AC-coupled differential drive circuit for opto-electronic modulators
View Patent ↗An AC-coupled differential drive circuit for an optical modulator is utilized, where a common “node” is defined between top (or bottom) plates of the modulator arms themselves (the “arms” of a modulator taking the form of MOS capacitors). A low pass filter is disposed between the differential driver output and the modulator's common node to provide the desired AC coupling by filtering out the DC bias voltage of the driver circuit itself without the need for a separate, external AC coupling capacitor. An independent, adjustable DC potential can then be applied to the common node, and will appear in a balanced manner across each arm of the modulator to provide the desired DC bias for the modulator independent of the DC bias of the driver circuit.
1. An integrated circuit for coupling an electrical driver to an optical modulator comprising an input waveguiding section and an output waveguiding section with a pair of parallel waveguide modulator arms disposed therebetween, the modulator arms responsive to the outputs from the electrical driver and coupled together so as to define a common node, the integrated circuit comprising
a low pass filter coupled between the driver output and the modulator common node and configured to extract a DC bias voltage associated with the electrical driver; and
an adder for combining a separate, independent DC voltage source with the output of the low pass filter at the modulator common node to provide a desired DC bias voltage to each waveguide modulator arm of the optical modulator.
2. The integrated circuit as defined in claim 1 wherein the electrical driver is a differential driver and the low pass filter is coupled between both outputs of the differential driver and the modulator common node.
3. An integrated circuit as defined in claim 1 wherein the low pass filter comprises a resistor and a capacitor coupled in series between the electrical driver output and ground potential, the intermediate connection between the resistor and the capacitor coupled to the input of the adder.
4. An integrated circuit as defined in claim 1 wherein the adder is configured to receive as an input an adjustable DC bias voltage from the separate DC voltage source.
5. An integrated circuit as defined in claim 1 wherein each modulator arm comprises a capacitor structure including a top plate, intermediate dielectric and a bottom plate, the bottom plates coupled together to define the common node.
6. An integrated circuit as defined in claim 1 wherein the optical modulator comprises an SOI-based optical modulator.