Methods for adjusting a modulator for optimal power
Aspects of the present disclosure provide an optical modulator with linearly distributed active circuitry coupled to a signal electrode to compensate for loss or attenuation of a high frequency modulation signal in the signal electrode. In one embodiment, negative resistance cells are attached to the signal electrode at various points, and have tunable negative resistances to compensate for the loss. In another embodiment, a segmented bias electrode is provided along the length of the optical waveguide in the optical modulator. Each segmented bias electrode may have a pre-determined bias voltage that can reduce impedance mismatches along the length of the signal electrode to reduce echoes and ripples in the modulation signal.
1. An optical modulator, comprising:
an optical waveguide configured to transmit a light signal along a length of the optical waveguide;
a signal electrode configured to receive a modulation signal, the signal electrode elongated alongside the length of the optical waveguide and coupled to the optical waveguide, wherein a first end of the signal electrode is coupled to a signal input terminal, a second end of the signal electrode is coupled to a termination resistor having a tunable resistance;
a multi-segment bias electrode having segments arranged alongside the length of the optical waveguide and coupled to the optical waveguide, wherein
some or all segments of the multi-segment bias electrode are configured to each receive a bias voltage of a plurality of bias voltages, and
the plurality of bias voltages and the tunable resistance selected such that a value of a characteristic of the light signal is within a predetermined range.
2. The optical modulator of claim 1 , further comprising:
a plurality of negative resistance cells coupled to respective points of a plurality of points between the first end and the second end along the length of the signal electrode, each negative resistance cell of the plurality of negative resistance cells having a tunable negative resistance.
3. The optical modulator of claim 2 , wherein negative resistances of the negative resistance cell are configured such that an attenuation of a voltage between the first end and the second end of the signal electrode is no more than 50% when the modulation signal has a frequency of between 30 and 100 GHz.
4. The optical modulator of claim 1 , wherein the characteristic is a bit error rate (BER).
5. The optical modulator of claim 1 , wherein each bias electrode is coupled to a respective bias terminal of a plurality of bias terminals, each bias terminal configured to provide a bias voltage to the corresponding bias electrode.
6. The optical modulator of claim 1 , wherein each bias electrode comprises a contact that is directly coupled to the optical waveguide at a plurality of bias contact points arranged along the first direction.
7. The optical modulator of claim 1 , wherein
a segment of the multi-segment bias electrode is aligned with the second end of the signal electrode along the length of the optical waveguide.
8. The optical modulator of claim 7 , wherein a bias voltage at the segment is selected such that an impedance of a portion of the signal electrode at the second end is matched with an impedance of the termination resistor.
9. The optical modulator of claim 3 , wherein the bias voltages provided to each bias terminal are selected such that a ripple amplitude at an end of the signal electrode at a frequency of 5 GHz or above is reduced compared to when the bias voltages at each bias terminal are identical.
10. The optical modulator of claim 3 , further comprising a voltage driver coupled to the plurality of bias terminals and configured to generate the bias voltage for each corresponding bias electrode.
11. The optical modulator of claim 10 , wherein the voltage driver comprises a digitally controlled voltage supply circuit.
12. A method for calibrating an optical modulator having an optical waveguide, a signal electrode having a terminal end coupled to a tunable terminal resistance, and a multi-segment bias electrode, the method comprising: providing a plurality of bias voltages to respective segments of the multi-segment bias electrode; providing a modulation signal to the signal electrode; monitoring a characteristic of a light signal transmitted through the optical waveguide; and iteratively adjusting one or more of the plurality of bias voltages and the terminal resistance such that a value of the monitored characteristic is within a predetermined range.
13. The method of claim 12 wherein the characteristic is a bit error rate (BER), and wherein adjusting one or more bias voltage comprises minimizing the value of the BER.
14. The method of claim 12 , wherein the signal electrode is coupled to a plurality of negative resistance cells at a plurality of points along a length of the signal electrode, each negative resistance cell having a tunable negative resistance, and the method further comprises: adjusting the tunable negative resistance for one or more negative resistance cells such that a value of the monitored characteristic is within a predetermined range.
15. The method of claim 12 , further comprising: monitoring a ripple amplitude at the signal input terminal at a frequency of 5 GHz or above, and wherein iteratively adjusting one or more bias voltages comprises minimizing the ripple amplitude.