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The first demonstration of an inline coherent optical amplifier with a noise figure which is lower than the 3 dB quantum limit is presented. The amplifier is based on optical phase-sensitive amplification using a nonlinear fibre Sagnac interferometer pumped by an injection-locked semiconductor laser with a compensation phase-locked loop. A noise figure of 2.0 dB with a high signal gain of 16 dB has been successfully obtained.
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Imajuku et al. (2000) studied this question.
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