L-BAND OPTICAL AMPLIFIER

An optical amplifier for amplifying an L-band optical signal is disclosed. T he amplifier has a first and a second stage and has an optical isolator disposed between the two stages for preventing light from propagating from the second stage to the first stage. The the first stage has a first length of rare earth doped amplifying fibre and has a pump for generating a pump with wavelength of more than 1400 nm coupled to the first length of rare earth doped optical fibre. This input end of the first length of rare earth doped optical fibre being coupled to receive and pre-amplify the L-band signal; A second stage has second length of rare earth doped amplifying fibre for receiving residual pu mp power from the first stage via the optical isolator and for receiving C-band ASE that has been generated within the first stage and for receiving the pre-amplified L-band signal. The second stage utilizes the residual pump power and the C-band ASE energy for constructively for further amplifying the pre-amplified L-band signal, in th e absence of a pump signal of less than 1400 nm being applied to the second stage.