Method and device for hitless tunable optical filtering

1. A method for filtering an optical signal comprising a plurality of channels lying on a grid of optical frequencies equally spaced by a frequency spacing and occupying an optical bandwidth, the method comprising:

operating an optical filter comprising resonators, each resonator having a respective free spectral range, wherein a first resonator is optically coupled to the optical signal and the remaining one or more resonators are optically coupled in series to the first resonator, so that a respective resonance of each one of the resonators falls within a first frequency band having bandwidth less than or equal to 15 GHz;

first tuning at least one resonator with respect to at least another resonator to obtain a separation between a resonance of the at least one resonator and a resonance of the at least another resonator greater than or equal to 25 GHz; and

second tuning the at least one resonator and the at least another resonator such that the respective resonance of each one of the resonators falls within a second frequency band, different from the first frequency band, having bandwidth less than or equal to 15 GHz;

wherein during a period between and including the first tuning and the second tuning, at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator is tuned outside a frequency region spanning between, and including, the first and the second frequency band.

2. The method of claim 1 , wherein the at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator belongs to a resonator different from the first resonator.

3. The method of claim 1 , wherein both the respective resonance of the at least one resonator and the respective resonance of the at least another resonator are also moved outside the frequency region comprised between, and including, the first and the second frequency band.

4. The method of claim 3 , wherein one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator is moved outside the frequency region either during the first tuning and the second tuning and the other among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator is moved outside the frequency region respectively either during the second tuning and the first tuning.

5. The method of claim 1 , further comprising, after the second tuning:

third tuning all the resonators of the optical filter so as to tune all respective resonances of the resonators by a frequency interval greater than the frequency spacing while maintaining a distance between the resonance of the at least one resonator with respect to the respective resonance of the at least another resonator not less than the separation.

6. The method of claim 5 , wherein during the third tuning, all the resonators are tuned substantially in unison so as to equally and contemporarily move all the respective resonances of the resonators.

7. The method of claim 1 , wherein the at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator is moved outside the frequency region within less than one channel spacing from a proximal edge of the frequency region.

8. The method of claim 1 , wherein all the resonators of the optical filter have the same free spectral range.

9. The method of claim 1 , wherein the separation is greater than or equal to 150 GHz.

10. The method of claim 1 , wherein the separation is less than or equal to 1000 GHz.

11. The method of claim 1 , wherein the resonators of the optical filter are optically coupled in series between a first optical path propagating the optical signal and a second optical path.

12. The method of claim 1 , wherein the resonators of the optical filter comprise two and no more than two resonators.

13. The method of claim 1 , wherein during the first tuning, the optical filter obtains the separation between each resonance of the at least one resonator falling within the optical bandwidth with respect to a respective closest resonance of the at least another resonator.

14. The method of claim 1 , wherein a first channel belonging to the optical signal at least partially overlaps the first frequency band and a second channel belonging to the optical signal at least partially overlaps the second frequency band.

15. The method of claim claim 1 , wherein the at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator belongs to a resonator different from the first resonator.

16. The method of claim 2 , wherein both the respective resonance of the at least one resonator and the respective resonance of the at least another resonator are also moved outside the frequency region comprised between, and including, the first and the second frequency band.

17. The method of claim 2 , further comprising, after the first tuning and before the second tuning:

intermediate tuning all the resonators of the optical filter so as to move all respective resonances of the resonators by a frequency interval greater than the frequency spacing while maintaining a distance between the resonance of the at least one resonator with respect to the respective resonance of the at least another resonator not less than the separation.

18. An optical device comprising:

an optical filter having:

an input port for receiving an optical signal comprising a plurality of channels lying on a grid of optical frequencies equally spaced by a frequency spacing and occupying an optical bandwidth, and an output port;

a first optical path optically connecting the input port to the output port; and

resonators, each having a respective free spectral range, wherein a first resonator is optically coupled to the first optical path and the remaining one or more resonators are optically coupled in series to the first resonator; and

a control system operatively connected to the resonators of the optical filter, the control system being configured to:

operate the optical filter so that a respective resonance of each one of the resonators falls within a first frequency band having bandwidth less than or equal to 15 GHz;

first tune at least one resonator with respect to at least another resonator to obtain a separation between a resonance of the at least one resonator and a resonance of the at least another resonator greater than or equal to 25 GHz; and

second tune the at least one resonator and the at least another resonator such that the respective resonance of each one of the resonators falls within a second frequency band, different from the first frequency band, having bandwidth less than or equal to 15 GHz;

wherein during a period between and including when the control system first tunes and second tunes, the control system tunes at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator outside the frequency region between, and including, the first and the second frequency band.

19. The device of claim 18 , wherein the resonators of the optical filter are optically coupled in series between the first optical path and a second optical path.

20. An optical communication system comprising:

a transmitter,

a receiver,

an optical line optically connecting the transmitter and the receiver; and

an optical device coupled along the optical line, the optical device comprising:

an optical filter having:

an input port for receiving an optical signal comprising a plurality of channels lying on a grid of optical frequencies equally spaced by a frequency spacing and occupying an optical bandwidth, and an output port;

a first optical path optically connecting the input port to the output port; and

resonators, each resonator having a respective free spectral range, wherein a first resonator is optically coupled to the first optical path and the remaining one or more resonators are optically coupled in series to the first resonator; and

a control system operatively connected to the plurality of resonators of the optical filter, the control system configured to:

operate the optical filter so that a respective resonance of each one of resonators falls within a first frequency band having bandwidth less than or equal to 15 GHz;

first tune at least one resonator with respect to at least another resonator to obtain a separation between a resonance of the at least one resonator and a resonance of the at least another resonator greater than or equal to 25 GHz; and

second tune the at least one resonator and the at least another resonator such that the respective resonance of each one of the resonators falls within a second frequency band, different from the first frequency band, having bandwidth less than or equal to 15 GHz;

wherein during a period between and including when the control system first tunes and second tunes, the control system tunes at least one among the respective resonance of the at least one resonator and the respective resonance of the at least another resonator outside the frequency region comprised between, and including, the first and the second frequency band.