Method and apparatus for fiber delivery of high power laser beams

Patent metadata

Field	Value
Publication number	US-9535211-B2
Application number	US-201113308789-A
Country	US
Kind code	B2
Filing date	Dec 1, 2011
Priority date	Dec 1, 2011
Publication date	Jan 3, 2017
Grant date	Jan 3, 2017

Abstract

In various embodiments, an optical fiber includes a core having a relatively large area selected so as to raise a threshold of stimulated Raman scattering or stimulated Brillouin scattering, or both, the core having a high aspect ratio elongated cross-section and having a first refractive index. The core is narrower in a fast-axis direction and wider in a slow-axis direction, such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

First claim

What is claimed:

1. An optical fiber, comprising:
a core having a high aspect ratio elongated cross-section and having a first refractive index;
first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; and
a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

2. The optical fiber of claim 1 , wherein a difference between the first refractive index of the core and the third refractive index of the third cladding is less than about 500 ppm to provide a loss for any higher-order transverse electromagnetic modes that is greater than a loss for lower-order transverse electromagnetic modes so as to substantially remove the higher-order transverse electromagnetic modes along the slow-axis direction within the fiber.

3. The optical fiber of claim 1 , wherein a width of the core along the slow-axis direction is selected so as to be greater than a width of a laser beam input into the fiber.

4. The optical fiber of claim 1 , wherein a width of the core exceeds a width of a diffracted laser beam at an output end of the fiber.

5. An optical fiber, comprising:
a core having a high aspect ratio elongated cross-section and having a refractive index;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; and
wherein a width w b of a laser beam input into the fiber is such that a Fresnel length L Fr that is proportional to w b 2 n/λ exceeds a fiber length L, where n is the refractive index of the core and λ is a wavelength of the laser beam input into the fiber.

6. An optical fiber comprising:
a core having a high aspect ratio elongated cross-section and having a first refractive index;
first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; and
a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; and
wherein the third refractive index of the third cladding is selected so that a numerical aperture of the third cladding at an outside boundary surface is less than 0.06 so as to allow laser radiation leaking into the third cladding to leak out of the third cladding.

7. An optical fiber comprising:
a core having a high aspect ratio elongated cross-section and having a first refractive index;
first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index;
a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index; and
a collar attached to the third cladding, wherein a fourth refractive index of the collar is essentially equal to the third refractive index of the third cladding;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

8. An optical fiber comprising:
a core having a high aspect ratio elongated cross-section and having a first refractive index;
first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index;
a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index; and
a coating covering the third cladding, wherein the coating is configured to be transparent to laser radiation leaking into the third cladding;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

9. The optical fiber of claim 1 , wherein the third cladding substantially surrounds the core and the first and second signal claddings.

10. The optical fiber of claim 1 , wherein the fiber is configured to transmit a laser beam having a power greater than or equal to about 10 kW.

11. The optical fiber of claim 1 , wherein the core has a substantially rectangular shape elongated in the slow-axis direction.

12. The optical fiber of claim 1 , wherein the fiber has a substantially rectangular external shape elongated in the slow-axis direction so as to allow mechanical flexing in the fast-axis direction and to resist mechanical flexing in the slow-axis direction.

13. An optical fiber, comprising:
a core having a high aspect ratio elongated cross-section and having a refractive index; and
a twisted portion occurring over a fiber length near an output end of the fiber, wherein the fiber length near the output end of the fiber is shorter than a characteristic distortion length that is related to a twist-induced focal length;
wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

14. The optical fiber of claim 13 , wherein:
the characteristic distortion length L dist is proportional to w core /δθ F ;
δθ F =(w b /2F) is a twist-induced divergence angle;
w core is a width of the core;
w b is a width of an input laser beam; and
F=δL/φ 2 is the twist-induced focal length associated with a twisting angle φ.

15. The optical fiber of claim 1 , wherein a beam divergence is conserved from an input end of the fiber to an output end of the fiber.

16. An optical fiber, comprising:
a core having a high aspect ratio elongated cross-section and having a refractive index, wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; and
a bent portion bent in the fast-axis direction and a twisted portion, wherein the twisted portion is distinct from the bent portion so as to maintain substantially a same beam quality throughout the fiber.

17. The optical fiber of claim 16 , wherein the optical fiber further comprises:
first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; and
a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index.

18. An optical fiber, comprising:
a core having a high aspect ratio elongated cross-section and having a refractive index, wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis

Assignees

• Rockwell David A
• Mulroy James Randolph
• Shkunov Vladimir V
• Raytheon Co

Inventors

• Rockwell David A
• Mulroy James Randolph
• Shkunov Vladimir V

Classifications

• G02B2006/02161

  Grating written by radiation passing through the protective fibre coating · CPC title

• G02B6/32

  having lens focusing means {positioned between opposed fibre ends (with lens being an integral part of the single fibre end G02B6/262)} · CPC title

• G02B6/4208

  using non-reciprocal elements or birefringent plates, i.e. quasi-isolators (optical isolators per se G02F1/093, G02F1/0955) · CPC title

• G02B6/02009 Primary

  Large effective area or mode field radius, e.g. to reduce nonlinear effects in single mode fibres · CPC title

• G02B6/02138

  based on illuminating a phase mask · CPC title