CLADDING MODE SPATIAL FILTER

• US 20120262938A1
• Filed: 12/28/2011
• Published: 10/18/2012
• Est. Priority Date: 04/15/2011
• Status: Active Grant

• Current Assignee: Nlight Inc.
• Original Assignee: nLIGHT Corporation
• Inventors: Price, R. Kirk, Karlsen, Scott, Martinsen, Rob, Mehl, Ronii C.
• Granted Patent: US 9,494,739 B2
• Time in Patent Office: Days
• US Class Current: 362/553
• CPC Class Codes: G02B 6/262 Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
• CPC Class Codes: H01S 5/005 Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping H01S5/026, H01S5/18388 take precedence

Abstract

A laser system can include an optical fiber having a spatial filter defined as a core extension coupled to or integrally formed in an optical fiber so as to reduce the coupling of optical radiation into a fiber cladding. Such a core extension can be formed by removing a length of the cladding from the optical fiber, leaving a portion of the core exposed at the end of the fiber. Alternatively, a core extension can be formed by coupling an end cap to the core of the optical fiber at a fiber end surface. By selecting a length of the core extension based on a beam divergence and beam diameter, radiation coupling into the fiber core can be reduced.

Claims

1. An optical illumination apparatus, comprising:

an optical radiation source configured to produce an optical beam having at least one of a beam diameter D_beam or numerical aperture NA_beam configured for coupling to an optical fiber;

a section of optical fiber that includes a core and a cladding that extend to a first fiber end, the optical fiber having a core diameter D_core, a cladding diameter D_clad, and a numerical aperture NA_fiber; and

a core extension optically coupled to the core at the first fiber end and extending from the first fiber end a distance of at least D_clad/2NA_fiber or at least D_clad/NA_beam, wherein an input surface of the core extension is situated so as to receive the optical beam.

2. The apparatus according to claim 1, wherein the core extension comprises a cladding-stripped region at the first fiber end, wherein substantially all of the cladding is removed from the cladding-stripped region.

3. The apparatus according to claim 2, wherein the cladding tapers so as to define the core extension.

4. The apparatus according to claim 2, further comprising a cladding terminal surface at the core extension that is substantially perpendicular to a longitudinal axis of the optical fiber.

5. The apparatus according to claim 4, further comprising a cladding terminal surface at the core extension that is tilted at an angle of between about 45 degrees and 90 degrees with respect to a longitudinal axis of the optical fiber.

6. The apparatus according to claim 1, wherein the core extension comprises a reduced-cladding region at the first fiber end, wherein a portion of the cladding is removed along the reduced-cladding region.

7. The apparatus according to claim 6, wherein a reduced cladding thickness is less than about 1/10 of a cladding diameter D_clad.

8. The apparatus according to claim 7, wherein the reduced cladding thickness is less than about 1/20 of the cladding diameter D_clad.

9. The apparatus according to claim 1, wherein the core extension comprises an end cap secured to at least the core of the optical fiber.

10. The apparatus according to claim 9, wherein the diameter of the end cap is substantially equal to the diameter of the core of the optical fiber at the first fiber end.

11. The apparatus according to claim 1, wherein the optical fiber is a single mode optical fiber for a wavelength λ

, and the core extension extends from the first fiber end a distance of at least D_coreD_clad/λ

.

12. The apparatus according to claim 11, wherein the core extension extends from the first fiber end a distance of at least 2, 5, 10, or 50 times D_core D_clad/λ

.

13. The apparatus according to claim 1, wherein the optical fiber is a multimode optical fiber, and the core extension extends from the first fiber end a distance of at least 2, 5, or 10 times (D_clad−

D_core)/2NA_fiber.

14. The apparatus according to claim 1, wherein the optical fiber is a multimode optical fiber, and the core extension extends from the first fiber end a distance of at least 2, 5, or 10 times D_clad/2NA_fiber.

15. The apparatus according to claim 1, wherein the core diameter is between about 8 and 500 μ

m, and the cladding diameter is between about 50 μ

m and 2 mm.

16. A system for delivering optical radiation to a target, comprising:

an optical radiation source;

a beam shaping optical system configured to produce an optical beam having a beam diameter and a beam numerical aperture;

a core-extended optical fiber situated to receive the optical beam at a core extension, wherein a length of the core extension is greater than a distance corresponding to a ratio of a cladding diameter of the core-extended optical fiber to the beam numerical aperture or a ratio of a cladding diameter of the core-extended optical fiber to twice a fiber numerical aperture.

17. The system of claim 16, wherein the optical radiation source includes at least one laser diode array, and the beam shaping optical system is configured to produce a combined optical beam.

18. The system of claim 17, wherein the length of the core-extended optical fiber is greater than 2, 5, 10, or 20 times the distance corresponding to a ratio of a cladding diameter of the core-extended optical fiber to the beam numerical aperture or a ratio of a cladding diameter of the core-extended optical fiber to twice a fiber numerical aperture.

19. The system of claim 16, wherein the core extension comprises a cladding-thinned portion of the optical fiber.

20. The system of claim 16, wherein the core extension comprises a cladding stripped portion of the optical fiber.

21. The system of claim 16, wherein the core extension comprises an optically transmissive end cap secured to the optical fiber.

22. A method, comprising:

providing an optical beam having a beam numerical aperture and a beam diameter;

directing the optical beam to an input surface of a core-extension on an optical fiber so as to couple the optical beam into a fiber core and so that the optical beam at an exit surface of the core-extension has a diameter that is greater than a fiber cladding diameter; and

directing the optical beam to a target using the optical fiber.

23. The method of claim 22, further comprising providing the optical beam so as to have a beam numerical aperture and a beam diameter corresponding to at least one of a core diameter and a numerical aperture of the optical fiber.

24. The method of claim 22, wherein a length of the core extension is greater than a distance corresponding to a ratio of a cladding diameter of the core-extended optical fiber to the beam numerical aperture or a ratio of a cladding diameter of the core-extended optical fiber to twice a fiber numerical aperture.