Patent ID: 7684711

Claim:
An optical transmission system comprising: a first optical fibre extending between a first terminal and a second terminal for carrying optical signal traffic from the first terminal to the second terminal; a second optical fiber extending between the first terminal and the second terminal for carrying optical signal traffic from the second terminal to the first terminal; and a plurality of optical repeaters coupled to the first and second optical fibers between the first terminal and the second terminal, each repeater having a permanently connected passive high loss loop back circuit between the first and second fibres; the first terminal including a transmitter and a receiver, wherein in use, the transmitter launches a pulsed supervisory signal on a dedicated supervisory wavelength on the first optical fibre, and the receiver detects the portion of the supervisory signal looped back from each repeater into the second optical fibre in order to identify the existence and location of faults in the transmission system, and wherein the pulsed supervisory signal is of sufficiently short duration such that portions of the signal returned from each repeater do not overlap with one another and the supervisory wavelength is different to that of any optical signal traffic on the first and second optical fibres, wherein the receiver comprises a heterodyne receiver, the heterodyne receiver comprising a local oscillator coupled to a first mixer for mixing the received loopback signal portion with a signal from the local oscillator and a second mixer coupled to both the local oscillator and a transmit oscillator in the transmitter for mixing signals from the two oscillators to provide a reference frequency for synchronous demodulation of the received signal.