Mode scrambling apparatus for multimode fiber

1. A multimode optical fiber device, comprising:

a multimode fiber that supports light in two or more optical waveguide modes;

a plurality of fiber squeezers engaged to the multimode fiber at different locations along a lengthwise direction of the multimode fiber to squeeze the multimode fiber along two or more different squeezing directions perpendicular to the lengthwise direction, the fiber squeezers responsive to respective control signals to modulate a degree of squeezing at selected squeezing frequencies, respectively, to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber; and

a control unit that produces the respective control signals to the fiber squeezers, respectively, and controls the fiber squeezers to produce either two or more different degrees of squeezing or to squeeze at two or more different selected squeezing frequencies.

2. The device as in claim 1 , wherein:

each fiber squeezer comprises a piezo-electric transducer that responds to a respective control signal to squeeze the multimode fiber.

3. The device as in claim 2 , wherein:

wherein the piezo-electric transducer comprises a stack of piezo-electric elements.

4. The device as in claim 1 , wherein:

two adjacent fiber squeezers are oriented to squeeze the multimode fiber at two different squeezing directions.

5. The device as in claim 4 , wherein:

the two different squeezing directions of the two adjacent fiber squeezers are at 45 degrees to each other.

6. The device as in claim 1 , wherein:

each selected squeezing frequency ranges from 100 Hz to 10000 Hz.

7. The device as in claim 6 , wherein:

each selected squeezing frequency ranges from 1000 Hz to 10,000 Hz.

8. The device as in claim 1 , wherein:

the fiber squeezers are operated at different selected squeezing frequencies, respectively.

9. The device as in claim 1 , wherein:

a contact surface of each fiber squeezer in contact with the multimode fiber has a surface roughness less than 0.1 mm.

10. The device as in claim 1 , wherein:

the plurality of the fiber squeezers include at least four fiber squeezers.

11. A method for scrambling light in different optical waveguide modes in a multimode optical fiber, comprising:

providing a multimode fiber that supports light in two or more different optical waveguide modes;

engaging a plurality of fiber squeezers to the multimode fiber at different locations along a lengthwise direction of the multimode fiber to squeeze the multimode fiber along two or more different squeezing directions perpendicular to the lengthwise direction; and

controlling the fiber squeezers to modulate a degree of squeezing at selected squeezing frequencies, respectively, to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber.

12. The method as in claim 11 , comprising:

controlling the fiber squeezers to produce either two or more different degrees of squeezing or to squeeze at two or more different selected squeezing frequencies.

13. The method as in claim 11 , comprising:

orienting two adjacent fiber squeezers to squeeze the multimode fiber at two different squeezing directions.

14. A multimode optical fiber device, comprising:

at least one linear actuator responsive to a control signal to cause a dimensional change along a straight line;

a fiber stretcher frame comprising at least one expandable slot that is engaged to the actuator so that the dimensional change along the straight line is across the expandable slot to change a width of the expandable slot, the fiber stretcher frame structured to amplify the dimensional change of the actuator to produce an amplified change in a circumference of an exterior surface of the fiber stretcher frame;

a multimode fiber that supports light in two or more optical waveguide modes and wraps around the exterior surface of the fiber stretcher frame to form a fiber loop; and

a control unit that produces a control signal to the linear actuator and controls the linear actuator to modulate a degree of stretching of the fiber loop at a selected frequency to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber.

15. The device as in claim 14 , wherein:

the linear actuator is a piezo-electric transducer that responds to the control signal to change a dimension.

16. The device as in claim 14 , wherein:

the fiber stretcher frame is partially divided into a first frame part and a second frame part by the expandable slot, wherein the first frame part and the second frame part are integral to each other and are connected at a pivot point around which the first and second frame parts move relative to each other under an operation of the actuator.

17. The device as in claim 16 , comprising:

a first actuator anchor on the first frame part to hold a first end of the actuator;

a second actuator anchor on the second frame part to hold a second end of the actuator and spaced from the first actuator anchor to exert an initial load on the first and second frame parts to set an initial width of the expandable slot which produces an initial stretch on the fiber loop,

wherein the actuator is operable to expand or contract to increase or decrease the initial stretch on the fiber loop, respectively.

18. The device as in claim 17 , comprising:

a screw mounted to the first actuator anchor and engaged to press against the first end of the actuator, the screw being adjustable to change a position of the actuator relative to the first actuator anchor to change the initial stretch.

19. A method for scrambling light in different optical waveguide modes in a multimode optical fiber, comprising:

providing a multimode fiber that supports light in two or more different optical waveguide modes; and

modulating a dimension of at least one linear transducer to cause spatial disturbances at multiple locations in the multimode fiber to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber.

20. The method as in claim 19 , comprising:

controlling the modulating the dimension of the at least one linear transducer at a modulation frequency from 100 Hz to 10000 Hz.