Amplifier having an amplifying optical fiber

An optical amplifier comprising an amplifying optical fiber having a first end and a

• second end and a length between said first and second ends substantially equal to an optimum length producing a maximum gain,
• means for coupling an optical signal and a pumping signal to said first end, and
• an optical isolator inserted in said amplifying optical fiber between said first and second ends, for transmitting said optical and pumping signals in a forward direction from said first end to said second end and for attenuating amplified spontaneous emissions travelling in a return direction opposite said forward direction,
• wherein said optical isolator is inserted at about the halfway point of said optimum length to thereby maximize said maximum gain by minimizing the accumulated effects of said amplified spontaneous emissions.