Electrically configurable optical filters
Aspects and examples are directed to programmable optical finite impulse response filters and optical infinite impulse response filters, which may be implemented as photonic integrated circuits.
1. An optical infinite impulse response filter comprising:
an input configured to receive an input optical signal;
a first substrate on which at least a portion of the filter is implemented;
a second substrate coupled to the first substrate;
an output configured to provide an output optical signal that is a filtered version of the input optical signal;
a plurality of input filter taps coupled to the input and configured to provide a corresponding plurality of weighted signal taps of the input optical signal;
a plurality of output filter taps coupled the output and configured to provide a corresponding plurality of weighted signal taps of the output optical signal, each of the plurality of input filter taps and each of the plurality of output filter taps including a variable delay implemented in the second substrate and an individually addressable attenuator; and
a plurality of summers configured to receive and sum the plurality of weighted signal taps of the input optical signal and the plurality of weighted signal taps of the output optical signal to produce the output optical signal at the output.
2. The optical infinite impulse response filter of claim 1 wherein the the second substrate is a piezoelectric substrate, and the optical infinite impulse response filter further includes a control input coupled to the piezoelectric substrate and configured to receive a control voltage that controls an amount of phase delay provided by the piezoelectric substrate.
3. The optical infinite impulse response filter of claim 2 wherein the piezoelectric substrate is a Lithium Niobate substrate.
4. The optical infinite impulse response filter of claim 2 wherein the variable delay further includes at least one delay line.
5. The optical infinite impulse response filter of claim 4 wherein the at least one delay line includes a plurality of switchable delay lines.
6. The optical infinite impulse response filter of claim 1 further comprising a gain element coupled to the input and configured to compensate for loss introduced by the individually addressable attenuators.
7. The optical infinite impulse response filter of claim 1 wherein a delay value of a variable delay is a function of a bit rate of the input optical signal, the optical infinite impulse response filter further comprising a control input configured to receive a control voltage that sets the delay value, the control voltage being adjustable to accommodate varying bit rates of the input optical signal, wherein the control voltage provides phase control of the output optical signal and the control signal on a control bus provides amplitude control of the output optical signal.
8. The optical infinite impulse response filter of claim 1 , wherein the first substrate is a silicon substrate, further comprising:
a control bus coupled to each individually addressable attenuator and configured to provide a control signal to each individually addressable attenuator to set an attenuation value of the individually addressable attenuator; and
wherein the optical infinite impulse response filter is implemented on the silicon substrate as a photonic integrated circuit.
9. The optical impulse filter of claim 1 further comprising one or more delay lines, implemented on the second substrate, switchably coupled to at least one filter tap of the plurality of input filter taps or the plurality of output filter taps.
10. The optical impulse filter of claim 1 wherein at least the input, the output, and the summers are implemented on the first substrate.
11. The optical impulse filter of claim 10 wherein every element of the filter except the variable delay is implemented on the first substrate.
12. A method of operating an optical infinite impulse response filter, comprising:
receiving, at an input coupled to a first substrate, an input optical signal;
operating a plurality of input filter taps to process the input signal to provide a corresponding plurality of weighted signal taps of the input optical signal, each of the plurality of input filter taps including a variable delay implemented on a second substrate and an individually addressable attenuator; and
operating a plurality of output filter taps to provide a corresponding plurality of weighted signal taps of the output optical signal, each of the plurality of output filter taps including a variable delay implemented on a second substrate and an individually addressable attenuator; and
operating a plurality of summers to receive and sum the plurality of weighted signal taps of the input optical signal and the plurality of weighted signal taps of the output optical to produce an output optical signal, the output optical signal being a filtered version of the input optical signal.
13. The method of claim 12 , further comprising:
operating a control bus to provide a control signal to each individually addressable attenuator to set an attenuation value of the individually addressable attenuator.
14. The method of claim 12 , wherein the second substrate is a piezoelectric substrate, and
wherein the method further comprises controlling an amount of phase delay provided by the piezoelectric substrate via a control voltage applied to the piezoelectric substrate.
15. The method of claim 12 further comprising switchably operating one or more delay lines coupled to at least one filter tap of the plurality of input filter taps or the plurality of output filter taps to adjust the variable delay implemented on the second substrate.
16. The method of claim 12 wherein at least the input, output, and the summers are implemented on the first substrate.
17. The method of claim 16 wherein every element of the filter is implemented on the first substrate except the variable delay.