Technique for power control in optical networks

1. A method for maintaining a required value of power per optical channel in a chain of optical nodes connected by optical fiber spans, wherein at least some of said nodes including optical amplifiers with controllable gains, the method comprising:

performing automatic gain control at said optical amplifiers so that, in the frame of power gain of an optical amplifier, to compensate a preceding span loss; said span loss being power loss on an optical fiber span incoming said amplifier;

obtaining information from a preceding node in the chain about a value of measured output power (MOP) of the preceding node;

measuring input power (MIP) at a following node;

obtaining a difference between the MOP of the preceding node and the MIP of the following node to calculate the span loss between the two nodes,

wherein the automatic gain control is performed at the amplifier of the following node in response to the calculated span loss; and

obtaining, from said preceding node, information about power loss (ΔPL) which has not been compensated at the preceding node, thereby ensuring distributed compensation of power loss changes if occur in one or more spans of the chain.

2. The method according to claim 1 , wherein the nodes include said optical amplifiers and one or more Optical Add Drop Multiplexers (OADMs).

3. The method according to claim 2 , further comprising a step of preliminary calibration of power of add channels of OADMs constituting nodes of the chain.

4. The method according to claim 2 , further comprising the following steps when a fiber cut is detected:

if the fiber cut is detected in the incoming fiber span of a node being an amplifier, switching the amplifier off and sending a signal Loss Of Signal (LOS) instead of information on the output power;

if the fiber cut is detected in the incoming fiber span of a node being OADM, keeping add channels of the OADM as adjusted at a previous monitoring session.

5. The method according to claim 1 , further comprising a step of preliminary calibration of power per optical channel at a first node in the chain.

6. The method according to claim 1 , comprising performing the following steps for an intermediate node “i” of the chain:

obtaining information on two parameters MOP(i−1) and ΔPL(i−1), the MOP(i−1) being said MOP of the preceding node, and the ΔPL(i−1) being said power loss which has not been compensated at the preceding node;

obtaining parameter MIP(i) being said MIP of the node “i”,

processing the parameters MOP(i−1), ΔPL(i−1) and MIP(i) to obtain a non-compensated power loss accumulated before the node “i”:

NCPLi=ΔPLi− 1+( MOPi− 1− MIPi );

in case the node “i” is an optical amplifier, controlling its gain so as to at least partially compensate the obtained non-compensated power loss accumulated before the node “i”(NCPLi),

producing two parameters MOPi and ΔPLi to be used for the next node “i+1”, wherein MOPi is said MOP of the node “i”, and ΔPLi is power loss which has not been compensated at the node “i”.

7. The method according to claim 6 , further comprising a step of pre-setting a particular extra gain value ΔGV for a node comprising an optical amplifier.

8. The method according to claim 7 , wherein the step of controlling the gain of the amplifier comprises changing it in the frame of its gain range so as to at least partially cover a gain sum (NCPLi+ΔGV), and

the step of producing the parameter ΔPLi comprises considering the ΔPLi to be equal to a portion of said gain sum not covered by the change of the amplifier's gain.

9. The method according to claim 6 , further comprising, if the node “i” is OADM, considering the parameter ΔPLi to be equal to value NCPLi+TCA, wherein TCA represents through-channels' attenuation of the OADM.

10. The method according to claim 6 , further comprising, if the node “i” is OADM, a step of adjusting power of add channels of the OADM in response to obtained value of NCPLi.

11. The method according to claim 10 , wherein the step of adjusting power of add channels of OADM comprises comparing the obtained value of NCPLi with a reference value of NCPLi.

12. The method according to claim 10 , comprising providing the adjustment of the add channels based on a periodic monitoring and using pre-selected thresholds.

13. The method according to claim 1 , wherein the step of obtaining the information and the step of controlling the gain are performed on the basis of periodical monitoring and by using pre-selected thresholds.

14. The method according to claim 1 , comprising transmitting said information via OSC (Optical Supervisory Channel).

15. A system for maintaining a required value of power per optical channel in a chain of optical nodes connected by optical fiber spans, wherein at least some of said nodes including optical amplifiers with controllable gains, the system being capable of providing automatic gain control at said optical amplifiers so that, in the frame of power gain of an optical amplifier, to compensate a preceding span loss, wherein said span loss being power loss on an optical fiber span incoming said amplifier,

the system being operative to:

obtain information from a preceding node in the chain about a value of measured output power (MOP) of the preceding node,

measure input power (MIP) at a following node,

obtain a difference between the MOP and the MIP at the following node to calculate the span loss between the two nodes,

wherein the automatic gain control is performed at the amplifier of the following node in response to the calculated span loss;

the system being additionally adapted to:

provide, for at least one of said optical nodes,

information from a preceding node in the chain concerning a value of power loss (ΔPL) which has not been compensated at the preceding node, thereby ensuring distributed compensation of power loss changes if occur in one or more spans of the chain.

16. A system for maintaining required power per channel in an optical chain comprising nodes connected by optical fiber spans, wherein the nodes comprise optical amplifiers having gain controllable within the gain range and comprising one or more Optical Add Drop Multiplexers (OADM) capable of regulating power of add channels according to power of incoming channels, wherein an intermediate node “i” being operative to:

receive information on two parameters MOPi−1 and ΔPLi−1 from its preceding node “i−1”, the MOPi−1 being Measured Output Power of the preceding node, and ΔPLi−1 being Power Loss which has not been compensated at the preceding node;

measure parameter MIPi being Measured Input Power of the node “i”,

process the parameters MOPi−1, ΔPLi−1 and MIPi at the node “i” to obtain a non-compensated power loss accumulated before the node “i”:

NCPLi=ΔPLi− 1+( MOPi− 1 −MIPi );

in case the node “i” is an optical amplifier, to control its gain within its range so as to at least partially compensate the obtained non-compensated power loss accumulated before the node “i”(NCPLi),

to produce two parameters MOPi and ΔPLi to be sent to the next node “i+1”, wherein MOPi is Measured Output Power of the node “i”, and ΔPLi is Power Loss which has not been compensated at the node “i”, said system thereby enabling distributed compensation of power loss changes if occurred in one or more of said fiber optic spans.

17. The system according to claim 16 , capable of pre-setting an extra gain value ΔGV at a particular amplifier node to allow said particular amplifier to provide a specific output power value in the frame of its gain range; said node being operative to control the gain of said particular amplifier so as to at least partially cover a gain sum (NCPLi+ΔGV), and operative to produce the parameter ΔPLi equal to a portion of said gain sum not covered by the change of the amplifier's gain.