Repeater and loopback mode switching method

Provided are a repeater and a loopback mode switching method. The repeater (10) comprises: a first optical coupler (201), a first optical amplifier (OA1) and a second optical coupler (202) that are connected in sequence in an uplink direction of an underwater cable, and a third optical coupler (203), a second optical amplifier (OA2) and a fourth optical coupler (204) that are connected in sequence in a downlink direction of the underwater cable. The repeater further comprises a switcher (100). The switcher comprises a first uplink terminal (101), a second uplink terminal (102), a first downlink terminal (103) and a second downlink terminal (104). The first uplink terminal is coupled to the first optical coupler, the second uplink terminal is coupled to the second optical coupler, the first downlink terminal is coupled to the third optical coupler, and the second downlink terminal is coupled to the fourth optical coupler. The switcher changes the connection relationship between the four terminals correspondingly according to a received switch indication, so that the repeater switches between an egress-to-ingress loopback mode and an ingress-to-egress loopback mode. Since the repeater can switch between the egress-to-ingress loopback mode and the ingress-to-egress loopback mode, a monitoring device is enabled to monitor a longer distance and obtain a larger dynamic range, and no co-channel interference is caused to service light in the downlink, so that a high signal-to-noise ratio of a service is obtained.