The high power (5 dBm) BP light is fed into the fiber ring through the 3 dB coupler and amplified by EDFA. Then it enters SMF 1 via cir 1. After entering SMF1, once the pre-amplified BP
power exceeds the Brillouin threshold of SMF 1, the first-order Brillouin Stokes (BS 1) light will be generated and propagates in the backward direction.