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Abstract

An optical fiber (12a) with a large effective core area and a large chromatic dispersion value is disposed on an input side of signal light, and an optical fiber (12b) with a small effective core area and a small chromatic dispersion value or a chromatic dispersion value of negative polarity is disposed on an output side of the signal light. A pumping light source (14) generates pumping light of 1450 nm to cause Raman amplification of 1550 nm in the optical fiber (12b). The output light from the pumping light source (14) enters the optical fiber (12b) from the back through a WDM optical coupler (16). Provided that y=(Pin-.alpha.)/(Pp.multidot.10 Log L) where input power of the optical fiber (12a) (i.e. output power of an optical transmitter (10)) is Pin, a total loss of the optical fibers (12a and 12b) is .alpha., pumping power for Raman amplification is Pp, and a total length of the optical fibers (12a and 12b) is L, the relation between ratio x (0.ltoreq.x.ltoreq.1) of the optical fiber (12b) and y is expressed as y=6.63.times.10.sup.-2 x-0.077. Here, the satisfactory x value is in a range from +0.2 to -0.2 of the y value.

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