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In this paper, studies are made of the information efficiencies of optical amplifiers, attenuators, and frequency convertors. Attenuators always reduce the amount of information carried by a signal. In contrast, parametric amplifiers and frequency convertors redistribute information between the signal and idler sidebands, but preserve the total information. Homodyne detection of the sideband quadratures degrades their signal-to-noise ratios. Detection always reduces the back-to-back and converted information capacities by moderate amounts, which do not depend on how the input information is encoded. The effects of detection on the amplified information capacity can be small or large, depending on how the input information is encoded. For (standard) two-sideband amplifiers, it is best to encode correlated information in both sidebands.