Systems Compensate for Polarization Mode Dispersion
Dispersion, particularly polarization mode dispersion, becomes increasingly problematic at higher fiber optic data rates because the resulting signal distortion scales as the square of the data rate. Although there are several techniques to compensate for ordinary chromatic dispersion, compensating for polarization mode dispersion is more difficult because of the randomness of the process. As light propagates through a fiber, mechanical stresses and other effects induce a birefringence whose fast and slow axes vary randomly along the fiber's length, and there is no straightforward technique to compensate for the dispersion and the polarization-state loss that result.
Figure 1. Researchers have developed a technique to compensate for polarization mode dispersion in optical fiber links. In one variant of the approach, a polarization controller regulates the orientation and degree of elliptical polarization. The feedback system maximizes the power transmitted through the polarizer.
Researchers at Osaka University in Japan have addressed this problem by devising a distributed approach to polarization mode dispersion compensation that increases transmission quality. At the heart of the technique are active polarization mode dispersion compensators using either a polarization controller or a Faraday rotator.
In the case of the polarization controller, the degree and orientation of elliptical polarization are adjusted to maximize transmission through the linear polarizer, with the two loops of fiber analogous to a pair of rotatable wave plates in free-space optics (Figure 1). In the case of the Faraday rotator, the linear polarization is rotated to maximize the transmission through the linear polarizer (Figure 2).
Figure 2. In another scheme, a Faraday rotator controls the orientation of linear polarization, and the feedback system again maximizes the power transmitted through the polarizer.
The researchers placed a number of these compensators along a fiber optic link, injected a digital signal into one end and monitored the eye pattern at the other end. The quality of the transmitted signal, indicated by the openness of the eye pattern, increased with the number of compensators in the link.
Inserting linear polarizers into the link necessarily reduces the power transmitted through the link. The loss can be as great at 3 dB per polarizer, if the incoming signal has been completely depolarized. To compensate for this signal loss, the researchers added a saturated erbium-doped fiber amplifier after each compensator. Because the amplifiers were saturated, the output power was constant, independent of input power.
Most existing fiber optic links operate at data rates of 2.5 Gb/s or less, at which polarization mode dispersion compensation is largely of academic interest. With state-of-the-art 10-Gb/s systems, polarization mode dispersion compensation is important, and with future 40-Gb/s systems, it will be essential. The techniques described here may enable the evolution of these future systems.
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