Abstract: An optical amplifier repeater is provided having a first optical amplifier that amplifies a main signal light and supervisory signal light transmitted on an up optical-fiber transmission line and a second optical amplifier that amplifies the main signal light and supervisory signal light transmitted on a down optical-fiber transmission line. Further, a transfer unit that is provided transfers part of the supervisory signal light transmitted on the up optical-fiber transmission line onto the down optical-fiber transmission line and transfers part of the supervisory signal light transmitted on the down optical-fiber transmission line onto the up optical-fiber transmission line.

Abstract: An optical transmission system having an optical regeneration circuit for regenerating a signal with a deteriorated signal to noise ratio. A signal light at wavelength &lgr;1 which includes a noise light is composed with a continuous light at wavelength &lgr;2 outputted from a light source to obtain a composed signal light in an optical coupler. The composed signal light is passed through a saturable absorption type optical element which transmits only the signal light having a light intensity equal to or greater than a threshold value, thereby removing the included noise light. From the signal light, only the signal at the same wavelength &lgr;2 as the wavelength of the continuous light is extracted by a wavelength divider and data signal of the incident signal light is replicated in the continuous light and outputted. Thus, a signal light having no deterioration of the signal to noise ratio at wavelength &lgr;2 can be regenerated from the signal light having the deteriorated signal to noise ratio.

Abstract: An optical amplifying and relaying system capable of easily and highly accurately monitor troubles in optical amplifiers provided in an up and a down optical fiber transmission line opposing each other is disclosed. Monitoring light signal folding-back lines including variable optical attenuators 4a and 4b and wavelength selective reflecting means 5a and 5b, respectively, are provided between optical transmission lines L1 and L2, which oppose each other and on which optical amplifiers 4a and 4b are disposed each other.

Abstract: Flatness of a gain (optical output level) is maintained even when a wavelength division multiplex signal light (WDM signal light) is amplified in an optical amplification utilizing Raman amplification function. The optical amplifier comprises: two (first and second) pumping light sources for Raman amplification for outputting first and second pumping lights; the wavelength multiplexer for Raman amplification for allowing the first and second pumping lights to be incident on optical fiber transmission paths, where the WDM signal light propagates, in the direction opposite to the propagation direction of the signal light. The signal lights are Raman amplified by the both pumping lights. A wavelength of the first pumping light is set such that the gain of the Raman amplified signal light declines in the right direction, and, on the other side, the wavelength of the second pumping light is set such that the gain declines in the left direction.