received by the International Bureau on 30 July 2019 (30.07.2019)

1. Apparatus for laser processing a material, which apparatus comprises an optical fibre (2), at least one squeezing mechanism (3), and a lens (4),

wherein;

• the optical fibre (2) is a multimode optical fibre;

• the optical fibre (2) is such that laser radiation is able to propagate along the optical fibre in a first optical mode (21) and in a second optical mode (22);

• the squeezing mechanism (3) comprises at least one periodic surface (6)

defined by a pitch (7); and

• the periodic surface (3) is located adjacent to the optical fibre;

and the apparatus is characterized in that;

• the pitch (7) couples the first optical mode (21) and the second optical mode (22) together;

• the first optical mode (21 ) is defined by a first mode order (24), and the second optical mode (22) is defined by a second mode order (25) which is higher than the first mode order (24);

• the squeezing mechanism (3) is configured to squeeze the periodic surface (6) and the optical fibre (2) together with a squeezing force (12), thereby coupling the first optical mode to the second optical mode;

• the lens (4) is defined by a front focal plane ( 14) and a rear focal plane (15);

• the first optical mode (21) is defined by a Rayleigh length, wherein the

Rayleigh length is defined as the distance from the distal end (16) of the optical fibre (2) to the plane where the radius of the first optical mode (21) has increased by a factor of the square root of two; and

• the lens (4) is located within two of the Rayleigh lengths from the distal end (16) of the optical fibre (2).

2. Apparatus according to claim 1 wherein the lens (4) is positioned such that a distal end (16) of the optical fibre is located at the front focal plane (14).

3. Apparatus according to claim 1 or claim 2 and including an optical lens arrangement (50) configured to image the rear focal plane (15) of the lens onto or near a surface of the material.

4. Apparatus according to claim 1 or claim 2 and including an optical lens arrangement (50) configured to image the rear focal plane in a plane between the optical lens arrangement and the surface of the material.

5. Apparatus according to claim 1 or claim 2 and including an optical lens arrangement (50) configured to image the rear focal plane beyond the surface of the material

6. Apparatus according to any one of the preceding claims wherein the lens (4)

comprises a graded index lens.

7. Apparatus according to any one of the preceding claims wherein the lens (4) is formed on the distal end (16) of the optical fibre (2).

8. Apparatus according to any one of the preceding claims wherein the squeezing

mechanism (3) is configured for switching the laser radiation from the first optical mode (21) to the second optical mode (22) by varying the squeezing force (12),

9. Apparatus according to any one of the preceding claims wherein the squeezing

mechanism (3) comprises an actuator (55).

10. Apparatus according to any one of the preceding claims wherein the squeezing

mechanism (3) comprises at least two of the periodic surfaces (6) arranged at an angle to each other.

11. Apparatus according to claim 10 wherein the periodic surfaces (6) have the same pitch.

12. Apparatus according to claim 10 or claim 1 1 wherein the angle is a right angle.

13. Apparatus according to claim 1.0 or claim 1 1 wherein the angle is sixty degrees.

14. Apparatus according to any one of claims 10 to 13 wherein the squeezing mechanism (3) is configured such that one of the periodic surfaces (6) is able to be squeezed against the optical fibre with a different squeezing force (12) than another of the periodic surfaces.

15. Apparatus according to any one of claims 10 - 14 wherein spatial phases of the

periodic surfaces (6) are configured such that the optical fibre (2) is deformed in a helical manner when the squeezing forces (12) are applied.

16. Apparatus according to any one of the preceding claims wherein the squeezing

mechanism (3) is configured such that the optical fibre is able to be pulled through the squeezing mechanism with a force less than IN.

17. Apparatus according to any one of the preceding claims wherein the first optical mode (21) is a fundamental LPQI mode of the optical fibre, and the second optical mode (22) has an azimuthal mode number of at least three. I S, Apparatus according to claim 17 wherein the second optical mode (22) has an azimuthal mode number between three and nine.

19- Apparatus according to any one of the preceding claims wherein the pitch (7) is a variable pitch which is chirped along the length of the periodic surface (6).

20. Apparatus according to claim 19 wherein the optical fibre supports a third optical mode (23), and the variable pitch has a first pitch and a second pitch, wherein the first pitch couples the first optical mode (21) and the second optical mode (22) together, and the second pitch couples the second optical mode (22) and the third optical mode (23) together.

21. Apparatus according to claim 20 wherein the third optical mode (23) has a higher azimuthal mode number than the azimuthal mode number of the second optical mode (22).

22. Apparatus according to claims 20 or claim 21 wherein the second pitch is longer than the first pitch.

23. Apparatus according to any one of claims 20 to 21 wherein the squeezing mechanism (3) is orientated such that the first pitch receives the laser radiation before the second pitch.

24. Apparatus according to any one of the preceding claims and including a second

squeezing mechanism (129), and in which the second squeezing mechanism is positioned between the squeezing mechanism (3) and the distal end (16) of the optical fibre (2).

25. Apparatus according to any one of the preceding claims wherein the lens (4) is a negative lens.

26. Apparatus according to any one of the preceding claims wherein the optical fibre (2) comprises a core (31) that is substantially homogeneous, thereby avoiding unintentional mode coupling between the optical modes.

27. Apparatus according to any one of the preceding claims and including a laser (1), and wherein the laser is connected to the optical fibre (2).

28. A method for laser processing a material, which method comprises:

• providing a laser (1), an optical fibre (2), at least one squeezing mechanism (3), and a lens (4);

• connecting the laser (1 ) to the optical fibre (2);

causing the laser (1) to emit laser radiation (13); and • propagating the laser radiation (13) along the optical fibre (2) in a first optical mode (21);

wherein

• the optical fibre (2) is a multimode optical fibre;

• the squeezing mechanism (3) comprises at least one periodic surface (6)

defined by a pitch (7); and

• the periodic surface (6) is located adjacent to the optical fibre (2); and and the method is characterized in that:

• the first optical mode (21 ) is defined by a first mode order (24), and the second optical mode (22) is defined by a second mode order (25) which is higher than the first mode order (24);

• the squeezing mechanism (3) is configured to squeeze the periodic surface (6) and the optical fibre (2) together with a squeezing force ( 12), thereby coupling the first optical mode (21 ) to the second optical mode (22);

• the lens (4) is defined by a front focal plane (14) and a rear focal plane (15);

• the first optical mode (21 ) is defined by a Rayleigh length;

• the lens (4) is located within two of the Rayleigh lengths from a distal end (16) of the optical fibre (2), wherein the Rayleigh length is defined as the distance from the distal end (16) of the optical fibre (2) to the plane where the radius of the first optical mode (21) has increased by a factor of the square root of two; and

• the squeezing force ( 12) is adjusted in order to couple the first optical mode (21) to the second optical mode (22); and

• laser processing the material with the laser radiation (13).

29. A method according to claim 28 wherein the lens (4) is positioned such that the distal end (16) of the optical fibre (2) is located at the front focal plane (14).

30. A method according to claim 28 or 29 wherein the lens (4) comprises a graded index lens.

31. A method according to any one of claims 28 to 30 wherein the lens (4) is formed on the distal end (16) of the optical fibre (2).

32. A method according to any one of claims 28 to 31 wherein the squeezing mechanism (3) includes an actuator (55).

33. A method according to claim 32 and including the step of providing a computer (75), and controlling the actuator (55) with the computer (75).

34. A method according to claim 33 wherein the computer (75) comprises a memory (76) comprising information concerning material parameters.

35. A method according to any one of claims 28 to 34 and including the step of providing an optical lens arrangement (50) that comprises a focusing lens 53, and imaging the rear focal plane ( 15) onto the material.

36. A method according to claim 35 and including the step of selecting the first optical mode (21) by adjusting the squeezing mechanism (3), and piercing the material with the laser (1),

37. A method according to claim 36 wherein the first optical mode (21) is a fundamental mode of the optical fibre (2).

38. A method according to any of claims 34 to 37 and including the step of selecting the second optical mode (22) by adjusting the squeezing mechanism (3), and cutting the material with the laser (1).

39. A method according to any one of claims 34 to 38 wherein the second optical mode (22) has an azimuthal mode number of at least three, and a radial mode number of at least one,

40. A method according to any one of claims 34 to 39 and including the steps of selecting a top hat profile and cutting the material with the laser (1).

41. A method according to any one of claims 28 to 40 and including the steps of

providing an optical lens arrangement (50), and imaging the rear focal plane (15) using the optical lens arrangement such that the image forms between the optical lens arrangement and the material, and optimizing the spot size and the intensity profile of the image by adjusting the squeezing force (12).

42. A method according to claim 41 and including the step of welding the material with the laser (1).

43. A method according to claim 42 and including the step of sintering the material with the laser (I ), wherein the material prior to sintering is in the form of a metallic powder.

44. A method according to claim 41 and including the step of drilling the material with the laser (l).

45. A method of cutting a material using the apparatus of claim 1 comprising providing a laser (1), coupling laser radiation (13) from the laser (1) into the optical fibre (2); focusing the laser radiation (13) onto the material with an optical lens arrangement (50) and selecting a Gaussian profile to pierce the material and atop hat profile to cut the material

46. A method of welding a material comprising using the apparatus of claim 1 , providing a laser (1), coupling laser radiation (13) from the laser (1) into the optical fibre (2); projecting the laser radiation (13) using an optical lens arrangement (50) away from focus and using the apparatus of claim 1 to vary the working spot size to optimize the weld process by variation of the spot size and profile.