heismann and Whalen 1992).

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