Low-loss waveguiding structures, in particular modulators

1. An optical modulator comprising:

a first waveguide arm for transmitting at least a first portion of a beam of light, the first waveguide arm comprising a first single mode waveguide section and a first multimode waveguide section;

a first set of electrodes for modulating the beam of light; and

an adiabatic taper coupling the first single mode waveguide section and the first multimode waveguide section;

wherein at least a portion of the first multimode waveguide section is disposed adjacent to the first set of electrodes; and

wherein the first single mode waveguide section and the first multimode waveguide section comprise Lithium Niobate or Lithium Tantalate.

2. An optical modulator comprising

a first waveguide arm for transmitting at least a first portion of a beam of light, the first waveguide arm comprising a first single mode waveguide section and a first multimode waveguide section;

a first set of electrodes for modulating the beam of light; and

an adiabatic taper coupling the first single mode waveguide section and the first multimode waveguide section;

wherein at least a portion of the first multimode waveguide section is disposed adjacent to the first set of electrodes;

wherein the first waveguide arm further comprises a second multimode waveguide section coupled to the first multimode waveguide section by a second single mode waveguide section; and

further comprising a second set of electrodes adjacent to at least a portion of the second multimode waveguide section.

3. The optical modulator according to claim 2 , wherein the second single mode waveguide section comprises a single-mode waveguide bend in the first waveguide arm between the first multimode waveguide section and the second multimode waveguide section.

4. The optical modulator according to claim 3 , wherein the single-mode waveguide bend comprises a substantially 180° bend.

5. The optical modulator according to claim 3 , wherein the single-mode waveguide bend comprises a substantially 180° Euler bend.

6. An optical modulator comprising:

a first waveguide arm for transmitting at least a first portion of a beam of light, the first waveguide arm comprising a first single mode waveguide section and a first multimode waveguide section;

a first set of electrodes for modulating the beam of light; and

an adiabatic taper coupling the first single mode waveguide section and the first multimode waveguide section;

wherein at least a portion of the first multimode waveguide section is disposed adjacent to the first set of electrodes;

wherein the first single mode waveguide section and the first multimode waveguide section comprise an electro-optic material with an electro-optic constant>10 pm/V.

7. The optical modulator according to claim 1 , wherein the first single mode waveguide section comprises a cross sectional area<3 μm 2 .

8. The optical modulator according to claim 1 , wherein the first single mode waveguide section comprises a cross sectional area<1 μm 2 .

9. The optical modulator according to claim 1 , where the first multimode waveguide section comprises a cross sectional area>0.2 μm 2 and <10 μm 2 .

10. The optical modulator according to claim 1 , wherein the first single mode waveguide section is configured to only support one TE mode and one TM mode with optical propagation loss<0.6 dB/cm, and optical propagation loss>ldB/cm for higher modes.

11. The optical modulator according to claim 1 , wherein the first multimode waveguide section supports more than one TE mode and more than one TM mode with optical propagation loss<0.6 dB/cm for both TE and TM modes.

12. The optical modulator according to claim 1 , further comprising:

a splitter configured to split the beam of light into a first sub-beam and a second sub-beam;

a second waveguide arm coupled to the splitter for transmitting the second sub-beam, wherein the first waveguide arm is coupled to the splitter for transmitting the first sub-beam; and

a combiner configured for combining the first sub-beam and the second sub-beam to form a modulated output beam;

wherein the second waveguide arm comprises a second single mode waveguide section and a second multimode waveguide section; and

wherein at least a portion of the second multimode waveguide section is disposed adjacent to the first set of electrodes.

13. The optical modulator according to claim 12 , wherein the first waveguide arm includes a third multimode waveguide section, and a first single mode waveguide bend between the first multimode waveguide section and the third multimode waveguide section; and

wherein the second waveguide arm includes a fourth multimode waveguide section, and a second single mode waveguide bend between the second multimode waveguide section and the fourth multimode waveguide section.

14. The optical modulator according to claim 13 , further comprising:

a second set of electrodes adjacent to at least a portion of the third multimode waveguide section and a portion of the fourth multimode waveguide section.

15. An optical modulator comprising:

a first waveguide arm for transmitting at least a first portion of a beam of light, the first waveguide arm comprising a first single mode waveguide section and a first multimode waveguide section;

a first set of electrodes for modulating the beam of light;

an adiabatic taper coupling the first single mode waveguide section and the first multimode waveguide section;

a bus waveguide optically coupled to an input port at one end and to an output port at an opposite end; and

a coupler coupled along the bus waveguide and configured to split the beam of light into a first sub-beam and a second sub-beam;

wherein at least a portion of the first multimode waveguide section is disposed adjacent to the first set of electrodes

wherein the first waveguide arm comprises a ring resonator coupled to the coupler for transmitting the first sub-beam, and recombining the first sub-beam back into the bus waveguide;

wherein the ring resonator includes curved sections and substantially straight or less curved sections;

wherein at least a portion of some of the curved sections comprise the first single mode waveguide section; and

wherein one of the substantially straight or less curved sections comprise the first multimode waveguide section.

16. The optical modulator according to claim 15 , wherein another one of the substantially straight or less curved sections comprises a second multimode waveguide section disposed adjacent to the first set of electrodes.

17. The optical modulator according to claim 1 , wherein the adiabatic taper is configured to maintain the at least a first portion of the beam of light traversing the adiabatic taper in fundamental TE or TM modes.

18. The optical modulator according to claim 1 , wherein the adiabatic taper is configured to include at least one of linear tapering, cubic tapering, exponential tapering, and nonlinear tapering, of waveguide width.

19. An optical modulator comprising:

a first waveguide arm for transmitting at least a first portion of a beam of light, the first waveguide arm comprising a first single mode waveguide section and a first multimode waveguide section;

a first set of electrodes for modulating the beam of light; and

an adiabatic taper coupling the first single mode waveguide section and the first multimode waveguide section;

wherein at least a portion of the first multimode waveguide section is disposed adjacent to the first set of electrodes;

wherein the adiabatic taper is configured to adiabatically convert a single mode of the first single mode waveguide section to and from a fundamental TE or TM mode of the first multimode waveguide section.