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IntroductionIn high-speed optical transmission systems beyond 10 Gbitls, signal distortion caused by polarization-mode dispersion (PMD) is a major limitation on transmission distance. PMD of the installed fibers fluctuates with time due to environmental influences, for example, changes in temperature and stress. Therefore, adaptive PMD compensation during system operation is indispensable [l]. There are two key requirements in realizing an automatic PMD compensation system. One is to monitor the state of polarization of the received optical signals without large-scale PMD measurement equipment. The other is the realization of an automatically controlled PMD compensator. In our 10-Gbitls NRZ transmission experiments, we already confirmed that the intensity of the half-frequency components of the bit rate in the received baseband signals can be used for PMD monitoring [2]. In this paper, we successfully demonstrated automatic PMD compensation in 40-Gbitls N E transmission for the first time. Using a PMD monitor of 20-GHz intensity extracted from the received 40-Gbitls NRZ baseband signals, we performed the feedback control of an optical PMD compensator consisting of a polarization controller and a polarizationmaintaining fiber.

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A semiconductor optical amplifier fiber ring laser (SOAFRL) utilizing a Fiber Bragg Grating (FBG) is demonstrated. The laser operates at a wavelength of 1547.64 nm, equal to the Bragg wavelength. By removing FBG, the SOAFRL shows a multi‐modes output due to the resonant characteristics of the semiconductor optical amplifiers (SOA's) gain. The output power ripple is observed around 5 dB. The experimental results show that when the saturation level is reached, the SOAFRL with the FBG shows a higher saturation current and peak power compared to that of the SOAFRL without the FBG. Mode competition in the cavity for the free‐running ring SOAFRL results in a lower peak power as well as a broader bandwidth as the current is increased. By incorporating the FBG in the SOAFRL, a lasing wavelength is generated and as such the side mode suppression ratio (SMSR) of the laser is significantly improved, resulting in a higher peak power at higher bias currents. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3101–3103, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23895