Pulsed high loss loop back signalling scheme

A method of monitoring the performance of 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 fibre extending between the first terminal and the second terminal for carrying optical signal traffic from the second terminal to the first terminal, and at least one optical repeater coupled to the first and second optical fibres 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, comprising the steps of:

• launching a pulsed supervisory signal on a dedicated supervisory wavelength on the first optical fibre, and
• detecting the portion of the supervisory signal looped back from each repeater into the second fibre in order to identify the existence and location of faults in the transmission system, 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 step of detecting the portion of the supervisory signal further comprises heterodyne reception of the detected loopback signal portion and synchronous demodulation of the received signal.

An optical transceiver comprising:

• means to produce a pulsed supervisory signal on a dedicated supervisory wavelength;
• and
• means to detect portion of the supervisory signal looped back to the transceiver from repeaters in an optical transmission system coupled to the transceiver, in order to identify the existence and location of faults in the transmission system,
• 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 optical transmission system
• wherein a receiver section of the transceiver 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.