ASYMMETRIC DIRECTIONAL COUPLER HAVING A REDUCED DRIVE VOLTAGE

Abstract

The invention relates to an electro-optic directional coupler suitable for use as a variable optical attenuator at reduced voltages compared to those known in the prior art. The present invention has found that by careful selection of an asymmetric directional coupler geometry, the transfer function of the device can be shifted so that it has an operating point between maximum and minimum transmission. Signal electrodes driven in push pull configuration advantageously use this operating point to achieve significant reduction in operating voltages for switching to maximum or minimum transmission. Asymmetry is created in the directional coupler by forming the waveguides to have different propagation constants by a difference in waveguide width, depth, index of refraction or index profile. Asymmetry can alternatively be created by causing mechanical stress in the waveguides through the placement, number, or dimensions of the electrodes, or through asymmetric dielectric structures between the waveguides and the electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

FIG. 1 is a schematic top view illustration of a channel waveguide layout for a Mach Zehnder with a variable optical attenuator;

FIG. 2 is a schematic top view illustration of the VOA of FIG. 1 alone;

FIG. 3 is a top view illustration of a Prior Art pattern of electrodes for a VOA, and showing a portion of the underlying waveguides;

FIG. 4 is a top view of electrodes for a directional coupler in accordance with the present invention;

FIG. 5 is a cross-section of the directional coupler of FIG. 4 showing the electrodes and waveguides as positioned on the electro-optic substrate;

FIG. 6 is an enlarged view of the waveguides of FIG. 4 showing the waveguide widths in closer detail;

FIG. 7 is an enlarged view of the waveguides of FIG. 4 showing the waveguide widths in closer detail for an alternative embodiment where waveguide width varies between two widths in one of the waveguides;

FIG. 8 is an alternate embodiment of the present invention illustrating an electrode offset with respect to the waveguides for creating a difference in propagation constants;

FIG. 9 is a cross-section of the embodiment of FIG. 8;

FIG. 10 is an enlarged view of the cross-section of FIG. 9 showing the electrode offset in closer detail;

FIG. 11 is a graphic illustration of a transfer curve for a VOA in accordance with the present invention in which a ΔW (delta W) of −0.2 microns demonstrates a shift in transfer curve of approximately +13 volts;

FIG. 12 is a graphic illustration of a transfer curve for a VOA in accordance with the present invention in which a ΔW (delta W) of −0.1 microns demonstrates a shift in transfer curve of approximately +6 volts;

FIG. 13 is a graphic illustration of a transfer curve for a Prior Art symmetric directional coupler VOA;

FIG. 14 is a graphic illustration of a transfer curve for a VOA in accordance with the present invention in which a ΔW (delta W) of +0.1 microns demonstrates a shift in transfer curve of approximately −7 volts; and,

FIG. 15 is a graphic illustration of a transfer curve for a VOA in accordance with the present invention in which a ΔW (delta W) of +0.2 microns demonstrates a shift in transfer curve of approximately −13 volts.

Claims

1. An asymmetric directional coupler formed in an electro-optic material for directing light transmitted into the directional coupler to be coupled from a first optical waveguide to a second optical waveguide at any selected optical power from full coupling to full attenuation comprising: a first optical waveguide having a first propagation constant;a second optical waveguide having a second propagation constant different from the first propagation constant sufficient to cause an accumulated optical phase difference between them;the first and second optical waveguides being disposed in close proximity to each other over a selected length to permit evanescent coupling between them;a first signal electrode associated with the first optical waveguide for receiving a control voltage to determine a percentage of optical coupling between the first and second optical waveguides;a second signal electrode associated with the second optical waveguide for receiving a control voltage approximately complementary to the first electrode control voltage for determining a percentage of optical coupling between the first and second waveguides;

2. An asymmetric directional coupler as defined in claim 1, wherein the first signal electrode and the second signal electrode have independent controllers for creating voltages approximately equal in magnitude but opposite in electrical polarity.

3. An asymmetric directional coupler as defined in claim 2 wherein the directional coupler comprises a variable optical attenuator (VOA) for controlling optical output power to any selected percentage of an operating range including electrode controllers adapted to provide complementary voltages to create any selected percentage of optical coupling.

4. An asymmetric directional coupler as defined in claim 3, wherein the operating range of optical power is a select range less than the total input power, and wherein the operating point is selected to be intermediate the select range such that less than 81% of the total switching voltage is required to obtain a minimum or maximum of the select range.

5. An asymmetric directional coupler as defined in claim 1 wherein the first waveguide and the second waveguide are substantially parallel over a coupling length at a center wavelength.

6. An asymmetric directional coupler as defined in claim 1, wherein the first optical waveguide has a first optical output and the second optical waveguide has a second optical output and the control voltages to the first and second signal electrodes determine the ratio of optical power output from the first and second optical outputs.

7. An asymmetric directional coupler as defined in claim 1 wherein the asymmetry is created by forming the first optical waveguide and the second optical waveguide to have different propagation constants by a difference in: waveguide width, waveguide depth, index of refraction, or index profile.

8. An asymmetric directional coupler as defined in claim 1 wherein the asymmetry is created by an asymmetric position of the first and second signal electrodes relative to the first and second waveguides.

9. An asymmetric directional coupler as defined in claim 1 wherein the asymmetry is created by the first and second electrodes having different widths.

10. An asymmetric coupler as defined in claim 1 wherein the asymmetry is created by a first buffer material structure between the electro-optic material and the first signal electrode and a second different buffer material structure between the electro-optic material and the second signal electrode.

11. An asymmetric directional coupler as defined in claim 1 wherein the asymmetry is created by causing different mechanical stress in the first and second waveguides.

12. An asymmetric directional coupler as defined in claim 11, wherein the mechanical stress differential is created by one or more asymmetric structures selected from: electrode positions; electrode widths and electrode thicknesses of signal and ground electrodes; number of ground electrodes; dielectric material structures, and one or more etched grooves in the substrate.

13. An asymmetric directional coupler as defined in claim 2 further comprising at least one ground electrode at mid-potential of the first and second signal electrodes to reduce the required voltage.

14. An asymmetric directional coupler as defined in claim 3 where the VOA is formed on lithium niobate substrate as an integrated device with a Mach Zehnder interferometer, and wherein the first optical waveguide is optically coupled to an output of the integrated MZ.

15. A variable optical attenuator (VOA) for reducing optical power to any selected optical power from full transmission power to full attenuation, having reduced operating voltage comprising: an asymmetric directional coupler comprising: a first optical waveguide and a second optical waveguide disposed in an electro-optic substrate for evanescent coupling between them;a first signal electrode and a second signal electrode operated in an approximately push-pull configuration for creating an electronic field through the first and second optical waveguides with approximately complementary applied voltages;wherein the first and second optical waveguides each have a different propagation constant over a substantial portion of their length selected to permit a percentage intermediate full transmission and full attenuation of optical power to couple between the first and second optical waveguides when zero voltage is applied,such that approximately complementary voltages applied to the first and second signal electrodes change the percentage of optical power coupled to increase or decrease the percentage from the zero volts condition to couple the selected optical power.

16. A VOA as defined in claim 15 wherein the asymmetry is created by forming the first optical waveguide and the second optical waveguide to have different propagation constants by a difference in: waveguide width, waveguide depth, index of refraction, or index profile.

17. A VOA as defined in claim 15 wherein the asymmetry is created by an asymmetric position of the first and second signal electrodes relative to the first and second waveguides.

18. A VOA as defined in claim 15 wherein the asymmetry is created by first and second signal electrodes causing different mechanical stress in the first and second waveguides.

19. A VOA as defined in claim 18, wherein the mechanical stress differential is created by one or more asymmetric structures selected from: electrode positions, electrode widths, and electrode thicknesses of signal and ground electrodes, number of ground electrodes, dielectric material structures, and one or more etched grooves in the substrate.

20. A VOA as defined in claim 19 wherein the asymmetric structure of one or more etched grooves in the substrate comprises a number of grooves, position of the grooves, groove widths, and groove depths to create the mechanical stress differential.

21. A VOA as defined in claim 15 wherein the difference between propagation constants of the two waveguides varies between zero and one or more non-zero values along the length of the coupler, the net asymmetry being determined by the average difference in propagation constants along the length of the coupler.

22. VOA as defined in claim 21, wherein the variation of the propagation constant is selected from discrete variation and continuous variation.