Spectral gain hole burning and modulation instability in a Brillouin fiber amplifier

We investigate theoretically the gain spectrum of a Brillouin fiber amplifier in the saturated region. When the amplifier is operated in the saturated region a hole is burned in the single-pass Brillouin gain spectrum, and peaks appear at frequencies determined mainly from the round-trip time of the amplifier, causing modulation instability in the amplified signal. This type of modulation instability originates intrinsically from signal amplification by the counterpropagating pump beam, which is depleted by the strong signal beam.