Patent ID: 7359637

Claim:
A passive optical network comprising: a central office; a plurality of optical network units, each of the optical network unit including: a first upstream light source for outputting an upstream optical signal; a circulator having a first to a third port, for outputting the upstream optical signal input to the first port to the second port connected to a distribution optical fiber, for outputting the returning upstream optical signal input to the second port to the third port; an optical receiver for receiving the upstream optical signal output from the third port of the circulator, and converting the received upstream optical signal into an electrical signal to be output; a second upstream light source for outputting an upstream optical signal; and a first switch for connecting the first upstream light source to the circulator in a normal state, for connecting the circulator to the second upstream light source when an abnormality has occurred at the first upstream light source; a remote node connected to the central office through a main optical fiber, the remote node transmitting one portion of power of the upstream optical signal, which has input from each of the optical network units, to the central office, and returning a remaining portion of the power of the upstream optical signal to the optical network unit; and a plurality of distribution optical fibers connecting the plurality of optical network units to the remote node, wherein each of the optical network units transmitting the upstream optical signal to the remote node through a corresponding distribution optical fiber, and detecting abnormality occurrence from a state of the upstream optical signal returning from the remote node.