Tunable optical filter based on a physically-deformable diffractive element

1. A tunable optical filter, comprising: an elastic

substrate having a major surface;

a diffractive element defined in the major surface of the elastic substrate, the diffractive element having a pitch;

actuator means for stretching the elastic substrate to control the pitch of the diffractive element;

a first optical path disposed non-parallel to the major surface; and

a second optical path optically coupled to the first optical path by the diffractive element at an optical frequency determined by the pitch of the diffractive element.

2. The tunable optical filter of claim 1 , additionally comprising a third optical path optically coupled to the first optical path by the diffractive element at an optical frequency different from that at which the first optical path is optically coupled to the second optical path at a given pitch of the diffractive element.

3. The tunable optical filter of claim 1 , additionally comprising a controller connected to control the actuator means.

4. The tunable optical filter of claim 3 , additionally comprising a feedback arrangement coupled to control to the controller.

5. The tunable optical filter of claim 1 , in which the actuator means includes a piezoelectric chip comprising a major surface to which the elastic substrate is bonded.

6. The tunable optical filter of claim 1 , in which the actuator means includes a piezoelectric chip mechanically coupled to the elastic substrate.

7. The tunable optical filter of claim 1 , in which the actuator means includes a linear motor mechanically coupled to the elastic substrate.

8. The tunable optical filter of claim 7 , in which the linear motor is an electrostatic linear motor.

9. The tunable optical filter of claim 7 , in which the linear motor is an electromagnetic linear motor.

10. The tunable optical filter of claim 1 , in which:

the actuator means includes a rotary motor and a rotary-to-linear motion converter;

the rotary motor provides motive power to the motion converter; and

the motion converter includes a linear motion output mechanically coupled to the elastic substrate.

11. The tunable optical filter of claim 1 , in which the actuator means includes:

a thermally-expansive element mechanically coupled to the elastic substrate; and

a heating element thermally coupled to the thermally-expansive element.

12. The tunable optical filter of claim 11 , in which:

the thermally-expansive element includes a major surface; and

the elastic substrate is bonded to the major surface of the thermally-expansive element.

13. The tunable optical filter of claim 11 , in which the heating element is integral with the thermally-expansive element.

14. The tunable optical filter of claim 13 , in which the elastic substrate is additionally integral with the thermally-expansive element.

15. The tunable optical filter of claim 1 , in which the second optical path is angularly separated from the first optical path.

16. A tunable optical filter, comprising:

an elastic substrate;

a diffractive element in the elastic substrate, the diffractive element having a pitch;

actuator means for stretching the elastic substrate to control the pitch of the diffractive element;

a first optical path;

a second optical path optically coupled to the first optical path by the diffractive element at an optical frequency determined by the pitch of the diffractive element; and

a feedback arrangement coupled to control to the controller, the feedback arrangement comprising:

a light source arranged proximate to the first optical pat to illuminate the diffractive element with a reference beam of light;

a light detector array arranged proximate to the second optical path to receive the reference beam diffracted by the diffractive element; and

an electrical connection from the light detector array to the controller.

17. A tunable optical filter, comprising:

an elastic substrate;

a diffractive element in the elastic substrate, the diffractive element having a pitch;

actuator means for stretching the elastic substrate to control the pitch of the diffractive element;

a first optical path;

a second optical path optically coupled to the first optical path by the diffractive element at an optical frequency determined by the pitch of the diffractive element; and

a beam reverser and selector located to receive light diffracted by the diffractive element as received light and structured to select an angular range of the received light as selected light and to return the selected light to the diffractive element for diffraction into the output optical path.

18. The tunable optical filter of claim 17 , additionally comprising a half-wave plate located between the beam reverser and selector and the diffractive element.

19. The tunable optical filter of claim 17 , in which the beam reverser and selector includes:

a corner reflector comprising two reflective surfaces; and

a slit located between the reflective surfaces.

20. The tunable optical filter of claim 19 , in which the beam reverser and selector additionally includes:

a first converging element structured to focus the received light on the slit; and

a second converging element structured to collimate the selected light diverging from the slit.

21. The tunable optical filter of claim 17 , in which the beam reverser and selector includes a substantially U-shaped length of optical fiber.

22. The tunable optical filter of claim 21 , in which the beam reverser and selector additionally includes:

a first converging element structured to focus the received light on the optical fiber, and

a second converging element structured to collimate the selected light diverging from the optical fiber.