Diffractive grating coupled whispering gallery mode resonators

1. An optical device, comprising:

an optical resonator structured to support a whispering gallery mode circulating in a plane of the optical resonator; and

a diffractive grating coupler comprising a substrate and a phase-type diffractive grating formed in the substrate, the diffractive grating coupler being located close to the optical resonator to evanescently couple an input optical beam incident at a normal incidence to one side of the substrate of the diffractive grating into the optical resonator located on the other side of the substrate of the diffractive grating,

wherein the diffractive grating is structured to suppress diffracted light along the normal incidence and diffract light of the input optical beam into a first diffraction order at a first side of the normal incidence that matches in phase with light in the whispering gallery mode along a first circulating direction and a second diffraction order at a second side of the normal incidence that matches in phase with light in the whispering gallery mode along a second circulating direction opposite to the first circulating direction to create a standing wave in the whispering gallery mode inside the optical resonator.

2. The device as in claim 1 , wherein:

the optical resonator is a spheroidal resonator.

3. The device as in claim 1 , comprising:

an optical beam shaping device placed in an optical path of the input optical beam to shape the input optical beam at the diffractive grating coupler to assist the phase matching of the first diffraction order and the second diffraction order with the light in the whispering gallery mode in the optical resonator.

4. The device as in claim 3 , wherein:

the optical beam shaping device comprises a GRIN lens.

5. The device as in claim 3 , wherein:

the optical beam shaping device comprises an optical mode expander.

6. The device as in claim 3 , comprising:

a laser located near the optical beam shaping device to direct a laser beam as the optical input beam into the optical beam shaping device and to receive feedback light from the optical beam shaping device and the diffractive grating coupler that is received by the diffractive grating via evanescent light of light in the whispering gallery mode at the surface of the optical resonator, wherein the laser is locked to an optical frequency of the received feedback light.

7. The device as in claim 6 , wherein:

the laser is a semiconductor diode laser.

8. The device as in claim 6 , wherein:

the laser is a distributed feedback laser.

9. The device as in claim 6 , comprising:

a second diffractive grating coupler comprising a second substrate and a second phase-type diffractive grating formed in the second substrate, the second diffractive grating coupler being located close to the optical resonator at a location different from the diffractive grating coupler and operable to evanescently couple light out of the optical resonator to produce an output optical beam; and

an optical detector that receives the output optical beam from the second diffractive grating coupler to convert the received output optical beam into an RF signal.

10. A method for providing optical coupling for a whispering gallery mode optical resonator, comprising:

placing a diffractive grating coupler close to an optical resonator which supports a whispering gallery mode circulating in a plane of the optical resonator to effectuate optical evanescent coupling therebetween, wherein the diffractive grating coupler comprises a substrate and a phase-type diffractive grating formed in the substrate and evanescently couples input light incident at a normal incidence to one side of the substrate of the diffractive grating into the optical resonator located on the other side of the substrate, and the diffractive grating is structured to suppress diffracted light along the normal incidence and diffract light of the input optical beam into a first diffraction order at a first side of the normal incidence that matches in phase with light in the whispering gallery mode along a first circulating direction and a second diffraction order at a second side of the normal incidence that matches in phase with light in the whispering gallery mode along a second circulating direction opposite to the first circulating direction; and

directing an input optical beam at an incident direction into the diffractive grating coupler to couple light into the optical resonator.

11. The method as in claim 10 , wherein:

the incident direction is normal to the substrate of the diffractive grating coupler to diffract light of the input optical beam into the first diffraction order that matches in phase with light in the whispering gallery mode along the first circulating direction and the second diffraction order that matches in phase with light in the whispering gallery mode along the second circulating direction to create a standing wave in the whispering gallery mode inside the optical resonator.

12. The method as in claim 10 , comprising:

using an optical fiber to guide the input optical beam into the diffractive grating coupler.

13. The method as in claim 12 , wherein:

the optical fiber includes a fiber core that has a fiber core grating section.

14. The method as in claim 13 , wherein:

the phase-type diffractive grating covers an area of an end facet of the optical fiber over the fiber core and a surrounding area of the end facet of the optical fiber over fiber cladding outside the fiber core.

15. The device as in claim 1 , comprising:

an optical fiber placed in an optical path of the input optical beam to guide and direct the input optical beam to the diffractive grating coupler.

16. The device as in claim 15 , wherein:

the optical fiber includes a fiber core that has a fiber core grating section.

17. The device as in claim 16 , wherein:

the phase-type diffractive grating covers an area of an end facet of the optical fiber over the fiber core and a surrounding area of the end facet of the optical fiber over fiber cladding outside the fiber core.