Over the past several years, non-repeated transmission distances using coherent detection have been longer than those using direct detection. It is noticed that minimum detectable photon numbers of the direct detection schemes are no more than those of the coherent schemes, under shot-noise limited condition. Optical amplification techniques play important roles to improve the receiver sensitivity in the direct detection system [1] and to increase the fiber launched power.

© 1989 Optical Society of America

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