A repeater, comprising: a first optical amplifier, a second optical amplifier, a first gating unit, a second gating unit, a first coupler, a second coupler, a third coupler, and a fourth coupler, wherein: the first optical amplifier is configured to amplify optical signals transmitted by an uplink fiber; the second optical amplifier is configured to amplify optical signals transmitted by a downlink fiber; characterized in that the first gating unit is configured to gate a first probe light, wherein one end of the first gating unit and an input end of the first optical amplifier are both connected with the first coupler, the other end of the first coupler is connected with an uplink fiber of a previous span, the other end of the first gating unit and an input end of the second optical amplifier are both connected with the fourth coupler, and the other end of the fourth coupler is connected with a downlink fiber of a next span, wherein said other end of the fourth coupler is configured to receive a scattered and/or reflected part of the first probe light, and the scattered and/or reflected part of the first probe light is returned to the fourth coupler from the downlink fiber of the next span; and the second gating unit is configured to gate a second probe light, wherein one end of the second gating unit and an output end of the first optical amplifier are both connected with the second coupler, the other end of the second coupler is connected with an uplink fiber of the next span, the other end of the second gating unit and an output end of the second optical amplifier are both connected with the third coupler, and the other end of the third coupler is connected with a downlink fiber of the previous span, wherein said other end of the second coupler is configured to receive a scattered and/or reflected part of the second probe light, and the scattered and/or reflected part of the second probe light is returned to the second coupler from the uplink fiber of the next span. the first optical amplifier is configured to amplify optical signals transmitted by an uplink fiber; the second optical amplifier is configured to amplify optical signals transmitted by a downlink fiber; characterized in that the first gating unit is configured to gate a first probe light, wherein one end of the first gating unit and an input end of the first optical amplifier are both connected with the first coupler, the other end of the first coupler is connected with an uplink fiber of a previous span, the other end of the first gating unit and an input end of the second optical amplifier are both connected with the fourth coupler, and the other end of the fourth coupler is connected with a downlink fiber of a next span, wherein said other end of the fourth coupler is configured to receive a scattered and/or reflected part of the first probe light, and the scattered and/or reflected part of the first probe light is returned to the fourth coupler from the downlink fiber of the next span; and the second gating unit is configured to gate a second probe light, wherein one end of the second gating unit and an output end of the first optical amplifier are both connected with the second coupler, the other end of the second coupler is connected with an uplink fiber of the next span, the other end of the second gating unit and an output end of the second optical amplifier are both connected with the third coupler, and the other end of the third coupler is connected with a downlink fiber of the previous span, wherein said other end of the second coupler is configured to receive a scattered and/or reflected part of the second probe light, and the scattered and/or reflected part of the second probe light is returned to the second coupler from the uplink fiber of the next span.