Optical supervisory channel

A method of providing a supervisory channel on an optical path between an output of an optical amplifier of a first node of an optical communication network and an input of an optical amplifier of a second node of the optical communication network, the first and second nodes being arranged to use a set of wavelengths allocated for carrying traffic channels, the method comprising:

• removing from the optical path at a point located upstream of the optical amplifier of the second node an optical signal that carries a supervisory channel, wherein the optical signal was added to the optical path a point located between the output of the optical amplifier of the first node and the input of the optical amplifier of the second node;
• receiving the optical signal at a supervisory channel receiver;
• and
• measuring at least one impairment parameter of the received optical signal, wherein
• the optical signal has a wavelength which is one of the set of wavelengths allocated for carrying traffic,
• the method is performed at a time when the wavelength is not being used to carry traffic, and
• the step of removing the optical signal from the optical path at a point located upstream of the optical amplifier consists of removing the optical signal from the optical path at a point that is located between the input of the optical amplifier and the output of the first node, wherein
• the supervisory channel receiver is a coherent receiver and the at least one impairment parameter is one of:
• chromatic dispersion (CD) and polarization mode dispersion (PMD).

An apparatus for use at a first node of an optical communication network for providing a supervisory channel on an optical path between an output of an optical amplifier of the first node and an input of a second node of the optical communication network, the first node being arranged to use a set of wavelengths allocated for carrying traffic channels, the apparatus comprising:

• a supervisory channel transmitter for generating an optical signal carrying the supervisory channel;
• an optical coupler for adding the optical signal to the optical path at a point of the optical path located between the output of the optical amplifier and the input of the second node, wherein
• the optical signal has a wavelength which is one of the set of wavelengths normally used for carrying traffic, and
• the supervisory channel transmitter is arranged to generate the optical signal when the wavelength is not being used to carry traffic.

An apparatus, the apparatus comprising:

• an optical filter for removing an optical signal from an optical path optically connecting an output of an optical amplifier of a first node and an input of an optical amplifier of a second node, wherein the optical signal carries a supervisory channel, and wherein the optical filter is configured to remove the optical signal from the optical path at a position of the optical path that is located between the output of the first node and the input of the optical amplifier;
• and
• a supervisory channel receiver arranged to receive the optical signal and to measure at least one impairment parameter of the received optical signal, wherein
• the optical signal has a wavelength which is one of the set of wavelengths allocated for carrying traffic,
• the supervisory channel receiver is arranged to receive the optical signal at a time when the wavelength is not being used to carry traffic, and
• the optical signal was added to the optical path a point located between the output of the optical amplifier of the first node and the input of the optical amplifier of the second node, wherein
• the supervisory channel receiver is a coherent receiver, and
• the at least one impairment parameter is one of:
• chromatic dispersion (CD) and polarization mode dispersion (PMD).