Abstract: A passive optical network (PON) system with loop detection capability is disclosed. The PON system may include an optical line terminal (OLT) and an optical network unit (ONU). The PON system may also include a prober residing in at least one of the OLT and the ONU. The prober may be configured to send a probe frame through a port in the PON system. The probe frame may include at least a logical identifier of the port. Existence of a loop going through the port may be determined based on at least one of a first condition and a second condition. The first condition may represent that the probe frame returns to the prober at least once. The second condition may represent that the probe frame is received by the port at least twice.