Bidirectional Fiber Amplifiers

We propose new bidirectional erbium-doped fiber amplifier repeaters for bidirectional fiber networks and OTDR fault detection. At each repeater the counterpropagating signals are separated, amplified separately, then recombined. Signal separation is done by directional couplers with intrinsic 6 dB coupling loss, or by optical circulators with no intrinsic loss. Although the 3 dB input directional coupler loss can be compensated with a few milliwatts extra pump power, it increases the amplifier noise figure by 3 dB. The pump power required to compensate for the output directional coupler loss increases with the output signal power to 3 dB when the amplifier is saturated. The saturation output power of the directional coupler module is approximately 3 dB lower than the optical circulator. Multiple reflection induced relative intensity noise is reduced by assigning non-overlapping optical bands to the signals in the two directions.