Thermally tuneable optical modulator adapted for differential signaling

1. Apparatus for optical modulation, comprising:

a modulator structure comprising a ring or disk optical resonator having a closed curvilinear periphery and a pair of oppositely doped semiconductor regions within and/or adjacent to the optical resonator and conformed to modify the optical length of the optical resonator upon application of a bias voltage;

a doped semiconductor modulation contact region situated within an area enclosed by the modulator structure, wherein said contact region is juxtaposed to at least one of the oppositely doped semiconductor regions;

a heater structure comprising a relatively resistive annulus of semiconductor material enclosed between an inner disk and an outer annulus of relatively conductive semiconductor material, the inner disk and outer annulus being adapted as contact regions for a heater activation current, the heater structure being situated within the periphery of the optical resonator such that in operation, at least a portion of the resonator is heated by radial conductive heat flow from the heater structure; and

a substantially annular isolation region of dielectric or relatively resistive semiconductor material interposed between the outer annulus of the heater structure and the said doped semiconductor modulation contact region, the isolation region being effective to electrically isolate the bias voltage from the heater activation current.

2. The apparatus of claim 1 , wherein the isolation of the bias voltage from the heater activation current, by the isolation region, is sufficient to permit the modulator to be driven by differential signaling.

3. The apparatus of claim 1 , further comprising a driver circuit electrically connected to the modulator structure and configured to drive the modulator by differential signaling.

4. The apparatus of claim 1 , wherein the modulator structure and the heater structure are formed of silicon.

5. The apparatus of claim 4 , wherein the modulator structure and the heater structure are formed on an SOI wafer.

6. The apparatus of claim 4 , wherein the isolation region comprises intrinsic silicon.

7. The apparatus of claim 4 , wherein the isolation region comprises silicon oxide.

8. The apparatus of claim 1 , wherein the modulator structure and the heater structure are substantially circular and concentric.

9. The apparatus of claim 1 , wherein the optical resonator comprises a substantially circular waveguide, the heater structure is substantially circular, and the heater structure is non-concentric with the resonator waveguide.

10. The apparatus of claim 1 , wherein a horizontal semiconductor junction is defined in the modulator structure.

11. The apparatus of claim 1 , wherein a vertical semiconductor junction is defined in the modulator structure.

12. The apparatus of claim 1 , comprising in order of increasing radius:

(a) a contact disk region for the heater structure;

(b) a resistive heating region for the heater structure;

(c) a contact annular region for the heater structure;

(d) an isolation annular region;

(e) an inner annular contact region for modulator contact;

(f) an inner annular modulator region doped for forming a semiconductor junction in the modulator structure;

(g) an annular waveguide;

(h) an outer annular modulator region, doped oppositely to the inner annular modulator region, for forming a semiconductor junction in the modulator structure; and

(i) an outer modulator contact region.

13. The apparatus of claim 12 , wherein elements (a)-(h) collectively occupy a region having a maximum lateral dimension of 10 μm or less.

14. The apparatus of claim 12 , wherein elements (a)-(h) collectively occupy a region having a maximum lateral dimension of 4 μm or less.

15. The apparatus of claim 1 , wherein the modulator structure, the heater structure, and the isolation region collectively occupy a region having a maximum lateral dimension of 10 μm or less.

16. A method for modulating an optical signal, comprising:

coupling the optical signal to a ring or disk resonator via evanescent optical coupling, wherein the resonator describes a closed curvilinear shape having an interior;

tuning the resonator with heat flowing radially outward from a heating element situated in the interior of the closed curvilinear shape; and

modifying an optical length of the resonator through application of a bias voltage, wherein the bias voltage is applied by differential signaling; wherein:

the heating element comprises an inner disk and an outer annulus of relatively conductive semiconductor material and a relatively resistive annulus of semiconductor material enclosed between the inner disk and outer annulus;

the tuning of the resonator comprises passing an electric heater activation current between the inner disk and the outer annulus and through the relatively resistive annulus of semiconductor material;

the bias voltage is applied between a pair of oppositely doped semiconductor regions within and/or adjacent to the optical resonator;

the bias voltage is applied to at least one of the oppositely doped semiconductor regions through a doped semiconductor modulation contact region situated within an area enclosed by the modulator structure,

said doped semiconductor contact region is juxtaposed to at least one of the oppositely doped semiconductor regions;

the radial heat flow for tuning the resonator passes through a substantially annular isolation region of dielectric or relatively resistive semiconductor material interposed between the outer annulus of the heating element and the doped semiconductor modulation contact region; and

the applying of the bias voltage is electrically isolated, by the interposed isolation region, from the passing of the electric heater activation current.