Power balancing in DWDM optical networks

1. A method of balancing the power level of an optical signal within an aggregate of optical signals in an amplified dense wavelength division multiplexing DWDM optical network having an optical link extending between an optical signal transmitter and an optical signal receiver, said optical signal transmitter associated with a multiplexer following it on along said optical link, said optical signal receiver associated with a multiplexer preceding it along said optical link, said optical link having one or more optical fibers spans, some of the optical fiber spans having an optical amplifier and variable optical attenuator VOA, said method comprising:

measuring the power level of the optical signal received at the receiver through the optical link and referencing said measured receive power level to a nominal power level in the multiplexer immediately preceding the receiver;

measuring the power level of the optical signal at the optical transmitter and referencing said measured transmit power level to a nominal power level in the multiplexer immediately following the transmitter;

calculating a mean power level of the optical signal from said optical measurements, the mean power level of the optical signal being the mean of the power levels of the optical signal at the transmitter and at the receiver referenced to the nominal power levels in the associated multiplexers; and

adjusting the mean power level of the optical signal independently of the power levels of other optical signals carried on the same optical link to the receiver so that a desired OSNR at the receiver is achievable.

2. A method according to claim 1 , wherein the mean power level of the optical signal is a logarithmic mean power level.

3. A method according to claim 1 , wherein adjusting the mean power level of the optical signal is conducted using a first-order closed control loop in log space so as to maintain network stability.

4. A method according to claim 1 , wherein adjusting the mean power level comprises:

calculating an amount of transmitter optical power adjustment of the optical transmitter from said optical power level measurements; and

adjusting the power level of the optical signal at the transmitter by an amount that sets the mean power level of the optical signal to substantially the nominal value.

5. A method according to claim 1 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.

6. A method according to claim 2 , wherein adjusting the mean power level of the optical signal is conducted using a first-order closed control loop in log space so as to maintain network stability.

7. A method according to claim 2 , wherein adjusting the mean power level comprises:

calculating an amount of transmitter optical power adjustment of the optical transmitter from said optical power level measurements; and

adjusting the power level of the optical signal at the transmitter by an amount that sets the mean power level of the optical signal to substantially the nominal value.

8. A method according to claim 2 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.

9. A method according to claim 3 , wherein adjusting the mean power level comprises:

calculating an amount of transmitter optical power adjustment of the optical transmitter from said optical power level measurements; and

adjusting the power level of the optical signal at the transmitter by an amount that sets the mean power level of the optical signal to substantially the nominal value.

10. A method according to claim 3 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.

11. A method according to claim 4 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.

12. A method according to claim 6 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.

13. Power balancing apparatus for balancing the power level of an optical signal within an aggregate of optical signals in an amplified dense wavelength division multiplexing DWDM optical network having an optical link extending between an optical signal transmitter and an optical signal receiver, said optical signal transmitter associated with a multiplexer following it on along said optical link, said optical signal receiver associated with a multiplexer preceding it along said optical link, said optical link having one or more optical fibers spans, some of the optical fiber spans having an optical amplifier and variable optical attenuator VOA, said apparatus comprising:

a first-order closed mean power control loop extending between the receiver and the transmitter, said control loop operable to calculate a mean power level of the optical signal and to adjust the mean power level of said optical signal so that a desired OSNR for said optical signal at the receiver is achieved, the mean power level being the mean of the power levels of said optical signal at the transmitter and at the receiver referenced to the nominal power levels in the associated multiplexers.

14. Power balancing apparatus according to claim 13 , further comprising:

receive power measuring circuitry operable to measure the power level of the optical signal being received by the optical receiver through the optical link and to reference said measured receive power level to a nominal power level in the multiplexer immediately preceding the receiver;

transmit power measuring circuitry operable to measure the power level of the optical signal being transmitted by the optical transmitter and to reference said measured transmit power level to a nominal power level in the multiplexer immediately following the transmitter; and

said first-order closed mean power control loop comprises:

a receiver for receiving the optical power level measurements of the optical signal;

a processor operable to calculate the mean power level of the optical signal, and an amount of transmitter optical power adjustment of the optical transmitter from said optical power level measurements; and

a communication line operable to communicate the amount of transmitter optical power adjustment of the optical transmitter.

15. Power balancing apparatus according to claim 13 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

an amplifier mean power control loop extending between downstream of the amplifier and upstream of the VOA, said amplifier mean power control loop operable to determine the span loss of the preceding optical fiber span and to control the VOA to adjust the power level of the aggregate of optical signals being input to the amplifier, said adjustment being dependent on the number of optical signals comprising the aggregate and on the amplifier gain.

16. An optical communications system comprising:

a dense wavelength division multiplexing DWDM optical network having an optical link extending between an optical signal transmitter and an optical signal receiver, said optical signal transmitter associated with a multiplex following it on along said optical link, said optical signal receiver associated with a multiplexer preceding it along said optical link, said optical link having one or more optical fibers spans, some of the optical fiber spans having an optical amplifier and variable optical attenuator VOA; and

a first-order closed mean power control loop extending between the receiver and the transmitter, said control loop operable to calculate a mean power level of the optical signal and to adjust the mean power level of said optical signal so that a desired OSNR for said optical signal at the receiver is achieved, the mean power level being the mean of the power levels of said optical signal at the transmitter and at the receiver referenced to the nominal power levels in the associated multiplexers.

17. An optical communications system according to claim 16 , further comprising:

receive power measuring circuitry operable to measure the power level of the optical signal being received by the optical receiver through the optical link and to reference said measured receive power level to a nominal power level in the multiplexer immediately preceding the receiver;

transmit power measuring circuitry operable to measure the power level of the optical signal being transmitted by the optical transmitter and to reference said measured transmit power level to a nominal power level in the multiplexer immediately following the transmitter; and

said first-order closed mean power control loop comprises:

a receiver for receiving the optical power level measurements of the optical signal;

a processor operable to calculate the mean power level of the optical signal, and an amount of transmitter optical power adjustment of the optical transmitter from said optical power level measurements; and

a communication line operable to communicate the amount of transmitter optical power adjustment of the optical transmitter.

18. An optical communications system according to claim 16 , further comprising

an amplifier mean power control loop extending between downstream of the amplifier and upstream of the VOA, said amplifier mean power control loop operable to determine the span loss of the preceding optical fiber span and to control the VOA to adjust the power level of the aggregate of optical signals being input to the amplifier, said adjustment being dependent on the number of optical signals comprising the aggregate and on the amplifier gain.

19. A computer readable storage medium storing instructions that, when executed by a computer, cause the computer to perform a method of balancing the power level of an optical signal within an aggregate of optical signals in an amplified dense wavelength division multiplexing DWDM optical network having an optical link extending between an optical signal transmitter and an optical signal receiver, said optical signal transmitter associated with a multiplexer following it on along said optical link, said optical signal receiver associated with a multiplexer preceding it along said optical link, said optical link having one or more optical fibers spans, some of the optical fiber spans having an optical amplifier and variable optical attenuator VOA, said method comprising:

measuring the power level of the optical signal received at the receiver through the optical link and referencing said measured receive power level to a nominal power level in the multiplexer immediately preceding the receiver;

measuring the power level of the optical signal at the optical transmitter and referencing said measured transmit power level to a nominal power level in the multiplexer immediately following the transmitter;

calculating a mean power level of the optical signal from said optical measurements, the mean power level of the optical signal being the mean of the power levels of the optical signal at the transmitter and at the receiver referenced to the nominal power levels in the associated multiplexers; and

adjusting the mean power level of the optical signal independently of the power levels of other optical signals carried on the same optical link to the receiver so that a desired OSNR at the receiver is achievable.

20. A computer readable storage medium according to claim 19 , further comprising in respect of each optical fiber span having an optical amplifier and variable optical attenuator VOA:

configuring the VOA for setting the mean power into each amplifier on start-up to a nominal value when the fiber loss of the preceding optical fiber span is less than a design maximum value; thereafter

at a low number of optical signals in the multiplexer, controlling the VOA with low gain; and

at a high number of optical signals in the multiplexer, controlling the VOA with high gain.