Continuously tunable optical filter

A tunable filter device for filtering an optical signal, comprising:

• a first ring filter comprising a first input, a first output bus and a first output drop;
• a first tuning element configured to tune a first characteristic wavelength of the first ring filter;
• a first optical coupler configured to split the optical signal into (i) an input optical signal that is input to the first ring filter via the first input and (ii) a first reference optical signal;
• a second optical coupler configured to combine the first reference signal with a band stop filtered optical signal, output from the first output bus of the first ring filter, to provide a first coupled optical signal and a second coupled optical signal;
• a first pair of photodiodes configured to detect the first and second coupled optical signals, respectively, and to provide corresponding first and second photocurrents;
• and
• a first control loop comprising:
• a first adder configured to determine a difference between the first photocurrent and the second photocurrent, and to provide first error signal indicating the difference;
• and
• a first control circuit configured to provide a first control signal, based at least in part on the first error signal, for controlling the first tuning element to automatically tune the first characteristic wavelength of the first ring filter to substantially equal a wavelength of the input optical signal, wherein the first characteristic wavelength of the first ring filter corresponds to a maximum of a wavelength dependent ratio of input optical power of the input optical signal and output optical power of a band pass filtered optical signal from the first output drop.

A tunable filter device for filtering an optical signal, the filter device comprising:

• a first filter bank comprising:
• a first ring filter configured to receive a first input optical signal corresponding to a portion of the optical signal, the first ring filter having a first tuning element for tuning a first characteristic wavelength of the first ring filter, wherein the first characteristic wavelength corresponds to a maximum of a wavelength dependent ratio of input optical power of the optical signal and output optical power of a band pass filtered optical signal output by the first ring filter;
• and
• a first control loop configured to provide a first control signal, based at least in part on a first reference signal corresponding to a reflected portion of the first input optical signal, that controls the first tuning element to automatically tune the first characteristic wavelength of the first ring filter to a wavelength of the optical signal;
• a second filter bank comprising:
• a second ring filter configured to receive a second input optical signal corresponding to a portion of the optical signal, the second ring filter having a second tuning element for tuning a second characteristic wavelength of the second ring filter, wherein the second characteristic wavelength corresponds to a maximum of a wavelength dependent ratio of input optical power of the optical signal and output optical power of a band pass filtered optical signal output by the second ring filter;
• and
• a second control loop, independent of the first control loop, configured to provide a second control signal, based at least in part on a second reference signal corresponding to a reflected portion of the second input optical signal, that controls the second tuning element to automatically tune the second characteristic wavelength of the second ring filter to a wavelength of the optical signal;
• an input optical switch configured to switch the optical signal between the first filter bank and the second filter bank to select an active filter bank, such that the second filter bank is reset when the first filter bank is selected as the active filter bank, and the first filter bank is reset when the second filter bank is selected as the active filter bank by switching the input optical switch;
• and
• an output optical switch configured to switch between the first filter bank and the second filter bank, in correspondence with the input optical switch, such that the band pass filtered optical signal output by one of the first ring filter in the first filter bank and the second ring filter in the second filter bank is output from the tunable filter device, respectively.