the circulator 760 outputs optical signals received therein via the second end and forward them to the third end which is connected to the fiber 710 the dispersion compensating fiber 770 compensates optical signal output from the circulator 760 as well as the optical signals that are reflected from the second wavelength selective coupler 780 at the same time the dispersion compensating fiber 770 performs a raman amplification on the optical signals passing therethrough the dispersion compensating fiber 770 is comprised of a 40 km dispersion compensating fiber of which insertion loss is 6 db and it is pumped by the pumping light outputted from the second wavelength selective coupler 780 the second pumping light source 790 pumps the dispersion compensating fiber 770 and the second erbium doped fiber 810 a preferred second pumping source 790 is a laser diode that outputs a 1450 nm wavelength pumping light the second wavelength selective coupler 780 couples optical signals that are reflected from the reflector 800 with the pumping light that is inputted from the second pumping light source 790 then outputs them to the dispersion compensating fiber 770 moreover the second wavelength selective coupler 780 outputs optical signals that are inputted from the dispersion compensating fiber 770 to the reflector 800 a preferred second wavelength selective coupler 780 is a 1450 1550 nm wavelength selective coupler the reflector 800 reflects optical signals inputted from the second wavelength selective coupler 780 and forward them back into the dispersion compensating fiber.